insulator semiconductor (mis) nanojunction structures for...

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Nano Energy (2015) 12, 347–373

http://dx.doi.org/12211-2855/& 2015 E

nCorresponding auBandar Sunway, Sela

E-mail address: C

REVIEW

Prospects of metal–insulator–semiconductor(MIS) nanojunction structures for enhancedhydrogen evolution in photoelectrochemicalcells: A review

Tao Zhua, Meng Nan Chonga,b,n

aSchool of Engineering, Chemical Engineering Discipline, Monash University Malaysia, Jalan LagoonSelatan, Bandar Sunway, Selangor Darul Ehsan 46150, MalaysiabSustainable Water Alliance, Advanced Engineering Platform, Monash University Malaysia, Jalan LagoonSelatan, Bandar Sunway, Selangor Darul Ehsan 46150, Malaysia

Received 19 September 2014; received in revised form 21 December 2014; accepted 1 January 2015Available online 9 January 2015

KEYWORDSH2 evolution;Metal–insulator–semi-conductor (MIS);Water splitting;Photocatalysts

0.1016/j.nanoen.2lsevier Ltd. All rig

thor at: School ongor Darul Ehsanhong.Meng.Nan@

AbstractThis review paper discussed recent developments in and prospects of metal–insulator–semiconductor (MIS) nanojunctions for hydrogen evolution from water splitting in photoelec-trochemical (PEC) cells. The basic principles of MIS that functionalise upon the criticalintermediate ultrathin insulator layer in the sandwiched MIS structure are explained in detail,which is followed by a summary and discussion on the generalised approaches for synthesisingMIS-based nanojunctions, including specific details on the preparation of metal, insulator andsemiconductor layers. Key challenges associated with the application of MIS nanojunctions insemiconductor photocatalysts for water splitting are also addressed in addition to thetechnique’s advantages and disadvantages. Recent developments of photovoltaic cell structuresbased on MIS principles are also reviewed because PEC cells are usually coupled withphotovoltaic cells and electrolysers for enhanced efficiency in water splitting. Finally, theprospects of MIS nanojunctions for water-splitting applications in the context of addressingenergy and environmental concerns are discussed.& 2015 Elsevier Ltd. All rights reserved.

015.01.001hts reserved.

f Engineering, Chemical Engineering Discipline, Monash University Malaysia, Jalan Lagoon Selatan,46150, Malaysia. Tel.: +60 3 5516 1840; fax: +60 3 5514 6207.monash.edu (M.N. Chong).

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T. Zhu, M.N. Chong348

Contents

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 348Basic principles of metal–insulator–semiconductor (MIS) nanojunctions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 349Generalised approaches for synthesising MIS nanojunctions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 358

Semiconductor pretreatment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 358Insulator preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 358Metal deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 360

Key challenges in the development of MIS nanojunctions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 361Semiconductor substrate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 361Ultrathin insulator layer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 362Metallic collector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 363MIS design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 365

Other photovoltaic structures based on MIS nanojunctions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 365Summary and future outlooks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 369Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 371References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 371

Introduction

Due to increasing energy and environmental concerns, therehas been considerable attention devoted to the developmentof clean and renewable energy sources [1–3]. Utilisation ofsolar energy has the potential to create a green, environmen-tally friendly and sustainable society. Much research has beendevoted to enabling the practical conversion of solar energyinto usable renewable energy sources. Hydrogen is a goodpotential candidate as a renewable energy source because thefinal combustion products are water and heat without produ-cing any environmentally deleterious greenhouse effect gases.In 1972, Fujishima and Honda proposed a green approach tohydrogen energy production using solar-assisted photoelectro-chemical (PEC) cells [4]. To date, hydrogen energy productionvia solar-assisted PEC water splitting is still an attractivemeans of converting intermittent solar radiation into storableand non-polluting chemical energy in the form of hydrogen.

Recent techno-economic analysis showed that to producecost-competitive hydrogen energy via the solar-assisted PECapproach, the solar-to-hydrogen (STH) efficiency of photo-catalysts used should be at least 15%, preferably greater than20% [5]. Based on recent developments in PEC semicond-uctor photocatalysts for water splitting, however, the routeto practical adoption and realisation is still significantlyimpeded by the low efficiency and stability of photocatalystissues. To make the PEC process more economical, both theissues of low efficiency and of stability of PEC semiconductorphotocatalysts must be addressed and improved significantly.Many previous studies have focused on introducing foreigndopants [6–11], p–n junctions [12–16] and phase junctions[17–21] to improve the photo-efficiency of PEC semiconduc-tor photocatalysts but have found limited success. Of late,one promising approach to achieving higher STH efficiencyand stability is through a metal–insulator–semiconductor(MIS) nanojunction design for a PEC water-splitting applica-tion [22–26].

The most important characteristic of MIS nanojunctiondesign is that the stability and light-harvesting character-istics of semiconductor photocatalysts are decoupled. Thisenables semiconductor photocatalysts with a narrower band

gap, which are well suited for absorbing solar radiation tobe used without being corroded by the electrolyte. Withinthe MIS nanojunction structure, the ultrathin insulator layer,which is typically a metal–oxide layer, plays a critical role inprotecting the semiconductor from corrosive electrolyte, aswell as efficiently mediating the minority carrier transportacross the MIS junction with minimal recombination [27]. Inthis instance, the MIS nanojunction presents a conceptuallysimpler approach when compared with photovoltaic cells. Inthe past, photovoltaic cells attracted much interest due totheir more economical approach to solar cell production. Inprinciple, the structuration of a MIS nanojunction resemblesthe illuminated side of a photovoltaic junction, allowing forbetter solar radiation collection. This is especially true forshort wavelength solar radiation compared with standardp–n junction solar cells [28].

Since the inception study on MIS nanojunction structurereported on Si solar cells in 1976, many studies have exploredboth the fundamental and application aspects of MIS struc-turation in solar-related devices [24]. In the past, MIS-basedsolar cells have received much attention due to their facilelow-temperature fabrication process and the potential forachieving high solar conversion efficiency [29]. To date,however, few studies have reported on the application ofMIS nanojunctions for hydrogen production from a water-splitting reaction in PEC cells. This review paper discussesrecent developments and prospects of MIS nanojunctions forhydrogen evolution from water splitting in PEC cells. The basicprinciples of MIS that functionalise upon the critical inter-mediate ultrathin insulator layer in the sandwiched MISstructure are explained in detail. This is followed by asummary and discussion of the generalised approaches forsynthesising MIS-based nanojunctions including specific detailson the preparation of metal, insulator and semiconductorlayers. Key challenges associated with the application of MISnanojunctions in semiconductor photocatalysts for watersplitting are also addressed along with its advantages anddisadvantages. Recent developments of photovoltaic cellstructures based on the MIS principles are also reviewedbecause the PEC cells are usually coupled with photovoltaiccells and electrolysers for enhanced efficiency in watersplitting. Finally, the prospects of MIS nanojunctions for

Fig. 1 Conventional view of MIS-based photoelectrode mechanism. (a) Schematic side view of the MIS-based photoelectrode withmetallic collector situated on the insulator-covered p-type semiconductor. (b) Energy band diagram for a standard MIS nanojunctionwith p-type semiconductor. Reproduced from Ref. [27].

349Prospects of metal–insulator–semiconductor (MIS) nanojunction structures for enhanced hydrogen evolution

water-splitting applications in the context of addressingenergy and environmental concerns are discussed.

Basic principles of metal–insulator–semiconductor (MIS) nanojunctions

The MIS nanojunction structure was initially proposed as avoltage-controlled varistor (i.e., variable capacitor) in 1959by Moll and Garrett [30]. Subsequently, the characteristics ofMIS were analysed by Frankl and Lindner [31]. The firstsuccessful MIS nanojunction structure was made of silicondioxide (SiO2) grown thermally on a silicon surface by Ligenzaand Spitzer [32]. The first application of MIS-based diodes asphotosensitive devices was solar cells. Within the MIS nano-junction structure, it was discovered that by using a tunnel-transparent insulator layer (i.e., approximately 20 Å of SiO2

in silicon Schottky-barrier solar cells), both the energyconversion efficiency and electromotive force could beincreased. The net effect of enhancing energy conversionefficiency is mainly due to the potential barrier of insulatorlayer that reduces the recombination current of majoritycharge carriers, whereas the electromotive force increasesbecause of the built-in charge by the insulator layer. Forinstance, the structure of an MIS-based diode consists of anintermediate insulator layer on the bottom wide-band semi-conductor layer and a highly doped amorphous or polycrystal-line semiconductor metal top layer [33]. In the early version[29], the MIS-based diode consisted of a continuous low-work-function metal that sandwiched an intermediate insu-lator layer with a p-type semiconductor bottom layer. Withthe structuration of a MIS nanojunction, a collecting barrier isinduced at the semiconductor surface due to the differencein work function. However, the major disadvantage of a MIS-based solar cell is that a very thin metal layer must be used.In this instance, the thickness of the transparent metal layeris a compromise between the degree of transparency andvoltage drop caused by the serial resistance. To overcome theissues with the thin metal layer, the structuration of the MISnanojunction was restricted to the contact grid where theactive area between grid fingers was covered by a transpar-ent dielectric layer (e.g., grating-type MIS solar cells). If an

inversion layer is induced by the presence of insulatorcharges, the device is called a MIS-inversion layer (MIS-IL)solar cell. Previously, different thermally assembled SiO2 andtitanium oxides (TiOx) materials, as well as chemical vapour-deposited SiO2, silicon oxide (SiO) and tantalum oxide (TaxOy)materials were used as the transparent dielectric for inver-sion layer formation at the silicon surface [34,35]. A majorshortcoming of early MIS-IL solar cells was that they sufferedfrom rapid degradation of the inversion layer. This could beovercome via the introduction of silicon nitride as thecharged dielectric and anti-reflection layer, with its highlystable positive charge densities achieved by the incorpora-tion of alkaline species [27].

Fig. 1 shows the typical MIS-based photoelectrode struc-ture consisting of a metallic collector layer situated at thesurface of an insulator-covered semiconductor layer. Whenthe MIS photoelectrode is illuminated, photogeneratedminority carrier electrons are created and subsequentlydiffused (tunnelling) through the insulator-to-metal collec-tor to initiate the hydrogen evolution reaction [33]. Asshown in Fig. 1, the photocurrent may only be producedwhen the photogenerated electrons are created within adistance less than the sum of the depletion width (W) andeffective minority-carrier diffusion length (Le) of the col-lector. In this instance, the potential barrier of an insulatorlayer can reduce (or block) the recombination current ofmajority carrier electrons, as the minority charge carriersare able to flow through the thin insulator layer via quantummechanical tunnelling [36]. This was observed for the Si-based MIS solar cells when a thin SiOx layer was inserted in-between the metal and Si semiconductor layers [37]. In awell-behaved p-type MIS nanojunction, the photogeneratedminority electrons pass directly from the semiconductorconduction band edge (Ec) to the Fermi level of the metalliccollector (Ef,metal), as illustrated in Fig. 1b. The key adv-antage of MIS-based photoelectrode design is that thesemiconductor stability and light-harvesting efficiency aredecoupled, enabling narrower bandgap semiconductorstypically used for absorbing solar radiation to be applicablein pH-extreme electrolyte without causing corrosion pro-blems. This is achieved by the critical insulator layer in theMIS structure, which is typically an ultrathin metal oxide

Fig. 2 Transmission electron micrographs of three ALD-TiO2 films of thickness 2 nm, 5 nm and 12 nm (from left to right).Reproduced from Ref. [38].

Fig. 3 Electrical equivalent circuits of MIS structures for various bias ranges: (a) Ugo�5 V, (b) �5 VrUgr�3 V, (c) Ug4�3 V,Cox—insulator capacitance, Rs—serial resistance, L—circuit inductance, Csc—bias-dependent space charge layer capacitance, CD—additional bias-dependent capacitance, CPE1,2—constant phase elements characterized by Q1,2 and n1,2 parameters, R1,2—resistances. Reproduced from Ref. [39].


layer that protects the semiconductor from corrosive elec-trolyte. The insulator layer will also efficiently mediate theminority carrier electron transport across the MIS nanojunc-tion with minimal recombination.

To understand the physics behind a MIS-based diode, it isuseful to analyse the band diagram of the MIS structure. Ingeneral, it was found that reverse applied bias (i.e., whichgives rise to the depletion layer) was distributed betweenthe insulator layer and space charge region. The height ofthe potential barrier on the semiconductor–insulator inter-face (SII) consists partly of the applied bias and is partlydetermined by the differences in (i) work functions betweenthe metal and semiconductor layers; (ii) summary charge ofthe interface states; and (iii) built-in insulator charge andthe charge of minority carriers accumulated near the SII.Once the reverse bias is applied to the MIS structure, theregime of non-equilibrium depletion is established [33].

For example, Scheuermann et al. [38] synthesised a MISstructure based on Si with uniform TiO2 films of thickness

between 1 nm and 12 nm through the ALD method on adegenerately doped p-type Si semiconductor wafer sub-strate (Fig. 2). They reported that the ALD-TiO2 thin filmsyield water oxidation overpotential between 300 mV and600 mV at 1 mA/cm2 in aqueous solution. Subsequently, ananalysis of an electrical equivalent circuit of MIS structurebased on the admittance measurements can be carried out[39]. Kochowski et al. [39] proposed electrical equivalentcircuit designs for the analysis of a MIS structure based onAl–SiO2-(n) Si. Fig. 3 shows the electrical equivalent circuitdesigns capable of measuring the frequency dispersion ofadmittance characteristics in a broad range of signalfrequencies and gate voltages from inversion to accumula-tion. The elements in these circuits for different gatevoltages are illustrated in Fig. 3. The serial resistancegenerally is from the resistances of the semiconductormaterials as well as of the contacts and electrical connec-tions. The origin of the additional bias-dependent capaci-tance CD is not currently clear. The estimated space charge

Table 1 Summary of recent developments of MIS nanojunction structures.

Nos. Figure Synthesis method MIS Devices Characterisation Materials/thickness /efficiency/stability

Advantages Disadvantages Ref.

1 1. SiO2 grown in a furnace byannealing at 500 1C for 2 min 30 s inan oxygen atmosphere

The threshold voltage of resonanttunnelling can be controlledaccording to the energy levels ofthe molecules. The carriers wereinjected into the respectivemolecules at correspondingthreshold voltages in the binarymolecules. These findings point to anew multilevel operation ofresonant tunnelling through organicmolecules in a practical MIS devicestructure

Materials: F16CuPc; CuPc Incorporating organic molecules inMIS structure has many advantages:The molecules have a uniform sizeat the nanometre scale; Thesenanometre-scale sizes permithigher densities of dots; Theirenergy levels are tunable throughthe attachment of functional groupsuch as electron-withdrawing (or —donating) groups; These featuresmake organic molecules superior toinorganic molecules for makingquantum dots

Although it proposed a quite newmethod that incorporates organicmolecules in an MIS structure,there are no test results describingSTH efficiency or the stability ofthis new-type MIS structure.Further research could bedeveloped on this new method

[50]2013Thickness: SiO2-1.2 nm, Al2O3-

3.5 nmEfficiency: –

2. F16CuPc and CuPc deposited onSiO2 from Knudsen cells under highvacuum

Stability: –

3. Al2O3 deposition in ALD reactorwithout exposure to air4. Au deposited on top of the Al2O3

through electron-beam deposition

2 1. P-doped CZ Si(1 0 0) was cleanedand dried

Recombination within the inversionlayer is significantly reduced,allowing electron transport oververy long distancesc(Le+D). TheLSV for the MIS photoelectrode with23 μm diameter collector ischaracterised by a photoelectrodeefficiency of 2.9%, assuming anideal counter-electrode. Thisefficiency is about 15 times largerthan previously reported for p-SiMIS photocathodes

Thickness: SiO2-2 nm; Pt-20 nm;Ti-30 nm

1. High-quality thermal SiO2 layer The efficiency is still low in meetingthe solar-to-hydrogen efficiencytarget of 10%; The metal collectordimensions, optimisation of the MISjunction and inversion layer couldfurther improve the efficiency

[27]20132. Both photo- and electrolyte-

induced formation of an inversionchannel beneath the SiO2/Siinterface can extend the transportdistance of minority carrier3. The bilayer collector structurecan decouple the collector’sfunction as catalyst from itsfunction as a Schottky junctionmetal in the MIS structure. Pt usedas the top layer to ensure goodreaction kinetics, while the lowfunction Ti layer enables thegeneration of a large photovoltageacross the MIS junction. Ti servedas good adhesion layer for the topPt layer

Efficiency: 20 mA at appliedpotential 0.15 V in 0.5 M H2SO4

solution

2. SiO2 grown by RTOn at 950 1C in8% O2/N2, cooled to 250 1C andthen annealed to 1000 1C for 60 s ina pure N2 followed by 60 s in 10%H2/N2

Stability: Excellent stability over2 h of continuous operation

3. Ti and Pt were evaporatedthrough shadow masks onto oxide-covered wafers at 1 A/s by e-beamevaporation in a Denton infinity 22thermal evaporator system with abase pressure of 1.0� 10�7 Torr

3 1. Ni films were deposited on as-received phosphorous-doped [100]n-type Si wafers (0.3–0.5 Ω cm) at adeposition rate of �0.2 Å/s

High photocurrent density wasmeasured at approximately 1.07 Vwithout sign of decay even after80 h of continuous PEC wateroxidation. The high photovoltagewas attributed to the high built-inpotential in a MIS-like device withan ultrathin, incomplete screeningof Ni/NiOx layer from theelectrolyte. In 1 M aqueous KOH,the Ni/n-Si photoanodes exhibitedhigh PEC activity with a low onsetpotential (�1.07 V vs RHEn) andhigh photocurrent density (60 mA/cm2)

Thickness: Ti-20 nm; SiO2-native;Ni-2 nm

1. Form MIS junction, favouringcharge separation and motion ofphotoexcited holes toward the OERcatalyst–electrolyte interface

1. The bandgap (1.1 eV) of Si limitsthe amount of photovoltage thatcan be generated for watersplitting, although it is a low-costand abundant earth material

[96]2013

2. Ni served as the activeelectrocatalyst for OER

2. The use of semiconductors withwider band gaps than Si undersimilar surface protection/electrolyte conditions should resultin a shift of the OER photocurrentonset to more negative potentials

3. Ni/NiOx film provided excellentprotection to Si

Efficiency: In 1 M aqueous KOH, thephotoanodes exhibited highphotocurrent density (60 mA/cm2)with a low onset potential (about1.07 V vs RHE) under 225 mW/cm2

Stability: The electrode showed nosign of decay even after 80 h ofcontinuous PEC water oxidation in amixed lithium borate–potassiumborate electrolyte

2. Ohmic contact was made to thebackside of the Si wafer by e-beamdeposition of Ti. Copper tape wasused to contact the Ti to thebackside for electrochemicalexperiments

351Prospects

ofmetal–insulator–sem

iconductor(M

IS)nanojunction

structuresfor

enhancedhydrogen

evolution

Table 1 (continued )



4 1. SiO2 layer was annealed in afurnace at 500 1C under an oxygenatmosphere

The change in the SET (single-electron tunnelling) could be dueto the alternation between open-and closed-ring isomers induced bylight irradiation. This means thatinformation about the thresholdvoltage in SET was memorised as areversible change in the molecularorbitals along with thephotoisomerisation and reveals thatdiarylethene molecules work as anoptical memory in a practicaldevice structure. Theseachievements represent uniquefeatures of organic molecules thatstrongly outweigh those ofinorganic quantum dots

Thickness: ITO-30 nm; Al2O3-3.5 nm; SiO21.2 nm; p-Si(1 0 0);diarylethene molecules

1. The molecules have a uniformsize at nanometre scale and thus,achieving quantum dots with alarge number density. The numberdensity of the molecules can reacharound 1013 cm�2, which is twoorders of magnitude higher thanthat of inorganic dots

This research introduced theorganic molecules into the device,which is a new approach for thepreparation of MIS structures.However, there is no metalcollector used in this device

[97]2013

2. Diarylethene molecules weredeposited on the SiO2 surfaces bythermal evaporation in a vacuumwith background pressure of5� 10�7 Pa

2. The tenability of the molecularorbitals. Photochromic moleculessuch as diarylethene andazobenzene molecules, permit themolecular orbitals to be reversiblychanged by light irradiation. This isa unique feature of organicmolecules that makes them verydifferent from inorganic dots

Efficiency: –Stability: –

3. Al2O3 layer was formed byintroducing trimethyl aluminiumand water vapor alternately asprecursors4. ITO electrode was deposited ontop of Al2O3 by radio frequencymagnetron sputtering system

5 1. TiO2 films were deposited on topof the FTO electrode

This MIS heterostructure forplasma-electric energy conversionis a novel architecture to harvesthot-electrons derived fromplasmonic excitations. The externalquantum efficiency (EQE) of 4% wasattained at 460 nm using a Agnanostructured electrode, whilethe EQE of 1.3% was attained at550 nm employing a Aunanostructured electrode. Theinsulator interfacial layer plays acrucial role in interfacepassivation, a requisite inphotovoltaic applications toachieving both high open-circuitvoltages (0.5 V) and fill-factors(0.5). However, its introductionsimultaneously modifies hot-electron injection and transport.The influence passivation has onthese processes for differentmaterial configurations, andcharacterise different types oftransport depending on the initialplasmon energy band, reportingpower conversion efficiencies of0.03% for nanopatterned silverelectrodes

Thickness: (anatase)TiO2-50 nm;TiO2 and ITO were used as controldevices

This device is a novel architectureto harvest hot-electrons derivedfrom plasmonic excitations. Unlikesemiconductors where light isabsorbed within a thickness of 1 μm(for direct band gapsemiconductors) to 100 μm (forindirect-band gap semiconductors),appropriately designed metalnanostructures exhibit total lightabsorption within thicknessesbelow 90 nm or showomnidirectional coupling. Thedirect use of LSPRs to harvest lightwould open the exciting possibilityof creating artificial materials witha tailored absorption, but withoutthe band gap limitations of bulksemiconducting materials

The major roadblock towardshigher performance stems from thelow quantum efficiency of thisarchitecture. This initially modestnumber, far from the injection yieldobtained in similar process, mighthave its origin in a combination ofthe following: (i) a poor efficiencyof electron–hole generation overphoton re-radiation for LSPRdamping in the current structure,(ii) small tunnelling probability, and(iii), cooling and recombinationrates of electrons in the metalfaster than the tunneling rate intothe semiconductor

[51]2013

2. TiO2 deposited over ITOelectrode via the ALD method

Efficiency: The external quantumefficiency (EQE) of 4% at 460 nm byusing a Ag nanostructued electrodeand EQE of 1.3% at 550 nm by a Aunanostructured electrode

3. TiO2 and Al2O3 films weredeposited by ALD method

4. 300 nm of metal deposited usinga Kurt J. Lesker Nano 36 system

Stability: Silver devices are stablewithin 22% performancedegradation after 2 months ofstorage

T.Zhu,

M.N.Chong

352

6 1. p-Si(1 0 0) wafers werechemically cleaned using RCAcleaning method

The lifetime increased more thanthree times compared to thosewithout Al2O3 films as a result ofpassivated surface effects. Theinterface state density with SiNx/Al2O3 structure was about oneorder higher in magnitude than thatwith only Al2O3 film because of thecharges incorporated in the SiNx

film during the process. Theefficiency increased from 0.9% to8.2% because of the increase in thenumber of inversion carriers andthe emergence of the anti-reflection coating effect as the SiNx

film was used

Thickness: Al2O3-1.5 nm The RPALD method was superiorover the conventional ALD methodbecause it incurred lower plasmadamage and can be carried out at alower process temperature. Thismethod is expected to deposit high-quality insulating layers andimprove the interface properties inMIS devices. The insulator Al2O3

films grown on Si structures usingthe RPALD method exhibited goodinterface properties. Theresearchers were able to offerexcellent atomic-level control ofthe deposited film thickness.Smooth and uniform Al2O3/Siinterfaces were obtained

This device required theincorporation of SiNx film fordouble insulator layers, making thestructure more complicated. Thefabricated cell efficiency withoutSiNx was only approximately 0.9%.However, with SiNx of 91 nm, theefficiency reached approximately8.2%. The efficiency increased from0.9% to 8.2% because of theincrease in the number of inversioncarriers and the emergence of theanti-reflection coating effect as theSiNx film was used

[24]2011Si-p-type (1 0 0) SiNx-91 nm

Efficiency: PEC is approximately8.21% under AM 1.5 (100 mW/cm2)at 25 1C

2. The ultra-thin Al2O3 gatedielectric film was deposited usingthe RPALD system

Stability: –

3. The SiNx film was sputtered ontothe surface as an anti-reflectioncoating4. Al was evaporated onto the rearsurface and annealed at 400 1Cusing N2 to complete the formationof MIS solar cells

7 1. n-Type liquid-phase epitaxial(LPE) GaAs wafers (Te doped) withorientation [100] and 1.5–7.3 mΩ cm resistivity were used

The barrier heights increased in therange of 0.23–0.63 eV with theideality factor decreased in therange of 5.92–1.66 with increasingtemperature

Thickness: GaAs-n type (1 0 0)TiO2-191 nm

GaAs is an III–V semiconductormaterial that has electricalproperties superior to those of Si.Since GaAs has a higher saturatedelectron velocity and higherelectron mobility, it is commonlyused in devices for high frequencyand low power applications. TiO2/n-GaAs MIS structure showedpromising than other insulatormaterials currently being used inhigh-performance devicetechnology

Parameter such as barrier heightexhibited strong temperaturedependence. Some of the synthesisparameters are still not reasonablyexplained. Also, both the efficiencyand stability of this GaAs based MISjunction are absent

[45]2012

Cu-high-purity copper layer with1 mm diameterEfficiency: –

The variation in electricalcharacteristics as a function oftemperature for this MIS structuredemonstrated it has the potentialto be used in solar cells. TiO2/n-GaAs MIS structure showedpromising than other insulatormaterials currently being used inhigh-performance devicetechnology

Stability: –2. The samples were dipped intothe TiO2 solution ten times. Aftereach dipping process, samples weresubjected to repeated annealingprocess at 300 1C for 5 min3. A high-purity Cu layer as dotswith diameter of approximately1 mm was coated on the surfaceunder a high vacuum pressure of10�7 Torr

8 1. GaN layer was grown on Al2O3

sapphire substrate by MOCVDmethod

The calculated values of the barrierheight for MS and MIS Schottkydiodes were found to be 0.79 eV (I–V), 0.87 eV (C–V) and 0.86 eV (I–V),0.99 eV (C–V), respectively. Theincrease in barrier height isascribed to the negative charge atthe interface, while recombinationin the oxide is presumed to be thecause of the latter. The dominantinterface trap was found to belocated at 0.76 eV below theconduction band

Thickness: Semiconductor: GaN(n-type); Insulator: SiO2 (20 nm);Metal: Au (50 nm)

GaN based semiconductor possessesmany good properties such as highlyefficient, long-lasting light-emitting capability especially in thevisible range, chemical stability,mechanical strength and hightemperature endurance

The saturation current was reducedin the MIS structure, which may becaused by the thin oxide layer, anda combination of increased barrierheight and reduced velocity ofcharge carriers. The increase inbarrier height is ascribed to thenegative charge at the interface,while recombination in the oxide ispresumed to be the cause of thelatter

[42]2011

2. A 20 nm thick SiO2 layer wasdeposited on another piece of GaNfilm followed by 50 nm thick Au byelectron beam evaporation system.Au evaporation processes werecarried out in a vacuum coatingunit at a pressure of about 5–6� 10�6 mbar


9 1. Si substrates were degreasedusing standard techniques andetched with HF solution (20%) toremove the SiO2 layer from thesurface

The as-deposited MIS devicesshowed characteristic diodebehaviour with a turn-on voltage atapproximately 0.5 V but presenteda breakdown field of only 0.2 MV/cm Si interface as well as in thebulk of the film. After annealing, anincrease in the dynamic impedancewas observed. There was also

Thickness: – AlN has a high dielectric constant,excellent thermal conductivity,wide bandgap and high velocity ofsound amongst others. Insulatorswith high dielectric constant suchas AlN are particularly suitable forapplication in MIS devices, wherethermal SiO2 cannot be grown. Theperformance of these devices will

The molecular beam epitaxy (MBE)techniques contain high density ofcolumnar defects, grain boundariesand dislocations amongst others.High densities of threading defectswhich extend through the film andinto the substrate may beobserved. These structural defectsrepresent paths for atomic/ionictransport through the insulating

[60]2014Efficiency: –

Stability: –

2. The AlN thin films in the MISstructure were grown by MBE onn-type (1 1 1) Si substrates3. Finally, Au metal disks (1.8 mmin diameter) were sputterly

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evidence of trapped charges at theinsulator-semiconductor interface

strongly depend on the insulator–semiconductor interface quality

material resulting in a poorelectrical performance of thedevice

deposited on top while a Ga ohmiccontact was applied on the back ofthe substrate

10 Si wafers were used as received,with a thin (o2 nm) SiO2 layer asprepared by the wafer vendor. TiO2

was deposited by ALD method at200 1C with tetrakisdimethylamidotitanium as the titanium source andH2O as the oxygen source. Metaldeposition was performed bye-beam evaporation. The backsidecontacts for the n-Si and p+-Sisubstrates were e-beamevaporated Al and Pt, respectively

PEC water oxidation was observedto occur below the reversiblepotential whereas darkelectrochemical water oxidationwas found to have low-to-moderateoverpotential at all pH values,resulting in an inferredphotovoltage of 550 mV. Wateroxidation was sustained at theseanodes for many hours under harshpH and oxidative environments,whereas comparable Si anodeswithout the TiO2 coating quicklyfailed

Thickness: Si was protected byALD-TiO2 of 2 nm thick and thencoated with an opticallytransmitting layer of known Ircatalyst (3 nm)

As a result of its stability over arange of pH and potentials, TiO2 is auseful photoanode material. TheTiO2 and Si can be combined well tosynthesize an oxidatively robustand efficient photoanode for wateroxidation via the ALD method. ATiO2 layer of 2 nm thick was foundto prevent Si from oxidation whilebeing thin enough to allow facileelectron tunnelling between anoverlying catalyst layer and thebase substrate. Ir, one of many wellknown metal and metal oxidecatalysts that promote efficientwater oxidation over a range of pH,was deposited on top of the TiO2

layer by physical vapour deposition(PVD) method

This MIS nanojunction structureremains a challenge when it isbeing used on other crystalstructures or high profilephotoelectrodes as earth abundantmaterials are usually preferred forlarge-scale production of solar fuels

[41]2011

Efficiency: 16 mA at 0.6 V vs NHE in1 M NaOH under 1 sun illuminationStability: It can last at least 8 hunder 1 sun illumination duringwater splitting process

11 1. Thermal oxide grown Si waferswere patterned with Au byphotolithography (top contact andetch mask)

The SWNT/Si solar cells with arecord high PCE of 411% for CNT/Sisolar cells. By employingtemperature-dependent electricalcharacterisations, it was found thatdiffusion-dominated p–n junctiontransport and dominantphotocarrier generation in Si. Thesuperior photovoltaic properties ofsingle-crystalline Si can be realisedby a simple, low-temperatureprocess, thus implying a greatpotential for low-cost, high-efficiency solar cells

Thickness: Semiconductor: n-typeSi; Insulator: SiO2; Metal: Au; Backcontact: Al

Carbon nanotube (CNT)/Si hybridsolar cells are a new class ofphotovoltaic devices which benefitsfrom the superior opto-electronicproperties of CNTs coupled withwell-established Si photovoltaictechnologies. SWNTs are moresuitable than double- (or multi-)walled carbon nanotubes owing totheir tunable/direct band gapenergies matching with a widerange of the solar spectrum andbetter charge carrier transportproperties. By employingtemperature-dependent electricalcharacterisations, it is known thatdiffusion-dominated p–n junctiontransport and dominantphotocarrier generation in Si. Thisstudy suggests that the superiorphotovoltaic properties of single-crystalline Si could be realised by asimple, low-temperature process,thus implying a great potential forlow-cost, high-efficiency solarcells. The fact that high PCE can beachieved without a need forcomplicated processing of CNTs

Besides the limited PCEs, theunderlying transport mechanism ofSWNT/Si solar cells is not well-established and remains underdebate. This is mainly becauseSWNTs are mixtures of bothsemiconducting and metallicnanotubes with inhomogeneousdiameters and chiralities, whichlead to the complexity in thenature of the electronic junction ofSWNT/Si

[36]2012

2. Al was used for the back contactof Si

Efficiency: The optimised cellsshowed power conversion efficiency(PCE) of 411%Stability: –3. High quality single-walled carbon

nanotubes (SWNT) thin films werefabricated by the “superacid slidingcoating method” and thendeposited on n-type Si

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(such as soring-out of nanotubetype) is very encouraging forfurther utilisation of CNTs forvarious photovoltaic applications

12 1. Si was initially cleaned followedby ALD deposition of TiO2 layers oncleaned Si

For the thickness of ALD-TiO2 filmsgreater than �2.0 nm, theeffective overpotential for wateroxidation increased linearly whenthe TiO2 thickness was increased inaccordance with a bulk-limitedconduction mechanism thatrequired a characteristic E-field inthe TiO2 layer to maintain a givenwater oxidation current across theMIS anode. For ultrathin TiO2 filmsin which direct tunnelling candominate charge transport, theminimum overpotential andreaction rate were relativelyindependent of TiO2 thickness andwere apparently dominated by thekinetics of the water oxidation onthe catalyst surface

Thickness: Semiconductor: p-typeSi(1 0 0); Insulator: SiO2 (�1.5 nmthickness) and TiO2 (1.2–11.6 nm)

1. Total overpotentials are quitelow for these ALD-grown MIS anodestructures

An analysis of different metalcatalysts for water oxidation hasunderlined the challenge ofengineering efficient wateroxidation catalysts as theoverpotentials increase greatlywhen using non-noble metals. Incontrast, reducing the thickness ofIr by three-fold had very littleeffect on the overpotential,indicating the possibility to reducecosts with a minimal effect ondevice performance

[38]2013

2. The overpotential penalty pernanometre of added TiO2 thickness(e.g. for increased chemicalstability) is modest

2. The metal precursor used in theALD method was tetrakis titanium(TDMAT)

Efficiency: The ALD-TiO2 films withthickness range 1–12 nm can yieldwater oxidation overpotential at1 mA/cm2 of 300 mV to 600 mV inaqueous solution (pH 0 to 14).

3. Ir was deposited by EBE with athickness of 2 nm

Stability: –

13 1. The Si wafer was immersed in HFsolution to remove the native SiOx,and then cleaned

The solution-processable GO can beused as an effective insulating layerfor MIS Si solar cells. The Voc of theGO-based MIS Si solar cells leads toa PCE enhancement of 88% ascompared with that of referencedevice (Schottky solar cells). It isbecause the overall reduced carrierrecombination arising from boththe increase in built-in potentialand the reduced interface defects.The fabricated r-GO based Si solarcells exhibited inferiorperformance than that of GO basedMIS Si solar cells, mainly owing tothe larger series resistance fromthe additional SiOx layer formedduring the thermal reductionprocess

Thickness: Semiconductor: n-typeSi(1 0 0); Insulator: GO; Metal: Au(about 12 nm) and Al(about150 nm)

The insulator GO is solutionprocessable and the work functionis tunable. They can be worked aseffective hole and/or electrontransport layers in both organicsolar cells (OPV) and organic lightemitting diodes (OLEDs). It can bealso used as an effective insulatinglayer for MIS Si solar cells.Compared with the conventionalinsulating SiOx layer, thepreparation of GO via solutionprocessing requires a far lowertemperature, which can minimisethe grain boundary diffusion indoped crystalline Si, preserveminority carrier lifetime andreduce the energy consumption. Inaddition, the highly water-solubleGO nano-sheets can produce a goodcontact with the hydrophilic Sisurfaces, which is critical forsurface passivation and carriertransportation

1. The metal collector Au in thisstructure is not cost-effective.

[44]2013

2. The uniform GO insulator withthickness more than 4 nm was hardto obtain2. GO was synthesized by a

modified Hummers method. The GOthin film was deposited on Si by spincasting from GO solution

Efficiency: Power conversionefficiency (PCE) 88%Stability: –

3. Both the top Au bottom contactsAl were deposited by thermalevaporation

14 1. HF solution was used to removethe native oxide formed on Sisurface

Thin Al2O3 layer deposited by ALDwas not only very effective for theprotection of Si photocathodeagainst oxidation, but alsobeneficial for the overpotentialreduction via surface passivation.The study also found that the Sinanostructuring, that has been

Thickness: Semiconductor: Si;Insulator: Al2O3 (2.3 nm); Metal: Pt

The insulator Al2O3 layer on p- andn-type Si has long beenimplemented in the photovoltaicindustry, because it can provide astate-of-art level of surfacepassivation. In addition, unlike thesputtering method reported, whereonly a relatively thick TiO2 layer

With the nanoporous Siphotocathode overcoated by theultra-thin Al2O3 layer, a steady andstable photocurrent was generatedduring continuous operation for12 h

[23]2014

2. Nanoporous Si was fabricated bya modified metal-catalysedelectrodeless etching (MCEE)method and then washed withdeionised water

Efficiency: 28 mA/cm2 at �0.9 V vsRHE under AM1.5 G (100 mW/cm2)

Given the poor long-term durabilityof nanostructured Si

Stability: It can generate a steady,stable photocurrent during

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widely performed to suppress lightreflection and reduceoverpotential, turned out to have asignificant negative effect on thelong-term stability

can be deposited (which is notsuitable for nanostructured Si), theALD method allows for conformalcoating, with a sub-nanometreprecision. Thus, the ALD methodenables a uniform andhomogeneous surface protection,even when applied tonanostructured Si photocathodes

photoelectrodes, more attentionshould be brought to thedevelopment of a new strategy forelectrode protection. We speculatethat a proper choice of protectionmaterials, other than Al2O3, couldfurther improve the PECperformance of Si photocathodes

continuous operation for at least12 h

3. Thin Al2O3 layer was deposited ina layer-by-layer fashion onto the Sielectrode via the ALD method4. The Pt was deposited bysputtering at a current of 20 μA for5 s

15 1. Si with native oxide was cleanedand made hydrophilic by treatmentwith SCI solution (NH4OH/H2O2/H2O=1:2:8) for 15 min

GO was deposited on Si to form anAl/GO/SiO2/Si MIS tunneling diode.With GO insertion, theaccumulation dark current andinversion photocurrent weregreater than those measured in thecontrol device. The photocurrent ofthe GO MIS tunneling diode was1.34� 10�6 A, while thephotocurrent of the control diodewas 1.94� 10�7 A. Thus, the GO-based device is a promisingcandidate for detector applications

Thickness: Substrate: p-type Siwith native oxide; A 100-nm-thickAl gate with a circular area of5� 10�4 cm2 on the GO-depositedside of the Si substrate. Large-scaleAl was sputtered to form an ohmiccontact with native SiO2

This method involves chemicaloxidation of graphite followed byexfoliation of GO. These GO layersare insulating and could bethermally reduced to beelectrically conductive.Nevertheless, the insulatingproperty of GO may be utilised tocertain applications. Each GO layerconsists of the pure two-dimensional honeycomb latticebearing functional groups, and thetwo-dimensional structure is notpresent in commonly employedinsulators such as amorphous SiO2

Although thick GO flakes mayprevent the tunnelling of carriers,carriers can still flow through theSiO2 that it is not covered withthick GO flakes for the GO sample.In other words, the GO sample isonly partially covered with GO, andcarriers can tunnel via theuncovered regions

[58]2012

2. The GO sheets were deposited onsubstrates by dip-coating3. Al gate was sputtered onto theGO-deposited side of the Sisubstrate. Al was also sputtered ona large-scale onto the backside of Sisubstrate


16 1. Al substrate was washed withNaOH or KOH pellets to removenative alumina layer and othersoluble salts

The energy density distributionprofile of the interface states wasobtained from the forward bias I–Vdata by taking into account the biasdependence ideality factor (n(V))and effective barrier height (φe)for MIS structure. Nss valuesincreased with the increasingapplied bias voltage values. Thevalues of Nss for MIS were lowerthan those of MS structure due tothe presence of insulator layer inMIS. The ideality factor, seriesresistance and barrier heightdepend on the existence of theinterfacial insulator layer betweenthe metal and semiconductor

Thickness: Substrate: Al metal;Insulator: Al2O3 (50 Å);Semiconductor: PVA: n-CdS (30 μm)

Organic semiconductors with thediscovery of organic conductingpolymers can be used as activecomponents in MIS structures.These devices have the advantagesof low production costs,mechanical flexibility, large areaelectronic and optoelectronicdevices application

The insulator layers sometimesdegraded the rectifying propertiesof a metal contact deposited on thesurface and hence resulted in ahigh value of n and leakage currentat the metal–semiconductorinterface. The presence of aninterfacial layer and surface stateson MS and MIS diode degrade thequality of diode

[46]2013

Polyvinyl alcohol (PVA) as polymermatrix. PVA is a poor electricalconductor, and the conductivity ofthe polymer is of major importancein constructing a Schottky barrier.When a polymer is doped withsemiconductor, especially II–VIsemiconductor such as CdS and ZnSin various quantities and forms,their incorporation within apolymeric system may be expectedto improve the conductivity.

The insulator will increase thecontact resistant value of metaland semiconductor

2. Al2O3 was prepared byelectrolytic anodisation

Efficiency:-Stability:-

3. PVA: n-CdS was prepared bytaking thiourea as a sulphur sourceand cadmium sulphate as acadmium source and deposited bythe casting method4. The Al metal was deposited bydirect evaporation on PVA: n-CdSfilms

T.Zhu,

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17 1. Substrates were surface polishedin a HF: HNO3 solution and thencleaned

In both cases an increasingdielectric thickness leads to areduction in surface recombinationand is accompanied by an increasein contact resistivity. Optimumthicknesses of ALD Al2O3 andthermal SiO2 were found to be�22 Å and �16 Å, respectively.This amounts to a maximumpotential Voc gain of 15 mV. Thesegains are found to diminishsignificantly after annealing at300 1C. The Al–SiO2–Si MIS typecontacts exhibit a lower maximumVoc gain of 6 mV but greaterthermal stability

Thickness: Semiconductor: p-type(1 0 0) silicon; Insulator: Al2O3;Metal: Al (10 nm)

This study investigated theoptimum dielectric thickness andpotential benefit of applying MIScontacts to conventional diffusedjunction Si solar cells

The Al2O3 MIS contact has aninferior thermal stability than thatof the SiO2 one

[47]2013

2. The Al2O3 was deposited at about200 1c (Beneq TFS200 ALD) usingtrimethylaluminium and water asalternating precursors

Efficiency: Maximum potential Vocgain of 15 mVStability: – The Al2O3 passivated contacts

demonstrated could also be applieduniformly to the n+ rear side of a p+ nn+ solar cell. In that case, thetolerable contact resistivity for a100% metal contact fraction is farhigher than a partial metal grid. Anincreasing dielectric thicknessleads to a reduction in surfacerecombination and is accompaniedby increase in contact resistivity

3. A thin Al layer was evaporated ontop of the thin passivating layers

18 1. Eliminate organic impurities andnative oxides on the surface of Si

Utilising metal-assisted chemicaletching resolved the field-effectpassivation effect induced by ALDAl2O3. The passivation ability ofALD Al2O3 was determined by thetrend in Dit (Fixed Charge Density)rather than Qf (Interface StateDensity) with increasing surfaceroughness. The use of forming gas(H2/N2) annealing was not sufficientto improve the passivationperformance. To effectively restorethe passivation performance ofhighly nanostructured silicon,improving Dit would be morerealistic than controlling Qf

Thickness: Semiconductor: p-typeSi(1 0 0); Insulator: SiO2 and Al2O3;Al2O3: 10 nm Thin Al2O3 layerdeposited onto a bare p-Si surfaceforms a built-in potential at the Sisurface due to the presence ofnegative fixed charges in the Al2O3

film

The use of anti-reflective Sinanostructures is essential toimprove the performance of thin-crystalline Si solar cells. The field-effect passivation effect induced byALD Al2O3 was resolved utilisingmetal-assisted chemical etching,according to the evolution ofsurface roughness in the range of1.7–11.7 nm. Significantnanostructuring of a Si surfaceresulted in an increase of positivefixed charges according to theformation of a SiO2 1 L

The use of forming gas (H2/N2),annealing was not sufficient toimprove the passivationperformance because, in the caseof highly nanostructured surfaces,the positive Qf in the SiO2 1 L washardly affected when compared tothe improvement of Dit

[25]2014

2. The substrates were thenimmersed in diluted solution of HFwith AgNO3 for galvanicdisplacement

3. Ag particles were removed byusing HNO3

Metal: Al layer was deposited toreduce ion damage by the FIB(Focused-ion beam)

4. About 10 nm thick Al2O3 filmswere deposited via ALD at 230 1Cusing trimethylaluminum as metalprecursor and H2O as the oxygensource


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layer capacitance Csc coincides with theoretical Csc corre-sponding to the inversion and depletion region of the MISstructure. The constant phase elements CPE1 and CPE2connect in series with resistors R1 and R2, which relate toelectron processes with a broad distribution of time con-stants. These time constants characterise the exchange ofelectrons between the SiO2–Si interface and semiconductorstates. The constant phase element CPE2 in series with R2resistance describes the dispersion phenomena evoked bythe SII states. The time constant τ1 is practically gate-voltage independent. This type of behaviour points to thepresence of deep traps in the semiconductor space chargeregion. The CPE1 element with parameter n1=0.95 forUgo�5 V has “capacitive” character and, together withR1 resistance, they characterise the time constant ofmonoenergetic electron traps in a lower part of the energygap of the semiconductor. For Ug4�3 V, which correspondsto the traps located in an upper part of the energy gap, theCPE1 element with parameter n1=0.46 connected with R1resistance describes electron processes with a broad spec-trum of time constants. Previous studies also estimated thebasic parameters of MIS structure from the analysis ofcapacitance–voltage and conductance–voltage characteris-tics. Levin et al. [40] found that the photoemission currentsin MIS structure, which was determined by the externalfield, were derived when the space charge was randomlydistributed over the insulator layer.

Generalised approaches for synthesising MISnanojunctions

Overall, the synthesis of a MIS nanojunction includes threeparts, namely the semiconductor, insulator and metalliccollector. In this section, we summarise the approaches forsynthesising a MIS nanojunction. Table 1 shows the summary ofthe generalised approaches for synthesising MIS nanojunctions.

Semiconductor pretreatment

Silicon (Si) is the most widely used semiconductor substratefor the synthesis of MIS nanojunctions [41]. Prior to thesynthesis of MIS nanojunctions, pretreatment of the semi-conductor substrate usually based on the RCA standard isnecessary. The standard RCA procedure consists of immersionsteps in standard clean 1 (SC1) (5:1:1 water (H2O):ammoniumhydroxide (NH4OH):hydrogen peroxide (H2O2)), 2% hydrogenfluoride (HF) and SC2 (5:1:1H2O:hydrochloric acid (HCl):H2O2)solutions [27]. In the pretreatment procedure, semiconduc-tor Si is immersed in the 2% HF solution for 5 min to removethe native SiOx on the surface. It was also reported that thesemiconductor substrates could be dipped in aqua regia for10 min to remove the native oxides on the surface [42,43].After that, the Si semiconductor wafer was rinsed thoroughlyusing deionised water and dried by a flow of nitrogen gasbefore use [44]. Apart from the Si semiconductor, othersemiconductor substrates such as gallium arsenide (GaAs)[45], gallium nitride (GaN) [42] and cadmium sulphide (CdS)[46] were also reported useful for the synthesis of MISnanojunctions. For instance, Sönmezoğlu et al. [45] reportedthe MIS nanojunction structure based on copper (Cu)/TiO2/n-GaAs fabricated using n-type liquid-phase epitaxial (LPE)

tellurium (Te)-doped GaAs wafers. They found variation inelectrical characteristics of the Cu/TiO2/n-GaAs MIS struc-ture as a function of temperature, which was of great help inimproving the quality of TiO2 grown on the GaAs semicon-ductor substrate. Reddy et al. [42] reported a 2 nm thick Si-doped GaN layer grown on a c-plane aluminium oxide (Al2O3)sapphire semiconductor substrate by using the metal organicchemical vapour deposition (MOCVD) method. They found thebarrier heights for this MIS structure to be 0.86 eV (I–V) and0.99 V (C–V). Lee et al. [43] reported the epitaxial structureof MIS hydrogen gas sensors consisting of a 750 nm thick GaNbuffer layer and a 0.8 nm thick undoped GaN layer. The GaNlayer was grown on a c-plane sapphire substrate using theMOCVD system. They found that when MIS hydrogen–gassensors are exposed to dilute hydrogen ambience, hydrogendipoles are formed at the platinum (Pt)/zinc oxide (ZnO)interface, with electrons released back to the ZnO layer. Thehydrogen adsorption reaction leads to the reduction ofbarrier height and series resistance.

Insulator preparation

The insulator layer plays an important role in the synthesisof efficient MIS nanojunctions. To date, the most widelyused insulator layer is native SiO2, which was reported inprevious studies [38,42,47]. This is because the SiO2/Siinterface acts as recombination centres to facilitate tunnel-ling, whereas the lower trap density decreases the reversesaturation current and, thus, increases the overall photo-voltages in the MIS structure [27]. The native oxide layer ofSiO2 could be formed by exposing the cleaned n-Si surfaceto air for 10 consecutive days [48]. The most widely usedmethod for artificial synthesis of an insulator layer is atomiclayer deposition (ALD). Overall, ALD is mostly used for thefabrication of thin films through the reaction of twochemical precursors on a semiconductor surface in sequen-tial and repeated exposure to the precursors over time.Therefore, it has a feature of angstrom-resolution due tothe layer-by-layer growth of thin films. Although the natureof deposition speed in ALD is very slow, it unarguablyproduces good quality, uniform and ultrathin films over awide range of materials [49]. In a study by Seo et al. [50],the researchers synthesised thin SiO2 insulator films 1.2 μmthick by annealing at 500 1C for 150 s in an oxygen-richenvironment. Additionally, the organic molecules of copperhexadecafluorophthalocyanine (F16CuPc) and copper phtha-locyanine (CuPc) were deposited as insulators on the SiO2

surfaces from highly vacuum-conditioned Knudsen cells(i.e., 5� 10�7 Pa). They found that using ALD actuallyallows the organic molecules to be embedded in high-quality SiO2 insulator thin films, as shown in Fig. 4.

TiO2 film is another alternative insulator for MIS struc-ture. It is reported that different thicknesses of TiO2

between 1 nm and 12 nm have been deposited through theALD method on degenerately doped p-type Si semiconductorwafer substrate [38]. The researchers reported that theALD-TiO2 thin films yield water oxidation overpotentialbetween 300 mV and 600 mV at 1 mA/cm2 in aqueoussolution. Apart from the commonly used native oxides ofSiO2 and TiO2, another native oxide such as Al2O3 was alsoused as an insulator layer to provide efficient interfacial

Fig. 4 Schematic illustration of MIS nanojunction structure with F16CuPc and CuPc molecules. Reproduced from Ref. [50].


tunnelling effects within the MIS nanojunction structure.For instance, Arquer et al. [51] employed the ALD methodto assemble thin-film layers of TiO2 and Al2O3 over theindium-doped tin oxide (ITO) photoelectrode substrate. Theprecursors used for Al2O3 thin films during the ALD methodwere H2O and trimethylaluminium Al (CH3)3, whereas thetitanium isopropoxide (C12H28O4Ti) was the precursor usedfor TiO2 thin films. They found that without the Al2O3

insulator layer, the built-in field vanished and that thisdirectly resulted in the absence of open-circuit voltage. Inanother study, Kim et al. [24] synthesised the Al2O3 thinfilms on p-type Si semiconductor substrates by using theremote plasma atomic layer deposition (RPALD) method, asdepicted in Fig. 4. The RPALD method is superior to typicaltypes of ALD techniques because it incurs lower plasmadamage and can used at a low process temperature.Therefore, this technique is expected to deposit high-quality insulating layers and improve the interface proper-ties in MIS devices. The RPALD method utilises an alternativeAl (CH3)3 precursor and oxygen radicals to obtain goodinterfacial properties for MIS-IL solar cell applications.Apart from the ALD-related synthesis methods, other meth-ods for the structuration of efficient MIS nanojunctionstructure were also reported. For instance, Voitsekhovskiiet al. [52] investigated the MIS nanojunction structurebased on mercury cadmium telluride (HgCdTe) grown byusing the molecular-beam epitaxy method. To match thecrystal lattices of GaAs and HgCdTe on the semiconductorsubstrate, buffer layers of zinc telluride (ZnTe) and cad-mium telluride (CdTe) are grown, respectively. In thisinstance, the respective CdTe and ZnTe layers match thelattice constants between the semiconductor substrate andbottom HgCdTe layer. The authors found that the upperHgCdTe thin film on the working layer is necessary to(i) produce a potential barrier for charge-carriers; (ii)reduce surface recombination velocity and (iii) influencehot electron lifetime in the bulk photocarrier.

Silicon–nitride (SiNx:H) films have been widely used asinsulators in MIS structures due to their good mechanicalstrength, good dielectric properties, and good barriercapability against moisture and mobile ions [53,54]. Thereis a variety of deposition techniques that have been used todeposit SiNx:H films over a wide range of plasma pressuresand temperatures[55,56]. However, the most general tech-nique for the deposition of SiNx:H films is plasma-enhanced

chemical vapour deposition (PECVD) using reactive gases ofsilane (SiH4) and ammonia (NH3) in the temperature rangeof 200–400 1C [57]. Lin et al. [58] developed a synthesismethod to deposit graphene oxide (GO) thin films on Sisemiconductor substrate, as depicted in Fig. 5. A 100 nmthick Al-gate with a circular area of 5� 10�4 cm2 defined bya shadow mask was sputtered onto the GO-deposited side ofSi semiconductor substrate. The Al was also sputtered ontothe backside of the Si semiconductor substrate to form anohmic contact. The researchers found that the accumula-tions of dark current and inversion photocurrent in the GO-device were superior to those in the control device. Apartfrom these materials, it was reported that the use of III–Vnitrides thin films as insulating materials have been exten-sively studied for their potential applications in optoelec-tronics and microelectronics [59]. One of the promisinginsulating materials is aluminium nitride (AlN), which has ahigh dielectric constant, excellent thermal conductivity andwide band gap. The high dielectric constant in AlN isparticularly suitable for application in MIS devices, wherea thermal SiO2 layer cannot be grown [60]. Because thethickness of the insulator layer is also a critical parameter indetermining an efficient MIS nanojunction structure, it isthus important to understand how to control the thicknessof the insulator layer. For instance, Esposito et al. [27]found that the thickness of SiO2 tunnelling layers could beadjusted by varying the duration of rapid thermal oxidation(RTO) treatment. In another study, Liu et al. [44] fabricatedMIS-based solar cells on an n-type c-Si(1 0 0) semiconductorwafer substrate and found that the thickness of insulator GOthin film varies with the spin-casting rate. The synthesistemperature can also affect the physical characteristics ofthe MIS nanojunction structure. Previous studies have shownthat the SiO2/Si interfaces formed at high temperaturecontain lower trap densities than do the interfaces formedat low temperature [27].

During the water-splitting process in a PEC cell, theinsertion of an ultrathin insulator layer between the metaland semiconductor layers can substantially reduce the Fermilevel pinning, which has a deep effect on the separation ofelectron–hole pairs. According to the MIS mechanism illu-strated earlier, to obtain high performance on the MISstructure application in water splitting, the design of theinsulator is quite important. Recently, some research hasfocussed on the application of SiO2 and TiO2 MIS structure

Fig. 5 Schematic illustration of Al2O3 thin films fabricated via the RPALD method. Reproduced from Ref. [24].


for water splitting, but there have been few reports concern-ing the Al2O3 and AlN for example. In this part, we summarisedmost types of insulators, such as SiO2, Al2O3, and TiO2, and thesynthesised methods including RPALD, PECVD, etc. Somecharacterisations can be applied to the PEC water splitting.For example, various thicknesses of the insulator will lead todifferent results during the water-splitting process. Moreover,the synthesis temperature, spin-casting rate, etc. can alsoaffect the trap densities; currently no result is reported in thePEC water-splitting research.

Metal deposition

As for the metal layer synthesis in MIS nanojunctionstructures, thermal evaporation deposition (TED) is suitablefor metals due to their relatively low melting pointscompared with those of metal oxides. In addition, a veryhigh vacuum condition in the thermal evaporation chamberenables the evaporated atoms or molecules from thetargeted metals to be kept in line-of-sight deposition onthe semiconductor substrates due to almost no other gasmolecules colliding with them. For instance, metals withlow melting points that can be deposited by TED are gold(Au) [61–63], silver (Ag) [64,65], copper (Cu) [66–69], nickel(Ni) [70–72], iron (Fe) [73–76] and cobalt (Co) [77,78].Through the deposition method, different-ordered nanos-tructured arrays including nanoparticle, nanochain, nano-gap, and nanoring arrays can be achieved based on colloidalmonolayer masks by controlling the synthesis parameters,i.e., angle (θ) between the normal direction of the semi-conductor substrate with the colloidal monolayer, deposi-tion direction and rotation of semiconductor substrate [49].For instance, Liu et al. [44] deposited both Au and Al filmsas top and bottom layers by using the TED method,respectively. They found that the thicknesses of Au and Almetal layers were approximately 12 nm and 150 nm, respec-tively, and the area of each device was 0.09 cm2, asdetermined by the area of a gold electrode [44]. Similarly,in another study by Yoo et al. [57], researchers reported onthe use of the TED method to evaporate the Al metalfollowed by annealing on the back surface of a Si semi-conductor wafer to provide ohmic contact for the bulk.Meanwhile, the front Al dots with diameters rangingbetween 450 μm and 850 μm and with thickness of 0.4 μm

were evaporated onto the annealed SiNx:H film by using ametal mask [57]. Apart from the TED method, the electronbeam evaporation (EBE) method is also widely used for thepreparation of metal layers in efficient MIS nanojunctionstructures. During the EBE synthesis process, the targetedmetal is usually bombarded with an electron beam of highenergy that results in the formation of an atomic gaseousphase. These gaseous atoms are then deposited onto thedesired substrates within line-of-sight under a high-vacuumcondition in the chamber. This EBE method can be used todeposit most metals such as Au [79–81], Ag [82–84] andpalladium (Pd) [85–87], as well as semiconductors orceramics such as ZnO [85–87], ZnS [88–90], TiO2 [84,91]and Al2O3 [88–90] at a relatively high deposition rate andlow substrate temperature. Due to the potential sight-linemovement of evaporated metal or semiconductor atoms,the colloidal monolayer can be used as a marker to fabricatethe periodic array via the EBE deposition method [49]. Oflate, the EBE is an emerging deposition method used for thefabrication of Si thin-film devices in microelectronics andphotovoltaics at high growth rates of up to a few micro-metres per minute. This is by far the highest growth ratesfor Si thin films and has even exceeded rates achievable byusing the conventional plasma enhanced chemical vapourdeposition (PECVD) method by more than a factor of 20.With the EBE method, PECVD will further contribute tosubstantial cost reduction potential by enabling industrial-scale production [92]. The schematic of the EBE system isshown in Fig. 6. A number of previous studies reported theapplication of EBE in the synthesis of thin metal layers[93–95]. For instance, Kenney et al. [96] reported thefabrication of MIS-based electrodes, wherein the Ni thinfilms were initially deposited on raw P-doped (1 0 0) n-typeSi semiconductor wafers using the EBE method at a deposi-tion rate of 0.2 Å/s. This was followed by the formation ofohmic contact on the backside of the Si semiconductorwafers via the EBE deposition of Ti metals. The researchersmeasured the thicknesses of Ni and Ti films at approxi-mately 2–20 nm and 20 nm, respectively. Similarly, Seo et al.[50] reported the EBE deposition of Au thin films with aneffective area of 3.1 mm2 on top of an Al2O3 layer through ametal shadow mask. With reference to the MIS nanojunctionstructure, generally the Au and Si layers are functionalisedas the top metal and bottom semiconductor layers, respec-tively [50]. More interestingly, it was reported that ordered

Fig. 7 Schematic of the electron-beam evaporation (EBE)system.

Fig. 6 Synthesis flow diagram for graphene oxide depositionon Si semiconductor substrate. Reproduced from Ref. [58].


arrays of metallic collectors such as Pt and Ti could besequentially evaporated through shadow masks onto oxide-covered semiconductor wafers via the EBE method [27].Reddy et al. [42] also reported on the EBE of a Au Schottkycontact of 50 nm thickness and 0.7 mm diameter on GaNthin film through a stainless steel mask. This was followedby a sequential deposition of a 20 nm-thick SiO2 layer and a50 nm-thick Au layer on the GaN film using the same EBEsystem. It was reported that the Au evaporation process wascarried out in a vacuum coating unit at a pressure ofapproximately 5� 10�6 mbar [42].

Key challenges in the development of MISnanojunctions

Semiconductor substrate

For the synthesis of MIS nanojunctions, Si is the most widelyused semiconductor substrate, as it is an earth-abundantelement and a suitably efficient photovoltaic material.However, the major disadvantage of the use of Si as asemiconductor substrate is that it can be easily corroded inelectrolyte. The corrosion in electrolyte issue can beresolved by depositing an insulator layer such as TiO2 orAl2O3 to improve its long-term stability. Previously, Chenet al. [41] synthesised an ultrathin TiO2 layer of 2 nmthickness via the ALD method to protect the Si semiconduc-tor wafers from corrosion. This was followed by the coatingof an optically transmitting layer of Iridium (Ir) catalyst of3 nm thickness to catalyse the oxygen evolution reaction(OER) (Fig. 7). In general, PEC water oxidation occurs belowthe reversible potential, whereas dark electrochemicalwater oxidation exhibits low-to-moderate overpotentialsat all pH values, resulting in an inferred photovoltage of550 mV. In Chen et al. [36], the presence of an ultrathinTiO2 layer was found to improve the corrosion-resistantproperties of the MIS nanojunction structure. From thestudy, it was observed that the water oxidation wassustained at the MIS-based anodes for a number of hoursunder harsh pH and oxidative environments, whereas theblank controls of pure Si anodes without the TiO2 layerswere observed to quickly fail. The researchers concludedthat the desirable electrochemical properties for theseMIS-based anodes are made possible by the low electrontunnelling resistance (o0.006 Ω cm2 for p+-Si) and uniformthickness of ALD-TiO2 [41].

Another study by Chen et al. [38,41,97] found that theirMIS nanojunction structure of Si/SiO2/TiO2/Ir showed asolar-to-oxygen conversion efficiency (SOCE) of 0.37%. Theyattributed the high SOCE to the high barrier-height defined bythe MIS nanojunctions and the high activity of the Ir noblemetal layer. However, this MIS nanojunction structure remains achallenge when it is being used on other crystal structures orhigh-profile photoelectrodes, as earth-abundant materials areusually preferred for large-scale production of solar fuels [98].Kim et al. [24] found that the lifetime of MIS nanojunctions canbe prolonged by protecting them using an ultrathin Al2O3 layer.This finding was further supported by Choi et al. [23], whoreported that the presence of an ultrathin Al2O3 layer preparedusing the ALD method is very effective for the protection of a Siphotocathode against oxidation. In this instance, the ALDmethod allows for conformal coatings with sub-nanometreprecision and, thus, lends itself to the protection of nanos-tructured Si photoelectrodes. The presence of an ultrathinAl2O3 layer was also reported to be beneficial for overpotentialreduction via surface passivation. The ultrathin Al2O3 layer canhelp generate a steady and stable photocurrent during con-tinuous operation for 12 h. However, the key challenge still liesin the poor durability of long-term protection of photoelec-trodes in the PEC system. Recently, the most noteworthy findingis the ultrathin Ni film that finish-protects Si photoanodes forthe water-splitting application [5]. Kenney et al. [96] foundthat the deposition of an ultrathin Ni film of 2 nm thickness onn-type Si/SiO2 surfaces resulted in a high-performance MIS-based photoanode for PEC water oxidation even under extremeaqueous borate buffer and KOH solutions of pH9.5 and pH14,respectively (Fig. 8). In this instance, the ultrathin Ni film actsas a surface-protection layer against corrosion in addition tofunctioning as a non-precious metal electrocatalyst forenhanced oxygen evolution. From the study, the resultant Ni/n-Si photoanodes exhibited high PEC water oxidation activitywith low onset potential (�1.07 V vs RHE), high photocurrentdensity and durability in 1 M aqueous KOH solution. In addition,the Ni/n-Si photoanodes showed no sign of decay even after80 h of continuous PEC water oxidation in a mixed lithiumborate-potassium borate electrolyte solution. The measuredhigh photovoltage was attributed to the high built-in potentialin the MIS-based device with an ultrathin and incompletescreening Ni/NiOx layer from the electrolyte. According to thediscussion by Turner [5], however, the current finding by Kenney

Fig. 8 Ir/TiO2/Si nanocomposite photoanode for water oxidation. (a) Schematic diagram; (b) water electrolysis using n-Sisubstrates in the dark under acidic (green dots), neutral (red dots) and basic (blue dots) solutions and 1 sun solar simulated light foracidic (green solid line), neutral (red line), and basic (blue line) solutions. Dashed vertical lines in (b) represent the thermodynamicredox potential for water oxidation at the appropriate pH. Scan rates were 0.1 V/s and potentials were corrected for solutionresistance as measured by impedance spectroscopy. Reproduced from Ref. [41].


et al. [87] is still preliminary and requires further study using aMIS nanojunction structure to enable feasible integration into aPEC water-splitting device. They also suggested that effortshould be devoted towards reconsideration of a long-held beliefconsidering n-type Si the most efficient photoanode for oxygenevolution. Because the recent techno-economic analysisshowed that STH efficiency of at least 15% is required toproduce cost-competitive hydrogen, this approach has openedup possibilities for the structuration of a MIS-based solar-assisted PEC water-splitting system [5].

Ultrathin insulator layer

In accordance with the basic principles of MIS nanojunctions,the ultrathin insulator layer plays an important role in ensuringhigh performance of solar conversion efficiency. A previousstudy showed that the insertion of an ultrathin insulator layerbetween the metal and semiconductor layers in a Schottkyjunction can substantially reduce Fermi level pinning [99]. Asimilar trend in the reduction of Fermi level pinning was alsoreported in other studies [38,100]. Esposito et al. [27] proposeda novel MIS-based photoelectrode architecture (Fig. 9) thatallows for stable and efficient water-splitting activity usingnarrow-band gap semiconductors. In the study, the perfor-mance of Si-based MIS photocathodes was substantiallyimproved through the combination of a high-quality thermalSiO2 layer and bilayer metal catalysts of Ti and Pt layers,respectively. Under light illumination, the thin and negativelycharged inversion layer induced by the electrolyte wouldsignificantly reduce photogenerated charged carriers recombi-nation, allowing electrons to be transported over a very longdistancec(Le+D). The benefits of reduced recombination havebeen fully established in previous studies [101,102] for MIS-typephotovoltaic cells based on inversion layers (Fig. 10). Thelateral transport of electrons within the inversion layer canbe ascribed to the differences in the band alignment of MIS andliquid junctions, as well as the lateral gradients in electronconcentration bolstered by non-uniform illustration from

shading. These findings have important implications for thefurther development of MIS-based photoelectrodes and offerthe possibility of achieving a highly efficient PEC water-splittingreaction.

Arquer et al. [51] also investigated the effects of aninsulator layer on MIS nanojunction structures. They devel-oped a MIS nanojunction heterostructure for plasmo-electricenergy conversion, which is a novel architecture to harvesthot electrons derived from plasmonic excitations. In thestudy, the insulator interfacial layer was found to play acrucial role in the interface passivation. This is a requisite inphotovoltaic applications to achieve higher open circuitvoltages of up to 0.5 V and fill factors of 0.5 (i.e., thecoefficient parameter). The introduction of an insulatorlayer is observed to modify simultaneously the hot-electroninjection and transport, respectively. In addition, they alsoinvestigated the influences of passivation on differentmaterial configurations and characterised different typesof electron transport depending on the initial plasmonenergy band. It was found that the PCE of nanopatternedsilver electrodes was 0.03%. This design of nanostructurescan increase the poor efficiency of electron–hole generationof photon radiation, which surmounts the limited applica-tion of complex metallic nanostructures in the past.

However, the key challenge to achieving a higher perfor-mance MIS nanojunction structure is the low quantumefficiency of this novel architecture. The low quantumefficiency of the novel MIS nanojunction structure, which isfar from the theoretical efficiency, can be attributed to thefollowing limitations: (i) poor efficiency of electron–holegeneration over photon reradiation for localised surfaceplasmon resonance (LSPR); (ii) small tunnelling probability;and (iii) cooling and recombination rates of electrons in themetal are faster than the tunnelling rate into the semicon-ductor. These limitations can be surmounted with the use ofmore-complex metallic structures with tailored photonicdensity of states [51]. This enables an increase in plasmon-to-hot electron conversion efficiency by manipulating theinterfacial electronic properties. A proper tuning in the

Fig. 10 Cross-sectional high-resolution transmission electron microscopy (HRTEM) image of a standard bilayer 20/30 nm Pt/Ticollector deposited on a 2 nm thick RTO SiO2|p-Si(1 0 0) substrate. Reproduced from Ref. [27].

Fig. 9 (a) Doubling up for solar hydrogen production. A design configuration is shown where two separate semiconductors withdifferent band gaps are illuminated in series to form a tandem system for water splitting. Sunlight illuminates the p-type electrode,which absorbs the visible light and transmits the red and near-infrared light that then illuminates the n-type electrode. (b) Cyclicvoltammograms of 2-, 5-,10-, and 20-nm Ni-coated n-Si anodes in 1 M KOH under illumination with a xenon lamp (150 W; 225 mW/cm2).Reproduced from Ref. [5].


interfacial electronic properties of the resultant plasmonicMIS nanojunction structure is crucial to vary the rates ofradiative and Landau damping mechanisms, which can resultin favourable hot-electron generation. In addition, the use ofplasmonic nanostructures supporting Fano resonances also canreduce re-radiation channels to increase hot electron genera-tion. Overall, the engineered MIS nanojunction structures areemployed to improve hot electron collection. Additionally, theresearch on novel passivating layers will open up excitingopportunities for precise control of the transport and electro-nic properties of these MIS-based devices. This will drivefuture developments towards achieving high quantum effi-ciency, open circuit voltage and fill-factor.

Scheuermann et al. [38] discussed the potential effects onwater oxidation performance by varying (1) the nanoscale TiO2

thickness and (2) different catalysts in the catalyst/TiO2/SiO2/SiMIS-based anodes, as shown in Fig. 11(a). Their study found thatuniform ALD-TiO2 films with 1–12 nm thickness on degeneratelydoped p+-Si semiconductor substrate are capable of yieldingwater oxidation overpotentials of 300–600 mV at 1 mA/cm2 inaqueous solution (pH0-14). In addition, they investigated theelectron–hole transport through the Schottky tunnel junctionstructures of varying TiO2 thickness using the reversible redox

couple of ferri/ferrocyanide. Through their study, it was foundthat the dependence of water oxidation overpotential on ALD-TiO2 thickness exhibits a linear trend that corresponds toapproximately 21 mV of added overpotential at 1 mA/cm2 forevery nanometre of TiO2 thickness (42 nm). For thinner ALD-TiO2 films (o2 nm), an approximate thickness-independentwater oxidation overpotential was observed. The linear beha-viour for photoanodes with thicker TiO2 films (42 nm) isconsistent with the predicted effect of bulk TiO2-limitedelectronic conduction on the voltage required to sustain thecurrent density across the TiO2/SiO2 insulator stack (Fig. 11b).In a similar study, eight different metal catalysts of 1–3 nmthickness were studied: iridium (Ir), ruthenium (Ru), platinum(Pt), aluminium (Al), nickel (Ni), gold (Au), and cobalt (Co). Thestudy concluded that both the ultrathin Ir and Ru films of 3 nmthickness resulted in the highest water oxidation performance.

Metallic collector

Based on the MIS structure mechanism, the metal layerplays an important role in ensuring high performance onsolar conversion efficiency. Under light exposure at the

Fig. 11 (a) Schematic side-view of Pt/Ti/SiO2/p-Si(1 0 0) photocathode illustrating the proposed scheme for lateral transport ofphotogenerated electrons through an inversion channel and hydrogen spillover-assisted H2 evolution off the SiO2 surface by aVolmer–Heyrovsky mechanism. H* represents the split over species. (b) Current–voltage curves recorded for various samples undersimulated AM 1.5 illumination (100 mW/cm2). The sudden changes in limiting current at negative potentials in the LSV curves aredue to the build-up and detachment of H2 bubbles from the electrode surface. Reproduced from Ref. [27].


inversion bias, excess electron–hole pairs are generated insemiconductors and contribute to the photocurrent. For theMIS structure with a non-transparent metal gate, theincident light is incorporated into the semiconductor fromthe edge of the metal gate. To increase the photocurrent, ingeneral, a transparent gate could be utilised and theincident light would be incorporated into the semiconductordirectly from the gate. Transparent gates can be roughlyclassified into two categories. In the first category are thetransparent conducting oxides (TCOs) such as indium tinoxide (ITO) or zinc oxide (ZnO). However, the traditional n-type TCO has a large-hole effective mass and a low-holeconductivity. In the second category are the metal filmswith thicknesses of approximately 10 nm. Because themetal film is so thin, absorption of incident light in themetal is suppressed. A suitable selection of the metal in theMIS structure may reduce the dark current. If high work-function metal were used, the dark current could bedecreased with the suppression of electron tunnelling fromthe metal to n-Ge. Dark inversion current reduction hasbeen achieved with the high work-function metal, Pt(5.65 eV), instead of the typical low work-function metal,Al (4.1 eV), for Ge MIS structure. On the other hand, using ahigh work-function metal contact would cause significantlight attenuation from the thick protection coatings, whichwould lead to decoupling of light absorption and anelectrochemical reaction. It would be interesting to exam-ine the earth-abundant transition metals with a high workfunction (such as Cu, Ni, Mo, etc.).

The suitable gate electrode selected here is Al instead ofPt, which has been suggested. If the semiconductor isn-type, then the selection of the high work-function metal,Pt, can suppress the electron-tunnelling current from themetal to the semiconductor at the inversion (negative) bias.However, for this SiGe/Si QDIP structure, the semiconductoris p-type. Pt will lead to a hole-tunnelling current at theinversion bias, which should be avoided. It is well knownthat Ni-based catalysts exhibit the highest OER activity inbasic conditions among first-row transition metals. Forexample, in Kenney’s report on the Ti/n-Si/SiOx/Ni MISstructure, the thin Ni film on Si played multiple roles duringPEC water oxidation. First, the metallic Ni at the n-Si

interface formed a junction to afford band bending, therebyfavouring charge separation and motion of photoexcitedholes towards the OER catalyst–electrolyte interface. The Nifilm thickness provides a degree of tuning of the built-inpotential and photovoltage. Second, surface Ni speciesserved as the active electrocatalyst for OER. At highpotentials, oxidised Ni species are formed in situ and actas OER active sites, and the OER activity is only slightlyinferior to that of precious metal catalysts. Finally, the Ni/NiOx film provides excellent protection to Si. In particular,remarkable stability was conferred by a 2-nm Ni coating onthe Si photoanode in a mixed aqueous K-borate/Li-borate(pH=9.5) electrolyte.

Ir film is also suitably employed as a metal collector; forexample, Chen et al. [97]used a high-quality ultrathin TiO2

within the tunnelling range to protect a n-Si photoanodeand a thin Ir film for catalysing an OER reaction. Thisstructure showed the champion solar-to-oxygen conversionefficiency (SOCE) of 0.37% due to the high barrier heightdefined by the MIS junction and the high activity of Ir noblemetal. This technique remains a challenge when used onother crystal structures or high-profile photoelectrodes.Earth-abundant materials are preferred for large-scaleproduction of solar fuels. A few other recent studies havedemonstrated the capability of using transition metal oxidesor a single layer of graphene to protect and catalyse n-Siphotoanodes for water oxidation. The uniform coating ofmetal oxide does not provide a large surface area, which isimportant for non-noble metal catalysts with inferior cata-lytic activity. Due to the pinholes developed during fabrica-tion or operation, long-term stability of the thin oxide isalso a concern. An insulating layer can almost fully passivatethe semiconductor surface. In this case, the semiconduc-tor’s equilibrium band bending is dominated primarily bythe difference between the (n-type) semiconductor elec-tron affinity and the metal work-function, following theSchottky–Mott model. Then, with the appropriate semicon-ductor–metal combination (n-semiconductor/high work-function metal; p-semiconductor/low work-function metal)the semiconductor interface will be strongly depleted oreven inverted. In the latter case, a p–n homojunction formsunderneath the insulating layer.


MIS design

To be utilised in a commercial process, a photoelectrochemicalwater-splitting device should have an affordable catalyst thatminimises the overpotential and increases the efficiency of thewater oxidation half-reaction, and the MIS device must becapable of absorbing a large fraction of the solar spectrum. Incomparison with Schottky barrier solar cells, MIS Si solar cellswith an insulator layer (typically SiOx) between the metal andthe Si show higher open circuit voltage (Voc) and superior deviceperformance. The purpose of the insulator layer in the MIS solarcells is to block the majority carriers injected into the metal atforward bias to reduce surface recombination. However, as forthe conventional MIS Si solar cells, elevated temperature isusually needed to deposit a 2–3 nm thick SiOx layer, which notonly complicates the fabrication process but also increases theenergy consumption and cost to a certain extent.

Therefore, investigation of high quality but low-costinsulating layers is of great importance for future MIS Sisolar cells. Graphene oxide (GO), a derivative of graphene,attracts much attention in optoelectronic devices due to itssolution processability as well as its tuneable work function.These studies experimentally demonstrated that solution-processed GO can be used as an effective insulating layerfor MIS Si solar cells. Compared with the conventionalinsulating SiOx layer, the preparation of GO via solutionprocessing has a far lower temperature, which can minimisethe grain boundary diffusion in doped crystalline Si, pre-serve minority carrier lifetime and reduce energy consump-tion. In addition, the highly water-soluble GO nanosheetscan produce a good contact with the hydrophilic siliconsurfaces, which is critical for surface passivation and carriertransportation. A possible improvement is to passivate themetallised surface regions with an ultra-thin dielectric,allowing a lighter (either local or global) dopant diffusionto be used. This dielectric must be sufficiently thin topresent negligible resistance to current flow (possibly viaquantum mechanical tunnelling) whilst being sufficientlythick to provide appreciable surface passivation. The sameultra-thin layer can be applied to the entire wafer surfacewith a capping layer applied in the non-metallised regions.

The metal layer is vital, however, as very little charge willpass without this layer acting as a tunnel current mediatorbetween the silicon substrate and redox species in solution; aseveral order-of-magnitude decrease in current was previouslyobserved when the oxygen-evolution-reaction metal wasomitted. Without a sufficient density of states, tunnelling willlikely be dramatically reduced, thus limiting the overall deviceefficiency. Changing the metal is anticipated to have twoprimary effects on the performance of a Schottky junctionphotoanode device: (1) altering the built-in potential essentialfor separating the photo-generated excitons by changing thecatalyst work function, and (2) altering the water oxidationkinetics by changing the nature of the catalytic site. If theFermi level in the MIS device were pinned, the photovoltagecould no longer be manipulated to improve device perfor-mance. The thickness of the metal was also varied to determinehow this affected water oxidation and charge transfer effi-ciency. However, even-thinner metal layers could be employedin photoelectrochemical water splitting devices, as efficiencymay not suffer in basic and neutral solutions and should

decrease only slightly in acid, whereas the amount of preciousmetal catalyst, and thus the cost, could be reduced.

In addition to varying the thickness of the metal layer, theidentity of the catalyst should be varied. Water-oxidationcatalysts that are more earth-abundant are desirable.Moreover, different catalysts may be optimal for differentelectrochemical reactions. It is reported that Pt is not asgood a water oxidation catalyst as Ir or Ru. Despite its lowerefficiency for catalysing water oxidation, the Pt layer stilldemonstrates a comparable ability to mediate chargetransfer to and from the silicon substrate as evidenced bythe ferri/ferrocyanide half peak-to-peak splitting result.Gold is not a good water oxidation catalyst unless exten-sively nanostructured and combined with other materials.The search for a cheap, efficient and robust water oxidationcatalyst made from earth-abundant elements has led to thediscovery of a cobalt–phosphate (Co–Pi) catalyst found tooxidise water in neutral pH.

Other photovoltaic structures based on MISnanojunctions

The defining similarity between PEC and PV devices is thatboth are designed to harness the energy of hole–electronpairs created by light absorption (usually in a semiconductoror a molecule), by separating them and causing them torecombine through a work-producing route. The definingdifference is that a PEC device contains an electrolytephase, in which ions carry the moving charge, and elec-trode/electrolyte interfaces at which electrochemical reac-tions occur. PEC cells can also be configured as ‘wet PVdevices’. A number of different approaches are possiblewith semiconductors as the photo converter. The mostdirect, brute force approach employs a solid-state photo-voltaic solar cell to generate electricity. The electrolysis ofwater at a reasonable rate in a practical cell requiresapplied voltages significantly larger than the theoreticalvalue (1.23 V at 25 1C), and electrolysis energy efficienciesof approximately 60% are typical. Thus, the efficiency of thecombined solar/electrolyser system using commerciallyavailable components is close to the desired 10% definedfor solar hydrogen generation. Moreover, the componentsare rugged and should be long-lived. The problem with sucha system is its cost. Solar photovoltaics cannot currentlyproduce electricity at competitive prices, and hydrogenfrom water electrolysers is significantly more expensivethan that produced chemically from coal or natural gas.Another alternative system involves the semiconductorphotovoltaic cell immersed directly in the aqueous system.At the least, this eliminates the costs and mechanicaldifficulties associated with separate construction and inter-connection of solar and electrochemical cells. In one suchsystem, the electrodes are composed of single or multiplejunctions based on semiconductors that are irradiated whilethey are within the cell. This simpler apparatus is attainedat the cost of encapsulating and coating the semiconductorsto protect them from the liquid environment and most likelywith a more limited choice of electrocatalyst for O2 or H2

evolution as shown in Fig. 13.


In principle, the direct PEC water-splitting device formolecular cleavage of water molecules into hydrogen andoxygen [5] combines a photovoltaic cell and an electrolyserinto a single device. Because MIS-based structures arewidely used in the fabrication of photovoltaic cells, recentdevelopments in photovoltaic cells are also reviewed, asthey can be potentially applicable in the development ofphotoanodes for water-splitting applications.

As previously discussed, the ultrathin insulator layer playsan important role in the MIS nanojunction structure. In thisstudy, we focused on the different insulator layers used inthe MIS-based photovoltaic cells that are replicable for PECwater-splitting devices. This finding is observed because thecharacteristics of an MIS Schottky junction and a p–njunction are similar, as they are based on similar minoritycarriers transport and activation energies [103,104]. In theprevious section, the significance of a native oxide layer ofSiOx in the Si-based MIS solar cells on efficient chargecarriers’ separation and minority carriers’ transport throughtunnelling was elaborated and discussed in detail. Thetransport mechanism was proven on the carbon nanotube(CNT)/silicon (Si) solar cells, where improved device char-acteristics were observed after the insertion of a thin SiOx

layer on the Si surface [105]. In contrast, Jung et al. [32]found that single-walled carbon nanotube (SWCNT)/Si solarcells showed that enhanced power conversion efficiency(PCE) was achieved when the ultrathin insulator layer ofSiOx was removed by using HF. In this instance, it wasreported that the presence of an insulator SiOx layer in theSWCNT/Si solar cells could significantly degrade the shortcircuit current density (Jsc), which is inconsistent with theMIS Schottky nanojunction model.

Kim et al. [24] fabricated Si-based MIS solar cells withultrathin Al2O3 films of 1–6 nm gate dielectric and SiNx filmsby using RPALD at 300 1C and room temperature, respec-tively. The resultant Si-based MIS solar cells were fabricatedwith a resistivity of 1 Ω cm on p-Si semiconductor waferspassivated with ultrathin Al2O3 and SiNx films and were ableto achieve a relatively high energy conversion efficiency of8.21%. As Al2O3 films formed by RPALD indicate a self-limiting surface reaction, they were able to offer excellentatomic-level control of the deposited film thickness. Smoothinterface and uniformity of the Al2O3/silicon interfaceswere observed. After the RPALD deposition of Al2O3 filmson Si surfaces, the lifetime increased more than three timescompared with those without Al2O3 films due to passivatedsurface effects. The interface state density with the SiNx/Al2O3 structure was approximately one order higher inmagnitude than that with only the Al2O3 film because ofthe charges incorporated in the SiNx film during the process.Song et al. [25] postulated that the ultrathin Al2O3 filmforms a built-in potential on the bare p-Si surface due to thepresence of negative fixed charges within the Al2O3 film.These negative fixed charges will attract the majoritycarriers (holes for p-Si) to form an accumulation layer ofholes close to the surface of p-Si, leading to an upward bandbending. In this instance, the height of the band bending isdefined as the surface potential (i.e., barrier) of the bare p-Si. When the minority carriers (i.e., electrons for p-Si) arerepelled from the p-Si surface, an improvement in thelifetime of minority carriers that is called the field-effectpassivation can be realised (Fig. 12).

Bullock et al. [47] investigated the contact properties ofAl2O3 and SiO2 passivating dielectrics in MIS-typed contacts onthe phosphorus-diffused regions. In both cases, the increase indielectric thickness leads to a reduction in surface recombina-tion but an increase in the contact resistivity. The optimumthickness of ALD-Al2O3 and thermal SiO2 layers were found tobe 22 Å and 16 Å, respectively. The SiO2 MIS contact was foundto have a greater thermal stability than that of the Al2O3.However, it is worth noting that the Al2O3 and SiO2 passivatedcontacts also could be applied uniformly to the n+ rear side ofa p+nn+ MIS structure. In that case, the tolerable contactresistivity for a 100% metal contact fraction is far higher than apartial metal grid. Liu et al. [44] reported that the solution-processable GO thin film was initially utilised as an insulatorlayer in the construction of Si-based MIS solar cells as shown inFig. 13. It was found that the efficient water-soluble GOnanosheets with controlled thicknesses produced a goodcontact with the hydrophilic Si surfaces. The average opencircuit voltage (Voc) of the GO-incorporated Si-based MIS solarcell was increased by 0.2 V, whereas the PCE was found to be88% higher than that of the corresponding Schottky solar cells.These improvements via the incorporation of GO nanosheets inthe Si-based MIS solar cell are attributed to the increasedbuilt-in potential, as well as the reduced interface defectsthat result in reduced carrier recombination. The ability toestablish a low-cost and solution-processable thin insulatinglayer will open the door for wide application in photovoltaicand other optoelectronic devices. This GO insulator can alsobe applied as an effective insulating layer for MIS structure inwater-splitting PEC cells. These results open up a newapproach to increasing built-in potential and reducing inter-face defects, which is useful in the MIS water-splittingapplication.

In recent years, many studies have focused on utilisinggraphene [6,44,58,106], particularly the GO because it is agood insulator material and an intermediate product to formgraphene. Thus, it has been significantly used to form agraphene insulator layer for applications in MIS nanojunctionstructures [58]. The insulator thickness decreases as thedimension of a transistor becomes smaller, resulting in asignificant gate tunnelling current. Such a tunnelling currentin the vertical direction (from metal to semiconductor or fromsemiconductor to metal) of a MIS structure has beenemployed in a number of applications, such as solar cells.For solar cells, the thickness of the insulator layer in the MIStunnelling diode is critical. For example, if the insulator is toothick, only limited tunnelling can occur, leading to a smallresponsivity. In contrast, if the insulator layer is too thin,Fermi level pinning may degrade the device current–voltagecharacteristics. In addition to the thickness dependence,insulator composition also affects the IV characteristics ofMIS devices. The GO layers were deposited onto Si substratesto form a MIS tunnelling structure and the optoelectronicproperties were studied. The accumulation of dark currentand inversion photocurrent from the GO-incorporated MIS-based device were found to be superior to the control MIS-based device. The incorporation of GO also improved therectifying characteristic of the diode and enhanced itsresponsiveness as a photodetector. At the gate voltage of2 V, the photo-to-dark current ratio for the GO-incorporatedMIS-based device was 24, which was higher than the measuredratio in the control MIS-based device, as shown in Fig. 14. GO

Fig. 13 Schematic diagrams of different types of semiconductor-based systems proposed for solar water splitting: (a) solid statephotovoltaic cell driving a water electrolyser; (b) cell with immersed semiconductor p/n junction (or metal/semiconductor Schottkyjunction) as one electrode.

Fig. 12 (a) Water oxidation i–E curves in basic solution using Ir/TiO2/SiO2/p+-Si anodes with different thicknesses of insulator TiO2

films. The vertical dashed line indicates the thermodynamic potential for water oxidation in base, 0.404 V, and the horizontal solidline indicates 1 mA/cm2, the characteristic current density at which all overpotentials are measured. (b) Band diagram of Ir/TiO2/SiO2/p

+-Si stack biased anodically to �0.2 V. The range of trap energies is indicated by the grey shading centered 1 eV below theconduction band. Reproduced from Ref. [38].


was deposited onto the Si substrate to form a Al/GO/nativeoxide/Si MIS tunnelling diode. Yoo et al. [57] fabricated theMIS-based devices using SiNx:H thin films as insulator layers byPECVD, where the insulator layers were analysed in thetemperature range of 100–400 K by using capacitance–voltage(C–V) and current–voltage (I–V) measurements. In addition,the annealed SiNx:H thin films were evaluated by using theelectrical properties at different temperatures to determinethe effect of surface passivation. From the study, theyreported that the energy conversion efficiency of 18.1% wasachieved under one-sun standard testing conditions for large-area (156 mm� 156 mm) Si-crystalline MIS-based solar cells.SiNx:H films function well as gate insulators in MIS devices dueto their good dielectric properties, being excellent barriersagainst moisture and mobile ions. Moreover, the depositionmethod PECVD in this report is a quite cost-effective,performance-enhancing technique that can provide an anti-reflection coating, as well as a surface passivating layer, forcrystalline-silicon solar cells. Wadhwa et al. [102] proposed anew type of Si-crystalline MIS-based solar cell as shown inFig. 15. In comparison, the newly proposed Si-crystalline

MIS-based solar cell is similar to the MIS-based PEC water-splitting cell. Within the Si-crystalline MIS-based solar cell,the liquid electrolyte creates a depletion (inversion) layer inthe n-type Si wafer substrate. However, no regenerativeredox couple is present to facilitate the charge transferbetween the Si and the counter electrode. Instead, the holestrapped within the electrolyte-induced inversion layer willdiffuse along the layer until they come to widely spacedgridlines for extraction. The gridlines consist of a SWCNT filmetched to cover only a fraction of the n-Si surface. Furthermodelling and simulation studies showed that the inversionlayer is a natural consequence of the device electrostatics.The electronic gating was demonstrated to boost the effi-ciency whereby the cell achieved a PCE of 12%, exceedingthat of dye-sensitised solar cells. Such cells can be applied totrap more of the light reflected from a surface by thetexturing of the Si. Additionally, this MIS junction cell withSiOx layer replaced by the ionic liquid electrolyte may providethe solution to the previous degradation problem even with-out the need for active gating (although gating incurs littleenergetic penalty, it does add to the device complexity). The


large spacing permitted between the grid lines in thiselectrolyte-coated device indicates that by filling the nano-holes with electrolyte, we would obtain the benefit of boththe inversion layer and the additional light trapping.

Previously, it was known that the annealing temperaturealso has an important effect on the characterisation of MISnanojunction structures. For instance, Ortiz et al. [60]

Fig. 15 (a) Device architecture of GO-based MIS Si solar cells,photocurrents versus voltage characteristics of GO and control devThe accumulation (negative bias) dark current and inversion (posdevice. Reproduced from Refs. [44, 58].

Fig. 14 Band diagrams of (a) Al2O3/p-Si and (b) Al2O3/SiO2 1L/p-including interface states; red open and filled rectangles denote accnear the valence band, respectively. (d) Calculated surface potentifor a σrms value of approximately 11.7 nm; solid black, red, and bluewhile the blue dashed line denotes the applied voltage of �0.65 V

investigated the electrical properties of Au/AlN/n-Si/GaMIS-based diodes by capacitance–voltage (CV) and current–voltage (IV) techniques. In the study, the MIS-based diodessynthesised via the molecular beam epitaxy approach showedimproved electrical properties after annealing treatment at200 1C under nitrogen ambient, which were attributed topartial out-diffusion of mobile ions from the thin films. The

with the inset: a schematic of GO structure; (b) dark andices. The inset shows the structures of GO and control devices.itive bias) photocurrent of the GO device are superior to the

Si structures. (c) Band diagram of Al2O3/SiO2 1L/p-Si structureeptor-like states near the conduction band and donor-like statesals of Si as a function of the applied voltage to an MIS capacitorlines represent the surface potential in panels a–c, respectively,for a flat-band condition. Reproduced from Ref. [25].


use of II–V nitrides thin films as insulating materials has beenextensively studied for its potential applications in optoelec-tronics and microelectronics. One of these promising materi-als is AlN, which has a high dielectric constant, excellentthermal conductivity, wide bandgap and high velocity ofsound amongst others. Insulators with high dielectric constantsuch as AlN are particularly suitable for applications inMIS devices where thermal SiO2 cannot be grown. Theperformance of these devices will strongly depend on theinsulator–semiconductor interface quality. In another study,Sönmezoğlu et al. [45] reported on the fabrication of a Cu/TiO2/n-GaAs MIS nanojunction structure. The variation inelectrical characteristics of this MIS nanojunction structurewas analysed as a function of temperature using current–voltage (I–V) measurements in the temperature range of 50–290 K with a step increase of 40 K. It was shown that theideality factor (n) decreases with increasing temperature,whereas the zero-bias barrier height (ϕb,0) increases withincreasing temperature. In this instance, the Gaussian poten-tial model was used to explain the barrier height inhomo-geneity observed in the MIS nanojunction structure. Thisdiscrepancy can be explained by the local inhomogeneity atthe metal–semiconductor interface by considering the fluc-tuation in the local surface potential. According to theGaussian distribution, the value of the Richardson constantthat was obtained from fitting the modified Richardson plot inthe first 290–170 K region is in very good agreement with thetheoretical value for n-type GaAs. The result showed thatunderstanding the temperature dependence of electrical char-acteristics on this MIS nanojunction structure may be of greathelp in improving the quality of TiO2 grown on GaAs substratesto aid the future development of device technology.

In a separate study, Voitsekhovskii et al. [52] found thatthe resistance in epitaxial film can significantly affect themeasured electrical characteristics of MIS nanojunctionstructures based on HgCdTe by molecular beam epitaxy(MBE) at a high-frequency test signal. The formation of anear-surface graded-gap layer with high CdTe content in theepitaxial HgCdTe film leads to changes in both the low- andhigh-frequency capacitance–voltage (C–V) characteristics inthe inversion mode. In addition, the formation of a near-surface graded-gap layer also leads to an increase in thedensity of mobile charges for the MIS nanojunction struc-tures with anodic oxide film (AOF) and SiO2/Si3N4 insulators.

Liu et al. [107] reported a novel MIS nanojunctionstructure configuration that can be used to measure thesurface photo-voltage (LS) spectroscopy of p-GaAs and p-AlxGa1�xAs/p-GaAs structures, respectively. In the study,the p-AlGaAs layer was assembled on a p-type GaAssubstrate grown by the MOCVD method. The SPV was foundto be dependent on the incident photo-intensity, whereasthe band-to-band excitation was adopted for the calculationof ideality factor. The ideality factor of this MIS nanojunc-tion structure configuration was found to be 0.008 forsamples with both sides polished under ambient air. As forthe p-AlxGa1�xAs/p-GaAs MIS nanojunction structure, theminority carrier diffusion length in the GaAs substrate wasdetermined from a linear plot of inverse SPV against theinverse absorption coefficient by intercepting the line withthe x-axis. The SPV spectrum of the p-Al0.63Ga0.37/p-GaAsMIS nanojunction structure showed that the incident light isabsorbed in the AlGaAs layer when the wavelength is

λo580 nm but is absorbed in the GaAs substrate when thewavelength is between 580 nmoλo870 nm.

Furthermore, Armin et al. [108] adopted a novel methodof injection-charge extraction by linearly increasing thevoltage in an MIS structure (MIS-CELIV) to measure both theelectron and hole mobilities of organic semiconductors in adiode structure of relevance to organic solar cells and otherorganic optoelectronic applications (Fig. 16). The method isbased upon the total blocking of one carrier type using asuitably placed wide-gap insulator. The standard CELIV pro-tocol (i.e., reverse bias voltage ramp) is used to generateextraction transients of the forward-bias injected carriers,from which the transit time and mobility are measured. Theyconcluded that other effects such as the photo-carriergeneration efficiency or charge trapping outweigh any issuesassociated with charge carrier mobility in the operationalorganic solar cells. Seo et al. [50] achieved multilevelmanipulation of resonant tunnelling in a MIS nanojunctionstructure in which the heterogeneous molecules of F16CuPcand CuPc were embedded in the insulator layer to form adouble-tunnelling junction (Fig. 17). In the study, the carrierswere injected into the respective molecules at correspondingthreshold voltages. This finding demonstrated a new multi-level operation of resonant tunnelling through organic mole-cules in a practical MIS-based device structure. Recently, Sunet al. [109] proposed a theoretical model on the electricalcharacteristics of metal–ferroelectric–insulator–semiconduc-tor field-effect transistors (MFIS-FET) by considering thehistory-dependent electric field-effect and the mobilitymodel. It was concluded that the improved theoretical modelis suitable for simulating the electrical characteristics ofMFIS-FET. This improved theoretical model is expected toprovide some guidance to the design and performanceimprovement of MFIS structure devices in the near future.Arquer et al. [51] introduced a MIS heterostructure of ITO/TiO2/Al2O3/Ag for plasma-electric energy conversion, whichis a novel architecture to harvest hot-electrons derived fromplasmonic excitations. A hot-carrier photocurrent could beobtained when energetic electrons derived from LSPR damp-ing are emitted over or tunnel to the semiconductor(Fig. 18). In this instance, the LSPR excited by an incomingphoton can result in the creation of an electron–hole pair,which can be split by the built-in field in the interface. Un-passivated interfaces contain traps that can result in Fermilevel pinning, deteriorating the junction from the ideal case.This has resulted in a suppression of open-circuit voltage(Voc) and no PCE, as shown in Fig. 18a. Depending on theirenergy level, the excited electron can either tunnel to theTiO2 layer or undergo Schottky emission over the barrier. Theheight of the barrier determines the difference of metalwork-function (Wf) and insulator electron affinity, as shownin (Fig. 18b).

Summary and future outlooks

Because current mass production methods for hydrogen areunable to enhance its cost-competitiveness over conventionalfossil fuels, new and novel concepts must be introduced thatcombine very high PEC cell efficiency with cost-effectivematerials. In this instance, the integration of MIS nanojunctionstructure into a PEC water-splitting cell is a good approach that

Fig. 17 (a) Schematic of device. Not shown is the EMI-BTI ionic liquid electrolyte that extends across both the gate electrodesSWCNT film and the n-Si junction. (b) Photograph of a SWCNT film across the exposed n-Si within the gold electrode window in whichthe SWCNT film was etched to form the grid pattern shown. The seeming break in the gridlines at the bottom edge of the window isan illustration. The lines run continuously up onto the gold electrode. Reproduced from Ref. [103].

Fig. 18 Schematic of MIS-based diodes in the case of (a) electron-only and (b) hole-only configurations. Electron (hole) mobilitiescan be measured from electron only (hole only) MIS-based diodes. The injected or induced charge carriers are distributed near theinterface of the insulator/semiconductor. Reproduced from Ref. [108].

Fig. 16 Effect of interface on the junction properties and resultant I–V characteristics. LSPR excited by an incoming photon canresult in the creation of an electron–hole pair, which can be splitted by the built-in field in the interface. Un-passivated interfaces(a) contain traps that can result in Fermi level pinning deterioration the junction from the ideal case (solid versus dashed lines,respectively). This results in a suppression of open-circuit voltage (Voc) and no power conversion efficiency (PCE). (b) Depending ontheir energy, the excited electron can either tunnel to the TiO2 or undergo Schottky emission over the barrier. The height of thebarrier determines the difference of metal work-function (Wf) and insulator electron affinity. Reproduced from Ref. [51].


has achieved some successes in recent years. In this reviewpaper, we have discussed recent developments and prospects ofMIS nanojunctions for hydrogen evolution from water splittingin PEC cells in the hope that readers could find new views inthis niche area. Other important aspects such as the basic

principles of MIS, generalised approaches for synthesising MISnanojunctions and the key application challenges of PEC water-splitting cells are also addressed to close the knowledge loopand present holistic interconnections among the variousaspects. Furthermore, recent developments of photovoltaic


cell structures based on MIS principles have also been reviewedbecause the PEC cells are usually coupled with photovoltaiccells and electrolysers for enhanced efficiency in water split-ting. Previously, it was also reported that the MIS-basedtechnology has proven to be particularly suitable for usingthinner Si wafers for harnessing the visible light in the PEC cells[110]. Thus, it is favourably viewed that the Si MIS-basedelectrode could be a promising candidate to improve signifi-cantly the cost-competitiveness of hydrogen energy producedvia the PEC water-splitting route due to the potential massproduction of low-cost Si MIS-based electrodes. Moreover, theALD method is widely used in the semiconductor deviceindustry, indicating the method’s potential to be employed ona large-scale basis in energy applications such as solar fuelsynthesis [29].

The MIS structures open up some additional possibilitiesfor a solar water-splitting system with high efficiencies of15% or greater. However, this is still a long distance frombeing integrated into a viable water-splitting device. Forinstance, the low cost of Si and its instability under anodicconditions make it an ideal material to demonstrate theeffectiveness of a protection scheme. However, its band gap(1.1 eV) limits the amount of photovoltage that can begenerated for water splitting. The use of semiconductorswith larger band gaps than Si under similar surface protec-tion/electrolyte conditions should result in a shift of theOER photocurrent onset to more-negative potentials. Othercriteria also need to be considered in the successful large-scale synthesis of hydrogen from PEC water splitting, suchas the stability and durability of photoactive materialsagainst photocorrosion, regenerability of the surface activ-ity of spent photoactive materials and others. In the longterm, there are also many challenges in the commercialisa-tion and upscaling of the PEC technology, such as separa-tion, purification, transportation and utilisation of solarhydrogen energy.

Acknowledgement

The authors are grateful to the financial support provided bythe eScience fund (Project no: 03-02-10-SF0121) fromMinistry of Science, Technology and Innovation (MOSTI),Malaysia. Similar gratitude also goes to the AdvancedEngineering Platform and School of Engineering of MonashUniversity Malaysia.

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Tao Zhu received his Bachelor degree fromthe College of Resources and EnvironmentalEngineering, Wuhan University of Technol-ogy. Thereafter, he also pursued a Mastersdegree in Chemical Engineering from Huaz-hong University of Science and Technology.Currently, he is pursuing his PhD degreeunder the main supervision of Dr. MengNan Chong at Monash University Malaysia.His research interests mainly focus on the

electrochemical synthesis and improvement of photocatalyst forapplication in water splitting.

Meng Nan Chong is a Senior Lecturer atMonash University Malaysia. He is also theprincipal investigator for this research pro-ject funded by Ministry of Science, Technol-ogy and Innovation (MOSTI), Malaysia. Hereceived his first class honours Bachelordegree in Chemical Engineering and a PhDin Engineering, both from the University ofAdelaide. Prior to Monash University, heworked as a Research Engineer and Project

Leader at Commonwealth Scientific and Industrial Research Orga-nisation (CSIRO), Australia. His research interests are synthesis andapplication of photocatalysts for environmental applications, aswell as integrated water resources management.












insulator semiconductor (mis) nanojunction structures for...

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