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Investigation of ZnO nanorods for

UV detection

Boban Gavric

Master of Science Thesis Supervisor: Dr. Qin Wang (Acreo AB)

Examiner: Prof. Mamoun Muhammed (KTH) Academic Supervisor: Assoc.Prof. Muhammet S. Toprak (KTH)

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Postal Address Royal Institute of Technology (KTH) Functional Materials Division, School of ICT Electrum 229, Isafjordsgatan 22 SE-16 440 Stockholm, Sweden

Supervisor Dr. Qin Wang

Acreo AB qin.wang@acreo.se

Academic-Supervisor Assoc.Prof. Muhammet S. Toprak toprak@kth.se Examiner Prof. Mamoun Muhammed

mamoun@kth.se Tutor Abhilash Sugunan

abhilash@kth.se TRITA-ICT-EX-2011: 57

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Contents

Abstract ........................................................................................................................................... 6 List of abbreviations and symbols ................................................................................................ 7 Acknowledgements ......................................................................................................................... 8 1 Introduction ................................................................................................................................. 9

1.1 Background .......................................................................................................................... 9 1.2 Applications ......................................................................................................................... 9 1.3 Project goal ......................................................................................................................... 10 1.4 Basic properties of ZnO materials ...................................................................................... 10 1.4.1 ZnO bulk material .................................................................................................... 10 1.4.2 ZnO 2D, 1D and 0D nanostructures ......................................................................... 10 1.5 ZnO nanorods UV photodetectors ...................................................................................... 12 1.6 Important device parameters of UV photodetectors .......................................................... 12 1.7 Thesis outline ..................................................................................................................... 13

2 Experimental .............................................................................................................................. 14 2.1 Growth of ZnO nanorods by chemical synthesis method .................................................. 14

2.1.1 ZnO seedlayer ......................................................................................................... 14 2.1.2 Vertical ZnO nanorods formation ........................................................................... 14 2.1.3 Lateral ZnO nanorods formation ............................................................................. 15 2.1.4 Nanoflowers ............................................................................................................ 17 2.1.5 II-VI quantum dots attachment ............................................................................... 17

2.2 Fabrication of ZnO nanorods UV detectors ....................................................................... 18 2.2.1 Interdigitated electrodes .......................................................................................... 18 2.2.2 UV detectors by ZnO casting on the top of IDEs ................................................... 19 2.2.3 UV detectors fabrication by direct vertically growth of ZnO nanorods arrays ....... 21 2.2.4 UV detector using covalent assemble QDs and ZnO nanorods .............................. 21 2.2.5 PN junction UV detector ......................................................................................... 22

2.3 Device characterization of fabricated UV detectors .......................................................... 23 2.3.1 I-V characteristics in dark, visible light and UV light ............................................ 23 2.3.2 Spectral photocurrent response ............................................................................... 24 2.3.3 Time response ......................................................................................................... 24

3 Results and discussions ............................................................................................................. 25 3.1 Seed layer chrystalisation ................................................................................................... 25 3.2. Formation of ZnO nanorods ................................................................................................ 25 3.2.1 Vertical ZnO nanorods ............................................................................................ 25 3.2.2 Nano flowers ........................................................................................................... 26 3.2.3 Quasi type II QDs .................................................................................................... 27 3.3 ZnO nanorods UV detectors ............................................................................................... 28 3.3.1 ZnO nanorods without and with QDs casted on the top of IDEs detectors ............ 28 3.3.2 Direct grown ZnO nanorods without and with QDs detectors ................................ 31 3.2.3 pn junction UV detectors ......................................................................................... 33 3.4 Carriertransport model ....................................................................................................... 34

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3.5 Spectral photocurrent and time response results ................................................................ 38 3.6 Time response results analysis using XPS ......................................................................... 39 3.7 Device stability ................................................................................................................... 40 4 Summary and conclusions ........................................................................................................ 42 5 Future work ............................................................................................................................... 43 6 References .................................................................................................................................. 44

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Abstract

In recent years, there has been increasing interest in ZnO semiconductors for optoelectronic application in the UV wavelength region due to its large exciton binding energy of 60 meV and wide bandgap energy of 3,37 eV at room temperature. One-dimensional ZnO nanostructures such as nanorods or nanowires have been considered as promising candidates for UV sensing and detecting applications owing to its high surface to volume ratio.

In this thesis work 5 different types of UV photodetectors based on ZnO nanorods were fabricated successfully. The basic technique is to utilize a chemical solution synthesis method for forming ZnO nanorods on different types of substrate, and then fabricate simple metal semiconductor metal (MSM) and P-doped/N- doped (PN) photodetectors using the nanorods as UV detecting material. The MSM photodetectors are accomplished using interdigitated electrodes forming by the thin gold fingers on the desired substrates, while the PN photodetector were fabricated using ZnO nanorods sandwiched between a top metal contact and a heavy P-doped Si substrate.

So far various lateral and vertical ZnO nanorods/nanowires with different dimensions (60 nm to 80 nm in diameter) and length (1 µm to 7 µm) have been synthesized. Also, the ZnO nanoflower (about 5 µm in height) arrays based on ZnO nanorods were demonstrated. The nanoflowers were mainly formed by three process steps. First a seed layer was spin coated on Si substrate, and then a positive photoresist layer with thickness of 1.5 µm was patterned for allowing the growth of ZnO nanorods precisely in desired locations. Finally the photoresist was stripped away.

The structural and composition properties of the ZnO nanostructures (seed, nanorods and nanoflowers) were inspected and characterized by implement of optical microscope, atomic force microscopy (AFM), scanning electron microscope (SEM), x-ray photoelectron spectroscopy (XPS). The device performance was characterized by current-voltage (I-V) characteristics in the dark, visible light and UV light, spectral photoresponse and time response as UV light switching on and off.

Demonstration of cost-effective ZnO nanorods UV photodetectors through this work reveals a promising potential for extending nano- and micro-technologies beyond the lab bench for future commercialization of such components, thus facilitate entry to new markets.

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List of abbreviations and symbols ZnO Zinc Oxide Quantum dots (QDs) Materials with the three dimension

smaller than its exciton Bohr radius ITO Indium Tin Oxide Hexamine (HMT) Hexamethylenetetramine MPA Mercapto propionic acid I-V Current-Voltage UV Ultraviolet Dip-coating Method for creating a thin film Nanomaterial Crystals with at least one

dimension smaller than 100 nm Photolithography Technique used to define

Micro-scale devices SEM Scanning Electron Microscopy XPS X-ray Photoelectron Spectroscopy 0-D, 1-D, 2-D, 3-D The degrees of freedom of charge

Carriers

Quantum efficiency R Responsivity

Wavelength of incident light h Planck constant c Speed of light q Charge of electron

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Acknowledgements This master thesis work was carried out at Acreo AB and Functional Material Division (FNM) KTH, and financed by IMAGIC center, and ECOC 2004 foundation.

I would like to thank my supervisor at Acreo AB Dr. Qin Wang for her help and care for me and for encouraging and leading through this project and for showing me a secret ways of mystery of science.

A lot of thanks to Associate Professor Muhammet S. Toprak from FNM KTH for supervising and helping me with my thesis work.

Thanks to Suzanne Almqvist for all help during the clean room processes. Special thanks to my fellow master thesis student Mr. Xuran Yang, PhD student Mr. Abhilash Sugunan and PhD student Ms.Yichen Zhao (providing quantum dots) at FNM KTH for sharing their ideas and works with me.

to Susan Savage and Jan Andersson for project support through IMAGIC center.

Thanks to all employees at Acreo AB making my time here at Acreo enjoyable and pleasant.

I would also like to thank my colleagues (master thesis students) at Acreo AB, Anneli Xun Li, Ludwig Östlund, Ali Asadollahi, Sandrine van Frank and PhD student Norbert K.

I want to thank Professor Mats Göthelid and Assistant Professor Emmanuelle Göthelid from KTH and Ångström laboratory Uppsala, respectively, for XPS measurements.

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1 Introduction 1.1 Background Ultraviolet (UV) light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV. ZnO II-VI semiconductors are very promising materials for optoelectronic applications in the (UV) region due to its large exciton binding energy of 60 meV and wide band gap energy of 3.37 eV at room temperature. In particular one dimensional ZnO nanostructures so called nanorods are believed to provide better performance for UV detection and sensing due to their large surface-to-volume ratio. It has been substantial efforts to developing synthetic methodologies for 1D oriented ZnO nanostructures [1-3].

Different methods including thermal evaporation and condensation, vapor-liquid-solid (VLS), metal organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), sol-gel methods, hydrothermal process, and chemical bath deposition (CBD). A variety of ZnO nanostructures can be obtained simply by changing the precursor chemicals, the concentration, the growth temperature, and time [4-10]. The solution chemical route is one of the most promising options due to its simplicity, low cost, and high efficiency [11].

1.2 Applications ZnO nanostructures based UV lasers, light emitting diodes, solar cells, nanogenerators, field-effect transistors etc have been reported intensively. The wide area of applications makes these nanorods competitive and appropriate material in present and future optoelectronics fabrication. Demand on the non-toxic material in industry gives ZnO nanorods a broad application area in everyday use and in the medicine.

The forecast for ZnO emerging applications are that at year 2016 will be around 2 billion dollar investment in emerging ZnO based devices. This is about four times more investment than it is today [12].

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1.3 Project goal The goal of this master thesis is to investigate and develop a low cost approach to fabricate UV photodetectors based on ZnO nanorods. The basic technique is to utilize a chemical solution synthesis method for forming ZnO nanorods on different types of substrates, and then fabricate simple metal semiconductor metal (MSM) and pn junction detectors using the nanorods as UV detecting material. 1.4 Basic properties of ZnO materials 1.4.1 ZnO bulk material Zinc oxide crystallizes in three forms hexagonal wurtzite, cubizincblend, and the rarely observed cubic rocksalt. The wurtzite structure is the most stable at ambient conditions, and thus the most common form. The zincblende can be stabilized by growing ZnO on substrates with cubic lattice structure. In both cases, the zinc and oxide centers are tetrahedral. The rocksalt (NaCl-type) structure is only observed at relatively high pressures about 10 GPa. As it is already mentioned the ZnO has a relatively large direct bandgap of 3.37 eV at room temperature. Large exciton binding energy 60 meV is also one of the properties of the ZnO, and electron mobility of ZnO strongly varies with temperature and has a maximum of ~2000 cm2/(V·s) at 80 K. The hole mobility are in the range 5 30 cm2/(V·s). One of the most important properties of ZnO is that it is nontoxic material [22].

Most ZnO has n-type characters. Controllable n-type doping is achieved by substituting Zn with group-III elements such as Al, Ga, In or by substituting oxygen with group-VII elements chlorine or iodine [1, 13].

P-type doping of ZnO is difficult. The problem originates from low solubility of p-type dopants and their compensation by abundant n-type impurities [1, 13, 14].

1.4.2 ZnO 2D, 1D and 0D nanostructures When the piece of material is so small that the surface bonds dominate over the bulk bonds, the material promotes the nano-size effect due to the unsatisfied bonds of the outermost atoms at any crystal. In the bulk those effects are compensated with in a crystal by strained lattice

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parameters of the surface atoms with their next to last atoms. One of the other nano-size effects is the changes in the quantum states of electrons in nano-sized materials. Electrons in an atom can populate only specific orbitals regarding to their specific energies and electrons can loose or absorb the specific discrete quanta of energies to move from it present energy state to next allowed energy state Fig.1.

In the bulk the energy states are very closely placed Fig.1. Density of the states is the number of allowed energy states per unit of the volume and depends on the size of the material. In the

es to form a continuum so called conduction band. The corresponding ground state is responsible of forming the valence band. In the very small pieces of crystal material (nanocrystals) the availability of energy states for electrons become discreet and far apart due to sparse of atoms Fig.1. The crystals with those properties are characterised as atomic clusters and are called quantum confinement systems. Definition of quantum confinement is that the size of the nanocrystal should be smaller than the exciton Bohr radius of that material. This means that the Bohr radius of the electron is residing in the first allowed excited state. This means that an excited electron is physically prevent by the small dimensions of the nanocrystal. The spatial confinement is recognised in only one dimension, two dimensions and in all three dimensions. The band gap energy and similarly a distance in between allowed density states of nanocrystal are depended by size of nanocrystal

of nanocrystal. This is another nano-size effect.

Fig. 1 Schematic of excited states available to valence electrons in a single atom, atoms in the bulk chrystal, and atoms in a nanocrystal [15].

If crystals have at least one dimension smaller than 100 nm is called nanomaterial and crystal material having at least one dimension smaller than the exciton Bohr radius for same material, is called quantum confined material. Nanostructures can be nano-sized in only one, two or all three dimensions. Depending of dimensions exist the different forms as film, rod or dots. The thin film (quantum well) is quantum confined in 1D but it is a 2D nanomaterial, same the quantum wire is quantum confined in 2D, but it is a 1D nanomaterial and the quantum dot (QD) is quantum confined in 3D and it is 0D nanomaterial [15].

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1.5 ZnO nanorods UV photodetectors An MSM photodetector consists of interdigitated Schottky metal contacts on top of an active (absorption) layer. Under applied electric field photogenerated carriers will be drifted to the electrode. The MSM photodetector is planar and requires only a single photolithography step. The MSM photodetectors are very high speed devices due to their low capacitance, and they typically have very low dark currents (current produced without incident light). The main causes for the low responsivity is the reflection from the surface metals and semiconductor surface, the finite carrier lifetime as the carriers traverse the gap between the electrodes before being collected, absorption of incident light outside the region in which photogenerated carriers can be collected by the electrodes, and surface recombination currents and deep traps within the semiconductor material which may lower the detected optical signal.

As known there are two different types of metal-semiconductor contacts: rectifying contacts that are known as Schottky diodes and ohmic contact. The ZnO nanorods in contact with metal in our case gold (Au) are creating a Schottky diode.

A pn photodetector is a typical photodiode that is capable of converting light into photocurrent.

1.6 Important device parameters of UV photodetectors UV photodetectors are targeting components in this work, therefore important device parameters are briefed as follows:

(1) Wavelength cut-off: measured by absorption and transmittance spectra (2) Photoresponse (photocurrent) at different biases, I-V measurement under UV light,

visible light and dark conditions. (3) Time response, turning on-off UV light, sensitivity of device (4) Spectral response, the cut-off of wavelength showing the range of devices application (5) Quantum efficiency

reactive surface that will produce an electron- can be calculated by: where R is the measured responsivity at incident light wavelength , h Planck constant, c is the speed of light and q is the charge of electron [16].

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1.7 Thesis outline This master thesis is organized in the following way:

Chapter 1 provides the general information about this master thesis work and basic properties of ZnO nanorods and UV photodetector.

Chapter 2 describes experimental details of formation of ZnO nanostructures and fabrication of UV photodetectors.

Chapter 3 shows results of experiments and brief discussion of the results.

Chapter 4 summarizes results and conclusions

Chapter 5 suggests future work

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2 Experimental 2.1 Growth of ZnO nanorods by chemical synthesis method There are many fabrication techniques producing ZnO nanorods for optoelectronic devices. Only a chemical bath approach is used in this work. It is a two step process. First step is deposition of ZnO seed layer on the substrate followed by growth of ZnO nanorods addressing it with a precursor Hexamethylenetetramine (hexamine).

Chemicals used in the growth process are: acetone [C3H6O], ethanol [C2H6O, Solveco AB, 98%], zinc acetate dihydrate [Zn(CH3COO)2 ·2H2O, Sigma/Aldrich, 98%], hexamethylenetetramine (hexamine) [C6H12N4, Merck, 99.5%] and zinc nitrate hexahydrate [Zn(NO3)2· 6H2O]. 2.1.1 ZnO seed layer Coating seed layer process is started with cleaning the substrate surface, usually used substrate is silicon (Si) wafer or Indium Tin Oxide (ITO) coated glass, with acetone followed by immersing substrate in an ultrasonic bath for 5-10 minutes. The seed layer was prepared by mixing 15mM ethanol and Zinc Acetate Dihydrate Zn(CH3COO)2 ·2H2O, (98% A.C.S. reagent Sigma Aldrich (0,067g)), and then stirring with magnetic stirrer until the Zn Acetate particles totally disolved within the ethanol. Deposition of the seed layer on the substrate was done in two different ways. One was that the well mixed seed layer solution was drop-casted on the surface of substrate, and such process was repeated until a thin uniform layer was formed. The second way was done by spinning the seed layer mixture on the top of wafer. Similarly, this process was repeated as previous one until a thin film layer was formed on the top of wafer. Substrate covered with Zinc Acetate was placed on hot-plate at 300min. 2.1.2 Vertical ZnO nanorods formation After the ZnO seed layer was crystallized on the substrate surface, the substrate covered with ZnO seed layer was droped in the grow solution with following compositions:

- First, Zn(NO3)2· 6H2O Zinc nitrate hexahydrate - Second distillated water - And third, C6H12N4 Hexamethylenetetramine (hexamine)

Order of mixing of reagents is very important. The amount of chemicals was optimized at FNM KTH division. Actually it is possible to increase the amount of growth solution, but it is very important to ensure the same concentration by keeping the same ratio between above three compositions. Table 1 shows examples for two different amounts of growth solutions.

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Table 1. Different amounts of growth solution.

Amount Zn(NO3)2· 6H2O (g) Water(ml) C6H12N4 (g) Solution 1 0.045 15 0.021 Solution 2 0.59 200 0.28 The growth solution was mixed with magnetic stirrer for 15 min. Growth approach was carried using a method so called downward, namely the substrate was placed seed layer facing down in the deposition solution. The sample, substrate with seed layer on the surface was immersed in the constant heated growth chemical bath precursor solution at 95Fig. 2 shows schematically the process steps of ZnO nanorods growth.

Fig.2 Schematic of ZnO nanorods growth process flow

2.1.3 Lateral ZnO nanorods formation Size, form and orientation of the nanorods are strongly dependent on growth parameters. In order to achieve a more controllable lateral growth for defining the ZnO nanorods at designed locations, a classical photolithography process can be used. First, the seed layer was deposited, and followed by baking it on hotplate at 300 °C for 30 min. Then, a photoresist was coated on the surface of ZnO thin seed film, as shown in Fig. 2.1.6 left panel.

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With mask, standard photolithography was performed to define openings in the photoresist on the surface of ZnO thin film as shown in the right panel of Fig.3. The rest of the wafer was still covered with photoresist.

Fig.3 Photolithography process step light exposure and developing The process was followed by baking the sample with defined photoresist at 110°C for 10 min in oven, and thereby etching of ZnO seed layer using hydrochloric acid (HCl). Etching solution is mix of hydrochloric acid (HCl) and water (H2O) in ratio 1:20. The etching proceeded for 30 sec followed by washing sample with DI water. Fig.4 a) is showing schematic image of etching process.

Fig.4 a) Etching of seed layer and b) Growth of ZnO nanorods Then, the growth of the ZnO nanorods laterally from the sidewalls of the seed layer beneath the photoresist was carried by the chemical bath process described in provious section. However, the growth time was decreased to 6h instead for 12h for avoiding the nanorods crossed over each other in the gaps between the seed layer sidewalls. Fig. 2.1.3 c) illustrates the principle of the method for getting fully controllable lateral grown ZnO nanorods.

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2.1.4 Nano flowers Utilizing the process principle as mentioned before for patterning the surface of substrate and then growing the nanorods, nanoflowers were where formed. First the round holes were patterned on the p-doped Si- wafer as shown in Fig.5 a), then carried a ZnO nanorods growth procedure as the same as described in previous section. The nanorods were allowed to grow only inside of the photoresist holes as shown in Fig.5 a), their length can be controlled by growth time. When the heights of the nanorods were longer than the thickness of the photoresist, the rods became tilted Fig.5 b).

Fig.5 a) patterned surface of photoresist b) grown ZnO nano

flower trough the patterned holes 2.1.5 I I -VI QDs attachment Attaching quantum dots (QDs) with ZnO nanorods can improve the device performance. To investigate the different detection mechanisms in comparison with detectors based on the pure ZnO nanorods, two types QDs CdS and CdSe/CdS core/shell QDs were attached on the surface of ZnO nanorods. Process used is so called dip coating to connect QDs to the nanorods. The substrate with the ZnO nanorods was immersed in the mixture of CdSe/CdS core/shell QDs and chloroform in ratio of 1:5, and followed by immersing the same substrate in the mercapto propionic acid (MPA) in order to improve contact in between QDs and ZnO nanorods. The next step is cleaning the substrate from the non stuck pieces of MPA by immersing, washing a substrate in the chloroform. Between each step is drying time, until the surface is visible dry. The whole process is repeated for three times. Schematic image in Fig.6 is showing the whole process flow. By similar procedure, attaching CdS QDs to the ZnO nanorods were also achieved.

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Fig.6 Process flow to attach QDs to ZnO nanorods

After the ZnO nanorods samples/chips were attached by QDs, they were inspected utilizing their fluoresce effect under UV light illumination. Fig. 2.1.5 a) shows the comparison between three samples, one was coated by only seed layer, and the second one had ZnO nanorods grown on the chip surface, and the third one was covered by ZnO nanorods attached by CdSe/ CdS core/shell QDs. The fluoresce from the QDs excited by the UV light was observed clearly. 2.2 Fabrication of ZnO nanorods UV detectors The chapter is focused on the different method to obtain the ZnO nanorods photodetectors. In total the following five type ZnO UV photodetectors were fabricated:

Metal semiconductor metal (MSM) photodetector fabricated by drop-casting ZnO nanorods on top of interdigitated electrodes (IDEs)

MSM photodetectors formed by drop-casting ZnO nanorods with coated CdS or CdS/ CdSe core/shell QDs

MSM photodetectors utilized ZnO nanorods directly grown on the IDE chip MSM photodetectors produced by directly grown ZnO nanorods with attached CdSe/

CdS core/shell QDs pn photodiode using the nanof lowers as detecting media

2.2.1 Interdigitated electrodes To fabricate ZnO nanorods MSM photodetectors, the IDE are needed. In this work various

Si substrate coated with SiO2 thin film using chemical vapor deposition (CVD). The IDEs were defined by photolithography, Ti/Au (50nm/300nm) metal layers evaporation and lift off process steps. Fig.7 2/Si wafer.

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Fig.7 Photo of the IDEs fabricated using a wafer-scale process

The IDEs were made in different configurations with a varied distance in between fingers from 2 to 10 micrometer, a few examples are shown in Fig.8 a) and b), respectively. Worth to mention that a double layer of photoresist was used regarding to the very small distance between the fingers for enabling metal lift-off easier. In fact, the distance between fingers defines the active detecting area of the MSM detectors.

Fig.8 (a) fabricated IDE and (b) fabricated IDE array.

2.2.2 UV detectors by ZnO casting on top of IDEs As described above, the IDEs were deposited on the SiO2 formed by CVD. The SiO2 layer can be acting as insulator between the interdigitated contact fingers. The ZnO nanorods were grown separately on the Indium Tin Oxide (ITO) coated glass substrate using the chemical

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bath deposition as the same procedure detailed in section 2.1.2. The growth time was 12h. After the ZnO nanorods growth, the backside of the glass substrate was washed with Nitric acid (HNO3). Then the cleaned substrate with grown ZnO nanorods was immersed in the bottle containing chloroform, which was placed inside the ultrasonic bath in order to break of ZnO nanorods from the glass substrate. Finally, the solution including chloroform mixed with the ZnO nanorods was obtained. In a schematic as shown in Fig.9 the process steps for separating the ZnO nanorods from glass substrate were illustrated.

Fig.9 Process flow for getting a solution contenting ZnO nanorods

The mixture of chloroform and ZnO nanorods was deposited using a pipette on the top of the interdigitated electrodes in order to link the contact in between the finger electrodes. Dropping-cast deposition shown in Fig.10.

Fig.10 Schematic illustration of the ZnO nanorods MSM photodetector via drop casting

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2.2.3 UV detector fabrication by direct vertically growth ZnO nanorods arrays Detection made with the drop-cast of the droplet mixture ZnO nanorods and chloroform is tried to be improved by growing the ZnO nanorods vertically on the top of gold (Au) interdigitated electrodes. The ZnO seed layer is deposit direct on the top of SiO2 substrate with golden (Au) contacts followed by baking at 300ºC for 30 min as described before continuing with standard procedure of growth by precursor composure. The grown time has been as before 12h. The mechanically separation of every component is done with help of probe station needle after the growth in order to observe each component separately. Schematic image in Fig.11 is showing the vertically direct grown ZnO nanorods device.

Fig.11 Illustration of vertical direct grown ZnO nanorods MSM photodetector

2.2.4 UV detector using covalent assemble QDs on ZnO nanorods It has been found during this work, attaching the quantum dots CdSe/ CdS core/shell QDs or CdS QDs on the grown ZnO nanorods can enhance photoresponse of the detectors. The QDs where provided by another MSc. student at FNM KTH Ms. Yichen Zhao, division is leaded by Professor Muhammet Toprak. Two different QDs, the core shell QDs, CdSe QD core, inside in CdS QD shell, and o CdS QD only were used in this work. The process of growing ZnO nanorods was carried as usual in chemical bath precursor solution. Before the last step of baking the sample with ZnO nanorods grown on its surface a dip coating step was done. By immersing the sample into the mixture of QDs and chloroform, and followed with immersing in the MPA. Finally the sample was washed using the chloroform to remove the rest of MPAs particles. In between coating and washing, the sample was needed to be dried at room temperature until the surface looked dried visually. The process was repeated for three times according to an optimized approach developed by FNM KTH. The detector was then characterized by I-V measurement in dark, room light and UV illumination.

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Fig.12 Principle of detector utilizing ZnO nanorods with QDs attachment

Schematic image Fig.12 is showing a simplified cartoon of the UV detector based on the ZnO nanorods with attached core/shell QDs CdSe/CdS. Such principle is also working for the detector based on ZnO nanorods with attached CdS QDs. 2.2.5 pn junction UV detector A joint of p-semiconductor and n-semiconductor to form a pn junction UV-detector was also explored in this work. The detectors are fabricated by the chemical bath grown of n-type ZnO nanorods flowers on p-doped Si substrate. Arrays of flowers were defined with classical photo lithography. In the beginning on the process, an oxygen plasma cleaned p-doped Si wafer was first performed, and then a thin film of ZnO seed layer was coated for creating a contact between p-doped Si semiconductor wafer and ZnO known as an n-type semiconductor material to form the pn junction. On the top of seed layer a positive photo-resist was spin coated, and followed by UV light exposure to define places with circle shape for growth of ZnO nanorods. The diameter of the holes was about 5 m. In those holes, the ZnO nanorods could be grown at same way as described before with growth time 12h, with such confined way growing the nanorods the array of nanoflowers were formed successfully. A thin Ti electrode was formed by using a shadow mask and Ti evaporation on the top of ZnO nanoflowers. The thickness of Ti was 50 nm, which conducted good transparent for UV light. Shining with UV light through the thin titanium, photo carriers were generated in ZnO nanorods, and a photocurrent signal was observed from such pn junction diode.

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Fig.13 Detecting device based on pn junction Schematic image Fig.13 is showing a detection principle of pn junction detector. 2.3 Device characterizations of fabricated UV detectors

s are described.

2.3.1 I -V characteristics in dark, visible light and UV light All fabricated devices were measured by current-voltage (I-V) measurement. The I-V measurement was performed using Acreo AB probe station at the three different ambient, dark, visible light and under UV light illumination. Following five types devices were characterized:

(1) Device made by drop-cast of ZnO nanorods on IDEs (2) Device made by drop-cast of ZnO nanorods + CdSe/CdS core/shell QDs and ZnO

nanorods + CdS QDs (3) Device made by direct grown of ZnO nanorods on the top of gold interdigitated

electrodes (4) Device made by direct grown of ZnO nanorods on the top of electrodes + CdSe/CdS

core/shell QDs (5) Device based on PN junction.

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2.3.2 Spectral photocurrent response

Spectral photoresponse measurement was done in the Imagic Lab at Acreo AB. The setup used is made by a Xenon (Xe) 150 Watt lamp controllable with Oriel Cornerstone 130 1/8 m monochromator and driven by software in the Labview programmed by Acreo. With monochromator enabled to choose desirable wavelengths. A light was then modulated with chopper at 1000 Hz making measurement with lock-in technique. The photoresponse was characterized by using the measured photocurrent dividing with input light power, which was recorded using a UV enhanced Si photodetector v.s. wavelengths. In fact, the intensity of the incident light was also measured by Karl Suss UV intensity meter-model 1000 I.D.#1006 at

2.3.3 Time response To investigate the response of the device to the on-off UV light the time response

the measurement started and swept up to 50 seconds, during the UV illumination the photocurrent response is measured under reverse bias of -1 V. After 150 s from the measurement starting the light, then the UV light was switched off, and the relaxation time was observed. Three different light emitting diodes (LEDs) with different wavelengths and different powers were used to illuminate devices. The two of those LEDs have a wavelength of 365 nm but different powers 75W respectively 40W, and the last one has a wavelength of 255 nm.

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3 Results and discussions All the results presented in this chapter are statistical data based on a large number of experiments. 3.1 Seed layer crystallization Crystallization of seed layer is necessary for formation of uniform and dense nucleation sites for nanorods growth [17]. The images of seed layers formed by different way are presented in Fig.14 (with drop-cast formed seed layer [17-21]) and Fig.15 (with spinning formed seed layer).

Fig.14 SEM seed layer formed Fig.15 SEM seed layer formed by spinning by the drop cast To get uniform grown of ZnO nanorods in this work are used drop cast of Zn acetate for 3 times before baking. 3.2 Formation of ZnO nanorods 3.2.1 Vertical and lateral ZnO nanorods The formation of vertical has well as lateral ZnO nanorods is completed after baking a sample with ZnO nanorods on surface for 2h at 120°C, Fig.16 and Fig.17.

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Fig.16 SEM image of vertical ZnO nanorods Fig.17 SEM image of lateral type ZnO nanowires formed in this work

Fig.18 Microscope image of ordered lateral ZnO nanorods formed utilizing combination of

photolithography technique and chemical synthesis approach. The dark areas in the optic microscope image as shown in Fig.18 are areas with photoresist and the white areas are substrate, it is clear to see that. The ZnO nanorods were grown laterally from the sides of earlier etched ZnO seed layer. 3.2.2 Nano flowers The result of pattering a substrate with holes, apparently the nanoflowers was formed as shown in Fig.19.

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Fig.19 SEM image of nano flower array formed in this work.

3.2.3 Quasi type I I QDs After the ZnO nanorods samples/chips were attached by QDs, they were inspected utilizing their fluoresce effect under UV light illumination. Fig.20 shows the comparison between three samples, one was coated by only seed layer, and the second one had ZnO nanorods grown on the chip surface, and the third one was covered by ZnO nanorods attached by CdSe/CdS core/shell QDs. The fluoresce from the QDs excited by the UV light was observed clearly.

Fig.20 Fluoresce from QDs obtained under UV illumination

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3.3 ZnO nanorods UV detectors 3.3.1 ZnO nanorods without and with QDs casted on top of IDEs detectors Fig.21 shows the ZnO nanorods were located at desired area between the interdigitated electrodes. When the UV light was illuminating the device, photo generated carriers in ZnO nanorods would be drifted to the finger electrodes under bias. In order to improved contact quality between the ZnO nanorods and gold metal electrodes, annealing process was used at different temperatures. The results indicated that annealing at 300 °C for 30 min can provide better attachment for the ZnO nanorods on the IDEs. With increasing the density of ZnO nanorods inside the droplets mixture can also help to improve the device performance, which was achieved by spinning the bottle with chloroform and ZnO nanorods composed inside. Using such mixture with higher density of ZnO nanorods to drop cast several times on the IDEs, the yield of the detector can be enhanced significantly. Nevertheless the use of chemical solution in the shape of mercapto propionic acid (MPA) molecule gave a very good contact between ZnO nanorods and the IDEs.

Fig.21 SEM image of ZnO bridged in between two finger electrodes. Fig.22 and Fig.23 show dark current and photocurrents at visible light and UV light illumination conditions, together with the background curve of the same component. The background signal was obtained before drop-cast of ZnO nanorods solution without and with CdSe/CdS core/shell QDs, namely at open-circuit case.

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Fig.22 I-V characteristics of a detector formed by drop-cast ZnO nanorods on IDEs

Fig.23 I-V results of a detector fabricated by drop-cast of ZnO nanorods with CdSe/CdS core/shell QDs

30

The other type of drop cast detector utilizing ZnO nanorods attached with CdS QDs was also characterized. The QDs (CdS) were assembled on the ZnO nanorods and cast dropped on the target, active detecting area. Fig.24 and Fig.25 show its I-V measurement result in the linear scale and log scale, respectively.

Fig.24 Dark and photocurrents of ZnO+CdS detector in linear scale

Fig.25 Dark and photocurrents of ZnO+CdS detector in linear scale

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3.3.2 Directly grown ZnO nanorods UV detectors without and with QDs Devices made by direct grown of ZnO nanorods on the top of gold interdigitated electrodes were measured using the same method above: Fig.26 is showing the current response of direct grown ZnO nanorods on the top of interdigitated gold electrodes in the linear scale under dark, light and UV illumination conditions. And Fig.27 is showing the same result in the logarithmic scale.

Fig.26 I-V measurement results of a direct ZnO nanorods direct grown detector

Fig.27 I-V measurement results of direct ZnO nanorods

direct grown detector in log scale

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By using the same measurement procedure the I-V measurement graphs were obtained as shown in Fig.28 and Fig.29 in the different scales.

Fig.28 I-V characteristics of device made by direct grown of ZnO nanorods on the top of electrodes + core shell QDs (CdSe/CdS)

Fig.29 I-V characteristics of device made by direct grown of ZnO nanorods on the top of

electrodes + core shell QDs (CdSe/CdS) in log scale

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3.3.3 pn junction UV detector

Fig.30 The nano flowers covered by thin film of titanium

Microscope image Fig.30 is showing a white square, which is a thin titanium electrode, deposited on the top of the ZnO nanoflowers as shown like black spots under the white square.

Photoresponse of the device based on PN junction was verified by I-V measurement. Fig.31 is showing a typical pn diode I-V characteristic, and Fig.32 indicates the photoresponse of the pn device measured in the reverse bias region.

Fig.31 I-V characteristic of pn junction device

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Fig.32 Photoresponse of the pn junction detector at reverse biases

3.4 Carrier transport model From the result obtained from I-V measurements, it is obvious that the MSM type photodetectors behaved as typical Schottky diode. The Schottky contact is achieved at interface between metal and semiconductor material. Such devices operate with the single type of carrier so called majority carriers. In our case ZnO nanorods are n-type material, therefore the majority carriers are electrons.

High photoresponse of ZnO nanorods photodetctors, and over 100% quantum efficiency were reported by other group [16]. To understand it, a carrier transport model was illustrated in Fig.33 a) and b). On the surface of semiconductor ZnO nanorods under dark condition, the free electrons from the ZnO nanorods are captured by oxygen molecules, which are then absorbed on the surface of ZnO nanorods forming a low conducting depletion layer near the surface as shown in Fig.33 (a).

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Fig.33 Carrier transport at dark (a) and UV illumination (b). Schematic image at Fig.34 shows principle of carrier transport mechanism on the surface of ZnO nanorods with attached QDs. In such case, the surface is only partially opened for oxygen to be absorbed. The large part of the ZnO nanorods surface is covered with QDs. The carrier mechanism shown in Fig.34 is partially similar with that in Fig.33 regarding to the oxygen absorption and disabsorbing process, however huge numbers of extra carriers were generated that benefited from attached QDs. It can be understood by their band alignment as shown in Fig.35, where photo generated electrons in QDs will be conducted to ZnO freely and result in enhancement of device sensitivity.

Fig.34 Principle of carrier transport mechanism of ZnO nanorods with attached QDs

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Fig.35 Band diagram of ZnO nanorods + CdSe/CdS core/shell QDs and their electron and hole transitions

To investigate difference of carrier transport mechanism in ZnO devices without and with QDs, we compared their photoresponse carefully using the same component on the following way:

(1) Deposition seed layer and measuring I-V characteristics (2) Grown ZnO nanorods and measuring I-V characteristics (3) Attaching QDs and measuring I-V characteristics

All three steps of process were done separately based on the same component and the obtained results are shown in Fig.36 and Fig.37 as follows:

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Fig.36 Photocurrent as the sample consistent of ZnO seed, nanorods and QDs step by step in linear scale.

Fig.37 Photocurrent as the sample consistent of ZnO seed, nanorods and QDs step by step in log scale.

From results shown in Fig.36 and Fig.37nanorods covered with QDs has higher response. Using bandgap diagram illustrated in Fig. 3.1 d) it can be explained. Photon energy of UV light can excite the electrons from both ZnO nanorods and QDs. Owing to the band-gap of QDs is smaller than band-gap of ZnO nanorods

ZnO nanorods covered with QDs.

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3.5 Spectral photocurrent and time response results Two measured results are shown in Fig.38 and Fig.39 as examples, one is corresponding to direct grown ZnO nanorods device, and another one obtained from direct grown ZnO nanorods device with attached CdS/CdSe core/shell QDs.

Fig.38 Spectral photoresponse at different biases of a

direct grown ZnO nanorods device

Fig.39 Spectral photoresponse at reverse bias 1.5 V of a

direct grown ZnO nanorods + QDs device

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The Fig.40 is showing obtained graph for a device made by direct grown of ZnO nanorods on the top of golden electrodes.

Fig.40 Time response of a direct grown ZnO nanorods detector

3.6 Time response results analysis using XPS From time response measurement results shown in Fig.41, it is clear that there was persistent current in the ZnO nanorods photodetector. To analyze such effect, and also obtain the surface composition of devices, the X-ray photoelectron spectroscopy (XPS) measurement was performed at Ångström laboratory in Uppsala University. The measurement was done for three devices in order to compare their surface compositions, these three are: thin film ZnO (seed layer), ZnO nanorods and ZnO nanorods with attached QDs. QDs were quasi type II QDs core-shell (CdSe/CdS).

Fig.41 XPS spectra of three samples with ZnO seed, nanorods and QDs

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From the graph of XPS spectra shown in Fig.41, the devices made by ZnO thin film and ZnO nanorods have the same peaks of zinc Zn 2p and oxygen O/1s. The curve in green color is obvious that the surface was covered by QDs, and dominated peaks came from cadmium Cd/3d 3/2 and Cd/3d 5/2. The peak of selenium Se and sulfide S/2p was difficult to distinguish from each other because they are on the same energy level. In order to further analyze time response of devices, the XPS on the oxygen specie was specially measured with higher spectral resolution, the results of oxygen O/1s are shown in Fig.42.

Fig.42 O/1s XPS spectra of three samples with ZnO seed, nanorods and core/shell QDs coated ZnO nanorods

The large surface to volume ratio of ZnO nanorods compared with ZnO thin film gives a more sites for oxygens to be adsorbed. However, probability of obtaining oxygens is lower for the sample with ZnO nanorods covered with QDs, which resulted in the smallest peak of oxygen as shown in Fig.42.

Very large area to volume gives us an explanation about the long relaxation time observed in Fig.40. The negatively charged oxygen ions absorbed on the surface of ZnO nanorods were oxidized by UV light generated holes. While the remaining electrons in the conduction band increased conductivity. The hole-trap states related to oxygen at the ZnO nanorods surface discourage charged carrier recombination and prolong the photo-carrier life time. 3.7 Device stability Device stability was verified by measuring their I-V characteristics under UV illumination, namely their UV photoresponse during the time of two months. The measurement was carriedonce per month, the experimental results from three example samples are shown in

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Figures: 43, 44 and 45 respectively. Under the time of investigation the devices were stored at the room ambient and the surface of devices was not passivate.

Fig.43 Time stability I-V measurement of ZnO nanorods

Fig.44 Time stability I-V measurement of ZnO seed layer

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Fig.45 Time stability I-V measurement of ZnO nanorods + QDs (CdSe/CdS)

The obtained results indicate a degradation of UV response in all three cases, however it is less pronounced for the device made by ZnO nanorods attached QDs. It is worth to note that the degradation in the first month was not so significant for all three type devices. This results point out that a device passivation process is needed for device reliability consideration.

4 Summary and conclusions The investigation of possibilities to fabricate low cost UV photodetectors based on ZnO nanorods were performed in this master thesis work. Non-ordered and ordered lateral and vertical ZnO nanorods (wires) were formed by chemical bath deposition method, which ensured simple process and cost-effective advantages. During this work five different UV devices were fabricated and characterized: First device of ZnO nanorods drop-casted on the top of IDEs showed god photocurrent response, after having problems of getting a connection at the SM interface. The second one ZnO nanorods drop-casted on IDEs + two different types of QDs responsing with even better photocurrent then with Zn nanorods only. The third one direct grown on the top of IDEs shows clearly better photocurrent response then that one with Zn nanorods drop-casted on the top of IDEs. With fourth device made by direct grown ZnO nanorods + quasi type II CdSe/CdS core/shell QDs shown possibilities of improving the photocurrent response even more regarding the device made by direct grown on the top of IDEs. Fifth ZnO nanorods UV detector is made by pn junction and that is showing good photocurrent response as well.

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Good UV photoresponse were obtained for all types devices. The methods to improve device performance were also explored utilizing different ZnO nanorods depositions and their differences are compared. Improvement of detector quantum efficiency by attaching the CdSe/CdS core-shell QDs to ZnO nanorods was demonstrated.

5 Future work Optimization of ZnO nanorods formulation process will be one important step for fabricating better performance ZnO nanorods based UV photodetectors. To be able to fabricate detectors in a larger scale, transferring ZnO nano structure formation process from chip to a wafer-scale should be conducted as well. According to results obtained from this master thesis work, improvement of the device performance can be expected if the ZnO directly grown on locations that are defined by photolithography. Also a proper surface passivation should be addressed to improve device stability and to prolong device persistent current. In addition, making ZnO nanorods devices using nano interdigitated electrodes formed by nano-imprint techniques at Acreo AB will strengthen the device performance. Finally the novel devices utilizing ZnO nano structures for the other applications, like solar cell, bio-sensing and others should also be explored.

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