nanoscale schottky behavior of au islands on tio2 probed ......nanoscale schottky behavior of au...

Nanoscale Schottky behavior of Au islands on TiO2 probed with conductive atomicforce microscopyHyunsoo Lee, Young Keun Lee, Trong Nghia Van, and Jeong Young Park Citation: Applied Physics Letters 103, 173103 (2013); doi: 10.1063/1.4826140 View online: http://dx.doi.org/10.1063/1.4826140 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/103/17?ver=pdfcov Published by the AIP Publishing

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Nanoscale Schottky behavior of Au islands on TiO2 probed with conductiveatomic force microscopy

Hyunsoo Lee,1,2 Young Keun Lee,1,2 Trong Nghia Van,1,2 and Jeong Young Park1,2,a)1Center for Nanomaterials and Chemical Reactions, Institute for Basic Science, Daejeon 305-701,South Korea2Graduate School of EEWS and NanoCentury KI, KAIST, Daejeon 305-701, South Korea

(Received 3 July 2013; accepted 23 September 2013; published online 21 October 2013)

Electrical properties of nanoscale Au islands on n-type TiO2, which form a Schottky junctionnanodiode, have been investigated using conductive atomic force microscopy at ultra-high

vacuum. The Au islands were formed using colloidal self-assembled patterns on an n-type TiO2semiconductor film using the Langmuir-Blodgett process. Characteristics of the nanoscale

Schottky contact were determined by fitting the local current–voltage plot to the thermionic

emission equation, which reveals the Schottky barrier height and the ideality factor of the Au

islands on n-type TiO2, and were compared with electrical characters of the conventionalmacroscale diode. VC 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4826140]

As metal–semiconductor interfaces continue to scale

down, electrical characterization of nanometer-scale Schottky

diodes has become increasingly important for applications in

low-dimensional electric devices.1–5 Since most diodes have

an upper metal–semiconductor micron-scale interface, con-

ventional spectroscopy methods (e.g., X-ray photoelectron

spectroscopy (XPS) and Raman spectroscopy) cannot be

employed to characterize the local electric properties of indi-

vidual metal islands at submicron-scale, but rather give aver-

aged information for the whole area measured.6 Atomic force

microscopy (AFM) can provide ample nanometer-scale infor-

mation, specifically electrical, magnetic, mechanical, and

chemical properties.7–12 Morphological properties can be

obtained simultaneously by using an AFM probe coated in a

conductive material with conductive AFM (c-AFM), while a

bias voltage is applied to the tip or sample.13–16 In previous

reports, c-AFM has been widely employed to investigate elec-

trical properties, such as the conductivity of self-assembled

monolayer films on a gold substrate,17–19 electronic properties

of InAs quantum dots on n-type GaAs,20 CdTe tetrapods,21

and TiSi2 islands on a Si substrate.6

The titanium dioxide (TiO2) thin film has lower den-

sities of the interface states and high dielectric permittivity,

which can be generally utilized to create a Schottky diode

with a noble metal, such as Au.22–27 The Schottky barrier

height of these macroscale Au/TiO2 diodes is given by the

difference between the conduction band minimum and the

Fermi energy, EF, at the interface. On the other hand, a nano-scale Schottky diode would give rise to a significant devia-

tion from this bulk value due to the enhanced portion at the

interface and edge of the metal islands. Therefore, it is

important to understand the differences in electrical behavior

of macroscale and nanoscale Schottky diodes.

In this paper, we report both the fabrication of Au/TiO2Schottky diodes formed using colloidal self-assembled pat-

terns and characterization of the electrical properties at the

nanometer-scale interfaces of these diodes. The nanoscale

Schottky characteristics were studied using the current detec-

tion mode of AFM in ultra-high vacuum (UHV). We found

that the Schottky barrier of the Au/n-type TiO2 nanodiodehas a lower value than that of conventional Au film/n-typeTiO2 diodes due to the image-force and tunneling effects.

Here, we suggest that c-AFM can be a facile method

to understand electrical transport through the metal–

semiconductor interface of Schottky nanodiodes.

The n-type TiO2 was deposited using the multitargetco-sputtering technique on a silicon substrate (the ratio of

TiO2 to Ti was fixed at 3:1). Annealing of the deposited TiO2film was performed by heating at 300 �C for 4 h at ambientpressure. The sputtered TiO2 film was 250 nm thick, carrier

concentration of 2� 1018 cm�3, and dielectric constant of30–40.28,29 The mono-dispersed silica spheres on a Si wafer

were synthesized via the St€ober process30 using 10 ml ethanol(Merck, 99.8%), 3.29 ml distilled water, 0.55 ml ammonium

hydroxide (Daejung Chemicals, 25%–28%), and 2.3 ml tetra-

ethyl orthosilicate (TEOS) (Aldrich). The reaction to form

the silica spheres was run for 12 h at room temperature. After

washing in ethanol, the silica spheres were diluted in metha-

nol. Sodium dodecyl sulfate (Aldrich) was added to the solu-

tion to induce the hydrophilicity–hydrophobicity of the silica

spheres, which improves the suspension of the silica spheres

(after ultrasonic dispersion for 30 min with heat). After add-

ing 3 ml of chloroform, the silica suspension (250 ll) wasapplied to the Langmuir-Blodgett (LB) trough at an initial

water surface area of 300 cm2. For stabilization of the initial

surface pressure before the isotherm process, a 20 min delay

time was given. The monolayer of silica spheres was com-

pressed at a barrier speed on the water surface of 20 cm2/min,

up to the target surface pressure of 11 mN/m.31 The mono-

layer of hexagonal close packed (hcp) silica spheres was

transferred to the n-type TiO2 substrate by lifting the verticalsubstrate at a velocity of 1 mm/min.

The scheme in Fig. 1(a) shows that a 10 nm thick layer

of Au was deposited on both the hcp silica particle mono-

layer and the residue surface of the TiO2 substrate using

e-beam evaporation under ultra-high vacuum (10�9 Torr).

The silica spheres were subsequently removed by ultrasonic

a)Author to whom correspondence should be addressed. Electronic mail:

[email protected]

0003-6951/2013/103(17)/173103/4/$30.00 VC 2013 AIP Publishing LLC103, 173103-1

APPLIED PHYSICS LETTERS 103, 173103 (2013)


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treatment. Triangle-shaped Au islands were formed at the

vacancies between the silica spheres on the n-type TiO2 sub-strate. The size of the triangle-shaped Au can be adjusted by

changing the size of the silica spheres. Here, we used 225 nm

silica spheres to form the arrays of hcp Au triangles. The

triangle-shaped Au islands (the work function of Au is 5 eV)

showed Schottky contact behavior with the n-type TiO2 (theelectron affinity for TiO2 is approximately 3.9 eV)

32 without

the after treatment process; Ti/Au was used for the ohmic

contacts on the other side of the diode.

A commercial ultra-high vacuum AFM system (chamber

with a base pressure of 1.0� 10�9 Torr; RHK-Tech STM/AFM) was used to measure the topographical image and elec-

trical properties of the Au triangles on the n-type TiO2 sub-strate with the normal load of 2.0 nN for the elastic

deformation region. A titanium nitride (TiN) coated AFM tip

(CSG10/TiN, NT-MDT) with a cantilever force constant of

0.11 N/m was used in the UHV system for these experiments.

Figure 1(b) shows the scheme for probing the electrical charac-

teristics of the Au/TiO2 nanodiode utilizing this setup. The

Schottky barrier was formed at the interface between the Au

islands and the n-type TiO2 substrate, while the ohmic contactsare formed where the Au film contacts the Ti intermediate.

In Fig. 2(a), we show a scanning electron microscope

(SEM) image of the Au triangle nanostructure for confirma-

tion of its morphology. The hcp Au islands (bright parts) are

clearly placed on the n-type TiO2 substrate. The topographi-cal image of the Au islands on the n-type TiO2 substratewas obtained via a contact-mode AFM scan under UHV

(Fig. 2(b)). Figure 2(c) shows that the conductance mapping

was obtained simultaneously with the topographical image

using the TiN-coated metallic AFM tip during application of

a 1.0 V sample dc bias, as demonstrated by the images

matching along the dotted line. The average height and

detected current value of the Au islands on the n-type TiO2are �10 nm and �600 nA, respectively (Fig. 2(d)). Theregions of higher current flow (bright parts) in the conduct-

ance image are equal to the Au parts on the n-type TiO2 sub-strate in the topographical image.

I–V spectroscopy was performed on the Au island on

TiO2 substrate marked by a cross in the topographical image

(Fig. 3(a)). The Schottky contact behavior of the I–V curve

was obtained on the Au/TiO2 nanoscale diode using a 61 Vsweep bias, as shown in Fig. 3(b). The I–V curve measured

directly on the TiO2 substrate (Fig. 3(c)) showed a relatively

low current flow compared to the Au island/TiO2 because

the contact area of the electrode on the Au island is higher

than that of the AFM tip, but it also showed Schottky contact

behavior between the TiN-coated AFM tip and the n-typeTiO2 substrate.

To determine the Schottky barrier height and ideality

factors for the Au/TiO2 nanodiode, we fit the measured I–V

curve to the thermionic emission equation.33,34 The current

FIG. 1. (a) Scheme of the sample preparation steps to build triangular-shaped

Au islands on a TiO2 substrate. (b) Scheme for probing the Schottky contact

behavior on a nanometer-scale Au/TiO2 diode under ultra-high vacuum.

FIG. 2. (a) SEM image showing the hcp triangular-shaped Au islands on

TiO2. (b) The topography (780� 780 nm2) was obtained simultaneouslywith (c) spatial mapping of the conductance of the Au islands/TiO2 nano-

diode. (d) The height and current line profiles of a Au island.

FIG. 3. (a) Topography (960� 960 nm2) showing Au islands on a TiO2 sub-strate using contact mode AFM. (b) I–V curve obtained on a Au island (posi-

tion marked with a cross in (a)) while applying a 61 V sweep bias. (c) I–Vcurves measured on the Au islands/TiO2 and TiO2 substrate using 2.0 nN for

the normal force.

173103-2 Lee et al. Appl. Phys. Lett. 103, 173103 (2013)


143.248.118.122 On: Tue, 21 Jan 2014 06:31:52

density of the Au/TiO2 Schottky contact as a function of

applied bias is given by

I ¼ A A� T2 exp � UnkBT

� �exp

e0ðVa � RsIÞgkBT

� �� 1

� �; (1)

where A is the active area, A� is the effective Richardsonconstant, T is the temperature of the diode, Un is theSchottky barrier height, g is the ideality factor, and Rs is theseries resistance. The active area of the Au/TiO2 nanodiode

is 5.9� 10�11 cm2 and the temperature is 300 K.Figure 4(a) shows a semi-logarithm plot of the I–V curve

measured on the Au/TiO2 nanodiode and fitted to thermionic

Eq. (1). The Schottky barrier height and ideality factor were

found to be 0.8 eV and 6.45, respectively, for the Au/TiO2nanodiode. To compare the Schottky barrier height of the Au

islands/TiO2 diode with the Au film/TiO2 diode, we prepared

a Au film (10 nm thick) on n-type TiO2. For the fabrication ofmacroscale Au/TiO2 Schottky diodes, the first step is deposit-

ing a 4� 6 mm and 200 nm thick film of titanium oxide ontoan insulating n-type (100) silicon wafer covered by 200 nmSiO2 through an aluminum shadow mask using multitarget

sputtering. The wafer is annealed at 300 �C for 4 h to controlthe sheet resistance of the titanium oxide film in air. The next

step is deposition of a 50 nm film of titanium and a 150 nm

film of gold, which constitutes the nanodiode’s two ohmic

electrodes. Finally, a thin gold film (10 6 2 nm thick) is de-posited to form the Schottky contact.27 The Au film has

Schottky contacts with the bottom TiO2, marked by the red

squares with a dashed line, while the other Au electrode has

an ohmic contact via Au/Ti. The active area of the Au/TiO2nanodiode is 0.06 cm2 and the temperature is 295 K. The

measured I–V curve of the Au film/TiO2 diode was fitted to

the thermionic emission equation, resulting in a Schottky bar-

rier height of 1.19 eV and ideality factor of 1.33, as shown

Fig. 4(b). The Schottky barrier height of the Au islands/TiO2diode is somewhat lower than that of the Au film/TiO2 diode.

The energy band diagram of the metal–semiconductor

Schottky barrier diode with nanometer-scale contacts is

shown in Fig. 4(d). The image charge in the Au island can be

generated by the existence of electrons, which induce an elec-

tric field in the n-type TiO2. The image force, �eE, inducedby the coulomb attraction between the electron and image

charge distort the potential barrier by applying a constant

external electric field. Therefore, the ideal Schottky barrier

height, eØB0, drops to the actual Schottky barrier height,eØBn, as the barrier decreases by DØ. As a result, theAu/TiO2 nanodiode is vulnerable to image force and

Fowler-Nordheim tunneling from the diode’s edges, which

induces a decrease in the Schottky energy barrier for charge

carrier thermionic emission.35,36 In addition, we note that

electron tunneling through the narrow Schottky barrier can

take place and become an additional current pathway.37 The

interfacial oxygen vacancies in the TiO2 can contribute to

electron tunneling, which could be affected under UHV or

low-oxygen-pressure atmosphere.38–40 We plan to investigate

this aspect by changing the environmental conditions and

chemistry of the interface of nanoscale junctions.

Nevertheless, the metal–semiconductor nanoscale Schottky

diode fabricated using a colloidal self-assembled pattern tech-

nique combined with c-AFM provides with an intriguing plat-

form that permits us to study the charge transport properties

of nanoscale interfaces.

In conclusion, we have shown spatial mapping of the con-

ductance and Schottky contact behavior based on I–V spectros-

copy of the Au/TiO2 nanodiode that was formed by colloidal

self-assembled nanopatterning. The nanoscale junction of the

Au islands and TiO2 exhibits a lower Schottky barrier height

and higher ideality factor in comparison with the conventional

macroscale diode, presumably due to the enhanced role of

impurities localized at the nanoscale interface.

This work was supported by 2012R1A2A1A01009249

through the National Research Foundation and the Research

Center Program (CA1201) of IBS (Institute for Basic

Science), Republic of Korea.

FIG. 4. The semi-logarithmic plots

were obtained by fitting the I–V curve

to the thermionic emission equation

for the (a) Au islands/TiO2 diode and

(b) Au film/TiO2 diode. (c) Scheme of

the cross section showing the configu-

ration of the Au film/n-type TiO2diode. (d) The energy band diagram

illustrates the mechanism of the

image-force-induced lowering of the

Schottky barrier height and the occur-

rence of tunneling current.



143.248.118.122 On: Tue, 21 Jan 2014 06:31:52

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nanoscale schottky behavior of au islands on tio2 probed ......nanoscale schottky behavior of au...

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