asymmetric contacts on a single sno nanowire device: an...

Published: July 20, 2011

r 2011 American Chemical Society 3097 dx.doi.org/10.1021/am2006096 |ACS Appl. Mater. Interfaces 2011, 3, 3097–3102

RESEARCH ARTICLE

www.acsami.org

Asymmetric Contacts on a Single SnO2 Nanowire Device: AnInvestigation Using an Equivalent Circuit ModelJunghwan Huh,†,§ Junhong Na,†,§ Jeong Sook Ha,‡ Sangtae Kim,*,§ and Gyu Tae Kim*,†

†School of Electrical Engineering and ‡Department of Chemical and Biological Engineering, Korea University, Seoul 136-701,South Korea§Department of Chemical Engineering and Materials Science, University of California, Davis, California 95616, United States

bS Supporting Information

1. INTRODUCTION

Over the past decade, nanometer-scale one-dimensional func-tional materials (hereafter nanomaterials) have been of particularinterest mainly because these materials often present uniquecharacteristics suitable for a wide variety of advanced deviceapplications, ranging from biomedical/chemical sensors1,2 tooptoelectronic devices.3,4 To fabricate such devices, however,nanomaterials should be interconnected with the external circuitsso that the inclusion of various junctions and/or contacts in thedevices is inevitable. This fact implies that the actual performanceof the fabricated devices may not necessarily be determinedsolely by the characteristics of the nanomaterials. In fact, it hasbeen reported that extra complications (e.g. Schottky barriers)caused by the contacts between the nanomaterial and metalelectrodes often becomes substantial, leading to the deviceperformance rather inconsistent with the intrinsic property ofthe materials.5 On the other hand, because of this reason, it mayprovide one opportunity to further improve the device perfor-mance by manipulating, e.g., the Schottky contacts as recentlydemonstrated for photovoltaic and sensor devices.6�9 It is thuscritically important to have a precise knowledge of the nature ofsuch contacts and the corresponding electrical behaviors at afundamental level to better understand the actual device perfor-mance and thus to eventually develop nano-scale devices that aremore reliable for practical uses.

To date, electrical behavior of conventional Si-based electronicdevices has been characterized by employing dc-measurement

techniques almost exclusively, whereas analyses using ac-impe-dance measurements have been rarely reported. One of theadvantages of using an electrochemical impedance spectroscopy(EIS) technique over the dc-characterization methods for elec-trical characterizations is that, in principle, EIS allows for decon-volution of dc resistance into the local resistances that compose thedc resistance, if an appropriate fitting model (i.e., an equivalentcircuit model) is available for the analysis.10,11 Thismeans that onecan possibly investigate electric characteristics of Schottky contactsin a device directly as well as exclusively using the EIS technique.On the other hand, it is also true that the equivalent circuit modelswhich can accurately represent electrical behavior of the device arerarely available. We previously proposed an equivalent circuitmodel to describe the change in conduction mechanism at thecontact between a Ti/Au electrode and a single individual ZnOnanowire from tunneling to thermionic emission as temperatureincreases.12,13

In this contribution, we report the results of our investigationon electrical nature of the Schottky contacts which we deliber-ately fabricated on a single SnO2 nanowire device by usingcombined dc and EIS analysis. As discussed below, we firstdeveloped an advanced equivalent circuit model which preciselyreproduces the dc current-voltage(I�V) characteristics measured

Received: May 16, 2011Accepted: July 20, 2011

ABSTRACT: Electrical contacts between the nanomaterial andmetal electrodes are of crucial importance both from fundamentaland practical points of view. We have systematically compared theinfluence of contact properties by dc and EIS (Electrochemicalimpedance spectroscopy) techniques at various temperatures andenvironmental atmospheres (N2 and 1%O2). Electrical behaviors aresensitive to the variation of Schottky barriers, while the activationenergy (Ea) depends on the donor states in the nanowire rather thanon the Schottky contact. Equivalent circuits in terms of dc and EISanalyses could be modeled by Schottky diodes connected with aseries resistance and parallel RC circuits, respectively. These results can facilitate the electrical analysis for evaluating the nanowireelectronic devices with Schottky contacts.

KEYWORDS: Schottky contact, asymmetric contact, equivalent circuit, nanowire, electrochemical impedance spectroscopy (EIS),activation energy

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3098 dx.doi.org/10.1021/am2006096 |ACS Appl. Mater. Interfaces 2011, 3, 3097–3102

ACS Applied Materials & Interfaces RESEARCH ARTICLE

by using a dc technique even at relatively high levels of appliedbias (( 200 mV) where the conventional circuit model withwhich a single diode often fails. More importantly, on the basis ofbias-dependent EIS analysis employing the advanced equivalentcircuit model newly developed, we for the first time were ableto determine the resistance at the Schottky contact separatelyfrom the total resistance in the device and thus to gain deeperunderstanding of the electrical characteristics of such contacts ata fundamental level.

2. EXPERIMENTAL DETAILS

Device Fabrication. A detailed method for synthesis of SnO2

nanowires can be found elsewhere.14 The synthesized SnO2 nanowireswere suspended in isopropyl alcohol (IPA) by sonication, and then thedispersed nanowire in the solution was dropped onto a Si/Si3N4

substrate with pre-defined probe pads. The probe pads were fabricatedby using a photo lithography technique followed by an electron beamevaporation (Cr/Au) process. For the fabrication of the individualnanowire devices, an electron beam lithography technique was appliedusing an Elphy Quantum, Raith. To make the Ohmic and the Schottkycontact on the nanowire device, Ti (20 nm)/Au (100 nm) and Au(120 nm) were deposited on the SnO2 nanowire devices, respectively,using an electron beam evaporation technique followed by a lift-offprocess.Nanowire and Device Characterizations. The morphology of

the nanowire devices was characterized by using a field emissionscanning electron microscope (FE-SEM, Philips XL 30). The acce-leration voltage was either 5 or 10 kV. The dc current-voltage char-acteristics were recorded by a source-measure unit (Keithley 236).The EIS analyses were obtained by employing an impedance analyzer(Novocontrol).

3. RESULTS AND DISCUSSIONS

Figure 1(a) shows a representative SEM image of a singleSnO2 nanowire device with the asymmetric contacts (i.e., onewith Ti/Au and the other with Au electrodes) we fabricated(hereafter the device). As clearly demonstrated in Figure S1 inthe Supporting Information, the I�V characteristics are indeedasymmetric to present more typical rectifying behavior of thedevice upon applying forward bias when compared with thebehavior observed at the reverse bias. It should be noted that theforward bias is defined as a positive voltage applied to the Au anda negative voltage to the Ti/Au contacts. The asymmetric I�Vcharacteristics observed are attributed to the dc-bias dependenceof the Schottky barrier formed at the Au contact. It is wellconceived that a Schottky barrier forms at the contact between asemiconductor and metal to compensate the difference betweenthe electron affinity of the semiconductor (e.g., χSnO2

= 4.9 eV)and the work function of metal (e.g.,ΦAu =5.1 eV) as illustratedin the inset of Figure 1b. The I�V characteristics in Figure 1bshow weak rectifying behaviors. Unlike other significant asym-metric device characteristics, less attention has been paid to theweak rectifying behavior. In fact, the observation of variouselectrical characteristics in seemingly identical one-dimensionalnanostructures is not rare. Although it is essential to deal with thecase of different behaviors to elucidate the nature of electricalcontacts, the weak rectifying behavior of the asymmetric contacthas been rarely reported. Herein, we report a combined study ofdc and EIS on asymmetric contacts fabricated on a singlenanowire device with the weak rectifying behavior. It is realizedfrom Figure 1b that the current level measured at the reversebias is relatively higher compared with those previously reportedfor such asymmetric contacts,15,16 implying that the dopingconcentration in the SnO2 nanowire used in the device is

Figure 1. (a) Representative FESEM image of an asymmetric device. A forward bias denotes a positive bias at the gold electrode and a negative voltageapplied to Ti/Au contacts. (b) Current�voltage characteristics of the SnO2 nanowire device with an asymmetric contact. Inset: Energy band diagram ofthe Schottky contact, indicating that the contact barrier height (0.2 eV) is attributed to the difference between theΦAu (5.1 eV) and χSnO2

(4.9 eV). (c)Room-temperature dV/dI versus 1/I plot for the series resistance model with a forward bias. The linear slope (red solid line) indicates the validity of theequivalent circuit model with a contact barrier. Inset: The equivalent circuit of the device with a contact barrier. (d) I�V plots corresponding to theexperimental data (black dot) and the simulation data using an equivalent model (red solid line) and the conventional resistance model (black dash line).Inset: The suggested equivalent circuit.



relatively higher: According to the thermionic emission (TE)theory in the reverse direction and the image force loweringeffect for Schottky barriers, the correlation between the currentdensity (J), and the doping concentration (ND) can be describedby the following equation8,17

Jreverse ¼ JS, TE exp

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiq7NDðV þ Φbi � kBT=qÞ=8π2ε3S

4p

kBT

!

ð1Þ

JS, TE ¼ A�T2exp � qΦEB

kBT

� �ð2Þ

ΦEB ¼ ΦIB �ffiffiffiffiffiffiffiffiffiffiqE4πεS

rð3Þ

E ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2qND

εSV þ Φbi � kBT

q

!vuut ð4Þ

where A* is the effective Richardson constant, ΦEB the effectivebarrier height, ΦIB the ideal barrier height without the imageforce lowering effect, E the maximum electric field at theSchottky junction, εS the dielectric constant of the SnO2

nanowire, ND the doping concentration of the tin oxide nano-wire, V the bias voltage, andΦbi the built-in potential. Accordingto above equations, J should be in proportion to ND so that thecurrent flow at the reverse bias is expected to increase with theincrease of ND in the SnO2 nanowires. ND in the SnO2 isestimated to be 4.6�1018/cm3 (see the Supporting Informationfor details) and indeed higher than the typical values of 1.5 �1014/cm3 previously reported, as we suspected.18

A device with a Schottky barrier at the contact exhibitsrectifying characteristics which have typically been modeledusing a single diode circuit (Figure 1b). The current, I, througha diode can be given by eq 5

ID ¼ ID0 expqVηkBT

� �ð5Þ

where ID0 and η denote the saturation current and the idealityfactor, respectively. On the other hand, it is worth pointing outthat the results of the fits using eq 5 are often dissatisfactory atrelatively higher biases, leading to substantial errors in η.We speculate that such inconsistency at the high biases mayresult from the fact that the conventional diode model focusesonly on the rectifying behavior of the junction, neglectingadditional invariant (i.e., bias-independent) resistance whichmay exist at the contact as well as in the channel of the device.To verify such a hypothesis, we construct an alternative circuitmodel which includes a resistor connected to a diode in seriesthat concerns such resistance.19 According to this model, theapplied voltage to the circuit is now shared between the resistorand the diode. Equation 5 should then be modified to yield eq 6.

V ¼ IR þ ηkBTq

lnII0

� �ð6Þ

By differentiating eq 6, one can yield

dVdI

¼ R þ ηkBTqI

ð7Þ

In view of eq 7, dV/dI should be linearly proportional to I�1, andthe expected linear behavior is indeed confirmed by Figure 1c.Although such a relationship is also expected from the conven-tional diode model (see eq 2), eq 7 results in a much better fit(see fitting errors indicated in Figure 1d) and thus these resultsclearly verify our hypothesis. The ideality factor, ηD1 and theserial resistance, RS1, determined from the linear fit of Figure 1cusing eq 7 are 2.71 and 190 kΩ, respectively. It is noted that thevalue of the ηD1 (2.71) now is reduced from that (3.03)determined using the conventional diode model.

It is realized from Figure 1d that the I�V curve measured atthe reverse bias also presents a rectifying behavior. Hence for thecomplete fitting, it is necessary to construct a back-to-back circuitmodel (see the inset in Figure 1d). From the best fit using eq 7,the ηD2 and the RS2 on the reverse bias were determined to be2.67 and 424 kΩ, respectively. The ηD2 is nearly identical to ηD1,whereas RS2 is found to be greater than RS1.

As mentioned above, EIS allows for deconvolution of dc-resistance into the local resistances that comprise the dc-resis-tance. The total resistance (in the circuit model we proposed(Figure 1d) consists of the resistance at the contact and that inthe channel. To investigate the local electrical properties exclu-sively and to check the applicability of our equivalent circuitmodel of the asymmetric contact device, we analyzed impedancespectra (Nyquist plot or Cole�Cole plot) of the device mea-sured at a frequency range of 1 to 1 � 106 Hz with an acamplitude of 0.03 V under both nitrogen (N2) and 1% oxygen(1% O2, balanced N2) ambient atmospheres as a function oftemperatures (20�100 �C). Figure 2a exhibits the Nyquist plotsof the SnO2 nanowire device under both the 1% O2 and the N2

environments, respectively. The diameter of each semicirculararc that corresponds to the total impedance of the Nyquist plotreduces as the temperature increases, implying that the conduc-tance in the semiconducting nanowire increases with increasingtemperature. It should be noted that the impedance in the devicedepends on not only the temperature but also the oxygen partialpressure of ambient gas. In the oxygen-rich environment, thewidth of the Schottky barriers in the SnO2 nanowire becomeswider since the oxygen molecules absorbed on the surface of thenanowire can trap the electrons in the vicinity of the surface1,20

(see below for the details). As a result, the impedance of thenanowire becomes higher under the 1% O2 compared with thatin N2. The thermal activation energy can be determined from theslope of the Arrhenius plot of the impedance measured as afunction of temperature, as shown in Figure 2b.10 The functionalform of the thermal activation energy is given by

RðTÞ ¼ R0 expEakBT

� �ð8Þ

where kB denotes the Boltzmann constant, and Ea the activationenergy. The Ea of the conduction in the SnO2 nanowire under N2

environment was determined to be about 46.3 meV, which isconsistent with the value previously reported.1 On the otherhand, the value of the Ea estimated under the 1% O2 was foundto be greater (79.3 meV). Such high Ea may be attributed tothe interactions between the oxygen molecules and the surface ofthe nanowire. The adsorbed oxygen leads to take electrons from



the nanowire, so the shallow donor states are depleted by theoxygen chemisorptions while the oxygen gas is absorbed into thesample.20 The increasing tendency of Ea is similar to the resultfor the individual SnO2 nanowire device without the Schottkycontact.1 Note that Ea of the asymmetric contact device showsthe same value or the similar tendency although our devices havethe Schottky contact. This result suggests that the activationenergy depends on the donor states in the nanowire rather thanon the contact barrier.

To determine the effect of the Schottky contact in terms of theEIS analysis, we measured the impedance spectra of the deviceunder identical conditions with different dc biases. Figure 3ashows the representative Cole�Cole plots of the device withdifferent dc biases (from �0.3 to 0.3 V) under 1% O2 environ-ment atT = 20 �C. A dc bias induces the change in the impedance

by modulating the height or the width of the Schottky barrier atthe contact.

As analyzed before, the Schottky barrier is subject to thethermionic emission theory so that the barrier height, in terms ofthe bias voltage, can be explained by the image force loweringeffect.17,21 The effective Schottky barrier height (ΦEB) is loweredwith the increase of the bias voltage, so that the current density

Figure 2. (a) Impedance spectra of the SnO2 nanowire device with anasymmetric contact at various temperatures (20�100 �C) under thenitrogen or the 1% oxygen (inset) ambient. Numbers (0, 4, and 6)indicate the index of the frequencies in a logarithmic scale. (b) Arrheniusplot of the resistance at different temperatures under the nitrogen (blacksquare dot) and the 1% oxygen (red circle dot) ambient. The slopes witha linear fit denote the activation energy (Ea) of the device under thedifferent ambient.

Figure 3. (a) Nyquist plots of the device with a DC bias from -0.3 to0.3 V under the 1% oxygen ambient. Numbers (0, 4, and 6) denote theindex of the measured frequencies in a logarithmic scale. Inset: 1/C2

versus DC bias voltage (V) for the device in nitrogen and 1% oxygen atroom temperature. (b) Impedance spectra corresponding to the experi-mental data (black square dot) and the simulation data (red solid line)obtained from the suggested equivalent circuit model in the nitrogenambient at room temperature. Upper inset: The plots of the best fit ofthe capacitance as a function of the bias voltage. The connected solidlines are only for guidance. Lower inset: The equivalent electrical circuitmodel composed of the two serially connected R//Cs.



increases in accordance with the above eqs 2�4. Therefore, theimpedance has the largest value at a zero bias, as shown inFigure 3a. On the other hand, the impedance at the appliedreverse bias has a larger value than that of the forward bias, whichcan be attributed to an asymmetric contact configuration andto an increase of the depletion width. When a reverse bias isapplied to the Au electrode, the width of the depletion layer willincrease, whereas it is suppressed at the forward junction (Ti/Auelectrode).22 The widening of the depletion layer at the Auelectrode will impede the conductance of the SnO2 nanowire,leading to an asymmetric behavior. For the same reason theCole�Cole plots under the N2 ambient condition also show thebias dependence.

It is well-known that the relationship between the capacitancedepending on the depletion layer and the bias voltage in aSchottky diode is as follows

1C2

¼ 2ðqεSε0A2NDÞ Vbi � V � kBT

q

!ð9Þ

where εS is the dielectric constant of tin oxide, εo the permittivityof the vacuum, A the area of the metal contacts, ND the carrierconcentration, Vbi the built-in potential, kB the Boltzmann con-stant, and V the applied external bias.17 The inset of Figure 3aexhibits a linear correlation between 1/C2 and V, which followseq 9 well. As shown in this figure, the capacitance of the device isvaried by two variable factors: one is the width of depletion layerat the Schottky junction depending on the bias voltage and theother is that of the channel which can be affected by ambient gas(N2 and 1% O2). Note that the variation of the capacitancecaused by the gas is greater than that caused by the external bias.This result strongly suggests that the electrical equivalent circuithas to include two capacitors to explain the impedance in thenanowire electronic devices.

As discussed earlier, the equivalent circuit used for the ACanalysis can be modeled by a scheme considering two series ofRC parallel circuits, as depicted in Figure 3b. Each impedancespectrum of the device can be well-fitted on the basis of theequivalent circuit model. The values of the variable parameterswere determined by the best fit (see Tables 1 and 2, including thedV/dI terms). In view of the fact that the ac signal is super-imposed on the dc level, the total resistance (R1 + R2) has to beidentical to the inverse of the slope of the tangent in the I�Vcharacteristics at a certain dc bias. It should be noted that thevalues of the total resistance of the AC analysis are consistentwith those of the dV/dI determined by the DC analysis. Inter-estingly, C2 depends on the applied bias, whereas C1 is lesssensitive to it. To better understand this observation, we assumed

that C1 corresponds to the capacitance of the channel and C2

corresponds to the Schottky junction of the SnO2 nanowire.If this is correct, then the values of C1 measured under the N2

ambient should be larger than that measured under the 1% O2,because of the depletion region being enhanced by adsorption ofoxygen molecules on the nanowire surface. The C1 value inTable 1 is about 10 times greater than that in Table 2. On theother hand, the C2 values do vary much regardless of the ambientgas. Therefore, the decrease in C1 values under 1% oxygenenvironment confirms the widening of the depletion layer dueto the decrease in the carrier concentration in the channel. Theseresults clearly demonstrate that indeed the equivalent circuitmodel should be composed of both the channel and the Schottkycontact parts.

4. CONCLUSIONS

Individual SnO2 nanowire devices with asymmetric contactshave been thoroughly studied through dc measurements and EISanalyses. Electrical behaviors at the Schottky contact varysensitively owing to variations of the Schottky barrier height orwidth, which is demonstrated using dc measurements and EISanalyses at various temperatures and atmospheres (N2 and 1%-O2). Through the atmospheres experiments, on the other hand,it is found that Ea of the SnO2 nanowire device depends on thedonor states in the nanowire rather than on the Schottky contact.We found that the relevant equivalent circuits in terms of dc andEIS analyses can be equivalent to a back-to-back diode modelconnected with a series resistance and to two series of RC parallelcircuits, respectively. The model and the analysis can be utilizedfor the evaluation and the simulation on the potential per-formance of an asymmetric contact device at the fundamentaldevice level.

’ASSOCIATED CONTENT

bS Supporting Information. Representative I�V character-istics of a single SnO2 nanowire device with the asymmetriccontact; Determination of dopant concentration (ND); outputand transfer characteristics of a SnO2 nanowire FET with asymmetric contact device. This material is available free of chargevia the Internet at http://pubs.acs.org/.

’AUTHOR INFORMATION

Corresponding Author*Tel: +82-2-3290-3250 (G.-T.K.); +1-530-754-2254 (S.K.).E-mail: [email protected] (G.-T.K.); [email protected](S.K.).

Table 1. Equivalent Circuit with the Best Fitting Values(R1, R2,C1, and C2) from the Impedance Spectra and dV/dI inTerms of DC Analysis Under the N2 Ambient Condition

Vbias (V) R1 (kΩ) C1 (pF) R2 C2 (pF) R1 + R2 dV/dI

0 464 86.3 1.61 MΩ 88.7 2.07 MΩ

0.1 456 78.6 914 kΩ 83.8 1.37 MΩ 1.39 MΩ

�0.1 424 72.3 1.01 MΩ 127 1.43 MΩ 1.49 MΩ

0.2 307 53.2 231 kΩ 235 538 kΩ 585 kΩ

�0.2 334 56.9 485 kΩ 192 819 kΩ 820 kΩ

0.3 221 46.4 101 kΩ 617 322 kΩ 315 kΩ

�0.3 280 46.7 319 kΩ 371 599 kΩ 586 kΩ

Table 2. Equivalent Circuit with the Best Fitting Values(R1,R2,C1, AndC2) from the Impedance Spectra and dV/dI inTerms of DC Analysis under the 1% O2 Ambient Condition

Vbias (V) R1 C1 (pF) R2 C2 (pF) R1 + R2 dV/dI

0 1.69 MΩ 6.90 1.00 MΩ 87.8 2.69 MΩ

0.1 1.52 MΩ 6.81 664 kΩ 83.1 2.18 MΩ 2.17 MΩ

�0.1 1.51 MΩ 6.57 737 kΩ 149 2.25 MΩ 2.31 MΩ

0.2 1.16 MΩ 6.43 366 kΩ 298 1.53 MΩ 1.46 MΩ

�0.2 957 kΩ 8.02 713 kΩ 81.9 1.67 MΩ 1.67 MΩ

0.3 782 kΩ 6.88 325 kΩ 97.3 1.11 MΩ 1.11 MΩ

�0.3 1.1 MΩ 6.19 372 kΩ 266.3 1.47 MΩ 1.34 MΩ



’ACKNOWLEDGMENT

This research was supported by the National Research Foun-dation of Korea (NRF) funded by the Ministry of Education,Science and Technology (2009-0083380, 2005-2002369, R01-2008-000-20185-0 and WCU, R32-2008-000-10082-0).

’REFERENCES

(1) Kolmakov, A.; Zhang, Y. X.; Cheng, G. S.; Moskovits, M.Adv. Mater. 2003, 15, 997.(2) Kong, J.; Franklin, N. R.; Zhou, C. W.; Chapline, M. G.; Peng, S.;

Cho, K. J.; Dai, H. J. Science 2000, 287, 622.(3) Chen, Z. H.; Appenzeller, J.; Lin, Y. M.; Sippel-Oakley, J.;

Rinzler, A. G.; Tang, J. Y.; Wind, S. J.; Solomon, P. M.; Avouris, P.Science 2006, 311, 1735.(4) Duan, X. F.; Huang, Y.; Cui, Y.; Wang, J. F.; Lieber, C. M.Nature

2001, 409, 66.(5) Jeon, D. Y.; Kim, K. H.; Park, S. J.; Huh, J. H.; Kim, H. Y.; Yim,

C. Y.; Kim, G. T. Appl. Phys. Lett. 2006, 89, 023108.(6) Chen, C. X.; Lu, Y.; Kong, E. S.; Zhang, Y. F.; Lee, S. T. Small

2008, 4, 1313.(7) Wei, T. Y.; Yeh, P. H.; Lu, S. Y.; Lin-Wang, Z. J. Am. Chem. Soc.

2009, 131, 17690.(8) Hu, Y.; Zhou, J.; Yeh, P. H.; Li, Z.; Wei, T. Y.; Wang, Z. L. Adv.

Mater. 2010, 22, 3327.(9) Yeh, P. H.; Li, Z.; Wang, Z. L. Adv. Mater. 2009, 21, 4975.(10) Barsoukov, E.;Macdonald, J. R. Impedance Spectroscopy: Theory,

Experiment, and Applications, 2nd ed.; JohnWiley & Sons: Hoboken, NJ,2005.(11) Huh, J.; Kim, G. T.; Lee, J. S.; Kim, S. Appl. Phys. Lett. 2008,

93, 042111.(12) Lee, S.; Yu, Y.; Hwang, S.; Ahn, D. International Conference on

Nanoscience and Nanotechnology; Gwangju, South Korea; AmericanScientific Publishers: Stevenson Ranch, CA, 2006, p 4089.(13) Yim, C. Y.; Jeon, D. Y.; Kim, K. H.; Kim, G. T.;Woo, Y. S.; Roth,

S.; Lee, J. S.; Kim, S. 4th International Conference on Advanced Materialsand Devices/6th Symposium on the Nano-Technology and Plasma Applica-tion for Next Generation Processing; Jeju, South Korea; Korean PhysicalSociety: Seoul, South Korea, 2005; p 1565.(14) Kim, D.; Kim, Y. K.; Park, S. C.; Ha, J. S.; Huh, J.; Na, J.; Kim,

G. T. Appl. Phys. Lett. 2009, 95, 043107.(15) Lu, C. G.; An, L.; Fu, Q. A.; Liu, J.; Zhang, H.; Murduck, J. Appl.

Phys. Lett. 2006, 88, 133501.(16) Yang, M. H.; Teo, K. B. K.; Milne, W. I.; Hasko, D. G. Appl.

Phys. Lett. 2005, 87, 253116.(17) Sze, S. M. Physics of Semiconductor Devices, 2nd ed.; Wiley:

New York, 1998.(18) Liu, Z. Q.; Zhang, D. H.; Han, S.; Li, C.; Tang, T.; Jin, W.; Liu,

X. L.; Lei, B.; Zhou, C. W. Adv. Mater. 2003, 15, 1754.(19) Kim, G. T.; Muster, J.; Burghard, M.; Roth, S. Symposium on

Nanophase and Nanocomposite Materials IV held at the 2001 MRSFall Meeting; Boston; Komarneni, S., Parker, J. C., Vaia, R. A., Lu,G. Q., Matsushita, J. I., Eds.; Materials Research Society: Warrendale,PA, 2001; p 505.(20) Park, C. O.; Akbar, S. A. J. Mater. Sci. 2003, 38, 4611.(21) Nam, C. Y.; Tham, D.; Fischer, J. E. Nano Lett. 2005, 5, 2029.(22) Xu, J. H.; Wu, N. Q.; Jiang, C. B.; Zhao, M. H.; Li, J.; Wei, Y. G.;

Mao, S. X. Small 2006, 2, 1458.

asymmetric contacts on a single sno nanowire device: an...

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