Electrical Contact Tunable Direct Printing Route for a ZnO Nanowire

Aug 13, 2010 - Address: 134 Shinchon-dong, Seodaemoon-gu, Yonsei University, Seoul, Postal #120-749. Telephone: (82-2)2123-2843. Fax: (82-2)365-2680 ...
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Electrical Contact Tunable Direct Printing Route for a ZnO Nanowire Schottky Diode Tae Il Lee,† Won Jin Choi,† Jyoti Prakash Kar,† Youn Hee Kang,† Joo Hee Jeon,† Jee Ho Park,† Youn Sang Kim,‡ Hong Koo Baik,† and Jae Min Myoung*,† †

Department of Materials Science and Engineering, Yonsei University, Seoul, Korea, and ‡ Department of Nano Science and Technology, Graduate School of Convergence Science and Technology, Seoul National University, Seoul, Korea ABSTRACT Although writing was the first human process for communication, it may now become the main process in the electronics industry, because in the industry the programmability as an inherent property is a necessary requirement for next-generation electronics. As an effort to open the era of writing electronics, here we show the feasibility of the direct printing of a high-performance inorganic single crystalline semiconductor nanowire (NW) Schottky diode (SD), including Schottky and Ohmic contacts in series, using premetallization and wrapping with metallic nanofoil. To verify the feasibility of our process, SDs made of Al-premetalized ZnO NWs and plain ZnO NWs were compared with each other. Even with cold direct printing, the Al-premetalized ZnO NW SD showed higher performance, specifically 1.52 in the ideality factor and 1.58 × 105 in its rectification ratio. KEYWORDS Direct printing, ZnO nanowire, Schottky diode, premetallization

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he single crystalline inorganic semiconductor nanowire (NW) has been considered an attractive building block for high-performance nanoscale electronic devices.1-4 Over the past decade, various synthesis routes of NWs have been introduced and well developed for mass production.5-12 Compared with the successful advancement in the NWs synthesis techniques, the state of the art in integration techniques for the NWs is not good enough to drive them into the electronic industry because of the difficulty in controlling each nanoscale electronic component. There have been many integration strategies based on the conventional thin-film-based integration paradigm depending on lithography, for example, flow-assisted alignment,13 selective chemical patterning,14 Langmuir-Blodgett method,15 and blown bubble films technique.16 However, these efforts will not be necessary for device integration with NWs that are inherently dispersed and isolated from each other at the nanoscale in a solution, if there is a technology that can handle NWs individually, such as an inkjet printer head moving and separately positioning separately liquidphase materials on the intended sites. The main issues in the direct writable integration of the nanoscale building block are how to register electrodes on it and position the device on the intended site without lithography. As a unique solution for these issues, in our previous work the programmable direct printing route for NW devices was introduced through the successful integration of p-type silicon NW field effect transistors with the intended gate system.17

The Schottky diode (SD) occupies an important status in various functional electronic circuit design technologies as a high-speed rectifier. Unlike the fabrication of the common NW field effect transistor, in the case of SDs two kinds of metal electrodes with different work functions should be registered on both edges of the NW to achieve Ohmic and Schottky contacts in series for the rectifying function. Whatever fabrication method is used, the electrodes’ gap formation with the different materials is a more complex process than that using the same materials. Additionally, thermal treatment for electrical contact improvement between the metal and semiconductor NW should be excluded in the device fabrication process if the device is set down on the cheap and commercial organic substrates that will be preferred in future electronics. Unfortunately, when low work function metals such as Al, Ti, and so forth are used in the direct printing process based on the cold welding for Ohmic contact with n-type semiconductor NWs, a serious problem will be generated; the formation of a thin oxide layer on the surface of the n-type Ohmic metal will disturb carrier transportation between the metal and the n-type semiconductor. Therefore, for direct writing to be a general route in the integration of NW devices, a strategy satisfying the above-mentioned needs related to the various electric contacts between the electrode materials and semiconductor NWs should be created. To satisfy these needs and then obtain a high-performance single NW SD through the direct printing route, we introduced the concept of the Ohmic premetallization of the n-type NW; its feasibility was verified by the successful fabrication of a high-performance single ZnO NW SD without additional thermal treatment.

* To whom correspondence should be addressed. Address: 134 Shinchon-dong, Seodaemoon-gu, Yonsei University, Seoul, Postal #120-749. Telephone: (82-2)2123-2843. Fax: (82-2)365-2680. E-mail: [email protected]. Received for review: 05/12/2010 Published on Web: 08/13/2010 © 2010 American Chemical Society

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FIGURE 1. The overall process of the direct printing of Al-premetalized and plain ZnO NW Schottky diodes. The devices are fabricated using a sequential process consisting of NW preparation, dielectrophoretic array, visual inspection, and decal printing.

In relation to the premetallization of nanoscale semiconductors, Sheldon et al.18 recently reported on the soluble Au premetallization of colloidal-grown CdSe nanorods to reduce contact resistance dramatically with an Au electrode pad. They formed the symmetrical Au parts on both edges of the CdSe nanorod through a soluble method based on chemical reactions. However, for high-performance SD, extremely asymmetric electrical contacts (a series combining Ohmic and Schottky) are needed, and there has been no report about Ohmic premetallization on the edge of an n-type semiconductor NW. As a unique strategy for the preparation of the asymmetrical Ohmic premetalized n-type semiconductor NWs in a solution, we employ the Ohmic metallization of the overall top side of NWs that are vertically grown on a substrate; we then cut them off from the mother substrate and dispersed them into a solution. This easy but efficient way to prepare the special NW for the SD is actually illustrated through the case of Al deposition on vertically grown n-type ZnO NWs in the NW preparation section in Figure 1. The overall process of direct printing the premetalized and plain ZnO NW SD is summarized in Figure 1. First, the vertically grown ZnO NWs were prepared; then, the Al metal electrode was deposited by using thermal evaporation. After the deposition, the premetalized and plain ZnO NWs were cut and dispersed in isopropyl alcohol (IPA). With the prepared ZnO NW solutions, a dielectrophoretic (DEP) array was conducted between the Au electrode pads on the © 2010 American Chemical Society

polydimethylsiloxane (PDMS) substrate. After the DEP process, single ZnO NW bridges on the PDMS were chosen using visual inspection. Finally, the single ZnO NW bridges were decaled on the target substrate and the intended devices were then complete. Plain ZnO NWs were synthesized using metal organic chemical vapor deposition (MOCVD) at 620 °C for 1 h on a c-axis-oriented sapphire substrate. A cross-sectional scanning electron microscope (SEM) image of the ZnO NWs that were grown is shown in Figure 2a. The length of the ZnO NW is about 13 µm and the diameter is about 300 nm. Top images of the ZnO NW before and after Al deposition are displayed in Figures 2b,c. An Al-premetalized single ZnO NW lying on a bare Si wafer was observed using high-resolution SEM and energy dispersive X-ray spectroscopy (EDX), as shown in Figures 2d-f, and Supporting Information 1. From the EDX data, near the top of the Al-premetalized single ZnO NW in Figure 2e Al was determined have to 3.28 at. %; on the other hand, near the bottom of the NW in Figure 2f on Al was not detected. On a cleaned 1 × 1 cm glass slide, a 7 mm thick PDMS Dow Corning Silgard 184 (10:1, cured for 4 h at 80 °C) layer was formed as a decal printing agent. To define the electrode gaps on this elastomeric matrix layer, a 3 µm tungsten wire stencil was used. With a thermal evaporator, arrays of gold 6 µm gap electrodes were deposited onto each block of the PDMS surface. After attaching the stencil shadow mask, gold was thermally evaporated up to 80 nm. Suspended NWs dispersed in IPA were dropped 3518

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FIGURE 2. Vertically aligned ZnO NWs and their Al premetallization. (a) The cross sectional SEM image of the vertically MOCVD-grown ZnO NWs on a c-axis oriented sapphire substrate, 45°-tilted image. (b) Top-side image of the plain ZnO NWs as grown, 45°-tilted image. (c) Top side image of the Al-premetalized ZnO NWs after Al deposition, 45°-tilted image. (d) An Al-premetalized ZnO NW lying on bare Si substrate. (e) Magnified image of the top side of the Al-premetalized ZnO NW displayed in panel d; the roughness of the surface comes from deposited Al. (f) Magnified image of the bottom side of the Al-premetalized ZnO NW displayed in panel d.

into well-defined electrode patterns, and then a 1 kHz unipolar direct current pulse of 10 V was applied using a digital-synthesized function generator (Protek-9300 Series) on the electrode gap until the intended number of NWs crossed this gap. After the DEP array, a 1000-power magnifying lens (Olympus optical microscope, BX41) was used to inspect the condition of the NW attachment. To gain a high direct printing yield, poly(4-vinylphenol) (PVP) was used as a polymeric adhesive agent. The PVP solution (10 wt %) with a cross-linking agent, poly(melamine-co-formaldehyde) (PMCF), in propylene glycol monomethyl ether acetate (PGMEA) was coated onto the glass substrate using a spin coater. The ZnO nanobridge was decaled on the 10 wt % PVP-coated gate dielectric layer on glass substrate. Currentvoltage (I-V) data were measured using an Agilent semiconductor parameter analyzer (Model 4145B) with contacts to the devices created using a probe station (Desert Cryogenics, Model TTP4). The top view of the device for measuring the active channel width was characterized by using SEM (JEOL, JSM-7001F). Using optical microscopic preliminary inspection, single NW bridges were chosen on the basis of the width, active length, and Schottky contact area for both of the premetalized and plain case; these were then decaled on the target substrate. Because PVP is a soft polymeric layer, the printed ZnO NWs were partially embedded into the PVP layer. (This is confirmed by the device top image after burning out the ZnO NW through the high current-allowing test; see Supporting Information 2). Top view images of the printed Alpremetalized and plain ZnO NW SD are shown in Figure © 2010 American Chemical Society

3a,b. The width, active length, and Schottky contact area of Al-premetalized ZnO NW were about 320 nm, 6.48 µm, and 1.170 µm2. In the case of Al-premetalized ZnO NW, in view of the growth direction the metalized top side should be the Ohmic contact and nonmetalized bottom side should be the Schottky contact with the Au electrodes. As shown in Figure 2a, the width of ZnO NW increased from bottom to top; hence, it is confirmed that the left side is the bottom and the right side is the top from the ZnO NW shape in Figure 3a, and the Schottky barrier should be the left side Au/ZnO NW. The measured Al-premetalized ZnO NW SD polarity accorded with this expectation. On the other hand, the width, active length, and Schottky contact area of plain ZnO NW were about 340 nm, 6.93 µm, and 1.135 µm2. The diode polarity direction is random regardless of the direction in which the c-axis lies in plain ZnO NW and is dominated by contact asymmetry during DEP.19 In this experiment, the plain ZnO NW SD showed the main Schottky barrier between right side Au electrode and plain ZnO NW as shown in Figure 3b. The I-V characteristics of both the Al-premetalized and plain cases are displayed in Figure 3c. There were clear differences in diode parameters between plain and Alpremetalized ZnO NW SDs, and these are summarized in Table 1. The characteristics of plain ZnO NW SD are comparable with the results of the Lao et al.19 By using the plot of the ln(I) vs V, the ideality factor was calculated within the voltage range for each device following 0.04 V < V < 0.4 and 0.16 V< V < 0.6 V, as shown in Figures 3d,e. The ideality factors for each device were 1.52 and 2.02. The Al-premet3519

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FIGURE 3. Top view images of the Al-premetalized and plain ZnO NW SD after decal printing, and their I-V characteristics. (a) Al-premetalized ZnO NW SD; the left side of the image was determined to play a rectifying role as Schottky contact. (b) Plain ZnO NW SD; the right side of image was determined to play a rectifying role as Schottky contact. (c) I-V characteristics for both Al-premetalized and plain ZnO NW SD, the solid black square belongs to the Al-premetalized ZnO NW SD and the solid red square belongs to the plain ZnO NW SD. (d) Linear fitting from the ln(I) vs V of the Al-premetalized ZnO NW SD, the ideality factor is calculated from the slope.(e) Linear fitting from the(ln(I) vs V of the plain ZnO NW SD; the ideality factor is calculated from the slope. TABLE 1. List of the Parameters of the Plain and Al-premetalized ZnO NW Schottky Diodea

plain ZnO NW SD AI premetalized ZnO NW SD

ideality factor, n

reverse current at -1 V

2.02 1.52

3.01 × 10-12 A 2.12 × 10-11 A

In the reverse bias case, according to the theory of thermionic emission in a metal-semiconductor contact,21 the band diagrams for each case have been generated and are illustrated in Figure 4a,b. The reverse currents at -1 V for plain and Al-premetalized ZnO NW SDs were 3.01 × 10-12 and 2.12 × 10-11 A. Considering the trend of the increase in the reverse current with an increasing reverse bias for each device, we found a clear difference between the two cases as shown in Figure 3c. The resistive interface of the secondary Schottky barrier is in a forward bias situation, as described in Figure 4a; this contributes to the low level of the reverse current in the plain ZnO NW SD.21 For the total R ) JAu1fZnONW - JZnONWfAu1 - JAu2fZnONW, current density, Jnet where JAu1fZnONW is the current density from Au1 to ZnO NW, JZnONWfAu1 is the current density from ZnO NW to Au1, and JAu2fZnONW is the current density from Au2 to ZnO NW. On the other hand, for the Al-premetalized ZnO NW SD, a rapid increase of the reverse current in comparison with the plain ZnO NW case was observed as shown in Figure 3c. The R current density in this case can be expressed as follows: Jnet ) JAlfZnONW - JAufZnONW, where JAlfZnONW is the current density from Au to ZnO NW, and JAufZnONW is the current density from ZnO NW to Au. Because the barrier against the carrier transportation is the only Au/ZnO NW Schottky contact described in Figure 4b,20 the carriers that cross over this barrier freely drift toward the Al electrode without any disturbance unlike in the plain ZnO NW SD case. There are two Schottky barriers as described in Figure 4c related to the plain ZnO NW SD. If one barrier is higher than the other, this difference will generate contact asymmetry,

turn-on rectification voltage ratio 0.75 V 0.55 V

6.61 × 104 1.58 × 105

a All the parameters were calculated from the I-V characteristics of both devices. The ideality factors were calculated by using the slope of the linear fitting of the plot of the ln(I) vs V. The turn-on voltages were estimated as the value of voltage at zero current from a function linearly fitted within the linearity range of the decimal scale of the forward I-V relationship. Rectification ratios were measured by dividing the absolute value of the forward current at 1 V by the absolute value of the reverse current at -1 V.

alized ZnO NW SD is nearer to the ideal diode than plain ZnO NW SD, because the secondary Schottky barrier in plain ZnO NW SD may be a resistive barrier that reduces the exponentially rising parameter of the forward current. In addition to the ideality factor, the turn-on voltage of the premetalized device is about 30% lower than that of the plain one. Here, only as a carrier transportation mechanism, the thermionic emission model was considered according to the report on the nano-Schottky barrier of Smit et al.20 This was because the width of the ZnO NWs in this work was about 300 nm, making the thermionic emission mechanism more dominant than the tunneling one. The work functions of the Au, Al, and ZnO NW are known to be 5.3, 4.06, and 4.1 eV, respectively. © 2010 American Chemical Society

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FIGURE 4. Band diagrams related to the I-V characteristics of both of Al-premetalized and plain ZnO NW SD. Band diagrams including band bending and carrier transportation mechanism in the case of reverse bias being applied to (a) the plain ZnO NW SD and (b) the Al-premetalized ZnO NW SD. Band diagrams including a band bending and carrier transportation mechanism in the case of the forward bias applied to (c) the plain ZnO NW SD, (d) the Al-premetalized ZnO NW SD.

which will in turn generate the rectification property.19 When the forward bias is applied to the higher barrier, the reverse bias is simultaneously applied to the lower barrier. In this situation, the total current density derived by the large forward and the small reverse bias situations in series can F be explained as follows: Jnet ) JAu2fZnONW - JZnONWfAu2 JAu1fZnONW, where JAu2fZnONW is the current density from Au2 to ZnO NW, JZnONWfAu2 is the current density from ZnO NW to Au2, and JAu1fZnONW is the current density from Au1 to ZnO NW. Through this lower Schottky barrier effect under the forward bias in view of the large barrier, a higher turn-on voltage is observed; this is shown as the red curve in Figure 3c. The reason for this phenomenon may be that the lower Schottky barrier is acting as a resistive barrier to reduce the total current at a certain forward bias. On the other hand, for the Al-premetalized ZnO NW SD, there is one Schottky contact and one Ohmic contact, as illustrated in Figure 4d. When the forward bias is applied to the Schottky barrier, the total current density is derived by a forward bias situation, F ) JAlfZnONW - JAufZnONW, where JAlfZnONW is the current Jnet density from Al to ZnO NW, and JAufZnONW is the current density from Au to ZnO NW. From the forward I-V characteristics, it was found that the turn-on voltage of the Alpremetalized ZnO NW SD was smaller than that of plain ZnO NW SD, because the current increases exponentially from the reverse saturation current in the forward bias situation with a single Schottky barrier. © 2010 American Chemical Society

To determine the difference of the series resistance and Schottky barrier height as key parameters related to the electrical contact type of the Al-premetalized and plain ZnO NW SD from the forward I-V characteristics, the analysis method reported by Cheung et al.22 was employed. As shown in Figure 5a,b, from the slope and the vertical axis intercept in the plot of (dV/d ln I) vs I, the total series resistance R, and the ideality factor n were determined as follows: 119 kΩ and 2.2 for the Al-premetalized ZnO NW case, and 270 kΩ and 2.7 for the plain ZnO NW case. The n’s were calculated to be larger than those of the standard method by the plot of ln(I) vs V, as illustrated in Figures 3d,e. For the Schottky barrier height, we first measured the effective Schottky contact area, Aeff from the SEM images in Figures 3a,b for each SD case as follows: 1.170 and 1.135 µm2. The barrier heights were then calculated using the reverse saturation current equation, Is ) AeffA**T2 exp(-qφB/ kBT), where Is is reverse saturation current, A** () 8.6 cm-2K-2A) is the Richardson constant, kB is the Boltzmann constant, and φB is the Schottky barrier height of the diode, as follows: 0.61 eV for the Al-premetalized ZnO NW case and 0.69 eV for plain ZnO NW case. The difference in the values of the Schottky barrier heights means that both devices have different electronic junctions. In addition, it has been determined that the difference in the R is mainly dependent on the electric contacts, except for the main Schottky contacts, of the devices, because the Al-premetalized and plain ZnO 3521

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FIGURE 5. The plots of (dV/d ln I) vs I and the linear fittings to determine total diode resistances and ideality factors. (a) Linear fitting result for the Al-premetalized ZnO NW SD, and (b) linear fitting result for the plain ZnO NW SD.

NW having similar active lengths, the Aeffs, and NW widths were selected using visual inspection. The reason for the 2-fold higher resistance of the plain ZnO NW SD is determined to be that the lower Schottky barrier contact is more resistive than the Ohmic contact of Al-premetalized ZnO NW SD. This result implies that the premetallization of NW is effective in enhancing forward bias performance through the reduction of the contact resistance of the diode. When we decaled the double bridges of ZnO NWs, symmetrical I-V characteristics were often observed (see Supporting Information 3). There is no reverse current characteristic, but only a forward current characteristic; this is similar to the case of a reverse directional connection of two diodes in parallel. For both the Al-premetalized and plain ZnO NW, it was difficult to obtain SD with the double bridges successfully, because the probability is very low of attaching NWs with the equivalent electrical contacts in the multibridge case. Therefore, before the integration of the NW SD device, a preliminary inspection is important and has to be employed to obtain high process yield. Although the reverse current level of the plain ZnO NW SD is lower than that of the Al-premetalized one, the rectification ratio of the plain ZnO NW case, considering the forward and reverse current levels simultaneously, is not superior to that of the Al-premetalized case. The value of the rectification ratio as shown in Table 1; 2.4-fold higher rectifying ability of the Al-premetalized ZnO NW SD is thought to come from the ten times higher forward current level due to the excellent Ohmic contact between Al and ZnO NW. Interestingly, when we do the cold welding between Al and ZnO NW, there must be an insulating oxide of Al or other defects in the contact interface; hence, without thermal treatment, a good electrical Ohmic contact is impossible to achieve. On the other hand, in the cold welding between the Al-premetalized ZnO NW and Au, the Ohmic contact is easily achieved with only mechanical contact because of the formation of the local Ohmic path through the Al to Au contact. Therefore, the premetalized side of ZnO NW with Al can be the electrical Ohmic contact with Au due to the mechanical contact, which is the main electrical contact © 2010 American Chemical Society

mechanism of the semiconductor NW with metal in our direct printing method. In summary, for our direct printing integration process of NW SD accompanying the cold welding between n-type semiconductor NWs and metal electrodes, to achieve good Ohmic contact, Al-premetalized ZnO NWs were synthesized using MOCVD and thermal evaporation. Without heat treatment for good electric contact between the metal and the semiconductor NW, a high-performance single ZnO SD was successfully obtained using the direct printing route. Through DEP array and sorting by visual inspection, a single ZnO nanobridge was structured on PDMS and chosen for the direct printing. To confirm the premetallization effect, the SDs with the premetalized and the plain ZnO NW, which have similar diameters, active lengths, and effective Schottky contact areas, were simultaneously fabricated using a sequential process that included DEP, visual inspection, and decal direct printing. The excellence of the Ohmic to Schottky series combination in Al-premetalized ZnO NW SD was confirmed by the enhancements of the ideality factor, rectification ability, and electric conductance through the reduction of the contact resistance; this can be compared with the Schottky to Schottky series combination in plain ZnO NW SD. Like the NW field effect transistor presented in our previous work,17 the NW SD can feasibly be included in our programmable direct printing fabrication route through the strategy of the premetallization of the NW. Our direct printing for various NW electronic devices gives opportunities in the field of direct writing electronics when it comes to obtaining high performance, flexibility, and transparency due to the electrical, mechanical, and optical properties of NW. Additionally, the concept of the premetallization of NW will be useful in reducing the number of steps in maskprocess in the lithographic fabrication of SD based on NW. Acknowledgment. This work was supported in part by the WCU (World Class University) program through the National Research Foundation of Korea funded by the Ministry of Education, Science, and Technology (R32-20031) 3522

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in part by the “System IC 2010” project of the Ministry of Knowledge Economy of Korea (2008-8-1511), and Y.S.K. acknowledges support from National Research Foundation of is Korea (NRF) Grant (2010-0000378) that was funded by Korea government (MEST), and H.K.B. acknowledges support from the Seoul RNBD program (ST090835) that was funded by Seoul city.

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Supporting Information Available. The contents include the following: (1) EDX data table and spectroscopic diagrams, (2) SEM image of the trace of PVP pressed by decal printing ZnO NW after burning out the devices, and (3) the I-V characteristics of the dual ZnO NW SD. This material is available free of charge via the Internet at http://pubs.acs.org.

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