Crystal Phase- and Orientation-Dependent Electrical Transport

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Crystal phase- and orientation-dependent electrical transport properties of InAs nanowires Mengqi Fu, Zhiqiang Tang, Xing Li, Zhiyuan Ning, Dong Pan, Jianhua Zhao, Xianlong Wei, and Qing Chen Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b00045 • Publication Date (Web): 22 Mar 2016 Downloaded from http://pubs.acs.org on March 23, 2016

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Crystal phase- and orientation-dependent electrical transport properties of InAs nanowires Mengqi Fu, † Zhiqiang Tang, † Xing Li, † Zhiyuan Ning, † Dong Pan, ‡ Jianhua Zhao, ‡ Xianlong Wei† and Qing Chen*,†

†

Key Laboratory for the Physics and Chemistry of Nanodevices and Department of Electronics, Peking University, Beijing 100871, China ‡

State Key Laboratory of Superlattices and Microstructures, Institute of

Semiconductors, Chinese Academy of Sciences, Beijing 100083, China

Abstract We report a systematic study on the correlation of the electrical transport properties with the crystal phase and orientation of single-crystal InAs nanowires (NWs) grown by molecular-beam epitaxy. A new method is developed to allow the same InAs NW to be used for both the electrical measurements and transmission electron microscopy characterization. We find both the crystal phase, wurtzite (WZ) or zinc-blende (ZB), and the orientation of the InAs NWs remarkably affect the electronic properties of the field effect transistors based on these NWs, such as the threshold voltage (VT), ON-OFF ratio, subthreshold swing (SS) and effective barrier height at the off-state (ΦOFF). The SS increases while VT, ON-OFF ratio and ΦOFF decrease one by one in the sequence of WZ , ZB , ZB , ZB and ZB . The WZ InAs NWs have obvious smaller field-effect mobility, conductivities and electron concentration at VBG = 0V than the ZB InAs NWs, while these parameters are not sensitive to the orientation of the ZB InAs NWs. We also find the diameter ranging from 12 nm to 33 nm shows much less effect than the crystal phase


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and orientation on the electrical transport properties of the InAs NWs. The good ohmic contact between InAs NWs and metal remains regardless of the variation of the crystal phase and orientation through temperature dependent measurements. Our work deepens the understanding of the structure-depended electrical transport properties of InAs NWs and provides a potential way to tailor the device properties by controlling the crystal phase and orientation of the NWs. Key words: InAs nanowires, electrical properties, crystal phase, crystal orientation, nano-manipulation Semiconductor nanowires (NWs) are attractive building blocks for nanoelectronics owing to their favorable electrostatic geometry.1-6 Among the class of NWs, InAs NWs have shown great potential for further promoting the performance of electronic devices and have attracted intense research interest, mainly because of their high electron mobility as compared with Si and most of other III-V NWs and their ohmic contacts with metals.1, 2, 7-12 Besides, InAs NWs have also shown rich applications in quantum devices for researches on novel and exciting physical phenomena owing to their strong spin-orbit coupling and large electron Landé g factor.13-16 The energy band structure of InAs NWs has been theoretically predicted to be different in various low index growth directions.17-19 For instance, a smaller effective mass and higher mobility have been predicted along some particular directions in InAs NWs with small diameter,17-19 so that fabricating devices along these orientations might improve the speed of the devices. While, when different calculation methods, geometry sizes and surface passivating conditions were used in the theoretical works, discrepant results have been obtained for some key parameters of the energy band structure, such as the bandgap (Eg) and the effective mass in different oriented InAs NWs.17-19 And without relevant solid input parameters from experimental works for modeling, it is very difficult to


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precisely estimate the band structure of the NWs used in experiments. On the other hand, the relation between the orientations and electrical transport properties of InAs NWs has not been experimentally clarified to date. Differences on electron mobility (μe) have been observed experimentally between and oriented zinc-blende (ZB) InAs NWs,20 and the lower mobility in oriented ZB InAs NWs has been attributed to the high density of stacking faults in that work. However, to our best knowledge, rare experimental work has been reported regarding the electrical transport properties of InAs NWs with the growth direction apart from the ZB and . Moreover, crystal phases, wurtzite (WZ) and ZB, have been well studied theoretically and experimentally to influence the band structure (e.g., the wider Eg in WZ phase InAs) and device performance (e.g., higher ON-OFF ratio in WZ InAs NW transistors) of InAs NWs.14,

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However, the impact of crystal phase on some key parameters including μe and resistivity is not very clear yet in consideration of a few incompatible results in the previous works.17, 22, 24, 25 One of the main difficulties is the influence of crystal phase is not easily distinguished from the effects of some other factors, such as the stacking faults and mixed phases,22, 24 uncertain background carbon doping,14, 25 and geometric scale.22, 25 Therefore, a clean system (e.g., doping free, similar geometry and size of the studied InAs NWs) is needed to achieve clear results. In this letter, we report for the first time an explicit correlation between the electrical transport properties and the orientation as well as the crystal phase of individual single-crystal InAs NWs. We first develop a method to establish the connection between the structure and the physical properties of the same individual InAs NWs by combining the use of nano-manipulation and microlithography techniques. We then quantitatively study the electrical transport properties


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of InAs NWs with different phase and orientation. Our study shows that both the crystal phase and the orientation significantly influence the electrical transport properties of the InAs NWs. Furthermore, ohmic contacts are confirmed to still maintain in all kinds of InAs NWs we studied. All the InAs NWs studied in this work were grown on a two-inch Si (111) substrate by molecular-beam epitaxy (MBE) using Ag particles as catalysts without intentionally doping.27 The conventional one-step catalyst annealing process was used for catalyst deposition, with a 0.2 nm thick starting Ag film annealed at 600 °C for 20 min in the growth chamber directly. The growth temperature and V/III ratio were 505 °C and 30, respectively. InAs NWs were observed to grow homogeneous on the whole Si wafer. The InAs NWs for device fabrication in this work are from a small area (smaller than 1.5×1.2 cm2) near the center of the wafer. The MBE-grown InAs NWs have high crystal quality and no carbon background doping, providing us a clean system for the investigation of intrinsic transport properties without the influence of doping, mixed phases and stacking faults. To investigate the correlation between the structural and the electrical transport properties of the same individual InAs NWs, we develop the following experimental method and process, as shown schematically in Figure 1(a). Disordered InAs NWs are mechanically transferred from the growth substrate (Figure 2(a)) to a n-doped Si substrate coated with a layer of 1.2 μm-thick S1813 photoresist, which are used as the back-gate electrode and the dielectrics, respectively. The photoresist has been hard baked at 200℃ before the NWs are transferred so that it will not be dissolved by the organic solvents used in the following processes. Patterned marks have also been fabricated on the S1813 layer to precisely label the dispersed NWs. Individual NWs with enough length (longer than 2.3 μm) and homogeneous diameter along their growth axis are located by


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scanning electron microscopy (SEM) and selected for the device fabrication. The source and drain (S/D) electrodes are patterned by electron beam lithography. The section of the InAs NW at the contact area is treated by diluted (NH4)2Sx solution to remove the native oxide layer. A film of Cr/Au/Cr (25/60/20nm) is deposited by electron-beam evaporation followed by lift-off in acetone to form the S/D electrodes. To exclude possible effect of the channel length (Lg) to the electrical properties, all the field effect transistors (FETs) studied here are designed to have the same channel length of around 1.5 μm. The electrical transport properties of the FETs are measured in an evacuated chamber of cryogenic probe-station. After all the room temperature and low temperature electrical measurements are finished, the InAs NWs in the devices are suspended through etching the photoresist under the channel by O2 plasma treatment. Then, through nano-manipulation in SEM,28, 29 the suspended NWs are transferred onto a holey carbon film supported by Cu grids with patterned marks for high-resolution transmission electron microscopy (HRTEM) investigation, as shown in Figure 1(b). To ensure the correspondence between TEM characterization and electrical measurements, we place the NWs from different devices in different areas of the micro-grids that are labeled by different marks (such as “E3” shown in the middle image in Figure 1(b)). More importantly, a series SEM images are taken at different magnification showing the NWs on the holey carbon film and are used to identify the position in TEM. The crystal phase and orientation of these InAs NWs are characterized by HRTEM. More than twenty InAs NWs FETs are studied in this work and five types of NW are observed, including oriented WZ NWs, , , and oriented ZB NWs, as shown in Figure 2(b-f). HRTEM study shows that most of the NWs investigated in this work are pure-phased single crystal with few stacking faults.


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First, we compare the electrical transport properties of the NWs with similar diameters (20 ± 2 nm) in order to avoid the influence of the diameter. As shown in Figure 3, different transfer curves and output curves of the FETs are obtained from different types of NWs at room temperature. It can be observed that both the crystal phase (WZ or ZB) and the orientation of the axis of the NWs affect the electrical transport properties. The threshold voltage (VT) of the FETs based on the , and oriented ZB InAs NWs are negative; that of the oriented ZB NW is slightly above zero and the FETs based on the oriented WZ InAs NW has the most positive VT among these five types of NWs, as shown in Figure 3(a). Also, the subthreshold swing (SS) of the FETs increases one by one in the sequence of oriented WZ NW, , and oriented ZB NWs, as shown in Figure 3(b). The SS for the oriented ZB NWs is not reliable to be extracted because of the very small ON-OFF ratio. The ON-OFF ratio of the FETs is also significantly influenced by both the crystal phase and the orientation, as shown in Figure 3(b). In this work, the on-state current (ION) denotes the source-drain current (IDS) at the backgate voltage VBG = VT + 10V and the off-state current (IOFF) is the IDS when IDS in the transfer curve reaches the minimum at an enough negative VBG. The changes of the electrical transport properties with the crystal phase and orientation of the NWs are also observed in the FETs based on the InAs NWs with their diameters ranging from 12 nm to 33 nm (that is the diameter range we study presently). Figure 4 shows a statistical analysis on the electrical properties of 21 devices we have studied and all their transfer curves are displayed in the supporting information (Figure S1-S3). Although the diameter of InAs NWs has been reported to affect the electrical properties of the FETs,8, 30, 31 the present results show that comparing with the crystal phase and the orientation, the effect of the diameter is not obvious,


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partly due to the small diameter range of NWs studied here. As can be seen in Figure 4(a), the ON-OFF ratio of the FETs decreases one by one in the sequence of oriented WZ, , , and oriented ZB NWs. All the oriented NWs present high ON-OFF ratio above 104 due to the larger Eg of WZ InAs NWs21,

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that could be helpful to suppress the leakage current at the OFF-state. The nice

switch-off properties of oriented InAs NWs claims that the back-gate structure used in our work owns enough electrostatic efficiency to the channel InAs NWs. Among four types of ZB InAs NWs, only oriented ZB NW has ON-OFF ratio higher than 103 but is still lower than 104. The other three types of ZB NWs with their axes along , and show the value around 103, 102 and 10, respectively, within a small spread. Half of oriented ZB NWs show ON-OFF ratio less than 5. This great distinct ON-OFF performance in the InAs NWs with various axis directions provides a possible method to prejudge the crystal phase and orientation of the NWs in the conditions that are unable to do TEM characterization. On the other hand, we find that the field-effect mobility (μFE), conductivity (σ) and effective electron concentration (ne) show no obvious dependence on the orientations, but are largely influenced by the crystal phase, as shown in Figure 4(b-d). Here, we extract μFE by using 2

FE  g m Lg /(C gV DS ), where Cg is the back-gate capacitance to the NW and gm is the maximum transconductance at VDS = 0.1 V. Cg is calculated by the metallic cylinder on an infinite metal plate model, permittivity,

C g / Lg  2 0 r / cosh 1(t / r )32, where  0 denotes the vacuum

 r and t are the relative dielectric constant (  r of S1813 is 2.5 33) and the thickness

of the photoresist, and r is the radius of the InAs NWs. Figure 4 (b) shows the μFE extracted from different InAs NWs. The extracted μFE of oriented WZ NWs is less than 1800 cm2/V•s,


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while most of the ZB-structured NWs show higher electron mobility with the mean value being around 2200 cm2/V•s. Values for σ and ne are extracted via

  Lg / r 2R and ne   / eFE ,25, 34 where R

is the NW resistance at VBG = 0V obtained from the linear region of the Ids-Vds curves as depicted in Figure 3(c). As shown in Figures 4 (c) and 4(d), both σ and ne of the WZ NWs are of orders of magnitude smaller than that of the ZB NWs. The value of ne of the four different ZB structured NWs at VBG = 0V is around or above 1018 cm-3 (1012 cm-2 normalized by the perimeter), which is much higher than the intrinsic carrier concentration of bulk ZB InAs (1015 cm-3 at room temperature)35 and comparable to the reported value of surface 2-dimensional electron gases (2DEGs) on InAs NWs.36 This result might suggest that the high density surface donor states near the bottom of conduction band reported in bulk ZB InAs37 and oriented ZB NWs24 might also exist in the ZB structured InAs NWs. And these surface donor states contribute to an electron accumulation layer36,

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on the surface. in which electron concentration increases and more

electrons contribution to the current under the electric field induced by VDS, consequently enhancing the σ and ne of the ZB InAs NWs with different orientations in this work. Thus in most of the ZB-structured InAs NWs, negative VBG is needed to deplete the electrons and pinch off the devices even that the present MBE-grown InAs NWs are undoped. In contrast, the highest extracted ne in oriented WZ NWs is about 1017 cm-3, which is one order of magnitude lower than those in ZB NWs and the lowest value is in the level of 1015 cm-3, that is about the same level as the intrinsic bulk ZB InAs.35 It might suggest that the surface of the WZ NWs is smoother and lack of dangling bonds, so that has less surface donor states. Although higher growth temperature would favor the formation of ZB structure as well as an increased


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indium content, as the present NWs are picked from the same small area on the same piece of chip, both the ZB and WZ NWs studied here are grown on the same temperature and in the same gas environment. Besides, we also study the composition of the NWs (3 ZB NWs and 6 WZ NWs) through X-ray energy dispersive spectrum (EDS) in TEM (the results are shown in Figure S4 and Table S1 in Supporting information). We find that the stoichiometry of both the WZ and ZB InAs NWs are the same within the accuracy of EDS analysis. Therefore, we can exclude the possibility that the difference on the electrical transport properties between ZB and WZ NWs is originated from the difference stoichiometry in the two crystal phases. Previous work on MOCVD-grown InAs NWs with 80 nm in diameter suggests that WZ NWs have larger μFE than the ZB NWs at low temperature.25 While extended WZ segments in predominantly ZB NWs (thicker than 40 nm) have been observed to increase the resistivity significantly.24 The present results on MBE NWs thinner than 33 nm indicate the WZ-structured NWs have smaller μe and larger resistivity than the ZB-structured NWs. As to the effect of the orientation, ZB MBE-grown InAs NWs with diameter around 40 nm have been reported to have obviously larger μe than ZB InAs NWs.20 While a theoretical work suggests that Eg and μe of ZB NWs are higher in direction and lower in direction compared with that in direction, and all of them in the three directions decrease as the diameter of the NWs increases, and the difference in different orientation become ignorable when the diameter increases to 10 nm.19 However, another theoretical simulation on the NWs thinner than 3 nm has suggested that ZB NWs have larger electron effective mass than ZB, ZB, WZ and WZ NWs, and the later four types of NWs have about the same electron effective mass.17 Presently, we study the InAs NWs with the diameter being 12-33 nm, located


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between that in the theoretical works and that in the previous experimental works. The orientation of the ZB NWs does not show obvious effect on the mobility, conductivity and effective electron concentration, but these three parameters of WZ NWs are obviously smaller than that of the ZB NWs. More theoretical works are needed to explain our experimental observations. To investigate the influence of the crystal phase and orientation on the OFF-state and contact properties, low temperature measurements (Figure 5) are performed to extract the effective barrier height (ΦB) and Schottky barrier heights (ΦSB) at the Cr-contact area in oriented WZ, , and oriented ZB NWs. In high-temperature region, conventional thermionic emission theory is used to extract ΦB : 8, 38, 39

I DS  A *T 2 exp( where

q B )[1  exp((qV DS ) / k BT )] k BT

(1)

A * is the Richardson’s constant, kB is the Boltzmann constant, q is the charge of an

electron, T is the temperature and VDS is the voltage bias across the device.8, 39 We applied a VDS = 0.2V, which is higher than 3kBT, on the devices in order to exclude the effect of the drain electrode and to simplify the relationship between ln (IDS/T2) and 1/T to linearity. Corresponding Arrhenius plots for various VBG values of the four types of NWs are shown in Figure 5(b, e, h, k). The effective potential barrier ΦB can then be extracted from the slope of the linear parts of the plots at high-temperature region (generally T>180K in our work), and the ΦB - VBG relations are obtained and shown in Figure 5(c, f, i, l). In the subthreshold region (VBG < VT), the ΦB follows a linearly response to VBG because the thermionic emission dominates the conduction mechanism. The flat-band voltage (VFB) is extracted from the end of this linear relation to mark the transition from the thermionic emission regime to the thermally assisted tunneling regime when ΦB is lower than ΦSB. We observe in Figure 5 that both WZ and ZB NWs have flat-band voltage (VFB) close to VT


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and the extracted ΦB at VFB (which is the Schottky barrier height at the contact) are all smaller than zero, indicating ohmic contact at the Cr-InAs NWs contacts for all the crystal phases and orientations we have studied in this work. Bulk InAs is well known to form ohmic contact with metals. However, a Schottky barrier has been reported between Ni and MOCVD-grown InAs NWs independent of the NW diameter down to 10 nm.39 While our recent work has proved that Cr and Ni form ohmic contact with the MBE-grown InAs NWs with the diameter ranging from sub-7 nm to 16 nm (should be WZ NW according to our experience).8 The present results further demonstrate that Cr forms ohmic contact with MBE-grown InAs NWs with diameter ranging from 12 nm to 33 nm independent of the crystal phase and the orientation. The ohmic contacts are greatly beneficial to the applications of InAs NWs on both traditional electrical devices and novel quantum devices. When VBG is turned to be more negative than VFB, the effective barrier height ΦB at the contact area gets higher and blocks the thermionic emission current, thus the conductance of the devices decreases and the current finally reaches the lowest value of the whole transfer curve. By applying a more negative VBG, the transfer curves show a plateau at the off-state or even a slight p-branch when the Fermi level aligns with or goes into the valence band, as shown in the inset of Figure 5(c). In this regime, the concentration of the hole greatly increases and exceeds the concentration of the electrons in the NWs. The hole tunneling current, rather than the electron current, plays the main role in the whole IDS. From Figure 5, we observe that the effective barrier extracted at negative VBG does not increase continuously, but reaches a peak value and then decreases a little. The peak value of ΦB in the ΦB versus VBG curve here should be the effective barrier at the off-state (ΦOFF). Notably, the value of ΦOFF follows the same changing tendency with


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the ON-OFF ratio. It monotonously increases from about 120 meV to 500 meV by the order of , , oriented ZB and oriented WZ InAs NWs. High ΦOFF can efficiently hinder the current flowing from the source to the drain electrodes so that suppresses the Ioff and improves the ON-OFF ratio. According to previous theoretical work,40 the ΦOFF of ideal junctionless FETs based on one-dimensional narrow Eg nanomaterial is related to the Eg of the channel material and the ΦSB at contact. Regarding the ohmic contacts at the Cr-InAs NWs contact for electron transport and the very slight p-branch in the transfer curves in our work, the ΦOFF is predicted to be closely related to the Eg of the InAs NWs. In the cases of oriented WZ NWs, the extracted ΦOFF are generally equal to their Eg predicted by previous works.8, 39 That the ΦOFF of the WZ InAs NW in our work is higher than all the ZB NWs is consistent with previous theoretical and experimental works which have verified that WZ structured InAs materials have larger band gap than ZB structured InAs.14, 22, 24, 26 However, in the case of ZB NWs, except oriented ZB NWs, the other two types of ZB NWs both show ΦOFF smaller than the Eg of 20 nm thickness ZB InAs NWs, which have been predicted theoretically to have no obvious distinction between different orientations.18 One possibility is that the surface properties in different orientated ZB NWs might lead to this inconformity between the extracted ΦOFF and the Eg. Since the high surface-volume ratio of NWs (especially the thin NWs) emphasizes the important role of the surface to the NWs, the band structure of NWs might be altered by the imperfect surface or interface with oxides.15, 17 The steps and defects on the surface of oriented ZB InAs NWs have been experimentally confirmed to induce states that lead to no signs of a band gap region in the measurements by scanning tunneling microscopy.15, 26 In addition, the bare ZB NW


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without pseudo-hydrogen passivation have been reported theoretically to have metallic character.17 Here, it is possible that different growth directions might result in various surface conditions in ZB InAs NWs and thus lead to different ΦOFF. Meanwhile, the surface-state capacitor induced by the high surface-state density might act as series capacitors with the back-gate capacitor, and consequently weaken the gate electrostatic coupling with the InAs NWs,34 thus SS increases by the reversed order of the ON-OFF ratio and ΦOFF in the InAs NWs with different orientations. In conclusion, we develop a method to investigate and establish the connection between the structure and the physical properties of the same individual InAs NWs by the combination of nano-manipulation and microlithography techniques. The individual NWs inside the FETs are characterized at atomic level by TEM characterization after the electrical measurements. The relation between crystal phase and electrical properties of NWs can be clearly identified through this method. This method is also applicable to other nanomaterials. By using high-quality, undoped MBE-grown InAs NWs with similar diameter, we exclude the effects of unintentionally background doping, geometric scale and crystal defects and explore the exact effects of the crystal phase and orientation on the electrical properteis of InAs NWs. We find that both the crystal phase and the orientation of the NWs intensively influence the electronic properties of the FETs based on InAs NWs. The SS increases while VT, ON-OFF ratio and ΦOFF decrease one by one in the sequence of WZ , ZB , ZB , ZB and ZB InAs NWs. While μFE, σ and ne of the InAs NWs are very sensitive to the crystal phases but are not obviously affected by the growth directions of the ZB InAs NWs. The ZB NWs generally have higher μFE, σ and ne than the WZ NWs. The diameter ranging from 12 nm to 33 nm shows much less effect than the crystal phase and orientation on the electrical transport properties of the


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InAs NWs. Furthermore, we observe that the good ohmic contact between InAs NWs and metal remains regardless of the variation of the crystal phase and orientation through temperature dependent measurements. However, the effective barrier at the off-state (ΦOFF) is found to monotonously increases by the order of , , oriented ZB and oriented WZ InAs NWs. The results of our work are important for deeper understanding and further development of NW devices, such as FETs and tunnel transistors.

Associated content Supporting information: Transfer curves of 21 InAs NWs studied in this work; EDS analysis of WZ and ZB InAs NWs.

Acknowledgments This work was supported by the MOST (Nos. 2012CB932702 and 2012CB932701) and NSF of China (Nos. 11374022 and 61321001). Dr. D. Pan is supported by NSF of China (Nos. 61504133).

References (1) Johansson, S.; Memisevic, E.; Wernersson, L.-E.; Lind, E.; IEEE Electron Device Lett. 2014, 35, 518-520. (2) Borg, M.; Schmid, H.; Moselund, K. E.; Signorello, G.; Gignac, L.; Bruley, J.; Breslin, C.; Das Kanungo, P.; Werner, P.; Riel, H. Nano Lett. 2014, 14, 1914-1920. (3) Xiang, J.; Lu, W.; Hu, Y.; Wu, Y.; Yan, H.; Lieber, C. M. Nature 2006, 441, 489-493. (4) Cui, Y.; Zhong, Z.; Wang, D.; Wang, W. U.; Lieber, C. M. Nano Lett. 2003, 3, 149-152.


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Figures and Captions

Figure 1. (a) Schematic diagrams showing the nanofabrication procedure of suspended InAs NW FETs. (b) SEM images showing an InAs NW transferred by nano-manipulation from the FET to a holey carbon film supported by Cu grid for HRTEM characterization.


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Figure 2. (a) SEM image of the InAs NWs grown by molecular-beam epitaxy. Five types of InAs NWs, including oriented WZ (b), (c), (d), (e), and (f) oriented ZB NWs, are observed by TEM in our work. The thick amorphous layer (several tens of nm) outside the NWs is the amorphous carbon deposited during the TEM observation and nano-manipulation in SEM chamber. HRTEM images show the single-phase structure of InAs NWs. The insets of (b) to (f) are the fast Fourier transform (FFT) patterns of the HRTEM showing the axis direction of the NWs. All these five types of InAs NWs are transferred from the FETs by nano-manipulation after they have been characterized by electrical measurements.

Figure 3. IDS - VBG characteristics of the back-gated NW FETs at VDS = 0.1 V for five different oriented InAs NWs in the linear (a) and logarithmic (b) coordinates. (c) The IDS-VDS characteristics of these FETs.


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Figure 4. The statistical data of several key electrical parameters, including the ON-OFF ratio (a), field-effect mobility (b), conductivity (c) and effective electron concentration (d). These data were measured from 21 InAs NWs with various phases and orientations.


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Figure 5. (a, d, g, j) Temperature-dependent transfer characteristics of the back-gated FETs based on WZ , ZB , ZB and ZB oriented InAs NWs, respectively. Corresponding Arrhenius plots for various gate voltages are shown in (b, e, h, k) and the extracted effective barrier height vs. VBG are plotted in (c, f, i, l).


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Crystal Phase- and Orientation-Dependent Electrical Transport

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