Selective-Area MOCVD Growth and Carrier-Transport-Type Control of

Nov 29, 2016 - Devices, Institute of Semiconductors, Chinese Academy of Sciences, ... transport type of core−shell nanowires can be tuned by the GaS...
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Selective-Area MOCVD Growth and Carrier-Transport-Type Control of InAs(Sb)/GaSb Core−Shell Nanowires Xianghai Ji,† Xiaoguang Yang,†,‡ Wenna Du,† Huayong Pan,§ and Tao Yang*,†,‡ †

Key Laboratory of Semiconductor Materials Science, Beijing Key Laboratory of Low Dimensional Semiconductor Materials and Devices, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, People’s Republic of China ‡ College of Materials Science and Optoelectronic Technology, University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China § Key Laboratory for the Physics and Chemistry of Nanodevices and Department of Electronics, Peking University, Beijing 100871, People’s Republic of China S Supporting Information *

ABSTRACT: We report the first selective-area growth of high quality InAs(Sb)/ GaSb core−shell nanowires on Si substrates using metal−organic chemical vapor deposition (MOCVD) without foreign catalysts. Transmission electron microscopy (TEM) analysis reveals that the overgrowth of the GaSb shell is highly uniform and coherent with the InAs(Sb) core without any misfit dislocations. To control the structural properties and reduce the planar defect density in the selfcatalyzed InAs core nanowires, a trace amount of Sb was introduced during their growth. As the Sb content increases from 0 to 9.4%, the crystal structure of the nanowires changes from a mixed wurtzite (WZ)/zinc-blende (ZB) structure to a perfect ZB phase. Electrical measurements reveal that both the n-type InAsSb core and p-type GaSb shell can work as active carrier transport channels, and the transport type of core−shell nanowires can be tuned by the GaSb shell thickness and back-gate voltage. This study furthers our understanding of the Sb-induced crystal-phase control of nanowires. Furthermore, the high quality InAs(Sb)/GaSb core−shell nanowire arrays obtained here pave the foundation for the fabrication of the vertical nanowire-based devices on a large scale and for the study of fundamental quantum physics. KEYWORDS: Core−shell nanowires, InAs(Sb)/GaSb, selective-area growth, crystal structure, metal−organic chemical vapor deposition, electrical properties

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Thus far, InAs/GaSb or GaSb/InAs core−shell nanowires have been mainly grown with the assistance of Au catalytic particles.6−8,10,11 However, the introduction of Au may degrade the properties of III−V nanowires and result in the formation of unwanted deep-level recombination centers in the Si band gap.12−14 In contrast, few Au-free methods for the growth of these nanowires on Si substrates have been reported.15,16 Furthermore, to the best of our knowledge, all reported InAs/ GaSb core−shell nanowires have been grown on bare substrates. To facilitate the fabrication of vertical nanowirebased devices and realize the monolithic integration of these nanodevices with complementary metal−oxide−semiconductor (CMOS) technology, the position of nanowires should be located accurately. In particular, for the vertical-gate-all-around nanowire devices, due to the parallel relationship between each nanowire, the density and uniformity of the nanowires should be controlled precisely to obtain the excellent device character-

ecause of the principle of thermal carrier injection, decreasing the subthreshold swing (SS) of traditional metal−oxide−semiconductor field-effect transistors (MOSFETs) below 60 mV/decade at room temperature is difficult. However, by a band-to-band tunneling mechanism, tunneling FETs (TFETs) can avoid this mechanistic limit and achieve SS lower than 60 mV/decade.1,2 As a result, TFETs have been considered as strong potential candidates for next-generation low-power and high-speed devices.3 Among III−V materials, InAs and GaSb are with small effective masses and extremely high carrier (electrons and holes) mobilities. In addition, the low lattice mismatch of 0.6% and unique type-II-broken band alignment between InAs and GaSb make this heterostructure system very suitable for the fabrication of TFETs. Moreover, the unique nanowire geometry can be used to form verticalgate-all-around devices, resulting in further improvement of the device performance.4,5 Apart from tunneling-based applications, InAs/GaSb core−shell nanowires enable a new platform for the design of other novel nanowire-based devices and fundamental quantum physics investigations, such as frequency multipliers,6,7 electron−hole hybridization,8 and exciton and spin physics.9 © XXXX American Chemical Society

Received: August 15, 2016 Revised: October 31, 2016 Published: November 29, 2016 A

DOI: 10.1021/acs.nanolett.6b03429 Nano Lett. XXXX, XXX, XXX−XXX

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off, and the substrates were cooled to 440 °C for the growth of GaSb shells under AsH3 flux. The growth of the GaSb shell was initiated by switching the AsH3 to TMSb and opening the TMGa flux simultaneously. The TMSb and TMGa flow rates were 1.7 × 10−6 and 0.3 × 10−6 mol/min, respectively, which resulted in the corresponding V/III ratio of ∼5.7. After growth, the samples were cooled to room temperature using TMSb as a protective agent. Characterization Methods. The morphology of the nanowires was characterized by scanning electron microscopy (SEM) (Nova Nano SEM 650), and transmission electron microscopy (TEM) (FEI Tecnai F30 TEM; 300 kV) in conjunction with X-ray energy-dispersive spectroscopy (EDS) was used to investigate the crystal structure and composition, respectively. For TEM observations, the nanowires were mechanically transferred from the samples to copper grids coated with a carbon film. Device Fabrication. To characterize the basic electrical properties of the grown nanowires, single-nanowire back-gated FETs were fabricated. The nanowires were transferred onto a p-doped Si substrate with a 300 nm SiO2 layer and located by SEM. The patterns of the contact electrodes were defined by EBL. Prior to contact metal deposition, the native oxide of the nanowires in the contact area was etched by diluted (NH4)2Sx solution. Subsequently, a Ti/Au (5/90 nm in thickness) layer was evaporated followed by a lift-off process. The fabricated devices were measured on a probe station using an Agilent B1500 semiconductor analyzer at room temperature. Results and Discussion. Figure 1 shows a schematic illustration of the growth of InAs/GaSb core−shell nanowires

istics (e.g., high cutoff frequency and maximum oscillation frequency). Clearly, nanowires grown by self-assembly are not preferable. Therefore, achieving the position-controlled growth of these nanowires on patterned substrates without foreign catalysts is highly desirable. However, without the assistance of foreign catalysts, InAs nanowires grown by self-catalyzed or catalyst-free mechanisms always display a polytypic wurtzite (WZ)/zinc-blende (ZB) crystal phase and tend to contain considerable planar defects.17−19 These structural defects can cause an undesirable inhibition of the carrier mobility,20,21 and for the core−shell nanowire structure, defects that exist in the core can even penetrate into the shell.22,23 These phenomena highlight the need to improve the crystal quality of InAs core nanowires. Recently, the incorporation of a small amount of Sb into nanowires has been reported as an effective approach to control the crystal phase and reduce the planar defect density.24−29 Because the antimonide alloys possess very low ionicity values among III−V materials, the incorporation of Sb can induce a WZ-to-ZB phase transition in InAs nanowires, thereby greatly increasing the potential of crystal-phase engineering of the InAs nanowires without foreign catalysts. Whereas the reported InAsSb nanowires were also mostly grown by self-assembled mechanism, the reports on the selective-area growth of high quality InAsSb nanowire arrays by metal−organic chemical vapor deposition (MOCVD) are very limited.26 In this paper, based on the selective-area MOCVD growth of InAs1−xSbx core nanowires, we present the first growth of crystal-phase-controllable InAs1−xSbx/GaSb core−shell nanowire arrays on patterned Si substrates. The nanowire arrays we obtained are highly uniform and without parasitic growth. As the Sb content increases from 0 to 9.4% in the core, the crystal structure changes from a mixture of WZ and ZB structures to a pure ZB phase, and the defect density is reduced dramatically. Electrical measurements revealed that the InAsSb core and GaSb shell can work as electron and hole transport channels, respectively. Moreover, the transport type of nanowires can be tuned by varying the GaSb shell thickness and back-gate voltage. The high quality InAs(Sb)/GaSb core−shell nanowire arrays obtained here lay the groundwork for the fabrication of nanowire-based devices (e.g., vertical TFETs) and represent a step toward the monolithic integration of these nanodevices on Si platforms. Experimental Details. Nanowire Growth. The patterned substrates were prepared by electron beam lithography (EBL) on p-type Si(111) substrates with 20 nm thick SiO2 films. These patterns were transferred onto the SiO2 layer via the wetchemical hydrofluoric acid (HF) etching of SiO2. The periodic nanoholes have a diameter of approximately 160 nm and a pitch length of 500 or 1000 nm. The InAs(Sb)/GaSb core− shell nanowire arrays were grown by a close-coupled shower head MOCVD system (AIXTRON Ltd., Germany) at a chamber pressure of 133 mbar. Trimethylindium (TMIn) and trimethylgallium (TMGa) were used as group III precursors, and arsine (AsH3) and trimethylantimony (TMSb) were used as group V precursors. Ultrahigh-purity hydrogen (H2) was used as a carrier gas, and the total flow rate of H2 was 12 SLM. Prior to growth, the substrates were heated to 635 °C for annealing and then cooled to 400 °C under AsH3 flux to form (111)B-like surfaces. For the growth of InAs/GaSb core−shell nanowires, the InAs core nanowires were grown at 565 °C for 2 min with TMIn and AsH3 flow rates of 0.8 × 10−6 and 2.0 × 10−4 mol/min, respectively. Subsequently, TMIn was switched

Figure 1. Schematic illustration of the selective-area growth of InAs/ GaSb core−shell nanowires and the source-supply sequences for the growth of the nanowires.

on a patterned Si (111) substrate and the source-supply sequences for the growth of the nanowires. InAs core nanowires grow via a self-catalyzed mechanism,15 and In droplets are consumed during the process of reducing the temperature under a AsH3 atmosphere (region 3 in Figure 1). Figure 2a shows a typical SEM image of a bare InAs nanowire array. All InAs nanowires are vertically aligned with fairly smooth side facets along the entire length but without visible tapering. Sb has a heavier atomic mass and lower volatility than typical group V elements, e.g., As and P. Thus, it is expected to significantly affect the surface mobility of group III adatoms. The growth of the GaSb shell is very sensitive to the TMSb flow rate. Therefore, to obtain the desired morphology of the GaSb shell by the vapor−solid (VS) mechanism, the TMSb flow rate should be controlled carefully. Details about the influence of Sb adatoms on the growth of B

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Figure 2. 30°-tilted SEM images of the bare InAs (a) and InAs/GaSb core−shell nanowire arrays (b). (c) Diameter distributions of bare InAs (red histogram) and InAs/GaSb core−shell nanowires (blue histogram).

Figure 3. (a) Low-magnification TEM image of an InAs/GaSb core−shell nanowire. (b) EDS line scan along the red line marked in (a). (c) Highresolution TEM (HRTEM) image of the nanowire acquired from the ⟨211⟩ zone axis. (d) Inverse fast Fourier transform (IFFT) image of the blue rectangular region of the nanowire in (c). (e) HRTEM image of the top region of the InAs/GaSb core−shell nanowire. (f) IFFT image of the blue rectangular region in (e). The red dashed lines indicate the interface between the InAs core and GaSb shell in (c, d). The insets in (c, e) are FFT patterns from the green rectangular regions in (c, e), respectively.

GaSb shell are reported elsewhere.30 A typical SEM image of the InAs/GaSb core−shell nanowire array with GaSb shell growth time of 15 min is shown in Figure 2b. Clearly, a uniform GaSb shell formed around the InAs core. Figure 2c shows the statistical distributions of the diameters of bare InAs and InAs/ GaSb core−shell nanowires over 60 counts. After the subsequent growth of the GaSb shell, the average diameter of the nanowires increases from ∼79 to ∼97 nm, and the thickness of the GaSb shell can be changed by varying the shell growth time (see Supporting Information Figure S1). To further verify the existence of the core−shell structure, TEM analysis was performed on the nanowire samples. Figure 3a shows a bright-field (BF) low-resolution TEM image of a

typical InAs/GaSb core−shell nanowire represented in Figure 2b. From the EDS line scan across the nanowire in Figure 3b, Ga and Sb signals can be clearly identified in the spectra, indicating the existence of the GaSb shell around the InAs core (more EDS measurements are available in Figure S2). The results of HRTEM investigations are shown in Figure 3c−f. A HRTEM image viewed from the ⟨211⟩ zone axis is shown in Figure 3c. Typically, because the six sidewalls of the InAs core nanowire are {110} planes, a clearer interface can form between the InAs core and the GaSb shell from this direction rather than from the ⟨110⟩ zone axis; this abrupt contrast enables precise measurements of the shell thickness16,31 (see Figure S3). Clearly, a GaSb shell with a thickness of approximately 10 nm C

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nanowire length is quite homogeneous. The relationship between the Sb content, x, and the TMSb vapor-phase composition, Xv, is also shown in Table 1. As Xv increases from 5% to 11% at 545 °C, the average Sb content increases from 4.6% to 7.9%. However, when the growth temperature was decreased to 520 °C, a higher Sb incorporation of 9.4% was achieved; this phenomenon has been observed in the growth of InAsSb nanowires by molecular beam epitaxy (MBE).20 To analyze the crystal properties of the InAs 1−x Sb x nanowires, TEM measurements were conducted. Representative HRTEM images of InAs1−xSbx nanowires with varying Sb contents are shown in Figure 4. Pure InAs nanowires show a mixture of ZB and WZ structures. However, the situation is changed when Sb is incorporated into the InAs crystal. As shown in Figure 4b, even for Sb content as low as 4.6%, the crystal structure is modified to a ZB-dominant one. When the Sb content is further increased to over 6.8%, the nanowires display a ZB crystal structure. In addition, we quantified the planar defect densities of the InAs1−xSbx nanowires. In general, to avoid ambiguity between ZB and WZ crystal phases, a section of ZB (or WZ) needs at least four planes in the stacking sequence. Typically, the planar defects observed in nanowires can be classified into three main categories: stacking faults (SFs), twin planes (TPs), and polytype boundaries (PBs). (Details about stacking sequences of these planar defects are available in Supporting Information Figure S6.) Clearly, pure InAs and the InAs1−xSbx (x = 4.6%) nanowires contained all three types of defects (Figure 4a,b). However, only SFs and TPs were found in the InAs1−xSbx (x = 6.8% and 7.9%) nanowires (Figures 4c,d). This lack of PBs also implies that InAs1−xSbx nanowires with Sb contents over 6.8% have a ZB structure without WZ phase. In addition, the total defect density as a function of the Sb content, x, is plotted in Figure 4f. Interestingly, for a Sb content of 4.6%, the average planar defect density is slightly higher than that of the InAs nanowires (from ∼577/μm to ∼612/μm). This difference can be explained by the fact that the incorporation of a small amount of Sb into InAs nanowires will change the crystal structure of the WZ segments to the ZB phase through intermediate planar defects in the initial transition stage. However, as the Sb content increases further, the average defect density in InAs1−xSbx nanowires gradually decreases. In particular, the InAs1−xSbx nanowires with Sb content of 9.4% display a perfect ZB structure that is completely free of SFs and TPs. In addition, the distribution of defect density along the InAs(Sb) nanowires is quite uniform (see Figure S7), which is different from the results obtained in some types of GaAs nanowires grown by MBE where the defects are dominated in the top regions.35,36 The main reason could be the difference in the nanowire growth kinetics between the MOCVD and MBE systems. Other explanations may be related with the difference in the diffusion length of the III adatoms under different growth conditions, the change in temperature, and effective local V/III ratio along the nanowire in the MBE system. However, the homogeneous distribution of the defect density along the nanowires obtained here indicates a quite stable local ambient during the growth process. Therefore, both the modification of the crystal phase (from a polytypic WZ/ZB structure to a ZB phase) and the sharp reduction in the defect density indicate that the incorporation of Sb is an effective way to improve the crystal quality of self-catalyzed InAs nanowires grown by MOCVD.

forms around the InAs core; this thickness is in agreement with the diameter statistics shown in Figure 2c. Figure 3d shows an IFFT image from the blue rectangular region of the nanowire in Figure 3c. Usually, the existence of misfit dislocations can be indicated by the terminating planes.32 However, no terminating planes were found by following the {111} planes from the InAs core to the GaSb shell. Therefore, the as-grown GaSb shell was coherent with the InAs core without any misfit dislocations. Moreover, Raman measurements also indicate that the strain between the InAs core and the GaSb shell can be ignored (see Figure S4). In addition to radial growth, the axial growth of the GaSb also occurred. A HRTEM micrograph of the upper part of an InAs/GaSb core−shell nanowire is shown in Figure 3e. Two regions of the upper GaSb and InAs/GaSb core−shell structure can be identified clearly (more HRTEM images are available in Figure S5). The GaSb on the nanowire tip is slightly thicker than that around the nanowire sidewalls because of preferential growth along the ⟨111⟩ direction compared to along the ⟨110⟩ direction under the same growth conditions. Moreover, the crystal structure of the region where GaSb grows radially around the InAs core is composed of a polytype of WZ and ZB structures because the GaSb shell inherits the crystal structure of the InAs core. In contrast, the FFT pattern in the inset of Figure 3e and the stacking sequence (···ABCABC···) in the IFFT image in Figure 3f reveal that the axially grown GaSb has a pure ZB crystal structure free of planar defects; this perfect ZB crystal structure commonly arises in the growth of binary GaSb or InSb nanowires along the ⟨111⟩ direction.33,34 The difference between the crystal structures of the radially and axially grown GaSb further confirms that the crystal structure of the GaSb shell is reproduced from that of the InAs core, and this behavior highlights the need to improve the crystal quality of InAs core nanowires. Next, to control the structural properties of the InAs core nanowires, Sb was introduced during the growth of the InAs core nanowires, and ternary InAs1−xSbx alloy nanowires were formed. For the growth of InAs1−xSbx core nanowires, the growth temperature was reduced to 545 or 520 °C, the growth duration was extended to 4 min, and the flow rates of TMIn and AsH3 were not changed relative to those used for the growth of pure InAs nanowires. The incorporation of Sb was controlled solely by varying the TMSb flow rate. The TMSb vapor-phase composition, Xv, which was defined as the ratio of the flow rates TMSb/(TMSb + AsH3), was varied from 5% to 11%. More detailed growth parameters for different core nanowire samples are summarized in Table 1. The Sb content, x, in the ternary InAs1−xSbx core nanowires was estimated by EDS spot measurements using the Lα emission signals of In, As, and Sb elementals. For each nanowire, the measurements of Sb content were performed at the middle, near top, and bottom regions, respectively. The Sb content distribution along the Table 1. Growth Parameters for InAs and InAs1−xSbx Core Nanowire Samples sample

T (°C)

time (min)

Xv (%)

InAs InAsSb

565 545 545 545 520

2 4 4 4 4

0 5 8 11 11

Sb content (%) 0 4.6 6.8 7.9 9.4

± ± ± ±

0.3 0.2 0.3 0.4 D

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Figure 4. HRTEM micrographs of the InAs1−xSbx nanowires with Sb contents, x, of (a) 0%, (b) 4.6%, (c) 6.8%, (d) 7.9%, and (e) 9.4%. (f) Defect density as a function of the Sb content, x.

Figure 5. (a, d, g) 30°-tilted SEM images of the InAs1−xSbx/GaSb core−shell nanowire arrays with Sb contents of 6.8% (a), 7.9% (d), and 9.4% (g). HRTEM images of the corresponding InAs1−xSbx/GaSb core−shell nanowire acquired from the ⟨211⟩ (b, e, h) and ⟨110⟩ zone axes (c, f, i). The red dashed lines indicate the interface between the core and the shell in (b, e, h). The insets are FFTs of the corresponding HRTEM images.

E

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Figure 6. (a) Schematic illustration of the single-nanowire back-gated FET and a top-view SEM image of a final device. (b−d) Transfer characteristics of bare InAs1−xSbx and InAs1−xSbx/GaSb core−shell nanowire FETs with GaSb shell thicknesses of ∼6 and ∼13 nm for different applied Vds = 50−200 mV, in steps of 50 mV.

increasing TMSb flow rate, resulting in an enhancement of the radial growth rate of the InAs1−xSbx nanowires via the vapor− solid (VS) mechanism.38,41 Thus, with the increase of Sb content, the axial growth of the InAs1−xSbx core nanowires was suppressed while the lateral size was increased, which led to the reduction in the aspect ratio of the nanowires. Notably, for the condition of Figures 5d,g, apart from the influence of Sb, the decrease of growth temperature can also play a role in the reduction in the aspect ratio of the nanowires. Therefore, the addition of Sb to the InAs core nanowires significantly modifies both the crystal quality and the nanowire morphology (especially the aspect ratio). All of these aspects need to be considered to determine the optimal amount of Sb incorporation. The basic electrical properties of the grown nanowires were evaluated by the electrical measurements in the fabricated single-nanowire back-gated FETs. Figure 6a shows schematic illustration of the single-nanowire back-gated FET and a topview SEM image of a final device. All devices have similar contact electrode width (∼400 nm) and channel length (∼550 nm). Figures 6b−d show the typical transfer characteristics (drain current, Ids, as a function of gate voltage, Vgs) of bare InAs1−xSbx and InAs1−xSbx/GaSb (x = 7.9%) core−shell nanowire FETs with GaSb shell thicknesses of ∼6 and ∼13 nm at drain−source bias, Vds = 50−200 mV, in steps of 50 mV. Clearly, the transfer characteristic curve in Figure 6b indicates that as-grown InAsSb nanowires exhibit n-type transport behavior, which is consistent with previous results.25,42 The undoped InAsSb nanowires exhibiting n-type transport behavior might be related with the pinned surface Fermi level induced by surface states and the large difference between the mobility of electrons and holes in InAsSb material. However, for the device of the nanowire with the GaSb shell thickness of ∼6 nm, ambipolar transport behavior is observed, as shown in Figure 6c. Ids first decreases and then increases with increasing Vgs from −25 to 25 V. For the reversed GaSb/InAs core−shell nanowires, it has been reported that the InAs shell can be

Figures 5a,d,g show the as-grown InAs1−xSbx/GaSb (x = 6.8%, 7.9%, and 9.4%) core−shell nanowire arrays. The HRTEM images from the ⟨211⟩ zone axis in Figures 5b,e,h show GaSb shells with thickness of approximately 13 nm. However, when the incident direction of electron beam was turned to the ⟨110⟩ zone axis, the interface between the core and shell could not be clearly observed in the HRTEM images, as shown in Figures 5c,f,i. We speculate that this phenomenon may be caused by further reduction of the lattice mismatch between the InAs1−xSbx (x = 6.8%−9.4%) core and the GaSb shell (e.g., for x = 9.4%, the lattice mismatch between InAs1−xSbx and GaSb was reduced to 0.04%). Whereas the crystal phase property could be clearly observed from this direction. The crystal quality of the GaSb shell is significantly improved by the incorporation of Sb into the InAs core nanowires. In addition, by comparing the nanowires shown in Figures 5a and 5d, as the Sb content increased from 6.8% to 7.9%, it can be seen that the length of the nanowires decreases as the diameter increases (see Figure S8). We ascribe this striking difference in the morphology of the InAs1−xSbx/GaSb core− shell nanowires primarily to the varying Sb content in the InAs1−xSbx core nanowires. On the one hand, for the selfcatalyzed growth mechanism, the adsorbed Sb atoms tend to segregate onto the surface of the In catalyst droplet and then reduce its surface energy, which will lower the axial growth rate of the nanowires.37 In addition, because of the reduction in the surface energy, the introduction of Sb will cause the lateral expansion in the In catalyst droplets and result in the grown nanowires with larger diameter eventually.38 Furthermore, it has been reported that the segregated Sb atoms have a poisoning effect by blocking the surface sites and inhibiting the group V atoms from incorporating into the catalyst droplets,39 which further lowers the axial growth rate of the nanowires by reducing the supersaturation (Δμ) during the growth process.40 On the other hand, due to the surfactant effect of Sb, the diffusion length of the In adatoms is greatly reduced under F

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utilized as an active n-channel and the transport type of GaSb/ InAs core−shell nanowires can be tuned by the InAs shell thickness and applied gate voltage.6,7 Therefore, this ambipolar behavior can be attributed to the hole transport in the ∼6 nm GaSb shell. In addition, the current level is higher for the nanowire with the ∼6 nm GaSb shell than the bare InAsSb nanowire, which is primarily attributed to the suppression of the surface scattering of the InAsSb core nanowires. However, the situation is changed for the device of the nanowire with GaSb shell thickness of ∼13 nm. The device shows p-type transport characteristics at negative Vgs, but Ids is almost independent of Vgs at positive range, indicating almost no dependence of the electron density in the InAsSb core nanowires on the studied back-gate voltage with thick GaSb shell. The ION/IOFF ratio is relatively low for the devices based on the InAsSb/GaSb core−shell nanowires due to the existence of the conducting n-type InAsSb core. Overall, the basic electrical results obtained here confirm that both the n-type InAsSb core and p-type GaSb shell with appropriate thickness can work as active carrier transport channels, and we anticipate that the as-grown InAsSb/GaSb core−shell nanowires can enable applications in tunneling-based nanowire devices and fundamental quantum physics investigations. Conclusions. In summary, we describe the first selectivearea MOCVD growth of high quality InAs1−xSbx/GaSb core− shell nanowires on Si substrates without foreign catalysts. Detailed TEM analysis reveals that the overgrowth of the GaSb shell is highly uniform and coherent with the InAs core without misfit dislocations. As the Sb content in the core nanowires increased from 0 to 9.4%, the structure of the nanowires changes from a mixed WZ/ZB structure to a pure ZB phase. Electrical measurements reveal that both the n-type InAsSb core and p-type GaSb shell can work as active carrier transport channels, and the transport type of InAsSb/GaSb core−shell nanowires can be tuned by the GaSb shell thickness and backgate voltage. This study furthers our understanding of the Sbinduced crystal-phase control of nanowires, and the as-grown high quality InAs1−xSbx/GaSb core−shell nanowire arrays have strong potential in the fabrication of future nanowire-based devices and in the study of fundamental quantum physics.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (T.Y.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Ministry of Science and Technology of China (Grant No. 2012CB932701) and National Natural Science Foundation of China (Grant No. 91433206).

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.6b03429. GaSb shell thickness as a function of the shell growth time (Figure S1), more EDS measurements of the InAs/ GaSb core−shell nanowires (Figure S2), the comparison of the difference in the visibility of the core−shell structure from ⟨110⟩ and ⟨211⟩ zone axes (Figure S3), Raman measurements of the corresponding nanowires (Figure S4), HRTEM images of the top region of the InAs/GaSb core−shell nanowires (Figure S5), stacking sequences of stacking faults, twin planes, and polytype boundaries (Figure S6), TEM images from different sections of the InAsSb nanowires with Sb contents of 6.8% and 9.4% (Figure S7), and statistics analyses of the InAs1−xSbx/GaSb core−shell nanowire morphology (Figure S8) (PDF) G

DOI: 10.1021/acs.nanolett.6b03429 Nano Lett. XXXX, XXX, XXX−XXX

Letter

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DOI: 10.1021/acs.nanolett.6b03429 Nano Lett. XXXX, XXX, XXX−XXX