InAs-mediated growth of vertical InSb nanowires on Si substrates
© Li et al.; licensee Springer. 2013
Received: 27 March 2013
Accepted: 6 July 2013
Published: 24 July 2013
In this work, InSb nanowires are grown vertically on Si (111) with metal organic chemical vapor deposition using InAs as seed layer, instead of external metal catalyst. Two groups of InSb nanowires are fabricated and characterized: one group presents Indium droplets at the nanowire's free end, while the other, in contrast, ends without Indium droplet but with pyramid-shaped InSb. The indium-droplet-ended nanowires are longer than the other group of nanowires. For both groups of InSb nanowires, InAs layers play an important role in their formation by serving as a template for growing InSb nanowires. The results presented in this work suggest a useful approach to grow catalyst-free InSb nanowires on Si substrates, which is significant for their device applications.
KeywordsCatalyst-free MOCVD InSb Nanowire Electron microscopy
Indium antimonide (InSb), a kind of III-V semiconductor with a narrow bandgap (0.17 eV), a large bulk electron mobility (≈7.7×104 cm2/V/s) , and a high thermoelectric figure of merit (0.6) , has been an attractive material for various applications such as high-speed and low-power electronics, infrared optoelectronics, quantum-transport studies, and thermoelectric power generation [3–5]. The heteroepitaxial growth of InSb films on Si surface has attracted much attention due to the potential of integrating InSb devices on Si substrate. However, because of the large lattice mismatch between InSb films and Si substrate (approximately 19.3%) , it is difficult to directly grow InSb film heteroepitaxially on Si substrate without generating defects.
Nanowires (NWs) are kinds of materials with a size in the range of nanometers. The lattice mismatch/strain in NWs is one of the most important features of NWs, in which the lattice mismatch/strain can be significantly relaxed due to their high surface/volume ratio and small lateral size, providing an opportunity to integrate InSb materials and devices on Si platform. It should be noted that gold, the most used seed particles for NW growth, is known to create detrimental midgap defects in silicon and should therefore be avoided in Si-compatible technological processes. So far, though some work has been devoted to external metal catalyst-free growth of InAs and GaAs NWs on Si [7–9], very few information is available on external metal catalyst-free growth of InSb NWs on silicon. In this work, we investigate the external metal catalyst-free growth of InSb NWs on Si substrates. Our results show that it is hard to grow InSb NWs directly on Si. However, using InAs as seeding layer, vertical InSb NWs can be readily achieved on Si substrates. The structural characteristics of InSb NWs are systematically studied and their underlying growth mechanisms are discussed as well.
Vertical InSb NWs were grown on n-type Si (111) substrates in a close-coupled showerhead metal-organic chemical vapor deposition (MOCVD) system (Thomas Swan Scientific Equipment, Ltd., Cambridge, England) at a pressure of 100 Torr. Trimethylindium (TMIn), trimethylantimony (TMSb), and AsH3 were used as precursors and ultra-high purity H2 as carrier gas. Before being loaded into the growth chamber, Si substrates were first cleaned (ultrasonicated in trichloroethylene, acetone, isopropanol, and deionized water, sequentially), and etched in buffered oxide etch solution (BOE, six parts 40% NH4F and one part 49% HF) for 30 s to remove the native oxide, then rinsed in deionized water for 15 s and dried with N2. After that, the substrates were loaded into the MOCVD reactor chamber for NW growth. The InSb NWs were grown using the following layer sequence: first, InAs seed layer was grown at 550°C by simultaneously introducing TMIn (2×10−6 mole/min) and AsH3 (2×10−4 mole/min) into the reactor chamber for 2 min, as reported in previous work [10, 11]; then, the growth temperature was reduced to 440°C with only AsH3 flowing. After that, the AsH3 flow was removed from the chamber, while TMSb (6.75×10−5) and TMIn (4.5×10−6) flows were simultaneously introduced into the reactor chamber to initiate the growth of InSb NWs. The InSb NWs were grown for 40 min and then cooled down with the protection of only hydrogen flow (TMIn and TMSb flows were removed from the reactor chamber during cooling). For comparison, InSb layers were also grown directly on Si (111) under the same growth conditions but without InAs seed layer. The morphology of InSb structures was characterized with field-emission scanning electron microscopy (FE-SEM; JSM-6700 F, JEOL, Akishima-shi, Japan)and transmission electron microscopy (TEM; Tecnai G20, 200 keV, FEI, Hillsboro, OR, USA). Raman scattering measurements were performed in a backscattering geometry at room temperature with a Jobin Yvon HR800 confocal micro-Raman spectrometer (HORIBA, Kyoto, Japan), in which a 514.5-nm line of an Ar-ion laser was used as the excitation source with the focus size around 1 μm and excitation power of 0.5 mW.
Results and discussion
Figure 1c,d shows the side view SEM images of InSb NWs obtained with InAs seed layer. Two groups of NWs are observed on the sample surface. The first group (as shown in Figure 1c) clearly shows a droplet-like end at the NW top. These NWs are about 2 μm in length, and 200 to 300 nm in diameter. Combined with the inset of Figure 1c, it is observed that the indium droplet on the NW top shows an identical (or slightly smaller) diameter to that of InSb NWs, which is a typical phenomenon for NWs grown with the vapor–liquid-solid (VLS) growth model and has also been observed in InSb NWs grown on InAs substrates . The second group of InSb NWs (as shown in Figure 1d), however, do not present droplet-like end at the NW top, and these NWs present a little small length (about 1 μm), but a similar sectional diameter to that of the first group. These two groups of NWs are observed in different areas of the sample surface.
A similar analysis is performed on the other group of NWs without droplet-like ends, where the TEM image and the related EDS spectra are shown in Figure 2c. Note the EDS spectra are obtained in the area indicated by the arrow in Figure 2c. The EDS spectra measured on the free end of InSb NW shows the same stoichiometry as the NW body with InSb. Similarly, arsenic signal is also observed at the bottom of InSb NW (composition spectra not shown here). This indicates that except the indium droplet end, the second group of NWs shows a similar chemical composition distribution to the first group of NWs. The absence of In droplets on the NW top end might be related to the catalyst self-consumption during the growth, which has been observed in other catalyst-assisted NWs . Such catalyst self-consumption during the NW growth will lead to a smaller axial growth rate for the NWs [12, 14], which is confirmed by the relatively small length of the second group of NWs. All the second group of InSb NWs (without In droplet on the top end) present a length less than 1 μm, while the first group of InSb NWs (with indium droplet on the top end) are all longer than 2 μm. It should be noted that that catalyst self-consumption during the NW growth will lead to the formation of randomly located NWs with wide distributed lengths, which, however, does not agree with the morphology observed for the second group of InSb NWs. As shown in Figure 1, the second group of NWs has a narrow length size distribution and is homogeneously located in well-defined parts of the substrate surface, which does not accord with the catalyst consumption dependence on the catalyst dimension. This suggests that the growth process of the second group of InSb NWs is more complicated compared with that of the first group InSb NWs, and some other factors except VLS model might take effect. Although it is difficult to present a complete growth mechanism for the second group of InSb NWs, the important role of InAs seed layer is clearly evidenced, which serves as a template for initiating the growth of InSb NWs.
In conclusion, InSb NWs have been grown on Si substrates using an InAs seed layer instead of external metal catalyst. The deposition of InAs seed layer leads to the growth of InAs NWs, which serve as a template for the subsequent initiation and growth of InSb NWs. Two different groups of InSb NWs are observed: one with indium droplet top end and the other without indium droplet top end. Though the growth of the first group of InSb NWs is evidenced to follow VLS mode, the growth of the second group of InSb NWs is more complex, the complete picture of which is not clear yet. Despite this, the work demonstrates a method towards the realization of Au catalyst-free InSb NWs, which is important for their ultimate device applications.
Metal-organic chemical vapor deposition
Selected-area electron diffraction
Energy dispersive spectroscopy
Scanning electron microscopy.
The work was supported by the 973 Program (no. 2012CB932701) and the National Natural Science Foundation of China (nos. 60990313, 60990315 and 21173068).
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