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Two Different Growth Mechanisms for Au-Free InAsSb Nanowires Growth on Si Substrate Wenna Du,† Xiaoguang Yang,† Huayong Pan,‡ Xiaoye Wang,† Haiming Ji,† Shuai Luo,† Xianghai Ji,† Zhanguo Wang,† and Tao Yang*,† †

Key Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083, People’s Republic of China ‡ Key Laboratory for the Physics and Chemistry of Nanodevices, Department of Electronics, Peking University, Beijing 100871, People’s Republic of China ABSTRACT: We present a study of Au-free InAsSb nanowire (NW) growth on Si (111) substrate under different growth parameters including V/III ratio, group Sb flow rate fraction (Sb-FRF, TMSb/(TMSb+AsH3)), and temperature. It was found that two different kinds of growth mechanisms for the Au-free InAsSb NW growth may be dominant depending on the growth parameters. At low V/III ratio and relatively high Sb-FRF, the NWs grow via vapor−liquid−solid (VLS) mode, while at high V/III ratio and relatively low Sb-FRF, they grow via vapor−solid (VS) mode. The NWs obtained by the two growth modes show clear differences in morphology, growth direction, and crystal quality. Under VS mode, the NWs exhibit unified growth direction and a uniform composition distribution, which are beneficial to integration devices of multiple NWs. On the other hand, under VLS mode, the NWs are first reported with pure crystal phase, which will be useful for the development of single NW devices.

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In fact, the growth mechanism of indium compound NWs without foreign metal assistance is still an open question. At the moment, there is still the dispute24−26 that mainly focuses on the In-droplet-catalyzed vapor−liquid−solid (VLS) mode or the droplet-free vapor−solid (VS) growth mode. It is important to understand the growth mechanism of the NWs because not only it does significantly influence the properties of resulting NWs, but also the NW’s position, diameter, and crystal structure can be controlled by the growth mechanism. In this paper, we report on the Au-free growth of InAsSb NWs on Si (111) using metal−organic vapor phase epitaxy (MOVPE). The effects of growth parameters such as V/III ratio, Sb flow rate fraction (Sb-FRF, defined as TMSb/ (TMSb +AsH3)), and temperature on the growth of InAsSb NWs have been investigated in detail. We found the presence of two different mechanisms on the growth of Au-free InAsSb NWs under different conditions. Detailed analyses reveal that InAsSb NWs grown under VS mode have more uniform morphology and composition distribution, while InAsSb NWs obtained under VLS mode have a higher crystal quality.

emiconductor nanowires (NWs) have attracted a great deal of attention for the prospective use in nanometer-scale devices because of their unique properties and various potential applications such as in NW transistors, nanosensors, and NW photodetectors. 1−3 In addition, for narrow gap III−V compounds such as InAs and InSb exhibiting high electron mobility and small effective mass, their NWs have a number of potential functional advantages over elemental semiconductor NWs in the applications of electronics and photonics devices.3−13 Furthermore, the bandgap engineering is a more powerful tool for NW-based devices by exploiting the modulated bandgap and then modulating electronic properties with ternary alloy semiconductors (such InAsSb,14 InAsP,15 and AlGaAs16). Among them, InAsSb alloy is attractive for application in optoelectronic devices, especially photodetectors, because of its tunable bandgap throughout mid-infrared spectrum (2−8 μm).17,18 However, the fabrication of highquality InAsSb alloy is difficult due to the lack of suitable substrate. Thanks to the characteristic of NWs, the large mismatch between InAsSb and its substrate can be released by the stretching in diameter of NWs, which provides a feasible way of realizing high quality InAsSb alloy.19 So far, there are several reports20,21 on InAsSb NW growth with Au-assisted method, but the growth is not suitable on a CMOS platform because of the contamination effect of Au. To overcome this problem, Au-free growth is deemed to an optimal solution. Nevertheless, very few studies14,22,23 have been reported on the Au-free InAsSb NW growth. © 2015 American Chemical Society

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EXPERIMENTAL METHODS

Growth Details. The growth of InAsSb NWs on Si (111) substrates was carried out in a close-coupled shower head MOVPE Received: February 9, 2015 Revised: March 20, 2015 Published: March 30, 2015 2413

DOI: 10.1021/acs.cgd.5b00201 Cryst. Growth Des. 2015, 15, 2413−2418

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system (AIXTRON Ltd., Germany) at a pressure of 133 mbar. Trimethylindium (TMIn), arsine (AsH3), and trimethylantimony (TMSb) were used as gas precursors and H2 as the carrier gas. The substrates were first loaded into the reactor followed by annealing at 635 °C and cooled down to 400 °C in H2 ambient. Then the temperature was raised to the growth temperature in an AsH3-rich environment. After stabilized, the NW growth was initiated by introducing TMIn and TMSb into the reactor chamber. During the growth, the TMIn flow rate was kept constant at 2.0 × 10−6 mol/min. The Sb-FRF and V/III ratios were varied respectively between 0.2−0.8 and 5−50. After the growth, the samples were cooled down to room temperature with the protection of AsH3 and TMSb flow. Characterization Methods. The morphologies of the InAsSb NWs were investigated by a scanning electron microscope (SEM) (a Nova Nano SEM 650). The crystal structure of the NWs was characterized by FEI Tecnai F30 transmission electron microscopes (TEM) operated at 300 kV. For high-resolution transmission electron microscope (HRTEM) analysis, NWs were removed from the growth substrate via sonication in ethanol and then drop-cast onto Cu grids coated with carbon film. The compositions of the NWs were determined by X-ray diffraction (XRD) measurements and confirmed on a few compositions by scanning TEM (STEM) in point analysis mode.

straight and uniform in diameter, mainly with a hexagonal cross section. The height of NWs decreases with increasing Sb-FRF. The distribution of NWs on the substrate surface is also even. It is important to note that the top of NWs is plain facet without any droplets. The uniformity of the NW morphology and the absence of droplets on the top suggest a catalyst-free growth mechanism;27,28 we define this kind of NWs as Type I NWs, which should be grown via VS mode. In contrast, for the growth under low V/III ratio and relatively high Sb-FRF, as displayed at the upper left side of the red dashed line Figure 1, the obtained NWs are quite different, where the InAsSb NWs tend to grow in multiple orientations and most of them appear to be more or less close to [1 1 1] orientation, and are nearly kink-free. It is noted that when V/III ratio is fixed at a low value but Sb-FRF increases, the yield of vertically grown [1 1 1]-oriented NWs decreases. The issue of NW’s multiple orientations has also been discussed by Bakkers et al. They suggest that a large yield of nonvertical NWs may relieve strain at the NW−substrate interfaces in highly latticemismatched materials systems.29 So when the Sb content increases, the growth of the InAsSb NWs may preferentially not be all vertical epitaxial growth.30 In the meantime, it is also discovered that when Sb-FRF is high, the density of the NWs is lower, and the top of each NW terminates with an In−Sb alloy droplet, which is completely different from the case of Type I NWs. The droplets on top of the NWs have a hemispherical shape with no defined facets. It was found that whether NWs have nontapered or tapered cross sections depends mainly on Sb-FRF. At a lower Sb-FRF, the NWs have a larger base area perhaps because of a relatively excessive As, which can gradually consume In droplets and make the base part lager. While at the higher Sb-FRF, the NWs have a larger top as a result of the accumulated In-rich droplets. At a proper Sb-FRF, the NWs may have a uniform diameter. The tapered NWs and the existence of the droplets on the top suggest a catalyst growth mechanism;31 we define this kind of NWs as Type II NWs, which should be grown via VLS mode. Furthermore, it was found that the growth rate of the two types NWs is quite different. As an example, one series of samples framed by a blue rectangle in Figure 1 has been chosen to study the variation of NW growth rate with changing V/III ratio from 5 to 50 at a fixed Sb-FRF of 0.4. The change in growth rate is shown in the inset of Figure 1. From the inset, a remarkable decrease in growth rate can be clearly observed when the V/III ratio increases from 5 to 10, which implies a change in growth mode from Type II to Type I. Focusing on the regimes at V/III > 5, which are mainly of Type I mode, it was found that the growth rate for NWs in this region almost does not depend on the As flux. A similar behavior was reported32 for the self-induced GaN NW growth in the VS mode previously. In contrast, the growth rate increases by three orders of magnitude when V/III < 5 for NWs growing in the regime, which has much more typical features of Type II mode. This is consistent with the expectation that the rate of deposition via a vapor phase-dependent VLS process is much higher than via noncatalyzed VS deposition, as the latter is presumably a result of surface reactivity.33,34 To further check the differences between the two types of NWs, we perform the test of STEM analysis with energy dispersive X-ray spectroscopy (EDS). As shown in Figure 2, the composition distribution of the two types of NWs is also significantly different. For type I NW (Figure 2a), the composition of NW from the bottom to the top is

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RESULTS AND DISCUSSION Existence of Two Growth Mechanisms. A set of InAsSb NWs was grown by MOCVD under varying conditions. Their SEM images are displayed in Figure 1 to show the influence of

Figure 1. Images of InAsSb NWs as a function of Sb-FRF and V/III ratio. The inset in the top right shows the variation of growth rate with V/III ratio when Sb-FRF = 0.4 (as highlighted by the blue rectangular frame). The solid line is used for eye guide, indicating the changing trend of growth rate. The red dashed line in the figure indicates the transition boundary between Type I and Type II growth modes.

both V/III ratio (as the abscissa) and Sb-FRF (as the ordinate) on the growth of InAsSb NWs. Two remarkably different growth morphology characteristics can be clearly recognized from an inspection of these SEM images, as demarked by the red dashed line in the figure. For the growth under the high V/ III ratio and relatively low Sb-FRF, as displayed in the bottom right side of red dashed line in Figure 1, the obtained NWs prefer to epitaxially grow perpendicular to the (111) substrate plane, and there are very few nonvertical NWs. The NWs are 2414

DOI: 10.1021/acs.cgd.5b00201 Cryst. Growth Des. 2015, 15, 2413−2418

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droplets will be maintained up on the top of NWs during the whole NW growth process. Then the In-rich alloy droplet captures the precursor materials and catalyzes crystal growth further at the liquid−solid interface. The VLS process occurs. At high of V/III ratio and low Sb-FRF, however, since the supply of group V flow is large enough to consume all the supply of In flow, the initial droplets crystallize quickly with forming InAsSb rather than gathering to stay as In-rich droplet. In this way, InAsSb NW growth is assumed to obey the VSmode, and the crystal growth takes place along the entire surface of the NW in a layer-by-layer mode.35 In addition, the two growth modes nevertheless could coexist on one substrate, especially under the intermediate conditions. Analysis of VS Mode Grown NWs. A range of growth parameters including growth temperature, Sb-FRF, and V/III ratio were explored for modulating the growth of InAsSb NWs via VS mode. It was found that InAsSb NWs can grow over a temperature range of 450−510 °C at the Sb-FRF of 0.2. The lateral and axial growth rates of the NWs, as well as Sb composition incorporated into the NWs, vary with growth temperature. The lateral growth rate (along the diameter) of the NWs increases with decreasing growth temperature from 510 to 450 °C, while the axial growth rate (in the length direction) changes in a contrary manner, as can be seen in Figure 4, panel a. Such temperature dependences of the growth

Figure 2. EDS result of alloy composition distribution for (a) Type I and (b) Type II NWs showing two different characteristic shapes.

homogeneous and nearly constant (In, 36 ± 1%; Sb, 8 ± 1%; As, 55 ± 1%) as reported by Shin,28 while for the type II NW (Figure 2b), the composition at the top of the NW shows a significant difference from those in the other parts, that is, the atomic percentage in spot 1 (In, 81%; Sb, 16%; As, 3%) is significantly different from the other two (In, 42%; Sb, 19%; As, 39% and In, 43%; Sb, 20%; As, 37%, respectively). Moreover, EDS results show that the top of this NW is an In-rich In−Sb alloy top, which is a typical feature for NWs grown with VLS growth mode. On the basis of the above experimental results, we can confirm that the Type I NWs grow via VS mode, while the Type II ones form via VLS mode. Then, we depict a scheme for the nucleation and growth of InAsSb NWs on Si (111) that can consistently account for two different growth modes, as illustrated in Figure 3. First, we need to note that an essential difference between the two growth modes occurs in the initial nucleation period. At low V/III ratio and high Sb-FRF, In-rich droplets forming spontaneously on Si in the beginning of the growth mediate the nucleation of InAsSb. Because of excessive supply of In flow, the In-rich

Figure 4. (a) Diameter and length and (b) Sb content and corresponding lattice constant of the InAsSb NWs as a function of temperature. The error bars indicate the standard deviation.

rates can be explained by the following: the limiting step for axial growth of VS mode NWs is the desorption of the alkyl groups from the Si substrate surface. Thus, the axial rate obeys an Arrhenius relationship and has an exponential dependence on temperature.35,36 The activation energy is calculated to be ∼272 kJ/mol according to the Arrhenius equation. However, any increase of the radial growth rate will reduce the possible supply of reactants available for axial growth37 and therefore will slow down the axial growth rate. Thus, the contrary dependences of axial and radial growth rates on the temperature take place. Figure 4, panel b also shows that the Sb composition incorporated into the NWs increases substantially with decreasing growth temperature from 510 to 450 °C. The lattice constant was calculated by XRD 2θ−ω

Figure 3. Schematic diagram of two growth modes for InAsSb NWs. 2415

DOI: 10.1021/acs.cgd.5b00201 Cryst. Growth Des. 2015, 15, 2413−2418

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and the ⟨001⟩-oriented ones have no any stacking faults, which is similar to what we observed in our NWs. However, the yield of ⟨001⟩ growth is normally much lower in comparison to ⟨111⟩ direction growth.4,44 A more in-depth theoretical investigation of this issue is needed but will not be discussed here because it is out of the scope of the present paper. Analysis of VLS Mode Grown NWs. With regard to the VLS-mode NWs, we have also investigated the influence of temperature on the growth direction of NWs. Two kinds of NWs can be obtained by tuning the growth temperature from 470 to 450 °C, as shown in Figure 6 ((a) vertical or nonvertical

analysis, and the Sb content was subsequently extracted by using Vegard’s law. The maximum of Sb composition at the SbFRF of 0.2 reaches up to 0.37 at the growth temperature of 450 °C. In addition to the temperature, it was found that the SbFRF has also a remarkable influence on the growth rate and Sb composition of NWs, which has been reported14 elsewhere. It was found that under the optimal condition, that is, at the growth temperature of 510 °C and Sb-FRF of 0.2, all NWs grown in the VS mode have a unified [111] direction and uniform composition distribution, which make them the best candidate for multiple NWs integration device. To determine the structural characteristics, TEM measurements were performed on the NWs grown in VS mode under the optimum growth conditions mentioned above. Figure 5

Figure 6. SEM images of NWs grown in VLS mode at two different growth temperatures: (a) 470 °C; (b) 450 °C.

NWs that grow at an angle with respect to the substrate surface, (b) planar NWs that grow along the substrate). Detailed discussions about this phenomenon will be presented elsewhere in another article. Considering the effect of both V/III ratio and Sb-FRF from Figure 1, the growth window for the vertical VLS mode NWs is relatively narrow, mainly limited in a small region with very low V/III ratio condition. Meanwhile, the structural characteristics of the VLS-mode NWs are investigated. Figure 7, panel a is a typical bright-field

Figure 5. Crystal structures of NWs with different diameters: (a) 34 nm; (b) 46 nm; (c) 50 nm; (d) 55 nm; (e) 78 nm; (d) 85 nm. The insets are FFT of the HRTEM images.

shows TEM images of several InAsSb NWs with diameters ranging from 30−90 nm. It was proposed38−40 that for many III−V compounds, whether the NW structure is zinc blend (ZB) or wurtzite (WZ) has a significant dependence on NW diameter. However, from Figure 5, we can see that all of these six NWs are mainly of ZB crystal phase independent of their diameter. It suggests41 that the Sb flux in the VS growth mode may be too low to achieve enough supersaturation to form stacking faults, and the formation of WZ structure is normally being associated with a high Sb supersaturation. Second, all of the current theoretical models38−40 are discussed based on VLS-mode rather than VS-mode; the crystal structure results of VS mode growth cannot be explained well by these models. However, it is amazing to find that most of these NWs, no matter how large or small their diameter, are not very pure ZB phase, and their growth direction is mainly [111] orientation, but only when the NWs with a diameter of 50−60 nm (Figure 5b,c) have a pure ZB crystal phase, free of stacking faults, and the growth direction is [001] instead of [111], as shown by the inset fast Fourier transform (FFT) results in the insets of Figure 5, panels b and c. To the best of our knowledge, this is the first report of successful growth of phase-pure InAsSb NWs. About the diameter dependence of the growth direction, Schmidt and Wu42,43 have reported a diameter-dependent growth direction of epitaxial silicon NWs, but their diameter scale is much smaller than our NWs. In addition, the ⟨111⟩oriented NWs are usually with a high density of stacking faults,

Figure 7. (a) Representative bright-field TEM of an InAsSb NW grown by VLS-mode and the corresponding SAED pattern indexed as ZB in [011] zone axis. (b−d) HRTEM image of the three rectangular regions in panel a and their corresponding FFT.

TEM image of such InAsSb NW. The smoothly curved feature of the top part with a darker contrast is typical in VLS modegrown NWs.45 The HRTEM images are acquired in three regions highlighted by the solid rectangles in Figure 7, panel a and presented in Figure 7, panels b−d, respectively. Clearly, the HRTEM image and corresponding FFT pattern from each position of the NW show a pure ZB crystal structure without 2416

DOI: 10.1021/acs.cgd.5b00201 Cryst. Growth Des. 2015, 15, 2413−2418

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stacking faults. According to the index marked in the FFT patterns, the growth direction of the NW is along [1−11]. The ZB crystal structure is confirmed by SAED patterns (for example, the inset in Figure 7a) acquired along the zone axis [1−1 2] for the entire NW shown in Figure 7, panel a. However, the detailed HRTEM observation of the curved top reveals that this part is polycrystalline instead of monocrystalline, which can be proved by the Moiré stripes appearing in the green rectangular frame shown in Figure 7, panel b. EDX spot scan analysis of the NW top part in Figure 7, panel a shows that the In-rich In−Sb alloy contains 81% In and 16% Sb (with a negligibly small amount of 3% As). The Sb composition is a little smaller than the 24% of the In−Sb solution obtained at the temperature of 470 °C, being in accord with the trend predicted by the phase diagram for the cooling down alloy. As a comparison, we have found that the defect density of VLS-mode NWs is normally less than the VS-mode ones. The only exceptional case is VS-mode ⟨001⟩-oriented NWs with diameters of 40−50 nm, which can be grown free of stacking faults. This advantage of VLS mode can be mainly attributed to the catalysis of In-rich droplets. In fact, an absence of the heterocatalyst would result in the ⟨111⟩-oriented NWs displaying a very high density of defects including stacking faults, twin boundaries, and polytypism, that is, uncontrolled axial modulation of the crystal phase between ZB and WZ polytypes of InAsSb, as reported by Mandl and Wei.27,46 Therefore, the In-rich droplet can be used to effectively improve crystal quality of ⟨111⟩-oriented NWs without introducing any Au-contaminant during the growth process. The high crystal quality of the NWs grown via VLS mode is very promising for improving the performances of single NW devices.

REFERENCES

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CONCLUSIONS We exhibited a controllable method of Au-free InAsSb NW growth chosen between the VLS and VS modes by modifying the growth conditions (SB-FRF and V/III ratio). At low V/III ratio and high Sb-FRF, the VLS process occurs. On the contrary, at high V/III ratio and low Sb-FRF, the NWs grow via the VS-mode. Under different mechanism, the grown NWs show very clear differences in morphology, growth direction, and crystal quality. For VS mode, all NWs have unified axial [111] direction and a uniform composition distribution, which makes them the best candidate for multiple NWs integration device, while the high crystal quality of the NWs grown via VLS mode is promising for improving the performances of single NW device. Therefore, by simply adjusting growth conditions, the obtained NWs may meet different needs.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +86 10 8230 4529. Fax: +86 10 8230 4529. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge D. S. Jiang and J. Y. Wang for their valuable discussions. T.Y. thanks the Ministry of Science and Technology (MOST) of China (Grant No. 2012CB932701). 2417

DOI: 10.1021/acs.cgd.5b00201 Cryst. Growth Des. 2015, 15, 2413−2418

Crystal Growth & Design

Article

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DOI: 10.1021/acs.cgd.5b00201 Cryst. Growth Des. 2015, 15, 2413−2418