NANO LETTERS
Control of InAs Nanowire Growth Directions on Si
2008 Vol. 8, No. 10 3475-3480
Katsuhiro Tomioka,*,‡ Junichi Motohisa,‡ Shinjiroh Hara,† and Takashi Fukui‡,† Graduate School of Information Science and Technology, Research Center for Integrated Quantum Electronics (RCIQE), Hokkaido UniVersity, North14 West9, 060-0814, Sapporo, Japan Received August 7, 2008
ABSTRACT We report on control of growth directions of InAs nanowires on Si substrate. We achieved to integrate vertical InAs nanowires on Si by modifying initial Si(111) surface in selective-area metal-organic vapor phase epitaxy with flow-rate modulation mode at low temperature. Crosssectional transmission electron microscope and Raman scattering showed that misfit dislocation with local strains were accommodated in the interface.
Semiconductor nanowires have attracted much attention as building blocks for nanoscale electronics and optoelectronics because of their one-dimensional nanoarchitecture, functional properties with heterostructures, and unique optical properties.1-6 Now, epitaxy of III-V compound semiconductor nanowires on silicon (Si) are attracting much interest because advantages of III-V nanowires can be integrated with Si platforms directly. Great efforts have been made to grow III-V nanowires directly on Ge7 and Si8-14 without fundamental issues such as mismatch due to lattice and thermal expansion, and the studies in such integration with Si are exceedingly common. However, there remains the control of vertical nanowire growth directions on Si surface. III-V nanowires favor to grow in the B direction, thus vertically independent nanowires grow on III-V(111)B substrates. Inclined growth directions that are three equivalent direction of B occur as well as vertical nanowire growth in the case of the growth on Si because of its nonpolar nature. This tilted nanowire growth direction is not appropriate to future high integrated devices on Si. Here, we propose an effective process of controlling nanowire growth directions on Si substrate and demonstrate integration of vertical InAs nanowires array on Si by using selective-area metal-organic vapor epitaxy (SA-MOVPE). Indium arsenide is a narrow-band gap semiconductor and has high electron mobility (20 times higher than that of Si at room temperature) because of its small electron effective mass. This material is less effective against surface depletion that usually resulted from surface states, because the surface * To whom correspondence should be addressed. E-mail: tomioka@ rciqe.hokudai.ac.jp. ‡ Graduate School of Information Science and Technology. † RCIQE. 10.1021/nl802398j CCC: $40.75 Published on Web 09/11/2008
2008 American Chemical Society
Fermi level is within the conduction band. This means that conductive InAs nanowires can be obtained easily without any surface passivation. Using the vapor-liquid-solid (VLS) method, InAs nanowires can be grown on Si although the crystal lattice mismatch between InAs and Si is very large (11.6%).11,14 It is difficult to control the growth direction of InAs nanowires because of the polar/nonpolar nature. In addition, as much as the lattice mismatch grows high, control of equivalent growth direction of III-V nanowires becomes difficult more.13 Moreover, site-controlled vertical InAs nanowire growth on Si has not been demonstrated although future integration using nanowires require precise placement. We controlled growth directions of InAs nanowire and its placement on Si(111) substrates by SA-MOVPE in this letter, and we discussed the optimum growth conditions to grow vertical InAs nanowires and their interface. In SAMOVPE, partially open amorphous films are used as template masks. We have already reported hexagonal-shaped III-V nanowire growths with six vertical sidewalls, that is, facets.15-18 We have also confirmed that InAs nanowires can be grown on partially masked InP and GaAs(111)B substrates. Thus, we should be able to form nanowires on (111)B-oriented surfaces with nanometer-scale areas. Also, selective-area growth can reduce the threading dislocations that occur due to large differences in the thermal expansion coefficient.19 Moreover, MOVPE is suitable for mass production for mature Si integrated devices. Fabrication processes for SA-MOVPE in details were reported elsewhere.16-18 In short, InAs growth was performed using a low-pressure (0.1 atm) horizontal-reactor MOVPE system. The carrier gas used in this growth was pure hydrogen (H2) that was purified through Pd film. The total flow rate of the gases was maintained at 5.75 standard liters
Figure 1. (a) Overview of InAs nanowire arrays on patterned substrate. (b) 45°-tilted view showing vertical InAs nanowire array. (c) Plan-view of InAs nanowire array. Side facets are {-110} planes and hexagonal-shaped cross-section is (111)B plane.
Figure 2. (a) 45°-tilted SEM image of InAs nanowire growth under conventional growth condition. Coexistence of vertical InAs nanowire and 19.6°-tilted nanowires on Si(111) was shown here. Inset shows schematic illustration of the nanowires. Illustrations of chemical structures: (b) InAs (111)B surface, (c) InAs(111)A surface, (d) As-incorporated Si3+ structure, (e) In-terminated Si1+ surface, (f) As-terminated Si1+ surface, and (g) In-incorporated Si3+ structure. These are viewed from direction. Yellow arrows indicate InAs nanowire-growth direction.
per minutes (SLM). The III-atom precursor was trymethylindium [TMIn] and the V-atom precursor was 5% hydrogendiluted arsine (AsH3) gas. The InAs growth was performed at 540 °C for 20 min. The partial pressures of TMIn and AsH3 were 4.7 × 10-7 and 1.3 × 10-4 atm, respectively. Prior to growth, thermal cleaning in an H2 ambient was carried out at 925 °C to remove the native oxide that had formed on the opening areas of the masked substrates when transferring the samples into the reactor. Approximately 1 nm thick native oxide was formed on the opened patterns, and the oxide disrupted selective-area growth. High-resolution TEM images were acquired using a transmission electron microscope (HITACHI H-9000UHR). The acceleration voltage was 300 kV. The incident electron beam was along the direction. The samples were sliced into thin sections by using focused-ion-beam (FIB) and Ar-ion-milling techniques. Raman scattering measurements were conducted to confirm that the materials grown on the Si were those we had designed materials and the strain was included in the nanowires. An He-Ne laser, whose wavelength was 632.8 nm, was focused on an approximately 3476
2 µm spot on the substrates with the nanowires grown on them. The laser power was about 0.1 mW. The incident direction of the excitation light was along the B direction in (111) back scattering geometry. First, we demonstrate typical placement of the vertical InAs nanowires on Si(111) under optimized conditions (Figures 1a-c). Figure 1a exhibits an overview of InAs nanowire array on Si(111) substrate. The position-controlled InAs nanowires, shown in Figure 1b, were formed within the gray square prepatterned regions (each 50 × 50 µm2) in Figure 1a. The prepatterned regions were readily fabricated by using electron-beam lithography and wet chemical etching. Here, we used the patterns whose opening diameter was 60 nm and whose pitch ranged from 400 to 800 nm. The InAs nanowires were only grown on the opening circles and oriented perpendicular to the surface, and they were an average of 60 nm in diameter and average of 3 µm in height. Size fluctuation of nanowire diameters was ( 4 nm as standard deviation. The length fluctuation of InAs nanowires in Figure 1b resulted from the size fluctuation of diameters because the nanowire height was proportional to diameter.18 Nano Lett., Vol. 8, No. 10, 2008
Figure 1c shows a plan view of scanning electron microscopy (SEM) image. All the nanowires had a hexagonal-shaped cross-section with surrounding {-110} side facets. The results shown in Figure 1 were achieved under special growth conditions. This is because 19.6°-tilted nanowire growth usually occurs on the Si(111) surface in addition to the vertical nanowire growth as shown in Figure 2a. In III-V compound semiconductors, the group V-atom-exposed (111) plane is (111)B (Figure 2b); on the other hand, that of IIIatoms is exposed on (111)A surface (Figure 2c). Thus, InAs nanowires on the (111)A-oriented surface grow along three equivalent B directions (see Supporting Information, Figure S1). However, Si has no polar nature, that is, both (111) oriented surface can exist in terms of III-V nanowire growth. Therefore, vertical and tilted nanowires are formed on the same surface. Differences in nanowire growth directions have been observed in case of Si nanowires20 as well as III-V nanowires on Si.7,8,11-13 The simple crystallographic issues have complicated a situation more and prevent us from applying III-V nanowires to the Si platform. An (111)B-oriented surface should be formed on Si(111) to simply align the four equivalent directions of III-V nanowires only in the vertical direction. The growth of SAMOVPE initiates from atomically flat surface without catalysts. Therefore, the coexistence of vertical and tilted nanowires indicates that the chemical structures in Figure 2d-g are formed on the same surface before InAs growth. Conversely, the growth directions of the InAs nanowires can be controlled by optimizing the initial surface and initial growth conditions. Si(111) surface usually has Si1+, Si2+, and Si3+ structures.21,22 Once As-incorporated Si3+ and/or In-terminated Si1+ has formed on the Si surface as shown in Figure 2d,e, only vertical InAs nanowires should grow on the Si(111) substrates. For this purpose, we studied the relation between the growth yields of InAs nanowires and AsH3 supply conditions prior to the growth. In these sequences, we used commonly a thermal cleaning at above 925 °C for 5 min to remove native oxide on opening area of patterns prior to the growth. The openings were 60 nm in diameter and the partial pressure of AsH3 prior to the growth was 2.5 × 10-4 atm. Figure 3e shows the percentage of growth yields in each sequence. We confirmed the reproducibility in each percentage of Figure 3e with 15 wafers and its standard deviation was within (1%. Under the conventional sequence shown in Figure 3a, fraction of vertical InAs nanowires is approximately 31%, and that of tilted nanowire growth is 13%. Also, no growth yield is 52%. The result suggests that the formation of (111)B-oriented surface is insufficient due to thermal desorption and surface reconstruction of Si(111). The MOVPE system uses pure H2 as carrier gas, but no hydrogen terminates on Si surface because the Si-H is thermally weak at 540 - 925 °C. Also, surface reconstructions should have occurred on the bare Si(111) at 925 °C. A complex As atom termination of the reconstructed Si(111) surface occurred prior to the growth, and this complex surface reconstruction stems the selective-area growth. Nano Lett., Vol. 8, No. 10, 2008
Figure 3. (a-c) Schematic illustrations of gas-flow and temperature sequence for InAs nanowire growth on Si. (d) Schematic diagram of flow-rate modulation mode. Values in the diagram are partial pressures and time. (e) Percentage of growth results in each growth sequence.
In case of ultrahigh-vacuum (UHV) studies, various kinds of reconstructed Si(111) surface have been reported, such as 1 × 1 structure at above 830 °C and metastable reconstructed surface of n × n (n ) 3, 2) and c2 × 4, from 925 to 540 °C. The AsH3 supply during cooling sequence in Figure 3a is thought to form As-incorporated, -reconstructed Si surface. These complex surfaces disturb nucleation of InAs on openings. Although the formation of the complex surface is assumed, the small fraction of the vertical InAs nanowires (31%) in Figure 3b indicated reduction of nucleation of InAs on (111)B-oriented Si surface, and it proves formation of As-incorporated, -reconstructed Si surface. Therefore, such surface reconstructions 3477
Figure 4. (a) High-resolution TEM image of InAs nanowire on Si(111) substrate. Incident direction of electron beam is in direction. (b) High magnification image of panel a. (c) Fast Fourier transformation (FFT) image calculated from panel b. Both Bragg spots of Si and InAs nanowire can be seen in image. (d) εxx strain mapping estimated from filtered image of panel b. (e) Raman spectra of grown InAs nanowires (black solid line) and InAs(111)B substrate (red solid line).
should be controlled in appropriate conditions of forming (111)B-oriented Si(111) surface. The promising way to control the surface reconstructions without metastable surface reconstructions is to cool Si(111) surface to 400 °C in H2 ambient, because 1 × 1 reconstructed surface that was formed at high temperature can regenerate at 400 °C. And also, As atoms should be replaced to the outermost Si atoms of 1 × 1 reconstructed surface because it is equivalent to As-terminated Si3+ surface and (111)B-oriented surface. Conveniently, Si(111)/ As 1 × 1 reconstructed surface was found to be formed at low temperatures below 430 °C in As ambiemt.23 Thus, we cooled the sample to 400 °C in hydrogen and supply AsH3. The results show the fraction of vertical nanowire growth increase to 60%, while that of 19.6°-tilted InAs nanowires is 11%. Also the no growth yield is 28%. The small fraction of the tilted InAs nanowires indicates that the formation of In-terminated Si1+ or As-incorporated Si3+ that are crystallographic equivalent to (111)B-oriented surface is still insufficient. The reason is thought to be decomposition or desorption of As adatoms from the Si surface during rising temperature to the growth temperature in Figure 3b, and 3478
(111)A-oriented surfaces are formed on Si surface prior to the InAs growth. We used the specific growth sequence shown in Figure 3c,d to avoid decompositions and desorption of As atoms. First, the substrate was cooled down to 400 °C after thermal cleaning. Next, AsH3 was supplied at this temperature to form the As-incorporated Si3+ surface shown in Figure 3c. Moreover, As atoms and In atoms should be efficiently supplied to the Si(111) surface to form a (111)B-oriented surface just before InAs nanowire growth. We therefore introduced the flow-rate modulated epitaxy (FME) mode24 at 400 °C. FME is a method of alternating group III- or V-precursor supply during MOVPE. The purpose of the FME is to enhance the termination of In atoms to Asincorporated Si3+ and bare Si1+ surfaces because the In termination to a bare Si1+ surface also forms (111)B-like surface. We also introduced an H2 interval between the TMIn and AsH3 supply to enhance the exchanges of supplied materials. Figure 3d outlines the optimum gas-flow sequence of the FME. The FME mode was carried out for 20 cycles at 400 °C. After the FME mode, typical InAs nanowire growth was carried out at 540 °C. As a result, 95% of all Nano Lett., Vol. 8, No. 10, 2008
nanowires were controlled to vertical direction and 5% were tilted growth. The results suggest that a (111)B-oriented surface was effectively formed on Si(111) by using this growth sequence. Figure 4a,b are cross-sectional TEM images of a InAs nanowires grown on Si(111). They indicate that InAs nanowires were epitaxially grown on the Si substrates. The interface between the InAs and Si substrate is almost atomically flat and shows strain-induced lamellar dark contrast, whose thickness is 5 MLs. The InAs nanowire contains random rotational twins with 1 to 3 MLs periods. Selected area electron diffraction spots showed that the nanowires were composed of both zincblende and wurtzite structures (see in Supporting Information, Figure S2). The crystallographic structure is totally same as for InAs nanowires grown on InAs(111)B by SA-MOVPE.25 This means that InAs nanowires can be grown on Si substrates in the same manner as homoepitaxial growth in SA-MOVPE regardless of large lattice mismatch. Figure 4b is a magnified image of TEM. The atomic layer stacking of the substrate and InAs occur as twinning across the interface. Moreover, a lamellar dark contrasts whose thickness is 5 MLs can be observed at the interface. We first filtered the image by using Bragg-spot filtering because the dark contrast due to strain and the twinning at the interface blurred the characterization of the interface. The FFT image is shown in Figure 4c. Bragg spots can be seen in addition to those of Si substrate. We next calculated the displacements in the bright spots on the Bragg-spot filtered image into a strain map by using a peak-pair finding technique.26 The calculated strain mapping is shown in Figure 4d. It describes εxx strain fields whose x vector is in the directions. In Figure 4d, periodical strains along the directions are observed at the interface. The average periodic strain measured from Figure 4d is 29.5 ( 0.6 Å. The Burgers vector due to misfit dislocations on the (111) plane is in the directions, and the period of the misfit dislocations calculated from the lattice-mismatch (11.6%) is 28.8 Å. Similar strain fields due to dislocations were calculated by Ertekin et al.27 Thus, the periodical strains in Figure 4d are originated from misfit dislocations. We presumed that coherent epitaxy without such dislocations occur in the lattice-mismatched systems when the opening diameters become smaller than that of Figure 4d.28 Further investigation is required to control the dislocations at interface. Periodical strains at interface are not observed within the InAs nanowire. This means that the strains generated from the lattice mismatch accommodate only at the interface and does not affect upper InAs nanowire growth. Raman scattering spectra of this sample and a reference InAs(111)B substrate are shown in Figure 4e. TO and LO phonon spectra from the nanowires and a strong Si LO phonon were observed. The TO and LO phonon spectra have no peakshift as compared to those of the bulk InAs(111)B surface. The FME mode at low growth temperature plays an important role as buffer layer for accommodation of strains at the interface of InAs and Si. Nano Lett., Vol. 8, No. 10, 2008
In conclusion, we controlled growth direction of InAs nanowires into vertical direction on Si(111) and sitecontrolled of the nanowire array successfully. The grown nanowires had no relation from strains with large lattice mismatching because the misfit dislocations with localized strains were accommodated in the interface. These results are first steps that we have to progress first to apply nanowires into functional devices on Si substrate. As a next step, we will expand the technique in this letter to fabrication of electrical and optical devices on Si platforms in future. Acknowledgment. We acknowledge Professor Kenji Hiruma and Professor Tamotsu Hashizume for the kind discussions we had with them and for sharing their insights. We also acknowledge Dr. J. Takeda, Dr. L. Yang, and Dr. Y. Ding for their support with the experiments on MOVPE growth. This work was financially supported by a Grant-inAid for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), Japan. K.T. would like to acknowledge the financial support provided by a Research Fellowship from the Japan Society for the Promotion of Science (JSPS). Supporting Information Available: Figure S1. (a) 45°tilted SEM image of InAs nanowires grown on InAs(111)B substrate. (b) Top-view SEM image of InAs nanowires grown on InAs(111)A substrate. Figure S2. TEM image of InAs nanowire grown on (a) InAs(111)B and (b) Si(111). (c,d) SAED pattern of the InAs nanowire grown on InAs(111)B and Si(111). This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Hiruma, K.; Yazawa, M.; Katsuyama, T.; Ogawa, K.; Haraguchi, K.; Koguchi, M.; Kakibayashi, H. J. Appl. Phys. 1995, 77, 447. (2) Gudiksen, M. S.; Lauhon, L. J.; Wang, J.; Smith, D. C.; Lieber, C. M. Nature 2002, 415, 617. (3) Lauhon, L. J.; Gudiksen, M. S.; Wang, D.; Lieber, C. M. Nature 2002, 420, 57. (4) Huang, Y.; Duan, X.; Cui, Y.; Lauhon, L. J.; Kim, K-H.; Lieber, C. M. Science 2001, 294, 1313. (5) Huang, M. H.; Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. Science 2001, 292, 1897. (6) Johnson, J. C.; Choi, H-J.; Knutsen, K. P.; Schaller, R. D.; Yang, P.; Saykally, R. J. Nat. Mater. 2002, 1, 106. (7) Bakkers, E. P. A. M.; Dam, J. A. V.; Franceschi, S. D.; Kouwenhoven, L. P; Kaiser, M.; Verheijen, M.; Wondergem, H.; Sluis, P. V. D. Nat. Mater. 2004, 3, 769. (8) Ma˚rtensson, A. T.; Svesson, C. P.; Wacaser, B. A.; Larsson, M. W.; Seifert, W.; Deppert, K.; Gustafsson, A.; Wallenberg, L. R.; Samuelson, L. Nano Lett. 2004, 4, 1987. (9) Roest, A. L.; Verheijen, M. A.; Wunnicke, O.; Serafin, S.; Wondergem, H.; Bakkers, E. P. A. M. Nanotechnology 2006, 17, S271. (10) Tateno, K.; Hibino, H.; Gotoh, H.; Nakano, H. Appl. Phys. Lett. 2006, 89, 033114. (11) Park, H. D.; Prokes, S. M.; Twigg, M. E.; Cammarata, R. C.; Gaillot, A-C. Appl. Phys. Lett. 2006, 89, 223125. (12) Ihn, S-S.; Song, J-I.; Kim, T-W.; Leem, D-S.; Lee, T.; Lee, S-G.; Koh, E. K.; Song, K. Nano Lett. 2007, 7, 39. (13) Bakkers, E. P. A. M.; Borgstro¨m, M. T.; Verheijen, M. A. MRS Bulletin 2007, 32, 117. (14) Ma˚rtensson, T.; Wagner, J. B.; Hilner, E.; Mikkelsen, A.; Thelander, C.; Stangl, J.; Ohlsson, B. J.; Gustafsson, S.; Lundgren, E.; Samuelson, L.; Seifert, W. AdV. Mater. 2007, 19, 1801. (15) Motohisa, J.; Noborisaka, J.; Takeda, J.; Inari, M.; Takashi, F. J. Cryst. Growth 2004, 272, 180. (16) Noborisaka, J.; Motohisa, J.; Fukui, T. Appl. Phys. Lett. 2005, 86, 213102. 3479
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