Zinc Blende and Wurtzite Crystal Phase Mixing ... - ACS Publications

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Zinc Blende and Wurtzite Crystal Phase Mixing and Transition in Indium Phosphide Nanowires Keitaro Ikejiri,*,†,‡ Yusuke Kitauchi,‡ Katsuhiro Tomioka,†,‡,§ Junichi Motohisa,‡ and Takashi Fukui†,‡ †

Research Center for Integrated Quantum Electronics, Hokkaido University, North 13 West 9, Sapporo 060-8628, Japan Graduate School of Information Science and Technology, Hokkaido University, North 14 West 9, Sapporo 060-0814, Japan § JST-PRESTO, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi 332-0012, Japan ‡

ABSTRACT: Indium phosphide (InP) nanowires, which have crystal phase mixing and transition from zinc blende (ZB) to wurtzite (WZ), are grown in intermediate growth conditions between ZB and WZ by using selective-area metalorganic vapor phase epitaxy (SA-MOVPE). The shape of InP nanowires is tapered unlike ZB or WZ nanowires. A growth model has been developed for the tapered nanowires, which is simply described as the relationship between tapered angle and the ratio of ZB and WZ segments. In addition, the peak energy shift in photoluminescence measurement was attributed to the quantum confinement effect of the quantum well of the ZB region located in the polytypic structure of ZB and WZ in nanowires. KEYWORDS: Nanowire, selective-area metalorganic vapor phase epitaxy, crystal structure, InP, zinc blende, wurtzite

R

ecently, semiconductor nanowires have attracted much attention because of their unique electrical and optical properties and application in various nanoscale devices.1
3 In particular, indium phosphide (InP)-based nanowires are promising for high-speed electron, optelectronic, and photovoltaic devices because of their superior material property and possibility to develop various kinds of heterostructures. Until now, InPbased nanowire devices including field effect transistors,4 photodetectors,5 light-emitting devices,4 waveguides,6 and solar cells7 have been demonstrated, and studies on heterostructures including quantum dots8
10 or core
shell structures11 have been reported. One of the key issues for creating such applications is to control the growth structure of nanowires such as shape and crystal structures. Various shapes of InP nanowires have been proposed such as hexagonal pillar, tapered pillar, and irregular wire structures. As for the crystal structure, a zinc blende (ZB) structure of InP nanowires has been reported by Duan and Lieber12 and Bhunia et al.,13 whereas Mattila et al. reported that they developed wurtzite (WZ) structures that differ from that in bulk crystals.14 Moreover, the transition of crystal structures from ZB to WZ in InP nanowires is also reported by using catalyst-assisted vapor
liquid
solid (VLS) growth15
18 and catalyst-free growth of selective-area MOVPE.19 These transitions of crystal structure strongly affect electronic states in nanowires. Both theoretical20
22 and experimental23,24 results have shown that the band gap energy of InP differs by about 80
90 meV between ZB and WZ crystal structures. Furthermore, if they are mixed together, they exhibit type II superlattice structures. In a recent study, a carrier confinement effect and emission peaks from photoluminescence spectra originating from the superlattice on the mixed crystal phase of InP nanowires were observed.17,18 Thus, when InP nanowires are applied for r 2011 American Chemical Society

electrical and optical devices, a comprehensive understanding of the growth mechanism is required to control both structure and electronic states. We have reported on the catalyst-free growth of III
V semiconductor nanowires by selective-area MOVPE25,26 and shown that the growth mode, morphology, shape, and crystal structures of nanowires can be controlled by the growth condition. We have reported that crystal structures of InP nanowires are observed for both ZB and WZ unlike GaAs or InAs nanowires. The crystal structures are controlled by changing growth parameters including growth temperature (Tg), ratio of partial pressure of the supply gas, [TBP]/[TMIn] (V/III ratio), mask opening size, and pitch of the nanowire array.19 We have shown that the transition of crystal structure corresponds to the termination of the surface with constituent atoms regarding with the growth conditions. Under the high Tg and low V/III ratio condition, we obtain ZB InP nanowires, whereas the crystal structure changes from ZB to WZ when Tg is decreased and V/III ratio is increased. These results indicate that quite a different growth mode of InP nanowires exists, and this mode clearly has opposite dependency on growth conditions due to the VLS growth mechanism being used for the crystal phase transition in InP nanowire.17,27 Thus, this is a completely new mechanism for controlling crystal structures of nanowires by growth conditions. Now, the next issue is that what happens to InP nanowire growth in intermediate growth conditions between ZB and WZ. In this study, we investigated the transition of the crystal structures of InP nanowires by growing them under intermediate Received: July 12, 2011 Revised: August 8, 2011 Published: August 29, 2011 4314

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Figure 1. Summary of InP nanowires on InP (111)A obtained under three different growth conditions. Upper images are 45
tilt and top view of SEM images of InP nanowires at each growth condition. Middle images are high-resolution TEM images and selected area electron diffraction (SAED) of InP nanowires. Electron beam projection is parallel to Æ
110æ direction of substrate. Bottom images show cross sectional views of TEM images of whole InP nanowire. Vertical sidewalls are formed on NW1 and NW3, while sidewalls of nanowires are tapered on NW2.

conditions between ZB and WZ nanowire growth and by analyzing their crystal structures using scanning electron microscopy (SEM), transmission electron microscope (TEM), and photoluminescence (PL) measurement. In this growth method, a 20 nm thick layer of SiO2 is formed on III
V semiconductor substrate, and the layer is partially removed using electron beam lithography and a wet chemical etching technique to form mask openings as a template for growth. When MOVPE is carried out under appropriate growth conditions, crystal growth starts from the opening of the mask where the bare semiconductor surface of the substrate is exposed. With SA-MOVPE, size and position of nanowires are precisely controlled, because the growth is inhibited in the SiO2 mask area. For InP, we use trimethylindium (TMIn) and tertiarybutylphosphine (TBP) as source materials and InP (111)A as a substrate. We obtained three types of InP nanowires under three different growth conditions: grown for ZB, an intermediate condition of ZB and WZ, and WZ, hereafter called NW1, NW2, and NW3, respectively. As a standard condition for ZB nanowires (NW1), the growth temperature Tg and ratio of partial pressure of the supply gas, [TBP]/[TMIn] (V/III ratio), were 660
C and 18, respectively. WZ nanowire (NW3) was grown at 600
C and at a V/III ratio of 55. As an intermediate condition between NW1 and NW3, Tg and V/III ratio were set to be 660
C and 55 for NW2 growth condition, respectively. For NW2, Tg is higher than for NW1 and the same as for NW3, and the V/III ratio is the same as that of NW1 and lower than that of NW3. For each growth condition, we use the same mask designed substrate for which the initial diameter, d, of the mask opening was 100 nm and the opening pitch, a, of the

mask was 0.5 μm. Figure 1 shows SEM images and high-resolution TEM images of nanowires grown under three different conditions. Birds eye views and top views of NW1 and NW3 in SEM images show 6-fold symmetric vertical sidewalls without tapering. However, the hexagonal pillars rotate 30
from each other. From TEM images and diffraction patterns, these sidewalls for NW1 and NW3 are parallel to [
211] and [110] directions on InP (111)A surfaces, respectively. Therefore, these growth modes of the two types of InP nanowires are different. On the other hand, tapered nanowires were observed for NW2, diameters of which gradually change from 340 nm at the bottom to 120 nm at the top. We cannot clearly observe the facet of sidewalls for a whole nanowire. However, cross-sectional shapes change along the growth direction. At the bottom, six sidewalls of these nanowires are parallel to [
211], which is the same as NW1. At the top, sidewalls are parallel to [
110], which is the same as NW3. These results indicate the nanowire growth mode of NW2 is different from those of NW1 and NW3, and the growth mode changes from ZB to WZ during the growth. In the high-resolution TEM images and selected area electron diffraction pattern (SAED) in Figure 1, the crystal structures of nanowires are ZB and WZ for NW1 and NW3, respectively. In addition, NW1 includes the high density of stacking faults and twinning dislocations, while the crystal structure of NW3 is basically WZ and ZB segments are partially observed. On the other hand, we observed the crystal phase mixing of ZB and WZ in NW2. We note that an appearance ratio of WZ segments (P_WZ) change from the bottom to top part of NW2. For detailed analysis, 4315

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Figure 2. TEM images of each part of InP nanowires grown for NW2. High-resolution TEM images and selected area electron diffraction (SAED) from top, middle, and bottom part of InP nanowires are shown in right side of the figure. Red lines in high-resolution TEM images show ZB segments of the nanowires. The probabilities of WZ segments (P_WZ) are 95%, 79%, and 57% at top, middle, and bottom part of the nanowires, respectively.

we carried out the statistical analysis of the probability distribution of WZ segments. Hereafter, the appearance ratio of WZ segments (P_WZ) is defined as shown here. P_WZ ¼ ðthe number of WZ layersÞ=ðthe total number of layersÞ

where the total number of layers that correspond to resolution is about 200 monolayers. The results show that the density of WZ segments is low at the bottom. The P_WZ is 57%, 79%, and 95% at the bottom, middle, and top part of NW2, respectively (Figure 2). According to the growth results of NW1 and NW3, the crystal symmetry strongly affects the formation of sidewall facets for nanowires. This is because facets, which have lower surface free energy, differ between ZB and WZ. {
110} and {
1100} preferentially appear in ZB and WZ, respectively. At the bottom of the tapered nanowires are high-density ZB segments, so {
110} facets preferentially appear as a sidewall for the hexagonal pillar because of the low surface free energy of ZB. On the other hand, at the top, WZ crystal structure is dominant, so that {
211} facets, which correspond to {
1100} for WZ, easily appear, because the surface free energy of {
211} ({
1100} for WZ) is lower than that of {
110} ({
2110} for WZ) for WZ crystal structure. From these discussions, the cross-sectional shapes for the nanowire are clearly reflected in the distribution of P_WZ along the growth direction, because the surface free

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Figure 3. (a, c) High-magnification of TEM images of sidewall of nanowire and (b, d) schematic illustrations of atomic arrangement of the growth model. (a) and (b) show ZB segments located between WZ segments contribute to the formation of tapered sidewalls, and angle of tapered sidewall is 19.5
from Æ111æ vertical direction to the InP (111)A substrate. (c) and (d) show vertical sidewall toward [
211] is formed at the opposite side of a tapered sidewall on the ZB segment.

energies in each facet vary with the crystal structures of ZB or WZ. This polytypic crystal structure clearly indicates that NW2 corresponds to ZB and WZ structural mixing and transition phase. Next, we propose a possible model for tapering growth of nanowires. Figure 3 shows the high-magnification TEM image of sidewall of the nanowire and structural model of the tapering formation. In panels a and b of Figure 3, in the WZ segment, {
1100} facets are vertical to the substrate, while in the ZB segment, {
111} facets are inclined 19.5
from (111)A. This tilted part of ZB contributes to tapering the shape of nanowires. On the other hand, {
110} vertical sidewalls are observed at the opposite side of inclined sidewalls on ZB segment in panels c and d of Figure 3. This is because once {111}A inverse mesa facets are formed at the sidewalls of ZB segments, lateral growth proceeds until vertical sidewalls are formed. We have proposed a mechanism in which the hexagonal pillar shaped GaAs nanowires are formed while twin boundaries appear in the ZB crystal structure.28 The existence of twin boundaries involves the growth toward the 3-fold symmetry directions of Æ
1
12æ, Æ
12
1æ, and Æ2
1
1æ being promoted from the corner of a tetrahedron, 4316

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Figure 4. (a) PL spectra of nanowires of condition NW1
NW3 and (b) band energy diagram corresponding to the ZB WZ polytypic InP nanowire. The peak position at 1.48 eV of NW2 is located between that at NW1 and NW3. The band gap energy of InP differs by about 84 meV between ZB and WZ crystal structures. When a thin ZB segment is sandwiched between WZ segments, it exhibits type II superlattice structures, and electrons are strongly confined to the ZB regions with 129 meV higher WZ barriers.

which is the initial stage of nanowire growth. Thus, a hexagon pillar nanowire is formed along the vertical to the substrate by piling up twins one on top of another. In the TEM image shown in Figure 1, note that there are many twins in InP nanowires, like GaAs nanowires, for NW1. Hence, ZB InP nanowire growth is contributed to by formation of rotational twins during growth. Here, we assume that same growth mechanism occurs in ZB segments among tapering InP nanowires. When a few monolayers of ZB segments are sandwiched between WZ stacking, the crystal structure become unstable near the boundary. Therefore, on the {110} facets of ZB segment and sidewall facets of WZ segments, many steps are supplied to prompt the lateral growth toward the 3-fold symmetry directions of Æ
1
12æ, Æ
12
1æ, and Æ2
1
1æ. Finally, lateral growth stops when {
111} inclined facets are formed in ZB segments. Therefore, average inclined angle (α) of NW2 can be estimated from the probability of ZB layers. The relationship between α and the P_WZ is expressed in the following equation. tan

α 2

¼

1
P_WZ  tan 2

19:5


In Figure 1, the 3.1 μm high nanowire has about 23% ZB layers on average. When these results are factored into the equation, the average inclined angle is about 4.7
. This result corresponds with measured data of a tapered angle, 4.1
, from a low magnified TEM image. Therefore, this growth model well explains the

formation and crystal structure of tapered nanowires from the crystal growth point of view. Figure 4a shows the results of PL measurements at 4.2 K for NW1
NW3. About 15 nanowires were irradiated by the excitation laser on each measurement. The peak position at 1.48 eV of NW2 is located between 1.42 eV (NW1) and 1.54 eV (NW3). We assume the quantum well of ZB segments among WZ segments in NW2. Figure 4b shows the band alignment between ZB and WZ InP. Along with the 84 meV change in band gap, theoretical considerations have suggested a 45 meV band offset between the ZB and WZ phases in a staggered type II band alignment.20 When a thin ZB segment is sandwiched between WZ segments, the electrons are strongly confined to the ZB regions with 129 meV higher WZ barriers. We calculate the subband separation of conduction band. When the number of layers of ZB regions is 4.3 monolayers on average, the ZB region is 1.7 nm thick, because the thickness of one InP monolayer is about 0.39 nm. The results show the energy level of subband from the bottom of the conduction band is 0.11 eV for every 1.7 nm in thickness of the quantum well. When we assume the indirect transitions from the subband level of conduction band of ZB region to the valence band of WZ region, the energy difference is 1.48
1.49 eV. The calculated value reasonably agrees with the peak energy of tapered nanowires (NW2). Therefore, the reason for the peak energy shift is attributed to the quantum confinement effect of the quantum well of the ZB region of nanowires. Further investigation such as excitation power dependence is required. In summary, we investigated the structural mixing and transition from ZB to WZ of InP nanowires from the dependence on the growth conditions, such as growth temperature and ratio of partial pressure of the supply gas. The crystal structure of InP nanowires can be controlled by changing growth conditions. We also found that under specific growth conditions, tapered nanowires that have polytypic superlattice structure of ZB and WZ were formed. The tapered angle simply reflected the mixing ratio of ZB (19.5
) and WZ (0
). These results clearly show that the InP nanowires lay at the structural transition boundary of ZB and WZ.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ REFERENCES (1) Huang, M. H.; Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. Science 2001, 292, 1897–1899. (2) Ng, H. T.; Han, J.; Yamada, T.; Nguyen, P.; Chen, Y. P. Nano Lett. 2004, 4, 1247–1252. (3) Xiang, J.; Lu, W.; Hu, Y.; Wu, Y.; Yan, H.; Lieber, C. M. Nature 2006, 441, 489–493. (4) Duan, X.; Huang, Y.; Cui, Y.; Wang, J.; Lieber, C. M. Nature 2001, 409, 66–69. (5) Wang, J.; Gudiksen, M. S.; Duan, X.; Cui, Y. Science 2001, 293, 1455–1457. (6) Ding, Y.; Motohisa, J.; Hua, B.; Hara, S.; Fukui, T. Nano Lett. 2007, 7, 3598–3602. (7) Goto, H.; Nosaki, K.; Tomioka, K.; Hara, S.; Hiruma, K.; Motohisa, J.; Fukui, T. Appl. Phys. Express 2009, 2, 5004. (8) Minot, E. D.; Kelkensberg, F.; Kouwen, M. van; Dam, J. A. van; Kouwenhoven, L. P.; Zwiller, V.; Borgstr€om, M. T.; Wunnicke, O.; Verheijen, M. A.; Bakkers, E. P. A. M. Nano Lett. 2007, 7, 367–371. 4317

dx.doi.org/10.1021/nl202365q |Nano Lett. 2011, 11, 4314–4318

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(9) Fuhrer, A.; Fr€oberg, L. E.; Pedersen, J. N.; Larsson, M. W.; Wacker, A.; Pistol, M.-E.; Samuelson, L. Nano Lett. 2007, 7, 243–6. (10) Zwiller, V.; Akopian, N.; Weert, M. van; Kouwen, M. van; Perinetti, U.; Kouwenhoven, L.; Algra, R.; Gomez Rivas, J.; Bakkers, E.; Patriarche, G.; Liu, L.; Harmand, J.-C.; Kobayashi, Y.; Motohisa, J. C. R. Phys. 2008, 9, 804–815. (11) Mohan, P.; Motohisa, J.; Fukui, T. Appl. Phys. Lett. 2006, 88, 133105. (12) Duan, X.; Lieber, C. M. Adv. Mater. 2000, 12, 298–302. (13) Bhunia, S.; Kawamura, T.; Fujikawa, S.; Nakashima, H.; Furukawa, K.; Torimitsu, K.; Watanabe, Y. Thin Solid Films 2004, 464, 244–247. (14) Mattila, M.; Hakkarainen, T.; Mulot, M.; Lipsanen, H. Nanotechnology 2006, 17, 1580–1583.  (15) Bao, J.; Bell, D. C.; Capasso, F.; Wagner, J. B.; Martensson, T.;  Tr€agardh, J.; Samuelson, L. Nano Lett. 2008, 8, 836–841. (16) Algra, R. E.; Verheijen, M. A.; Borgstr€om, M. T.; Feiner, L. F.; Immink, G.; van Enckevort, W. J. P.; Vlieg, E.; Bakkers, E. P. A. M. Nature 2008, 456, 369–372. (17) Akopian, N.; Patriarche, G.; Liu, L.; Harmand, J.-C.; Zwiller, V. Nano Lett. 2010, 10, 1198–1201. (18) Pemasiri, K.; Montazeri, M.; Gass, R.; Smith, L. M.; Jackson, H. E.; Yarrison-Rice, J.; Paiman, S.; Gao, Q.; Tan, H. H.; Jagadish, C.; Zhang, X.; Zou, J. Nano Lett. 2009, 9, 648–654. (19) Kitauchi, Y.; Kobayashi, Y.; Tomioka, K.; Hara, S.; Hiruma, K.; Fukui, T.; Motohisa, J. Nano Lett. 2010, 10, 1699–1703. (20) Murayama, M.; Nakayama, T. Phys. Rev. B 1994, 49, 4710–4724. (21) Zhang, L.; Luo, J. W.; Zunger, A.; Akopian, N.; Zwiller, V.; Harmand, J. C. Nano Lett. 2010, 10, 4055–4060. (22) Akiyama, T.; Sano, K.; Nakamura, K.; Ito, T. Jpn. J. Appl. Phys. 2006, 45, L275–L278. (23) Mishra, A.; Titova, L.; Hoang, T.; Jackson, H.; Smith, L.; Yarrison-Rice, J.; Kim, Y.; Joyce, H.; Gao, Q.; Tan, H.; Jagadish, C. Appl. Phys. Lett. 2007, 91, 263104. (24) Kobayashi, Y.; Fukui, M.; Motohisa, J.; Fukui, T. Phys. E (Amsterdam, Neth.) 2008, 40, 2204–2206. (25) Motohisa, J.; Noborisaka, J.; Takeda, J.; Inari, M.; Fukui, T. J. Cryst. Growth 2004, 272, 180–185. (26) Noborisaka, J.; Motohisa, J.; Hara, S.; Fukui, T. Appl. Phys. Lett. 2005, 87, 093109. (27) Dick, K. A.; Caroff, P.; Bolinsson, J.; Messing, M. E.; Johansson, J.; Deppert, K.; Wallenberg, L. R.; Samuelson, L. Semicond. Sci. Technol. 2010, 25, 024009. (28) Ikejiri, K.; Sato, T.; Yoshida, H.; Hiruma, K.; Motohisa, J.; Hara, S.; Fukui, T. Nanotechnology 2008, 19, 265604.

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