Letter pubs.acs.org/NanoLett
Bidirectional Growth of Indium Phosphide Nanowires Keitaro Ikejiri,*,† Fumiya Ishizaka,† Katsuhiro Tomioka,†,‡ and Takashi Fukui† †
Graduate School of Information Science and Technology, and Research Center for Integrated Quantum Electronics, Hokkaido University, North 13, West 8, Sapporo, 060−8628, Japan ‡ Japan Science and Technology Agency-PRESTO, Kawaguchi 332−0012, Japan S Supporting Information *
ABSTRACT: We present a bidirectional growth mode of InP nanowires grown by selective-area metalorganic vapor-phase epitaxy (SA-MOVPE). We studied the effect of the supply ratio of DEZn ([DEZn]) on InP grown structure morphology and crystal structures during the SA-MOVPE. Two growth regimes were observed in the investigated range of the [DEZn] on an InP(111)B substrate. At low [DEZn], grown structures formed tripod structures featuring three nanowires branched toward the [111]A directions. At high [DEZn], we obtained hexagonal pillar-type structures vertically grown on the (111)B substrate. These results show that the growth direction changes from [111]A to [111]B as [DEZn] is increased. We propose a growth mechanism based on the correlation between the incident facet of rotational twins and the shapes of the grown structures. Our results bring us one step closer to controlling the direction of nanowires on a Si substrate that has a nonpolar nature. They can also be applied to the development of InP nanowire devices. KEYWORDS: Nanowire, selective-area metalorganic vapor-phase epitaxy, crystal structure, InP, zinc doping, growth mechanism
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InP(111)B substrates; that is, these growth directions are [111]B,16,17 which is completely opposite to the nanowires grown by SA-MOVPE. These reports imply that InP nanowire growth has a “bidirectional” property. Understanding and controlling the bidirectional mechanisms of InP nanowires are key challenges for application to future electrical/optical devices because these devices require well-aligned nanowires vertically grown on substrates. Moreover, these mechanisms might be a significant component of the formation of III−V nanowires on Si substrates, which are promising for use as building blocks in next-generation III−V/Si-integrated devices. Si has a nonpolar nature, that is, both (111) oriented surfaces can exist in terms of III−V nanowire growth. It is difficult but imperative to control the growth direction of III−V nanowires on Si substrates because of this polar/nonpolar nature.18 However, no systematic approaches for clarifying the bidirectional property have been available for understanding the effects of growth conditionsincluding material supply ratio, growth temperature, and supply amount of the dopanton nanowire formation, until now. In VLS growth, an effect of diethylzinc (DEZn) on the crystal structure of InP has been reported.22 Here, we report that the InP growth direction can be controlled by supplying DEZn as a dopant via SA-MOVPE and propose a mechanism of bidirectional growth behavior.
ecently, free-standing semiconductor nanowires have been produced by using crystal growth techniques, which has attracted considerable interest due to the potential application of the nanowires in various nanoscale devices.1,2 In particular, indium phosphide (InP)-based nanowires are promising building blocks for future nanoscale electrical and optical devices because of their superior material properties and the possibility of developing various kinds of heterostructures. Until now, InP-based nanowire devices including field effect transistors,3 photodetectors,4 light-emitting devices,3 and solar cells5 have been demonstrated, and heterostructures including quantum dots6−8 and core−shell structures9 have been reported. We have previously reported on the catalyst-free growth of III−V semiconductor nanowires by selective-area metalorganic vapor-phase epitaxy (SA-MOVPE).10,11 We have found that the growth mode, morphology, shape, and crystal structures of InP nanowires can be controlled by controlling the growth conditions, such as growth temperature and V/III ratio.12−15 As for the growth direction, InP nanowires preferentially grow in the A direction by SA-MOVPE, and vertically aligned InP nanowires have been grown on an InP (111)A substrate.13 We have also found that the growth behavior of InP crystals is quite different between (111)A- and (111)B-oriented substrates.15 For example, on a (111)B surface, the InP nanowires tend to shape a three-fold symmetric tripod grown in three equivalent 19.5° tilted to the (111)B surface in each rod. However, by using the vapor−liquid−solid (VLS) method, which is currently the most popular approach to producing nanowires, vertical InP nanowires have been formed on © 2012 American Chemical Society
Received: June 11, 2012 Revised: July 23, 2012 Published: August 13, 2012 4770
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Figure 1. InP grown structures on InP (111)B substrates under 0 and 2.2 of II/III ratio ([DEZn]/[TMIn]) conditions: (a,d) 45° tilt views of SEM images; (b,e) top views of SEM images, showing different growth directions and side facets; and (c,f) schematic illustrations of grown structures. Tripod structures toward [111]A direction are mainly obtained under II/III ratio = 0 condition, while hexagonal pillar structures toward B direction are obtained under II/III ratio = 2.2 condition.
After a 20 nm-thick SiO2 layer is formed on a semiconductor substrate, the SiO2 layer is partially removed using electron beam lithography and a wet chemical etching technique to form mask openings as a template for the SA-MOVPE. Crystal growth starts only at the opening area of the mask where the bare semiconductor surface of the substrate is exposed. It is possible to control the size and position of the grown nanowires because the growth is inhibited in the SiO2 mask area. For InP, we used trimethylindium (TMIn) and tertiallbutylphosphine (TBP) as the source materials and InP(111)A and (111)B as the growth substrates. We investigated the dependence of growth mode on the substrate orientations using both (111)A and (111)B substrates at the same growth run. DEZn was used as a p-type dopant. We defined a partial pressure ratio of DEZn to TMIn as a II/III ratio, which we varied from 0 to 2.2 in this experiment. We analyzed the grown structures with a scanning electron microscope (SEM) and transmission electron microscope (TEM). Figure 1 shows SEM images of InP structures grown under different II/III ratios on (111)B substrates. (A summary of InP structures grown on (111)A and (111)B substrates is shown in Supporting Information S1.) The grown structures were dramatically changed by introducing DEZn. On the InP (111)B substrates, under low II/III ratio conditions, we
obtained tripod structures that had three nanowires pointing toward three-fold symmetric [111]A directions. A similar tripod structure has already been reported on InP15 and II−VI materials.19,20 The growth mode dramatically changed under a high II/III ratio condition, and hexagonal pillar structures with vertical sidewalls were formed in the towered B direction. These pillar structures have six-fold side facets equivalent to {−110}. These results indicate that the “bidirectional” behavior, in which the growth directions of the hexagonal pillar structures switch from [111]A to [111]B, can be observed simply by changing the DEZn concentration. Figure 2 shows an appearance ratio of tripod, hexagonal pillar, and irregular polycrystalline structures as a function of II/III ratios. Under the condition of no DEZn (II/III = 0), almost all grown structures are tripod shapes; however, as the Zn supply is increased, the ratio of hexagonal pillar structures increases. Under intermediate conditions, around II/III = 0.6, we obtained many irregular polycrystalline structures, which indicate that the crystal growth mode is located at the structural transition boundary from tripod to hexagonal pillar structures. Figures 3 and 4 show high-resolution TEM images and a selected area electron diffraction (SAED) pattern for tripod and hexagonal pillar structures at II/III ratios of 0 and 2.2, respectively. The crystal structures of the tripod and hexagonal 4771
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Figure 4. (a) Cross-sectional TEM image of hexagonal pillar structures obtained under 2.2 of II/III ratio condition. (b) High-resolution TEM images and SAED. Arrows indicate positions of rotational twins. Inset scale bar: 10 nm. Electron beam projection is parallel to directions.
Figure 2. Appearance ratio of each grown structure plotted as a function of II/III ratio. A transition phase from the tripod to the hexagonal pillar structure is observed around II/III = 0.6. The phase transition indicates that the incident facet of twins is changed from (111)A to (111)B when II/III ratio is increased.
agreement with vertical InP nanowires grown on the (111)A substrate under the same conditions.12 This means that both wire structures obtained on (111)A and (111)B are based on the same growth mode regardless of the orientation of the substrates. Moreover, there are a few rotational twins near the interface between the substrate and the grown structure. This means that, in the initial stages of crystal growth, a rotational twin will occasionally appear, and after that, a stable ZB crystal island without any twins can be grown successively. This is why tripod structures with inverse directions can be seen in the SEM images (Figure 1a). Next, we propose a possible growth model for tripod structures obtained at low II/III ratios. We assume that the tripod growth starts on three {111}A slanted sidewall surfaces of truncated tetrahedral structures with an (111)B top because we know from the TEM observations that the boundary of the island and wire areas are (111)A surfaces in the tripod structure. It is easy to form rotational twins on the (111)A plane under a non-doped condition because InP nanowires grown on InP(111)A substrates under similar growth conditions contain many rotational twins.12 During the formation of a truncated tetrahedral structure, tripod wire structures also start to grow on {111}A-inclined sidewalls by introducing rotational twins (see Supporting Information S2). Figure 5 shows schematic illustrations of the truncated tetrahedral structures (a), the rotational twins on the {111}A sidewalls (b), and those on the (111)B top surface (c). In the initial stage of growth of the wire structure on the (111)A, when a rotational twin is formed, the crystal axis is rotated 60° around [111]A, and the bonds are separated from each other at the ridgeline of the tetrahedral structures (Figure 5b). This bond separation makes it difficult to absorb the growth materials in the ridgelines of the tetrahedron, which results in the formation of tripod structures because the growth structure is branched to three directions along [111)A. In contrast, under highly Zn-doped conditions, we obtained hexagonal pillar structures with crystal structures that were mainly ZB structures. We have previously proposed a mechanism in which hexagonal pillar-shaped GaAs nanowires are formed while twin boundaries appear in the ZB crystal
Figure 3. (a) Cross-sectional TEM image of whole tripod structures obtained under II/III ratio = 0 condition. The blue dashes indicate an island area. (b) TEM image and SAED of boundary area between island and wire areas. Arrow indicates position of rotational twins. Inset scale bar: 5 nm. (c,d) High-resolution TEM images of island and wire areas, respectively. Inset scale bar: 5 nm. Electron beam projection is parallel to directions.
pillar structures are clearly different. Those of the tripod can be divided into two areas, island and wire areas, located at the center and outer of the structures, respectively. The crystal structure of the island area is a zincblende (ZB) structure without rotational twins, while the wire area has mainly a wurtzite (WZ) structure with partially observed ZB segments. The TEM images show that the probability of the WZ structure is approximately 85%. This crystal structure is in good 4772
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Figure 5. Schematic illustrations of atomic arrangements and grown structures of the (a) truncated tetrahedral structures, (b) that with rotational twins on {111}A sidewalls, and (c) that on (111)B top surface. Blue and red dashed lines in (b,c) show the rotational twins on {111}A and (111)B planes, respectively.
structure.11 The existence of twin boundaries on an (111)B surface involves growth toward the three-fold symmetry directions of , , and being promoted from the corner of a tetrahedron, which is the initial stage of grown structure. Finally, lateral growth stops when {−110} vertical facets are formed. Thus, a hexagonal pillar structure is formed along the vertical direction to the substrate by piling up twins one on top of another.21 In the TEM image shown in Figure 4b, several twins in InP hexagonal pillar structures were observed, similar to GaAs nanowires. This demonstrates that the hexagonal pillar growth of Zn-doped InP on the (111)B substrate is affected by the formation of rotational twins on (111)B during growth, as shown in Figure 5c. We concluded that the key factor of the bidirectional phenomenon can be explained by the orientation of facet that rotational twins is likely to occur on (111)A (non-doped) or (111)B (Zn-doped) surfaces. Finally, we discuss the mechanism behind the appearance of rotational twins as it relates to surface orientation, including (111)A and B. As mentioned earlier, the occurrence facet of twins is switched from (111)A to (111)B when we increase the Zn supply ratio. The reason for this phenomenon is the increase in the effective V/III ratio (material supply ratio of TBP and TMIn) with increasing partial pressure of DEZn. Zn and In precursors are competitively adsorbed on the group III lattice sites of InP crystal structures and Zn atoms which are incorporated into the crystal site of In act as a p-type acceptor dopant. By increasing the II/III ratio, Zn precursors preferentially adsorb onto the InP surface and obstruct the adsorption of In precursors, and as a result, the effective V/III ratio is relatively high (low) under a high (low) Zn supply ratio. The effect of V/III ratio and growth temperature on the crystal structures of InP nanowires has previously been discussed on InP(111)A surfaces.12,14 Here, we expand these discussions to (111)B and propose a model that is consistent for both (111)A and (111)B, as shown in Figure 6. To start with, we point out that the termination of the surface with constituent atoms depends on the growth conditions related to the effective V/III ratio. Let us recall that the difference between WZ and ZB is in the relative position of the second-nearest pair of groups of III
Figure 6. Atomistic models for structural transition in InP grown structures. Side (top row) and top (bottom row) views of atom arrangements in (a,d) WZ and (b,c) ZB. A represents adsorption sites of In (a,b) and P (c,d) for WZ stacking, and B represents sites for ZB stacking. (a) If surface is terminated with In and desorption of P is significant, additional In atoms are needed for growth to proceed in [111]A direction. Binding of In at site A is stronger than at site B due to larger Coulomb interaction through second-nearest pair of In and P (enclosed with blue dotted circles, and indicated by arrows). As a result, WZ stacking is favorable. On the other hand, P atoms preferentially sit on site B as the ZB of InP bulk crystal structures without rotational twin if surface is terminated with P, as schematically shown in (b).
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and V atoms. For example, in WZ, group III (V) atoms sit atop the second-nearest group V (III) atoms (site A), as shown in Figure 6a,d. This gives rise to an enhanced Coulomb interaction in the second-nearest pairs compared with site B of the ZB stacking. Under low II/III ratio conditions, the supply of In is sufficient compared with highly doped conditions, and this condition is an effective low V/III ratio condition. For growth on the (111)A surface, the P atoms could be desorbed easily, and thus the (111)A surface is terminated with In, and simultaneous In adsorption is required to enable P to be incorporated into the crystal. Moreover, the binding is stronger if In atoms sit on site A (Figure 6a) due to the influence of the Coulomb interaction with the third-nearest neighbor P atoms. Hence, WZ stacking is preferentially formed on the (111)A surface under non-doped conditions. In such a situation, a rotational twin is likely to appear on the (111)A surface because the twin plane in a ZB structure can be considered a monolayer of the WZ structure. In contrast, for growth on the (111)B surface to proceed on the P-terminated surface, the surface must be terminated with more In because the supply of In is relatively sufficient (Figure 6c). In such a situation, P atoms preferentially sit on site B as the ZB of InP bulk crystal structures without rotational twins. This difference of atomic structure formation on (111)A and (111)B surfaces results in the separating growth direction to [111]A and the formation of tripod structures on the (111)B substrate. In contrast, under high II/III ratio conditions, we should take into account that the effective V/III ratio is relatively high due to the introduction of Zn, and In atoms are difficult to absorb due to the existence of sufficient Zn atoms compared with low II/III ratio conditions. On the (111)A surface, In atoms preferentially sit on site B as the ZB structure due to the high surface coverage ratio of P atoms (Figure 6b). In contrast, on the (111) B surface, P atoms preferentially sit on site A due to the Coulomb interaction in the second-nearest pairs (Figure 6d). As a result, under highly doped conditions, rotational twins are likely to form only on the (111)B surface, and hexagonal pillar structures grow on the (111)B substrate. The promotion of lateral growth under high II/III ratio conditions can be explained by the higher effective V/III ratio due to the existence of Zn. In summary, we analyzed the effect of Zn doping during InP crystal growth from the viewpoint of the appearance of shapes and the structure of the grown crystals. Bidirectional behavior of InP growth was observed within the investigated range of the DEZn supply ratio on the InP(111)B substrate. We proposed a growth model for the bidirectional behavior and showed that the bidirectional phenomenon can be explained by the relationship between the substrate orientation and the orientation of facet that a rotational twin is likely to occur. We also found that the twin incident facet can be controlled by the DEZn supply ratio. These results bring us one step closer to controlling nanowire direction on Si substrates and ultimately to developing better InP nanowire devices on Si.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by a Grant-in-Aid for Scientific Research provided by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. The authors would like to thank to Prof. Junichi Motohisa, Prof. Shinjiro Hara, Dr. Young Joon Hong, and Mr. Shogo Yanase for stimulating discussions, and Mr. Masatoshi Yoshimura, Mr. Eiji Nakai, and Mr. Takahito Endo for helping with the MOVPE experiments.
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ASSOCIATED CONTENT
S Supporting Information *
Experimental details, SEM images of grown structures on InP(111)A and B (S1), and early stage of tripod structures (S2). This material is available free of charge via the Internet at http://pubs.acs.org. 4774
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