Nonpolar GaAs Nanowires Catalyzed by Cu5As2: Insights into As Layer Epitaxy.

Controlled synthesis of GaAs nanowires (NWs) with specific phases and orientations is important and challenging, which determines their electronic performances. Herein, single-crystalline GaAs NWs are successfully synthesized by using complementary metal-oxide semiconductor compatible Cu2O catalysts via chemical vapor deposition at an optimized temperature of 560 °C. In contrast to typically Au catalyzed GaAs NWs, the Cu2O catalyzed ones are found to grow along nonpolar orientations of zincblende <110> and <211> and wurtzite <1̅100> and <2̅110>. The Cu2O catalysts are found to change into orthorhombic Cu5As2 after the NW growth, which is also significantly distinguished from the Au-Ga catalyst alloy in the literature. The Cu5As2 alloy plays the epitaxy role in the nonpolar GaAs NW growth due to the lattice matching with the nonpolar planes of GaAs, which is verified by the atomic stack model. These nonpolar oriented GaAs NWs have minimized stacking faults, promising for the other semiconductor synthesis as well as electronic applications.

Introduction

In the past decade, one-dimensional (1D) semiconductor nanowires (NWs) such as gallium arsenide (GaAs) and gallium antimonide (GaSb) have attracted great attention as fundamental building blocks for electronic, power generation, conversion and storage devices, etc.[1−6] due to the unique electronic and optical properties. Their performances are highly affected by their crystal phases and growth orientations; for example, the wurtzite (WZ) domains in zincblende (ZB) InAs NWs would act as electron scattering centers, which evidently degrade the electron mobility.[7] The <111> oriented GaAs NWs show relatively higher photon-to-electricity conversion efficiency than the <110> ones due to the different surface Fermi level pinning effect on varied facets.[8] Furthermore, the ZB<110> and ZB<211> GaSb NWs show relatively lower hole mobilities compared with the <111> ones due to the varied surface roughnesses.[6] Therefore, it is necessary to control the growth direction of NWs in the synthesis process for their wide technological applications.

In the literature, Au was typically utilized as the catalyst for the synthesis of high-quality semiconductor NWs with excellent yields via the vapor–liquid–solid (VLS) mode,[9] and the growth orientation can be tuned by the substrate orientation and/or by the tailored diameter due to the varied surface/interface energy. For example, the orientations of GaAs NW are highly dependent on the orientation of the adopted single-crystalline GaAs substrate.[10,11] Furthermore, the crystal phase can be tuned by the V/III ratio in molecular beam epitaxy.[12] It should also be noted that the atomic alignments are the same in the hexagonal WZ {0001} plane and ZB {111} plane in which the plane stacking is different, i.e., “...” for ZB and “abab...” for WZ. Therefore, both the <0001> and <111> NWs will have mixed crystal phases having a detrimental effect on the application performance of NW-based optoelectronic devices and corresponding studies have been developed to explore high-quality pure WZ GaAs NWs.[13,14] Unfortunately, Au is notorious for its complementary metal-oxide semiconductor (CMOS) incompatibility, not only forming foreign impurities and deep level dopants but also degenerating the electronic properties.[15−17] Hence, the use of an Au catalyst may restrict the deployment of these NWs for large-scale semiconductor device integration.[15,18,19] Therefore, it is crucial to explore alternative means to synthesize CMOS-compatible NWs with well-controlled physical properties.

To make the III–V NWs more CMOS-compatible, new catalysts such as Ni, Ag, Pd, and Cu materials are evolved to obtain semiconductor NWs in VLS and/or VSS modes (vapor–solid–solid).[20−22] For example, Arbiol et al. synthesized Si NWs with Cu as a catalyst via the VSS process with arbitrary NW diameters.[23] In our previous study, a catalyst epitaxy strategy was adopted to synthesize single-crystalline III–V NWs with a tuned crystal phase and orientations with the III group atomic epitaxy in the catalyst/NW interface.[21] Furthermore, <1̅100> and <2̅110> oriented nonpolar InP NWs can be synthesized via In atomic epitaxy in the PdIn/InP interface with a high electron mobility of >2000 cm2/V s.[24] However, most catalysts are prepared by annealing the e-beam or thermal deposited metal films, which induces a randomly distributed catalyst particle size.[25] Furthermore, Renard et al. evidenced the complete oxidation of the original copper-based seed layer into Cu2O under the optimal oxygen pressure for the obtained Si NWs with morphological evolution of their tips in an ambient air.[17] Herein, CMOS-compatible chemically synthesized uniform Cu2O catalysts are used for the crystal phase and orientation tuning of GaAs NWs grown on a noncrystalline SiO2 substrate. Other than the common ZB nonpolar <110> GaAs NWs, WZ nonpolar <1̅100> and <2̅110> NWs are also synthesized. Characterizations infer that the growth mechanism is the V group (As) atomic epitaxy growth of these NWs by the Cu5As2 catalyst derived from Cu2O nanoclusters. These results show the promise of the NW orientation tuning by the V group atomic epitaxy growth in III–V NW synthesis.

Experimental Section

Cu2O Catalyst Synthesis

The Cu2O nanocubes were synthesized according to the literature,[26] as detailed in the Supporting Information. Typically, 0.015 M sodium dodecyl sulfate (SDS; 99%, J.T. Baker) solution was mixed with 0.1 M CuSO4 and 1.0 M NaOH to obtain the Cu(OH)2 precipitate. Then, 0.2 M l-(+)-sodium ascorbate solution was added to reduce Cu(OH)2 into Cu2O, with particle sizes of 25, 53, 63, and 86 nm by varying different precursor ratios, as listed in Table S1 in the Supporting Information. The particles were collected by centrifugation and washed three times with an ethanol and water mixture (1:1 volume ratio).

GaAs NW Growth

A dual-zone horizontal tube furnace, one zone for the solid source (upstream) and one zone for the NW growth (downstream), was used as the reactor for the synthesis of GaAs NWs. In detail, the solid source (0.5 g GaAs powder, 99.9999% purity) was evaporated at the center of the upstream zone, while the growth substrate (various chemically synthesized Cu2O as the catalyst dispersed on SiO2/Si) was placed in the middle of the downstream zone with a tilt angle of ∼20° and a distance of 10 cm away from the source. After the system was pumped down to ∼10–3 Torr, H2 gas (100 standard cm3/min or sccm, 5% in Ar) was provided through control of the mass flow controller for 30 min, and then, the substrate zone began to ramp up to 450–640 °C at a rate of 60 °C/min. During the NW growth, the source was heated to the required temperature (800 °C), while the substrate was controlled at various growth temperatures. After 30 min of growth, the source and substrate heaters were stopped together and cooled to room temperature under the flow of Ar/H2 gas. Finally, NWs can be harvested in the substrate for further characterization.

Characterization of GaAs NWs

Surface morphologies of the grown NWs were examined with scanning electron microscopy (SEM; JEOL JSM-6700F, Japan, 15 kV, 10 mA) and high-resolution transmission electron microscopy (HRTEM) images, and corresponding energy-dispersive spectroscopy (EDS) of NWs were obtained on a JEOL JEM-2100F microscope with an accelerating voltage of 200 kV. For the elemental mapping and TEM, the GaAs NWs were first suspended in the ethanol solution by ultrasonication and drop-casted onto a nickel grid for the corresponding characterization.

Results and Discussion

The chemically synthesized Cu2O nanocubes are adopted as the catalyst for GaAs NW growth, which have varied particle sizes of 25–177 nm by tuning the synthesis conditions, as shown in Table S1 and Figure S1 in the Supporting Information. Furthermore, the typical 25 nm Cu2O nanocubes are shown in the SEM images in Figure a, where a uniform particle size distribution is shown in the particle size histogram with a standard deviation of 3.95 nm shown in Figure b. At a higher temperature of 800 °C, the GaAs powders are decomposed into Ga and As4 precursors, with a high As/Ga ratio because of the higher vapor pressure of As.[27] The precursors are then transported by H2 gas into the growth catalyst. The growth temperature is first optimized using the 25 nm Cu2O catalyst, as shown in the SEM images of GaAs NWs grown at various temperatures from 450 to 640 °C in Figure c–f and Figure S2. It is clear that 560 °C is the optimum temperature, where the yielded GaAs NWs have the most uniform surface morphology and length. At low temperatures from 450 to 520 °C (Figure S2a–c), the catalysts are insufficiently supersaturated with the Ga and As precursors, leading to minimized nucleation and low growth rates. On the other hand, the twists, bends, and lateral growth structures are favored when the growth temperatures increase to 620–640 °C (Figure S2d,e). These results can be explained by the Gibbs–Thomson model in eq (28)where v is the growth rate, b is a kinetic coefficient of crystallization, k is Boltzmann’s constant, Ω is the atomic volume of the growth species, Δμ0 is the supersaturation in the planar limit, avs is the average surface energy density of the NW surface facets, and T is the temperature. A higher temperature would favor As diffusion into the catalyst and thus improve Δμ. It is therefore that the growth rate is proportional with Δμ and inversely proportional to the growth temperature, and thus, the optimum growth temperature would be a compromise of the two factors.

Figure 1

SEM surface images of Cu2O nanocube catalysts and GaAs NWs grown at various temperatures. (a) Cu2O nanocubes, (b) Cu2O size distribution, and GaAs grown at (c) 540, (d) 560, (e) 580, and (f) 600 °C. The scale bars are 200 nm.

Quantitatively, the relationship between the NW length, density, and growth temperature is plotted in Figure b, where it is clear that a 6 μm length and 90 NW/μm2 NW density can be obtained at the optimal 560 °C growth temperature. To shed light on how the varied sizes of Cu2O catalysts can affect the GaAs NW diameter, the NW diameter distributions are compared in detail in Figure a. It is noted that there is a limited size dependence of the NW diameter (16–20 nm) on the catalyst size (25–177 nm). This result would be attributable to the limited dissolubility (Cd) of precursors in the large-size catalyst, as shown in the inverse relationship of Cd with the catalyst size (d) by ln(Cd/C0) = 4γVm/(dRT), where C0 is the equilibrium concentration in the flat surface (d → ∞), γ is the surface energy, Vm is the molar volume of the catalyst, R is the constant, and T is the growth temperature. Furthermore, this phenomenon is also observed in GaAs growth catalyzed by thick Au catalysts.[11,29]

Figure 2

(a) Relationship between the NW diameter grown at 560 °C and the average size of the catalyst. (b) Change of the GaAs NW growth length and density with different temperatures. (c) Number statistics of grown WZ-GaAs NW directions. (d) Growth direction statistics of grown GaAs NWs. The NWs in the diameter range of 20–30 nm are mostly grown in the <1̅100>, <2̅110>, and <110> directions while only having little mixed orientations of ZB<211> and ZB<311>.

The crystal phase and growth orientations of GaAs NWs are then investigated in detail. Both ZB and WZ NWs are observed in the HRTEM observations, as typically shown in Figures S3 and S4. X-ray diffraction (XRD) is performed, as shown in Figure S5. Two dominant diffraction peaks are found at 2θ angles of 27.30 and 45.41° without considering the substrate peaks. It should be noted that most of the ZB GaAs NWs are in the nonpolar <110> and <211> orientations, with no commonly reported polar <111> NWs and a little amount of polar <311> NWs observed. More importantly, all the observed WZ GaAs NWs are grown along nonpolar orientations of <1̅100> and <2̅110>, without any commonly reported polar <0001> NWs. The NW growth orientation statistics are then plotted in Figure c, where it is clear to see that of the 51 NWs observed, majority (34 NWs) are in the WZ phase along nonpolar orientations, while the minority are in the ZB phase. Furthermore, in contrary to the Au catalyzed Si and GaAs NWs, which grow along different phases and orientations at different diameters, the GaAs NW phase and orientation here shows no significant diameter dependence, as shown in Figure d.

To further dig out the growth mechanism of these nonpolar GaAs NWs, EDS mapping of one typical GaAs NW is carried out, as shown in Figure . It can be noticed that the Ga and As elements are distributed along the NW, while the Cu and As elements are concentrated on the catalyst tip, inferring a catalyst-seeded VLS or VSS growth mechanism. There is also a small content of oxygen, as shown in Figure S6, which might be attributed to the surface oxidation of the NWs. Then, HRTEM images are explored in the catalytic tip and NW body. It is noted that there are Cu5As2 alloy catalyst seeds clearly observed at the tip region of the GaAs NW in Figures and 5. The Cu5As2 phase has a melting point of about 709 °C, much higher than the growth temperature of 560 °C, inferring the solid state in the growth and thus favoring the VSS growth mechanism. It should be also noted that this Cu–As catalyst is distinguishingly different from the previously observed M–Ga alloy catalysts in GaAs NW growth such as Au–Ga, Ni–Ga, etc., which infers that the As elements might play an important role in the epitaxial growth of the GaAs NWs.

Figure 3

EDS mapping of GaAs NWs grown at 560 °C. The scale bars are 50 nm.

Figure 4

HRTEM images and the corresponding fast Fourier transform (FFT) images (inset) of the top and body regions of representative GaAs NWs. (a) WZ GaAs {2̅110} | Cu5As2{1̅10} and (b) WZ GaAs {1̅100} | Cu5As2{240}. Schematic illustration of the proposed atomic modeling. As-plane As atom alignments of the catalytic seed/NW interfaces of (c) Cu5As2 {110} | WZ GaAs {2̅110} and (d) Cu5As2 {240} | WZ GaAs {1̅100} showing the minimal lattice mismatch.

Figure 5

HRTEM images and the corresponding FFT images (inset) of the top and body regions of representative GaAs NWs. (a) ZB GaAs {311} | Cu5As2 {100} and (b) ZB GaAs {110} | Cu5As2 {240}. Schematic illustration of the proposed atomic modeling. As-plane As atom alignments of the catalytic seed/NW interfaces of (c) Cu5As2 {100} | ZB GaAs {311} and (d) Cu5As2 {240} | ZB GaAs {110} showing the minimal lattice mismatch, making the <110> orientated and <311> orientated GaAs NWs relatively easy to be epitaxially grown from Cu5As2<240> and Cu5As2<100> tips.

The catalyst/NW interfaces are then further explored to investigate the role of the As elements. As shown in Figure a and Figure b, the catalyst tips are identified as Cu5As2 in the orthorhombic phase, with the seed/NW interface orientation relationship determined to be Cu5As2<1̅10>||GaAs<2̅110> and Cu5As2<240>||GaAs<1̅100>, respectively. At the same time, the atomic arrangement of As in the Cu5As2 {1̅10} and {240} planes and in the <2̅110> and <1̅100> oriented GaAs crystals are shown in Figure c and Figure d, respectively, exhibiting excellent epitaxial characteristics with the smallest in-plane lattice mismatches (calculated as (aNW – aCat)/aNW × 100%)) of −5.3 (x axis) and 17.5% (y axis) and 4.5 (x axis) and −5.8% (y axis), as shown in Table . However, the lattice mismatches between typical Cu5As2 {240}, {110}, {011}, and {001} planes and WZ GaAs {0001} are as large as 35.4% and even 74.4%, as shown in Table and Figure S7, resulting in no WZ GaAs NW growth along this polar <0001> direction. It is also noted that a certain NW is not straight with some kinks, which might be attributed to the change of growth orientations due to similar epitaxy planes in the Supporting Information (Figure S8).

Table 1

Calculation and Comparison of the Lattice Mismatch (%)a,b

	NW body
	WZ GaAs {1̅100}		WZ GaAs {2̅110}		WZ GaAs {0001}		ZB GaAs {110}		ZB GaAs {311}		ZB GaAs {111}		ZB GaAs {211}
catalyst seed	x	y	x	y	x	y	x	y	x	y	x	y	x	y
Cu5As2{240}	–5.3	17.5	25.7	17.8	–5.5	–35.4	–6.8	2.6	38.3	18.2	6.5	27.18	–6.92	43.9
Cu5As2{110}	–35	–5.8	–4.5	–5.8	–35.4	–74.4	–76.8	2.8	–2.1	–37.4	–37.4	–76.8	–37.4	27.8
Cu5As2{011}	–47.5	–5.8	–3.9	–5.8	–47.9	–74.4	–48.5	–25.4	1.4	–76.8	–50	–76.8	–50.0	27.8
Cu5As2{100}	–42.8	42.0	0.70	42	–42.9	4.3	–2.8	–2.6	16.4	3.1	–44.8	3.2	–44.9	60.5

Note that the lattice mismatch can be calculated by the equation of [(aNW – aCat)/a NW × 100%], where “a” represents the lattice constant of each described species.

The As atom alignment difference between the Cu5As2 catalyst seed and the GaAs NW existed in both WZ and ZB structures.

Meanwhile, similarly, nonpolar ZB<110> GaAs NWs are also obtained, as shown in Figure b, with a seed/NW interface of Cu5As2 {240}||GaAs<110>. Again, these interface lattice mismatches (−6.8% in the x axis and 2.6% in the y axis) are small enough to enable the catalyst epitaxy growth (Figure d). Due to this small lattice mismatch between ZB GaAs {311} and Cu5As2 {100} (16.4% in the x axis and 3.1% in the y axis), a small amount of polar <311> NWs is also obtained (Figure a,c). From Table and Figure S7a, it is also clear that the lattice mismatches of Cu5As2 planes with GaAs {111} are so large as not to lead any ZB GaAs<111> NW growth. Medium lattice mismatch exists between Cu5As2 {110} and GaAs {211}, which might lead to some <211> NW growth, which however still needs further identification from the HRTEM observations of the interfaces.

The catalyst and GaAs NW orientations are then listed and compared, as shown in Table . It is clear that, by using both CMOS-incompatible Au and other CMOS-compatible Pd, Ni, and Ag as a catalyst, the GaAs NWs are prone to grow in the ZB phase in a larger diameter (>10 nm) while in a smaller diameter in the WZ phase (<10 nm). If in the ZB phase, then the favorable growth orientation is polar <111> because this plane has the highest atomic density and thus the lowest surface energy, so does the WZ<0001> orientation. However, in this study, nonpolar WZ<1̅100> and WZ<2̅110> NWs are successfully synthesized in a large diameter range of 10–40 nm, and the main difference is that the catalyst seed here is the Cu–As alloy (shown in Figures –5) instead of the M–Ga alloy (M = Au, Pd, Ni, and Ag) used in the literature. From the Cu–As–Ga ternary phase diagram in Figure S9, Ga is less soluble (2 atomic%) in the Cu–As alloy, and Cu5As2 serves as one of the most stable alloy phases with an eutectic temperature of >700 °C. Therefore, the relatively lower growth temperature of 560 °C favors the VSS growth mode of the GaAs NW, similar to those of Al catalyzed Si NWs,[30] Pd catalyzed InP NWs,[24] Ni catalyzed GaAs NWs,[31] and even Au catalyzed GaAs NWs with lower growth temperatures than the melting point of the Au–Ga catalyst.[32]

Table 2

Comparison of the Synthesis Parameters and the Growth Orientations of Different Catalysts

catalyst	catalyst seed	growth mechanism	diameter (nm)	crystal phase	growth orientations	ref
Au	Au7Ga2 Au7Ga3	VLS	70–90	ZB	<111>, <110>	(9)
Pd	PdGa5	VLS	10–30	ZB	<111>	(18)
Ni	NiGa, Ni2Ga3	VSS	10–20	ZB	<111>, <110>, <101̅0>	(16)
				WZ
Ag	AgxGay	VLS	30	ZB	<111>	(17)
Au	unknown	VLS	∼10	WZ	<0001>	(21)
Cu2O	Cu5As2	VSS	10–40	WZ	<1̅100>, <2̅110>	this study
				ZB	mainly <110>

All these M–Ga catalyzed and Cu–As catalyzed GaAs NW growth are schematically compared in Figure , taking Au as an example. Typical in Figure a, Au films are thermally deposited onto the growth substrate, which will be annealed into nanoparticles to serve as the catalyst. The Au nanoparticles are then alloyed with the Ga precursors (atomic Ga vapor or decomposed metal–organic precursors such as trimethylgallium) to form the real catalytic seed because As (either As vapor or decomposed AsH3) is less soluble in Au.[33] Finally, Ga rather than As precursors are diffused both through the Au–Ga alloy, as evidenced in the molecular epitaxy growth,[32] and from the NW sidewalls, as evidenced by the tapering of the NWs.[34] Meanwhile, As precursors are directly impinging into the growth frontier for the GaAs NW growth as it has a far higher vapor pressure (1 Pa As at 280 °C while 1 Pa Ga at 1037 °C)[35] to stick on the NW surface. On the contrary, when chemically synthesized uniform Cu2O nanoparticles serve as the catalyst, they alloy with As to serve as the catalyst seeds. Then, the As precursors rather than Ga would be diffused directly through the catalyst seeds and lead to nonpolar GaAs NW growth, enabled by the perfect lattice matching of the Cu5As2 seed and the nonpolar GaAs planes.

Figure 6

Atomic stacking in (a) WZ GaAs {1̅100}, (b) ZB GaAs {111}, and (c) WZ GaAs {0001} NW orientations, showing that no bond rotation is allowed in WZ GaAs {1̅100}; therefore, no stacking fault occurred as compared with the easily rotated Ga–As bond, leading to the transformation between ZB {111} and WZ {0001}. (d) Schematics of the proposed growth mechanism of the successive GaAs NW obtained by the Cu2O and Au seeds.

One of the advantages of the nonpolar <2̅110> and <1̅100> oriented GaAs NWs is the minimized crystal defects, as shown in the HRTEM images in Figures and 5 and Figure S10. This is attributable to the special atomic bonding of Ga and As, with two bonds linking in the plane and the other two bonds linking in the backside and frontside planes, as shown in the atomic model in Figure and Figure S11. On the other side, if GaAs NWs are stacked in the polar ZB<111> and WZ<0001> orientations, then the stacking planes have the same atomic arrangement while stacking in the abab... sequence in WZ<0001> and in the abcabc... sequence in ZB<111>. The Ga and As atoms have three bonds with the frontplane and one bond with the backplane, with no bonding in the planes. Therefore, a rotation would be easy to occur, which will make the faults of the stacking sequence and lead to crystal defects, as shown in Figure . Therefore, single-crystalline, nonpolar GaAs NWs are successfully prepared by using chemically synthesized Cu2O nanoparticles as the catalyst, promising for the further electronic applications.

Conclusions

Single-crystalline and nonpolar oriented GaAs nanowires (NWs) are successfully synthesized by using CMOS-compatible Cu2O catalysts at an optimized growth temperature of 560 °C in a chemical vapor deposition system. The NW diameter is slightly dependent on the Cu2O nanocube size due to the Gibbs–Thomson effect. Furthermore, the nonpolar ZB<110>, ZB<211>, WZ<1̅100>, and WZ<2̅110> NWs are found to be epitaxially grown from a Cu5As2 catalyst seed, with good lattice matchings of Cu5As2 {240}||ZB GaAs<110>, Cu5As2<240>||WZ GaAs<1̅100>, Cu5As2 {110}||ZB GaAs<110>, and Cu5As2<1̅10>||WZ GaAs<2̅110>. The Cu5As2 catalyst epitaxy growth is significantly different from the commonly used Au catalyst, where the Au–Ga alloy rather than the Au–As alloy serves as the catalyst. This special catalyst alloy has good lattice matching with nonpolar GaAs planes, serving as an alternative catalyst for high-performance nonpolar GaAs NW synthesis as well as other semiconductor NWs.