Letter pubs.acs.org/NanoLett
Demonstration of Defect-Free and Composition Tunable GaxIn1−xSb Nanowires Sepideh Gorji Ghalamestani,*,† Martin Ek,‡ Bahram Ganjipour,† Claes Thelander,† Jonas Johansson,† Philippe Caroff,§ and Kimberly A. Dick†,‡ †
Solid State Physics, Lund University, Box 118, SE-22100 Lund, Sweden Polymer and Materials Chemistry, Lund University, Box 124, SE-22100 Lund, Sweden § Institut d’Electronique, de Microélectronique et de Nanotechnologie, UMR CNRS 8520, Avenue Poincaré, B.P. 60069, 59652 Villeneuve d’Ascq, France ‡
ABSTRACT: The GaxIn1−xSb ternary system has many interesting material properties, such as high carrier mobilities and a tunable range of bandgaps in the infrared. Here we present the first report on the growth and compositional control of GaxIn1−xSb material grown in the form of nanowires from Au seeded nanoparticles by metalorganic vapor phase epitaxy. The composition of the grown GaxIn1−xSb nanowires is precisely controlled by tuning the growth parameters where x varies from 1 to ∼0.3. Interestingly, the growth rate of the GaxIn1−xSb nanowires increases with diameter, which we model based on the Gibbs−Thomson effect. Nanowire morphology can be tuned from high to very low aspect ratios, with perfect zinc blende crystal structure regardless of composition. Finally, electrical characterization on nanowire material with a composition of Ga0.6In0.4Sb showed clear p-type behavior. KEYWORDS: III−V semiconductor, nanowire, antimonide, GaInSb, zinc blende structure, MOSFET
D
concentrations.23−25 Despite the clear motivation for small bandgap ternary antimonide nanowires, highlighted in a recent review by Zhuang et al. on ternary nanowires,26 there is to date no report on growth of GaxIn1−xSb nanowires by any synthesis technique. Here we report on the successful growth of GaxIn1−xSb nanowires using a wire-on-wire stem technique, using MOVPE. The choice of a two-step InAs−InSb stem results in uniform nucleation and simplifies interpretation of results by avoiding a simultaneous change in group III and V species. We demonstrate control over the alloy composition homogeneity over a large compositional range, and show that morphology is tunable from high aspect ratio to distorted diamond-shaped morphology. As is commonly observed for Sb-containing nanowires, the GaxIn1−xSb nanowires shown here have perfect crystal quality (free from any extended defect) for all compositions. The growth rate of all the nanowires studied showed a relatively unusual direct diameter dependence, which we explain with a model including mass transport and the Gibbs−Thomson effect. Finally, we show results of electrical characterization of GaxIn1−xSb nanowires processed into fieldeffect transistors (FETs). Growth is performed on InAs (111)B substrates. The nanowires are seeded by size-selected Au aerosol nanoparticles
ue to their notable properties, such as high carrier mobility and narrow direct band gap, Sb-based nanowires are gaining considerable attention. Among III−V materials, InSb and GaSb present the highest bulk electron and hole mobilities, respectively, attractive for high speed n- and p-type devices. There have been several reports on the epitaxial growth of InSb1−5 and GaSb6−8 nanowires by metalorganic vapor phase epitaxy (MOVPE) as well as InSb nanowires by chemical beam epitaxy (CBE)9,10 and molecular beam epitaxy (MBE).11 Promising electrical properties have been demonstrated in several fundamental physics and demonstrator device applications, such as observation of Majorana fermion signatures,12 InSb and GaSb single electron transistors,13,14 In(As)Sb field effect transistors (FET),15,16 infrared and gas sensors,17,18 and GaSb-based tunnel diodes.19 Growth of GaxIn1−xSb ternary material in the form of nanowires is expected to give access to additional device opportunities, while maintaining a material quality that may not be possible in layer growth, due to its critical dependence on lattice mismatch.20 The GaxIn1−xSb composition range covers a large range of bandgaps from 0.18 eV (InSb) to 0.725 eV (GaSb) which allows several applications including IR detectors, optical and thermophotovoltaic devices, and highspeed terahertz electronics.21,22 Moreover, the combined high electron and hole mobility in ternary GaxIn1−xSb nanowires could offer an ideal platform for a vertical C-MOS technology where both n-type and p-type very high mobility channels could be grown sequentially by tuning dopant type and alloy © 2012 American Chemical Society
Received: July 6, 2012 Revised: August 12, 2012 Published: August 27, 2012 4914
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Figure 1. (a) Schematic illustration of the InAs/InSb/GaInSb nanowires grown on the InAs substrate. (b) SEM image of the grown InAs/InSb/ Ga0.6In0.4Sb nanowires seeded with 50 nm diameter Au nanoparticles, demonstrating the three clear segments. The scale bar is 200 nm. (c) XEDS radial line scan of a nanowire similar to the one shown in part b. The scale bar is 0.1 μm. (d) SEM image of the diamond-shaped nanowires seeded with 40 nm diameter Au nanoparticles. The scale bar is 200 nm. (e) SEM image of a similar sample (to d) with lithographically defined Au particles. The scale bar is 1 μm. The inset shows a schematic illustration of a nanowire with indexed side facets. (f) Top and side view schematic illustrations of the structures in parts d and e, showing the appearance for different rotation angles.
The axial heterostructure geometry of the grown nanowires is schematically illustrated in Figure 1a. Figure 1b shows a SEM image of representative InAs/InSb/GaxIn1−xSb nanowires with three distinct segments. There are two apparent diameter increases along the nanowire axis, related to the InAs to InSb and InSb to GaInSb transitions, respectively. The first is wellknown, as the diameter always increases from InAs to InSb, due to the increased In content in the Au particle2,3,5,27 and also related to the rotation of the facets with respect to the stem.27 This facet rotation occurs due to the transition from the WZ InAs stem with {11̅00}-type side facets to the ZB InSb segment with {110}-type side facets. The second diameter increase from InSb to GaxIn1−xSb is primarily attributed to the higher radial growth rate on GaxIn1−xSb compared to InSb, as shown in Figure 1b and discussed further below. An XEDS line scan of a nanowire (similar to Figure 1b) is shown in Figure 1c where the red Ga and blue In signals confirm uniform composition of the Ga0.6In0.4Sb segment. Note that the apparent Sb in the InAs stem is due to a partial overlap between the In Lβ1 and Sb Lα peaks used for recording the line scan. Postgrowth XEDS analysis of the Au alloy particles shows substantial quantities of both Ga and In and smaller quantities of Sb, with precise levels depending on growth conditions. This is rather different than GaxIn1−xP and GaxIn1−xAs nanowires, which typically show only In in the Au postgrowth.28,29 TEM characterization on all the grown samples regardless of their compositions showed defect-free zinc-blende crystal structure (Figure 3g), consistent with pure InSb and GaSb nanowires.1−11 Note that the crystal phase stability, which is highly desired for applications, is not a general feature in ternary III−V NWs, where composition-dependent crystal structure evolutions are observed, for instance, in GaxIn1−xAs30 or InAs xSb 1−x nanowires.11 Convergent beam electron diffraction was used to confirm that the growth proceeds in [1̅ 1̅ 1]̅ B for the InSb and GaInSb segments, by analyzing the asymmetric contrast in the 002 discs.30 Parts d and e of Figure 1 show SEM images of similar samples with 40 nm Au aerosol nanoparticle and litho-
with 30, 40, 50, 60, and 70 nm diameters and a surface density of 1 μm−2. Electron beam lithography (EBL, Raith 150) is also used to prepare ordered Au nanoparticles. The growth is done in a standard low pressure (100 mbar) horizontal MOVPE reactor (Aixtron 200/4). Trimethylindium (TMIn), trimethylgallium (TMGa), trimethylantimony (TMSb), and arsine (AsH3) are used as the precursors. Hydrogen is used as a carrier gas with a total flow of 13 L/min. Directly after loading the samples into the reactor, InAs substrates are annealed at 550 °C. Then, a short InAs stem is grown for 5 min with respective TMIn and AsH3 molar fractions of 4.88 × 10−6 and 3.85 × 10−4. Afterward, a segment of InSb is grown for 5 min following a simultaneous switch between AsH3 and TMSb (molar fraction 4.33 × 10−4). Finally, to form the GaxIn1−xSb segment, the second group III precursor (TMGa) is introduced into the reactor following a 30 s pause to vary TMGa, TMIn, and TMSb molar fractions to the desired values. For the GaxIn1−xSb segment, the TMGa, TMIn, and TMSb molar fractions are varied in the ranges of 3.55 × 10−5 to 9.1 × 10−5, 3.87 × 10−7 to 4.88 × 10−6, and 2.17 × 10−5 to 4.33 × 10−4, respectively. The growth time is varied from 20 to 60 min. The growth temperature is set to 450 °C and is kept constant all the time. The choice of a single constant growth temperature is intended to avoid complicated temperature effects on growth kinetics (including precursor decomposition and diffusion rates on the surface and through the alloy particle) which would impede understanding of the details of the growth process. The temperature 450 °C was chosen, since it can be used to give good morphology for the component materials InSb and GaSb, and is also suitable for the stem material InAs. The grown nanowires were characterized by SEM (Zeiss LEO 1560) and TEM (JEOL 3000F). Prior to TEM characterization, the nanowires were broken off from the substrate and mechanically transferred onto carbon Cu grids. Also, X-ray energy dispersive spectroscopy (XEDS) together with high angle annular dark field scanning TEM (HAADFSTEM) was used to determine the composition of the nanowires. 4915
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Figure 2. (a) Ga/In ratio of the grown GaxIn1−xSb nanowires, calculated by XEDS analysis, with respect to the Ga/In ratio in the vapor phase (based on TMGa and TMIn precursor molar fractions). The green and red curves correspond to the Sb molar fractions of 4.33 × 10−4 and 6.72 × 10−5, respectively. (b) Ga content measured from XEDS analysis versus TMSb molar fraction (TMSb mf), indicating Ga-rich GaxIn1−xSb nanowire formation by lowering TMSb. The black and magenta curves correspond to Ga/In ratios of 235 and 131, respectively.
Figure 3. SEM images of the InAs/InSb/GaInSb nanowires grown via (a) 30, (b) 40, (c) 50, (d) 60, and (e) 70 nm Au nanoparticle diameters. The red color overlay highlights the GaxIn1−xSb segment which scales directly with the Au nanoparticle diameter. This sample is grown with a TMSb molar fraction of 6.72 × 10−5 and a Ga/In ratio of 192, corresponding to the second dot from the right in Figure 2a. (f) GaxIn1−xSb growth rate versus nanowire−particle interface diameter (measured postgrowth, corresponding to a nominal Au diameter of 40, 50, 60, and 70 nm) for three samples grown with different conditions, showing longer GaxIn1−xSb segment for larger Au particle size. The solid curve represents a fit to the Gibbs−Thomson model described by eqs 1 and 2. (g) TEM image of a sample grown with a TMSb molar fraction of 4.33 × 10−4 and a Ga/In ratio of 235, demonstrating perfect zinc-blende crystal structure. The inset shows the fast Fourier transform of part of the upper segment confirming zincblende structure.
graphically defined Au particles, respectively. These nanowires are grown under conditions leading to high radial overgrowth and low aspect ratio, yielding a distorted morphology similar to the so-called nanocubes reported for InSb grown under high V/ III ratio.5 Figure 1f shows top and tilted view schematic structures of the facet orientation, illustrating the deduced 3D morphology of these structures viewed from different angles. A more detailed schematic structure with indexed facets is shown in the inset of Figure 1e. We note that the structures are not in fact cube-shaped but resemble a {111}-bound octahedron with narrow {001} facets at the edges; henceforth, we will refer to these structures as “diamond-shaped”. Significant radial over-
growth ultimately leads to the formation of low-energy facets as high-energy, fast-growing facets disappear. The result is a structure bound only by low-energy facets where the ratio between surface facet energy and area is constant and minimized, represented by a kinetic Wulff construction. The low growth rates of these facets will result in reduced overall radial growth rate and a rather stable 3D structure. As will be discussed below, the high radial/low axial growth rates leading to these diamond-shaped structures are observed for high Ga/ In ratio, very low Sb molar fraction (
