Letter Cite This: Nano Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/NanoLett
Compositional Varied Core−Shell InGaP Nanowires Grown by Metal−Organic Chemical Vapor Deposition Han Gao,†,⊥ Wen Sun,†,⊥ Qiang Sun,† Hark Hoe Tan,‡ Chennupati Jagadish,‡ and Jin Zou*,†,§ †
Materials Engineering, The University of Queensland, St. Lucia, QLD 4072, Australia Department of Electronic Materials Engineering, Research School of Physics and Engineering, The Australian National University, Canberra, ACT 2601, Australia § Centre for Microscopy and Microanalysis, The University of Queensland, St. Lucia, QLD 4072, Australia ‡
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ABSTRACT: In this study, we report the growth of core− shell InGaP nanowires with compositional varied cores/shells using metal−organic chemical vapor deposition. These core− shell InGaP nanowires exhibit Ga-enriched cores attributed to the strong affinity between Au and In, and In-enriched shells due to In-rich vapor ambient. Detailed electron microscopy investigations indicate that the In and Ga concentrations in the nanowire cores and shells varied along the growth direction of InGaP nanowires. It is found that the strain relaxation through Ga diffusion outward and In diffusion inward leads to the decrease of compositional difference between the nanowire core and shell from top to bottom. This study offers a possibility to grow structural complex ternary nanowires that can be used for future applications. KEYWORDS: InGaP nanowire, core−shell structure, GaAs (111) substrate, electron microscopy, semiconductor
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nanowires are commonly observed in Au-assisted grown ternary nanowires, especially in III−III−V nanowire systems.14,20−22 From detailed structural and chemical characterization, Guo et al.13 found that the formation of spontaneous core−shell structured InGaAs nanowires was attributed to the different growth mechanisms between the nanowire core and shell, and competitive alloying between In/Ga with Au catalysts. Besides, Zhou et al.23 studied core−shell InGaAs nanowires grown by molecular beam epitaxy in which the observed unique core−multishell structure was found to be caused by the difference in adatom diffusion and affinity with catalyst between In and Ga. The structural and compositional complexities of such heterostructures make the controllable growth of ternary III−V nanowires challenging and the growth mechanism has not been fully understood. On the other hand, the controllability of the structure is crucial for designing structural complex nanowires for future device applications. Therefore, understanding the formation mechanism of spontaneous core−shell formation in nanowires of different ternary alloy compositions is essential for achieving the desired properties. In this study, ternary InGaP nanowires were epitaxially grown on GaAs (111) substrates by Au-assisted vapor−liquid−
he III−V epitaxial semiconductor nanowires have been considered as building blocks for advanced functional devices, primarily due to their great potential for electronic and optoelectronic applications.1−3 The ternary III−V epitaxial nanowires have the advantage that their bandgap can be finely tuned by varying their compositions,4 which opens up a new direction for device applications, such as InGaAs for field-effect transistors,5 InGaP for light-emission diodes (LEDs),6 GaAsP for near-infrared lasers,7 and InAsP for infrared photodetectors.8 Among them, the InGaP material system is an ideal candidate for efficient LED in red to yellow wavelength range and photovoltaic applications due to its direct tunable bandgap in the range from 1.35 to 2.26 eV.9,10 So far, solar cells fabricated by core−multishell heterostructure GaAs/ InGaP nanowires11 and LEDs based on core−shell structured GaAs/InGaP nanowires6 have been demonstrated. Up to now, investigations have been devoted to the epitaxial growth of ternary III−V nanowires, such as AlGaAs,12 InGaAs,13−15 GaAsP,16,17 and GaAsSb,18,19 from which the fabrications of ternary nanowires with desired composition have become possible through the manipulation of growth conditions, and the resultant ternary nanowires have exhibited promising applications for future devices.12,20 However, unlike their counterparts of elemental Si and Ge nanowires as well as binary GaAs and InAs nanowires, nonuniform compositions across nanowires are often observed in ternary nanowires. Interestingly, the spontaneous formation of core−shell heterostructures induced by elemental segregation in ternary © XXXX American Chemical Society
Received: March 4, 2019 Revised: May 2, 2019
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DOI: 10.1021/acs.nanolett.9b00915 Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters
Figure 1. SEM images of InGaP nanowires grown on a GaAs (111) substrate: (a) top-view image with the inset of enlarged area showing individual nanowires and orientation stretch showing ⟨110⟩ and ⟨112⟩ in blue lines and red dashed lines, respectively; (b) 20°-tilted view image; (c) side-view image; and enlarged images of a single nanowire at the top region (d), middle region (e), and bottom region (f).
contaminants from their surfaces. The temperature was then decreased to 460 °C for nanowire growth, initiated by simultaneously introducing TMGa, TMIn, and PH3 with flow rates of 6.248 × 10−6, 5.288 × 10−6, and 4.464 × 10−3 mol/min, respectively. These flow rates resulted in a nominal Ga/(Ga + In) molar ratio of 55% and a V/III ratio of 387. It is of interest to note that a low V/III ratio (e.g.,
