Selective Area Epitaxy of GaAs Microstructures by Close-Spaced Vapor Transport for Solar Energy Conversion Applications Ann L. Greenaway,† Meredith C. Sharps,† Jason W. Boucher,‡ Lyndi E. Strange,† Matthew G. Kast,† Shaul Aloni,§ and Shannon W. Boettcher*,† †
Department of Chemistry and Biochemistry, University of Oregon, Eugene, Oregon 97403, United States Department of Physics, University of Oregon, Eugene, Oregon 97403, United States § The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ‡
S Supporting Information *
ABSTRACT: Close-spaced vapor transport is a plausibly low-cost, high-rate method to grow III−V materials for photovoltaic and photoelectrochemical device applications. We report the first homoepitaxial growth of GaAs microstructures on (100)- and (111)B-oriented GaAs substrates using patterned SiOx and Al2O3 masks and show that the resulting microstructured GaAs is an efficient semiconductor absorber for photovoltaic and photoelectrochemical applications. Cross-sectional transmission electron microscopy reveals an unusually low density of twin-plane defects in the (111)-oriented microstructures and the occurrence of stacked twin-plane defects in the (100)-oriented microstructures. Nonaqueous photoelectrochemical measurements show similar short-circuit currents of 9.7 and 9.1 mA cm−2 for (100)- and (111)-oriented microstructures, respectively, with promising external quantum efficiencies. Together, the low twin density and good electronic properties indicate that micro- or nanostructures grown by selective area epitaxy in close-spaced vapor transport are promising for device applications that take advantage of their three-dimensional structure.
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nanoscale structures. Such three-dimensional structures, a major area of research,7−10 are particularly important for envisioned one-sun PV and PEC devices, especially if such structures could be directly integrated on Si to access low-cost tandem PEC/PV architectures.11−13 Vapor−liquid−solid (VLS) growth and selective area epitaxy (SAE) are the major techniques for controlling three-dimensional growth in the dominant III−V deposition technologies. VLS growth uses metal nanoparticles to catalyze precursor decomposition and direct growth,10 while SAE allows for control of position, dimension, and shape through a patterned dielectric mask that confines epitaxial growth to the exposed crystalline substrate.7 III−V structure growth generally proceeds in the ⟨111⟩ direction as a result of its high growth rate regardless of substrate orientation,14,15 although a few examples of VLS and SAE growth in the ⟨100⟩ direction can be found in the literature.16,17 However, twin-plane defects, which have been shown theoretically18−20 and experimentally21−23 to degrade
lose-spaced vapor transport (CSVT) is an alternative method for the rapid growth (≥1 μm/min) of III−V semiconductors that are used in the highest-efficiency solar photovoltaic and photoelectrochemical devices. CSVT uses safe, solid sources rather than vapor-phase precursors and typically operates at ambient pressure. Volatile species are generated as the heated source reacts with a transport agent (water vapor or a halide) introduced to initiate growth. A small thermal gradient between the source and substrate drives diffusion of the precursors across a narrow (
