Abstract
Achieving bulk-like charge carrier mobilities in semiconductor nanowires is a major challenge facing the development of nanowire-based electronic devices. Here we demonstrate that engineering the GaAs nanowire surface by overcoating with optimized AlGaAs shells is an effective means of obtaining exceptionally high carrier mobilities and lifetimes. We performed measurements of GaAs/AlGaAs core-shell nanowires using optical pump-terahertz probe spectroscopy: a noncontact and accurate probe of carrier transport on ultrafast time scales. The carrier lifetimes and mobilities both improved significantly with increasing AlGaAs shell thickness. Remarkably, optimized GaAs/AlGaAs core-shell nanowires exhibited electron mobilities up to 3000 cm(2) V(-1) s(-1), reaching over 65% of the electron mobility typical of high quality undoped bulk GaAs at equivalent photoexcited carrier densities. This points to the high interface quality and the very low levels of ionized impurities and lattice defects in these nanowires. The improvements in mobility were concomitant with drastic improvements in photoconductivity lifetime, reaching 1.6 ns. Comparison of photoconductivity and photoluminescence dynamics indicates that midgap GaAs surface states, and consequently surface band-bending and depletion, are effectively eliminated in these high quality heterostructures.
Highlights
G aAs nanowires and associated heterostructures are currently inspiring a host of new device concepts,[1−3] ranging from nanowire-based photovoltaics[4−6] to single photon emitters.[7]
Room temperature operation of GaAs/ AlGaAs core−shell nanowire lasers was demonstrated,[8,9] and studies of single-nanowire devices revealed the potential of GaAs nanowire-based solar cells to achieve efficiencies exceeding the Shockley−Queisser limit.[4]
Two obstacles have arisen in the development of GaAs nanowire-based devices
Summary
Letter introduced by the gate capacitance term in field-effect mobility measurements,[25] and the nanowires’ quasi one-dimensional geometry which precludes conventional Hall effect measurements unless specialized procedures are employed for making electrical contacts to the nanowires.[26]. For a given AlGaAs shell thickness, core−shell−cap nanowires exhibit higher electron mobilities than core−shell nanowires which lack the GaAs cap (see Supporting Information Figure S3). Description of experiments (nanowire growth, terahertz timedomain spectroscopy, photoluminescence spectroscopy), transmission electron microscopy data, photoluminescence spectral data, mobility data comparing core−shell−cap and core−shell nanowires, and photoconductivity spectra and decays for annealed bare GaAs nanowires. This material is available free of charge via the Internet at http://pubs.acs.org
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