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S7.01 - Accurate Ultrafast and Contact-Free Measurements of Charge Carrier Transport and Dynamics in III�V Nanowires 
April 9, 2015   1:30pm - 2:00pm

III�V nanowires exhibit outstanding potential as nanocomponents for future electronic and optoelectronic devices. In order to develop these nanowire-based technologies further, it is essential to control the electronic properties of the nanowires, and to understand the limitations on charge carrier transport and dynamics. Unfortunately, measuring nanowire electronic properties using traditional contact-based techniques has proved challenging, because forming electrical contacts to nanoscale structures is technically difficult and introduces artefacts. To avoid these problems, non-contact probes of nanowire conductivity are highly desirable. Optical pump�terahertz probe (OPTP) spectroscopy is one such non-contact probe, capable of measuring carrier transport and dynamics with sub-picosecond temporal resolution. It is therefore ideally suited to studies of nanowires at room temperature. OPTP spectroscopy provides two principal measurements: photoconductivity decays and spectra. Photoconductivity decays provide information on the charge carrier lifetime and surface recombination velocity. Photoconductivity spectra of nanowires exhibit a pronounced surface plasmon mode, and from these spectra, electron mobilities can be readily extracted. This study spans a variety of technologically important III�V nanowires and heterostructure nanowires, including GaAs, InAs, InP, and GaAs/AlGaAs core�shell nanowires. Of all nanowires studied, InAs nanowires exhibited the highest electron mobilities of over 6000 cm2V-1s-1. InP nanowires exhibited the lowest surface recombination velocity of just 170 cm/s. This makes InP nanowires promising for applications which require long charge carrier lifetimes, such as photovoltaics. However, the electron mobility, measured as below 600 cm2V-1s-1, was strongly limited by the high density of stacking faults in these predominantly wurtzite InP nanowires. This points to the importance of controlling the crystal phase of InP nanowires for future device applications. Bare GaAs nanowires featured the highest surface recombination velocities (> 105 cm/s), and photoconductivity lifetimes below 5 ps. Bare GaAs nanowires also exhibited significantly lower charge carrier mobilities than their bulk GaAs counterparts, due to carrier scattering at the nanowire surface. To improve the lifetimes and mobilities, we investigated engineering the GaAs nanowire surface by overcoating with AlGaAs shells. This increased the photoconductivity lifetime in the GaAs core by almost 3 orders of magnitude to up to 1.6 ns. These GaAs/AlGaAs core�shell nanowires achieved room temperature electron mobilities above 2500 cm2V-1s-1 in the GaAs core. This electron mobility is significantly higher than in bare GaAs nanowires, and is approaching values typical of high quality bulk GaAs. The long photoconductivity lifetime and high electron mobility suggest the immediate suitability of these nanowires for optoelectronic devices.

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