III-V semiconductor nanowires have already shown great promise for use in a variety of applications, from solar cells¹ to light-emitting diodes², with some prototype optoelectronic devices having already been developed³. Thus, the need for a greater understanding of the electronic properties and the effects of surface states on these nanowires has become ever more apparent. In particular, controllable and effective doping of nanowires is a key requirement of most devices. Previously, GaAs nanowires have been shown to exhibit extremely short lifetimes of a few picoseconds due to high recombination surface velocities at the GaAs surface⁴. The effects of adding optimised AlGaAs shells has already been investigated and shown to improve both the carrier lifetime and mobility of these nanowires⁵. However, achieving reliable doping of III-V nanowires without loss of other electrical properties, such as mobility, has proven difficult.

MBE-grown GaAs/AlGaAs core-shell nanowires with modulation n-type silicon doping within the shell have therefore been investigated (Figure 1). By using optical pump-probe terahertz spectroscopy to allow non-contact measurements to be taken, the carrier lifetimes of these nanowires could then be examined. Figure 1 plots the photoconductivity decay at different fluences for these modulation-doped nanowires, showing extremely long lifetimes of over 3.92ns.⁶ The photoconductivity spectra (Figure 1) exhibit a Lorentzian response, suggesting the presence of localised surface plasmon modes. Through fitting of a Lorentzian function to these spectra, an electron mobility of 2200cm²V^-1s^-1 was extracted and the doping density measured to be 1.1 x 10¹⁶ cm^-3. Notably, the doping did not affect the high mobility seen for undoped core-shell GaAs/AlGaAs nanowires.⁶ The long lifetime and maintained high electron mobility exhibited by these nanowires and indicate their suitability for use in optoelectronic devices.

Bio: Mike Johnston received his bachelor and PhD degrees in physics from the University of New South Wales in 1996 and 2000 respectively.  He was a postdoc in the Cavendish Laboratory, University of Cambridge from 1999 until October 2002 when he joined the faculty at the Department of Physics, University of Oxford.  Dr Johnston's research interests include terahertz science and technology, semiconductor physics and photovoltaics.