Investigation of core-shell nanowires via electron-beam-induced current

Synopsis

Core-shell semiconductor nanowires (NWs) have gained increasing attention since the last decade for their advances in multiple applications, including field-effect transistors, photovoltaic devices, light-emitting diodes, and lasers. This core-shell geometry is advantageous because of the relatively short distance required for excited electron-hole pairs (EHPs) to travel before being collected and the potential to eliminate surface recombination in the core. It is essential to fully understand the electrical properties, including the minority carrier diffusion length, depletion width, and doping level, in the shell as well as in the core for optimization of growth and improving the optoelectronic performance. For this purpose, a characterization technique with high lateral and vertical spatial resolution, is needed. In this thesis, two types of core-shell NWs, both with n-type GaAs NW cores but with shells of either a metal, Fe, or p-type GaAs, were investigated using electron-induced-beam current (EBIC) measurements. This is a complementary technique carried out in situ in a scanning electron microscope (SEM) using a nano-size tungsten tip, collecting excess electron-excited carriers in a biased diode as a function of beam position and accelerating voltage.

Epitaxial Fe shells were grown onto GaAs NWs via electrodeposition, potentially acting as spin injectors or detectors. Both the axial and radial EBIC currents as a function of beam position exhibit oscillations that were reproducible regardless of the scan direction, beam spot size, dwell time (duration of the beam on one spot), and accelerating voltage. These oscillations were attributed to defects or oxides at the Fe/GaAs interface as recombination centers, showing the capability of extracting highly-spatially-resolved information from the radial junction via EBIC.

In addition, axial and radial EBIC scans were carried out on unprocessed, free standing core-shell GaAs NW tunnel diodes, showing high sensitivity to the three-dimensional shape of the structure. The axial variation in current was due to the high resistance of the very thin {112}B shell, or to interfacial traps at the junction. The carrier kinetics in both the n-type core and the p-type shell were determined by analyzing radial EBIC profiles as a function of beam energy and beam direction. These profiles are highly sensitive to changes in depletion widths and minority carrier diffusion lengths due to geometric effects. Due to the complex core-shell geometry of our NWs, numerical calculations (Monte Carlo simulations) were employed to estimate the minority carrier diffusion length and depletion width. By comparing the radial profiles to simulations, minority carrier diffusion lengths were found to be 15 ± 5 nm and 50 ± 10 nm in the shell and the core, respectively. The relatively short hole diffusion length in the core, can be attributed to bulk point defects originating from low-temperature growth (400 °C). The depletion width was found to be smaller in <112>B directions than in <112>A, due to different dopant corporation efficiencies of Te and Zn in the core and the shell, respectively, and to the very thin shell on the {112}B surface.