Photocurrent spectroscopy of single ZB GaAs and GaAs/AlGaAs core-shell nanowires*

We investigate the band structure of single ZB GaAs nanowires and GaAs/Al$_{0.5}$Ga$_{0.5}$As core shell nanowires using photocurrent spectroscopy at room and low temperatures. The single nanowire devices were fabricated photolithographically to define Ti (20nm)/Al (300nm) metal contacts on either end the nanowire. Photocurrent measurements were performed using CW excitation from a tunable CW Ti-Sapphire laser (775nm-890nm) and a broadly tunable (550-960 nm) pulsed excitation from a coherent super continuum photonic crystal fiber. At room temperature we observe an Urbach tail near the absorption edge at 1.42 eV for both GaAs and GaAs/ Al$_{0.5}$Ga$_{0.5}$As core-shell nanowires. In the core shell structure, we also observe the exponential tail from the Al$_{0.5}$Ga$_{0.5}$As superimposed on the GaAs absorption in the core. The 2eV onset is consisant with 50{\%}. At low temperature, 10K, similar measurements were performed and a peak is observed near the band edge $\sim$ 1.50-1.51 eV for both bare and core-shell structure for GaAs reflecting the contribution of excitons to the photocurrent.