Persistent Photoconductivity Studies in Nanostructured ZnO UV Sensors

The phenomenon of persistent photoconductivity is elusive and has not been addressed to an extent to attract attention both in micro and nanoscale devices due to unavailability of clear material systems and device configurations capable of providing comprehensive information. In this work, we have employed a nanostructured (nanowire diameter 30–65 nm and 5 μm in length) ZnO-based metal–semiconductor–metal photoconductor device in order to study the origin of persistent photoconductivity. The current–voltage measurements were carried with and without UV illumination under different oxygen levels. The photoresponse measurements indicated a persistent conductivity trend for depleted oxygen conditions. The persistent conductivity phenomenon is explained on the theoretical model that proposes the change of a neutral anion vacancy to a charged state

Detection of surface states in GaAs and InP by thermally stimulated exoelectron emission spectroscopy

The energy distribution and relative densities of electronically active surface defects have been studied using thermally stimulated exoelectron emission (TSEE) spectroscopy. This novel and relatively simple technique has high sensitivity for detecting the surface states which are difficult to assess by other techniques. Here this technique is successfully used for detecting the pinned positions of the Fermi level in n-GaAs and n-InP which are, respectively, 0.91 and 0.43 eV below the conduction bands corresponding to 2Eg/3 and Eg/3 as expected. Antisite and oxygen related defects in these semiconductors are also identified at the surface. The relative TSEE peak intensities correlate very closely to the reported surface recombination velocities for these materials which are two to three orders of magnitude higher for GaAs. The effect of chromium on the surface states in these semiconductors, studied using semi-insulating GaAs, showed partial passivation of the surface defects in semi-insulating GaAs resulting in unpinning of the Fermi levels. Fe doped InP did not, however, show any sign of dopant induced deep levels