Conductive atomic force microscopy has been used to investigate the local conductivity in hydride vapor-phase epitaxy and molecular-beam epitaxy GaN films, focusing on the effect of off-axis facet planes. We investigated two different types of samples, in which the facet planes were either present on the perimeters of as-grown islands, or on the edges of etch pits created by post-growth chemical etching. The results show that crystallographic planes tilted with respect to the c-plane growth direction show a significantly higher conductivity than surrounding areas. The n-type (or p-type) samples required a negative (or positive) sample bias for current conduction, consistent with the formation of a Schottky barrier between the metallized atomic force microscope tip and sample. The time dependence of this enhanced conductivity was different for the two types of samples, possibly indicating different conduction mechanisms.

We present an optical characterization of as-grown and thermally annealed InGaNAs/GaAs multiple quantum well samples. In both samples, from the analysis of the photoluminescence spectra we can infer that the low-temperature photoluminescence emission is related to carriers localized in the alloy potential fluctuations; with increasing temperature, we observe their gradual delocalization and then the transition towards a completely different type of lineshape, typical of free carrier recombinations. The comparison between the photoluminescence spectra of the as-grown and the annealed samples shows that a remarkable improvement of the optical properties occurs after the thermal annealing. This improvement is related to an important reduction in the density of the defects and in the depth of the alloy potential fluctuations.

No abstract

No abstract

In this work we have studied the role of excitons in two different low barrier metal-semiconductor-metal (MSM) GaN-based UV photodetectors at high temperature. The active material of the two MSM devices consists of bulk GaN grown by metal organic chemical vapour deposition (MOCVD), and of an AlGaN/GaN heterostructure forming a two-dimensional electron gas (2DEG) grown by molecular beam epitaxy/magnetron sputtering epitaxy (MBE/MSE) system. The response of the devices has been characterized at the wavelength of 325 nm (He-Cd laser) as a function of the temperature in the range between room temperature and 700 K. The Arrhenius plot obtained by the decay times of the photocurrent allowed us to calculate the activation energies of some trapping mechanisms responsible of the persistent photocurrent (PPC). This shows that in both device the GaN excitonic resonances provide the most important contribution to the PPC on the millisecond time scale at low temperature.