Professor Robert Taylor

Abstract:

The saturation of exciton resonances with increasing carrier density has been measured in the case of resonant and non-resonant excitations by using a pump-probe technique. We observed that the bleaching density for resonantly excited excitons is considerably higher than that for non-resonantly excited excitons and that with increasing excitation energy, the exciton bleaching density converges to the Mott density of free carrier model. This results suggest that resonant excitons show weaker screening than an unbound electron-hole plasma mixed with excitons and that the transient optical nonlinearities associated with screening and bandgap renormalization are governed for the ratio of excitons to free carriers with different interactions. Also, we observed that the non-resonantly excited excitons had longer lifetimes than the resonantly excited excitons, which we attributed to the relaxation of free carriers and to the polariton bottle-neck effect.

Abstract:

The surface morphologies of AlN and GaN on Si(111) and Si(100) were studied using atomic force microscopy. The AlN buffer layer was deposited using RF sputtering for 30 minutes on Si(111) and Si(100) substrates, and then the specimens were annealed at 850 °C, 950 °C and 1050 °C for 60 minutes in an NH3 atmosphere. It was found that the grains of AlN became larger by increasing the annealing temperature. GaN was grown on AlN/Si(111) and AlN/Si(100) substrates by using a hydride vapor phase epitaxy technique. The surface of GaN grown on the AlN/Si substrate showed thick- thread and fine-thread patterns. The microstructure of GaN grown on the buffer-GaN/Al 2O3 had a step-flow pattern. These phenomena could be explained by a lattice mismatch and the growth temperature. A power spectral density analysis was performed on the GaN surfaces and the results were compared with the experimental data.

Abstract:

We present a systematic investigation of free-carrier screening in coupled asymmetric GaN quantum discs with embedded AlGaN barriers using time-integrated and time-resolved micro-photoluminescence measurements, supported by three-dimensional multi-band k.p computational modeling. Free-carrier screening effects decreased with the barrier thickness, which indicates that the source of the free-carriers were carriers tunneling through the barrier.