The activation of inhomogeneous electric field-induced Raman scattering (IEFIRS) at ZnSe/GaAs interfaces is investigated. The consideration of the inhomogeneous character of the electric field brings additional conditions on top of those predicted by the microscopic and perturbative approaches. These conditions concern the relative orientations of (i) the wave vector of the vibrational modes in the matter, (ii) the direction of the electric field at the junction, and (iii) the gradient vector of the electric field magnitude in the space charge regions. As latter vector has opposite directions in the layer and the substrate, IEFIRS is only observed from one side of the junction for a given scattering geometry. (C) 1999 American Institute of Physics. [S0021-8979(99)03604-X].

We report the fabrication of submicon-scale structures using high resolution etching to transfer patterns from PMMA into GaN with an intermediate mask consisting of a bilayer of titanium and SiNx. Atomic force microscopy measurements showed the high quality of the structures etched in CH4/H-2 as well as an erosion of the mask. The low temperature photoluminescence measured on the etched structures was almost as strong as that from the unetched surface. (C) 1999 Elsevier Science S.A. All rights reserved.

Deep ultraviolet micro-Raman scattering was employed to monitor high-temperature processing of AlGaN films under resonant excitation conditions, giving rise to enhanced first and second-order Raman scattering. High-temperature treatments at 1100 degrees C result in changes in the second-order Raman scattering signal and monitor the emergence of microscopic defects during the high-temperature processing. The second-order Raman spectrum was analyzed to gain insight into the AlGaN phonon density of states. For annealing temperatures higher than 1150 degrees C, the Al0.72Ga0.28N film decomposes: a low- and a high-aluminum composition AlxGa1-xN phase emerge. At 1100 degrees C, prior to the Al0.72Ga0.28N decomposition, deep UV Raman scattering detects the built-up of strain in the Al0.72Ga0.28N film.

We study the influence of defect states (shallow donors and deep acceptors) on the carrier relaxation dynamics of gallium nitride in the picosecond regime for different excitation intensities and different lattice temperatures. Time-resolved luminescence, degenerate, and nondegenerate four-wave mixing experiments show a saturation threshold in the blue and yellow spectral region, which is found to disappear for lattice temperatures below 200 K. When analyzing all these results in the frame of a rate-equation model, we give a relaxation scenario for the carriers, the lifetimes of the population of the different states, and identify radiative and nonradiative transitions. After filling defect states by an optical excitation, the ambipolar diffusion coefficient of GaN is measured through degenerate four-wave mixing experiments. A law value of 0.16 cm(2)/s at room temperature is determined, indicating that defect states still influence the diffusion. Nondegenerate four-wave mixing experiments exhibit a competition between an electronical and a thermal contribution to the nonlinear susceptibility in GaN. [S0163-1829(99)00340-9].

We report a low temperature and room temperature photoluminescence (PL) study of a series of two-dimensional arrays of holes and pillars in a triangular lattice arrangement, fabricated in a 1.8 mu m thick GaN/sapphire epilayer. The sample was patterned by electron beam lithography and reactive ion etching in a CH4/H-2 plasma. Such structures with periodic variation of the dielectric constant may give rise to photonic crystals that can create a range of forbidden frequencies called a photonic bandgap. We have used the PL peak to evaluate the damage introduced onto the semiconductor by the etching procedure, and to determine the magnitude of the relaxation of the strain in the GaN film. Theoretical computations predict the photonic bandgap positions for future experimental investigations.

We have illustrated the use of ultraviolet (UV) Raman scattering to investigate the thermal stability of AlGaN layers with high-aluminum content. The degradation pathway of Al0.72Ga0.28N was monitored for high-temperature treatments up to 1200 degrees C. For annealing temperatures higher than 1150 degrees C, the Al0.72Ga0.28N film decomposes: a low- and a high-aluminum composition AlxGa1-xN phase emerge. At 1100 degrees C, prior to the Al0.72Ga0.28N decomposition, UV Raman scattering detects the buildup of a large strain in the Al0.72Ga0.28N film The crystalline quality of Al0.72Ga0.28N is unaffected up to 1000 degrees C. (C) 1999 American Institute of Physics. [S0003-6951(99)00704-4].

We have developed a model which describes the incorporation of aluminum in the solid phase of Ga1-xAlxM where M is As or N, taking into account the diffusion of the group In species through the gas phase in the vicinity of the substrate. In order to compare our model with experimental data, GaAlAs and GaAlN alloys were grown by low pressure (76 Torr) MOVPE. Art excellent agreement is obtained in the case of GaAlAs alloys, considering TEGa and TMA1 as diffusing species in a hydrogen ambient. For AlGaN alloys, we discuss here the nature of the species involved in the diffusion mechanism since the precursors pyrolysis occurs at lower temperature than the growth temperature. We thus use this model to discuss the nature of the diffusing species, which are clearly not TEGa and TMA1, but rather unstable species, like diethyl and monomethyl, resulting: from the thermal decomposition of the organometallics.