Bragg mirrors are highly interesting structures for a large set of applications including vertical cavity lasers and the upcoming range of devices based on microcavities. Although the nitride semiconductors are performing fairly well in optoelectronic applications, it is not straightforward to realize Bragg mirrors based on this material system, due to the low optical index differences between GaN and AlN. Moreover, the lattice parameter difference between these materials will generate crystal defects, which prevent the stacking of a large number of periods, adding to the difficulty. In this work, we have grown high reflectivity Bragg mirrors, with a band centered in the visible blue range. The structures were first modelled, then grown by low pressure MOCVD, and were optimised using an in-situ reflectivity system.;This in-situ reflectivity measurement was compared to a calculated profile, to enable real-time control of the structures. The samples were characterized by transmission electron microscopy and reflectivity. It was possible to realize samples with 90% reflectivity near 400 nm.

Double heterostructure diodes were widely studied in the 80's; based on the use of "classical" III-V materials, i.e. based on (Ga, Al, In, As, P). Negative differential conductivities were predicted in these systems, making them highly interesting for hyperfrequency applications, but experimental results were always far from theoretical predictions, which resulted in a loss of interest for these systems. Recently, there has been a lot of efforts devoted to Nitride semiconductors, based on (Al,Ga,In)N alloys. These materials have been regarded as potential candidates for high frequency, high power applications, since they both exhibit high electron saturation velocities and high thermal and chemical stability. Moreover, although they usually contain a lot of growth defects, device properties were surprisingly good with regards to defect densities. In this work, we have modeled double heterostructure tunnel diodes based on AlGaN/GaN system. The extremely high conduction band offset in these materials, along with the quite low dielectric constants, and built-in electric fields (originating from both the spontaneous and piezoelectric polarization), are extremely favorable parameters, which may renew the interest for such devices in this material system. It will be shown that the built-in electric field leads to a symmetric potential profile under a given external applied bias, which optimizes the transmission coefficient in the structure. We have calculated extremely high peak/valley ratios, which suggests that even mid quality samples could still exhibit interesting device properties.

We present an experimental work on the Raman analysis of single InN dots grown by metal-organic vapor phase epitaxy on GaN buffer layer. InN islands of controlled sizes have been fabricated by taking advantage of the Stranski-Krastanov growth mode. Atomic force microscopy and micro-Raman spectroscopy have been employed to investigate their morphology and internal strain. The usual shape of the dots corresponds to truncated pyramids with a hexagonal base as revealed by atomic force microscopy measurements. The E-2 phonon frequency, detected in micro-Raman spectra recorded from single dots of sizes ranging from 480 nm down to 30 nm in height, allows a rough evaluation of the residual strain field. Careful analysis of these data reveals that the islands are weakly strained, which is likely due to the formation of dislocations near the InN/GaN interface. Nevertheless, the reduction of the size of InN islands leads to an increasing strain field. Finally, we observed a surprisingly small strain increase when the dots are capped with a thin GaN top surface layer. This demonstrates the major role played by the plastic strain relaxation.

Self ordered InN dots are grown by Metal Organic Vapor Phase Epitaxy onto GaN, with very low surface densities. Such nano-objects may be employed in the realization of single photon emitters, for quantum cryptography applications. The growth parameters, like growth temperature, V/III molar ratio and deposition time are investigated. First "large" dots are grown, in order to easily assess the material quality, and are studied both by X-ray diffraction and Raman spectroscopy. After optimising the growth conditions, we decrease the deposition time, in order to obtain nanometer size structures. (C) 2003 WILEYNCH Verlag GmbH & Co. KGaA, Weinheim.

Photonic crystals are a new class of materials where photonic band gaps, large dispersion and anisotropy occur. By exploiting these properties GaN photonic crystals should have important potential for optoelectronic applications, principally in the areas of high-efficiency light emitters and second-harmonic generators. We present measurements of the equifrequency surfaces of the radiative Bloch modes for a photonic crystal etched in a GaN/sapphire film. The photonic band structure is calculated by using a scattering matrix method that reproduces well the anisotropy of the equifrequency surfaces exhibited by the photonic crystal. (C) 2002 Elsevier Science B.V. All rights reserved.

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.