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.

For a few years now, it has been possible to dope GaN layers using molecular beam epitaxy (MBE) growth. This aims to use the wide band gap semiconductors as hosts to rare earth ions and exploit the sharp emission lines from atomic shell transitions. It was shown that the whole visible spectrum can be covered by addressing different exited states of various rare earth ions. It is also suspected that as in some systems like Si(nano)/SiO2, the energy coupling could involve defects. In this work, we carry out TEM and HREM analysis on MBE doped GaN layers grown on metal organic chemical vapour deposition (MOCVD) GaN templates. Er concentrations of 1, 6, and 16 at.% were subsequently measured by wavelength dispersive X-ray (WDX) in an electron probe. We discuss the results on the spatial distribution of the rare earth atoms at a nanometer scale. (C) 2003 Elsevier B.V. All rights reserved.

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 report the successful infilling of opal which consists of a fee arrangement of silica spheres with the semiconductor GaN by MOCVD technique. The angular dispersion of the Bragg diffracted light was studied for bare opal and GaN infilled opal by measuring reflection spectra at different external angles with respect to the [1 1 1] direction. The Bragg's law gives an estimation of 4.5% of void volume filled by the GaN. The crystal inhibits the yellow emission of GaN in the energies of the photonic gap. This decrease in the PL signal has one strong sensibility to the collection angle variation. This dependence is a signature of the GaN inside the opal voids.

GaN quantum wells embedded in Al0.13Ga0.87N layers have been studied by conventional, High Resolution and Energy Filtered Transmission Electron Microscopy. This set of quantum wells was analysed in order to determine the quality, the exact size and the chemical composition of the heterostructures. Such characteristics are crucial to analyse the optical measurements of such layers. It turns out that these quantum wells are also perfect test objects to determine the strength and limits of some transmission electron microscopy techniques. This last point is the subject of this paper.