We have studied the optical properties of Eu doped GaN thin films. We have grown high quality Eu doped GaN thin films by using Gas Source Molecular Beam Epitaxy (GSMBE), with 1.4% Eu concentration. The Full Width at Half Maximum (FWHM) of the X-ray diffraction in an omega scan was found to be 288 arcsecs. Low Eu concentration (0.08%) doped GaN thin films were grown, where Eu-related photoluminescence at 622 and 613 nm was detected using above band-gap excitation at 2 K. For high Eu concentration of 30% GaN:Eu crystal photoluminescence (PL) and cathodoluminescence (CL) spectra show strong and intense transitions at 622 and 664 nm, but also at 593 nm for CL spectra, with a similar transition observed from the low Eu concentration sample. (C) 2004 Published by Elsevier Ltd.

We have demonstrated the giant enhancement of second-harmonic generation in a one-dimensional GaN photonic crystal structure caused by the simultaneous achievement of strong fundamental field confinement and quasi-phase matching conditions. We have firstly measured the linear dispersive properties of the resonant modes for various azimuthal directions to predict the quasi-phase matching conditions. The theory based on a rigorous scattering matrix method has provided a good description of the equi-frequency surfaces. We have then shown that the equi-frequency surfaces may be used to identify the angular situations for which the quasi-phase matching conditions can be satisfied. The SHG enhancement achieved due to the double resonance was 5000 times greater than for the unpatterned GaN layer. (C) 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Since the work of Favennec et al.[1] it is well known that the quenching of luminescence from rare earth ions decreases with the host band gap. This has led to a large activity with silicon implanted or doped with RE, and then GaAs was used, in hope to realize simple, cheap light emitters. With a band gap of 3.4 eV at room temperature, GaN is even better suited to such applications. As a matter of fact, Steckl et al.[2] have demonstrated a green light emitting device based on Er doped GaN. This resulted in a renewed effort in this direction, but the crystal quality still have to be mastered and the physical phenomenon involved to be understood. In this work, GaN and Er-doped GaN with various Er concentrations were grown by gas source molecular beam epitaxy on high quality GaN templates grown by metalorganic chemical vapour deposition. In order to understand the influence of the Er incorporation on the crystal quality of GaN, Er-doped GaN were grown with a concentration between 0.1% and 5%. High quality undoped GaN were also grown, as a reference material, to show how the smallest amount of Er may affect drastically the structural and optical properties. All the samples were characterized by scanning electron microscopy, atomic force microscopy and X-ray diffraction. With these measurements, we demonstrate a strong correlation between the Er concentration and the surface roughness and the crystalline quality. This study shows that the activation of the Erbium luminescence is not improved with improving crystal quality. This assumption supports the idea that Er luminescence should be related to defect center in GaN.

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 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.

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].