Wide bandgap nitride semiconductors have recently attracted a great level of attention owing to their direct bandgaps in the visible to ultraviolet regions of the spectrum as emitters and detectors. However, this material system with its favorable heterojunctions and transport properties began to produce very respectable power levels in microwave amplifiers. If and when the breakdown fields achieved experimentally approaches the predicted values, this material system may also be very attractive for switching power devices. In addition to the premature breakdown, a number of scientific challenges remain including a clear experimental investigation of polarization effects. In this paper, transport properties as pertained to electronic devices and potential switching devices, and polarization effects will be treated.

The 2-K recombination dynamics of coupled GaN-AlGaN multiple quantum wells reveals a composite nature in terms of the joint contributions of short-lived direct excitons and long-lived oblique ones. The possibility to observe these oblique excitons, which produce a unusual blueshift of the photoluminescence at long decay times is found to be in straightforward correlation with the presence of internal electric fields together with the existence of a slight disorder at the monolayer scale. [S0163-1829(99)09815-X].

Room temperature photoreflectance investigations have been performed on a series of AlGaN layers grown both by metalorganic vapor phase epitaxy and molecular beam epitaxy on c-plane sapphire substrates. The aluminum composition was ranging between 0% and 20%, and was determined independently in the different growth laboratories, by various methods. It is found that within the experimental uncertainty, there is no detectable bowing parameter in these alloys. This contradicts some previous experimental investigations and confirms other ones. (C) 1999 American Institute of Physics. [S0003-6951(99)01122-5].

Microcalorimetric measurements of small absorption coefficients have been performed on thin GaN-AlGaN quantum wells grown by Reactive Molecular Beam Epitaxy on Al2O3 substrates. In addition to strong absorption at the energy of the GaN buffer and at the energy of the thick AlGaN barrier layers, we could also readily detect transitions associated to the quantum well. These measurements which pave the way to a precise determination of the gap mismatch between the well and the barrier layers are combined with self consistent excitonic and envelope function calculations in the context of a model for the band line-ups which includes the piezoelectric effect. (C) 1999 Elsevier Science S.A. All rights reserved.

Numerical simulation of light reflection from a lambda/2 GaN microcavity with Ga0.8Al0.2N/Ga0.5Al0.5N Bragg mirrors grown on the A-surface of Al2O3 revealed a Rabi-splitting of the order of 50 meV and remarkable optical anisotropy. In a separate Bragg mirror, the exciton interference results in a 95% modulation of reflection spectra. These effects originate from the giant exciton oscillator strength in GaN and a pronounced uniaxial strain in the structure. (C) 1999 Elsevier Science S.A. All rights reserved.

GaN epilayers grown on A-plane sapphire experience an orthorhombic strain field giving an in-plane anisotropy of the optical response. By varying the polarisation conditions of reflectivity measurements, we measure the effects of the in-plane anisotropy of the strain field. Also, from a very careful lineshape fitting of the reflectivity spectra, we obtain the splittings between Gamma(2) and Gamma(4) excitons and report the first determination of the electron-hole exchange energy in wurtzite GaN, 0.6 +/- 0.1 meV. This value is compared to the data obtained for other III-I and II-VI semiconductors, taking into account the length of the chemical bonds. (C) 1999 Elsevier Science S.A. All rights reserved.

Very strong coefficients for spontaneous and piezoelectric polarizations have recently been predicted for III-V nitride semiconductors with natural wurtzite symmetry. Such polarizations influence significantly the mechanisms of radiative emissions in quantum-confinement heterostructures based on these materials. The photoluminescence decay time of excitons is used as a probe of internal electric fields in GaN-(Ga, Al)N quantum wells in various configurations of strain, well widths, and barrier widths. The measured decays are not only controlled by radiative lifetimes, which depend on the fields inside GaN wells but also on the nonradiative escape of carriers through Ga1 - xAlyN barriers, which depends on their widths and on the electric field in these layers. It is shown in particular that the magnitude of the held in the wells is not a simple function of the strain of these layers via the only piezoelectric effect, but rather the result of the interplay of spontaneous and piezoelectric polarizations in both well and barrier materials. [S0163-1829(99)02923-9].