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].

Electrostatic effects which take place in group-m nitrides in their wurtzite crystallographic phase have important consequences on the optical properties of (Al,Ga)N/GaN multiple quantum wells. A low-temperature photoluminescence study shows that the behavior of the transition energy versus the barrier width is the opposite of that currently obtained for more usual m-V semiconductor compounds like arsenides: when decreasing the barrier thickness, a blueshift is observed. This behavior is attributed to a redistribution across the samples of the huge built-in electric field (several hundreds kV/cm) induced by the polarization difference between wells and barriers. The trend of the barrier-width dependence of the electric field is reproduced by using simple electrostatic arguments. However, the field magnitude is higher than that predicted laking account only piezoelectric effects. This result points out the role of the spontaneous polarization in wurtzite nitrides. [S0163-1829(99)13627-0].

We combined the self-consistent procedure of solving the Schrodinger and Poisson equations for electron and hole wave functions with the variational calculation of exciton states in strained GaN/AlGaN quantum wells. The procedure accounted properly for the free-carrier effects on the excitonic wave function, namely, bleaching and quantum exclusion effects and allowed to quantify the dependence of the exciton energy and oscillator strength on the optical pumping density. The calculation revealed an interesting interplay between the screening of the polarisation fields which leads to the increase of the electron-hole overlap and the screening of the electron-hole interaction which affects the exciton Bohr radius. Peculiar non-monotonic behaviour of the exciton binding energy as function of the density of electron-hole plasma results from these effects.

We calculate the original properties of excitons in GaN-AlGaN quantum wells by a variational approach in the envelope function formalism. The separation of electrons and holes by huge internal electric fields induces an enhanced dependence of exciton binding energy and oscillator strength on the well width. We demonstrate the necessity,to perform excitonic calculations for obtaining, in particular, reliable values of oscillator strengths (thus radiative lifetimes), which are extremely sensitive to the well width.

Optical anisotropy in the growth plane has been observed by reflectance and photoluminescence (PL) spectroscopy on nitride epilayers grown on A-plane sapphire. This anisotropy results from the strongly anisotropic thermal strain which is such that the wurtzite symmetry goes from Cg, to C-2v Time-resolved PL reveals less marked anisotropy for PL decay times, due to the contributions of both radiative and non-radiative recombination channels. Nevertheless, by using a multilevel modelling of the time-dependent PL accounting for the optical anisotropy, we are able to separate the two types of recombinations. Last, we present cw and time-resolved PL results on GaN-AlGaN single quantum wells grown on A-face sapphire.

Picosecond time-resolved photoluminescence is used to investigate the recombination dynamics of excitons in samples which ail contain four GaN-AlxGa1-xN quantum wells of respective widths of 4, 8, 12 and 16 molecular monolayers, grown by molecular beam epitaxy. The compositions and thicknesses of the barriers have been varied, in order to change the electric fields induced by piezo- and pyro-electric effects. The dependences of experimental decay times with barrier characteristics indicate the presence of efficient inter-well carrier escaping. Calculations of electronic tunneling times versus barrier width show that the present carrier escaping is enhanced by some additional process, such as scattering by impurities or composition fluctuations in the barriers.

We present the first experimental study of the exciton spin relaxation dynamics in InGaN/GaN multiple quantum well structures after quasi-resonant picosecond excitation with linearly polarized light. Whereas short spin-relaxation times are generally expected in GaN-based bulk structures, for multiple quantum well structures we found long spin-relaxation times, around 100 ps, when the indium content was not too high. Our results suggest that the energy relaxation toward the radiative state can preserve the linear polarization of the exciton.