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.

Room-temperature photoreflectance spectroscopy is performed on a series of GaN-AlGaN quantum wells grown by molecular beam epitaxy. We show that the potentialities of this powerful investigation method previously demonstrated on many low-dimensional systems can be extended to nitride-based quantum wells for accurate large-scale characterisation. In particular, this technique allows us to get rid of optical interferences that usually prevent the observation of free-exciton transitions below the band-gap of GaN. Such transitions occur in wide quantum wells, due to large built-in electric fields which also quench the oscillator strength of the transitions. Also, this technique allows us to magnify the features of confined excited states, which are difficult to observe by standard reflectance.

Quenching of the exciton luminescence of GaN epitaxial films and GaN/AlGaN quantum wells in an electric field is observed and attributed to impact ionization of excitonic states by hot electrons. It is found that impurity scattering rules the momentum relaxation of electrons rather than the acoustic phonon scattering. The effective mean free path of hot electrons l(eff) is estimated. The value of l(eff) for quantum wells appears to be one order of magnitude larger than that for epitaxial films.

A mechanism for exciton Bose condensation is suggested. We show that the interaction of localized exciton magnetic polarons (EMP's) can lead to the formation of a free collective exciton magnetic polaron (CEMP) able to move as a whole particle. Critical conditions for this transition and characteristics of the CEMP are found. We interpret the recent data on giant magnetic polaron mobility in ZnSe/ZnMnxSe1-x quantum wells [G. A. Balchin, C. D. Poweleit, and L. M. Smith, in Proceedings of the 23rd International Conference on the Physics of Semiconductors, edited by M. Scheffler and R. Zimmermann (World Scientific, Singapore 1996), p. 2055] as a manifestation of the free CEMP formation due to clusterization of long-living indirect EMP's. [S0163-1829(98)52308-9].

The exciton binding energies are measured in GaN grown on A-plane sapphire. A value of 25+/-1 meV is found to be consistent with reflectance and photoreflectance experiments. This 25 meV value matches very well the results obtained on GaN epilayers grown on C-plane sapphire with strong biaxial compression, and seems to indicate that exciton energies in GaN epilayers are predominantly influenced by strain fields. From the in-plane anisotropy of optical response, we deduce a value of -2.4 eV for the C-5 deformation potential in GaN. [S0163-1829(98)03508-5].