We report continuous-wave and time-resolved optical spectroscopy of ZnO nanorods and nanowires of varying diameters grown by MOVPE, under varying thermo-dynamical conditions. We discuss the influence of these conditions on the different spectral components. Two families of photoluminescence lines are identified. The first one is related to defect-bound excitons and to their two-electron replica. The second one is correlated to near surface states. The relative intensities of various contributions appear to be strongly dependent on the growth conditions and of the diameter of the nanostructures. Photoluminescence decay time of the different lines has been measured on one of the samples.

We analyze the low temperature photoluminescence properties of two multi-layer stacking of GaN/AlN quantum dots. We report drastic differences of linewidths between continuous wave and time-resolved photoluminescence experiments. After the pulsed excitation, time-resolved photoluminescence reveals a substantial red shift of the line, which keeps a fairly constant width. In continuous wave experiments, the screening of internal electric fields by accumulation of e-h pairs in quantum dot planes induces a blue-shift. For samples with large number of quantum dot planes an unexpected narrowing of the emission line is observed when the laser intensity is increased. We assign the observed behaviors of energies and linewidths to the contributions of the in-depth decrease of the degree of excitation of the different planes. Our interpretation is supported by the use of a model based on a self-consistent solution of the Schrodinger and Poisson equations within the envelope function approximation.

The composition dependence of the band-gap reduction of GaAs1-xNx grown by molecular beam epitaxy and metal organic vapor phase epitaxy was investigated using transmission, reflection, and low-temperature photoluminescence (PL) spectroscopy for N incorporations ranging from doping concentrations up to x=5 × 10-2. We identified four different regimes of N incorporation with distinctly different band-gap scaling. N-doped GaAs shows sharp PL lines due to N cluster states, but no significant change in the band gap. In the ultradilute region (10-5≤x≤ 1.5 × 10-3) a strong band-gap reduction was observed which scales according to x, irrespective of the local distribution of N atoms in the As sublattice. The same band-gap scaling was observed for ultradilute InGaAsN after corrections for strain and In alloying. In an intermediate compositional region (1.5 × 10-3≤x≤2.5×10-2) ΔEg scales according to x2/3. At higher concentrations (x>2.5 × 10-2) ΔEg weakens due to effects connected with N oversaturation of the As sublattice.

Piezoelectric effects on the optical properties of GaN/AlN quantum dots have been investigated by both continuous-wave and time-resolved photoluminescence (TRPL) measurements. The increase in excitation density in CW-PL leads to a blue shift of the transition energy. The TRPL measurements reveal a very large blue-shift (0.6 eV) of the PL peak energy, which reduces with time delay in a complex way, due to the strong transition-energy dependence of the carrier recombination time. The results are discussed in terms of respective roles played by the population of excited levels and by the screening of internal electric fields by accumulation of electron-hole dipoles in the quantum boxes. (C) 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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.

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.