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.

Time-resolved photoluminescence studies have been performed on GaN/Ga0.93Al0.07N multiple quantum wells grown by molecular-beam epitaxy on a sapphire substrate. Radiative recombinations of excitons in both the wells and barriers exhibit average decay times of similar to 330 ps, at T = 8 K, with a significant spectral distribution of times. This distribution is interpreted in terms of localization of carriers by potential fluctuations due to alloy disorder and to well width and depth variations. The temperature dependence of the radiative lifetime tau(r). Of excitons in the wells is deduced from the measurement of both the photoluminescence decay time and intensity. We find that tau(r) increases linearly with temperature dependence partial derivative tau(r)/partial derivative T=20.5+/-0.7 ps K-1, for T <60 K. From this result, we deduce the radiative lifetime of free excitons in the low-temperature limit (related to the oscillator strength) to be of 2.4 ps, i.e., half one order of magnitude smaller than for GaAs/Ga-Al-As quantum wells. [S0163-1829(98)50916-2].

(Al,Ga)N/GaN quantum wells have been studied by temperature-dependent luminescence and reflectivity. The samples were grown by molecular beam epitaxy on (0001) sapphire substrates, and well widths were varied from 3 to 15 monolayers (ML's) with a 2-ML increment, thus providing a reliable data set for the study of the well width dependence of transition energies. The latter shows a strong quantum confined Stark effect for wide wells, and an internal electric-field strength of 450 kV/cm is deduced. X-ray diffraction performed on the same samples shows that the GaN layers are nearly unstrained, whereas the (AI,Ga)N barriers are pseudomorphically strained on GaN. We conclude that the origin of the electric field is predominently due to spontaneous polarization effects rather than a piezoelectric effect in the well material. [S0163-1829(98)50944-7].

We report CW and time-resolved optical investigations performed on two different InGaN/GaN multiple quantum well structures grown by MOVPE on sapphire substrates. In both cases, we find at low temperature (5K) strong "blue" luminescence bands, of which energy position does not change very much with temperature and corresponds with the low energy side of the. stimulated emission when exciting the sample with a pulsed nitrogen laser. From the time-resolved PL data, we show that these "blue" bands correspond with very long decay times, which can reach up to 4.5 ns at 8K. When increasing either the energy or the temperature, the decay time decreases and, at room temperature, we get for the lasing wavelength values in the range of 100 ps.

We present low-temperature-reflectance experiments, performed on GaAs-Ga1-xAlxAs asymmetrical double quantum wells, under in-plane uniaxial stress. rhe results, when compared to what is obtained from single quantum wells, exhibit some peculiar behaviors, which are typical effects of the asymmetry of the structures. A careful examination of these results is proposed in light of theoretical predictions about subband-to-subband transition energies and oscillator strengths. Our conclusion is that asymmetry makes it possible to obtain novel couplings between excitonic states, which cannot be accounted for without an accurate theoretical treatment of the Coulombic interaction, including intersubband mixings of valence wave functions.

Quantum-well ground and excited states are observed using wavelength-modulated reflectivity in InxGa1-xAs/AlyGa1-yAs single-quantum-well structures. To account for the dependence of their energies on indium and aluminum concentrations, we find that it is necessary to include the coupling between the light-hole GAMMA(8) valence band and the spin-orbit split-off GAMMA(7) valence band. The calculation is performed using a two-band envelope-function approximation, and good agreement with experiment is obtained.