Law pressure MOVPE has been used to grow a series of GaN epilayers, deposited onto (0001) sapphire substrates. All the growth parameters have been kept constant, except for the V/III molar ratio, which has been changed from sample to sample by changing the ammonia flow rate. The growth rate has been measured vs the V/III molar ratio at both low temperature (550 degrees C, growth of the buffer layer) and normal process temperature. A strong dependence of the growth rate vs the V/III molar ratio is found at both temperatures. Systematic photoluminescence and reflectivity experiments have been performed at 2 K on all samples. The photoluminescence is dominated by the donor bound exciton and donor acceptor pair are visible on samples grown under the lowest V/III molar ratio, indicating higher impurity incorporation. An optimum V/III molar ratio of 5000 is proposed, which corresponds to the highest growth rate observed and to the lowest full width at half maximum for the photoluminescence line. (C) 1997 Elsevier Science Ltd.

We address combined utilization of temperature dependent reflectance, photoluminescence, and Raman spectroscopy measurements to optimize the structural and electronic properties of GaN epilayers deposited on sapphire. Last, we study residual strain fields in such epilayers.

In this article, we report on the characterization of a photovoltaic detector based on an n-type GaN Schottky barrier. We first present the photovoltaic responsivity above the gap. Its spectrum is explained by the combined effects of absorption and diffusion. The hole diffusion length is estimated to be in the 0.1 mu m range with a numerical model. The photoresponse below the gap is also investigated and it is shown that the current generated by the internal photoemission is the major contribution to the photocurrent at reverse biases at 80 K. At room temperature, an additional component to the photocurrent is clearly demonstrated and identified. This extra current stems from the existence of traps. Several spectroscopy techniques are used to characterize those traps. The supplementary current emitted from the traps in the depletion region accounts for the spectral and the temporal behavior of the Schottky photodetector at room temperature. (C) 1997 American Institute of Physics.

The samples studied are thin GaN epilayers grown by metalorganic vapor phase epitaxy (MOVPE). At liquid helium temperature, the linear luminescence, transmission and reflection spectra of the films has been recorded, as well as the nonlinear emission spectra under high UV excitation, By using the variable strip length method, we have measured the gain coefficient as a function of the excitation intensity and the temperature. Pump and probe experiments have been performed in the femtosecond regime, showing crossed two-photon absorption without long lasting response. This allows to envisage the use of GaN for the characterization femtosecond pulses in the blue spectral range. (C) 1997 Elsevier Science S.A.

The low pressure MOVPE growth process of GaN deposited onto GaN buffer layers on sapphire substrates is studied in detail. The relevant growth parameters are identified, and their influences are studied. These include the influence of the sapphire nitridation, the growth of the GaN buffer layer and its subsequent thermal treatment, and the growth parameters of the GaN epilayer (growth temperature, precursor flow rates, V/III ratio, etc.). Low temperature photoluminescence was mainly used to investigate the layer quality and, as a result of this optimization study, we have been able to reproducibly grow layers which have a 2 K photoluminescence dominated by the free exciton. (C) 1997 Elsevier Science S.A.

Raman spectroscopy is used to analyze the effect of electrons on the lattice dynamics of unintentionally heavily doped GaN. The deposition temperature of GaN buffer layers on sapphire substrates is found to have an important influence on the presence of free carriers in GaN layers, evidenced by plasmon coupling to the A(1)(LO) phonon. Data from infrared measurements are used to calculate the Raman line shape of q= 0 coupled A(1)(LO)-plasmon modes in a dielectric approach and give a good fit of the L-(q= 0) component observed in Raman spectra. In particular, the fitting procedure applied to spatially resolved micro-Raman measurements reveals an inhomogeneous concentration of electrons on the scale of hexagonal microcrystallites. Partial screening of phonons with wave vectors differing from the q=0 transfer of incident and scattered photons is invoked to explain LO-like scattering over the whole spectral range of optical phonons, attributed to charge density fluctuations on account of its polarization properties. (C) 1997 American Institute of Physics.

Raman measurements on ZnSe/GaAs heterostructures show unusual behaviour of the optical phonon modes from ZnSe. High epitaxy rates are responsible for both poor structural quality at the interface and Fermi level pinning by carrier traps, The first effect gives rise, beyond a critical thickness, to polycrystalline growth associated with the activation of a theoretically forbidden TO-ZnSe mode, The second effect induces a spectacular enhancement of the interfacial LO-ZnSe mode strength, The competition between the latter local electric field effect and absorption in the upper disoriented part of the layer may result in the quasi-total extinction of the LO-ZnSe mode. (C) 1997 John Wiley & Sons, Ltd.