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.

Drastic changes in Raman spectra from GaN epitaxial layers are shown to depend on the GaN buffer layer deposition temperature: for temperatures higher than 600 degrees C, non intentional n-doping is evidenced by the screening of the allowed A(1)(LO) phonon by free carriers. Raman measurements at liquid nitrogen temperature confirm this interpretation and speak in favor of. degenerate carrier gas. 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. Defects correlated to three-dimensional growth and to non-radiative recombination processes in the layers are proposed as the origin of heavy n-doping and of the wave-vector non-conservation. (C) 1997 Elsevier Science S.A.

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.

We characterize by transmission electron microscopy (TEM), GaN layers deposited by metal organic chemical vapor deposition (MOCVD) on (0001) sapphire. Different GaN films with different surface morphologies have been observed and their crystallographic quality determined. Polarity and surface diffusion are the important factors that determine the surface morphology. The lack of an adapted buffer layer leads to a layer with a dominant N-polarity that contains many inversion domains (IDs) (of Ga-polarity) that grow faster than the surrounding material and form pyramids. All the flat unipolar GaN films we have observed have a Ga-polarity. Unipolarity (Ga-polarity) is realized with the recrystallization of the low temperature buffer layer or/and of the nitridation of the sapphire substrate. An intermediate cubic phase has been observed at the sapphire/buffer layer interface of optimized nitridated samples. In non optimized samples, IDs (of N-polarity) can remain near the buffer layer, but they tend to disappear during the growth of the Ga-polar GaN layer. A high growth temperature (about 1000 degrees C) was necessary to obtain flat GaN layers. (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.