We have studied the fabrication of Pt/Au Schottky diodes on n-type GaN. We show that the electrical characteristics of the diodes are strongly dependent on the surface chemical treatment before the metal deposition. Lowest leakage currents were obtained by the use of a HC1 solution. We also show that annealing the diode at a moderate temperature (400°C) leads to reduced reverse currents. In order to explain these results, we measured the density of deep levels in the Schottky diode depletion region before and after the annealing process. We did not observe any significant difference in the bulk density of defects due to the anneal. We also studied the temperature dependence of the reverse currents and found a low activation energy. Our results are interpreted in terms of electrical defects at the metal-GaN surface.

Photoluminescence under intense excitation is studied in GaN. As the excitation density increases, we show the Mott transition between an excitonic recombination to a plasma-type recombination. The carrier density at the Mott transition is given. At and above the Mott density, we show that the carrier temperature is higher than the lattice temperature. The energy relaxation of the hot plasma is shown to be dominated by LO-phonon emission. Coulomb screening and band-gap renormalization are observed from the photoluminescence peak position and the measured renormalization factor is in good agreement with elementary many-body theory. Finally the dependence of the Mott density on carrier temperature is shown to follow a Debye-Huckel model. [S0163-1829(99)10727-6].

Two new aspects of photoluminescence in GaN and alloys are presented. First, quantitative photoluminescence is carried out in a double InGaN/GaN quantum well structure. By comparing the luminescence intensities from both wells, we could extract the recombination velocities in both wells. We show that the capture is more efficient in a deeper well. Second, photoluminescence under strong excitation density is studied. Hot carrier phenomena are clearly demonstrated. From the high energy tail, we determine the electron temperature and we show that the main energy relaxation mechanism is the optical phonon emission. The effect of carrier temperature on the phonon replica on the low energy side of the luminescence peak is also emphasized.

We report on the realization of etched mirror facets in GaN cavities by chemically assisted ion-beam etching. The etching conditions are adjusted to obtain a high degree of verticality and smoothness. Optical pumping experiments and gain measurements are performed in etched GaN cavities of various geometries. Stimulated emission and lasing are observed. The study of the value of the gain at threshold as a function of the cavity length allows a determination of the reflection coefficient of the etched mirror. The measured value of 15% is in good agreement with the one expected for a perfect air–GaN interface. This demonstrates the high quality of the etched mirror facets.

When additional carriers are introduced in a material with a non uniform concentration, they tend to diffuse on a scale given by their diffusion length. This parameter can be measured by different methods. Depending on the conditions, different values can be found as the recombination mechanisms differ. In this paper, we present the situation in GaN with various experiments including the photocarrier grating method, photoluminescence and the spectral response in photoconductors. We show that the diffusion length varies from 0.1 μm to a few μm depending on experimental conditions. The interpretation is given based on the diffusion equations and on the analysis of the recombinations.

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 μm 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.

We present the fabrication of mirror facets by chemically assisted ion beam etching. Optical pumping experiments are performed to characterize the facets. The reflection coefficient is measured and its value confirms the high quality of the etched mirrors. Optical resonators are then realized in a double heterostructure composed of InGaN and GaN. Stimulated and laser emissions are investigated in this heterostructure and we show that population inversion can occur in both materials.

Steady-state and transient responses of a nonintentionally doped GaN photodetector are investigated. The kinetics of the photoresponse demonstrate the existence of deep levels in the gap, acting as recombination centers with an acceptor character. The photoresponse displays two competing processes: a bimolecular recombination, dominating at high optical power range, and a monomolecular recombination involving long response times. The observed persistent photoconductivity and the huge photoconductive gain are due to the small electron capture cross section and a much faster hole capture rate.