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The metal-organic vapour phase epitaxy (MOVPE) growth of GaN on 6H-SiC with a conductive AlGaN buffer layer has been studied. With only 6% Al incorporation, a continuous 200 nm thick AlGaN layer n-type doped in the 1018 cm-3 range was obtained. A crack-free 1.5 µm thick GaN layer was subsequently grown. The conductivity of the buffer was checked by I(V) measurements with Ni/Au contacts deposited on the back of SiC and on the GaN surface. The layer exhibits strong low temperature (10 K) band edge luminescence at 3.465 eV related to donor bound exciton with a ratio to deep level about 103 and an FWHM of 3.6 meV. Only threading dislocations are evidenced by TEM with a density of ~109 cm-2. The FWHM of the (0002) line of GaN in rocking curve scan by x-ray diffraction is 150 arc sec. These properties are similar to the ones of GaN layers deposited on an insulating AlN buffer layer.

We report on the fabrication and characterisation of ultraviolet photoconductive detectors based on Si-doped AlxGa1-xN (0 x0.35) epitaxial layers grown on sapphire. The peak responsivity shifts from 365 nm to 300 nm for increasing Al contents. The devices present high responsivities (~100 A W-1 for Popt = 1 W m-2), which strongly depend on the incident optical power. This behaviour is explained by a light-induced modulation of the conductive area. In addition, the responsivity increases with carrier concentration and mobility, as also predicted by this model. Persistent photoconductivity decreases with doping, whatever the Al content, because of the enhancement of tunnel recombination across the potential barriers generated by surface and defects. Photocurrent decays have been successfully modelled by taking into account both the thermoionic and tunnel relaxation mechanisms, the latter being dominant even for undoped GaN.

The fabrication and characterization of ultraviolet photodetectors based on GaN p-n junctions is reported. The devices are grown by metalorganic vapour phase epitaxy on basal-plane sapphire substrates. These detectors are visible-blind with a sharp wavelength cut-off at 360 nm. The photocurrent is linear with incident power from up to , with a responsivity of at 360 nm. The device time response is dominated by the effective resistance-capacitance time constant, and a 105 ns response is estimated for very low load resistances. A comparison with the response of GaN photoconductor detectors is also presented. The application of these high-performance photodetectors for solar ultraviolet monitoring is described.

Gallium nitride epitaxial layers were grown by metalorganic vapour phase epitaxy (MOVPE) on bulk GaN crystals. These substrates were grown at high temperature (about 1800 K) and high pressure (about 15 kbar). MOVPE was carried out in a vertical growth chamber at atmospheric pressure. was used as the nitrogen precursor. The layers were examined using x-ray diffraction and optical photoluminescence. It was found that most of the layers exhibit smaller lattice constants than the substrates, which indicates a smaller concentration of point defects and free electrons. The photoluminescence spectra consist of sharp (down to 1.2 meV) near-band-edge peaks attributed to the excitons bound to neutral donors and broad yellow peaks centred at 2.3 eV.

The low-temperature photoluminescence of GaAs co-doped with Se and Ge by neutron transmutation is studied as a function of pressure in order to investigate the deepening of these donor states. Above 3 GPa, donor-acceptor pairs and free hole to neutral donor transitions attributed to Se donors linked to the X1c valleys are observed, and an indirect Se donor depth of 100 meV is deduced. No new deep donor level photoluminescence is observed in the vicinity of 1 GPa, as should be expected for deep Ge Al states. This is explained by the low radiative efficiency of these levels.

The ionization energy of DX centres in Ga(As,P) is shown to depend upon the chemical nature of the substitutional n-type impurity. Tellurium in Ga(As,P) introduces a DX centre lying at about 100 meV higher than the DX centres associated with other n-type impurities (Si or Sn). A comparative study between Ga(As,P) and (Al,Ga)As alloys confirms that the ionization energy of the tellurium-related DX centre is very sensitive to the local environment of the impurity.

In a thick (100 mu m) Sn-doped Al0.35Ga0.65As layer and an 11 mu m Si-doped Al0.41Ga0.59As layer the magnetic circular dichroism (MCDA) of the optical absorption, the optically detected electron spin resonance (ODEPR) and the photoconductivity (only in the Sn layer) have been measured simultaneously. All signals appear together after the photoionization of the DX centre and are directly correlated to each other. The ODEPR spectra measured in the MCDA bands show the hyperfine doublets due to the magnetic 117Sn and 119Sn isotopes (I=1/2) in Sn-doped samples and an ODEPR line (I=0) in Si-doped layers. The 4.7% abundant 29Si (I=1/2) isotopes could not be observed. The authors show that the MCDA is due to a neutral (Si0, Sn0) donor, a deep level which is stable at 1.6 K in the dark. This DX0 state needs thermal energy or light to capture an electron to form the negative-U DX- ground state which contains one donor dopant in a large lattice configuration.

MOVPE-grown, Te-doped AlxGa1-xAs samples (doping level (2-7)*1018 cm-3), with 0

Reports on low-temperature (5 K), high-pressure photoluminescence of GaAs doped with S, Si and Sn. The behaviour of shallow donor levels is compared when possible. Particular attention is given to deep-level photoluminescence. A photoluminescence band deepens with a L-like pressure coefficient in Si-doped GaAs, but such a feature is not observed in the samples doped with the other donors investigated. On the other hand the pressure dependence of the room-temperature hole lifetime in Si-, Sn- and Te-doped GaAs is studied, showing that a hole recombination centre appears in the 1.5-2 GPa range.

Photoluminescence decay measurements on n-GaAs under pressure indicate the appearance of a hole recombination centre at p approximately=2 GPa. This can be ascribed to DX centres with a hole capture cross section of 10-16 cm2.