By a combination of conventional, HREM and CBED TEM experiments we have studied wurtzite GaN layers grown by Metal-Organic Chemical Vapour Deposition (MOCVD) on (0001)Al2O3. We experimentally determine the structure of the macroscopic hexagonal pyramids that are visible at the surface of the layers when no optimised buffer is introduced. These pyramids look like hexagonal volcanoes with one hexagonal microscopic chimney (up to 75nm wide) at their core. The crystal inside the chimney is a pure GaN crystal with a polarity opposed to the one of the neighbouring material : the GaN layers grown on (0001)Al2O3 are everywhere Ga-terminated except in the chimneys where they are N-terminated. Some of the N-terminated chimneys grow faster and form macroscopic hexagonal pyramids. Chimneys bounded by Inversion Domains Boundaries (IDBs) originate from steps at the surface of the substrate and may be suppressed by an adapted buffer layer.
The temperature and excitation power dependence of a bound exciton photoluminescence line S with a localization energy Q=11.5 meV has been studied in undoped and moderately Mg-doped wurtzite GaN of high resistivity. The data provide strong evidence that line S is due to recombination of excitons bound to ionized shallow donors. The consistency of this assignment with theoretical predictions is demonstrated
Oxygen doped GaN has been grown by metalorganic chemical vapor deposition using N2O as oxygen dopant source. The layers were deposited on 2" sapphire substrates from trimethylgallium and especially dried ammonia using nitrogen (N2) as carrier gas. Prior to the growth of the films, an AlN nucleation layer with a thickness of about 300 AA was grown using trimethylaluminum. The films were deposited at 1085 degrees C at a growth rate of 1.0 mu m/h and showed a specular, mirrorlike surface. Not intentionally doped layers have high resistivity (>20 kW/square). The gas phase concentration of the N2O was varied between 25 and 400 ppm with respect to the total gas volume. The doped layers were n-type with carrier concentrations in the range of 4*1016 cm-3 to 4*1018 cm-3 as measured by Hall effect. The observed carrier concentration increased with increasing N2O concentration. Low temperature photoluminescence experiments performed on the doped layers revealed besides free A and B exciton emissi on an exciton bound to a shallow donor. With increasing N2O concentration in the gas phase, the intensity of the donor bound exciton increased relative to that of the free excitons. These observations indicate that oxygen behaves as a shallow donor in GaN. This interpretation is supported by covalent radius and electronegativity arguments
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