Exafs Studies of Group III-Nitrides
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Mat. Res. Soc. Symp. Proc. Vol. 512 01998 Materials Research Society
set using the in-situ beam monitoring ion-gauge, assuming the relative sensitivity for In relative to Ga was 1.4. We have recently demonstrated that this method can be used to accurately control the composition over a wide temperature range from 400'C to 600°C. A growth rate of 0.3pim/hour was used and this was determined by the arrival rate of the Group III elements, since excess atomic nitrogen was provided. X-ray 0-20 and (o scans were carried out on each thin film sample using a Philips Xpert powder diffractometer. Electron Probe Micro-analysis (EPMA) also provided quantitative chemical analysis. The samples were investigated at both 5 and 10 kV primary beam voltages with the depths of the EPMA analysis estimated to be about 0.40 pm for In, 0.50 ptm for Ga and 0.32 ýtm for N at 10kV beam energy. To calculate both the thickness and composition of the layers it was necessary to employ the thin-film EPMA-method 9,due to the small thickness' of the samples. SIMS studies were performed in a Cameca 4f system using a Cs primary beam. Implantation standards were not available for the alloy films, so the results have been normalised to nitrogen and therefore the peak intensities do not give a quantitative indication of the alloy composition, only the variation of concentration of each species with depth. EXAFS studies were performed in total electron yield mode on station 9.3 at the UK Daresbury SRS facility. The data was analysed using standard data analysis packages (EXCURV92, EXBACK and EXCALIB) to model interatomic distances, along with number and type of nearest neighbour in each successive atomic shell. GaN and InN powders have also been studied as standards, results for interatomic distances for the powder samples agree well with literature values. Data from both the In-edge, and the Ga-edge of the (InGa)N alloys is analysed in this publication and will be compared in detail in a later publication with similar data obtained from the Ga-edge taken in REFLEXAFS mode reported previously 2 . RESULTS The X-ray 0-20 data for the whole range of (InGa)N alloys grown on oxidised Si substrates was reported elsewhere"0 and clearly showed a progressive variation in peak position as the composition changes from GaN through to InN. For the alloy samples only a single X-ray peak is observed, the (0002) Bragg reflection, with the higher order reflections being buried by the noise, indicating that only one composition is obtained for the crystalline fraction of the material. The low peak intensities indicated that the alloys appear to contain highly orientated crystallites with the rest of the film being of poor long range order (either very small crystal grains or amorphous material). The X-ray spectra of the pure InN sample showed a remarkable similarity to the X-ray powder diffraction pattern of InN, although again some differences in the ratio's of the peak intensities indicated that preferred orientations were evident. A comparison of the nominal composit
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