Photoelectric Properties Study Of AlN And BN Films Grown With Laser Molecular Beam Epitaxy

Group-III nitride semiconductors including AlN, BN, GaN and AlGaN have potential applications in ultraviolet even deep ultraviolet light sources and photodetectors. They have attracted great interest because of their large band gaps, outstanding physical and chemical properties. Based on a detailed analysis on the deposition process and mechanism of laser molecular beam epitaxy, the main parameters for film deposition were found. Hexagonal BN and AlN films were successfully prepared by optimizing experimental conditions. To characterize the films, XRD, Raman, FTIR, SEM, EDS, XPS, UV-V-NIR, IV and resistivity measurements were performed. The main results are as follows:1.Based on a detailed analysis on the deposition process and mechanism of laser molecular beam epitaxy, we found that the average kinetic energy of the particles before arriving at the substrate is determined by the laser power density, pulse duration and the ambient gas pressure. The film structure and crystalline quality were attributed to the substrate temperature together with those three factors above-mentioned.2.BN thin films were grown on Si (100) substrates by laser molecular beam epitaxy under various temperatures, pulse energy and nitrogen pressures. To characterize the films, FTIR, Raman and XPS measurements were performed. The results indicate that all the films deposited are h-BN. The crystallization improved with increasing the nitrogen pressure and pulse energy. For the films deposited under 700℃, 600 mJ/pulse and 2×10-2 Pa nitrogen pressure, a obvious Raman peak at ~1365 cm-1 were investigated , with which a N/B radio at 0. 958 was given by a quantitative XPS analysis. This agrees well with the chemical stoichiometric relation between B and N.3.AlN thin films were prepared under 300mJ/pulse, without nitrogen, while various temperatures with laser molecular beam epitaxy. The c-axis oriented AlN thin films have been deposited successfully even at a substrate temperature as low as 100°C. However, the diffraction peak of the substrate can hardly be seen under 300°C. Combined with the results of reflectance spectra, we can infer that metallic Aluminum particles are formed in the films. The inference was verified with resistivity measurements.4.AlN thin films were prepared at 300mJ/pulse, 250°C, under various nitrogen pressure. The grain size from XRD shows a maximum at 2×10-4 Pa, but the films are amorphous under nitrogen pressure above 2×10-2 Pa. The diffraction peak of the substrate can be seen only under 2×10-2 Pa. The resistivity of the films increased with the nitrogen pressure. This shows that the higher nitrogen pressure compensates the deficiency of nitrogen in the growing films, but it also led to average kinetic energy of the particles and poor crystallinity. The results from optical characterization are consistent with the XRD results.5.AlN thin films were prepared at 500mJ/pulse, 700°C, under various nitrogen pressure. From the XRD spectra we found that, the films deposited under 2×10-2 Pa nitrogen pressure are c-axis oriented, but the Al diffraction peak also appears; under 1 Pa, the films are also c-axis oriented with the best crystallinity; under 10 Pa, the films turn polycrystalline. From the SEM image, we found that there are many particles (diameter 1 - 3μm) on the surface of the film deposited under 2×10-2 Pa. The EDS spectra of the films and the elements distribution in the same places show the particles were rich in aluminum, but poor in nitride. Combined with the results of XRD, we confirmed that the particles are formed by the metallic aluminum. The electrical properties of the films have been significantly influenced by the presence of aluminum particles. Resistivity measurement shows that the AlN films deposited under 1 Pa nitrogen pressure are typical for insulators with the resistivity in the order of 1011~ 1012 ?·cm, while, they become conductors under 2×10-2 Pa. The electrical conductivity of the AlN films can be explained from percolation theory. The presence of metallic aluminum also makes the transmittance of the AlN films worse, while it does not damage the structure or crystallinity of them.