Preparation Of Aln Films And Gan Films. Oxide Substrate Induced Growth Study

As the wide-bandgap semiconductors,Ⅲ-Ⅴgroup nitrides are under intense investigation owing to their excellent characteristics such as wide band gap, high electron mobility, high thermal conductivity, good chemical and thermal stability, and so on. They have wide potential application and development in manufacturing LED and LD in the range from blue to ultraviolet and other optoelectronic devices. They also suit for manufacturing high-temperature, high-frequency and high power electronic or microwave devices. However, because of the mismatch of lattice constants and thermal expansion coefficients between GaN and the substrates, there are high defect densities and residual strain in the heteroepitaxial GaN thin films. In order to relax the strain and reduce the defects, the buffer layers such as AlN are often used in the epitaxial growth of GaN thin films due to the small mismatch of 2.4% in the in-plane lattice parameters between AlN and GaN. In this paper, the growth of GaN films on Al₂O₃ and SrTiO₃ substrates using different AlN buffers has been systematically studied.Firstly, AlN films were deposited on Al₂O₃ by laser molecular beam epitaxy (L-MBE). The effect of growth temperature, pulse laser energy intensity and deposition rate on crystal quality of AlN films was studied, and the optimum conditions were found to be as follows: growth temperature 650℃; pulse laser energy intensity 200mJ/pulse and deposition rate 2Hz. It was found that there was a 30°in-plane rotation between the AlN film and the sapphire substrate in order to reduce the lattice mismatch and the interface energy. Therefore, the epitaxial relationship between AlN and Al₂O₃ substrate was AlN[1-210]//Al₂O₃[1-100] and AlN(0001)//Al₂O₃(0001).Secondly, cubic AlN thin films were fabricated on SrTiO₃ (100) substrates at different substrate temperatures, ambient nitrogen pressures and pulse laser energy intensities. The epitaxial relationship of cubic AlN film on SrTiO₃ substrate was AlN[010]//SrTiO₃[010] and AlN(100)//SrTiO₃(100). AlN films fabricated at 450℃had polycrystalline structures. Epitaxial cubic AlN films were obtained when the deposition temperature was increased to 650℃. However, the degradation of the film crystalline quality was found if the growth temperature was further increased to 800℃. AFM investigation revealed that surface morphology of AlN films strongly depended on N₂ partial pressure. The root mean square roughness values (RMS) for AlN film deposited at 650℃and at 10 Pa N₂ was about 0.674nm. It was also found that amorphous films were obtained when the pulse laser energy intensity was lower than 150mJ/pulse, while crystalline films were obtained when the pulse laser energy intensity was higher than 150mJ/pulse. In addition, XPS results demonstrated that no oxide phase existed in epitaxial AlN films.Finally, GaN thin films were deposited on Al₂O₃ and SrTiO₃ substrates using AlN buffers. The GaN target pellets were fabricated by cold isostatic pressing at 200 Mpa and then sintering at 850℃in order to improve its mechanical strength. It revealed that the qualities of GaN films on different substrates can be increased with the increase of the deposition temperature. The epitaxial relationship of hexagonal wurtzite GaN film on Al₂O₃ substrate using hexagonal AlN layer was GaN[1-210]//AlN[1-210]//Al₂O₃ [1-100] and GaN(0001)//AlN(0001)//Al₂O₃(0001), and the relationship among cubic zincblende GaN film, cubic AlN buffer layer and SrTiO₃ substrate was GaN[010]// AlN[010]//STO[010] and GaN(100)//AlN(100)//STO(100), respectively. It was also found that the full width at half maximum (FWHM) value of hexagonal GaN(0002) crystalline orientation distribution was 0.907°and the RMS was about 0.442nm, and the FWHM value of cubic GaN(200) was 0.053°and the RMS was 2.518nm. It showed that both the GaN films were of good qualities.