III-nitrides have been considered to be the most favorable materials in the field of white light illumination due to their direct band gaps[1, 2, 3]. This is because they can form a continuous range of solid solution and also, they allow a wide wavelength distribution from the visible to the deep ultraviolet region. Beside the lighting and display applications, III-nitride (GaN) material can also be used in the field of microwave devices[5, 6, 7]. However, the principal difficulty for GaN application is the absence of a suitable homogeneous substrate. The most popular substrates for the GaN epitaxial are sapphire and SiC[8, 9]. However, the mismatches of the crystal lattice and the thermal expansion coefficient between GaN and these heterogeneous substrates will lead to a large density of defects in the GaN epilayer[10, 11]. To reduce the density of defects, first a buffer layer has to be inserted and annealed at a high temperature before growing the GaN layer[12, 13]. In addition, the growth parameters for GaN needed to be optimized to achieve an improvement of the GaN epilayer's crystal quality. This is especially important in the early stage of GaN layer growth. In the past several years, a lot of work on the growth of high quality GaN epilayers has been done. Li et al. investigated the influence of the initial growth rate of the GaN layer at high temperature on the quality of GaN. Their studies found that the quality of GaN can be improved by decreasing the growth rate of the initial stage. However, if the growth rate was decreased to some extent, the quality of the GaN layer started to deteriorated again. Yang et al. studied the impact of the V/III ratio on the GaN layer quality during the initial stage of GaN growth. They declared that a low V/III ratio could reduce the nuclei density of GaN, and hence, an improvement in the GaN's quality. All researchers have come to a similar conclusion, that the control of the growth mode at the initial stage of GaN growth plays a significant role in improving GaN quality. Furthermore, a smooth surface morphology could also enhance the quality of GaN grown afterwards[16, 17].
Pits are the most widely observed defects on the surface of GaN film. They are caused by edge-type dislocations, which will lower the reliability of the GaN device and shorten the device lifetime. So, controlling the density of dislocations is significant in improving the performance of GaN devices.
In this paper, the influence of the growth rate and V/III ratio, at the high temperature (HT) initial stage of the growth of GaN, on the quality of GaN film and the effect of Mg-surfactant on the density of dislocation was studied. The results indicate that a high V/III ratio and a low growth rate at the initial stage of HT GaN growth could improve the surface morphology of GaN. In addition, the Mg-surfactant could reduce the density of edge dislocation significantly.