Hexagonal Gan Mocvd Epitaxial Growth

Ⅲ nitrides have become one of the forefront research topics in semiconductor materials fields due to their successful application to fabricating of highly efficient optoelectronic devices operating in the ultraviolet and visible spectral regions. In this thesis, we grow hexagonal GaN on c-plane sapphire substrates in a horizontal MOCVD reactor equipped with an in situ normal incidence reflectance monitoring, and the focus has been turned to improve the quality of unintentionally doped GaN epilayer. Listed below are the main contents of this thesis.(1)A single-wavelength normal incidence reflectance monitoring system was installed. A reasonable explanation has been found to explain the reflectance transitions in the monitoring curves. By means of quantitative analysis, we accessed growth rate and film thickness of GaN epilayer, and even determined in real time the thickness of buffer layer from in situ measurements of normal incidence reflectance. By multi-process curve fitting of monitoring data, we extracted optical constant of GaN under growth temperature and the surface evolutionary process during GaN MOCVD growth, and the latter shows good agreement with results from atomic force microscope observations.(2) With the aid of in situ monitoring tool, we have investigated the effects of substrate nitridation. We have studied the influence of growth parameters of buffer layer under large flux of reactant materials and found that parameters such as V/Ⅲ ratio, growth temperature and ammonia flux during annealing have evident influences on the growth of buffer layer, while other parameters like growth pressure, pressure during annealing and ramping rate show little influence. In experiments carried out under small reactant source flux, it was found that parameters such as V/Ⅲ ratio, growth pressure, thickness of buffer layer, ammonia flux during annealing and ramping rate have strong influence on the growth of buffer layer, while the influence of TMGa flux and pressure during annealing can be omitted. With optimized buffer layer growth parameters, GaN epilayer with improved quality has been grown, whose FWHM of(0002) plane DC-XRD rocking curve is 6 arcmin.(3) The effects of growth pressure of buffer layer on the growth of buffer layer and GaN epitaxy, and the morphological evolution during high temperature growth have been investigated. It was found that growth pressure of buffer layer exerts a strong influence on the surface state of annealed buffer layer. Larger growth pressure of buffer layer led to the dramatic increase of structural and optical quality of GaN epilayer, and the improvement in quality was attributed to the transition of growth mode from 3D to quasi 2D, which was revealed by scanning electron microscope. There exists a strong correlation between initial surface morphology prior to HT growth and HT GaN growth mechanism and eventual threading dislocation density in epilayers. A possible dislocation reduction mechanism has been proposed.