Graduate Student:
K. Wang
Professor
D. Pavlidis and Professor J. Singh
Office of Naval Research N10014-92-J1552
Gallium Nitride (GaN) is one of the most promising wide bandgap semiconductors for applications in optoelectronic devices in the blue and ultraviolet (UV) wavelengths, and in high-temperature and high-power electronic devices. In the past several years, a number of notable advances have been reported in the growth of GaN. However, the success of GaN-based electronic and optoelectronic devices has been limited due to the presence of large unintended donor concentrations, the lack of high quality lattice-matched substrates, and the difficulty in controlling electronic properties. Little has been done on the systematic study of growth and incorporation processes in GaN. This research addresses the theoretical study of growth mechanics of GaN.
The dynamics of crystal growth from the vapor could be simulated by several basic events: impingement, surface reaction, surface migration, and evaporation. The thermodynamics, kinetics, and the microscopic details of incorporation of atoms play very important roles in controlling the quality of growth. To fully understand these important issues, an atomistic understanding of GaN growth must be developed. In this study, a model for MBE-like growth of GaN has been developed. A theoretical study based on Monte Carlo techniques was carried out.
Growth front contours simulated for GaN growth at 400C with V/III=10. The legend to the right
shows the number of surface monolayers. The results display rough (4 monolayers) growth front
which can be improved at higher growth temperatures.
Growth rate dependence on Ga flux. As the Ga flux increases the growth rate increases but the
growth front becomes rougher. As it turns out this is related to initiation of 3D growth for
increased gallium incorporation.
The vapor pressure of N2 on GaN at 800C is close to one atmosphere and becomes about 1000 atmospheres at 1200C. This feature not only causes difficulties in experimental growth of GaN, but also creates a special problem in computer simulation. Due to the high evaporation and migration rates of nitrogen, it is required to study more than 1012 events to simulate the growth of 10 monolayers of GaN for a 30 x 30 site area if conventional Monte Carlo approaches were used. This makes it prohibitively long for practical simulations. To circumvent this problem, a model for nitrogen incorporation has been developed.
Based on this model, a special Monte Carlo technique was developed. The importance of V/III ratios, growth temperatures, and Ga flux on the growth quality of GaN examined. It shows that at a given temperature and V/III ratio, the lower the Ga flux, the better the quality of the growth front. At a given growth rate there is a temperature ``window''; within which a high quality growth front can be obtained. Finally, for given growth temperature, the lower the V/III ratio, the better the growth front is. These theoretically estimated trends are evidenced by 2D and 3D growth front contours evaluated under various growth conditions. The goal of this work is to provide information on how certain growth conditions control incorporation of nitrogen and eventually to understand the growth mechanism of GaN.
