Theoretical and experimental studies of MBE growth of zinc-selenide

With the advent of the ultrahigh vacuum crystal growth technique of Molecular Beam Epitaxy (MBE), there is a surge in the research activity related to developing wide gap ZnSe for opto-electronic device applications in the blue range of the visible spectrum. The performance of devices fabricated critically depends on the quality of the doped and undoped ZnSe epilayers grown. To achieve device quality ZnSe by MBE, it is of great importance to understand the growth kinetics of ZnSe which will aid in optimizing the growth conditions. The object of the present study is to perform theoretical computer modeling and experimental studies on the growth kinetics of ZnSe. Realizing the criticality of anion to cation flux ratio, flux calibration experiments were performed to remove uncertainties from the fluxes measured by the quartz crystal monitor. With the calibrated fluxes, ZnSe epilayers were grown at unity flux ratio and the following inferences related to growth kinetics were made: (1) high sticking coefficients for Zn and Se, 0.64 and (2) Se stabilized growth fronts were observed based on RHEED reconstruction patterns. To explain the experimental observations plausibly, a growth kinetic model is proposed. It is proposed in this model that Se atoms arriving on Se islands, surface migrate in their physisorbed state and get incorporated at energetically favorable kink sites at the edges of the island. Its validity is tested out using a phenomenological theory. A Monte Carlo simulation program is developed to simulate the growth kinetic model proposed. Results of the simulation such as growth rate, sticking coefficients and RHEED intensity oscillation of specular spot from the simulations are compared with experimental observations and the agreement is good. The MC simulation model developed in this study is limited by the small size of the model and therefore, can not be employed for studies of non-stoichiometry and doping. To obviate these difficulties, an analytical model of MBE growth based on non-equilibrium statistical mechanics is developed for zincblende crystals. A preliminary growth kinetic study is performed to investigate the effect of surface migration on growth rate, and roughness parameter. It is observed that with increasing surface migration length, the growth rate increases and the growth front roughness decreases. The potential use of the analytical theory in doping studies and that of Monte Carlo simulation in ordering and clustering are also discussed