A Modeling Study of GaN Growth by MOVPE
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Mat. Res. Soc. Symp. Proc. Vol. 395 01996 Materials Research Society
absent from the gas phase. The adduct-species decomposed at slightly higher temperatures than that associated with TMG decomposition. In this study we will describe a vertical reactor used to grow MOVPE GaN films. The emphasis will be on studying different chemistries that may be feasible. Predicted growth rates are compared to experimentally observed rates to determine the chemistry underlying epitaxial GaN growth. REACTOR MODEL & EXPERIMENTAL A schematic of the reactor is shown in Figure 1. TMG in hydrogen carrier gas is supplied in the inner tube while ammonia and hydrogen are supplied in the outer tube. The outer wall of the reactor is water cooled. The graphite susceptor is heated inductively with a RF unit. The ID of the reactor is 85 mm and the OD of the two inlet tubes are 6.4 mm and 25.4 mm respectively. The inner tube is at a distance of 114 mm above the susceptor which is 70 mm in diameter. All runs were performed at a reactor pressure of 100 Torr and a susceptor temperature of 1000 °C. The coolant and the inlet gases were assumed to be at room temperature. The total flowrate was kept constant at 12 slm (2 slm of NH 3 and 10 slm of H2) and the flowrate of TMG through the inner tube was kept at 0.748 sccm. The films were grown on 2" basal plane sapphire for a period of 1.5 hours. Two extreme NH + H TMG cases were studied. In Case I, there was a high hydrogen 3 2 + H2 flowrate of 5 slm through the inner tube. In case II, there was Radiation a comparatively low hydrogen flowrate of 0.2 slm in the inner tube. The balance of hydrogen in both cases flowing through the outer tube. Cylindrical coordinates have been used in the model and ".Water the computational domain extends from 20 cm upstream of cooled the substrate to 40 cm downstream. The fundamental equations of continuity, momentum, and energy balances and species conservation are used to describe the system [5]. With the assumption of no variation in circumferential direction, the Figure 1: Diagram of the reactor flow, temperature, and concentrations are obtained in two dimensions. The properties of the gas mixture are determined at any point using the concentrated species and applying ideal mixing rules. Susceptor
Physical and Transport Properties of Gaseous Species: Experimental values of thermal conductivity, specific heat, and viscosity of hydrogen [6] and ammonia [7] were fitted to obtain their functions of temperature listed elsewhere [8]. The Lennard Jones parameters obtained from experimental data were used whenever possible. For intermediate species, they were estimated using the method of group contributions [9]. Binary diffusion coefficients of gas phase species were either obtained from literature or estimated from their Lennard-Jones parameters. The thermal diffusion ratio of all the species were estimated using their Lennard-Jones parameters [10]. The values of the binary diffusion coefficient, Lennard-Jones parameters, and thermal diffusion ratios of all gaseous s
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