Engineering Considerations to Optimize Growth

Based on the study by Giling, as well as MOCVD growth practices (Ref 52), certain parameters are recommended for the growth of high-quality Group III-V semiconductor materials with sharp interfaces. First, laminar flows that are free of convection should exist by using a horizontal reactor, working at low pressure, and decreasing the reactor diameter. Second, temperature gradient should be present across the susceptor. Third, the memory effect should be eliminated by reactor geometries that prevent the development of vortices, and present no dead volumes. These conditions are designed to avoid parasitic reactions in the gas phase and condensation at the inlet of the deposition zone. Dead volumes should be avoided because they and the vortices will act as sources of unwanted materials that cannot be removed easily.

There have been at least 60 publications since the mid-1970s on the design of reactor cells and configurations. The essential factors in the design of horizontal cells have been established more recently (Ref 78, 79, 80, 81, 82, 83, 84). The ability to grow highly uniform compound semiconductors by MOCVD is influenced by a number of parameters in the gas-handling manifold and the growth cell. Several reactor design modifications have been reported to improve uniformity. First, the manifold must incorporate a pressure-balanced vent-run system (Ref 78). In addition, gas mixing must occur on the atomic level to ensure sufficient homogeneity across the susceptor. Compositional nonuniformity is eliminated by using a variety of gas-mixing devices (Ref 85), including vanes, meshes, mixing jets, spray tubes, silica frits and orifices, and perforated plates that are either stationary or rotating (Ref 78). The expansion region must prevent flow separation from the cell walls that leads to recirculation, upstream deposition, and nonuniform layers (Ref 70, 84, 86, 87, 88, 89, 90). The space between the substrate and reactor wall must be small enough to prevent return flow (Ref 70).

To overcome gas-phase depletion and geometrical effects, the substrate can be rotated (Ref 91, 92, 93, 94). Another approach to this engineering problem is to levitate the wafer carrier on a gas foil (Ref 92). To avoid parasitic gas reactions and to improve layer quality (Ref 95), some reactors use two separate inlets.

The understanding of the MOCVD process, including gas flow patterns and chemical-reaction pathways, is still immature. Growth parameters, such as temperature, pressure, total gas flow, and Group III/V ratio, are optimized empirically. Material is grown and characterized by ex-situ techniques. These results are correlated with the deposition conditions to predict the best processing parameters.

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