THE GROWTH, CHARACTERIZATION, AND THERMAL STABILITY OF HIGH-ELECTRON-MOBILITY SEMICONDUCTOR HETEROJUNCTIONS

As GaAs-GaAlAs heterojunction technology matures, demands upon the epitaxial crystal-growth techniques increase. Molecular Beam Epitaxy (MBE) has answered with high material quality and precise dimensional and compositional control, and has produced the highest-quality selectively-doped heterojunctions (SDHJs) to date. Devices using the SDHJ are candidates for high-speed logic applications.

This thesis addresses the growth and characterization of the SDHJ. SDHJs require an atomically abrupt GaAs-GaAlAs heterojunction, high crystal quality, and an abrupt dopant profile. High-quality GaAlAs is grown at low rates and high substrate temperatures, but diffusion mechanisms identified herein make growth under these conditions detrimental to heterointerface and dopant-profile abruptness.

Reduced surface mobility caused by low substrate temperatures was thought to limit crystal quality at high growth rates, but common observations contradict this belief. This thesis observes that Ga(,2)O is emitted by the gallium effusion cell, and that Ga(,2)O incorporation is caused by decreasing the substrate temperature or increasing the growth rate. Both the phenomenon of abruptly deteriorating material quality and the growth conditions under which the deterioration is observed are predicted by the onset of Ga(,2)O incorporation. Ga(,2)O also affects the electrical activity of magnesium, and suppression of Ga(,2)O enables Mg doping of GaAs at unity efficiency, under conditions where little or no doping effect had been observed before.

In the growth of the SDHJ, aluminum and silicon evaporation sources are used at similar high temperatures. The Si flux cannot be directly measured due to impurity evolution. The Al source has more contamination because Al chemically attacks the crucible material. An improved Si source is constructed and record doping ability is observed for GaAs, suggesting that the conventional Al and Si sources limit material quality. It is concluded that if growth of high-quality GaAlAs at low temperatures and high rates is possible, a higher-purity Al source is required.