ALBERT

Notes: The molecular-beam epitaxial growth of (331)-oriented GaAs and (100)-oriented GaAs were compared. Silicon-doping behavior was studied as a function of As/Ga flux ratio and substrate temperature at constant Si cell temperature. GaAs layers grown on (100) and (331)B were always n type. For growths on the (100) substrates, the carrier concentration increased by about a factor of 5 as the As/Ga ratio was increased from about 1 to 50. On (331)B GaAs, the carrier concentration started out 3 times higher than on GaAs(100) and remained relatively level up to a flux ratio of 20; it then began to rise, becoming about 30% higher at a V/III ratio near 50. (331)A layers were invariably p type at low As/Ga ratios. As the flux increased, the acceptor concentration dropped rapidly. At high flux ratios, the (331)A GaAs became n type, with a donor concentration approaching that on (100). Photoluminescence (PL) measurements taken on crystals grown side by side showed pronounced defect exciton peaks for GaAs(100), less pronounced peaks for (331)A crystals, and no peaks for (331)B crystals. The carbon acceptor peaks for (331)A crystals were at least an order of magnitude more intense than the exciton peaks. PL measurements were also taken on multiquantum wells grown on the (100) and (331)A orientations. PL data also indicated that carbon was incorporated more readily in the (100) and (331)A layers than in the (331)B layers. The results of these experiments can be explained on the basis of the crystal structure of the surface, the As/Ga flux ratios, and the amphoteric nature of silicon. At the surface of the (331)GaAs, both cation and anion dangling bonds extend into the vacuum: two Ga dangling bonds for every As dangling bond on (331)A, and the reverse for (331)B. In contrast, there is only one site offered in the (100) orientation: either a gallium or arsenic double dangling bond. Thus, the number of sites available for As bonding should be a determinant of carbon incorporation. Since the (331)B has the fewest such sites, it should incorporate the least carbon, as, indeed, was confirmed by our PL and capacitance-voltage data.