Growth and Characterization of GaAs Single Crystals

From a commercial viewpoint, gallium arsenide (GaAs) is currently the leading member of the III-V compound family. Oriented substrates, cut and polished from single-crystal boules, form the materials foundation for a rapidly maturing technology of high speed and high frequency electronic devices and circuits. The initial thrust of GaAs applications was in high powered lasers and light-emitting diodes (LEDs) fabricated on n-type (Si-doped) GaAs wafers grown by the horizontal Bridgman technique. One of the important benefits of using GaAs is the high electron mobility compared to Si. This property has driven the development of low noise and power field-effect transistors (FETs) for microwave applications. The semi-insulating substrate requirement (>107 Ω-cm) was initially met by chromium doping. Currently, the interest is focused on MMIC (microwave monolithic integrated circuits), MIMIC (millimeter microwave ICs), analog ICs for lightwave transmitters and receivers, and digital ICs. The digital circuits have been realized with ion-implanted FETs, selectively doped heterostructure transistors (SDHTs), and heterostructure bipolar transistors (HBTs). Presently, most of the semi-insulating (SI) material processed by the industry is nominally undoped, and grown by the liquid encapsulated Czochralski (LEC) technique. The SI behavior is attained via a delicate balance of deep EL2 donors and carbon acceptors, avoiding chromium in order to eliminate the anomalous out-diffusion and type-conversion associated with this dopant.GaAs wafers up to 4 inches in diameter, with electrical properties homogenized by whole ingot annealing, are currently available from U.S. domestic and overseas suppliers. However, the overall quality is compromised by the large dislocation densities, varying 104 – 105/cm2.