Large semi-insulating gallium arsenide (GaAs) wafers with low dislocation density are required for the next generation of very high-speed integrated circuits. There is a current lack of availability of GaAs substrates with low dislocation density, particularly undoped, semi-insulating, and of size comparable to that of silicon wafers.
The GaAs growth techniques generally utilized in industry are Horizontal Bridgman (HB) growth and Liquid Encapsulated Czochralski (LEC) growth. Vertical Bridgman growth has never proved commercially practical, among other reasons because of crystal cracking.
Horizontal Bridgman growth typically proceeds from presynthezide GaAs in a sealed crucible or ampoule, and in commercial practice typically produces crystals having dislocation densities (etch pit densities) somewhat less than 10.sup.4 (e.g., about 5,000) dislocations per square centimeter. However, HB growth results in D-shaped ingots making it difficult to obtain wafers as large as 3 inches in diameter; further the HB process usually produces &lt;111&gt; oriented crystals, which must be sliced off-axis to obtain wafers having the desired &lt;100&gt; orientation. HB wafers often also exhibit silicon contamination and, if uniform semi-insulating properties are to be achieved, must be doped with chromium. This presents further problems since chromium often migrates (redistributes) during subsequent processing, thus degrading performance.
LEC-grown GaAs has more consistent semi-insulating properties, but higher (typically greater than 5.times.10.sup.4 per cm.sup.2) dislocation densities. To obtain consistent semi-insulating properties, it is also generally necessary to grow the crystals in expensive pyrolitic boron nitride (PBN) crucibles. To reduce dislocation density, efforts have been made to reduce temperature gradients, the melt has been doped with indium, and an extra thick boron oxide encapsulant layer has been used; none of these attempts have been particularly successful since each causes its own complications.
There is, in the industry, a need for a process that can consistently produce crack-free, semi-insulating GaAs having low dislocation densities.