One of the most significant developments in semiconductor technology in recent years has been the increased use and importance of compound semiconductors, particularly the group III-V compounds composed of elements III and V of the periodic table such as gallium arsenide and indium phosphide. Such materials are used, for example, for making lasers, light emitting diodes, microwave oscillators and light detectors. Also promising are the group II-VI compounds such as cadmium telluride which may be used for making light detectors and other devices.
Most commercial use of compound semiconductors requires the growth of large single-crystal ingots from which wafers can be cut for the subsequent fabrication of useful devices. One of the more promising methods for such crystal growth is the vertical gradient freeze (VGF) method, particularly the VGF method described in the U.S. patent of W. A. Gault, U.S. Pat. No. 4,404,172, granted Sept. 13, 1983, and the paper, "The Novel Application of the Vertical Gradient Freeze Method to the Growth of High Quantity III-V Crystals," by W. A. Gault et al., Journal of Crystal Growth, Vol. 74, pp. 491-506, 1986, both of which are hereby incorporated herein by reference. According to this method, raw semiconductor material is placed in a vertically extending crucible, typically of pyrolytic boron nitride, which includes a small crystal seed well portion at its bottom end snugly containing a monocrystalline seed crystal. Initially, the raw material and a portion of the seed crystal are melted. An encapsulant material such as boric oxide can be included to aid in containing volatile vapors within the melt. The temperature of the system is then reduced in such a manner that freezing proceeds vertically upwardly from the seed crystal, with the crystal structure of the grown ingot corresponding to that of the seed crystal.
It is known that the high thermal conductivity of the pyrolytic boron nitride (PBN) crucible relative to the semiconductor melt creates a concave solidliquid interface near the crucible wall during crystal growth. It is believed that this shape, in combination with chemical interaction at the crucible wall and other parameters such as the stability of the growth rate, can be a cause of a crystallographic dislocation defect known as "twinning" when one attempts to grow the crystal, in particular, an indium phosphide crystal, in the &lt;100&gt; crystallographic orientation. Growth in the &lt;100&gt; orientation is desired because wafers can thereafter be cut perpendicularly to the ingot axis to obtain the appropriate crystallographic surface for device processing.
A common solution to the problem of twinning is to orient the seed crystal in a PBN crucible such that the semiconductor crystal grows in the &lt;111&gt; crystallographic direction (or, more specifically, the &lt;111&gt;.sub.B direction), which has been recognized to avoid or to reduce the incidence of twinning. When this is done, however, the wafer must be cut at an angle with respect to the ingot central axis of about 35.3 degrees so that the upper surfaces of such wafers lie in a crystallographic plane that is appropriate for device fabrication. Since the ingot is cylindrical, the slices or wafers cut at this angle are elliptical. It is difficult to use elliptically shaped semiconductor wafers efficiently and a significant wastage of usable semiconductor wafer area inherently accompanies their use.
Workers have alternatively tried to overcome the twinning problem by using quartz crucibles which have a much lower thermal conductivity and a different surface chemistry than that of the PBN crucible. However, quartz crucibles raise a new set of problems. During ingot cooling, there is a strong adherence between the glass crucible and the ingot by way of an intermediate layer of boric oxide, the material used as an encapsulant. The cooling results in a differential thermal contraction of the crucible, the boric oxide, and the ingot, which creates stresses that tend to fracture the ingot, rendering it in most cases useless.
Accordingly, there has been a long-felt need for a method for reducing the number of defects in grown compound semiconductor ingots. There has also been a need for such a method that is consistent with the growth of ingots in the &lt;100&gt; crystallographic direction.