One of the most significant developments in semiconductor technology in recent years has been the increased use and importance of compound semiconductors. Particularly significant are the Group III-V compound semiconductor devices composed of elements of Groups III and V of the periodic table, such as gallium arsenide (GaAs) and indium phsophide (InP). Compound semiconductors are used in such devices as lasers, light-emitting diodes, microwave oscillators and amplifiers, and various types of light detectors.
Most such 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 defined in the U.S. pat. No. of W. A Gault, 4,404,172, granted Sept. 13, 1983 and assigned to Western Electric Company, Inc., which is hereby incorporated herein by reference. According to this method, polycrystalline starting material is placed in a vertically extending crucible including a small cylindrical seed well portion at its bottom end which snugly contains a monocrystalline seed crystal. Initially, the starting material and a portion of the seed crystal are melted. The power to the system is then reduced in such a manner that freezing proceeds vertically upwardly from the seed crystal. One major advantage of the VGF method is that, by using low thermal gradients, crystals with very low dislocation densities can be produced. Another advantage is that the crystal growth rate be changed with no effect on the crystal diameter.
It is well-known that the Group III-V compounds tend to dissociate at higher temperatures, with the more volatile Group V element escaping into the vapor phase. Several approaches have been developed to prevent or retard this tendency during crsytal growth. In one approach, escape of the more volatile Group V component is retarded by providing a vapor pressure of Group V vapor over the melt from a sperately heated reservoir of Group V material within the sealed growth container. It is also knwon that Group V material loss from the melt may be retarded with the use of any of various materials such as boric oxide (B.sub.2 O.sub.3), barium chloride (BaCl.sub.2), or calcium chloride (CaCl.sub.2) which act as diffusion barriers. Such additives, having a lower density than the molten indium phosphide, rise to the surface, encapsulate the melt, and, together with an inert gas pressure in the vessel, can contain the volatile vapors; see, for example, the paper of "Growth of Single Crystals of GaAs in Bulk and Thin Film Form," by B. A. Joyce, included in the book, Crystal Growth, edited by B. R. Pamplin, Pergamon Press, 1975, pp. 157-184 at p. 165.
The use of boric oxide as a diffusion barrier can sometimes lead to a fully stoichiometric crystal. But in most cases there is some loss of the volatile Group V material, resulting in excess Group III inclusions in the last-to-freeze portion of the crystal. "Inclusion," as is known in the art, is a small volume within the crystal structure having an excess of one of the constituents or of impurity atoms, e.g., an excess of indium in indium phosphide.
The use of a separately heated reservoir of Group V material has been found to require relatively large amounts of the Group V material for each crystal growth run. The pressure vessel must be cleaned frequently because most of the Group V material sublimes and deposits on the cold surface of the pressure vessel. Also, the requirement of a second heater consitutes an added cost and complication to the system.