Gaseous organic/inorganic thermal cracker for vacuum chemical epitaxy

#This invention relates to an apparatus capable of disassociating gaseous compounds. More specifically, this invention relates to a thermal cracking apparatus capable of disassociating organic or inorganic gaseous compounds and particularly to an apparatus capable of disassociating organic or inorganic compounds in the gaseous state by vacuum chemical epitaxy, (VCE).

#Various processes of fabricating III-V compound semiconductor layers require the use of arsenic and phosphorous in, for example, gallium arsenide, gallium phosphide, gallium arsenide phosphide, and the like. With molecular beam epitaxy, (MBE), elemental arsenic and phosphorous are often heated to obtain the species necessary to grow the desired semiconductor layers. However, the species of arsenic, i.e., As.sub.4, derived from heating elemental arsenic or phosphorous are difficult to handle and the polymer from leads to point defects or regions of high phosphorous or arsenic concentrations in the growing layer. To avoid these problems, phosphine, Ph.sub.3, and arsine, AsH.sub.3, have been used in chemical vapor deposition, (CVD), and MBE growth processes. These materials are generally broken down into smaller molecules or components by heating the molecule above its bond breaking temperature, i.e., "cracked" into useful species of P.sub.2 and As.sub.2 by passing them through a heated zone to liberated the hydrogen as H.sub.2 gas. The process of using the gas in a heated atmosphere to break down the arsine and phosphine is generally referred to as thermal cracking.

#Thermal crackers in the past have involved a tube through which the gas is passed coming into direct contact with a heating wire such as a material made from tungsten and tantalum. Although these devices are capable of cracking the gases, the process of subjecting the gases to direct contact with the heater wire permits the incorporation of impurities in the gas, both tantalum or tungsten and other impurities during the process of heating the filament to a temperature sufficiently high to crack the gas. In addition, the heating filament lifetime is shortened by coming into direct contact with the gas or gases during the heating operation.

#The drawing illustrates a cross-sectional view of an embodiment of a thermal cracker according to our invention.

#The invention will be more clearly illustrated by referring to the FIGURE. The FIGURE illustrates an organic/inorganic thermal cracker 100.

#Thermal cracker 100 includes a central elongate tube 10 of graphite or other suitable material which is stable at cracking temperatures and does not add deleterious impurities to the cracked gas stream. Prior art heaters have used quartz tubes which disassociate at thermal cracking temperatures to inject silicon and oxygen atom impurities into the gaseous stream during the semiconductor layer-fabrication. Graphite is stable in a reducing environment. The tube 10 is connected to a vacuum seal and flange 12 which fits into the wall of the vacuum chamber, the tube 10 connects to the outside of the chamber through a suitable gas-carrying tube 14 such as a stainless steel tube or a graphite tube. Tube 14 connects to the arsine or phosphine gaseous sources. Surrounding the graphite tube is a heating coil 16 of tungsten-rhenium or other suitable heating wires. The heating-coil wire 16 is sandwiched between insulating tubes 20 and 22. Suitable insulating tubes can be fabricated from Al.sub.2 O.sub.3 or like materials which are stable at the temperatures necessary to crack the gas and will not introduce any deleterious gaseous or ionic species into the vacuum chamber atmosphere.

#The length of the heating coil 16 and the number of windings is a function of the amount of gas to be cracked and the rapidity with which the gas needs to be cracked. Preferably, the heating coil should be as short as possible to minimize the amount of heat injected into the vacuum chamber portion of the VCE apparatus. The heating wire 16 is connected to the outside through a suitable vacuum seal 12 via wire 16a.