Patent GB-1015708-A (GB)
Title: Production of epitaxial films
IPC: C01B25/08
Filed: 1962-07-31
Abstract: 1,015,708. Semi-conductor devices. MONSANTO CO. July 31, 1962 [July 31, 1961], No. 29333/62. Addition to 1,011,979 Heading H1K. [Also in Division C1]  An epitaxial film having a graded forbidden energy gap is formed on a substrate with a cubic lattice structure consisting of a A III B V , A II B VI  or A I B VII  compound, or of silicon, germanium or a metal conductor by reacting in the vapour phase at least one volatile compound of an element from Group III b  with at least one element from Group V or with a volatile compound or compounds thereof. The total number of elements from Groups III b  and V involved in the reaction at any time is at least three and the relative proportions of either the Group III b  or V b  elements are varied as the reaction proceeds to deposit a material of changing composition on the substrate. Deposition is carried out in the presence of free hydrogen or a hydride of one of the III b  or Vb elements and in the absence of oxidizing gases. Metals suitable for use as substrates are gold, silver, calcium, cerium, cobalt, iron, irdium, lanthanum, nickel, palladium, rhodium, platinum, strontium, thorium, copper, and alloys such as aluminium-zirconium, antimony - cobalt - manganese, boron -titanium, and cobalt-titanium. The following A II B V  compounds are also stated to be suitable; sulphides, selenides, and tellurides of beryllium, zinc, cadmium and mercury, and the oxides, sulphides, selenides and tellurides of magnesium, calcium, strontium, barium, and cadmium. In a typical embodiment an N-type gallium arsenide substrate is first placed in a reaction tube and heated to 800 C. for 15 minutes in flowing hydrogen to remove oxide. Afterwards separate hydrogen streams flowing over heated sources of gallium tribromide, indium trichloride, antimony trichloride, arsenic and selenium are fed to the chamber. Initially the indium and antimony trichloride flows are suppressed and selenium doped N-type gallium arsenide deposits on the substrate. The flow of indium trichloride is then gradually increased at the expense of the gallum trichloride flow to give a film of changing composition. When the gallium tribromide flow has been reduced to zero the antimony trichloride is increased at the expense of the arsenic flow until the deposited material is N-type indium antimonide. The edges of the resulting crystal are lapped and a pair of electrodes attached to them. Such a device, Fig. 2, when subjected to a magnetic field as shown acts as a photovoltaic cell when illuminated on the gallium arsenide face. Another type of photovoltaic cell is formed in a similar manner on an N-type gallium arsenide substrate. In this case convergent streams of hydrogen chloride bromide or iodide flowing over heavily-doped P-type gallium arsenide and phosphide respectively are used, the relative flows being charged to produce a P-type film grading from gallium arsenide to gallium phosphide. Electrodes are attached to opposite faces to leave part of the sensitive phosphide face exposed. A PNP transistor may be constructed by epitaxial deposition on a P-type germanium substrate in a similar way using five hydrogen streams flowing over selenium, arsenic and the chlorides of gallium, indium and zinc respectively. The flows are controlled to give the composition and doping shown in Fig. 4. In another method a film changing progressively from indium antimonide to indium arsenide is deposited on a silver iodide crystal, using sources of indium chloride, antimony chloride and arsenic in separate hydrogen streams. An alternative way of controlling the film composition is by variation of the temperatures of the various sources. With this method of control a closed tube system may be employed. Linear, exponential and sinusoidal variations of energy gap may be achieved.