Patent GB-803430-A (GB)
Title: Improvements in zone-melting processes
IPC: H01L21/761
Filed: 1955-06-21
Abstract: A molten zone is caused to progress through a body of semi-conductor or other material by contacting the surface of the body with a substance soluble in it and setting up a temperature gradient such that the body is everywhere below its melting-point but that the contacted surface is above the melting-point of the lowest melting mixture of substance and body and that a positive temperature gradient exists away from this surface.  Figs. 1A-1C show schematically the principle of the method; Fig. 1A shows the solidus and liquidus curves of the relevant part of the phase diagram of two-component system S (material, solvent) and R (substance, solute), the curves being straight lines so that k, the ratio between the proportion of solute in the frozen material to that in the liquid from which it freezes, is constant.  Fig. 1C shows the zone 1 progressing through a body 2 of solvent which is subjected to the temperature gradient of Fig. 1B; zone 1 could only remain stationary if it could maintain a concentration difference C1 - C2 between its left-hand and right-hand faces. Diffusion in liquid zone 1, however, causes the solute concentration at the left-hand face to fall towards C2, thus raising its melting-point and causing freezing at this face, simultaneously melting of solvent takes place at the right-hand face so that the concentration gradient is maintained and the zone progresses to the right.  The rate of travel of the zone may be controlled since it depends on the temperature gradient (TG); lower rates of travel being obtained by lower TG'S at which a more uniform distribution of frozen material is obtained since a smaller part of the curves of <PICT:0803430/III/1> <PICT:0803430/III/2> <PICT:0803430/III/3> <PICT:0803430/III/4> <PICT:0803430/III/5> <PICT:0803430/III/6> <PICT:0803430/III/7> <PICT:0803430/III/8> Fig. 1A is used; using a low temperature gradient may result in the zone length decreasing, possible to zero so that the zone solidifies before traversing the body.  Fig. 3B shows the use of heaters 27 and 29 to set up the TG shown in Fig. 3A in the material 28-31-30 so that liquid zone 31 moves to the right, movement is maintained by moving the heaters themselves.  Fig. 4 illustrates the refining of a charge 37 by the travel through it from heat sink 35 to source 36 of zones 39 of a selected solute; these are initially interleaved with slabs 40 of charge material and stacked at the low temperature end. Insulating walls 38 ensure parallel heat flow lines; the use of a solute in which an undesired impurity is more soluble than it is in the charge material enables substantial removal of the impurity.  The method may be used to add desired conductivity determining materials to a body of semi-conductor; Fig. 6 shows the formation of a P-type region 57 by sweeping a layer 55 of indium through a block 56 of N-type semi-conductor, and Fig. 8 shows in section the passage of a wire 75 of acceptor-containing material through block 76 of N-type material.  Lead wire containing 2 per cent antimony or gallium, or aluminium wire containing 2 per cent gallium, or gold containing 0.1 per cent antimony may be used as the traversing substance.  The zones may, in the case of silicon or germanium, contain somewhat more than about 0.1 per cent of aluminium, gallium, boron or indium for high conductivity P-type; somewhat less than 0.1 per cent of the above materials or may contain gold, copper or zinc for low conductivity P-type; about 0.1 per cent or higher of phosphorus, arsenic, antimony, bismuth or lithium for high conductivity N-type; and about 0.1 per cent or less of these for low conductivity N-type.  In the case of "area" zones swept through a block from a seed crystal, the orientation of the seed crystal is retained during the zone pass, while in the case of "dot" or "line" zones the orientation of the traversed block will communicate itself to the swept zone.  The use of very short length zones is advantageous in growing single crystals to reduce random nucleation, the zone lengths may be of the order of 1000th of an inch and non-parallel heat flow lines may be used to prevent increase of zone width (due to an oblique preferred crystal plane, for example the (1,1,1) plane in the case of germanium or silicon).  Specifications 734,650, [Group XXXIX], 769,673 and 769,674, [Group XXXVI], are referred to.