Patent GB-769673-A (GB)
Title: Processes for the control of solute segregation by zone melting
IPC: H01L21/00
Filed: 1952-11-14
Abstract: <PICT:0769673/III/1> <PICT:0769673/III/2> <PICT:0769673/III/3> <PICT:0769673/III/4> A method of producing a desired distribution of a minor ingredient in a body of material, in particular a desired distribution of a conductivity determining impurity in germanium or silicon, and also of growing crystals of germanium, comprises forming at least one molten zone in the body and causing the zone to traverse the body by melting the solid at one solid-liquid interface and simultaneously freezing liquid at the other solid-liquid interface, the arrangement being such that the distribution of minor ingredient obtained is substantially different from that which would be obtained by melting the whole body and allowing normal freezing to take place. The invention is also applicable to the refining of systems of metals, and other treatments of systems of metals and salts, in which the rate of diffusion of the minor ingredient in the solid is low compared to that in the liquid.  The method may be used to concentrate unwanted impurities at the end of a bar by zone-melting, that is by setting up a small molten zone at one end and causing it slowly to traverse the bar, carrying with it the impurity.  This method requires that the distribution co-efficient d , defined as the ratio of the concentration of solute in the freezing solid to that in the liquid from which it freezes, be less than 1.  Figs. 4 and 5 illustrate the result of a single zone-pass on a body of material represented diagrammetically in Fig. 4, which body has initially a mean concentration Co of a minor ingredient which is substantially uniformly distributed on a macroscale.  A molten zone of length L is set up at the left-hand-end of the body and caused to traverse the entire body sufficiently slowly to ensure that diffusion in the molten zone keeps the composition of this zone uniform along its length.  Fig. 5 shows the variation in concentration of the freezing solid (i.e. the fiinal concentration distribution) with X, the distance along the body, for a system in which g <1.  As shown in Fig. 13b a molten zone 17 is set up in material 15 in a tube or boat 16 of quartz by a heater 12 in the field of induction coil 13, the tube being moved through the heater from left to right to form a single crystal of germanium on seed crystal 41, hydrogen being passed through tube 16 by pipes 20.  Fig. 11 shows a method of arranging a zone 3 of varying size in conjunction with such apparatus, and Fig. 14b shows a system of refining by a multipass method in which a open toroidal body of the material is rotated about its axis through heaters 27 to obtain a refined body with a small impurity-containing zone at the end, which zone may then be removed.  Alternative systems use a straight rod with a number of heaters through which it is passed to set up a number of moving zones, or a closed ring otherwise similar to that shown in Fig. 14b may be used in order to obtain uniform impurity distribution. Reverse zone-melting may be employed, in which after one or more passes in a forward direction, and optionally after cropping the solute from the end of the body, a zone-melting pass in the reverse direction is effected; this increases the uniformity of the distribution of impurity.  In the case of a semi-conductor with two conductivity-type-determining impurities of different type (e.g. members of Groups III and V in the case of germanium or silicon), the freezing solid may be arranged to have the difference between the concentrations of the two impurities constant along the body by adding appropriate starting charges to the molten zone when formed at one end of a rod (this method being used to prepare starting material for re-melt junctions to be obtained by methods described in Specification 769,674 [Group II].  Alternatively the conductivity type may be made to change from one type to the other by altering the zone length during a zone melting pass, by adding pills of one or the other impurity type to the zone as it passes along the body, or allowing an impurity-containing gas to contact the zone and give up impurity thereto.  Instead of melting a zone across a complete cross-section, a surface layer molten zone only may be employed. Rates of zone travel up to about 0.006 inches/second but preferably below 0.003 inches/second may be used to obtain single crystals of germanium by the method.  Specification 310,812 [Class 41], 359,731, 456,085, 456,086, 477,861 [all in Group II], 590,458, 592,303, 549,121, 694,021 and 700,231 [both in Group II] also are referred to.