GB853979A - Improvements in or relating to the preparation of monocrystalline structures - Google Patents

Abstract

<PICT:0853979/III/1> <PICT:0853979/III/2> A monocrystalline body of a substance is prepared by condensing the substance in its gaseous phase on the surface of a substrate which is initially provided by a surface which is part of and which lies in a principal plane of crystallization of a crystal of a material different from the substance and having beneath the surface an internal crystalline structure whose lattice constant approximates that of the substance, and which substrate is later provided by a monocrystalline deposit of the substance already condensed on the crystal surface, the temperature of the substrate being adjusted during the condensation to maintain the substrate temperature within the critical range at which the substance will condense in monocrystalline form on the substrate. In a method for preparing monocrystalline germanium the substrate is a polished single crystal of sodium chloride 19 (Fig. 1) attached to a tantalum plate 20 in an evacuated chamber 1a. Water vapour is introduced into the chamber 1a and water vapour particles are ionized by an electrical discharge between two electrodes 18 in front of and behind crystal 19, and the surface of crystal 19 is bombarded with ions of water vapour which remove an amorphous layer and expose the monocrystalline structure of the crystal. The chamber 1a is again evacuated, and a crucible 8a on a plate 4 is heated to 1500 DEG C. to evaporate germanium contained therein, some condensing on a tantalum plate 9, which is then heated to 1200 DEG C. to evaporate the germanium thereon so that some condenses on a plate 10, which is heated in turn, and so on with plates 11 and 12, purified germanium being obtained on plate 12. Plate 12 is heated to 900 DEG C. to evaporate the germanium which passes through an aperture 15, when a shutter 14 is opened, and through an aperture 17c in a plate 17, and deposits on crystal 19. Monochromatic light from a source 26 (Fig. 2) is passed through a collimator 27, a Nicol prism 28, and a quarter-wave plate 29, is reflected from the condensed layer on crystal 19, passes through a quarter-wave plate 30 and a Nicol prism 31, and is received by a photocell 33. The signal from the photocell 33 is amplified in an amplifier 34 and then applied to the vertical deflecting plates of a cathode ray oscilloscope 35. The Nicol prism 31 is rotated about the axis of the optical path by a synchronous motor 32 energised by an alternating current source 36 which also synchronizes the horizontal time base generated by the oscilloscope 35. Before the deposition commences the quarter-wave plates and Nicol prism 28 are adjusted so that two different maxima appear on the trace of the oscilloscope. The tantalum plate 20 (Fig. 1), and thus crystal 19, is heated to 430 DEG C. by passing high frequency electric current through a pair of separate coils 23 and 24 from a high frequency oscillator (not shown) and two adjustable attenuators (not shown), so that the currents in the coils set up electrical fields which thread the plate 20 in opposite directions. When the germanium begins to deposit on the crystal 19, the two maxima on the oscilloscope become weaker and the relative strengths of the currents in coils 23 and 24 are adjusted to stabilize the two maxima, this state being kept throughout the evaporation. When it is desired to add conductivity-determining additives to the germanium, plate 4 is rotated so that a crucible 8b containing the additive is beneath aperture 15, and the additive is evaporated therefrom on to the deposited germanium, which causes the maxima on the oscilloscope to disappear. The crystal is then heated to 350 DEG C. to allow the additive to diffuse into it, which, when complete, causes the maxima to appear again. Different additives from several crucibles may be deposited on the germanium to obtain several N and P type layers in the germanium single crystal. The several additives are passed through differently shaped apertures in plate 17 (which is rotatable), so that several layers each having a portion which does not overlap with the other layers, are obtained in the monocrystal, to enable independent connections to the several layers to be made when the crystal is for use as a semi-conductor device. The germanium crystal is removed from the sodium chloride substrate by dissolving the latter. Specification 841,154 is referred to.