DE2817285A1 - Extensive monocrystalline film prodn. at high growth rate - by passing temp. profile along polycrystalline or molten strip to give convex growth junction - Google Patents

Abstract

Prodn. of monocrystalline films of large surface area at high growth rate is achieved by passing a temp. profile along a polycrystalline or molten strip of the starting material, so that the transition into the monocrystalline zone takes place with a growth surface, the intersections of which, parallel to the plane of the film, have convex curature. The process can be used for making films up to ca. 1 mm thick. Thin films of Ag halides are useful in photography and holography and, with suitable doping, for ionising corpuscular radiation detectors, whilst thin semiconductor films are also of interest. The growth rate can be increased by a factor of 100, even with substances having poor thermal conductivity.

Description

Process for the production of large-area, monocrystalline foils The invention relates to a method for growing large-area, monocrystalline layers - foils or countries - in thicknesses @is about 1 millimeter by converting a polycrystalline Initial structure with increased growth rate of the monocrystalline area, by creating it with a convex growth surface.

Monocrystalline foils are interesting in several ways, e.g.

a) as thin layers of silver halides n for use as optical information storage (photography or holography), or with suitable Doping for recording the traces of ionizing corpuscular beams (particle track detectors).

It has been shown that monocrystalline layers of doped Silver chloride advantageously in place of the silver halide core emulsions Can be used with new properties such as the dry development arc UV light, the ability to influence the particle recording - yes or no - by accompanying Illumination with visible light and the constancy of form as non-hygroscopic Solid-state system, with the spatial resolution of the microsp.

b) as large-area, monocrystalline semiconductor layers.

It is important for these applications that the layers with sufficient Homogeneity, even with high doping, in larger areas in a simpler, more rational way Way can be made.

To grow crystals or thin crystalline layers exist a number of processes:,) Epitaxial vapor deposition on a crystalline substrate.

This method is the one mentioned for the creation of splints Thick too slow and unsuitable.

2) Growing from the melt according to the methods or Kyropoulos.

They lead to massive cylindrical blocks from which thin layers are made must be made by sawing and subsequent processing of the surface.

Apart from the mechanical difficulties of making larger ones thin layers, the process has the disadvantage of a slow growth rate - a factor of 100 less than the procedure on which the registration is based. this brings the emergence of concentration gradients to the doping components and Difficulties in doping with high concentrations above the solubility limit in the crystal with itself.

3) Has a way of making crystalline thin films R. Mitchell for silver chloride shown; that located between carrier plates (glass) Silver halide is melted from a hot base onto a cold base moved and froze. This creates monocrystalline areas of random size, up to about an inch square.

A certain variant is a method agar, in which a series parallel carrier plates from the melt, guided at a suitable distance from one another to be pulled. Low pulling speed leads to the formation of larger ones monocrystalline areas, but with high doping concentration for segregation. This effect occurs in particular in the case of the silver chloride particle track detectors mentioned cadmium used in concentrations of up to 5000 ppm.

4) There is another possibility of producing monocrystalline foils in the conversion of band-shaped pre-formed polycrystalline structures into monocrystalline structures by remelting, e.g. in zones, and subsequent monocrystalline solidification.

The polycrystalline structures in film form can be known in a Way, for example by melting powder between plates BEZW. in flat Containers (pots or tubs) or preformed by sintering, pressing or rolling getting produced.

When converting the polycrystalline structure into u e monocrystalline the following conditions must be met: a) the formation of different orientations to suppress when the layer solidifies, b) changes in the doping concentration, even at high concentrations, both along the plane of the film (x, y) and in Avoid transverse direction (z).

As is known (Hurle 1962), condition (a) can best be fulfilled, when the monocrystalline region with a convex growth surface is formed.

The flat geometry is used for the formation of the convex growth surface the film is essential, de according to the invention a mutually independent setting the temperature gradients and their local course in the x, y-foil plane allow, without restriction by the breeding speed.

This is not possible with a non-planar, e.g. cylinder geometry, because the temperature of volume areas close to the axis via their heat exchange with the outside areas is coupled.

The temperature gradient in the z-coordinate - perpendicular to the plane of the film (Fig.1) - is chosen so that one foil surface solidifies last. Through this will tle formation of two differently oriented film layers from the surfaces avoided.

The increase in the achievable breeding speed by about the factor 100 - also with substances with poor heat conduction - allowed, condition (b) as well to meet and prevent changes in concentration.

An embodiment for the production of Cd-doped AgCl single crystal foils as particle track detectors is described below; it allows manufacture from AgCl foils of e.g. 50 to 400 µm thickness, 70 mm width and 800 mm length.

The AgCl layer fused between quartz glass plates (Fig. 1), first solidified polycrystalline, i @ a tube furnace with three Kanthal belts (Fig.2 and laterally and above by means of radiation stimulation so that (Fig.1) one in the drawing direction The enamel zone is about three centimeters in length, with one also in the case of toner Growing speed adjustable, convex solidification isotherm.

Fig. 3 shows the arrangement of the three heating bands in a longitudinal section. Outside of each about 4 cm long annealing zone i cr cross-section reinforced by three times. The side glow zones are opposite to the upper one by about three centimeters moved to the left in the direction of drag. You have the task of the solidification isotherm to "bend" from the concave to the convex course (see Fig.1).

The Ztchtofen with the three heating bands is vibration-free the cultivation tube filled with protective glass with z.3.

40 cm / h moved, compared to 1-3 mm / h in the normal Bridgman method. The high growth rate in the hocn - over the solubility limit - with CdCl doped AgCl detectors is essential in view of the otherwise zone cleaning effects caused doping inhomogeneity.

5) The application of the convex growth surface according to the invention Ribbon-shaped geometry can also be applied to the drawing in the sense of Nacken, Czochralski of monocrystalline tapes. Fig. 4 shows a schematic of execution games: From a vessel with a correspondingly curved slot-shaped outlet (Fig. 4) becomes the melt is placed on a base made of quartz glass, for example, and on this with set thickness abe -strips. The solidification on the plate takes place briefly behind the scraper edge with a convex temperature profile, generated by appropriately designed Emitters of light or high frequency.

A prefabricated polycrystalline tape can also be used in an analogous manner crystallizes in a convex solid-liquid boundary that runs along it (Fig.5) or monocrystalline close to the melting temperature due to recrystallization tempering being transformed.

The transition to a rotating cylindrical base allows this Pulling endless crystal ribbons.

L e r s e i t e

Claims (1)

Claims Claim 1: Method for the production of large-area, monocrystalline molds with increased growth rate, characterized in that that along the ribbon-shaped polycrystalline or as a melt existing starting substance such a temperature profile is carried out that the transition into the monocrystalline Area with a growth area takes place whose parallel to the film plane Cutting lines are convexly curved. Claim 2: Process for the production of large-area, ronocrystalline Foils according to Claim 1, characterized in that a Temperature gradient is set so that the solidification from a surface to other runs. Claim 3: Process for the production of large-area, monocrystalline Films according to Claims 1 and 2, characterized in that the monocrystalline The substance to be transferred is preformed as a polycrystalline film. Claim 4: Process for the production of large-area, monocrystalline Films according to Claims 1 and 2, characterized in that the monocrystalline Slide to be transferred substance is in powder form in a shallow depth. Claim 5: A method for producing large, monocrystalline Foil according to claims 1 and 2, characterized in that the transition to monocrystalline state from the plastic or solid state close to below the melting temperature he follows. Attachments: 5 drawings