US3491004A - Method of manufacturing crystalline bodies consisting of one or more chalcogenides of elements from the group ii-b of the periodic system or of mixed crystals thereof and to bodies obtained by these methods - Google Patents

Description

"United States Patent f U.S. Cl. 204143 5 Claims ABSTRACT OF THE DISCLOSURE Epitaxial deposits of Group IIB chalcogenides are grown on a single crystal of germanium having substantially greater electrical conductivity than the epitaxial deposit. The germanium is then connected directly to an electrical source and anodically dissolved in an electrolyte which is chemically inert to the chalcogenide.

The invention relates to a method of manufacturing crystalline bodies consisting of one or more chalcogenides of one or more elements from the Group II-B of the Periodic System of the Elements or of mixed crystals thereof and to a crystalline body obtained by the use of this method. According to such a known method, cadmium sulphide single crystals were obtained by sublima tion in a sealed tube. In order to obtain a single crystal of comparatively large dimensions, for example, of a diameter of 1 cm., the rate of growth should be very low and comparatively solid crystals could then be obtained. The use of such crystals in semi-conductor devices, for example photocells, preferably requires comparatively thin plate-shaped bodies. Moreover, the crystal obtained by the known method should be subdivided by sawing or by other operations into plate-shaped bodies. This operation should be carried out very carefully in order to pre vent the comparatively brittle material from breaking or cracking.

The invention has inter alia for an object to provide a method of manufacturing self-supporting crystalline bodies, preferably single-crystal bodies, consisting of the said chalcogenides in which the afore-mentioned disadvantages are avoided. Furthermore, an object of the present invention is to obtain in a'simple manner plate-shaped bodies of the said chalcogenides which nevertheless may have comparatively large dimensions in the longitudinal direction and in the direction of Width. The invention further provides the possibility of manufacturing thin plate-shaped bodies the composition of which may vary in the direction of thickness and which, for example, may consist of zones of various compositions, the material throughout the thickness of the plate nevertheless consisting of one or more of the compounds associated with the said chalcogenides. According to the invention, the chalcogenides is deposited epitaxially on a single-crystal germanium substrate and the substrate material is then electrolytically removed at least for the major part. Germanium can readily be obtained in single-crystal form and in comparatively large dimensions, for example, by drawing from a melt or by zone levelling and subdivision of the rods obtained into plate-shaped bodies.

It should be noted that it is known per se to obtain bodies having so-called hetero-junctions, that is to say junctions between two semi-conductor materials of differice ent compositions, in that one semi-conductor material is epitaxially deposited on the substrate of the other material in single-crystal form. Layers of given compounds of the type A B have already been deposited on a single-crystal substrate of another compound of the type A B or of silicon or germanium. Furthermore, epitaxial layers consisting of a cadmium chalcogenide have been formed on a single-crystal substrate of an A B compound. In these known cases, the substrate material was not removed afterwards. It has now been found that chalcogenides of elements from Group IIB of the periodic system, more particularly cadmium chalcogenides, can grow epitaxially on a substrate of single-crystal germanium.

A difliculty involved in efiicaciously removing germanium from the epitaxial chalcogenide layer obtained is that it is diflicult to keep intact the comparatively brittle chalcogenide layer when the germanium is mechanically removed. Especially in the case of comparatively thin layers of these chalcogenides, for example, of the order of IOU/1., such mechanical operations can hardly be used. For selectively removing the germanium chemically, the difiiculty is met that cadmium chalcogenides are generally much more reactive than germanium. However, the fact can be utilized that germanium has a much higher specific conductivity than the said chalcogenides. Therefore, the germanium can be selectively removed advantageously by means of an electrolytic etching treatment in which substantially no etching current flows through the chalcogenide portion of the semi-conductor body obtained, since this current substantially entirely flows through the germanium of higher conductivity.

Since many chalcogenides are not resistant to acids, use is preferably made of a neutral or alkaline electrolyte bath. The bath preferably contains a cyanide, for example, a complex cyanide such as an alkali ferricyanide. Cyanide ions are capable of dissolving in a complex form many cations, but they generally do not displace chalcogen ions in an insoluble chalcogenide.

The invention will now be described with reference to an example.

The substrate consists of a monocrystalline p-type germanium body having a thickness of approximately 500p and a diameter of approximately 20 mm. with the major surfaces parallel to planes of the crystal. The resistivity of the germanium is 1 ohm cm. The germanium plate is previously treated by etching in a mixture of 30 parts by volume of boiling fuming nitric acid to Which 1 part by volume of concentrated hydrofluoric acid is added, whereupon the plate is rinsed with de-ionized water and then with alcohol in a manner, known as such, and is then dried in a nitrogen atmosphere.

At one area in a horizontally arranged quartz tube there is arranged a quartz tray containing 10 gms. of pure cadmium sulphide powder and the germanium plate is disposed at another area. The quartz tube is placed in a two-temperature furnace so that the tray containing the cadmium sulphide can be heated at a temperature different from that of the germanium substrate. A hydrogen current is passed through the tube, which current fiows along the cadmium sulphide before reaching the germanium plate. When a tube is used having an inner diameter of approximately 30 mm., a hydrogen current of 200 ml./min. is passed. In order to permit the cadmium sulphide to grow epitaxially on the germanium substrate, the cadmium sulphide is heated at 800 C. and the germanium body at 700 C. The heated cadmium sulphide then reacts with the hydrogen forming hydrogen sulphide and cadmium vapour. At the substrate an inverse reaction takes place in which cadmium sulphide is deposited epitaxially on the germanium body. The rate of growth is approximately 40n/hour. After two hours, the process is terminated and a germanium plate with an epitaxial cadmium sulphide having a layer thickness of 80, is then obtained.

The plate obtained is now stuck by the side on which the epitaxial layer of 80 thickness has formed to a glass plate, for example, by means of a thermoplastic synthetic resin insoluble in water, whereupon cadmium sulphide that may have grown on the other side is removed by means of diluted hydrochloric acid. By means of a contact clamping member provided with an electrical connection wire, a platinum strip is pressed on said side at the periphery of the germanium plate. The assembly is immersed in an electrolyte bath consisting of a solution of 0.3 g./mol of potassium ferricyanide in one litre of 1 N KOH. The contact clamping member is connected to the positive terminal and a platinum electrode also immersed in the electrolyte bath is connected to the negative terminal of a voltage source. The voltage applied is approximately 6 v. and the current strength is approximately 800 ma. When the germanium has dissolved completely, substantially no current flows through the bath. It has been found that the cadmium sulphide has not been noticeably attacked during the electrolytic etching treatment.

The cadmium sulphide can be removed from the glass plate by softening or dissolving the adhesive and any residual adhesive can then be removed by means of a suitable solvent. The cadmium sulphide obtained has the form of a single-crystal plate having the same diameter as the original germanium plate and having a thickness of 80 Its resistivity is about 200 ohm cms.

In the present example cadmium sulfide was deposited onto the germanium substrate by means of a transport reaction, using hydrogen. In principle other methods of epitaxially depositing cadmium sulfide may be used, such as sublimation, by leading H S-gas over heated cadmium to form a mixture of H S-gas and cadmium vapour, and by a transport reaction using iodine.

In the present example the electrolytic bath comprises a dissolved cyanide. A suitable example of an electrolytic bath which, however, does not comprise a cyanide was an aqueous solution comprising 0.15 gr. mol p tassium sulfate and 0.4 gr. mol hydrogenperoxide per liter. The electrolytic etching treatment was carried out with an applied voltage of about 10V and current densities of about 0.2-0.6 A. cm.

Instead of cadmium sulphide, also other chalcogenides of elements from the Group II-B of the periodic system can be epitaxially deposited on germanium, whereupon the germanium can be removed electrolytically in the manner described above. It will be appreciated that many modifications of the given example are possible without departing from the scope of the invention. The invention is further not limited to the epitaxial growth of cadmium sulphide.

Cadmium selenide is grown epitaxially on a singlecrystal germanium plate in the same manner as in the above example. The hydrogen current used in this case is 400 mL/min. and the source of cadmium selenide is heated at 840 C. and the germanium plate at 620 C.

Zinc selenide may also be applied in a corresponding manner with a hydrogen current of 500 ml./min. a temperature of the source of zinc selenide of 860 C. and a temperature of the substrate of 650 C.

Alternatively, layers of chalcogenides of elements from Group II-B of the periodic system of various compositions may successively be deposited epitaxially on a singlecrystal germanium substrate. For example, first a cadmium sulphide layer has been epitaxially deposited on a single-crystal germanium substrate and then a mercury selenide layer has been applied epitaxially to the cadmium sulphide layer obtained.

In all the cases stated above, the germanium substrate may be removed electrolytically, for example, in the same manner as described in the above example. Furtherm re, impurities that may influence the conductivity or the conductivity type of the epitaxially growing material may be incorporated during the epitaxial growth. In certain cases, it has even been found that given impurities could have a favourable effect on the crystal perfection of the epitaxially grown layer. For example, the crystal perfection of cadmium sulphide layers epitaxially growing on germanium was favourably influenced by simultaneously doping with gallium or indium.

As far as the epitaxially growing material had a hexagonal crystal structure, the orientation of the hexagonal c-axis of the epitaxially growing material corresponded with the orientation of a lll-axis of the substrate material. It should be noted that it has been found that the epitaxial layer need not be formed on a lll-face of the substrate which also has been carried out successfully. As stated above, in the example described above use has successfully been made of a substrate consisting of singlecrystal germanium in the form of a plate having a flat side oriented in the direction of a -plane of the crystal.

In the electrolytic process described in the example first all germanium which is not covered by the platinum strip is removed and only a small part of the original germanium disc has remained underneath said strip. After a prolonged etching treatment also this part is dissolved. However, this part may be left onto the remaining chalcogenide disc and may serve for mechanically handling the disc. Further the local p-type germanium part onto the cadmium-sulfide disc was found to form a suitable ohmic connection to the cadmium sulfide and may be used as an ohmic contact.

What is claimed is:

1. A method of manufacturing crystalline bodies consisting essentially of at least one chalcogenide of an element from Group II-B of the periodic table or mixed crystals thereof, comprising the steps of providing a single crystal germanium substrate of relatively high electrical conductivity, epitaxially depositing on the germanium substrate the said chalcogenide with a substantially lower electrical conductivity than that of the germanium crystal, making a positive electrical connection to the germanium substrate, immersing the substrate with the epitaxial deposit in a bath of an electrolyte incapable of chemically attacking the chalcogenide, and passing current thr ugh the bath via said electrical connection and said germanium substrate to remove electrolytically at least the major part of the germanium substrate.

2. The method as set forth in claim 1, wherein the electrolyte used is alkaline and contains a dissolved cyanide.

3. The method as set forth in claim 2, wherein the electrolyte contains a dissolved complex cyanide.

4. The method as set forth in claim 3, wherein the electrolyte contains an alkali metal-ferricyanide.

5. The method as set forth in claim 1, wherein the chalcogenide is epitaxially deposited by means of a transport reaction with hydrogen.

References Cited UNITED STATES PATENTS 3,042,593 7/1962 Michlin 204-14l 3,081,418 3/1963 Manintveld et al 204143 ROBERT K. MIHALEK, Primary Examiner U.S. Cl. X.R.