THIN FILMS OF COMPOUNDS AND ALLOY COMPOUNDS OF GROUP III AND GROUP V ELEMENTS

Abstract: Method of producing a thin film of a compound of Group III and Group V elements, which comprises forming a thin film of a selected Group III element on a suitable substrate (1), for example by dip coating (2, 3) and exposing the thin film in a chamber (8) to a gaseous atmosphere comprising a selected Group V element in gaseous form, at an appropriate temperature to result in production of the desired compound.

Description: THIN FILMS OF COMPOUNDS AND ALLOY COMPOUNDS OF GROUP III AND GRODP V ELEMENTS This invention relates to thin films of compounds (which term is intended to include alloy compounds) of. elements drawn from Group III and Group V of the periodic table, and is concerned particularly, but not exclusively, with the production of thin films of semiconductor compounds such as gallium arsenide or gallium arsenide compounds, for example for use in terrestrial solar cells. In bulk form gallium arsenide or gallium arsenide compounds, can be used as the active semiconductor in solar cells with high conversion efficiency. Gallium arsenide is expensive, thus in order to reduce cell cost it is desirable to produce cells which use only very thin films of gallium arsenide while nevertheless retaining high conversion efficiency. Accordingly, there has sought a method of producing photovoltaic cells with thin films of gallium arsenide using a manufacturing process which enables low cost mass production of large areas of cells. Various methods are known for producing thin films of semiconductor compounds such as gallium arsenide and these known methods have used either chemical vapour deposition (CVD) processes or liquid phase epitaxy (LPE) techniques to produce the films. The CVD processes are similar to those used for the. manufacture of expensive small area opto-eleetronic components such as laser emitters. The CVD processes use at least two gaseous species separately containing say, gallium and arsenic-carrying molecules which react together when passed over a heated substrate to produce a thin film of gallium arsenide. The substrates are small and heated to a high temperature. Typically the reactant species are gaseous trimethylgalliu and arsine gas, or gaseous gallium chloride and arsine gas. OMPI -2- The gaseous reactants used in such known methods are difficult to contain, meter and control in the reaction area. In addition, the particular reactions employed require high substrate temperatures to produce thin film gallium arsenide solar cells with suitable performance characteristics. These problems have resulted in the CVD manufacturing process requiring expensive and complex equipment which produces solar cells of relatively small area. LPE techniques involve depositing by precipitation the solid semiconductor compound from a saturated solution in a metal. Saturation is achieved by dissolving solid compound in the metal, and then cooling the solution. These techniques, like CVD techniques, have been developed for the production of small area opto-electronic and similar devices. Using LPE techniques, it is expensive to produce an area larger than a few square centimetres, and further¬ more the process is difficult to control and slow in production. It is accordingly an object of the present invention to provide a method of producing thin films of compounds, including alloy compounds, of Group III and Group V elements suitable for low cost production of large areas. According to one aspect of the present invention, there is provided a method of producing a thin film of a compound of Group III and Group V elements, comprising forming a thin film of a selected Group III element on a suitable substrate, and exposing the thin film to a gaseous atmosphere comprising a selected Group V element in gaseous form, at an appropriate temperature to result in production of the desired compound or alloy compound. When practising the method of the invention, the Group V element in gaseous form reacts with the thin film of the Group III element to produce a thin film of the desired co pound or alloy compound. A typical reaction, in the case where the Group III element is gallium and the Group V element is arsenic in the form of arsine, would be as follows: 2 Ga + 2 AsH 3 > 2 GaAs + 3 H 2 The invention may be used for producing various types of thin film suitable for various different applications, including for example, the production of thin films of GaAs, GaP or their compounds for use in large area light- emitting diodes. Use of the invention for this purpose enables the production of large area displays at low cost. The invention also finds particular application in the production of thin films of gallium arsenide or gallium arsenide compounds, eg for use in solar cells. Hence, in a preferred aspect, the present invention provides a method of producing a thin film selected from the group comprising gallium arsenide, gallium phosphide, a gallium arsenide compound and a gallium phosphide compound, comprising forming a thin film of gallium on a suitable substrate, and exposing the thin film to a gaseous atmosphere comprising at least one of arsenic and phosphorus in gaseous form, at an appropriate temperature to result in production of a thin film of the desired composition. If desired the characteristics of the resulting film can be controlled, eg to produce a film having suitable properties for good solar cell operation, by introducing suitable dopant impurities. This can be effected either by incorporating a suitable dopant into the thin film of the Group III element via the Group III element, or by adding a suitable gas, such as hydrogen sulphide, to the gaseous atmosphere. A sui table s ubs trate i s selected depending upon the intended use of the thin f ilm. For example , in the case of a thin film for use in a solar cell , the substrate is preferably conductive on at least on one surface thereof. The substrate may either be of conductive material, or may comprise an insulator with a conductive, eg metallic, coating. Suitable subs trates for this -purpose include tungsten coated graphite and gold covered glass. The sub¬ strate is conveniently flexible to facilitate treatment. The thin f i lm of the G roup II I element, eg gallium, typically has a thickness in the range 0.5 to 50 microns. The th in f i l m may be f ormed on the subs trate by any suitable technique, such as evaporation, sputtering , dip coating, spreading or electroplating. The gaseous atmosphere conveniently comprises the selected Group V element in gaseous form (eg arsenic in the form of ars ine , or phosphorous in the form of phosphine) and an inert gas, such as nitrogen or helium. Suitable reaction conditions depend on the reactants. For instance, in cases where the Group III element is gallium, a suitable temperature at which the thin film and gaseous atmosphere will react is in the range 150°C to 750°C. Suitable operating conditions are conveniently achieved by heating the substrate coated with the thin film to an appropriate temperature and passing the gaseous atmosphere over the heated substrate until all of the Group III element has reacted to produce the desired compound. The method is preferably conducted in a continuous manner. This is conveniently achieved by producing a suitable thin film coating on the substrate, eg by dipping or continous electroplating, and then pass ing , the coated subs trate through a gas seal into a gaseous reaction chamber through which the subs trate is passed and through which is also passed the gaseous atmosphere. The substrate is conveniently heated in the reaction chamber, eg by means of a suitable heater located in the chamber. In a further aspect, the present invention provides apparatus for producing a thin film of a compound, including an alloy compound, of Group III and Group V elements, comprising means for forming a thin layer of a selected Group III element on a suitable substrate, and a gaseous reaction chamber wherein the substrate and thin film are exposed to a gaseous atmosphere comprising a selected Group V element in gaseous form. The means for forming a thin layer of the element on the substrate may comprise for example, a dip tank for dip coating the substrate, an evaporation chamber, sputtering means, spreading means, or electroplating means. The apparatus conveniently includes feed means, eg drive rollers, for causing continuous feeding of the substrate through the apparatus. The gaseous reaction chamber conveniently comprises a generally closed chamber with a gap inlet and outlet and also a substrate inlet and outlet for permitting the substrate to be passed into and out of the chamber in a manner which does not permit significant escape of the gaseous atmosphere from the chamber. A substrate heater is conveniently located in the gaseous reaction chamber. The present invention also includes within its scope a thin film produced by the method of the invention or using apparatus in accordance with the invention. In yet a further aspect the present invention provides a solar cell incorporating a thin film of gallium arsenide or OMFI gallium arsenide compound produced by the method of the invention or us ing- apparatus in accordance w ith the invention. The invention will be further described by way of example, with reference to the accompanying drawing, the single Figure of which is a schematic view illustrating one embodiment of apparatus in accordance with the invention. In the Figure, a substrate 1 in the form of a flexible band of material, typically 20 cm wide and 0.2 cm thick, made from graphite film coated with tungsten is shown being passed continuously through reaction apparatus under the action of drive rollers 4. The substrate 1 is first passed through a dip tank 3 where it is passed by the action of guide roller 5 through a bath containing gallium metal 2 at a temperature above its melting point of 30°C. This serves to produce a thin film of gallium on the substrate. The substrate is then passed through a reaction chamber 8. The substrate enters via inlet 6 , which includes sealing means for preventing any significant escape of gas from the chamber. The chamber 8 also includes inlets 7 f or introducing gases , including inert gas , ars ine and dopant gas to the chamber. In the chamber, the subs trate is moved past a substrate heater 9 which heats the substrate to a temperature typically in the range 150°C to 750°C. The heated substrate reacts with the gaseous atmosphere in the chamber to produce a thin film of gallium arsenide. The substrate with this film exits the chamber 8 via outlet 11 which includes seal ing means and is of s i mi lar construction to inlet 6. The chamber also includes a gas outlet 10.

Claims: CLAIMS 1. A method of producing a thin film of a compound of Group III and Group V elements, characterised by the steps of forming a thin film of a selected Group III element on a suitable substrate (1), and exposing the thin film to a gaseous atmosphere comprising a selected Group V element in gaseous form, at an appropriate temperature to result in production of the desired compound. 2. A method of producing a thin film of gallium 0 arsenide, gallium phosphide, a gallium arsenide compound or a gallium phosphide compound, characterised by the steps .of forming a thin film of gallium on a suitable substrate (1), and exposing the thin film to a gaseous atmosphere comprising arsenic or phosphorus in gaseous form, at an appropriate temperature to result in production of a thin film of the desired composition. 3. A method as claimed in claim 2, characterised in that dopant impurities are added to the thin film to control the properties of the film. 4. A method as claimed in claim 3, characterised in that the dopant is added to the Group III element. 5. A method as claimed in claim 3, characterised in that the dopant is added to the gaseous atmosphere. 6. A method as claimed in claim 5, characterised in that the dopant is introduced in the form of hydrogen sulphide gas into the gaseous atmosphere. ■ 7. A method as claimed in claim 2, characterised in that the substrate (1) is formed of a conductive material. 8. A method as claimed in claim 7, characterised in that the substrate (1) is formed of a flexible material. -8- 9. A method as claimed in claim 8, characterised in that the substrate is formed of graphite coated with tungsten. 10. A method as claimed in claim 2, characterised in that the substrate is formed of an insulating material provided with an electrically conductive coating. 11. A method as claimed in any of claims 2 to 10, charac¬ terised in that the thin film of the Group III element has a thickness in the range from 0.5 to 50 microns. 12. A method as claimed in any of claims 2 to 11, charac- terised in that the thin film of Group III element is formed by dip coating (2, 3). 13. A method as claimed in any of claims 2 to 12, charac¬ terised in that the gaseous atmosphere comprises the selec¬ ted Group V element in gaseous form and an inert gas. 14. A method as claimed in claim 2, characterised in that the thin film of Group III element and the gaseous atmos¬ phere are made to react with one another at a temperature in the range from 150°C to 750°C. 15. A method as claimed in claim 14, which comprises heating the substrate coated with the thin film to an appropriate temperature and passing the gaseous atmosphere over the heated substrate until all of the Group III element has reacted to produce the desired compound. 16. A method as claimed in any of claims 2 to I6,charac- terised in that a thin film coating is formed continuously on the substrate, by dip coating, and the coated substrate is passed through a gas seal into a gaseous reaction chamber into which the gaseous atmosphere is introduced. -9- 17. A method as claimed in claim 16, characterised in that the substrate is heated in the reaction chamber, by means of a heater located in the chamber. 18. Apparatus for producing a thin film of a compound of Group III and Group V elements, characterised by coating means (2, 3) for forming a thin layer of a selected Group III element on an elongate substrate (1), a gaseous reaction chamber (8) wherein the substrate (1) and thin film are exposed to a gaseous atmosphere comprising a selected Group III element in gaseous form, heating means (9) in the reaction chamber (8) for heating the substrate (1), and drive means (4) for continuously driving the substrate (1) through the coating means (2, 3) and the reaction chamber (8). 19. A thin film of a compound of Group III and Group V elements produced by the method claimed in claim 2. 20. A solar cell including a thin film produced by the method claimed in claim 2. OMPI

Inventors: DAVEY KEITH SIMON, DAVEY, KEITH, SIMON
Applicants: PRUTEC LTD, PRUTEC LIMITED
Application year: 1982
Main classification: C30B1
IPC codes: C30B1, C30B23, C30B25, C30B29