Method of producing monocrystalline semiconductor films utilizing an intermediate water dissolvable salt layer
BACKGROUND OF THE INVENTION 
     The present invention relates to a method of producing semiconductor films. It is particularly applicable for producing monocrystalline semiconductor films of silicon or germanium, and is therefore described below with respect to such applications. 
     Monocrystalline films of silicon and germanium are basic materials for the fabrication of photovoltaic solar cells and integrated circuits. In the current state of the technology, the semiconductor crystals are usually produced according to the Czolchvalski technique, wherein the crystals are pulled from the melt and are then sliced into layers which subsequently receive the appropriate treatment for the fabrication of the appropriate junctions, contacts, and the like, according to the specific application. Semiconductor crystal slices produced in this way are expensive because the process is not amenable to automated mass production. In addition, this process requires considerable amounts of energy because of the high temperatures required. Both of the above drawbacks in the present technology are serious limitations to the wide-spread use of such semiconductor films, particularly in large scale solar cell applications where the cost of the semiconductor films is a critical factor. 
     BRIEF SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a method of producing semiconductor films, particularly monocrystalline films of silicon and germanium, which process is more amenable to automated mass production and requires less energy than the above-described presently-used processes. 
     According to a broad aspect of the present invention, there is provided a method of producing semiconductor films, particularly monocrystalline films of silicon and germanium, characterized by the steps of (a) epitaxially-growing on a substrate at least one layer of a salt dissolvable in water; (b) epitaxially-growing on the dissolvable salt a layer of the semiconductor; and (c) dissolving the salt layer with water, thereby separating the semiconductor layer from the substrate. 
     As known, epitaxial growth is a process in which a layer is deposited on a substrate with the same crystal orientation as exists in the substrate. Thus, if the substrate is of monocrystalline structure, the layer epitaxially-grown thereon will also be of monocrystalline structure. This process is commonly used for extending or modifying the surface of a single crystal structure, e.g., silicon, by the addition of dopants or more silicon for creating circuit elements, the dopants and more silicon being passed over a slice of the silicon substrate in an epitaxial furnace in which the slice is heated. Thus, monolithic integrated circuits are commonly produced according to this technique wherein the circuit elements are applied to a single crystal (e.g., p-type silicon doped with boron) by placing same in a high-temperature epitaxial furnace for growing thereon another silicon layer (e.g., n-type) with the same crystal orientation as the substrate. In producing integrated circuits, the grown layer is covered by a thin coating of silicon dioxide (e.g. by placing the substrate in an oxidizing atmosphere such as steam at high temperature) and then by a masking layer having windows for producing the circuit components. 
     One of the advantages of the present invention is that the substrate is reusable as a matrix, and therefore its original cost is not a significant factor when used in the mass-production of such films. The growth of the semiconductor layer is effected by evaporation in a high vacuum, at least 10 -6  Torr, in which the substrate need only be moderately heated, while the semiconductor source of evaporation is heated by a low power electron gun. Accordingly, the energy requirements for such a process are considerably less than the present Czolchvalski technique, which is another advantage of the present invention. Further, the semiconductor layers may be deposited at the desired thickness, and impurities can be readily introduced during the deposition. This permits not only very close control of the thickness of the semiconductor film, but also close control of the amount of impurities introduced into it, as well as of the impurity profile of the film, i.e., of the concentration of the impurities with respect to location in the semiconductor. Further, the process is highly amenable to mass production; moreover, it permits a plurality of the semiconductor films to be epitaxially-grown on a substrate with each film separated by an epitaxially-grown dissolvable layer, all the latter layers being dissolved to produce a plurality of separate semiconductor films. 
     A number of embodiments of the invention are described below for purposes of example. In some described embodiments, the epitaxially-grown semiconductor layer is silicon, in which case it would be preferable to first grow a thin film of germanium on the dissolvable material layer before epitaxially-growing the silicon layer thereover. In the other described embodiments, the epitaxially-grown semiconductor film is germanium. 
     Further, in some described embodiments, the substrate is silicon, and in such embodiment it is also preferred to first grow a thin film of germanium on the silicon substrate before epitaxially-growing the dissolvable layer thereon since the silicon tends to oxidize. In other described embodiments, the substrate is sapphire, which does not oxidize and which is also stable at high temperatures. 
     As examples of the material of the dissolvable layer, there may be used sodium fluoride, sodium chloride, or a composite layer including e.g., a film of sodium chloride and an overlying film of silver. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further features and advantages of the invention will be apparent by a description of a number of examples taken in conjunction with the annexed drawings, wherein: 
     FIG. 1 diagrammatically illustrates a known high vacuum system which may be used in the method of the present invention; 
     FIG. 2 illustrates the various layers epitaxially grown on the substrate in accordance with Example 1 of the method as described below; and 
     FIG. 3 illustrates the semiconductor film produced by the method of Example 1 (and FIG. 2) after it has been separated from the substrate. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     With reference first to FIG. 1, there is diagrammatically shown a well known high vacuum system commonly used for evaporating thin films. It includes an outer housing 2 in which a high vacuum is produced via a pumping outlet 4. A substrate 6 is supported within the furnace and is heated by an electrical heater 8. 
     The semiconductor, e.g. silicon or germanium, to be epitaxially grown on the substrate 6 is introduced into a container 10 and is heated by an electron beam generated from a filament 12, such that the semiconductor is evaporated and is deposited on the substrate 6. Cooling of the electron gun is effected by a water-cooling conduit 14. 
     The apparatus illustrated in FIG. 1 further includes Knudsen cells 16, 18, for evaporating the p-n impurities which are introduced into the semiconductor film as it is epitaxially-grown on the substrate 6. A thickness monitor 20 monitors the thickness of the film. 
     Such an apparatus is well known, and therefore further details of its construction and operation are available from the literature. 
     In the present invention, the housing 2 further includes holders or boats 22, 24, for evaporating the material of the dissolvable layer applied between the substrate and the epitaxially-grown semiconductor film. Such holders may also be used for the anti-reflection coating material, and for the electrodes applied to the semiconductor film before the film is separated from the substrate by dissolving the dissolvable layer. 
     Following is a description of a number of examples of the invention. 
     EXAMPLE 1 (Si-Ge/NaF/Ge-Si) 
     In this example, the substrate 6 in FIG. 1 is monocrystalline silicon protected by a thin film of germanium because of the tendency of silicon to oxidize. 
     First, the silicon substrate 6 is cleaned according to a standard chemical treatment. Then, the entire chamber including the silicon substrate 6 is baked at a temperature of about 200° C. to obtain a vacuum of at least 10 -6  torr, preferably in the order of a few 10 -8  torr. This may require a few hours or up to a day or two, depending on pumping capacity and previous exposure to air. A germanium film 30 (FIG. 2) is then epitaxially-grown on the silicon substrate by being bombarded by the electron beam from gun 12 while the furnace is at a temperature of about 750° C. Preferably, the germanium is evaporated at a rate of the order of 100 A/min, and to a thickness of about 1,000 A. 
     The germanium film 30 is applied to the silicon substrate 6 because of the tendency of silicon to oxidize. Once made, this substrate including the germanium film serves as a reusable matrix for producing many semiconductor films. 
     A layer 32 (FIG. 2) of a dissolvable material is epitaxially grown on the germanium film 30 of the silicon substrate 6. In this example, the dissolvable layer is sodium fluoride. This material is evaporated at room temperature, at a rate of 100 A/min, and to a thickness of about 1,000 A. 
     Next, a germanium film 34 is epitaxially-grown on the sodium fluoride film 32. This is accomplished at a temperature of about 500° C. to 600° C., at a rate of about 100 A/min, and to a thickness of about 1,000 A to 2,000 A. 
     Next, the silicon semiconductor layer 36 is epitaxially-grown on the germanium film 34. This is preferably effected at a temperature of about 650° C., and at a rate of 100 A to a few 1,000 A/min, to the desired thickness, typically about 2 μm to 20 μm. The epitaxial deposition of the silicon layer 36 is effected by evaporation from the electron source 12. At the same time, the impurities included within cells 16 and 18 may be evaporated at the appropriate rate to obtain the desired impurity-concentration profile, including the p/n junction and desired backfield for lowering of back surface recombination. 
     Following the application of the silicon layer 36, the contacts 38 and antireflection coating 40 may be evaporated from material contained in the appropriately-provided holders 22, 24. 
     The substrate 6, including the above-described epitaxially-grown layers thereon, is then removed from the furnace, and the sodium fluoride layer 32 is dissolved in water. This effects the separation of the silicon layer 36 including its germanium film 34, from the matrix 6 including its germanium film 30. As described above, the matrix may then be reused for epitaxially-growing additional semiconductor films. 
     EXAMPLE 2 (Si-Si/NaF/GeSi) 
     This example is the same as Example 1 described above, except that the silicon matrix 6 is protected by a silicon film instead of the germanium film 30 (FIG. 2). This silicon film is preferably epitaxially-grown on the silicon matrix 6 by evaporation at a temperature of 750° C., at a rate of 100 A/min, and to a thickness of about 1,000 A. 
     EXAMPLE 3 (Si-Ge/NaCl-Ag/Si) 
     This example is the same as Example 1 described above, except that instead of using the sodium fluoride dissolvable layer 32, there is used first a layer of sodium chloride and then a layer of silver. The sodium chloride layer is evaporated at a temperature of 250° C.-350° C., at a rate of about 100 A/min, and to a thickness of a few hundred A&#39;s. The silver is evaporated at a temperature of 250° C.-350° C., at a rate of about 100 A/min, and to a thickness of about 1,500 A. In this example 3 the silicon layer 36 is preferably evaporated at a temperature of about 350° C. increasing to about 600° C. during film growth. 
     The remainder of the procedure is the same as described above in Example 1. 
     EXAMPLE 4 (GERMANIUM SEMICONDUCTOR FILM) 
     This example is the same as Example 1 described above, except that instead of growing the silicon layer 36, the underlying germanium film 34 is grown to the desired thickness, typically 2 μm to 20 μm. 
     EXAMPLE 5 (GERMANIUM-SILICON COMPOSITE FILM) 
     This example may follow Examples 1 and 2 above, except that the final layers of germanium 34 and silicon 36 are of about equal thickness, namely the thickness of the silicon layer 36, to provide a double gap for better spectral efficiency. 
     EXAMPLE 6 (SAPPHIRE/Ag/Si) 
     In this example, the substrate (layers 6, 30 in FIG. 2) is of sapphire, which may be cleaned according to any standard chemical treatment, and then baked as described in Example 1. In addition, the dissolvable layer (32 in FIG. 2) is silver, which may be applied by evaporation at a temperature of 350° C., at a rate of about 100 A/min, and to a thickness of about 1,500 A. The remainder of the procedure is the same as described above in Example 1, except that the dissolvable layer of silver is dissolvable in nitric acid. 
     It will be appreciated that, while the above examples describe only the application of one semiconductor film, a plurality of the semiconductor films may be epitaxially-grown on the common substrate, with each film separated by an epitaxially-grown dissolvable layer. After the plurality of layers have been grown, the substrate including the alternating layers of semiconductor may then be treated in the solvent so as to dissolve all the dissolvable layers at one time and thereby to produce a plurality of semiconductor films. 
     While the invention has been described with respect to several preferred embodiments, it will be appreciated that these are described purely for purposes of example, and that many other variations, modifications and applications of the invention may be made.