Patent Grant
9722130
The present invention is related to producing large grained to single crystal semiconductor films, such as silicon films, for producing articles such as photovoltaic and other electronic devices.
None
It is widely known that radiation from the sun striking earth provides enough energy to supply all of mankind's needs for energy for the indefinite future. Such a source of energy can be clean and environmentally benign.
It is also widely known that global warming is associated with the use of fossil fuels, such as coal, oil, and natural gas. It is accepted by the scientific community that global warming can have severe adverse effects around the planet. There are numerous efforts around the world, combined with a sense of urgency, to cut down emissions from the usage of fossil fuels. A dominant factor in favor of the continual use of fossil fuels is their cost per unit of available energy. If, for example, the cost of producing photovoltaic cells can be reduced by a factor of approximately three while maintaining efficiency of conversion, the photovoltaic technology would become cost competitive with fossil fuels.
A major cost component in photovoltaic cells is the cost of the substrate on which the semiconductor film capable of converting sunlight into electricity is placed. The most widely used substrate is single crystal silicon (Si). These substrates developed for the microelectronics industry have been modified for application in photovoltaic technology. If a silicon film could be deposited on an inexpensive substrate, such as glass, and with comparable quality as that found in silicon single crystals used in the microelectronics industry, the cost of photovoltaic technology would drop significantly.
Epitaxial growth of thin films is a very well established process. It has been investigated by hundreds of researchers. Epitaxial deposition provides a very viable way of growing very good quality films. Many single crystal semiconductors and insulator surfaces are used to study the epitaxial growth of metallic films; for example, the growth of silver on silicon, sapphire, or a mica surface. Epitaxial metallic films have also been grown on other metallic films, such as gold on silver. In contrast to metals, semiconductors, such as silicon, are difficult to grow epitaxially. For example, heteroepitaxial films of silicon have been successfully grown only on sapphire but at temperatures that are relatively high for the applications we disclose here, such as the growth of silicon on glass substrates.
In order to take advantage of highly textured large grained films for photovoltaic technology two problems need to be solved: inexpensive growth of high quality films and the availability of an inexpensive substrate on which desirable properties can be achieved. Here, we disclose a method for growing semiconductor films, such as silicon, satisfying the two requirements listed above and suitable for photovoltaic technology and other electronic applications.
The thermodynamic stability and formation temperature of two or more elements is described by a composition versus temperature diagram, called a phase diagram. In this invention we shall make use of phase diagrams. These phase diagrams are available in the scientific literature (Massalski et al). The phase diagram provides information on the behavior of different phases, solid or liquid as a function of temperature and composition. For example, the liquidus in a simple binary eutectic system, such as Au and Si, shows how the relative composition of the liquid and solid, it is in equilibrium with, changes with temperature. It is therefore possible to choose an average composition, different from the eutectic composition, and cool the mixture in such a way as to precipitate out one phase or the other. If the composition is chosen to be richer in silicon than the eutectic composition then on cooling through the liquidus boundary between the single phase liquid and the two phase liquid plus solid, silicon will nucleate and form a solid phase. If on the other hand it is gold rich relative to the eutectic composition the first solid phase to nucleate is gold rather than silicon.
At and below the eutectic temperature the two components, in this case, Au and Si solidify from the liquid phase to phase separate into the two components Au and Si. The interface energy between the two components is generally positive and therefore drives the two components to aggregate into distinct phases with a minimum of surface area between the two rather than a fine mixture of the two. There is, however, the energetics of two other interfaces to consider also: one with the substrate and the other with vacuum or gas. In considering energetics it is not only the chemical interaction of the metal or Si with the substrate that is important but also its crystallographic orientation, for the surface or interface energy depends upon orientation of the grains. Another concern is the difference in lattice match between the nucleating film and the substrate which can lead to strain induced energy that is minimized by either inducing defects or not growing uniformly in thickness across the substrate surface. These factors determine if silicon is likely to deposit on the substrate (heterogeneous nucleation) or nucleate and forms small crystals in the liquid (homogeneous nucleation).
An advantage of using eutectics compositions is that the eutectic temperature is lower than the melting temperature of the constituent elements. For example, the eutectic temperatures of Au, Al, and Ag with Si are 363, 577, and 835 degrees Centigrade (° C.), respectively. In contrast the melting temperatures of the elements are 1064, 660, and 961° C., respectively. The melting temperature of silicon is 1414° C. The eutectics then offer the possibility of nucleating a silicon crystal from the liquid far below the temperature at which pure liquid silicon crystallizes. By a proper choice of the substrate surface exposed to the nucleating silicon, it is possible to nucleate and grow single crystal or large grained silicon films.
We have discussed silicon eutectics using elements such as Au, Ag, and Al. However, it is possible to replace the elements by silicon based compounds. For example, the compound nickel silicide forms a eutectic with Si. There are numerous other examples of silicide compounds forming a eutectic with Si (Massalski et al). An advantage of using a silicide is that frequently the electrical contact of the silicide with silicon has very desirable properties, such as a good ohmic contact or a Schottky barrier. Some silicides are also known to have an epitaxial relationship with silicon. In this case, by appropriately choosing either a silicide rich or silicon rich melt either the silicon can be induced to grow epitaxially on the silicide or the silicide on silicon. A disadvantage in this approach is the eutectic temperature, which is generally high.
Low temperature solutions can also be formed with some elements, For example, gallium (Ga) and Si have a eutectic temperature of less than 30° C., very close to that of the melting point of Ga. There are other elements, such as indium or tin that form low temperature liquid solutions with silicon. Si can be nucleated from these solutions at very low temperatures relative to pure silicon (Girault et al, Kass et al). These temperatures are sufficiently low that it opens up the possibility of using organic materials as substrates on which large grained to single crystal films can be grown. While this is an advantage, there is also a serious disadvantage; at these low temperatures, the silicon film can contain defects and hence are not very useful as a photovoltaic material. However, these very low temperature deposits can be used to initiate the nucleation of a very thin silicon film, which is subsequently thickened by using higher temperature processes to optimize its photovoltaic properties.
The choice of a particular system (phase diagram) is not only determined by temperature and energetics of the interfaces, but also by the solubility of the second element in Si. It is desirable to have precise control of the doping of Si in order to optimize its semiconductor properties for photovoltaic applications. It is also important to select the composition of the substrate and temperature of processing such that there is minimal or no chemical interaction between the silicon film and the surface of the substrate on which it is being deposited.
From the preceding description, we can extract five common points which are relevant to this invention. First, one end of the phase diagram always has the semiconductor we wish to nucleate and use to produce a film, we have used silicon in the preceding examples but it could be germanium or a compound such as gallium arsenide or cadmium selenide. Second, the thermodynamically predicted concentration of the second element or phase in the semiconductor is minimal. If there is solubility then it must be a desirable dopant. For example aluminum (Al) in silicon behaves as a p-type dopant and experience in the semiconductor industry has shown that trace amount of Al can be desirable. Third, the liquidus curve has the highest temperature on the semiconductor side. In other words, the melting point of the semiconductor is greater than the liquidus for all compositions in equilibrium with the semiconductor. Fourth, the homogeneous nucleation energy of silicon crystal from the melt is greater than that for heterogeneous nucleation on the substrate. This latter condition promotes heterogeneous nucleation. And, fifth, the temperature for epitaxial growth is low enough to use inexpensive substrates such as glass but high enough to promote a good quality silicon film. For example, a growth temperature above approximately 550 degrees Centigrade (550° C.) is desirable to make a good quality silicon film. The softening temperature of ordinary glasses is around 600° C. The softening temperature of borosilicate glasses is higher. However it is not high enough to use conventional deposition temperature of greater than 750 degrees Centigrade for silicon on insulator, such as a sapphire substrate.
In order to take full advantage of the invention disclosed here the semiconductor material has to be deposited on a substrate material which is inexpensive, and the surface of which enables heterogeneous nucleation and growth. In the following we shall discuss two specific methods for producing substrates suitable for heterogeneous deposition of films for photovoltaic technology. Both of these methods have been described in the scientific literature and we do not claim to invent them. We include them here for completeness.
The use of rolled and textured Ni and Ni-alloy sheets has been proposed as substrate material for superconducting films and, more recently, for films for photovoltaic devices (Findikoglu et al). In order to facilitate the growth of epitaxial superconducting films on such substrates, there have been two approaches described in the scientific literature: in one the sharp rolling texture produced in a rolled and annealed Ni alloy is used as a template on which various epitaxial buffer layers are deposited followed finally by an epitaxial film of a high temperature cuprate superconductors (Goyal et al). In the second approach (Findikoglu et al), the nickel ribbon is used as a substrate for ion beam assisted deposition of a wide variety of highly textured ceramics, for example, magnesium oxide (MgO). The ion beam aligns the growing MgO film, which provides a template for the subsequent deposition of the cuprate superconductor. The latter approach is not limited to using metal tapes but can be extended to other inexpensive substrates such as glass (Teplin et al). It has been found that texture can also be induced in MgO by depositing the film on a substrate that is inclined to the normal from the oncoming vapor of MgO.
One limitation of the use of glass as a substrate has been its softening temperature, which is generally lower than the conventional processing temperatures required for the growth of large grained or single crystal films of silicon. With the method of depositing silicon films at low temperatures, described in this invention, the use of buffered glass becomes an option for we can deposit highly textured and large grained silicon on MgO at or below the softening temperature of glass. Similarly, researchers have grown crystalline aluminum oxide (Al2O3) on inexpensive substrates (Findikoglu et al). We shall use MgO and Al2O3 as illustrative examples. However, it is understood to those skilled in the art that a variety of other materials can also work. Both Findikoglu et al and Goyal et al describe other buffer layers, including conducting ceramic layers, such as TiN.
It is an object of the present invention to provide single crystal or highly textured relatively large grained good quality semiconductor films and, in particular silicon films, for photovoltaic technology or other semiconductor devices, such as field effect transistors used, for example, in displays.
It is yet another object of this invention to provide single crystal or highly textured relatively large grained good quality semiconductor films and, in particular silicon films, at low temperatures. For example, if silicon films are used, the growth temperature is between 450 and 750 degrees Centigrade.
It is yet another object of this invention to provide single crystal or highly textured relatively large grained good quality semiconductor films and, in particular silicon films, on inexpensive substrates, for example, substrates such as glass on which buffer layers such as MgO and/or Al2O3 have been deposited.
In accordance with one aspect of the present invention, the forgoing and other objects can be achieved by alloying a semiconductor and, in particular silicon, with elements or compounds that form an eutectic system, and increasing slowly the concentration of the semiconductor, such as silicon, through the liquidus line to reach the two phase region in which the semiconductor, in particular silicon, nucleates out of the melt and on the surface of a substrate.
In accordance with another aspect of the present invention, the forgoing and other objects can be achieved by alloying a semiconductor and, in particular silicon, with elements or compounds that form an eutectic system, and increasing slowly the concentration of the semiconductor, such as silicon, through the liquidus line to reach the two phase region in which the semiconductor, in particular silicon, nucleates on the surface of a substrate to produce a highly textured relatively large grained or single crystalline film.
In accordance with yet another aspect of the present invention, the forgoing and other objects can be achieved by alloying a semiconductor and, in particular silicon, with elements or compounds that form an eutectic system, and increasing slowly the concentration of the semiconductor, such as silicon, through the liquidus line to reach the two phase region in which the semiconductor, in particular silicon, nucleates on the surface of a substrate made of a buffered tape in which texture is produced by mechanical deformation and the buffer layers are epitaxial to the texture of the metal tape. The buffer layer exposed to the melt comprises of compounds, such as Al2O3 or MgO.
In accordance with yet another aspect of the present invention, the forgoing and other objects can be achieved by alloying a semiconductor and, in particular silicon, with elements or compounds that form an eutectic system, and increasing slowly the concentration of the semiconductor, such as silicon, through the liquidus line to reach the two phase region in which the semiconductor, in particular silicon, nucleates on the surface of a substrate made of a buffered tape, a glass substrate, or any other material suitable for inexpensive manufacture of photovoltaic cells in which strong texture is produced by ion beam assisted deposition. The final layer, which is exposed to the silicon melt, comprises of compounds, such as Al2O3or MgO.
In accordance with still another aspect of the present invention, the forgoing and other objects can be achieved by using a solid phase composition comprising a semiconductor and, in particular silicon, with elements or compounds that form an eutectic system, and in which a thin film of the element or compound is deposited first followed by the semiconductor, such as silicon, and depositing at a temperature where the semiconductor atoms diffuse through the element or compound to heterogeneously nucleate on the substrate and propagate this crystallinity to the semiconductor film remaining on top of the element or compound.
The method of manufacture of materials suitable for photovoltaic technologies described in this invention are much less expensive in the conversion of sunlight into electricity than those practiced in the prior art.
We have used the phase diagram of the Au—Si eutectic. The Al—Si eutectic is very similar. Here we can heterogeneously nucleate silicon from the Al—Si melt on a single crystal sapphire substrate to form a single crystal heteroepitaxial silicon film.
As described above, we have disclosed a method to produce low cost single crystal or large grained epitaxially aligned good quality semiconductor films, in particular silicon, for photovoltaic technology. We have also suggested the use of tapes or glass slabs as substrate materials. The tapes provide strong texture on which buffer layers suitable for silicon growth are present. Our method can produce silicon epitaxy at substantially lower temperatures than those commonly practiced, hence not only minimizing interaction with the surface of the substrate but also enabling the use of glass substrates.
We shall be using the eutectics of silicon with gold and aluminum in describing the details of the invention. It is, however, understood that one skilled in the art can extend the methodology to other semiconductors such as germanium, gallium arsenide, or the cadmium selenide class of photovoltaic materials.
We have started with vapor deposition of the metallic film and added silicon to it to traverse the phase diagram from point marked 11 in the figure. However, the metallic element and silicon can be co evaporated to reach any concentration between the points marked 12 and 13 in the figure and subsequently silicon added to reach the desired thickness, before cooling to room temperature.
When the desired thickness of the silicon film is obtained, the substrate with the film is cooled to room temperature. Even though the amount of gold required to catalyze a silicon film is small, it can be further reduced by etching the gold away, for example, by using iodine etch, available commercially. This gold can be recycled
The following non-limiting examples are used as illustrations of the various aspects and features of this invention.
A good high vacuum system with two electron beam guns, is used to deposit gold and silicon independently. A glass substrate coated with ion beam assisted deposited MgO film is held at temperatures between 575 and 600° C. These are nominal temperatures. It is understood to one skilled in the art that lower or higher temperatures can also be used depending upon the softening temperature of the glass substrate or the reaction kinetics of either gold or silicon with the metallic tape or its buffer layers when used as substrates. A thin gold film of approximately 10 nm thickness is deposited first. This is followed by a silicon film deposited at a rate of 2 nm per minute on top of the gold film. The ratio of the thickness of the gold and silicon films is chosen such that the final composition ensures that a point, marked 13, in
By relatively large grained it is understood to imply a grain size larger than would have been achieved if a silicon film had been deposited under the same conditions but without Au. In the example discussed above the crystallographic texture is strongly [111]. Instead of an insulating substrate such as MgO, it is possible to choose stable and electrically conducting nitrides, such as TiN.
The gold diffuses to the surface of the silicon film, driven by its lower surface energy relative to the silicon surface. The film is etched in a solution, such as a commercially available iodine based chemical, which removes the gold from the two phases, gold and silicon, leaving behind a silicon film.
This silicon film can now be used as the surface on which a thicker silicon film appropriately doped to form a p-n junction, suitable for applications such as photovoltaics, can be deposited. Alternatively, the thin silicon film can be used for heteroepitaxial deposition of other semiconductors, which might be more efficient convertors of sunlight to electricity.
We have used two electron beam guns as an illustrative example. It is understood to one skilled in the art that other methods such as a single gun with multiple hearths, chemical vapor deposition, thermal heating, or sputtering can also be used.
A good high vacuum system with two electron beam guns is used to deposit aluminum and silicon independently. A glass substrate or a Ni based substrate coated with a buffer layer of Al2O3 is held at temperatures between 600 and 615 degree ° C. These are nominal temperatures. It is understood to one skilled in the art that lower or higher temperatures can also be used depending upon the softening temperature of the glass substrate or the reaction kinetics of either aluminum or silicon with the metallic tape or its buffer layers when used a substrates. The eutectic Al—Si is used instead of the Au—Si example above. A thin Al film 6 nm thick is deposited on the Al2O3 followed by a 100 nm thick silicon deposition, and as described in example 1, above, the two phase region comprising of solid silicon and a liquid Si—Al mixture is reached. The deposition is stopped and the sample is slowly cooled to room temperature. Aluminum diffuses through the silicon film, driven by its lower surface energy relative to silicon. The silicon film is heteroepitaxially aligned by the Al2O3 surface. The aluminum film on the surface can be etched chemically by well known processes to leave behind a silicon film. The surface of this film can now be used for further growth of epitaxial films either for photovoltaic devices or for field effect transistors.
We note, as stated earlier, that silicon can be grown epitaxially on sapphire but at temperatures higher than 750° C. This is a well established commercial process. However, in the absence of aluminum, silicon deposition at, say, 600° C. produces a fine grained film rather than a heteroepitaxial film, as described above.
We describe in this example how different methods of deposition can be combined to take advantage of highly textured films as described in example 1, above. The Si film produced from the deposition of example 1 is etched to remove the Au and then placed back into the vacuum chamber and p+-Si is deposited on this film. This latter layer serves two purposes: it provides a conducting layer for a photovoltaic device to be subsequently built on it and can be the starting point for a variety of differently configured photovoltaic devices as, for example, a nanowire photovoltaic device. Here a 2-3 nm thick gold film is deposited on the silicon using an electron gun. This 2-3 nm thick gold film breaks up into nanoparticles and is the starting point used by a number of investigators to use chemical vapor deposition to grow nanowires and use these nanowires for photovoltaic devices. The difference is that we show how an inexpensive buffered glass can be used rather than a relatively expensive single crystal Si substrate.
A second possibility is to deposit a Au film of thickness 5 nm as islands on a MgO buffered glass substrate, using lithographic or other means known in the art. A heavily doped silicon (p+or n−) film is now deposited on the surface followed by a p- or n-type silicon using electron beam deposition, as described in example 1. The thickness of the heavily doped film is in the micron range whereas the lightly doped film is of the order of 100 nm. The deposition process is now changed and chemical vapor deposition is used for subsequent deposition of suitably doped films of silicon, practiced in the art to grow silicon nanowire photovoltaic devices. The heavier doped silicon film serves the purpose of a conducting layer. Using gold islands has the advantage of controlling the nanowires diameter and length in order to maximize the efficiency of the photovoltaic cell (Kayes et al). Instead of using the insulating MgO buffer layer, a conducting material such as TiN can be used.
We describe how different methods of deposition and temperature can be combined to take advantage of films grown as described in Examples 1 and 2, above to produce desirable device structures. Instead of depositing the Al and Si films described in Example 2 above, by two electron beam heated sources in a vacuum, we deposit the Si by decomposition of silane using a chemical vapor deposition chamber. This is a well known industrial process. As in Example 2, we deposit a 6 nm thin film of Al on to a sapphire substrate held at 600° C. We then introduce silane gas into the chamber. At these temperatures, the silane decomposes to form a Si film which reacts with the Al to produce a eutectic solution and when this solution is saturated with Si (the equivalent of point marked 13 in
While the principles of the invention have been described in connection with specific embodiments, it should be understood clearly that the descriptions, along with the examples, are made by way of example and are not intended to limit the scope of this invention in any manner. For example, a variety of suitable substrates different from the examples given above can be utilized or a different variety of deposition methods and conditions can be employed as would be understood from this invention by one skilled in the art upon reading this document.
The present invention is a continuation of U.S. patent application Ser. No. 12/903,750, now U.S. Pat. No. 8,491,718 issued Jul. 23, 2013, which is a continuation-in-part of U.S. patent application Ser. No. 12/774,465, now U.S. Pat. No. 9,054,249 issued Jun. 9, 2015, which is a continuation of U.S. patent application Ser. No. 12/154,802, now abandoned, filed May 28, 2008 all of which are hereby incorporated by reference in their entirety.
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| 20130284258 A1 | Oct 2013 | US |
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