1. Field of the Invention
This invention relates to a semiconductor device and its manufacturing method and, more particularly, to a semiconductor device using a plastic substrate suitable for application to a thin-film solar battery, for example.
2. Description of the Related Art
In case of using fossil fuel like coal and petroleum as an energy source, carbon dioxide as its exhaust product is considered to invite global warming. Using an atomic energy involves a danger of radioactive contamination. In these days where environmental issues are being discussed, it is not desirable to rely on these energies.
Solar batteries that are photoelectric conversion elements for converting sunlight to electric energy have almost no effects on the earth environment, and their further diffusion is anticipated. Currently, however, there are some problems that disturb their diffusion.
There are a lot of materials of solar batteries. Among them, solar batteries using silicon are commercially available. They are generally classified to crystalline silicon solar batteries using mono-crystalline silicon or polycrystalline silicon and amorphous silicon solar batteries. Heretofore, mono-crystalline or polycrystalline silicon has been often used for solar batteries. However, although these crystalline silicon solar batteries have a higher conversion efficiency that indicates the performance of converting photo (solar) energy into electric energy than amorphous silicon, much energy and time were required for crystalline growth. Therefore, it was difficult to mass-produce them and provide them inexpensively.
Amorphous silicon solar batteries currently have a lower conversion efficiency than crystalline silicon solar batteries. However, they have advantageous features, such as the need for only a small thickness of less than one hundredth of the thickness that a crystalline silicon solar battery needs for photoelectric conversion, which exhibits a high photo absorption property of amorphous silicon solar batteries and enables formation of a solar battery by stacking a relatively thin film; the capability of selecting as a substrate various materials like glass, stainless steel, polyimide plastic films, and so on, making use of the amorphous quality; readiness of making a battery of a much larger extension; and so on. Furthermore, it is considered that the manufacturing cost can be lowered than that of crystalline silicon solar batteries, and future diffusion over a wide range from the home use level to a large-scaled power plant level is anticipated.
In the structure of an amorphous silicon solar battery, development of CVD technologies has made it possible to produce cells by sequentially stacking semiconductor thin films of desired compositions and thicknesses. In general, often used cells have a structure having a potential gradient from the photo detecting surface to the back surface, which is made by sequentially stacking on a substrate of glass, for example, n-type hydrogenated amorphous silicon (hereinafter called xe2x80x9ca-Si:Hxe2x80x9d) thin film containing phosphorus, a [p-type] i-type a-Si:H thin film containing no impurity, and a p-type a-Si:H thin film containing boron.
In addition to such structure having a potential gradient produced by impurities, also known are hetero junction type solar battery cells that have a structure including a multi-layered film made by stacking two or more kinds of semiconductor materials different in band gap and are capable of efficient photoelectric conversion matching with different wavelengths, as a technology for fabricating a high-efficiency amorphous solar battery.
Regarding hetero junction type solar battery cells, there is a trial of effectively using light by employing hydrogenated amorphous silicon germanium (hereinafter called xe2x80x9ca-SiGe:Hxe2x80x9d) thin film, for example. This a-SiGe:H has a high photo absorptance, and allows an increase in short-circuit current. However, since a-SiGe:H has more levels in a band gap than a-Si:H, it has the drawback that slope factors decrease. Thus, the band gap is continuously changed by changing the composition ratios of a-SiGe:H, a-Si:H or the like of the i-type layer, to overcome those drawbacks. In case of this structure, as the minimum value portion of the band gap of the i-type layer comes closer to the p-type layer on the part of incidence of light, light deteriorates less and the device can be improved in reliability. This is because along with an increase of the photo absorption distribution near the p-type layer, collection of holes is improved more. However, making the minimum band gap portion near the p-type layer involved the problem that the band gap of the i-type layer near the p-type layer became smaller and rendered the open circuit voltage lower. Further, although this method decreases the band gap of the i-type layer and increases the optical absorption, decreasing the band gap of the i-type layer to about 1.4 eV or less causes slope factors to decrease, and the efficiency is not improved even with an increase of the amount of photo absorption. Furthermore, there is known the method of interposing hydrogenated amorphous silicon carbide (hereinafter called a-SiC:H) film having a wide gap around 2.1 eV between the p-type layer and the i-type layer for the purpose of further improving the open circuit voltage.
On the other hand, an amorphous film fabricated at a substrate temperature of or below 200?C contains a number of elements like local energy levels in the energy band gap, which can be nucleus of recombination of minority carriers, and its carrier length is shorter than those of single crystals and poly-crystals. If the dark conductivity becomes small in doped a-Si:H, a-SixGel-x:H, a-Ge:H, a-SiC:H and other like films, conversion efficiency of solar batteries using these films as their p-type layers and/or n-type layers of pin diodes forming the solar batteries become lower, and this is a bar to fabrication of high-quality solar batteries at low temperatures. However, also proposed is the technology of increasing the dark conductivity by using laser annealing which crystallizes only p-type layers and/or n-type layers of pin diodes while keeping substrates at lower temperatures.
Appropriate combination of these technologies is expected to improve the efficiency of amorphous silicon solar batteries, and further diffusion of amorphous silicon solar batteries in the future is anticipated also from the standpoint of their manufacturing cost.
In order to provide for various future uses of solar batteries for wide-spreading amorphous silicon solar batteries, decreasing the weight of products, improvement of their productivity, reduction of the curvature processing cost, and others, are required. Many of materials having low melting points and plastic materials can be configured into desired shapes at low temperatures, and are therefore advantageous in readiness to reduce the processing cost. Plastic materials have further advantages that products are light and not fragile. Therefore, it is desirable to make high-quality amorphous silicon solar batteries or hetero junction type solar batteries on substrates of those materials.
If plastics, especially general-purpose plastics like polyester films, can be employed as base bodies, those requirements can be met in combination with roll-to-to-roll production facilities using elongated base bodies.
However, when films are stacked on a plastic substrate, the substrate is liable to curve or warp after growth of the films due to a stress in films caused by difference in thermal expansion coefficient between the plastic and films, swelling of the plastic, and so on. In this case, if the films grown on the plastic substrate insufficiently adhere one another, films will peel off at their boundaries. Additionally, although the stress of the films exerted to the substrate can be relaxed by simultaneously stacking films on opposite surfaces of the substrate, if the films do not adhere well, it is not possible to make the most of flexibility of the plastic.
Plasma CVD has been typically used heretofore as a technology for fabricating photovoltaic devices using amorphous silicon films or other like films. Plasma CVD typically uses SiH4 as the source material gas. In the case where the film is stacked on ITO as a transparent electrode by plasma enhanced CVD (PE-CVD) using SiH4, SiH4 gas is decomposed in the plasma into hydrogen ions and damages the ITO surface. In case of a solar battery using a conventional glass substrate, the substrate does not warp with a stress of the film after deposition of the film, and the solar battery is not bent in practical use. Therefore, separation of the substrate and the film did not occur. A plastic substrate, however, warps with a stress from a film after deposition of the film. An a-Si film stacked thereon will undesirably peel off at the boundary with ITO.
Taking account of photovoltaic property of a film and its deposition rate, deposition of a film by PE-CVD is indispensable, and this is very serious problem upon fabricating a solar battery on a plastic substrate.
Toward a solution of the above-indicated problems involved in the conventional techniques, the Inventor made various researches that are summarized below.
As already explained, conventional manufacturing method of photovoltaic devices using amorphous silicon films, or the like, usually provide very good thin films if the films are stacked by using plasma CVD.
There is also a deposition method by sputtering as one of deposition methods of amorphous silicon films. In case of stacking a film by plasma CVD, since it uses SiH4 as the source material gas, it inevitably exposes the substrate surface to H2 plasma. However, sputtering is conducted without introducing H2 gas, this problem can be avoided.
This invention has been made through further researches by the Inventor based on the knowledge reviewed above.
To attain the above-indicated object, according to the first aspect of the invention, there is provided a semiconductor device comprising:
a base body made of an organic high polymer material;
an oxide electrode film on the base body; and
a semiconductor thin film on the oxide electrode film, which contains at least one kind of group IV elements,
wherein no granular products each having a diameter not smaller than 3 nm are substantially contained at the boundary between the oxide electrode film and the semiconductor thin film.
From the viewpoint of further improving the adherence between the oxide electrode film and the semiconductor thin film, it is preferable that the boundary between the oxide electrode film and the semiconductor thin film does not contain granular products with a diameter not smaller than 1 nm.
According to the second aspect of the invention, there is provided a semiconductor device comprising:
a base body made of an organic high polymer material;
an oxide electrode film on the base body; and
a semiconductor thin film on the oxide electrode film, which contains at least one kind of group IV elements,
wherein the semiconductor thin film is stacked in a non-reducing atmosphere in an initial period of deposition thereof.
According to the third aspect of the invention, there is provided a manufacturing method of a semiconductor device having a base body made of an organic high polymer material; an oxide electrode film on the base body; and a semiconductor thin film on the oxide electrode film, which contains at least one kind of group IV elements, comprising:
a step of stacking the semiconductor thin film in a non-reducing atmosphere in an initial period of deposition thereof.
In the present invention, the base body is typically a transparent base body, and more specifically, a film of a transparent organic high polymer material such as polyester (PET), for example, is used. The oxide electrode film is typically a transparent electrode film, and more specifically, it is, for example, ITO (indium tin oxide), tin oxide, tin oxide doped with fluoric acid, zinc oxide, zinc oxide-aluminum oxide, or the like.
In the first aspect of the invention, a portion of the semiconductor thin film near the boundary between the oxide electrode film and the semiconductor thin film is preferably stacked in a non-reducing atmosphere, and more particularly, in an atmosphere not containing hydrogen gas. Typically, the portion of the semiconductor thin film near the boundary between the oxide electrode film and the semiconductor thin film is stacked by sputtering not using hydrogen gas, and at least another portion of the semiconductor thin film is stacked by plasma enhanced chemical vapor deposition (PE-CVD).
In the present invention, the semiconductor thin film is typically an amorphous semiconductor thin film, and more specifically, it is a hydrogenated amorphous silicon, a hydrogenated amorphous germanium, a hydrogenated amorphous silicon germanium, a hydrogenated amorphous silicon carbide.
In the present invention, the semiconductor device may basically be any that uses a semiconductor thin film. Specifically, however, it is a thin film photovoltaic device, for example, and more particularly, a thin film solar battery, for example.
According to the first aspect of the invention having the above-summarized structure, since the boundary between the oxide electrode film and the semiconductor thin film does not contain granular products having a diameter of 3 nm or larger, their adherence is improved. Therefore, when using a base body of a general-purpose plastic like a polyester film having formed thereon an oxide electrode film like ITO, and stacking thereon a semiconductor thin film by PE-CVD using a source material gas containing hydrogen, the semiconductor thin film can be effective prevented from peeling off from the base body even if the base body curves or warps after deposition of the film.
According to the second and third aspects of the invention arranged as summarized above, by stacking the semiconductor thin film in a non-reducing atmosphere in an initial period of deposition, it is ensured that no granular products having a diameter of 3 nm or larger are contained at the boundary between the oxide electrode film and the semiconductor thin film, and their adherence is improved. Therefore, when using a base body of a general-purpose plastic like a polyester film having formed thereon an oxide electrode film like ITO, and stacking a semiconductor thin film by PE-CVD using a source material gas containing hydrogen, the semiconductor thin film can be effective prevented from peeling off from the base body even if the base body curves or warps after deposition of the film.