Method for forming bond between different elements   

Abstract: The present invention provides a doping technique that forms a stable amorphous silicon film and a stable polycrystalline silicon film at a low temperature and simultaneously that imparts conductivity in an atmospheric pressure environment. A method for producing a compound containing a bond between different elements belonging to Group 4 to Group 15 of the periodic table, the method included: applying, at a low frequency and atmospheric pressure, high voltage to an inside of an electric discharge tube obtained by attaching high-voltage electrodes to a metal tube or an insulator tube or between flat plate electrodes while passing an introduction gas, so as to convert molecules present in the electric discharge tube or between the flat plate electrodes into a plasma; and applying the plasma to substances to be irradiated, the substances to be irradiated being two or more elementary substances or compounds.

TECHNICAL FIELD

The present invention relates to a technique for forming a bond between different metals using low-temperature atmospheric-pressure plasma jet and, in particular, relates to a technique useful for doping with a Group 13 element or a Group 15 element into a silicon film using a polysilane compound and an oligosilane compound that can form a film through coating.

BACKGROUND ART

A silicon semiconductor is a material that has been studied as materials for a thin film transistor (TFT) and a solar cell since a long time ago. In particular, the TFT has been studied for a long time. In 1930, Lilienfeld et al. developed it as a current control device, and then in 1945, a vacuum deposited silicon thin film was observed to have a slight TFT action. Bardeen et al. examined the characteristic evaluation to lead to a suggestion in which an electric field to a semiconductor surface due to a gate potential is shielded by surface level, causing surface carriers not to be substantially changed (Bardeen\'s model). Based on the model, a point-contact transistor (1947) and a junction transistor (1948) that act as a transistor by injection of a minority carrier into a bulk crystal were developed. In 1950, a bipolar transistor was actively studied, and various inexpensive discrete elements have become popular to take the place of a vacuum tube.

However, at that time, a silicon TFT exhibited no characteristic, and compound semiconductors such as CdS were used, but the characteristics of such semiconductors were not stabilized at all. Then, RCA improved the characteristics and developed a TFT using a CdS thin film exhibiting fine characteristics in 1962. Furthermore, from 1960 to 1970, Westinghouse produced operative TFTs on Mylar (polyethylene terephthalate), Kapton (polyimide), and paper that are precursors of a flexible display. Up to then, II to VI compound semiconductors, tellurium (Te), and the like had been used as semiconductor materials. CdSe as a compound semiconductor readily forms a polycrystalline structure through a low temperature process such as deposition, can achieve a hole electron mobility of about 50 cm2/Vs and has characteristics of a high ion current and a low off-state current, and consequently has been used as a TFT material. However, it is a two-dimensional compound semiconductor. This raises an intrinsic problem of stoichiometry and a problem of unstable transistor characteristics due to the contact condition between a gate insulating film and a CdSe interface.

To address this, in 1975, Prof. Spear et al. at Dundee University in the UK successfully developed a hydrogenated amorphous silicon (a-Si:H) thin film that exhibits good semiconductor characteristics by glow discharge. Good TFT characteristics using the thin film were reported in 1979, and then the TFT material has shifted to silicon at once. The (a-Si:H) TFT has a small off-state current and can ensure a high on/off current ratio but is considered to have a disadvantage of a low mobility. In contrast, it has been pointed out that a thin film of silicon deposited at a high temperature or a poly-Si TFT composed of a (poly-Si) thin film that is obtained by heating of an (a-Si:H) thin film has a high mobility but a large off-state current. However, the silicon has finally been selected as the TFT material from the viewpoints of low cost, operability, and performance.

The (a-Si:H) film is excellent in uniformity, reproducibility, and microfabrication properties of the film in the formation of a thin film having a large area and is a material suited for a device having a large area. The a-Si (amorphous silicon) can be treated at a comparatively low temperature. The obtained TFT is a highly resistant material and can work with low voltage alternating-current. Hence, such a TFT is used as a basic switch element for picture elements of LCDs. As a preparation method of a commonly used amorphous silicon thin film, a plasma CVD method, a photo-CVD method, a thermal CVD method, and a reactive sputtering method are known.

There is a production method that includes forming an electron flow from a negative electrode to a positive electrode through a minute substance, inducing plasma on a surface of the minute substance based on the potential difference, and producing a reaction substance by a plasma reaction (see Patent Document 1).

There is a method for forming a thin film that includes introducing a reaction gas that is to be a material of a thin film into a reaction furnace, applying a voltage between electrodes to form plasma and decompose the reaction gas, and causing a chemical reaction to deposit a thin film on a substrate (see Patent Document 2).

In the plasma CVD method, electric energy is applied to a Si-containing material gas such as silane gas (SiH4) to generate plasma, and active radicals or ions are generated to cause a chemical reaction in an active environment.

The generated plasma has two temperatures of an electron temperature and a gas temperature. There are a high temperature plasma having high electron and gas temperatures reaching several tens of thousands Kelvin and a low temperature plasma having a high electron temperature reaching several tens of thousands Kelvin and a low gas temperature ranging from room temperature to hundreds of degree Celsius. In particular, the used of the low temperature plasma enables the formation of a thin film while maintaining a substrate temperature at a low temperature. In contrast to the conventional thermal CVD method in which the temperature of a substrate must be raised to about 1,000° C., the plasma CVD method has an advantage of being capable of forming a thin film while maintaining a substrate at a low temperature.

The formed active species reaches a substrate surface mainly due to diffusion, and undergoes processes such as adsorption, desorption, drawing, insertion, and surface diffusion on the surface to form a film as an a-Si thin film. The electric power used for generating plasma is a direct current, a radio wave, a high-frequency wave, a microwave, and the like. Among them, a high-frequency wave in a frequency band of 13.56 MHz is typically used. In an actual plasma reaction apparatus, electrodes are disposed opposite to each other in a vacuum chamber. The high-voltage electrode is connected to a high-frequency power supply through a dielectric material and the like to form a cathode. The other electrode is grounded together with the vacuum chamber to form an anode. Almost all the high-frequency electric power charged is consumed near the cathode. Thus, in such region, SiH4 is actively excited and decomposed. In addition, the thin film deposition rate is larger on the cathode side. However, the film is formed in a large electric field, the electrode surface is accordingly subjected to a strong impact of positive ions, and it is very difficult to obtain a smooth surface. Therefore, a substrate on which a deposition film is formed is typically installed on the anode side. However, it seems to be disadvantageous for the plasma CVD method that the substrate is affected by the ion impact even in such installation manner. This impact effect is more remarkable when the pressure is lower and when the charged electric power is higher.

With respect to the film growth, the generated active species come into collision in SiH4 gas to be deactivated. However, among them, SiH3 is stable against the deactivation by the collision. In other words, SiH3 has low reactivity, and thus SiH3 alone cannot participate in the formation of a network as long as the growth surface has no dangling bond. Actually, on a substrate having a temperature of 100 to 300° C., it is shown that the surface of a growth film is almost covered with hydrogen. SiH3 that has reached the surface moves to a site from which hydrogen is drawn while diffusing on the surface. When the surface diffusion is active in this manner, Si atoms are arranged in an energetically more stable site to form a dense amorphous film having high relaxivity.

In recent years, new low-defect film preparation techniques have been developed. For example, there are a method in which reaction at a high temperature (350° C. or more) increases the diffusion coefficient of a reaction species and dangling bonds generated by thermal desorption of hydrogen covering a surface are integrated into a film without clearance by supplying a large amount of SiH3, a method in which thermal energy is supplied to a reaction dominating species, and a method in which a growth surface is photoexcited to accelerate surface diffusion. There are also developed a growth/hydrogen plasma treatment repeating method (chemical annealing method) in which an amorphous structure that is present in a several atom layer from a growth surface and that has not been solidified is intended to be relaxed by atomic hydrogen and a method of reducing a thin film growth rate to provide a time needed for relaxation. It has been demonstrated that thin films prepared by the above methods are especially useful as an (a-Si:H) solar cell.

In contrast, in the photo-CVD method, SiH4 is directly (so-called direct photo-CVD) or indirectly (so-called mercury sensitizing or indirect photo-CVD) degraded using light energy, and these methods are collectively called the photo-CVD method. In these growth methods, it is supposed that a growth film surface is not subjected to the impact of ion species and electrons having high energy, and hence mild growth conditions can be obtained. Furthermore, for example, in the mercury sensitizing method, it is considered that a SiH3 reaction species having a large surface diffusion coefficient is selectively generated to achieve conditions suited for the formation of a good film. Generally, a low film formation rate causes a problem, but the use of a high-order silane having a high degradation efficiency, such as Si2H6 and Si3H8 in place of SiH4 compensates for the problem.

As doping for such a thin film, a gas phase doping method is commonly performed. The doping can be readily performed by reaction while adding an impurity gas such as PH3, AsH3 (n-type), and B2H6 (p-type) to a SiH4 source.

By the plasma CVD method and the photo-CVD method as above, an amorphous silicon film can be obtained at a comparatively low temperature (about 300° C.) and the use of SiH4 in combination with another mixing gas enables the reaction of these two or more molecules on a solid phase surface or in a gas phase to form a bond between different elements.

Meanwhile, use of polycrystalline silicon intends to reduce cost by the reduction in cost needed for crystallization which accounts for a large proportion of the raw material cost of single crystalline silicon, and the purity of the raw material and a substrate preparation process including crystallization are improved. As shown in FIG. 1, preparation methods of a polycrystalline silicon wafer are classified into three methods; that is, an ingot slicing method, a sheet method with a substrate, and a sheet method without a substrate. For these three methods, methods shown in FIG. 1 are developed. First, a metallurgical grade silicon (an impurity concentration of about 10−2) as a raw material includes heavy metals (life time killer) that provide a deep level in a bulk crystal, elements that provide a donor and an acceptor, and a large amount of oxygen and carbon. In semiconductor-grade silicon, impurities are chemically removed or removed by segregation to a level not affecting a product. In contrast, a solar cell is not required to have such a high purity that is required for the semiconductor-grade silicon, because it is a large area single junction device. For example, for a solar cell, the necessary impurity level for achieving a conversion efficiency of about 10% is as shown in Table 1. For the solar cell-grade silicon, acid washing, a method using segregation at the time of crystallization, and the like are considered to be effective. The metallurgical-grade silicon includes carbon that is used in a reduction process of silicon oxide, and hence a decarbonizing process is also important.

TABLE 1 Concentration in metallurgical Allowable Impurity grade silicon (ppm) amount (ppm) Dopant Al 1,500 to 4,000 B 40 to 80 P 20 to 50 Life time killer Ti 160 to 250 0.001 V  80 to 200 0.002 Fe 2,000 to 3,000 0.02 Cr  50 to 200 0.1 Ni 30 to 90 0.8

For the preparation of polycrystalline silicon, a crystal grain size is firstly considered. This is because, especially in a solar cell, when the crystal grain size is large as compared with a film thickness, a minority carrier that flows into a junction to effectively contribute to electric power generation is sufficiently larger than the flow into a grain boundary exhibiting a short carrier lifetime, and this can suppress effects on the crystal grain boundary. An actual silicon solar cell needs a crystal grain boundary of 50 μm or more. The crystal grain boundary greatly depends on a production process and a film thickness, and in general, the production process is broadly divided into a liquid phase method and a gas phase method. The methods shown in FIG. 1 correspond to the liquid phase method. The ingot slicing method is a method in which a molten silicon is poured into a mold and cooled to prepare an ingot and the ingot is sliced.

Silso by Wacker and HEM (Heat exchange method) by Crystal System are known, and a crystal grain boundary prepared by the methods reaches several millimeters. Such a liquid phase method needs high temperature treatment because a molten silicon is typically used, and hence the method needs a large system. In contrast, the preparation method from a gas phase includes a vacuum deposition method, a sputtering method, and a gas-phase chemical reaction (CVD) method as described above. However, the crystal grain boundary obtained by such a method is normally very small, and hence the product cannot be used without treatment. Therefore, in order to obtain a crystal having a large grain boundary, it is necessary to subject the gas phase grown crystal to a crystal growth process in a liquid phase state once again, and this needs treatment with electron beam, laser, lamp heating, and the like.

Patent Document 1: Japanese Patent Application Publication No. JP-A-2005-238204 Patent Document 2: Japanese Patent Application Publication No. JP-A-6-314660

SUMMARY

In related arts, for example, for the formation of an amorphous silicon film, silane gas and a dopant gas are mixed in an ultrahigh vacuum chamber and a silicon film is formed on a substrate. However, such a method needs the vacuum chamber, which causes a problem in which a substrate area is limited by the chamber volume.

Furthermore, a dangling bond generated during film formation is bonded to impurity elements during crystal growth to provide an electron state having energy that corresponds to a forbidden band in a single crystalline silicon. This works as a trap site or a recombination center with respect to a minority carrier and usually shortens the service life of a device. In the plasma CVD method and the like, short circuit must be developed using hydrogen with respect to such a dangling bond for inactivation. This operation must be carried out in a CVD system and this also needs an ultrahigh vacuum chamber, which causes a problem in process.

In contrast, in the production of a polycrystalline silicon, as described above, the liquid phase method needs the operation in an environment at high temperature for melting silicon and without oxygen and also needs a reduction operation using carbon for removing oxygen that is contained in the silicon in a large amount. In this manner, the polycrystalline silicon needs huge thermal energy for crystal growth and has a problem in production process.

Furthermore, doping with respect to such a silicon film is carried out in a manner similar to that in the production process of a single crystalline silicon. In contrast, when a polycrystalline silicon is produced by the gas phase method, after the film formation of an amorphous silicon by CVD described above, the amorphous silicon is further heated and melted to make a single crystal. This method also has a problem in which heat treatment that is substantially the same degree as in the liquid phase method is finally required.

The present invention overcomes the above disadvantages and provides a doping technique that forms, at a low temperature, a stable amorphous silicon film and a stable polycrystalline silicon film, and at the same time imparts conductivity in an atmospheric pressure environment.

The present invention provides, as a first aspect, a method for producing a compound containing a bond between different elements belonging to Group 4 to Group 15 of the periodic table, the method being characterized by including applying, at a low frequency and atmospheric pressure, high voltage to an inside of an electric discharge tube obtained by attaching high-voltage electrodes to a metal tube or an insulator tube or between two flat plate electrodes provided with high-voltage electrodes while passing an introduction gas, so as to convert molecules present in the electric discharge tube or between the flat plate electrodes into a plasma; and applying the plasma to substances to be irradiated, the substances to be irradiated being two or more elementary substances belonging to Group 4 to Group 15 of the periodic table, two or more compounds containing the element, or a combination of the elementary substance and the compound,

as a second aspect, the production method according to the first aspect, in which the compound containing a bond between different elements is a compound containing a bond between different elements belonging to Group 13 to Group 15 of the periodic table,

as a third aspect, the production method according to the first aspect or the second aspect, in which the plasma or a radical of a surrounding gas excited by the plasma is applied to the substances to be irradiated and ultraviolet light is also applied to the substances to be irradiated,

as a fourth aspect, the production method according to any one of the first aspect to the third aspect, in which the compound containing a bond between different elements is obtained as a coating on a substrate,

as a fifth aspect, the production method according to any one of the first aspect to the fourth aspect, in which the substances to be irradiated are two or more substances selected from the group consisting of the elementary substances, the compounds, solutions containing the elementary substances, solutions containing the compounds, gases of the elementary substances, and gases of the compounds,

as a sixth aspect, the production method according to any one of the first aspect to the fifth aspect, in which one of the substances to be irradiated is a compound containing a Group 14 element and another is a gas of an elementary substance belonging to Group 4 to Group 15 or a gas of a compound containing the element,

as a seventh aspect, the production method according to any one of the first aspect to the sixth aspect, in which one of the substances to be irradiated is a compound containing a Group 14 element, another is an elementary substance belonging to Group 13, a compound containing the element, an elementary substance belonging to Group 15, or a compound containing the element, and the elementary substance belonging to Group 13, the compound containing the element, the elementary substance belonging to Group 15, or the compound containing the element is included in a ratio of 0.2 to 10 mol with respect to 1 mol of the compound containing a Group 14 element,

as an eighth aspect, the production method according to the first aspect, in which the compound containing a bond between different elements contains a Si—Si bond and a Si—B bond or a Si—P bond,

as a ninth aspect, the production method according to any one of the sixth aspect to the eighth aspect, in which the compound containing a Group 14 element is at least one silane compound selected from the group consisting of a chain silane compound of Formula (1):

SinH2n+2  Formula (1)

(in Formula (1), n is an integer of 2 to 40), a cyclic silane compound of Formula (2):

SihH2h  Formula (2)

(in Formula (2), h is an integer of 3 to 10), a cyclic silane compound of Formula (3):

SihH2h−2  Formula (3)

(in Formula (3), h is an integer of 3 to 10), and a cage silane compound of Formula (4):

SimHm  Formula (4)

(in Formula (4), m is an integer of 6, 8, or 10),

as a tenth aspect, the production method according to any one of the sixth aspect to the eighth aspect, in which the elementary substance belonging to Group 13 or the compound containing the element is an elemental boron or a boron-containing compound of Formula (5):

BiHj  Formula (5)

(where i is an integer of 1 to 10, and j is an integer of 0 to 12),

as an eleventh aspect, the production method according to any one of the sixth aspect to the eighth aspect, in which the elementary substance belonging to Group 15 or the compound containing the element is an elemental phosphorus or a phosphorus-containing compound of Formula (6):

PwXu  Formula (6)

(where w is an integer of 1 to 10, u is an integer of 0 to 12, and X is a hydrogen atom or a monovalent organic group), an elemental arsenic, an arsenic-containing compound, a nitrogen molecule, or a nitrogen-containing compound,

as a twelfth aspect, the production method according to any one of the first aspect to the eleventh aspect, in which the gas introduced into the electric discharge tube or between the flat plate electrodes is at least one type of gas selected from the group consisting of helium, neon, argon, krypton, xenon, nitrogen molecules, oxygen molecules, hydrogen molecules, carbon dioxide, nitric oxide, nitrogen dioxide, ammonia, halogen molecules, hydrogen halides, sulfur dioxide, hydrogen sulfide, and water vapor,

as a thirteenth aspect, the production method according to any one of the first aspect to the eleventh aspect, in which the gas introduced into the electric discharge tube or between the flat plate electrodes is helium gas alone or a mixed gas of helium with at least one type of gas selected from the group consisting of hydrogen molecules, oxygen molecules, nitrogen molecules, carbon dioxide, carbon monoxide, fluorine molecules, and chlorine molecules,

as a fourteenth aspect, the production method according to any one of the first aspect to the thirteenth aspect, in which the two flat plate electrodes formed of a metal or an insulator are disposed facing each other, one of the electrodes is connected to a high-voltage electrode, the other is not connected to an earth lead for air discharge or is connected to a grounding electrode, and a gas is passed through between the electrodes so as to convert molecules present between the electrodes into a plasma,

as a fifteenth aspect, the production method according to any one of the first aspect to the fourteenth aspect, in which the two flat plate electrodes formed of a metal or an insulator are in a decompressed container, the introduction gas is passed after decompression, and a high voltage is applied at a low gas pressure and a low frequency to convert molecules present between the electrodes into a plasma,

as a sixteenth aspect, the production method according to any one of the first aspect to the fifteenth aspect, in which the electric discharge tube formed of a metal tube or each of the flat plate electrodes formed of a metal is formed of an elementary substance belonging to Group 4 to Group 14 or a mixture containing the elementary substance,

as a seventeenth aspect, the production method according to any one of the first aspect to the fifteenth aspect, in which the electric discharge tube formed of an insulator tube or each of the flat plate electrodes formed of an insulator is formed of a synthetic polymer, a natural polymer, glass, or ceramic, and

as an eighteenth aspect, the production method according to any one of the first aspect to the seventeenth aspect, in which a power supply used for plasma generation has a frequency of 10 Hz to 100 MHz and an output voltage of 1,000 V to 30,000 V and the plasma is applied at a low temperature.

The present invention enables doping that forms a stable amorphous silicon film and a stable polycrystalline silicon film at a low temperature and simultaneously that imparts conductivity in an atmospheric pressure environment.

The present invention relates to a technique for forming a bond between different metals using low-temperature atmospheric-pressure plasma jet, and in particular, relates to a technique useful for doping with a Group 13 element or a Group 15 element into a silicon film using an oligosilane compound that can form a film through coating.

In other words, chemical reaction by plasma is controlled by the control of plasma jet that is in an ionized gas state and has high energy by gas pressure and electric field, and this enables doping of a silicon thin film with a different element near atmospheric pressure and at low temperature.

As a whole, plasma that is at low temperature but has a high energy component with high reactivity and is in a non-equilibrium state, is generated at a pressure not less than the steam pressure of a liquid, that is, at a pressure around atmospheric pressure. By application of a mixed solution of a boron hydride compound and an oligosilane compound and subsequent spin coating, a thin film is formed. The plasma is applied to the thin film to remove hydrogen so that silicon and boron are directly bonded. The treatment can be continuously performed because an ambient pressure process using the ambient pressure (atmospheric pressure) plasma does not need vacuum. Furthermore, helium gas that is readily ionized is used for plasma generation. In addition, the treatment can be performed at a low temperature, and hence the treatment can be performed without damage to a substrate. Therefore, the technique can be applied to various substrates. The technique is used for producing a solar cell, a transistor, and various sensors including a silicon semiconductor as a basic component in an atmospheric pressure environment through a low temperature wet process. The technique enables the formation in a low temperature process. Therefore, the technique is useful for weight reduction of devices and useful as a technique for production of plastic devices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing types of the preparation method of a polycrystalline silicon.

FIG. 2 is a schematic view showing a thermal non-equilibrium atmospheric-pressure plasma jet apparatus.

FIG. 3 is a view showing emission spectra from plasma.

FIG. 4 is a schematic view showing a thermal non-equilibrium atmospheric-pressure plasma apparatus using flat plate electrodes.

FIG. 5 is a view showing the results of XPS analysis of a Si—N bond formed by doping of an oligosilane with nitrogen.

FIG. 6 is a view showing the results of XPS analysis of a Si—B bond formed by doping of an oligosilane with boron.

FIG. 7 is a view showing voltage-current characteristics of a boron-doped amorphous silicon.

FIG. 8 is a view showing an ESR spectrum showing a hole generated in a valence band derived from boron by plasma exposure.

FIG. 9 is a view showing an ESR spectrum showing a hole generated in a valence band derived from boron by plasma exposure.

FIG. 10 is a view showing an ESR spectrum showing a hole generated in a valence band derived from boron by plasma exposure.

FIG. 11 is a view showing an ESR spectrum showing a hole generated in a valence band derived from boron by plasma exposure.

FIG. 12 is a view showing an ESR spectrum showing an electron generated near a conductor derived from phosphorus by plasma exposure.

The present invention is a method for producing a compound containing a bond between different elements belonging to Group 4 to Group 15 of the periodic table. The compound is obtained by applying, at a low frequency and a pressure near atmospheric pressure, high voltage to an inside of an electric discharge tube that is obtained by attaching high-voltage electrodes to a metal tube or an insulator tube or between two flat plate electrodes provided with high-voltage electrodes while passing an introduction gas, so as to convert molecules present in the electric discharge tube or between the flat plate electrodes into a plasma (see FIG. 2 and FIG. 4), and applying the plasma to two or more elementary substances belonging to Group 4 to Group 15 of the periodic table or compounds containing the element.

The application of the plasma (ionized substance) and radicals of a surrounding gas excited by the plasma to metals, elements to be a semiconductor, or compounds of them achieves the production of the compound containing a bond between different elements.

The compound containing a bond between different elements contains a bond between different elements between Group 4 elements and Group 15 elements of the periodic table and in particular, contains a bond between different elements between Group 13 elements to Group 15 elements of the periodic table. Containing a bond between different elements means being composed of a bond between the same elements and a bond between different elements. In a compound containing a bond between different elements, a Group 14 element doped with a Group 13 element has a ratio (Group 13 element)/(Group 14 element) ranging from 1×10−1 to 1×10−6 and includes hydrogen in a total amount of about 15% by atom to 25% by atom, while a Group 14 element doped with a Group 15 element has a ratio (Group 15 element)/(Group 14 element) ranging from 1×10−2 to 1×10−5 and includes hydrogen in a total amount of about 15% by atom.

In particular, the compound containing a bond between different elements may contain a Si—Si bond in combination with a Si—B bond or a Si—P bond. Each doping amount of boron and phosphorus is not limited, but the amount of hydrogen in a p-type a-Si:H film doped with boron can be determined by a hydrogen heat release method and an infrared absorption method (Z. E. Smith, Glow-discharge Hydrogenated Amorphous Silicon, KTK/Kluwer, Tokyo, Boston, 1989, 127). The hydrogen heat release spectrum reveals that (B)/(Si) is in a range of 1×10−1 to 1×10−6 and the total hydrogen amount is about 15% by atom to 25% by atom. Phosphorus can be analyzed in a similar method and the measurement reveals that (P)/(Si) is in a range of 1×10−2 to 1×10−5 and the total hydrogen amount is about 15% by atom.

The atmospheric pressure is a pressure in an environment where plasma is generated, and is not particularly limited but, for example, is in a range not less than the steam pressure of a solution and 10 atmosphere or less, and more preferably an atmospheric pressure where a reaction is carried out without pressure, and the atmospheric pressure is preferred because such an environment does not need a decompressor and a compressor.

The compound containing a bond between different elements can be obtained as a coating on a substrate by applying two or more elementary substances or compounds of them as substances to be irradiated on the substrate and applying plasma to the substances.

The substances to be irradiated can be used in a form of the elementary substance itself, the compound itself, a solution containing them, a gas of the elementary substance, a gas of the compound, or a combination of some of them.

For the substances to be irradiated, a compound containing a Group 14 element may be used as one of the substances, and a gas may be used as the other.

In addition, for the substances to be irradiated, an elementary substance belonging to Group 14 or a compound containing the element may be used as one of the substances, and an elementary substance belonging to Group 13 or Group 15 or a compound containing the element may be used as the other. The elementary substance belonging to Group 13 or Group 15 or the compound containing the element may be used as a gas.

As the compound containing a Group 14 element, at least one silane compound selected from the group consisting of Formula (1), Formula (2), Formula (3), and Formula (4) may be used.

Formula (1) represents a chain silane compound (including linear and branched silane compounds), and n is an integer of 2 to 40 or an integer of 2 to 30. Formula (2) and Formula (3) represent cyclic silane compounds, and h is an integer of 3 to 10. Formula (4) represents a cage silane compound, and m is 6, 8, or 10.

The silane compound can be obtained, for example, by the following reaction as an example.

Cp is a cyclopentadienyl group, and Ph is a phenyl group. The silane compound can also be obtained by the following reaction as an alternative method.

where Ph is a phenyl group.

As the elementary substance belonging to Group 13 or the compound containing the element, Formula (5) can be exemplified.

A boron component can be exemplified by Formula (5) where i is an integer of 1 to 10 and j is an integer of 0 to 12. When j is zero, it is an elemental boron. When j is 1 to 12, it is a boron hydride (borane). Examples of the boron hydride include monoborane (BH3), diborane (B2H6), tetraborane (B4H10), and decaborane (B10H14).

In addition, as the elementary substance belonging to Group 13 or the compound containing the element, gallium, indium, and compounds of them can be used.

As the elementary substance belonging to Group 15 or the compound containing the element, Formula (6) can be exemplified.

An elemental phosphorus or a phosphorus-containing compound can be exemplified by Formula (6) where w is an integer of 1 to 10 and u is an integer of 0 to 12. When u is zero, it is an elemental phosphorus. When u is 1 to 12, it is a phosphorus hydride. Examples of the elemental phosphorus include white phosphorus, red phosphorus, yellow phosphorus, and black phosphorus, and examples of the compound include phosphine (PH3).

In addition, as the elementary substance belonging to Group 15 or the compound containing the element, nitrogen gas, a nitrogen-containing compound, arsenic, and an arsenic-containing compound can be used. Examples of the nitrogen-containing compound include an amine, nitric acid, and a diazo compound. Examples of the arsenic-containing compound include arsenic hydride.

The elementary substance belonging to Group 13, the compound of the element, the elementary substance belonging to Group 15, or the compound of the element may be used in a ratio of 0.2 to 10 mol and preferably 1 to 5 mol with respect to 1 mol of the compound containing a Group 14 element.

A plasma is generated in an electric discharge tube by applying, at a low frequency and a pressure of atmospheric pressure (near atmospheric pressure), high voltage to the electric discharge tube that is obtained by attaching high-voltage electrodes to a metal tube or an insulator tube while passing a gas through the electric discharge tube (FIG. 2a). The generated plasma is applied to a metal, a compound that is to be a semiconductor, or a solution of them, so as to achieve the formation of a metal thin film.

When a metal tube is used as the electrode, the high-voltage electrode alone is connected to the metal tube and grounding is air (FIG. 2b). When a plastic tube is used, earth leads may be attached to front and back of the high-voltage electrode (at positions not being in contact with each other and keeping a distance from each other not causing arc discharge), and grounding may be air as with the metal tube. As the power supply needed for plasma generation, an alternating-current high voltage power supply is used. The alternating-current is 10 Hz to 100 MHz, preferably 50 Hz to 100 kHz, and more preferably 5 kHz to 20 kHz. An alternating voltage in a range of 1,000 V to 30,000 V can generate plasma, but the alternating voltage is preferably 1,000 V to 20,000 V and more preferably 5,000 V to 8,000 V.

A nozzle used for release the plasma is formed of an elementary substance belonging to Group 4 to Group 14 of the periodic table or a mixture containing the elementary substances. To the nozzle, a high-voltage electrode is connected and grounding is air. The application of a high voltage at a low frequency while passing a gas enables the generation of ionized gas and radical gas.

The electrode material is not particularly limited. As a metal, any electric discharge tube of a metal tube having a gas flow path, such as (an) aluminum (tube), (a) stainless steel (tube), (a) copper (tube), (an) iron (tube), and (a) brass (tube) or a metallic electrode can be used.

As the electric discharge tube of an insulator tube or the electrode of an insulator, any plastic may be used. Examples of general-purpose plastics include polyethylene (high-density polyethylene, medium-density polyethylene, and low-density polyethylene), polypropylene, polyvinyl chloride, polyvinylidene chloride, polystyrene, polyvinyl acetate, an acrylonitrile butadiene styrene resin (ABS resin), an acrylonitrile styrene resin (AS resin), an acrylic resin, and polytetrafluoroethylene. Examples of engineering plastics include, but are not necessarily limited to, polyamide, nylon, polyacetal, polycarbonate, modified polyphenylene ether (m-PPE or modified PPE), polybutylene terephthalate, polyethylene terephthalate, a polyethylene terephthalate glass resin (PET-G), cyclic polyolefin, and glass fiber reinforced polyethylene terephthalate (FRP). Examples of super engineering plastics include, but are not necessarily limited to, polyphenylene sulfide, polysulfone, polyethersulfone, amorphous polyarylate, liquid crystal polyester, polyether ether ketone, polyamide imide, polyimide, and polyamide.

In addition to the plastics, an inorganic ceramic material may be used as the insulator tube or the insulator electrode. Specific examples of the inorganic ceramic material include, but are not necessarily limited to, glass, silicon, zirconia, ceramics, alumina, titania, silicon carbide, and silicon nitride.