JPH0267719A - Formation of silicon carbide crystallite thin film - Google Patents

Abstract

PURPOSE:To make possible the formation of an Si carbide crystallite thin film having a wide optical band gap and moreover, having a high conductivity by a method wherein mixed gas containing an Si compound having an unsaturated group capable of performing a radical polymerization, silane fluoride and at least hydrogen is decomposed by glow discharge. CONSTITUTION:Mixed gas containing an Si compound having an unsaturated group capable of performing a radical polymerization, silane fluoride and at least hydrogen is decomposed by glow discharge. As the mixing ratio of U-silane to the silane fluoride, a ratio of using the silane fluoride at 0.1 times to 20 times or thereabouts the volume of the U-silane is desirable. The hydrogen to be used as raw gas is used at two times or more the volume of the Si compound, desirably is used at 4 times or more and 50 times or less. The mixed gas is decomposed by glow discharge, but a plasma CVD method is used for the glow discharge decomposition. Plasma can be generated by feeding power such as DC power, AC power, high-frequency power, microwave power and the like to a raw gas mixture under a reduced pressure.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field] The present invention relates to a method for forming a silicon carbide microcrystalline thin film, and particularly to a method for forming a silicon carbide microcrystalline thin film having a wide bandgap.

[Background technology]

It is known that a silicon microcrystalline thin film can have properties of a P-type semiconductor or an n-type semiconductor, and its application to thin-film semiconductor devices including silicon solar cells is being considered.

Therefore, a silicon carbide microcrystalline thin film (hereinafter abbreviated as μc-3iCx) is being studied in order to change or optimize the optical characteristics of this silicon microcrystalline thin film. Conventional μ
c-3iCx is formed by electron cyclotron resonance microwave discharge decomposition of monosilane and methane.
international PVSEC-3 (Tokyo
, 1987). This formation condition requires high hydrogen dilution and high discharge power, and the LOeV is a harsh condition in which high energy electrons are present in the thin film formation atmosphere. It is thought that chemical reactions caused by hydrogen atoms and collisions with high-energy electrons and ions may cause damage to the parts. In addition, in glow discharge decomposition in the conventional technology, even under film formation conditions of high discharge power and high hydrogen dilution, only microcrystalline silicon is formed and silicon carbide remains amorphous.
international PVSEC-3 (Tokyo.

1987) was reported at the same time. As a result of this,
Improvements in the performance of the thin film alone have not significantly contributed to improving the characteristics of semiconductor devices, and problems still remain. Furthermore, electron cyclotron resonance microwave discharge decomposition has problems in practicality due to the high cost of manufacturing equipment for increasing the area of semiconductor devices such as amorphous silicon solar cells and amorphous thin film transistors.

For this reason, a method of forming a silicon carbide microcrystalline thin film using glow discharge that can be applied to a large area is desired.

[Disclosure of the invention]

As a result of intensive studies in view of this point of view, the present inventors have found that μc-SiCx We were able to solve this problem by discovering that it is possible to form .

That is, the present invention provides a silicon carbide microcrystalline thin film (hereinafter referred to as μc -3iCx).

The present invention will be explained in detail below.

The silicon compound having a radically polymerizable unsaturated group used in the present invention, that is, the silane having a radically polymerizable unsaturated hydrocarbon group in the molecule (hereinafter referred to as U-
(abbreviated as silane) has the general formula R@S l waHz
m*z-s (where, R is an unsaturated hydrocarbon group having about 2 to 10 carbon atoms that is capable of radical polymerization, m and n are natural numbers, 0m is 1 or 2, and when m=1, n
-1, 2, 3° or 4 and m-2, n=1.
2. 3゜4.5. or 6).

Specific examples of this U-silane include vinylsilane, divinylsilane, trivinylsilane, vinyldisilane, divinyldisilane, trivinyldisilane, 1-propenylsilane, 2-propenylsilane, isopropenylsilane, 1-butenylsilane, -butenylsilane, 2-
Preferred examples include silicon compounds such as pentenylsilane and ethynylsilane.

Fluorinated silanes useful in the present invention include SiaH
zm+t-bF b (a is 1 or 2, b is a
When =l11, l, 2, 3. Or 4, and when a-2, 1, 2, 3, 4, 5. or 6) is a silicon compound having the general formula: Specific examples of this fluorinated silane include monofluorosilane, difluorosilane, trifluorosilane, tetrafluorosilane, monofluorodisilane, difluorodisilane, trifluorodisilane, tetrafluorodisilane, pentafluorodisilane, and hexafluorodisilane. etc. can be used effectively.

The mixing ratio of U-silane and fluorinated silane is not particularly limited, but fluorinated silane is used in a relatively large amount, that is, about 0.1 to 20 times the volume of U-silane. is preferable in order to achieve the object of the present invention.

In the present invention, hydrogen is further used as a raw material gas together with U-silane and fluorinated silane.
Used with more than double the capacity.

Preferably, the capacity is 4 times or more and 50 times or less. Even under conditions where the amount of hydrogen is more than 50 times the volume, μC-
3iCx is formed, but in the production of semiconductor devices, the conditions for producing μc-SiCx with such a high hydrogen content are
This is not a preferable condition because it will reduce the characteristics of the semiconductor device.On the other hand, under conditions less than 2 times the capacitance, the characteristics of μC-3iCx will be significantly deteriorated (which gives p-type or n-type conductivity to Therefore, the effectiveness of introduced impurities decreases.

In the present invention, such a mixed gas is decomposed by glow discharge, but the glow discharge decomposition is performed by plasma CVD.
(plasma chemical deposition method) can be effectively used. Plasma uses DC power, AC power, high frequency power,
It can be generated by supplying electrical power, such as microwave power, to the feed gas mixture under reduced pressure.

In the present invention, the conditions for forming a thin film are as follows, in addition to using a mixture of silicon compound and hydrogen as raw materials.
In the present invention, which is not limited, μC51Cx can be obtained at low discharge power and low substrate temperature. This is another feature of the invention. Furthermore, by using hydrogenated silicon such as monosilane or disilane mixed with U-silane, it is possible to change the carbon content of μc-3iCx, and the optical bandgap can be changed. Furthermore, by doping with impurities, P type, n
It is also possible to impart semiconductor properties to the type. In order to impart p-type and n-type properties, compounds containing boron and phosphorus such as diborane and phosphine may be decomposed by glow discharge together with the raw material gas.

In the present invention, the optimum conditions for specific film forming conditions such as substrate temperature, reaction pressure, raw material gas flow rate, discharge power, etc. during μc-3iCx fabrication differ depending on the glow discharge decomposition method used, and the decomposition method Although the decision will be made as appropriate depending on the situation, the following points are listed below that can serve as effective guidelines in such cases.

Substrate temperature changes the optical or electrical properties of μc-3iCx. The substrate temperature is 100°C or higher and 350°C or lower, preferably 150°C or higher and 300°C or lower, and more preferably 175°C or higher and 250°C or higher.
below °C. When imparting p-type and n-type semiconductor properties, a decrease in temperature increases the light transmittance, but the electrical resistance increases too much, making it undesirable for use in semiconductor devices. This is undesirable because it reduces the target band gear, and furthermore, there is a tendency for μc-3iCx to peel off from the substrate.

On the other hand, the reaction pressure is from 0.01 torr to 10 torr.
In a range of 0.02 torr to 2.0 torr, preferably.
The range is approximately 0 torr.

Further, the flow rate of the raw material gas and the discharge power are appropriately selected depending on the size of the film forming chamber of the film forming apparatus, the size of the discharge electrode, the size of the substrate, the target carbon content, the film forming rate, etc. The raw material gas flow rate is 1 to 1010 for silicon compounds.
00 sc is sufficient, and hydrogen is used in a range of 2 to 50 times.

Discharge power can be lowered from 5W to 150W
is sufficient, preferably about 100 W from LOW, and more preferably about 60 W from IOW.

(Action) It is generally recognized that various silicon-related radical species are formed during plasma decomposition of silicon compounds.The U-silane of the present invention works particularly effectively on these silicon The relevant radicals and the unsaturated hydrocarbon groups in the U-silanes of the present invention can undergo radical reactions, so that fewer hydrocarbon groups remain as pendants, and carbon is effectively incorporated into the silicon network. Therefore, it is believed that μc 5iCx can be formed without using the harsh conditions as in the prior art.

In particular, the ability to significantly reduce the discharge power required for film formation (as described above) compared to the conventional technology provides vitally favorable conditions when forming semiconductor devices, such as thin-film solar cells, light-emitting elements, optical sensors, etc. We will be able to provide this. However, semiconductor devices are usually formed from electrodes and several types of thin film semiconductors, and as in the prior art, when forming μc-3iCx, it is necessary to adopt formation conditions that require high discharge power. It is a well-known fact to those skilled in the art that in situations where the decomposition is not performed, large plasma damage is inevitable to the electrodes and semiconductor thin films that have already been formed. This is because, although various attempts are desperately being made to achieve this goal, no technological breakthrough has been achieved until now.

〔Example〕

Hereinafter, embodiment B of the present invention will be explained in more detail with reference to Examples.

[Example 1] The thin film forming apparatus includes (1) a raw material supplying means having a raw material flow rate control means, (2) a means for generating electric discharge to decompose the raw material, and (4) maintaining the thin film forming atmosphere at a pressure of 10 torr or less. vacuum evacuation means, ■ μc-Si on it
A thin film forming device comprising at least a means for inserting a substrate on which a Cxi film is formed into a thin film forming chamber, a means for holding the substrate, a means for heating the substrate, and a means for measuring formation conditions such as pressure and temperature. Using the device with a chamber, μ
Formation of c-3iCxfl film was carried out. A mixed gas of monovinylsilane, difluorosilane, and hydrogen was used as the raw material gas. For measuring the characteristics of μc-5iCx, quartz glass was used as the substrate. yi film formation temperature is 200
It was set to °C. While supplying monovinylsilane at a flow rate of 5 seconds, difluorosilane at a flow rate of 5 seconds, and hydrogen at a flow rate of 40 seconds to the thin film forming chamber, the pressure in the thin film forming chamber was maintained at 50 mtorr by a vacuum evacuation means. The frequency was 13.56 MHz and the discharge power was 60 W. The thin film was formed to a thickness of 1 μm. The optical band gap was 2.35 eV. In addition, peaks were observed at 740 cm-' and 520 cm-' in the Raman scattering spectrum. These peaks are Raman peaks specific to cubic SiC crystals and crystalline silicon, respectively, and it was confirmed that silicon carbide microcrystals (microcrystals 5iC) are mainly generated in the present invention. That is, this thin film is a thin film with high visible light transmittance that also contains silicon microcrystals.

[Example 2] In Example 1, the formation conditions were changed as follows, and p
A thin film with high transmittance and high conductivity was formed for the mold.

Discharge power is 20W, F! The film formation temperature was 250°C. While monovinylsilane was supplied at a flow rate of 5 seconds, difluorosilane was supplied at a flow rate of 5 seconds, and hydrogen was supplied at a flow rate of 101005e, the pressure of the thin film formation chamber was maintained at 70 mtorr by a vacuum evacuation means. Further, 1% by volume of diborane was added to the raw material gas to form p-type μc-3i.
An attempt was made to form a Cx thin film. The optical bandgap is approximately 2.
2 eV, and the conductivity was 25 S/cm. in this way,
A p-type thin film that is optically transparent and has low resistance was formed.

[Example 3] In Example 2, it was revealed that the p-type thin film had the ability to be used on the light incident side of a solar cell, so a pin-type amorphous solar cell was formed using the p-type thin film. I tried. After forming 150 P-type μc-SiCx films on a transparent electrode, an i-type amorphous film and an n-type μc-3i film were formed in order of 5000 films and 300 films by plasma decomposition (plasma CVD). Furthermore, a metal electrode was formed, AMI, and simulated sunlight of 100 mW/d was irradiated to measure the performance of the solar cell. The open-circuit voltage of the solar cell was 0.98V, which was found to be extremely high compared to O and aV levels that are common in the prior art. This is because the μc-3iCX formation conditions of the present invention are mild, so that the influence on the transparent electrode is reduced.

(Example 4) In Example 2, in order to investigate the influence of hydrogen dilution rate,
μc-5iCx by reducing the amount of hydrogen mixed by 2 times the volume.
was formed. The optical bandgap of the obtained thin film is about 2.35 eV, and the dark conductivity is 8 × 10-'S
/cra, which still showed good doping characteristics. In this way, being able to reduce the amount of hydrogen dilution without significantly reducing the conductivity of the thin film is extremely advantageous in reducing damage caused by discharges or reactions caused by hydrogen atoms during the fabrication of semiconductor devices. This is a great advantage of the present invention.

[Example 5] In Example 2, phosphine was used instead of diborane. The amount of phosphine supplied was 0.5% by volume relative to the silicon compound. The discharge power was 15W.

The optical band gap of the obtained thin film is approximately 2.35 eV
It was revealed that the dark conductivity was 7057 cm or more, and that it exhibited extremely excellent doping characteristics as an n-type semiconductor thin film.

[Example 6] In Example 2, in order to further investigate the influence of the mixing ratio of U silane and fluorinated silane, monovinyl silane was mixed at 0.
5 sccm, difluorosilane was changed to 10105e to form pc-3iCx. Although the optical bandgap of the obtained thin film was slightly narrowed to about 2.0 eV, the dark conductivity was 30 S/c+a, which still had good doping characteristics.

[Comparative Example 1] Methane was used instead of monovinylsilane in Example 1. The optical bandgap of the thin film is about 1.9 eV, and the Raman scattering spectrum shows 480 c
Only a peak characteristic of amorphous silicon was obtained at m-'. As described above, it has become clear that changing the raw material to one outside the range defined by the present invention makes it difficult to form μc-3iCx.

[Comparative Example 2] In Comparative Example 1, a thin film was formed by changing the discharge power to 200 W as performed in the prior art. By setting the discharge power to 200 W, not only was it not possible to form μc-3iCx, but the thin film peeled off from the substrate.

[Effects and industrial applicability of the invention] As is clear from the above examples, by using the raw material mixture of the present invention, it is possible to obtain excellent electrical properties with a wide optical bandgap and high conductivity. This method provides a silicon carbide microcrystalline thin film. Furthermore, it is possible to reduce the discharge power with respect to plasma damage during discharge decomposition. As described above, the method using the raw material mixture of the present invention can be applied to semiconductor devices using thin films, such as thin film solar cells, light emitting devices, light sensor thin film transistors, P
It is clear that this method is an excellent method for forming photoreceptors for PCs and the like.

Claims (3)

[Claims] (1) A method for forming a silicon carbide microcrystalline thin film, which comprises decomposing a mixed gas containing at least a silicon compound having a radically polymerizable unsaturated group, a fluorinated silane, and hydrogen by glow discharge. (2) A method for forming a silicon carbide microcrystalline p-type thin film according to claim 1, wherein an impurity imparting p-type conductivity is added to the mixed gas and decomposed. (3) A method for forming a silicon carbide microcrystalline n-type thin film according to claim 1, wherein an impurity imparting n-type conductivity is added to the mixed gas and decomposed.