JP2659400B2 - Formation method of carbon-containing silicon thin film - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for forming a carbon-containing silicon thin film, and more particularly to a method for forming a carbon-containing silicon thin film having a wide band gap.

(Background technology)

As is well known, amorphous silicon thin films have been put to practical use in advanced technologies such as solar cells, photoconductors and thin film transistors. Further, a carbon-containing silicon thin film (hereinafter abbreviated as SiCx) is being studied for changing and optimizing the optical characteristics of such an amorphous silicon thin film. Then
SiCx is usually formed by using silanes such as monosilane and disilane and hydrocarbons such as methane, ethane, propane, ethylene and acetylene as raw materials and decomposing them using plasma or light. Further, an alkylsilane having a saturated hydrocarbon group such as a methyl group or an ethyl group in a molecule has also been used as a raw material. However, in the former case, it is difficult to control the carbon content in SiCx due to the different decomposability of silane and hydrocarbon when these raw materials are decomposed.As a result, it is difficult to control the optical bandgap of SiCx. There was a limit. In the latter alkylsilane, the proportion of the alkyl group remaining as a pendant in SiCx was large, and the improvement of the photoelectric characteristics of SiCx was hindered.

The inventors of the present invention have conducted intensive studies in view of the above points, and have found that these problems can be solved by using an alkylsilane having an unsaturated hydrocarbon group in a molecule as a raw material for plasma decomposition. Thus, the present invention has been completed.

[Disclosure of the Invention]

That is, the present invention decomposes a silicon compound having a polymerizable unsaturated group in a molecule, that is, a silane having a radical polymerizable unsaturated hydrocarbon group in a molecule (hereinafter abbreviated as U-silane) by plasma. Then, a method of forming SiCx is summarized.

U- silanes used in the present invention have the general formula _{R n Si m H 2m + 2} -n ( wherein, R is an unsaturated hydrocarbon group having about 2 to 10 carbon atoms are possible radical polymerization, m, n
Is a natural number. m is 1 or 2, when m = 1, n = 1, 2, 3, or 4, and when m = 2, n = 1, 2,
3, 4, 5, or 6).

Specific examples of the U-silane include vinyl silane, divinyl silane, trivinyl silane, vinyl disilane, divinyl disilane, trivinyl disilane, 1-propenyl silane, 2-propenyl silane, isopropenyl silane, 1-butenyl silane, Butenyl silane, 2
-Silicon compounds such as pentenyl silane and ethenyl silane are preferred.

As the method of plasma decomposition used in the present invention, any method can be effectively used as long as it can generate various radical species having silicon during the decomposition reaction. Specifically, plasma CVD (plasma chemical deposition) using glow discharge decomposition, microwave CVD using microwaves for decomposition, ECRCVD (electron cyclotron resonance chemical deposition), or the like can be used.

In the present invention, the conditions for forming a thin film are not particularly limited, and can be determined as appropriate. In this case, the following points can be effective guidelines. First, the present inventors have found that, when U-silane is decomposed alone, SiCx having a large carbon content and a wide optical band gap can be obtained. Further, by using a silicon hydride such as monosilane or disilane mixed with U-silane, the carbon content in SiCx can be changed, and the optical band gap can be arbitrarily changed. Furthermore, using U.sub.-silane and a mixed gas of U-silane and silicon hydride in a diluted state using a diluent gas such as hydrogen, helium, argon, or the like, film formation conditions that can be effectively used in the present invention are also applicable. It is.
By using a diluent gas, the stability of decomposition conditions is improved, and the reproducibility of the film characteristics of the obtained SiCx is improved, which is practically preferable. In addition, p-type and n-type
The semiconductor properties of the mold can also be imparted. In order to impart p-type and n-type properties, a compound containing boron or phosphorus such as diborane or phosphine may be plasma-decomposed together with U-silane.

The substrate temperature at the time of producing a thin film employed in the present invention,
Operational factors such as reaction pressure, source gas flow rate, and discharge power are generally as follows, but more specific film forming conditions are slightly different in optimum conditions depending on the plasma decomposition method used. Yes, it may be appropriately determined according to the decomposition method.

The substrate temperature is not lower than room temperature and not higher than about 500 ° C., preferably not lower than 100 ° C. and not higher than about 400 ° C. As the substrate temperature increases, the optical band gap decreases. In plasma CVD (plasma chemical deposition) using glow discharge decomposition or microwave CVD using microwaves for the decomposition, the reaction pressure is in the range of about 0.01 to 10 torr, preferably about 0.02 to 2.0 torr. Range. In the ECRCVD method using electron cyclotron resonance for excitation even in the method using microwaves for decomposition, lower pressure conditions are sufficient, and about 0.01 mtorr to 50 mtorr is sufficient. The flow rate of the source 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 substrate, the target carbon content, the film forming speed, and the like. The raw material gas flow rate is U
For silane, about 1 to 1000 sccm is sufficient, and when silicon hydride or a diluting gas is used, it is used in a range of about 0.1 to 100 times that of U-silane.

[Action]

It is generally recognized that various plasma-related radical species are formed during the plasma decomposition of silicon compounds. The U-silane of the present invention works particularly effectively because these silicon-related radicals and the unsaturated hydrocarbon group in the U-silane of the present invention can cause a radical reaction. It is believed that the groups are reduced and carbon is effectively incorporated into the silicon network.

Therefore, in the case of the decomposition method using U-silane of the present invention, the discharge power may vary depending on the equipment used, but is usually in the range of about 1 W to 50 W, and carbonization of methane, ethane, propane, etc. under the same conditions. It can be much lower than the decomposition of hydrogen. In most cases, semiconductor devices are formed of electrodes and several types of thin film semiconductors.
It is a well-known fact to those skilled in the art that plasma damage is caused to already formed electrodes and semiconductor thin films. For this reason, various attempts to reduce the power required for decomposition are being studied.
For example, it is possible to provide extremely favorable conditions when forming a thin-film solar cell, a light-emitting element, an optical sensor, and the like.

〔Example〕

Example 1 As a thin film forming apparatus, a raw material supply means having a raw material flow rate control means, a means for generating a discharge for decomposing a raw material, a vacuum exhaust means for maintaining a thin film forming atmosphere at a pressure of 10 torr or less, At least means for inserting a substrate on which a SiCx thin film is formed into a thin film forming chamber, means for holding the substrate, means for heating the substrate, and means for measuring formation conditions such as pressure and temperature The SiCx thin film was formed using the apparatus having the thin film forming chamber. Monovinylsilane alone was used as a source gas. For optical band gap measurement, a quartz glass substrate was used. A silicon single crystal substrate was used for measuring an infrared absorption spectrum. The thin film formation temperature was 250 ° C. While supplying monovinylsilane to the thin film formation chamber at a flow rate of 10 sccm, the pressure in the thin film formation chamber was maintained at 0.1 torr by vacuum evacuation means. Discharge power is 1
Performed at 0W. The thin film was formed to a thickness of 1 μm. The optical band gap was more than 2.7 eV. Further, it was confirmed from the infrared absorption spectrum that the number of pendant alkyl groups was small.

Example 2 The same experiment as in Example 1 was performed, except that monovinylsilane and disilane were used as source gases and hydrogen was used as a diluent gas. Here, monovinylsilane was supplied at a ratio of 1 sccm, disilane at 9 sccm, and hydrogen at 50 sccm. The pressure in the thin film formation chamber is reduced by 0.5 to
orr. It was found that the film formation rate was 10 ° / sec or more, which was higher than the film formation rate of disilane alone of 9 ° / sec. The optical band gap of the thin film is about 2.1e
V, the photoconductivity was 1 × 10 ⁻⁵ S / cm or more, and it was revealed that the composition exhibited extremely excellent photoelectric characteristics. This is because the photoactive layer of a photoreceptor or a solar cell using an amorphous thin film,
It has the performance that it can be used on the light incident side of the solar cell by doping with the type impurities.

Example 3 In Example 2, it was clarified that the solar cell had a performance that could be used on the light incident side of the solar cell, so an attempt was made to form a p-type thin film. In Example 2, diborane was further added to the source gas, and a thin film was formed in the same manner.
The supply amount of diborane was 1% by volume of the supply amount of monovinylsilane and disilane. With the introduction of diborane,
The optical bandgap dropped slightly to about 2.0 eV. The dark conductivity was 1 × 10 ⁻⁵ S / cm or more, which revealed that the p-type semiconductor thin film exhibited extremely excellent doping characteristics.

Example 4 In Example 3, in order to investigate the influence of discharge power,
The same experiment was conducted by reducing the power to 2 W, at which stable glow discharge could be maintained with this device. The optical band gap of the obtained thin film is about 2.15 eV, and the photoconductivity is
8x10 ^-6 S / cm still showed good photoelectric characteristics. As described above, the ability to lower the discharge power without significantly lowering the photoelectric characteristics of the thin film is extremely advantageous in reducing discharge damage when a semiconductor device is manufactured.
This is a great advantage of using silane as a raw material.

Example 5 In Example 3, the same experiment was performed using monosilane as silicon hydride. The supply amount of diborane is 2% by volume of the sum of the supply amounts of monovinylsilane and monosilane.
It was carried out in. The discharge power was 5W. The optical band gap of the obtained thin film was about 2.2 eV and the dark conductivity was 7 × 10 ⁻⁶ S / cm or more, and it was revealed that the p-type semiconductor thin film exhibited extremely excellent doping characteristics.

Comparative Example 1 Instead of the monovinylsilane of Example 5, methane was used. Although the photoconductivity of the thin film was 2x10 ^-5 S / cm or more, the optical band gap was about 1.6 eV, which was about the same as that of amorphous silicon containing no carbon. . This is probably due to the fact that the discharge power was as low as 5 W, and monosilane was mainly decomposed, and the proportion of methane incorporated in the thin film was reduced.

Comparative Example 2 In Comparative Example 1, since the optical band gap was small, only the discharge power was increased to 40 W to form a thin film. By setting the discharge power to 40 W, a thin film having an optical band gap of about 2.1 eV was obtained. As described above, when a carbon-containing silicon thin film is formed using the raw materials used in the related art, it is necessary to increase the power during decomposition. However, the photoconductivity of the thin film is
As low as 2 × 10 ⁻⁹ S / cm, high photoconductivity was not obtained as in the present invention.

〔The invention's effect〕

As is clear from the above embodiments, according to the present invention,
By using U-silane as a raw material and performing plasma decomposition, it is possible to obtain a thin film having a wide optical band gap and excellent in photoelectric characteristics showing high photoconductivity at a high speed. Further, the discharge power can be reduced as compared with the case where other carbon sources such as methane, ethane, acetylene, etc. are used, so that plasma damage at the time of discharge decomposition can be reduced.

As described above, the method of the present invention for decomposing U-silane to form a thin film is used for forming a semiconductor device using the thin film, for example, a thin-film solar cell, a light-emitting element, an optical sensor, a thin-film transistor, a photoconductor for PPC, and the like. As a method of
It is very good and its industrial applicability has to be extremely large.

Claims (1)

(57) [Claims] 1. A method for forming a carbon-containing silicon thin film, comprising decomposing a silicon compound having an unsaturated group capable of radical polymerization using plasma.