JP2723548B2 - Formation method of carbon-containing silicon microcrystalline thin film - Google Patents

Description

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

(Background technology)

It is known that a silicon microcrystalline thin film can impart properties of a p-type semiconductor and an n-type semiconductor, and application to a thin-film semiconductor device such as a silicon solar cell is being studied.

Thus, a carbon-containing silicon microcrystalline thin film (hereinafter abbreviated as μc-SiCx) has been studied for changing and optimizing the optical characteristics of the silicon microcrystalline thin film. Conventionally, μc
It is reported in International PVSEC-3 (Tokyo, 1987) that -SiCx is formed by RF glow discharge decomposition of monosilane and methane or electron cyclotron resonance microwave discharge decomposition. Since these formation conditions are all severe conditions requiring high hydrogen dilution and high discharge power, other parts of the semiconductor device using the thin film are subjected to a chemical reaction due to hydrogen atoms or high energy. It has been considered that adverse effects such as damage due to collisions of electrons and ions are considered. As a result, the performance improvement of the thin film alone does not greatly contribute to the improvement of the characteristics of the semiconductor device.
There are still issues left.

[Disclosure of the Invention]

The present inventors have conducted intensive studies in view of such a viewpoint,
To solve this problem by finding that μc-SiCx can be formed without using such harsh conditions by using a mixed gas of an alkylsilane having an unsaturated hydrocarbon group in a molecule and a fluorinated silane as a raw material. Was completed.

The present invention uses a plasma to decompose a mixed gas containing a silicon compound having a radically polymerizable unsaturated group, a fluorinated silane, and at least 2 times and 50 times or less hydrogen of the silicon compound. A method for forming a carbon-containing silicon microcrystalline thin film (μc-SiCx), characterized in that:

 Hereinafter, the present invention will be described in detail.

The silicon compound having a radically polymerizable unsaturated group used in the present invention, that is, a silane having a radically polymerizable unsaturated hydrocarbon group in its molecule (hereinafter referred to as U)
- abbreviated as silane) of 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, and when m = 1, n = 1, 2, 3, or 4 and 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.

Fluorinated silanes useful in the present invention include Si _a H
_{2a + 2-b} F _b (a is 1 or 2, b is 1, when a = 1
2, 3, or 4, and when a = 2, 1, 2, 3, 4, 5, or 6
It is. ) Is a silicon compound represented by the general formula: Specific examples of the fluorinated silane include monofluorosilane, difluorosilane, trifluorosilane, tetrafluorosilane, monofluorodisilane, difluorodisilane, trifluorodisilane, tetrafluorodisilane, pentafluorodisilane, and hexafluorodisilane. Can be used effectively.

Although the mixing ratio of U-silane and fluorinated silane is not particularly limited, it is preferable to use fluorinated silane at about 0.1 times to 2 times the volume of U-silane.

In the present invention, hydrogen is used as a raw material gas together with U-silane and fluorinated silane,
Hydrogen is preferably, for the compound of silicon described above,
It is used at 2 times or more and 50 times or less. More preferably, it is 4 times or more and 10 times or less. Even under conditions where the amount of hydrogen is greater than 50 times the volume, μc-SiCx is formed in the same manner as in the prior art. This is not a preferable condition because the characteristics are deteriorated. On the other hand, under the condition that the amount of hydrogen is smaller than twice the capacity, the characteristics of μc-SiCx are remarkably deteriorated. In particular, the effect of impurities introduced for imparting p-type or n-type conductivity is reduced.

In the present invention, such a mixed gas is subjected to plasma decomposition, and any method of plasma decomposition may be used as long as it can generate various radical species having silicon during the decomposition reaction. It can be used effectively. 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 the thin film are not particularly limited, other than the mixing ratio between the silicon compound as the raw material and hydrogen. In the present invention, compared with the prior art, μc-Si at much lower discharge power and lower substrate temperature is used.
You can get Cx. This is another feature of the present invention. Furthermore, according to the findings of the present inventors, it is possible to change the carbon content of μc-SiCx by using silicon hydride such as monosilane, disilane, and the like mixed with U-silane, so The band gap can be changed. Further, by doping impurities, p
The properties of a type and n-type semiconductor can also be imparted. p
In order to impart the type and n-type properties, compounds containing boron and phosphorus, such as diborane and phosphine, may be plasma-decomposed together with the raw material gas.

In the present invention, specific film forming conditions such as a substrate temperature, a reaction pressure, a source gas flow rate, and a discharge power during the production of μc-SiCx are different depending on the plasma decomposition method used, and the optimum conditions are different. Although it is determined appropriately according to the situation, points that can be effective guidelines in that case are described below.

The substrate temperature changes the optical or electrical properties of μc-SiCx. The substrate temperature is usually 100 ° C or higher and 350 ° C or lower, preferably 150 ° C or higher and 300 ° C or lower,
Preferably, it is 175 ° C or higher and 250 ° C or lower. When imparting the properties of p-type and n-type semiconductors, a decrease in temperature increases the light transmittance, but the electrical resistance becomes too large, which is not preferable for use in a semiconductor device. In addition, the optical band gap decreases with an increase in temperature, and further, μc-SiCx tends to peel off from the substrate, which is not preferable.

On the other hand, the reaction pressure is plasma using glow discharge decomposition
In the case of CVD (plasma chemical deposition or microwave CVD using microwaves for decomposition, the pressure is in the range of about 0.01 torr to 10 torr, preferably in the range of 0.02 torr to 2.0 torr. ECRCVD using electron cyclotron resonance for excitation
In the method, even lower pressure conditions may be used, from 0.01 mtorr to 5
About 0mtorr is enough.

In addition, the flow rate of the source gas and the discharge power are appropriately selected depending on the size of the film forming chamber and the discharge electrode 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 1 to 1000 sc for silicon compounds.
cm is sufficient, and hydrogen is twice as large as described above.
Used in a range of 50 times. The discharge power can be reduced as compared with the decomposition of methane and monosilane in the prior art, and 5 W to 150 W is sufficient, preferably 10 W to 100 W, more preferably about 10 W to 60 W.

[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 μc-SiCx is formed without the harsh conditions as in the prior art due to the reduced number of groups and the effective incorporation of carbon into the silicon network.

In particular, the fact that the discharge power required for film formation can be significantly reduced as compared with the related art as described above provides a vitally favorable condition when forming a semiconductor device, for example, a thin film solar cell, a light emitting element, an optical sensor, and the like. You can do it. However, most semiconductor devices are formed of electrodes and several types of thin film semiconductors.However, as in the prior art, when forming μc-SiCx, it is necessary to adopt a forming condition with a high discharge power. It is well known to those skilled in the art that, in some situations, the electrodes and semiconductor thin films that have already been formed must be severely damaged by plasma, and therefore, various methods of reducing the power required for decomposition are required. Despite the desperate search for an attempt, the technological breakthrough that has not been achieved has been achieved.

〔Example〕

Hereinafter, embodiments of the present invention will be described more specifically with reference to examples.

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, and a vacuum exhaust for maintaining a thin film forming atmosphere at a pressure of 10 torr or less. Means, means for inserting a substrate on which a μc-SiCx thin film is to be formed into a thin film forming chamber, means for holding the substrate, means for heating the substrate, means for measuring the formation conditions such as pressure and temperature. Using the apparatus having at least a thin film forming chamber,
A SiCx thin film was formed. As a raw material gas, monovinylsilane was used as U silane; a mixed gas of difluorosilane and hydrogen was used as fluorinated silane. μc−
For specific measurement of SiCx, quartz glass was used as the substrate.
The thin film forming temperature was 200 ° C. Monovinylsilane 5sccm,
While supplying difluorosilane at 5 sccm and hydrogen at 40 sccm (flow rate of hydrogen mixed with the silicon compound by 4 times the volume of the silicon compound), the pressure in the thin film formation chamber was maintained at 50 mtorr by vacuum evacuation means. Use frequency 13.56MHz
And the discharge power was 20 W.

The obtained thin film was formed to a thickness of 1 μm. The optical band gap was 2.3 eV. Further, the Raman scattering spectra, it was possible to observe a peak in the 740 cm ^-1 and 520 cm ^-1. Each of these peaks is cubic Si
It is a Raman peak peculiar to C crystal and crystalline silicon, and μc-SiCx of the present invention is microcrystalline silicon carbide (microcrystalline silicon).
It was confirmed to be a carbon-containing silicon microcrystalline thin film containing SiC) and microcrystalline silicon.

Example 2 In Example 1, 1% by volume of diborane was further added to the raw material gas to try to form a p-type μc-SiCx thin film.
Other conditions were the same. Optical band gap is 2.2e
V, conductivity was 25 S / cm. Thus, a light-transmissive and low-resistance p-type thin film could be formed.

Example 3 In Example 2, it was clarified that the solar cell had performance that could be used on the light incident side of the solar cell. Therefore, a pin-type amorphous solar cell was formed using the n-type thin film. Tried. After forming p-type μc-SiCx on the transparent electrode at 150 °, the thickness of each of the i-type amorphous film and the n-type μc-Si is set to 5000 °, 30 μm by plasma decomposition (plasma CVD).
0 ° formed. Furthermore, a metal electrode is formed and AM1,100mW
The performance of the solar cell was measured by irradiating artificial sunlight at / cm ² . The open-circuit voltage of the solar cell was 0.98 V, which was significantly higher than that of the conventional technology of 0.8 V. This indicates a result that the influence on the transparent electrode was reduced because the conditions for forming μc-SiCx of the present invention were mild.

[Example 4] In Example 2, in order to investigate the influence of the hydrogen dilution rate, the hydrogen mixing amount was reduced to twice the volume, and μc-SiCx
Was formed. The optical band gap of the obtained thin film is about
2.35 eV, and the dark conductivity was still 8 × 10 ⁻¹ S / cm, showing good photoelectric characteristics. As described above, the ability to reduce the dilution amount of hydrogen without significantly lowering the conductivity of the thin film is extremely advantageous in reducing damage due to discharge or reaction due to hydrogen atoms when manufacturing a semiconductor device. Yes, a great advantage of the present invention.

[Example 5] In Example 2, phosphine was used instead of diborane. The supply of phosphine was carried out at 0.5% by volume with respect to the silicon compound. The discharge power was 15 W. The obtained thin film had an optical band gap of about 2.35 eV and a dark conductivity of 70 S / cm or more, and it was revealed that the n-type semiconductor thin film exhibited extremely excellent doping characteristics.

[Comparative Example 1] Instead of monovinylsilane as U silane in Example 1, methane was used. The optical band gap of the thin film was about 1.9 eV, and only a peak specific to amorphous silicon was obtained at 480 cm ⁻¹ in the Raman scattering spectrum. Thus, it became clear that the formation of μc-SiCx becomes difficult by changing the raw material.

[Comparative Example 2] In Comparative Example 1, as performed in the related art,
The discharge power was changed to 200 W to form a thin film. Discharge power
By setting the power to 200 W, not only was it impossible to form μc-SiCx, but also the thin film was peeled off from the substrate.

[Effect of the invention and industrial applicability]

As is clear from the above examples, by using the raw material mixture of the present invention, with a wide optical band gap,
Further, the present invention provides a carbon-containing silicon microcrystalline thin film having high electrical conductivity and excellent electrical characteristics. Also, regarding the plasma damage at the time of discharge decomposition, it is possible to remarkably reduce the discharge power. Thus, the method of using the raw material mixture of the present invention is a semiconductor device utilizing a thin film, for example, a thin-film solar cell, a light-emitting element, an optical sensor,
It is clear that this method is excellent as a method for forming a thin film transistor, a photoconductor for PPC, and the like.

Claims (3)

(57) [Claims] 1. A method comprising the steps of: decomposing a mixed gas containing a radically polymerizable silicon compound having an unsaturated group, a fluorinated silane, and hydrogen at least 2 times and up to 50 times the volume of the silicon compound using plasma. Forming a carbon-containing silicon microcrystalline thin film. 2. The method of forming a thin film according to claim 1, wherein said mixed gas is decomposed by adding an impurity imparting p-type conductivity to said mixed gas. 3. The method of forming a thin film according to claim 1, wherein said mixed gas is decomposed by adding an impurity imparting n-type conductivity to said mixed gas.