JPH0654762B2 - Method for producing P-type carbon-doped amorphous silicon film - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a p-type film of a carbon-doped amorphous silicon film used because it has a wider bandgap than amorphous silicon.

[Conventional technology]

P-type carbon-doped hydrogenated amorphous silicon (hereinafter a-SiC:
The thin film (referred to as H) is used as a material for the window layer (layer on the light incident side) of the p-i-n type amorphous silicon solar cell, and contributes greatly to improving the photoelectric conversion efficiency of the amorphous silicon solar cell. I've been Figure 2 shows Tawada et al.
Of Applied Physics (Journal of Applie
The characteristics of an amorphous silicon solar cell using a-SiC: H for a p-layer, which was published in d Physics) 53, 5273-5281 (1982), are shown. As shown in FIG. 3, the structure of this solar cell is such that a tin oxide (SnO ₂ ) thin film 32,
A p-type a-SiC: H layer 33, an i-type hydrogenated amorphous silicon (hereinafter referred to as a-Si: H) layer 34, and an n-type a-Si: H layer 35.
And an aluminum electrode 36 are laminated. The tin oxide thin film 32 is formed by a vacuum evaporation method or a thermal CVD method, and the aluminum electrode 36 is formed by a vacuum evaporation method or a sputtering method. The a-Si: H layer is a so-called plasma C that decomposes monosilane gas by glow discharge.
It is formed by the VD method. The substrate temperature at this time is 250 ° C.
It is a degree. When forming the n-type a-Si: H layer, phosphine gas (PH ₃ ) is mixed at a flow rate ratio of about 1% with respect to the monosilane gas. Further, at the time of forming the p-type a-SiC: H layer, methane gas (CH ₄ ) and diborane gas (B ₂ H ₆ ) are mixed.

When light 37 enters such a solar cell from the glass substrate side, a positive potential is generated on the p-side tin oxide electrode 32 positive and on the n-side aluminum electrode 36 due to the photovoltaic effect of the pin junction. .
Fig. 2 shows the power generation characteristics under sunlight irradiation with 100 mW / cm ² intensity. The black circles indicate the product of output current density and output voltage, that is, the point where the output power per 1 cm ^{2 of} solar cell is the maximum, and the output power at this point is generally called the output of the solar cell. , The ratio to the amount of incident light is called conversion efficiency.

In such an amorphous solar cell, since the photoelectric conversion action is limited to the i layer, it is necessary to increase the light transmittance of the p layer so that as much light as possible reaches the i layer in order to increase the output. is important. For that purpose, the optical absorption coefficient of the p layer must be lowered. Optical absorption coefficient of p layer α
Is the band gap E expressed in electron volts.
It is calculated from the following equation using g (provided that E <Eg).

α = B ² (E−Eg) ² / E Here, B ² is a constant, and E is light energy in electron volts. From the above equation, it is understood that Eg should be increased in order to lower the absorption coefficient. In the above-mentioned magazine, Tawada et al.
It has been reported that by using a-SiC: H larger than Si: H, the conversion efficiency of an amorphous solar cell is improved from 5 to 6% to about 8%.

[Problems to be solved by the invention]

Figure 4 shows the p-type a-Si reported in the aforementioned magazine.
The band gap dependence of the photoconductivity of the C: H film is plotted again. As described above, the higher the band gap, the higher the light transmittance of the p-layer, but at the same time, the electrical conductivity under light irradiation (photoconductivity) becomes lower as shown in the figure, and the series resistance component of the solar cell is reduced. To increase. For example, the p-layer thickness is 10
^{At -6} cm (10 nm), the photoconductivity of 10 ^-6 S / cm is 1 area.
This gives a 1 Ω series resistance increase for a cm ² solar cell. As described above, the conventional p-type a-SiC: H film formation method has a drawback that the band gap cannot be increased to 2.0 eV or more when the photoconductivity of 10 ^-6 S / cm or more is required. was there.

An object of the present invention is to solve the above problems and provide a method for producing a p-type a-SiCH film having a band gap of 2.1 eV or more and a photoconductivity of 10 ^-3 / cm or more.

[Means for solving problems]

In order to achieve the above object, the method of the present invention is a silane-based gas, a hydrocarbon gas and a raw material gas containing diborane gas are diluted with hydrogen 100 times or more of the silane-based gas flow rate, the diluted raw material gas Plasma CVD using
Therefore, the film is formed on the substrate held at a temperature of 150 ° C. or less.

[Action]

When the hydrogen dilution ratio of the silane-based gas in the source gas is 100 times or more when the p-type a-SiC: H film is formed, the photoconductivity of the a-SiC: H film is increased by one digit or more. Board temperature 15
By keeping the temperature below 0 ° C, the bandgap becomes wider.

〔Example〕

Next, examples of the present invention will be described. As the source gas,
Monosilane, methane, diborane and hydrogen were used. The flow rate of monosilane is 1.0 cc / min, the flow rate of methane is 2.0 cc / min.
Min, diborane flow rate is 0.03cc / min, hydrogen flow rate is 200cc
/ Min. In other words, the hydrogen dilution ratio of monosilane gas is 200 times, and the doping amount of diborane with respect to monosilane is 3%. This raw material gas was introduced into a tank 1 having a volume of 10 as shown in FIG. The vacuum exhaust amount is adjusted by the conductance valve 4 provided between the vacuum chamber 1 and the vacuum pump 3,
The pressure of the raw material gas was adjusted to 100 Pa. Inside the vacuum chamber, the heater block 5 for heating the substrate and the area 15
A pair of circular electrodes 61 and 62 for 0 cm ² glow discharge are arranged opposite to each other. Next, switch on the heater block 5,
The temperature of the glass substrate 7 on the electrode 61 was set to 100 ° C. Then, the circular electrode 62 is fed with a high frequency power source to generate a frequency 1
Applying high frequency power of 3.56MHz and power of 2W, circular electrode 6
Glow discharge was generated between 1 and 62 to decompose the raw material gas and form a p-type a-SiC: H film on the glass substrate 7. The production rate was 0.05Å / sec.

The p-type a-SiC: H film thus produced has a bandgap of 2.1 eV and a photoconductivity of 10 ^-5 S / cm, which is higher than that of the conventional device shown in FIG. It can be seen that the photoconductivity was increased by two digits. Fig. 1
(a) shows the substrate temperature dependence of the band gap of the p-type a-SiC: H film thus produced. The photoconductivity was around 10 ⁻⁵ S / cm, which was almost independent of the substrate temperature. From FIG. 1 (a), it is understood that the substrate temperature of 150 ° C. or lower is important for increasing the band gap of the p-type a-SiC: H film.

Fig. 1 (b) is a p-type a-Si prepared by keeping the temperature of the glass substrate constant at 100 ° C and changing the hydrogen dilution ratio of monosilane gas.
The photoconductivity of the C: H film is shown. At this time, the flow rates of monosilane, methane, and diborane are 1.0 cc / min and 2.0 cc / min, respectively.
Min, 0.03cc / min, and the flow rate of hydrogen is 25cc / min-30
The hydrogen dilution ratio was changed by changing in the range of 0 cc / min. As can be seen from the figure, the photoconductivity increases as the hydrogen dilution rate increases up to 100 times the hydrogen dilution rate. Hydrogen dilution rate is 10
Above 0 times, the photoconductivity hardly depends on the hydrogen dilution rate. The band gap was 2.1 eV, which hardly depended on the hydrogen dilution ratio. From these results, in order to increase the photoconductivity of the p-type a-SiC: H film to 10 ⁻⁵ S / cm or more, in addition to the substrate temperature of 150 ° C. or less, a silane-based gas such as monosilane gas is used. Hydrogen dilution rate of 10
It turns out that it is necessary to be 0 times or more.

〔The invention's effect〕

According to the present invention, when the p-type a-SiC: H film is formed, the hydrogen dilution ratio of the silane-based gas is increased 100 times or more, and the substrate temperature is set to 150 ° C. or less, so that the band gap is 2.1 eV. Above, p-type a- with a photoconductivity of 10 ^-5 S / cm or more
A SiC: H film can be produced. Therefore, the series resistance component of the solar cell using this film is reduced and the output is improved. Further, according to the present invention, since the substrate temperature at the time of formation may be low, the p-type a-SiC of tin or oxygen, which is a constituent element thereof, when forming on a transparent electrode such as tin oxide: There is also an advantage that contamination of the H film is significantly reduced.

[Brief description of drawings]

FIG. 1 shows the effect of the present invention. FIG. 1 (a) is a relationship diagram between band gap and substrate temperature, and FIG. 1 (b) is a relationship diagram between photoconductivity and hydrogen dilution rate of silane gas. FIG. 2 is a characteristic diagram of a conventional solar cell using a-SiC: H for the p layer, FIG.
The figure is a cross-sectional view of a solar cell showing the characteristics of FIG. 2, and FIG.
FIG. 5 is a cross-sectional structural diagram of the plasma CVD apparatus used in the examples of the present invention, wherein the relationship between the photoconductivity and band gap of the a-SiC: H film is shown. 1: vacuum tank, 2: gas introduction pipe, 3: vacuum pump, 61, 6
2: electrode, 7: substrate, 8: high frequency power supply.

Claims (1)

[Claims] 1. A raw material gas containing a silane-based gas, a hydrocarbon gas and diborane as a component is diluted with hydrogen at a flow rate of 100 times or more that of the silane-based gas, and the diluted raw material gas is subjected to plasma CVD at a temperature of 150 ° C. or lower. A method for producing a p-type carbon-doped amorphous silicon film, which comprises forming the film on a substrate kept at a temperature.