JPH0669029B2 - Method for forming p-type semiconductor thin film - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for forming a p-type semiconductor thin film made of amorphous silicon, and particularly to the formation of the thin film used for an amorphous silicon solar cell having a wide optical band gap. Regarding the method.

[Background technology]

Amorphous silicon (hereinafter referred to as aS _i : H) thin films have been well studied recently, and their applications are open to solar cells, photosensitive drums, scanning circuits of image reading devices, drive circuits of image display devices, and the like.

Therefore, when the aS _i : H thin film is used as a window material of a solar cell, that is, a thin film with low incident light loss, it is desirable that the optical forbidden band width (hereinafter referred to as Eopt) of the silicon thin film is as large as possible.

This is because light having an energy higher than Eopt is cut by the aS _i : H layer before reaching the photoactive layer and is not effectively used.

In the conventional technology, it was difficult to form such a window material using only a silicon raw material. Therefore, a raw material in which monosilane (S _i H ₄ ) is mixed with hydrocarbon, ammonia, etc. Amorphous silicon carbide (hereinafter referred to as aS _i C _x ) and amorphous silicon nitride (hereinafter referred to as aS _i N _y ) in which carbon or nitrogen is introduced into a matrix have been studied.

At present, a part of p-type aS _i C _{x obtained} by doping aS _i C _x with boron is practically used as a window material.

However, (i) aS _i C _x and aS _i N _y originally have a property of being difficult to conduct electricity, and when these are used as a window material, there is a problem due to low electric conductivity. In aS _i C _x and aS _i N _y , if the amount of C or N is large, Eopt becomes large, but the electrical conductivity is remarkably reduced, so the value of x or y is increased unnecessarily to increase Eopt. I couldn't help. If the values of x and y are too large, the window material acts as a resistance component in a photoelectric conversion element such as a solar cell, which is disadvantageous in improving efficiency.
(Ii) Furthermore, the hydrocarbon used to form aS _i C _x causes carbon deposition in the glow discharge chamber.
There is a new problem that this deposited carbon must be removed. (Iii) Since the conductivity is greatly influenced by the value of x of aS _i C _x , the composition of the silicon raw material and the carbon raw material must be strictly controlled in addition to the prevention of the deposition carbon. The process becomes complicated. (Iv) In addition to this, the presence of elements such as carbon, nitrogen and oxygen in the amorphous silicon promotes the photo-induced effect (deterioration effect). Introduction is not preferable.

In addition, a method for producing a wide-gap a-Si: H film from disilane is described in "Supplement to the Extended abstracts of
the 15th Conf.on Solid State Devices and Material
s ", (1983-8-30) pp.42-43 [C-3-8LN], it decomposes disilane with extremely low RF power at 220 ° K (about -53 ° C). However, even if a wide-gap a-Si: H film is formed under a film forming condition such that the forming temperature is less than 100 ° C., the p value useful in an amorphous silicon solar cell is increased. The performance as a type semiconductor thin film was insufficient.

The present invention has been made in view of the above points, and an object thereof is to provide a method for forming a semiconductor thin film suitable as a window material from only a silicon raw material.

As a result of intensive studies from the above viewpoint, the present inventors have found that the silicon thin film formed by glow discharge decomposition using disilane gas has an increased Eopt (Japanese Patent Application No.
-134709), and as a result of further study of discharge conditions, surprisingly, a p-type aS _i : H film having an Eopt of more than 1.9 eV was obtained without introducing elements such as carbon, nitrogen and oxygen. The present invention has been completed by finding a possible forming method.

[Disclosure of Invention]

The p-type aS _i : H film of the present invention is a low resistance thin film having dark conductivity and photoconductivity of 10 ⁻⁶ and 10 ⁻⁵ S · cm ⁻¹ or more, respectively, and has suitable properties as a window material. It is a thing.

That is, the present invention relates to a method of forming a p-type semiconductor thin film on a substrate by decomposing it by glow discharge using disilane and a p-type doping gas as raw materials.
A method for forming a p-type semiconductor thin film, characterized in that the thin film is formed by applying glow discharge energy of 40 KJ or less, wherein diborane (B ₂ H ₆ ) diluted with hydrogen or helium is used as a doping gas. Is the way.

The constituent features of the present invention will be described in detail below.

In the present invention, disilane is a polysilane compound represented by the general formula S _in H _{2n + 2} , where n = 2.
Of course, high purity is preferable, but it is possible to include high-order substances such as trisilane and tetrasilane (n = 3, 4 in the general formula) that behaves similarly to disilane in glow discharge. Absent. The higher the content of the higher order, the smaller the glow discharge energy, but if the content of the higher order is 10 vol% or less relative to disilane, no special consideration is required as a raw material gas. Can be handled as the disilane of.

Disilane may be one obtained by a physical synthesis method in which monosilane is silently discharged and partially converted to disilane.
From the viewpoint of industrial production of solar cells, it is more preferable to use a chemical synthesis method (for example, a method of reducing hexachlorodisilane) in which a large quantity of stable quality is obtained.

The use ratio of the doping gas to disilane is 10 times or less, preferably 3 times or less. If the doping gas amount exceeds 10 times and increases, Eopt falls below 1.9 eV. The required amount of B ₂ H ₆ contained in the doping gas is
S as a percentage of _i2 H ₆ is _{B 2 H 6 / S i2 H} 6 = 1 / 1000~1 / 30. If this value is less than 1/1000, the dark conductivity does not exceed 10 ^-7 S · cm ^-1 , the activation energy is also larger than 0.6 eV, and the p-type property appears weak. When it is used in excess of 1/30, the adverse effect due to the decrease in Eopt is larger than the improvement in conductivity, which is not preferable. The doping gas is B ₂ H ₆ and hydrogen or
Since it is composed of He, the remaining amount is the required amount of hydrogen or He.

The glow discharge energy is 40 KJ / gS _i2 H ₆ or less, preferably 20 KJ / gS _i2 H ₆ or less, and particularly preferably 10 KJ / gS _i2 H ₆ or less. When glow discharge energy exceeding 20 KJ / gS _i2 H ₆ is applied, the conductivity is improved, but Eopt is lower than 1.9 eV, which is not preferable as a window material. 40KJ
By applying energy exceeding / gS _i2 H ₆ , Eopt became 1.8 eV or less as shown in the comparative example described later.

This glow discharge energy is calculated by the following formula.

The unit of the film forming conditions used in the above formula is RF power (W) and raw material gas flow rate (standard state flow rate per minute = SCCM).
= 60 (min) x 22.4 (l / mol). For example, when the disilane is 10 SCCM and the doping gas (500 ppm hydrogen containing diphorane) is 20 SCCM, the average molecular weight is (10 × 62.2 +
20 × 2) /30≈22.0, disilane weight fraction = 10 × 62.2 / 22.0 =
0.94, and the glow discharge power is 5 W, the above (I)
Substituting into the expression Becomes

The lower limit of the glow discharge energy is 8 kJ
Is / _g-Si 2 _{H 6} or more. This lower limit value is adopted in order to form a stable glow discharge state and to sufficiently exert the performance as a p-type semiconductor thin film useful in an amorphous silicon solar cell. A preferred specific example is 8 kJ / g-Si ₂ in any case where helium and hydrogen are used as diluent components of the doping gas.
It is H ₆ or more.

In the present invention, when forming a-Si: H, it is preferable to increase the raw material gas flow rate, especially disilane. This is because, in the above energy calculation formula, the flow rate is in the denominator, and the higher the flow rate, the larger the glow discharge power that can be applied, and the larger the allowable range of power variation, which facilitates discharge management. However, if the flow rate is too high, the formation rate of the p-type a-Si: H film increases, which makes it difficult to control the film thickness of the p-type a-Si: H film. In the case where the film thickness is strictly controlled, such as a solar cell, the increase in the flow rate should be selected so that the formation rate is appropriate.

The reaction pressure is not particularly limited, but 0.5 to 2 Tor
It is carried out in a pressure range slightly higher than r. Lowering the pressure improves conductivity but decreases Eopt.

The p-type a-Si: H film is formed at a temperature of 300 ° C. or lower.
It is particularly preferable to carry out at 250 to 150 ° C. Although it is possible to form a p-type a-Si: H film even below 100 ° C., Eopt becomes larger than 2.1 ev and becomes large, and the conductivity is remarkably lowered, so that it can be used as a window material for an amorphous silicon solar cell. It doesn't work well. If the temperature exceeds 300 ° C, the conductivity will improve, but the Eopt will decrease.

[Preferred modes for carrying out the invention]

Next, embodiments of the present invention will be described. A substrate on which a semiconductor thin film is to be formed is placed in a reaction chamber capable of glow discharge. It is heated and kept at a temperature of 100-300 ° C under reduced pressure. Then, a raw material gas is introduced and glow discharge energy of 40 KJ or less per unit mass of disilane is applied to form an a-Si: H film containing hydrogen at a pressure of 0.5 to 2 Torr.

P-type a-Si obtained under the manufacturing conditions according to the present invention:
The band gap of the H film is 2.1 eV to 1.9 eV, which is sufficiently wide corresponding to visible light even though it is doped with diborane, and is sufficiently useful as a window material for an amorphous silicon solar cell. It functions as a material.

Since the p-type semiconductor thin film obtained in the present invention has a wide optical band gap, it is used as a window material of an amorphous silicon photoelectric conversion device such as an a-Si solar cell, a back contact material, and a light incident side of a tandem junction device. In addition to being suitable for use as a material of the element arranged in the above, it can be effectively used for various devices using amorphous silicon.

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

Example-1. Capacitively coupled high frequency plasma CVD (Chemical) having a substrate heating means, an evacuation means, a gas introduction means, and a glow discharge chamber having a parallel plate electrode on which a substrate can be installed.
To V apor D eposition) substrate installation part of the device, set up a Corning 7059 glass substrate. While evacuating to 10 ⁻⁷ Torr or less with an oil diffusion pump, heating was performed so that the temperature of the substrate installation portion (thin film formation temperature) was 150 ° C.

Disilane flow rate 10SCCM, hydrogen dilution 500vppm diborane 20SCC
M was flowed to form a p-type-a-Si: H film at a discharge power of 5W. The pressure during film formation was 0.5 Torr, and the supplied energy was about 9.6 KJ / gS _i2 H ₆ . The film thickness measured by a stylus type film thickness meter is 6000Å, and the dark conductivity is 6 × 10 ^-7 S ・ cm ^-1 , AMI, 100
The photoconductivity under irradiation with mW / cm ² was 1.3 × 10 ⁻⁵ S · cm ⁻¹ . After measuring the light absorption coefficient α of the thin film with a spectrophotometer, Eopt is (αhν) for the energy (hν) of the incident light.
1/2 was plotted, and the straight line portion was extrapolated to obtain a value of an intercept with the hν axis, and a value of 1.93 eV was obtained.

Examples-2 to 6 Using the plasma CVD apparatus used in Example 1, an amorphous silicon film was formed with the raw material gas flow rate, discharge power, and formation temperature shown in Table 1. The Eopt and dark conductivity of the resulting amorphous silicon film are also shown in Table 1.
The photoconductivity (AMI, under irradiation of 100 mW / cm ² ) was in the order of 10 ⁻⁵ S · cm ⁻¹ for all films.

Using hydrogen dilution 0.5v% B ₂ H ₆ in place of the hydrogen dilution 500vppm B ₂ H ₆ in the embodiment -7 Example 1. The obtained a-Si: H film with B ₂ H ₆ / S _i2 H ₆ = 1/100 has Eopt of 1.90 eV and dark conductivity of 4 × 10 ^−6.
It was S · cm ⁻¹ .

Comparative Example-1 This is an example in which the discharge power was 25 W and the supply energy value was 47.7 KJ / gS _i2 H ₆ in Example ₆ .

Comparative Example-2 This is a case where the formation temperature was raised to 350 ° C. in Example 8.

Comparative Examples-3 to 5 The amorphous silicon film shown in Table 1 was formed by the plasma CVD apparatus used in Example 1. The results are shown in Table 1.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (56) References Appl. Phys. Lett. , 42 [4], (1983-2-15), p. 369 to 371 Supplement to the Extended Abstract of the 15th Conf. on Solid state Devices and Materials, (1983-8-30), P.P. 42-43 Proceedings of the 31st Annual Meeting of the Japan Society of Applied Physics, (1984-3-29), p. 378, No. 1a-Q-1 to 1a-Q-2

Claims (2)

[Claims] 1. A method for forming a p-type semiconductor thin film on a substrate by decomposing a disilane and a p-type doping gas as raw materials by glow discharge.
Apply glow discharge energy of KJ or less and 8 KJ or more,
A method for forming a p-type semiconductor thin film for a window material of a solar cell, which comprises forming the thin film having a band gap of 1.9 eV or more and 2.1 eV or less at a temperature of 200 ° C. or less and 100 ° C. or more. 2. The method according to claim 1, wherein diborane diluted with hydrogen or He is used as the p-type doping gas.