JPS6216513A - Semiconductor thin film - Google Patents

Abstract

PURPOSE:To obtain an amorphous silicon carbide semiconductor thin film, especially, a P-type amorphous silicon carbide semiconductor thin film (P-aSiC:H film), having excellent characteristics at a low temperature using low discharge power by a method wherein the carbon content of the thin film, to be obtained by performing glow discharge decomposition on the specific methyl silane, silane and impurity gas, is specified. CONSTITUTION:The content of carbon in the thin film obtained by glow discharge decomposition of methyl silane indicated by the formula Si(CH3)nH4-n (n=1-4), the silane indicated by the formula SimH2m+2 (m=1-3) and impurity gas, is set at 10at% or below. There are monomethylsilane, dimethyl silane, trimethyl silane and tetramethyl silane corresponding to n=1-4 in the methyl silane expressed by the formula Si(CH3)nH4-n. There are monosilane, disilane and trisilane in the silane expressed by the formula SimH2m+2 corresponding to m=1-3. There is diborane (B2H6) which gives P-type conductivity, for example, and phosphine (PH3) which gives N-type conductivity as the impurity gas with which the conductive type of the semiconductor thin film can be changed.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field] The present invention relates to semiconductor thin films, and particularly to amorphous silicon carbide exhibiting r-type conductivity.

[Background technology]

Amorphous silicon carbide (a-8iCX, 0<X)
Thin films have been extensively studied recently, and their applications are open to solar cells, photosensitive drums, etc. A-3iCX has a larger optical bandgap than amorphous silicon (a-8i) and can be doped with p-type and n-type impurities, making it suitable for short wavelength light conversion in tandem solar cells. It is used for solar cells and, with p- or n-type doping, for the light incident film (window layer) of solar cells.

Conventionally, a widely known method for forming a-8iCX is to decompose a mixed gas of monosilane and hydrocarbon by glow discharge. However, in the method using this mixed gas, the energy required for decomposition of monosilane and hydrocarbon is greatly different, so the a-8i obtained is
It has been difficult to control the composition and properties of CX. That is, since monosilane is easily decomposed and hydrocarbons are difficult to decompose, decomposition conditions must be extremely strictly controlled in order to obtain a film with a desired S/C ratio.

In addition, increasing the optical band gap has the problem of decreasing dark conductivity and photoconductivity, and furthermore, it has been difficult to effectively impart p- or n-type conductivity by doping with p- or n-type impurities. .

The present inventors previously proposed an amorphous silicon carbide semiconductor thin film with a wide optical bandgap and high photoconductivity by glow discharge decomposition of a compound having a silicon-carbon bond together with disilane. (Patent application No. 59-247577).

As a result of further study, the present inventor found that an amorphous silicon carbide semiconductor thin film, particularly an r-type amorphous silicon carbide semiconductor thin film (hereinafter referred to as p
-asic:H film) was successfully obtained.

[Disclosure of the invention]

The present invention is based on the general formula 5i(CH,)nH,-n(n=1-4
), the general formula SimH2m+2
The carbon content in the thin film obtained by glow discharge decomposition of silane and impurity gas represented by (m = 1 to 3) is 10
It is a semiconductor thin film having a concentration of at% or less.

General formula S i (CH,)n used in the present invention
Methylsilanes represented by H, -9 (n=1 to 4) include monomethylsilane, dimethylsilane, trimethylsilane and tetramethylsilane corresponding to n=1.2.5 and 4, respectively.

Furthermore, in the present invention, the general formula SimH, m+2 (m=1
The silanes represented by ~3) include monosilane, disilane, and trisilane corresponding to m=1.2 and 3, respectively.

The impurity gas is one that changes the conductivity type of the semiconductor thin film; for example, diborane (B2H6) is used to impart r-type conductivity, and phosphine (PH3) is used to impart n-type conductivity. ). It is preferable to use these B2H and PH after diluting them with hydrogen or helium.

The present invention is characterized by a 1)-asic:H film having good optical and electrical properties obtained by glow discharge decomposition at low temperature and low discharge power. For this purpose, preferably monomethylsilane, dimethylsilane, trimethylsilane is used as the methylsilane; disilane as the silane; diborane diluted with hydrogen or helium as the impurity gas. Particularly preferably, the invention provides glow discharge decomposition of dimethylsilane, disilane and diborane diluted with hydrogen or helium at low discharge power.
ASIC formed on a substrate kept below 0°C
:H film.

The f-asic:H film of the present invention contains 10 at% or less of carbon. Furthermore, it is known to those skilled in the art that hydrogen is incorporated into the amorphous thin film obtained from glow discharge and is bonded to unpaired electrons of silicon atoms. It has been confirmed from the IR spectrum that the bonding mode is dominated by SiH or (SiH2)n. Furthermore, the thin film of the present invention is 20a
It contains a large amount of bonded hydrogen exceeding t%. In the prior art, an amorphous silicon film containing <5IH2 or (siH2)n and a large amount of hydrogen as in the present invention was not put to practical use because it had poor electrical properties. However, the present inventor has discovered a very unique effect by selecting the composition of the raw materials as described above. That is, its properties are clear from the fact that, although the carbon content is at most 3 at%, the optical bandgap is wide, exceeding 2.2 eV, and furthermore, the photoconductivity is 1
Some reach as high as 0''S/cm, which is 10% of the dark conductivity.
Some exhibit photoconductivity that is more than twice as high. In the present invention, the optical bandgap is determined by the carbon content in the film being 1
~10 at% and can be varied between 1.8 eV and 2.5 e'V by varying the hydrogen content between 15 and 60%.

In this case, the hydrogen content can be changed by changing the temperature of the substrate. In order to efficiently incorporate a large amount of hydrogen, the temperature of the substrate is maintained below 250°C. The temperature of the substrate that gives a favorable effect is 80°C or higher, 20°C
The temperature of the substrate is preferably 0° C. or lower, and a particularly preferable temperature of the substrate is 100° C. or higher and 160° C. or lower. [Preferred Mode 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, and heated and maintained at a temperature of 250° C. or less under reduced pressure. Next, dimethylsilane, disilane and diborane diluted with hydrogen or helium are introduced, and the pressure is 0.05~
The present invention can be carried out by glow discharge decomposition at a discharge power of 1 to 10 W at 2 Tom, and the amount of diborane etc. to be added can be determined as appropriate in the same manner as for ordinary dopants.

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

Example 1 A capacitively coupled high-frequency plasma CVD (chemi
Place a crystalline silicon plate and a Corning Co., Ltd. 7
A 059 glass substrate was installed. 10 by oil diffusion pump
The substrate was heated to a temperature of 150° C. (thin film formation temperature) while evacuating to a temperature below -' Torr.

Dimethylsilane, disilane, hydrogen dilution 10 Ooll
Xfi diborane at 7, 2.5 and 100 se
cM was introduced, and glow discharge was performed at a pressure of 0.56 Torr and a discharge power of 1 W. After leaving it for 45 minutes, stop the discharge,
After cooling, the substrate on which the thin film was formed was taken out, and the film thickness, optical band gap, degree of light guide, dark conductor, and infrared absorption spectrum were measured. It was found that the film thickness was 5860 A and the film formation rate was about 2.2 A/sec. The optical bandgap is determined by measuring the light absorption coefficient α of a thin film using a spectrophotometer, and then calculating the optical band gap by (αhν) with respect to the energy of incident light (hν).
) 1/2 was plotted, and the straight line portion was extrapolated to find the value of the intercept with the hν axis, and 2.78v was obtained. Also A
The light guiding degree under Ml, 100 mW/cd irradiation is 1.
4X10 87cm, dark conductor 6.2X10 87c
It showed p-type conductivity at m.

Example 2 In this example, the dimethylsilane flow rate was changed to 1 in Example 1.
/2-3. (Assuming 5sccM, the temperature of the board installation part is 10
This is an example where the temperature was set to 0°C.

After discharging for 25 minutes at a discharge power of 3 W and a pressure of 0.55 Torr, the discharge was stopped, and after cooling, the same measurements as in Example 1 were carried out. As a result, the deposition rate was 4.7 A/sec, and the optical band gap was 2.
．． 16 eV, a light conductivity of 8×10-88/c*, and a dark conductor of 5×1087 cm were obtained.

In Examples 1 and 2, the carbon content was determined by secondary ion mass spectrometry and was 3 at% and 1.0%, respectively.
It was 6 at%. The amount of hydrogen determined from the infrared absorption spectrum was larger in Example 2 than in Example 1, about 32 at.
%Met. Now, the thin film obtained in Example 2 has a larger optical bandgap than Example 1, which indicates that the thin film of the present invention is different from the silicon carbide film of the conventional method. .

That is, the present invention provides the convenience of controlling the optical bandgap, which is an important optical property of a thin film, by controlling both the carbon content and the hydrogen content.

[Effect]

Since the semiconductor thin film obtained by the present invention has a wide optical gap, it can be used as a window material for amorphous photoelectric conversion elements such as amorphous solar cells, a back contact material, and a tandem junction solar cell on the light incident side. In addition to being suitable for use as a material for photoelectric conversion parts with good short wavelength sensitivity, it is also useful for various devices using amorphous silicon.

Claims (5)

[Claims] (1) General formula Si(CH_3)_nH_4_-_n(n
= 1 to 4), general formula Si_m
1. A semiconductor thin film obtained by glow discharge decomposition of silane and impurity gas represented by H_2_m_+_2 (m=1 to 3) and having a carbon content of 10 at% or less. (2) The semiconductor thin film according to claim 1, wherein the methylsilane is dimethylsilane where n=2. (3) The semiconductor thin film according to claim 1, wherein the silane is disilane represented by m=2. (4) The semiconductor thin film according to claim 1, wherein the impurity gas is diborane or phosphine diluted with hydrogen or helium. (5) The semiconductor thin film according to claim 1, wherein the thin film is formed on a substrate maintained at a temperature of 250° C. or less.