JP2000058889A - Silicon based thin film and silicon based thin film photoelectric conversion device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the performance of a crystalline silicon-based thin-film photoelectric conversion device manufactured by a low-temperature process by a plasma CVD method, high quality in which residual impurity atoms in the film are controlled. A silicon-based thin film and a photoelectric conversion device exhibiting high photoelectric conversion characteristics using the same are obtained. SOLUTION: This is a silicon-containing thin film containing a crystalline material deposited at a base temperature of 400 ° C. or lower using a plasma CVD method,
The average concentration of oxygen atoms in the thin film is 1 × 10 ¹⁹ cm ^-3 or more 1 × 10
²⁰ cm ^-3 or less, and a silicon-based thin film characterized in that the average concentration of carbon atoms in the thin film is 1 × 10 ¹⁹ cm ^-3 or less, and having this as a photoelectric conversion layer And one conductivity type layer 111 and the photoelectric conversion layer and the opposite conductivity type layer 113
A silicon-based thin-film photoelectric conversion device, characterized in that the photoelectric conversion unit 11 is formed by a plasma CVD method.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a silicon-based thin film that can be used for a high-performance thin-film photoelectric conversion device.
In the specification of the present application, the terms “polycrystal” and “microcrystal” also mean those partially including an amorphous state.

[0002]

2. Description of the Related Art In recent years, photoelectric conversion devices using a thin film containing crystalline silicon, for example, a polycrystalline silicon or microcrystalline silicon thin film have been vigorously developed. These are attempts to form a high-quality silicon-based thin film on an inexpensive substrate by a low-temperature process to achieve both low cost and high performance, and are used not only for solar cells but also for various photoelectric conversion devices such as optical sensors. The application of is expected.

[0003] As a method of forming these silicon-based thin films, for example, a silicon-based thin film containing a crystalline material is directly deposited by a CVD method or a sputtering method, or an amorphous film is once deposited by a similar process and then thermally annealed. There is a method such as crystallization by performing laser annealing, but in any case, in order to use the above-mentioned inexpensive substrate, it is necessary to perform it by a low-temperature process.

Among them, the method of directly depositing a silicon-based thin film by the plasma CVD method is expected to be able to obtain a high-quality film by a relatively simple process, in which the temperature can be easily reduced and the area can be increased. Have been. When a polycrystalline silicon-based thin film is obtained by this method, a high-quality silicon-based thin film having crystalline properties is once formed on a substrate by some process and then formed as a seed layer or a crystallization control layer. High quality films can be formed even at very low temperatures. On the other hand, it is widely known that a microcrystalline silicon thin film can be obtained by diluting a silane-based source gas by 10 times or more with hydrogen and forming a film by a plasma CVD method.
Microcrystallization can be easily performed even at temperatures before and after. For example,
Photoelectric converter with microcrystalline silicon pin junction is App
l. Phys. Lett. 65 (1994) p.860 (New Chatel University). This is a p-layer, which is one conductivity type layer,
All of the junction components composed of the intrinsic layer, which is a photoelectric conversion layer, and the n-type layer, which is a reverse conductivity type layer, are microcrystalline silicon, and are characterized in that they are easily and sequentially deposited by a plasma CVD method.

[0005] By the way, in a thin film semiconductor, the amount of a trace amount of residual impurity atoms present in the film is often discussed. For example, in the case of a silicon-based thin film, the impurity atoms referred to here are not conductivity-type determining atoms such as phosphorus and boron, but oxygen,
It is an element such as carbon or nitrogen, which is mixed in a very small amount during the formation of a thin film from various places inside and outside a semiconductor manufacturing apparatus and consequently remains in the film. Except in exceptional cases, in the case of silicon-based thin films that can be generally applied to devices such as photoelectric conversion devices, it is considered that the amount of this residual impurity should be as small as possible. Devices have been devised.

[0006]

However, there is a limit to lowering the concentration of the above-mentioned residual impurity atoms, and the more the amount of all impurities is reduced as much as possible, the more expensive thin-film forming apparatuses and expensive manufacturing equipment and manufacturing lines are manufactured. , The production cost increases and is not practical. On the other hand, the behavior of impurity atoms in a silicon-based thin film in a thin-film silicon-based photoelectric conversion device has many unclear points, and more specifically, what kind of atoms should be kept in a certain concentration range. Until now, it was not revealed empirically.

In view of the above-mentioned problems of the prior art, an object of the present invention is to improve the quality of a silicon-based thin film formed by a low-temperature plasma CVD method by controlling the amount of the remaining impurities and to improve the performance of a photoelectric conversion device. Is also to improve.

[0008]

Means for Solving the Problems As a result of repeated studies to solve the above-mentioned problems, the present inventors have found that a silicon-based thin film applied to a photoelectric conversion device has an appropriate concentration of specific residual impurity atoms. It has been found that a high-performance photoelectric conversion device can be obtained by controlling the range.

That is, the present invention relates to a crystalline silicon-containing thin film deposited at a base temperature of 400 ° C. or lower using a plasma CVD method, wherein the average concentration of oxygen atoms in the thin film is 1%.
× 10 ¹⁹ cm ^-3 to 1 × a 10 ²⁰ cm ^-3 in the range, and silicon-based film, wherein the average concentration of carbon atoms in the thin film is 1 × 10 ¹⁹ cm ^-3 or less And a silicon-based thin-film photoelectric conversion having the same as a photoelectric conversion layer, and wherein a photoelectric conversion unit comprising a layer of one conductivity type, a layer of the photoelectric conversion layer and a layer of opposite conductivity type is formed by a plasma CVD method. An apparatus is provided.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The silicon-based thin film applied in the embodiment of the present invention is formed by the following method. The silicon-based thin film is formed by a parallel plate RF plasma CVD method which is generally widely used, or may be a RF-VHF band having a frequency of 150 MHz or less, or a high-frequency power supply in a microwave band. The film formation temperature is set to 400 ° C. or lower because an inexpensive substrate can be used. A silane-based gas and a diluent gas such as hydrogen are used as main components of the gas introduced into the reaction chamber. The flow rate of the diluent gas with respect to the silane gas is
It is preferably 20 times or more, but other film forming conditions (discharge
Power and the pressure in the reaction chamber) determine the optimal amount of dilution. As the silane-based gas, monosilane, disilane and the like are preferable. In addition, a silicon halide-based gas such as silicon tetrafluoride, silicon tetrachloride and dichlorosilane may be used. As a diluting gas, an inert gas such as a rare gas, preferably helium, neon, argon, or the like may be used in addition to hydrogen.

The silicon-based thin film is thin-film polycrystalline silicon or microcrystalline silicon having a volume crystallization fraction of 80% or more, and is a non-doped intrinsic semiconductor or a weak p-type or weak n-type photoelectric layer containing a small amount of impurities. It is desirable to have a sufficient conversion function. The invention is not limited to these, and silicon germanium or the like, which is an alloy material, may be used. 400
Since it is formed at a low temperature of ℃ or less, it contains many hydrogen atoms for terminating and inactivating defects at crystal grain boundaries and in grains, and the hydrogen content in the film is 1 atomic% or more and 20 atomic% or less. is there. Many of the crystal grains contained in the silicon-based thin-film photoelectric conversion layer extend upward from the base layer in a columnar shape and grow.
The crystal grains have a (110) crystal orientation as a predominant orientation plane with respect to the vertical direction of the film, and were determined by X-ray diffraction spectrum (22).
The ratio of the (111) diffraction peak to the 0) diffraction peak is 0.2 or less.

In addition to the above features, the silicon-based thin film according to the present invention has an average oxygen atom concentration in the film of 1 × 10 ¹⁹ cm ^−3.
It is characterized in that it is within the range of 1 × 10 ²⁰ cm ⁻³ or less and the average concentration of carbon atoms in the film is 1 × 10 ¹⁹ cm ⁻³ or less. The concentration of these residual impurity atoms depends on the plasma CV
It also changes depending on the specifications and capabilities of the film forming chamber and the exhaust device in the D apparatus, the purity of the source gas to be introduced, and the film forming conditions. In order to maintain a photoelectric property parameter above a certain level, it is necessary that the residual impurities have a value below a certain concentration. This is 1 × 10 ²⁰ cm ^{−3 for} oxygen and 1 × 10 ¹⁹ cm ⁻³ for carbon. On the other hand, the oxygen concentration is 1 × 10
^The reason that the density is preferably ¹⁹ cm ^-3 or more is because oxygen atoms are thermally activated and the carrier concentration of the n-type silicon-based thin film increases, so that the effect of improving the open-circuit voltage of the photoelectric conversion device is obtained. It is considered something.

A method of manufacturing a thin-film silicon-based photoelectric conversion device applied in another embodiment of the present invention is as follows. A metal such as stainless steel, an organic film, a low-melting-point inexpensive glass, or the like is used for the substrate. First, a thin film layer composed of one or more of the following (A) and (B) is formed as a back electrode portion disposed on the substrate by, for example, an evaporation method or a sputtering method.

(A) A metal thin film comprising at least one material selected from Ti, Cr, Al, Ag, Au, Cu, and Pt, or a combination of alloy layers thereof. (B) A transparent conductive oxide film comprising at least one layer selected from ITO, SnO ₂ , and ZnO. Next, a photoelectric conversion unit composed of a nip or pin junction is formed. Here, all the layers constituting the photoelectric conversion unit are deposited by a plasma CVD method at a base temperature of 400 ° C. or lower. Here, the parallel plate type which is usually widely used
In addition to the RF plasma CVD method, a method using a VHF band or microwave band high frequency power supply may be used.

First, one conductivity type layer of the photoelectric conversion unit is deposited. For this, for example, an n-type silicon-based thin film doped with phosphorus atoms, which are conductivity type determining impurity atoms, or a p-type silicon-based thin film doped with boron atoms, is used. These conditions are not limiting,
For example, nitrogen or the like may be used as the impurity atom in the n-type layer. As a specific material and form of the one conductivity type layer, an alloy material such as amorphous silicon carbide or amorphous silicon germanium may be used in addition to amorphous silicon, and polycrystalline or microcrystalline silicon including partially crystalline, Alternatively, the alloy-based gold material may be used. In some cases, pulsed laser light is irradiated after the deposition of the one conductivity type layer to control the crystallization fraction and the carrier concentration due to the conductivity type determining impurity atoms.

Subsequently, as the photoelectric conversion layer of the bottom cell,
The silicon-based thin film according to the present invention is deposited. The photoelectric conversion layer has a thickness of 0.5 to 20 μm, and has a necessary and sufficient thickness as a silicon-based thin film photoelectric conversion layer. Subsequent to the deposition of the photoelectric conversion layer, a silicon-based thin film serving as a conductivity type layer of the opposite type to the one conductivity type layer in the photoelectric conversion unit is deposited. As the reverse conductivity type layer, for example, a p-type silicon-based thin film doped with boron atoms, which are conductivity type-determining impurity atoms, or an n-type silicon-based thin film doped with phosphorus atoms is used. These conditions are not limited, and the impurity atoms may be, for example, aluminum in the p-type layer. As a specific material and form of the one conductivity type layer, an alloy material such as amorphous silicon carbide or amorphous silicon germanium may be used in addition to amorphous silicon, and polycrystalline or microcrystalline silicon including partially crystalline, Alternatively, the alloy-based gold material may be used.

After depositing the photoelectric conversion unit, the IT
A transparent conductive oxide film including at least one layer selected from O, SnO ₂ , and ZnO is formed by, for example, an evaporation method or a sputtering method. A which will be the grid electrode after this
In some cases, a comb-shaped metal electrode made of at least one or more materials selected from l, Ag, Au, Cu, and Pt, or a combination of these alloy layers may be formed.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a thin-film silicon solar cell as a photoelectric conversion device according to some embodiments of the present invention and a comparative example will be described with reference to FIG. (Example 1) On a glass substrate 1, first,
The film thickness was set to 0, the Ti film 101 was formed to 50 nm, the Ag film 102 to 300 nm, and the ZnO film 103 to 100 nm by sputtering. Next, the phosphorus-doped n-type silicon layer 111 is 10 nm, the non-doped photoelectric conversion layer 112 is 1.6 μm, and the p-type silicon layer 11 is
3 is 10nm, each film is formed by RF plasma CVD method.
Silicon-based photoelectric conversion unit 11 containing -p junction crystalline material
Was formed. Further, a transparent conductive film ITO2 having a thickness of 80 nm and a comb-shaped Ag electrode 3 for extracting a current were formed as an upper electrode.

Here, the thin-film polycrystalline silicon photoelectric conversion layer 11
2 silicon film is RF using 13.56MHz high frequency power supply
It was deposited by the plasma CVD method. This film formation chamber is equipped with an exhaust line with a mechanical booster pump and a dry pump, and the back pressure before the start of film formation is 2 × 10 ^-3
It was Torr. The reaction gas at the time of film formation was mixed at a flow ratio of silane and hydrogen of 1:90, and the pressure in the reaction chamber was set to 5.0 Torr. The discharge power density was 100 mW / cm ² and the deposition temperature was 300
° C. X of the silicon film formed under these film forming conditions
(11) for the (220) diffraction peak determined by X-ray diffraction spectrum
1) The ratio of diffraction peaks was 0.11.

The concentration of the element in the silicon-based thin film in this photoelectric conversion device was determined by secondary ion mass spectrometry.
As a result, the hydrogen atom content in the film was 2.5 atomic%.
FIG. 2 shows the distribution of oxygen and carbon atom concentrations in the film in the depth direction. The vertical axis represents the concentration of each atom, the horizontal axis represents the depth of the film, and the origin position represents the sample surface. When the average concentration in the range indicated by the double-ended arrows in the figure excluding the region of 0.1 μm from the front and back surfaces was determined, the oxygen atom concentration was
1.1 × 10 ¹⁹ cm ⁻³ and the carbon atom concentration were 4.6 × 10 ¹⁸ cm ⁻³ .

The output characteristics of this thin-film polycrystalline silicon solar cell when AM1.5, 100 mW / cm ² light intensity is used as incident light 4 are: open-end voltage 0.536 volt, short-circuit current density 25.2 mA / cm ² , and fill factor 76.1. % And a conversion efficiency of 10.3%. Comparative Example 1 Similarly, a thin-film silicon solar cell having a structure as shown in FIG. 1 was formed. The device structure including the thickness of each layer is exactly the same as in the first embodiment. The silicon film serving as the thin-film polycrystalline silicon-based photoelectric conversion layer 112 was deposited by an RF plasma CVD method using a 13.56 MHz high frequency power supply. This film formation chamber is equipped with an exhaust line with a turbo molecular pump and an oil rotary pump, and exhausts by a combination of these two vacuum pumps before the start of film formation.
× 10 ^-7 Torr. On the other hand, when a film was formed by introducing a raw material gas, only an oil rotary pump was used.

The concentration of elements in the silicon-based thin film in this photoelectric conversion device was determined by secondary ion mass spectrometry.
FIG. 3 shows the distribution of oxygen and carbon atom concentrations in the film in the depth direction. The vertical axis represents the concentration of each atom, the horizontal axis represents the depth of the film, and the origin position represents the sample surface. When the average concentration in the range indicated by the double-ended arrows in the figure excluding the region of 0.1 μm from the front and back surfaces was determined, the oxygen atom concentration was
8.8 × 10 ¹⁸ cm ^-3 and the carbon atom concentration were 5.1 × 10 ¹⁹ cm ^-3 . It was found that although the back pressure before film formation was low, the oxygen concentration was slightly lower, but the carbon concentration was higher than in Example 1 using a vacuum pump without using oil.

The output characteristics of this thin-film silicon solar cell when AM1.5 and 100 mW / cm ² are used as the incident light 4 are: open-end voltage 0.438 volt, short-circuit current density 22.7 mA / cm ² , and fill factor 72.
The conversion efficiency was 7% and the conversion efficiency was 7.2%. (Examples 2 to 6 and Comparative Examples 2 to 4) A thin film silicon solar cell as in Example 1 and Comparative Example 1 by changing the film forming chamber, the vacuum pump, the value of Back Pressure and the film forming conditions variously. Was formed. Table 1 shows the values of the average contents of oxygen and carbon atoms in the silicon film obtained by the secondary ion mass spectrometry in the same manner as described above. In addition, results as shown in Table 2 were obtained as characteristics of the solar cell.

[0024]

[Table 1]

[0025]

[Table 2]

From these Examples and Comparative Examples, the average concentration of oxygen atoms in the film is in the range of 1 × 10 ¹⁹ cm ^{−3 to} 1 × 10 ²⁰ cm ⁻³ and the average concentration of carbon atoms in the film. Is 1 × 10 ¹⁹ c
It has been found that a high conversion efficiency can be obtained if m ⁻³ or less.

[0027]

As described above, according to the present invention, the quality of a thin film silicon film formed at a low temperature by a plasma CVD method can be improved, which can greatly contribute to the enhancement of the performance of a thin film silicon photoelectric conversion device.

[Brief description of the drawings]

FIG. 1 is a structural cross-sectional view of a silicon-based thin-film photoelectric conversion device applied in Embodiment 1 of the present invention.

FIG. 2 is a depth profile of oxygen and carbon atom concentrations in a silicon film in Example 1 measured by secondary ion mass spectrometry.

FIG. 3 is a depth profile of oxygen and carbon atom concentrations in a silicon film in Comparative Example 1 measured by secondary ion mass spectrometry.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1: Glass substrate 2: Transparent conductive oxide film (ITO) 3: Comb electrode (Ag) 4: Incident light 10: Backside electrode part 11: Thin film silicon-based photoelectric conversion unit 101: Ti film 102: Ag film 103: Transparent Conductive oxide film (ZnO) 111: n-type silicon layer 112: non-doped crystalline silicon-based photoelectric conversion layer 113: p-type silicon layer

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 4K030 BA27 BA29 BB01 BB03 BB04 BB12 CA06 FA10 HA02 JA01 JA06 LA04 LA16 5F045 AA08 AB02 AB03 AB04 AC01 AC05 AC14 AC16 AC17 AD07 AE21 AF07 AF10 BB04 CA13 5F051 AA03 AA04 AA05A DA17 FA02 FA03 FA04 FA06 GA02 GA03 GA05

Claims (3)

[Claims] Claims: 1. A crystalline silicon-containing thin film deposited at a base temperature of 400 ° C. or lower using a plasma CVD method,
The average concentration of oxygen atoms in the thin film is 1 × 10 ¹⁹ cm ^-3 or more 1 × 10
A silicon-based thin film characterized by being in a range of ²⁰ cm ^-3 or less and having an average concentration of carbon atoms in the thin film of 1 × 10 ¹⁹ cm ^-3 or less. 2. The silicon-based thin film containing crystalline material is polycrystalline silicon or a microcrystalline silicon thin film having a volume crystallization fraction of 80% or more, and the hydrogen content in the thin film is 1 atomic% or more.
20 atomic% or less, having a (110) crystal orientation as a dominant orientation plane with respect to the vertical direction of the thin film, the ratio of the (111) diffraction peak to the (220) diffraction peak in the X-ray diffraction spectrum is 0.2 or less. The silicon-based thin film according to claim 1, wherein 3. The semiconductor device according to claim 1, wherein the semiconductor is substantially an intrinsic semiconductor and has a thickness of 0.5.
3. The method according to claim 1, wherein the thickness is in a range of 5 μm to 20 μm.
Wherein the silicon-based thin film according to the above is used as a photoelectric conversion layer, and a photoelectric conversion unit comprising one conductivity type layer and the photoelectric conversion layer and the opposite conductivity type layer is formed by a plasma CVD method. Thin film photoelectric conversion device.