JPH04320323A - Semiconductor thin film - Google Patents

Abstract

PURPOSE:To obtain a semiconductor thin film with a crystallizability in an extremely thin-film state for improving performance of an amorphous solar cell. CONSTITUTION:A process for allowing a thin film to be grown while emitting ion onto a growth surface and that for performing only irradiation of ion are repeated and then a title item is a crystallizability semiconductor thin film where a thickness of the thin film which is formed by each repetition is 1 to 100Angstrom and an entire film thickness is 300Angstrom or less, thus enabling a crystallizability semiconductor thin film with a film thickness of 300Angstrom or less which could not be produced by conventional technologies to be formed. Photoelectric conversion efficiency can be improved by applying this thin film to a conductive thin film of an amorphous solar cell.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to improving the performance of amorphous solar cells, and more particularly to a technique for forming an extremely thin crystalline semiconductor thin film.

0002

2. Description of the Related Art Amorphous solar cells have already been put into practical use as a low-output energy supply source for driving calculators and watches. However, as an energy supply source with a large output of 0.1W or more, such as in photovoltaic power generation applications, it cannot be said that the performance and stability are sufficient, and various studies have been carried out with the aim of improving performance. ing. Therefore, the photoelectric conversion efficiency of a solar cell is expressed as the product of open circuit voltage, short circuit photocurrent, and fill factor. As a result of various studies, the currently achieved values for the short-circuit photocurrent and fill factor are approaching the theoretically expected values, but the open-circuit voltage, in particular, has not yet been sufficiently improved.

In order to improve the reliability of solar cells, pin-type amorphous solar cells in which a p-layer is provided on the light incident side have been studied in recent years. In order to improve the open circuit voltage of this amorphous solar cell, it is necessary to improve the photoelectric properties of the p-type semiconductor thin film, and in particular, it is necessary to expand the optical band gap and improve the electrical conductivity at the same time. This was not possible due to technical difficulties.

A microcrystalline thin film has been proposed as a material that satisfies these requirements, but it is difficult to form a thin film using conventional techniques such as plasma CVD under film-forming conditions to form a p-type microcrystalline thin film on a transparent electrode. Despite attempts to form amorphous solar cells, the open-circuit voltage of amorphous solar cells has not improved. The reason for this is
50~ which is necessary and sufficient for the p layer of a pin type amorphous solar cell
It has been reported that this is because it is difficult to form a p-type microcrystalline thin film on a transparent electrode at a film thickness of 300 Å (for example, B. Goldstein et al. “Characteristics of p-type SiC:H microcrystalline thin films”, Applied Physics Letters, 53
Volume, pp. 2672-2674, published 1988 (B.Go
ldstein et al. ,”Property
s of p+ microcrystalline
films of SiC：H deposited
by conventional rf glow d
Applied Physics
s letters, 53, p2672-2674
(1988)). In other words, at a film thickness of 50 to 300 Å, the film does not crystallize, has a high resistance, has a low fill factor, and cannot improve the open circuit voltage. On the other hand, ECR(E
lectron Cyclotron Resonane
) When the p layer is formed using the plasma CVD method,
Improvements in open-circuit voltage and photoelectric conversion efficiency have been reported, but this method causes severe damage to the underlying material due to ion bombardment, and the characteristics of the p-type microcrystalline thin film obtained by this method cannot be fully exploited. (For example, Hattori Yutaka et al., "High-efficiency amorphous heterojunction solar cell using p-type microcrystalline SiC film formed by ECR-CVD," Technical Digest of International PVSEC-3, pp. 171-174, 1987. Published in 2013 (Y. Hattori et al.
, ”High Efficiency Amorph
ous Heterojunction Solar
Cell Employing ECR-CVD Pr
oduced p-Type Microcrysta
lline SiC Film”, Technica
Digest of the International
onal PVSEC-3, p. 171 to 174 (1
987)).

In recent years, it has been reported that microcrystalline thin films can be obtained by repeating treatment with atomic hydrogen generated by microwave plasma, high-frequency plasma, etc. after film formation. Total film thickness is 300Å
It is not mentioned whether microcrystalization is possible in the following (
For example, A. Asano, “Influence of hydrogen atoms on the network structure of hydrogenated amorphous silicon and microcrystalline silicon thin films,” Applied Physics Letters, 56.
Volume, pages 533-535, published in 1990 (A.A.
sano, ``Effects of hydrogen
atoms on the network str
structure of hydrogenated am
orphous and microcrystall
ine silicon thin films”,
Applied Physics letters,
56, p. 533-535 (1990)), Isamu. Shimizu et al. “Chemical reaction control for crystalline silicon growth at low substrate temperatures”, Materials Research Society Symposium Proceedings, 1
Volume 64, pages 195-204, published in 1990 (I
．． Shimizu et al. ,”CONTROLO
F CHEMICAL REACTIONS FOR
GROWTH OF CRYSTALLINE Si
AT LOW SUBSTRATE TEMPERAT
Materials Research
Society Symposium Proceedings
ing Vol. 164, p. 195 ~ 204 (
1990))).

It has also been reported that a microcrystalline thin film can be obtained by forming a silicon thin film with a thickness of 900 to 1000 Å and then treating it with an ion beam. ions must be irradiated (for example, J.S. Im et al., "Crystal Nucleation by Ion Irradiation in Amorphous Silicon Thin Films," Applied Physics Letters, Vol. 57, pp. 1766-1768, published in 1990 (
J. S. Imet al. , ”Ion irradi
ation enhanced crystal nu
creation in amorphous Si
thin films”, Applied Phys
ics letters, 57, p. 1766 ~
1768 (1990)), C. Spinella et al. “Grain growth mechanism under ion beam irradiation of amorphous silicon by chemical vapor deposition” Applied Physics
Letters, Vol. 57, pp. 554-556, published in 1990 (C. Spinella et al., “Gr.
ain growth kinetics durin
g ion beam irradiation of
chemical vapor deposited
amorphous silicon”, Appl
ied Physics letters, 57,
p. 554-556 (1990))), damage to the underlying material occurs and the device becomes extremely large and expensive, making it impractical.

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to obtain a thin film having crystallinity in an extremely thin film state (300 Å or less), which was previously impossible.

[0008]

[Means for Solving the Problems] In order to solve the above problems, the present inventors have made extensive studies and have found that
100Å while irradiating with low energy ions below V
After forming the ultra-thin film shown below, crystallization of the film is promoted by continuing ion irradiation, and by repeating this process, it is possible to form a semiconductor thin film with crystallinity even at a film thickness of 300 Å or less. They found that this is the case, and completed the present invention.

The present invention comprises a step of forming a thin film in an atmosphere containing charged particles (hereinafter referred to as a film forming step) and a step of stopping the supply of the thin film forming raw material to guide only the charged particles to the surface on which the thin film is to be formed. (hereinafter abbreviated as "modification step"), and the thickness of the thin film formed in each repetition of both steps is 1 to 100 Å, and the total film thickness is 30 Å.
It is a crystalline semiconductor thin film of 0 Å or less.

The film forming process is characterized in that the thin film is grown in an atmosphere containing charged particles, and the means for supplying the thin film forming raw material, that is, the film forming means itself is not particularly limited. Specifically, this is carried out by a physical film forming method such as vacuum evaporation, sputtering, or ion plating, or a chemical vapor deposition (CVD) method such as optical CVD or plasma CVD. The atmosphere containing charged particles is one in which an atmosphere containing cations or anions is generated and guided (collided) with the thin film surface of the substrate by means such as applying a bias voltage to the substrate.

On the other hand, in the modification process, the supply of the thin film forming raw material in the film forming process is stopped, and only the atmosphere containing charged particles is continued, which is introduced (collided) with the thin film forming surface of the substrate to change the properties of the semiconductor thin film. It is a process to improve

An effective physical film forming method will be explained below. As mentioned above, the atmosphere containing charged particles in the film formation process is performed in conjunction with the physical film formation method and chemical vapor deposition method described below, but it can be controlled independently. Since this is the same generation method as the atmosphere containing charged particles in the modification process, effective generation methods will be explained in the specific section on the modification process described later.

As starting materials for film formation, silicon,
Elements, compounds, and alloys such as silicon carbide, silicon nitride, silicon-germanium alloy or composite powder, silicon-tin alloy or composite powder can be effectively used. Other than these, elements, compounds, and alloys such as carbon, germanium, and tin can also be used.

The film forming conditions are not particularly limited, except that ions are irradiated during thin film growth, and rare gases such as argon, xenon, helium, neon, and krypton, hydrogen, hydrocarbons, fluorine, and nitrogen are used. The film can be formed in an atmosphere of , oxygen gas, or the like. As specific conditions, the gas flow rate is 0.1 to 100 sccm, and the reaction pressure is 0.1 to 100 sccm.
The range is from 0001 mtorr to 100 mtorr. Further, film forming conditions such as flow rate, pressure, and electric power are appropriately selected depending on the film forming rate.

Regarding the film formation temperature, film formation is performed by controlling the substrate temperature. Although the temperature range is basically not subject to any restrictions, it is preferable to set the temperature in accordance with the reforming process. Specifically, it is selected within a temperature range of 500°C or less.

Next, a specific example of an effective chemical vapor deposition method will be shown below.

As a raw material gas for film formation, general formula Si
Monosilane represented by n H2n+2 (n is a natural number),
Silane compounds such as disilane, trisilane, and tetrasilane, fluorinated silanes, organic silanes, hydrocarbons, and germane compounds are used. Further, a gas such as hydrogen, deuterium, fluorine, chlorine, helium, argon, neon, xenon, krypton, or nitrogen may be introduced together with the raw material gas. When using these gases, for the raw material gas,
It is effective when used in a range of 0.01 to 100% (volume ratio), and is appropriately selected in consideration of the film formation rate and film characteristics.

[0018] As with the physical film forming method, there are no particular limitations on the film forming conditions, other than that the thin film is grown in an atmosphere containing charged particles. The specific conditions are disclosed below.

In optical CVD, film formation is performed by decomposing a source gas using an ultraviolet light source with a wavelength of 350 nm or less, such as a low-pressure mercury lamp, deuterium lamp, or rare gas lamp. Conditions during film formation include gas flow rate of 1 to 100 sccm, reaction pressure of 15 mtorr to atmospheric pressure, and substrate temperature of 200 to 60 mtorr.
0°C, the heat resistance of the substrate, the film formation time considered from the film formation rate, the temperature of the modification process, etc., more preferably 3.
The temperature is appropriately selected within the range of 00 to 500°C.

Further, plasma CVD will be specifically described below. As the discharge method, methods such as high frequency discharge, direct current discharge, microwave discharge, and ECR discharge can be effectively used. Flow rate of raw material gas: 1 to 900 sccm,
Reaction pressure 0.001 mtorr to atmospheric pressure, power 1 mW/
A range of cm2 to 10 W/cm2 is sufficient. These film forming conditions are changed as appropriate depending on the film forming rate and the discharge method. The substrate temperature is 200-600℃,
More preferably, it is 300 to 500°C.

In the present invention, the atmosphere containing charged particles in the modification step and the film forming step is an atmosphere in which charged particles are generated by discharge using a non-depositing gas and exposed to the growth surface. Even if non-ionized substances exist in the atmosphere, this does not impede the effects of the present invention in any way. A more preferable modification method is to expose the substrate to this discharge atmosphere and apply a bias voltage to the substrate to effectively guide the ionized components onto the substrate. As a method for generating discharge, high frequency discharge, direct current discharge, microwave discharge, ECR discharge, etc. can be effectively used. Furthermore, it is also a useful method in the present invention to effectively generate ions using an ion generator and guide them to the substrate surface. Specifically, various ion generators such as a Kauffman ion gun and an ECR ion gun are used. Non-depositional gases include reactive gases such as hydrogen, fluorine, and fluorine compounds, and rare gases such as argon, neon, helium, xenon, and krypton. Gases such as nitrogen trifluoride, carbon tetrafluoride, argon gas, neon gas, helium gas, xenon gas, and krypton gas can be effectively used. Additionally, mixtures of these gases can also be used effectively.

Next, specific conditions for the modification step will be disclosed. When using discharge, the discharge power is 1 to 500 W, the flow rate of non-depositing gas is 5 to 500 sccm, and the pressure is 0.00.
It is generated and maintained in the range of 1 mtorr to atmospheric pressure. When using an ion gun, the flow rate of non-depositional gas is 0
．． 1~50sccm, pressure 0.0001mtorr
~100 mtorr, a pressure range is used that provides ion generation and sufficient lifetime. Further, the ion energy is sufficient in the range of 10 to 1000 eV, preferably 100 to 600 eV. If the ion energy is increased beyond this range, the damage caused by the ions and the sputtering phenomenon will be more severe than the reforming effect, making it ineffective.

[0023] The temperature conditions in the modification step are controlled by the temperature of the substrate. This substrate temperature is the same as or lower than the substrate temperature in the film forming process, and ranges from room temperature to 600°C.
, preferably 200 to 500°C.

[0024] In one film forming process, 1 to 100
The film thickness is preferably 3 to 50 Å. When the film thickness exceeds 100 Å, the effect of the present invention decreases. Further, a film thickness of less than 1 Å is not preferable because the number of repetitions of film formation and modification increases from the viewpoint of practicality. The time required for one cycle is not particularly limited, but is within 1000 seconds.

The substrate on which the semiconductor thin film of the present invention is formed is:
There are no limiting conditions other than withstanding the process temperatures of the present invention. Translucent materials such as blue plate glass, borosilicate glass, and quartz glass, metals, ceramics, heat-resistant polymer materials, and the like can be used as the substrate. Moreover, it goes without saying that substrates on which electrodes used for solar cells, sensors, etc. are formed can also be effectively used in the present invention.

[0026]

EXAMPLES Example 1 An apparatus for carrying out the present invention is shown in FIG. The apparatus consists of an electron beam evaporator (1) for depositing silicon and an ion generator (2) for generating ions. A shutter (3) is provided on the electron beam evaporation device and the ion generator, and by opening and closing this shutter,
Film formation and modification can be repeated. As a starting material,
High-purity silicon was placed in a crucible, and an electron beam was applied to it to evaporate it. The substrate temperature was set at 300°C, which is the temperature for the next modification step. Introducing hydrogen gas at 10 sccm and pressure at 1 x 10-3 torr into the ion generator,
Applying an ion beam voltage of 300 V and an acceleration voltage of 50 V,
An ion beam was generated, the ion beam shutter was opened, and the substrate (4) was irradiated with the ion beam. at the same time,
The shutter of the electron beam evaporator was opened for 20 seconds to deposit a 20 Å silicon thin film on the substrate. Next, the shutter of the electron beam evaporator is closed and only ion beam irradiation is performed.
It took 0 seconds. In this way, the shutter of the electron beam evaporator was repeatedly opened and closed at each time interval. A thin film of about 200 Å was obtained by repeating this process 10 times. The substrate used here was a quartz glass substrate or a glass substrate coated with tin oxide. In order to investigate the presence or absence of crystallization in these samples, evaluation was performed using Raman scattering spectra. The results are shown in FIG. As is clear from the figure, a Raman scattering spectrum of 520 cm-1 due to silicon crystals was observed in this sample, and it was confirmed that a crystalline thin film could be obtained even at a film thickness of 200 Å.

Example 2 In Example 1, only the deposition thickness and modification time per one time were changed to approximately 4 Å and 6 seconds, respectively. The deposition thickness was changed by changing the time for opening the shutter of the electron beam evaporator. In Example 1, the deposition rate by electron beam evaporation was found to be about 1 Å/sec, so in this example, the time for one film formation was set to 4 seconds. A thin film of about 200 Å was obtained by repeating the vapor deposition process and the modification process 50 times. As a result of measuring the same Raman scattering spectrum as in Example 1, 52
A Raman scattering spectrum unique to silicon crystals at 0 cm-1 was obtained. Although this example is very effective, the film formation
The number of times of modification is more than 5 times that of Example 1, and
Since the ratio of the Raman scattering spectrum at 520 cm −1 to the Raman scattering spectrum at 480 cm −1 due to amorphous silicon decreased by half, it was determined that the crystallinity rate decreased.

Example 3 In Example 1, only the deposition thickness and modification time per one time were changed to approximately 80 Å and 10 seconds, respectively. The deposition thickness was changed by changing the opening time of the shutter of the electron beam evaporator. In Example 1, since the deposition rate by electron beam evaporation was found to be approximately 1 Å/sec, the time for one film formation was set to 80 seconds in this example. A thin film of about 240 Å was obtained by repeating the vapor deposition process and the modification process three times. As a result of measuring the same Raman scattering spectrum as in Example 1, 520
A Raman scattering spectrum specific to silicon crystals at cm-1 was obtained. This example is very effective and can shorten the process by reducing the number of times of film formation and modification to about 1/3 of Example 1, but the Raman scattering spectrum of 520 cm due to amorphous silicon is Since the ratio of the Raman scattering spectrum of -1 decreased to 1/4,
It is judged that the crystallization rate has decreased.

Comparative Example 1 In Example 1, the film was formed to a thickness of 200 Å without going through the modification process using only ion irradiation. That is,
The shutters of the ion beam evaporator and the electron beam evaporator were opened at the same time, and since the deposition rate by electron beam evaporation was found to be about 1 Å/sec in Example 1, both shutters were closed after 200 seconds to complete the formation. When the Raman scattering spectrum of this sample was measured in the same manner as in the example, as shown in FIG. 3, no Raman scattering spectrum peculiar to silicon crystal at 520 cm −1 was observed. This comparative example makes it clear that repeating film formation and modification by ion irradiation is essential for obtaining a crystalline thin film with a total film thickness of 300 Å or less.

Comparative Example 2 In Example 1, after forming the Si thin film to a thickness of 200 Å, only ion irradiation was continued. The time for irradiation alone was 1000 seconds. In the Raman scattering spectrum of the thin film obtained under these conditions, no spectrum peculiar to silicon crystals at 520 cm-1 was observed. This comparative example shows that there is an upper limit to the film thickness per repetition in the film formation-modification process.

Comparative Example 3 In Example 1, film formation was performed without ion irradiation in the film formation process. That is, the shutter of the electron beam evaporator was opened for 20 seconds to deposit a 20 Å silicon thin film on the substrate, and then the shutter of the electron beam evaporator was closed, the ion beam shutter was opened, and ion beam irradiation was performed for 10 seconds. Next, the shutter of the ion beam is closed, and the shutter of the electron beam evaporation apparatus is opened again to form a film. Repeat this film formation and modification for 10
By repeating this process several times, a thin film of about 200 Å was obtained. As a result of Raman scattering spectrum measurement of the thin film formed under these conditions, a Raman scattering spectrum peculiar to silicon crystals at 520 cm-1 was not observed. This comparative example clearly shows that even if the film formation-modification process is repeated by selecting the conditions for the film formation thickness per time, the effect will not appear if there is no ion irradiation in the film formation process. It is something to do.

[0032]

[Effects of the Invention] As is clear from the above Examples and Comparative Examples, the semiconductor thin film produced using the present method has 3
It was confirmed that the thin film had crystallinity even at a film thickness of 00 Å or less. This makes it possible to apply microcrystalline semiconductor thin films to the p-layer of amorphous solar cells, which was previously difficult, and improves the photoelectric conversion efficiency and reliability of amorphous solar cells. This is something that leads to. Therefore, the present invention can provide a technology that enables high conversion efficiency and high reliability required for power solar cells, and is an extremely useful invention for the energy industry.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an example of a semiconductor thin film manufacturing apparatus for implementing the present invention.

FIG. 2 is a diagram showing a Raman scattering spectrum of a semiconductor thin film formed by the method shown in Example 1.

FIG. 3 is a diagram showing a Raman scattering spectrum of a semiconductor thin film formed under the conditions shown in Comparative Example 1.

[Explanation of symbols]

1　Electron beam evaporation equipment 2 Ion beam generator 3 Shutter 4 Board 5　Substrate heater 6 Turbo molecular pump, 7 Oil rotary pump, 8 Gas flow meter, 9 Substrate bias power supply

Claims (1)

[Claims] Claim 1: Repeating the step of forming a thin film in an atmosphere containing charged particles and the step of stopping the supply of the thin film forming raw material and further modifying the thin film forming surface in an atmosphere containing charged particles, and The thickness of the thin film formed in one repetition is 1 to 100 Å, and the total film thickness is 30 Å.
Crystalline semiconductor thin film of 0 Å or less