JP5502210B2 - Microcrystalline semiconductor thin film manufacturing method - Google Patents

Description

  The present invention relates to a method for producing a microcrystalline semiconductor thin film, and more particularly to a method for producing a microcrystalline semiconductor thin film such as microcrystalline silicon or microcrystalline silicon germanium used for a photoelectric conversion layer or the like of a silicon-based thin film solar cell.

As one of photoelectric conversion layers of silicon-based thin film solar cells, for example, a microcrystalline silicon thin film is widely used. As a method for producing this microcrystalline silicon film, a plasma CVD (Chemical Vapor Deposition) method using a mixed gas of silane (SiH ₄ ) and hydrogen (H ₂ ) is used on a glass substrate at a low temperature (˜200 ° C.). It is common to deposit (for example, refer patent document 1, nonpatent literature 1).

In a plasma CVD apparatus, a plasma electrode that generates discharge plasma and a stage electrode (electrically grounded) on which a substrate is placed are disposed in a vacuum container so as to face each other. When manufacturing a microcrystalline silicon thin film, a mixed gas of silane (SiH ₄ ) and hydrogen (H ₂ ) is supplied to the vacuum vessel at a constant flow rate, and the gas pressure in the vacuum vessel is adjusted to a desired value. Thereafter, a high frequency voltage of RF (Radio Frequency: 10 to 30 MHz) or VHF (Very High Frequency: 30 to 300 MHz) band is applied to the plasma electrode, and the gap between the plasma electrode and the stage electrode (gap length: 3 to 10 mm) A low-pressure glow discharge is caused to generate SiH ₄ / H ₂ mixed plasma.

At this time, chemically active SiH ₃ , SiH ₂ , SiH molecules, or Si, H atoms generated in the plasma are transported to the substrate, and these adhere to and react with the surface. A silicon thin film is deposited.

In general, in order to deposit a microcrystalline silicon thin film by this plasma CVD method, a film forming technique called “high pressure depletion method” is widely used (see, for example, Patent Document 1 and Non-Patent Document 1). Specifically, the SiH ₄ gas flow rate F [SiH ₄ ] to be supplied is made sufficiently small (in other words, under conditions of relatively high gas pressure (up to several hundred Pa or higher) and high high frequency power (up to several hundred W or higher). H ₂ gas flow rate F [H ₂ ] is increased sufficiently), and the SiH ₄ flow rate ratio = F [SiH ₄ ] / (F [SiH ₄ ] + F [H ₂ ]) is lowered to about 1 to 2%, By depleting the SiH ₄ , a microcrystalline silicon thin film can be deposited. Conversely, when SiH ₄ flow rate ratio = F [SiH ₄ ] / (F [SiH ₄ ] + F [H ₂ ]) is larger than ˜2%, the deposited film becomes an amorphous silicon thin film. .

  The microcrystalline silicon thin film obtained by this method exists as a mixed state of silicon crystal grains having a grain size of several nanometers to several tens of nanometers and silicon in an amorphous state. In the long wavelength region (600 nm or more), the spectral sensitivity characteristics are good and the carrier mobility is high.

  In the solar cell using this microcrystalline silicon thin film for the photoelectric conversion layer, good cell characteristics with a photoelectric conversion efficiency of about 9% are obtained. Also proposed is a tandem cell structure in which a photovoltaic cell whose photoelectric conversion layer is made of a microcrystalline silicon thin film and a solar cell whose photoelectric conversion layer is made of an amorphous silicon film are connected in series. A tandem solar cell using such an amorphous silicon photoelectric conversion layer and a microcrystalline silicon photoelectric conversion layer has a light absorption in a wide wavelength range from ultraviolet to infrared, so that the photoelectric conversion efficiency is 12 to 12%. A practical solar cell module reaching 15% has been reported.

  When a microcrystalline silicon thin film having a film thickness of about 2 to 3 μm is formed as a photoelectric conversion layer of a solar cell, a thin amorphous film called an incubation layer is formed at the initial stage of film formation, regardless of which film formation method is used. A quality silicon film is formed. When nucleation occurs near the surface of the amorphous silicon film, microcrystalline silicon continues to grow from this nucleation. Here, the thickness of the incubation layer is several tens nm to several hundreds nm, and occupies a ratio of about several% of the film thickness of the photoelectric conversion layer.

  An amorphous silicon thin film has a lower charge mobility and a higher electric resistance of the film than a microcrystalline silicon thin film. For this reason, if the incubation layer is formed thick when the microcrystalline silicon thin film is formed, the resistance of the photoelectric conversion layer (that is, the incubation layer + microcrystalline silicon thin film) increases. For this reason, in the photovoltaic cell which has such a photoelectric converting layer, the value of the fill factor (Fill Factor) of an electric current-voltage characteristic became large, and there existed a problem of causing the fall of photoelectric conversion efficiency.

  Further, after the generation of silicon nuclei, the microcrystalline silicon thin film grows in a columnar shape above the base film. The crystallinity of the deposited silicon film is not uniform in the growth direction of the film, that is, the perpendicular direction of the base film, and the crystallinity tends to increase as the film grows. Since crystal growth occurs unevenly in this way, even if the crystallization rate of the deposited microcrystalline silicon film (average value of the entire film) is a value suitable for the photoelectric conversion layer, the film is not near the interface with the base film. The crystallinity of the film is too low, or the crystallinity may be too high near the surface of the film.

  When the crystallinity of the microcrystalline silicon film is too low, since the proportion of amorphous silicon in the film is large, the electric resistance of the film is large, and the photoelectric conversion efficiency is low as described above. On the other hand, when the crystallinity of the microcrystalline silicon film is too high, many crystal grain boundaries exist in the film. When the cross-section of the deposited microcrystalline silicon thin film is observed using a transmission electron microscope, if the crystallinity of the microcrystalline silicon film is too high, the crystal grain boundary passes through the film or along the grain boundary. There are often places where cracks occur. Therefore, after the microcrystalline silicon thin film is deposited on the substrate, when the substrate is taken out from the vacuum container to the atmosphere, impurities such as oxygen, nitrogen, hydrocarbons, etc. from the atmosphere enter the deep part along the crystal grain boundary from the film surface. Until it becomes contaminated. Thus, in the microcrystalline silicon thin film obtained by the conventional method, not only the surface of the film but also the inside is oxidized or contaminated with impurities such as carbon, which adversely affects the characteristics of the solar cell. There was a problem of affecting.

As a method for solving such a problem, a technique for forming a microcrystalline silicon thin film called a SiH ₄ profiling method is known (for example, see Non-Patent Document 2). This technique is characterized in that when depositing a microcrystalline silicon thin film, the flow rate of the SiH ₄ gas to be supplied is not constant during film formation but is changed over time. Hereinafter, the SiH ₄ profiling method will be specifically described.

In the example of SiH ₄ profiling shown in Non-Patent Document 2, the H ₂ gas flow rate is kept constant at F [H ₂ ] = 600 sccm during the film formation of silicon, and the SiH ₄ gas flow rate is gradually changed to F [SiH ₄ ] = It is increased from 4 sccm to 12 sccm. In the normal film formation method, the SiH ₄ gas flow rate is kept constant from the start to the end of film formation, but in this document, the SiH ₄ gas flow rate is F [SiH ₄ ] = 12 sccm (SiH It is shown that a relatively good microcrystalline silicon film can be obtained when the ₄ flow rate ratio is set to F [SiH ₄ ] / (F [SiH ₄ ] + F [H ₂ ]) = 2%).

As an example of the SiH ₄ profiling method, in the initial stage of film formation where an incubation layer is formed (0 <t <20 seconds), in order to easily cause nucleation of crystals, a SiH ₄ gas flow rate condition with a high crystallization rate, That is, F [SiH ₄ ] = 4 sccm is set. At this time, the SiH ₄ flow rate ratio is F [SiH ₄ ] / (F [SiH ₄ ] + F [H ₂ ]) = 0.7%. Next, in the subsequent middle stage of film formation (20 seconds <t <40 seconds), the SiH ₄ gas flow rate is increased to F [SiH ₄ ] = 8 sccm, and the SiH ₄ flow rate ratio is F [SiH ₄ ] / (F [ SiH ₄ ] + F [H ₂ ]) = 1.3% The condition shifts to a condition where relatively high crystallinity is obtained. Thereafter (t> 40 seconds), the SiH ₄ gas flow rate is increased to F [SiH ₄ ] = 12 sccm, and the normal SiH ₄ flow rate ratio F [SiH ₄ ] / (F [SiH ₄ ] + F [H ₂ ]) The film is formed to the end under the condition of = 2%.

According to Non-Patent Document 2, the microcrystalline silicon deposited by increasing the SiH ₄ flow rate ratio F [SiH ₄ ] / (F [SiH ₄ ] + F [H ₂ ]) stepwise in the vicinity of the interface with the base film. It has been shown that the reproducibility of film characteristics can be improved and the uniformity of the crystallization rate distribution in the film thickness direction can be improved. It is also shown that the characteristics of the prototyped solar battery cell are improved in response to the improvement in crystallinity uniformity.

JP 2001-237187 A

T. Matsui, M. Kondo, A. Matsuda, “Origin of the improved performance of high-deposition-rate microcrystalline silicon solar cells by high-pressure glow discharge”, Jpn. J. Appl. Phys., Vol.42, pp .L901-903 (2003) A.H.M. Smets, T. Matsui, M. Kondo, “High-rate deposition of microcrystalline silicon p-i-n solar cells in the high pressure depletion regime”, J. Appl. Phys., Vol.104, 034508 (2008)

However, in the microcrystalline silicon film formation using the conventional SiH ₄ profiling method, the SiH ₄ gas flow rate of the raw material is set to a relatively low value in the initial to intermediate period of the film formation. For this reason, during these film formation periods, the film formation rate of silicon was lowered, resulting in a decrease in the throughput of the manufacturing process of the solar cell, which hindered cost reduction.

In addition, even if the SiH ₄ flow rate is changed rapidly during film formation, the time response of the mass flow controller controlling the flow rate is about 1 second, and in the piping path from the mass flow controller to the vacuum vessel The time for the gas to be transported is about several seconds. This makes it difficult to adjust the flow rate in a short time and to switch the flow rate every short time. As a result, there is a limit to fine crystal control at the initial stage of film formation, and further improvement in uniformity of the crystallization rate has been demanded.

  The present invention has been made in view of the above, and a microcrystalline semiconductor capable of preventing a decrease in film formation speed while maintaining uniform crystallinity along the film thickness direction when forming a microcrystalline semiconductor thin film It aims at obtaining the thin film manufacturing method.

In order to solve the above-described problems and achieve the object, a microcrystalline semiconductor thin film manufacturing method of the present invention is a microcrystalline semiconductor thin film manufacturing method for manufacturing a microcrystalline semiconductor thin film by a plasma CVD method, and includes a plasma electrode . and disposing a board of depositing a microcrystalline semiconductor film in the vacuum vessel, while continuously supplying a gas containing a main component of hydrogen into the vacuum chamber, a semiconductor material gas containing at least silicon or germanium with intermittent supply, and a microcrystalline semiconductor film forming step of forming a microcrystalline semiconductor film by generating a plasma between the plasma electrodes by feeding the high frequency power to the plasma electrode and the substrate, wherein the higher the microcrystalline semiconductor film forming Engineering, said semiconductor material gas periodically supplying the supply of the semiconductor material gas is turned on / off modulation, wherein the semiconductor material gas The high-frequency power when the supply of the semiconductor material gas is turned off is smaller than the high-frequency power when the supply of the semiconductor material gas is turned off, and the modulation frequency of the on / off modulation or the duty of the on / off modulation A thin film is formed while changing the ratio with time.

  According to the present invention, it is possible to obtain a good microcrystalline semiconductor thin film having a uniform crystallization rate in the film thickness direction at high speed, and to achieve high performance and low cost of the thin film solar cell. Have.

FIG. 1 is a diagram schematically showing an example of the configuration of the microcrystalline semiconductor thin film manufacturing apparatus according to the first embodiment of the present invention. FIG. 2 is a diagram showing how H ₂ gas and SiH ₄ gas flow during thin film formation and how the high frequency power in the high frequency power supply changes with time. FIG. 3 is a diagram for explaining the dependency of the film formation rate and the crystallization rate on the modulation frequency. FIG. 4 is a diagram for explaining the dependency of the deposition rate and the crystallization rate on the modulation duty ratio. FIG. 5 is a diagram showing an impurity concentration profile in the microcrystalline silicon thin film according to the first embodiment. FIG. 6 is a cross-sectional view schematically showing a film structure of the microcrystalline silicon thin film solar cell according to the first and second embodiments.

  Hereinafter, preferred embodiments of a method for producing a microcrystalline semiconductor thin film according to the present invention will be described in detail with reference to the drawings. In addition, this invention is not limited to the following description, In the range which does not deviate from the summary of this invention, it can change suitably.

Embodiment 1 FIG.
FIG. 1 is a diagram schematically showing an example of the configuration of the microcrystalline semiconductor thin film manufacturing apparatus according to the first embodiment of the present invention. This microcrystalline semiconductor thin film manufacturing apparatus is based on a conventional plasma CVD apparatus, and a substrate stage 12 and a plasma electrode 13 are provided in a vacuum vessel 10 in which an atmosphere for forming a semiconductor thin film is formed. The plasma electrodes 13 and the substrate stage 12 are arranged so that the opposing surfaces are parallel to each other. A gas exhaust pipe 11 is provided in the vacuum container 10, and the gas in the vacuum container 10 is exhausted by a vacuum pump (not shown) connected to the gas exhaust pipe 11, so that the inside of the vacuum container 10 has a predetermined degree of vacuum. Is set.

  The substrate stage 12 is electrically grounded and has a structure on which a substrate 100 to be subjected to film formation is placed. In addition, a heater is built in the substrate stage 12, and the substrate temperature is set to a predetermined temperature, for example, about 150 to 250 ° C. during the film forming process. Here, the substrate stage 12 is provided on the lower side of the vacuum vessel 10.

  The plasma electrode 13 is composed of a cylindrical side surface portion 131 and a bottom surface opposite to the upper surface portion 133 at one end of the cylindrical structure, and a gas shower head 132 having a plurality of through holes formed on the bottom surface. Have. Further, the vacuum electrode 10 is arranged such that the plasma electrode 13 is disposed above the substrate placement surface of the substrate stage 12 by a predetermined distance so that the gas shower head 132 is parallel to the substrate placement surface of the substrate stage 12. Fixed inside. Here, in the vacuum vessel 10, the cylindrical side surface portion 131 and the upper surface portion 133 of the plasma electrode 13 are electrically insulated from the vacuum vessel 10 by an insulating spacer 14 such as alumina or Teflon (registered trademark). It is fixed.

  Further, the cylindrical upper surface portion 133 of the plasma electrode 13 is electrically connected to the high-frequency power source 40 via a shield box 20 provided corresponding to the position where the plasma electrode 13 is installed and an impedance matching device (not shown). Yes. As a result, a high frequency voltage is applied to the plasma electrode 13. The oscillation frequency of the high frequency power supply 40 is generally 13.56 MHz or 27.12 MHz, but a frequency of 30 to 150 MHz, that is, a VHF band is used in order to increase the plasma density and increase the film formation speed. Sometimes. Since the high-frequency voltage is applied to the plasma electrode 13 in this way, a shield box 20 grounded so as to surround the plasma electrode 13 outside the vacuum vessel 10 is disposed in order to prevent high-frequency radiation and leakage.

The upper surface portion 133 of the plasma electrode 13, SiH ₄ and H ₂ gas supply port 22a and the SiH ₄ gas supply ports 22b for supplying the gas and H ₂ gas are provided separately, they H ₂ Ya SiH ₄ gas to H ₂ gas supply pipe 21a and the SiH ₄ gas supply pipe 21b for supplying, are connected. When plasma is being generated, a high-frequency voltage and a direct current self-bias voltage are simultaneously applied to the upper surface portion 133 of the plasma electrode 13, so that the gas supply pipes 21a and 21b are made of metal (for example, made of SUS). In order to electrically insulate the plasma electrode 13 from the gas supply pipes 21a and 21b, the gas supply ports 22a and 22b have gas flow paths formed inside an insulator block such as alumina or Teflon (registered trademark). Things are used. Further, the gas valve 23a of the air-driven type or an electromagnetic type is on the H ₂ gas supply pipe 21a for turning on / off the flow of H ₂ gas from the H ₂ gas supply unit for supplying the H ₂ gas, not shown, the H ₂ gas A mass flow controller 24a for controlling the flow rate of The gas valve 23a is disposed outside the shield box 20.

SiH ₄ and the gas valve 23b of the air-driven type or an electromagnetic type for turning on / off the flow of SiH ₄ gas from a SiH ₄ gas supply unit for supplying the SiH ₄ gas (not shown) on the gas supply pipe 21b, the flow rate of SiH ₄ gas And an air-driven gas valve 25 for turning on / off the supply of SiH ₄ gas to the vacuum vessel 10 during thin film formation. The gas valve 23 b is disposed outside the shield box 20, and the air-driven gas valve 25 is disposed near the SiH ₄ gas supply port 22 b in the shield box 20. The gas valve 25 is an air drive type, and an air supply pipe 30 for sending compressed air to the gas valve 25 is disposed. The air supply pipe 30 is provided with an electromagnetic gas valve 31 for turning on / off the supply of compressed air outside the shield box 20.

By opening and closing the gas valve 31, the supply of compressed air to the gas valve 25 is turned on / off. When the gas valve 31 is turned on and opened, and compressed air is supplied to the gas valve 25, the gas valve 25 is turned on (opened), and SiH ₄ gas flowing through the SiH ₄ gas supply pipe 21b is supplied into the vacuum vessel 10. can do. Further, it becomes closed gas valve 31 is turned off, when stopped the supply of compressed air to the gas valve 25, gas valve 25 is OFF (closed state), the SiH ₄ gas vacuum vessel 10 through the SiH ₄ gas supply pipes 21b Supply to the inside stops. Thus, since the gas valve 25 is opened and closed by the compressed air, the supply of SiH ₄ gas into the vacuum vessel 10 can be turned on / off at high speed.

  Further, since a high frequency voltage is applied to the plasma electrode 13 during the formation of the thin film, a high frequency is radiated into the shield box 20. For this reason, although a high-frequency voltage is also applied to the metal part of the gas valve 25, malfunction and damage of the gas valve 25 can be suppressed because of the air-driven type that does not have any electromagnetic operation. Further, the compressed air for driving the gas valve 25 is supplied at high speed by an electromagnetic gas valve 31 placed outside the shield box 20.

  An optical window 15 capable of observing the state of the plasma is disposed on the side surface of the vacuum vessel 10 corresponding to the space (plasma generation space) in which the plasma between the substrate stage 12 and the plasma electrode 13 is generated. The optical window 15 is provided with a light emission intensity observation unit 50 for observing the intensity of light emission (for example, Si: 288 nm, SiH: 414 nm) from Si atoms or SiH molecules in the generated plasma. Here, as the emission intensity observation unit 50, an interference filter 51 that selects light emission from Si atoms or SiH molecules from the emission spectrum of the plasma, and light of the Si atoms or SiH molecules selected by the interference filter 51 is converted into an electrical signal. A photomultiplier tube 52.

The thin film manufacturing apparatus also includes a control unit 60 that controls the opening and closing of the gas valve 31 and the high-frequency power source 40. The control unit 60 supplies a valve opening / closing signal for switching on / off of introduction of the SiH ₄ gas into the vacuum vessel 10 at a predetermined cycle to the gas valve 31 and is synchronized with on / off of the supply of SiH ₄ gas. Then, a power modulation signal is supplied to the high frequency power supply 40 so that the output of the high frequency power supplied to the plasma electrode 13 is amplitude-modulated.

Note that a light emission intensity signal indicating the light emission intensity from the Si atoms or SiH molecules observed by the light emission intensity observation unit 50 is sent to the control unit 60. The controller 60 outputs the valve open signal to the gas valve 31 and outputs the delay time until the emission intensity of Si atoms or SiH molecules actually increases and the valve close signal to the gas valve 31, and then actually A delay time until the emission intensity of the Si atom or SiH molecule decreases is obtained, and a power modulation signal is output based on the ON / OFF period of the SiH ₄ gas supply determined by taking this delay time into account.

Next, a microcrystalline semiconductor thin film manufacturing method in the microcrystalline semiconductor thin film manufacturing apparatus having such a configuration will be described. First, after the substrate 100 is set on the substrate stage 12 in the vacuum vessel 10, the inside of the vacuum vessel 10 is evacuated through the gas exhaust pipe 11 to make the inside of the vacuum vessel 10 have a predetermined degree of vacuum. Further, the substrate 100 is heated by the heater of the substrate stage 12 so as to reach a predetermined temperature. In this state, opening the gas valve 23a provided on the H ₂ gas supply pipe 21a, the H ₂ gas is supplied at a predetermined flow rate from the H ₂ gas supply port 22a into the vacuum chamber 10. In this case, H ₂ gas flowing from the H ₂ gas supply port 22a into the vacuum container 10 flows through the cylindrical plasma electrode 13 through the gas shower head 132 at the bottom of the plasma electrode 13, is supplied to the plasma generating space The

On the other hand, with respect to SiH ₄ gas, the gas valve 23b provided on the SiH ₄ gas supply pipe 21b is opened and kept open constantly, but the air-driven gas valve 25 in the shield box 20 is repeatedly opened and closed at a predetermined cycle. Thus, the supply of SiH ₄ gas into the vacuum vessel 10 is turned on / off at high speed. Specifically, in response to a valve opening signal from the control unit 60, the gas valve 31 is first operated to be in an open state, and compressed air is supplied to the gas valve 25. The gas valve 25 is opened by the action of the air pressure, and SiH ₄ gas is supplied into the vacuum vessel 10 (plasma generation space) through the gas shower head 132 of the plasma electrode 13. In response to the valve closing signal from the control unit 60, the gas valve 31 is first operated to be in a closed state, and the supply of compressed air to the gas valve 25 is stopped. The gas valve 25 is closed by the action of air pressure due to the fact that the compressed air is not supplied, and the supply of SiH ₄ gas into the vacuum vessel 10 is stopped.

FIG. 2 is a diagram showing how H ₂ gas and SiH ₄ gas flow during thin film formation and how the high frequency power in the high frequency power supply changes with time. As shown in this figure, but always into the vacuum chamber 10 irrespective of the H ₂ gas is the time supplied at a predetermined flow rate, SiH ₄ gas is supplied into the vacuum chamber 10 during the T _on, It is not supplied into the vacuum vessel 10 during the period of _Toff . The high-frequency power is applied to the plasma electrode 13 with a P _on output during the period of T _on , that is, while the SiH ₄ gas is on, and P _off (> during the period of T _off , that is, when the SiH ₄ gas is off. P _on ) is applied to the plasma electrode 13.

Since SiH ₄ gas is easily dissociated by electron collision in the plasma, if the plasma electron density is too high, not only SiH ₃ that is a precursor of the silicon thin film but also SiH that causes generation of particles due to collision with gas particles. ₂ , SiH and Si are also produced in large amounts, generating particles in the gas phase and forming a silicon film with many defects. Therefore, in order to generate preferable SiH ₃ molecules more selectively, it is effective to set the high frequency power low to keep the plasma density low when the SiH ₄ gas supply is turned on. On the other hand, H ₂ gas is known to be a gas species that is relatively difficult to dissociate. This is because H atoms generated by electron impact dissociation in plasma easily recombine in the gas phase or on the wall of the vacuum vessel 10 or the electrode surface, and thus return to H ₂ molecules. For this reason, in order to increase the density of H atoms in the plasma, it is necessary to increase the electron density in the plasma.

As already mentioned, the film can be crystallized by exposing the amorphous silicon thin film to H ₂ plasma. It is also known that the time required for crystallization can be shortened as the density of H atoms in the plasma increases. Therefore, in order to quickly crystallize an amorphous silicon film using H ₂ plasma, it is necessary to increase the density of H atoms, and the high-frequency power for generating plasma should be as high as possible. Therefore, the time T _{off when} the supply of SiH ₄ gas is turned off and the film surface is crystallized by H ₂ plasma (in FIG. 2, T ₁ <t <T ₂ , T ₃ <t <T ₄ ,...) Then, the high frequency power is set high (P _on <P _off ). For the above reason, during the thin film formation, the flow rate of SiH ₄ gas and the output of high frequency power are controlled as shown in FIG.

Here, in the period T _on, SiH ₄ gas and H ₂ gas is supplied into the vacuum chamber 10, the high frequency power P low SiH ₄ / H ₂ mixed plasma electron density containing many SiH ₃ molecules by _on is generated, An amorphous silicon thin film is formed on the surface of the substrate 100 on the substrate stage 12. Then, the period T _on is completed, at a time T _off, only H ₂ gas is supplied into the vacuum chamber 10, a high H ₂ plasma electron density by the high frequency power P _off (> P _on) are generated, the substrate The amorphous silicon thin film formed on 100 is crystallized in a short time.

As described above, generation of a low density SiH ₄ / H ₂ mixed plasma suitable for silicon deposition and a high density H ₂ plasma suitable for silicon crystallization can be alternately performed at high speed. Then, a period T _on (= T ₁ = T ₃ −T ₂ =...) For turning on the supply of SiH ₄ gas and a period T _off (= T ₂ −T ₁ = T ₄ −T ₃ =) for turning off the supply. ..)) Is adjusted so that microcrystalline silicon can be formed even under film formation conditions (ie, high SiH ₄ flow rate ratio) that enable high-speed film formation by the conventional film formation method but become an amorphous silicon film. A thin film can be deposited.

Here, in the microcrystalline semiconductor thin film manufacturing method according to the first embodiment, when the microcrystalline silicon thin film is deposited on the substrate using the SiH ₄ / H ₂ mixed plasma, on / off modulation of the supply of SiH ₄ gas is performed. Time modulation is applied to the supply (feeding) of high-frequency power, both time modulations are synchronized, and the modulation frequency or duty ratio of on / off modulation is changed over time during film formation. The duty ratio of the on / off modulation is T _on / (T _on + T _off ). In the first embodiment, the on / off modulation of the supply of SiH ₄ gas into the vacuum chamber 10 is performed using the gas valve 25 that is opened and closed by compressed air, so that the modulation frequency of the on / off modulation or the on / off modulation is performed. The modulation duty ratio can be accurately performed at high speed. Thereby, the crystallinity of the film can be controlled with almost no change in the film forming speed, and a microcrystalline silicon thin film having a uniform crystallization rate in the film thickness direction can be deposited at a high speed. As a result, the throughput of the manufacturing process of the microcrystalline silicon thin film can be improved. For example, the throughput of the manufacturing process of the solar cell used as the photoelectric conversion layer can be improved.

Further, when the on / off time of the SiH ₄ gas supply becomes relatively short, it becomes difficult to modulate the high-frequency power in synchronization with this accurately. Therefore, from the time when the control unit 60 issues a valve opening / closing signal to the gas valve 31, the time delay in which the flow rate of SiH ₄ gas actually increases / decreases into the vacuum vessel 10 is correctly known, and the time delay is taken into consideration. It is desirable to determine the timing for amplitude modulation of the high frequency power. Therefore, as shown in FIG. 1, the emission intensity observation unit 50 monitors the temporal change in the emission intensity of the Si atoms or SiH molecules caused by SiH ₄ .

By monitoring the temporal change in the light emission intensity of Si atoms or SiH molecules caused by SiH ₄ , for example, when the valve opening signal is output from the control unit 60 at t = 0, the time point t ₁ when the SiH light emission intensity starts increasing. From this, it is possible to correctly know the time delay (= t ₁ ) until the SiH ₄ gas actually flows into the plasma. Conversely, for example, after the valve closing signal is output from the control unit 60 at t = t ₂ , the time when the supply of SiH ₄ gas is turned off (= t ₃ −t ₂₎ from the time point t ₃ at which the SiH emission intensity starts to decrease. ) Correctly. As described above, it is possible to accurately know the time delay using the monitoring result by the light emission intensity observation unit 50, and it is possible to determine the timing for amplitude-modulating the high-frequency power in consideration of the time delay.

After the microcrystalline silicon thin film having a predetermined thickness is formed as described above, the gas valve 23a provided on the H ₂ gas supply pipe 21a and the gas valve 23b provided on the SiH ₄ gas supply pipe 21b are closed. Then, after the heater of the substrate stage 12 is turned off and the inside of the vacuum vessel 10 is sufficiently evacuated, the pressure is returned to atmospheric pressure, the substrate 100 is transferred from the substrate stage 12 to the outside of the vacuum vessel 10, and the thin film forming process is completed.

Next, modulation of characteristics (deposition rate DR, crystallization rate I _c / I _a ) of the microcrystalline silicon film formed on the glass substrate using the above-described microcrystalline semiconductor thin film manufacturing method (SiH ₄ gas pulse method) Evaluation of frequency dependency and modulation duty ratio dependency will be described with reference to FIGS. FIG. 3 is a diagram for explaining the dependency of the film formation rate and the crystallization rate on the modulation frequency. FIG. 3A is a characteristic diagram showing the relationship between the modulation frequency F and the film formation rate DR. FIG. 3B is a characteristic diagram showing the relationship between the modulation frequency F and the crystallization rate I _c / I _a . FIG. 4 is a diagram for explaining the dependency of the deposition rate and the crystallization rate on the modulation duty ratio. FIG. 6 is a characteristic diagram showing a relationship between _a modulation duty ratio R, a film formation rate DR, and a crystallization rate I _c / I _a . FIG. 4A is a characteristic diagram showing the relationship between the modulation duty ratio R and the film deposition rate DR. FIG. 4B is a characteristic diagram showing the relationship between the modulation duty ratio R and the crystallization rate I _c / I _a .

The film forming conditions are as follows: the SiH ₄ gas flow rate is F [SiH ₄ ] = 20 sccm, the H ₂ gas flow rate is F [H ₂ ] = 980 sccm (that is, the SiH ₄ flow rate ratio is F [SiH ₄ ] / (F [SiH ₄ ] + F [H ₂ ]) = 2%), the pressure is 1000 Pa, the high frequency power when the SiH ₄ gas supply is on (frequency 60 MHz) is P _on = 100 W, the power when _off is P _off = 300 W, and the plasma The distance between the electrode 13 and the substrate 100 was 6 mm, the temperature of the substrate stage 12 was set to 200 ° C., and a silicon thin film was deposited on the substrate.

Further, in the evaluation of the frequency dependence, the frequency F of the on / off modulation of the SiH ₄ gas supply was changed in the range of F = 1 to 5 Hz (see FIG. 3). The modulation duty ratio R at this time was fixed to R = 50%. On the other hand, for the evaluation of the duty ratio dependency, the duty ratio R of the on / off modulation of the SiH ₄ gas supply was changed in the range of R = 10 to 80% (see FIG. 4). The modulation frequency at this time was fixed at F = 2 Hz. The SiH ₄ gas flow rate F [SiH ₄ ] = 20 sccm is a time average value.

When the silicon film is formed under the above conditions, the film is formed even if the on / off modulation frequency F and the duty ratio R of the SiH ₄ gas supply are changed as shown in FIGS. 3 (a) and 4 (a). The speed was almost constant and DR was in the range of 1.4 to 1.8 nm / s. This is almost the same as the film formation rate value (DR = 1.5 nm / s) at the time of normal continuous wave (CW) film formation in which SiH ₄ gas is continuously supplied. Here, in the case of CW film formation, the high-frequency power is set to P _on = 100 W constant.

On the other hand, regarding the crystallinity of the film, the film deposited at the time of CW film formation was in a completely amorphous state. However, when the SiH ₄ gas supply or high-frequency power was modulated, the deposited film became amorphous. It turned out to be microcrystalline silicon, and its crystallinity was strongly dependent on the modulation frequency F and the duty ratio R.

The peak intensity ratio I _c / I _a (which is defined as the crystallization rate) of the crystalline silicon peak I _c at 520 cm ⁻¹ to the amorphous silicon peak I _{a at} 480 cm ⁻¹ measured by Raman spectroscopy. As shown in FIGS. 3B and 4B, the crystallinity of the amorphous silicon film during the CW film formation is I _c / I _a = 0.44. However, when the modulation frequency is set to F = 1.5 Hz and the duty ratio is set to R = 50%, a microcrystalline silicon film is obtained, and the crystallization rate is I _c / _Ia ~ 11 has been reached. The peak intensity ratio I _c / I _a represents the degree of crystallization in the silicon thin film. When this value is 5 or more and 10 or less, sufficient crystallization is possible when used as a solar cell. Is believed to have a rate.

Further, as shown in FIG. 3B, when the modulation duty ratio R is fixed to R = 50% and the modulation frequency F is increased from F = 1.5 Hz to 5 Hz, the crystallization rate of the silicon film is increased. Decreased monotonically from I _c / I _a ˜11 to ˜0.5, and was almost the same value as that during CW film formation at F = 5 Hz.

Further, as shown in FIG. 4B, when the modulation frequency is fixed to F = 2 Hz and the modulation duty ratio R is increased from R = 10% to 80%, R = 10-30% The crystallization rate I _c / I _a hardly changes at about -10, but when R exceeds that, the crystallization rate I _c / I _a decreases monotonously to ˜0.5, and CW formation is achieved at R = 80%. The value was almost the same as the film.

As described above, in the microcrystalline silicon film formation using the SiH ₄ gas pulse method, the SiH ₄ gas supply on / off modulation modulation frequency and duty are fixed even though the SiH ₄ and H ₂ gas flow rates are fixed. By adjusting the ratio, it can be said that only the crystallinity of the film can be controlled without substantially affecting the silicon deposition rate. The modulation frequency F is preferably greater than 1 Hz and less than or equal to 5 Hz. The duty ratio R is preferably in the range of 10% to 80%.

Next, an example of the microcrystalline semiconductor thin film manufacturing method according to the first embodiment described above, that is, an example of the microcrystalline silicon thin film by the SiH ₄ gas pulse method that changes the modulation frequency of on / off modulation with time during film formation will be described. To do. In the microcrystalline silicon film formation by the SiH ₄ gas pulse method described above, the high frequency power is turned on at t = 0 seconds, and the on / off modulation of the SiH ₄ gas supply is performed until t = 5 seconds after the film formation starts. The modulation frequency F is set to F = 1.5 Hz (step 1), and then F = 2.0 Hz (step 2) for 5 seconds <t <15 seconds and F = 2 for 15 seconds <t <60 seconds. The film was formed at .5 Hz (step 3) and after t = 60 seconds, F = 3.0 Hz (step 4). Here, the duty ratio R of the on / off modulation is constant at R = 50%. Further, the high frequency power when the SiH ₄ gas supply is on is P _on = 100 W, and the power when the SiH ₄ gas supply is _off is P _off = 300 W.

As described above, the modulation frequency F is increased stepwise from F = 1.5 Hz to 3.0 Hz in 60 seconds after the film formation starts, but the microcrystalline semiconductor thin film manufacturing according to the first embodiment is performed. In the apparatus, the values of the modulation frequency F and the high frequency power can be changed instantaneously. Further, the time required to switch each step is the residence time of the SiH ₄ gas in the plasma generation region between the plasma electrode 13 and the substrate 100, which can be estimated to be approximately ˜tens of ms. Therefore, in this film formation method, the time required for switching between each step of film formation (up to several tens of ms) is sufficiently smaller than the step time (minimum 5 seconds), and the switching time can be ignored.

Other conditions for film formation are the same as in the case of FIG. 3, SiH ₄ gas flow rate F [SiH ₄ ] = 20 sccm, H ₂ gas flow rate F [H ₂ ] = 980 sccm (the SiH ₄ flow rate ratio is F [SiH ₄ ] / [F [SiH ₄ ] + F [H ₂ ]] = 2%), pressure: 1000 Pa, high frequency power when SiH ₄ gas supply is on: P _on = 100 W, power when _off : P _off = 300 W, plasma electrode The distance from the substrate was 6 mm, and the substrate temperature was 200 ° C.

By the way, in the present embodiment, since the film forming conditions and the pulse conditions are determined as described above, from the relationship between the crystallization rate I _c / I _a and the modulation frequency F (see FIG. 3B), step 1 is performed. (0 <t <5 seconds), I _c / I _a ˜11, step 2 (5 seconds <t <15 seconds), I _c / I _a -10, step 3 (15 seconds <t <60 seconds), I _c / I _a ˜8, step 4 (60 seconds <t) is expected to be I _c / I _a ˜7. As described above, in step 1 and step 2 in the initial stage of film formation, conditions are set such that crystal nucleation and crystal growth are likely to occur on the film surface in order to suppress the generation of the incubation layer as much as possible.

When film formation was performed for a total of 1200 seconds under the above conditions, a silicon thin film having a thickness of ˜2.0 μm was obtained. When the average film formation rate for 1200 seconds was calculated from the film thickness, the film formation rate DR = 1.67 nm / s (= 2 μm / 1200 s). This value is larger than the film formation speed (film formation speed DR = 1.5 nm / s) at the time of CW film formation in which SiH ₄ gas is continuously supplied, which is convenient for improving the throughput of the film formation process. In this embodiment, the modulation frequency F is increased from F = 1.5 Hz to 3.0 Hz during the film formation, but corresponds to the value of the frequency F at each step as shown in FIG. The film forming speed is slightly larger than the value at the time of CW film forming, and the film forming speed DR is in the range of 1.7 to 1.5 nm / s. For this reason, it is considered that the average film formation rate during film formation is slightly higher than that in the case of CW film formation.

Further, the crystallization rate of the entire silicon film obtained by Raman spectroscopy, that is, the value of the peak intensity ratio of a peak of crystalline silicon in 520 cm ^-1 to the peak of the amorphous silicon at _{^{480cm -1, I c / I a}} = It was 7.2, and a good-quality microcrystalline silicon thin film that could be used as a solar cell could be formed.

  Since it is difficult to directly evaluate the uniformity of crystallinity in the film thickness direction, the concentration profile of impurities (oxygen and carbon) in the film was examined by SIMS (Secondary Ion Mass Spectrometry) analysis. As already described, since the microcrystalline silicon thin film grows in a columnar shape above the base film, the crystallinity tends to increase as the film grows. When the crystallinity near the surface is too high, impurities such as oxygen, nitrogen, hydrocarbons, etc. enter the deep part of the film from the atmosphere along the crystal grain boundaries that exist near the surface and contaminate the inside of the film. It becomes like this. Therefore, the uniformity of crystallinity of the film can be indirectly known from the concentration profile of impurities such as oxygen.

FIG. 5 shows the SIMS analysis results of the microcrystalline silicon thin film obtained by the film forming method of this example. FIG. 5 is a diagram showing an impurity concentration profile in the microcrystalline silicon thin film according to the first embodiment. From FIG. 5, the oxygen (O) and carbon (C) concentrations of impurities are almost constant in the film, and the values detected in the film (oxygen concentration [O] = 4 × 10 ¹⁸ cm ⁻³ , carbon The concentration [C] to 10 ¹⁷ cm ⁻³ ) is sufficiently low. Note that it is said that a device grade microcrystalline silicon thin film has an oxygen concentration [O] of 10 ¹⁸ cm ⁻³ and a carbon concentration [C] of 10 ¹⁷ cm ⁻³ . Thus, in the microcrystalline silicon thin film obtained in this example, intrusion of impurities such as oxygen from the film surface is sufficiently suppressed, and the distribution of crystallinity from the film surface to the inside is uniform. It can be said. That is, it can be said that the microcrystalline silicon thin film obtained by the present invention is suitable for a photoelectric conversion layer of a solar battery cell.

  Using this microcrystalline silicon thin film, a solar battery cell was produced and used as the solar battery cell of Example 1. FIG. 6 is a schematic cross-sectional view showing the structure of the thin-film solar cell of Example 1 manufactured using the microcrystalline silicon thin film formed by the above-described method as a photoelectric conversion layer.

In FIG. 6, a transparent surface electrode 202 having translucency is formed on a glass substrate 201 positioned on the light receiving surface side. The surface electrode 202 is formed by depositing an aluminum-doped zinc oxide (AZO) film 202b on the surface of a tin oxide (SnO ₂ ) film 202a. The AZO film 202b was deposited by DC sputtering, and the film thickness was 45 nm. On the surface electrode 202, a photoelectric conversion unit 203 which is a so-called pin battery cell unit is formed. The photoelectric conversion unit 203 includes, from the surface electrode 202 side, p-type microcrystalline silicon (p layer) 203a doped with boron (B), i-type microcrystalline silicon (i layer) 203b that is a photoelectric conversion layer, phosphorus (P ) Doped n-type microcrystalline silicon (n layer) 203c.

  Carriers generated in the i layer 203b by light incident through the glass substrate 201 and the surface electrode 202 drift to the n layer 203c side and holes to the p layer 203a side by an internal electric field. As a result, an electromotive force is generated between the p layer 203a and the n layer 203c.

  On the photoelectric conversion unit 203, a back electrode 204 is formed to reflect the light that has passed through the photoelectric conversion unit 203 and return it to the photoelectric conversion unit 203 again, and collect electrons drifting to the n layer 203c. . The back electrode 204 is configured by sequentially laminating a translucent AZO thin film 204a and a silver (Ag) thin film 204b from the photoelectric conversion unit 203 side. Here, the film thickness of the AZO thin film 204a was 90 nm.

In the film formation of the p layer 203a and the n layer 203c of the photoelectric conversion unit 203, the film thickness of the p layer 203a is set by using a normal plasma CVD method (that is, supplying all SiH ₄ gas and high frequency power by CW). The film thickness of 15 nm and the n layer 203c was 40 nm. Here, in the formation of the p-layer 203a, a small amount of B ₂ H ₆ gas is added to SiH ₄ / H ₂ gas (F [SiH ₄ ] / F [H ₂ ] = 10/1000 sccm) (concentration˜0. The p layer 203a was formed under the conditions of about 002%), total pressure: 800 Pa, high frequency power: 200 W, and substrate temperature: 200 ° C.

Further, in forming the n layer 203c, a small amount of PH ₃ gas is added to SiH ₄ / H ₂ gas (F [SiH ₄ ] / F [H ₂ ] = 10/1000 sccm) (concentration of about 0.002%). ), N layer 203c was formed under the conditions of total pressure: 1000 Pa, high frequency power: 200 W, substrate temperature: 200 ° C.

On the other hand, in the film formation of the i layer 203b that is a photoelectric conversion layer, the above-described SiH ₄ gas pulse method is used while changing the modulation frequency of on / off modulation from F = 1.5 Hz to 3.0 Hz during film formation. A microcrystalline silicon thin film having a thickness of ˜2 μm was formed.

Further, during the period from the start to the end of film formation (1200 seconds), except that the modulation frequency F of the on / off modulation of the SiH ₄ gas supply is fixed to F = 3 Hz and the duty ratio R is fixed to R = 50%. Produced a microcrystalline silicon thin film of a comparative example in the same manner as in the above example. And the photovoltaic cell was produced using the microcrystalline silicon thin film of the obtained comparative example, and it was set as the photovoltaic cell of the comparative example 1. And as a characteristic of the photovoltaic cell of Example 1 and Comparative Example 1 obtained in this way, the short circuit current density Jsc (mA) when irradiated with artificial sunlight of AM1.5 (light amount: 100 mW / cm ² ). / Cm ² ), open circuit voltage Voc (V), fill factor F.V. F. (%) And photoelectric conversion efficiency η (%) were measured. The results are shown in Table 1.

  As shown in Table 1, the solar cell of Example 1 has a short circuit current density Jsc, an open circuit voltage Voc, a fill factor F.V. F. It can be seen that the photoelectric conversion efficiency η is larger than that of Comparative Example 1, good cell characteristics are obtained, and good solar cells are realized. In particular, the fill factor F.I. F. Is greatly improved as compared with Comparative Example 1 because the thickness of the amorphous incubation layer formed in the initial stage of film formation is reduced and the series resistance value of the microcrystalline silicon layer is reduced. Conceivable.

As described above, according to the first embodiment, when the microcrystalline silicon thin film is deposited on the substrate using the SiH ₄ / H ₂ mixed plasma, the on / off modulation of the supply of SiH ₄ gas and the high-frequency power are controlled. Time modulation is applied to the supply, both time modulations are synchronized, and the modulation frequency or duty ratio of the on / off modulation is changed over time during film formation. Thereby, the crystallinity of the film can be controlled with almost no change in the film forming speed, and a microcrystalline silicon thin film having a uniform crystallization rate in the film thickness direction can be deposited at a high speed. As a result, the throughput of the manufacturing process of the microcrystalline silicon thin film can be improved. For example, the throughput of the manufacturing process of the solar cell used as the photoelectric conversion layer can be improved.

In the conventional SiH ₄ profiling method, the time required to change the SiH ₄ flow rate is determined by the response time of the mass flow controller that controls the gas flow rate and the transport time of the gas from the mass flow controller to the vacuum vessel. Needed. For this reason, each step time of film formation profiling has to be set to a time of at least ˜5 seconds or more, more preferably ˜10 seconds or more. Therefore, when the film formation rate is high and a considerable amount of film is deposited even in a short time, it is difficult to increase the number of film formation profiling steps in order to precisely control the distribution of the crystallization rate.

  On the other hand, in the thin film manufacturing method of the present invention, the modulation frequency and duty ratio of the gas supply and the high-frequency power supply can be changed at high speed, and the time required for switching the modulation frequency for on / off modulation of the gas supply is approximately ˜several. 10 ms. For this reason, each step time of film formation profiling can be shortened to ˜1 second or less, and more stages of film formation profiling can be performed at the initial stage of film formation in which high-precision crystal control is required. Thereby, it has the effect that the uniformity of the crystallinity of a film thickness direction can be improved more.

  Note that in the above, parameters such as gas flow rate, pressure, and power are fixed, but the deposition conditions for the microcrystalline silicon film are not limited to these values.

In the above description, a method for manufacturing a microcrystalline silicon film using H ₂ as a crystallization promoting gas and SiH ₄ as a semiconductor material gas has been described. However, an inert gas such as He, Ne, or Ar is used as the H ₂ gas. May be added. Further, the material gas is not limited to SiH ₄ but may be other gas containing Si, for example, Si ₂ H ₆ , diborane (B ₂ H ₆ ), phosphine (PH ₃ ), arsine (AsH). A dopant gas represented by ₃ ) may be added.

In addition to the microcrystalline silicon, the same effect can be obtained in the formation of microcrystalline silicon germanium (Si _x Ge _1-x ). In this case, a mixed gas of SiH ₄ and GeH ₄ may be used as the semiconductor material gas. Further, the emission intensity observation unit 50 may observe light emission from Si or SiH or Ge or GeH in the plasma.

Embodiment 2. FIG.
In the first embodiment described above, in the initial stage of deposition of the microcrystalline silicon thin film, the duty ratio R of the on / off modulation of the SiH ₄ gas supply time varying the frequency F of the SiH ₄ gas supply on / off modulation as constant has described the case where not a film is formed is, even over time changing the duty ratio R of the SiH ₄ gas supply on / off modulation frequency F on / off modulation of the SiH ₄ gas supply a constant reversed The uniformity of crystallinity can be improved. In the second embodiment, an example in which the duty ratio R is changed at the initial stage of film formation of a microcrystalline silicon thin film using the microcrystalline semiconductor thin film manufacturing apparatus shown in FIG. 1 will be specifically described.

In the microcrystalline silicon thin film by the microcrystalline semiconductor thin film manufacturing method (SiH ₄ gas pulse method) according to the first embodiment described above, for example, t = 5 seconds after the high frequency power is turned on at t = 0 seconds and the film formation starts. The duty ratio of ON / OFF modulation of the SiH ₄ gas supply is set to R = 20% during the period up to R = 20% (step 1), and then R = 50% (step 2) for 5 seconds <t <15 seconds, 15 seconds Film formation was performed with R = 60% (step 3) for <t <60 seconds and R = 70% (step 4) after t = 60 seconds. Here, the modulation frequency F of the on / off modulation of the SiH ₄ gas supply was constant at F = 2 Hz. Other film forming conditions are the same as those in the first embodiment, and a description thereof is omitted here.

By the way, in this embodiment, since the film forming conditions and the pulse conditions are determined as described above, from the relationship between the crystallization rate I _c / I _a and the duty ratio R (see FIG. 4B), step 1 is performed. (0 <t <5 seconds) I _c / I _a -10, step 2 (5 <second t <15 seconds) I _c / I _a -8, step 3 (15 seconds <t <60 seconds) I _c / I _a ˜7, step 4 (60 seconds <t) is expected to be I _c / I _a ˜6. As in the first embodiment, in steps 1 and 2 at the initial stage of film formation, in order to suppress the generation of the incubation layer as much as possible, conditions are set such that crystal nucleation and crystal growth are likely to occur on the film surface.

  Further, from the relationship between the film formation speed DR and the duty ratio R (see FIG. 4A), the film formation speed corresponding to the value of the duty ratio R of each step (R = 20 to 70%) is DR = The range is 1.7 to 1.5 nm / s, which is slightly larger than the value at the time of CW film formation.

Under the above conditions, a microcrystalline silicon thin film having a thickness of ˜2 μm was formed. And the solar cell using this microcrystalline silicon thin film for the photoelectric converting layer was produced, and it was set as the solar cell of Example 2. The film structure of the battery cell and the film formation method are the same as those described in Example 1, and are omitted here. Further, in the film formation of the i layer 203b which is a photoelectric conversion layer, the modulation frequency F of the on / off modulation of the SiH ₄ gas supply is set to F = 2 Hz until the film formation starts and ends (1200 seconds). A microcrystalline silicon thin film of a comparative example was manufactured in the same manner as in Example 2 except that the duty ratio R was fixed to R = 70%. And the photovoltaic cell was produced using the microcrystalline silicon thin film of the obtained comparative example, and it was set as the photovoltaic cell of the comparative example 2. Then, as the characteristics of the solar cells of Example 2 and Comparative Example 2 obtained in this way, the short-circuit current density Jsc (mA) when irradiated with AM1.5 pseudo-sunlight (light amount: 100 mW / cm ² ). / Cm ² ), open circuit voltage Voc (V), fill factor F.V. F. (%) And photoelectric conversion efficiency η (%) were measured. The results are shown in Table 2.

  As shown in Table 2, the solar cell of Example 2 has a short circuit current density Jsc, an open circuit voltage Voc, a fill factor F.I. F. It can be seen that the photoelectric conversion efficiency η is larger than that of Comparative Example 2 and good cell characteristics are obtained, and a favorable solar cell is realized. In particular, the fill factor F.I. F. Is significantly improved as compared with Comparative Example 2 because the thickness of the amorphous incubation layer formed in the initial stage of film formation is reduced and the series resistance value of the microcrystalline silicon layer is reduced. Conceivable.

  Note that in the above, parameters such as gas flow rate, pressure, and power are fixed, but the deposition conditions for the microcrystalline silicon film are not limited to these values.

In the above description, a method for manufacturing a microcrystalline silicon film using H ₂ as a crystallization promoting gas and SiH ₄ as a semiconductor material gas has been described. However, an inert gas such as He, Ne, or Ar is used as the H ₂ gas. May be added. Further, the material gas is not limited to SiH ₄ but may be other gas containing Si, for example, Si ₂ H ₆ , diborane (B ₂ H ₆ ), phosphine (PH ₃ ), arsine (AsH). A dopant gas represented by ₃ ) may be added.

In addition to the microcrystalline silicon, the same effect can be obtained in the formation of microcrystalline silicon germanium (Si _x Ge _1-x ). In this case, a mixed gas of SiH ₄ and GeH ₄ may be used as the semiconductor material gas. Further, the emission intensity observation unit 50 may observe light emission from Si or SiH or Ge or GeH in the plasma.

  As described above, the method for producing a microcrystalline semiconductor thin film according to the present invention is useful for producing a solar cell having a crystalline semiconductor thin film such as microcrystalline silicon or microcrystalline silicon germanium in a photoelectric conversion layer. Moreover, it can contribute to the performance improvement and cost reduction of a thin film solar cell.

DESCRIPTION OF SYMBOLS 10 Vacuum vessel 11 Gas exhaust pipe 12 Substrate stage 13 Plasma electrode 14 Insulating spacer 15 Optical window 20 Shield box 21a, 21b Gas supply pipe 22a, 22b Gas supply port 23a, 23b, 31 Gas valve 24a, 24b Mass flow controller 25 Gas valve 30 Air supply Tube 40 High-frequency power supply 50 Emission intensity observation unit 51 Interference filter 52 Photomultiplier tube 60 Control unit 100 Substrate 131 Side surface 132 Gas shower head 133 Upper surface portion 201 Glass substrate 202 Surface electrode 202a Tin oxide film 202b Zinc oxide doped with aluminum ( AZO) film 203a p-type microcrystalline silicon film (p layer)
203b i-type microcrystalline silicon film (i layer)
203c n-type microcrystalline silicon film (n layer)
204 Back electrode 204a AZO film 204b Silver (Ag) thin film

Claims (7)

A microcrystalline semiconductor thin film manufacturing method for manufacturing a microcrystalline semiconductor thin film by a plasma CVD method,
Disposing a board of depositing a microcrystalline semiconductor film in a vacuum vessel having a plasma electrode,
While continuously supplying a gas containing a main component of hydrogen into the vacuum container, thereby intermittently supplying the semiconductor material gas containing at least silicon or germanium, the plasma by feeding high frequency power to the plasma electrode and a microcrystalline semiconductor film forming step of forming a microcrystalline semiconductor thin film by generating plasma between the electrode substrate,
The higher the microcrystalline semiconductor film forming Engineering, the high frequency power when the supply of the semiconductor material gas on / off modulating the semiconductor material gas periodically supplied, and to turn on the supply of the semiconductor material gas Is made smaller than the high-frequency power when the semiconductor material gas supply is turned off, and the thin film is formed while changing the modulation frequency of the on / off modulation or the duty ratio of the on / off modulation over time about,
A method for producing a microcrystalline semiconductor thin film. The modulation frequency is greater than 1 Hz;
The method for producing a microcrystalline semiconductor thin film according to claim 1. Increasing the modulation frequency over time;
The method for producing a microcrystalline semiconductor thin film according to claim 1 or 2 . The modulation frequency is greater than 1 Hz and in the range of 5 Hz or less;
The method for producing a microcrystalline semiconductor thin film according to claim 3. Increasing the duty ratio over time;
The method for producing a microcrystalline semiconductor thin film according to claim 1 or 2 . The duty ratio is in the range of 10% to 80%;
The method for producing a microcrystalline semiconductor thin film according to claim 5. Prior to the microcrystalline semiconductor thin film formation step, the timing of switching on / off modulation control of the semiconductor material gas supply and the timing of on / off modulation of the semiconductor material gas supply into the vacuum chamber are shifted. Further including a deviation detecting step for obtaining
In the microcrystalline semiconductor thin film forming step, there is a time lag between the intensity modulation timing of the high frequency power applied to the plasma electrode and the switching timing of the on / off modulation control of the semiconductor material gas supply based on the deviation. Setting
The method for producing a microcrystalline semiconductor thin film according to any one of claims 1 to 6 .

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