JPH02119125A - Manufacture of amorphous silicon germanium thin film - Google Patents

Abstract

PURPOSE:To improve and stabilize film quality, and make it suitably applicable to a solar cell and the like by precisely controlling the film formation speed at the time of forming an a-SiGe:H thin film. CONSTITUTION:An electric potential A on substrate side and an electric potential B on electrode side are monitored, and a power supply for applying a bias voltage is so controlled that the potential difference (B-A) keeps always a specified value (a practical range is +40V or more and +100V or less). As to material gas, SiH4, Si2H6, Si3H3, SiF4, etc., are used for an Si source, and GeH4, GeF4, etc., are used for a Ge source. These are diluted by H2 and used. A parallel flat plate type plasma CVD system 1 is provided with a reaction chamber 2 and a holder 4 which is positioned at the upper part of the chamber and retains a substrate 3. A lower electrode 5 arranged at the lower part of the chamber is connected with a high frequency power supply 6. Plasma is generated by applying a high frequency voltage to the electrodes, and reaction is accelerated. A power supply 7 for applying a DC bias voltage is connected with the electrode 5 in parallel to the power supply 6. A control unit 8 controls the potential difference between the substrate holder 4 and the electrode 5 so as to keep a specified value.

Description

[Detailed description of the invention]

Industrial Application Field The present invention is directed to amorphous silicon germanium (hereinafter referred to as
a-3iGe (denoted as H) thin film manufacturing method. More specifically, the present invention is a method for producing an amorphous silicon germanium thin film as a narrow bandgap material useful in multilayer structure amorphous silicon solar cells, etc. The present invention relates to a method for manufacturing an amorphous silicon germanium thin film. Conventional technology The band gap (E9) of amorphous silicon (hereinafter referred to as a-3i:H) conventionally used in solar cells has extremely low sensitivity to light with an energy of about 1.756 V or less. (Attempts have been made to lower the band gap by adding group II elements such as Ge.
a-3i:H, that is, a-3iGe:
Since H has excellent long-wavelength sensitivity, it is expected to be used in various applications such as solar cells and imaging devices. a-3iGe:H thin film is Blaz? CVD method, reactive sputtering method, ion blating method, optical C
Production using methods such as the VD method has already been attempted. However, an a-3iGe:H film with photoconductivity excellent enough to be used in solar cells has not yet been realized, and therefore, a practically usable a-3iGe:H product has not been obtained. As a proposal for these fields, a-3iGe
: Patent application No. 60-21342 as a method for producing H thin film
No. 2 (filed on September 26, 1985), or Patent Application No. 1988-6885 (filed on January 16, 1985) for a method for manufacturing amorphous silicon germanium solar cells.
etc. have already been filed by the applicant. Problem to be solved by the invention Figure 2 shows that a-3iG is produced by plasma enhanced chemical vapor deposition.
FIG. 2 is a diagram schematically showing the configuration of an apparatus for producing an e:)i thin film. That is, this parallel plate type plasma CVD apparatus 11 includes a reaction chamber 12, a substrate holder 14 provided above the reaction chamber 12 and supporting a substrate 13, and a lower electrode plate similarly provided below within the reaction chamber 12. 15. The lower electrode 15 is connected to a high frequency power source 16, and is configured to generate plasma in the reaction chamber 12 by applying high frequency to promote the reaction. Further, a DC power supply 16 for applying a bias voltage is connected to the substrate holder 14 . The operation for forming an a-3iGe:H film using this apparatus is as follows. That is, first, the substrate 13 is set on the substrate holder 14, the inside of the reaction chamber 12 is brought to a predetermined high vacuum by operation of a vacuum evacuation system (not shown), and then the raw material gas mixture 18 is brought into the reaction chamber at a predetermined pressure (flow rate). On the other hand, a high frequency power supply 17 and a bias application power supply 16
is operated to generate high-frequency plasma in the reaction chamber, decomposing the source gas and depositing reaction products on the substrate. By the way, the a-3iG in the conventional device as mentioned above
e: In the process of forming the H thin film, a-3iGe
Although there is almost no change in the film properties of the :H film, it is known that the film formation rate changes by about ±15%. This is considered to be because the plasma state fluctuates depending on the state of film attachment to the electrode plate 15 because the electrode plate 15 is in a floating state. Such unexpected fluctuations in film formation rate significantly reduce film thickness controllability when producing multilayer thin film devices such as solar cells, resulting in a problem of reduced reproducibility of device characteristics. Therefore, the purpose of the present invention is to solve the problems of the above-mentioned conventional technology and to develop a novel a-3iGe film that improves the reproducibility of device characteristics of products by enabling precise film thickness control:
An object of the present invention is to provide a method for producing a H thin film. Means for solving the problem, namely, according to the present invention, by plasma enhanced chemical vapor deposition method,
In a method of introducing a low-pressure gas mixture of raw materials including a Si source, a Ge source, and hydrogen into a reaction chamber, applying an external alternating current electric field to obtain plasma, and causing a reaction to form an amorphous silicon germanium thin film on a substrate. , during the film forming operation, a potential difference between a first potential that is a DC potential on the substrate side and a second potential that is a DC potential of an electrode section to which the external AC electric field is applied is kept constant at a predetermined value. Accordingly, there is provided a method for manufacturing an amorphous silicon germanium thin film characterized by controlling a DC bias voltage applied from the outside to the electrode and/or a DC bias voltage applied from the outside to the substrate holder. Further, according to a preferred aspect of the present invention, it is advantageous that the potential difference between the first potential and the second potential is within a range of 40 V or more and 100 V or less. Further, according to one aspect of the invention, the frequency of the external alternating electric field is preferably in the range of I MH2 to 100 MHz. Effect In order to improve the reproducibility of this device characteristic, the present inventors conducted various studies on the causes of plasma state fluctuations, and found that even if the film is formed with the same applied bias voltage, the self-bias potential itself It was found that the value fluctuated by about ±10%, and that it also fluctuated during the -th film formation. Then, it was discovered that this variation in self-bias potential caused the width of the sheath formed on the substrate to be varied by applying a negative DC bias voltage to the substrate holder, and as a result, the rate of film formation on the substrate was varied. reached. That is, according to the present invention, the potential A on the substrate side and the potential B on the electrode side are monitored, and the power supply for bias application is controlled so that the potential difference C (=B-A) is always a predetermined value. There is. Therefore, the width of the sheath formed on the substrate depending on the potential difference C can be made constant, and the width of the sheath on the substrate can be kept constant.
e: Fluctuations in the deposition rate of the H film can be suppressed. As a result, film thickness controllability during device formation is improved,
Reproducibility of device characteristics can be ensured. In addition, here, the value of the potential difference C is specifically +40V or more,
It is preferable that the voltage is +100 V or less. That is, when the potential difference C is lower than +40V, the sheath width of the plasma formed depending on the potential difference C is small and the amount of 5i-H2 in the a-5iGe:H film increases, as will be described later. A good a-3iGe:H film with a high ΔσPh is not formed. Furthermore, if the potential difference C is higher than +100V, the film formation rate will be extremely reduced, which is not practical. In addition, the frequency of the external AC electric field is I MHz ~ 100M
Preferably, it is within the range of Hz. This is because when the frequency is lower than 1 MHz, the ions in the plasma can follow this frequency, so even if the potential difference C mentioned above is made larger than OV, a high quality a-3iGe:H film can be formed on the substrate. This is because no sheath is formed. Also,
When the frequency is higher than 100 MHz, electrodeless discharge is possible and no sheath is formed. In the method of the present invention, as the raw material gas, 5IH4, 5i2H, 5i3He, 5iFs, etc. are used as a Si source, and GeHg, GeFs, etc. are used as a Ge source, and these are generally H2 (dangling bond terminator). It is used diluted with. The present invention will be described in more detail below with reference to the drawings, but the following disclosure is only one example of the present invention and does not limit the technical scope of the present invention in any way. Embodiment FIG. 1 is a diagram schematically showing the configuration of a parallel plate plasma CVD apparatus that can be applied to the method for producing an a-5iGe:H thin film according to the present invention. That is, this parallel plate type plasma CVD apparatus 1 has a reaction chamber 2
, a substrate holder 4 that is provided above the reaction chamber 2 and supports the substrate 3, and a lower electrode plate 5 that is similarly provided below the reaction chamber 2. The lower electrode 5 is connected to, for example, a high frequency power source 6, and is configured to generate plasma in the reaction chamber 2 by applying high frequency to promote the reaction. Further, a DC bias voltage applying power source 7 is connected to the lower electrode 5 in parallel with the high frequency power source 6, and a DC bias voltage control device 8 measures the potential difference between the potential of the substrate holder 4 and the potential of the lower electrode 5. However, the output voltage of the bias application power source 7 can be controlled so that the potential difference becomes a predetermined voltage. The operation for forming an a-3iGe:H film using this apparatus is as follows. First, the substrate 3 is set on the substrate holder 4, and the inside of the reaction chamber 2 is brought to a predetermined high vacuum by operating a vacuum evacuation system (not shown). Next, the raw material gas mixture 9 is supplied into the reaction chamber at a predetermined pressure (flow rate), while the high frequency power source 6, bias application power source 7, and bias voltage control device 8 are operated to generate high frequency plasma in the reaction chamber. The source gas is decomposed and the reaction products are deposited on the substrate. Preparation Example (1) Application of DC bias voltage to substrate and change in plasma state Using the apparatus shown in FIG. 2, application of DC bias voltage to the substrate and change in plasma state by a conventional method was investigated. The film forming conditions are shown in Table 1. Table 1 Gas flow rate 10%Si Hs/H2200 (sccm
]10%GeHn/Hz 40 (sccm) pressure
1 (Torr) Substrate temperature
200 (t)RF power density
0.05 (W/cJ) RF frequency 1
3.56 CM) lz) Figures 3 (a) to (d) show how to vary the DC bias voltage applied to the substrate from +100V to -200V.
up to v, i.e. +100V, OV, -10 respectively
It is a figure which shows the change of a plasma state when it changes to 0V and -200V. As is clear from these figures, as the negative bias voltage is applied, the plasma moves away from the substrate, and the sheath width above the substrate increases. On the other hand, the deposition rate decreases as the sheath width increases;
Compared to the case where the bias voltage OV was applied, the voltage decreased to about 70% when -200V was applied. That is, as the sheath width increases, the film formation rate decreases. This is because the plasma moves away from the substrate, and the number of film-forming species that reach the substrate decreases. Next, film formation was performed several times with the substrate DC bias voltage fixed at -100V, and fluctuations in the DC potential of the lower electrode portion 15 were determined. FIG. 4 is a graph plotting the results. As shown in the figure, although the bias voltage is constant, the potential of the lower electrode fluctuates by more than ±10%. Furthermore, FIG. 5 shows the film deposition rate and the potential difference between the electrodes during the film deposition operation shown in FIG. 4, that is, the difference C (= B-A
) is a graph showing the correlation between As shown in FIG. 5, the film formation rate decreases as the potential difference C increases. This corresponds to an increase in the resheath width due to an increase in the potential difference C. (2) RF electrode, substrate holder potential difference C and a -9i
Ge:H Film Quality Next, the correlation between the potential difference C between the DC potential B of the RF electrode and the DC potential A of the substrate holder portion and the a-3iGe:H film quality was evaluated. The film forming conditions shown in Table 1 were used as they were, the film forming time was 90 minutes, and the substrate was C-3i. 5i-H2 bond and 5i-H bond determined from infrared absorption □
Figure 6 shows the ratio to the total. As the potential difference C increases, the SI H2 bond decreases, and when C≧40V, a good film quality with less SI H2 bond was obtained. That is, when the potential difference C is smaller than 40V, the width of the plasma sheath becomes narrower, and a-3iGe:
The SI H2 bond in the H film increases, Δσ,. This is not preferable because it reduces the Furthermore, when the potential difference C becomes larger than 100 V, the width of the plasma sheath increases rapidly, and the film formation rate decreases significantly, making it impractical. From the above results, it is confirmed that the potential difference C is preferably 40 V or more and 100 V or less. (3) Solar cell output characteristics and reproducibility Film deposition was performed 15 times with the same deposition conditions using the method according to the present invention and the conventional method using the apparatus shown in Figure 2, and there was no variation in the deposition rate. was evaluated. The film forming conditions were the same as those in Table 1, and in the example according to the method of the present invention, the potential difference between the lower electrode 5 and the substrate holder 4 was controlled to be +60V. In the conventional method, a DC bias voltage of -100V was applied to the substrate holder 14. Furthermore, 5US304 was used for the substrate. As a result, the variation in film deposition rate according to the present invention was less than ±5%, whereas the variation in the conventional method was ±10% or more, confirming that the method according to the present invention is effective in controlling the film deposition rate. Ta. Furthermore, similarly, a-5iGe:H solar cells were produced using the method according to the present invention and the conventional method under the same conditions, and their characteristics were evaluated. The a-3iGe:H solar cell was formed in a pin structure on a glass substrate on which a transparent conductive film was deposited. The pS'n layer was formed under the conditions shown in Table 2 for both the method of the present invention and the conventional method. In the present invention, the film forming conditions at the time of forming the i-layer were such that the potential difference between the lower electrode 5 and the substrate holder 4 was controlled to be +60V. On the other hand, in the conventional method, a DC bias voltage of -100V was applied to the substrate holder 4 so that the potential difference between the lower electrode 15 and the substrate holder 14 was initially +60V. Other film forming conditions were set to the values shown in Table 1, and the film forming time was kept constant at 90° so that the film thickness of the i-layer was 2000 Å. Furthermore, in order to improve solar cell characteristics, p/iG
An e-graded layer and an i/n[ye-graded layer that are useful for reducing defects at the i/n interface were provided. The film thickness of this p/iS i/nGe graded layer is such that the first 10 minutes of the above 90 minutes of i-layer deposition time are the p/iGe graded layer and the last 10 minutes are the i/nGe graded layer.
This was controlled by using a graded layer, and the method of the present invention and the conventional method were made the same. Table 2

[P layer] Gas flow rate 10% SiH4/H2100 (sccm: 1
10%CH,/Ha 75 (sccm) 500
1] pmB*Hs/Hz 100 (sccm) Pressure 1 (Torr) Substrate temperature
200 (t) RF power density 0.05 CW/cat) RF frequency 13.56 CM) lz) Film formation time
3 [minutes] (~200 people) [N layer] Gas flow rate 1O%SI H4/ H2100 (sccm
]10001) p+++PH3/Hz 100 (s
ccm) Pressure 2 (Torr) Substrate temperature 200 [℃] RF power density 0.50 CW/c++f) RF frequency 13.56 CM) Iz) Film formation time
6 [minutes] (~500 people) Figure 7 (a) and (
b) is a diagram showing the distribution of output characteristics of the produced solar cells, FIG. 7(a) shows the characteristics of the solar cell produced by the method according to the present invention, and FIG. 7(b) shows the distribution of the output characteristics of the produced solar cells Each corresponds to the characteristics of the solar cell. The measurements were conducted under AM (air mass) conditions of 1.5.100 mW/cd. In the conventional method, due to large variations in film formation speed, p/]
, it can be seen that the thickness controllability of the Ge graded layer or the single layer itself at the i/n interface is poor, and the FF variation is large. This is due to the fact that although the a-3iGe:H film properties have been improved, it is inferior to the a-3i:H film, and the FF fluctuation is large with respect to changes in the film thickness of one layer, and the p/i
This is because the thickness of the I/N interface Ge graded layer greatly affects the FF of the solar cell. In contrast, with the method of the present invention, the variation in film formation rate is small, so solar cells can be manufactured with good reproducibility.
It can be seen that the variation in FF is reduced and the reproducibility is improved. Effects of the Invention As detailed above, a-3iGe:H according to the present invention
The thin film production method allows precise control of the film formation rate, which results in good film quality and stable properties, making it suitable for use in devices such as solar cells.
-5iGe: H film can be manufactured with good reproducibility. The effect is particularly great when used to form multilayer thin film devices such as solar cells that require fine control of film thickness.

[Brief explanation of the drawing]

FIG. 1 is a diagram schematically showing the configuration of a parallel plate electrode type plasma CVD apparatus that can be applied to the implementation of the a-3iGe:H manufacturing method according to the present invention, and FIG. Conventional method using vapor phase deposition method
-3iGe: This is a diagram schematically showing the configuration of an apparatus used when producing a H thin film. FIG. 4 is a diagram schematically showing changes in the plasma state; FIG. 4 is a diagram showing changes in the DC potential of the lower electrode portion 15 when film formation is performed under the same conditions by a conventional method; FIG. This is a diagram showing the correlation between the difference C (=B-A) between the DC potential of the lower electrode part and the DC potential A of the substrate holder and the film formation rate. (infrared absorption characteristics), and FIGS. 7(a) and 7(c) show the case where a-5iGe:H solar cells were produced under the same conditions using the method of the present invention and the conventional method. FIG. 7(a) is a diagram showing the distribution of cell output characteristics of
) corresponds to the method of the present invention, and FIG. 7(b) corresponds to the conventional method. Figure 1 [Main reference numbers] 1111 Parallel plate plasma CVD apparatus, 2.1
2 Reaction chamber, 3.13 Substrate, 4.14 Substrate holder, 5.15 Lower electrode plate, 6.17 High frequency power supply, 7.16 DC power supply for applying DC bias voltage, 8
...Bias voltage control device, 9.18
Raw material gas, 5... Electrode plate patent applicant Sumitomo Electric Industries, Ltd.

Claims (1)

[Claims] By plasma enhanced chemical vapor deposition, a low-pressure gas mixture of raw materials including Si source, Ge source and hydrogen is introduced into a reaction chamber, an external alternating current electric field is applied to obtain a plasma, and a reaction is caused to deposit amorphous silicon on the substrate. In the method for forming a germanium thin film, during the film forming operation, a potential difference between a first potential that is a DC potential on the substrate side and a second potential that is a DC potential of an electrode section to which the external AC electric field is applied is Production of an amorphous silicon germanium thin film characterized by controlling the DC bias voltage applied from the outside to the electrode and/or the DC bias voltage applied from the outside to the substrate holder so that it is kept constant at a predetermined value. Method.