JPS6273624A - Manufacture of amorphous silicon germanium thin film - Google Patents

Abstract

PURPOSE:To contrive the remarkable improvement in photoelectric characteristic by applying a negative D.C. bias voltage of -100V or over to a holder part of a substrate during the film formation. CONSTITUTION:An Si source, a Ge source and a low-pressure gas mixture of row materials including hydrogen are introduced into a reactor by plasma CVD technique. An external electric field is applied to obtain plasma and a reaction is promoted to form an a-SiGe:H thin film. During the operation of forming the film, a negative D.C. bias voltage of -100 or over is applied to a substrate holder part. As a material gas, SiH4, Si2H6, Si3H6, and SiF4 for an Si source and GeH4 and GeF4 for a Ge source are used and these are usually dilluted by H2 (dangling bond terminator). It can be carried out by using a parallel flat type plasma CVD device. The photoelectric characteristic of a-SiGe: H thus obtained is improved exceedingly and the practical application of that becomes possible.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a method for producing amorphous silicon germanium thin films. More specifically, the present invention relates to a method for improving the film quality of an amorphous silicon germanium thin film as a narrow bandgap material useful in multilayer amorphous silicon solar cells and the like.

2. Description of the Related Art A typical example of an electronic device that utilizes the photovoltaic effect is a solar cell. These solar cells convert solar energy or other light energy into electrical energy, and are attracting attention as a clean energy source as part of future energy measures.

The above energy conversion by solar cells utilizes the photovoltaic effect, which is one of the basic characteristics of semiconductor heterojunctions, pn or pln junctions, Schottky junctions, etc., and absorbs incident light. It is based on a mechanism in which electron-hole pairs are generated there and taken out to the outside.

Amorphous semiconductors, such as amorphous silicon (hereinafter referred to as a-3i:H), can be made into thin films and have a large area, have a large degree of freedom in composition, and have electrical and optical properties that can be controlled over a wide range. It is attracting attention as a material for devices. In particular, the absorption coefficient for light near the peak of the solar energy distribution is larger than that of Si crystals, the thin film formation temperature is low, the film can be formed directly from raw materials by glow discharge, and bond formation is easy. Because of this, it is attracting attention as a solar cell material.

Design and design of photovoltaic devices such as solar cells
What is important in manufacturing is to carefully consider the selection and combination of materials that can achieve high photoelectric conversion efficiency, and also to devise ways to increase absorption efficiency over a wide wavelength range. That's true. This makes it possible to realize highly efficient and commercially viable products.

By the way, the a that was conventionally used in this solar cell
-3i: The bandgap (E,) of H is approximately 1.75
eν, and the sensitivity to targets with energy below 1.75e is extremely poor, so G is added to a-81:H.
Attempts have been made to lower the band gap by adding group II elements such as e.

This Ge-doped a-3i:H, that is, a-3iGe
:H has excellent long-wavelength sensitivity, so it is expected to be applied to solar cells as well as image pickup devices as described above. This a-3iGe:H film is plasma C
VD method, reactive sputtering method, ion blating method,
Although it is formed by a photo-CVD method, an a-3iGe:H film with good photoconductivity that can be applied to photovoltaic devices such as solar cells has not yet been obtained, and therefore it is difficult to obtain sufficient device performance. Product not obtained.

A-3+G manufactured by conventional, for example, plasma CVD method
e: The photoelectric properties of the H film are as follows: at energy bandgap (E9) -1.5 eV, photoconductivity (Δ(7Ph)
is 9 xlo-'S-cm-', the dark conductivity (σd) is 3 x 10-1 O3-cm=, and the a-3i used in amorphous solar cells in practical use is
: l(Δσph of film (E9-1, 75eV)=
2 Xl0-'S-cm-', a, =3.5x1o
-S-0m-', the value of electrical conductivity (Δσph) especially when irradiated with light was about three orders of magnitude lower.

As a result, a
When -5iGe:H film is used, the generated current is too small, so such a -3iGe:I
] The film could not be applied as a type 1 layer.

In order to make this application possible, it is necessary that the value of Δσph is at least equivalent to that of the a −3i :H film.

In order to investigate the reason why the a-5iGe:H film has poor photoelectric properties, we investigated the bonding state between H and Si atoms by infrared absorption, and found that the relationship was SI82 amount > Si-H amount. It was found that the amount of H incorporated into the film was larger than that incorporated in the form of H, resulting in a fairly non-uniform film. by the way,
a-3i: In the case of H film, the amount of 5l-1-12>Si
It is known that a -H amount causes a decrease in photoconductivity, and it is expected that a similar phenomenon occurs in the a -3iGe:H film during its film formation.

Problems to be Solved by the Invention As mentioned above, a-8 has good photosensitivity in the long wavelength range.
+Ge: H is a constituent material of multilayer solar cells (type 1 layer)
If it can be used as a solar cell, it would be possible to greatly improve the conversion efficiency of the resulting solar cell.

However, those obtained by conventional methods only have poor photoelectric properties. For example, one of the solar cells
In order to incorporate it as a mold layer, the photoconductivity Δσph must be at least a −8i : H value (2XIO−′S・c
m-').

For this purpose, an attempt was made to apply a negative bias voltage using a screen electrode during the preparation of the a-3iGe:H film (see Proceedings of the Japan Society of Applied Physics Autumn 1983 Conference, p. 356). However, it has been found that when a DC bias voltage is applied in the presence of a screen electrode (metal mesh electrode), there are problems particularly in the following points.

(1) When forming a film for a long time, use a-3i on the mesh.
Fine powder of Ge:H is likely to be generated and mixed into the film, making it impossible to achieve homogeneity in the film thickness direction.

(ii) Using the same growth chamber, p-type layer -3i:l(
When forming dry pin solar cells on a substrate, thin films left on the mesh after previous deposition operations result in contamination of the thin films formed in subsequent operations. That is, for example, the thin film (p
The impurity element (for example, B) in the type layer-3i:H) is bombarded by ions generated by plasma during the subsequent formation of the type-1 a-3iGe:H film, and seeps into the atmosphere. It eventually mixes into the L-type film being formed. As a result, the purity of the l-type layer is significantly impaired, and the resulting solar cell properties are also inadequate.

Therefore, the purpose of the present invention is to improve the various drawbacks of the above-mentioned conventional methods,
The object of the present invention is to provide a method for producing an a-5iGe:H film that can improve the photoelectric properties to 1] without the above-mentioned contamination.

Means for Solving the Problems In view of the above-mentioned current state of the manufacturing method of a-3iGe:H film, which is expected to be used as a material for photoelectric conversion elements such as solar cells, the present inventors have developed a method for producing high-quality a-3iGe:H films. 3iGe: H
As a result of various studies to develop a membrane manufacturing method, a-3i
The basic idea is that it is most advantageous to apply a predetermined bias voltage during film formation in order to control the bonding state between Si and Ge in the film, which has the greatest influence on the quality of Ge:H film properties. By applying a bias voltage to a specific position, the problem of contamination that occurs during continuous film deposition operations and the problem of nonuniformity in the film thickness direction that occurs when film deposition operations are continued over a long period of time can be solved. The present invention has been completed based on the discovery that an advantageous solution can be achieved.

That is, the method for manufacturing the aS+Ge: A method for producing an a-3iGe:H thin film by accelerating the reaction and forming a film on a substrate, the method comprising applying a negative DC bias voltage of -100V or more to the substrate holder during the film formation operation. It is characterized by

In the method of the present invention, the raw material gas includes 5IH4, 5i2Hs, 5laHe, SiF, etc. as a Si source.
Also, GeH4, GeF, etc. are used as a Ge source,
These are generally used diluted with H2 (dangling bond terminator).

The method of the present invention can be carried out using, for example, a parallel plate type plasma CVD apparatus as shown in the attached FIG. 1. That is, this parallel plate type plasma CVD apparatus 1 includes a reaction chamber 2, a substrate holder 4 provided above the reaction chamber 2 and supporting a substrate 3, and a lower electrode plate similarly provided below within the reaction chamber 2. 5, the substrate holder 4 is connected to a power source 6 for applying a DC bias voltage so that a predetermined bias voltage can be applied to the substrate, and the lower electrode 5 is connected to, for example, a high frequency power source 7 to apply a high frequency voltage. reaction chamber 2 by applying
The structure is such that plasma can be generated within the chamber to promote reactions.

In the operation of forming an a-5iGe:H film using this device, first, the substrate 3 is set, the inside of the reaction chamber 2 is brought to a predetermined high vacuum by operation of the evacuation system (not shown), and then the raw material gas is A mixture 8 is supplied into the reaction chamber at a predetermined pressure (flow rate), while a high frequency power source 7 and a DC voltage for bias application are supplied.
This can be carried out by operating the power supply 6 to generate high-frequency plasma in the reaction chamber, decomposing the source gas and depositing the reaction product on the substrate.

A-3iGe as a narrow bandgap material for photovoltaic devices including multilayer A-8i solar cells
In order to make the :H film usable, it was necessary to increase its photoconductivity Δσph to at least the same level as the a −3i :H film (2xio−′ S −cm−′). In the a-3iGe:H film obtained by the conventional plasma CVD method, Δσ is 9×10'-1
It was only about lS-Cm-1. This was because the amount of H atoms incorporated in the form of 5i-H2 in the formed a-3iGe:H thin film was greater than the amount of H atoms incorporated in the form of 5i-H.

Therefore, in the method of the present invention, a
-3iGe: ) (during film formation, 100%
■By applying a negative DC bias voltage of
The bonding state was changed without reducing the amount of H atoms in the film. That is, as is clear from the infrared absorption characteristics shown in Fig. 3, when the DC bias voltage (■8) is -100 volts, the amount of H atoms in the film does not decrease and is based on the stretching mode of 5i-82. The absorption near <2090 cm-' decreases, while the absorption near 2000 cm-' due to the stretching mode of 5i-H increases, which is the opposite of the case when Vs=0 volts.

This is a -3iGe obtained according to the method of the present invention.
FiA,l can also be determined by considering the measurement results of the photoelectric properties of the :H film (Fig. 2). That is, the Δσph of the film obtained at ■I]−0 volts is significantly smaller than that obtained when a negative Vs is applied (about two orders of magnitude lower than the Δσph at V, = −200 volts).

As explained above, according to the method of the present invention, by applying a predetermined negative DC bias voltage to the substrate, 5i
-82 formation is suppressed, while the formation of 5i-H, which is advantageous for photoelectric properties, is promoted;
In particular, Δσpb is improved to 1].

In addition, since the method of the present invention does not use a metal mesh electrode as in the conventional method, the film obtained is uniform in the thickness direction even if the film is formed over a long period of time. Since there is no risk of contamination due to deposition of reaction products, the formation of relatively thick films, even the continuous formation of multilayer thin film structures, can be carried out advantageously without the problems encountered heretofore.

EXAMPLES Hereinafter, the method of the present invention will be explained in more detail using examples. However, the scope of the present invention is not limited in any way by the following examples.

First, a parallel plate electrode type plasma C as shown in Fig.
A Corning 7059 glass for measuring photoelectric properties and a crystalline silicon wafer for measuring the bonding state of Si and H were attached to the substrate holder part of the VD apparatus.

As source gases, 5IH4 and GeH4 diluted to 10% with H2 were mixed at a predetermined gas flow rate ratio, GeH4/ (SI
H4+GeH<) was supplied to the film forming chamber at an ultimate pressure of 0.5 Torr. At a substrate temperature of 250°C, a high frequency of 13.56 MHz (output 5 W) was applied to the electrode plate to generate high frequency plasma between the substrate holder part and the electrode, while a DC power source was used to close the substrate holder part. +100V to
A DC bias voltage in the range of ~-200 cm was applied to decompose and deposit the raw material gas. The film formation time is 0 when the film thickness is 0.
．． The thickness was changed to 5 μm depending on each case.

A-3 thus formed on Corning 7059 glass
An AI metal electrode was formed on the iGe:H film and its photoelectric properties were measured, and a-3i formed on crystalline silicon was
The infrared absorption spectrum of the Ge:H film was measured. In addition, gas flow 1 ratio GeH</ (Sit(4+GeH
4) in the range of 0 to 1, a-3iGe:H films with different film compositions were fabricated.

In the thus obtained a-3iGe:H film, Ge
Figure 2 shows the dependence of the photoelectric properties on the applied DC bias voltage (■,) for the film formed under the conditions of H4/(SI H4+GeH4) = 0.21, and the vI of the infrared absorption properties.
The l dependence is shown in Figure 3.

First, from the results in Figure 2, for Vn = 0 volts, Va
= Band gap at -200 volts (E9oPt)
photoconductivity △σ without significantly changing.

It can be seen that this has improved by about two orders of magnitude. Also, from Fig. 3, for VB-0 volts, V, = (St at 00 volts)
Peak of H2 stretching mode (2090cm-')
The intensity decreases and the peak of the 5i-H stretching mode (200
0 cm-') It can be seen that the intensity increases. This peak intensity is approximately 5i-82ff, which exists in each mode.
It is proportional to the amount of i and 5i-H, and from the measurement results of infrared absorption characteristics, the amount of H bound in the film does not change to that extent, but the main bond state changes from 5i-82 mode to 5i-H.
- It can be understood that the mode has changed to H mode. It is believed that this change in the bonding state between Si and H made it possible to improve the photoelectric characteristics.

Furthermore, an a-5iGe:H film obtained by keeping other film-forming conditions constant and changing only the gas flow rate ratio had the same E9°2. It was confirmed that the photoelectric characteristics were improved.

In addition, in order to improve the film quality, we also tried installing a metal mesh electrode between the substrate holder and the RF electrode and applying a negative DC bias voltage to the substrate holder, but this did not have a particularly large film quality improvement effect. I couldn't see it. Furthermore, positive DC
When a bias voltage was applied, the photoelectric characteristics were almost the same as when no substrate bias voltage was applied.

Effects of the Invention As explained in detail above, according to the method of the present invention, by applying a negative DC bias voltage of a predetermined intensity to the substrate holder part during the formation of the a-3iGe:H film by the plasma CVD method, , obtained a-3iGe: H
This greatly improved the photoelectric properties of the film, making it possible to put it into practical use. That is, the application of the bias voltage reduces the proportion of Si dangling bonds in the grown film,
The photoconductivity Δσph was improved by about two orders of magnitude.

In addition, when using conventional metal mesh electrodes,
A- 3
An iGe:H film can be advantageously obtained, and various elements with a multilayer thin film structure and thick layers can be produced without problems.

Thus, according to the method of the present invention, an a-3iGe:H film having at least the same level of photoelectric properties (particularly Δσph) as an a-Si:H film can be advantageously produced.
Not only various semiconductor devices having 8 layers of a-3iGe as type 1 layer, including photoelectric conversion elements such as solar cells and photoreceptors, but also solar cells obtained using the method of the present invention and other solar cells. It is extremely useful for producing tandem solar cells or multi-color solar cells, etc., which are connected in series.

[Brief explanation of drawings]

FIG. 1 is a schematic configuration diagram for explaining an example of an apparatus (parallel plate electrode type plasma CVD apparatus) useful for carrying out the method of the present invention, and FIG. Conductivity (Δσp
h), dark conductivity (σd) and energy gap (E
9° pt) is plotted against the DC bias voltage applied to the substrate holder part, and FIG.
3 is a spectrum chart showing changes in infrared absorption characteristics of 3iGe:H. (Main reference numbers) 1. Parallel plate electrode type plasma CVD device, 2. Reaction chamber, 3. Substrate, 4. Substrate holder, 5. Electrode plate, 6. DC bias power supply,

Claims (2)

[Claims] (1) Using the plasma enhanced chemical vapor deposition method, a low-pressure gas mixture of raw materials including a Si source, a Ge source, and hydrogen is introduced into a reaction chamber, an external electric field is applied to obtain plasma, and a reaction is caused to occur on the substrate. The method for producing an amorphous silicon germanium thin film, which comprises applying a negative DC bias voltage of -100V or more to the holder portion of the substrate during the film formation. (2) The above Si source is SiH_4, Si_2H_6, Si
3. The method for producing amorphous silicon germanium according to claim 1, wherein the Ge source is GeH_4 or GeF_4.