JPH0590172A - Manufacture of polycrystalline germanium film and photoelectric converter using same - Google Patents

Abstract

PURPOSE:To form a film in excellent quality regardless of the low heat treatment temperature by a method wherein an amorphous germanium film is formed within the substrate temperature range of specific values to be heat- treated for polycrystallization. CONSTITUTION:An amorphous germanium film 3000Angstrom thick is formed by a plasma CVD on a supporting substrate 1. Such an amorphous film 2 is to be a little bit thicker starting material to form a polycrystalline germanium film. At this time, the substrate temperature as an important factor in such a step is to be adjusted within the range of 315 deg.C-345 deg.C. Next, the supporting substrate 1 where the amorphous germanium film 2 is formed is to be left intact in nitrogen atmosphere to be heat-treated at 350 deg.C later. By such a heat treatment step, the amorphous germanium film 2 is turned into excellent polycrystalline germanium film 3. That is, the later heat treatment temperature can be lowered by specifying the formation temperature of the amorphous germanium film within range of 315 deg.C-345 deg.C.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a polycrystalline germanium film and a photoelectric conversion device using the polycrystalline germanium film as a photoactive layer.

[0002]

2. Description of the Related Art Since germanium semiconductors have a characteristic of having a narrow band gap, they have been applied with various characteristics that conventional silicon does not have.

The narrow band gab is superior to silicon in sensitivity to long-wavelength light. For example, it has been proposed to comprehensively use various wavelengths from short-wavelength light to long-wavelength light region. In a stacked solar cell, this germanium semiconductor is used as the final semiconductor material for solar cells when viewed from the light incident side (Reference Proc. Of the 21st IEEE photovoltaic Spec).
ialist Conf. Florida (May 1991) p.73-78). Further, the sensor is used as a photosensitive portion of a light receiving element in the infrared region.

In addition to this, the germanium semiconductor has a large gauge factor, and has a small thermal conductivity when alloyed with another metal.
It is also expected to be used as a strain sensor or a thermoelectric conversion element. In recent years, studies have been actively promoted to use such germanium semiconductor in a thin film state as a device material.

[0005]

However, as with other semiconductor materials, there are many problems in forming a germanium semiconductor as a thin film and having high quality characteristics.

That is, the formation of the germanium film in the thin film state results in the formation of the germanium film in the amorphous state or the polycrystalline state, which is inferior to the single crystal germanium in terms of characteristics. can not deny.

In terms of manufacturing, although it does not require a high temperature as high as that of a single crystal germanium, improvement of the film quality leads to a tendency of higher temperature in the manufacturing process.
This narrows the scope of selection of the substrate material that supports the germanium film in the thin film state, and raises the cost of the substrate material.

For this reason, even in the so-called solid phase growth method, in which so-called solid phase growth method, it is important to lower the temperature of the heat treatment step, in which an amorphous semiconductor, which has been actively studied in recent years as a method for producing a polycrystalline germanium film, is polycrystallized. It has become a challenge.

[0009]

The feature of the present invention resides in that an amorphous germanium film is formed on a supporting substrate at a substrate temperature of 315 ° C. or higher and 345 ° C. or lower; A method for producing a polycrystalline germanium film, which comprises a step of polycrystallizing a germanium film by heat-treating it, and particularly, the substrate temperature range is 320 ° C.
Above 340 ° C. is the most suitable manufacturing method, and there is a photoelectric conversion device provided with a photoactive layer in which the polycrystalline germanium film formed by this manufacturing method is used at least in part.

[0010]

According to the present invention, the substrate temperature is set to 31 on the supporting substrate.
A polycrystalline germanium film obtained by heat-treating an amorphous germanium film formed within the range of 5 ° C. or higher and 345 ° C. or lower is a high-quality semiconductor film even though the heat treatment temperature is lower than that of a conventional one. Obtainable.

As a characteristic feature, the amorphous germanium film formed in that temperature range has a sharp peak of the crystal face (111) as a diffraction peak after heat treatment and a large polycrystalline grain. Be done.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is an element structure diagram for each step for explaining the manufacturing steps of the method for manufacturing a polycrystalline germanium film according to the present invention. At the first step shown in FIG.
Plasma CVD on supporting substrate (1) such as alkali glass
The amorphous germanium film (2) formed by the
000Å form. Such an amorphous germanium film (2) serves as a starting material for forming a polycrystalline germanium film.

The substrate temperature, which is an important factor in such a process, is adjusted to be in the range of 315 ° C. or higher and 345 ° C. or lower. The plasma CVD method used in this example includes the following conditions: germane (G
H ₄ ) Flow rate 40 SCCM, high frequency discharge power 30 mW / c
m ² , and the degree of vacuum during discharge is 0.15 Torr.

Next, in the second step shown in FIG.
The supporting substrate (1) on which the amorphous germanium film (2) is formed is left in a nitrogen atmosphere and heat-treated at 350 ° C.

By this heat treatment, the amorphous germanium film (2) is transformed into a good quality polycrystalline germanium film (3).

[0016]

[Table 1]

Table 1 shows the plasma CV used in this example.
The conditions for forming the amorphous germanium film by the D method are shown below.
In this example, the plasma CVD method was used as the method for forming the amorphous germanium film, but the manufacturing method of the present invention is not limited to this, and other sputtering methods, vapor deposition methods, etc. can be used for forming. By setting the substrate temperature in the above range, the same operation can be performed.

Further, in this example, the heat treatment temperature was 350 ° C. and the heat treatment time was 2 hours. Although the heat treatment conditions were set in consideration of the treatment in a relatively short time, for example, the heat treatment time may be lengthened. If allowed, the heat treatment temperature may be set to the same value as the formation temperature of the amorphous germanium film.

Nitrogen was used as the atmospheric gas during the heat treatment in this example, but an inert gas such as argon or helium may be used in addition to this, or a high vacuum state may be used without using these inert gases. A similar polycrystalline germanium film can be obtained even if heat treatment is carried out in the above tank.

In the following, this polycrystalline germanium film is
The correlation between the heat treatment temperature for forming (3) and the formation temperature of the amorphous germanium film (2) as a starting material will be described. In both cases, an amorphous germanium film by plasma CVD is used as a sample. used.

FIG. 2 is a characteristic diagram showing the relationship between the formation temperature of the amorphous germanium film (2) and the heat treatment temperature when forming the polycrystalline germanium film (3). This sample uses the same amorphous germanium film as in the above-mentioned embodiment, and its film thickness is 3000 Å as well.

It should be noted that the heat treatment temperature for forming the polycrystalline germanium film referred to in the description of the figure means that the amorphous germanium film, which is the starting material, is a sign of polycrystallization as a result of such heat treatment. It means the minimum heat treatment temperature necessary for observing the X-ray diffraction peak. Therefore, it indicates that the lower the heat treatment temperature is, the easier polycrystallization can be performed.

According to the figure, as the forming temperature of the amorphous germanium film as the starting material becomes higher, the heat treatment temperature required for polycrystallization is lower. Specifically, the formation temperature of the amorphous germanium film is 150 ° C.
The heat treatment temperature for polycrystallization decreases from 450 ° C. to about 350 ° C. as the temperature rises from 290 ° C. to 290 ° C.

On the other hand, when the formation temperature of the amorphous germanium film is 290 ° C. or higher, the peak of X-ray diffraction is always observed at the heat treatment temperature of 350 ° C., and the heat treatment temperature is not further lowered. ..

According to the present experiment, when the starting amorphous germanium film is formed at 350 ° C., the X-ray diffraction peak is already observed without performing such heat treatment.

The results shown in the figure can be seen as simply showing that if a large amount of energy is supplied in advance during the formation of the starting material, the heat treatment temperature required for the subsequent polycrystallization is relatively low. According to the inventors, the amorphous germanium film formed in the range of 290 ° C. or higher and 350 ° C. or lower, which does not exhibit this gradual decrease tendency, rather than the polycrystalline germanium film formed in the region in which this heat treatment temperature gradually decreases It is considered that the polycrystalline germanium film using as a starting material has an important meaning.

The details will be described below. FIG. 3 shows the light absorption coefficient after heat treatment when the amorphous germanium film as the starting material is formed at various forming temperatures, with the horizontal axis representing the wavelength. However, the heat treatment temperature for polycrystallization is constant at 350 ° C. For comparison, the light absorption coefficient of single-crystal germanium is shown in the same figure, but it is shown that the film quality is better as a polycrystalline germanium film exhibiting characteristics similar to those of this single-crystal germanium. Becomes

There are three types of temperatures for forming the amorphous germanium film: 290 ° C., 330 ° C. and 350 ° C.

According to the figure, it is in the case of using an amorphous germanium film having a formation temperature of 330 ° C. that the characteristics are very similar to those of the single crystal germanium. On the other hand, the formation temperature of the amorphous germanium film is 290 ° C or 3
At 50 ° C., the characteristics are significantly different from those of the single crystal germanium.

This characteristic difference is 290 ° C. or 350 ° C.
8 for the amorphous germanium film with the formation temperature as
Light absorption coefficient is 1 in the region from 00 nm to 1200 nm
While it greatly decreases from 0 ⁵ cm ^-1 to 10 ³ cm ^-1 , the change is within the order region of 10 ⁴ cm ^-1 between the single crystal germanium and the amorphous germanium film having the formation temperature of 330 ° C. Is loose.

From these results, it can be seen that the polycrystalline germanium film formed of the amorphous germanium film formed at around 330 ° C. exhibits unique polycrystallization in terms of good film quality.

FIG. 4 shown below shows X after the heat treatment of the amorphous germanium film formed at various forming temperatures used in FIG.
It is a characteristic view of line diffraction peak intensity. In the figure, in addition to the formation temperature of the amorphous germanium film shown in FIG. 3, 315 ° C. and 345 ° C., which are the critical temperatures of the manufacturing method of the present invention, are also shown.

However, since the amorphous germanium film formed at 350 ° C. shows an X-ray diffraction peak even if it is not heat-treated as described above, this sample is not heat-treated. The heat treatment temperature of other samples was 3
The temperature is constant at 50 ° C.

In the same figure, in order to clarify the difference in the characteristics of each sample, the data of the samples at 290 ° C. correspond to the horizontal axis indicating the angle, but the data of the other samples correspond to the crystal planes. In consideration of the overlap of peaks, they are shown by being shifted along the horizontal axis. However, the crystal planes indicated by the respective peaks are indicated by, for example, (111) and (220) as indicated in the figure.

First, regarding an amorphous germanium film formed at 290 ° C. as a starting material, no X-ray diffraction peak intensity can be observed at the time of forming the amorphous germanium film (not shown). Peaks of the crystal plane (111) and the crystal plane (220) are observed by the heat treatment. However, these peaks are very small and extremely small as compared with the signal (a) due to the substrate itself, which indicates that the film is insufficient as a polycrystalline germanium film.

On the other hand, when the amorphous germanium film formation temperature was 330 ° C., no remarkable X-ray diffraction peak was observed before the heat treatment, but after the heat treatment, among the X-ray diffraction peaks. And a steep crystal plane (11
1) The peak will be observed.

And, as a particularly characteristic feature, the peak of the crystal plane (111) is much larger than that of the crystal plane (220).

On the other hand, in the amorphous germanium film formed at 350 ° C., a steep peak of the crystal plane (220) is observed without heat treatment. Even in such a situation, the crystal plane (111) is observed, but is much smaller than the peak of the crystal plane (220).

It should be noted that the crystal plane (11
1) and the peak of the crystal plane (220) mean a polycrystalline state, but a polycrystalline semiconductor that usually shows a large peak of the crystal plane (111) is remarkably seen when it is composed of large crystal grains. The crystal plane (2
In the case of the peak of 20), the peak is composed of relatively small crystal grains, and is particularly observed in a microcrystalline semiconductor.

Incidentally, the size of the crystal grains of each sample under an electron microscope is 1 to 2 μm at 290 ° C. and 350 ° C., respectively.
In contrast to 200 Å to 500 Å, at 330 ° C. according to the production method of the present invention, crystal grains having a large particle size of 10 to 20 μm were observed. Although 350 ° C. is a sample that has not been subjected to heat treatment, this sample is one in which polycrystallization is observed even before the heat treatment as described above. Hardly changes.

From the above, the amorphous germanium film formed at a substrate temperature of around 330 ° C. is a polycrystalline germanium film having excellent characteristics that have not been obtained in the past, and is very similar to the single crystal germanium. It is a film.

The critical temperature at which a polycrystalline germanium film having such excellent characteristics can be formed can be remarkably known from the X-ray diffraction peak. 315 ° C shown in the figure
In the X-ray diffraction peak of (3), the peak of the crystal plane (111) starts to become particularly large as compared with the signal (b) due to the substrate itself.

At 345 ° C., 330
Although the peak of the crystal plane (111) is smaller than that in the case of ° C, it is still observed remarkably. However, the intensity of the peak of the crystal plane (111) and that of the crystal plane (220) are almost the same, and deterioration of the film quality is observed as compared with the case of 330 ° C.

According to the inventors of the present invention, such a polycrystalline germanium film having good film quality is suitable as the formation temperature of the amorphous germanium film in the range of 315 ° C. to 345 ° C., but the optimum temperature range. It is assumed that the temperature is 320 ° C. to 340 ° C. by observing the intensity of the X-ray diffraction peak described above.

Therefore, according to the manufacturing method of the present invention, by setting the formation temperature of the amorphous germanium film as the starting material in the range of 315 ° C. to 345 ° C., good characteristics can be obtained without increasing the subsequent heat treatment temperature. It is possible to form a polycrystalline germanium film having

Next, the heat treatment temperature will be described. In the production method of the present invention, as described above, excellent film quality is exhibited by controlling the substrate temperature during the formation of the amorphous germanium film, but the heat treatment temperature for the polycrystallization in the subsequent step and the time required therefor are The relationship is as shown in FIG. As the sample in the figure, an amorphous germanium film formed at 330 ° C. was used.

As a general tendency, the higher the heat treatment temperature, the shorter the time required for polycrystallization. The results shown in the figure show that the heat treatment temperature is 330 ° C. for 8 hours or longer, 340 ° C. for 5 hours or longer, and 350 ° C.
Then, it takes more than 2 hours. 40 higher temperature
At 0 ° C. and 500 ° C., polycrystallization can be achieved in a shorter time.

Therefore, even after polycrystallization by heat treatment, as a verification method therefor, the X-ray diffraction peak intensity is observed and the magnitudes of the crystal plane (111) and the crystal plane (220) are compared. Can easily be done by. That is,
As long as the production method of the present invention is used, the peak intensity of the crystal plane (111) of this polycrystalline germanium film is the crystal plane (220).
It is at least equal to or more than that.

FIG. 6 is an element structure diagram of the photoelectric conversion device of the present invention in which the polycrystalline germanium film formed by the above-described manufacturing method of the present invention is used as a photoactive layer.

In the figure, (61) is a transparent substrate made of quartz or glass, (62) is a transparent electrode formed on the transparent substrate (61), and (63) is amorphous silicon carbide. A p-type semiconductor layer (thickness 100 Å to 200 Å) composed of (64) is the first buffer layer composed of intrinsic amorphous silicon (thickness 10
(0Å to 200 Å), (65) is a polycrystalline germanium film (thickness 1000 Å to 3000 Å), which is a photoactive layer that is a feature of the photoelectric conversion device of the present invention, and (66) is an amorphous silicon The second buffer layer (100 Å to 200 Å), (67) is an n-type semiconductor made of amorphous silicon (200 Å to 300 Å), and (68) is made of metal such as aluminum. This is the back electrode.

Other than the polycrystalline germanium film (65), it is formed by a conventionally known method. In particular, all the amorphous silicon-based manufacturing methods are formed by the well-known plasma CVD method, and the first buffer layer (64) has a band gap between the polycrystalline germanium film (65) and the p-type semiconductor (63). The second buffer layer (66) is provided so that the band gaps of the polycrystalline germanium film (65) and the n-type semiconductor (67) are gradually continuous.

In the following, this polycrystalline germanium film (6
The formation procedure of 5) will be described. When forming the polycrystalline germanium film (65), first, the p-type semiconductor layer (63) and the first buffer layer (64) are sequentially laminated on the transparent electrode (62).
Then, the amorphous germanium film is applied at a substrate temperature of 330 ° C.
It is formed by the plasma CVD method under the above condition. The forming conditions other than the substrate temperature were set within the range of the forming conditions shown in Table 1.

Next, the film laminated on the transparent substrate (61) is heat-treated at 350 ° C. for 2 hours. The second buffer layer (66) and the like that are subsequently performed may be formed by a method similar to the conventional method.

Typical characteristics of the photoelectric conversion device manufactured in this example are as follows: an open voltage of 0.20 V and a short circuit current of 50 mA / c under light irradiation conditions of AM 1.5 and 100 mW / cm ^2.
m ² , fill factor 0.55, conversion efficiency 5.5%.
These characteristics are almost the same as those of a photoelectric conversion device using a polycrystalline germanium film as a substrate.

Further, in the photoelectric conversion device of this example, since the polycrystalline germanium film is used as the photoactive layer, the sensitivity to long wavelength light is large. FIG. 7 shows the spectral sensitivity characteristics of this photoelectric conversion device, and it can be seen that it has a maximum peak of 7 × 10 ² mA / W at a wavelength of 1.4 μm. Such characteristics are almost the same as those of the infrared detection element using a thick film single crystal germanium.

When the polycrystalline germanium film formed by the manufacturing method of the present invention is applied to a strain detection element and a thermoelectric conversion element, typical characteristics are a gauge factor of 80,
The thermal conductivity was 50 W / mK (at room temperature). These characteristic values were almost the same as those of the thick film single crystal germanium, and it was found that the polycrystalline germanium film had excellent characteristics.

From the above, the polycrystalline germanium film formed by the manufacturing method of the present invention, when used as a strain detecting element and a thermoelectric conversion element, is 1/10 to 1 in comparison with the conventional thick film. A film thickness of / 100 can sufficiently function.

[0058]

In the manufacturing method of the present invention, a polycrystalline germanium film obtained by heat-treating an amorphous germanium film formed at a substrate temperature of 315 ° C. or higher and 345 ° C. or lower becomes a single crystal germanium in terms of film quality. This is a very high quality film in which a sharp peak of the crystal plane (111) is observed and the crystal grains are large.

Therefore, according to the manufacturing method of the present invention, by setting the formation temperature of the amorphous germanium film as the starting material in the range of 315 ° C. to 345 ° C., good characteristics can be obtained without increasing the subsequent heat treatment temperature. It is possible to form a polycrystalline germanium film having

This makes it possible to use a material having relatively poor heat resistance such as amorphous silicon as the substrate.

Further, since it is a high quality film, when the polycrystalline germanium film produced by the present production method is applied to a device, it has excellent characteristics comparable to those of the device using the single crystal germanium. Indicates.

According to the photoelectric conversion device of the present invention, since it has excellent photosensitivity up to a long wavelength region, a high total conversion efficiency can be obtained.

[Brief description of drawings]

FIG. 1 is a sectional view of an element structure for each step for explaining the manufacturing method of the present invention.

FIG. 2 is a characteristic diagram illustrating a relationship between a heat treatment temperature when forming a polycrystalline germanium film and a forming temperature of an amorphous germanium film that is a starting material.

FIG. 3 is a light absorption characteristic diagram after heat treatment of an amorphous germanium film formed at various forming temperatures.

FIG. 4 is an X-ray diffraction peak characteristic diagram after heat treatment of an amorphous germanium film formed at various forming temperatures.

FIG. 5 is a characteristic diagram for explaining the relationship between the heat treatment temperature for polycrystallization and the time required for it.

FIG. 6 is a cross-sectional view of an element structure for explaining the photoelectric conversion device of the present invention.

FIG. 7 is a spectral sensitivity characteristic diagram of the photoelectric conversion device.

[Explanation of symbols]

(1) ... Support substrate (2) ... Amorphous germanium film (3) ... Polycrystalline germanium film

Claims (3)

[Claims] 1. A step of forming an amorphous germanium film on a supporting substrate at a substrate temperature within a range of 315 ° C. or higher and 345 ° C. or lower, and a step of polycrystallizing the amorphous germanium film by heat treatment. And a method for producing a polycrystalline germanium film. 2. The method for producing a polycrystalline germanium film according to claim 1, wherein the substrate temperature is 320 ° C. or higher and 340 ° C. or lower. 3. A substrate temperature of 315 ° C. or higher on a supporting substrate 3
A photoelectric conversion device comprising a photoactive layer, at least a part of which is used for a polycrystalline germanium film formed by heat-treating an amorphous germanium film formed within a range of 45 ° C. or lower.