JP3059796B2 - Method for producing polycrystalline germanium film and photoelectric conversion device using the same - Google Patents

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 Germanium semiconductors have the characteristic of having a narrow band gap, and thus have been applied to various characteristic applications not found in conventional silicon.

[0003] The narrow bandgap is superior to the sensitivity of long-wavelength light as compared with silicon. For example, it has been proposed to comprehensively use various wavelengths from short-wavelength light to long-wavelength light. In a stacked solar cell, this germanium semiconductor is used as a semiconductor material for a solar cell at the final stage when viewed from the light incident side (Proc. Of the 21st IEEE photovoltaic Spec).
ialist Conf. Florida (1990.5) p.73-78). Further, the sensor is used as a photosensitive portion of a light receiving element in an infrared region.

[0004] In addition, germanium semiconductors are characterized by having a large gauge factor and exhibiting small thermal conductivity when alloyed with other metals.
It is also expected to be used as a strain sensor, a thermoelectric conversion element, and the like. In recent years, research to use such a germanium semiconductor in a thin film state as a device material has been actively advanced.

[0005]

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

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

On the manufacturing side, high temperature is not required as much as single crystal germanium, but improving the film quality follows the tendency of high temperature during the manufacturing process.
This narrows the room for selection of a substrate material that supports the germanium film in a thin film state, and increases the cost of the substrate material.

For this reason, as a method for producing a polycrystalline germanium film, an amorphous semiconductor, which has been particularly studied in recent years, is subjected to heat treatment to polycrystallize it. It has become a challenge.

[0009]

A feature of the present invention is that a step of forming an amorphous germanium film on a supporting substrate at a substrate temperature of 315 ° C. to 345 ° C. The germanium film is placed on the crystal face (111).
Has a lower X-ray diffraction peak intensity than that of the crystal plane (220).
A polycrystallized germanium film by heat-treating under conditions that are at least equal to or higher than the above, wherein the optimal substrate temperature range is 320 ° C. to 340 ° C. And a photovoltaic device provided with a photoactive layer using at least a part of the polycrystalline germanium film formed by this manufacturing method.

[0010]

According to the present invention, the temperature of the substrate is set at 31 on the supporting substrate.
The amorphous germanium film formed in the range of 5 ° C. or more and 345 ° C. or less is subjected to an X-ray diffraction peak of a crystal plane (111).
The crystal strength is at least equal to that of the crystal plane (220).
Alternatively, the polycrystalline germanium film obtained by performing the heat treatment under the above conditions can obtain a high-quality semiconductor film even though the heat treatment temperature is lower than the conventional temperature.

As a characteristic feature, in the amorphous germanium film formed in the temperature range, a sharp peak of the crystal plane (111) is obtained as a diffraction peak after the heat treatment, and a large polycrystalline grain is obtained. Can be

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a diagram showing an element structure for each process for explaining a manufacturing process of a polycrystalline germanium film manufacturing method according to the present invention. In the first step shown in FIG.
Plasma CVD on a support substrate (1) such as alkali glass
The amorphous germanium film (2) formed by
000 mm. Such an amorphous germanium film (2) is 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 more and 345 ° C. or less. The formation conditions of the plasma CVD method employed in this example include, besides the above-mentioned substrate temperature, 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 a second step shown in FIG.
The support 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 high-quality polycrystalline germanium film (3).

[0016]

[Table 1]

Table 1 shows the plasma CV used in this embodiment.
The conditions for forming an amorphous germanium film by Method D are shown.
In this example, a plasma CVD method was used as a method for forming an amorphous germanium film. However, the manufacturing method of the present invention is not limited to this, and other sputtering methods, vapor deposition methods, and the like may be used. By setting the substrate temperature within the above range, the same can be achieved.

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

As the atmosphere gas for the heat treatment, nitrogen was used in this embodiment, 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 by performing a heat treatment in a reduced tank.

Hereinafter, the polycrystalline germanium film will be described.
The correlation between the heat treatment temperature when forming (3) and the formation temperature of the amorphous germanium film (2) as a starting material will be described. In each case, an amorphous germanium film formed by a plasma CVD method is used as a sample. used.

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

The heat treatment temperature for forming a polycrystalline germanium film in the description of FIG. 1 means that the amorphous germanium film as a starting material is a sign of polycrystallization as a result of the heat treatment. Is the minimum heat treatment temperature required for observing the X-ray diffraction peak. Therefore, the lower the heat treatment temperature is, the easier the polycrystallization can be.

According to the figure, it is understood that as the forming temperature of the amorphous germanium film as the starting material becomes higher, the heat treatment temperature required for polycrystallization becomes lower. Specifically, the formation temperature of the amorphous germanium film is 150 ° C.
As the temperature rises from 300 ° C. to 290 ° C., the heat treatment temperature for polycrystallization decreases from 450 ° C. to around 350 ° 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 no further decrease in the heat treatment temperature is observed. .

According to this experiment, when an amorphous germanium film as a starting material was formed at 350 ° C., an X-ray diffraction peak was already observed without performing such a heat treatment.

The results shown in the figure simply show 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 can be relatively low. According to the present inventors, an amorphous germanium film formed in a range of 290 ° C. to 350 ° C., which does not show this tendency, rather than a polycrystalline germanium film formed in a region showing the tendency of the heat treatment temperature to decrease gradually. It is considered that the polycrystalline germanium film starting from is more important.

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

The formation temperature of the amorphous germanium film is 290 ° C., 330 ° C., and 350 ° C.

According to the figure, the characteristics are very similar to those of a single crystal germanium when an amorphous germanium film having a formation temperature of 330 ° C. is used. On the other hand, when the formation temperature of the amorphous germanium film is 290 ° C. or 3 ° C.
In the case where the temperature is set to 50 ° C., the characteristics are significantly different from those of the single crystal germanium.

This characteristic difference is caused by 290 ° C. and 350 ° C.
8 for an amorphous germanium film with
The light absorption coefficient is 1 in the region from 00 nm to 1200 nm.
In contrast to a large decrease from 0 ⁵ cm ^-1 to 10 ³ cm ^-1 , the change between a single crystal germanium and an amorphous germanium film formed at a formation temperature of 330 ° C. is within a range of 10 ⁴ cm ^-1. Is gradual.

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

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

However, since the amorphous germanium film formed at 350 ° C. shows an X-ray diffraction peak without heat treatment as described above, this sample was not heat-treated. The heat treatment temperature of the other samples was 3
The temperature is kept constant at 50 ° C.

In the same figure, the data of the sample at 290 ° C. correspond to the horizontal axis indicating the angle in order to clarify the difference between the characteristics of each sample, but the data of the other samples correspond to the crystal plane of each crystal plane. In consideration of the overlap of the peaks, each is shifted along the horizontal axis. However, the crystal plane indicated by each peak is indicated by, for example, (111) or (220) as shown in FIG.

First, when the amorphous germanium film formed at 290 ° C. was used as a starting material, no X-ray diffraction peak intensity could be observed at the time of forming the amorphous germanium film (not shown). The 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, indicating that the film is insufficient as a polycrystalline germanium film.

On the other hand, in the case where the formation temperature of the amorphous germanium film was set to 330 ° C., no remarkable X-ray diffraction peak was observed before the heat treatment, but after the heat treatment, the Steep crystal plane (11
1) Peaks are observed.

The characteristic feature is that the peak of the crystal plane (111) is much larger than that of the crystal plane (220).

On the other hand, in the case of the amorphous germanium film formed at 350 ° C., a steep peak of the crystal plane (220) is observed without heat treatment. 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
The meaning of the polycrystalline state indicated by 1) and the peak of the crystal plane (220) means that the polycrystalline semiconductor usually showing the peak of the large crystal plane (111) is remarkably seen when the crystal is composed of large crystal grains. The crystal plane (2
In the case of the peak 20), the peak is composed of relatively small crystal grains, and is particularly observed in a microcrystalline semiconductor.

Incidentally, the crystal grain size of each sample measured by an electron microscope was 1 to 2 μm at 290 ° C. and 350 ° C., respectively.
In contrast to 200 ° to 500 °, in the case of 330 ° C. according to the production method of the present invention, large crystal grains of 10 to 20 μm were observed. At 350 ° C., the sample was not subjected to the heat treatment. However, as described above, polycrystallization was observed even before the heat treatment. Hardly changes.

From the above, the amorphous germanium film formed with the substrate temperature around 330 ° C. can obtain a polycrystalline germanium film having unprecedented excellent characteristics and is very similar to a single crystal germanium. It is a membrane.

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 FIG.
In the X-ray diffraction peak, the peak of the crystal plane (111) has begun to be particularly large as compared with the signal (b) of the substrate itself.

In the case of 345 ° C.,
Although the peak of the crystal plane (111) is smaller than that in the case of ° C., it is still remarkably observed. However, the intensity of the peak of the crystal plane (111) and that of the crystal plane (220) are almost the same, and a lower film quality is observed than at 330 ° C.

According to the present inventors, a polycrystalline germanium film having such a good film quality is suitable for forming an amorphous germanium film in a temperature range of 315 ° C. to 345 ° C., but an optimum temperature range. Is from 320 ° C. to 340 ° C. according to the observation of the X-ray diffraction peak intensity 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., it is possible to obtain good characteristics without increasing the subsequent heat treatment temperature. Can be formed.

Next, the heat treatment temperature will be described. As described above, the manufacturing method of the present invention shows excellent film quality by controlling the substrate temperature during the formation of the amorphous germanium film, but the heat treatment temperature for the polycrystallization in the post-process and the time required for it The relationship is as shown in FIG. The sample in the figure used an amorphous germanium film formed at 330 ° C.

As a general tendency, as the heat treatment temperature becomes higher, the time required for polycrystallization becomes shorter. The results shown in the figure show that the heat treatment temperature is 8 hours or more at 330 ° C., 5 hours or more at 340 ° C., and 350 ° C.
Requires more than two hours. Even hot 40
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 intensity of the crystal plane (111) and the crystal plane (220) should be compared by observing the X-ray diffraction peak intensity. Can easily be achieved by That is,
According to the production method of the present invention, the peak intensity of the crystal plane (111) of the polycrystalline germanium film is the same as that of the crystal plane (220).
Is at least equal to or higher than that of As described above, the substrate temperature is set to 315 ° C. on the supporting substrate.
Amorphous germanium formed within the range of not less than 345 ° C.
X-ray diffraction peak intensity of crystal plane (111)
At least equal to or higher than that of the crystal plane (220)
By performing heat treatment under the above conditions, good characteristics can be obtained.
Can form a polycrystalline germanium film
Wear.

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

In the figure, (61) is a light-transmitting substrate made of quartz, glass, or the like, (62) is a transparent electrode formed on the light-transmitting substrate (61), and (63) is amorphous silicon carbide. (64) is a first buffer layer made of intrinsic amorphous silicon, and (64) is a first buffer layer made of intrinsic amorphous silicon.
(65) is a polycrystalline germanium film (thickness: 1000 to 3000) which is a photoactive layer characteristic of the photoelectric conversion device of the present invention, and (66) is a film made of intrinsic amorphous silicon. (67) is an n-type semiconductor made of amorphous silicon (thickness 200 to 300) and (68) is made of metal such as aluminum. It is a back electrode.

Except for the polycrystalline germanium film (65), it is formed by a conventionally well-known method. In particular, all amorphous silicon-based manufacturing methods are formed by a well-known plasma CVD method, and the first buffer layer (64) increases the 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 gap between the polycrystalline germanium film (65) and the n-type semiconductor (67) becomes a gradual continuous state.

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

Next, the film laminated on the translucent substrate (61) is subjected to a heat treatment at 350 ° C. for 2 hours. The subsequent second buffer layer (66) and the like may be formed by a method similar to the conventional method.

Typical characteristics of the photoelectric conversion device manufactured in this example include an open circuit voltage of 0.20 V and a short circuit current of 50 mA / c under a light irradiation condition of AM 1.5 and 100 mW / cm ^2.
m ² , fill factor 0.55, and 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 embodiment, since a polycrystalline germanium film is used as the photoactive layer, the sensitivity to long wavelength light is high. FIG. 7 shows the spectral sensitivity characteristics of this photoelectric conversion device. It can be seen that the photoelectric conversion device has a maximum peak of 7 × 10 ² mA / W at a wavelength of 1.4 μm. Such characteristics are substantially the same as those of an infrared detecting 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 detecting element and a thermoelectric conversion element, typical characteristics include a gauge factor of 80,
Thermal conductivity was 50 W / mK (at room temperature). These characteristic values were almost the same as those of a thick single crystal germanium film, and it was found that the film was a polycrystalline germanium film having excellent characteristics.

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

[0058]

According to the manufacturing method of the present invention, the amorphous germanium film formed at a substrate temperature of 315 ° C. or more and 345 ° C. or less has an X-ray diffraction peak intensity of the crystal plane (111).
At least equal to or higher than that of the crystal plane (220)
The polycrystalline germanium film obtained by heat treatment under the above conditions is very similar to a single crystal germanium in film quality, and the crystal plane (111)
Is observed, and the crystal grains are large and a high quality film is obtained.

Therefore, according to the production method of the present invention, the formation temperature of the amorphous germanium film as the starting material is set in the range of 315 ° C. to 345 ° C., and the X (X) of the crystal plane (111) is
Line diffraction peak intensity less than that of crystal plane (220)
By heat treatment under the condition to be equal to or greater than both, it is possible to form a polycrystalline germanium film having good properties without the heat treatment temperature and high temperature.

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

Further, since the film is of high quality, when the polycrystalline germanium film manufactured by the present manufacturing method is applied to a device, the characteristics are as excellent as those of a device using a single crystal germanium. Is shown.

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

[Brief description of the drawings]

FIG. 1 is a sectional view of an element structure for each process for explaining a 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 as a starting material.

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

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

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

FIG. 6 is a sectional view of an element structure for explaining a 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

────────────────────────────────────────────────── (5) References JP-A-4-133368 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01L 21/205 H01L 21/324 H01L 31 / 0248

Claims (3)

(57) [Claims] And 1. A process for the amorphous germanium layer over the supporting substrate is formed within the substrate temperature is 315 ° C. or higher 345 ° C. or less, the amorphous germanium layer, X of the crystal plane (111)
Line diffraction peak intensity less than that of crystal plane (220)
And a step of performing a heat treatment under conditions that are equal to or higher than the above, thereby performing a polycrystallizing process. 2. The method for producing a polycrystalline germanium film according to claim 1, wherein said substrate temperature is set to 320 ° C. or higher and 340 ° C. or lower. 3. A substrate temperature of 315 ° C. or higher on a supporting substrate.
The amorphous germanium film formed within the temperature range of 45 ° C. or less was converted to a crystal face (111) having an X-ray diffraction peak intensity of the crystal face.
(220) At least equal or better than that of (220)
1. A photoelectric conversion device comprising a photoactive layer at least partially using a polycrystalline germanium film formed by heat treatment under the following conditions .