JPH05259096A - Method of forming amorphous semiconductor - Google Patents

Abstract

PURPOSE:To form an amorphous semiconductor high in photoconductivity and low in deterioration by light. CONSTITUTION:An amorphous semiconductor hydride thin film 8 is formed on a prescribed substrate 3 through a vapor reaction method where discharge decomposition is carried out under a reduced pressure in an amorphous semiconductor forming method, where a first deposition process where an amorphous semiconductor hydride thin film 8 is deposited on the substrate 3 at a deposition rate of over 3Angstrom /second keeping the substrate 3 at a temperature of 400-550 deg.C and a dehydrogenation process where hydrogen is partially dissociated from the amorphous semiconductor hydride thin film 8 making the substrate stand at the same temperature after discharge decomposition is stopped are alternately repeated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for forming an amorphous semiconductor thin film using a gas phase reaction.

[0002]

2. Description of the Related Art In recent years, for example, photovoltaic elements (solar cells)
Hydrogenated amorphous silicon (a-Si: H) used for
As a method for forming an amorphous semiconductor such as plasma CV
The D method (PCVD method) is often used. In the PCVD method, a low-pressure gas gas as a raw material is introduced, electric energy is applied to create a plasma state, and a reaction is performed to form a semiconductor thin film on a substrate. As a method for applying electrical energy, a glow discharge method such as a direct current glow discharge method in which a direct current voltage is applied between electrodes to perform glow discharge and a high frequency glow discharge method in which high frequency power is applied are generally used.

Hydrogenated amorphous silicon produced by the glow discharge method is called GDa-Si, and the most frequently used gas for producing this GDa-Si is monosilane (SiH ₄ ).
Therefore, a gas obtained by diluting this SiH ₄ with an inert gas such as argon (Ar) or helium (He) or hydrogen (H ₂ ) is often used.

GDa-Si using SiH ₄ contains a large amount of hydrogen, and this hydrogen contains 40% as shown in FIG. 1 by annealing (annealing).
Free hydrogen, which is easily separated at about 0 ° C to 500 ° C,
It is known that there is dispersed hydrogen that is separated at about 550 ° C to 800 ° C.

As free hydrogen, polysilane (SiH
₂ ) n or clustered hydrogen is included, and if strong light is continuously applied, these free hydrogen move in the film, defects increase and photoconductivity etc. decrease, so-called phenomenon called photodegradation It is known to occur.

Typical examples of dispersed hydrogen are isolated SiH, isolated SiH ₂ (silylene), and isolated SiH ₃ (silyl), which play a role of terminating bond breaking defects.

As a method for preventing photodegradation, conventionally, a method has been adopted in which the substrate temperature is set high to reduce the total amount of hydrogen in the film. According to this conventional method, the network can be made dense and the phenomenon of optical deterioration can be achieved.

[0008]

However, according to this conventional method, the amount of dispersed hydrogen as hydrogen in the film is reduced, the optical bandgap is lowered, and the photoelectromotive force accompanying the lowering of the optical bandgap is reduced. There are problems that the open circuit voltage of the device is lowered and that the defect level density in the amorphous semiconductor is increased with the decrease of the total hydrogen amount.

An object of the present invention is to provide a method for forming an amorphous semiconductor thin film capable of preventing photodegradation without lowering the open circuit voltage and increasing the defect level density. And

[0010]

The present invention is a method for forming an amorphous semiconductor in which a hydrogenated amorphous semiconductor thin film is formed on a predetermined substrate by using a gas phase reaction by discharge decomposition under reduced pressure. Then, the temperature of the substrate is set to 400 ° C. or higher and 550 ° C. or lower, and the hydrogenation amorphous semiconductor thin film is deposited on the substrate at a deposition rate of 3Å or more per second; It is characterized by alternately repeating a dehydrogenation step of leaving a part of hydrogen to be desorbed.

[0011]

In the present invention, by increasing the substrate temperature and increasing the film deposition rate to 3Å / sec or more, an amorphous semiconductor is formed at high speed, and accordingly, a large amount of hydrogen, particularly The dispersed hydrogen is taken in and a dense film structure is formed. In addition, free hydrogen such as clustered hydrogen can be released by performing a dehydrogenation process subsequent to the deposition process.

Further, by repeating these steps alternately, it becomes possible to perform dehydrogenation efficiently in a short time by keeping the deposited film thickness of one time less than a certain value, and to perform the non-deposition process performed in each deposition step. The crystalline semiconductor thin film can be dehydrogenated uniformly to obtain an amorphous semiconductor thin film that has been uniformly dehydrogenated over the entire film thickness.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A method for forming an amorphous semiconductor according to an embodiment of the present invention will be specifically described below with reference to the drawings.

This method is, for example, a high-frequency plasma C used in a known high-frequency glow discharge method shown in the schematic view of FIG.
It is performed using a VD device (RF plasma CVD).

That is, a pair of electrodes 2 facing each other is arranged in a chamber 1, and one electrode 2 on which a substrate 3 is mounted is placed.
While discharging the raw material gas from the gas inlet ring 4 arranged around, the exhaust gas is discharged from the central portion of the electrode 2 to the exhaust system 5, while a high frequency current is supplied from a high frequency power source 6 between the electrodes 2. A heater 7 for heating the substrate 3 is provided below the one electrode 2.

SiH ₄ was used as a source gas, and the substrate temperature was 450 ° C. as shown in the flow chart of FIG. 3 and Table 1.
Manufacturing pressure (gas pressure) 13 Pa, SiH ₄ flow rate 50 SCC
M, RF power of 50 W, deposition rate of 5 Å / sec, deposition process of forming a predetermined film thickness once, and the same substrate temperature (450 ° C.) for a predetermined time after stopping discharge decomposition
The dehydrogenation step of leaving a part of hydrogen to be left for 2 hours is alternately repeated, and it took about 4 hours to form the amorphous silicon film 8 having a thickness of 1 μm.

Here, the film thickness of the amorphous silicon formed in one deposition step is preferably 10 Å or more and 300 Å or less in order to obtain a certain or higher photoconductivity.
As shown in FIG. 4, when the film thickness of the amorphous silicon formed in one deposition step is less than 10Å, the effect of plasma damage at the initial stage of discharge becomes large and the characteristics (photoconductivity) deteriorate. Is not preferred. Further, the film thickness of amorphous silicon formed in one deposition step is 30
If it exceeds 0Å, hydrogen cannot be sufficiently desorbed in a short time and the characteristics (photoconductivity) are deteriorated, which is not preferable.

In this example, the film thickness formed in one deposition step was set to 50Å where the best characteristics were obtained. When the film thickness formed in one deposition step is 50 Å, the dehydrogenation step has a photodeterioration of 500 hours or less after irradiation with light for a certain time as shown in FIG. It was confirmed that the value was suppressed to 1. Therefore, it was set to 1 minute in this example.

[0019]

Conventional Example 1 SiH ₄ was used as a source gas, the substrate temperature was 450 ° C., the fabrication pressure (gas pressure) was 13 Pa, the SiH ₄ flow rate was 50 SCCM, the RF power was 50 W, and the film formation rate was 5 Å / sec. Then, an amorphous silicon film having a film thickness of 1 μm was formed.

[0020]

Conventional Example 2 SiH ₄ was used as a source gas, the substrate temperature was 200 ° C., the fabrication pressure (gas pressure) was 13 Pa, the SiH ₄ flow rate was 50 SCCM, the RF power was 50 W, and the film formation rate was 1 Å / sec. Then, an amorphous silicon film having a film thickness of 1 μm was formed.

[0021]

Conventional Example 3 SiH ₄ was used as a source gas, the substrate temperature was 450 ° C., the fabrication pressure (gas pressure) was 13 Pa, the SiH ₄ flow rate was 50 SCCM, the RF power was 50 W, and the film formation rate was 1 Å / sec. Then, an amorphous silicon film having a film thickness of 1 μm was formed.

[0022]

Conventional Example 4 SiH ₄ was used as a source gas, the substrate temperature was 200 ° C., the fabrication pressure (gas pressure) was 13 Pa, the SiH ₄ flow rate was 50 SCCM, the RF power was 50 W, and the film formation rate was 5 Å / sec. Then, an amorphous silicon film having a film thickness of 1 μm was formed.

[0023]

Conventional Example 5 The amorphous silicon film obtained in Conventional Example 1 was left at a substrate temperature of 450 ° C. for 5 hours to be thermally released.

[0024]

[Table 1]

The photoconductivity after irradiation of AM-1.5, 100 mW / cm ² of light for 500 hours on each of the amorphous silicons of Example 1 and Conventional Examples 1 to 5 was determined as the photoconductivity before irradiation. It is shown in Table 2 together with the rate.

[0026]

[Table 2]

From Table 2, low temperature conditions (substrate temperature 200 ° C.)
It can be seen that in Conventional Examples 2 and 4, the optical deterioration is obviously greater than that under the high temperature condition. Further, in the conventional example 1 simply under the conditions of high temperature and high speed film formation (substrate temperature 450 ° C., film formation rate 5 Å / sec), photodegradation was observed as a result of the inclusion of clustered hydrogen in the film. Therefore, when comparing the photoconductivity after irradiation, this example has the highest value.

Further, in Conventional Example 5, photodegradation was still observed even though it took a long time for desorption of hydrogen.
It seems that more time is needed. However, since further thermal desorption is difficult in the process, it was not performed.

Only when the temperature exceeds 400 ° C. (Si-H
₂ ) Thermal desorption of free hydrogen such as n and clustered hydrogen occurs, but when it exceeds 550 ° C, necessary hydrogen (dispersed hydrogen) such as Si-H is desorbed. Further, as shown in Conventional Example 3, when the film formation rate is lower than 3 Å / s when the film is formed at 400 ° C. or higher, the photoconductivity is 1 × 10 ⁻⁵ S / that can be applied to the photovoltaic device. I could not get cm.

[0030]

As described above, according to the present invention,
By setting the substrate temperature to 400 ° C. or more and 550 ° C. or less and the film thickness growth rate to 3 Å or more per second, the amorphous semiconductor is formed at a high speed, a large amount of hydrogen is taken into the amorphous semiconductor, and the amount of dispersed hydrogen is increased. Therefore, it is possible to suppress a decrease in open circuit voltage and an increase in defect level density of an amorphous semiconductor as a photovoltaic element.

Further, since the dehydrogenation step is carried out subsequent to the deposition step, free hydrogen taken into the amorphous semiconductor in the deposition step can be efficiently released, and photodegradation can be reduced.

Moreover, by alternately performing the deposition process and the dehydrogenation process, it is possible to obtain a homogeneous amorphous semiconductor which is uniformly dehydrogenated over the entire film thickness even when the final film thickness is large.

In particular, in the present invention, the amorphous semiconductor is mainly composed of silicon, and the film thickness produced in the film deposition step is 10 Å or more and 300 Å per deposition step.
When the step of desorbing hydrogen is 1 minute or more, a photovoltaic element having high photoconductivity and less photodegradation can be obtained.

[Brief description of drawings]

FIG. 1 is a dehydrogenation characteristic diagram showing a relationship between a dehydrogenation amount and an annealing (annealing) temperature.

FIG. 2 is a schematic diagram showing a configuration of a high frequency plasma CVD apparatus.

FIG. 3 is a flow chart of the present invention.

FIG. 4 is a photoconductivity characteristic diagram showing a relationship between a film thickness deposited in one deposition process and photoconductivity after light irradiation.

FIG. 5 is a photoconductive characteristic diagram showing the relationship between the process time of the dehydrogenation process and the photoconductivity after light irradiation.

[Explanation of symbols]

 3 Substrate 8 Amorphous silicon film

Claims (2)

[Claims] 1. A method for forming an amorphous semiconductor in which a hydrogenated amorphous semiconductor thin film is formed on a predetermined substrate by using a gas phase reaction by discharge decomposition under reduced pressure, wherein the temperature of the substrate is 400 ° C. The deposition step of depositing the hydrogenated amorphous semiconductor thin film on the substrate at a deposition rate of 3 Å or more and the temperature of 550 ° C. or higher and 550 ° C. or lower, and the discharge decomposition is stopped and the same substrate temperature is used to remove a part of hydrogen. A method for forming an amorphous semiconductor, characterized by alternately repeating a dehydrogenation step of desorbing. 2. The amorphous semiconductor is mainly composed of silicon, the film thickness produced in the film deposition step is 10 Å or more and 300 Å or less per deposition step, and the step of desorbing hydrogen is 1 minute. The method for forming an amorphous semiconductor according to claim 1, wherein the method is as described above.