JP2004031551A - Method for manufacturing compound semiconductor thin film - Google Patents

Abstract

A compound semiconductor thin film comprising a group IB element, a group IIIB element, and a group VIB element and containing In and Ga as a group IIIB element has good Ga distribution uniformity in the film thickness direction, or has a good film thickness. Provided is a method for manufacturing a compound semiconductor thin film that can arbitrarily control Ga distribution in a direction.
A precursor thin film is formed by alternately repeating a step of supplying In to a substrate and a step of supplying Cu and Ga at least twice each using a sputtering method. The substrate (17) is made of glass (1) on the surface of which a Mo conductive film (2) is formed, on which an In layer (3) and a Cu-Ga layer (4) are alternately repeated to form a precursor thin film (5). ) Is formed. Thereafter, the precursor thin film is heat-treated in an atmosphere containing Se to form a Cu (In, Ga) Se ₂ thin film.
[Selection diagram] FIG.

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for producing a compound semiconductor thin film, and more particularly to a method for producing a compound semiconductor thin film containing a Group IB element, a Group IIIB element, and a Group VIB element, which is used particularly in the field of thin-film solar cells.
[0002]
[Prior art]
A compound semiconductor thin film having a chalcopyrite structure containing a group IB element, a group IIIB element, and a group VIB element is used as a light absorption layer constituting a thin film solar cell. As such a compound semiconductor thin film, a CuInSe ₂ film (hereinafter, also referred to as “CIS film”) or a Cu (In, Ga) Se ₂ film (hereinafter, “CIS film”) in which a part of In is substituted by Ga in the CIS film. CIGS film "). It is known that in the case of a solar cell using a CIGS film as a light absorbing layer, the band gap is slightly larger than in the case of a CIS film, and the efficiency of absorbing sunlight is improved. In addition, the CIGS film having a composition distribution in the film thickness direction in which Ga is gradually increased toward the rear surface (electrode film) in the direction of the rear surface (electrode film) has a large band gap from the front surface to the rear surface. Such a band structure is called a graded band gap. At this time, the conduction band level mainly increases. However, since an internal electric field is generated in such a band gap profile, carriers excited by light move from the back surface to the front surface. That is, there is an effect that the carrier collection efficiency is improved in the graded band gap.
[0003]
As a method for manufacturing a compound semiconductor effective for forming a film having a large area and a uniform thickness, a method using sputtering can be considered. For example, the following method has been proposed as a method of forming a CIGS film. As a first step, Cu and Ga are supplied to a substrate at room temperature using a Cu—Ga compound target, and as a second step, In is supplied using an In target to prepare a precursor thin film. Subsequently, by heating the precursor thin film at a high temperature in a hydrogen selenide atmosphere, Se is supplied to the precursor thin film, and a CIGS crystal grows to form a CIGS film. In this method for producing a CIGS thin film, hydrogen selenide used for supplying Se to the precursor thin film is very toxic, and therefore, is very difficult to handle. Therefore, as a method of supplying Se to the precursor thin film, a method of using Se vapor without using hydrogen selenide has been proposed. After heating the substrate to about 400 ° C., supply In and Se using an In—Se compound target as a first step, and supply Cu and Ga using a Cu—Ga compound target as a second step. To form a precursor thin film. Subsequently, the precursor thin film is heat-treated at a high temperature while irradiating Se vapor to form a CIGS film. FIG. 5 shows the structure of the precursor thin film. The precursor thin film 33 has a structure in which a first layer 31 containing In and Se and a second layer 32 containing Cu and Ga are sequentially laminated on a substrate 17 composed of, for example, glass 1 and a conductive film 2 on the surface thereof. Have.
[0004]
[Problems to be solved by the invention]
However, in order to supply Se to the precursor thin film. In the CIGS film manufactured by the above method using Se vapor, the Ga diffusion rate is slower than other elements constituting CIGS, so it is difficult to control the Ga distribution in the film thickness direction. There was a tendency to segregate near the center in the direction. FIG. 6 shows the Ga distribution in the thickness direction of the CIGS film as the ion count ratio of Ga to (Ga + In) from the ion count numbers of Ga and In measured by SIMS. Such non-uniformity of the CIGS film composition has caused a problem that good battery characteristics cannot be obtained when a solar cell is manufactured using the CIGS film as a light absorbing film layer.
[0005]
The present invention includes a Group IB element, In and Ga as a Group IIIB element, and a Group VIB element, and has good Ga distribution uniformity in the film thickness direction or arbitrary Ga distribution in the film thickness direction. It is an object of the present invention to provide a method for producing a compound semiconductor thin film having the following.
[0006]
[Means for Solving the Problems]
In order to achieve the above object, a method for producing a compound semiconductor thin film of the present invention comprises the steps of: forming a precursor thin film for supplying a group IB element, a group IIIB element, and a group VIB element to a substrate;
Heat treating the precursor thin film in a group VIB element atmosphere,
Forming the precursor thin film comprises: supplying (A) at least In as a group IIIB element to the substrate (A);
A step (B) comprising at least a group IB element and a group IIIB element and supplying Ga as a group IIIB element;
The method is characterized in that the steps (A) and (B) are alternately repeated, and the steps (A) and (B) are performed at least twice each.
[0007]
BEST MODE FOR CARRYING OUT THE INVENTION
In the present invention, the group IB element is Cu, Ag, or the like. The group IIIB element is In, Ga, or the like. The VIB group element is Se, S, or the like.
[0008]
In the method of the present invention, a particularly effective compound semiconductor thin film can be manufactured when the thickness of the film formed in one step (A) is 0.01 μm to 0.50 μm.
[0009]
The supply amount of the element supplied in the step (A) may be constant. Further, the supply amount of the element supplied in the step (B) may be constant. Further, the supply amount of the element supplied in the step (A) may be changed at least once. Preferably, the supply amount of the element supplied in the step (A) is increased at least once. Further, the supply amount of the element supplied in the step (B) may be changed at least once. Preferably, the supply amount of the element supplied in the step (B) is reduced at least once.
[0010]
Preferably, the group VIB element is supplied simultaneously with the step (A). Preferably, the VIB group element is supplied simultaneously with the step (B). Preferably, the group IB element is Cu or Ag. Preferably, the VIB group element is Se or S.
[0011]
Next, an embodiment of the present invention will be described. In the following description, a case where a Cu (In, Ga) Se ₂ film is manufactured using Cu as a Group IB element, using Ga and In as a Group IIIB element, and using Se as a Group VIB element. However, the present invention is not limited to this.
[0012]
1 and 2 show a first embodiment of the present invention.
[0013]
(Embodiment 1)
Hereinafter, Embodiment 1 of the present invention will be described with reference to FIGS. FIG. 1 is a schematic diagram of an apparatus for manufacturing a compound semiconductor thin film used in the present embodiment, and FIG. 2 is a schematic diagram showing the arrangement of a sputtering source and a vapor deposition source as a plan view as viewed from above a vacuum vessel 11.
[0014]
In this manufacturing apparatus, a sputtering source 12a provided with an In target, a sputtering source 12b provided with an In-Se compound target, and a sputtering source provided with a Cu-Ga compound target are provided inside a vacuum vessel 11 provided with an exhaust port. A source 12c and a Se deposition source 13 are provided. Although only the sputter source 12a is shown in FIG. 1, the sputter source is actually arranged as shown in FIG. Each sputter source has a shutter 14. The shutter 14 is controlled so as to cover the target surface while the discharge of the sputter source is stopped. Each sputter source is configured to receive power from the DC power supply 15 or the RF power supply 16.
[0015]
A substrate susceptor 18 for holding a substrate 17 is arranged in the vacuum vessel 11, and the substrate susceptor 18 is connected to a substrate rotating device 19. Further, a lamp heater 20 as a heater for heating the substrate 17 is arranged in the vacuum vessel 11. The substrate 17 can be controlled to a desired temperature by the lamp heater 20. Further, an Ar gas supply line 21 with a valve 22 is connected to the vacuum vessel 11.
[0016]
Although FIG. 2 illustrates an apparatus provided with three sputtering sources, the apparatus that can be used in the manufacturing method of the present invention is not limited to this.
[0017]
(Example 1)
Next, a specific example of the method of manufacturing a compound semiconductor according to the present embodiment, which is performed using the above-described apparatus, will be described.
[0018]
First, a substrate provided with a conductive film on the surface is manufactured. As the conductive film, for example, a Mo film can be used, and as the substrate 17, for example, a glass substrate can be used. As a method for forming the conductive film, for example, a sputtering method can be employed. The preferred film thickness is in the range of 0.05 to 1.00 μm.
[0019]
After attaching the substrate 17 to the substrate susceptor 18, the inside of the vacuum vessel 11 is evacuated. Subsequently, the substrate 17 is heated via the substrate susceptor 18 using the lamp heater 20. The heating is controlled so that the temperature of the surface of the conductive film formed on the substrate 17 is, for example, 20 to 450 ° C, preferably 250 to 400 ° C. Further, the valve 22 of the Ar gas supply line 21 is opened to supply Ar gas into the vacuum vessel 11.
[0020]
When the temperature of the substrate 17 reaches a predetermined temperature and becomes substantially constant, the substrate rotating device 19 is operated to rotate the substrate susceptor 18, thereby rotating the substrate 17. At this time, the supply and exhaust of the Ar gas are controlled so that the pressure in the vacuum vessel 11 is kept substantially constant. The pressure in the vacuum vessel 11 is maintained at, for example, 0.1 to 5.0 Pa, or preferably 0.1 to 3.0 Pa.
[0021]
Subsequently, a precursor thin film is prepared by supplying raw material elements onto the substrate 17. The production of this precursor thin film is performed by alternately repeating the following steps (A) and (B).
[0022]
Step (A) is a step of supplying In onto the substrate 17. First, the power is applied by connecting the sputter source 12a provided with the In target to a power supply. At this time, if necessary, the In target surface may be cleaned by pre-sputtering. When the release of In is stabilized, the shutter 14 provided in the sputtering source 12a is opened to supply In to the surface of the substrate 17. After a lapse of a predetermined time, the shutter 14 provided in the sputter source 12a is closed, and the supply of In ends.
[0023]
Step (B) is a step of supplying Cu and Ga onto the substrate 17. First, a sputter source 12c provided with a Cu-Ga compound target is connected to a power source to apply power. At this time, if necessary, the surface of the Cu—Ga compound target may be cleaned by pre-sputtering. When the release of Cu-Ga is stabilized, the shutter 14 provided in the sputtering source 12c is opened to supply Cu-Ga to the surface of the substrate 17. After a lapse of a predetermined time, the shutter 14 provided in the sputter source 2c is closed to terminate the supply of Cu and Ga.
[0024]
In order to prepare a precursor thin film, the steps (A) and (B) are alternately and repeatedly performed. The number of times of performing the step (A) and the step (B) is, for example, two times or more, and preferably five times or more.
[0025]
Further, the thickness of the layer containing In supplied to the substrate 17 while performing the step A once is not particularly limited. However, considering the thickness of the layer containing In that can sufficiently diffuse Ga, for example, It is desirably 0.01 to 0.50 μm, preferably 0.05 to 0.20 μm.
[0026]
The thickness of the In-containing layer supplied to the substrate 17 during the single step (A) may be the same every time, but the composition ratio of Ga and In in the thickness direction of the CIGS film is sufficient. May be changed when the band structure in the film thickness direction is controlled by controlling arbitrarily.
[0027]
Further, by gradually decreasing the composition ratio of Ga to In in the thickness direction of the CIGS film from the substrate 17 side to the surface side, it is possible to obtain a band structure called a graded band gap which improves carrier collection efficiency. It is desirable to increase the thickness of the layer containing In supplied to the substrate 17 in at least one step (A) at least once.
[0028]
In the step (A), Se may be supplied to the substrate 17 while supplying In. In this case, the power may be applied to the sputtering source 12a and the evaporation source 23 provided with the Se evaporation material may be heated. Alternatively, instead of the sputtering source 12a, a sputtering source 12b provided with an In-Se compound target may be used.
[0029]
Further, the thickness of the layer containing Cu and Ga supplied to the substrate 17 while performing the step (B) once is not particularly limited, but is, for example, 0.01 μm to 0.50 μm, preferably, It is 0.01 μm to 0.20 μm.
[0030]
The thickness of the layer containing Cu and Ga supplied to the substrate 17 during one step (B) may be the same every time, but the thickness of Ga with respect to In in the thickness direction of the CIGS film is sufficient. When controlling the band structure in the film thickness direction by arbitrarily controlling the composition ratio, it may be changed.
[0031]
Also, by gradually decreasing the Ga distribution in the thickness direction of the CIGS film from the substrate 17 side to the surface side, a band structure called a graded band gap that improves carrier collection efficiency can be obtained. It is desirable to reduce the thickness of the layer containing Cu and Ga supplied to the substrate 17 in B) at least once.
[0032]
In the step (B), Se may be supplied to the substrate 17 while supplying Cu—Ga. In this case, the power is applied to the sputtering source 12c, and the evaporation source 23 provided with the Se evaporation material may be heated.
[0033]
A precursor thin film is formed by alternately repeating the steps (A) and (B). FIG. 3 is a cross-sectional view illustrating an example of the structure of the precursor thin film formed in the present embodiment. The precursor thin film 5 is formed on the conductive film 2 on the surface of the substrate 1, and has a structure in which the In-containing layers 3 and the Cu-Ga-containing layers 4 are alternately stacked in order from the substrate 17. However, in producing the precursor thin film 5, the first step may be either the step (A) or the step (B), and the last step may be either the step (A) or the step (B). . That is, the first layer formed on the substrate 17 may be either the In-containing layer 3 or the Cu-Ga-containing layer 4, and the outermost surface layer of the precursor thin film 5 is the In-containing layer 3 and the Cu-containing layer. Any of the Ga layer 4 may be used.
[0034]
Also, the boundaries between the layers in the precursor thin film 5 need not always be clear, and the boundaries may be unclear due to the diffusion of each element during the formation of the precursor thin film.
[0035]
The ratio of each element contained in the precursor thin film 5 is not particularly limited, and can be appropriately set according to the desired composition of the compound semiconductor thin film. The ratio of the element amounts of Cu to In and Ga in the precursor thin film, that is, the value of Cu / (In + Ga) is, for example, 0.3 to 0.5, or preferably 0.45 to 0.50. In addition, the ratio of the element amount of Ga to In and Ga, that is, the value of Ga / (In + Ga) is, for example, 0 to 0.5, or preferably 0.2 to 0.3. In addition, the ratio of each element contained in the precursor thin film 5 is adjusted by the supply amount of each element in the step (A) and the step (B).
[0036]
After preparing the precursor thin film 5, the application of power to each sputtering source is stopped, and the supply of Ar gas is preferably stopped.
[0037]
Subsequently, the precursor thin film 5 is heated using the lamp heater 20 while heating the Se deposition source 13 and irradiating the precursor thin film 5 with Se. The heating temperature is set so that the surface of the conductive film 2 on the substrate 17 is at a temperature higher than that at the time of preparing the precursor thin film, for example, 300 to 600 ° C, preferably 450 to 580 ° C. The heating time is, for example, 1 second to 30 minutes, preferably 1 second to 10 minutes. After a lapse of a predetermined time, overheating of the lamp heater 20 to the substrate 17 is stopped. After the heating is stopped or after a lapse of a predetermined time from the stopping of the heating, the supply of Se is stopped. The time from the stop of the heating to the stop of the supply of Se is not particularly limited, but in consideration of the time until the temperature of the substrate 17 is cooled to a temperature at which Se does not re-evaporate, for example, 1 second to 20 minutes. Preferably, it is 1 second to 10 minutes.
[0038]
By this step, Se is taken into the precursor thin film, and nucleus elements constituting the precursor thin film are thermally diffused to form a compound semiconductor thin film. The ratio of the element amounts of Cu to In and Ga in the compound semiconductor thin film, that is, the value of Cu / (In + Ga) is, for example, 0.3 to 0.5, or preferably 0.45 to 0.50. Further, the ratio of the element amount of Ga to In and Ga, that is, the value of Ga / (In + Ga) is, for example, 0 to 0.5, preferably 0.2 to 0.3. The ratio of Se is, for example, 50 to 60 atomic%, and preferably 50 to 55 atomic%.
[0039]
The Ga distribution in the thickness direction of the CIGS film in the case where the supply amounts of Cu and Ga supplied in the step (B) are gradually reduced in the method of manufacturing the CIGS film described above was measured for Ga and In measured by SIMS. FIG. 4 shows the ion count ratio of Ga to (Ga + In) based on the ion count number. As is clear from FIG. 4, the Ga distribution in the thickness direction of the CIGS film gradually decreases from the conductive film side to the surface side of the CIGS film, and the graded band, which is a band structure desirable for a high-efficiency solar cell, is obtained. The distribution resulted in a gap.
[0040]
In the above embodiments, the steps of supplying In to the substrate and the step of supplying Cu and Ga are alternately repeated at least twice each to form a precursor thin film by a sputtering method, and then the precursor thin film is formed in an atmosphere containing Se. The precursor thin film was heat-treated to form a Cu (In, Ga) Se ₂ thin film.
[0041]
【The invention's effect】
As is apparent from the above description, according to the method for manufacturing a compound semiconductor thin film of the present invention, segregation of Ga in the thickness direction of the compound semiconductor thin film is suppressed, and Ga distribution in the thickness direction is made substantially uniform. Can be.
[0042]
In addition, since the precursor thin film is obtained by alternately laminating In and Cu-Ga at least twice, the elements are present more homogeneously than in the conventional case where In and Cu-Ga are laminated once each. When heat treatment is performed in an atmosphere of a group VIB element, the diffusion distance of these elements can be shorter than in the prior art, and the time required for this heat treatment can be shortened. As a result, the manufacturing cost and the energy cost can be reduced. Also, in the step (B) of the step of forming the precursor thin film, the supply amount of the element to be supplied to the substrate is reduced every time the step (B) is repeated. Thereby, the Ga distribution in the thickness direction of the compound semiconductor thin film can be gradually reduced from the conductive film side to the surface side, and a graded band structure that improves carrier collection efficiency can be obtained. A solar cell can be manufactured.
[Brief description of the drawings]
FIG. 1 is a schematic sectional view showing a manufacturing apparatus according to a first embodiment of the present invention.
FIG. 2 is a schematic view showing an example of an arrangement of a sputtering source and a vapor deposition source in the apparatus shown in FIG.
FIG. 3 is a cross-sectional view illustrating a structure of a precursor thin film in Example 1 of the present invention.
FIG. 4 is a graph showing a Ga distribution in a film thickness direction of a CIGS film manufactured by SIMS by gradually decreasing the supply amounts of Cu and Ga in a step B in Example 1 of the present invention. FIG. 5 is a graph showing the ion count ratio of Ga to (Ga + In).
FIG. 5 is a cross-sectional view showing a structure of a precursor thin film in a conventional manufacturing method.
FIG. 6 is a diagram showing the Ga distribution in the film thickness direction of a CIGS film manufactured by a conventional manufacturing method as the ion count ratio of Ga to (Ga + In) from the ion count numbers of Ga and In measured by SIMS.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Substrate 2 Conductive film 3 In-containing layer 4 Including Cu-Ga layer 5 Precursor thin film 11 Vacuum container 12a, 12b, 12c Sputtering source 13 Deposition source 14 Shutter 15 DC power supply 16 RF power supply 17 Substrate 18 Substrate susceptor 19 Substrate rotating device 20 Lamp type heater 21 Ar gas supply line 22 Valve 31 First layer 32 Second layer 33 Precursor thin film

Claims (12)

Forming a precursor thin film for supplying a group IB element, a group IIIB element and a group VIB element to the substrate;
Heat treating the precursor thin film in a group VIB element atmosphere,
Forming the precursor thin film comprises: supplying (A) at least In as a group IIIB element to the substrate (A);
A step (B) comprising at least a group IB element and a group IIIB element and supplying Ga as a group IIIB element;
A method for producing a compound semiconductor thin film, wherein the steps (A) and (B) are alternately repeated, and the steps (A) and (B) are each performed at least twice. The method for producing a compound semiconductor thin film according to claim 1, wherein the thickness of the film formed in one step of the step (A) is in a range of 0.01 µm to 0.50 µm. 3. The method for producing a compound semiconductor thin film according to claim 1, wherein the supply amount of the element supplied in the step (A) is constant. The method for producing a compound semiconductor thin film according to claim 1, wherein the supply amount of the element supplied in the step (B) is constant. 5. The method for producing a compound semiconductor thin film according to claim 1, wherein the supply amount of the element supplied in the step (A) is changed at least once. The method for producing a compound semiconductor thin film according to claim 1, wherein the supply amount of the element supplied in the step (A) is increased at least once. The method for producing a compound semiconductor thin film according to claim 1, wherein the supply amount of the element supplied in the step (B) is changed at least once. The method for producing a compound semiconductor thin film according to claim 1, wherein the supply amount of the element supplied in the step (B) is reduced at least once. The method for producing a compound semiconductor thin film according to claim 1, wherein the group VIB element is supplied simultaneously with the step (A). The method for producing a compound semiconductor thin film according to claim 1, wherein the group VIB element is supplied simultaneously with the step (B). The method for producing a compound semiconductor thin film according to claim 1, wherein the IB group element is at least one element selected from Cu and Ag. The method for producing a compound semiconductor thin film according to claim 1, wherein the VIB group element is at least one element selected from Se and S.

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