KR100819121B1 - Method for formation of cadmium sulfide thin film and cadmium sulfide diode by chemical bath deposition - Google Patents

Abstract

A method for forming a cadmium sulfide thin film and a cadmium sulfide diode by using chemical bath deposition is provided to form a Cd(Cu)S thin film and a CdS thin film in thick and uniform by additionally using ammonium chloride and triethylamine. A substrate(11) is immersed in a solution containing at least one Cd salt, at least one Cu salt, at least one sulphur source, ammonia, ammonium chloride and triethylamine to form a Cd(Cu)S thin film. The Cd salt comprises cadmium chloride(CdCl2), cadmium sulfide(CdSO4) or cadmium acetate(Cd(CH3COO)2). The Cu salt comprises copper sulfate(CuSO4). The solution is stored in a reactor, and the reactor is heated by a temperature of 45 to 65 °C during 20 to 60 minutes.

Description

Method for formation of cadmium sulfide thin film and cadmium sulfide diode by chemical bath deposition method

1 is a schematic cross-sectional view of a Cd (Cu) S / CdS thin film diode formed according to an embodiment of the present invention.

2 is a schematic cross-sectional view of a reactor for forming a Cd (Cu) S thin film or a CdS thin film according to an embodiment of the present invention.

Description of the main parts of the drawing

11, 33: substrate 12: lower electrode

13: Cd (Cu) S thin film 14: CdS thin film

15: upper electrode 20: reactor

21: thermostat 22: reaction vessel

25: Jig 27: Magnetic Stirrer

31: reaction solution

The present invention relates to a method for forming a cadmium sulfide thin film and a method for manufacturing a Cd (Cu) S / CdS thin film diode.

CdS is a II-VI compound semiconductor and is a material being studied in the field of optical electronics such as solar cells and light emitting diodes. CdS generally has n-conducting semiconductor properties and it is very difficult to fabricate p-conducting semiconductor thin films. However, it has been known by B. Ullrich et al that a p-conductive CdS thin film doped with copper (Cu) can be manufactured by vacuum laser deposition (Materials Science and Engineering B35 (1995) 117-159). Since the development of cadmium sulfide-based diodes by the homo-junction (homo-junction) rather than hetero-junction was made. T. Abe et al. Developed a Cd (Cu) S / CdS light emitting diode manufactured by vacuum deposition (phys.stat. Sol. (B) 229, No. 2, 1015.1018 (2002)). As described above, conventional cadmium sulfide diodes are generally manufactured using a vacuum process.

Meanwhile, cadmium sulfide thin films are widely used in compound solar cells. Recently, when a cadmium sulfide thin film is deposited by vacuum deposition such as sputtering, it is known to cause fine cracks in the underlying thin film to deteriorate the properties of the solar cell. Thus, a method of forming a cadmium sulfide thin film by chemical bath deposition is known. Has been developed.

Chemical bath deposition method is a thin film manufacturing method using a chemical reaction that takes place in a solution, attracting attention because the manufacturing process is simple and can produce a large area thin film quickly and inexpensively at low temperature. In addition, the stoichiometry of the thin film is Cd: S = 1: 1, which has the advantage that the film quality is dense and homogeneous. However, the chemical bath deposition method is mainly used to raise thin cadmium sulfide thin films of several tens of nanometers, and it is very difficult to raise a thick film of several hundred nanometers or more by using the chemical bath deposition method.

In chemical bath deposition, metal salts, chalcogenide sources (sulfur sources) and complex media, which are generally soluble salts of metal ions, are reacted by dissolving in water. Metal salts usually select salts that are soluble in water, such as chlorides, nitrates, sulfur oxides and acetate salts. Chalcogenide sources use thiourea and the like, and water provides oxygen. The complex medium providing the ligand is ammonia (NH ₃ ), triethanolamine (N (C ₂ H ₄ OH) ₃ ), citrate (C ₃ H ₅ O (COO) ₃ ^3- ), tartrate ((CHOH) ₂ (COO) ^2- ) and the like.

In the chemical vapor deposition of cadmium sulfide, the reaction takes place in the following order.

1) Equilibrium of Complex Media with Water

2) Formation and separation of metal complexes between metal ions and complex media

3) Hydrolysis of Sulfur Sources

4) Formation of Solid Membrane

The hydrolysis step of the third sulfur source of the above step is more likely to occur at higher concentrations of hydroxide ions (OH ⁻ ), that is, at higher pH. Therefore, generally, the reaction proceeds by increasing the concentration of ammonia, which is a complex medium that can raise the pH. On the other hand, when the OH ⁻ ions are excessive, surface complexes are generated on the surface of the substrate, and the surface complexes are regenerated into CdS films through decomposition of CdS and ligand ions. However, this process is very slow, causing the film to be thinner.

For this reason, it is very difficult to form a thin film of cadmium sulfide thickly by chemical bath deposition or chemical wet method. Therefore, until now, a method of fabricating p-conductive and n-conductive cadmium sulfide thin films using chemical bath deposition and forming a homojunction is not known.

An object of the present invention is to provide a method for producing a copper-doped p-conductive Cd (Cu) S thin film of 80 nm or more by a chemical wet method.

Another technical problem to be achieved by the present invention is to provide a method for producing an n-conductive CdS thin film of 200 nm or more by a chemical wet method.

Another object of the present invention is to provide a method for manufacturing a homojunction diode of Cd (Cu) S / CdS by a chemical wet method.

In order to achieve the above technical problem, a method of forming a Cd (Cu) S thin film by chemical vapor deposition according to an embodiment of the present invention includes forming at least one Cd salt, at least one Cu salt, and at least one sulfur. Dipping in an aqueous solution comprising a source, ammonia, ammonium chloride, and triethylamine (TEA).

The Cd salt may include cadmium chloride (CdCl ₂ ), cadmium sulfate (CdSO ₄ ) or cadmium acetate (Cd (CH ₃ COO) ₂ ). The Cu salt may comprise copper sulfate (CuSO ₄ ). The sulfur source may comprise thiourea (SC (NH ₂ ) ₂ ).

Wherein the concentration of ammonium chloride is 10 ⁻³ It is preferable that it is ^-10-2 M, and the concentration of the triethylamine is ^10-4 It is preferable that it is-5x10 <^-3> M.

It is preferable to heat the temperature of the said aqueous solution to 45-65 degreeC for about 20 to 60 minutes.

In another aspect, a method of forming a CdS thin film by chemical bath deposition according to an embodiment of the present invention is to provide a substrate with at least one Cd salt, at least one sulfur source, ammonia, ammonium chloride and triethyl Immersing the substrate in an aqueous solution comprising an amine.

The Cd salt may include cadmium chloride (CdCl ₂ ), cadmium sulfate (CdSO ₄ ) or cadmium acetate (Cd (CH ₃ COO) ₂ ). The sulfur source may comprise thiourea (SC (NH ₂ ) ₂ ).

The concentration of ammonium chloride is 10 ^-3 It is preferable that it is -10 ^-2 M, and the concentration of the triethylamine is 10 ^-4 It is preferable that it is-5x10 <^-3> M.

In the forming of the CdS thin film, the aqueous solution is preferably heated to 55 to 75 ° C. for about 20 to 60 minutes.

According to another aspect of the present invention, there is provided a method of manufacturing a Cd (Cu) S / CdS diode, the method including: forming a lower electrode on a substrate; At least one Cd salt, copper sulfate (CuSO ₄ ), thiourea (SC (NH ₂ ) ₂ ), ammonia (NH ₃ ), ammonium chloride (NH ₄ Cl) and triethylamine were added to the substrate on which the lower electrode was formed. Immersing in an aqueous solution containing (TEA: triethylamine) to form a p-conductive Cd (Cu) S thin film on the lower electrode; The substrate on which the Cd (Cu) S thin film is formed is immersed in an aqueous solution containing at least one Cd salt, thiourea, ammonia, ammonium chloride, and triethylamine in a reaction tank to form an n-type conductive film on Forming a CdS thin film; And forming an upper electrode on the CdS thin film.

The material of the lower electrode may include ITO or SnO ₂ (F).

The method may further include cleaning the substrate after forming the lower electrode.

In the forming of the Cd (Cu) S thin film, it is preferable to heat the aqueous solution to 45 to 65 ° C. for about 20 to 60 minutes.

The method may further include cleaning a surface of the Cd (Cu) S thin film after formation of the Cd (Cu) S thin film.

In the forming of the CdS thin film, it is preferable to heat the aqueous solution to 55 to 75 ℃ for about 20 to 60 minutes.

After the formation of the CdS thin film may further comprise the step of cleaning the surface of the CdS thin film.

The method may further include forming an insulator thin film after forming the Cd (Cu) S thin film and before forming the CdS thin film. The insulator thin film may be formed of a thermal oxide film. Alternatively, the insulator thin film may be formed of silicon oxide (SiO ₂ ) or titanium oxide (TiO ₂ ).

Alternatively, it may be formed of an aluminum oxide film (Al ₂ O ₃ ).

Hereinafter, with reference to the accompanying drawings will be described a preferred embodiment of the present invention; In the following description, when a component is described as being on top of another component, it may be directly on top of another component, and a third component may be interposed therebetween. In addition, in the drawings, the thickness or size of each component is omitted or exaggerated for convenience and clarity of description, and the same reference numerals in the drawings refer to the same element. On the other hand, the terms used are used only for the purpose of illustrating the present invention and are not used to limit the scope of the invention described in the meaning or claims.

1 is a schematic cross-sectional view of a Cd (Cu) S / CdS thin film diode formed according to an embodiment of the present invention. Referring to FIG. 1, a lower electrode 12, a p-conductive Cd (Cu) S thin film 13, an n-conductive CdS thin film 14, and an upper electrode 15 are sequentially stacked on the substrate 11. It is. Here, the substrate material may include quartz, glass or polymer film. The lower electrode 12 may use a transparent electrode such as indium tin oxide (ITO) or SnO ₂ (F). The Cd (Cu) S thin film 13 on the lower electrode 12 contains Cu and has a p conductivity type, and the CdS thin film 14 is undoped and has n conductivity type. As the upper electrode, a metal material such as copper, aluminum, tungsten, or the like may be used. The diode of this embodiment may be used as a light emitting diode, but the present invention is not limited to the light emitting diode.

2 is a schematic cross-sectional view of a reactor 20 for forming a Cd (Cu) S thin film or a CdS thin film according to an embodiment of the present invention. Referring to FIG. 2, the reaction vessel 20 contains a reaction solution 31 for forming a thin film in the reaction vessel 22 in the thermostat 21. The substrate 33 on which the thin film is to be formed is fixed to the jig 25 and completely immersed in the reaction solution 31. The magnetic stirrer 27 may be used to uniformly mix the reaction solution 31 during thin film formation.

How to Form a CdS Thin Film

A method of forming a CdS thin film by chemical bath deposition according to an embodiment of the present invention will be described.

Cadmium chloride (CdCl ₂ ) or cadmium sulfate (CdSO ₄ ) 0 .003 to 0.03 M, thiourea (SC (NH ₂ ) ₂ ) 0 .01 to 0.3 M, ammonia (NH ₃ ) 1. Submerge the substrate completely in an aqueous solution containing 0-1. 6 M, ammonium chloride (NH ₄ Cl) 0.001-0.01 M and triethylamine (TEA: 0.01) -0.005 M. The reactor is then heated to 55-75 ° C. for about 20-60 minutes and then cooled. Thereafter, the substrate on which the CdS thin film is formed is cleaned to remove coarse particles on the surface of the thin film.

Where cadmium chloride or cadmium sulfate is the source of cadmium and thiourea is the source of sulfur. Ammonia can prevent cadmium from forming and forming Cd (OH) ₂ by forming a complex with cadmium ions (Cd ²⁺ ). However, if the concentration of ammonia is too high, the pH is increased to accelerate the complex formation reaction on the surface of the substrate and to promote the colloid formation reaction in the solution, thereby slowing the formation of the thin film on the surface of the substrate, causing a thin CdS thin film.

As described above, CdS thin films are difficult to thicken by conventional chemical bath deposition. In the present invention, ammonium chloride and triethylamine were additionally used to obtain a more uniform and thick thin film. In the solution composition of the above example, when 0.11 to 0.01 M ammonium chloride was added, a CdS thin film having a thickness of 200 nm or more was formed. However, when more than 0.01 M of ammonium chloride alone was added, the CdS thin film began to become thinner, and when added in excess, the CdS thin film was not formed. When triethylamine was added alone, the thickness of the thin film tended to be slightly thin, but when 0.000 to 0.005 M was added with ammonium chloride, a cadmium sulfide thin film having a thickness of 200 nm or more was uniformly deposited.

The cause of this effect can be thought of as follows. That is, ammonium chloride, like ammonia, can serve as a buffer to control pH while providing a ligand to form a complex with cadmium. In addition, triethylamine can also form a complex compound with cadmium in an aqueous solution to lower the solidification rate of Cd (OH) ₂ , which competes with CdS thin film formation, and to suppress formation of large particles of Cd (OH) ₂ . Therefore, it is believed that colloidal particles are rapidly formed and the thin film can be uniformly formed by suppressing the mechanism by which the particles adhere to the substrate.

Immediately after formation of the CdS thin film by chemical vapor deposition, large particles adhere to the surface of the CdS thin film, and these particles can be removed through a cleaning process. The cleaning process can be carried out, for example, by ultrasonic cleaning in distilled water.

How to Form a Cd (Cu) S Thin Film

Next, a method of forming a Cd (Cu) S thin film by chemical bath deposition according to an embodiment of the present invention will be described.

Cadmium chloride (CdCl ₂ ) or cadmium sulfate (CdSO ₄ ) 0 .001 to 0.01 M, copper sulfate (CuSO ₄ ) 0.005 to 0.0001 M, thiourea (SC (NH ₂ ) ₂ ) 0. 01 to 0.5 M, ammonia (NH ₃ ) 1.1 to 1. 8 M, ammonium chloride (NH ₄ Cl) 0.001 to 0.01 M and triethylamine (TEA) 0.001 to 0 Submerge the substrate completely in an aqueous solution containing 005 M. The reactor is then heated to 45-65 ° C. for about 20-60 minutes and then cooled. Thereafter, the substrate on which the Cd (Cu) S thin film is formed is washed to remove coarse particles on the surface. As in the formation process of the CdS thin film, ultrasonic cleaning may be performed using distilled water.

Cd (Cu) S thin films are very difficult to deposit thick by conventional chemical bath deposition. In the absence of ammonium chloride and triethylamine, it is generally observed that very thin films of 20 nm or less form unevenly. However, Cd (Cu) S thin films of more than 80 nm were formed more uniformly when 0.001-0.005 M triethylamine was added together with 0.11-0.01 M ammonium chloride in the embodiment of the present invention. It became.

As in the method of forming a CdS thin film, cadmium chloride or cadmium sulfate is a source for supplying cadmium, and thiourea is a source for supplying sulfur. Copper sulfate is a source of copper. As with the method for forming the CdS thin film according to the present invention, it is believed that ammonium chloride can act as a buffer for controlling pH while providing a ligand to form a complex with cadmium. In addition, it is considered that triethylamine also forms a complex compound with cadmium in an aqueous solution to lower the solidification rate of Cd (OH) ₂ , which competes with CdS thin film formation, and to suppress formation of large particles of Cd (OH) ₂ .

How to Form a Cd (Cu) S / CdS Thin Film Diode

The Cd (Cu) S / CdS thin film diode described in FIG. 1 may be formed by forming the Cd (Cu) S and CdS thin films by the chemical bath deposition method described above. First, the lower electrode 12 is formed on the substrate 11. The substrate 11 may be formed of, for example, quartz, glass or a polymer film. The lower electrode 12 may be formed of a transparent material having high conductivity, such as ITO or SnO ₂ (F). A material such as ITO or SnO ₂ (F) is deposited on the substrate 11 by using a vacuum deposition method, and then the lower electrode 12 is patterned by using a photolithography method. The substrate 11 on which the lower electrode 12 is formed may be cleaned using, for example, alcohol or acetone.

Subsequently, a p-conductive Cd (Cu) S film 13 is formed on the lower electrode 12. The p-conductive Cd (Cu) S film 13 can be formed by the chemical bath deposition method described above. That is, the substrate 11 on which the lower electrode 12 is formed is fixed to the jig 25 and completely immersed in the aqueous solution of the reaction tank 20. Aqueous solution is cadmium chloride (CdCl ₂ ) or cadmium sulfate (CdSO ₄ ) 0.11 to 0.01 M, copper sulfate (CuSO ₄ ) 0.005 to 0.001 M, thiourea (SC (NH ₂ ) ₂ ) 0. 01 to 0.5 M, ammonia (NH ₃ ) 1.1 to 1. 8 M, ammonium chloride (NH ₄ Cl) 0.001 to 0.01 M and triethylamine (TEA) 0.001 to 0 Contains 005 M. The aqueous solution in which the substrate 11 is completely submerged is heated to 45 to 65 ° C. for about 20 to 60 minutes and then cooled to form a Cd (Cu) S thin film. Subsequently, coarse particles formed on the Cd (Cu) S thin film may be removed by ultrasonic cleaning using, for example, distilled water at room temperature. Subsequently, the substrate 11 may be dried, for example, in an oven.

Next, the n-conductive CdS film 14 on the p-conductive Cd (Cu) S film 13 can be formed by the chemical bath deposition method described above. The substrate 11 on which the p-conductive Cd (Cu) S film 13 is formed is fixed to the jig 25 and completely immersed in the aqueous solution of the chemical reactor 20. Aqueous solution is cadmium chloride (CdCl ₂ ) or cadmium sulfate (CdSO ₄ ) 0.03 to 0.03 M, thiourea (SC (NH ₂ ) ₂ ) 0.01 to 0.3 M, ammonia (NH ₃ ) 1. 0-1. 6 M, ammonium chloride (NH ₄ Cl) 0.001-0.01 M and triethylamine (TEA) 0.001-0.005 M. The aqueous solution in which the substrate 11 is completely submerged is heated to 55 to 75 ° C. for about 20 to 60 minutes and then cooled. Subsequently, the coarse particles formed on the CdS thin film 14 may be removed by ultrasonic cleaning using distilled water at room temperature as in the case of the Cd (Cu) S thin film 13 and dried in an oven.

The Cd (Cu) S and CdS thin films 13 and 14 can be patterned using lift off or etching. In the lift-off method, a photoresist pattern is formed on a portion where a thin film is not formed, and the Cd (Cu) S and CdS thin films 13 and 14 are formed. The thin film on the photoresist is then removed while the photoresist pattern is removed using an organic solvent. In the etching method, after forming the Cd (Cu) S and CdS thin films 13 and 14, a photoresist pattern is formed, the photoresist pattern is used as a mask, and the thin films 13 and 14 are etched and patterned. In order to prevent edge shorting, the size of the pattern of the CdS thin film 14 may be smaller than that of the Cd (Cu) S thin film 13 pattern.

Meanwhile, an insulator thin film may be formed between the Cd (Cu) S thin film 13 and the CdS thin film 14 to form a PIN diode. The insulator thin film may be formed by thermally oxidizing the Cd (Cu) S thin film 13 or by raising a heterogeneous insulator thin film such as SiO ₂ , TiO ₂ , Al ₂ O _3, or the like. In the first method, a Cd (Cu) S thin film 13 is raised and subjected to a cleaning process, and then heat-treated at 400 to 500 ° C. for 10 to 60 minutes to form an insulator thin film. It is preferable that the temperature increase rate be high at the time of heat treatment, and at least 10 ° C / min. In the second method, after the Cd (Cu) S thin film 13 is raised, heterogeneous insulating films such as SiO ₂ , TiO ₂ , Al ₂ O _3, or the like are deposited to a thickness of 20 nm or less by chemical vapor deposition, spin coating or electrophoresis. Oligo-high temperature heat treatment may be used to form an insulator thin film.

In addition, in order to remove impurities at the Cd (Cu) S / CdS interface to improve diode characteristics, the surface of the Cd (Cu) S thin film may be acid treated with nitric acid, hydrochloric acid, sulfuric acid, phosphoric acid, acetic acid, or a mixture thereof. Can be. The acid concentration is preferably not more than 0.5 M. The substrate after the acid treatment is preferably carried out in a CdS thin film forming process as soon as possible to prevent recontamination of the interface.

Next, the upper electrode 15 is placed on the CdS thin film 14. The upper electrode 15 may be formed of a conductive material such as copper, aluminum, tungsten, or the like by using a vacuum deposition method or an electroplating method. Patterning of the upper electrode 15 may use a lift off or an etching method.

The Cd (Cu) S / CdS light emitting diode uses a transparent substrate and a transparent lower electrode, and forms a thin p-conductive Cd (Cu) S thin film on the lower electrode and has a thick n-conductive CdS thin film thereon. It is preferable to form However, it is not necessary to limit the materials of the substrate and the lower electrode in the diode other than the light emission, and it is not necessary to limit the order of formation of the p-conductive Cd (Cu) S thin film and the n-conductive CdS thin film.

So far, the present invention has been described with reference to the embodiments shown in the drawings, which are merely exemplary, and those skilled in the art will understand that various modifications and equivalent other embodiments are possible therefrom. will be. Therefore, the true technical protection scope of the present invention will be defined by the technical spirit of the appended claims.

As described in detail above, according to the present invention, by additionally using ammonium chloride and triethylamine in the chemical bath deposition method, the Cd (Cu) S thin film and the CdS thin film can be formed thick and uniformly. The Cd (Cu) S / CdS homojunction thin film diode may be formed using the Cd (Cu) S thin film and the CdS thin film formation method.

Claims (20)

Chemical bath deposition comprising immersing the substrate in an aqueous solution comprising at least one or more Cd salts, at least one Cu salt, at least one sulfur source, ammonia, ammonium chloride (NH ₄ Cl) and triethylamine (TEA) A method of forming a Cd (Cu) S thin film by the method. The Cd (Cu) S thin film according to claim 1, wherein the Cd salt comprises cadmium chloride (CdCl ₂ ), cadmium sulfate (CdSO ₄ ), or cadmium acetate (Cd (CH ₃ COO) ₂ ). How to form. The method of claim 1, wherein the Cu salt forms a Cd (Cu) S thin film by chemical bath deposition including copper sulfate (CuSO ₄ ). The method of claim 1, wherein the sulfur source comprises thiourea (SC (NH ₂ ) ₂ ). The method of claim 1, wherein the concentration of ammonium chloride is 10 ^-3 A method of forming a Cd (Cu) S thin film by chemical bath deposition method of ˜10 ^-2 M. The method of claim 1, wherein the concentration of triethylamine is 10 ^-4 A method of forming a Cd (Cu) S thin film by chemical bath deposition method of ˜5 × 10 ⁻³ M. The method of claim 1, wherein the Cd (Cu) S thin film is formed by a chemical bath deposition method comprising heating the reactor to a temperature of 45-65 ° C. for about 20-60 minutes. A method for forming a CdS thin film by chemical vapor deposition comprising immersing a substrate in an aqueous solution comprising at least one Cd salt, at least one sulfur source, ammonia, ammonium chloride, and triethylamine (TEA). . The method of claim 8, wherein the Cd salt comprises cadmium chloride (CdCl ₂ ), cadmium sulfate (CdSO ₄ ), or cadmium acetate (Cd (CH ₃ COO) ₂ ). . The method of claim 8, wherein the sulfur source comprises thiourea (SC (NH ₂ ) ₂ ). The method of claim 8, wherein the concentration of ammonium chloride is 10 ^-3 A method of forming a CdS thin film by chemical bath deposition method of ˜10 ^-2 M. The method of claim 8, wherein the concentration of triethylamine is 10 ^-4 A method of forming a CdS thin film by chemical bath deposition method of ˜5 × 10 ⁻³ M. The method of claim 8, wherein the forming of the CdS thin film comprises heating the reactor to 55 to 75 ° C. 10. Forming a lower electrode on the substrate; At least one Cd salt, copper sulfate (CuSO ₄ ), thiourea (SC (NH ₂ ) ₂ ), ammonia (NH ₃ ), ammonium chloride (NH ₄ Cl) and triethylamine were added to the substrate on which the lower electrode was formed. Immersing in an aqueous solution containing (TEA: triethylamine) to form a p-conductive Cd (Cu) S thin film on the lower electrode; The substrate on which the Cd (Cu) S thin film is formed is immersed in an aqueous solution containing at least one Cd salt, thiourea, ammonia, ammonium chloride, and triethylamine in a reactor to form an n-type conductive film on the Cd (Cu) S thin film. Forming a CdS thin film; And Diode manufacturing method comprising the step of forming an upper electrode on the CdS thin film. The method of claim 14, wherein the lower electrode is formed of a transparent conductive film including an ITO electrode or a SnO ₂ (F) electrode. 15. The method of claim 14, wherein heating the reactor to 45 ~ 65 ℃ in the step of forming the Cd (Cu) S thin film. The method of claim 14, wherein the forming of the CdS thin film comprises heating the aqueous solution to a temperature of 55 to 75 ° C. for about 20 to 60 minutes. The method of claim 14, further comprising forming an insulator thin film after forming the Cd (Cu) S thin film and before forming the CdS thin film. The method of claim 18, wherein the insulator thin film is formed of a thermal oxide film. The method of claim 18, wherein the insulator thin film is formed of a silicon oxide film (SiO ₂ ), a titanium oxide film (TiO ₂ ), or aluminum oxide (Al ₂ O ₃ ).

Images