JP2958448B2 - Method and apparatus for manufacturing compound semiconductor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the field of manufacturing sulfide compound semiconductors such as CdS and ZnS, and such compound semiconductors are suitable for use in solar cells, optical sensors, light-emitting panels and the like. It is.

[0002]

2. Description of the Related Art There are various techniques for producing a semiconductor thin film, and a chemical vapor deposition method, a molecular beam epitaxy method, a liquid phase epitaxy method and the like have been put to practical use. However, since these methods are generally expensive, it is not practical to apply them to a semiconductor element used for a large area such as a solar cell.

For this reason, as a technique for producing a compound semiconductor thin film used for a solar cell or the like at a low cost and in a large area, a method of depositing a semiconductor from an aqueous solution has been attempted, and the following (a) electrodeposition method (Electrochemical deposition; ECD method),
(b) A number of two types of solution growth (chemical bath deposition; CBD) have been reported.

The electrodeposition method (a) is described, for example, in E. Fatas, R. Duo, P.
Herrasti, F. Arjona and E. Garcia-Camarero: J. Electroc
As published in hem. Soc. 131 (1984) p. 2243,
By applying a current to an aqueous solution, the component ions in the solution are reduced by the current, and a compound semiconductor is deposited on the cathode. While this method has the advantage that the reaction can be easily controlled by turning on / off the current, the absolute condition is that the substrate to be deposited is in principle conductive, and the choice of the substrate is limited. If conditions such as the deposition voltage are not properly set, only one of the elements (particularly, a metal element) may be excessively precipitated, resulting in a compound having a stoichiometric composition.

On the other hand, the solution growth method (b) described above
NRPavaskar, CAMenezes and APBSinba: J.Electro
Chem. Soc. 124 (1977) p. 743, and instead of using an electric current as in the electrodeposition method described above, a desired compound is produced by a chemical reaction in a solution until it becomes supersaturated, This is a method of depositing the supersaturated compound on a substrate. In this method, deposition can be performed on a non-conductive substrate regardless of the substrate, but the deposition reaction cannot be artificially controlled. In other words, once the solution is in a uniform supersaturated state, the start of the reaction, the reaction rate, and the stop of the reaction cannot be controlled. In addition, since the reaction takes place in the whole solution, it is deposited not only on the substrate but also on the side of the container, and the efficiency of the raw material is low. Furthermore, the solution must be used immediately after it is made, and of course cannot be used more than once.

[0006]

SUMMARY OF THE INVENTION It is an object of the present invention to enable mass production at low cost, to facilitate the control of the amount and quality of semiconductor film deposition, to make effective use of raw materials, and to determine the material of the substrate. An object of the present invention is to propose a novel method and apparatus for manufacturing a compound semiconductor, which has the advantage that the degree of freedom of the manufacturing process is large because of the absence of such a method.

[0007]

According to a method of manufacturing a compound semiconductor of the present invention, a substrate is immersed in an aqueous solution containing a metal ion and a thiosulfate ion of a semiconductor to be produced, and light in an ultraviolet region is directed toward the substrate. Irradiation causes a compound semiconductor consisting of metal sulfide to be formed on the substrate by a photochemical reaction. Here, in the manufacturing method of the present invention, the compound semiconductor is CdS, ZnS, Cu ₂ S, SnS
Alternatively, it is advantageously suitable to anneal the compound semiconductor at 300 ° C. or higher after being CuInS ₂ or after being deposited on a substrate. In addition, the apparatus for producing a compound semiconductor of the present invention includes a processing tank containing an aqueous solution containing metal ions and thiosulfate ions, and a supporting device for a substrate immersed in a bath of the processing tank. A light source for irradiating light in the ultraviolet region toward the substrate attached to the substrate. Here, in the manufacturing apparatus of the present invention, it is advantageous to provide a condenser lens between the substrate and the light source.

The present invention proposes a completely new method of forming a semiconductor thin film using light, which overcomes the above-mentioned drawbacks of the electrodeposition method and the solution growth method. This is a method called photochemical deposition (PCD).

In the photochemical deposition method (hereinafter, referred to as “PCD method”), the reaction of forming a compound is caused only by photoexcitation of ions in a solution (photochemical reaction).
Therefore, in the PCD method, the substrate is not required to be electrically conductive because electricity is not used.
The film thickness can be easily controlled by turning off and changing the intensity. In addition, the semiconductor film formed by the above-described electrodeposition method (ECD method) may contain a large amount of a single metal due to reduction of metal ions on the substrate. There is also an advantage that a single metal is not contained in a large amount due to the absence of the metal. Furthermore, the solution itself is stable unless irradiated with light containing a specific wavelength, and the reaction can be neglected in room lighting. In addition, since the photochemical reaction is limited to only the light irradiation region, it is possible to selectively form a semiconductor in a specific region.

As described above, the PCD method uses the above-described electrodeposition (ECD).
) Method and solution growth (CBD) method. In addition, the equipment for performing the PCD method is as simple and inexpensive as the electrodeposition method and the solution growth method, and can be easily enlarged, so that a large-area product can be manufactured. . From this viewpoint, the PCD method is a very advantageous method for manufacturing a semiconductor device such as a solar cell.

In the meantime, there has been an experimental example in which light irradiation was performed in the metal ECD (plating) method so far (for example, I. Zo
uari, F.Lapocque, M, Calvo and M.Cabrera: J.Electroche
m.Soc. 139 (1992) p.2163). In this case, the role of light was mainly to generate heat, and was only an auxiliary function of increasing the deposition rate by this heat.
Therefore, the role of light in the photochemical deposition method of the present invention is
It is clearly different.

[0012]

FIG. 1 shows an essential part of an example of an apparatus suitable for carrying out the manufacturing method of the present invention. In the figure, reference numeral 1 denotes a processing tank containing an aqueous solution containing metal ions and thiosulfate ions, 2 denotes a substrate immersed in a bath of the processing tank, and 3 denotes a supporting device for the substrate 2. 2
And a light source (not shown) disposed opposite to the substrate 2, a condenser lens 4 is provided, and a rotator 5 for stirring the aqueous solution is provided. Reference numeral 6 denotes a region in which a compound semiconductor is generated, and reference numeral 7 denotes an aqueous solution containing metal ions and thiosulfate ions.

The deposition mechanism of the sulfide semiconductor according to the present invention will be described by taking CdS as an example. In FIG. 2, using the above-described apparatus, 2 mM of cadmium sulfate (CdSO ₄ ) and sodium thiosulfate (Na ₂ S ₃ O ₃ ) were added to pure water as an aqueous solution.
Irradiate with light from a mercury lamp using 100 mM
The figure shows the light transmittance of an aqueous solution before and after PCD treatment when a compound semiconductor is formed by the PCD method.

FIG. 2 shows that the solution before the deposition of the compound semiconductor has an absorption edge (wavelength at which the transmittance starts to fall) around 300 nm. This means that only the shortest wavelength (about 250 nm) of the mercury lamp is absorbed by the solution.
The main chemical species in the solution are Cd ²⁺ , Na ⁺ , SO ₄ ²⁻ , and S ₂ O ₃ ²⁻ , including H ⁺ when the solution is acidic. Where Cd
When the transmittance of an aqueous solution in which only SO ₄ and NaOH were dissolved was measured, the absorption edge was much shorter than 250 nm in each case, and the spectrum in FIG. 2 did not depend on the pH value. from the absorption edge in the vicinity of 300 nm of the PCD pretreatment solution is by S ^₂ O ₃ ^2-, it can be said that only this ion is excited by light. The solution after deposition has another absorption edge near 500 nm as shown in FIG. The energy corresponding to this wavelength is the band gap of CdS at room temperature.
(2.42eV). This indicates that CdS particles were formed in the solution by light irradiation.

In order for CdS to be formed from the sample preparation solution as in the following formula (1), S is dissociated from S ₂ O ₃ ^2-
And supplying electrons to reduce Cd ²⁺ .

Embedded image Here, it is known that S dissociates in an acidic solution according to the following equation (2).

Embedded image However, in practice, the PCD process of the present invention proceeds even at pH values above 7.0, although the deposition rate is slower than acidic solutions. Therefore, it can be said that the dissociation process of S by light as in the following equation (3) exists.

Embedded image

Further, in the PCD method, since electrons are not supplied from the outside and only ions excited by light are S ₂ O ₃ ^2- , electrons are also supplied by excited S ₂ O ₃ ^2-. it is conceivable that. It is well known that S ₂ O ₃ ^2- works as a reducing agent, and the above formula (1) is obtained by a reaction such as the following formulas (4) and (5).
Is considered to be supplying electrons.

Embedded image

The SO ₃ ^{2- in the} formula (5) is generated by the formula (3).
In an acidic solution, S is supplied according to the formula (2), but without light irradiation, electrons are not supplied and CdS is not formed. Therefore, to reduce Cd ²⁺ as in equation (1), the equation
It can be said that the photoexcitation reactions (4) and (5) are indispensable. A major portion of the CdS molecules formed in solution are deposited on a substrate that serves as a site for heterogeneous nucleation. Others will drift in the solution between the substrate and the liquid surface and will be observed in the deposited solution as an absorption edge near 500 nm.

In the compound deposition technique of the present invention based on the above mechanism, an aqueous solution is used, and the compound formation reaction in the aqueous solution is controlled not by the current or the amount of the solvent but by light. Thus, the deposition method according to the present invention can deposit a sulfide semiconductor thin film on any substrate. Most of the sulfide semiconductors are direct transition semiconductors and are suitable for light-emitting and light-receiving photoelectric conversion devices. Further, the deposition apparatus can be realized with a simple structure, and a light source such as a mercury lamp that is easily and inexpensively available is sufficient.
Therefore, there is no obstacle to increasing the size of the device. Taking advantage of such advantages, it is possible to apply the semiconductor thin film to a low-cost, large-area deposition apparatus. Specifically, it can be used for manufacturing solar cells, optical sensors, light emitting panels, and the like.

An aqueous solution in which thiosulfate ions and raw material metal ions are present is essential for the formation of the compound. The aqueous solution used for the deposition is prepared by dissolving an appropriate salt containing each ion (thiosulfate ion is Na ₂ S ₂ O ₃ , and metal ion is a metal sulfate or chloride) in pure water. The amount of such thiosulfate ions and metal ions may be an amount sufficient to form a required semiconductor film thickness. The pH of the aqueous solution may be acidic, neutral or alkaline, and is preferably acidic from the viewpoint of the film formation rate. However, in the case of CdS, the pH is 3.
If it is less than 5, the composition deviates from the stoichiometric composition. It is at pH 3.5-6.0 that the stoichiometric composition can be obtained as-deposited. It should be noted that even if the pH is lower than 3.5 and the stoichiometric composition deviates from the stoichiometric composition as it is deposited, there is no particular problem since a sulfide semiconductor having a stoichiometric composition can be obtained by subsequent annealing.

When a substrate is immersed in this aqueous solution and irradiated with light having a wavelength of 300 nm or less toward the substrate, thiosulfate ion acts as a source of sulfur S and also acts as a reducing agent for metal ions, and sulfide Forming the semiconductor is as described above. Depending on the concentration in the aqueous solution, S ₂
O ₃ ^2- ions are excited to cause a photochemical reaction. The wavelength required for this reaction does not change depending on the type of semiconductor to be deposited. As a light source having such a wavelength, a light source whose emission spectrum extends to the ultraviolet region, such as a mercury lamp, is used.

The film forming speed depends on the concentration of the solution, pH, the depth of immersion of the substrate from the bath surface, the intensity of the solution stirring, and the like.
Can be easily controlled. Further, the substrate on which the sulfide semiconductor is deposited may be of any material such as glass or metal. Thus, it is possible to produce CdS, ZnS, Cu ₂ S, a sulfide semiconductor _2, such as SnS or CuInS. After forming the sulfide semiconductor on the substrate, annealing at 300 ° C. or higher improves the composition and crystallinity.

An apparatus for manufacturing a compound semiconductor according to the present invention includes a processing tank for containing an aqueous solution containing metal ions and thiosulfate ions, and a device for supporting a substrate immersed in a bath of the processing tank. It has a light source for irradiating light including a wavelength of 300 nm or less to the substrate attached to the supporting device. The light source may be placed on the upper part of the processing tank so as to face the substrate, and the light may be irradiated directly toward the substrate.However, if the light is reflected and reflected by an Al-coated mirror or the like, the degree of freedom of the arrangement of the light source is increased. Increase. Providing a condensing lens between the substrate and the light source allows light to be condensed by the lens according to the size of the substrate and the area to be deposited, and causes a compound generation and deposition reaction only in a desired range. It is preferred. By condensing light with a lens, a sulfide semiconductor can be selectively formed in a partial region of the substrate. Alternatively, a sulfide semiconductor can be selectively formed in a partial region of the substrate by providing a mask material between the light source and the substrate. The treatment tank may be provided with a means for stirring the solution to promote the reaction of the source ions on the substrate, for example, a rotor. Further, an elevating means of the substrate supporting device may be provided so that the depth at which the substrate is immersed from the bath surface can be adjusted.

[0023]

EXAMPLES As sample preparation solutions, 2 mM cadmium sulfate (CdSO ₄ ) and 10% sodium thiosulfate (Na ₂ S ₂ O ₃ ) were added to pure water.
A solution in which 0 mM was dissolved was used. The pH of the solution is adjusted to sulfuric acid (H ₂ S
O ₄ ) was adjusted to various values from 3.0 to 8.0. A 1.5 cm × 1.0 cm square degreased glass substrate was immersed in this solution at a depth of about 5 mm from the solution surface as shown in FIG. 1, and light from a high-pressure mercury lamp was collected by a lens and irradiated from above. The diameter of the irradiation area was about 10 mm. During the deposition, the solution was stirred by a rotor, and the deposition time was 1 hour. The solution temperature increased slightly during the deposition, but did not exceed 30 ° C. By these treatments, the CdS
Was deposited. After the deposition process, the sample is washed with pure water,
After air drying, 300 ℃, 500 ℃ in nitrogen atmosphere
For 30 minutes at each temperature in a quartz tube.

As a result, a CdS thin film was deposited on the light-irradiated side of the glass substrate by the PCD method regardless of the pH value. Therefore, the following mainly describes samples deposited at pH = 3.0. This CdS thin film was deposited in a circular shape on a glass substrate. The thickness of the sample increases rapidly from the outer periphery of the circle toward the center, but immediately becomes almost constant and reaches the center.
The diameter of the flat part near the center is about 8mm and the thickness is as-dep
It was about 1.5 μm (deposition time: 1 hour) for the esited sample. The color of the sample was yellow when as-depesited and approached orange after annealing.

For comparison, samples were prepared by the electrodeposition (ECD) method. The sample was prepared by adjusting the pH value of the sample preparation solution to 3.0 and the following method. 2 mM CdSO ₄ and Na ₂ S in pure water
_A solution in which 100 mM of ₂ O ₃ was dissolved was used as a sample preparation solution. The saturated electrode is used as a reference electrode, and the cathode potential is kept at -1.0 vs SCE by potentiostat. 2.0 for anode
A 1.5 cm × 1.0 cm square Nesa glass (In ₂ O ₃ ) substrate (sheet resistance 10 Ω / cm ² ) was used for the stainless steel plate of each cm × 2.0 cm and the cathode. During the deposition, the solution was stirred by a rotor, and the deposition time was 1 hour.

The samples obtained by the PCD method and the ECD method were subjected to Raman scattering measurement, X-ray diffraction (XRD) and composition analysis. Raman scattering measurements were performed at 488 nm Ar
The measurement was performed at room temperature using an ion laser as a light source in a back scattering arrangement. XRD was measured using Kα radiation of Cu tube. Also,
Composition analysis was performed by Auger electron spectroscopy (AES).

FIG. 3 shows a Raman spectrum of a thin film produced by the PCD method. In both the as-deposited sample and the annealed sample, a peak appeared around 305 cm ^-1 . Since this peak position coincides with the position of the LO phonon peak of CdS, it can be confirmed that the thin film deposited by the PCD method is CdS. Further, the peak intensity decreases as the annealing temperature increases. On the other hand, in the Raman spectrum of the thin film deposited by the ECD method, although not shown, the peak intensity of CdS increased by annealing from 300 ° C to 500 ° C.

FIG. 4 shows the change in the half width of the peak of the Raman spectrum of the sample obtained by the PCD and ECD methods with respect to the annealing temperature. As-deposited for PCD method
The sample had a half width of 18 cm ^-1 . The annealing at 300 ° C. and 500 ° C. was 14.5 and 13.5 cm ⁻¹ , respectively, indicating that the half-width decreased as the temperature increased. This indicates that the higher the annealing temperature, the better the crystallinity. On the other hand, since the half-width of the as-deposited CdS thin film by the ECD method is 20 cm ^-1 which is larger than that produced by the PCD method, it can be said that the production by the PCD method is superior in quality. Furthermore, the film produced by the PCD method also gave better results in improving the crystallinity by annealing.

FIG. 5 shows Cd produced by the PCD method.
This is the result of X-ray diffraction (XRD) of the S thin film. as-depos
In the ited sample, peaks are seen at 2θ = 26.2 °, 45.3 ° and 51.6 °. These are the hexagonal CdS (00
2), (110), and (112) correspond to reflection. The result is P
The sample as-deposited by the CD method shows that it is polycrystalline. In addition, a small peak exists at 22.6 °, which corresponds to the (131) reflection of orthorhombic S, indicating that there is a single S that is not bonded to Cd. This single S causes the as-deposited sample to appear yellow in color, and has a lower transmittance than the annealed sample. Peak around 2θ = 30 ° and 2θ = 15-40
The broad background peak over ° is due to the substrate.

As can be seen from FIG. 5, after annealing at 300 ° C., the (002) peak of CdS became slightly sharper than that of as-deposited, and the S peak disappeared.
This is because excessive S vaporized as a simple substance was vaporized by annealing. Thus, it is considered that the color of the sample was changed to orange. In the sample annealed at 500 ° C., the peak at 2θ = 26.2 °, which was broad in as-deposited, was 2θ = 24.6 °, 26.2 ° and 2θ.
Divided into three sharp peaks at 7.9 °, 2θ = 26.2
° peak intensity increased. The peaks at 2θ = 24.6 and 27.9 ° correspond to the (100) and (101) reflections of hexagonal CdS, respectively, and the peaks due to the (110) and (112) reflections also became narrower and the intensity increased. Therefore, the crystallinity was significantly improved by annealing at 500 ° C.

On the other hand, FIG. 6 shows the result of XRD of the sample prepared by the ECD method. The orientation of CdS and the improvement of crystallinity by annealing are the same as those by the PCD method. But as-d
In eposited, 2θ = 31.8, 34.8 and 38.2 °
There are considerably large peaks due to (002), (100), and (101) reflections, and the Cd peak at 2θ = 38.2 ° remains even after annealing at 500 ° C. This single Cd makes the as-deposited sample gray, and is considered to be blocking light transmission. Since the increase in the peak intensity of CdS due to annealing is larger than that produced by the PCD method, it is considered that the bonding between Cd and S is promoted by annealing. That is, a part of CdS in the semiconductor film is generated for the first time by annealing, which causes an increase in Raman peak intensity. From the above, in the production by the ECD method, there are a lot of single Cd and S that are not bonded to each other in the as-deposited state, but in the production by the PCD method according to the present invention, the as-d
At the time of eposited, it can be said that all of Cd are already bonded to S.

FIG. 7 shows the Cd / S ratio obtained from the AES measurement result of the CdS thin film prepared by the PCD method. Elements other than Cd and S were not detected in this AES measurement. pH = 3.0
It can be seen that the as-deposited sample prepared in step 2 is S-rich. However, after annealing at 300 ° C. or more, excess S vaporized and was improved to a film having almost stoichiometric composition.

FIG. 8 shows the Cd / S ratio obtained by AES measurement for as-deposited samples prepared under various conditions from pH = 3.0 to 8.0. At pH = 3.0, the film is considerably S-rich, and it can be seen that the film is close to the stoichiometric composition from pH = 3.5 to 6.0. At pH = 8.0, it becomes slightly Cd-rich. When the acidity reaches about pH = 3.0, the more the composition of the fabricated film deviates from the stoichiometric one, the more the formula
It is considered that the dissociation of S by (2) is active. On the other hand, at pH = 8.0, since the supply of S is performed only by the formula (3), the supply amount of S is smaller than that of the acidic solution, and the formula (1)
It is considered that there is a reaction containing no S.

Next, a solution prepared by dissolving 2 mM ZnSO ₄ in place of CdSO ₄ was prepared by the PCD method. FIG. 9 shows the Raman spectrum of the ZnS thin film thus obtained. The LO phonon peak position of ZnS is 35
A peak appeared at 0 cm ⁻¹ , confirming that ZnS was formed. The deposition mechanism is considered to be similar to that of CdS.

[0035] Cu ₂ S thin film is CuSO ₄ instead of CdSO _4, SnS
Is also SnSO _4, CuInS ₂ can be formed by using a CuSO ₄ and In ₂ (SO _{4) 3.}

[0036]

According to the present invention, a substrate is immersed in an aqueous solution containing a metal ion and a thiosulfate ion of a raw material of a semiconductor to be made, and the substrate is irradiated with light in the ultraviolet region, and the metal is produced by a photochemical reaction. By forming a compound semiconductor made of sulfide on a substrate, the following effects can be obtained. (a) The equipment is extremely simple and inexpensive, and can be easily scaled up. (b) The use of light enables temporal and spatial control of the reaction. That is, the reaction can be started and stopped by irradiating / blocking the light, and the reaction can be caused locally (only on the substrate) by condensing the light. (c) The substrate is not limited to a conductive one, and various substances can be used. (d) A reaction in which only metal elements are selectively deposited hardly occurs. (e) The solution used for the reaction is stable without spontaneous reaction unless exposed to light, and its management is easy. In this way, the disadvantages of the conventional method can be overcome while maintaining the advantage of low cost and large area deposition.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example of an apparatus suitable for carrying out a manufacturing method of the present invention.

FIG. 2 is a diagram showing light transmittance of an aqueous solution before and after PCD treatment.

FIG. 3 is a diagram showing a Raman spectrum of a thin film manufactured by a PCD method.

FIG. 4 is a diagram in which the half width of the peak of the Raman spectrum of a sample obtained by the PCD and the ECD method is plotted with respect to the annealing temperature.

FIG. 5 is a view showing an X-ray diffraction pattern of a CdS thin film produced by a PCD method.

FIG. 6 is a diagram showing an X-ray diffraction pattern of a sample prepared by an ECD method.

FIG. 7 is a diagram showing the temperature dependence of the Cd / S ratio obtained from the result of AES measurement of a CdS thin film produced by the PCD method.

FIG. 8 is a diagram showing the pH dependence of the Cd / S ratio obtained by AES measurement for an as-deposited sample.

FIG. 9 is a diagram showing a Raman spectrum of a ZnS thin film formed by a PCD method.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Processing tank 2 Substrate 3 Supporting device 4 Condensing lens 5 Rotor

──────────────────────────────────────────────────続 き Continuation of the front page (56) References Fumitaka Goto, "Preparation and evaluation of sulfide semiconductors by photochemical deposition", IEICE Technical Report, Vol. 97, No. 59, May 23, 1997, p. 87-92 (58) Field surveyed (Int. Cl. ⁶ , DB name) C30B 1/00-35/00 CA (STN) JICST file (JOIS)

Claims (5)

(57) [Claims] 1. A compound comprising a metal sulfide by a photochemical reaction by immersing a substrate in an aqueous solution containing metal ions and thiosulfate ions of a raw material of a semiconductor to be formed, irradiating the substrate with light in the ultraviolet region. A method for manufacturing a compound semiconductor, comprising forming a semiconductor on a substrate. 2. The compound semiconductor is CdS, ZnS, Cu ₂ S, SnS.
_{2. The} method according to claim 1, wherein the compound semiconductor is CuInS2. 3. The method according to claim 1, further comprising annealing the compound semiconductor at 300 ° C. or more after depositing the compound semiconductor on the substrate. 4. A processing tank containing an aqueous solution containing metal ions and thiosulfate ions, and a device for supporting a substrate immersed in a bath of the processing bath, and facing a substrate attached to the supporting device. An apparatus for manufacturing a compound semiconductor, comprising a light source for irradiating light in an ultraviolet region. 5. The compound semiconductor manufacturing apparatus according to claim 4, further comprising a condenser lens between the substrate and the light source.