Patent Application
20090217969
The present invention relates to a method for manufacturing a photoelectric converter and a photoelectric converter, and more particularly, it relates to a photoelectric converter such as a photosensor or a solar battery employing a compound semiconductor thin film having a chalcopyrite structure.
A thin-film solar battery employing CuInSe2 (CIS thin film) which is a semiconductor thin film with a chalcopyrite structure made of a group Ib element, a group IIIb element and a group VIb element or Cu(In,Ga)Se2 (CIGS thin film) prepared by solid soluting Ga therein as a light absorptive layer advantageously exhibits high energy conversion efficiency and has small deterioration of the efficiency resulting from photoirradiation or the like.
a) to 8(d) are sectional views of a device for illustrating a conventional method for manufacturing cells of a CIGS thin-film solar battery.
As shown in
Then, a light absorptive layer 3 made of a CIGS thin film exhibiting a p type by composition control is formed on the Mo electrode layer 200, as shown in
Then, a buffer layer 400 of CdS or the like is formed on the light absorptive layer 3, and a light-transmitting electrode layer 500 made of ZnO (zinc oxide) for forming a minus-side upper electrode doped with an impurity to exhibit an n+ type is formed on the buffer layer 400, as shown in
Then, the layers from the light-transmitting electrode layer 500 made of ZnO up to the Mo electrode layer 200 are collectively scribed with a mechanical scriber, as shown in
Laser scribing (scribing technique of partially removing a thin film by applying a laser beam) can also be utilized for this scribing step.
In this case, however, the laser beam is so concentrically applied that high heat is locally generated, whereby the characteristics of the cells are disadvantageously deteriorated.
Therefore, the cells of the solar battery are generally electrically isolated from one another with a mechanical scriber.
A mechanical scriber described in Patent Document 1 performs scribing by vertically pressing a blade tapered at a prescribed cone angle and having a flat tip against scribed portions of a workpiece with a prescribed pressure and moving the blade along the worked surface.
A CIS thin film and a CIGS thin film are mainly utilized as a solar battery.
The inventors of this invention have noted the high optical absorption coefficient of this compound semiconductor thin film material and the characteristic of high sensitivity over a wide wavelength range from visible light to near infrared light, and conducted a study as to utilization of this compound semiconductor thin film as an image sensor for a security camera (camera sensing visible light by day and sensing near infrared light by night), a personal identification camera (camera for personal identification with near infrared light not influenced by external light) or an onboard camera (camera loaded on a vehicle for nightly visual aid or distant visual field assurance).
As a result, it is clarified that a dark current (current flowing in a P-N junction in reverse bias) is unexpectedly large in the case of the CIS thin film (CuInSe2) for example.
In other words, a dark current of about 1×10−3 A/cm2 flows in a reverse bias of −0.8 V, for example, and this value is greater than that of a silicon material by six orders of magnitude. If nothing is done, it leads to a consequence that weak light of less than 1000 Lux is hidden by noise and cannot be detected.
Therefore, the dark current must be remarkably reduced. The inventors have detailedly investigated the factor increasing the dark current in the compound semiconductor thin film. The result has confirmed that damages or defects are caused in the crystals of the compound semiconductor material in mechanical scribing thereby forming a plurality of energy levels on the interface of the P-N junction to cause a large dark current.
Referring to
The present invention has been proposed on the basis of this consideration, and an object thereof is to remarkably reduce a dark current in a photoelectric converter employing a compound semiconductor thin film.
A method for manufacturing a photoelectric converter according to the present invention is a method for manufacturing a photoelectric converter configured by laminating a lower electrode layer on a substrate, a compound semiconductor thin film having a chalcopyrite structure serving as a light absorptive layer and a light-transmitting electrode layer, the method including the step of patterning the compound semiconductor thin film having the chalcopyrite structure by photolithography.
According to this structure, the compound semiconductor thin film having the chalcopyrite structure is patterned by photolithographic etching in place of mechanical scribing. In this case, the respective ones of the lower electrode layer and the light-transmitting electrode layer are also patterned by etching using photolithography. Thus, neither damages nor defects are caused in the compound semiconductor thin film unlikely the case of performing mechanical scribing, and the dark current can be remarkably reduced.
In the method for manufacturing a photoelectric converter according to the present invention, the step of patterning the compound semiconductor thin film having the chalcopyrite structure includes a first step of patterning the compound semiconductor thin film by dry etching, and a second step of removing etching residues resulting from the first step by wet etching.
In patterning of the compound semiconductor thin film having the chalcopyrite structure serving as the light absorptive layer, two-stage etching combining dry etching and wet etching with each other is performed, thereby implementing precise pattern formation with no residues. While a p-type compound semiconductor thin film having a chalcopyrite structure serving as a light absorptive layer can be etched by dry etching, complete etching cannot be performed but a large number of columnar residues remain in this case. In wet etching, on the other hand, an undercut is formed by side etching (phenomenon that lateral etching progresses as to a portion located immediately under a resist pattern), and the pattern cannot be precisely formed. Therefore, the compound semiconductor thin film is patterned by dry etching with no side etching, and only residues resulting from dry etching are removed by subsequent short-time wet etching, whereby a precise pattern can be formed without causing residues. Thus, neither damages nor defects are caused in the crystals of the compound semiconductor unlikely to a case of employing mechanical etching, and the dark current can be remarkably reduced.
The method for manufacturing a photoelectric converter according to the present invention includes that in the first step performing dry etching with a chlorine gas employed as an etchant, and in the second step treating the compound semiconductor thin film having the chalcopyrite structure with a mixed solution of bromine and methanol or a mixed solution of water and ammonia and thereafter treating the same with a mixed solution of hydrochloric acid and nitric acid.
The compound semiconductor thin film constituting the light absorptive layer can be etched at a high rate by employing the chlorine gas as the etchant for dry etching. Also wet etching, residues can be efficiently and completely removed.
In the method for manufacturing a photoelectric converter according to the present invention, the compound semiconductor thin film having the chalcopyrite structure is Cu(Inx,Ga(1-x))Se2 (0≦x≦1).
Widening of the band gap of a CIS thin film (CuInSe2) is effective for reduction of the dark current, whereby the CIGS thin film prepared by substituting gallium for part of In (indium) is used. The recombination process of carriers can be reduced and the dark current can be reduced by widening the bandwidth.
As the compound semiconductor thin film having the chalcopyrite structure, other compound semiconductor thin film is also applicable, such as a film with CuAlS2, CuAlSe2, CuAlTe2, CuGaS2, CuGaSe2, CuGaTe2, CuInS2, CuInSe2, CuInTe2, AgAlS2, AgAlSe2, AgAlTe2, AgGaS2, AgGaSe2, AgGaTe2, AgInS2, AgInSe2, AgInTe2.
In the method for manufacturing a photoelectric converter according to the present invention, the step of forming the compound semiconductor thin film having the chalcopyrite structure includes the step of forming a Cu(Inx,Ga(1-x))Se2 (0≦x≦1) thin film by CVD, and the step of forming the light-transmitting electrode layer includes the steps of forming a non-doped ZnO film on the compound semiconductor thin film and forming an n+-type ZnO film on the non-doped ZnO film.
According to this structure, by providing the non-doped ZnO film (i-ZnO) as the light-transmitting electrode layer thereby filling up voids and pinholes formed in the underlayer CIGS thin film with a semi-insulating layer and forming an i-p junction with the CIGS thin film, a leakage resulting from a tunnel current can be prevented that is caused when a conductive ZnO film (n+) is directly brought into contact with a CIGS thin film. Therefore, the dark current on the P-N junction interface can be reduced by increasing the thickness of the non-doped ZnO film (i-ZnO). Here, PVD denotes a method of forming a film by depositing a raw material evaporated in a vacuum.
A photoelectric converter according to the present invention is a photoelectric converter manufactured by the method for manufacturing a photoelectric converter according to the present invention.
The photoelectric converter according to the present invention is precisely patterned by photolithography without leaving residues so that neither damages nor defects are caused in the crystals of the compound semiconductor unlike the case of employing mechanical scribing. Therefore, no unnecessary energy levels are formed on the interface of the P-N junction, and the dark current can be dramatically reduced. In other words, the dark current can be improved on the order of 103 by changing the manufacturing process and optimizing the device structures of the light absorptive layer and the light-transmitting electrode layer. Further, the dark current can be reduced on the order of 102 by band gap control in Cu(Inx,Ga(1-x))Se2
According to one aspect, the photoelectric converter according to the present invention is a photosensor having a sensitivity also in a near infrared region.
A sensor according to the present invention obtained by forming a photosensor (image sensor) using a compound semiconductor thin film in which the dark current is remarkably reduced has high a sensitivity also for near infrared light. As a result, the sensor of the present invention is sufficiently utilizable as a security camera (camera sensing visible light by day and sensing near infrared light by night), a personal identification camera (camera for personal identification with near infrared light not influenced by external light) or an onboard camera (camera loaded on a vehicle for nightly visual aid or distant visual field assurance).
The photoelectric converter according to the present invention is a solar battery.
In the photoelectric conversion according to the present invention, photoelectric conversion loss on the interface of the P-N junction is sufficiently reduced as compared with the prior art. Therefore a solar battery having a high efficiency of collection for charges resulting from light and high photoelectric conversion efficiency can be implemented.
According to the present invention, neither damages nor defects are caused in the compound semiconductor thin film unlike to the case of performing mechanical scribing but the dark current can be remarkably reduced by employing the technique of patterning the lower electrode layer and the compound semiconductor thin film with the chalcopyrite structure serving as the light absorptive layer by photolithographic etching.
As to the light absorptive layer, precise patterning can be implemented without causing damages or defects in the crystals of the compound semiconductor thin film and without leaving residues by performing two-stage etching combining dry etching and wet etching with each other.
The compound semiconductor thin film constituting the light absorptive layer can be precisely etched at a high rate by employing the chlorine gas as the etchant for dry etching. Also in wet etching, residues can be efficiently and completely removed.
Further, the recombination process of carriers can be reduced and the dark current can be reduced (reduced to ⅕) by using the CIGS thin film prepared by substituting gallium for part of In (indium) and widening the bandwidth.
The photoelectric converter according to the present invention is precisely scribed by photolithography without leaving residues so that neither damages nor defects are caused in the crystals of the compound semiconductor unlike to the case of employing mechanical scribing. Therefore, no unnecessary energy levels are formed on the interface of the P-N junction, and the dark current can be remarkably reduced. In other words, the dark current can be improved on the order of 103 by changing the manufacturing process and optimizing the device structures of the light absorptive layer and the light-transmitting electrode layer. Further, the dark current can be reduced on the order of 102 by band gap control in Cu(Inx,Ga(1-x))Se2.
The sensor according to the present invention has high a sensitivity also for near infrared light, whereby the same is sufficiently utilizable as a security camera (camera sensing visible light by day and sensing near infrared light by night), a personal identification camera (camera for personal identification with near infrared light not influenced by external light) or an onboard camera (camera loaded on a vehicle for nightly visual aid or distant visual field assurance).
In the photoelectric conversion according to the present invention, photoelectric conversion loss on the interface of the P-N junction is sufficiently reduced as compared with the prior art. Therefore a solar battery having a high collection efficiency for charges resulting from light and high photoelectric conversion efficiency can be implemented.
The foregoing and other objects, features and effects of the present invention will become more apparent from the following detailed description of the embodiments with reference to the attached drawings.
Embodiments of the present invention are now described with reference to the drawings.
In this manufacturing process, the respective ones of a laminated/formed lower electrode layer, a p-type compound semiconductor thin film (hereinafter referred to as a CIGS thin film) having a chalcopyrite structure serving as a light absorptive layer and a light-transmitting electrode layer (including both of a non-doped portion and a portion doped with an impurity to exhibit an n+ type) are patterned by photolithography for minimizing damages to the crystals of the CIGS thin film.
In the method for manufacturing a photoelectric converter according to the present invention, a Mo (molybdenum) layer for forming a lower electrode is formed by sputtering (about 0.6 μm) on a glass substrate, and thereafter patterned by photolithography (step S1). Thus, an electrically isolated island region of Mo is formed.
Then, a CIGS thin film is formed and patterned by photolithography (step S2). This step S2 is the most important characteristic point of the method according to the present invention.
In other words, a compositionally-controlled p-type CIGS thin film (Cu(Inx,Ga(1-x))Se2 (0≦x≦1)) is formed by ion beam sputtering, for example (step S2a). The thickness of this film is about 1.7 μm.
Then, a CIGS thin film (p−) is patterned by two-stage etching employing both dry etching and wet etching (step S2b). Thus, an electrically isolated CIGS thin film (p−) is obtained.
Then, a thin CdS film (about 50 nm) serving as a buffer layer (window layer) is formed by solution growth (step S3).
Then, a non-doped ZnO film (i-ZnO) and a low-resistance ZnO (n+) film doped with an impurity are continuously formed by sputtering (step S4). The i-ZnO film has a small thickness of about 60 nm, while the low-resistance ZnO (n+) film has a sufficient thickness of about 1 μm.
The non-doped ZnO film (i-ZnO) functions to fill up voids and pinholes formed in the underlayer CIGS thin film with a semi-insulating layer and form an i-p junction with the CIGS thin film, thereby preventing a leakage resulting from a tunnel current caused when the low-resistance ZnO (n+) film is directly brought into contact with the CIGS thin film.
Therefore, the dark current on the P-N junction interface can be reduced by increasing the thickness of the non-doped ZnO film (i-ZnO). (Results confirmed that the effect of reducing the dark current was about ⅕). However, since the increased thickness is sufficiently small (60 nm, for example), a substantial p-n junction is conceivably formed between the low-resistance ZnO film (n+) serving as a light-transmitting electrode layer and the CIGS thin film (p−).
Then, the ZnO film (i-ZnO and ZnO (n+)) is patterned (step S5). In this step S5, the ZnO film is patterned that the position of an end thereof is inward beyond the side surface of the patterned CIGS thin film (p−) (in other words, the ZnO film having a smaller lateral width than that of the CIGS thin film (p−) is formed on the CIGS thin film (p−) to form a pyramidal structure).
Thus, an end of the interface (boundary between depletion layers) of the P-N junction formed by the ZnO (n+) film and the CIGS thin film (p−) does not reach the sidewall (surface exposed by etching) of the patterned CIGS thin film (p−), whereby an embedded structure of the P-N junction interface is implemented.
The numbers of crystal defects and damages on the side surface of the CIGS thin film (p−) patterned by photolithography are sufficiently small as compared with the case of mechanical scribing. However, dangling bonds and the like could be still present and it cannot be said that no unnecessary energy levels are formed. Therefore, a leakage current from the end of the interface of the P-N junction can be further reduced by preventing the end of the P-N junction interface from reaching the side surface (surface exposed by etching) of this patterned CIGS thin film (p−).
Finally, an extraction electrode is formed (step S6). Thus, a photoelectric conversion device is completed.
The method for manufacturing a photoelectric converter according to the present invention is now specifically described with reference to
a) to 2(j) are sectional views of the device along the respective principal steps for specifically illustrating the method for manufacturing a photoelectric converter according to the present invention.
In the following description,
First, a Mo (molybdenum) film is formed on a soda-lime glass substrate (SLG) 10 by sputtering with a thickness of 0.6 μm, as shown in
As shown in
As shown in
Therefore, the residues 35 are completely removed by performing treatment with a mixed solution of bromine and methanol or a mixed solution of water and ammonia, and thereafter performing treatment with a mixed solution of hydrochloric acid and nitric acid, as shown in
Thus, the CIGS thin film 30 constituting a light absorptive layer can be precisely etched at a high rate without causing an undercut by employing the chlorine gas as the etchant for dry etching. Thereafter short-time wet etching is performed for completely removing the columnar residues 35. Thus, the CIGS thin film can be precisely patterned without causing residues. In this case, neither damages nor defects are caused in the crystals of the CIGS thin film 30 unlike to a case of employing mechanical etching, and the dark current can be remarkably reduced.
Then, the resist pattern 40 is removed.
As shown in
The ZnO film 60 is formed by continuously forming a non-doped ZnO film (i-ZnO) and a low-resistance ZnO (n+) film (denoted as n-ZnO in the Figure) doped with an n-type impurity. The thickness of the i-ZnO film is about 60-nm, and the thickness of the low-resistance ZnO (n+) film is about 1 μm.
The non-doped ZnO film (i-ZnO) functions to fill up voids and pinholes formed in the underlayer CIGS thin film with a semi-insulating layer and form an i-p junction with the CIGS thin film 30, thereby preventing a leakage resulting from a tunnel current.
Therefore, the dark current on the P-N junction interface can be reduced by increasing the thickness of the non-doped ZnO film (i-ZnO). However, since the increased thickness is sufficiently small (60 nm, for example), a substantial p-n junction is conceivably formed between the low-resistance ZnO (n+) film functioning as the light-transmitting electrode layer and the CIGS thin film (p−).
Then, a resist pattern 70 is formed as an etching mask and the ZnO film (i-ZnO and ZnO (n+)) 30 is thereafter wet-etched, as shown in
In other words, wet etching is performed with a dilute acid of hydrochloric acid:water=1:10, for example. At this time, the etching time is adjusted for intentionally causing side etching and forming an undercut under the resist pattern 70. This is intended that the position of an end of the patterned ZnO film 60 is inward beyond the side surface of the patterned CIGS thin film (p−) 60.
Then, dry etching is performed using the resist pattern 70 for partially removing the CdS film 50 on the sidewall of the CIGS thin film 30.
Then, the resist pattern 70 is removed, as shown in
Consequently, an end of an interface 80 (boundary between depletion layers: shown by a dotted line in
The numbers of crystal defects and damages on the sidewall (surface exposed by etching) of the CIGS (p−) thin film patterned by photolithography are sufficiently small as compared with the case of mechanical scribing. However, dangling bonds and the like are still present and it cannot be said that no unnecessary energy levels are formed.
Therefore, the leakage current from the end of the interface of the P-N junction can be further reduced by preventing the end of the P-N junction interface 80 from reaching the sidewall of this patterned CIGS (p−) thin film 30.
Finally, extraction electrodes 90 and 95 made of aluminum or the like are formed, as shown in
This photoelectric converter is also applicable as a high-efficiency solar battery.
In this embodiment, an example of using the photoelectric converter according to the present invention as a photosensor having high sensitivity also in the near infrared region is described.
With attention drawn to this point, a composite image sensor is formed by laminating the photoelectric converter of the present invention made of a compound semiconductor thin film on a silicon substrate formed with MOS transistors or the like in this embodiment.
In the general CMOS image sensor, a photodiode 610 (having a p-i-n structure, for example) and n+ diffusion layers 616, 618 and 620 constituting MOS transistors are formed in a P−-type silicon substrate 600, while gate layers 612 and 614 of the MOS transistors, a wiring layer 622 and a protective film (including an interlayer dielectric film) 624 are formed on the P−-type silicon substrate 600, as shown in
In the composite image sensor according to the present invention, on the other hand, n+ diffusion layers 702, 704, 706 and 708 constituting MOS transistors are formed in a P−-type silicon substrate 700, while gate layers 703 and 705 of the MOS transistors, wiring layers 712 and 714 and an interlayer dielectric film 710 are formed on the P−-type silicon substrate 700.
The photoelectric converter (photosensor) made of a compound semiconductor thin film is laminated/formed on this interlayer dielectric film 710.
This photoelectric converter (photosensor) is constituted of a lower wiring layer 716, a CIGS thin film (PD) 718 (an electrode layer is omitted in the Figure) serving as a light absorptive layer and an upper wiring layer 720.
As described with reference to the above-mentioned embodiment, the CIGS thin film according to the present invention is precisely patterned without residues by dry and wet two-stage etching employing photolithography, and the numbers of damages to the crystals and crystal defects are reduced. Further, the end of the P-N junction interface is embedded by patterning of the ZnO film not to reach the etched sidewall of the CIGS thin film, and the dark current is dramatically reduced by about five orders of magnitude as compared with the conventional device.
The sensor according to the present invention has a high sensitivity also for near infrared light, whereby the same is sufficiently utilizable as a security camera (camera sensing visible light by day and sensing near infrared light by night), a personal identification camera (camera for personal identification with near infrared light not influenced by external light) or an onboard camera (camera loaded on a vehicle for nightly visual aid or distant visual field assurance).
While Cu(Inx,Ga(1-x))Se2 is employed as the compound semiconductor thin film (CIGS thin film) having the chalcopyrite structure in the above-mentioned embodiment, the present invention is not limited thereto.
A film having a composition of Cu(Inx,Ga(1-x))(Sey,S(1-y))2, x=0 to 1, y=0 to 1 is also known as the CIGS thin film, and a CIGS thin film having such a composition is also utilizable.
This CIGS thin film can be formed on a substrate by vacuum evaporation or sputtering. When vacuum evaporation is employed, the respective components (Cu, In, Ga, Se and S) of the compound are separately evaporated on the substrate as evaporation sources. In sputtering, the chalcopyrite compound is employed as a target, or the respective components thereof are separately employed as targets. When the chalcopyrite compound semiconductor thin film is formed on a metal substrate or a glass substrate, the substrate is heated to a high temperature, whereby re-evaporation results from heating of the chalcogenide elements (Se and S). Therefore, compositional deviation may be caused by desorption of the chalcogenide elements. In this case, Se or S is preferably replenished by performing heat treatment in a vapor atmosphere of Se or S at a temperature of 400 to 600° C. for 1 to several hours after the film formation (selenidation or sulfuration).
According to the present invention, as hereinabove described, neither damages nor defects are caused in the compound semiconductor thin film unlike to the case of performing mechanical scribing but the dark current can be remarkably reduced by employing the technique of patterning the respective ones of the lower electrode layer, the compound semiconductor thin film with the chalcopyrite structure serving as the light absorptive layer and the light-transmitting electrode layer by photolithographic etching.
As to the light absorptive layer, precise patterning can be implemented without causing damages or defects in the crystals of the compound semiconductor and without leaving residues by performing the two-stage etching combining dry etching and wet etching with each other.
Further, the compound semiconductor thin film constituting the light absorptive layer can be etched at a high rate by employing the chlorine gas as the etchant for dry etching. Also in wet etching, residues can be efficiently and completely removed.
The recombination process of carriers can be reduced and the dark current can be reduced by using the CIGS thin film prepared by substituting gallium for part of In (indium) as the compound semiconductor thin film forming the light absorptive layer and widening the bandwidth.
Further, the photoelectric converter according to the present invention is precisely scribed by photolithography without leaving residues, and neither damages nor defects are caused in the crystals of the compound semiconductor unlike the case of employing mechanical scribing. Therefore, no unnecessary energy levels are formed on the interface of the P-N junction, and the dark current can be remarkably reduced.
In other words, the dark current can be improved on the order of 103 by changing the manufacturing process and optimizing the device structure. Further, the dark current can be reduced on the order of 102 by band gap control in Cu(Inx,Ga(1-x))Se2.
The sensor according to the present invention has a high sensitivity also for near infrared light, whereby the same is sufficiently utilizable as a security camera (camera sensing visible light by day and sensing near infrared light by night), a personal identification camera (camera for personal identification with near infrared light not influenced by external light) or an onboard camera (camera loaded on a vehicle for nightly visual aid or distant visual field assurance).
In the photoelectric conversion according to the present invention, photoelectric conversion loss on the interface of the P-N junction is sufficiently reduced as compared with the prior art. Therefore, a solar battery having a high collection efficiency for charges resulting from light and high photoelectric conversion efficiency can be implemented.
While the present invention has been described in detail by way of the embodiments thereof, it should be understood that these embodiments are merely illustrative of the technical principles of the present invention but not limitative of the invention. The spirit and scope of the present invention are to be limited only by the appended claims.
This application corresponds to Japanese Patent Application No. 2005-316789 filed with the Japanese Patent Office on Oct. 31, 2005, the disclosures of which are incorporated herein by reference in its entirety.
The present invention can remarkably reduce the dark current in a photoelectric conversion element employing a compound semiconductor thin film, whereby the same is effective as a photosensor suitable for a security camera or a personal identification camera, a solid-state image sensor and a solar battery, and a method for manufacturing these photoelectric conversion elements.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2005-316789 | Oct 2005 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2006/321768 | 10/31/2006 | WO | 00 | 2/19/2009 |
