Method for Manufacturing a Solar Cell

Abstract: To better accommodate surface cracks a shaped layer or mask is provided during manufacture for controlling the generation of cracks (16) and to confine cracks (16) to a predetermined geometry relative to electrodes (6a,6b). As shown in figure 9, wedge shaped notches (15) are defined in a buffer layer. Alternatively pin-holes can be defined for the same purpose.

Description: The present application is divided out of EP 91303176.1 FIELD OF THE INVENTION The invention relates to a method of manufacturing a GaAs solar cell on an Si substrate. BACKGROUND OF THE INVENTION Figures 6(a) and 6(b) are views showing a structure of a conventional GaAs solar cell on a Si substrate, in which figure 6(a) is its plan view and Figure 6(b) is a sectional view taken along a line A-A in Figure 6(a). In these figures, reference numeral 1 designates a Si substrate. An n type GaAs layer 2 and a p type GaAs layer 3 which serve as active layers are laminated on a first main surface la of the Si substrate 1 and then an anode electrode (p type electrode) 6 is formed on the p type GaAs layer 3 and a cathode electrode (n type electrode) 5 is provided on a second main surface 1b of the Si substrate 1. Thus, a solar cell 21 is formed. The GaAs solar cell 21 on the Si substrate is normally manufactured by the following method. First, the n type GaAs layer 2 and the p type GaAs layer 3 are sequentially formed on the first main surface la of the n type Si substrate 1 having a surface orientation of approximately (100) by a method of crystal growth of a compound semiconductor, such as MOCVD method. Thus, a pn junction 4 which generates a photovoltaic effect is formed. Then, as an electrode for taking a photoelectromotive force out, the anode electrode (p side electrode) 6 is selectively formed on the p type GaAs and the cathode electrode (n side electrode) 5 is formed on the whole surface of the second main surface 1b of the Si substrate 1. The anode electrode 6 comprises collecting electrodes 6a for collecting a photoelectric current and a common electrode 6b for connecting the collecting electrodes to an outside circuit. In addition, these electrodes are formed by sputtering or by a vapour deposition method and Ti/Ag is normally used as their materials. However, the conventional GaAs solar cell on the Si substrate has the following problems. That is, since the n type GaAs layer 2 and the p type GaAs layer 3 are usually formed at a high temperature of 700 to 800°C, when the wafer on which the above GaAs layers are formed is taken out at a room temperature, a large warpage is generated because of a difference in thermal expansion coefficient between GaAs and Si as shown in Figure 6(b). As a thickness of the GaAs layer is increased, the degree of this warpage becomes large. When the thickness of the GaAs layer exceeds 3µm, a crack is generated. On the other hand, as the thickness of the GaAs layer is increased, the dislocation density in the GaAs layer is reduced. Therefore, in order to assure sufficient performance of the solar cell, the thickness should be 4 to 5µm. Thus, the crack is generated in the operation layer (active layer) of the conventional solar cell. Particularly, a region surrounded by the crack 7 on which the collecting electrode 6a is not provided, which region is shown by slanting lines in Figure 6(a), is a loss region because the generated photoelectric current cannot be collected. In addition, even if the crack is not generated, since considerably large thermal stress remains in the GaAs layer, when slight stress is applied from the outside, the crack is easily generated in the GaAs layer. Since the warpage is concave when the GaAs layer is on the upper side as shown in Figure 6(b), the crack is easily generated when a stress is applied from the direction of the second main surface 1b of the Si substrate, that is, when a tensile stress is applied to the GaAs layer. Therefore, when the wafer is flattened in a photolithography process for patterning the p type electrode 6 or when interconnector welding for modulation is performed on the first electrode 5 side in assembling process, the crack is obviously generated. Meanwhile, GaAs on an Si substrate in which the crack is prevented from being generated later by intentionally generating the crack at a predetermined position to relieve thermal stress is disclosed in Appl. Phys. Lett. 55(21), 20 November 1989, pp. 2187 to 2189. Figure 7 is a view showing a principle for manufacturing the above solar cell. In Figure 7, reference numeral 25 designates a mask formed on the Si substrate, which is used for selectively growing GaAs, reference numeral 26 designates a wedge, and reference numeral 27 designates a crack generated in the GaAs layer from the end of the wedge. According to this conventional example, when the GaAs active layer is selectively grown on the Si substrate using the mask having the wedge 26, a crack 27 is generated in the GaAs layer from the end of the wedge 26. Thus, thermal stress in the element is relieved by the crack, so that the crack is prevented from being generated later. Therefore, it is possible to prevent generation of an invalid region by forming the collecting electrode so as not to generate a region surrounded by the crack in which the collecting electrode does not exist with due regard to the position of the crack 27. However, the Si substrate surface is difficult to be satisfactorily clean in a case where GaAs is selectively grown by using the mask as shown in the conventional example, while it is to be desired that the Si substrate on which GaAs is to be grown should be clean when the GaAs solar cell on the Si substrate is manufactured. SUMMARY OF THE INVENTION The present invention was made to solve the above problems and it is an object of the present invention to provide a method of manufacturing a GaAs solar cell on a Si substrate capable of preventing arbitrary generation of cracks. Other objects and advantages of the present invention will become apparent from the detailed description given hereinafter. The invention, which is defined in the appended claims, will now be described with reference to the drawings, in which: Figure 1 is a plan view showing a GaAs solar cell on an Si substrate during manufacturing process in accordance with the present invention; Figure 2 is a plan view showing a GaAs solar cell on an Si substrate in accordance with an embodiment of the present invention; Figure 3 is a sectional view in the vicinity of the crack, whose generation is controlled, in the GaAs solar cell on the Si substrate in accordance with the above embodiment of the present invention; Figure 4 is a plan view showing a GaAs solar cell on an Si substrate during its manufacturing process in accordance with another embodiment of the present invention; Figure 5 is a plan view showing a GaAs solar cell on an Si substrate in accordance with still another embodiment of the present invention; Figures 6 and 7 are views each showing a conventional GaAs solar cell on an Si substrate. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring first to Figure 1, this figure is a plan view showing a method of manufacturing a solar cell in accordance with a first embodiment of the present invention. The n + GaAs buffer layer is formed on the Si substrate and then the mesa groove 13 is formed by selectively etching the n + GaAs layer. At this time, a wedge-shaped concave part 15 whose bottom reaches the Si surface is provided on at least one side of the mesa groove. When the pattern shown in Figure 1 is used as a mask for forming the mesa groove, this concave part can be formed at the same step for forming the above mesa groove 13. Then, the n type GaAs layer and the p type GaAs layer which serve as active layers are formed. Although they are selectively grown at a part in which the n + GaAs layer remains, since stress is concentrated in the vicinity of the grown layer as described above, when the wedge-shaped concave part is formed in accordance with this embodiment of the present invention, the crack 16 is generated from its end as shown in Figure 1. Then, the p type electrode 6 is formed and then finally the new mesa groove 14 is formed by removing a peripheral part of the mesa groove. At this time, the peripheral part of the wedge-shaped concave part is also etched away. As described above, since the wedge-shaped concave part is previously formed to intentionally generate the crack from this part in this invention, stress is relieved and the crack is prevented from being generated later. In addition, when the wedge-shaped concave part 15 is formed at the centre between the collecting electrodes 6a as shown in Figure 1, photoelectric current collected by one grid electrode is well balanced in the right and left directions and it is possible to prevent spreading resistance from being increased, which spreading resistance is caused by imbalanced collection of current due to the crack. In addition, when the crystal surface orientation is [001], the direction in which the collecting electrode 6a is arranged runs almost parallel with the direction in which the crack 16 is generated, which direction is [010]. As described above, according to the embodiment of the present invention shown in Figure 1, generation of the crack 16 is controlled and its influence can be minimum. However, it is not possible to completely prevent the crack from being newly generated for some reason between the collecting electrodes 6a. In this case, a ratio of increase of the spreading resistance is in proportion to the square of the ratio of a length of the maximum electrode 6b to an arrangement distance. For example, when the former is 2cm and the latter is lmm, the spreading resistance becomes 400-fold. In order to reduce the above influence, it is effective to shift the direction in which the crack is generated from the end of the wedge so as not to be parallel with the direction of the collecting electrode. As shown in the variation in Figure 2, it is most effective when both cross at an angle of 45°. According to this variation, an increase of the spreading resistance is twofold at the most. Figure 3 is a sectional view in the vicinity of the crack 16 whose generation is controlled in accordance with the present invention. Since the pn junction 4 is exposed because the crack 16 is generated, this part is coated with an insulating film 20 formed of a material such as Si 3 N 4 or SiO 2 . In general, when the exposed pn junction end face 4a is left in the air, a leak current is generated because water, a foreign material or the like is attached thereto, with the result that the characteristic of the solar cell could be degraded. Therefore, that part is passivated with the insulating film, so that the leak current can be prevented from being generated. In addition, after the crack is generated, when metal is deposited in the step of forming a current, the pn junction could be short circuited because the metal enters from the crevice of the crack. Therefore, when the exposed end face 4a of the pn junction is protected with the insulating film 20 before the electrode is formed and then the second electrode 6 is formed, the pn junction can be prevented from being short circuited. Meanwhile, when the insulating film is formed on the whole surface of the lower part of the second electrode, a current cannot be taken out. Therefore, the insulating film should be left only in the vicinity of the crack and a contact hole is formed other than that part. The above step is possible when generation of the crack 16 is controlled as described in this invention. More specifically, when generation of the crack is controlled, the position of the crack can be specified and then the pattern of the contact hole can be specified. As described above, according to the present invention, the electrode capable of protecting the pn junction can be formed. Figure 4 is a plan view showing another embodiment of the present invention. According to this embodiment, the n + GaAs buffer layer is formed on the Si substrate and then the mesa groove 13 is formed by selectively etching the n + GaAs layer. At this time, a plurality of rhombic concave parts 17 is formed in the n + GaAs layer which remains like an island. This concave part can be formed at the same step of forming the mesa groove 13 like the embodiment shown in Figure 1. Then, when the n type GaAs layer and the p type GaAs layer are formed, the crack 16 is generated from a part of the acute angle of the rhomb. Although the crack is rarely generated from a part of the obtuse angle of the rhomb, even if it is generated, it has almost no substantial influence because it crosses the collecting electrode at right angles. However, in order to prevent the latter crack, the concave part may have a configuration formed of two circular arcs which are circumscribed to the rhomb like still another embodiment of the present invention shown in Figure 5. According to this embodiment of the present invention, if the rhombic concave part is also provided at the centre of the collecting electrodes 6a, the same effect described in Figure 1 can be obtained. In addition, when a diagonal line connected to the acute angle parts crosses the collecting electrode 6a at an angle of 45 degrees as shown in Figure 5, the influence of the crack which is accidentally generated from the part other than this concave part can be minimum like the embodiment shown in Figure 2. According to the method for manufacturing the GaAs solar cell on the Si substrate of the present invention, the GaAs buffer layer to which an impurity of the first conductivity type of high concentration is applied is formed on the first main surface of the first conductivity type Si substrate and then a wedge-shaped notched part is formed on a side of the buffer layer or a pinhole of predetermined configuration is provided on the buffer layer and the active layer comprising the GaAs layers of the first and second conductivity types and having a crack generated from the end of the wedge shaped notched part or from the pinhole is formed on the buffer layer. As a result, the active layer can be formed on the clean Si substrate surface and the thermal stress can be relieved by intentionally generating the crack. Thus, the crack is prevented from being accidentally generated later and the characteristic of the solar cell can be prevented from being considerably degraded. Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the scope of the present invention being limited only by the terms of the appended claims.

Claims: A method of manufacturing a solar cell having a GaAs active layer (2,3) comprising a first conductivity type GaAs layer (2) and a second conductivity type GaAs layer (3) on a GaAs buffer layer (12) to which an impurity of the first conductivity type is applied in high concentration and a Si substrate (1) wherein one or more cracks are generated in said GaAs active layer (2,3) during the growth of said GaAs active layer; which method comprises the following steps of: forming one or more recesses in said buffer layer (12) which recesses are in the form of mesa grooves (13,14) having wedge-shaped notched parts (15) at the side edges of the buffer layer to the mesa grooves, or in the form of pinholes (17) of a predetermined configuration, by the successive steps of forming said GaAs buffer layer (12) and etching said GaAs buffer layer; and growing said GaAs active layers (2,3) preferentially on said GaAs buffer layer by selective deposition while said Si substrate (1) is exposed by said one or more recesses and while generating a corresponding one or more cracks (16) extending in a crystallographic direction of said GaAs active layers (2,3) from said one or more recesses. A method of manufacturing a solar cell according to claim 1 including a further step of forming an electrode (6) including collecting electrodes (6a) on the GaAs active layer and in which the wedge-shaped notched parts are substantially centrally positioned with respect to the collecting electrodes. A method of manufacturing a solar cell in accordance with claim 2, wherein the direction of said crack or cracks is neither parallel to said collecting electrodes nor crosses them at right angles. A method of manufacturing a solar cell in accordance with claim 1, wherein a pn junction, exposed when said crack is generated, is covered with an insulating film. A method of manufacturing a solar cell in accordance with claim 4, wherein said step of covering said insulating film is performed before forming the collecting electrodes. A method of manufacturing a solar cell in accordance with claim 5, wherein the collecting electrodes are formed on said insulating film at a region where they cross the crack. A method of manufacturing a solar cell in accordance with claim 1, wherein the configuration of said pinhole is a rhomb, or is formed of two circular arcs circumscribed on the rhomb, and the angle formed between said two circular arcs is 90 degrees or less. A method of manufacturing a solar cell in accordance with claim 7, wherein said pinhole is substantially centrally positioned with respect to the collecting electrodes. A method of manufacturing a solar cell in accordance with claim 7, wherein a diagonal line through acute vertices of said rhomb is parallel to the direction of cleavage of the GaAs active layer and is neither parallel to the collecting electrodes nor crosses it at right angles. A method of manufacturing a solar cell in accordance with claim 9, wherein the angle formed between said diagonal line and said collecting electrodes is approximately 45 degrees. A method of manufacturing a solar cell in accordance with claim 7, wherein said active layer and buffer layer in the vicinity of said pinhole are removed after said crack is generated.

Inventors: KADOTA YOSHIAKI, MITSUI KOTARO, NISHIMURA TAKASHI, OGASAWARA NOBUYOSHI, OHMACHI YOSHIRO, OOHARA TAKAHIKO, KADOTA, YOSHIAKI, MITSUI, KOTARO, NISHIMURA, TAKASHI, OGASAWARA, NOBUYOSHI, OHMACHI, YOSHIRO, OOHARA, TAKAHIKO, MITSUI, KOTARO, C/O MITSUBISHI DENKI K.K., NISHIMURA, TAKASHI, C/O MITSUBISHI DENKI K.K., OGASAWARA, NOBUYOSHI, C/O MITSUBISHI DENKI K.K.
Applicants: MITSUBISHI ELECTRIC CORP, NIPPON TELEGRAPH & TELEPHONE, MITSUBISHI DENKI KABUSHIKI KAISHA, NIPPON TELEGRAPH AND TELEPHONE CORPORATION
Application year: 1991
Main classification: H01L31
IPC codes: H01L27, H01L31, H01L21