JP6405784B2 - Photoelectric conversion element - Google Patents

Description

  The present invention relates to a photoelectric conversion element.

Following the recent global warming phenomenon and accidents at nuclear power plants, the promotion of the use of renewable energy has become a global trend. Among them, power generation using sunlight can be configured and mounted from portable devices to large systems as well as reducing CO ₂ gas emission, and has attracted a great deal of attention including high versatility. In addition, subsidy systems such as feed-in tariffs have been started in various countries around the world, and as a part of energy policy that assumes the substitution of fossil fuels, the demand for solar cells is expected to grow rapidly in the future. Has been.

  Conventionally, group IV semiconductors such as silicon (Si) have been used as materials for solar cells. However, since a single material photoelectric conversion element also has a single band gap energy, it has been difficult to efficiently convert sunlight into a wide wavelength range. Therefore, development of a solar cell having a multi-band structure in which a plurality of materials having different band gap energies are laminated has been active. In particular, GaN-based compound semiconductor materials based on gallium nitride (GaN) can be adjusted to the desired band gap energy in a wide wavelength range by mixed crystals with indium (In) and aluminum (Al). It is. Therefore, it is said that photoelectric conversion with an efficiency exceeding 60%, which cannot be achieved with a single material, can be theoretically realized by laminating an appropriate band gap composition.

  On the other hand, the power generation costs of renewable energy such as solar cells are still relatively high compared to fossil fuels and nuclear power, and in addition to improving basic performance, it is important to reduce manufacturing costs in order to ensure sufficient competitiveness. It becomes a problem. A compound semiconductor material is usually deposited on a support substrate of a different material by a MOVPE (metal organic vapor phase epitaxy) method or the like because a so-called “bulk” free-standing film is expensive and difficult to increase in area. It is common.

JP 2007-281478 A JP 2006-156529 A JP 2008-21889 A

  The use of an inexpensive Si substrate as a support substrate is promising from the viewpoint of manufacturing cost. In this case, since the lattice constants of the GaN-based material and Si are greatly different, warpage and cracks due to lattice mismatch are likely to occur. Furthermore, since Ga reacts easily with Si and melts Si, there is a problem that product quality deteriorates. Therefore, it has been proposed to avoid these problems by depositing an aluminum nitride (AlN) layer as a buffer layer on a Si substrate and forming a GaN-based material directly thereon.

However, AlN has a band gap of about 6.3 eV, has poor conductivity, and it is difficult to reduce resistance by doping. Therefore, in the above structure using AlN as the buffer layer, it is necessary to remove a part of the GaN-based material by etching to form a pair of electrodes, and the manufacturing process becomes complicated. As a result, there is a problem that the manufacturing cost is greatly increased.
In addition, when an AlN layer is formed as a buffer layer, many defects are generated in the GaN-based crystal grown on the Si substrate, and there is a problem that the photoelectric conversion efficiency is lowered.

  The present invention has been made in view of the above-mentioned problems, and can produce a high-quality nitride semiconductor on an inexpensive Si substrate to obtain excellent photoelectric conversion efficiency, and is manufactured with a simple configuration. It aims at providing the highly efficient photoelectric conversion element which suppressed cost.

One aspect of the photoelectric conversion element includes a silicon substrate, a silicon carbide layer formed on the silicon substrate, a light receiving unit including a laminate on the silicon carbide layer, a back surface of the silicon substrate, and the light receiving unit. And a pair of electrodes electrically connected to the surface, wherein the stacked body includes a first stacked body having a first n-type InGaN layer and a first p-type InGaN layer, and the silicon substrate from the first stacked body. A second stacked body having a second n-type InGaN layer and a second p-type InGaN layer that are spaced apart and have a smaller In composition ratio and a higher band gap energy than the first stacked body, and the silicon than the second stacked body And a third stacked body having an n-type GaN layer and a p-type GaN layer that are spaced apart from the substrate and have a larger band gap energy than the second stacked body .

  According to the above-described aspect, it is possible to obtain a high-quality nitride semiconductor on an inexpensive Si substrate to obtain excellent photoelectric conversion efficiency, and a highly efficient photoelectric conversion element that suppresses manufacturing costs with a simple configuration. Is realized.

It is sectional drawing which shows schematic structure of the photoelectric conversion element by 1st Embodiment. It is sectional drawing which shows schematic structure of the photovoltaic cell by 2nd Embodiment.

  Hereinafter, preferred embodiments of a photoelectric conversion element and a solar cell to which the photoelectric conversion element is applied will be described in detail with reference to the drawings.

(First embodiment)
In the present embodiment, a photoelectric conversion element is disclosed. FIG. 1 is a cross-sectional view illustrating a schematic configuration of the photoelectric conversion element according to the present embodiment.

-Configuration of photoelectric conversion element-
In this photoelectric conversion element, a light receiving portion 3 made of a nitride semiconductor, here a GaN-based semiconductor, is formed on a Si substrate 1 via a silicon carbide (SiC) layer 2, and the back surface of the Si substrate 1 and the light receiving portion 3. A pair of electrodes 4a and 4b are electrically connected to the surface of the substrate.

The Si substrate 1 is a versatile and inexpensive support substrate, and is doped with an n-type impurity such as P at a concentration of about 5 × 10 ¹⁹ / cm ³ . The plane direction of the Si substrate 1 is (111).

The SiC layer 2 inserted between the Si substrate 1 and the light receiving portion 3 is a buffer layer having crystallinity and conductivity, and having a small lattice mismatch difference with a GaN-based semiconductor, and a 3C crystal polymorph (Polytype). The SiC layer 2 has the same conductivity type as that of the Si substrate 1, here, N, which is an n-type impurity, is doped to a concentration of about 1 × 10 ¹⁹ / cm ³ and is n-type, and has a range of about 10 nm to about 5 μm. The inner thickness is formed, for example, about 1 μm. If the thickness of the SiC layer 2 is less than 10 nm, the morphology of the SiC layer 2 may be deteriorated and the defects of the GaN-based semiconductor formed immediately above may increase. If the thickness of the SiC layer 2 is greater than 5 μm, cracks may occur in the GaN-based semiconductor due to stress strain during or after the growth of the GaN-based semiconductor, or cracks may occur in the Si substrate 1. By forming the SiC layer 2 to a thickness in the range of about 10 nm to about 5 μm, the generation of defects and cracks in the GaN-based semiconductor formed immediately above it and the cracking of the Si substrate 1 are suppressed.

The light receiving unit 3 is configured by sequentially stacking an n-type InGaN layer 3a that is an n-type nitride semiconductor and a p-type InGaN layer 3b that is a p-type nitride semiconductor. The n-type InGaN layer 3a is, for example, doped with Si, which is an n-type impurity, at a concentration of about 5 × 10 ¹⁹ / cm ³ to be n-type, and has a thickness of, for example, about 1 μm. The p-type InGaN layer 3b is, for example, p-type doped with Mg, which is a p-type impurity, at a concentration of about 2 × 10 ¹⁹ / cm ^3, and has a thickness of, for example, about 1 μm.

  The pair of electrodes 4a and 4b are formed such that 4a is made of, for example, Al-Si, 4b is made of, for example, Ni, the electrode 4a is directly formed on the back surface of the Si substrate 1, and the electrode 4b is directly formed on the surface of the light receiving unit 3. ing. In the present embodiment, the Si substrate 1 and the SiC layer 2 have conductivity. Therefore, the electrode 4a on the back surface of the Si substrate 1 is electrically connected to the n-type InGaN layer 3a via the Si substrate 1 and the SiC layer 2, and the electrode 4b on the light receiving unit 3 is electrically connected to the p-type InGaN layer 3b. Connected to.

-Manufacturing method of photoelectric conversion element-
In order to manufacture the above photoelectric conversion element, first, as a buffer layer, an SiC layer having a polytype of n type conductivity type and 3C type on the surface of the Si substrate 1 having n type conductivity and (111) plane orientation. 2 is formed. The SiC layer 2 is formed by a thermal CVD method to a thickness of, for example, about 1 μm using silane (SiH ₄ ) and propane (C ₃ H ₈ ) as source gases.

Subsequently, the n-type InGaN layer 3 a and the p-type InGaN layer 3 b are sequentially stacked on the SiC layer 2 to form the light receiving unit 3.
The n-type InGaN layer 3a and the p-type InGaN layer 3b are grown by, for example, a metal organic vapor phase epitaxy (MOVPE) method. Instead of the MOVPE method, a molecular beam epitaxy (MBE) method or the like may be used.

Formation of n-type InGaN layer 3a, by MOVPE, trimethylgallium (TMG), trimethylindium (TMI), and NH ₃ as source gases, SiH ₄ as n-type doping gas of a (. which Si is doped) Use. Under the conditions of a growth temperature of 800 ° C. and a growth pressure of 20 kPa, n-type InGaN is grown to a thickness of about 1 μm.

The p-type InGaN layer 3b is formed by MOVPE using TMG, TMI, and NH ₃ as source gases and Cp ₂ Mg (Mg doped) as a p-type doping gas. Under the conditions of a growth temperature of 800 ° C. and a growth pressure of 20 kPa, p-type InGaN is grown to a thickness of about 1 μm.

Subsequently, the electrode 4a is formed on the back surface of the Si substrate 1, and the electrode 4b is formed on the surface of the light receiving unit 3 (p-type InGaN layer 3b). For the electrodes 4a and 4b, for example, a resist mask having an opening is formed, an electrode material is deposited on the resist mask so as to fill the opening by vapor deposition or the like, and then the resist mask and the electrode material thereon are removed by lift-off. By that, it is formed.
Thus, the photoelectric conversion element according to the present embodiment is formed.

In the present embodiment, since the SiC layer 2 has conductivity together with the Si substrate 1, one electrode 4 a can be formed on the back surface of the Si substrate 1. Therefore, unlike the case where a non-conductive AlN layer is formed as a buffer layer, it is not necessary to remove a part of the p-type InGaN layer by etching to expose the surface of the n-type InGaN layer in order to form an electrode, The manufacturing process is simplified.
Moreover, the surface of the SiC layer 2 is excellent in morphology, and the light-receiving part 3 made of a GaN-based semiconductor formed immediately above the surface is suppressed from generating defects, and the photoelectric conversion efficiency is improved.

-Experimental example-
Hereinafter, experimental examples in which various characteristics of the photoelectric conversion element according to the present embodiment are examined based on comparison with the comparative examples will be described.

(1) Experimental example 1
The sheet resistance of the buffer layer was examined.
A non-doped Si substrate having a (111) plane orientation was prepared, and a SiC layer having a 3C polytype was formed as a buffer layer on the Si substrate with a thickness of about 1 μm as a base substrate.

  When the sheet resistance of the surface of the base substrate was measured using a non-contact sheet resistance measuring device, it showed a low value of about 50 ohm / sq. Since the Si substrate used for the support substrate is non-conductive, the measured value is a value resulting from the SiC layer.

As a comparative example, a non-doped Si substrate having a plane orientation of (111) was prepared, and an AlN layer was formed as a buffer layer directly thereon to form a base substrate. The AlN layer is formed by MOVPE, using trimethylaluminum (TMA) and NH ₃ as source gases, with a growth temperature of 1000 ° C. and a growth pressure of 20 kPa, and AlN is grown to a thickness of about 200 nm.

  When the sheet resistance of the base substrate thus formed was measured using a non-contact sheet resistance measuring device, it showed an extremely high value of about 40000 ohm / sq. Since the Si substrate used as the support substrate is non-conductive, it is difficult to specify the attribution of the sheet resistance value. From this result, it can be seen that the AlN layer is a high resistance layer of 40000 ohm / sq. Or more.

  As described above, it was confirmed from Experimental Example 1 that the sheet resistance is significantly reduced by providing the SiC layer serving as the buffer layer on the Si substrate, unlike the case of providing the AlN layer as the buffer layer. .

(2) Experimental example 2
The crystallinity and surface state of the light receiving part were examined.
By the manufacturing method of the present embodiment described above, an n-type InGaN layer and a p-type InGaN layer are sequentially stacked on an n-type Si substrate having a (111) plane orientation via an SiC layer having a 3C polytype. A light receiving part was formed.

For the formed light-receiving part, the half-value width as an index of crystallinity was evaluated by the rocking curve method using an X-ray diffractometer (XRD). As a result, the (002) orientation was 370 arcsec and the (102) orientation was 710 arcsec. It showed good crystallinity with few defects.
Further, when the surface shape of the formed light receiving part was observed using an atomic force microscope (AFM), it was found that the center line average roughness (Ra) was 0.2 and a smooth surface.

  As a comparative example, a light receiving portion was formed by sequentially laminating an n-type InGaN layer and a p-type InGaN layer via an AlN layer on a non-doped Si substrate having a (111) plane orientation.

When the half-width, which is an index of crystallinity, was evaluated by the rocking curve method using an X-ray diffractometer (XRD), the formed light-receiving portion was dislocated with a (002) orientation of 600 arcsec and a (102) orientation of 950 arcsec. The crystallinity suggests that there are relatively many sex defects.
Further, when the surface shape of the formed light receiving part was observed using AFM, it was found that Ra was a smooth surface of 0.2.

  As described above, according to Experimental Example 2, unlike the case where the buffer layer is an AlN layer by forming a light receiving portion including an n-type and a p-type InGaN layer via a SiC layer as a buffer layer on the Si substrate. It was confirmed that the light-receiving part showed excellent crystallinity. Regarding the surface flatness of the light receiving portion, it was confirmed that a good state with no problem can be obtained with any buffer layer.

  As described above, according to the present embodiment, it is possible to form a high-quality nitride semiconductor on an inexpensive Si substrate to obtain excellent photoelectric conversion efficiency, and suppress the manufacturing cost with a simple configuration. A highly efficient photoelectric conversion element is realized.

(Second Embodiment)
In the present embodiment, a photoelectric conversion element including a plurality of stacked bodies of n-type nitride semiconductors and p-type nitride semiconductors is disclosed. This photoelectric conversion element is used as a solar battery cell. FIG. 2 is a cross-sectional view illustrating a schematic configuration of the solar battery cell according to the present embodiment. In addition, about the same thing as the structural member shown in FIG. 1 of 1st Embodiment, the same code | symbol is attached | subjected and detailed description is abbreviate | omitted.

  In this solar cell, a light receiving portion 10 made of a nitride semiconductor, here a GaN-based semiconductor, is formed on a Si substrate 1 via SiC 2 as a buffer layer, and the back surface of the Si substrate 1 and the front surface of the light receiving portion 10 are formed. A pair of electrodes 4a and 4b are electrically connected to each other.

The light receiving unit 10 includes an n-type In _x Ga _1-x N that is an n _- type nitride semiconductor and a p-type In _x Ga _1-x N (0 ≦ x ≦ 1) stack 11 that is a p-type nitride semiconductor. 12 and 13 are sequentially laminated.
The stacked body 11 is formed by sequentially stacking an n-type In _0.3 Ga _0.7 N layer 11a and a p-type In _0.3 Ga _0.7 N layer 11b having an In composition ratio of 30% and a Ga composition ratio of 70%. Yes.
The stacked body 12 includes an n-type In _0.1 Ga _0.9 N layer 12a and a p-type In _0.1 Ga _{0.9 in which the} In composition ratio is 10% smaller than that of the stacked body 11 and the Ga composition ratio is 90% larger than that of the stacked body 11. The N layer 13b is sequentially stacked.
The stacked body 13 is formed by sequentially stacking an n-type GaN layer 13a and a p-type GaN layer 13b in which the In composition ratio is 0% smaller than that of the stacked body 12 and the Ga composition ratio is 100% larger than that of the stacked body 12. Configured.

The n-type In _0.3 Ga _0.7 N layer 11a is, for example, doped with Si, which is an n-type impurity, at a concentration of about 5 × 10 ¹⁹ / cm ³ to be n-type, and has a thickness of, for example, about 1 μm. Yes. The p-type In _0.3 Ga _0.7 N layer 11b is, for example, p-type doped with Mg, which is a p-type impurity, at a concentration of about 2 × 10 ¹⁹ / cm ^3, and has a thickness of, for example, about 1 μm. Yes.

The n-type In _0.1 Ga _0.9 N layer 12a is, for example, doped with Si, which is an n-type impurity, at a concentration of about 5 × 10 ¹⁹ / cm ³ to be n-type, and has a thickness of, for example, about 1 μm. Yes. The p-type In _0.1 Ga _0.9 N layer 13b is, for example, p-type doped with Mg, which is a p-type impurity, at a concentration of about 2 × 10 ¹⁹ / cm ^3, and has a thickness of, for example, about 1 μm. Yes.

The n-type GaN layer 13a is, for example, doped with Si, which is an n-type impurity, at a concentration of about 5 × 10 ¹⁹ / cm ³ to be n-type, and has a thickness of, for example, about 1 μm. The p-type GaN layer 13b is, for example, p-type doped with Mg, which is a p-type impurity, at a concentration of about 2 × 10 ¹⁹ / cm ^3, and has a thickness of, for example, about 1 μm.

  The laminates 11, 12, and 13 are formed in the same manner as the manufacturing method described in the first embodiment. For InGaN, the composition ratios of In and Ga are controlled by adjusting the flow rates of TMG and TMI in the source gas.

  In the light receiving unit 10, the band gap energy is larger as the stacked body is separated from the Si substrate 1. In the present embodiment, the laminated body 12 is formed of a material having a band gap energy larger than that of the laminated body 11, and the laminated body 13 is larger than that of the laminated body 12. With this configuration, for example, the laminated body 13 can receive ultraviolet light, the laminated body 12 can receive visible light, and the laminated body 11 can receive infrared light.

  According to this embodiment, it is possible to form a high-quality nitride semiconductor on an inexpensive Si substrate to obtain excellent photoelectric conversion efficiency, and a highly efficient solar cell with a simple configuration and reduced manufacturing cost Is realized.

  Hereinafter, various aspects of the photoelectric conversion element are collectively described as supplementary notes.

(Appendix 1) a silicon substrate;
A silicon carbide layer formed on the silicon substrate;
A light-receiving unit including at least one layered body including an n-type nitride semiconductor layer and a p-type nitride semiconductor layer on the silicon carbide layer;
A photoelectric conversion element comprising: a back surface of the silicon substrate; and a pair of electrodes electrically connected to the surface of the light receiving portion.

  (Additional remark 2) The said silicon substrate and the said silicon carbide layer are both made into n-type conductivity type, The photoelectric conversion element of Additional remark 1 characterized by the above-mentioned.

  (Supplementary note 3) The photoelectric conversion element according to supplementary note 1 or 2, wherein the silicon carbide layer is a 3C polytype.

(Supplementary Note 4) The n-type nitride semiconductor layer and the p-type nitride semiconductor layer, according to Note 1 to 3, characterized in that it is formed by _{In x Ga 1-x N (} 0 ≦ x ≦ 1) The photoelectric conversion element of any one.

  (Appendix 5) The photoelectric conversion element according to any one of appendices 1 to 4, wherein the silicon substrate has a plane orientation of (111).

  (Additional remark 6) The said silicon carbide layer is formed in the thickness within the range of 10 nm-5 micrometers, The photoelectric conversion element of any one of Additional remark 1-5 characterized by the above-mentioned.

(Additional remark 7) The said light-receiving part is provided with the said some laminated body,
The photoelectric conversion element according to any one of appendices 1 to 6, wherein the stacked body has a larger band gap as it is separated from the silicon substrate.

1 Si substrate 2 SiC layer 3,10 light receiving portion 3a n-type InGaN layer 3b p-type InGaN layer 4a, 4b electrode 11, 12, 13 laminate 11a n-type In _0.3 Ga _0.7 N layer 11b p-type In _0.3 Ga _0.7 N layer 12a n-type In _0.1 Ga _0.9 N layer 12b p-type In _0.1 Ga _0.9 N layer 13a n-type GaN layer 13b p-type GaN layer

Claims (4)

A silicon substrate;
A silicon carbide layer formed on the silicon substrate;
A light receiving portion comprising a laminate on the silicon carbide layer;
A pair of electrodes electrically connected to the back surface of the silicon substrate and the front surface of the light receiving unit,
The laminate is
A first stack having a first n-type InGaN layer and a first p-type InGaN layer;
A second stacked body having a second n-type InGaN layer and a second p-type InGaN layer that are further away from the silicon substrate than the first stacked body and have a smaller In composition ratio and a larger band gap energy than the first stacked body. When,
A third stacked body having an n-type GaN layer and a p-type GaN layer that are separated from the silicon substrate than the second stacked body and have a larger band gap energy than the second stacked body;
A photoelectric conversion element comprising:   The photoelectric conversion element according to claim 1, wherein the silicon substrate and the silicon carbide layer are both of an n-type conductivity type.   The photoelectric conversion element according to claim 1, wherein the silicon carbide layer is a 3C polytype. It said silicon carbide layer includes a photoelectric conversion element according to any one of claims 1 to 3, characterized in that it is formed to a thickness in the range of 10 nm to 5 [mu] m.

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