WO2013038822A1 - Élément de conversion photoélectrique - Google Patents

Élément de conversion photoélectrique Download PDF

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WO2013038822A1
WO2013038822A1 PCT/JP2012/069323 JP2012069323W WO2013038822A1 WO 2013038822 A1 WO2013038822 A1 WO 2013038822A1 JP 2012069323 W JP2012069323 W JP 2012069323W WO 2013038822 A1 WO2013038822 A1 WO 2013038822A1
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nitride semiconductor
layer
type nitride
semiconductor layer
intervening
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佐野　雄一
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シャープ株式会社
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/18—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof
    • H01L31/184—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof the active layers comprising only AIIIBV compounds, e.g. GaAs, InP
    • H01L31/1844—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof the active layers comprising only AIIIBV compounds, e.g. GaAs, InP comprising ternary or quaternary compounds, e.g. Ga Al As, In Ga As P
    • H01L31/1848—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof the active layers comprising only AIIIBV compounds, e.g. GaAs, InP comprising ternary or quaternary compounds, e.g. Ga Al As, In Ga As P comprising nitride compounds, e.g. InGaN, InGaAlN
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/04—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices
    • H01L31/054—Optical elements directly associated or integrated with the PV cell, e.g. light-reflecting means or light-concentrating means
    • H01L31/056—Optical elements directly associated or integrated with the PV cell, e.g. light-reflecting means or light-concentrating means the light-reflecting means being of the back surface reflector [BSR] type
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/04—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices
    • H01L31/06—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices characterised by at least one potential-jump barrier or surface barrier
    • H01L31/075—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices characterised by at least one potential-jump barrier or surface barrier the potential barriers being only of the PIN type
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/50—Photovoltaic [PV] energy
    • Y02E10/52—PV systems with concentrators
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/50—Photovoltaic [PV] energy
    • Y02E10/544—Solar cells from Group III-V materials
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/50—Photovoltaic [PV] energy
    • Y02E10/548—Amorphous silicon PV cells

Definitions

  • the present invention relates to a photoelectric conversion element, and more particularly to a photoelectric conversion element with improved short-circuit current.
  • a photoelectric conversion element made of silicon is not excellent in sensitivity to light in a short wavelength region of 0.5 ⁇ m or less because silicon has a band gap energy of 1.1 to 1.8 eV. Therefore, the photoelectric conversion element made of silicon has a material-specific problem that it cannot effectively use all the wavelength regions of the sunlight spectrum.
  • the nitride semiconductor material represented by Al a In b Ga (1-ab) N (0 ⁇ a ⁇ 1, 0 ⁇ b ⁇ 1, 0 ⁇ a + b ⁇ 1) has a band gap energy of a composition ratio a.
  • b vary in a very wide range of 0.7 eV to 6.0 eV, so that the sensitivity is excellent even for light in a short wavelength region of 0.5 ⁇ m or less. Therefore, the nitride semiconductor material has attracted a great deal of attention for use in next-generation photoelectric conversion elements.
  • a layer made of a nitride semiconductor material is usually formed by metal organic chemical vapor deposition (MOCVD) method, hydride vapor phase epitaxy (HVPE) method, molecular beam vapor phase epitaxy (MBE). It is formed on the substrate by using a vapor phase growth method such as a beam epitaxy (PLD) method or a pulsed laser deposition (PLD) method.
  • MOCVD metal organic chemical vapor deposition
  • HVPE hydride vapor phase epitaxy
  • MBE molecular beam vapor phase epitaxy
  • nitride semiconductor materials are suitable as materials for light-emitting elements such as light-emitting diodes (LEDs) or laser diodes (LDs), there is a history of extensive development. In addition, nitride semiconductor materials have been elucidated as next-generation photoelectric conversion elements since the band gap has been elucidated as described above.
  • the photoelectric conversion element is composed of a plurality of layers having different lattice constants. For this reason, a piezoelectric electric field is generated because lattice defects are formed at the interface of each layer and / or compressive stress or tensile stress is generated in the layer due to a difference in lattice constant. Since the internal electric field is reduced due to the generation of the piezoelectric electric field, there is a possibility that the short-circuit current may be reduced when used as a photoelectric conversion element.
  • This invention is made
  • the photoelectric conversion element according to the present invention includes an n-type nitride semiconductor layer, an i-type nitride semiconductor layer provided on the n-type nitride semiconductor layer, and a p provided on the i-type nitride semiconductor layer.
  • the pin nitride semiconductor layer is pin-joined.
  • This photoelectric conversion element is provided between the n-type nitride semiconductor layer and the i-type nitride semiconductor layer, and the type of constituent elements is the same as that of the i-type nitride semiconductor layer, while the composition ratio of the constituent elements is An intervening layer different from the i-type nitride semiconductor layer is further provided.
  • the In composition ratio of the intervening layer is lower than the In composition ratio of the i-type nitride semiconductor layer.
  • the intervening layer may be configured by laminating a plurality of semiconductor layers having different band gap energies. At this time, the plurality of semiconductor layers may be arranged such that the band gap energy increases as it goes from the i-type nitride semiconductor layer to the n-type nitride semiconductor layer.
  • the intervening layer is made of Al x In z Ga (1-xz) N (0 ⁇ x ⁇ 1, 0 ⁇ z ⁇ 1, 0 ⁇ x + z ⁇ 1, 0 ⁇ z ⁇ y) or a single layer formed on the n-type nitride semiconductor layer Al x In zn Ga (1-x-zn) N layer, Al x In z (n-1) Ga (1-xz (n-1)) N layer, ..., Al x In z2 Ga (1 -x-z2) N layer, and Al x In z1 Ga (1-x-z1) N layer (0 ⁇ zn ⁇ z (n-1) ⁇ ... ⁇ z2 ⁇ z1 ⁇ y, where n is a natural number Yes, n ⁇ 4)
  • the intervening layer may be a single layer having a thickness of 1 nm to 5 nm, or may have a stacked structure in which layers having a thickness of 1 nm to 5 nm are stacked.
  • the photoelectric conversion element according to the present invention is provided between an i-type nitride semiconductor layer and a p-type nitride semiconductor layer, and the type of constituent elements is the same as that of the i-type nitride semiconductor layer, It is preferable to include a second intervening layer having a composition ratio different from that of the i-type nitride semiconductor layer. At this time, the In composition ratio of the second intervening layer is preferably lower than the In composition ratio of the i-type nitride semiconductor layer.
  • the second intervening layer is formed by laminating a plurality of second semiconductor layers having different In composition ratios.
  • the plurality of second semiconductor layers are preferably arranged such that the In composition ratio decreases as going from the i-type nitride semiconductor layer to the p-type nitride semiconductor layer.
  • the second intervening layer may be a single layer having a thickness of 1 nm to 5 nm, or may have a stacked structure in which layers having a thickness of 1 nm to 5 nm are stacked.
  • the second intervening layer may be made of Al x In w Ga (1-xw) N (0 ⁇ x ⁇ 1, 0 ⁇ w ⁇ 1, 0 ⁇ x + w ⁇ 1).
  • the Al composition ratio of the n-type nitride semiconductor layer is preferably higher than the Al composition ratio of the intervening layer, and the In composition ratio of the n-type nitride semiconductor layer is preferably lower than the In composition ratio of the intervening layer.
  • the absolute refractive index value of the n-type nitride semiconductor layer is preferably smaller than the absolute refractive index value of the intervening layer.
  • the Al composition ratio of the p-type nitride semiconductor layer is preferably higher than the Al composition ratio of the second intervening layer, and the In composition ratio of the p-type nitride semiconductor layer is lower than the In composition ratio of the second intervening layer. It is preferable.
  • the absolute refractive index value of the p-type nitride semiconductor layer is preferably smaller than the absolute refractive index value of the second intervening layer.
  • the band gap energy of the p-type nitride semiconductor layer is preferably larger than the band gap energy of the second intervening layer.
  • a transparent conductive film is provided on the upper surface of the p-type nitride semiconductor layer.
  • the transparent conductive film may be a single layer containing at least one of Zn, In, Sn, and Mg, or may have a stacked structure in which the single layers are stacked.
  • the transparent conductive film should just have an absolute refractive index value smaller than 2.3, and should just have a film thickness of 250 to 500 nm.
  • the n-type nitride semiconductor layer, the i-type nitride semiconductor layer, and the p-type nitride semiconductor layer may be grown on the upper surface of the substrate in this order.
  • the substrate is Al p In q Ga (1-pq) N (0 ⁇ p ⁇ 1, 0 ⁇ q ⁇ 1, 0 ⁇ p + q ⁇ 1), GaP, GaAs, NdGaO 3 , LiGaO 2 , Al 2 O 3 , MgAl Any of 2 O 4 , ZnO, Si, SiC, SiGe, and ZrB 2 may be used.
  • the light reflection layer may be a single layer made of Ag, and may have a film thickness of 10 nm or more and 1000 nm or less.
  • the amount of photocarriers generated itself increases, so that the short circuit current increases and the photoelectric conversion efficiency increases.
  • 2 is a graph showing a standardized PL (photo luminesence) spectrum of an i-type nitride semiconductor layer.
  • FIG. 1 is a side view of the photoelectric conversion element 1 according to the first embodiment of the present invention.
  • FIG. 2 is an energy band diagram (n side) in the photoelectric conversion element 1 according to this embodiment.
  • FIG. 3 is a diagram showing the direction of stress and electric field generated in the photoelectric conversion element 1 according to this embodiment.
  • FIG. 4 is a graph showing a normalized PL spectrum of the i-type nitride semiconductor layer.
  • FIG. 5 is an energy band diagram (p side) in the photoelectric conversion element 1 according to this embodiment. 2 and 3, the second intervening layer 19 is not shown.
  • the photoelectric conversion element 1 includes an n-type nitride semiconductor layer 13, an i-type nitride semiconductor layer 17, and a p-type nitride semiconductor layer 21 that are pin-joined. Specifically, as shown in FIG. 1, in the photoelectric conversion element 1, the n-type nitride semiconductor layer 13, the intervening layer 15, the i-type nitride semiconductor layer 17, and the second intervening layer are formed on the upper surface of the substrate 11. 19, a p-type nitride semiconductor layer 21 and a transparent conductive film 23 are laminated in this order, and a light reflecting layer 25 is provided on the lower surface of the substrate 11.
  • photoelectric conversion element 1 In such a photoelectric conversion element 1, light (for example, sunlight) 101 is incident from the p-type nitride semiconductor layer 21 side, and the incident light 101 is absorbed by the i-type nitride semiconductor layer 17. As a result, photocarriers (electrons and holes) are formed, electrons diffuse into the conduction band of n-type nitride semiconductor layer 13, and holes diffuse into the valence band of p-type nitride semiconductor layer 21.
  • photocarriers electrons diffuse into the conduction band of n-type nitride semiconductor layer 13
  • holes diffuse into the valence band of p-type nitride semiconductor layer 21.
  • the material of the substrate 11 is not particularly limited, but for example, Al p In q Ga (1-pq) N (0 ⁇ p ⁇ 1, 0 ⁇ q ⁇ 1, 0 ⁇ p + q ⁇ 1), GaP, GaAs, NdGaO 3 , LiGaO 2 , Al 2 O 3 , MgAl 2 O 4 , ZnO, Si, SiC, SiGe, and ZrB 2 , preferably consisting of Al p In q Ga (1-pq) N It is.
  • substrate 11 is not specifically limited to the film thickness, For example, what is necessary is just to have a film thickness of 0.3 mm or more and 0.5 mm or less.
  • n-type nitride semiconductor layer 13 only needs to have a larger band gap energy than each of the i-type nitride semiconductor layer 17 and the intervening layer 15, and the material is not particularly limited.
  • the n-type impurity concentration in the n-type nitride semiconductor layer 13 is not particularly limited, but may be 1 ⁇ 10 18 cm ⁇ 3 or more and 1 ⁇ 10 20 cm ⁇ 3 or less.
  • the i-type nitride semiconductor layer 17 absorbs light (incident light) 101 incident on the photoelectric conversion element 1 and generates photocarriers (electrons and holes).
  • Such i-type nitride semiconductor layer 17 is not particularly limited as long as it has a smaller band gap energy than each of intervening layer 15 and n-type nitride semiconductor layer 13.
  • the i-type nitride semiconductor layer 17 may be a single layer composed of Al x In y Ga (1-xy) N (0 ⁇ x ⁇ 1, 0 ⁇ y ⁇ 1, 0 ⁇ x + y ⁇ 1) layers.
  • Al x in y Ga (1- xy) N layer may be a MQW (multiple quantum well) structure for the light emitting layer, and Al x in y Ga (1- xy) N layer a light emitting layer SQW A (single quantum well) structure may be used.
  • MQW multiple quantum well
  • SQW A single quantum well
  • the film thickness of the i-type nitride semiconductor layer 17 is not particularly limited. However, if this film thickness is too large, the internal electric field created by the n-type nitride semiconductor layer 13 and the p-type nitride semiconductor layer 21 may not be sufficiently applied to the i-type nitride semiconductor layer 17. Therefore, the film thickness of i-type nitride semiconductor layer 17 is preferably 400 nm or less, for example.
  • the p-type nitride semiconductor layer 21 only needs to have a band gap energy larger than that of the second intervening layer 19 than that of the i-type nitride semiconductor layer 17, and the material thereof is not particularly limited.
  • a p-type impurity such as Be or Mg is an Al u In v Ga (1-uv) N (0 ⁇ u ⁇ 1, 0 ⁇ v ⁇ 1, 0 ⁇ u + v ⁇ 1) layer. It is sufficient that the layer is doped with Mg, and Mg is preferably doped with the GaN layer.
  • the p-type impurity concentration in the p-type nitride semiconductor layer 21 is not particularly limited, but may be, for example, 1 ⁇ 10 17 cm ⁇ 3 or more and 1 ⁇ 10 19 cm ⁇ 3 or less.
  • the intervening layer 15 is configured such that the type of constituent elements is the same as that of the i-type nitride semiconductor layer 17, while the composition ratio of constituent elements is different from that of the i-type nitride semiconductor layer 17.
  • the band gap energy Eg z of the intervening layer 15 is larger than the band gap energy Eg y of the i-type nitride semiconductor layer 17 and larger than the band gap energy Eg 0 of the n-type nitride semiconductor layer 13 as shown in FIG. small.
  • the n-type nitride semiconductor layer 13 is n-Al s In t Ga (1-st) N (0 ⁇ s ⁇ 1, 0 ⁇ t ⁇ 1, 0 ⁇ s + t ⁇ 1) and the i-type nitride semiconductor layer 17 is Al x In y Ga (1-xy) N (0 ⁇ x ⁇ 1, 0 ⁇ y ⁇ 1) , 0 ⁇ x + y ⁇ 1) layer
  • the intervening layer 15 is made of Al x In z Ga (1-xz) N (0 ⁇ x ⁇ 1, 0 ⁇ z ⁇ 1, 0 ⁇ x + z ⁇ 1, 0 ⁇ t ⁇ It suffices if z ⁇ y) layer.
  • the band gap energy (Eg y ) of the i-type nitride semiconductor layer 17 is lower than the band gap energy (Eg 0 ) of the n-type nitride semiconductor layer 13, and thus the i-type nitride semiconductor layer A potential barrier V (eV) is formed as viewed from the photocarriers (especially electrons) generated in FIG. Since the potential barrier V (eV) is relatively large, the movement of electrons is hindered.
  • the In composition of the n-type nitride semiconductor layer 13 is 0, that is, if the n-type nitride semiconductor layer 13 is made of (Al x Ga (1-x) N (0 ⁇ x ⁇ 1)), the n-type nitride semiconductor layer 13 Since the band gap energy of the nitride semiconductor layer is 3.4 eV to 6.0 eV, the potential barrier V (eV) viewed from the photocarriers (particularly electrons) generated in the i-type nitride semiconductor layer 17 is very large. Become. Therefore, the hindrance to the movement of electrons becomes remarkable.
  • the intervening layer 15 is not present, the n-type nitride semiconductor layer 13 and the i-type nitride semiconductor layer 17 are heterojunctioned.
  • the degree of lattice mismatch with the physical semiconductor layer 17 is large. Therefore, misfit dislocations may be formed in i-type nitride semiconductor layer 17.
  • the photocarriers generated in the i-type nitride semiconductor layer 17 may disappear near the interface between the n-type nitride semiconductor layer 13 and the i-type nitride semiconductor layer 17.
  • the potential barrier V ′ (V ′ Eg z ) as viewed from the photocarriers (particularly electrons) generated in the i-type nitride semiconductor layer 17. -Eg y ⁇ V) is formed. Therefore, photocarriers (especially electrons) generated in the i-type nitride semiconductor layer 17 can move to the n-type nitride semiconductor layer 13 along the stepped potential barrier. Therefore, hindering movement of electrons can be prevented.
  • the intervening layer 15 and the i-type nitride semiconductor layer 17 have the same type of constituent elements. Therefore, if the intervening layer 15 is provided, the degree of lattice mismatch between the n-type nitride semiconductor layer 13 and the i-type nitride semiconductor layer 17 is reduced. Therefore, misfit dislocations can be prevented from being formed in i-type nitride semiconductor layer 17, so that more photocarriers (electrons) can be diffused into the conduction band of n-type nitride semiconductor layer 13. Thereby, since a short circuit current improves, it is very preferable for the photoelectric conversion element to provide the intervening layer 15.
  • the In composition ratio z of the intervening layer 15 will be described.
  • the In composition ratio z of the intervening layer is 0, that is, when the intervening layer is made of Al x Ga (1-x) N (0 ⁇ x ⁇ 1)
  • the band gap energy of the intervening layer is 3.4 eV to 6. 0 eV, which is larger than the band gap energy when the intervening layer 15 contains In. Therefore, the potential barrier seen from the photocarriers generated in the i-type nitride semiconductor layer 17 becomes much larger than that in the case where the intervening layer 15 contains In, and the movement of electrons is hindered.
  • the band gap energy (Eg z ) of the intervening layer 15 is i-type nitride semiconductor.
  • the intervening layer 15 is sandwiched between the potential barrier of the n-type nitride semiconductor layer 13 and the potential barrier of the i-type nitride semiconductor layer 17, and the n-type nitride semiconductor layer 13, the intervening layer 15, and the i-type nitride semiconductor.
  • An SQW structure is formed with the layer 17. As a result, the generated photocarrier is confined in the intervening layer 15.
  • the In composition ratio z of the intervening layer 15 may be, for example, 1% lower than the In composition ratio y of the i-type nitride semiconductor layer 17 and is 0.02 times or more the In composition ratio y of the n-type nitride semiconductor layer 13. It is preferable that it is 0.99 times or less.
  • the film thickness d of the intervening layer 15 is not particularly limited, but is preferably 1 nm or more and 5 nm or less. Electrons that are photocarriers diffuse to the n-type nitride semiconductor layer 13 along the potential barrier of the intervening layer 15, but some of the electrons diffuse to the n-type nitride semiconductor layer 13 by the tunnel effect. Probability of tunneling occurs, i.e. the tunnel probability P is interposed between the difference between the band gap energy Eg y of the band gap energy Eg z and i-type nitride semiconductor layer 17 of the intermediate layer 15 (i.e. the magnitude of the potential barrier V ') This greatly depends on the film thickness d of the layer 15.
  • the tunnel probability P may be small because the time (recombination lifetime) in which electrons and holes as photocarriers recombine in the tunnel is short.
  • the tunnel probability P increases, but the coverage (coverage) on the surface of the n-type nitride semiconductor layer 13 because the thickness of the intervening layer 15 is too thin. Tends to be inadequate, which can make production control difficult.
  • the tunnel probability P is optimized, and sufficient coverage on the surface of the n-type nitride semiconductor layer 13 can be ensured. Therefore, a large amount of photocarriers can be taken out, so that the short circuit current is improved. Therefore, it is preferable for the photoelectric conversion element that the thickness d of the intervening layer 15 is 1 nm or more and 5 nm or less. Note that the recombination lifetime between electrons and holes in InGaN grown on the c-plane is discussed in, for example, Non-Patent Document 1 (APPLIED PHYSICS LETTERS vol.89 211907 (2006)).
  • the type of the constituent element or the configuration thereof It is known that there is a correlation between the composition ratio of elements and the film thickness.
  • an impurity was added Al s In t Ga (1- st) consisting N n-type nitride semiconductor layer and its straight upper The degree of lattice mismatch is given.
  • the In composition ratio t of the n-type nitride semiconductor layer is 0, that is, when the n-type nitride semiconductor layer is made of Al s Ga (1-s) N (0 ⁇ s ⁇ 1)
  • the n-type nitride Since the semiconductor layer and the layer immediately above it are heterojunction, the degree of lattice mismatch increases as the Al composition ratio s of the n-type nitride semiconductor layer or the In composition ratio of the layer immediately above increases. Therefore, compressive stress or tensile stress acts on the layer immediately above the n-type nitride semiconductor layer. In order to mitigate energy due to this strain, misfit dislocations are generated immediately above the n-type nitride semiconductor layer.
  • the degree of lattice mismatch at which misfit dislocations are formed varies depending on the thickness of the layer immediately above the n-type nitride semiconductor layer (the maximum thickness at which no misfit transition is formed is referred to as the critical thickness).
  • the critical thickness As the thickness of the layer immediately above the n-type nitride semiconductor layer increases, the stress applied to the entire heterostructure increases. Therefore, misfit dislocations are easily formed immediately above the n-type nitride semiconductor layer.
  • Non-Patent Document 2 PYSICAL REVIEW B vol.78 pp.233303-1 to 233303-4 (2008) describes a decrease in the critical film thickness accompanying an increase in the In composition ratio in the layer immediately above the n-type nitride semiconductor layer.
  • Non-Patent Document 3 Japanese Journal of Applied Physics vol. 1 describes the defects formed in the immediately above layer when the thickness of the layer immediately above the n-type nitride semiconductor layer reaches the critical thickness and the mechanism thereof. .45 No.22 2006 pp.L549-L551).
  • the generated photocarriers can be expected to diffuse into the n-type nitride semiconductor layer 13 along the potential barrier of the intervening layer 15. Thereby, since many photocarriers can be taken out, a short circuit current improves. Therefore, it is very preferable for the photoelectric conversion element 1 that the thickness of the intervening layer 15 is 1 nm or more and 5 nm or less.
  • the critical film thickness of the immediately above layer decreases, so that the intermediate layer 15 of the intermediate layer 15 depends on the value of the In composition ratio z of the intermediate layer 15. It is preferable to set the film thickness d appropriately in the range of 1 nm to 5 nm.
  • the In composition ratio z of the intervening layer 15 is preferably lower than the In composition ratio y of the i-type nitride semiconductor layer 17 and the thickness d of the intervening layer 15 is preferably 1 nm or more and 5 nm or less is further described.
  • the intervening layer 15 is not provided, the degree of lattice mismatch between the n-type nitride semiconductor layer and the i-type nitride semiconductor layer is large. Therefore, the i-type nitride semiconductor layer is subjected to compressive stress. As shown in FIG. 3, since this compressive stress generates a piezoelectric electric field in the opposite direction to the internal electric field, the internal electric field is weakened.
  • the thickness d of the intervening layer 15 is in the range of 1 nm to 5 nm and the In composition ratio z of the intervening layer 15 is lower than the In composition ratio y of the i-type nitride semiconductor layer 17, the n-type nitride The degree of lattice mismatch between the semiconductor layer 13 and the i-type nitride semiconductor layer 17 can be reduced. The reason is that the interstitial layer 15 and the i-type nitride semiconductor layer 17 have different composition ratios of the constituent elements but the same kind of constituent elements, so that the difference in lattice constant can be kept small.
  • the intervening layer 15 is thin, the i-type nitride semiconductor layer 17 grows while maintaining lattice continuity at the interface with the intervening layer 15. Therefore, formation of misfit dislocations in the i-type nitride semiconductor layer 17 can be prevented.
  • the intervening layer 15 functions as a buffer layer for the i-type nitride semiconductor layer 17 coherently grown on the intervening layer 15. For these reasons, the compressive stress received from the n-type nitride semiconductor layer 13 is relaxed, and the piezoelectric electric field is reduced.
  • the thickness d of the intervening layer 15 is in the range of 1 nm to 5 nm, and the In composition ratio z of the intervening layer 15 is lower than the In composition ratio y of the i-type nitride semiconductor layer 17. It is very preferable for the photoelectric conversion element.
  • E f is E s It is sufficient if> E i + E f is satisfied.
  • the intervening layer 15 functions as a buffer layer that reduces the degree of lattice mismatch between the n-type nitride semiconductor layer 13 and the i-type nitride semiconductor layer 17, the strain energy of the i-type nitride semiconductor layer 17 is reduced. Can be reduced. Therefore, since the growth energy E f of the i-type nitride semiconductor layer 17 becomes small, E s > E i + E f can be maintained, and the high-quality i-type nitride semiconductor layer 17 can be formed.
  • FIG. 4 is a PL spectrum of the i-type nitride semiconductor layer when the intervening layer 15 is not formed, and L2 in FIG.
  • the i-type nitride semiconductor layer 17 is composed of 6-cycle GaN / InGaN, and the intervening layer 15 moves from the n-type nitride semiconductor layer 13 toward the i-type nitride semiconductor layer 17.
  • the In 0.06 Ga 0.94 N layer having a thickness of 2 nm, the In 0.07 Ga 0.93 N layer having a thickness of 2 nm, and the In 0.08 Ga 0.92 N layer having a thickness of 2 nm are stacked in this order.
  • the second intervening layer 19 is configured such that the type of constituent elements is the same as that of the i-type nitride semiconductor layer 17, while the composition ratio of constituent elements is different from that of the i-type nitride semiconductor layer 17.
  • the band gap energy Eg w of the second intervening layer 19 is larger than the band gap energy E g y of the i-type nitride semiconductor layer 17 and the band gap energy E g of the p-type nitride semiconductor layer 21 as shown in FIG. Less than 0 .
  • the p-type nitride semiconductor layer 21 becomes p-Al u In v Ga (1-uv) N (0 ⁇ u ⁇ 1, 0 ⁇ v ⁇ 1, 0 ⁇ u + v ⁇ 1) and the i-type nitride semiconductor layer 17 is Al x In y Ga (1-xy) N (0 ⁇ x ⁇ 1, 0 ⁇ y ⁇ 1) , 0 ⁇ x + y ⁇ 1), the second intervening layer 19 has Al x In w Ga (1-xw) N (0 ⁇ x ⁇ 1, 0 ⁇ w ⁇ 1, 0 ⁇ x + w ⁇ 1, The layer may be 0 ⁇ v ⁇ w ⁇ y).
  • Some of the electrons generated in the i-type nitride semiconductor layer 17 by the incidence of the light 101 on the photoelectric conversion element 1 are intended to diffuse into the conduction band of the p-type nitride semiconductor layer 21 as shown in FIG. Exists.
  • the band gap energy of the second intervening layer 19 is larger than the band gap energy of the i-type nitride semiconductor layer 17, the electrons can be driven back to the conduction band of the n-type nitride semiconductor layer 13. Therefore, the number of electrons diffusing into the conduction band of n-type nitride semiconductor layer 13 can be increased, and the short-circuit current is improved.
  • the second intervening layer 19 and the i-type nitride semiconductor layer 17 have the same type of constituent elements. Therefore, if the second intervening layer 19 is provided, the degree of lattice mismatch between the i-type nitride semiconductor layer 17 and the p-type nitride semiconductor layer 21 is reduced. Therefore, formation of misfit dislocations at the interface between i-type nitride semiconductor layer 17 and p-type nitride semiconductor layer 21 can be prevented.
  • the electrons to be diffused into the conduction band of the p-type nitride semiconductor layer 21 are the p-type nitride semiconductor layer unless the second intervening layer 19 is provided. It is trapped by the interface state caused by misfit dislocations at the interface between 21 and the i-type nitride semiconductor layer 17 and disappears. However, if the second intervening layer 19 is provided, the formation of misfit dislocations at the interface between the p-type nitride semiconductor layer 21 and the i-type nitride semiconductor layer 17 can be prevented, so that the electrons are p-type nitride.
  • the photoelectric conversion element Even if it diffuses to the interface between the semiconductor layer 21 and the i-type nitride semiconductor layer 17, it can be prevented from disappearing due to misfit dislocations. Also from these things, it is preferable for the photoelectric conversion element to provide the second intervening layer 19.
  • the film thickness of the 2nd intervening layer 19 is not specifically limited, It is preferable that they are 1 nm or more and 5 nm or less. If there is Al x In w Ga (1- xw) N second intermediate layer 19 made of (0 ⁇ w ⁇ y) between the p-type nitride semiconductor layer 21 and the i-type nitride semiconductor layer 17, The band gap energy (Eg w ) of the second intervening layer 19 becomes larger than the band gap energy (Eg y ) of the i-type nitride semiconductor layer 17 (Eg y ⁇ Eg w ). A potential barrier V ′ (eV) is formed as viewed from the generated photocarriers (holes) (FIG. 5).
  • the In composition ratio z of the p-type nitride semiconductor layer 21 is 0, that is, when the p-type nitride semiconductor layer 21 is a p-Al x Ga (1-x) N (0 ⁇ x ⁇ 1) layer.
  • the band gap energy Eg 0 of the p-type nitride semiconductor layer 21 is as large as 3.4 eV to 6.0 eV.
  • the holes generated in the i-type nitride semiconductor layer 17 pass along the potential barrier V ′ generated by the formation of the second intervening layer 19 or through the second intervening layer 19 by the tunnel effect. Diffusion into the valence band of the p-type nitride semiconductor layer 21 is possible.
  • the film thickness of the second intervening layer 19 is desired to be a film thickness that allows holes to pass within the recombination lifetime.
  • the recombination lifetime is discussed in detail in Non-Patent Document 1, for example.
  • the thickness of the second intervening layer 19 is preferably 5 nm or less.
  • controlling the film thickness of the second intervening layer 19 to be less than 1 nm may be difficult in terms of production technology.
  • the In composition ratio z of the intervening layer 15 is lower than the In composition ratio y of the i-type nitride semiconductor layer 17. Therefore, the absolute refractive index n z of the intervening layer 15 is smaller than the absolute refractive index value n y i-type nitride semiconductor layer 17 (n z ⁇ n y) .
  • the In composition ratio t of the n-type nitride semiconductor layer 13 is preferably lower than the In composition ratio z of the intervening layer 15, and the Al composition ratio s of the n-type nitride semiconductor layer 13 is the Al composition ratio of the intervening layer 15. Preferably it is higher than x.
  • 0.4 .mu.m ⁇ 0 absolute refractive index n n the n-type nitride semiconductor layer 13 which is smaller than the absolute refractive index n z of the intervening layer 15 (n n ⁇ n z) the present invention is applied. In the region of 5 ⁇ m, it is 2.3 to 2.6.
  • the In composition ratio w of the second intervening layer 19 is lower than the In composition ratio y of the i-type nitride semiconductor layer 17. Therefore, the absolute refractive index n w of the second intervening layer 19 is smaller than the absolute refractive index n y of the i-type nitride semiconductor layer 17 (n w ⁇ ny ).
  • the In composition ratio v of the p-type nitride semiconductor layer 21 is preferably lower than the In composition ratio w of the second intervening layer 19, and the Al composition ratio u of the p-type nitride semiconductor layer 21 is the second intervening layer. It is preferable that the Al composition ratio x of the layer 19 is higher.
  • the absolute refractive index n p of the p-type nitride semiconductor layer 21 becomes smaller than the absolute refractive index n w of the second intervening layer 19 (n p ⁇ n w ), which is 0.4 ⁇ m which is the object of the present invention. In the region of 0.5 ⁇ m, it is 2.3 to 2.6.
  • the transparent conductive film 23 is not particularly limited in its material, but may be a single layer containing at least one of Zn, In, Sn, and Mg, or may be one of Zn, In, Sn, and Mg. A layer including at least one layer may be stacked.
  • the single layer containing Zn include an AZO (aluminum doped zinc oxide) layer in which ZnO is doped with Al, a GZO (gallium doped zinc oxide) layer in which ZnO is doped with Ga, and Mg in which ZnO is doped with Mg.
  • Examples thereof include an MZO (magnesium doped zinc oxide) layer or an IZO (indium doped zinc oxide) layer in which ZnO is doped with In.
  • Examples of the single layer containing Mg include a Mg (OH) 2 layer doped with C.
  • an AZO film having a different Al concentration in the thickness direction may be formed using ZnO targets having a different Al concentration, and a GZO layer and an ITO layer may be laminated. Also good.
  • the transparent conductive film 23 is not particularly limited in its film thickness, but preferably has a film thickness of 250 nm or more and 500 nm or less. If the film thickness of the transparent conductive film 23 is less than 250 nm, there is a tendency that an optimal ohmic contact between the p-type nitride semiconductor layer 21 and the transparent conductive film 23 cannot be formed. F tends to decrease.
  • the film thickness of the transparent conductive film 23 is preferably changed as appropriate according to the film thickness of the p-type nitride semiconductor layer 21.
  • the absolute refractive index value of the transparent conductive film 23 is larger than 1.5 and smaller than 2.3 and the film thickness of the transparent conductive film 23 is 250 nm or more and 500 nm or less, a short wavelength of 0.4 to 0.5 ⁇ m
  • the transmittance of the transparent conductive film 23 with respect to the region is increased, so that a large amount of light enters the i-type nitride semiconductor layer 17, so that more photocarriers are generated in the i-type nitride semiconductor layer 17.
  • the short circuit current of the photoelectric conversion element 1 is further improved, and the fill factor is further improved.
  • the photoelectric conversion element it is very preferable for the photoelectric conversion element that the absolute refractive index value of the transparent conductive film 23 is larger than 1.5 and smaller than 2.3, and the film thickness of the transparent conductive film 23 is 250 nm or more and 500 nm or less.
  • the light reflecting layer 25 is not particularly limited as long as it can be a layer that reflects light.
  • the light reflecting layer 25 is preferably an Al single layer or an Ag single layer in that it is easily available among metal elements that reflect light and has a high reflectance.
  • the light reflecting layer 25 is more preferably an Ag single layer having a high reflectance on the short wavelength side.
  • the light reflecting layer 25 is not particularly limited in its film thickness, but preferably has a film thickness of 10 nm or more and 1000 nm or less.
  • the film thickness of the light reflecting layer 25 is outside this range, the light reflecting layer 25 tends to be peeled off from the lower surface of the substrate 11. And if the photoelectric conversion element 1 which concerns on this embodiment is provided with the light reflection layer 25, the following (light confinement effect 5) can be obtained.
  • the light 101 incident from the p-type nitride semiconductor layer 21 side is confined in the i-type nitride semiconductor layer 17 by the intervening layer 15 and the second intervening layer 19 ( Light confinement effect 1).
  • the optical path length in the i-type nitride semiconductor layer 17 increases, a lot of photocarriers are generated, and thus the short-circuit current of the photoelectric conversion element is improved.
  • the optical confinement effect 1 is a decrease in the absolute refractive index value at the interface between the intervening layer 15 and the i-type nitride semiconductor layer 17 and the interface between the i-type nitride semiconductor layer 17 and the second intervening layer 19.
  • ⁇ Al composition ratio of the nitride semiconductor material and the In composition ratio and the relationship between the absolute refractive index value> is an n w ⁇ n y, a n z ⁇ n y.
  • light can be transmitted from a layer having a low absolute refractive index value to a layer having a high absolute refractive index value, but it is difficult to transmit light from a layer having a high absolute refractive index value to a layer having a low absolute refractive index value. Therefore, when the light 101 enters from the p-type nitride semiconductor layer 21 side, the incident light 101 enters the i-type nitride semiconductor layer 17 from the p-type nitride semiconductor layer 21 through the second intervening layer 19. .
  • the incident light 101 is returned as reflected light 102 is reflected at the interface between the i-type nitride semiconductor layer 17 and the intermediate layer 15 to the i-type nitride semiconductor layer 17, Absorbed by i-type nitride semiconductor layer 17.
  • the reflected light 102 the light that is not absorbed by the i-type nitride semiconductor layer 17 diffuses toward the second intervening layer 19.
  • the incident light 101 is reflected light 102
  • a part of the incident light 101 may not be reflected light 102 and may enter the intervening layer 15.
  • the photoelectric conversion element 1 2.3 ⁇ n n ⁇ n z ⁇ n as shown in the above ⁇ Relationship between Al composition ratio and In composition ratio of nitride semiconductor material and absolute refractive index value>. Since y , the light incident on the intervening layer 15 is reflected at the interface between the n-type nitride semiconductor layer 13 and the intervening layer 15 and returns to the i-type nitride semiconductor layer 17 as reflected light 104 (light confinement effect). 2). Thereby, since more photocarriers are generated, the short-circuit current of the photoelectric conversion element is further improved.
  • the light that could not be the reflected light 103 may enter the second intervening layer 19.
  • the photoelectric conversion element 1 according to the present embodiment 2.3 ⁇ n p ⁇ n w ⁇ n as shown in the above ⁇ Relationship between Al composition ratio and In composition ratio of nitride semiconductor material and absolute refractive index value>. Since y , the light incident on the second intervening layer 19 is reflected at the interface between the second intervening layer 19 and the p-type nitride semiconductor layer 21 to become reflected light 105, and the i-type nitride semiconductor layer 17. Return to (light confinement effect 3). Thereby, more photocarriers are generated and the short-circuit current of the photoelectric conversion element is further improved.
  • the band gap energy (Eg 0 ) of the p-type nitride semiconductor layer 21, the band gap energy (Eg w ) of the second intervening layer 19, and the i-type nitride semiconductor layer 17 Since the band gap energy (Eg y ) satisfies Eg y ⁇ Eg w ⁇ Eg 0 , absorption of incident light by the p-type nitride semiconductor layer 21 and the second intervening layer 19 is suppressed. A large amount of light can be incident on the type nitride semiconductor layer 17.
  • the incident light 101 is transmitted from the p-type nitride semiconductor layer 21 having a small absolute refractive index value through the second intervening layer 19 having a large absolute refractive index value, loss of light intensity due to reflection can be prevented. From these points, the short-circuit current of the photoelectric conversion element 1 is improved, which is preferable for the photoelectric conversion element 1.
  • the absolute refractive index value of the transparent conductive film 23 is less than 2.3, the absolute refractive index value of the transparent conductive film 23 is the absolute refractive index of the p-type nitride semiconductor layer 21. Less than the value. Therefore, the light incident on the p-type nitride semiconductor layer 21 is reflected at the interface between the p-type nitride semiconductor layer 21 and the transparent conductive film 23 to return to the i-type nitride semiconductor layer 17 as reflected light 106 ( Light confinement effect 4). Thereby, since more photocarriers are generated, the short-circuit current of the photoelectric conversion element is further improved.
  • a part of the light that could not be the reflected light 102 may not be the reflected light 104 and may pass through the n-type nitride semiconductor layer 13 and the substrate 11.
  • the light reflection layer 25 since the light reflection layer 25 is provided, the light transmitted through the substrate 11 is reflected at the interface between the substrate 11 and the light reflection layer 25 to become reflected light 107 i. Return to the type nitride semiconductor layer 17 (light confinement effect 5). Thereby, since more photocarriers are generated, the short-circuit current of the photoelectric conversion element is further improved.
  • the configuration of the intervening layer is not limited to the configuration shown in FIG. 2, and may be the configuration shown in the following first modification.
  • FIG. 6 is an energy band diagram (n side) in the photoelectric conversion element according to the first modification.
  • the second intervening layer 19 is not shown.
  • points different from the first embodiment will be mainly described.
  • the intervening layer 35 is composed of three semiconductor layers 35A, 35B, and 35C.
  • the first layer 35A, the second layer 35B, and the third layer are arranged in order from the i-type nitride semiconductor layer 17 to the n-type nitride semiconductor layer 13. This is referred to as a layer 35C.
  • the first layer 35A is provided immediately below the i-type nitride semiconductor layer 17 and has a larger band gap energy (Eg z1 ) than the i-type nitride semiconductor layer 17.
  • the second layer 35B is provided immediately below the first layer 35A, and has a larger band gap energy (Eg z2 ) than that of the first layer 35A.
  • the third layer 35C is provided between the second layer 35B and the n-type nitride semiconductor layer 13, and has a band gap that is larger than the second layer 35B and smaller than the n-type nitride semiconductor layer 13. It has energy (Eg z3 ).
  • the first layer 35A, the second layer 35B, and the third layer 35C have different band gap energies and travel from the i-type nitride semiconductor layer 17 to the n-type nitride semiconductor layer 13.
  • the band gap energy is arranged so as to increase sequentially as Eg z1 ⁇ Eg z2 ⁇ Eg z3 .
  • the band gap energy increases stepwise from the i-type nitride semiconductor layer 17 toward the n-type nitride semiconductor layer 13, so that it is seen from the photocarriers (electrons) generated in the i-type nitride semiconductor layer 17.
  • Step potential barriers V1, V2, V3, and V4 are formed in this order, and electrons diffuse along the step potential barrier into the conduction band of n-type nitride semiconductor layer 13. As a result, many photocarriers can be taken out, thereby improving the short-circuit current.
  • the first layer 35A is made of Al x In z1 Ga (1- x-z1) N
  • the second layer 35B is made of Al x In z2 Ga (1- x-z2) N
  • the third layer 35C is made of Al x in z3 Ga (1- x-z3) N
  • the 0 ⁇ z3 ⁇ z2 ⁇ z1 ⁇ y ( however, y is an in composition ratio of the i-type nitride semiconductor layer 17) Satisfies.
  • each of the first layer 35A, the second layer 35B, and the third layer 35C may be 1 nm or more and 5 nm or less.
  • the number of semiconductor layers constituting the interposition layer 35 is not particularly limited.
  • the plurality of semiconductor layers only have to be configured to have different In composition ratios, and are arranged such that the In composition ratio decreases from the i-type nitride semiconductor layer 17 toward the n-type nitride semiconductor layer 13. Just do it.
  • the photoelectric conversion element which concerns on this invention was shown by 1st Embodiment and a 1st modification, the photoelectric conversion element which concerns on this invention is not limited to description of 1st Embodiment and a 1st modification.
  • the method of adjusting each of the magnitude of the band gap energy and the absolute refractive index value is not limited to the control of the In composition ratio.
  • the metal layer may be provided on the p-type nitride semiconductor layer with the transparent conductive film interposed therebetween.
  • the intervening layer may have a non-uniform In concentration in the layer (that is, the portion other than the interface with the i-type nitride semiconductor layer and the interface with the n-type nitride semiconductor layer).
  • the absolute refractive index value may be non-uniform. This is also true for the second intervening layer.
  • the second intervening layer has substantially the same configuration as the intervening layer 35 in the first modification.
  • the second intervening layer is formed by stacking a plurality of second semiconductor layers having different In composition ratios, and the plurality of second semiconductor layers are made of i-type nitride semiconductor layers. What is necessary is just to arrange
  • the plurality of second semiconductor layers are arranged such that the band gap energy increases from the i-type nitride semiconductor layer toward the p-type nitride semiconductor layer.
  • the staircase-like potential barrier is sequentially formed as viewed from the photocarriers (holes) generated in the i-type nitride semiconductor layer, the holes are formed in the p-type nitride semiconductor layer along the staircase-like potential barrier. Diffuses into the valence band. As a result, many photocarriers can be taken out, thereby improving the short-circuit current.
  • the thickness of each of the plurality of semiconductor layers constituting the second intervening layer may be 1 nm or more and 5 nm or less.
  • FIG. 1 is a side view of a photoelectric conversion element 1 according to Example 1 of the present invention.
  • the photoelectric conversion element 1 according to the present embodiment is manufactured by the MOCVD method, and is operated by making incident light 101 incident from the surface of the transparent conductive film 23 toward the substrate 11 side.
  • a GaN substrate whose surface was cleaned with 47% hydrogen fluoride was used as the substrate 11.
  • the GaN substrate 11 is heated to 1100 ° C., more preferably to 1120 ° C., 125 ⁇ mol of tri-methyl gallium (TMG) is used, 270 mmol of ammonia (NH 3 ) is used, and an n-thickness of 1.5 ⁇ m is formed.
  • a type nitride semiconductor layer 13 was formed.
  • the n-type nitride semiconductor layer 13 is a GaN layer doped with 2 ⁇ 10 18 Si / cm 3 .
  • As the n-type impurity gas monosilane (SiH 4 ) with a supply amount of 2 mmol was used.
  • the n-type nitride semiconductor layer 13 can have a film thickness of, for example, 0.1 ⁇ m or more and 4 ⁇ m or less.
  • the temperature is lowered to 800 ° C., more preferably to 750 ° C., an In 0.05 Ga 0.95 N layer having a thickness of 2 nm, an In 0.06 Ga 0.94 N layer having a thickness of 2 nm, and the film thickness was formed intermediate layer 15 comprising three layers which are stacked in this order in 0.07 Ga 0.93 N layer of 2 nm (FIG. 1 describes the intervening layer 15 as a first layer).
  • the In 0.05 Ga 0.95 N layer was formed using 10 ⁇ mol of TMG, 29 ⁇ mol of tri-methyl indium (TMI), and 420 mmol of NH 3 .
  • the In 0.06 Ga 0.94 N layer was formed using 10 ⁇ mol of TMG, 33 ⁇ mol of TMI, and 420 mmol of NH 3 .
  • the In 0.07 Ga 0.93 N layer was formed using 10 ⁇ mol of TMG, 38 ⁇ mol of TMI, and 420 mmol of NH 3 .
  • an i-type nitride semiconductor layer 17 made of In 0.10 Ga 0.90 N was formed.
  • an In depth distribution was obtained by Auger electron spectroscopy. The result is shown in FIG. A point X shown in FIG. 7 indicates a growth start point of the In 0.10 Ga 0.90 N layer.
  • the In concentration suddenly became 11% from the point Y shown in FIG. This indicates that the critical film thickness of InGaN with an In concentration of 9% is about 50 nm.
  • the edge dislocations are formed to reduce the strain caused by the difference in lattice spacing, there are many dangling bonds on the surface where the dislocations are formed. Therefore, a large amount of In is adsorbed on the surface.
  • the critical thickness of the In 0.10 Ga 0.90 N layer formed by growth conditions of the present example was obtained experimentally, to form an In 0.10 Ga 0.90 N layer having a thickness of 45nm as a i-type nitride semiconductor layer 17.
  • the i-type nitride semiconductor layer 17 was formed using 10 ⁇ mol of TMG, 54 ⁇ mol of TMI, and 420 mmol of NH 3 .
  • a second intervening layer 19 made of an In 0.05 Ga 0.95 N layer having a thickness of 2 nm was formed.
  • the second intervening layer 19 was formed using 10 ⁇ mol of TMG, 38 ⁇ mol of TMI, and 420 mmol of NH 3 .
  • the intervening layer 15 and the second intervening layer 19 serve to confine the incident light 101 in the i-type nitride semiconductor layer 17 (light confinement effect 1).
  • the optical confinement effect 1 is caused by the discontinuity of the absolute refractive index value at the interface between the intervening layer 15 and the i-type nitride semiconductor layer 17 and at the interface between the i-type nitride semiconductor layer 17 and the second intervening layer 19.
  • the incident light 101 is first absorbed by the i-type nitride semiconductor layer 17.
  • Incident light 101 that reaches the interface between the intervening layer 15 and the i-type nitride semiconductor layer 17 without being completely absorbed by the i-type nitride semiconductor layer 17 is reflected at the interface to become reflected light 102 and is i-type nitrided. Returned to the semiconductor layer 17 and absorbed by the i-type nitride semiconductor layer 17 (light confinement effect 1).
  • Incident light 101 that has passed through the intervening layer 15 and has reached the interface between the intervening layer 15 and the n-type nitride semiconductor layer 13 without being reflected at the interface between the intervening layer 15 and the i-type nitride semiconductor layer 17 And is reflected back to the i-type nitride semiconductor layer 17 and absorbed by the i-type nitride semiconductor layer 17 (light confinement effect 2).
  • the reflected light 102 and the reflected light 104 are absorbed by the i-type nitride semiconductor layer 17 and contribute to the generation of photocarriers.
  • the reflected light 102 and the reflected light 104 that are not completely absorbed by the i-type nitride semiconductor layer 17 reach the interface between the i-type nitride semiconductor layer 17 and the second intervening layer 19, the reflected light 102 and the reflected light 104 are reflected. Is reflected at the interface, becomes reflected light 103, returns to the i-type nitride semiconductor layer 17, and is absorbed by the i-type nitride semiconductor layer 17 (light confinement effect 1).
  • the light transmitted through the second intervening layer 19 without being reflected at the interface between the i-type nitride semiconductor layer 17 and the second intervening layer 19 (without being the reflected light 103) is transmitted through the second intervening layer.
  • the temperature is raised to 1000 ° C., more preferably raised to 1070 ° C., and then the p-type nitride semiconductor layer 21 having a thickness of 50 nm using 125 ⁇ mol of TMG and 270 mmol of NH 3 is used. Formed.
  • the p-type nitride semiconductor layer 21 is a GaN layer doped with 2 ⁇ 10 19 Mg / cm 3 .
  • As the p-type impurity gas a supply amount of 0.3 ⁇ mol of biscyclopentadienyl magnesium (CP 2 Mg) was used.
  • the p-type nitride semiconductor layer 21 can have a film thickness of, for example, 50 nm or more and 2000 nm or less.
  • the p-type nitride semiconductor layer 21 was heat-treated using an annealing furnace.
  • the heat treatment temperature was 800 ° C. and held for 5 minutes.
  • the gas phase atmosphere of the heat treatment was composed only of nitrogen.
  • an AZO transparent conductive film 23 having a film thickness of 0.32 ⁇ m was formed from a ZnO target having an Al concentration of 2% by using magnetron sputtering.
  • the substrate temperature was 180 ° C.
  • the transparent conductive film 23 may be an AZO film formed from ZnO targets having different Al concentrations by changing the partial pressure from 3.0% to 10.0%.
  • a transparent conductive film 23 may be formed of a film having a composition different from that of AZO, such as a GZO film or ITO film having Ga as a dopant.
  • heat treatment was performed using an annealing furnace in order to improve crystallinity and adhesion.
  • the heat treatment temperature was 600 ° C. and held for 10 minutes.
  • the gas phase atmosphere of the heat treatment was configured in a vacuum with an oxygen partial pressure of 2.0%.
  • the absolute refractive index value of the transparent conductive film 23 is smaller than 2.3. Therefore, the light that reaches the interface between the p-type nitride semiconductor layer 21 and the transparent conductive film 23 without being reflected at the interface between the second intervening layer 19 and the p-type nitride semiconductor layer 21 is reflected at the interface.
  • the reflected light 106 is returned to the i-type nitride semiconductor layer 17 (light confinement effect 4). Due to the light confinement effect 4, more photocarriers are generated in the i-type nitride semiconductor layer 17.
  • a 150 nm-thick light reflecting layer 25 is formed on the back surface of the substrate 11 from an Ag target having an Ag purity of 99.9% by using magnetron sputtering. Formed.
  • a single Ag film is used in this embodiment, a single film of Al, Au, Ni, Ti or Pt may be used, for example, as long as it is a metal that can reflect light. May be.
  • the light reflecting layer 25 is formed by magnetron sputtering, but the film forming apparatus for the light reflecting layer 25 is not limited.
  • the light reflecting layer 25 is a film forming apparatus using a vacuum deposition method or an ion plating method. Also good.
  • FIG. 8 shows the reflectance of the light reflecting layer 25.
  • L3 indicates the wavelength dependence of the reflectance of the Ag film
  • L4 indicates the wavelength dependence of the reflectance of the Al film. Comparing L3 and L4, the Ag film has higher reflectance on the short wavelength side (0.35 ⁇ m or more and 0.5 ⁇ m or less). Therefore, it can be seen that an Ag film is preferably used as the light reflecting layer 25 of the photoelectric conversion element 1.
  • the reflectance of the light reflection layer 25 was measured by the automatic measurement of a commercially available optical element measuring apparatus. The Fresnel equation was used to derive the reflectivity.
  • the light reflecting layer 25 reflects light that is not absorbed by the i-type nitride semiconductor layer 17 and the n-type nitride semiconductor layer 13 in the incident light 101.
  • the reflected light 107 passes through the n-type nitride semiconductor layer 13 again and is incident again on the i-type nitride semiconductor layer 17 (light confinement effect 5). Due to the light confinement effect 5, more photocarriers are generated in the i-type nitride semiconductor layer 17.
  • a mask having a predetermined shape was formed on the surface, and then etching was performed from above the mask with an etching apparatus to expose a part of the n-type nitride semiconductor layer 13.
  • a mask (resist mask) having a predetermined pattern is formed on the upper surfaces of the transparent conductive film 23 and the n-type nitride semiconductor layer 13, and a metal film made of Ni / Pt / Au is sequentially stacked on the mask by an evaporation method. Then, a pad electrode (not shown) made of the metal film was formed by a lift-off method.
  • the pad electrode in the obtained photoelectric conversion element was connected to a lead frame with a gold wire, and a probe was brought into contact with the positive electrode and the negative electrode of the lead frame to form a circuit for measuring current and voltage.
  • the output characteristics of the photoelectric conversion element 1 are irradiated with 1 SUN pseudo-sunlight having a spectral distribution of AM1.5 and an energy density of 100 mW / cm 2 on the photoelectric conversion element, and the ambient temperature and the temperature of the photoelectric conversion element are set at 25 ° C. Was measured.
  • a photoelectric conversion element in a comparative example was produced according to the same method as in Example 1 except that the intervening layer 15 and the second intervening layer 19 were not formed. According to the method described in Example 1 above, the output characteristics of the obtained photoelectric conversion element were measured.
  • the open-circuit voltage V oc is 1.73 V
  • the short-circuit current J sc is 0.80 mA / cm 2
  • the curve factor F.I. F was 0.41 and the light conversion efficiency was 0.57%.
  • the open-circuit voltage V oc is 1.85 V
  • the short-circuit current J sc is 1.85 mA / cm 2
  • the fill factor F.V. F was 0.60
  • the short circuit current was improved and the fill factor was improved as compared with the comparative example.
  • Example 2 A photoelectric conversion element according to Example 2 was fabricated according to the same method as in Example 1 except that the configuration of i-type nitride semiconductor layer 17 was different. In the following, differences from the first embodiment will be mainly described.
  • the intervening layer 15 After forming the intervening layer 15, an MQW (multiple layer) in which six layers of well layers made of In 0.20 Ga 0.80 N with a thickness of 3.5 nm and barrier layers made of GaN with a thickness of 6 nm are alternately stacked on the intervening layer 15. An i-type nitride semiconductor layer 17 having a (quantum well) structure was formed.
  • the intervening layer 15 has an MQW structure in which six well layers and barrier layers are laminated.
  • the number of well layers and barrier layers may be larger than six, or the intervening layers may be more than six. 15 may have a single quantum well structure.
  • a buffer layer that relaxes lattice mismatch between the n-type nitride semiconductor layer 13 and the intervening layer 15 may be formed.
  • 20 buffer layers of In x Ga 1-x N (x ⁇ 0.1) with a thickness of less than 2 nm and 20 barrier layers of GaN with a thickness of less than 2 nm are alternately stacked. Any MQW structure may be used.
  • Example 2 Thereafter, according to the same method as in Example 1, the second intervening layer 19, the p-type nitride semiconductor layer 21, the transparent conductive film 23, the light reflecting layer 25, and the pad electrode were formed. In this way, a photoelectric conversion element according to Example 2 was obtained. According to the same method as in Example 1, the output characteristics of the obtained photoelectric conversion element were measured.
  • the open-circuit voltage V oc is 1.73 V
  • the short-circuit current J sc is 0.80 mA / cm 2
  • the curve factor F.I. F was 0.41 and the light conversion efficiency was 0.57%.
  • the open-circuit voltage V oc is 2.21 V
  • the short-circuit current J sc is 1.97 mA / cm 2
  • the fill factor F.I. F was 0.66 and the light conversion efficiency was 2.87%.
  • the short circuit current was improved and the fill factor was improved as compared with the comparative example.
  • 1 photoelectric conversion element 11 substrate, 13 n-type nitride semiconductor layer, 15 intervening layer, 17 i-type nitride semiconductor layer, 19 second intervening layer, 21 p-type nitride semiconductor layer, 23 transparent conductive film, 25 light Reflective layer, 35 intervening layer, 35A first layer, 35B second layer, 35C third layer, 101 incident light, 102-107 reflected light.

Abstract

La présente invention concerne un élément de conversion photoélectrique qui est conçu en connectant de façon goupillée une couche à semi-conducteur à nitrure de type n, une couche à semi-conducteur à nitrure de type i qui est prévue sur la couche à semi-conducteur à nitrure de type n, et une couche à semi-conducteur à nitrure de type p qui est prévu sur la couche à semi-conducteur à nitrure de type i. L'élément de conversion photoélectrique est également pourvu d'une couche intermédiaire, qui est prévue entre la couche à semi-conducteur à nitrure de type n et la couche à semi-conducteur à nitrure de type i, et possède les types des éléments constituants égaux à ceux de la couche à semi-conducteur à nitrure de type i mais présente des rapports de composition des éléments constituants différents de ceux de la couche à semi-conducteur à nitrure de type i. Le rapport de composition In de la couche intermédiaire est inférieur au rapport de composition In de la couche à semi-conducteur à nitrure de type i.
PCT/JP2012/069323 2011-09-13 2012-07-30 Élément de conversion photoélectrique WO2013038822A1 (fr)

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US9799689B2 (en) 2014-11-13 2017-10-24 Artilux Inc. Light absorption apparatus
JP5876562B1 (ja) * 2014-11-27 2016-03-02 日本電信電話株式会社 太陽電池の製造方法
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