JP5405545B2 - Photoelectric conversion element - Google Patents

Description

  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 μm 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.

However, _a 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. And b vary in a very wide range of 0.7 eV to 6.0 eV, and therefore the sensitivity is excellent even for light in a short wavelength region of 0.5 μm or less. Therefore, nitride semiconductor materials are attracting 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.

  A nitride semiconductor material is suitable as a material for a light emitting element such as a light emitting diode (LED) or a laser diode (LD), and thus has been actively developed. In addition, nitride semiconductor materials have been elucidated as next-generation photoelectric conversion elements since the band gap has been elucidated as described above.

APPLIED PHYSICS LETTERS vol.89 211907 (2006) PHYSICAL REVIEW B vol.78 pp.233303-1〜233303-4 (2008) Japanese Journal of Applied Physics vol.45 No.22 2006 pp.L549〜L551

  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 | formed in view of this point, The place made into the objective is to provide the photoelectric conversion element with a large short circuit current and high photoelectric conversion efficiency by increasing the generation amount itself of a photocarrier. is there.

  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 stacking 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.

When the i-type nitride semiconductor layer is made of Al _x In _y Ga _(1-xy) N (0 ≦ x ≦ 1, 0 <y ≦ 1, 0 <x + y ≦ 1), 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) may be laminated in order.

  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 preferably configured by stacking 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 intermediate layer, and the In composition ratio of the n-type nitride semiconductor layer is preferably lower than the In composition ratio of the intermediate 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 preferably 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. Moreover, 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.

  A light reflecting layer is preferably provided on the lower surface of the substrate. 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.

  In the photoelectric conversion element according to the present invention, since the amount of photocarriers generated itself increases, the short-circuit current increases and the photoelectric conversion efficiency increases.

It is a side view of the photoelectric conversion element which concerns on one Embodiment of this invention. It is an energy band figure of the photoelectric conversion element which concerns on one Embodiment of this invention. It is a figure which shows direction of the stress which arises in the photoelectric conversion element which concerns on one Embodiment of this invention, and an electric field. It is a graph which shows the normalized PL (photo luminesence) spectrum of an i-type nitride semiconductor layer. It is an energy band figure of the photoelectric conversion element which concerns on one Embodiment of this invention. It is an energy band figure of the photoelectric conversion element which concerns on another embodiment of this invention. It is a graph which shows the depth direction distribution of In concentration by Auger electron spectroscopy. It is a graph which shows the relationship of the reflectance of the light reflection layer with respect to a wavelength.

  Hereinafter, the photoelectric conversion element of the present invention will be described with reference to the drawings. In the drawings of the present invention, the same reference numerals represent the same or corresponding parts. In addition, dimensional relationships such as length, width, thickness, and depth are changed as appropriate for clarity and simplification of the drawings, and do not represent actual dimensional relationships. Furthermore, the present invention is not limited to the following items.

<First Embodiment>
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.

<Configuration of photoelectric conversion element>
The photoelectric conversion element 1 according to this embodiment 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. 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.

<Board>
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. Although the board | 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>
The 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. n-type nitride semiconductor layer 13, for example Si, P, As or n-type impurity is Al _s such as _{Sb In t Ga (1-st} ) N (0 ≦ s ≦ 1,0 ≦ t ≦ 1,0, < Any layer doped in the s + t ≦ 1) layer is preferable, and a layer in which Si is doped in the GaN layer is preferable. 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.

<I-type nitride semiconductor layer>
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. , to the _{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.

  The film thickness of 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.

<P-type nitride semiconductor layer>
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. In the p-type nitride semiconductor layer 21, for example, 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.

<Intervening layer>
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. Considering that the band gap energy decreases as the In composition ratio of the nitride semiconductor layer increases, the n-type nitride semiconductor layer 13 becomes 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.

When the intervening layer 15 is not present, 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. For example, if 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 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.

  In addition, if 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 (difference in lattice constant) is large. Therefore, misfit dislocations may be formed in i-type nitride semiconductor layer 17. As a result, 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.

On the other hand, if the intervening layer 15 is provided, as shown in FIG. 2, 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. When 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.

If the In composition ratio z of the intervening layer 15 is equal to or higher than the In composition ratio y of the i-type nitride semiconductor layer 17 (z ≧ y), the band gap energy (Eg _z ) of the intervening layer 15 is i-type nitride semiconductor. the band gap energy of the layer 17 (Eg _y) below. In this case, 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.

  However, if the In composition ratio z of the intervening layer 15 is higher than 0 and lower than the In composition ratio y of the i-type nitride semiconductor layer 17 (0 <z <y), electrons are smoothly transferred to the n-type nitride semiconductor layer 13. It is possible to prevent the problem that the photocarriers diffused and 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. When the thickness d of the intervening layer 15 is larger than 5 nm, 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. On the other hand, when the thickness d of the intervening layer 15 is less than 1 nm, 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. On the other hand, when the thickness d of the intervening layer 15 is 1 nm or more and 5 nm or less, 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 of electrons and holes in InGaN grown on the c-plane is discussed in Non-Patent Document 1, for example.

For the nitride semiconductor layer formed on the n-type nitride semiconductor layer (hereinafter referred to as “the layer immediately above the n-type nitride semiconductor layer” or simply “the layer immediately above”), 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. One of the factors that determine the quality of the direct upper layer of the crystals of the n-type nitride semiconductor layer, 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. For example, when 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). 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. For example, Non-Patent Document 2 discusses in detail the reduction in the critical film thickness accompanying the increase in the In composition ratio of the layer immediately above the n-type nitride semiconductor layer. Further, for example, Non-Patent Document 3 discusses in detail 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. .

Non-Patent Document 2 discloses that when the upper layer of the n-type nitride semiconductor layer is made of Al _x In _z Ga _(1-xz) N (x = 0, z = 0.25), the critical film of the immediately upper layer is formed. The thickness is disclosed to be 5 nm. That is, when x = 0 and z> 0.25, unless the film thickness of the layer immediately above the n-type nitride semiconductor layer is less than 5 nm, the distortion is alleviated and the thickness of the n-type nitride semiconductor layer is reduced. Misfit dislocations are formed in the upper layer. From the viewpoint of production technology, it is difficult to form a layer immediately above the n-type nitride semiconductor layer made of Al _x In _z Ga _(1-xz) N (x = 0, z = 0.25). As discussed in Non-Patent Document 2, when the In composition ratio z of the layer immediately above the n-type nitride semiconductor layer increases, the critical layer thickness of the layer immediately above decreases, so that the intervening layer 15 (this embodiment) In the embodiment, if the thickness d of the n-type nitride semiconductor layer immediately above the intervening layer 15 is 1 nm or more and 5 nm or less, it can be expected that a high-quality intervening layer 15 free from misfit dislocations can be formed. The generated photocarriers (electrons) 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.

Non-Patent Document 2 _discloses that when an upper layer of an n-type nitride semiconductor layer is made of Al _x In _z Ga _(1-xz) N (x = 0, z = 0.40), the critical film thickness of the upper layer is directly related. Is approximately 3 nm, and when the upper layer of the n-type nitride semiconductor layer is made of Al _x In _z Ga _(1-xz) N (x = 0, z = 0.50), the critical film of the upper layer It is described that the thickness is about 2 nm. Thus, as the In composition ratio z of the immediately above layer of the n-type nitride semiconductor layer increases, 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 reason why 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 will be further described. When 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.

  However, if 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. This is because since 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. In addition, since the degree of lattice mismatch is reduced by providing the intervening layer 15, 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. Therefore, it is possible to prevent the internal electric field from becoming weak, so that many photocarriers can be taken out, and the short-circuit current is improved. From the above, 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.

In order to form a high-quality i-type nitride semiconductor layer with few crystal defects, it is generally required to improve the flatness of the interface with the i-type nitride semiconductor layer. This is required because the formation of three-dimensional islands from nucleation formed by collision of atoms (atoms are supplied from the gas phase) or the like is suppressed during the growth of the i-type nitride semiconductor layer. It is mentioned that there is. In order to suppress the formation of the three-dimensional island, the surface energy E _{s of} the n-type nitride semiconductor layer, the interface energy E _{i at} the interface between the n-type nitride semiconductor layer and the i-type nitride semiconductor layer, and i-type nitridation Energy of the semiconductor layer (growth energy of the i-type nitride semiconductor layer = surface energy of the i-type nitride semiconductor layer + strain energy of the i-type nitride semiconductor layer + internal energy of the i-type nitride semiconductor layer) E _f is E _s It is sufficient if> E _i + E _f is satisfied. Here, since 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. This is apparent from 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. 4 is an i-type nitride semiconductor layer 17 when the intervening layer 15 is formed. It is a PL spectrum, and both spectra are standardized. The full width at half maximum of the PL spectrum was d ₁ = 214 meV in L1, but d ₂ = 174 meV in L2, and decreased when the intervening layer 15 was provided. Moreover, when the intervening layer 15 was provided, the intensity of the spectrum in the low energy region was lowered. These results mean that when the intervening layer 15 is provided, a high-quality i-type nitride semiconductor layer is formed. Therefore, when the intervening layer 15 is provided, a large number of photocarriers can be taken out, and therefore the short-circuit current is improved. Also from this, it is very preferable for the photoelectric conversion element to provide the intervening layer 15.

In the PL spectrum shown in FIG. 4, 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.

<Second intervening layer>
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 . Considering that the band gap energy decreases as the In composition ratio of the nitride semiconductor layer increases, 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. However, since 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. Here, among the electrons generated in the i-type nitride semiconductor layer 17, 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. 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.

Although 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). For example, when 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. However, 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. When holes move through the second intervening layer 19 to the valence band of the p-type nitride semiconductor layer 21 by the tunnel effect, the holes passing through the second intervening layer 19 are transferred to the p-type nitride semiconductor. When an electron to be diffused into the conduction band of the layer 21 is felt, it recombines with the electron in the second intervening layer 19 and disappears. Therefore, 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. According to the discussion, the thickness of the second intervening layer 19 is preferably 5 nm or less. On the other hand, controlling the film thickness of the second intervening layer 19 to be less than 1 nm may be difficult in terms of production technology.

About the critical film thickness of the 2nd intervening layer 19, the same thing as the intervening layer 15 can be said. That is, in Non-Patent Document 2, when the second intervening layer 19 is made of Al _x In _w Ga _(1-xw) N (x = 0, w = 0.40), the critical film of the second intervening layer 19 is disclosed. When the thickness is about 3 nm and the second intervening layer 19 is made of Al _x In _w Ga _(1-xw) N (x = 0, w = 0.50), the critical film of the second intervening layer 19 It is described that the thickness is about 2 nm. Thus, as the In composition ratio w of the second intervening layer 19 becomes higher, the critical film thickness of the second intervening layer 19 becomes thinner. Therefore, the second intervening layer 19 has a critical thickness according to the value of the In composition ratio w. It is preferable to set the thickness of the second intervening layer 19 as appropriate in the range of 1 nm to 5 nm.

<Relationship between Al composition ratio and In composition ratio of nitride semiconductor material and absolute refractive index value>
It is known that the absolute refractive index value of the nitride semiconductor layer increases as the In composition ratio of the nitride semiconductor layer increases.

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. Thus, the absolute refractive index n _n the n-type nitride semiconductor layer 13 is smaller than the absolute refractive index n _z of the intervening layer 15 _(n n <n _z), the present invention is directed 0.4Myuemu～0. In the region of 5 μm, it becomes 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. As a result, 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 μm which is the object of the present invention. It becomes 2.3 to 2.6 in the region of .about.0.5 .mu.m.

<Transparent conductive film>
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. In addition, examples of 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. Further, as the transparent conductive film 23, for example, 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. However, when 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 μm 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. Thereby, the short circuit current of the photoelectric conversion element 1 is further improved, and the fill factor is further improved. Therefore, 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.

<Light reflection layer>
The light reflecting layer 25 is not particularly limited as long as it can be a layer that reflects light. However, 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. In this embodiment in which excellent sensitivity is required for light in a short wavelength region of 0.5 μm or less, 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. If 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.

<Optical path in photoelectric conversion element>
In the photoelectric conversion element 1 according to this embodiment, 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). Thereby, since 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.

More specifically, 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. Caused by continuity. As indicated above <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. In general, 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. . Here, since it is n z _{<n y,} 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. Of 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. Here, since it is n w _{<n y,} this light to the i-type nitride semiconductor layer 17 and the second reflected at the interface between the intermediate layer 19 as reflected light 103 i-type nitride semiconductor layer 17 Return to the i-type nitride semiconductor layer 17 and absorbed. _Thus, since a n z _{<n y} and is _n w _{<n y,} the optical confinement effect 1 is obtained.

Although most of 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. In the photoelectric conversion element 1 according to the present embodiment, 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. In 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.

In addition, as shown in FIG. 5, 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. Further, since 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.

  Some of the light that could not be the reflected light 103 may not be the reflected light 105 and may enter the p-type nitride semiconductor layer 21. In the photoelectric conversion element 1 according to this embodiment, since 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.

  Some 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. In the photoelectric conversion element according to this embodiment, 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.

  In addition, the structure of an intervening layer is not limited to the structure shown in FIG. 2, The structure shown in the following 1st modification may be sufficient.

<First Modification>
FIG. 6 is an energy band diagram (n side) in the photoelectric conversion element according to the first modification. In FIG. 6, the second intervening layer 19 is not shown. Hereinafter, points different from the first embodiment will be mainly described.

In the photoelectric conversion element according to this modification, the intervening layer 35 is composed of three semiconductor layers 35A, 35B, and 35C. In order to distinguish the 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 ). Thus, 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 . As described above, 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.

Specifically, 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.

  The film thickness d of 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. Thereby, photocarriers (electrons) generated in the i-type nitride semiconductor layer 17 can be diffused to the conduction band of the n-type nitride semiconductor layer 13 along the stepped potential barrier. In addition, deterioration of the crystallinity of the i-type nitride semiconductor layer 17 can be prevented.

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. Specifically, on the n-type nitride semiconductor layer 13, an 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 and n ≧ 4) may be stacked in order.

  As mentioned above, although 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. . For example, 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.

Moreover, 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.

  Moreover, the 2nd intervening layer should just have the structure substantially the same as the intervening layer 35 in a 1st modification. Specifically, 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 | position so that In composition ratio may become low as it goes to a p-type nitride semiconductor layer. As a result, 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. Therefore, since 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.

  At this time, 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. Thereby, not only can the photocarriers (holes) generated in the i-type nitride semiconductor layer diffuse into the valence band of the p-type nitride semiconductor layer along the stepped potential barrier, but also the p-type nitride semiconductor layer The deterioration of crystallinity can be prevented.

<Example 1>
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.

In this example, 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 μmol of tri-methyl gallium (TMG) is used, 270 mmol of ammonia (NH ₃ ) is used, and an n-thickness of 1.5 μm 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 μm or more and 4 μm or less.

After the n-type nitride semiconductor layer 13 is formed, 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 μmol of TMG, 29 μmol of trimethylindium (TMI), and 420 mmol of NH ₃ . The In _0.06 Ga _0.94 N layer was formed using 10 μmol of TMG, 33 μmol of TMI, and 420 mmol of NH ₃ . The In _0.07 Ga _0.93 N layer was formed using 10 μmol of TMG, 38 μmol of TMI, and 420 mmol of NH ₃ .

After forming the intervening layer 15, an i-type nitride semiconductor layer 17 made of In _0.10 Ga _0.90 N was formed. In order to experimentally determine the critical thickness of the In _0.10 Ga _0.90 N layer, 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. After growing while maintaining the In concentration at 9% until the film thickness was 51 nm (around 84 nm on the horizontal axis in FIG. 7), 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 reason why the In concentration sharply increases when the thickness is greater than the critical film thickness cannot be determined, but when the dislocations are formed and the strain caused by the difference in lattice spacing is relieved, the dislocations are formed on the surface. This is probably because a large number of dangling bonds are present, and a large amount of In is adsorbed on the surface. After 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 μmol of TMG, 54 μmol of TMI, and 420 mmol of NH ₃ .

After the i-type nitride semiconductor layer 17 was formed, 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 μmol of TMG, 38 μmol 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.

  When 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). In addition, 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. 19 is reflected at the interface between the p-type nitride semiconductor layer 21 and reflected light 105 is returned to the i-type nitride semiconductor layer 17 and absorbed by the i-type nitride semiconductor layer 17 (light confinement effect). 3). Due to the light confinement effects 1, 2, and 3, many photocarriers are generated in the i-type nitride semiconductor layer 17.

After forming the second intervening layer 19, 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 μmol 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 μmol 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.

After the p-type nitride semiconductor layer 21 was heat-treated, an AZO transparent conductive film 23 having a film thickness of 0.32 μm was formed from a ZnO target having an Al concentration of 2% by using magnetron sputtering. The substrate temperature was 180 ° C., and the partial pressure during film formation was O ₂ /Ar=3.8%. In order to improve conductivity and transmittance, 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%. . For the same purpose, 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.

  After forming the transparent conductive film 23, 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.

  After the heat treatment of the transparent conductive film 23, as shown in FIG. 1, 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. Although 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. In this embodiment, the light reflecting layer 25 is formed by magnetron sputtering, but the film forming apparatus for the light reflecting layer 25 is not limited. For example, the light reflecting layer 25 is a film forming apparatus using a vacuum deposition method or an ion plating method. May be.

  FIG. 8 shows the reflectance of the light reflecting layer 25. L3 indicates the wavelength dependence of the reflectance of the Ag film, and 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 μm or more and 0.5 μm 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. In addition, 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.

  After heat treatment was performed on the transparent conductive film 23, 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 p-type nitride semiconductor layer 21 and the n-type nitride semiconductor layer 13, and a metal film made of Ni / Pt / Au is deposited on the mask. Then, a pad electrode (not shown) made of the metal film was formed by a lift-off method.

  Next, heat treatment was performed at 400 to 600 ° C. with a lamp annealing device, the substrate was ground and polished, and then the separation step was performed, and the obtained substrate was divided at predetermined positions. This obtained the photoelectric conversion element which concerns on a present Example.

The pad electrode in the obtained photoelectric conversion element was connected to the lead frame with a gold wire, and a probe was brought into contact with the positive and negative electrodes 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.

<Comparative example>
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.

In the photoelectric conversion element in the comparative example, the open-circuit voltage V _oc is 1.73 V, the short-circuit current J _sc is 0.80 mA / cm ² , and the curve factor F.I. F was 0.41 and the light conversion efficiency was 0.57%. On the other hand, in the photoelectric conversion element according to Example 1 shown in FIG. 1, the open-circuit voltage V _oc is 1.85 V, the short-circuit current J _sc is 1.85 mA / cm ² , and the fill factor F.V. F was 0.60, and the light conversion efficiency was 2.05%. As described above, in Example 1, 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.

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. In this embodiment, the intervening layer 15 has an MQW structure in which six well layers and barrier layers are laminated. However, 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. In addition, a buffer layer that relaxes lattice mismatch between the n-type nitride semiconductor layer 13 and the intervening layer 15 may be formed. For example, 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.

  Thereafter, in accordance with 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.

In the photoelectric conversion element in the comparative example, the open-circuit voltage V _oc is 1.73 V, the short-circuit current J _sc is 0.80 mA / cm ² , and the curve factor F.I. F was 0.41 and the light conversion efficiency was 0.57%. On the other hand, in the photoelectric conversion element according to Example 2, the open-circuit voltage V _oc is 2.21 V, the short-circuit current J _sc is 1.97 mA / cm ² , and the fill factor F.I. F was 0.66 and the light conversion efficiency was 2.87%. Thus, in Example 2, the short circuit current was improved and the fill factor was improved as compared with the comparative example.

  It should be understood that the embodiments and examples disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

  DESCRIPTION OF SYMBOLS 1 Photoelectric conversion element, 11 board | substrate, 13 n-type nitride semiconductor layer, 15 intervening layer, 17 i-type nitride semiconductor layer, 19 2nd 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 reflected light, 103 reflected light, 104 reflected light, 105 reflected light, 106 reflected light, 107 reflected light.

Claims (11)

An n-type nitride semiconductor layer, an i-type nitride semiconductor layer provided on the n-type nitride semiconductor layer, and a p-type nitride semiconductor layer provided on the i-type nitride semiconductor layer A photoelectric conversion element formed by pin junction,
Provided between the n-type nitride semiconductor layer and the i-type nitride semiconductor layer, the type of constituent elements is the same as that of the i-type nitride semiconductor layer, and the composition ratio of constituent elements is the i-type nitride semiconductor layer. Further comprising an intervening layer different from the nitride semiconductor layer,
The In composition ratio of the intervening layer is lower than the In composition ratio of the i-type nitride semiconductor layer,
The intermediate layer may have a layered structure in which film thickness is 5nm or less of the plurality of semiconductor layers above 1nm are stacked,
The plurality of semiconductor layers have different band gap energies, and are arranged such that the band gap energy increases from the i-type nitride semiconductor layer toward the n-type nitride semiconductor layer . The i-type nitride semiconductor layer is made of Al _x In _y Ga _(1-xy) N (0 ≦ x ≦ 1, 0 <y ≦ 1, 0 <x + y ≦ 1),
The intervening _{layer, Al x In zn Ga (1} -x-zn) N layer on the front Symbol n-type nitride semiconductor _{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, the photoelectric conversion element according to claim 1 having a laminated structure n ≧ 4) are stacked in this order. The i-type nitride semiconductor layer is provided between the i-type nitride semiconductor layer and the p-type nitride semiconductor layer, and the type of constituent elements is the same as that of the i-type nitride semiconductor layer. A second intervening layer different from the nitride semiconductor layer,
The In composition ratio of the second intermediate layer, a photoelectric conversion element according to claim 1 or 2 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,
4. The photoelectric conversion element according to claim 3 , wherein the plurality of second semiconductor layers are arranged such that an In composition ratio decreases from the i-type nitride semiconductor layer toward the p-type nitride semiconductor layer. Said second intermediate layer has a thickness which is less of a single layer 5nm or more 1nm, or photoelectric conversion device according to claim 3 or 4 having a laminated structure having a thickness are laminated following layers 5nm or more 1nm . Said second intermediate layer, according to any one of _{Al x In w Ga (1-} xw) N (0 ≦ x ≦ 1,0 ≦ w ≦ 1,0 <x + w ≦ 1) consists of claims 3-5 Photoelectric conversion element. The Al composition ratio of the n-type nitride semiconductor layer is higher than the Al composition ratio of the intervening layer,
The In composition ratio of the n-type nitride semiconductor layer is lower than the In composition ratio of the intervening layer,
The absolute refractive index value of the n-type nitride semiconductor layer, the photoelectric conversion device according to any one of the smaller claim than the absolute refractive index value of the intervening layer 1-6. The Al composition ratio of the p-type nitride semiconductor layer is higher than the Al composition ratio of the second intervening layer,
The In composition ratio of the p-type nitride semiconductor layer is lower than the In composition ratio of the second intervening layer,
The absolute refractive index value of the p-type nitride semiconductor layer is smaller than the absolute refractive index value of the second intervening layer,
The p-type band-gap energy of the nitride semiconductor layer, the photoelectric conversion device according to any one of the second claims 3 to greater than the band gap energy of the intermediate layer 6. A transparent conductive film is provided on the upper surface of the p-type nitride semiconductor layer,
The transparent conductive film is
A single layer containing at least one of Zn, In, Sn, and Mg, or a stacked structure in which the single layers are stacked;
Having an absolute refractive index value less than 2.3;
The photoelectric conversion element according to any one of claims 1 to 8 , which has a film thickness of 250 nm or more and 500 nm or less. The n-type nitride semiconductor layer, the i-type nitride semiconductor layer, and the p-type nitride semiconductor layer are crystal-grown on the upper surface of the substrate in this order,
The substrate is made of 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 a photoelectric conversion element according to any one of claims 1 to 9 comprising one of ZrB _2. A light reflecting layer is provided on the lower surface of the substrate,
The photoelectric conversion element according to claim 10 , wherein the light reflection layer is a single layer made of Ag and has a thickness of 10 nm to 1000 nm.

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