JP2016031984A - Light absorption material, manufacturing method thereof, light absorption material precursor, and surface structure - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide: a light absorption material easy to control an uneven structure in geometry; a method for manufacturing such a light absorption material; a light absorption material precursor; and a surface structure.SOLUTION: A light absorption material precursor comprises: a light absorbing part 1 formed on a substrate 2; a buffer layer 3 formed on the surface of the light absorbing part; and a surface structure-formed part 5 formed on the surface of the buffer layer. The light absorbing part is formed by laminating a first absorber layer 11 including a first material, and a second absorber layer 12 including a second material. The buffer layer includes the second material, and has a thickness larger than that of the second absorber layer in the light absorbing part. The surface structure-formed part includes the first material, and is formed by alternately laminating a first dot-like surface layer 51 having dot-like protrusions 53 dotted therein, and a second surface layer 52 including the second material. Concave portions are formed in parts of the buffer layer which underlie the dot-like protrusions by etching the surface structure-formed part, whereby a light absorption material can be obtained.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a light-absorbing material, a method for producing the same, a light-absorbing material precursor, and a surface structure.

  A solar cell is a device that converts light energy into electrical energy. Among these, silicon solar cells using silicon materials have high energy conversion efficiency, and are widely put into practical use as photovoltaic power generation systems.

  Silicon, which is a main raw material for silicon-based solar cells, is a semiconductor having a forbidden band of 1.1 eV. When the silicon material is irradiated with light having a wavelength of 1130 nm or less, electrons are separated from Si atoms in the silicon substrate. Electrons move from the valence band to the conduction band beyond the forbidden band width, and Si atoms in a state where electrons are insufficient remain as holes in the valence band. When the conduction electrons move to the negative electrode terminal side and the holes move to the positive electrode terminal side, an electromotive force is generated.

Such silicon-based solar cells may have reduced energy conversion efficiency due to the following factors.
1) Incident light is reflected on the surface of the silicon material, and the light absorption rate of the silicon material is reduced.
2) The long wavelength light is transmitted through the silicon material, and the energy of the long wavelength light cannot be used.
3) If there is a defect site in the silicon material, electrons and holes are recombined by this defect, and the electron energy that can be extracted decreases.
4) Light having a short wavelength is easily converted into thermal energy and is difficult to extract as electric energy.

  Therefore, in order to improve the performance of the silicon-based solar cell, for example, as shown in Non-Patent Document 1, a concavo-convex structure is formed on the surface of the silicon substrate so that light is multiply reflected in the concavo-convex structure. The light is taken into the silicon substrate to reduce the surface reflection of the light from the silicon substrate. As a method for forming an uneven structure on the surface of a silicon substrate, there is a method using an alkaline solution (Non-patent Document 1). This method is an anisotropic etching technique using the etching selectivity of the surface orientation of the silicon substrate surface. As another method, there is a method using reactive ion etching (Non-patent Document 2). In this method, an etching gas is turned into plasma and activated to collide with the surface of the substrate. When the concavo-convex structure is formed on the surface of the silicon material in this manner, the amount of light taken in by the silicon material increases, but the absorption of light having a long wavelength cannot be increased.

  Non-Patent Document 3 and Patent Document 1 propose a Ge—Si stack formed by alternately stacking silicon layers and Ge dots on a silicon substrate. Germanium has a forbidden band width of 0.7 eV, which is smaller than that of silicon. For this reason, by using silicon and germanium as the light absorbing material, it is possible to increase the absorption of light having a long wavelength as compared with the case where only silicon is used as the light absorbing material.

  However, in the Ge-Si laminate disclosed in Non-Patent Document 3 and Patent Document 1, the absorption of long-wavelength light is increased, but the generated carriers composed of holes and electrons generated by light irradiation are extracted. It is insufficient. Further, according to Non-Patent Document 4, it has been reported that the amount of generated carriers taken out from Ge dots increases nonlinearly by increasing the intensity of light absorbed by the Ge—Si stack.

  Therefore, as shown in FIG. 6 to be described later, as disclosed in Non-Patent Document 5, it has been proposed to form the concavo-convex structure 6 on the surface of the light absorbing portion 1 made of a Ge—Si laminate. The Ge—Si laminate is formed by alternately laminating first absorption layers 11 having Ge dot-shaped protrusions 13 and second absorption layers 12 of Si. In Non-Patent Document 5, reflection of light is suppressed by the concavo-convex structure 6 to increase the intensity of light absorbed by the Ge—Si laminate, thereby increasing the amount of generated carriers taken out. And it was thought that the light absorption efficiency of a Ge-Si laminated body can be improved by devising the shape of the uneven structure 6.

JP 2013-21203 A

H. Seidel et.al., J. Electrochem. Soc. 137, 3612 (1990) Y. Inomata et al., Sol. Energy mater. Sol. Cells 48, 237 (1997) A. Alguno et al., App. Phys. Lett. 83, 1258 (2003) T.Tayagaki et al., App.Phys.Lett.101,133905 (2012) N. Usami et al., Nanotechnology, 23, 185401 (2012)

  However, since the concave portion 61 is formed on the surface of the Ge—Si laminated body immediately above the dot-shaped convex portion 13 of Ge, the shape of the concavo-convex structure 6 is influenced by the arrangement of the dot-shaped convex portion 13. For this reason, it is difficult to control the shape of the uneven structure 6 independently of the Ge—Si laminated structure.

  This invention is made | formed in view of this situation, and makes it a subject to provide the light absorption material which can control the shape of an uneven structure easily, its manufacturing method, a light absorption material precursor, and a surface structure.

  (1) The light absorbing material of the present invention includes a substrate, a light absorbing portion formed on the substrate, a buffer layer formed on the surface of the light absorbing portion, and a surface formed on the surface of the buffer layer. A light-absorbing portion comprising a first absorption layer made of a first material and a second absorption layer made of a second material formed on the surface of the first absorption layer. One or more units are laminated, the buffer layer is made of the second material and has a thickness larger than the thickness of the second absorbing layer in the light absorbing portion, and the surface structure forming portion has the first structure. Light absorption formed by laminating one or more laminating units comprising a first dot-like surface layer made of a material and dotted with a plurality of dot-like convex portions, and a second surface layer made of the second material. A preparation step of preparing a material precursor, and less of the light absorbing material precursor Also characterized in that the surface structure forming part obtained and the recess forming step of forming a recess in a portion etched located below the dot-shaped convex portions of the buffer layer, by performing.

  The light absorbing material of the present invention is a light absorbing material having a substrate, a light absorbing portion formed on the substrate, and a buffer layer formed on the surface of the light absorbing portion, The portion is formed by laminating one or more laminating units composed of a first absorbent layer made of a first material and a second absorbent layer made of a second material formed on the surface of the first absorbent layer, and the buffer layer Is made of the second material and has a thickness larger than the thickness of the second absorbing layer in the light absorbing portion, and the buffer layer has a plurality of recesses.

  The light-absorbing material precursor of the present invention includes a substrate, a light-absorbing part formed on the substrate, a buffer layer formed on the surface of the light-absorbing part, and a surface structure formed on the surface of the buffer layer A laminated unit comprising: a first absorption layer made of a first material; and a second absorption layer made of a second material formed on the surface of the first absorption layer. And the buffer layer is made of the second material and has a thickness larger than the thickness of the second absorbing layer in the light absorbing portion, and the surface structure forming portion is made of the first material. 1 or more, and a plurality of dot units, and a plurality of dot units, and a second surface layer made of the second material. To do.

  The method for producing a light-absorbing material of the present invention includes a substrate, a light-absorbing part formed on the substrate, a buffer layer formed on the surface of the light-absorbing part, and a surface formed on the surface of the buffer layer A light-absorbing portion comprising a first absorption layer made of a first material and a second absorption layer made of a second material formed on the surface of the first absorption layer. One or more units are laminated, the buffer layer is made of the second material and has a thickness larger than the thickness of the second absorbing layer in the light absorbing portion, and the surface structure forming portion has the first structure. Light absorption formed by laminating one or more laminating units comprising a first dot-like surface layer made of a material and dotted with a plurality of dot-like convex portions, and a second surface layer made of the second material. A preparation step of preparing a material precursor, and a small amount of the light-absorbing material precursor Etching the Kutomo the surface structure forming unit, characterized in that with a recess forming step of forming a recess in a portion positioned below the dot-shaped convex portions of the buffer layer.

  (2) The light absorbing material of the present invention includes a substrate, a light absorbing portion formed on the substrate, a buffer layer formed on the surface of the light absorbing portion, and a surface formed on the surface of the buffer layer. The buffer layer is made of the second material and has a thickness of 100 to 500 nm, the surface structure forming portion is made of the first material, and a plurality of dot-shaped convex portions are dotted. A preparation step of preparing a light-absorbing material precursor formed by laminating one or more laminating units comprising a first dot-shaped surface layer and a second surface layer made of the second material; A recess forming step of etching at least the surface structure forming portion of the absorbent material precursor to form a recess in a portion of the buffer layer located below the dot-shaped protrusion. And

  (3) The uneven surface structure of the present invention includes a substrate, a stacked portion formed on the substrate, and a buffer layer formed on the surface of the stacked portion, and the stacked portion is a first material. And a buffer layer is made of the second material. The buffer layer is made of the second material. The first layer is made of a second layer made of the second material and formed on the surface of the first absorption layer. In addition, the buffer layer has a thickness larger than the thickness of the second layer in the stacked portion, and a plurality of recesses are formed in the buffer layer.

  Since the present invention has the above-described configuration, it is possible to provide a light-absorbing material and a manufacturing method thereof, a light-absorbing material precursor, and a surface structure that can easily control the shape of the concavo-convex structure.

It is sectional explanatory drawing of the light absorption material precursor of this invention. It is sectional explanatory drawing of the light absorption material of this invention. It is the SEM photograph which image | photographed the surface of the light absorption material of Example 1 from the diagonal direction. It is the SEM photograph which image | photographed the surface of the light absorption material of Example 1 from right above. 2 is a cross-sectional explanatory view of the solar cell of Example 1. FIG. 6 is a cross-sectional explanatory view of a light absorbing material of Comparative Example 1. FIG. It is a diagram which shows the reflection spectrum of Example 1 and Comparative Examples 1 and 2. FIG. It is the SEM photograph which image | photographed the surface of the light absorption material of Example 2 from the diagonal direction.

  The light-absorbing material and the manufacturing method thereof, the light-absorbing material precursor, and the surface structure according to the embodiment of the present invention will be described in detail.

  (1) The light-absorbing material of the present invention is obtained by forming an uneven structure on the surface by etching a light-absorbing material precursor. The light absorbing material precursor is obtained by sequentially forming a light absorbing portion, a buffer layer, and a surface structure forming portion on a substrate. When etching the light absorbing material precursor, at least the surface structure forming portion is etched.

  The surface structure forming portion is formed by laminating one or more lamination units with a first dot-like surface layer made of the first material and a second surface layer made of the second material as a lamination unit. The first dot-shaped surface layer has dot-shaped convex portions. In the surface structure forming portion, crystal distortion is locally generated in a portion immediately above the dot-shaped convex portion in the second surface layer due to the lattice mismatch between the first material and the second material. In the second surface layer, distortion is applied to the crystal in the portion of the first surface layer immediately above the dot-shaped protrusion, and in the portion other than the portion of the second surface layer directly above the dot-shaped protrusion, the crystal distortion is small. The composition ratio of the first material in the surface structure forming portion is large in the dot stacking region where the dot-like convex portions of the first dot-shaped surface layer are stacked, and the general region other than the dot stacking region in the surface structure forming portion. Then it gets smaller. When such a surface structure forming portion is etched, the etching rate changes between the dot stacking region and the general region due to the crystal strain and the distribution of the first material composition. The dot stacking region has a higher etching rate than the general region, and the surface structure forming portion is etched in a state where the dot stacking region has a recess. When etching is further performed, the buffer layer under the surface structure forming portion is etched. Etching reaches a portion immediately below the dot stacking region in the buffer layer faster than other portions in the buffer layer. For this reason, the portion immediately below the dot stacking region is etched prior to the other portions, and a recess is formed in the portion immediately below. Thus, the recessed part formed in the buffer layer is formed under the influence of the 1st dot-like surface layer of a surface structure formation part.

  Under the buffer layer, a light absorption part is formed by alternately stacking first absorption layers made of the first material and second absorption layers made of the second material. A lattice mismatch distortion occurs between the first absorption layer and the second absorption layer. However, the thickness of the buffer layer is larger than the thickness of the second absorption layer. For this reason, the influence of distortion of lattice mismatch between the first absorption layer and the second absorption layer in the absorption portion is cut off by the buffer layer and does not reach the surface structure formation portion. For this reason, when the surface structure forming portion is etched, the buffer layer is not affected by the distortion generated in the light absorbing portion, but is affected by the local strain derived from the first dot-shaped surface portion of the surface structure forming portion. A recess is formed on the surface. In the buffer layer, by adjusting the structure of the surface structure forming portion, the shape of the recess can be controlled without being affected by the structure of the light absorbing portion.

  The structure of the light absorbing portion and the concave portion on the surface of the light absorbing material can be controlled independently. For example, the light absorbing portion can have a structure suitable for absorbing light having a wide range of wavelengths. Moreover, the surface structure of the light absorbing material can be made into a shape suitable for light confinement.

  In the recess formed in the buffer layer, multiple reflection of light occurs, and the amount of light taken in increases. By making the concave portion a shape suitable for light confinement, light can be taken in efficiently. Further, in the light absorption part, electrons are separated from atoms contained in the second material and the first material by light energy. When the forbidden band width of the first material is smaller than the forbidden band width of the second material, electrons are separated from atoms in the first absorption layer mainly made of the first material, and electrons and holes are generated. . Electrons move from the valence band to the conduction band beyond the forbidden band width, and atoms in a state where electrons are insufficient remain as holes in the valence band. When the conduction electrons move to the negative electrode terminal side and the holes move to the positive electrode terminal side, an electromotive force is generated.

  The light absorbing material of the present invention can be used as a solar cell. The light absorbing material of the present invention can also be used as a light receiving element or a light emitting diode.

  The light-absorbing material of the present invention comprises a recess forming step of etching at least the surface structure forming portion of the light-absorbing material precursor to form a recess in a portion of the buffer layer located below the dot-shaped protrusion. To obtain.

  The light absorbing material precursor includes a substrate, a light absorbing portion formed on the substrate, a buffer layer formed on the surface of the light absorbing portion, and a surface structure forming portion formed on the surface of the buffer layer. The light absorption part includes one or more laminated units each including a first absorption layer made of a first material and a second absorption layer made of a second material formed on the surface of the first absorption layer. The buffer layer is made of the second material and has a thickness larger than the thickness of the second absorption layer in the light absorbing portion, and the surface structure forming portion is made of the first material and It is formed by laminating one or more laminating units composed of a first dot-like surface layer interspersed with a plurality of dot-like convex portions and a second surface layer made of the second material.

  FIG. 1 is a cross-sectional explanatory view of a light absorbing material precursor, and FIG. 2 is a cross-sectional explanatory view of the light absorbing material. As shown in FIG. 1, the light absorbing material precursor has a substrate 2, a light absorbing portion 1, a buffer layer 3, and a surface structure forming portion 5. The substrate 2 may include a second material, and may further include a second material. The second material used for the substrate 2 may include an element having a semiconductor property, and includes, for example, one or more selected from the group of silicon (Si), gallium (Ga), and arsenic (As). The substrate 2 may be a Si substrate made of silicon or a GaAs substrate made of gallium and arsenic. The crystal structure of the substrate 2 may be single crystal or polycrystal, but is preferably single crystal. The substrate 2 may be doped. The doping of the substrate 2 may be either p-type or n-type. The substrate 2 may not be doped.

  The light absorbing portion 1 is formed by laminating one or more laminating units composed of a first absorbing layer 11 and a second absorbing layer 12. The second material constituting the second absorption layer 12 preferably includes an element having a semiconductor property. For example, one or more selected from the group consisting of silicon (Si), gallium (Ga), and arsenic (As). Is mentioned. The second material is preferably made of Si or GaAs. The element constituting the second absorption layer 12 of the light absorbing portion 1 is preferably the same as the element constituting the substrate 2.

  It is preferable that the 1st material which comprises the 1st absorption layer 11 consists of a self-organization material. The self-organizing material refers to a material having a property of rising from a wetting layer of a first material by using surface energy as a trigger to spontaneously form dot-shaped convex portions. Examples of the self-organizing material include Ge, InAs, InSb, AlAs, and GaSb. When the second material is made of Si, the first material is preferably Ge. When the second material is GaAs, the first material is preferably InAs, InSb, AlAs, or GaSb.

  The forbidden band width of the first material is preferably smaller than the forbidden band width of the second material. The absorption wavelength region of light in the first absorption layer extends to a longer wavelength side than the absorption wavelength region of light in the second absorption layer, and the light absorption efficiency is increased. The first absorption layer made of such a first material serves as a place where carriers consisting exclusively of electrons and holes are generated.

  The light absorbing portion 1 is formed by alternately laminating two or more first absorbing layers 11 and second absorbing layers 12, and each first absorbing layer 11 is dotted with a plurality of dot-like convex portions 13. Therefore, it is preferable that the planar positions of the dot-shaped convex portions 13 in the two or more first absorption layers 11 are the same. In this case, the dot-shaped convex part 13 of the 1st absorption layer 11 continues in a lamination direction. This structure is formed by epitaxial growth of the first absorption layer 11. In order to increase the light absorption efficiency, the dot-like convex portions 13 are preferably arranged regularly.

  In order to form the light absorption part 1, it is preferable that the first absorption layer 11 and the second absorption layer 12 are sequentially epitaxially grown on the surface of the substrate 2 by molecular beam epitaxy. In order to form the light absorption part 1, the substrate 2 is put in a vacuum chamber and heated to a temperature of 500 to 900 ° C., and a hydride gas of the first material is supplied to form the first absorption layer 11. Thereafter, the second absorption layer 12 is formed by supplying a hydride gas of the second material to the vacuum chamber. After repeating this process several times to several tens of times, the last first absorption layer 11 is formed. The thickness of each stacked unit composed of the first absorption layer 11 and the second absorption layer 12 is preferably 5 nm to 100 nm.

  Since the second material and the first material have the same crystal structure and different lattice constants, distortion occurs in the second absorption layer 12 due to epitaxial growth. Further, when the supply amount of the first material to the surface of the second absorption layer 12 exceeds the critical value, the first absorption layer 11 is raised in a convex shape to form dots. The total free energy can be reduced by forming the dot-like convex portions 13 by elastic relaxation of strain. For this reason, the dot-shaped convex part 13 which consists of a 1st material is naturally formed on the wetting layer 14 of the 1st material of thickness of several atomic layers. Periodic distortion of micrometer or less reflecting the periodic structure caused by the dots of the first absorption layer 11 is generated in the second absorption layer 12, and the dot-like convex portions 13 of the upper first absorption layer 11 are in the lower first. The first absorbent layer 11 is formed immediately above the dot-like convex portion 13.

  When the 1st absorption layer 11 has the dot-shaped convex part 13, it is good that the height of the dot-shaped convex part 13 is 5-20 nm. When the height of the dot-shaped convex part 13 is too small, local distortion may not be applied in the second absorbent layer 12 immediately above the first absorbent layer 11. When the height of the dot-shaped convex part 13 is too high, the crystal distortion locally accumulated in the second absorption layer 12 immediately above the first absorption layer 11 is relaxed, and there is a possibility that a crystal defect is generated.

  When the 1st absorption layer 11 has the dot-shaped convex part 13, it is preferable that the pitch of the dot-shaped convex part 13 is 100-300 nm. When the pitch of the dot-shaped convex part 13 is too small, a crystal defect may generate | occur | produce in a light absorption part and there exists a possibility of degrading the characteristic of a solar cell. When the pitch of the dot-shaped convex part 13 is excessive, there exists a possibility that the light absorption in the dot-shaped convex part 13 may reduce.

  The thickness of the second absorption layer 12 is preferably 10 to 100 nm, and more preferably 10 to 20 nm. The thickness of the 2nd absorption layer 12 says the distance from the lowest part of the 2nd absorption layer 12 to the uppermost part. When the 1st absorption layer 11 has the dot-shaped convex part 13, the distance from the part which contact | connects the wetting layer 14 in the 2nd absorption layer 12 to the uppermost part of the 2nd absorption layer 12 is said. When the thickness of the second absorbent layer 12 is in the above range, the crystal quality is prevented from being lowered, and the effect of the distortion generated in the second absorbent layer 12 is inherited, so that the dot-like protrusions 13 of the first absorbent layer 11 are formed. They can be formed at substantially the same position in the layer thickness direction. On the other hand, if the thickness of the second absorption layer 12 is too thin, unevenness due to the dot shape of the first absorption layer 11 remains in the stacked unit composed of the first absorption layer 11 and the second absorption layer 12, and the light absorption portion There is a possibility that the entire 1 cannot be formed flat. When the thickness of the 2nd absorption layer 12 is too large, there exists a possibility that the position of the dot-shaped convex part 13 formed in the 1st absorption layer 11 may become irregular.

  In the light absorption unit 1, when the first absorption layer 11 and the second absorption layer 12 are the lamination units, the number of repetitions of the lamination units is preferably 10 to 50. When the number of stacked units is too small, the number of generated carriers per unit area of the light-absorbing material is small, and the amount of light energy generated may be reduced. When the number of stacked units is excessive, the crystal quality of the light absorption unit 1 may be deteriorated due to distortion due to mismatch of the lattice constant between the second material and the first material.

  A buffer layer 3 is formed on the surface of the light absorbing portion 1. The thickness of the buffer layer 3 is larger than the thickness of the second absorption layer 12 of the light absorption unit 1. Before performing the recess forming step, the thickness of the buffer layer 3 needs to be larger than the thickness of the second absorption layer 12 of the light absorption unit 1. Even after the recess forming step, the thickness of the buffer layer 3 is preferably larger than the thickness of the second absorption layer 12 of the light absorption unit 1. When a plurality of second absorption layers 12 are formed in the light absorption portion 1 and the thicknesses of the second absorption layers 12 are different, among all the second absorption layers 12 formed in the light absorption portion 1 The thickness of the buffer layer 3 is preferably larger than the thickness of the second absorption layer 12 having the largest thickness. Further, the thickness of the buffer layer 3 is preferably large enough that the influence of the strain caused by the first absorption layer in the light absorbing portion 1 does not reach the surface structure forming portion 5.

  Before performing the recess forming step, the thickness of the buffer layer 3 is preferably 100 nm or more. Since the thickness of the buffer layer 3 is larger than the thickness of the second absorption layer 12, the shape of the concavo-convex structure 6 of the buffer layer 3 can be absorbed without being affected by local crystal distortion of the light absorption portion 1. It can be controlled independently of the structure of the part 11. When the thickness of the buffer layer 3 is too small, the influence of the distortion caused by the first absorption layer 11 in the light absorbing portion 1 affects the surface structure forming portion 5, and when etching the surface structure forming portion 5, When the first absorption layer 11 has the dot-shaped convex portion 13, a concave portion is formed in a portion located immediately above the dot-shaped convex portion 13, and the shape of the concave portion formed in the buffer layer 3 is changed to the surface structure forming portion. There is a possibility that it cannot be controlled by the structure 5. The upper limit value of the thickness of the buffer layer 3 is not particularly limited, but may be, for example, 500 nm. When the thickness of the buffer layer 3 is excessively large, the thickness of the entire light absorbing material is too large, and there is a concern that downsizing of the device equipped with the light absorbing material may be hindered.

  The buffer layer 3 is made of the second material, and is the same as the second material constituting the second absorption layer 12 of the light absorption unit 1.

  The surface structure forming portion 5 is formed by laminating a plurality of lamination units composed of the first dot-like surface layer 51 and the second surface layer 52. The first dot-shaped surface layer 51 has a plurality of dot-shaped convex portions 53. By changing the shapes of the first dot-like surface layer 51 and the second surface layer 52, the shape of the recesses formed in the buffer layer 3 can be controlled.

  For example, the height, width, pitch, and thickness of the second surface layer 52 of the dot-like convex portions 53 of the first dot-like surface layer 51 may be changed. When the depth of the concave portion is increased, the thickness of the second surface layer 52 is decreased, or the height of the dot-shaped convex portion 53 is increased. When increasing the pitch of the concave portions, the pitch of the dot-shaped convex portions 53 of the first dot-shaped surface layer 51 is increased. When the opening width of the concave portion is increased, the size of the dot-like convex portion 53 of the first dot-like surface layer 51 in the planar direction is increased.

  The surface structure forming portion is formed by alternately laminating two or more layers of the first dot-like surface layer and the second surface layer, and each dot in the first dot-like surface layer of two or more layers. It is preferable that the positions of the convex portions in the plane direction are the same.

  Since the positions in the planar direction of the dot-shaped convex portions 53 in the plurality of first dot-shaped surface layers 51 are the same as each other, the dot-shaped convex portions 53 are continuous in the stacking direction of the stack unit. Etching proceeds faster in the dot arrangement portion 55 where the dot-like convex portions 53 in the surface structure forming portion 5 are continuous than in other portions, and a concave portion is formed in a portion of the buffer layer 3 located immediately below the dot arrangement portion 55. The shape of the recess is easy to control.

  The pitch of the dot-like convex portions 53 is preferably 100 nm or more and 1000 nm or less, and more preferably 150 nm or more and 1000 nm or less. When the pitch of the dot-shaped convex part 53 is too small or when the pitch of the dot-shaped convex part 53 is excessive, surface reflection in an appropriate wavelength region in the visible light region and the near infrared region of the light absorbing material is performed. There is a possibility that it cannot be reduced.

  The width of the dot-shaped convex portion 53 is preferably 100 nm or more and 1000 nm or less, and more preferably 150 nm or more and 1000 nm or less. When the width of the dot-like convex portion 53 is too small, the concave portion 61 formed in the buffer layer 3 is too small, and the light surface reflection reduction effect of the concave portion 61 may be reduced. When the width of the dot-shaped convex portion 53 is excessive, the opening width of the concave portion 61 formed in the buffer layer 3 is increased, and the light confinement effect may be reduced.

  The height of the dot-shaped convex part 53 is preferably 5 nm or more and 30 nm or less, and more preferably 10 nm or more and 20 nm or less. When the height of the dot-shaped convex portion 53 is too small, it is difficult to make a difference in etching rate, the depth of the concave portion 61 formed in the buffer layer 3 becomes shallow, and the light confinement effect may be reduced. When the height of the dot-shaped convex part 53 is too large, the unevenness | corrugation resulting from the shape of the dot-shaped convex part 53 remains, and there exists a possibility that the whole surface structure formation part 5 cannot be formed flat.

  The thickness of the second surface layer is preferably 5 to 100 nm or more, and more preferably 10 to 20 nm. The thickness of the second surface layer refers to the distance between the lowermost part and the uppermost part of the second surface layer. Since the first dot-shaped surface layer 51 has the dot-shaped projections 53, the lowermost portion of the second surface layer 52 is a portion in contact with the thin wet layer 54 of the first dot-shaped surface layer 51.

  It is preferable that the number of repetitions of the lamination unit composed of the first dot-shaped surface layer 51 and the second surface layer 52 is 5 to 50. If the number of repetitions of the lamination unit is too small, the recess 61 may not be formed in the buffer layer 3. If the number of repetitions of the stacking unit is too large, unevenness due to the shape of the dot-shaped convex portion 53 remains, and the entire surface structure forming portion 5 may not be formed flat.

  The thickness of the second surface layer 52 is preferably 5 nm or more and 100 nm or less. When the thickness of the second surface layer 52 is too small, crystal defects generated in the surface structure forming portion 5 may propagate to the light absorbing portion 1 and the crystal quality of the light absorbing portion 1 may be deteriorated. If the thickness of the second surface layer 52 is excessive, the dot-like convex portions 53 of the first dot-like surface layer 51 may not be easily epitaxially grown. For this reason, the position of the dot-shaped convex part 53 in the planar direction becomes random, and it may be difficult to control the shape of the concave part 61 formed in the buffer layer 3.

  Before performing the recess forming step, the thickness of the buffer layer 3 is preferably larger than the thickness of the second surface layer 52. A deep recess 61 having a high light confinement effect can be formed in the buffer layer 3. The thickness of the buffer layer 3 refers to the thickness from the top to the bottom of the buffer layer 3.

  In the light absorbing material precursor, the light absorbing portion 1, the buffer layer 3, and the surface structure forming portion 5 may be formed by, for example, a molecular beam epitaxy method, a sputter epitaxy method, or a chemical vapor deposition method. In the light absorbing material precursor, the light absorbing portion 1, the buffer layer 3, and the surface structure forming portion 5 are preferably single crystals. The dot-like convex portion 13 can be formed on the first absorption layer 11, or the dot-like convex portion 53 can be formed on the first dot-like surface layer 51.

At least the surface structure forming portion 5 of the light absorbing material precursor is etched. Etching is preferably performed by bringing an etching solution into contact with the surface of the surface structure forming portion 5. The etchant may contain nitric acid. The etchant preferably contains hydrofluoric acid. The etching solution may be a mixed solution containing hydrogen fluoride (HF) and nitric acid (HNO ₃ ). At this time, due to the first dot-like surface layer 51 and the distortion of the period of micrometer or less distributed in the second surface layer 52, the etching rate becomes periodic at micrometer or less. For this reason, in the area | region immediately above the dot-shaped convex part 53 of the 1st dot-shaped surface layer 51 in the 2nd surface layer 52, the local distortion has arisen. Due to the periodic arrangement of the dot-shaped convex portions 53, the second surface layer 52 is also periodically distorted. The etching rate is high at the strained portion. In the buffer layer 3, the portion immediately below the periodically strained portion of the second surface layer 52 is etched before the other portions to form recesses. For this reason, a periodic uneven structure is formed in the buffer layer 3.

  When the etching solution is brought into contact with the surface of the surface structure forming portion 5 and the surface structure forming portion 5 is chemically etched, the surface structure forming portion 5 is etched isotropically in the vertical and horizontal directions. If the concentration of the etching solution is reduced or the etching time is shortened, it is considered that the uneven structure formed in the buffer layer is also reduced.

  The light absorbing material formed by etching the light absorbing material precursor in this way includes a substrate, a light absorbing portion formed on the surface of the substrate, and a buffer layer formed on the surface of the light absorbing portion. The light absorption part includes a first absorption layer made of a first material and a stack unit formed of a second absorption layer formed on the surface of the first absorption layer and made of a second material. The buffer layer is formed of the second material and has a thickness larger than the thickness of the second absorption layer in the light absorption portion, and the buffer layer has a plurality of recesses. .

  As shown in FIG. 2, the light absorbing material includes a substrate 2, a light absorbing portion 1 formed by stacking a first absorbing portion 11 and a second absorbing portion 12, and a buffer layer 3 having a concavo-convex structure 6. Have The thickness of the buffer layer 3 is preferably larger than the thickness of the second absorption layer 12 in the light absorption unit 1. Here, the thickness of the buffer layer 3 refers to the thickness between the lowermost part and the uppermost part of the buffer layer 3. When the thickness of the buffer layer 3 is defined in more detail, the thickness between the uppermost part of the convex part 62 and the lowermost part in contact with the light absorbing part 1 in the concavo-convex structure 6 formed on the surface of the buffer layer 3 is defined. Say. The thickness of the buffer layer 3 after etching may be large or small with respect to the thickness of the second absorption layer 12 of the light absorption unit 1, but is preferably as large as possible. This is because when the thickness of the buffer layer 3 is larger than the thickness of the second absorption layer 12, it is easy to form a deep recess 61 having a high light confinement effect in the buffer layer 3.

  The position of the concave portion 61 of the buffer layer 3 in the planar direction may be different from the position of the dot-shaped convex portion 13 in the first absorption layer 11 in the planar direction. The shape of the concave portion 61 of the buffer layer 3 is a shape suitable for light confinement, and the shape of the dot-shaped convex portion 13 of the first absorption layer 11 of the light absorption portion 1 is a structure suitable for light absorption of long wavelength light. be able to.

  The depth of the recess 61 of the buffer layer 3 is preferably 10 nm or more and 300 nm or less, and more preferably 50 nm or more and 200 nm or less. When the depth of the recess 61 is too small, light is reflected on the surface of the buffer layer 3, which may reduce the light absorption efficiency. If the depth of the recess 61 is excessive, the light confinement effect may be suppressed.

  The opening width of the recess 61 is preferably 100 nm or more and 1000 nm or less, and more preferably 150 nm or more and 1000 nm or less. When the opening width of the recess 61 is too small or too large, visible light and near-infrared light are reflected on the surface of the buffer layer 3, which may reduce the light absorption efficiency.

  It is preferable that the first absorption layer is not exposed on the surface of the buffer layer 3 including the recess 61. If the first absorption layer 11 is exposed at the surface including the recess 61 of the buffer layer 3, electron / hole recombination is likely to occur in the exposed first absorption layer 11, and carrier hole extraction efficiency is increased. May decrease.

  The light-absorbing material of the present invention can be used not only for solar cells but also for light-receiving elements. In addition, depending on the shape of the surface concavo-convex structure of the buffer layer, there is a possibility that the efficiency of taking out the light generated in the first absorption layer from the inside of the light absorbing material to the outside may be improved. Application to light emitting diodes is also conceivable.

  (2) The light absorbing material of the present invention is formed on a substrate, a light absorbing portion formed on the substrate, a buffer layer formed on the surface of the light absorbing portion, and a surface of the buffer layer. The buffer layer is made of a second material and has a thickness of 100 to 500 nm, and the surface structure formation portion is made of a first material and has a plurality of dot-shaped convex portions. A preparatory step of preparing a light-absorbing material precursor formed by laminating one or more laminating units composed of interspersed first dot-like surface layers and the second surface layer made of the second material; A recess forming step in which at least the surface structure forming portion of the light absorbing material precursor is etched to form a recess in a portion of the buffer layer located below the dot-like projection. It is characterized by being .

  In this invention, unlike the invention of (1) above, the structure of the light absorbing portion is not specified. The concave portion of the buffer layer has a shape reflecting the shape of the dot-shaped convex portion of the surface structure forming portion. For this reason, the structure of the light absorption part can be designed independently of the structure of the surface structure forming part. In this invention, the light absorption part may not be a laminate of the first absorption layer and the second absorption layer as in the above invention.

  The light-absorbing material (2) can be used as a light-receiving material or a light-emitting diode as well as a solar cell, similarly to the light-absorbing material (1).

  (3) Further, the surface uneven structure according to the present invention includes a substrate, a stacked portion formed on the substrate, and a buffer layer formed on the surface of the stacked portion. The buffer layer is formed by stacking one or more stack units each including a first layer made of one material and a second layer made of a second material formed on the surface of the first absorption layer. And having a thickness larger than the thickness of the second layer in the stacked portion, and the buffer layer is formed with a plurality of recesses.

  In the invention of (3), unlike the inventions of (1) and (2), the laminated part formed on the substrate surface may not be a light absorbing part. In the present invention, the substrate may have a second material made of Si or GaAs.

  The recess provided in the buffer layer of the surface concavo-convex structure (3) can be formed by the same method as in (1) above. In the buffer layer, by adjusting the structure of the surface structure forming portion of the light absorbing material precursor (1) described above, the shape of the recess can be controlled without being affected by the structure of the laminated portion. The structure of the laminated portion and the recess of the buffer layer can be controlled independently.

  The surface concavo-convex structure can be used not only as a light absorbing material but also as a semiconductor electronic material, for example.

Example 1
<Board cleaning>
First, a p-type single crystal Si (100) substrate was prepared. The substrate was washed with running water to remove fine particles adhering to the substrate surface. Etching was performed for 5 minutes with a mixed solution of sulfuric acid: hydrogen peroxide = 2: 1 (volume ratio) to remove metal impurities and form a chemical oxide film on the substrate surface. The substrate was etched with a 2.5% by volume hydrofluoric acid solution for 30 seconds to form a hydrogen terminated surface on the substrate. This surface cleaning process was repeated twice.

(Formation of light absorption part)
As shown in FIG. 1, by using disilane (Si ₂ H ₆ ) as a Si source gas and germane (GeH ₄ ) as a Ge source gas, a light absorption portion is formed on the surface of the substrate 2 by gas source molecular beam epitaxy. 1 was formed.

Specifically, the Si (100) substrate was carried into the chamber of the molecular beam epitaxy apparatus. In the chamber, a heat treatment was performed at 900 ° C. for 10 minutes and the surface was cleaned to form a clean surface. Thereafter, the substrate temperature was lowered to 700 ° C., which is the crystal growth temperature, and then a source gas was supplied. The flow rate of the source gas was 2.5 sccm for GeH ₄ and 4.0 sccm for Si ₂ H ₆ . By alternately supplying each source gas, light absorption in which a plurality of stack units each including a first absorption layer 11 made of Ge (germanium) single crystal and a second absorption layer 12 made of Si (silicon) single crystal are stacked. Part 1 was formed. In order to suppress the mixing of residual gas in the chamber, an interruption time of 10 seconds was provided when the supply of GeH ₄ and Si ₂ H ₆ was switched.

  The 1st absorption layer 11 protruded periodically in the shape of a dot by self-organization of Stranski-Krastanov (SK) growth mode, and formed the dot-like convex part 13. The SK growth mode is a growth mode in which layer growth is performed at the initial stage of crystal growth, but three-dimensional dot growth is performed when the growth film thickness exceeds a certain critical value. The thickness of the 1st absorption layer 11 was made into the Ge coverage of an 8 atomic layer, and the thickness of the 2nd absorption layer 12 was 20 nm. The Ge coverage of the 8-atomic layer means that an amount of source gas is supplied on the underlying Si surface to a thickness corresponding to the 8-atomic layer, and corresponds to 1.12 nm. Actually, since the first absorption layer 11 is crystal-grown in a dot shape, the thickness varies depending on the location. The thickness of the wetting layer 14 of the first absorption layer 11 is several atomic layers (1 to 2 nm), and the thickness of the dot-shaped convex portion 13 is about 10 nm. The first absorption layer 11 and the second absorption layer having the dot-shaped protrusions 13 were used as a stack unit, and this stack unit was grown for 10 periods. Thereby, the light absorption part 1 which has a Ge dot laminated structure was formed.

(Formation of buffer layer)
In the same chamber, Si ₂ H ₆ gas was continuously supplied to the surface of the light absorption unit 1 at a flow rate of 4.0 sccm to form a buffer layer 3 having a thickness of 100 nm.

(Formation of surface structure forming part)
Further, GeH ₄ and Si ₂ H ₆ are alternately supplied onto the buffer layer 3, and the first dot-like surface layer 51 made of Ge single crystal and the second surface layer 52 made of Si single crystal are used as a lamination unit. The surface structure forming part 5 was formed by laminating these 10 times. The gas flow rate was 2.5 sccm for GeH ₄ and 4.0 sccm for Si ₂ H ₆ . In order to suppress the mixing of residual gas in the chamber, an interruption time of 10 seconds was provided when the supply of GeH ₄ and Si ₂ H ₆ was switched. The first dot-like surface layer 51 was periodically raised in a dot shape by the self-organization of the SK growth mode, and the dot-like convex portion 53 was formed. The thickness of the first dot-like surface layer 51 was an Ge atomic coverage of 8 atomic layers, and the thickness of the second surface layer 52 was 20 nm. The thickness of the dot-shaped convex part 53 of the 1st dot-shaped surface layer 51 was about 10 nm, and the thickness of the wetting layer 54 was several atomic layers (1-2 nm).

(etching)
The prepared light-absorbing material precursor was wet-etched using a mixed solution of HF and HNO ₃ (HF; HNO ₃ = 1; 100) (volume ratio) to form an uneven structure in the buffer layer. By controlling the wet etching time, only the surface structure forming portion was etched to form a light absorbing material having a light absorbing portion and an uneven structure of the buffer layer. In this embodiment, the etching time is 1 minute.

  SEM images of the obtained light-absorbing material are shown in FIGS. FIG. 3 is a surface SEM image of the light absorbing material taken from an oblique direction, and FIG. 4 is a surface SEM (scanning electron microscope) image of the light absorbing material taken from directly above. As shown in both figures, a regular uneven structure was formed on the surface of the light absorbing material. As shown in FIG. 2, the concavo-convex structure 6 was formed in the buffer layer 3. The concave portion 61 of the concavo-convex structure 6 was formed in a portion immediately below the dot-shaped convex portion 53 of the surface structure forming portion 5 in the buffer layer 3. The depth of the recess 61 was 40 nm, the opening width of the recess 61 was 160 nm, and the pitch of the recess 61 was 180 nm. The uneven structure 6 was covered with the second material constituting the buffer layer 3, and the first absorption layer 11 of the light absorbing portion 1 was not exposed on the surface of the uneven structure 6.

(Solar cell production)
As shown in FIG. 5, an n-type Si single crystal layer 71 was formed on the surface of the concavo-convex structure 6 by thermal diffusion of phosphorous glass. A single-layer antireflection film 7 (thickness 75 to 85 nm) using ITO (Indium Tin Oxide) was formed on the surface of the n-type Si single crystal layer 71 by sputtering. Further, an Al paste electrode is baked on the back surface of the p-type semiconductor substrate 2 to form a back electrode 81, and an Ag finger electrode is printed and baked on the surface of the antireflection film 7 to form a surface electrode 82. In this way, a solar cell can be manufactured. In addition, the surface of the buffer layer 3 becomes a light-receiving surface of the solar cell.

In this example, conductive ITO is used as the antireflection film 7, but the present invention is not limited to this, and non-conductive TiO ₂ (titanium oxide), Si ₃ N ₄ (silicon nitride) ) May be used.

(Comparative Example 1)
In Comparative Example 1, as shown in FIG. 6, the concavo-convex structure 6 is formed in the light absorbing portion 1 without forming the buffer layer and the surface structure forming portion.

  A light absorbing portion 1 was formed on the surface of a p-type single crystal Si (100) substrate in the same manner as in Example 1. The number of repetitions of the laminate unit composed of the first absorption layer 11 and the second absorption layer 12 of the light absorption layer 1 was 10.

The light absorbing portion 1 was wet etched to form a concavo-convex structure 6 on the surface of the light absorbing portion 1. In the wet etching, the same etching solution as in Example 1 was brought into contact with the light absorbing portion 1 for 30 seconds. The concave portion 61 was formed immediately above the dot-shaped convex portion 13 of the first absorption layer 11. The depth of the recess 61 was 40 nm, the opening width of the recess 61 was 160 nm, and the pitch of the recess 61 was 180 nm.

(Comparative Example 2)
In Comparative Example 2, the buffer layer and the surface structure forming part are not formed. Moreover, the uneven structure is not formed also in the light absorption part, and the surface of the light absorption part is flat. The light absorption part was formed in the same manner as in Comparative Example 1.

(Example 2)
In Example 2, a light-absorbing material was produced in the same manner as in Example 1 except that the thickness of the second surface layer of the surface structure forming portion was 5 nm. In FIG. 8, the SEM image which image | photographed the surface of the light absorption material from the diagonal direction was shown. As shown in FIG. 8, a concavo-convex structure having a concavo-convex difference larger than that of Example 1 was formed on the surface of the light absorbing material. The depth of the recesses of the concavo-convex structure was 150 nm, the opening width of the recesses 61 was 150 nm, and the pitch of the recesses 61 was 160 nm. From this, it was found that when the thickness of the second surface layer 52 of the surface structure forming portion 5 is reduced, the depth of the concave portion is increased.

<Measurement of reflection spectrum>
The reflection spectra of the light absorbing materials of Example 1 and Comparative Examples 1 and 2 were measured. In order to measure the reflection spectrum of the light absorbing material, the light absorbing material was irradiated with light having a wavelength of 400 nm to 1600 nm, the reflected light was collected, and the ratio of the intensity of the irradiated light to the intensity of the reflected light was shown as a percentage. The apparatus used for the measurement of the reflection spectrum is an ultraviolet-visible near-infrared spectrophotometer (trade name V-570, manufactured by JASCO Corporation). The reflection spectra of various light absorbing materials are shown in FIG.

  As shown in FIG. 7, it was found that Example 1 in which a concavo-convex structure was formed on the surface of the light-absorbing material had a lower reflectance than Comparative Examples 1 and 2 and was able to capture light efficiently. In Example 1, the concavo-convex structure suitable for light confinement could be formed because the concavo-convex structure was controlled independently of the structure of the light absorbing portion.

  In addition, when the thickness of the first dot-shaped surface layer of the surface structure forming portion was increased, the depth of the concave portion of the concavo-convex structure formed after etching was reduced. Moreover, when the pitch of the dot-shaped convex part of the 1st dot-shaped surface layer of a surface structure formation part was enlarged, the pitch of the recessed part formed after an etching became large. Increasing the width of the dot-shaped convex portion of the first dot-shaped surface layer of the surface structure forming portion increased the opening width of the concave portion formed after etching.

1: light absorbing portion, 11: first absorbing layer, 12: second absorbing layer, 13: dot-like convex portion, 14, wetting layer, 2: substrate, 3: buffer layer, 5: surface structure forming portion, 51: First dot-like surface layer, 52: second surface layer, 6: surface structure, 61: concave portion, 62: convex portion

Claims (27)

A substrate, a light absorbing portion formed on the substrate, a buffer layer formed on the surface of the light absorbing portion, and a surface structure forming portion formed on the surface of the buffer layer,
The light absorbing portion is formed by laminating one or more laminating units composed of a first absorbing layer made of a first material and a second absorbing layer made of a second material formed on the surface of the first absorbing layer,
The buffer layer is made of the second material and has a thickness larger than the thickness of the second absorbing layer in the light absorbing portion,
The surface structure forming portion is a laminated unit comprising the first dot-like surface layer made of the first material and interspersed with a plurality of dot-like convex portions, and the second surface layer made of the second material. A preparatory step of preparing a light-absorbing material precursor,
A recess forming step of etching at least the surface structure forming portion of the light absorbing material precursor to form a recess in a portion of the buffer layer located below the dot-like projection. Light-absorbing material characterized by   The surface structure forming portion is formed by alternately laminating two or more layers of the first dot-like surface layer and the second surface layer, and each dot in the first dot-like surface layer of two or more layers. The light-absorbing material according to claim 1, wherein the positions of the convex portions in the planar direction are the same.   The light absorbing material according to claim 1 or 2, wherein a pitch of the dot-shaped convex portions is 150 nm or more and 1000 nm or less.   The light-absorbing material according to any one of claims 1 to 3, wherein a width of the dot-shaped convex portion is 150 nm or more and 1000 nm or less.   The light-absorbing material according to any one of claims 1 to 4, wherein the height of the dot-shaped convex portions is 10 nm or more and 20 nm or less.   The light absorbing material according to any one of claims 1 to 5, wherein the thickness of the second surface layer is 5 nm or more and 100 nm or less.   The light-absorbing material according to claim 1, wherein the buffer layer has a thickness of 100 nm or more before performing the recess forming step.   The light-absorbing material according to any one of claims 1 to 7, wherein a thickness of the buffer layer is larger than a thickness of the second surface layer before performing the recess forming step. A light absorbing material having a substrate, a light absorbing portion formed on the substrate, and a buffer layer formed on a surface of the light absorbing portion,
The light absorbing portion is formed by laminating one or more laminating units composed of a first absorbing layer made of a first material and a second absorbing layer made of a second material formed on the surface of the first absorbing layer,
The buffer layer is made of the second material and has a thickness larger than the thickness of the second absorbing layer in the light absorbing portion,
A light-absorbing material, wherein the buffer layer has a plurality of recesses.   The depth of the said recessed part is 10 nm or more and 300 nm or less, The light absorption material of any one of Claims 1-9.   The light absorption material according to any one of claims 1 to 10, wherein an opening width of the recess is 100 nm or more and 1000 nm or less.   The light absorbing material according to any one of claims 1 to 11, wherein the first layer is not exposed on a surface including the concave portion of the buffer layer.   The light-absorbing material according to claim 1, wherein the forbidden band width of the first material is smaller than the forbidden band width of the second material.   The light absorbing material according to claim 1, wherein the second material is made of Si or GaAs, and the first material is made of GeInAs, InSb, AlAs, or GaSb.   The light absorbing material according to claim 14, wherein the second material is made of Si, and the first material is made of Ge.   The light absorption portion is formed by alternately laminating two or more layers of the first absorption layer and the second absorption layer, and each of the first absorption layers is dotted with a plurality of dot-shaped convex portions. The light-absorbing material according to claim 1, wherein the planar positions of the dot-shaped convex portions in the first absorption layer of two or more layers are the same.   The light absorbing material according to claim 16, wherein the position of the buffer layer in the planar direction of the concave portion is different from the position of the dot-shaped convex portion in the planar direction of the first absorbing layer.   The light absorption material according to claim 1, wherein the substrate has the second material.   A solar cell provided with the light absorption material of any one of Claims 1-18. A substrate, a light absorbing portion formed on the substrate, a buffer layer formed on the surface of the light absorbing portion, and a surface structure forming portion formed on the surface of the buffer layer,
The light absorbing portion is formed by laminating one or more laminating units composed of a first absorbing layer made of a first material and a second absorbing layer made of a second material formed on the surface of the first absorbing layer,
The buffer layer is made of the second material and has a thickness larger than the thickness of the second absorbing layer in the light absorbing portion,
The surface structure forming portion is a laminated unit comprising the first dot-like surface layer made of the first material and interspersed with a plurality of dot-like convex portions, and the second surface layer made of the second material. A light-absorbing material precursor characterized by laminating one or more of the above. A substrate, a light absorbing portion formed on the substrate, a buffer layer formed on the surface of the light absorbing portion, and a surface structure forming portion formed on the surface of the buffer layer,
The light absorbing portion is formed by laminating one or more laminating units composed of a first absorbing layer made of a first material and a second absorbing layer made of a second material formed on the surface of the first absorbing layer,
The buffer layer is made of the second material and has a thickness larger than the thickness of the second absorbing layer in the light absorbing portion,
The surface structure forming portion is a laminated unit comprising the first dot-like surface layer made of the first material and interspersed with a plurality of dot-like convex portions, and the second surface layer made of the second material. A preparatory step of preparing a light-absorbing material precursor,
A recess forming step of etching at least the surface structure forming portion of the light absorbing material precursor to form a recess in a portion of the buffer layer located below the dot-shaped protrusion. Absorbent material manufacturing method.   The method for producing a light-absorbing material according to claim 21, wherein the light-absorbing part and the surface structure forming part are formed by a molecular beam epitaxy method.   23. The method for producing a light-absorbing material according to claim 21, wherein the etching is performed by bringing an etching solution into contact with the surface of the surface structure forming layer.   The method for producing a light-absorbing material according to claim 23, wherein the etching solution contains nitric acid.   The method for producing a light-absorbing material according to claim 23 or 24, wherein the etching solution contains hydrofluoric acid. A substrate, a light absorbing portion formed on the substrate, a buffer layer formed on the surface of the light absorbing portion, and a surface structure forming portion formed on the surface of the buffer layer,
The buffer layer is made of a second material and has a thickness of 100 to 500 nm,
The surface structure forming section includes a stack unit composed of a first dot-like surface layer made of a first material and dotted with a plurality of dot-like convex portions, and a second surface layer made of the second material. A preparatory step of preparing a light-absorbing material precursor formed by laminating one or more;
A recess forming step of etching at least the surface structure forming portion of the light absorbing material precursor to form a recess in a portion of the buffer layer located below the dot-like projection. Light-absorbing material characterized by A substrate, a laminate formed on the substrate, and a buffer layer formed on the surface of the laminate,
The stacked portion is formed by stacking one or more stack units each including a first layer made of a first material and a second layer made of a second material formed on the surface of the first absorption layer,
The buffer layer is made of the second material and has a thickness larger than the thickness of the second layer in the stacked portion,
A plurality of concave and convex portions are formed in the buffer layer.