Photo detector

Abstract

To provide a photodetector capable of improving quantum efficiency.SOLUTION: A photodetector 1 includes: an avalanche photodiode 10 having a light incident surface 10a into which light enters; and a plasmon structure part 20 formed on the light incident surface 10a and diffracting light by surface plasmon resonance. The avalanche photodiode 10 has: a p-type first semiconductor region 15; and an n-type second semiconductor region 16 formed on the opposite side of the light incident surface 10a to the first semiconductor region 15 and forming the pn junction with the first semiconductor region 15. The plasmon structure part 20 has a plurality of unit structures 21 arranged on the first semiconductor region 15. Each of the plurality of unit structures 21 includes: a top surface 21a opposite the light incident surface 10a; a bottom surface 21b facing the light incident surface 10a; and a side surface 21c connected to the top surface 21a and bottom surface 21b. A height of each of the plurality of unit structures 21 is 100 nm or more and 250 nm or less.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a light detection element. [Background technology]

[0002] Patent Document 1 describes a solid-state imaging element that utilizes avalanche multiplication. Patent Document 1 describes that surface plasmon resonance is generated by arranging metal nanoparticles on the surface of a silicon substrate that constitutes the solid-state imaging element (FIG. 23). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2017 / 038542 Summary of the Invention [Problem to be solved by the invention]

[0004] The present inventors have found that there is room for improvement in the above-mentioned structure in terms of improving quantum efficiency. An object of the present invention is to provide a photodetector element capable of improving quantum efficiency. [Means for solving the problem]

[0005] The photodetector of the present invention is [1] "a photodetector comprising: an avalanche photodiode having a light incident surface onto which light is incident; and a plasmon structure formed on the light incident surface and diffracting the light by surface plasmon resonance, wherein the avalanche photodiode has a p-type first semiconductor region and an n-type second semiconductor region formed on the opposite side of the first semiconductor region from the light incident surface and forming a p-n junction with the first semiconductor region, the plasmon structure having a plurality of unit structures arranged on the first semiconductor region, each of the plurality of unit structures including a top surface opposite to the light incident surface, a bottom surface facing the light incident surface, and a side surface connected to the top surface and the bottom surface, and wherein the height of each of the plurality of unit structures is 100 nm or more and 250 nm or less."

[0006] In this photodetector, the avalanche photodiode has a p-type first semiconductor region and an n-type second semiconductor region that is formed on the opposite side of the light incident surface with respect to the first semiconductor region and forms a pn junction with the first semiconductor region. In a so-called PonN type configuration in which a p-type semiconductor region is formed on an n-type semiconductor region, the amount of light absorption in the p-type semiconductor region is likely to be small. In this respect, in this photodetector, the plasmon structure diffracts the incident light, thereby increasing the optical path length in the p-type first semiconductor region, and as a result, the amount of light absorption can be increased and the quantum efficiency can be improved. In addition, in this photodetector, each unit structure that constitutes the plasmon structure has a top surface, a bottom surface, and a side surface, and the height of each unit structure is 100 nm or more and 250 nm or less. This makes it possible to generate localized surface plasmon resonance in the plasmon structure, and to favorably diffract the incident light. As a result, the above-mentioned effect of increasing the amount of light absorption and improving the quantum efficiency can be significantly achieved. Therefore, according to this photodetector, the quantum efficiency can be improved.

[0007] The photodetector of the present invention may be [2] "the photodetector according to [1], wherein the side surface of each of the plurality of unit structures is formed perpendicular to the light incident surface, and the height of each of the plurality of unit structures is 100 nm or more and 150 nm or less." In this case, when the side surface is formed perpendicular, localized surface plasmon resonance can be generated, and incident light can be suitably diffracted.

[0008] The photodetector of the present invention may be [3] "the photodetector according to [1], in which the side surface of each of the plurality of unit structures is inclined so as to widen as it approaches the bottom surface." In this case, the wavelength range in which surface plasmon resonance occurs can be widened. In addition, for example, if a shape defect occurs at the boundary between the top surface and the side surface during the manufacturing process (for example, if a corner is chipped or rounded), the desired function may not be achieved. However, when the side surface is inclined, the effect of such a shape defect can be suppressed compared to when the side surface is formed vertically.

[0009] The photodetector of the present invention may be [4] "the photodetector according to [3], in which the height of each of the plurality of unit structures is 125 nm or more and 250 nm or less." In this case, when the side surface is inclined, localized surface plasmon resonance can be generated, and incident light can be suitably diffracted.

[0010] The photodetector of the present invention may be [5] "the photodetector according to any one of [1] to [4], in which the top surface and the side surface of each of the unit structures are connected to each other via a curved surface." In this case, the plasmonic structure is less likely to break even when subjected to an external force, and the stability of the photodetector can be improved.

[0011] The photodetector of the present invention may be [6] "the photodetector according to any one of [1] to [5], in which a silicon dioxide layer is formed between the plasmon structure and the first semiconductor region." In this case, by adjusting the thickness of the silicon dioxide layer, surface plasmon resonance can be generated in a desired wavelength range.

[0012] The photodetector of the present invention may be [7] "the photodetector according to [6], in which an adhesion layer made of a metal material is formed between the plasmon structure and the silicon dioxide layer." In this case, the bonding strength between the plasmon structure and the silicon dioxide layer can be increased.

[0013] The photodetector of the present invention may be [8] "the photodetector according to any one of [1] to [7], wherein the avalanche photodiode has a trench formed therein to surround the first semiconductor region when viewed from a direction perpendicular to the light incident surface, and the trench reflects the light diffracted by the plasmonic structure. In this case, the optical path length in the first semiconductor region can be further increased by reflecting the light by the trench.

[0014] The photodetector of the present invention may be [9] "the photodetector according to any one of [1] to [8], in which, when viewed from a direction perpendicular to the light incident surface, the unit structures are arranged along a first direction, and each of the unit structures has an elongated shape in a second direction perpendicular to the first direction." In this case, for example, one of P-polarized light and S-polarized light can be diffracted by the plasmon structure, while the other light can be reflected by the plasmon structure.

[0015] The photodetector of the present invention may be

[10] "the photodetector according to [1] or [2], wherein the plasmon structure is configured so that second-order diffracted light travels at an angle of 70° or more and less than 90° with respect to a direction perpendicular to the light incident surface." In this case, the size of the plasmon structure can be ensured, and manufacturing precision can be ensured.

[0016] The photodetector of the present invention may be

[11] "the photodetector according to [1] or [2], in which the plasmon structure is covered with a protective layer, and the protective layer is embedded between adjacent unit structures." In this case, the physical durability and chemical durability of the plasmon structure can be increased, and it becomes possible to further provide a predetermined optical element on the protective layer.

[0017] The photodetector of the present invention may be

[12] "the photodetector according to [6], in which the silicon dioxide layer has a thickness of 1 nm or more and 5 nm or less." In this case, the above-mentioned effect of being able to cause surface plasmon resonance in a desired wavelength range by adjusting the thickness of the silicon dioxide layer is significantly achieved.

[0018] The light-detecting element of the present invention may be the light-detecting element according to [7], wherein the adhesion layer is a titanium layer. In this case, the light absorption rate can be improved. Effect of the Invention

[0019] According to the present invention, it is possible to provide a photodetector capable of improving quantum efficiency. [Brief description of the drawings]

[0020] [Figure 1] FIG. 1A is a plan view of a light detection element, and FIG. 1B is a cross-sectional view taken along line BB in FIG. [Diagram 2] 1A is a perspective view of a unit structure, and FIG. 1B is a photograph showing the unit structure. [Diagram 3] FIG. 2 is a cross-sectional view of a unit structure of the first example. [Figure 4] FIG. 11 is a cross-sectional view of a unit structure of a second example. [Diagram 5] 1A is a cross-sectional view of a PonN type structure, and FIG. 1B is a cross-sectional view of a NonP type structure. [Figure 6]FIG. 1A is a cross-sectional view of a structure in which a plasmonic structure portion is arranged in a PonN type, and FIG. 1B is a diagram showing an electric field mode in the structure of FIG. [Figure 7] 7 to 11 are graphs showing the relationship between wavelength and absorptance, transmittance, and reflectance when the period P is 520 nm in the first example. Fig. 7(a) is a graph when the height H is 50 nm, and Fig. 7(b) is a graph when the height H is 60 nm. [Figure 8] (a) is a graph when the height H is 70 nm, and (b) is a graph when the height H is 80 nm. [Figure 9] (a) is a graph when the height H is 90 nm, and (b) is a graph when the height H is 100 nm. [Figure 10] (a) is a graph when the height H is 125 nm, and (b) is a graph when the height H is 150 nm. [Figure 11] (a) is a graph when the height H is 175 nm, and (b) is a graph when the height H is 200 nm. [Figure 12] 12 to 14 are graphs showing the relationship between wavelength and absorptance, transmittance, and reflectance when the period P is 250 nm in the first example. Fig. 12(a) is a graph when the height H is 100 nm, and Fig. 12(b) is a graph when the height H is 125 nm. [Figure 13] (a) is a graph when the height H is 150 nm, and (b) is a graph when the height H is 175 nm. [Figure 14] This is a graph when the height H is 200 nm. [Figure 15] 15 to 17 are graphs showing the relationship between wavelength and absorptance, transmittance, and reflectance when the period P is 260 nm in the first example. Fig. 15(a) is a graph when the height H is 100 nm, and Fig. 15(b) is a graph when the height H is 125 nm. [Figure 16](a) is a graph when the height H is 150 nm, and (b) is a graph when the height H is 175 nm. [Figure 17] This is a graph when the height H is 200 nm. [Figure 18] Figures 18 to 25 are graphs showing the relationship between wavelength and absorptance, transmittance, and reflectance when the angle θ is 80.5° and the period P is 520 nm in the second example. Figures 18 to 22 are graphs when the gap is 20 nm to 60 nm, and Figures 23 to 25 are graphs when the gap is 70 nm to 130 nm. Figure 18(a) is a graph when the height H is 100 nm, and Figure 18(b) is a graph when the height H is 125 nm. [Figure 19] (a) is a graph when the height H is 150 nm, and (b) is a graph when the height H is 175 nm. [Figure 20] (a) is a graph when the height H is 200 nm, and (b) is a graph when the height H is 210 nm. [Figure 21] (a) is a graph when the height H is 220 nm, and (b) is a graph when the height H is 230 nm. [Figure 22] (a) is a graph when the height H is 240 nm, and (b) is a graph when the height H is 250 nm. [Figure 23] (a) is a graph when the height H is 100 nm, and (b) is a graph when the height H is 125 nm. [Figure 24] (a) is a graph when the height H is 150 nm, and (b) is a graph when the height H is 175 nm. [Diagram 25] This is a graph when the height H is 200 nm. [Figure 26]Figures 26 to 32 are graphs showing the relationship between wavelength and absorptance, transmittance, and reflectance when the angle θ is 77.5° and the period P is 520 nm in the second example. Figures 26 to 29 are graphs when the gap is 20 nm to 60 nm, and Figures 30 to 32 are graphs when the gap is 70 nm to 130 nm. Figure 26(a) is a graph when the height H is 100 nm, and Figure 26(b) is a graph when the height H is 125 nm. [Figure 27] (a) is a graph when the height H is 150 nm, and (b) is a graph when the height H is 175 nm. [Figure 28] (a) is a graph when the height H is 200 nm, and (b) is a graph when the height H is 225 nm. [Figure 29] This is a graph when the height H is 250 nm. [Diagram 30] (a) is a graph when the height H is 100 nm, and (b) is a graph when the height H is 125 nm. [Diagram 31] (a) is a graph when the height H is 150 nm, and (b) is a graph when the height H is 175 nm. [Diagram 32] This is a graph when the height H is 200 nm. [Diagram 33] 33 to 35 are graphs showing the relationship between wavelength and absorptance, transmittance, and reflectance when the angle θ is 77.5° and the period P is 250 nm in the second example. Fig. 33(a) is a graph when the height H is 100 nm, and Fig. 33(b) is a graph when the height H is 125 nm. [Diagram 34] (a) is a graph when the height H is 150 nm, and (b) is a graph when the height H is 175 nm. [Diagram 35] This is a graph when the height H is 200 nm. [Diagram 36]36 to 38 are graphs showing the relationship between wavelength and absorptance, transmittance, and reflectance when the angle θ is 77.5° and the period P is 260 nm in the second example. Fig. 36(a) is a graph when the height H is 100 nm, and Fig. 36(b) is a graph when the height H is 125 nm. [Figure 37] (a) is a graph when the height H is 150 nm, and (b) is a graph when the height H is 175 nm. [Figure 38] This is a graph when the height H is 200 nm. [Figure 39] Figures 39 and 40 are graphs showing the relationship between wavelength and reflectance when the angle θ is 77.5° and the period P is 520 nm in the second example. The upper graph in Figure 39(a) shows the calculation results when the height H is 150 nm, and the lower graph in Figure 39(a) shows the measurement results when the height H is 150 nm. The upper graph in Figure 39(b) shows the calculation results when the height H is 175 nm, and the lower graph in Figure 39(b) shows the measurement results when the height H is 175 nm. [Diagram 40] The upper graph shows the calculation results when the height H is 200 nm, and the lower graph shows the measurement results when the height H is 200 nm. [Diagram 41] 41(a) to 41(c) are graphs showing the relationship between wavelength and light absorptance calculated based on the measurement results of Fig. 39 and Fig. 40. (a), (b), and (c) are graphs when the height H is 150 nm, 175 nm, and 200 nm, respectively. [Diagram 42] 42 to 44 are graphs showing the relationship between wavelength and absorptance, transmittance, and reflectance when the presence or absence of a silicon dioxide layer and its thickness are changed in the case where the angle θ is 77.5° and the period P is 520 nm in the second example. Fig. 42(a) is a graph when there is no silicon dioxide layer, and Fig. 42(b) is a graph when the silicon dioxide layer is 1 nm thick. [Diagram 43](a) is a graph when the silicon dioxide layer is 2 nm thick, and (b) is a graph when the silicon dioxide layer is 3 nm thick. [Diagram 44] (a) is a graph when the silicon dioxide layer is 4 nm thick, and (b) is a graph when the silicon dioxide layer is 5 nm thick. [Diagram 45] Fig. 45 and Fig. 46 are graphs showing the relationship between wavelength and absorptance, transmittance, and reflectance when the angle θ is 80.5°, the period P is 520 nm, and an adhesive layer made of titanium is provided in the second example. Fig. 45(a) is a graph when the height H is 125 nm, and Fig. 45(b) is a graph when the height H is 150 nm. [Figure 46] (a) is a graph when the height H is 175 nm, and (b) is a graph when the height H is 200 nm. [Figure 47] FIG. 1(a) is a perspective view of a unit structure of a first modified example, and FIG. 1(b) is a photograph showing the unit structure of the first modified example. [Figure 48] Graphs (a) and (b) are for the second example where the angle θ is 77.5°, the period P is 520 nm, and the height H is 200 nm. Graph (a) shows the relationship between the wavelength and the reflectance and optical absorptance for S-polarized light, and graph (b) shows the relationship between the wavelength and the reflectance and optical absorptance for P-polarized light. [Figure 49] Graphs (a) and (b) show the first modified example in which the angle θ is 77.5°, the period P is 520 nm, and the height H is 200 nm. Graph (a) shows the relationship between the wavelength and the reflectance and optical absorptance for S-polarized light, and graph (b) shows the relationship between the wavelength and the reflectance and optical absorptance for P-polarized light. [Figure 50] FIG. 13(a) is a diagram showing an example of an electric field mode in a first example, and (b) is a diagram showing an example of an electric field mode in a second example. [Figure 51]The upper graph in (a) shows the relationship between wavelength and reflectance when the height H is 50 nm in the first example, and the lower diagram shows the electric field mode when the wavelength is 905 nm in that case. The upper graph in (b) shows the relationship between wavelength and reflectance when the height H is 150 nm in the first example, and the lower diagram shows the electric field mode when the wavelength is 905 nm in that case. In Figure 51, the results for the first-order diffracted light output from the plasmonic structure are shown. [Figure 52] The upper graph in (a) shows the relationship between wavelength and reflectance when the height H is 100 nm in the second example, and the lower diagram shows the electric field mode when the wavelength is 905 nm in that case. The upper graph in (b) shows the relationship between wavelength and reflectance when the height H is 200 nm in the second example, and the lower diagram shows the electric field mode when the wavelength is 905 nm in that case. In the example of Figure 52, the results for the first-order diffracted light output from the plasmonic structure are shown. [Figure 53] The upper graph in (a) shows the relationship between wavelength and reflectance when the height H is 50 nm in the first example, and the lower diagram shows the electric field mode when the wavelength is 905 nm in that case. The upper graph in (b) shows the relationship between wavelength and reflectance when the height H is 125 nm in the first example, and the lower diagram shows the electric field mode when the wavelength is 905 nm in that case. In the example of Figure 53, the results for the second-order diffracted light output from the plasmonic structure are shown. [Figure 54] The upper graph in (a) shows the relationship between wavelength and reflectance when the height H is 100 nm in the second example, and the lower diagram shows the electric field mode when the wavelength is 905 nm in that case. The upper graph in (b) shows the relationship between wavelength and reflectance when the height H is 200 nm in the second example, and the lower diagram shows the electric field mode when the wavelength is 905 nm in that case. In the example of Figure 54, the results for the second-order diffracted light output from the plasmonic structure are shown. [Figure 55]FIG. 1A is a diagram for explaining localized surface plasmon resonance, FIG. 1B is a diagram for explaining propagating surface plasmon resonance, and FIG. 1C is a diagram for explaining grating surface plasmon resonance. [Figure 56] 52(a) and (b) correspond to FIG. 52(a) and FIG. 52(b), and are diagrams for explaining the difference between a dipole SLR and a quadrupole SLR. [Figure 57] FIG. 13 is a diagram for explaining an electric field mode in the case of a dipole SLR. [Figure 58] FIG. 13 is a diagram for explaining an electric field mode in the case of a quadrupole SLR. [Figure 59] FIG. 13 is a diagram for explaining an electric field mode in the case of a quadrupole SLR. [Figure 60] FIG. 11 is a cross-sectional view of a unit structure of a second modified example. [Figure 61] Figures 61 and 62 are graphs showing the relationship between wavelength and absorptance, transmittance, and reflectance when the angle θ is 71.6° and the period P is 520 nm in the second example. Figure 61(a) is a graph when the height H is 230 nm, and Figure 61(b) is a graph when the height H is 240 nm. [Figure 62] This is a graph when the height H is 250 nm. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the following description, the same or corresponding elements are designated by the same reference numerals, and duplicated descriptions will be omitted.

[0022] As shown in FIG. 1, the photodetector 1 includes an avalanche photodiode (hereinafter, also referred to as "APD") 10. The APD 10 has a light incident surface 10a on which light L is incident, and a surface 10b opposite to the light incident surface 10a. The light incident surface 10a and the surface 10b are, for example, flat surfaces parallel to each other. The APD 10 is a photodiode that utilizes avalanche multiplication, and absorbs light L and converts it into a photocurrent. In the APD 10, the photocurrent is multiplied by application of a reverse voltage. The photodetector 1 has sensitivity to light in, for example, the near-infrared region (for example, 750 nm to 2.5 μm).

[0023] The APD 10 has a plurality of pixel units 11. In the example of Fig. 1, three pixel units 11 are arranged along the X direction (first direction). The plurality of pixel units 11 may be arranged, for example, in a lattice pattern (matrix pattern), or may be arranged along each of the X direction and the Y direction (direction perpendicular to the X direction) (second direction). The number of pixel units 11 is not limited, and for example, only one pixel unit 11 may be provided, or four or more pixel units 11 may be provided.

[0024] Each pixel unit 11 is formed, for example, in a rectangular shape in plan view (when viewed from the Z direction). The Z direction is perpendicular to the X and Y directions and perpendicular to the light incident surface 10a. Each pixel unit 11 is fabricated on a semiconductor substrate 12 made of, for example, silicon (Si). Each pixel unit 11 has an n-type semiconductor region 13, an n-type semiconductor region 14, a p-type semiconductor region 15 (first semiconductor region), and an n-type semiconductor region 16 (second semiconductor region). The semiconductor regions 15 and 16 function as avalanche multiplication regions for generating avalanche multiplication.

[0025] The n-type semiconductor region 13 is, for example, a substrate region. The semiconductor region 13 constitutes the surface 10b of the APD 10. The n-type semiconductor region 14 is, for example, an epitaxial region (epitaxial layer) and has a lower impurity concentration than the semiconductor region 13. The semiconductor region 14, together with the semiconductor regions 15 and 16, functions as a sensitive region (light absorbing region) sensitive to light L. A portion of the semiconductor region 14 is exposed to the light incident surface 10a.

[0026] The p-type semiconductor region 15 and the n-type semiconductor region 16 are regions (layers) in which the impurity concentration is increased by, for example, ion implantation. The n-type semiconductor region 16 has an impurity concentration higher than that of the semiconductor region 14 and lower than that of the semiconductor region 13. The impurity concentrations of the semiconductor regions 15 and 16 are set to a concentration that can cause avalanche multiplication. The semiconductor region 15 is disposed on the light incident surface 10a side so as to be exposed to the light incident surface 10a. The semiconductor region 16 is formed on the opposite side of the semiconductor region 15 from the light incident surface 10a, and forms a pn junction with the semiconductor region 15. A part of the semiconductor region 14 is located on the opposite side of the semiconductor regions 15 and 16 from the light incident surface 10a. In a plan view, the semiconductor regions 15 and 16 are surrounded by the semiconductor region 14.

[0027] A trench 17 is formed around the entire outer edge of each pixel unit 11. When viewed from the Z direction, the trench 17 surrounds the semiconductor region 15. The trench 17 is formed to extend in the Z direction from the light incident surface 10a. The trench 17 functions as a low sensitivity region that has no sensitivity (or has low sensitivity) to the light L. The trench 17 is formed, for example, by embedding a metal material in a groove formed in the light incident surface 10a.

[0028] As shown in Fig. 1 to Fig. 4, the light detection element 1 further includes a plasmon structure 20 formed on the light incident surface 10a. As shown in Fig. 3 and Fig. 4, the plasmon structure 20 diffracts the light L by surface plasmon resonance, and causes the diffracted light to travel in a direction intersecting the Z direction. The angle of the traveling direction of the diffracted light with respect to the Z direction is, for example, 80°. Details of the surface plasmon resonance will be described later.

[0029] The plasmon structure 20 has a plurality of unit structures 21 arranged on the light incident surface 10a (semiconductor region 15) of the APD 10. The unit structures 21 are arranged, for example, in a lattice (matrix) shape, and are lined up along both the X and Y directions. In this example, the unit structures 21 are formed in a square shape in a plan view. The unit structures 21 are formed, for example, from a metal material, a dielectric material, or a semiconductor material (for example, silicon). An example of a dielectric material constituting the unit structures 21 is TiO 2 ,SiO 2 ,HfO 2 , SiN, and a-Si. In this example, the unit structure 21 is formed of gold (Au), which is a metal material. Other examples of the metal material constituting the unit structure 21 include silver (Ag) and aluminum (Al). Note that while the unit structure 21 is shown in a simplified form in FIG. 1(b), in reality many more small unit structures 21 than shown are arranged side by side.

[0030] The unit structure 21 has, for example, the shape of the first example shown in FIG. 3 or the second example shown in FIG. 4. The unit structure 21 of the first example includes a top surface 21a on the opposite side to the light incident surface 10a, a bottom surface 21b facing the light incident surface 10a, and a side surface 21c connected to the top surface 21a and the bottom surface 21b. The top surface 21a and the bottom surface 21b are flat surfaces parallel to the light incident surface 10a, for example. The side surface 21c is a flat surface perpendicular to the light incident surface 10a. That is, the unit structure 21 of the first example is formed in a substantially rectangular shape in a cross section perpendicular to the X direction and a cross section perpendicular to the Y direction. The boundary between the top surface 21a and the side surface 21c (the edge of the top surface 21a) is rounded in an R shape so as not to have a sharp corner over the entire circumference (FIG. 2(a)). That is, the top surface 21a and the side surface 21c are connected to each other via a curved surface 21d. The curved surface 21d is curved, for example, in an arc shape in a cross section perpendicular to the X direction and in a cross section perpendicular to the Y direction.

[0031] The unit structure 21 of the second example shown in Fig. 4 differs from the unit structure 21 of the first example in that the side surface 21c is inclined (tapered). In the second example, the side surface 21c is inclined so as to widen as it approaches the bottom surface 21b. In other words, the unit structure 21 of the second example is formed in a trapezoidal shape in a cross section perpendicular to the X direction and a cross section perpendicular to the Y direction. The angle θ of the side surface 21c with respect to the bottom surface 21b is, for example, 50° or more and less than 90°.

[0032] 2(a), the arrangement period of the unit structures 21 in the X direction is Px, the arrangement period of the unit structures 21 in the Y direction is Py, the distance between adjacent unit structures 21 in the X direction is Gx, the distance between adjacent unit structures 21 in the Y direction is Gy, and the height of the unit structures 21 is H. The period Px is the length obtained by adding 2×Gx to the width of the unit structures 21 in the X direction, and the period Py is the length obtained by adding 2×Gy to the width of the unit structures 21 in the Y direction. The height H is 100 nm or more and 250 nm or less. FIG. 2(b) is a photograph of a unit structure 21 when Px and Py are 250 nm, and Gx and Gy are 40 nm.

[0033] As shown in FIGS. 3 and 4, the photodetector element 1 is made of a silicon dioxide (SiO ) layer formed between the plasmonic structure 20 and the light incident surface 10a (semiconductor region 15). 2 The plasmon structure 20 further includes a silicon dioxide layer 31 and an adhesion layer 32 formed between the plasmon structure 20 and the silicon dioxide layer 31. That is, in this example, the plasmon structure 20 is formed on the light incident surface 10a via the silicon dioxide layer 31 and the adhesion layer 32. The silicon dioxide layer 31 has a thickness of, for example, 1 nm to 5 nm. The adhesion layer 32 is made of a metal material. In this example, the adhesion layer 32 is made of titanium (Ti). Another example of a metal material constituting the adhesion layer 32 is chromium (Cr). The adhesion layer 32 has a thickness of, for example, about 3 nm.

[0034] Although not shown in the figure, the photodetector 1 has a pair of electrodes for applying a voltage to the APD 10, for example, on the light incident surface 10a side and the surface 10b side. A reverse voltage is applied to the photodetector 1 via the electrodes. The APD 10 operates, for example, in a Geiger mode, in which a reverse voltage equal to or greater than the breakdown voltage is applied. The APD 10 may also operate in a linear mode, in which a reverse voltage smaller than the breakdown voltage is applied. The photodetector 1 may further include a lens that focuses the light L onto a sensitive region in the APD 10.

[0035] The photodetector 1 will be described with reference to Figs. 5 and 6. The avalanche photodiode may have a PonN type structure shown in Fig. 5(a) or a NonP type structure shown in Fig. 5(b). As shown in Fig. 5(a), in the PonN type structure, a p-type semiconductor layer 41 is formed on an n-type semiconductor layer 42 and is located on the light incident surface side. As shown in Fig. 5(b), in the NonP type structure, an n-type semiconductor layer 43 is formed on an n-type semiconductor layer 44 and is located on the light incident surface side. The above-mentioned photodetector 1 is a PonN type.

[0036] In the avalanche photodiode, the p-layer is used as the light absorption layer. For example, when an avalanche photodiode is configured to have sensitivity to light in the visible or near-ultraviolet region, the PonN type may be adopted. This is because the light absorption layer made of the p-layer easily absorbs light in those wavelength regions, so the p-type semiconductor layer 41 may be thin as shown in FIG. 5(a). On the other hand, when an avalanche photodiode is configured to have sensitivity to light in the near-infrared region, the NonP type may be adopted. This is because the light absorption layer made of the p-layer does not easily absorb light in the near-infrared region, and if the p-type semiconductor layer 41 is thin as in the PonN type, it cannot sufficiently absorb incident light, so it is necessary to absorb light by also using the p-type semiconductor layer on the substrate side as shown in FIG. 5(b).

[0037] In this respect, the photodetector 1 of the embodiment is sensitive to light in the near-infrared band, but is configured as a PonN type. This is because, as shown in FIG. 6(a), the plasmon structure 20 is provided, and light can be diffracted by surface plasmon resonance to cause the diffracted light to travel in a direction intersecting the Z direction, and the optical path length in the p-type semiconductor layer 41 (semiconductor region 15) can be increased to increase the amount of light absorption. By adopting the PonN type in a case where the photodetector 1 has sensitivity to light in the near-infrared band, the time resolution and sensitivity can be improved. The reason why the time resolution can be improved is that the effect of concentrating the diffracted light of the plasmon structure near the surface makes it possible to limit the excitation location to a thin layer (p layer) near the surface, and the fluctuation in the time until the excited electrons reach avalanche multiplication can be suppressed. In addition, since the photodetector 1 has a trench 17 provided so as to surround the sensitive region, the diffracted light from the plasmon structure 20 can be reflected by the trench 17, and the optical path length in the p-type semiconductor layer 41 can be further increased.

[0038] As shown in FIG. 6(b), in the light detection element 1, localized surface plasmon resonance occurs in the plasmon structure 20. In the localized surface plasmon resonance, an electric field mode is formed such that a dipole 45 consisting of a pair of a positive electrode and a negative electrode extends along the Z direction. This causes the light L to be diffracted so as to travel in a direction intersecting the Z direction. Meanwhile, the present inventors have found that in order to cause the localized surface plasmon resonance in the plasmon structure 20, it is necessary to devise a structure for the plasmon structure 20. This point will be described below.

[0039] 7 to 11 are graphs showing the relationship between wavelength and absorptance, transmittance, and reflectance when the period P (Px and Py described above) is 520 nm in the first example shown in FIG. 3 (side surface 21c is vertical). The absorptance, transmittance, and reflectance in each graph are those in APD 10. Each graph shows the results of a simulation using the FDTD method (Finite Difference Time Domain Method). 7(a), 7(b), 8(a), 8(b), 9(a), 9(b), 10(a), 10(b), 11(a), and 11(b) are graphs showing the results when the height H is 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, 100 nm, 125 nm, 150 nm, 175 nm, and 200 nm, respectively. Each graph shows the results when the gap (Gx and Gy described above) is 20 nm, 30 nm, 40 nm, 50 nm, and 60 nm. Also, the results when the plasmon structure 20 is not provided are shown as "W / o Au grating". In this simulation, the thickness of the silicon dioxide layer 31 was set to 3 nm. The adhesion layer 32 was not provided. The plasmonic structure 20 is configured so that the second-order diffracted light travels at an angle of 70° or more and less than 90° with respect to the Z direction (the direction perpendicular to the light incident surface 10a). Each figure shows the results for the second-order diffracted light output from the plasmonic structure 20.

[0040] In each figure, for example, if the reflectance at the target wavelength (in this example, 750 nm or more) is lower than when the plasmon structure 20 is not provided, it can be understood that reflection can be suppressed by providing the plasmon structure 20, and localized surface plasmon resonance can be generated in the plasmon structure 20.

[0041] 7 to 11 show that when height H is in the range of 100 nm to 150 nm, the reflectance at the target wavelength is lower than when plasmon structure 20 is not provided, and localized surface plasmon resonance can be generated in plasmon structure 20. In particular, when height H is in the range of 100 nm to 125 nm, the wavelength range in which reflectance is reduced is narrow, and it can be seen that localized surface plasmon resonance can be generated in a specific wavelength range.

[0042] 12 to 14 are graphs showing the relationship between the wavelength and the absorptance, transmittance, and reflectance when the period P is 250 nm in the first example. FIG. 12(a), FIG. 12(b), FIG. 13(a), FIG. 13(b), and FIG. 14 are graphs when the height H is 100 nm, 125 nm, 150 nm, 175 nm, and 200 nm, respectively. Other points are the same as those in FIG. 7 to FIG. 11. In FIG. 13(a), the symbols "20" and "30" indicate the results when the gap is 20 nm and 30 nm, respectively, and in FIG. 13(b), the symbol "w / o" indicates the results when the plasmonic structure 20 is not provided. This is the same in the following figures.

[0043] 12 to 14 show that when the height H is in the range of 100 nm to 150 nm, the reflectance at the target wavelength is lower than when the plasmon structure 20 is not provided, and that localized surface plasmon resonance can be generated in the plasmon structure 20.

[0044] Figures 15 to 17 are graphs showing the relationship between wavelength and absorptance, transmittance, and reflectance when the period P is 260 nm in the first example. Figures 15(a), 15(b), 16(a), 16(b), and 17 are graphs when the height H is 100 nm, 125 nm, 150 nm, 175 nm, and 200 nm, respectively. Other points are the same as in Figures 7 to 11.

[0045] 15 to 17, it can be seen that when the height H is in the range of 100 nm to 150 nm, the reflectance at the target wavelength is lower than when the plasmon structure 20 is not provided, and localized surface plasmon resonance can be generated in the plasmon structure 20.

[0046] 7 to 11, 12 to 14, and 15 to 17, it can be seen that in the first example in which the side surface 21c is vertical, localized surface plasmon resonance can be generated when the height H is in the range of 100 nm to 150 nm, and incident light can be suitably diffracted. Although not shown in the figures, a similar calculation was also performed in the first example in which the period P is 500 nm, and it was confirmed that similar results were obtained.

[0047] 18 to 25 are graphs showing the relationship between the wavelength and the absorptance, transmittance, and reflectance when the angle θ is 80.5° (Atan(6.0)) and the period P is 520 nm in the second example (side surface 21c is inclined) shown in FIG. 4. FIGS. 18 to 22 are graphs when the gap is 20 nm to 60 nm, and FIGS. 23 to 25 are graphs when the gap is 70 nm to 130 nm. FIGS. 18(a), 18(b), 19(a), 19(b), 20(a), 20(b), 21(a), 21(b), 22(a), and 22(b) are graphs when the height H is 100 nm, 125 nm, 150 nm, 175 nm, 200 nm, 210 nm, 220 nm, 230 nm, 240 nm, and 250 nm, respectively. Figures 23(a), 23(b), 24(a), 24(b), and 25 are graphs for heights H of 100 nm, 125 nm, 150 nm, 175 nm, and 200 nm, respectively. Other points are similar to those in Figures 7 to 11.

[0048] 18 to 25, when the height H is in the range of 125 nm to 210 nm, the reflectance at the target wavelength is lower than when the plasmon structure 20 is not provided, and it is understood that localized surface plasmon resonance can be generated in the plasmon structure 20. In particular, when the height H is in the range of 200 to 210 nm, the wavelength range in which the reflectance is reduced is narrow, and it is understood that localized surface plasmon resonance can be generated in a specific wavelength range. Furthermore, in the case of the second example in which the side surface 21c is inclined, the wavelength range in which localized surface plasmon resonance can be generated can be made wider than in the case of the first example in which the side surface 21c is vertical.

[0049] 26 to 32 are graphs showing the relationship between wavelength and absorptance, transmittance, and reflectance when the angle θ is 77.5 (Atan(4.5)) and the period P is 520 nm in the second example (side surface 21c is inclined) shown in FIG. 4. FIGS. 26 to 29 are graphs when the gap is 20 nm to 60 nm, and FIGS. 30 to 32 are graphs when the gap is 70 nm to 130 nm. FIGS. 26(a), 26(b), 27(a), 27(b), 28(a), 28(b), and 29 are graphs when the height H is 100 nm, 125 nm, 150 nm, 175 nm, 200 nm, 225 nm, and 250 nm, respectively. Figures 30(a), 30(b), 31(a), 31(b), and 32 are graphs for heights H of 100 nm, 125 nm, 150 nm, 175 nm, and 200 nm, respectively. Other points are similar to those in Figures 7 to 11.

[0050] From Figures 26 to 32, it can be seen that when the height H is in the range of 150 nm to 230 nm (225 nm), the reflectance at the target wavelength is lower than when the plasmon structure 20 is not provided, and localized surface plasmon resonance can be generated in the plasmon structure 20.

[0051] 33 to 35 are graphs showing the relationship between wavelength and absorptance, transmittance, and reflectance when the angle θ is 77.5° (Atan(4.5)) and the period P is 250 nm in the second example (side surface 21c is inclined) shown in FIG. 4. FIGS. 33(a), 33(b), 34(a), 34(b), and 35 are graphs when the height H is 100 nm, 125 nm, 150 nm, 175 nm, and 200 nm, respectively. Other points are similar to those in FIGS. 7 to 11.

[0052] From Figures 33 to 35, it can be seen that when the height H is in the range of 125 nm to 200 nm, the reflectance at the target wavelength is lower than when the plasmon structure 20 is not provided, and localized surface plasmon resonance can be generated in the plasmon structure 20.

[0053] Figures 36 to 38 are graphs showing the relationship between wavelength and absorptance, transmittance, and reflectance when the angle θ is 77.5° and the period P is 260 nm in the second example. Figures 36(a), 36(b), 37(a), 37(b), and 38 are graphs when the height H is 100 nm, 125 nm, 150 nm, 175 nm, and 200 nm, respectively. Other points are the same as in Figures 7 to 11.

[0054] From Figures 36 to 38, it can be seen that when the height H is in the range of 125 nm to 200 nm, the reflectance at the target wavelength is lower than when the plasmon structure 20 is not provided, and localized surface plasmon resonance can be generated in the plasmon structure 20.

[0055] 18 to 25, 26 to 32, 33 to 35, and 36 to 38, it can be seen that in the second example in which the side surface 21c is inclined, localized surface plasmon resonance can be generated when the height H is in the range of 125 nm to 230 nm, and incident light can be suitably diffracted. Although not shown in the figures, similar calculations were also performed for the third example in which the angle θ is 77.5° and the period P is 270 nm, 280 nm, and 500 nm, and it was confirmed that similar results were obtained.

[0056] 39 and 40 are graphs showing the relationship between wavelength and reflectance when the angle θ is 77.5° and the period P is 520 nm in the second example (side surface 21c is inclined) shown in FIG. 4. The upper graph in FIG. 39(a) shows the calculation result when the height H is 150 nm, and the lower graph in FIG. 39(a) shows the measurement result when the height H is 150 nm. The upper graph in FIG. 39(b) shows the calculation result when the height H is 175 nm, and the lower graph in FIG. 39(b) shows the measurement result when the height H is 175 nm. The upper graph in FIG. 40 shows the calculation result when the height H is 200 nm, and the lower graph in FIG. 40 shows the measurement result when the height H is 200 nm. The calculation result is the simulation result described above.

[0057] 39 and 40 show that the calculation results and the measurement results show similar trends for heights H of 150 nm, 175 nm, and 200 nm, respectively. This shows that the actual characteristics can be understood based on the simulation.

[0058] FIG. 41 is a graph showing the relationship between wavelength and light absorptance calculated based on the measurement results of FIG. 39 and FIG. 40. FIG. 41(a), FIG. 41(b) and FIG. 41(c) are graphs in the cases where the height H is 150 nm, 175 nm and 200 nm, respectively. In each graph, the light absorptance (enhancement) is the ratio of the light absorption amount calculated with the light absorption amount in the case where the plasmon structure 20 is not provided being 1 (reference). As shown in FIG. 41, in all cases where the height H is 150 nm, 175 nm and 200 nm, the light absorptance is improved by about 20% at the target wavelength. This shows that the light absorptance can be improved by providing the plasmon structure 20.

[0059] 42 to 44 are graphs showing the relationship between wavelength and absorptance, transmittance, and reflectance when the presence or absence of silicon dioxide layer 31 and its thickness are changed in the second example shown in FIG. 4 (side surface 21c is inclined), where angle θ is 77.5° and period P is 520 nm. FIG. 42(a) is a graph when there is no silicon dioxide layer, and FIG. 42(b), FIG. 43(a), FIG. 43(b), FIG. 44(a), and FIG. 44(b) are graphs when the thickness (T) of silicon dioxide layer 31 is 1 nm, 2 nm, 3 nm, 4 nm, and 5 nm, respectively. The height H of unit structure 21 is 200 nm. Other points are the same as those in FIG. 7 to FIG. 11.

[0060] 42 to 44 show that the reflectance decreases in different wavelength ranges when the thickness of the silicon dioxide layer 31 is different. This shows that localized surface plasmon resonance can be generated in a desired wavelength range by adjusting the thickness of the silicon dioxide layer 31.

[0061] Fig. 45 and Fig. 46 are graphs showing the relationship between wavelength and absorptance, transmittance, and reflectance in the second example shown in Fig. 4 (side surface 21c is inclined), where angle θ is 80.5°, period P is 520 nm, and adhesive layer 32 made of titanium is provided. Fig. 45(a), Fig. 45(b), Fig. 46(a), and Fig. 46(b) are graphs in the cases where height H is 125 nm, 150 nm, 175 nm, and 200 nm, respectively. Other points are the same as those in Figs. 7 to 11.

[0062] Comparing the corresponding graphs of height H between FIG. 45 and FIG. 46 (where the adhesion layer 32 is provided) and FIG. 18 to FIG. 22 (where the adhesion layer 32 is not provided), it can be seen that the light absorption rate is improved in the graphs of FIG. 45 and FIG. 46 compared to the corresponding graphs in FIG. 18 to FIG. 22. For example, comparing the portion indicated by the arrow in the graph of absorption rate (Absorption) at the top of FIG. 46(b) with the corresponding portion in the graph of absorption rate (Absorption) at the top of FIG. 20(a), it can be seen that the absorption rate is about twice as high. From this, it can be seen that the light absorption rate can be improved by providing the adhesion layer 32 made of titanium. Note that the bonding strength (degree of adhesion) between the plasmon structure 20 and the silicon dioxide layer 31 can also be increased by providing the adhesion layer 32.

[0063] The plasmon structure 20 may be configured as in one modified example shown in FIG. 47. In the first modified example, the unit structure 21 has a shape elongated in the Y direction. That is, the length of the unit structure 21 in the Y direction is longer than the length of the unit structure 21 in the X direction. In this example, the unit structure 21 is formed in a rectangular shape in a plan view. FIG. 47(b) is a photograph of the unit structure 21 when Px is 500 nm, Py is 3000 nm, Gx is 70 nm, and Gy is 100 nm. In the first modified example, the side surface 21c may be formed vertically as in the first example shown in FIG. 3, or may be inclined as in the second example shown in FIG. 4. Gy may be 0. That is, the unit structure 21 may be formed continuously along the Y direction without a break.

[0064] Fig. 48 is a graph showing the second example (side surface 21c is inclined) shown in Fig. 4 in which the angle θ is 77.5°, the period P is 520 nm, and the height H is 200 nm. Fig. 48(a) is a graph showing the relationship between the wavelength, reflectance, and optical absorptance for S-polarized light, and Fig. 48(b) is a graph showing the relationship between the wavelength, reflectance, and optical absorptance for P-polarized light. The vibration direction of the S-polarized light is parallel to the Y direction, and the vibration direction of the P-polarized light is parallel to the X direction.

[0065] Fig. 49 is a graph showing the relationship between the wavelength, reflectance, and optical absorptance for S-polarized light in the first modified example, and Fig. 49(b) is a graph showing the relationship between the wavelength, reflectance, and optical absorptance for P-polarized light. The vibration direction of the S-polarized light was parallel to the Y direction, and the vibration direction of the P-polarized light was parallel to the X direction (Fig. 47(a)).

[0066] As shown in FIG. 48, when the unit structure 21 is square, both S-polarized and P-polarized light are diffracted by the plasmon structure 20. On the other hand, as shown in FIG. 49, when the unit structure 21 is rectangular, the P-polarized light is diffracted by the plasmon structure 20, while the S-polarized light is not diffracted by the plasmon structure 20 and is reflected. In this way, in the first modified example, one of the P-polarized light and the S-polarized light can be diffracted by the plasmon structure 20, while the other light can be reflected by the plasmon structure 20. Such a function can be used to separate signal light and noise light, for example, in the field of LiDAR (Light Detection And Ranging).

[0067] Fig. 50(a) is a diagram showing an example of an electric field mode (electric field vector) in the first example, and Fig. 50(b) is a diagram showing an example of an electric field mode in the second example. As shown in Fig. 50(a), in the first example, an electric field mode is formed in the plasmon structure 20 such that the dipole 45 extends along the Z direction. As shown in Fig. 50(b), in the second example, an electric field mode is formed in the plasmon structure 20 such that the dipole 45 extends at an angle with respect to the Z direction. In either case, the light L is diffracted so as to travel in a direction intersecting the Z direction.

[0068] 51 to 54 are diagrams for explaining the relationship between whether or not an electric field mode in which dipoles 45 extend along the Z direction or at an angle (non-perpendicular to the Z direction) is formed in plasmon structure 20, and height H of unit structure 21. Fig. 51 and Fig. 52 show results regarding first-order diffracted light output from plasmon structure 20. Fig. 51 shows results in a first example where height H is 50 nm and 150 nm, and Fig. 52 shows results in a second example where height H is 100 nm and 200 nm.

[0069] As shown in Fig. 51(a), in the first example, when the height H is 50 nm, the dipole 45 extends perpendicular to the Z direction (parallel to the light incident surface). On the other hand, as shown in Fig. 51(b), when the height H is 150 nm, the dipole 45 extends at an incline with respect to the Z direction. As shown in Fig. 52(a), in the second example, when the height H is 100 nm, the dipole 45 extends perpendicular to the Z direction (parallel to the light incident surface). On the other hand, as shown in Fig. 52(b), when the height H is 200 nm, the dipole 45 extends at an incline with respect to the Z direction.

[0070] Figures 53 and 54 show the results regarding the second-order diffracted light output from the plasmon structure 20. Figure 53 shows the results when the height H is 50 nm and 125 nm in the first example, and Figure 54 shows the results when the height H is 100 nm and 200 nm in the second example.

[0071] As shown in Fig. 53(a), in the first example, when the height H is 50 nm, the dipole 45 extends perpendicular to the Z direction (parallel to the light incident surface). On the other hand, as shown in Fig. 53(b), when the height H is 125 nm, the dipole 45 extends at an angle to the Z direction. As shown in Fig. 54(a), in the second example, when the height H is 100 nm, the dipole 45 extends perpendicular to the Z direction (parallel to the light incident surface). On the other hand, as shown in Fig. 54(b), when the height H is 200 nm, the dipole 45 extends at an angle to the Z direction.

[0072] With reference to Figs. 55 to 59, localized surface plasmon resonance will be described. Surface plasmon resonance includes localized surface plasmon resonance, propagating surface plasmon resonance, and lattice surface plasmon resonance. In localized surface plasmon resonance (LSPR) shown in Fig. 55(a), collective oscillation of free electrons is excited in a specific wavelength band in a structure sufficiently smaller than the wavelength (λ) depending on the structure shape and size (W). As a result, scattered light (radiated light) is enhanced in a specific wavelength band. Propagating surface plasmon resonance (SPR) shown in Fig. 55(b) is based on a different principle from localized surface plasmon resonance, and when an evanescent wave (evanescent wave) couples with an interface as a boundary, a wave that propagates through the interface is generated. A prism or grating is required for this coupling.

[0073] The lattice surface plasmon resonance (SLR) shown in FIG. 55(c) occurs due to a combination of localized surface plasmon resonance and propagation surface plasmon resonance. Specifically, the lattice surface plasmon resonance occurs due to interference between diffracted light at the lattice interface and localized surface plasmon. A high Q value is realized by exciting the propagation surface plasmon via the localized surface plasmon. The photodetector 1 of the embodiment utilizes this lattice surface plasmon resonance. The localized surface plasmon resonance depends on the structure shape, and the propagation surface plasmon resonance depends on the structure period. The lattice surface plasmon resonance depends on both the structure shape and the structure period. That is, in the photodetector 1 of the embodiment, the propagation surface plasmon that depends on the period P is also excited, and the localized surface plasmon that depends on the shape of the plasmon structure portion 20 is excited, thereby generating the lattice surface plasmon resonance.

[0074] FIG. 56 corresponds to FIG. 52. The electric field mode in which the dipole 45 in FIG. 56(a) extends perpendicular to the Z direction (parallel to the light incident surface) is a dipole SLR (DSLR) generated by the coupling of a dipole and SPR. Since light is emitted perpendicular to the extension direction of the dipole in the plasmon structure, in this case, the diffracted light travels parallel to the light incident surface, and the light cannot be diffracted suitably. In contrast, the electric field mode in which the dipole 45 in FIG. 56(b) extends along the Z direction or at an angle is a quadrupole SLR (SLR) generated by the coupling of a quadrupole and SPR. Since light is emitted perpendicular to the extension direction of the dipole in the plasmon structure, in this case, the light can be diffracted suitably as shown by the arrow in the figure.

[0075] FIG. 57 is a diagram for explaining the electric field mode in the case of a dipole SLR, and FIG. 58 and FIG. 59 are diagrams for explaining the electric field mode in the case of a quadrupole SLR. FIG. 57 corresponds to FIG. 51(a). As shown in FIG. 57, when the height H is 50 nm in the first example, the dipole 45 extends perpendicular to the Z direction (parallel to the light incident surface) at any wavelength, and a dipole SLR occurs. FIG. 58 corresponds to FIG. 51(b). As shown in FIG. 58, when the height H is 150 nm in the second example, the dipole 45 extends along the Z direction or at an incline at any wavelength, and a quadrupole SLR occurs. FIG. 59 corresponds to FIG. 54(b). As shown in FIG. 59, when the height H is 200 nm in the first example, a dipole 57 extends along the Z direction or at an incline at any wavelength, and a quadrupole SLR occurs. In addition, RA in Fig. 57 means Rayleigh anomaly, which is an abnormal transmission phenomenon that occurs with the same period as the wavelength. As shown in Fig. 57 to Fig. 59, the resonance dip is excited in the order of SPR (propagating surface plasmon resonance), SLR (lattice surface plasmon resonance), and LSPR (localized surface plasmon) from the long wavelength side. In Fig. 57, the excitation wavelength of RA and SPR is 940 nm, the excitation wavelength of DSLR is 905 nm, and the excitation wavelength of LSPR is 820 nm.

[0076] As in a second modified example shown in FIG. 60, the photodetector 1 may further include a protective layer 50 that covers the plasmon structure 20. The protective layer 50 covers the entire plasmon structure 20 and is provided so as to be embedded between adjacent unit structures 21. The protective layer 50 is made of, for example, silicon dioxide or aluminum oxide (Al 2 O 3 ) film, which is an ALD film formed by atomic layer deposition. By providing the protective layer 50, it is possible to increase physical durability and chemical durability, and also to provide a predetermined optical element on the protective layer 50. Note that the protective layer 50 may be provided only between adjacent unit structures 21. [Action and Effects]

[0077] In the photodetector 1, the APD 10 has a p-type semiconductor region 15 (first semiconductor region) and an n-type semiconductor region 16 (second semiconductor region) that is formed on the opposite side of the semiconductor region 15 from the light incident surface 10a and forms a pn junction with the semiconductor region 15. In a so-called PonN type configuration in which the p-type semiconductor region is formed on the n-type semiconductor region, the amount of light absorption in the p-type semiconductor region is likely to be small. In this regard, in the photodetector 1, the plasmon structure 20 diffracts the incident light, thereby increasing the optical path length in the p-type semiconductor region 15, and as a result, the amount of light absorption can be increased to improve the quantum efficiency. In the photodetector 1, each unit structure 21 constituting the plasmon structure 20 has a top surface 21a, a bottom surface 21b, and a side surface 21c, and the height H of each unit structure 21 is 100 nm or more and 250 nm or less. This allows localized surface plasmon resonance to occur in the plasmon structure 20, and allows the incident light to be suitably diffracted. As a result, the above-mentioned effect of increasing the amount of light absorption and improving the quantum efficiency can be significantly achieved. Therefore, according to the light detection element 1, the quantum efficiency can be improved. As described above, in the first example, the localized surface plasmon resonance can be generated when the height H is in the range of 100 nm to 150 nm, in the second example, when the angle θ is 80.5°, the localized surface plasmon resonance can be generated when the height H is in the range of 125 nm to 210 nm, and in the second example, when the angle θ is 77.5°, the localized surface plasmon resonance can be generated when the height H is in the range of 150 nm to 230 nm. Therefore, if the angle θ is further reduced, it is considered that the range of the height H for generating the localized surface plasmon resonance will become higher. In addition, when the unit structure 21 is formed by lift-off, for example, the height H may be about 250 nm as the manufacturing limit. In addition, since the light absorption in the plasmon structure 20 may become large if the height H is too large, the height H may be set to 250 nm or less.

[0078] 61 and 62 are graphs showing the relationship between the wavelength and the absorptance, transmittance, and reflectance when the angle θ is 71.6° (Atan(3.0)) and the period P is 520 nm in the second example. FIG. 61(a), FIG. 61(b), and FIG. 62 are graphs when the height H is 230 nm, 240 nm, and 250 nm, respectively. From FIG. 61 and FIG. 62, it can be seen that when the angle θ is 71.6°, in the range of the height H of 230 nm to 250 nm, the reflectance at the target wavelength is lower than that in the case where the plasmon structure 20 is not provided, and the localized surface plasmon resonance can be generated in the plasmon structure 20. In addition, in light of this result and the above-mentioned result, it can be seen that in the second example in which the side surface 21c is inclined, the localized surface plasmon resonance can be generated in the range of the height H of 125 nm to 250 nm, and the incident light can be suitably diffracted.

[0079] 3, the side surface 21c of each unit structure 21 is formed perpendicular to the light incident surface 10a, and the height H of each unit structure 21 may be 100 nm or more and 150 nm or less. In this case, localized surface plasmon resonance can be generated, and the incident light can be suitably diffracted.

[0080] In the second example shown in Fig. 4, the side surface 21c of each unit structure 21 is inclined so as to widen as it approaches the bottom surface 21b. This makes it possible to widen the wavelength range in which surface plasmon resonance occurs. In addition, for example, if a shape defect occurs at the boundary between the top surface 21a and the side surface 21c during the manufacturing process (for example, if a corner is chipped or rounded), the desired function may not be achieved. However, when the side surface 21c is inclined, the effect of such a shape defect can be suppressed compared to, for example, when the side surface 21c is formed vertically.

[0081] In the second example, the height H of each unit structure 21 may be 125 nm or more and 250 nm or less. In this case, localized surface plasmon resonance can be generated, and incident light can be suitably diffracted.

[0082] In the light detection element 1, the top surface 21a and the side surface 21c of each unit structure 21 are connected to each other via the curved surface 21d. This makes the plasmon structure 20 less likely to break even when subjected to an external force, and improves the stability of the light detection element 1.

[0083] A silicon dioxide layer 31 is formed between the plasmon structure 20 and the semiconductor region 15. Thus, by adjusting the thickness of the silicon dioxide layer 31, surface plasmon resonance can be generated in a desired wavelength range.

[0084] An adhesion layer 32 made of a metal material is formed between the plasmon structure 20 and the silicon dioxide layer 31. This makes it possible to increase the bonding strength between the plasmon structure 20 and the silicon dioxide layer 31.

[0085] In the APD 10, a trench 17 that reflects light diffracted by the plasmonic structure 20 is formed so as to surround the semiconductor region 15 in a plan view. Thus, by reflecting light by the trench 17, the optical path length in the semiconductor region 15 can be further increased.

[0086] In the above-described first modified example, in a plan view, a plurality of unit structures 21 are arranged along the X direction (first direction), and each unit structure 21 has an elongated shape in the Y direction (second direction) perpendicular to the X direction. In this case, for example, one of the P-polarized light and the S-polarized light can be diffracted by the plasmon structure 20, while the other light can be reflected by the plasmon structure 20.

[0087] The plasmon structure 20 (unit structures 21) is made of a metal material, which allows the plasmon structure 20 to function as a wavelength filter that transmits only light of a specific wavelength.

[0088] In the above example, the plasmon structure 20 is configured so that the second-order diffracted light travels at an angle of 70° or more and less than 90° with respect to the Z direction (direction perpendicular to the light incident surface 10a). This makes it possible to ensure the size of the plasmon structure 20 and the manufacturing precision, as compared to the case where the first-order diffracted light is used, for example.

[0089] In the second modified example, the plasmonic structure 20 is covered with a protective layer 50, and the protective layer 50 is inserted between adjacent unit structures 21. This makes it possible to increase the physical durability and chemical durability of the plasmonic structure 20, and also makes it possible to further provide a predetermined optical element on the protective layer 50.

[0090] The thickness of the silicon dioxide layer 31 is not less than 1 nm and not more than 5 nm. This significantly exhibits the above-mentioned effect that the thickness of the silicon dioxide layer 31 can be adjusted to cause surface plasmon resonance in a desired wavelength range.

[0091] The adhesion layer 32 is a titanium layer, which can improve the light absorption rate.

[0092] The present invention is not limited to the above-mentioned embodiment and modified examples. For example, the material and shape of each component are not limited to the above-mentioned material and shape, and various materials and shapes can be adopted. The silicon dioxide layer 31 may be omitted, and the plasmonic structure 20 may be formed directly on the light incident surface 10a. The trench 17 may be omitted. The curved surface 21d may be omitted, and a sharp corner may be formed at the boundary between the top surface 21a and the side surface 21c.

[0093] In the above-mentioned example, the plasmon structure 20 is configured so that the second-order diffracted light (positive and negative second-order diffracted light) travels at an angle of 70° or more and less than 90° with respect to the Z direction, but the first-order diffracted light (positive and negative first-order diffracted light) may travel at an angle of 70° or more and less than 90° with respect to the Z direction. That is, the first-order diffracted light may be used instead of the second-order diffracted light. However, when the first-order diffracted light is used, the size of the plasmon structure 20 needs to be reduced, which may decrease the manufacturing accuracy. In other words, when the second-order diffracted light is used, the size of the plasmon structure 20 can be secured, and the manufacturing accuracy can be increased.

[0094] In the above embodiment and each modified example, the APD 10 may be configured as a NonP type. That is, similar to the structure shown in FIG. 5(b), in the APD 10, the semiconductor region 13 may be p-type, the semiconductor region 14 may be p-type, the semiconductor region 15 (first semiconductor region) may be n-type, and the semiconductor region 16 (second semiconductor region) may be p-type. That is, the APD 10 may be a PonN type or a NonP type. As described above, when the PonN type is adopted, the time resolution and the sensitivity can be improved. On the other hand, when the plasmon structure 20 is provided in the NonP type, the optical path length in the light absorption region can be increased compared to when the plasmon structure 20 is not provided in the NonP type, and the amount of light absorption can be increased. Specifically, it can be expressed in the form of L=Leff*sin(90°-θ). L is the optical path length in the Si film thickness direction, Leff is the effective optical path length, and θ is the diffraction angle. Moreover, the NonP type structure can be suitably used when the light diffracted by the plasmon structure 20 travels at, for example, 70° to 80° with respect to the Z direction. The reason for this will be described below. The diffraction angle at the plasmon structure 20 (the angle at which the light diffracted by the plasmon structure 20 travels) is determined according to the period P and the wavelength of the light L. For example, when the period P is 260 nm and the wavelength is 905 nm (effective wavelength is 905 nm / 3.6 (refractive index of Si)=251 nm), the diffraction angle is 75°. When the diffraction angle is small like this, in the case of the PonN type, the light absorption layer (semiconductor layer 41 in FIG. 5(a)) made of a p-layer is extremely thin at several hundreds of nm, so that the light may not be sufficiently absorbed. On the other hand, in the case of the NonP type, the light absorption layer (semiconductor layer 44 in FIG. 5(b) and the p-type semiconductor layer on the substrate side) made of a p-layer has a thickness of 1 μm or more, so that the light can be sufficiently absorbed. In this way, when the diffraction angle is large, for example, 70° to 80°, the NonP type structure can be suitably used. In the photodetector 1 of the embodiment, the detection sensitivity can be effectively improved by adjusting the configuration of the plasmon structure 20 according to the configuration of the APD 10 and the wavelength of the detection target. [Explanation of symbols]

[0095] 1...photodetection element, 10...avalanche photodiode, 10a...light incident surface, 15...semiconductor region (first semiconductor region), 16...semiconductor region (second semiconductor region), 17...trench, 20...plasmon structure, 21...unit structure, 21a...top surface, 21b...bottom surface, 21c...side surface, 21d...curved surface, 31...silicon dioxide layer, 32...adhesion layer, 50...protective layer, L...light.

Claims

1. An avalanche photodiode having a light incident surface on which light is incident, and a plasmon structure portion formed on the light incident surface and diffracting the light by surface plasmon resonance, the avalanche photodiode has a p-type first semiconductor region and an n-type second semiconductor region formed on the opposite side of the first semiconductor region from the light incident surface and forming a p-n junction with the first semiconductor region; the plasmon structure has a plurality of unit structures arranged on the first semiconductor region, Each of the plurality of unit structures includes a top surface opposite to the light incident surface, a bottom surface facing the light incident surface, and a side surface connected to the top surface and the bottom surface, A photodetector element, wherein the height of each of the plurality of unit structures is 100 nm or more and 250 nm or less.

2. the side surface of each of the plurality of unit structures is formed perpendicular to the light incident surface, The light detection element according to claim 1 , wherein the height of each of the plurality of unit structures is not less than 100 nm and not more than 150 nm.

3. The light-detecting element according to claim 1 , wherein the side surfaces of each of the unit structures are inclined so as to widen toward the bottom surface.

4. The light detection element according to claim 3 , wherein the height of each of the plurality of unit structures is not less than 125 nm and not more than 250 nm.

5. The light detection element according to claim 1 , wherein in each of the plurality of unit structures, the top surface and the side surface are connected to each other via a curved surface.

6. The light detection element according to claim 1 , further comprising a silicon dioxide layer formed between the plasmon structure and the first semiconductor region.

7. The light detection element according to claim 6 , further comprising an adhesion layer made of a metal material formed between the plasmon structure and the silicon dioxide layer.

8. 3. The photodetection element according to claim 1, wherein a trench that reflects the light diffracted by the plasmonic structure is formed in the avalanche photodiode so as to surround the first semiconductor region when viewed from a direction perpendicular to the light incident surface.

9. 3. The light detection element according to claim 1, wherein when viewed from a direction perpendicular to the light incident surface, the plurality of unit structures are arranged along a first direction, and each of the plurality of unit structures has an elongated shape in a second direction perpendicular to the first direction.

10. The light detection element according to claim 1 , wherein the plasmon structure is configured so that second-order diffracted light travels at an angle of 70° or more and less than 90° with respect to a direction perpendicular to the light incident surface.

11. The light detection element according to claim 1 , wherein the plasmon structure is covered with a protective layer, and the protective layer is embedded between adjacent ones of the unit structures.

12. The photodetector according to claim 6 , wherein the silicon dioxide layer has a thickness of 1 nm or more and 5 nm or less.

13. The light-detecting element according to claim 7 , wherein the adhesion layer is a titanium layer.

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