Photonic crystal surface emitting laser

Abstract

To provide a photonic crystal surface emitting laser capable of oscillating in a single mode.SOLUTION: A first semiconductor layer, an active layer, a photonic crystal layer laminated on the active layer, a second semiconductor layer provided on a surface of the active layer opposite to the first semiconductor layer, a first electrode 26 having an opening provided on a surface of the first semiconductor layer opposite to the active layer, a second electrode provided on a surface of the second semiconductor layer opposite to the active layer, and a dielectric film, in which the second electrode overlaps the opening in a lamination direction, in one of the central portion and the outer peripheral portion of the second electrode, a dielectric film is provided between a contact portion where the second electrode is in contact with the second semiconductor layer and the second semiconductor layer, and in the other of the non-contact portion where the second electrode is not in contact with the second semiconductor layer, the central portion 40, and the outer peripheral portion 42 of the second electrode, the dielectric film is not provided between the second electrode and the second semiconductor layer, and the second electrode is in contact with the second semiconductor layer.SELECTED DRAWING: Figure 3B

Description

[Technical Field]

[0001] The present disclosure relates to photonic crystal surface-emitting lasers. [Background technology]

[0002] Photonic-crystal surface-emitting lasers (PCSELs) are known, which are made by stacking a photonic crystal and an active layer with optical gain. Techniques for operating PCSELs in a single mode have been studied (see, for example, Non-Patent Documents 1 and 2). [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] Ryohei Morita et.al. “Photonic-crystal lasers with two-dimensionally arranged gain and loss sections for high-peak-power short-pulse operation”, NATURE PHOTONICS VOL.15, 311-318 (April 2021) [Non-patent document 2] Eiji Miyai et al., “Current Profile Control for Improving Robustness of Single-Mode Operation in Photonic Crystal Lasers,” Proceedings of the 83rd Autumn Meeting of the Japan Society of Applied Physics, 21a-A101-7 (2022) Summary of the Invention [Problem to be solved by the invention]

[0004] However, there is a risk that the threshold current and device resistance will increase, resulting in a decrease in efficiency. Therefore, an object of the present invention is to provide a photonic crystal surface-emitting laser capable of oscillating in a single mode. [Means for solving the problem]

[0005] A photonic crystal surface-emitting laser according to the present disclosure includes a first semiconductor layer, an active layer provided on one surface of the first semiconductor layer, a photonic crystal layer stacked on the active layer, a second semiconductor layer provided on the surface of the active layer opposite to the first semiconductor layer, a first electrode provided on the surface of the first semiconductor layer opposite to the active layer, a second electrode provided on the surface of the second semiconductor layer opposite to the active layer, and a dielectric film, wherein the photonic crystal layer has a first region and a plurality of second regions having a refractive index different from that of the first region, and the first electrode has an opening, In the direction in which the photonic crystal layer and the second semiconductor layer are stacked, the second electrode overlaps the opening, and one of the central and peripheral portions of the second electrode has a contact portion and a non-contact portion, where the second electrode contacts the second semiconductor layer at the contact portion, the dielectric film is provided between the second electrode and the second semiconductor layer at the non-contact portion, and the second electrode does not contact the second semiconductor layer, and where the other of the central and peripheral portions of the second electrode does not provide the dielectric film between the second electrode and the second semiconductor layer, and the second electrode contacts the second semiconductor layer. [Effects of the Invention]

[0006] According to the present disclosure, it is possible to provide a photonic crystal surface-emitting laser capable of oscillating in a single mode. [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 1 is a cross-sectional view illustrating a photonic crystal surface-emitting laser according to the first embodiment. [Figure 2A] FIG. 2A is an enlarged cross-sectional view of the photonic crystal layer. [Figure 2B] FIG. 2B is a plan view illustrating the photonic crystal layer. [Figure 3A] FIG. 3A is a bottom view illustrating a photonic crystal surface-emitting laser. [Figure 3B]FIG. 3B is a top view illustrating a photonic crystal surface-emitting laser. [Figure 4A] FIG. 4A is a cross-sectional view illustrating the center of an electrode. [Figure 4B] FIG. 4B is a plan view illustrating the outer periphery of the electrode. [Figure 4C] FIG. 4C is a cross-sectional view illustrating the outer periphery. [Figure 5A] FIG. 5A is a schematic diagram illustrating the distribution of light. [Figure 5B] FIG. 5B is a schematic diagram illustrating the distribution of light. [Figure 6A] FIG. 6A is a diagram illustrating the threshold gain. [Figure 6B] FIG. 6B is a diagram illustrating the threshold current. [Figure 7] FIG. 7 is a diagram illustrating the light intensity. [Figure 8A] FIG. 8A is a cross-sectional view illustrating a method for manufacturing a photonic crystal surface-emitting laser. [Figure 8B] FIG. 8B is a cross-sectional view illustrating a method for manufacturing a photonic crystal surface-emitting laser. [Figure 8C] FIG. 8C is a cross-sectional view illustrating a method for manufacturing a photonic crystal surface-emitting laser. [Figure 9] FIG. 9 is a top view illustrating the photonic crystal surface emitting laser according to the second embodiment. [Figure 10A] FIG. 10A is a cross-sectional view illustrating the center of an electrode. [Figure 10B] FIG. 10B is a cross-sectional view illustrating the outer periphery of the electrode. DETAILED DESCRIPTION OF THE INVENTION

[0008] [Description of the embodiments of the present disclosure] First, the contents of the embodiments of the present disclosure will be listed and described.

[0009] One aspect of the present disclosure provides a semiconductor device comprising: (1) a first semiconductor layer; an active layer provided on one surface of the first semiconductor layer; a photonic crystal layer stacked on the active layer; a second semiconductor layer provided on a surface of the active layer opposite the first semiconductor layer; a first electrode provided on a surface of the first semiconductor layer opposite the active layer; a second electrode provided on a surface of the second semiconductor layer opposite the active layer; and a dielectric film, wherein the photonic crystal layer has a first region and a plurality of second regions having a refractive index different from that of the first region, the first electrode has an opening, and the first semiconductor layer, the active layer, the photonic crystal layer, and and the second semiconductor layer are stacked, the second electrode overlaps the opening, and one of the central and peripheral portions of the second electrode has a contact portion and a non-contact portion, the second electrode is in contact with the second semiconductor layer at the contact portion, the dielectric film is provided between the second electrode and the second semiconductor layer at the non-contact portion, the second electrode is not in contact with the second semiconductor layer, and the other of the central and peripheral portions of the second electrode does not have the dielectric film provided between the second electrode and the second semiconductor layer, and the second electrode is in contact with the second semiconductor layer. In the photonic crystal surface-emitting laser, the threshold gain of the central portion is lower than the threshold gain of the peripheral portion. The fundamental mode is easily oscillated, and higher-order modes are difficult to oscillate. The photonic crystal surface-emitting laser is capable of oscillating in a single mode. (2) In the above (1), the reflection phase at the central portion of the second electrode may differ from the reflection phase at the peripheral portion by π / 2 or more and 3π / 2 or less. This increases the gain difference between the central portion and the peripheral portion of the photonic crystal surface-emitting laser. This allows for single-mode oscillation. (3) In the above (1) or (2), when the length of the second electrode is L, the width of the outer circumferential portion of the second electrode may be L / 4 or less. By increasing the gain difference, higher-order modes are suppressed, and it becomes easier to oscillate in the fundamental mode. (4) In any of the above (1) to (3), the ratio of the area of ​​the contact portion to the area of ​​the non-contact portion may be 5% or more and 30% or less. This can increase the threshold gain of the outer periphery of the photonic crystal surface-emitting laser. It can also suppress an increase in contact resistance. (5) In any one of the above (1) to (4), the plurality of contact portions may be periodically arranged, which makes it possible to make the current more uniform. (6) In any one of the above (1) to (5), the second electrode may have a circular planar shape, and the peripheral portion of the second electrode may have an annular planar shape. Since the threshold gain of the central portion of the photonic crystal surface-emitting laser is lower than the threshold gain of the peripheral portion, single-mode oscillation is possible. (7) In any of (1) to (6) above, the outer periphery of the second electrode may have the contact portion and the non-contact portion, and the dielectric film may not be provided between the second electrode and the second semiconductor layer in the central portion of the second electrode, and the second electrode may be in contact with the second semiconductor layer. Single-mode oscillation is possible. Contact resistance can be reduced. (8) In any of the above (1) to (6), the central portion of the second electrode may have the contact portion and the non-contact portion, and the dielectric film may not be provided between the second electrode and the second semiconductor layer at the outer periphery of the second electrode, and the second electrode may be in contact with the second semiconductor layer. Single-mode oscillation is possible. (9) In the above (1) to (8), the second semiconductor layer may include a cladding layer and a contact layer, and the cladding layer and the contact layer may be stacked in this order between the active layer and the second electrode. A voltage may be applied between the first electrode and the second electrode to inject carriers into the active layer.

[0010] [Details of the embodiments of the present disclosure] Specific examples of photonic crystal surface-emitting lasers according to embodiments of the present disclosure will be described below with reference to the drawings. Note that the present disclosure is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims.

[0011] First Embodiment (Photonic crystal surface-emitting laser) Fig. 1 is a cross-sectional view illustrating a photonic crystal surface-emitting laser 100 according to a first embodiment. As shown in Fig. 1, the photonic crystal surface-emitting laser (PCSEL) 100 includes a substrate 10, a cladding layer 12 (first semiconductor layer), a photonic crystal layer 14, a cladding layer 16, an active layer 18, a cladding layer 20 (second semiconductor layer), a contact layer 22 (second semiconductor layer), an electrode 24 (first electrode), and an electrode 26 (second electrode). Although not shown in Fig. 1, the photonic crystal surface-emitting laser 100 includes a dielectric film 50.

[0012] The semiconductor layers are stacked along the Z axis. A cladding layer 12, a photonic crystal layer 14, a cladding layer 16, an active layer 18, a cladding layer 20, and a contact layer 22 are stacked in this order on a substrate 10. The surfaces of each layer are parallel to the XY plane. The X, Y, and Z axes are perpendicular to one another. An electrode 24 is electrically connected to the substrate 10. An electrode 26 is electrically connected to the contact layer 22.

[0013] The substrate 10, the cladding layer 12, and the cladding layer 16 are formed of, for example, n-type indium phosphide (n-InP). The n-type dopant is, for example, silicon (Si). The thickness of the cladding layer 12 is, for example, 500 nm. The thickness of the cladding layer 16 is, for example, 100 nm.

[0014] Photonic crystal layer 14 is made of, for example, n-type indium gallium arsenide phosphide (InGaAsP) or aluminum indium gallium arsenide (AlInGaAs), and has a thickness of, for example, 300 nm.

[0015] The active layer 18 includes multiple well layers and barrier layers, and has a multi-quantum well (MQW) structure. The well layers and barrier layers are formed of, for example, undoped indium gallium arsenide phosphide (InGaAsP) or aluminum gallium indium arsenide (AlGaInAs). The above materials are examples, and each layer may be formed of other materials or a combination of the above materials with other materials.

[0016] The cladding layer 20 is made of, for example, p-type indium phosphide (p-InP) with a thickness of 3 μm. The contact layer 22 is made of, for example, p-type indium gallium arsenide (p-InGaAs) with a thickness of 300 nm. The p-type dopant is, for example, zinc (Zn).

[0017] The refractive index of active layer 18 is, for example, 3.5. The refractive index of the InP cladding layer is, for example, 3.2. The refractive index of InGaAsP, which is the base material of photonic crystal layer 14, is higher than the refractive index of the cladding layer, for example, 3.4.

[0018] FIG. 2A is an enlarged cross-sectional view of photonic crystal layer 14. FIG. 2B is a plan view illustrating photonic crystal layer 14. Photonic crystal layer 14 has a base material 30 (first region), voids 32 (second region), and voids 34 (second region). Base material 30 is an InGaAsP layer or the like, as described above. A plurality of voids 32 and a plurality of voids 34 are provided in base material 30.

[0019] As shown in FIG. 2A , the air holes 32 and 34 extend in the Z-axis direction. One end of each of the air holes 32 and 34 is located on one surface of the photonic crystal layer 14. The other end of each of the air holes 32 and 34 is located midway through the photonic crystal layer 14. The air holes 32 and 34 may extend to the cladding layer 12, for example. The air holes 32 are longer than the air holes 34. The interiors of the air holes 32 and 34 are filled with air. The refractive index of the air holes 32 and 34 is different from the refractive index of the base material 30. The refractive index varies periodically within the plane of the photonic crystal layer 14.

[0020] As shown in FIG. 2B, the plurality of voids 32 are arranged in a square lattice pattern. The plurality of voids 34 are arranged in a square lattice pattern. The plurality of voids 32 are arranged periodically in the X-axis direction and the Y-axis direction. The lattice constant of the square lattice is 400 nm. That is, the distance between adjacent voids 32 and the distance between adjacent voids 34 are 400 nm. The plurality of voids 34 are arranged circumferentially. The planar shape of the voids 32 is elliptical. The major and minor axes of the voids 32 are inclined from the direction in which the plurality of voids 32 are arranged. The planar shape of the voids 34 is circular.

[0021] FIG. 3A is a bottom view illustrating a photonic crystal surface-emitting laser 100. As shown in FIG. 3A, an opening 25 is provided in the electrode 24. The opening 25 has a circular planar shape. The diameter D1 of the opening 25 is, for example, 340 μm. The opening 25 penetrates the electrode 24. The substrate 10 is exposed through the opening 25. The opening 25 functions as an aperture for emitting light. The lengths L1 and L2 of one side of the photonic crystal surface-emitting laser 100 are, for example, 1000 μm.

[0022] The electrode 24 is an n-type electrode and is in contact with the surface of the substrate 10. The electrode 24 is made of metal, and is formed, for example, by stacking nickel (Ni), germanium (Ge), and gold (Au) from the side closest to the substrate 10.

[0023] 3B is a top view illustrating the photonic crystal surface-emitting laser 100. The planar shape of the electrode 26 is circular. The electrode 26 is a p-type electrode, and is formed, for example, by stacking titanium (Ti), platinum (Pt), and gold (Au) from the side closest to the contact layer 22. The center of the electrode 26 in the XY plane is designated by C.

[0024] The electrode 26 has a central portion 40 and an outer peripheral portion 42. The central portion 40 is located at the center of the electrode 26. The outer peripheral portion 42 is the shaded portion in FIG. 3B, and is located outside the central portion 40 and surrounds it. The planar shape of the central portion 40 is circular. The planar shape of the outer peripheral portion 42 is annular. The diameter L of the electrode 26 is, for example, 200 μm or more and 300 μm or less. The width W of the outer peripheral portion 42 is, for example, about 10% of the diameter L, and may be 20 μm or more and 30 μm or less. The diameter D2 of the central portion 40 is equal to L-2W.

[0025] 4A is a cross-sectional view illustrating a central portion 40 of the electrode 26, illustrating the area from the cladding layer 20 to the electrode 26. As shown in FIG. 4A, the central portion 40 of the electrode 26 has a solid structure. In the central portion 40, no dielectric film 50 is provided between the electrode 26 and the contact layer 22. The entire surface 26a of the electrode 26 is in contact with the contact layer 22.

[0026] 4B is a plan view illustrating an example of the outer peripheral portion 42 of the electrode 26. As shown in FIG. 4B, the outer peripheral portion 42 has a mesh structure and includes contact portions 44 and non-contact portions 46. The mesh structure means that a plurality of contact portions 44 are arranged. The non-contact portions 46 are located around the contact portions 44.

[0027] The planar shape of the contact portion 44 is rectangular. The length L3 of one side of the contact portion 44 is, for example, 1.6 μm. The multiple contact portions 44 may be arranged periodically or randomly. In the example of FIG. 4B, the multiple contact portions 44 are arranged periodically in the X-axis direction and the Y-axis direction. The distance (pitch) L4 between corresponding sides of adjacent contact portions 44 is, for example, 5.0 μm. The area sufficiency (Filling Factor: FF) of the contact portion 44 is calculated by the following formula (1) and is, for example, 5% or more and 30% or less. FF=(L3) 2 / (L4) 2 (1)

[0028] 4C is a cross-sectional view illustrating the outer peripheral portion 42, taken along line AA in FIG. 4B. As shown in FIG. 4C, in the outer peripheral portion 42, a dielectric film 50 is provided between the electrode 26 and the contact layer 22. The dielectric film 50 is made of an insulator such as silicon nitride (SiN). The thickness of the dielectric film 50 is designated as T1.

[0029] In the contact portion 44, an opening 52 is provided in the dielectric film 50. The opening 52 penetrates the dielectric film 50 in the Z-axis direction. The electrode 26 is provided within the opening 52. A surface 26b of the electrode 26 contacts the contact layer 22. In the non-contact portion 46, the dielectric film 50 does not have an opening 52. The electrode 26 in the non-contact portion 46 is provided on the upper surface of the dielectric film 50 and does not contact the contact layer 22. A surface 26c of the electrode 26 contacts the dielectric film 50.

[0030] The operation of the photonic crystal surface-emitting laser 100 will now be described. A voltage is applied to the photonic crystal surface-emitting laser 100 through the electrodes 24 and 26. Light is generated by injecting carriers into the active layer 18. The light is diffracted within the plane of the photonic crystal layer 14. A resonator is formed between the electrode 26 and the photonic crystal layer 14. The electrode 26 functions as one mirror of the resonator of the photonic crystal surface-emitting laser 100. The photonic crystal layer 14 functions as the other mirror of the resonator of the photonic crystal surface-emitting laser 100. Light resonates between the electrode 26 and the photonic crystal layer 14. Light having a wavelength corresponding to the period of the air holes 32 and 34 is amplified and laser oscillation occurs. The wavelength of the laser light is in the 1.3 μm band or the 1.55 μm band, for example.

[0031] The laser light is emitted in the Z-axis direction. Light propagating downward in FIG. 1 is emitted from opening 25 in electrode 24. Light propagating upward is reflected by the lower surface of electrode 26, propagates downward, and is emitted from opening 25.

[0032] 5A and 5B are schematic diagrams illustrating the distribution of light. FIG. 5A shows the fundamental mode. FIG. 5B shows the higher-order modes. The light is shown by dotted lines. As shown in FIGS. 5A and 5B, the fundamental mode and the higher-order modes are generated. As shown in FIG. 5A, the fundamental mode is strongly distributed in the central portion 40. As shown in FIG. 5B, the higher-order modes spread to the outer periphery 42.

[0033] To achieve single-mode laser oscillation, it is sufficient to oscillate in the fundamental mode and suppress oscillation in higher-order modes. By lowering the threshold gain of the resonator in the lower part of the central portion 40, oscillation in the fundamental mode becomes easier. By making the threshold gain in the lower part of the peripheral portion 42 higher than the threshold gain in the lower part of the central portion 40, oscillation in higher-order modes becomes more difficult. As shown in FIG. 3B, by having the central portion 40 as a solid structure and the peripheral portion 42 as a mesh structure, the threshold gain can be changed between the central portion 40 and the peripheral portion 42.

[0034] The threshold gain gth is expressed by the following equation (2). gth=(1+(√R)cosθ)α1+α2+α3 (2) R is the reflectivity of electrode 26 for light. α1 is the radiation coefficient of the oscillation band. α2 is the loss in the in-plane direction. α3 is the internal loss. θ is the phase difference (reflection phase) between the outgoing light emitted from photonic crystal layer 14 toward electrode 26 and the reflected light reflected by electrode 26.

[0035] As shown in FIG. 4A, in the central portion 40, surface 26a of electrode 26 is in contact with contact layer 22. Light that has passed through contact layer 22 is reflected by surface 26a. As shown in FIG. 4C, in the peripheral portion 42, electrode 26 has surfaces 26b and 26c in peripheral portion 42. Surface 26b is in contact with contact layer 22. Light that has passed through contact layer 22 is reflected by surface 26b. Surface 26c is provided on dielectric film 50. Light that has passed through contact layer 22 and dielectric film 50 is reflected by surface 26c.

[0036] Due to the influence of the thickness and refractive index of the dielectric film 50, the optical path length in the non-contact portion 46 differs from the optical path length in the contact portion 44 and the central portion 40. The change in the optical path length changes the phase of the reflected light, and the phase difference θ also changes. The threshold gain gth of the resonator in the lower portion of the central portion 40 can be made different from the threshold gain gth of the resonator in the lower portion of the peripheral portion 42.

[0037] FIG. 6A is a diagram illustrating the threshold gain. The horizontal axis represents the reflection phase, and the vertical axis represents the threshold gain. The threshold gain gth is calculated using the above-mentioned formula (2). The threshold gain gth changes periodically depending on the reflection phase θ. When the reflection phase θ is π and 3π, the threshold gain gth has a minimum value. When the reflection phase θ is 0 and 2π, the threshold gain gth has a maximum value. The minimum value is approximately 16 cm. -1 The maximum value is about 33 cm -1 is.

[0038] FIG. 6B illustrates the threshold current. The horizontal axis represents the thickness of the dielectric film 50. The right vertical axis and circles represent the reflection phase. The left vertical axis represents the threshold current. The triangles represent the threshold current of a mesh structure. The dashed line represents the threshold current of a solid structure. The thickness T1 of the dielectric film 50 is varied from 0 nm to 600 nm, and the reflection phase and threshold current are simulated. When T1 = 0 nm, no dielectric film 50 is provided between the contact layer 22 and the electrode 26, corresponding to the central portion 40 of the solid structure. When T1 is finite, a dielectric film 50 is provided between the contact layer 22 and the electrode 26, corresponding to the outer periphery 42 of the mesh structure.

[0039] The threshold current Ith and reflection phase θ change periodically depending on the thickness T1. As shown by the dashed line in Figure 6B, when T1 = 0 nm, the threshold current is approximately 180 mA. When the thickness T1 is approximately 250 nm, the reflection phase is π and the threshold current Ith reaches a maximum value. When the thickness T1 is around 0 nm and approximately 400 nm, the reflection phase is close to 0 and the threshold current Ith reaches a minimum value. The maximum value of the threshold current Ith is approximately 300 mA. The minimum value is approximately 170 mA.

[0040] FIG. 7 is a diagram illustrating light intensity. The horizontal axis represents the position within the plane of the photonic crystal surface-emitting laser 100. 0 on the horizontal axis represents the center C of the photonic crystal surface-emitting laser 100. The diameter L of the electrode 26 is 200 μm. The width of the outer peripheral portion 42 is 30 μm. The outer peripheral portion 42 extends from -70 μm to -100 μm and from 70 μm to 100 μm. The central portion 40 extends from -70 μm to 70 μm. The thickness of the dielectric film 50 is 200 nm. The vertical axis represents light intensity, which is normalized. The solid line represents the fundamental mode. The dotted line represents the first higher-order mode. The intensity of the fundamental mode is greatest at 0 (the center) and decreases with increasing distance from the center. The intensity of the higher-order mode is greatest at 50 nm and -50 nm and least at 0 and ±100 nm.

[0041] The threshold gain gth of the cavity at the bottom of the central portion 40 is 28 cm -1 The threshold gain gth of the resonator at the bottom of the outer peripheral portion 42 is set to 49 cm -1 The threshold gain for each mode is calculated by weighting the threshold gain according to the optical intensity of the fundamental mode and the higher-order mode. The threshold gain for the higher-order mode is 33.6 cm -1 The threshold gain of the fundamental mode is lower than that of the higher modes, at 31 cm -1 The fundamental mode is easily oscillated, and oscillation of higher modes can be suppressed.

[0042] (Manufacturing method) 8A to 8C are cross-sectional views illustrating a method for manufacturing photonic crystal surface-emitting laser 100. As shown in Fig. 8A, cladding layer 12 and photonic crystal layer 14 are epitaxially grown in this order on substrate 10, for example, by metal organic chemical vapor deposition (MOCVD). In this step, base material 30 (InGaAsP) of photonic crystal layer 14 is formed, but no voids are formed.

[0043] A mask (not shown) is provided on the upper surface of the photonic crystal layer 14. The mask is made of an insulator such as SiN. An insulating film is formed on the upper surface of the photonic crystal layer 14. A resist pattern is formed using an electron beam (EB) or the like, and the resist pattern is transferred to the insulating film to form the mask. The upper surface of the base material 30 is exposed through the openings in the mask. Air holes 32 and 34 are formed in the photonic crystal layer 14 by reactive ion etching (RIE) or the like. The etching proceeds, for example, halfway through the photonic crystal layer 14, but does not reach the lower surface of the photonic crystal layer 14. The planar shapes of the air holes 32 and 34 are determined by the planar shape of the openings in the mask. For example, elliptical air holes 32 and circular air holes 34 are formed as shown in FIG. 2B. After etching is completed, the mask is removed.

[0044] As shown in Figure 8B, cladding layer 16, active layer 18, cladding layer 20, and contact layer 22 are epitaxially grown on photonic crystal layer 14. Air holes 32 and 34 are blocked by cladding layer 16. The inside of the air holes is not filled with cladding layer 16, leaving a cavity. Active layer 18, cladding layer 20, and contact layer 22 are epitaxially grown on flat cladding layer 16.

[0045] 8C omits the cladding layer 20 to the substrate 10 and illustrates the contact layer 22 and above, showing the portion corresponding to the peripheral portion 42. In the peripheral portion 42, a dielectric film 50 is formed on the upper surface of the contact layer 22 by, for example, plasma enhanced CVD (PECVD). A plurality of openings 52 are formed in the dielectric film 50 by etching or the like. In the central portion 40, the contact layer 22 is exposed.

[0046] The electrode 26 is formed by vapor deposition and lift-off. For example, a Ti layer, a Pt layer, and an Au layer are laminated in this order. The electrode 26 contacts the upper surface of the contact layer 22 in the central portion 40. The electrode 26 is provided on the upper surface of the dielectric film 50 in the peripheral portion 42, and contacts the upper surface of the contact layer 22 inside the opening 52. An electrode 24 is provided on the lower surface of the substrate 10, as shown in FIG. 1, and an opening 25 is formed. For example, heat treatment is performed at a temperature of 300°C or higher to establish contact between the electrode and the semiconductor. Through the above steps, the photonic crystal surface-emitting laser 100 is formed.

[0047] According to the first embodiment, the central portion 40 of the electrode 26 has a solid structure and is in contact with the contact layer 22. The peripheral portion 42 has a mesh structure and includes a contact portion 44 and a non-contact portion 46. At the contact portion 44, the electrode 26 is in contact with the contact layer 22. At the non-contact portion 46, a dielectric film 50 is provided between the electrode 26 and the contact layer 22. The electrode 26 is provided on the dielectric film 50 and is not in contact with the contact layer 22. The threshold gain of the resonator in the lower portion of the central portion 40 is lower than the threshold gain of the resonator in the lower portion of the peripheral portion 42. The fundamental mode is easily oscillated because it is strongly distributed in the central portion 40, where the threshold gain is low. Higher-order modes are difficult to oscillate because they are spread to the peripheral portion 42, where the threshold gain is high. By cutting off higher-order modes, the photonic crystal surface-emitting laser 100 can oscillate in a single mode.

[0048] The thickness T1 of the dielectric film 50 determines the reflection phase θ, as well as the threshold gain gth and threshold current Ith. The thickness T1 is determined so that the threshold gain gth and threshold current Ith are low in the central portion 40 and high in the peripheral portion 42. Because the threshold gain gth and threshold current Ith of the resonator below the central portion 40 are lower than those in the peripheral portion 42, oscillation in the fundamental mode is facilitated and higher modes are cut off. Single-mode oscillation is possible.

[0049] As shown in Figures 6A and 6B, the phase difference increases the difference in threshold gain and threshold current between the central portion 40 and the peripheral portion 42. For example, the reflection phase in the central portion 40 may differ from the reflection phase in the peripheral portion 42 by more than π / 2, less than 3π / 2, or more preferably by π. The threshold gain of the resonator below the central portion 40 approaches a minimum value, and the threshold gain of the resonator below the peripheral portion 42 approaches a maximum value. This increases the gain difference between the central portion 40 and the peripheral portion 42. Oscillation in the fundamental mode is possible.

[0050] For example, the thickness T1 of the dielectric film 50 is set to 200 nm. As shown in FIG. 6B, the difference in reflection phase between the central portion 40 of the solid structure and the peripheral portion 42 of the mesh structure approaches π. The threshold current of the resonator below the peripheral portion 42 is high, and the threshold current of the resonator below the central portion 40 is small. The threshold gain of the fundamental mode is 31 cm. -1 The threshold gain of the higher-order mode is 33.6 cm -1 The gain difference between the fundamental mode and the higher-order mode is 2.6 cm. -1 and the higher modes can be suppressed.

[0051] 3A, the central portion 40 has a solid structure. No dielectric film 50 is provided between the electrode 26 and the contact layer 22. The entire lower surface 26a of the electrode 26 is in contact with the contact layer 22. Because the contact resistance is low, current can be effectively injected, resulting in high efficiency.

[0052] As shown in FIG. 3B , the outer peripheral portion 42 surrounds the central portion 40. If the width of the outer peripheral portion 42 is W and the diameter (length) of the electrode 26 is L, the width W is, for example, L / 4 or less, and may be L / 3 or less, or L / 5 or less. If the outer peripheral portion 42 is wide, the difference between the threshold gain of the fundamental mode and the threshold gain of the higher-order mode becomes small. By setting the width W to L / 4, the outer peripheral portion 42 becomes narrow, and the difference between the threshold gain of the fundamental mode and the threshold gain of the higher-order mode can be increased. This makes the fundamental mode more likely to oscillate. If the outer peripheral portion 42 is too narrow, the higher-order mode will also be distributed in the central portion 40, making the higher-order mode more likely to oscillate. The width W may be L / 10 or more, or L / 20 or more. This makes it possible to suppress the higher-order mode.

[0053] The outer peripheral portion 42 has a contact portion 44 and a non-contact portion 46. If the proportion of the contact portion 44 in the outer peripheral portion 42 is large, the gain difference between the outer peripheral portion 42 and the central portion 40 will be small. If the proportion of the contact portion 44 is small, the contact resistance will increase. The area ratio of the contact portion 44 in the outer peripheral portion 42 is, for example, 5% to 30%, and may be 10% to 20%. This can increase the threshold gain of the resonator in the lower portion of the outer peripheral portion 42. It can also suppress an increase in contact resistance.

[0054] As shown in Figure 4B, the multiple contact portions 44 are arranged periodically. This allows the current injected into the active layer 18 to be more uniform. The multiple contact portions 44 may also be arranged non-periodically. The planar shape of the contact portions 44 may be rectangular, circular, elliptical, or polygonal.

[0055] As shown in FIG. 3B, the planar shape of the electrode 26 is circular. The planar shape of the central portion 40 is also circular. The planar shape of the outer peripheral portion 42 is annular. The fundamental mode is circular and is distributed in the central portion 40, which has a low threshold. The fundamental mode is more likely to oscillate. The planar shapes of the electrode 26 and the central portion 40 may be elliptical or polygonal. The planar shape of the outer peripheral portion 42 is ring-shaped, and may also be an elliptical arc, etc.

[0056] The substrate 10, cladding layer 12, photonic crystal layer 14, and cladding layer 16 have n-type conductivity. The active layer 18 is an undoped layer. The cladding layer 20 and contact layer 22 have p-type conductivity. These layers are stacked to form a pin junction (positive-intrinsic-negative). An electrode 26 is provided on the p-type contact layer 22. Carriers can be injected into the active layer 18 by applying a voltage to electrodes 24 and 26. The conductivity types may be reversed. An n-type layer is provided on one side of the active layer 18, and a p-type layer is provided on the other side.

[0057] Although the photonic crystal layer 14 has two types of holes, it may have one type or three or more types. The planar shape of the holes may be elliptical, circular, or polygonal. Photonic crystal layer 14 has periodically arranged regions having a refractive index different from that of base material 30. These regions may be holes or may be made of a material different from base material 30. Photonic crystal layer 14 may be arranged between cladding layer 12 and active layer 18, or between active layer 18 and cladding layer 20.

[0058] Second Embodiment Fig. 9 is a top view illustrating a photonic crystal surface-emitting laser 200 according to the second embodiment. Fig. 10A is a cross-sectional view illustrating a central portion 40 of an electrode 26. Fig. 10B is a cross-sectional view illustrating a peripheral portion 42 of an electrode 26. The central portion 40 has a mesh structure. The peripheral portion 42 has a solid structure. Descriptions of the same configuration as in the first embodiment will be omitted.

[0059] According to the second embodiment, the outer peripheral portion 42 of the electrode 26 has a solid structure. The central portion 40 has a mesh structure and includes a contact portion 44 and a non-contact portion 46. In the non-contact portion 46, a dielectric film 50 is provided between the electrode 26 and the contact layer 22. By adjusting the thickness of the dielectric film 50, the threshold gain of the resonator in the lower portion of the central portion 40 becomes lower than the threshold gain of the resonator in the lower portion of the outer peripheral portion 42. The fundamental mode is likely to oscillate, and higher-order modes are unlikely to oscillate. By cutting higher-order modes, the photonic crystal surface-emitting laser 200 can oscillate in a single mode.

[0060] The reflection phase in the central portion 40 may differ from the reflection phase in the outer peripheral portion 42 by π / 2 or more, 3π / 2 or less, or more preferably by π. The phase difference allows for differences in threshold gain and threshold current between the central portion 40 and the outer peripheral portion 42. The threshold gain of the resonator in the lower portion of the central portion 40 is close to a minimum value, while the threshold gain of the resonator in the lower portion of the outer peripheral portion 42 is close to a maximum value, thereby effectively cutting off higher-order modes. Oscillation in the fundamental mode is possible.

[0061] Although the embodiments of the present disclosure have been described in detail above, the present disclosure is not limited to such specific embodiments, and various modifications and variations are possible within the scope of the gist of the present disclosure as described in the claims. [Explanation of symbols]

[0062] 10 Substrate 12, 16, 20 cladding layers 14 Photonic crystal layer 18 Active layer 22 Contact layer 24, 26 electrodes 25, 52 opening 26a, 26b, 26c sides 30 Base material 32, 34 Vacancies 40 Central part 42 Outer periphery 44 Contact area 46 Non-contact part 50 Dielectric film 100, 200 Photonic crystal surface-emitting laser

Claims

1. a first semiconductor layer; an active layer provided on one surface of the first semiconductor layer; a photonic crystal layer stacked on the active layer; a second semiconductor layer provided on a surface of the active layer opposite to the first semiconductor layer; a first electrode provided on a surface of the first semiconductor layer opposite to the active layer; a second electrode provided on a surface of the second semiconductor layer opposite to the active layer; a dielectric film; the photonic crystal layer has a first region and a plurality of second regions having a refractive index different from that of the first region; the first electrode has an opening; the second electrode overlaps with the opening in a direction in which the first semiconductor layer, the active layer, the photonic crystal layer, and the second semiconductor layer are stacked; one of the central portion and the outer circumferential portion of the second electrode has a contact portion and a non-contact portion; the second electrode contacts the second semiconductor layer at the contact portion; the dielectric film is provided between the second electrode and the second semiconductor layer in the non-contact portion, and the second electrode is not in contact with the second semiconductor layer; A photonic crystal surface-emitting laser in which the dielectric film is not provided between the second electrode and the second semiconductor layer at the other of the central and peripheral portions of the second electrode, and the second electrode is in contact with the second semiconductor layer.

2. 2. The photonic crystal surface-emitting laser according to claim 1, wherein the reflection phase at the central portion of the second electrode differs from the reflection phase at the peripheral portion by π / 2 or more and 3π / 2 or less.

3. When the length of the second electrode is L, 3. The photonic crystal surface emitting laser according to claim 1, wherein the width of the outer periphery of the second electrode is equal to or less than L / 4.

4. 3. The photonic crystal surface emitting laser according to claim 1, wherein the ratio of the area of ​​the contact portion to the area of ​​the non-contact portion is 5% to 30%.

5. 3. The photonic crystal surface emitting laser according to claim 1, wherein the plurality of contact portions are periodically arranged.

6. the second electrode has a circular planar shape, 3. The photonic crystal surface emitting laser according to claim 1, wherein the outer peripheral portion of the second electrode has a planar shape that is annular.

7. the outer periphery of the second electrode has the contact portion and the non-contact portion, 3. The photonic crystal surface-emitting laser according to claim 1, wherein the dielectric film is not provided between the second electrode and the second semiconductor layer in the central portion of the second electrode, and the second electrode is in contact with the second semiconductor layer.

8. the central portion of the second electrode has the contact portion and the non-contact portion; 3. The photonic crystal surface-emitting laser according to claim 1, wherein the dielectric film is not provided between the second electrode and the second semiconductor layer at the outer periphery of the second electrode, and the second electrode is in contact with the second semiconductor layer.

9. the second semiconductor layer includes a cladding layer and a contact layer; 3. The photonic crystal surface emitting laser according to claim 1, wherein the cladding layer and the contact layer are stacked in this order between the active layer and the second electrode.

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