Photonic crystal surface emitting laser and method of manufacturing the same

Abstract

To provide a photonic crystal surface emitting laser capable of enhancing optical output, and to provide a method of manufacturing the same.SOLUTION: A first semiconductor layer, an active layer stacked on the first semiconductor layer, a second semiconductor layer provided opposite to the first semiconductor layer of the active layer, a photonic crystal layer provided between the first semiconductor layer and the second semiconductor layer, a first electrode electrically connected to the first semiconductor layer, an insulating film provided on a surface of the second semiconductor layer opposite to the active layer, and a second electrode provided on a surface of the insulating film opposite to the second semiconductor layer, the photonic crystal layer includes a first region and a plurality of second regions having a refractive index different from a refractive index of the first region, the insulating film includes a plurality of openings, and the second electrode and the second semiconductor layer are electrically connected to each other in the plurality of openings.SELECTED DRAWING: Figure 3B

Description

[Technical Field]

[0001] The present disclosure relates to a photonic crystal surface-emitting laser and a method for manufacturing the same. [Background technology]

[0002] BACKGROUND ART Photonic-crystal surface-emitting lasers (PCSELs) are known in which a photonic crystal and an active layer having optical gain are stacked (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2016 / 031966 Summary of the Invention [Problem to be solved by the invention]

[0004] The electrodes are provided on the surface of the semiconductor layer. Light generated in the active layer can be extracted by reflecting it off the electrodes. However, light scattering reduces output power. Therefore, the objective of this study is to provide a photonic crystal surface-emitting laser that can increase optical output power, and a method for manufacturing the same. [Means for solving the problem]

[0005] The photonic crystal surface-emitting laser according to the present disclosure comprises a first semiconductor layer, an active layer stacked on the first semiconductor layer, a second semiconductor layer provided on the active layer opposite the first semiconductor layer, a photonic crystal layer provided between the first semiconductor layer and the second semiconductor layer, a first electrode electrically connected to the first semiconductor layer, an insulating film provided on the surface of the second semiconductor layer opposite the active layer, and a second electrode provided on the surface of the insulating film opposite the second semiconductor layer, 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 insulating film has a plurality of openings, and the second electrode and the second semiconductor layer are electrically connected through the plurality of openings. [Effects of the Invention]

[0006] According to the present disclosure, it is possible to provide a photonic crystal surface-emitting laser capable of increasing optical output and a method for manufacturing the same. [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 a plan view illustrating an example of a photonic crystal layer. [Figure 2B] FIG. 2B is an enlarged plan view of the photonic crystal layer region. [Figure 2C] FIG. 2C is an enlarged cross-sectional view of 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 4] FIG. 4 is a diagram illustrating the slope efficiency. [Figure 5A] FIG. 5A is a cross-sectional view illustrating a method for manufacturing a photonic crystal surface-emitting laser. [Figure 5B]FIG. 5B is a cross-sectional view illustrating a method for manufacturing a photonic crystal surface-emitting laser. [Figure 5C] FIG. 5C is a cross-sectional view illustrating a method for manufacturing a photonic crystal surface-emitting laser. [Figure 6A] FIG. 6A is a cross-sectional view illustrating a method for manufacturing a photonic crystal surface-emitting laser. [Figure 6B] FIG. 6B is a cross-sectional view illustrating a method for manufacturing a photonic crystal surface-emitting laser. [Figure 6C] FIG. 6C is a cross-sectional view illustrating a method for manufacturing a photonic crystal surface-emitting laser. [Figure 7A] FIG. 7A is a cross-sectional view illustrating a method for manufacturing a photonic crystal surface-emitting laser. [Figure 7B] FIG. 7B is a cross-sectional view illustrating a method for manufacturing a photonic crystal surface-emitting laser. [Figure 8] FIG. 8 is a cross-sectional view illustrating a photonic crystal surface-emitting laser according to the second embodiment. [Figure 9] FIG. 9 is a diagram illustrating the optical output. [Figure 10] FIG. 10 shows the calculation results of the reflectance. [Figure 11A] FIG. 11A is a graph showing the slope efficiency. [Figure 11B] FIG. 11B is a graph showing the slope efficiency. [Figure 11C] FIG. 11C is a graph showing the slope efficiency. [Figure 12A] FIG. 12A is a cross-sectional view illustrating a method for manufacturing a photonic crystal surface-emitting laser. [Figure 12B] FIG. 12B is a cross-sectional view illustrating a method for manufacturing a photonic crystal surface-emitting laser. [Figure 13A] FIG. 13A is a top view illustrating the photonic crystal surface-emitting laser according to the third embodiment. [Figure 13B] FIG. 13B is a top view illustrating the photonic crystal surface emitting laser according to the fourth embodiment. 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 is a photonic crystal surface-emitting laser comprising: (1) a first semiconductor layer; an active layer stacked on the first semiconductor layer; a second semiconductor layer provided on the active layer opposite the first semiconductor layer; a photonic crystal layer provided between the first semiconductor layer and the second semiconductor layer; a first electrode electrically connected to the first semiconductor layer; an insulating film provided on the surface of the second semiconductor layer opposite the active layer; and a second electrode provided on the surface of the insulating film opposite the second semiconductor layer, 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 insulating film has a plurality of openings, and the second electrode and the second semiconductor layer are electrically connected through the plurality of openings. The insulating film provided between the second electrode and the second semiconductor layer reduces the contact area between the second electrode and the second semiconductor layer. The lower surface of the second electrode is less likely to become rough, resulting in high reflectivity. The phase of light can be adjusted by the thickness of the insulating film. The optical output can be increased. (2) In the above (1), the plurality of openings may be periodically provided within the surface of the insulating film, allowing current to be injected uniformly. (3) In the above (1) or (2), the area filling rate of the plurality of openings may be 5% or more and 50% or less, which can suppress contact resistance and increase reflectance. (4) In any of the above (1) to (3), the opening may have a rectangular planar shape and a length of 1 μm or more and 10 μm or less. The opening can be easily manufactured. The current can be made nearly uniform. (5) In any one of the above (1) to (4), the second electrode may include a first metal layer and a second metal layer, the reflectivity of the second metal layer may be higher than the reflectivity of the first metal layer, the first metal layer may be provided in the plurality of openings in the insulating film, and the second metal layer may be provided on the surface of the insulating film. By increasing the reflectivity, it is possible to increase the light output. (6) In any of (1) to (5) above, a third semiconductor layer may be provided between the active layer and the second semiconductor layer, and the first semiconductor layer may have n-type conductivity, and the second semiconductor layer and the third semiconductor layer may have p-type conductivity. A pin junction is formed. A second electrode is connected to the second semiconductor layer through an opening. Carriers can be injected into the active layer. (7) A method for manufacturing a photonic crystal surface-emitting laser, comprising the steps of: stacking an active layer on a first semiconductor layer; forming a photonic crystal layer; forming a second semiconductor layer on the active layer opposite the first semiconductor layer; forming a first electrode electrically connected to the first semiconductor layer; forming an insulating film on the surface of the second semiconductor layer opposite the active layer; forming multiple openings in the insulating film; and forming a second electrode on the surface of the insulating film opposite the second semiconductor layer, wherein the photonic crystal layer has a first region and multiple second regions having a refractive index different from that of the first region, and the second electrode and the second semiconductor layer are electrically connected through the multiple openings. The insulating film provided between the second electrode and the second semiconductor layer reduces the contact area between the second electrode and the second semiconductor layer. The lower surface of the second electrode is less likely to become rough, resulting in higher reflectivity. The phase of light can be adjusted by the thickness of the insulating film. The optical output can be increased. (8) In the above (7), a step of stacking a third semiconductor layer on the active layer may be included, and the second semiconductor layer may be stacked on the third semiconductor layer. In the step of forming the insulating film, the insulating film may be formed to have a thickness based on a thickness from the active layer to the second semiconductor layer. By controlling the thickness of the insulating film, the phase of light can be adjusted and optical output can be increased.

[0010] [Details of the embodiments of the present disclosure] Specific examples of photonic crystal surface-emitting lasers and methods for manufacturing the same 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 (third semiconductor layer), a semiconductor layer 21, a contact layer 22 (second semiconductor layer), an electrode 24 (first electrode), and an electrode 26 (second electrode). The surface of each layer is parallel to the XY plane. The X-axis, Y-axis, and Z-axis are orthogonal to one another.

[0012] Semiconductor layers are stacked along the Z axis. On substrate 10, cladding layer 12, photonic crystal layer 14, cladding layer 16, active layer 18, cladding layer 20, semiconductor layer 21, and contact layer 22 are stacked in this order. Within the XY plane, the portion where semiconductor layer 21 and contact layer 22 are provided is defined as region 29. The length L0 of region 29 is, for example, 200 μm.

[0013] An insulating film is provided on the upper surface of the contact layer 22 and outside the region 29. The insulating film has a plurality of openings 27 in the region 29 and above the contact layer 22. The openings 27 penetrate the insulating film .

[0014] An electrode 26 is provided on the upper surface of the insulating film 23. The electrode 26 contacts the upper surface of the insulating film 23 and also contacts the upper surface of the contact layer 22 in the opening 27, and is electrically connected to the contact layer 22. The electrode 26 fills the inside of the opening 27. The electrode 24 contacts the lower surface of the substrate 10, and is electrically connected to the substrate 10 and the cladding layer 12.

[0015] 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.

[0016] 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.

[0017] 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 active layer 18 has optical gain.

[0018] The cladding layer 20 is formed of, for example, p-type indium phosphide (p-InP) with a thickness of 3 μm. The semiconductor layer 21 is formed of, for example, p-type indium gallium arsenide phosphide (p-InGaAsP) with a thickness of 100 nm. The contact layer 22 is formed of, for example, p-type indium gallium arsenide (p-InGaAs) with a thickness of 200 nm. The p-type dopant is, for example, zinc (Zn) or carbon (C). The insulating film 23 is formed of an insulator such as silicon nitride (SiN). The above materials are examples, and each layer may be formed of other materials or a combination of the above materials with other materials.

[0019] 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 that of the cladding layer, for example, 3.4.

[0020] 2A is a plan view illustrating photonic crystal layer 14. The lengths of one side, L1 and L2, are, for example, 1000 μm. Region 15 of photonic crystal layer 14 is located at the center of photonic crystal layer 14. Region 15 has a circular planar shape. Region 15 has a diameter, D1, of 300 μm, for example. Air holes are provided in region 15. When photonic crystal surface-emitting laser 100 is viewed from the Z-axis direction, region 15 overlaps with region 29 in FIG. 1.

[0021] FIG. 2B is an enlarged plan view of region 15 of photonic crystal layer 14. FIG. 2C is an enlarged cross-sectional view of photonic crystal layer 14, showing a cross section along line AA in FIG. 2B. 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.

[0022] As shown in FIG. 2B, the plurality of voids 32 and voids 34 are arranged two-dimensionally. 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 and voids 34 are arranged periodically in the X-axis direction and the Y-axis direction. The lattice constant is, for example, 400 nm. That is, the distance between adjacent voids 32 and the distance between adjacent voids 34 in the X-axis direction and the Y-axis direction is 400 nm. 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.

[0023] As shown in FIG. 2C , 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 through the photonic crystal layer 14 to the cladding layer 12. 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.

[0024] 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 D2 of the opening 25 is, for example, 340 μm. The opening 25 penetrates the electrode 24, and the substrate 10 is exposed through the opening 25. The opening 25 functions as an aperture for emitting light. The surface of the substrate 10 inside the opening 25 may be covered with an insulating film.

[0025] 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 may be formed by stacking, for example, nickel (Ni), germanium (Ge), and gold (Au) from the side closest to the substrate 10.

[0026] FIG. 3B is a top view illustrating the photonic crystal surface-emitting laser 100, seen through the electrode 26. The electrode 26 covers the entire upper surface of the insulating film 23. In region 29, a plurality of openings 27 are arranged two-dimensionally within the surface of the insulating film 23. The planar shape of the openings 27 is, for example, rectangular. The length L3 of one side of the openings 27 is, for example, 1.6 μm. The distance between corresponding sides of adjacent openings 27 (pitch L4) is, for example, 5 μm. The area filling factor (FF) of the openings 27 is calculated using the following formula and is approximately 10%. FF=(L3) 2 / (L4) 2

[0027] The contact layer 22 is exposed from the multiple openings 27, and the electrode 26 is in contact with the contact layer 22. That is, the electrode 26 has a mesh structure and is in periodic contact with the contact layer 22 in the XY plane. In the region between adjacent openings 27, an insulating film 23 is provided between the electrode 26 and the contact layer 22. 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 electrode 26 may be formed by stacking an Au layer on a Ti layer.

[0028] 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 when carriers are injected into the active layer 18. The light is diffracted and scattered within the plane of the photonic crystal layer 14, and light having a wavelength corresponding to the period of the air holes 32 and 34 is amplified, resulting in laser oscillation. The wavelength of the laser light is, for example, in the 1.3 μm band or the 1.5 μm band.

[0029] 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.

[0030] The light output depends on the reflectivity of the surface of the electrode 26 and the thickness of the insulating film 23. The light reflected by the electrode 26 is emitted, thereby increasing the light output.

[0031] Heat treatment is performed to ensure electrical continuity between the electrode 26 and the contact layer 22. Heat treatment may roughen the lower surface of the electrode 26, resulting in a decrease in reflectivity. If the electrode 26 is a solid electrode, the entire lower surface is in contact with the contact layer 22. Because the contact area is large, a rough surface is likely to form, and the reflectivity is likely to decrease. In the first embodiment, as shown in FIG. 1, the electrode 26 is provided on the insulating film 23 and contacts the contact layer 22 inside the opening 27. This reduces the contact area between the electrode 26 and the contact layer 22. The portion of the electrode 26 that contacts the insulating film 23 is less likely to roughen even after heat treatment. This increases the reflectivity.

[0032] The intensity of the emitted light changes depending on the phase of the light reflected from the lower surface of the electrode 26 and the phase of the light traveling from the active layer 18 toward the opening 25. When the phases match, the lights reinforce each other, improving the optical output. The phase of the reflected light depends on the refractive index and thickness of the insulating film 23. The phase can be adjusted by selecting an appropriate thickness.

[0033] (reflectance) The reflectance of four types of samples, A, B, C, and D, was measured. Sample A has a 180-nm-thick SiN layer, a 600-μm-thick GaAs substrate, a 160-nm-thick SiN layer, a Ti layer, and an Au layer stacked in that order. Sample B has the same structure as sample A, except that it does not have a SiN layer between the GaAs and Ti layers. Sample C has a 180-nm-thick SiN layer, a 250-μm-thick InP substrate, a 40-nm-thick InGaAsP layer, a 70-nm-thick C-doped InGaAs layer, a 100-nm-thick SiN layer, a Ti layer, a Pt layer, and an Au layer stacked in that order. Sample D has the same structure as sample C, except that it does not have a SiN layer between the InGaAs and Ti layers.

[0034] Light is irradiated through the SiN layer of each sample, the reflected light is measured, and the reflectance is obtained. The reflectance of Sample A is 96%. The reflectance of Sample B is 79%. The reflectance of Sample C is 62%. The reflectance of Sample D is 45%. Samples A and C have a SiN layer between the GaAs layer and Ti layer. Samples B and D do not have a SiN layer in this position. By providing a SiN layer between the semiconductor layer and metal, the reflectance increases.

[0035] (Slope efficiency) 4 is a diagram illustrating the slope efficiency. The horizontal axis represents the film thickness of the insulating film 23. The vertical axis represents the slope efficiency (SE) of the photonic crystal surface-emitting laser 100. The wavelength of light is 1310 nm. The reflectance of the electrode 26 at the opening 27 is 0.5. The slope efficiency is calculated assuming that the refractive index of the insulating film 23 is 1.99 and that of InP is 3.2.

[0036] The dashed line in Figure 4 corresponds to the case where the reflectance of the entire electrode 26 is 0. The higher the reflectance, the higher the slope efficiency. The higher the slope efficiency, the higher the optical output power obtained when the current flowing through the photonic crystal surface-emitting laser 100 is increased. The slope efficiency changes periodically with changes in the thickness of the insulating film 23. The period of the slope efficiency waveform, converted into the thickness of the insulating film 23, is approximately 320 nm. That is, the slope efficiency reaches its minimum value at a thickness of 0 nm and approximately 320 nm. The slope efficiency reaches its maximum value at a thickness of approximately 160 nm. When the slope efficiency reaches its maximum value, the reflectance of the entire electrode 26 is 0.5. The phase can be adjusted by appropriately adjusting the thickness of the insulating film 23. The optical output power can be increased by constructively combining the emitted light from the active layer 18 and the reflected light.

[0037] (Manufacturing method) 5A to 7B are cross-sectional views illustrating a method for manufacturing photonic crystal surface-emitting laser 100. As shown in Fig. 5A, 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.

[0038] 5B and 5C are enlarged views of photonic crystal layer 14. As shown in FIG. 5B, a mask 50 is provided on the upper surface of photonic crystal layer 14. Mask 50 is made of an insulator such as SiN. An insulating film is formed on the upper surface of 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 mask 50. Mask 50 has openings 51 and 52. The upper surface of base material 30 is exposed through openings 51 and 52. A plurality of openings 51 and 52 are arranged two-dimensionally.

[0039] As shown in FIG. 5C, holes 32 and 34 are formed in photonic crystal layer 14 by reactive ion etching (RIE) or the like. The etching proceeds, for example, partway through photonic crystal layer 14, but does not reach the bottom surface of photonic crystal layer 14. Holes 32 are formed at positions overlapping openings 51 of mask 50. Holes 34 are formed at positions overlapping openings 52. The planar shapes of holes 32 and 34 are determined by the planar shapes of openings 51 and 52. By making openings 51 elliptical and openings 52 circular, elliptical holes 32 and circular holes 34 are formed, as shown in FIG. 3B. After etching is completed, mask 50 is removed.

[0040] As shown in Figure 6A, cladding layer 16, active layer 18, cladding layer 20, semiconductor layer 21, 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, semiconductor layer 21, and contact layer 22 are epitaxially grown on flat cladding layer 16. By adjusting the growth conditions, contact layer 22 can be doped with, for example, carbon (C).

[0041] 6B, the outer peripheral portions of the contact layer 22, the semiconductor layer 21, and the cladding layer 20 are etched. The upper surface of the active layer 18 is exposed in the etched portion. The thickness from the upper surface of the active layer 18 to the upper surface of the contact layer 22 is measured.

[0042] 6C, the insulating film 23 is formed by, for example, plasma enhanced CVD (PECVD). The thickness of the insulating film 23 is determined based on the thickness from the active layer 18 to the contact layer 22.

[0043] As shown in FIG. 7A, a mask 54 is provided on the insulating film 23. For example, a resist is used as the mask 54. A plurality of openings are provided in the mask 54 by resist patterning. Portions of the insulating film 23 that are exposed from the mask 54 are removed by dry etching to form a plurality of openings 27. The mask 54 is then removed.

[0044] As shown in FIG. 7B, a mask 55 is provided on the outer periphery of the insulating film 23. For example, a resist is used as the mask 55. The portion of the mask 55 that overlaps the opening 27 is opened by resist patterning. An electrode 26 is formed by vapor deposition and lift-off. For example, a Ti layer, a Pt layer, and an Au layer are stacked in this order. The mask 55 is then removed. An electrode 24 is provided on the underside of the substrate 10 as shown in FIG. 1, and an opening 25 is formed. For example, a heat treatment is performed at a temperature of 300°C or higher to establish contact between the electrode and the semiconductor. The wafer is cut using the outer periphery as a scribe line. Through these steps, the photonic crystal surface-emitting laser 100 is formed.

[0045] According to the first embodiment, the insulating film 23 has a plurality of openings 27. As shown in FIG. 1 , the electrode 26 is provided on the upper surface of the insulating film 23, contacts the upper surface of the contact layer 22 through the openings 27, and is electrically connected to the contact layer 22. Because the insulating film 23 is provided between the contact layer 22 and the electrode 26, the contact area between the electrode 26 and the contact layer 22 is reduced. The lower surface of the electrode 26 is less likely to become rough even after heat treatment. The reflectivity of the lower surface of the electrode 26 to light is increased. Light is less likely to be scattered, is reflected by the electrode 26, and is emitted from the openings 25. The optical output is increased.

[0046] The phase of the reflected light is adjusted by adjusting the thickness of the insulating film 23 to a desired value. When the phase difference between the reflected light and the light emitted from the active layer 18 is 2nπ (n=0, 1, 2, etc.), the lights constructively interact with each other, increasing the optical output.

[0047] For example, in FIG. 6B, before depositing the insulating film 23, the thickness of the semiconductor layers from the active layer 18 to the contact layer 22 is measured. The thickness of the insulating film 23 is determined based on this thickness. By controlling the thickness of the insulating film 23 to an optimal value during manufacturing, the phase of the light can be adjusted and the optical output can be increased. For example, in FIG. 4, the thickness that maximizes the slope efficiency is selected. The thickness of the insulating film 23 is controlled, for example, by the deposition time.

[0048] As shown in FIG. 3B, the multiple openings 27 are periodically arranged in the X-axis and Y-axis directions. The electrode 26 and the contact layer 22 are electrically connected through the multiple openings 27. Current can be uniformly injected into the active layer 18. The openings 27 do not have to be arranged periodically, but this may result in biased current. By periodically arranging the openings 27 at a constant pitch L4, the current can be made more uniform.

[0049] If the area fill factor (FF) of the multiple openings 27 in the region 29 is small, the contact area between the electrode 26 and the contact layer 22 decreases, resulting in high contact resistance. If the FF is large, the contact area increases, reducing the area of ​​the electrode 26 on the insulating film 23. This results in a rough underside and a decrease in reflectivity. The FF is, for example, 5% to 50%, or alternatively, 10% to 20%, 40% to 45%, or may be used. This allows for suppressing contact resistance and increasing reflectivity.

[0050] If the opening 27 is small, it is difficult to manufacture. If the opening 27 is large, it is difficult to inject a current uniformly. The opening 27 is rectangular, and the length L3 of the opening 27 is, for example, 1 μm or more and 10 μm or less. The length L3 may be 2 μm or more, 3 μm or more, 8 μm or less, or 9 μm or less. The opening 27 is easy to manufacture, and allows for uniform current injection.

[0051] The insulating film 23 is made of SiN and has a refractive index of 1.99. The phase of light depends on the refractive index and thickness. The thickness is controlled so that the product of the thickness and refractive index of the SiN insulating film 23 divided by the wavelength of light is an integer multiple of 2π. This adjusts the phase and increases the reflectance.

[0052] The contact layer 22 is doped with C. The C concentration is 1×10 19 cm -3 By setting the C concentration to 1×10 or more, the contact resistance can be reduced. 20 cm -3 By increasing the concentration to this level or more, the contact resistance between the contact layer 22 and the electrode 26 is reduced to about 1 / 10 compared to Zn doping.

[0053] 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 connected to the p-type contact layer 22 through an opening 27. Carriers can be injected into the active layer 18 by applying a voltage to the electrode. 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.

[0054] Although two types of holes are used in the embodiment, one type or three or more types may be used. The planar shape of the holes may be elliptical, circular, or polygonal. Photonic crystal layer 14 is periodically provided with 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 is provided between cladding layer 12 and cladding layer 20, and may be provided between cladding layer 12 and active layer 18, or between active layer 18 and cladding layer 20.

[0055] Second Embodiment FIG. 8 is a cross-sectional view illustrating a photonic crystal surface-emitting laser 200 according to the second embodiment. Description of the same configuration as in the first embodiment will be omitted. The electrode 26 has a plurality of metal layers 40 (first metal layers) and a metal layer 42 (second metal layer). The metal layer 40 is provided in the opening 27 of the insulating film 23 and is in contact with the upper surface of the contact layer 22. The metal layer 42 is provided on the upper surfaces of the insulating film 23 and the metal layer 40 and is in contact with these surfaces.

[0056] The metal layer 40 is formed by laminating a Ti layer, a Pt layer, and an Au layer from the side closer to the contact layer 22. The metal layer 42 is formed by laminating a Ti layer, a Pt layer, and an Au layer from the side closer to the insulating film 23. The thickness of the Ti layer of the metal layer 42 is thinner than the thickness of the Ti layer of the metal layer 40. The electrode 26 may be formed of a metal other than the above.

[0057] (Slope efficiency) FIG. 9 is a diagram illustrating optical output. The horizontal axis represents the current flowing through the photonic crystal surface-emitting laser. The vertical axis represents the optical output. The dotted line represents Comparative Example 1. The dashed line represents Comparative Example 2. The solid line represents the second embodiment. In Comparative Example 1 and Comparative Example 2, the entire upper surface of the contact layer 22 is exposed from the insulating film 23. The electrode 26 is a solid electrode and is in contact with the entire upper surface of the contact layer 22. The electrode 26 in Comparative Example 1 has the same configuration as the metal layer 40 described above. The electrode 26 in Comparative Example 2 has the same configuration as the metal layer 42 described above.

[0058] As shown in FIG. 9, the slope of the second embodiment is greater than that of Comparative Example 1 and Comparative Example 2. The slope efficiency of Comparative Example 1 is 0.19 W / A, and the maximum optical output is 224 mW. The slope efficiency of Comparative Example 2 is 0.17 W / A, and the maximum optical output is 193 mW. The slope efficiency of the second embodiment is 0.41 W / A, and the maximum optical output is 365 mW. According to the second embodiment, the slope efficiency of the photonic crystal surface-emitting laser 200 is more than twice that of Comparative Example 1 and Comparative Example 2. The optical output of the photonic crystal surface-emitting laser 200 can be increased to 300 mW or more.

[0059] We will now examine the change in slope efficiency when the thickness of the insulating film 23 is changed. First, we calculate the change in reflectance. Figure 10 is a diagram showing the results of the reflectance calculations. The horizontal axis represents the thickness of the insulating film 23. The vertical axis represents the reflectance over the entire lower surface of the electrode 26. The black circle represents the total value of the reflectance in all directions. The white circle represents the reflectance in the vertical direction (downward in Figure 8). The difference between the total value and the vertical reflectance corresponds to the scattering loss of light. The higher the reflectance in the vertical direction, the higher the light output.

[0060] Calculate the slope efficiency. The following parameters are used to calculate the slope efficiency: absorption coefficient A, light wavelength λ, quantum efficiency ηi, reflectance R, light loss αv, α||, α0 that depends on the polarization direction of the light, and light phase θ. The phase θ is expressed by the following equation.

number

[0061] The thickness of the insulating film 23 is changed in three photonic crystal surface-emitting lasers 200 (chips E to G), and the slope efficiency is calculated. In the three chips, the refractive index n1 is 3.3, n2 is 1.78, the total film thickness T1 is 2700 nm, the wavelength is 1330 nm, and the loss α0 is 4.5 cm -1 The area filling rate of the opening 27 in the chip E is 10%, and the loss α is 11.5 cm -1 and the loss α|| is 9.5 cm -1 The area filling rate of chip F is 13%, and the loss αv is 12 cm -1 and the loss α|| is 11.5 cm -1 The area filling rate for chip G is 16.8%, and the loss αv is 11.5 cm -1 and the loss α|| is 18 cm -1 The reflectance R is the value in the vertical direction shown in Figure 10.

[0062] 11A to 11C are graphs showing the slope efficiency, showing the results for chip E, chip F, and chip G, respectively. The horizontal axis represents the film thickness of the insulating film 23. The vertical axis represents the slope efficiency. The solid line represents the calculation results of the slope efficiency. The slope efficiency changes periodically as the film thickness changes. By using the reflectance changes shown in FIG. 10 and Equation 1, the slope efficiency can be calculated with high accuracy.

[0063] (Manufacturing method) 12A and 12B are cross-sectional views illustrating a method for manufacturing a photonic crystal surface-emitting laser 200. The steps up to FIG. 7A are common to the second embodiment. As shown in FIG. 12A, a mask 56 is provided on the insulating film 23. Portions of the mask 56 that overlap with the openings 27 are opened by resist patterning. A plurality of metal layers 40 are formed in the openings 27 by vapor deposition and lift-off. The mask 56 is then removed.

[0064] A mask 57 is provided on the outer periphery, and resist patterning is performed. Metal layer 42 is formed on insulating film 23 and metal layer 40 by vapor deposition and lift-off. Mask 57 is then removed. Heat treatment is performed, for example, at a temperature of 300°C or higher, to establish contact between the electrodes and the semiconductor. Through these steps, photonic crystal surface-emitting laser 200 is formed.

[0065] According to the second embodiment, the insulating film 23 is provided between the contact layer 22 and the electrode 26, thereby reducing the contact area between the electrode 26 and the contact layer 22. This increases the reflectance and the optical output.

[0066] The electrode 26 has a metal layer 40 and a metal layer 42. The metal layer 40 contacts the contact layer 22. The metal layer 42 is located on the insulating film 23 and has a higher reflectivity than the metal layer 40. This allows for both electrical conductivity and high reflectivity. The Ti layer of the metal layer 42 is thinner than the Ti layer of the metal layer 40. The reflectivity of the metal layer 42 is high.

[0067] 11A to 11C, the phase of the reflected light is adjusted by changing the thickness of the insulating film 23. When the phase difference between the reflected light and the light emitted from the active layer 18 is 2nπ (n=0, 1, 2, etc.), the lights constructively interact with each other, increasing the optical output.

[0068] <Third embodiment> 13A is a top view illustrating a photonic crystal surface-emitting laser 300 according to the third embodiment, with the electrodes 26 shown in perspective. Description of the same configuration as in the first or second embodiment will be omitted. The electrodes 26 may be the same as in the first or second embodiment.

[0069] As shown in Figure 13A, the planar shape of the opening 27 is rectangular. The length L5 of the opening 27 in the X-axis direction is, for example, 1 μm or more and 10 μm or less. The length L6 of the opening 27 in the Y-axis direction is longer than L5 and is, for example, 200 μm. The openings 27 in the insulating film 23 are arranged periodically in the X-axis direction. The pitch L7 is, for example, 5 μm. The FF of the openings 27 is, for example, 5% or more and 10% or less.

[0070] According to the third embodiment, the electrode 26 contacts the upper surface of the contact layer 22 through the opening 27. This reduces the contact area between the electrode 26 and the contact layer 22. This increases the reflectivity and the optical output.

[0071] <Fourth embodiment> 13B is a top view illustrating the photonic crystal surface-emitting laser 400 according to the fourth embodiment, with the electrodes 26 shown in perspective. Description of the same configuration as in the first or second embodiment will be omitted. The electrodes 26 may be the same as in the first or second embodiment.

[0072] As shown in FIG. 13B, the multiple openings 27 are arranged concentrically. Of the multiple openings 27, one opening 27a is circular. Opening 27b is annular and surrounds opening 27a. The width L8 of openings 27 is, for example, 1 μm or more and 10 μm or less. The pitch L9 is, for example, 5 μm. The outer diameter of the outermost opening 27 is 200 μm. The FF of openings 27 is 5% or more and 10% or less.

[0073] According to the fourth embodiment, the electrode 26 contacts the upper surface of the contact layer 22 through the opening 27. This reduces the contact area between the electrode 26 and the contact layer 22. This increases the reflectance and the optical output.

[0074] The plurality of openings 27 are arranged periodically within the surface of the insulating film 23. The planar shape of the openings 27 may be polygonal, circular, or may have a shape including a curve.

[0075] 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]

[0076] 10 Substrate 12, 16, 20 cladding layers 14 Photonic crystal layer 15, 29 areas 18 Active layer 21 Semiconductor layer 22 Contact layer 23 insulating film 24, 26 electrodes 25, 27, 27a, 27b, 51, 52 opening 30 Base material 32, 34 Vacancies 40, 42 metal layer 50, 54, 55, 56, 57 Masks 100, 200, 300, 400 Photonic crystal surface-emitting laser

Claims

1. a first semiconductor layer; an active layer stacked on the first semiconductor layer; a second semiconductor layer provided opposite the first semiconductor layer of the active layer; a photonic crystal layer provided between the first semiconductor layer and the second semiconductor layer; a first electrode electrically connected to the first semiconductor layer; an insulating film provided on a surface of the second semiconductor layer opposite to the active layer; a second electrode provided on a surface of the insulating film opposite to the second semiconductor layer, 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 insulating film has a plurality of openings, The photonic crystal surface-emitting laser has the second electrode and the second semiconductor layer electrically connected to each other at the plurality of openings.

2. The photonic crystal surface-emitting laser according to claim 1 , wherein the plurality of openings are periodically provided within the surface of the insulating film.

3. 3. The photonic crystal surface emitting laser according to claim 1, wherein the area filling rate of the plurality of openings is 5% or more and 50% or less.

4. The opening has a rectangular planar shape, 3. The photonic crystal surface emitting laser according to claim 1, wherein the length of the opening is 1 μm or more and 10 μm or less.

5. the second electrode includes a first metal layer and a second metal layer; the reflectivity of the second metal layer is higher than the reflectivity of the first metal layer; 3. The photonic crystal surface-emitting laser according to claim 1, wherein the first metal layer is provided in the plurality of openings of the insulating film, and the second metal layer is provided on the surface of the insulating film.

6. a third semiconductor layer provided between the active layer and the second semiconductor layer; the first semiconductor layer has n-type conductivity; 3. The photonic crystal surface-emitting laser according to claim 1, wherein the second semiconductor layer and the third semiconductor layer have p-type conductivity.

7. depositing an active layer on the first semiconductor layer; forming a photonic crystal layer; forming a second semiconductor layer opposite the first semiconductor layer of the active layer; forming a first electrode electrically connected to the first semiconductor layer; forming an insulating film on a surface of the second semiconductor layer opposite to the active layer; forming a plurality of openings in the insulating film; forming a second electrode on a surface of the insulating film opposite to the second semiconductor layer, 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 method for manufacturing a photonic crystal surface-emitting laser includes electrically connecting the second electrode and the second semiconductor layer through the plurality of openings.

8. a step of stacking a third semiconductor layer on the active layer, the second semiconductor layer is stacked on the third semiconductor layer, 8. The method for manufacturing a photonic crystal surface-emitting laser according to claim 7, wherein in the step of forming the insulating film, the insulating film is formed to have a thickness based on a thickness from the active layer to the second semiconductor layer.

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