Vertical cavity light emitting device

Abstract

To provide a vertical resonator type light-emitting element having high light emission efficiency.SOLUTION: A vertical resonator type light-emitting element comprises: a semiconductor structure layer including a first semiconductor layer formed at an upper side of a gallium nitride-based semiconductor substrate, an active layer formed on the first semiconductor layer, and a second semiconductor layer formed on the active layer and having a conductivity type which is opposite to that of the first semiconductor layer; a first electrode layer which is electrically in contact with the first semiconductor layer; a second electrode layer which is formed on a top face of the semiconductor structure layer and electrically in contact with the second semiconductor layer in one region of a top face; a first multilayer film reflector disposed at the side of the first semiconductor layer in the semiconductor structure layer; and a second multilayer film reflector which is disposed at the side of the second semiconductor layer in the semiconductor structure layer and constitutes a resonator with the first multilayer film reflector. The semiconductor structure layer includes a first recessed structure including one or more recesses penetrating from the top face through the active layer in a region enclosing the one region, and the recessed structure is provided within 50 μm from an outer edge of the one region.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to vertical cavity light emitting devices such as vertical cavity surface emitting lasers (VCSELs). [Background technology]

[0002] A vertical cavity semiconductor surface emitting laser (hereinafter also simply referred to as a surface emitting laser) has been known as one type of semiconductor laser, and has a semiconductor layer that emits light when a voltage is applied thereto and multilayer film reflectors that face each other with the semiconductor layer sandwiched therebetween. For example, Patent Document 1 discloses a vertical cavity semiconductor laser having an n-electrode and a p-electrode connected to an n-type semiconductor layer and a p-type semiconductor layer, respectively. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2017-98328 Summary of the Invention [Problem to be solved by the invention]

[0004] For example, in a vertical cavity light emitting device such as a surface emitting laser, an optical resonator is formed by opposing reflecting mirrors. For example, in a surface emitting laser, when a voltage is applied to a semiconductor layer via electrodes, light emitted from the semiconductor layer resonates within the optical resonator, generating laser light.

[0005] However, one example of a problem with vertical cavity semiconductor laser elements is that they have lower light emission efficiency than horizontal cavity semiconductor lasers that have a cavity in the in-plane direction of a semiconductor layer including an active layer.

[0006] The present invention has been made in view of the above-mentioned points, and an object of the present invention is to provide a vertical cavity light emitting device having high luminous efficiency. [Means for solving the problem]

[0007] A vertical cavity light emitting device according to the present invention comprises a semiconductor structure layer including a gallium nitride semiconductor substrate, a first semiconductor layer made of a nitride semiconductor having a first conductivity type formed above the gallium nitride semiconductor substrate, an active layer made of a nitride semiconductor formed on the first semiconductor layer, and a second semiconductor layer formed on the active layer and made of a nitride semiconductor having a second conductivity type opposite to the first conductivity type, a first electrode layer electrically contacting the first semiconductor layer of the semiconductor structure layer, and a second electrode layer formed on an upper surface of the semiconductor structure layer and covering one region of the upper surface. the semiconductor structure layer has a recessed structure including one or more recesses penetrating the active layer from the top surface in a region surrounding the first region, the recessed structure being located within 50 μm of the outer edge of the first region. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a perspective view of a surface-emitting laser according to a first embodiment; [Figure 2] 1 is a top view of a surface-emitting laser according to a first embodiment. [Figure 3] 1 is a cross-sectional view of a surface-emitting laser according to a first embodiment. [Figure 4] FIG. 10 is a perspective view of a surface-emitting laser according to a second embodiment. [Figure 5] FIG. 10 is a cross-sectional view of a surface-emitting laser according to a second embodiment. [Figure 6] FIG. 10 is a perspective view of a surface-emitting laser according to a third embodiment. [Figure 7] FIG. 10 is a top view of the surface-emitting laser according to the third embodiment. [Figure 8] FIG. 10 is a cross-sectional view of a surface-emitting laser according to a third embodiment. [Figure 9] FIG. 10 is a perspective view of a surface-emitting laser according to a fourth embodiment. [Figure 10] FIG. 10 is a cross-sectional view of a surface-emitting laser according to a fourth embodiment. [Figure 11] FIG. 10 is a top view of a surface-emitting laser according to a modified example. [Figure 12] FIG. 10 is a top view of a surface-emitting laser according to a modified example. [Figure 13] FIG. 10 is a top view of a surface-emitting laser according to a modified example. [Figure 14] FIG. 10 is a perspective view of a surface-emitting laser according to a fifth embodiment. [Figure 15] FIG. 10 is a top view of the surface-emitting laser according to the fifth embodiment. [Figure 16] FIG. 10 is a top view of a surface-emitting laser according to a modified example. DETAILED DESCRIPTION OF THE INVENTION

[0009] The following describes in detail the embodiments of the present invention. In the following description, a semiconductor surface-emitting laser element is used as an example (semiconductor laser), but the present invention can be applied not only to surface-emitting lasers but also to various vertical-cavity light-emitting elements such as vertical-cavity light-emitting diodes. [Example]

[0010] FIG. 1 is a perspective view of a vertical cavity surface emitting laser (VCSEL, hereinafter also simply referred to as a surface emitting laser) 10 according to a first embodiment.

[0011] The substrate 11 is a gallium nitride semiconductor substrate, such as a GaN substrate. The substrate 11 has, for example, a rectangular top surface. A first multilayer film reflector 13 made of semiconductor layers grown on the substrate 11 is formed on the substrate 11.

[0012] The first multilayer reflector 13 is a semiconductor multilayer reflector in which low-refractive-index semiconductor films having an AlInN composition and high-refractive-index semiconductor films having a GaN composition and a higher refractive index than the low-refractive-index semiconductor films are alternately stacked. In other words, the first multilayer reflector 13 is a distributed Bragg reflector (DBR) made of semiconductor material. For example, a buffer layer having a GaN composition is provided on the upper surface of the substrate 11, and the first multilayer reflector 13 is formed by alternately depositing the high-refractive-index semiconductor films and the low-refractive-index semiconductor films on the buffer layer.

[0013] The upper surface of the substrate 11, i.e., the surface on which the buffer layer having a GaN composition is provided, is preferably the C-plane or a plane off-axis within 0.5° from the C-plane, because this improves the crystallinity of the semiconductor structure layer 15, which will be described later.

[0014] The semiconductor structure layer 15 is a laminated structure made up of multiple semiconductor layers formed on the first multilayer reflector 13. The semiconductor structure layer 15 has an n-type semiconductor layer (first semiconductor layer) 17 formed on the first multilayer reflector 13, a light-emitting layer (or active layer) 19 formed on the n-type semiconductor layer 17, and a p-type semiconductor layer (second semiconductor layer) 21 formed on the active layer 19.

[0015] The n-type semiconductor layer 17 is a semiconductor layer formed on the first multilayer film reflector 13. The n-type semiconductor layer 17 has a GaN composition and is doped with Si as an n-type impurity. The n-type semiconductor layer 17 has a prismatic lower portion 17A and a cylindrical upper portion 17B disposed thereon. Specifically, for example, the n-type semiconductor layer 17 has a cylindrical upper portion 17B protruding from an upper surface 17S of the prismatic lower portion 17A. In other words, the n-type semiconductor layer 17 has a mesa-shaped structure including the upper portion 17B.

[0016] The active layer 19 is formed on the upper portion 17B of the n-type semiconductor layer 17 and has a quantum well structure including a well layer having an InGaN composition and a barrier layer having a GaN composition. In the surface-emitting laser 10, light is generated in the active layer 19.

[0017] The p-type semiconductor layer 21 is a semiconductor layer having a GaN composition formed on the active layer 19. The p-type semiconductor layer 21 is doped with Mg as a p-type impurity.

[0018] N-electrode 23 is a metal electrode provided on upper surface 17S of lower portion 17A of n-type semiconductor layer 17 and electrically connected to n-type semiconductor layer 17. N-electrode 23 is formed in a ring shape so as to surround upper portion 17B of n-type semiconductor layer 17.

[0019] Insulating layer 25 is a layer made of an insulator and formed on p-type semiconductor layer 21. Insulating layer 25 is made of a substance, such as SiO2, that has a lower refractive index than the material that forms p-type semiconductor layer 21. Insulating layer 25 is formed in a ring shape on p-type semiconductor layer 21, and has an opening (not shown) in the center that exposes p-type semiconductor layer 21.

[0020] The p-electrode 27 is a metal electrode formed on the insulating layer 25. The p-electrode 27 is electrically connected to the upper surface of the p-type semiconductor layer 21 exposed from the opening in the insulating layer 25 via a transparent electrode (not shown) made of a metal oxide film such as ITO or IZO.

[0021] The second multilayer reflector 29 is a dielectric multilayer reflector in which low-refractive-index dielectric films made of Al2O3 and high-refractive-index dielectric films made of Ta2O5 and having a higher refractive index than the low-refractive-index dielectric films are alternately stacked. In other words, the second multilayer reflector 29 is a distributed Bragg reflector (DBR) made of a dielectric material.

[0022] 2 is a top view of the surface-emitting laser 10. As described above, the surface-emitting laser 10 has a semiconductor structure layer 15 including an n-type semiconductor layer 17 formed on a substrate 11 having a rectangular top surface shape, an active layer 19 having a circular top surface shape, and a p-type semiconductor layer 21 (see FIG. 1). An insulating layer 25 and a p-electrode 27 are formed on the p-type semiconductor layer 21. A second multilayer film reflector 29 is formed on the p-electrode 27.

[0023] The insulating layer 25 has an opening 25H, which is a circular opening that exposes the p-type semiconductor layer 21 of the insulating layer 25. As shown in Fig. 2, the opening 25H is formed in the center of the insulating layer 25 when viewed from above the surface-emitting laser 10, and is covered by the second multilayer film reflector 29 when viewed from above the surface-emitting laser 10. In other words, the opening 25H is covered by the second multilayer film reflector 29 on the upper surface of the p-type semiconductor layer 21. In other words, the opening 25H is formed in a region of the insulating layer 25 facing the lower surface of the multilayer film reflector 29.

[0024] The p-electrode 27 is formed in the center of the insulating layer 25 when viewed from above the surface-emitting laser 10, and has an opening 27H surrounding the opening 25H. That is, the opening 27H is larger than the opening 25H. For example, the shape of the opening 27H is a circle that is concentric with the shape of the opening 25H.

[0025] 2, an annular groove 15G is formed in the upper surface of the p-type semiconductor layer 21, i.e., the upper surface of the semiconductor structure layer 15. The groove 15G is formed in the region outside the openings 25H and 27H. That is, the groove 15G is an annular groove when viewed from a direction perpendicular to the in-plane direction of the semiconductor structure layer 15.

[0026] In this embodiment, the groove 15G is formed on the upper surface of the p-type semiconductor layer 21 so as to be covered by the second multilayer film reflector 29. That is, in this embodiment, the groove 15G is formed at a position facing the lower surface of the second multilayer film reflector 29.

[0027] Fig. 3 is a cross-sectional view of the surface-emitting laser 10 taken along line 3-3 in Fig. 2. As described above, the surface-emitting laser 10 has the substrate 11, which is a GaN substrate, and the first multilayer film reflector 13 is formed on the substrate 11. The lower surface of the substrate 11 may be coated with an AR coating.

[0028] A semiconductor structure layer 15 is formed on the first multilayer film reflector 13. The semiconductor structure layer 15 is a laminated body in which an n-type semiconductor layer 17, an active layer 19, and a p-type semiconductor layer 21 are formed in this order.

[0029] The groove 15G formed in the semiconductor structure layer 15 is formed so as to surround the protruding portion 21P that protrudes from the center of the upper surface of the p-type semiconductor layer 21, and extends from the upper surface of the p-type semiconductor layer 21 through the active layer 19 to the n-type semiconductor layer 17.

[0030] In this way, in the surface-emitting laser 10 of Example 1, the grooves 15G are formed so as to penetrate through the active layer 19. In other words, gaps are formed in the active layer 19 by the grooves 15G.

[0031] The insulating layer 25 is formed to cover the upper surface of the p-type semiconductor layer 21 and the inner surface of the groove 15G. As described above, the insulating layer 25 is made of a material having a lower refractive index than the p-type semiconductor layer 21. The insulating layer 25 has an opening 25H that exposes the protruding portion 21P. For example, as shown in FIG. 2, the opening 25H is circular. For example, the opening 25H and the protruding portion 21P have the same shape, and the inner surface of the opening 25H and the outer surface of the protruding portion 21P are in contact with each other.

[0032] The light-transmitting electrode layer 31 is a layer made of a light-transmitting conductor formed so as to cover the insulating layer 25 and the protruding portion 21P exposed from the opening 25H of the insulating layer 25. That is, the light-transmitting electrode layer 31 is in electrical contact with the p-type semiconductor layer 21 in a region of the upper surface of the p-type semiconductor layer 21 exposed by the opening 25H. The light-transmitting electrode layer 31 is formed of a metal oxide, such as ITO or IZO, that is light-transmitting to light emitted from the active layer 19.

[0033] As described above, the p-electrode 27 is a metal electrode and is formed to cover the translucent electrode layer 31. That is, the p-electrode 27 is in electrical contact with the translucent electrode layer 31. Therefore, the p-electrode 27 is in electrical contact or connection with the p-type semiconductor layer 21 via the translucent electrode layer 31 in a region exposed by the opening 25H on the upper surface of the p-type semiconductor layer 21. The p-electrode 27 has an opening 27H in the center that exposes the translucent electrode layer 31. The opening 27H is wider than the opening 25H.

[0034] The second multilayer reflector 29 is formed so as to cover the opening 27H and the groove 15G. The second multilayer reflector 29 is formed so as to fill the space formed by the opening 27H and to be in contact with the light-transmitting electrode layer 31. The second multilayer reflector 29 is also formed so as to fill the space formed by the groove 15G.

[0035] In the surface-emitting laser 10, the first multilayer reflector 13 has a reflectivity slightly lower than that of the second multilayer reflector 29. Therefore, part of the light resonating between the first multilayer reflector 13 and the second multilayer reflector 29 passes through the first multilayer reflector 13 and the substrate 11 and is extracted to the outside.

[0036] As described above, in the surface-emitting laser 10 of Example 1, the groove 15G is formed to penetrate the active layer 19. In other words, the groove 15G forms a gap in the active layer 19. The groove 15G is formed after the semiconductor structure layer 15 is formed. Thereafter, the insulating layer 25 is formed before the light-transmitting electrode layer 31, the p-electrode 27, and the second multilayer film reflector 29 are formed.

[0037] Therefore, after the semiconductor structure layer 15 is formed, the grooves 15G reaching the active layer 19 are formed, thereby forming spaces or gaps in the direction along the in-plane of the active layer 19. These gaps relieve strain that occurs in the in-plane direction of the active layer 19 or in the in-plane direction of the semiconductor structure layer 15 during the formation of the active layer 19.

[0038] Specifically, when the active layer 19 is formed, the difference in lattice constant between InGaN and GaN, which form the quantum well structure, causes distortion of the crystal structure, resulting in piezoelectric polarization and the generation of a piezoelectric field. The generation of this piezoelectric field reduces the recombination probability of electrons and holes injected into the light-emitting layer, which is one of the causes of a low internal quantum efficiency.

[0039] In the surface-emitting laser 10, a groove 15G is formed in the semiconductor structure layer 15, reaching the active layer 19. The gap created by this groove relieves strain that occurs in the active layer 19 in the in-layer direction during growth of the active layer 19, and it is believed that this improves the internal quantum efficiency of the active layer 19.

[0040] Here, we will explain the operation of the surface-emitting laser 10. When a voltage is applied between the n-electrode 23 and the p-electrode 27 in the surface-emitting laser 10, a current flows in the semiconductor structure layer 15 as shown by the thick dashed dotted line in the figure, and light is emitted from the active layer 19. The light emitted from the active layer 19 is repeatedly reflected between the first multilayer film reflector 13 and the second multilayer film reflector 29, and reaches a resonance state (laser oscillation).

[0041] In the surface-emitting laser 10, current is injected into the p-type semiconductor layer 21 only from the portion exposed by the opening 25H. Furthermore, because the p-type semiconductor layer 21 is very thin, current does not diffuse in the in-plane direction within the p-type semiconductor layer 21, i.e., in the direction along the in-plane direction of the semiconductor structure layer 15. Therefore, in the surface-emitting laser 10, current is supplied only to the region of the active layer 19 directly below the opening 25H, and light is emitted only from that region. That is, in the surface-emitting laser 10, the opening 25H forms a current confinement structure that limits the range of current supply in the active layer 19.

[0042] As described above, in this embodiment, the first multilayer reflector 13 has a reflectivity slightly lower than that of the second multilayer reflector 29. Therefore, part of the light resonating between the first multilayer reflector 13 and the second multilayer reflector 29 passes through the first multilayer reflector 13 and the substrate 11 and is extracted to the outside. In this way, the surface-emitting laser 10 emits light from the lower surface of the substrate 11 in a direction perpendicular to the in-plane direction of the lower surface of the substrate 11 and each layer of the semiconductor structure layer 15.

[0043] The protruding portion 21P of the p-type semiconductor layer 21 of the semiconductor structure layer 15 and the opening 25H of the insulating layer 25 define an emission center, which is the center of the light-emitting region in the active layer 19, and define a central axis (emission central axis) AX of the resonator OC. The central axis AX of the resonator OC passes through the center of the protruding portion 21P of the p-type semiconductor layer 21 and extends along a direction perpendicular to the in-plane direction of the semiconductor structure layer 15.

[0044] The light-emitting region of the active layer 19 is, for example, a region having a predetermined width in the active layer 19 from which light of a predetermined intensity or more is emitted, and its center is the light-emitting center. Also, for example, the light-emitting region of the active layer 19 is a region in the active layer 19 into which a current of a predetermined density or more is injected, and its center is the light-emitting center. A straight line passing through the light-emitting center and perpendicular to the upper surface of the substrate 11 or the in-plane direction of each layer of the semiconductor structure layer 15 is the central axis AX. The light-emitting central axis AX is a straight line extending along the cavity length direction of the cavity OC formed by the first multilayer reflecting mirror 13 and the second multilayer reflecting mirror 29. The central axis AX corresponds to the optical axis of the laser light emitted from the surface-emitting laser 10.

[0045] Here, we will explain exemplary configurations of the first multilayer reflector 13, the semiconductor structure layer 15, and the second multilayer reflector 29, as well as exemplary dimensions of the grooves 15G, in the surface-emitting laser 10. In this example, the first multilayer reflector 13 is made of a 1 μm thick GaN underlayer formed on the upper surface of the substrate 11, and 42 pairs of n-GaN and AlInN layers.

[0046] The n-type semiconductor layer 17 is an n-GaN layer with a thickness of 1580 nm. The active layer 19 has a multi-quantum well structure in which four pairs of 4 nm GaInN layers and 5 nm GaN layers are stacked. An Mg-doped AlGaN electron barrier layer is formed on the active layer 19, and a p-type semiconductor layer 21 made of a 50 nm p-GaN layer is formed on top of that. The second multilayer reflector 29 is made by stacking 10.5 pairs of Nb2O5 and SiO2. The resonant wavelength in this case was 440 nm.

[0047] The groove 15G formed in the semiconductor structure layer 15 has an outer diameter of 8 μm, a depth of 120 nm, and a width of 2 μm. The light-transmitting electrode layer 31 formed on the semiconductor structure layer 15 is a layer made of ITO with a thickness of 20 nm, and the second multilayer film reflector 29 is formed on the light-transmitting electrode layer 31 and the p-electrode 27 with a spacer layer of NbO5 with a thickness of 40 nm sandwiched therebetween.

[0048] The rear surface of the substrate 11 is a polished surface, and a two-layer AR coating of Nb2O5 and SiO2 is formed on the polished surface.

[0049] Furthermore, p-type semiconductor layer 21 may have a layer thickness of 50 nm in protruding portion 21P and a layer thickness of 30 nm in other regions. That is, p-type semiconductor layer 21 may have a different layer thickness in protruding portion 21P than in other portions. Furthermore, the upper surface of insulating layer 25 is configured to be located at the same height as the upper surface of protruding portion 21P of p-type semiconductor layer 21. Note that these are merely examples.

[0050] The following describes the optical characteristics inside the surface-emitting laser 10. As described above, in the surface-emitting laser 10, the insulating layer 25 has a refractive index lower than that of the p-type semiconductor layer 21. Furthermore, the thicknesses of the other layers between the first multilayer film reflector 13 and the second multilayer film reflector 29 are the same anywhere within the plane if they are within the same layer.

[0051] Therefore, the equivalent refractive index (the optical distance between the first multilayer reflector 13 and the second multilayer reflector 29, which corresponds to the resonant wavelength) within the resonator OC formed between the first multilayer reflector 13 and the second multilayer reflector 29 of the surface-emitting laser 10 differs between the cylindrical central region CA whose top surface shape is defined by the opening 25H and the cylindrical peripheral region PA around it, due to the difference in refractive index between the p-type semiconductor layer 21 and the insulating layer 25.

[0052] Specifically, between the first multilayer reflector 13 and the second multilayer reflector 29, the equivalent refractive index of the peripheral region PA is lower than the equivalent refractive index of the central region, i.e., the equivalent resonant wavelength in the central region CA is smaller than the equivalent resonant wavelength of the peripheral region PA. As described above, light is emitted from the region directly below the opening 25H in the active layer 19. That is, the light-emitting region from which light is emitted in the active layer 19 is the portion of the active layer 19 that overlaps with the central region CA, in other words, the region within the opening 25H of the insulating layer 25 in a top view.

[0053] Thus, the surface-emitting laser 10 is formed with a central region CA including the light-emitting region of the active layer 19, and a peripheral region PA surrounding the central region CA and having a lower refractive index than the central region CA. This suppresses optical loss caused by standing waves in the central region CA diverging (radiating) to the peripheral region PA. That is, much light remains in the central region CA, and laser light LB is extracted to the outside in this state. Therefore, much light is concentrated in the central region CA around the light-emitting central axis AX of the resonator OC, enabling the generation and emission of high-power, high-density laser light.

[0054] As described above, in the surface-emitting laser 10 of this embodiment, the semiconductor structure layer 15 is formed with the groove 15G extending from the upper surface of the p-type semiconductor layer 21 to the active layer 19. The groove 15G relieves strain generated in the active layer 19 in the in-layer direction, improving the internal quantum efficiency of the active layer 19 and enabling an improvement in light-emitting efficiency to be realized. [Manufacturing method] An example of a method for manufacturing the surface-emitting laser 10 is described below. First, a c-plane n-GaN substrate is prepared as the substrate 11. An n-GaN layer (1 μm thick) is formed as an underlayer on the substrate by metalorganic vapor phase epitaxy (MOVPE). Then, 42 pairs of n-GaN / AlInN layers are deposited on the underlayer to form the first multilayer reflector 13.

[0055] Next, Si-doped n-GaN (layer thickness 1580 nm) is formed on the first multilayer reflector 13 to form the n-type semiconductor layer 17, and four pairs of layers made of GaInN (layer thickness 4 nm) and GaN (layer thickness 5 nm) are stacked on top of that to form the active layer 19.

[0056] Next, an electron barrier layer made of Mg-doped AlGaN is formed on the active layer 19 (not shown), and a p-GaN layer (layer thickness 50 nm) is formed on the electron barrier layer to form the p-type semiconductor layer 21.

[0057] Next, the p-type semiconductor layer 21, the active layer 19, and the peripheral portions of the n-type semiconductor layer 17 are etched to form a mesa shape that exposes the top surface 17S of the n-type semiconductor layer 17 in the peripheral portions. In other words, this step completes the semiconductor structure layer 15 having a cylindrical portion made up of the n-type semiconductor layer 17, the active layer 19, and the p-type semiconductor layer 21 shown in FIG. 1.

[0058] Next, a trench 15G is formed by etching, penetrating the active layer 19 from the upper surface of the p-type semiconductor layer 21. Thereafter, an SiO2 film is formed on the semiconductor structure layer 15, and an opening 25H is formed by removing a portion of the SiO2 film, thereby forming the insulating layer 25.

[0059] Next, a 20 nm ITO film is formed on the insulating layer 25 to form the translucent electrode layer 31, and an Au film is formed on the translucent electrode layer 31 and on the upper surface 17S of the n-type semiconductor layer 17 to form the p-electrode 27 and the n-electrode 23, respectively.

[0060] Next, a 40 nm Nb2O5 spacer layer (not shown) is formed on the p-electrode 27 and the transparent electrode layer 31, and 10.5 pairs of layers, each consisting of Nb2O5 / SiO2, are formed on the spacer layer to form the second multilayer reflector 29.

[0061] Next, the rear surface of the substrate 11 is polished, and an AR coating made of Nb2O5 / SiO2 is formed on the polished surface, thereby completing the surface-emitting laser 10. [Example]

[0062] Second Embodiment A surface-emitting laser 40 according to a second embodiment of the present invention will now be described. The surface-emitting laser 40 differs from the surface-emitting laser 10 in that the grooves 15G in the semiconductor structure layer 15 are formed outside the second multilayer film reflector 29 when viewed from above.

[0063] Fig. 4 shows a perspective view of a surface-emitting laser 40 of Example 2. Fig. 5 shows a cross-sectional view of the surface-emitting laser 40 taken along the same cross section as that shown in Example 1. As shown in Figs. 4 and 5, in the surface-emitting laser 40, a groove 15G in the semiconductor structure layer 15 is formed on the upper surface of the p-type semiconductor layer 21, outside the second multilayer film reflector 29. That is, on the upper surface of the p-type semiconductor layer 21, the groove 15G is exposed from the second formation region.

[0064] A groove structure 27G, which is formed by inheriting the shape of the groove 15G, is formed on the upper surface of the p-electrode 27 formed on the p-type semiconductor layer 21 via the insulating layer 25. That is, the groove structure 27G is formed on the groove 15G in substantially the same shape as the groove 15G.

[0065] 5, in the surface-emitting laser 40, as in the surface-emitting laser 10, a groove 15G is formed in the semiconductor structure layer 15, extending from the upper surface of the p-type semiconductor layer 21 to the active layer 19. The groove 15G relieves strain generated in the active layer 19 in the in-layer direction, improving the internal quantum efficiency of the active layer 19 and enabling an improvement in light-emitting efficiency. Note that in the surface-emitting laser 10, the second multilayer reflector 29 does not need to be formed on the p-electrode 27. In other words, since the second multilayer reflector 29 does not need to be formed across a step, the precision of forming the second multilayer reflector 29 can be further improved. [Example]

[0066] Third Embodiment A surface-emitting laser 50 according to a third embodiment of the present invention will now be described. The surface-emitting laser 50 differs from the surface-emitting laser 10 in that an electrode connected to the n-type semiconductor layer 17 is located on the back surface of the substrate 11.

[0067] 6 is a perspective view of a surface-emitting laser 50. The substrate 51 is, for example, a substrate having a rectangular upper surface. The substrate 51 is made of a conductive material such as n-GaN. An n-electrode 52 made of metal is formed on the back surface of the substrate 51.

[0068] A first multilayer reflector 53 made of semiconductor layers grown on the substrate 51 is formed on the substrate 11. The first multilayer reflector 53 is a conductive semiconductor multilayer reflector in which low-refractive-index semiconductor films having an AlInN composition and high-refractive-index semiconductor films having an n-GaN composition and a higher refractive index than the low-refractive-index semiconductor films are alternately stacked. In other words, the first multilayer reflector 53 is a distributed Bragg reflector (DBR) made of semiconductor material. For example, a buffer layer having an n-GaN composition is provided on the upper surface of the substrate 51, and the first multilayer reflector 53 is formed by alternately depositing the high-refractive-index semiconductor films and low-refractive-index semiconductor films on the buffer layer.

[0069] The semiconductor structure layer 15 is a laminated structure made up of multiple semiconductor layers formed on the first multilayer reflector 53. The semiconductor structure layer 15 has an n-type semiconductor layer (first semiconductor layer) 17 formed on the first multilayer reflector 53, a light-emitting layer (or active layer) 19 formed on the n-type semiconductor layer 17, and a p-type semiconductor layer (second semiconductor layer) 21 formed on the active layer 19.

[0070] The n-type semiconductor layer 17 is a semiconductor layer formed on the first multilayer film reflector 53. The n-type semiconductor layer 17 has a GaN composition and is doped with Si as an n-type impurity.

[0071] The active layer 19 is formed on the n-type semiconductor layer 17 and has a quantum well structure including a well layer having an InGaN composition and a barrier layer having a GaN composition. In the surface-emitting laser 50, light is generated in the active layer 19.

[0072] The p-type semiconductor layer 21 is a semiconductor layer having a GaN composition formed on the active layer 19. The p-type semiconductor layer 21 is doped with Mg as a p-type impurity.

[0073] Insulating layer 25 is a layer made of an insulator and formed on p-type semiconductor layer 21. Insulating layer 25 is made of a substance, such as SiO2, that has a lower refractive index than the material that forms p-type semiconductor layer 21. Insulating layer 25 is formed in a ring shape on p-type semiconductor layer 21, and has an opening (not shown) in the center that exposes p-type semiconductor layer 21.

[0074] The p-electrode 27 is a metal electrode formed on the insulating layer 25. The p-electrode 27 is electrically connected to the upper surface of the p-type semiconductor layer 21 exposed from the opening in the insulating layer 25 via a transparent electrode (not shown) made of a metal oxide film such as ITO or IZO.

[0075] The second multilayer reflector 29 is a dielectric multilayer reflector in which low-refractive-index dielectric films made of Al2O3 and high-refractive-index dielectric films made of Ta2O5 and having a higher refractive index than the low-refractive-index dielectric films are alternately stacked. In other words, the second multilayer reflector 29 is a distributed Bragg reflector (DBR) made of a dielectric material.

[0076] 7 is a top view of a surface-emitting laser 50. As described above, the surface-emitting laser 10 has a semiconductor structure layer 15 including an n-type semiconductor layer 17, an active layer 19, and a p-type semiconductor layer 21 formed on a substrate 11 having a rectangular top surface shape (see FIG. 6). An insulating layer 25 and a p-electrode 27 are formed on the p-type semiconductor layer 21. A second multilayer film reflector 29 is formed on the p-electrode 27.

[0077] The insulating layer 25 has an opening 25H, which is a circular opening that exposes the p-type semiconductor layer 21 of the insulating layer 25. As shown in Fig. 2, the opening 25H is formed in the center of the insulating layer 25 when viewed from the top surface of the surface-emitting laser 10, and is covered by the second multilayer reflector 29 when viewed from the top surface of the surface-emitting laser 10. In other words, the opening 25H is covered by the second multilayer reflector 29 on the top surface of the p-type semiconductor layer 21. In other words, the opening 25H is arranged in a region on the top surface of the p-type semiconductor layer 21 that faces the bottom surface of the multilayer reflector 29.

[0078] The p-electrode 27 is formed in the center of the insulating layer 25 when viewed from the top surface of the surface-emitting laser 10, and has an opening 27H surrounding the opening 25H. That is, the opening 27H is larger than the opening 25H. For example, the shape of the opening 27H is a circle that is concentric with the shape of the opening 25H.

[0079] 7, an annular groove 15G is formed in the upper surface of the p-type semiconductor layer 21, i.e., the upper surface of the semiconductor structure layer 15. The groove 15G is formed in the region outside the openings 25H and 27H. In this embodiment, the groove 15G is formed so as to be covered by the second multilayer film reflector 29 on the upper surface of the p-type semiconductor layer 21. That is, in this embodiment, the groove 15G is formed at a position facing the lower surface of the second multilayer film reflector 29.

[0080] Fig. 8 is a cross-sectional view of the surface-emitting laser 50 taken along line 8-8 in Fig. 7. As described above, the surface-emitting laser 50 has the substrate 51, which is an n-GaN substrate, and the first multilayer film reflector 53 is formed on the substrate 51.

[0081] A protrusion 51P is formed on the rear surface 51A of the substrate 51. When viewed from the normal direction of the substrate 51, the protrusion 51P is formed in a region corresponding to the protrusion 21P. This protrusion 51P is a convex portion that remains after the rear surface 51A is polished and the area around the protrusion 51P is removed by dry etching. Therefore, the top surface of the protrusion 51P is a polished surface, and the area around the protrusion 51P on the rear surface 51A of the substrate 51 is a surface that is obtained by dry etching the polished surface. The n-electrode 52 is formed in the area around the protrusion 51P on the rear surface 51A of the substrate 51, i.e., the area excluding the protrusion 51P. The top surface of the protrusion 51P serves as an opening through which emitted light is released to the outside, and the emitted light is not blocked by the n-electrode 52. In other words, the protrusion 51P protrudes from the opening of the n-electrode 52.

[0082] The antireflection layer 55 is formed on the rear surface 51A of the substrate 51 so as to cover the protruding portion 51P. The antireflection layer 55 is made of, for example, a dielectric multilayer film and has a structure in which, for example, Ta2O5 layers and SiO2 layers are alternately stacked multiple times. The antireflection layer 55 is a so-called AR coating that suppresses reflection of light emitted from the active layer 19 on the top surface of the protruding portion 51P of the substrate 51.

[0083] A semiconductor structure layer 15 is formed on the first multilayer film reflector 53. The semiconductor structure layer 15 is a laminated structure in which an n-type semiconductor layer 17, an active layer 19, and a p-type semiconductor layer 21 are formed in this order.

[0084] The p-type semiconductor layer 21 has a cylindrical protruding portion 21P protruding upward in the center of its upper surface. The groove 15G formed in the semiconductor structure layer 15 is formed so as to surround the protruding portion 21P on the upper surface of the p-type semiconductor layer 21, and extends from the upper surface of the p-type semiconductor layer 21 through the active layer 19 to the n-type semiconductor layer 17.

[0085] As described above, in the surface-emitting laser 50 of Example 3, similarly to the surface-emitting lasers 10 and 40 of the above-described examples, the grooves 15G are formed so as to penetrate the active layer 19. In other words, gaps are formed in the active layer 19 by the grooves 15G.

[0086] The insulating layer 25 is formed to cover the upper surface of the p-type semiconductor layer 21 and the inner surface of the groove 15G. As described above, the insulating layer 25 is made of a material having a lower refractive index than the p-type semiconductor layer 21. The insulating layer 25 has an opening 25H that exposes the protruding portion 21P. For example, as shown in FIG. 7, the opening 25H is circular. For example, the opening 25H and the protruding portion 21P have the same shape, and the inner surface of the opening 25H and the outer surface of the protruding portion 21P are in contact with each other.

[0087] The light-transmitting electrode layer 31 is a layer made of a light-transmitting conductor formed so as to cover the insulating layer 25 and the protruding portion 21P exposed from the opening 25H of the insulating layer 25. The light-transmitting electrode layer 31 is formed of a metal oxide such as ITO or IZO that is light-transmitting to the light emitted from the active layer 19.

[0088] As described above, the p-electrode 27 is a metal electrode, and is formed so as to cover the translucent electrode layer 31. The p-electrode 27 has an opening 27H in the center that exposes the translucent electrode layer 31. The opening 27H is wider than the opening 25H.

[0089] The second multilayer reflector 29 is formed so as to cover the opening 27H and the groove 15G. The second multilayer reflector 29 is formed so as to fill the space formed by the opening 27H and to be in contact with the light-transmitting electrode layer 31. The second multilayer reflector 29 is also formed so as to fill the space formed by the groove 15G.

[0090] In the surface-emitting laser 50, the first multilayer reflector 53 has a reflectivity slightly lower than that of the second multilayer reflector 29. Therefore, part of the light resonating between the first multilayer reflector 53 and the second multilayer reflector 29 passes through the first multilayer reflector 53 and the substrate 51 and is extracted to the outside.

[0091] As described above, in the surface-emitting laser 50 of Example 3, the groove 15G is formed to penetrate the active layer 19. In other words, the groove 15G forms a gap in the active layer 19. The groove 15G is formed after the semiconductor structure layer 15 is formed. Thereafter, the insulating layer 25 is formed before the light-transmitting electrode layer 31, the p-electrode 27, and the second multilayer film reflector 29 are formed.

[0092] Therefore, after the semiconductor structure layer 15 is formed, the groove 15G is formed to reach the active layer 19, thereby forming a gap in the in-plane direction of the active layer 19. This gap relieves strain that occurs in the in-plane direction of the active layer 19 or the semiconductor structure layer 15 during the formation of the active layer 19.

[0093] Specifically, as mentioned above, the crystal structure of the active layer 19 is distorted due to the difference in lattice constants between the InGaN and GaN that form the quantum well structure during its formation, which causes piezoelectric polarization and generates a piezoelectric field. This piezoelectric field reduces the recombination probability between electrons injected into the light-emitting layer and electrons, lowering the internal quantum efficiency.

[0094] In the surface-emitting laser 50, a groove 15G is formed in the semiconductor structure layer 15, reaching the active layer 19. The gap created by this groove is thought to relieve strain that occurs in the active layer 19 in the in-layer direction during growth of the active layer 19, thereby improving the internal quantum efficiency of the active layer 19.

[0095] Here, we will explain the operation of the surface-emitting laser 50. When a voltage is applied between the n-electrode 52 and the p-electrode 27 in the surface-emitting laser 50, a current flows in the semiconductor structure layer 15 as shown by the thick dashed line in the figure, and light is emitted from the active layer 19. The light emitted from the active layer 19 is repeatedly reflected between the first multilayer film reflector 53 and the second multilayer film reflector 29, and reaches a resonance state (laser oscillation).

[0096] In the surface-emitting laser 50, similarly to the surface-emitting laser 10, current is injected into the p-type semiconductor layer 21 only from the portion exposed by the opening 25H. Furthermore, because the p-type semiconductor layer 21 is very thin, current does not diffuse in the in-plane direction within the p-type semiconductor layer 21, i.e., in the direction along the in-plane direction of the semiconductor structure layer 15. Therefore, in the surface-emitting laser 10, current is supplied only to the region of the active layer 19 directly below the opening 25H, and light is emitted only from that region. That is, in the surface-emitting laser 50, the opening 25H forms a current confinement structure that limits the range of current supply in the active layer 19.

[0097] As described above, in this embodiment, the first multilayer reflector 53 has a reflectivity slightly lower than that of the second multilayer reflector 29. Therefore, part of the light resonating between the first multilayer reflector 53 and the second multilayer reflector 29 passes through the first multilayer reflector 53 and the substrate 51 and is extracted to the outside from the protrusion 51P. In this way, the surface-emitting laser 10 emits light in a direction perpendicular to the lower surface of the substrate 51 and the in-plane direction of each layer of the semiconductor structure layer 15.

[0098] The explanation of the light emission central axis AX and the like is omitted since it is the same as that of the surface emitting laser 10 of the first embodiment.

[0099] The following describes exemplary configurations of the first multilayer reflector 53, the semiconductor structure layer 15, and the second multilayer reflector 29, as well as exemplary dimensions of the grooves 15G, in the surface-emitting laser 50. In this example, the first multilayer reflector 53 is made of a 1 μm n-GaN underlayer formed on the upper surface of the substrate 51, and 42 pairs of n-GaN and AlInN layers.

[0100] The n-type semiconductor layer 17 is a Si-doped n-GaN layer with a thickness of 1580 nm. The active layer 19 has a multi-quantum well structure in which four pairs of 4 nm GaInN layers and 5 nm GaN layers are stacked. An Mg-doped AlGaN electron barrier layer is formed on the active layer 19, and a p-type semiconductor layer 21 made of a 50 nm p-GaN layer is formed on top of that. The second multilayer reflector 29 is made by stacking 10.5 pairs of Nb2O5 and SiO2.

[0101] The groove 15G formed in the semiconductor structure layer 15 has an outer diameter of 20 μm, a depth of 120 nm, and a width of 1 μm. The light-transmitting electrode layer 31 formed on the semiconductor structure layer 15 is a layer made of ITO with a thickness of 20 nm, and the second multilayer film reflector 29 is formed on the light-transmitting electrode layer 31 and the p-electrode 27 with a spacer layer of NbO5 with a thickness of 40 nm sandwiched therebetween.

[0102] Furthermore, p-type semiconductor layer 21 may have a layer thickness of 50 nm in protruding portion 21P and a layer thickness of 30 nm in other regions. That is, p-type semiconductor layer 21 may have a different layer thickness in protruding portion 21P than in other portions. Furthermore, the upper surface of insulating layer 25 is configured to be located at the same height as the upper surface of protruding portion 21P of p-type semiconductor layer 21. Note that these are merely examples.

[0103] The optical characteristics inside the surface-emitting laser 50 are similar to those of the surface-emitting laser 10, and therefore a description thereof will be omitted.

[0104] As described above, in the surface-emitting laser 50 of this embodiment, similarly to the surface-emitting laser 10 of Example 1, the semiconductor structure layer 15 is formed with a groove 15G extending from the upper surface of the p-type semiconductor layer 21 to the active layer 19. The groove 15G relieves strain generated in the active layer 19 in the in-layer direction, improving the internal quantum efficiency of the active layer 19 and enabling an improvement in light-emitting efficiency to be realized. [Example]

[0105] A surface-emitting laser 60 according to a fourth embodiment of the present invention will now be described. The surface-emitting laser 60 differs from the surface-emitting laser 50 in that the grooves 15G in the semiconductor structure layer 15 are formed outside the second multilayer film reflector 29 when viewed from above.

[0106] Fig. 9 shows a perspective view of a surface-emitting laser 60 of Example 4. Fig. 10 shows a cross-sectional view of the surface-emitting laser 60 taken along the same cross section as that shown in Example 3. As shown in Figs. 9 and 10, in the surface-emitting laser 60, the grooves 15G of the semiconductor structure layer 15 are formed on the upper surface of the p-type semiconductor layer 21, outside the second multilayer film reflector 29. That is, on the upper surface of the p-type semiconductor layer 21, the grooves 15G are exposed to the outside of the second multilayer film reflector.

[0107] A groove structure 27G, which inherits the shape of the groove 15G, is formed on the upper surface of the p-electrode 27, which is formed on the p-type semiconductor layer 21 via the insulating layer 25. That is, the groove structure 27G is formed on the groove 15G in substantially the same shape as the groove 15G. As shown in FIG. 10 , in the surface-emitting laser 40, similar to the surface-emitting laser 10, a groove 15G is formed in the semiconductor structure layer 15, extending from the upper surface of the p-type semiconductor layer 21 to the n-type semiconductor layer 17. The groove 15G relieves strain generated in the active layer 19 in the layer direction of the active layer 19, improving the internal quantum efficiency, e.g., slope efficiency, and thus light-emitting efficiency. Note that in the surface-emitting laser 10, the second multilayer reflector 29 does not need to be formed on the p-electrode 27. That is, since the second multilayer reflector 29 does not need to be formed across a step, the precision of forming the second multilayer reflector 29 can be further improved. [Modification of semiconductor structure layer] In the above embodiment, the groove 15G formed in the semiconductor structure layer 15 has an annular shape. However, the groove 15G may have another shape.

[0108] For example, as shown in FIG. 11, the groove 15G may have a groove structure including recesses GV (or grooves GV) formed intermittently rather than in a complete ring in the annular region CR (the region surrounded by the dashed line in the figure). That is, the groove 15G may have a recess structure including a plurality of recesses. In other words, the groove 15G may have an intermittently formed annular structure. For example, the groove 15G may have an intermittently formed annular structure surrounding the region exposed by the opening 25H in the upper surface of the p-type semiconductor layer 21.

[0109] In addition, when the groove 15G is formed by a plurality of recesses, it is desirable that the recesses are formed in two or more directions as viewed from the above-mentioned light-emitting central axis AX in order to uniformly relieve strain in the active layer 19. That is, when the groove 15G is formed by a plurality of recesses, it is preferable that the recesses are formed in two or more directions as viewed from the light-emitting region including the light-emitting central axis AX in a top view of the surface-emitting laser 10, so that the light-emitting region is sandwiched between the recesses. In order to uniformly relieve strain in the active layer 19, it is more preferable that the recesses forming the groove 15G are arranged rotationally symmetrically with respect to the light-emitting central axis AX. In other words, it is preferable that the recesses forming the groove 15G are arranged rotationally symmetrically as viewed from a direction perpendicular to the in-plane direction of the semiconductor structure layer 15.

[0110] As described above, by changing the shape of the groove 15G, for example, by forming the groove 15G with a plurality of recesses, it is possible to provide anisotropy in the degree of strain relaxation in the active layer 19 and control the polarization of the light emitted from the surface-emitting laser of the above-described embodiment. For example, in the case of the surface-emitting laser shown in Fig. 11, the strain in the active layer 19 is further relaxed in the direction in which the recesses GV forming the groove 15G exist as viewed from the opening 25H, i.e., as viewed from the central region CA. This increases the optical gain in the direction in which the recesses GV forming the groove 15G exist as viewed from the opening 25H (the left-right direction in the figure), making it possible to obtain a large amount of light polarized along that direction.

[0111] In order to relax the strain in the active layer 19 and to control the polarization of the light emitted from the surface-emitting laser by providing anisotropy to the degree of relaxation of the strain, it is preferable to form the groove 15G near the light-emitting central axis AX. Therefore, for example, as in the surface-emitting laser 10 of Example 1 and the surface-emitting laser 50 of Example 3, it is preferable to form the groove 15G below the second multilayer film reflector 29. Note that, in order to relax the strain in the active layer 19, it is preferable to form the groove 15G or the plurality of grooves GV within a distance of, for example, 50 μm from the outer edge of the light-emitting region. That is, it is preferable that the annular region CR in which the groove 15G is formed is located within a distance of 50 μm from the outer edge of the light-emitting region or the central region CA.

[0112] In the above-described embodiment, the surface-emitting laser emits a circular laser beam, but the surface-emitting laser may emit an annular laser beam. That is, when viewed from above the surface-emitting laser of the above-described embodiment, a circular insulating layer may be formed in the center of the opening 25H of the insulating layer 25. In other words, the insulating layer 25 may have an annular hole, through which the upper surface of the p-type semiconductor layer 21 is exposed, and the light-transmitting electrode layer 31 and the p-type semiconductor layer 21 may be in electrical contact.

[0113] In this case, an inner recess IG may be further formed inside the groove 15G of the semiconductor structure layer 15. This recess IG also penetrates the active layer 19 from the upper surface of the p-type semiconductor layer 21, similar to the groove 15G.

[0114] Fig. 12 is a top view of a modified example in which a cylindrical inner recess IG is formed in the surface-emitting laser 10 of Example 1. Fig. 13 is a top view of a modified example in which a cylindrical inner recess IG is formed in the surface-emitting laser 10 of Example 1. By forming the inner recess IG in this way, it is possible to further reduce the strain in the active layer 19. [Example]

[0115] A surface-emitting laser 70 according to a fifth embodiment of the present invention will be described below. The surface-emitting laser 70 of this embodiment differs from the surface-emitting laser 10 of the first embodiment in that a C-plane GaN substrate inclined (offset) from the c-plane toward the m-plane (1-100) direction is used as the substrate 11, and in that the grooves 15G have a different configuration. The surface-emitting laser 70 can be formed by the same manufacturing method as the surface-emitting laser 10 of the first embodiment.

[0116] Fig. 14 is a perspective view of the surface-emitting laser 70 as seen obliquely from above and the front. Fig. 15 is a top view of the surface-emitting laser 70. As described above, in the surface-emitting laser 70, the top surface of the substrate 11 is offset from the c-plane of the GaN crystal to the m-plane.

[0117] Specifically, the top surface of substrate 11 is inclined at an angle of 0.4° from the c-plane toward the m-plane. In other words, substrate 11 is a GaN substrate having a primary surface that is offset from the c-plane toward the m-plane by 0.4°. In FIG. 15 , axis AX1 is an axis that extends along the m-axis direction perpendicular to the m-plane of substrate 11 and is located in a plane that includes the top surface of substrate 11, and axis AX2 is an axis that is perpendicular to axis AX1 and is located in a plane that includes the top surface of substrate 11.

[0118] In the surface-emitting laser 70, the groove 15G is made up of two grooves GV extending parallel to each other along the axis AX2. In other words, the groove 15G is made up of two grooves GV extending on either side of the axis AX2. That is, the grooves 15G are arranged in the direction along the axis AX1 so as to sandwich an opening 25H in the insulating layer 25 that overlaps with the light-emitting region of the active layer 19.

[0119] Moreover, each of the grooves GV extends on the upper surface of the p-type semiconductor layer 21 from a region directly below the second multilayer reflector 29 to the outside of the second multilayer reflector 29. In other words, both ends of each of the grooves GV are disposed outside the second multilayer reflector 29. Therefore, the groove structure 27G having a shape similar to that of the groove 15G also extends from the region directly below the second multilayer reflector 29 to the outside of the second multilayer reflector 29.

[0120] In the surface-emitting laser 70, the groove GV is not formed in a region extending along the axis AX2 as viewed from the opening 25H. That is, in the surface-emitting laser 70, the groove GV is formed only in a region extending along the axis AX1 as viewed from the opening 25H.

[0121] The polarization direction of the emitted light from the surface-emitting laser 70 will be described below. When the semiconductor layer is grown on a growth surface offset to the m-plane of the substrate 11 as in the surface-emitting laser 70 of this embodiment, the polarization direction of the emitted light is polarization Since the optical gain in the m-axis direction is larger than in other directions, laser light polarized in the m-axis direction is likely to oscillate. Therefore, the light emitted from the central region CA of the surface-emitting laser 70 is mostly light polarized in the m-axis direction.

[0122] Furthermore, as described above, the surface-emitting laser 70 has the groove structure 15G in which the grooves GV are formed only in the region along the axis AX1 that runs along the m-axis direction as viewed from the opening 25H. Therefore, strain in the active layer 19 is strongly relaxed in the direction in which the grooves GV are formed as viewed from the opening 25H, i.e., the central region CA that serves as the light-emitting region, i.e., in the direction along AX1.

[0123] As a result, in the surface-emitting laser 70, the gain increases mainly in the direction along the axis AX1 along the m-axis direction, and this, combined with the effect of the offset, makes it possible to obtain a large amount of laser light polarized in the direction along the m-axis direction. Note that, to obtain the offset effect, it is preferable that the top surface of the substrate 11 be inclined from the c-plane toward the m-plane by 0.1 to 0.5 degrees.

[0124] Furthermore, the surface-emitting laser 70 is formed by growing the semiconductor structure layer 15 in the same manner as the surface-emitting laser 10 of Example 1, but by performing a heat treatment after forming the grooves 15G, it is possible to obtain a surface-emitting laser that can obtain even more laser light polarized in the direction along the m-axis.

[0125] Specifically, after forming the groove 15G and before forming the insulating layer 25, a surface-emitting laser capable of obtaining even more laser light polarized along the m-axis direction can be formed by performing a heat treatment at a temperature of, for example, 500° C. This is thought to be because the heat treatment increases the conductivity of the current path along the axis AX1 in the p-type semiconductor layer 21, among the current paths to the active layer 19.

[0126] The increase in conductivity in the current path in the direction along axis AX1 is due to hydrogen being desorbed from the p-GaN forming the p-type semiconductor layer 21 exposed in groove 15G, thereby increasing the conductivity of the p-type semiconductor layer 21 around the portion exposed from groove 15G.

[0127] In addition to the increase in the laser light polarized along the axis AX direction due to the relaxation of the strain in the active layer 19 along the axis AX1 and the offset of the growth surface of the substrate 11 as described above, the heat treatment described above makes it possible to further increase the laser light polarized along the axis AX direction in the surface-emitting laser 70.

[0128] The heat treatment performed after forming the grooves 15G and before forming the insulating layer 25 may be performed at a temperature at which hydrogen is desorbed from the p-GaN that forms the p-type semiconductor layer 21. Specifically, for example, the heat treatment is preferably performed at 400° C. or higher to desorb hydrogen from the p-GaN.

[0129] In order to enhance the effect of increasing the amount of light polarized along the axis AX1, the grooves 15G are preferably formed symmetrically with respect to the axis AX2. The grooves 15G preferably have a shape that follows the outer edge of the light-emitting region. Therefore, in the surface-emitting laser 70, as shown in FIG. 16, the grooves 15G preferably have a shape that follows the arc shown in FIG. 11.

[0130] In the above embodiment, it is preferable that the groove 15G is formed in a circular region 50 μm or less from the outer edge of the light-emitting region in order to relieve distortion in the active layer 19. In order to control the polarization direction of the laser light, it is further preferable that the groove 15G is formed in a region where the emission intensity of the active layer when the surface-emitting laser is viewed from above is 1.8% or less of the emission peak of the active layer.

[0131] In other words, it is preferable that the region where the groove 15G is formed, for example, the annular region CR in FIG. 16, is located at a distance of 50 μm or less from the outer edge of the light-emitting region and is a region where the light-emitting intensity of the active layer when the surface-emitting laser is viewed from above is 1.8% or less of the light-emitting peak of the active layer.

[0132] This is to prevent the occurrence of the confinement effect of the grooves 15G, which confines the light emitted from the active layer 19 inside the formation region of the grooves 15G. If this confinement effect occurs, the polarization direction will vary in directions other than along the axis AX1 compared to when the confinement effect does not occur, so it is preferable not to cause the confinement effect.

[0133] Preferably, the groove 15G is formed so as not to overlap the opening 25H in the direction along the axis AX2, which suppresses the occurrence of relaxation of the strain of the active layer 19 in the direction along the axis AX2, and suppresses the generation of light polarized in the direction along the axis AX2.

[0134] In other words, by increasing the difference in strain relaxation between the direction along the axis AX1 and the direction along the axis AX2 of the active layer 19 and enhancing the anisotropy of strain relaxation, it is possible to increase the proportion of light emitted from the surface-emitting laser 70 that has a polarization direction along the axis AX1.

[0135] Although the fifth embodiment has described a case in which the upper surface of the substrate 11 is offset from the c-plane to the m-plane, which is a nonpolar plane, the upper surface of the substrate 11 may be offset from the c-plane to the a-plane, which is another nonpolar plane. In that case, in the surface-emitting laser 70 of the fifth embodiment, the axis AX1 is an axis along the a-axis, and the groove GV is formed in a region in a direction along the axis AX1 as viewed from the opening 25H.

[0136] As described above, the surface-emitting laser of Example 5 can obtain light that contains a large amount of linearly polarized light polarized in a specific direction, i.e., light with a uniform polarization direction. Since the light emitted from the surface-emitting laser 70 of Example 5 is itself light with a uniform polarization direction, it is possible to minimize the loss of light that would otherwise be caused by using an optical system such as a liquid crystal or a polarizing element, and easily obtain light with a specific polarization direction.

[0137] For example, a surface-emitting laser such as the surface-emitting laser 70 of the fifth embodiment is useful when obtaining sensor light that requires light with a uniform polarization direction, such as light for communication such as life-fibre.

[0138] In the above-described embodiment, the insulating layer 25 is provided to form an insulating region to cause current confinement and to form a region with a low refractive index. However, instead of providing the insulating layer 25, other methods may be used to create a region with a current confinement and a low refractive index. For example, the insulating region and the region with a low refractive index may be formed by etching the surface of the semiconductor structure layer 15 on which the insulating layer 25 is provided in the above-described embodiment. Alternatively, the insulating region and the region with a low refractive index may be formed by ion implantation into the surface of the semiconductor structure layer 15 on which the insulating layer 25 is provided, thereby achieving the same effect as that achieved by forming the insulating layer 25 in the above-described embodiment. When ion implantation is performed, for example, B ions, Al ions, or oxygen ions are implanted into the semiconductor structure layer 15.

[0139] In the surface-emitting laser element as described above, the semiconductor structure layer 15 may be formed by stacking a p-GaN layer, an active layer similar to that in the above embodiment, and an n-GaN layer in this order on the n-type semiconductor layer 17. In this case, the region of the p-GaN layer that contacts the n-type semiconductor layer 17 overlaps with the central region CA in the above embodiment in top view. + -GaN layer and p + A tunnel junction layer made of -GaN may be formed.

[0140] In the semiconductor structure layer having this configuration, current flows only from the tunnel junction layer from the p-GaN layer to the n-type semiconductor layer 17. This makes it possible to generate current confinement similar to that achieved by forming the insulating layer 25 described above.

[0141] The various numerical values, dimensions, materials, etc. in the above-described embodiments are merely examples, and can be appropriately selected depending on the application and the surface-emitting laser to be manufactured. [Explanation of symbols]

[0142] 10, 40, 50, 60, 70 Surface-emitting laser 11, 51 board 13, 53 First multilayer mirror 15 Semiconductor structural layer 17 n-type semiconductor layer 19 Active layer 21 p-type semiconductor layer 23, 52 n electrode 25 insulating layer 27p electrode 29 Second multilayer mirror 31 Transparent electrode layer

Claims

1. a gallium nitride semiconductor substrate; a semiconductor structure layer including: a first semiconductor layer made of a nitride semiconductor having a first conductivity type formed above the gallium nitride based semiconductor substrate; an active layer made of a nitride semiconductor formed on the first semiconductor layer; and a second semiconductor layer formed on the active layer and made of a nitride semiconductor having a second conductivity type opposite to the first conductivity type; a first electrode layer in electrical contact with the first semiconductor layer of the semiconductor structure layer; a second electrode layer formed on an upper surface of the semiconductor structure layer and electrically contacting the second semiconductor layer of the semiconductor structure layer in a region of the upper surface; a first multilayer reflector disposed on the first semiconductor layer side of the semiconductor structure layer; a second multilayer film reflector disposed on the second semiconductor layer side of the semiconductor structure layer and constituting a resonator between itself and the first multilayer film reflector, the semiconductor structure layer has one recessed structure including one or more recessed portions penetrating the active layer from the upper surface in a region surrounding the one region, the recessed structure is provided within 50 μm from the outer edge of the first region, a vertical cavity light emitting device, wherein the surface of the gallium nitride based semiconductor substrate on which the semiconductor structure layer is formed is a surface offset from the c-plane to one of the m-plane and the a-plane, and the one or more recesses extend in a region on the upper surface of the semiconductor structure layer in a direction along a first axis that is perpendicular to the first crystal plane as viewed from the first region, but are not formed in a region that is perpendicular to the direction along the first axis as viewed from the first region.

2. 2. The vertical cavity light emitting device according to claim 1, wherein the recessed structure is formed in a region where the luminous intensity of the active layer when viewed from above is 1.8% or less of the peak luminous intensity of the active layer.

3. 3. The vertical cavity light emitting device according to claim 1, wherein the recessed structure is formed so as to overlap the second multilayer film reflector when viewed in a direction perpendicular to the in-plane direction of the semiconductor structure layer.

4. 4. The vertical cavity light emitting device according to claim 1, wherein the recessed structure is formed around the second multilayer film reflector when viewed in a direction perpendicular to the in-plane direction of the semiconductor structure layer.

5. 5. The vertical cavity light emitting device according to claim 1, wherein the second semiconductor layer and the second electrode layer are electrically insulated from each other in a region surrounding the first region on the upper surface of the semiconductor structure layer.

6. 6. The vertical cavity light emitting device according to claim 5, wherein the second semiconductor layer and the second electrode layer in the region surrounding the first region are electrically insulated from each other by an insulating layer.

7. 6. The vertical cavity light emitting device according to claim 5, wherein the second semiconductor layer and the second electrode layer in the region surrounding the first region are electrically insulated by an etched surface of the second semiconductor layer.

8. 6. The vertical cavity light emitting device according to claim 5, wherein the second semiconductor layer and the second electrode layer in the region surrounding the first region are electrically insulated by an ion-implanted surface of the second semiconductor layer.

9. 9. The vertical cavity light emitting device according to claim 1, wherein the first multilayer reflector is a semiconductor multilayer reflector.

10. 10. The vertical cavity light emitting device according to claim 1, wherein the first multilayer reflector is disposed between the gallium nitride based semiconductor substrate and the semiconductor structure layer.

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