III-nitride semiconductor photonic crystal surface-emitting laser device

Abstract

To provide stable, high beam quality laser elements that suppress higher-order mode oscillation and maintain the fundamental mode even at high current injection.SOLUTION: The present invention provides a group III nitride semiconductor photonic crystal surface emitting laser element which has an n-type group III nitride semiconductor layer formed on a substrate with embedded photonic crystal layers with vacancies arranged in a two-dimensional periodic pattern, an active layer formed on an n-type group III nitride semiconductor layer, a p-type group III nitride semiconductor layer formed on the active layer, a mesa-shaped mesa portion formed by a translucent oxide conductor on the surface of a p-type group III nitride semiconductor layer, and an insulating film formed on the surface other than the mesa portion on the p-type III nitride semiconductor layer, and in which the refractive index of the insulating film is smaller than that of the mesa portion, and the mesa portion is formed inside the vacancy formation area when viewed perpendicular to the photonic crystal layer.SELECTED DRAWING: Figure 10

Description

[Technical Field]

[0001] The present invention relates to a group III nitride semiconductor photonic crystal surface-emitting laser device. [Background technology]

[0002] In recent years, development of photonic-crystal surface-emitting lasers using photonic crystals (PC) has been progressing.

[0003] For example, Non-Patent Document 1 discloses the in-plane diffraction effect and threshold gain difference of a photonic crystal laser, and Non-Patent Document 2 discloses a three-dimensional coupled wave model of a square lattice photonic crystal laser.

[0004] Also known is a photonic crystal surface emitting laser having a multi-lattice photonic crystal configured by arranging a plurality of holes of different sizes at lattice points.

[0005] For example, Patent Document 1 describes a two-dimensional photonic crystal surface-emitting laser light source having a two-dimensional photonic crystal in which a plate-shaped base material is made up of a plurality of regions with a refractive index different from that of the base material, and a large number of assemblies of modified refractive index areas, at least two of which have thicknesses different from each other, are periodically arranged within the base material.

[0006] Furthermore, Non-Patent Document 3 discloses that the hole size and lattice constant of a photonic crystal are changed to suppress multimode oscillation that causes beam quality degradation.

[0007] However, since photonic crystals have very small holes, it is difficult to manufacture them with precise hole sizes and lattice constants.

[0008] In such two-dimensional photonic crystal surface-emitting laser elements, it is important to suppress higher-order mode oscillation, maintain the fundamental mode, and realize a laser element with high beam quality that is stable even when high current is injected. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Patent No. 4294023 [Non-patent literature]

[0010] [Non-Patent Document 1] Tanaka et al., 2016 Autumn Applied Physics Society Proceedings 15p-B4-20 [Non-patent document 2] Y. Liang et al.:Phys. Rev.B Vol.84(2011)195119 [Non-patent document 3] M. Yoshida et al., Proceedings of the IEEE (2019). “Experimental Investigation of Lasing Modes in Double-Lattice Photonic-Crystal Resonators and Introduction of In-Plane Heterostructures.” Summary of the Invention [Problem to be solved by the invention]

[0011] The present invention aims to realize a photonic crystal surface-emitting laser device that suppresses higher-order mode oscillation, maintains the fundamental mode even when a high current is injected, and produces a stable laser element with high beam quality. [Means for solving the problem]

[0012] In order to achieve the above object, a III-nitride semiconductor photonic crystal surface-emitting laser element according to the present invention comprises: a light-transmitting substrate; an n-type III-nitride semiconductor layer formed on the substrate and having an embedded photonic crystal layer having voids arranged with two-dimensional periodicity; an active layer formed on the n-type III-nitride semiconductor layer; a p-type III-nitride semiconductor layer formed on the active layer; a mesa-shaped mesa portion formed on a surface of the p-type III-nitride semiconductor layer using a light-transmitting oxide conductor; and an insulating film formed on a surface of the p-type III-nitride semiconductor layer other than the mesa portion, wherein the refractive index of the insulating film is smaller than the refractive index of the mesa portion, and the mesa portion is formed inside the region where the voids are formed when viewed from a direction perpendicular to the photonic crystal layer. [Brief explanation of the drawings]

[0013] [Figure 1A] 1 is a cross-sectional view schematically illustrating an example of the structure of a photonic crystal laser device (PCSEL device) 10 according to a first embodiment. [Figure 1B] 1B is an enlarged cross-sectional view schematically showing photonic crystal layer 14P of FIG. 1A and hole pairs 14K arranged in photonic crystal layer 14P. FIG. [Figure 2A] FIG. 2 is a plan view schematically showing the upper surface of the photonic crystal laser 10. [Figure 2B] 1B is a cross-sectional view schematically showing a cross section of photonic crystal layer 14P in a plane parallel to n-guide layer 14 (cross section AA in FIG. 1B). [Figure 2C] FIG. 2 is a plan view schematically showing the bottom surface of photonic crystal laser 10. [Figure 3] FIG. 10 is a top view schematically showing a resist pattern for forming main openings K1 and sub-openings K2 in a two-dimensional arrangement in a square lattice pattern within a plane. [Figure 4] 10 is an SEM image showing the shape of holes in a photonic crystal layer 14P of this example. [Figure 5] FIG. 1 is a cross-sectional view schematically illustrating a PCSEL device 90 of Comparative Example 1. [Figure 6A]FIG. 1 is a diagram showing a far-field pattern of the PCSEL device 10 of Example 1. [Figure 6B] FIG. 10 is a diagram showing the far-field pattern of the PCSEL device 90 of Comparative Example 1. [Figure 7] 1 is a diagram schematically illustrating oscillation mode frequencies with respect to spatial positions, comparing Example 1 and Comparative Example 1. FIG. [Figure 8] FIG. 10 is a cross-sectional view schematically illustrating an example of the structure of a PCSEL device 20 according to Example 2. [Figure 9] FIG. 10 is a cross-sectional view schematically illustrating an example of the structure of a PCSEL device 30 according to Example 3. [Figure 10] FIG. 10 is a cross-sectional view schematically illustrating an example of the structure of a PCSEL device 40 according to Example 4. DETAILED DESCRIPTION OF THE INVENTION

[0014] In the following, preferred embodiments of the present invention will be described, but these may be modified and combined as appropriate. In the following description and accompanying drawings, substantially the same or equivalent parts are designated by the same reference numerals. [Example]

[0015] [Photonic crystal surface-emitting laser structure] A photonic crystal surface-emitting laser (hereinafter also referred to as PCSEL) is an element that has a resonator layer in a direction parallel to the semiconductor light-emitting structure layers (n-guide layer, light-emitting layer, p-guide layer) that constitute the light-emitting element, and emits coherent light in a direction perpendicular to the resonator layer.

[0016] Meanwhile, distributed Bragg reflector (DBR) lasers, which have a pair of resonator mirrors (Bragg reflectors) sandwiching a semiconductor light-emitting structure layer, are known, but photonic crystal surface-emitting lasers (PCSELs) differ from DBR lasers in the following respects: In a photonic crystal surface-emitting laser (PCSEL), light waves propagating in a plane parallel to the photonic crystal layer are diffracted by the diffraction effect of the photonic crystal to form a two-dimensional resonance mode, and are also diffracted in a direction perpendicular to the parallel plane. In other words, the light extraction direction is perpendicular to the resonance direction (in the plane parallel to the photonic crystal layer).

[0017] 1A is a cross-sectional view schematically illustrating an example of the structure of a photonic crystal laser device (PCSEL device) 10 according to Example 1. The PCSEL device 10 is configured by stacking multiple semiconductor layers on a substrate. The semiconductor layers are made of, for example, hexagonal nitride semiconductors such as GaN-based semiconductors.

[0018] More specifically, an n-clad layer (first clad layer of a first conductivity type) 13, an n-guide layer (first guide layer) 14, an active layer 15, a p-guide layer (second guide layer) 16, and a p-contact layer 17 are formed in this order on a substrate 11.

[0019] The n-clad layer (first clad layer of a first conductivity type) 13 and the n-guide layer (first guide layer) 14 constitute a first semiconductor layer 12, and the second guide layer 16 and the p-contact layer 17 constitute a second semiconductor layer 18. The first semiconductor layer 12 includes a semiconductor layer of a first conductivity type (e.g., n-type), and the second semiconductor layer 18 includes a semiconductor layer of a conductivity type opposite to the first conductivity type (e.g., p-type).

[0020] Furthermore, n-guide layer (first guide layer) 14 is made up of lower guide layer 14A, photonic crystal layer (air hole layer or PC layer) 14P, and buried layer 14B. Photonic crystal layer 14P has air holes arranged with two-dimensional periodicity in a plane parallel to the layer.

[0021] The second guide layer 16 is composed of a first p-side semiconductor layer 16A provided on the active layer 15, an electron blocking layer (EBL) 16B provided on the first p-side semiconductor layer 16A, and a second p-side semiconductor layer 16C provided on the electron blocking layer 16B.

[0022] The p-contact layer 17 formed on the second guide layer 16 is a semiconductor layer that improves ohmic contact with the metal electrode, and is formed of a semiconductor layer having a smaller energy band gap and / or a higher impurity concentration than the second p-side semiconductor layer 16C.

[0023] In this specification, a case will be described in which the first conductivity type is n-type and the second conductivity type, which is the opposite conductivity type to the first conductivity type, is p-type, but the first conductivity type and the second conductivity type may also be p-type and n-type, respectively.

[0024] In this specification, "n-" and "p-" mean "n-side" and "p-side," respectively, and do not necessarily mean that they have n-type or p-type. For example, an n-guide layer refers to a guide layer provided on the n-side of the active layer, and may be an undoped layer (or i-layer).

[0025] The n-cladding layer 13 may be composed of multiple layers, not just a single layer. In this case, all layers do not need to be n-layers (n-doped layers), and may include an undoped layer (i-layer). The same applies to the p-guide layer (second guide layer) 16.

[0026] Although the specific and detailed configuration of the semiconductor layers of photonic crystal laser device 10 has been described above, this is merely one example of the device structure. In essence, it is sufficient that the device has a first semiconductor layer (or first guide layer) having photonic crystal layer 14P, a second semiconductor layer (or second guide layer), and an active layer (light-emitting layer) sandwiched between these layers, and is configured to emit light when a current is injected into the active layer.

[0027] For example, a photonic crystal laser element does not necessarily have to have all of the semiconductor layers described above, and may alternatively have various semiconductor layers (e.g., hole barrier layers, light confinement layers, current confinement layers, tunnel junction layers, etc.) for improving element characteristics.

[0028] Furthermore, a circular n-electrode (cathode) 20A is formed on the back surface of substrate 11, and an anti-reflection film 22 is provided inside n-electrode 20A on the back surface of substrate 11, which is the laser light emission surface.

[0029] As shown in FIG. 1A, a groove 16G is formed on the upper surface of the second semiconductor layer 18, extending from the surface of the second semiconductor layer 18 (i.e., the surface of the p-contact layer 17) to the inside of the second p-side semiconductor layer 16C.

[0030] The groove 16G only needs to be formed to a depth that reaches the inside of the second guide layer 16, and may be formed as a groove with a depth that stops within the second p-side semiconductor layer 16C, for example.

[0031] 2A, groove 16G is formed as an annular groove surrounding the periphery of circular p-electrode 20B when viewed from a direction perpendicular to n-guide layer 14 (i.e., photonic crystal layer 14P) (hereinafter also referred to as a top view). That is, groove 16G is a cylindrical groove formed inside hole-formed region 14R of photonic crystal layer 14P. A cross section perpendicular to the circumferential direction of groove 16G has a rectangular or trapezoidal shape, and the bottom surface of groove 16G is an annular flat surface parallel to the semiconductor layer.

[0032] Groove 16G defines mesa portion (hereinafter also simply referred to as mesa) 16M having a cylindrical mesa shape in second guide layer 16. Mesa 16M is formed inside hole formation region 14R when viewed from a direction perpendicular to photonic crystal layer 14P. Mesa 16M is a cylindrical plateau that is coaxial with thin cylindrical hole formation region 14R.

[0033] In addition, a p-electrode (anode) 20B is formed on the p-contact layer 17 of the mesa 16M.

[0034] The side surfaces of the stacked semiconductor layers (i.e., the first semiconductor layer 12, the active layer 15, and the second semiconductor layer 18), the surfaces other than the top surface of the p-electrode 20B, and the inside (side surfaces and bottom surfaces) of the groove 16G are covered with an insulating film 21 such as SiO2. For clarity of illustration, the insulating film 21 is not hatched. In addition, a pad electrode 23 is formed and electrically connected to the p-electrode 20B.

[0035] Light directly emitted from photonic crystal layer 14P (directly emitted light Ld) and light emitted from photonic crystal layer 14P and reflected by p-electrode 20B (reflected emitted light Lr) are emitted to the outside from light emission region 20L on the back surface of substrate 11.

[0036] 1B is an enlarged cross-sectional view schematically showing photonic crystal layer 14P and hole (cavity) pairs 14K arranged in photonic crystal layer 14P in FIG. 1A. Hole pairs 14K (main holes 14K1 and sub-holes 14K2) are formed in a crystal growth plane (semiconductor layer growth plane), i.e., in a plane (cross section AA in the figure) parallel to n-guide layer 14, with a period PK in the shape of a square lattice, for example, and hole pairs 14K are two-dimensionally arranged at square lattice point positions and embedded in n-guide layer 14.

[0037] FIG. 2A is a plan view schematically showing the top surface of photonic crystal laser (PCSEL) 10, FIG. 2B is a cross-sectional view schematically showing a cross section of photonic crystal layer (PC layer) 14P in a plane parallel to n-guide layer 14 (cross section AA in FIG. 1B), and FIG. 2C is a plan view schematically showing the bottom surface of photonic crystal laser (PCSEL) 10.

[0038] As shown in Fig. 2B, in photonic crystal layer 14P, holes (cavities) 14K are provided, for example, in a circular hole-forming region 14R, and are periodically arranged. As shown in Fig. 2C, n-electrode (cathode) 20A is provided as a ring-shaped electrode outside hole-forming region 14R so as not to overlap with hole-forming region 14R when viewed perpendicularly to photonic crystal layer 14P (when viewed from above). The region inside n-electrode 20A is light-emitting region 20L.

[0039] 1A, mesa 16M of second guiding layer 16 has height HM. Also, as shown in FIGS. 1A, 2A, and 2B, diameter DM of mesa 16M is smaller than diameter DP of hole-formed region 14R of photonic crystal layer 14P (DM <DP)。

[0040] The annular n-electrode 20A is formed so as to be coaxial with the void formation region 14R, and the diameter DP of the void formation region 14R is smaller than the inner diameter DE of the n-electrode 20A (DP <DE)。 1. Fabrication process of photonic crystal laser (PCSEL) 10 The following describes in detail the process for fabricating the PCSEL device 10. Metalorganic Vapor Phase Epitaxy (MOVPE) was used as the crystal growth method, and semiconductor layers were grown and stacked on a substrate 11 (growth substrate) by normal pressure (atmospheric pressure) growth. In the process described below, Sn refers to step n.

[0041] Furthermore, the layer thickness, carrier concentration, Group 3 (Group III) and Group 5 (Group V) raw materials, temperature, etc. shown below are merely examples unless otherwise specified. [S1: Board preparation process] As the substrate 11, a GaN single crystal was prepared whose primary surface was the "+c" plane, which is the (0001) plane with Ga atoms arranged on the outermost surface. The primary surface may be just aligned, or may be offset by up to about 1° in the m-axis direction. For example, a substrate offset by up to about 1° in the m-axis direction can achieve mirror-finish growth under a wide range of growth conditions.

[0042] The substrate surface (back surface) on which the light emission region 20L is provided, facing the main surface, is the "-c" plane, which is the (000-1) plane with N atoms arranged on the outermost surface. The -c plane is resistant to oxidation and is therefore suitable as a light extraction surface.

[0043] In this example, an n-type GaN single crystal was used as the GaN substrate 11. The n-type GaN substrate 11 functions as a contact layer with the electrode. [S2: n-clad layer formation process] A 2-μm-thick n-type Al0.04Ga0.96N layer with an Al composition of 4% was grown as the n-cladding layer 13 on a +c-plane GaN substrate 11. The AlGaN layer was grown by supplying ammonia (NH3) as a source of group 5 atoms, and trimethylgallium (TMG) and trimethylaluminum (TMA) as sources of group 3 atoms to the GaN substrate heated to 1100°C.

[0044] Carrier doping was performed by simultaneously supplying silane (SiH4) with the above-mentioned raw materials (Si doping). At this time, the carrier concentration at room temperature was approximately 4 × 1018 cm-3. [S3a: Lower guide layer + hole preparation layer formation process] Next, TMG was supplied and n-type GaN was grown to a layer thickness of 250 nm as n-guide layer 14. Carrier doping was performed by simultaneously supplying silane (SiH4) as in the case of the AlGaN layer. The carrier concentration at this time was approximately 4 x 1018 cm-3. This growth layer was a preparatory layer for forming photonic crystal layer 14P in addition to lower guide layer 14A.

[0045] In the following, for the sake of simplicity and ease of understanding, it is assumed that such a growth layer is formed. The substrate 11 thus formed (substrate with growth layers) may be simply referred to as a substrate. [S3b: Hole formation process] After forming the above preparatory layer, the substrate was removed from the chamber of the MOVPE system, and fine holes (holes) were formed on the surface of the growth layer. After obtaining a clean surface by washing, a silicon nitride film (SixNy) was formed using plasma CVD. An electron beam lithography resist was spin-coated on top of this, and the substrate was placed in the electron lithography system to pattern a two-dimensional periodic structure.

[0046] As shown in Figure 3, a pattern was performed in which pairs of openings, each consisting of a rectangular main opening K1 and a square sub-opening K2 that is smaller than the main opening K1, were arranged two-dimensionally in the resist plane in a square lattice pattern with a period PK = 164 nm. For clarity of the drawing, the openings are shown hatched.

[0047] More specifically, the main apertures K1 are arranged in a square lattice pattern with their centers of gravity CD1 in two mutually orthogonal directions (x and y directions) at a period PK=164 nm, and the sub-apertures K2 are also arranged in a square lattice pattern with their centers of gravity CD2 in the x and y directions at a period PK=164 nm.

[0048] The x and y directions are inclined at 45° with respect to the long axis direction (<11-20> direction) and the short axis direction (<1-100> direction) of the main opening K1, respectively. In this specification, the x and y coordinates are also referred to as hole coordinates.

[0049] The long axis of the main aperture K1 and two sides of the sub-aperture K2 are parallel to the <11-20> direction of the crystal orientation, and the short axis of the main aperture K1 and the other two sides of the sub-aperture K2 are parallel to the <1-100> direction. In this specification, the "long axis or short axis" of a hole refers to the long axis or short axis of the cross section (opening plane) of the hole in a plane parallel to the photonic crystal layer 14P.

[0050] Furthermore, the center of gravity CD2 of the sub-aperture K2 is spaced apart from the center of gravity CD1 of the main aperture K1 by Δx and Δy. Here, Δx=Δy. That is, the center of gravity CD2 of the sub-aperture K2 is spaced apart from the center of gravity CD1 of the main aperture K1 in the <1-100> direction. Specifically, the distance Δx between the centers of gravity in the x direction and the distance Δy between the centers of gravity in the y direction were 65.6 nm (=PK×0.4).

[0051] After developing the patterned resist, the SixNy film was selectively dry-etched using an ICP-RIE (Inductive Coupled Plasma - Reactive Ion Etching) system, which resulted in the formation of main openings K1 and sub-openings K2 arranged in a square lattice pattern with a period of 164 nm, penetrating the SixNy film.

[0052] The period (hole spacing) PK is determined by setting the oscillation wavelength (λ) to 410 nm and the refractive index of GaN (n) was set to 2.5 and PK=λ / n=164 nm.

[0053] Next, the resist was removed, and holes were formed in the GaN surface using the patterned SixNy film as a hard mask. The GaN was dry-etched in the depth direction using a chlorine-based gas and argon gas in an ICP-RIE apparatus, forming pairs of holes (hole pairs) consisting of an elongated cylindrical hole and a cylindrical hole dug perpendicularly to the GaN surface. That is, elongated cylindrical holes and cylindrical holes were formed in the rectangular main opening K1 and the square sub-opening K2. In this process, the holes dug in the GaN surface by this etching are hereinafter referred to as "holes" to distinguish them from the holes (cavities) in the photonic crystal layer 14P. [S3c: Cleaning process] The substrate with the holes formed therein was degreased and washed, and then the SixNy film was removed with buffered hydrofluoric acid (HF). [S3d: Buried layer formation process] This substrate was again introduced into the reactor of the MOVPE apparatus, and ammonia (NH3) was supplied to raise the temperature to 950°C (first embedding temperature). Then, trimethylgallium (TMG) and NH3 were supplied to block the hole pairs (main hole and sub-hole), thereby forming the embedded layer 14B.

[0054] By the above embedding process, n-guide layer 14 was formed, which had photonic crystal layer 14P with a double lattice structure in which hole pairs 14K each consisting of a main hole 14K1 and a sub-hole 14K2 were arranged at each of the square lattice points. [S4: Light-emitting layer formation process] Next, a multiple quantum well (MQW) layer was grown as the active layer 15, which is the light-emitting layer. The barrier and well layers of the MQW were GaN and InGaN, respectively. The barrier layer was grown by cooling the substrate to 820°C and then supplying triethylgallium (TEG) as a group 3 atom source and NH3 as a nitrogen source. The well layer was grown at the same temperature as the barrier layer, supplying TEG and trimethylindium (TMI) as group 3 atom sources and NH3 as a nitrogen source. The center wavelength of the photoluminescence (PL) from the active layer in this example was 412 nm. [S5: First p-side semiconductor layer formation process] After the active layer is grown, the substrate is heated to 1050° C., and a GaN layer is grown as the first p-side semiconductor layer 16A. The first p-side semiconductor layer 16A was grown without doping with a dopant, by supplying TMG and NH 3 . [S6: Electron barrier layer formation process] After the growth of the first p-side semiconductor layer 16A, the electron barrier layer (EBL) 16B was grown while the substrate temperature was maintained at 1050° C. The growth of the EBL 16B was performed using TMG and T MA was performed by supplying NH3 as a nitrogen source and Cp2Mg as a p-dopant. As a result, an EBL16B with an Al composition of 18% and a layer thickness of 17 nm was formed. [S7: Second p-side semiconductor layer formation process] After the growth of the electron barrier layer (EBL) 16B, the second p-side semiconductor layer 16C was grown while maintaining the substrate temperature at 1050°C. The second p-side semiconductor layer 16C was grown by supplying TMG and TMA as group III atom sources and NH3 as a nitrogen source. Cp2Mg was also supplied as a p-dopant. This resulted in the formation of a second p-side semiconductor layer 16C with an Al composition of 6% and a layer thickness of 600 nm. After the growth, activation was performed in an N2 atmosphere at 850°C for 10 minutes, and the carrier concentration of the p-cladding layer (p-AlGaN) 18 was 2×1017 cm-3.

[0055] By forming the second p-side semiconductor layer 16C, the second guide layer 16 including the first p-side semiconductor layer 16A, the EBL 16B, and the second p-side semiconductor layer 16C was formed. [S8: p-contact layer formation process] After growing the second p-side semiconductor layer 16C, the p-contact layer 17 was grown to a thickness of 25 nm while maintaining the substrate temperature at 1050°C. The growth of the p-contact layer 17 was performed by supplying TMG as a group III atom source, NH as a nitrogen source, and CpMg as a dopant. [S9: Element isolation trench formation process] After the formation of the epitaxial growth layer was completed, SiO2 was applied to the surface of the substrate with the growth layer by the spin-on-glass (SOG) method. Element isolation grooves were patterned on the applied SiO2 film using photolithography. Using the SiO2 as a mask, etching was performed by vapor-phase etching until the n-side cladding layer 13 or the growth substrate 11 was exposed. The SiO2 mask was then removed with BHF, and the element isolation grooves were formed. [S10: Groove and mesa formation process] SiO2 was again applied to the surface using the SOG method. The applied SiO2 film was patterned using photolithography to form a mask. Using this SiO2 mask, a groove 16G was formed by vapor-phase etching, reaching the inside of the p-guide layer (second guide layer) 16. As shown in Figure 2A, as a result of forming the annular groove 16G, a cylindrical mesa 16M (mesa portion) of the p-guide layer 16 was formed inside the groove 16G.

[0056] Mesa 16M is a cylindrical plateau that is inside the outer edge of hole formation region 14R when viewed from a direction perpendicular to photonic crystal layer 14P and is concentric with hole formation region 14R. As shown in Fig. 1A, mesa 16M has a height HM. [S11: Anode electrode formation process] A 100 nm thick palladium (Pd) film was formed as a p-electrode metal layer on the surface of the epitaxial growth substrate by electron beam evaporation. The p-electrode metal layer was patterned by photolithography to form a p-electrode 20B with a diameter of 200 μm on the p-contact layer 17. [S12: Insulating film formation process] After forming a mask on the p-electrode 20B using photolithography, a 200 nm thick SiO2 insulating film (protective film) was formed by sputtering. The SiO2 insulating film 21 on the p-electrode 20B was removed by lift-off. [S13: p-side pad electrode formation process] After a mask was formed on the top surface of the PCSEL device 10 using photolithography, a metal layer consisting of titanium / platinum / titanium / gold (Ti / Pt / Ti / Au) was deposited in this order by electron beam evaporation using sputtering. The metal layer around the top surface of the PCSEL device 10 was removed by lift-off to form the pad electrode 23. [S14: Substrate polishing process] Next, the rear surface of the substrate was ground to a thickness of 150 μm and then mirror-polished using diamond slurry and chemical mechanical polishing (CMP). [S15: Process-affected layer removal process] Next, the emission surface was wet etched with KOH solution to remove the damaged layer. Alternatively, dry etching using a chlorine-based gas or a wet etching method may be used. [S16: Cathode electrode formation process] Subsequently, Ti and Au films were formed in this order on the back surface of the substrate 11 by electron beam evaporation, and patterned into a circular shape to form an n-electrode 20A. [S17: Anti-reflection film formation process] On the inside of the annular n-electrode 20A, which is the laser light emitting surface, SiO2 was deposited by sputtering to form an anti-reflection film 22. The anti-reflection layer may be a single layer or may be a multi-layer film combined with another dielectric film. [S12: Singulation process] Finally, laser scribing was performed along the center lines of the substrate separation grooves to obtain individual PCSEL devices 10. 2. Pore layer 4 is a scanning electron microscope (SEM) image showing the shape of holes in photonic crystal layer 14P of this example. The SEM image of photonic crystal layer 14P of Comparative Example 1, which will be described later, is similar.

[0057] To confirm the shape of the filled pores in this example, the laminated structure was processed from the surface using a focused ion beam (FIB) until the pores in photonic crystal layer 14P were exposed, and then SEM observation was performed.

[0058] A vacancy pair consisting of a long hexagonal prism-shaped vacancy (main vacancy) 14K1 and a regular hexagonal prism-shaped vacancy (subvacancy) 14K2 was observed. The main vacancy 14K1 had a long hexagonal prism shape with its long axis parallel to the <11-20> axis. The subvacancy 14K2 was smaller in size (e.g., at least one of the vacancy diameter and depth) than the main vacancy 14K1.

[0059] That is, it was confirmed that a photonic crystal layer 14P having a double lattice structure was formed in which a hole pair 14K consisting of a main hole 14K1 and a sub-hole 14K2 was arranged at each of the square lattice points of the period PK. 3. Device Characteristics The device characteristics were evaluated for the PCSEL device 10 of Example 1 and the PCSEL device 90 of Comparative Example 1, fabricated as described above. The PCSEL device 90 of Comparative Example 1 differed from the PCSEL device 10 of this example only in that it did not have the groove 16G, but was otherwise similar in configuration to the PCSEL device 10 of this example.

[0060] 5 showing the PCSEL device 90 of Comparative Example 1, the mesa 16M is not formed in the PCSEL device 90 of Comparative Example 1, and the surface of the second guide layer 16 is flat. That is, the thicknesses of the p-contact layer 17 and the second guide layer 16 are constant.

[0061] Furthermore, a p-electrode 20B that is coaxial with the void formation region 14R is formed on the surface of the p-contact layer 17. The diameter DC of the p-electrode 20B is equal to the diameter DM of the mesa 16M of the PCSEL device 10 of Example 1 (DC=DM).

[0062] A pulse current with a repetition frequency of 1 kHz and a pulse width of 100 ns (nanoseconds) was passed through the fabricated PCSEL device 10 (Example 1) and PCSEL device 90 (Comparative Example 1), and the beam shape was measured.

[0063] Figure 6A shows the far-field pattern of the PCSEL device 10 of Example 1, and Figure 6B shows the far-field pattern of the PCSEL device 90 of Comparative Example 1. The PCSEL device 10 of Example 1 produced a single-peaked beam with a divergence angle of 0.2 degrees or less. The PCSEL device 90 of Comparative Example 1 produced a single-peaked beam with a divergence angle of 0.2 degrees or less, as well as a linear (cross-shaped) higher-order mode.

[0064] That is, it was confirmed that the PCSEL device 10 of Example 1 suppresses higher-order modes (transverse modes) and produces laser light with a single-peaked beam and high beam quality. This point will be considered below.

[0065] 7 is a diagram showing a schematic diagram of the oscillation mode frequency versus spatial position. It shows a comparison between Example 1 and Comparative Example 1. In the diagram, the band edge frequency FBE (photonic band edge frequency) is shown by a dashed line, and the photonic band gap (PBG) is shown by hatching. It also shows the mesa region (Example 1) and the electrode region (Comparative Example 1), which are current injection regions.

[0066] Here, the photonic band gap (PBG) refers to the forbidden band of light, i.e., the photonic band gap indicates a frequency range in which light cannot exist, and light having a frequency within this range cannot be transmitted.

[0067] 7, in Comparative Example 1, (A) when no current is injected, the band edge frequency FBE is constant inside and outside the electrode region (p-electrode 20B). On the other hand, (B) when current is injected, the carrier density increases and the refractive index decreases in the semiconductor layer inside the electrode region, which is the current injection region, compared to the semiconductor layer outside the electrode region.

[0068] This results in a higher band edge frequency for the semiconductor layer inside the electrode region than for the semiconductor layer outside the electrode region. This means that light leakage into the region outside the electrode region (i.e., the non-transparent absorbing region) is reduced, and lateral light confinement is strengthened. As a result, the threshold gain difference between the fundamental mode and higher-order modes is reduced, making it easier for oscillation to occur in higher-order modes. This, in turn, reduces lateral mode controllability, making multi-mode oscillation more likely to occur.

[0069] On the other hand, in Example 1, the effective refractive index of the semiconductor layer outside the mesa region (i.e., the region of the groove 16G) is smaller than that of the semiconductor layer inside the mesa region. Therefore, (A) when no current is injected, the band edge frequency inside the mesa region is lower than the band edge frequency outside the mesa region.

[0070] Furthermore, (B) when current is injected, even if the carrier density increases in the semiconductor layer inside the mesa region and the refractive index decreases, the difference in band edge frequency between the semiconductor layer inside and outside the mesa region can be reduced. Note that Figure 7 shows a case where this difference is zero. Furthermore, stable fundamental mode oscillation and high-quality laser light beam can be obtained, especially during high current injection operation.

[0071] More specifically, the smaller difference in band edge frequencies results in greater light leakage outside the electrode region (mesa region) compared to Comparative Example 1. The fundamental mode has a peak in electric field intensity at the center of the electrode, while the higher-order modes have peaks in electric field intensity offset from the center of the electrode. Therefore, the effect of light leakage (i.e., loss) is greater in the higher-order modes than in the fundamental mode. As a result, the difference in threshold gain between the fundamental mode and the higher-order modes increases, suppressing the higher-order modes.

[0072] Therefore, it is possible to suppress a decrease in the threshold gain difference between the fundamental mode and the higher-order modes, and to obtain laser light with high beam quality in which higher-order mode oscillation is suppressed. [Example]

[0073] 8 is a cross-sectional view showing a schematic example of the structure of a PCSEL device 20 according to Example 2. The configuration of the semiconductor layers of the PCSEL device 20 according to Example 2 is similar to that described in Example 1. The following description will focus on the mesa structure of the PCSEL device 20 in detail.

[0074] In the PCSEL device 20 of Example 2, similarly to Example 1, a groove 16G is formed in the second semiconductor layer 18, extending from the surface of the second semiconductor layer 18 (i.e., the surface of the p-contact layer 17) to the inside of the second p-side semiconductor layer 16C. In addition, a mesa 16M is formed in the second guide layer 16 by the groove 16G.

[0075] Mesa 16M has a cylindrical shape coaxial with air hole formation region 14R, and is formed inside air hole formation region 14R when viewed from a direction perpendicular to photonic crystal layer 14P. That is, diameter DM of mesa 16M is smaller than diameter DP of air hole formation region 14R of photonic crystal layer 14P (DM <DP)。

[0076] Furthermore, the diameter DG of the periphery of groove 16G (or the periphery of the bottom surface of groove 16G) is equal to or larger than the diameter DP of hole formation region 14R of photonic crystal layer 14P (DP≦DG).

[0077] As explained with reference to FIG. 7 , even in the PCSEL device 20 of Example 2, when a current is injected, the carrier density increases in the semiconductor layer inside the mesa region, causing a decrease in the refractive index, but the difference in band edge frequency between the semiconductor layer inside and outside the mesa region can be reduced.

[0078] Furthermore, in the PCSEL device 20 of Example 2, the outer diameter DG of the groove 16G is equal to or larger than the diameter DP of the hole-formation region 14R (DP≦DG), so that the difference in refractive index (difference in band edge frequency) between the inside and outside of the mesa region during current injection can be reduced over the entire outer peripheral region of the hole-formation region 14R.

[0079] Therefore, it is possible to suppress the decrease in the threshold gain difference between the fundamental mode and the higher-order modes, thereby obtaining high-quality laser light with suppressed higher-order mode oscillation.Furthermore, it is possible to obtain high-quality laser light with stable fundamental mode oscillation even during high-current injection operation.

[0080] Although the above description has been given of the case where hole formation region 14R of photonic crystal layer 14P has a circular shape in top view, this is not limiting. When hole formation region 14R has a shape other than a circular shape, it is preferable that groove 16G be formed so that the outer periphery is outside hole formation region 14R, that is, so that the outer periphery of groove 16G encompasses hole formation region 14R in top view. [Example]

[0081] 9 is a cross-sectional view schematically illustrating an example of the structure of a PCSEL device 30 according to Example 3. Compared to the PCSEL device 10 according to Example 1, the PCSEL device 30 has a mesa 16M formed in the second guide layer 16, but the area of ​​the second guide layer 16 other than the cylindrical mesa 16M is flat (i.e., has the same thickness).

[0082] In the PCSEL device 30, similar to the PCSEL device 10 of FIG. , the mesa 16M is formed inside the hole formation region 14R when viewed from the vertical direction with respect to the photonic crystal layer 14P. Further, the mesa 16M is a cylindrical mesa coaxial with the thin cylindrical hole formation region 14R.

[0083] The mesa 16M has a diameter DM and a height HM. The diameter DM of the mesa 16M is smaller than the diameter DP of the hole formation region 14R of the photonic crystal layer 14P (DM < DP). Further, the diameter DP of the hole formation region 14R is smaller than the inner diameter DE of the n-electrode 20A (DP < DE).

[0084] As described with reference to FIG. 7, in the PCSEL device 30 of FIG. also, at the time of current injection, even if the carrier density increases and the refractive index decreases in the semiconductor layer inside the mesa region, the difference in the band-edge frequencies of the semiconductor layers inside and outside the mesa region can be made small. Therefore, it is possible to suppress a decrease in the threshold gain difference between the fundamental mode and the higher-order mode, and a high-quality laser beam with suppressed higher-order mode oscillation can be obtained. Further, a high-quality laser beam with stable fundamental mode oscillation can be obtained particularly in high current injection operation.

Example

[0085] FIG. 10 is a cross-sectional view schematically showing an example of the structure of a PCSEL device 40 of FIG. In the PCSEL device 40 of this example, the surface of the second semiconductor layer 18 (that is, the surface of the p-contact layer 17) is flat, and a mesa 20M as a mesa portion is formed on the p-contact layer 17.

[0086] The mesa 20M is a metal oxide and is formed of an ITO (Indium Tin Oxide) layer which is a translucent conductor. The mesa 20M functions as a p-electrode. Further, an Ag (silver) layer (not shown) is formed on the upper surface of the mesa 20M, and it has an ITO / Ag electrode structure, and a pad electrode 23 is provided thereon.

[0087] Mesa 20M is provided as a cylindrical layer coaxial with hole formation region 14R, and is formed inside hole formation region 14R when viewed from a direction perpendicular to photonic crystal layer 14P. That is, diameter DM of mesa 20M is smaller than diameter DP of hole formation region 14R of photonic crystal layer 14P (DM <DP)。

[0088] The surfaces other than the side surfaces of the stacked semiconductor layers (i.e., the first semiconductor layer 12, the active layer 15 and the second semiconductor layer 18), the top surface of the second semiconductor layer 18, and the top surface of the mesa 20M are covered with an insulating film 21 such as SiO2.

[0089] The PCSEL device 40 employs a mesa 20M (ITO layer) as a mesa-shaped p-electrode. Due to its transparency and refractive index (refractive index n=2.0 to 2.2), ITO functions not only as an electrode but also as part of the cladding layer.

[0090] In this embodiment, the second p-side semiconductor layer 16C (thickness: 200 nm) is employed, which is thinner than the second p-side semiconductor layer 16C (thickness: 600 nm) of the above embodiment.

[0091] The refractive index of the SiO2 insulating film 21 formed on the outside of the mesa 20M (ITO layer) is n=1.4 to 1.55, and the effective refractive index is (current injection region: mesa 20M (ITO layer))>(current non-injection region).

[0092] By providing a difference in the effective refractive index in this way, the same effect as that of the mesa in the above embodiment can be obtained, and mode stabilization can be achieved.

[0093] This structure can avoid damage to the crystal caused by dry etching during mesa formation, and also simplify the manufacturing process.

[0094] Furthermore, the ITO / Ag electrode structure allows the laser light emitted from the p-electrode side to be efficiently reflected back to the substrate side, achieving high efficiency. Furthermore, the second p-side semiconductor (p-AlGaN) layer, which has a relatively high resistance, can be made thinner, enabling higher efficiency and output.

[0095] Although the mesa 20M has been described as being made of ITO, it is not limited to indium oxide, and other translucent oxide conductors such as ZnO, ZrO, GaO, SnO, or alloys thereof can be used. Furthermore, although the ITO / Ag electrode structure has been described as an example, Ag can be replaced with a metal having high reflectivity, such as Al, Rh, Ru, Pt, Pd, or Au, or an alloy thereof. Alternatively, a reflective film made of a dielectric multilayer film can be used instead of a metal.

[0096] As described above in detail, the present invention can provide a photonic crystal laser (PCSEL) element with high beam quality in which higher-order mode oscillation is suppressed.

[0097] In the above-described embodiments, the circular shape is not limited to a perfect circle but includes elliptical shapes such as an oval shape and an elliptical cylinder shape, and the cylindrical shape includes an elliptical cylinder shape and an elliptical cylinder shape.

[0098] For example, in the above-described embodiment, the hole formation region 14R has a circular shape when viewed from a direction perpendicular to the photonic crystal layer (top view), but this is not limiting. For example, the hole formation region may have an oval shape or the like. Alternatively, the hole formation region may have an n-sided polygon (n is an integer of 4 or more) shape, including a square or a rectangle.

[0099] Alternatively, the hole formation region may be formed over the entire photonic crystal layer 14P.

[0100] In the above-described embodiments, the mesa has a cylindrical shape, but this is not limiting. For example, the mesa may have an n-sided prism shape (n is an integer of 4 or more), including a quadrangular prism shape.

[0101] Moreover, the mesa structure preferably has a shape corresponding to the void formation region, and is formed to have a columnar shape corresponding to the void formation region.

[0102] In the above-described embodiment, a photonic crystal laser element having a photonic crystal layer with a double lattice structure has been described, but the present invention is not limited to this and can also be applied to a photonic crystal laser element having a photonic crystal with a single lattice structure.

[0103] Furthermore, in the above-described embodiment, a photonic crystal laser made of a nitride semiconductor has been described, but the present invention is not limited to this and can also be applied to photonic crystal lasers made of semiconductors of other crystal systems.

[0104] Furthermore, the numerical values ​​in the above-described embodiments are merely examples and can be appropriately modified and applied. [Explanation of symbols]

[0105] 10, 20, 30, 40: PCSEL element, 11: Substrate, 12: first semiconductor layer; 13: first cladding layer, 14: first guide layer, 14A: Lower guide layer, 14P: photonic crystal layer, 14B: buried layer, 15: active layer, 16: second guide layer, 16A: first p-side semiconductor layer, 16B: electron barrier layer, 16C: second p-side semiconductor layer; 16G: groove, 16M, 20M: Mesa section, 17: p-contact layer, 18: second semiconductor layer; 20A: first electrode, 20B: second electrode, 20L: light emission area, 21: insulating film, 23: pad electrode, 90: PCSEL element (comparison example), DM: mesa diameter, DP: diameter of the pore-forming region.

Claims

1. a light-transmitting substrate; an n-type Group III nitride semiconductor layer formed on the substrate and including an embedded photonic crystal layer having a formation region formed by voids arranged with two-dimensional periodicity; an active layer formed on the n-type Group III nitride semiconductor layer; a p-type Group III nitride semiconductor layer formed on the active layer; a mesa portion having a mesa shape formed of a transparent oxide conductor on a surface of the p-type Group III nitride semiconductor layer; an insulating film formed on the surface of the p-type Group III nitride semiconductor layer other than the mesa portion; and the refractive index of the insulating film is smaller than the refractive index of the mesa portion; The III-nitride semiconductor photonic crystal surface-emitting laser element, wherein the mesa portion is formed inside the region where the holes are formed when viewed from a direction perpendicular to the photonic crystal layer.

2. 2. The III-nitride semiconductor photonic crystal surface-emitting laser element according to claim 1, wherein the region where the air holes are formed has a circular shape, and the mesa portion has a cylindrical shape coaxial with the region where the air holes are formed.

3. 3. The III-nitride semiconductor photonic crystal surface-emitting laser element according to claim 1, further comprising a light-reflective p-electrode on the mesa portion.

4. 4. The III-nitride semiconductor photonic crystal surface-emitting laser element according to claim 3, wherein the transparent oxide conductor is made of an indium oxide-based, ZnO-based, ZrO-based, GaO-based, SnO-based, or alloy-based material thereof.

5. 5. The III-nitride semiconductor photonic crystal surface-emitting laser device according to claim 3, wherein the light-reflective p-electrode is made of Ag, Al, Rh, Ru, Pt, Pd, Au, or an alloy thereof.

6. 4. The III-nitride semiconductor photonic crystal surface-emitting laser device according to claim 1, wherein an n-electrode is formed on a surface of the substrate opposite to the n-type III-nitride semiconductor layer, the n-electrode having an opening for passing laser light therethrough.

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