Group iii nitride semiconductor photonic crystal surface emitting laser element

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 Art

[0002] In recent years, the development of photonic-crystal surface-emitting lasers (PCSELs) using photonic crystals (PCs) has been underway.

[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] In addition, a photonic crystal surface-emitting laser having a multi-lattice photonic crystal formed by arranging a plurality of holes of different sizes at lattice points is known.

[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 large number of assemblies of birefringent regions composed of a plurality of regions having different refractive indices from the base material and at least two of the regions having different thicknesses are periodically arranged in a plate-like base material.

[0006] In addition, Non-Patent Document 3 discloses suppressing multi-mode oscillation that causes beam quality degradation by changing the hole size and lattice constant of a photonic crystal.

[0007] However, since a photonic crystal has very small holes, it is difficult to accurately manufacture the hole size and lattice constant.

[0008] In such a two-dimensional photonic crystal surface-emitting laser device, it is important to realize a laser device with high beam quality that suppresses high-order mode oscillation and maintains the fundamental mode even during high-current injection, while maintaining stability.

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Non-Patent Documents

[0010]

Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0011] An object of the present invention is to realize a laser device with high beam quality that suppresses high-order mode oscillation and maintains the fundamental mode until high-current injection in a photonic crystal surface-emitting laser, while maintaining stability.

Means for Solving the Problems

[0012] In order to solve the above problems, a group-III nitride semiconductor photonic crystal surface-emitting laser device according to the present invention includes a light-transmissive substrate, an n-type group-III nitride semiconductor layer formed on the substrate and embedded with a photonic crystal layer having two-dimensionally periodically arranged holes, 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-shaped mesa portion formed of a light-transmissive oxide conductor on the surface of the p-type group-III nitride semiconductor layer, and an insulating film formed on the surface of the p-type group-III nitride semiconductor layer other than the mesa portion. 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 formation region of the holes when viewed from a direction perpendicular to the photonic crystal layer.

Brief Description of Drawings

[0013]

Figure 1A

Figure 1B

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Figure 2C

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Figure 10

[0014] Hereinafter, preferred embodiments of the present invention will be described, but these may be appropriately modified and combined. Also, in the following description and the accompanying drawings, substantially the same or equivalent parts will be denoted by the same reference numerals and described. [Examples]

[0015] [Structure of Photonic Crystal Surface Emitting Laser] A photonic crystal surface emitting laser (hereinafter also referred to as a PCSEL) has a resonator layer in a direction parallel to a semiconductor light emitting structure layer (n-guide layer, light emitting layer, p-guide layer) constituting a light emitting element, and emits coherent light in a direction orthogonal to the resonator layer.

[0016] On the one hand, a distributed Bragg reflector (DBR) laser having a pair of resonator mirrors (Bragg reflectors) sandwiching a semiconductor light-emitting structure layer is known, but a photonic crystal surface-emitting laser (PCSEL) is different from the DBR laser in the following points. That is, 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. That is, the light extraction direction is perpendicular to the resonance direction (in the plane parallel to the photonic crystal layer).

[0017] FIG. 1A is a cross-sectional view schematically showing an example of the structure of a photonic crystal laser element (PCSEL element) 10 according to Example 1. The PCSEL element 10 is configured by laminating a plurality of semiconductor layers on a substrate. The semiconductor layer is made of a hexagonal nitride semiconductor such as a GaN-based semiconductor, for example.

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

[0019] The n-clad layer (the first clad layer of the first conductivity type) 13 and the n-guide layer (the 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 (for example, n-type), and the second semiconductor layer 18 includes a semiconductor layer of a conductivity type opposite to the first conductivity type (for example, p-type).

[0020] Further, the n-guide layer (the first guide layer) 14 is composed of a lower guide layer 14A, a photonic crystal layer (a hole layer, or a PC layer) 14P, and an embedded layer 14B. The photonic crystal layer 14P has 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-type semiconductor layer 16A provided on the active layer 15, an electron blocking layer (EBL) 16B provided on the first p-type semiconductor layer 16A, and a second p-type 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 a 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-type semiconductor layer 16C.

[0023] In this specification, the case where the first conductivity type is n-type and the second conductivity type, which is the opposite conductivity type of the first conductivity type, is p-type will be described. However, the first conductivity type and the second conductivity type may be p-type and n-type, respectively.

[0024] Note that in this specification, “n-” and “p-” mean “n-side” and “p-side”, respectively, and do not necessarily mean having n-type or p-type. For example, the n-guide layer means a guide layer provided on the n-side of the active layer, and may be an undoped layer (or i-layer).

[0025] Also, the n-clad layer 13 may be composed of a plurality of layers instead of a single layer. In that case, not all layers 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] Also, in the above, the specific and detailed semiconductor layer configuration of the photonic crystal laser element 10 has been described, but it is only an example of the element structure. In short, it may have a first semiconductor layer (or first guide layer) having a photonic crystal layer 14P, a second semiconductor layer (or second guide layer), and an active layer (light-emitting layer) sandwiched between these layers, and may be configured to emit light by injecting current into the active layer.

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

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

[0029] As shown in FIG. 1A, a groove (groove) 16G reaching 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 - type semiconductor layer 16C is formed on the upper surface of the second semiconductor layer 18.

[0030] Note that the groove 16G only needs to be formed to a depth reaching inside the second guide layer 16. For example, it may be formed as a groove having a depth staying inside the second p - type semiconductor layer 16C.

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

[0032] By the groove 16G, a mesa portion (hereinafter also simply referred to as a mesa) 16M having a cylindrical mesa shape is defined in the second guide layer 16. The mesa 16M is formed inside the hole formation region 14R when viewed from a direction perpendicular to the photonic crystal layer 14P. Also, the mesa 16M is a cylindrical mesa coaxial with the thin cylindrical hole formation region 14R.

[0033] Also, 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 upper surface of the p - electrode 20B, and the inside (side surfaces and bottom surface) of the groove 16G are covered with an insulating film 21 such as SiO2. Note that for clarity of the figure, the insulating film 21 is not hatched. Also, a pad electrode 23 electrically connected to the p - electrode 20B is formed.

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

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

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

[0038] As shown in FIG. 2B, in the photonic crystal layer 14P, the holes (cavities) 14K are provided to be periodically arranged, for example, within a circular hole formation region 14R. As shown in FIG. 2C, the n electrode (cathode) 20A is provided as an annular electrode outside the hole formation region 14R so as not to overlap the hole formation region 14R when viewed from a direction perpendicular to the photonic crystal layer 14P (in top view). The region inside the n electrode 20A is the light emission region 20L.

[0039] As shown in FIG. 1A, the mesa 16M of the second guide layer 16 has a height HM. Also, as shown in FIGS. 1A, 2A, and 2B, 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).

[0040] Also, the annular n electrode 20A is formed to be coaxial with the hole formation region 14R, and the diameter DP of the hole formation region 14R is smaller than the inner diameter DE of the n electrode 20A (DP < DE). 1. Fabrication process of the photonic crystal laser (PCSEL) 10 Hereinafter, the fabrication process of the PCSEL element 10 will be described in detail. Using the MOVPE (Metalorganic Vapor Phase Epitaxy) method as the crystal growth method, a semiconductor layer was grown and laminated on the substrate 11 (growth substrate) by atmospheric pressure growth. In the following steps, Sn means step n.

[0041] Also, the layer thickness, carrier concentration, group III and group V raw materials, temperature, etc. shown below are only examples unless otherwise specified. [S1: Substrate preparation step] As the substrate 11, a GaN single crystal with a “+c” plane whose main surface is a (0001) plane with Ga atoms arranged on the outermost surface was prepared. The main surface may be just right or, for example, a substrate offset by about 1° in the m-axis direction. For example, a substrate offset by about 1° in the m-axis direction can obtain mirror growth under a wide range of growth conditions.

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

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

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

[0045] In the following, for the sake of simplicity of explanation and ease of understanding, the substrate 11 (substrate with a grown layer) on which such a grown layer is formed may sometimes be simply referred to as the substrate. formed may sometimes be simply referred to as the substrate. [S3b: Hole formation process] After forming the above-mentioned preparation layer, the substrate was taken out from the chamber of the MOVPE apparatus, and fine pores (holes) were formed on the surface of the growth layer. After obtaining a clean surface by cleaning, a silicon nitride film (SixNy) was formed using plasma CVD. An electron beam lithography resist was spin-coated on this, and it was placed in an electron lithography apparatus to perform patterning of a two-dimensional periodic structure.

[0046] As shown in FIG. 3, patterning was performed in which an opening pair composed of a rectangular main opening K1 and a sub-opening K2 having a square shape and smaller than the main opening K1 was two-dimensionally arranged in a square lattice in the plane of the resist with a period PK = 164 nm. For the sake of clarity of the drawing, the openings are shown with hatching.

[0047] More specifically, the centers of gravity CD1 of the main openings K1 are arranged in a square lattice with a period PK = 164 nm in two directions (x direction and y direction) orthogonal to each other. Similarly, for the sub-openings K2, the centers of gravity CD2 are arranged in a square lattice with a period PK = 164 nm in the x direction and the y direction.

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

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

[0050] Also, the center of gravity CD2 of the sub-aperture K2 is separated 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 separated from the center of gravity CD1 of the main aperture K1 in the <1-100> direction. Specifically, the distance between the centers of gravity in the x-direction Δx and the distance between the centers of gravity in the y-direction Δy 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) apparatus. As a result, the main aperture K1 and the sub-aperture K2 arranged in a square lattice with a period of 164 nm were formed so as to penetrate the SixNy film.

[0052] Note that the period (pore spacing) PK was calculated with the oscillation wavelength (λ) being 410 nm and the refractive index (n) of GaN being 2.5, and PK = λ / n = 164 nm.

[0053] Subsequently, the resist was removed, and holes were formed in the GaN surface portion using the patterned SixNy film as a hard mask. By dry-etching GaN in the depth direction using a chlorine-based gas and argon gas in an ICP-RIE apparatus, a pair of holes (hole pairs) that are oblong-columnar and columnar holes dug perpendicularly to the GaN surface was formed. That is, oblong-columnar and columnar holes were formed in the rectangular main aperture K1 and the square sub-aperture K2. In this step, in order to distinguish the holes dug in the GaN surface portion by the etching from the holes (cavities) in the photonic crystal layer 14P, they are hereinafter referred to as "holes". [S3c: Cleaning process] The substrate with holes formed was subjected to degreasing cleaning and then the SixNy film was removed with buffered hydrofluoric acid (HF). [S3d: Embedded layer formation process] This substrate was introduced again into the reactor of the MOVPE apparatus, ammonia (NH3) was supplied, and the temperature was raised to 950 °C (the first implantation temperature). Then, trimethylgallium (TMG) and NH3 were supplied to close the hole pair (main hole and sub-hole), and the implanted layer 14B was formed.

[0054] Through the above implantation process, an n-guide layer 14 having a photonic crystal layer 14P with a double lattice structure in which a hole pair 14K composed of a main hole 14K1 and a sub-hole 14K2 was arranged at each of the square lattice points was formed. [S4: Light-emitting layer formation process] Subsequently, a multiple quantum well (MQW) layer was grown as the active layer 15 which is the light-emitting layer. The barrier layer and well layer of the MQW were GaN and InGaN, respectively. For the growth of the barrier layer, after the temperature of the substrate was lowered to 820 °C, triethylgallium (TEG) was supplied as a group III atom source and NH3 was supplied as a nitrogen source. Also, for the growth of the well layer, at the same temperature as the barrier layer, TEG and trimethylindium (TMI) were supplied as group III atom sources and NH3 was supplied as a nitrogen source. The center wavelength of the PL (Photoluminescence) emission from the active layer in this example was 412 nm. [S5: First p-side semiconductor layer formation process] After the growth of the active layer, the temperature of the substrate was raised to 1050 °C, and GaN was grown with a layer thickness of 120 nm. The first p-side semiconductor layer 16A was grown without doping a dopant by supplying TMG and NH3. [S6: Electron barrier layer formation process] After the growth of the first p-side semiconductor layer 16A, while maintaining the substrate temperature at 1050 °C, an electron barrier layer (EBL) 16B was grown. The growth of the EBL 16B was carried out by supplying TMG and T MA as a group III atom source and NH3 as a nitrogen source. Also, Cp2Mg was supplied as a p-dopant. As a result, an EBL 16B 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 blocking layer (EBL) 16B, while maintaining the substrate temperature at 1050 °C, the second p-type semiconductor layer 16C was grown. The second p-type semiconductor layer 16C was grown by supplying TMG and TMA as group III atomic sources and NH3 as a nitrogen source. Also, Cp2Mg was supplied as a p-dopant. As a result, a second p-type semiconductor layer 16C with an Al composition of 6% and a layer thickness of 600 nm was formed. Note that when activation was performed at 850 °C for 10 minutes in a nitrogen atmosphere after growth, the carrier concentration of the p-clad layer (p-AlGaN) 18 was 2×1017 cm-3.

[0055] By forming the second p-type semiconductor layer 16C, a second guide layer 16 composed of the first p-type semiconductor layer 16A, the EBL 16B, and the second p-type semiconductor layer 16C was formed. [S8: p-contact layer formation process] After growing the second p-type semiconductor layer 16C, while maintaining the substrate temperature at 1050 °C, a p-contact layer 17 with a layer thickness of 25 nm was grown. The growth of the p-contact layer 17 was performed by supplying TMG as a group III atomic source and NH3 as a nitrogen source. Also, Cp2Mg was supplied as a dopant. [S9: Element isolation groove formation process] SiO2 was applied to the surface of the substrate with the grown layer on which the epitaxial growth layer formation was completed by the spin-on glass (SOG) method. An element isolation groove was patterned on the applied SiO2 film using photolithography. Using SiO2 as a mask, etching was performed by vapor etching until the n-side clad layer 13 or the growth substrate 11 was exposed. Then, the SiO2 mask was removed with BHF to form an element isolation groove. [S10: Groove and mesa formation process] Again, SiO2 was applied to the surface by the SOG method. The applied SiO2 film was patterned using photolithography to form a mask. Using this SiO2 mask, a groove 16G reaching the inside of the p-guide layer (second guide layer) 16 was formed by vapor etching. As shown in Fig. 2A, as a result of the formation of the annular groove 16G, a columnar mesa 16M (mesa portion) of the p-guide layer 16 was formed inside the groove 16G.

[0056] The mesa 16M is a cylindrical mesa that is inside the outer edge of the hole formation region 14R and concentric with the hole formation region 14R when viewed from a direction perpendicular to the photonic crystal layer 14P. Also, as shown in FIG. 1A, the mesa 16M has a height HM. [S11: Anode electrode formation step] Palladium (Pd) with a thickness of 100 nm was deposited 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 step] After forming a mask on the p - electrode 20B using photolithography, a SiO2 insulating film (protective film) with a thickness of 200 nm was deposited by sputtering. The SiO2 insulating film 21 on the p - electrode 20B was removed by lift - off. [S13: p - side pad electrode formation step] After forming a mask on the upper surface of the PCSEL element 10 using photolithography, a metal layer composed of titanium / platinum / titanium / gold (Ti / Pt / Ti / Au) was deposited in this order by electron beam evaporation by sputtering. The metal layer around the upper surface of the PCSEL element 10 was removed by lift - off to form a pad electrode 23. [S14: Substrate polishing step] Next, the back surface of the substrate was ground to a thickness of 150 μm and then mirror - polished by a diamond slurry and chemical mechanical polishing (CMP) method. [S15: Process - affected layer removal step] Subsequently, the emission surface was wet - etched with a KOH solution to remove the process - affected layer. For the removal of the process - affected layer, wet etching with a NaOH solution, a 4 - methylammonium hydroxide (TMAH) solution, etc. Dry etching methods using chlorine - based gases may also be used. [S16: Cathode electrode formation step] Subsequently, Ti and Au were sequentially deposited on the back surface of the substrate 11 by electron beam evaporation, patterned in an annular shape, and an n - electrode 20A was formed. [S17: Anti-reflection film formation process] SiO2 was deposited by sputtering inside the annular n-electrode 20A which is the laser light emitting surface to form an anti-reflection film 22. The anti-reflection layer may be a single layer or a multilayer film combined with another dielectric film. [S12: Dicing process] Finally, laser scribing was performed along the center line of the substrate separation groove to obtain diced PCSEL elements 10. 2. Hole layer Fig. 4 is an SEM (Scanning Electron Microscope) image showing the shape of the holes in the photonic crystal layer 14P of this example. Note that the SEM image of the photonic crystal layer 14P in Comparative Example 1 described later is the same.

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

[0058] A pair of holes, a long hexagonal column hole (main hole) 14K1 and a regular hexagonal column hole (subsidiary hole) 14K2, was observed. The main hole 14K1 had a long hexagonal column shape with its major axis parallel to the <11-20> axis. Also, the subsidiary hole 14K2 was smaller in size (e.g., at least either the hole diameter or the depth) than the main hole 14K1.

[0059] That is, it was confirmed that a photonic crystal layer 14P having a double lattice structure in which a pair of holes 14K composed of a main hole 14K1 and a subsidiary hole 14K2 was arranged at each square lattice point of the periodic PK was formed. 3. Device characteristics The device characteristics of the PCSEL element 10 of Example 1 fabricated as described above and the PCSEL element 90 of Comparative Example 1 were evaluated. Note that the PCSEL element 90 of Comparative Example 1 differed from the PCSEL element 10 of this example only in that it did not have the groove 16G, and the other configurations were the same as those of the PCSEL element 10 of this example.

[0060] Referring to FIG. 5 showing the PCSEL element 90 of Comparative Example 1, a more specific description will be given. In the PCSEL element 90 of Comparative Example 1, the mesa 16M is not formed, 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] Also, on the surface of the p-contact layer 17, a p-electrode 20B coaxial with the hole formation region 14R is formed. The diameter DC of the p-electrode 20B is equal to the diameter DM of the mesa 16M of the PCSEL element 10 of Example 1 (DC = DM).

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

[0063] FIG. 6A shows the far-field image of the PCSEL element 10 of Example 1, and FIG. 6B shows the far-field image of the PCSEL element 90 of Comparative Example 1. In the PCSEL element 10 of Example 1, a single-peak-shaped beam with a divergence angle of 0.2 degrees or less was obtained. In the PCSEL element 90 of Comparative Example 1, in addition to a single-peak-shaped beam with a divergence angle of 0.2 degrees or less, a linear (cross-shaped) higher-order mode was observed.

[0064] That is, in the PCSEL element 10 of Example 1, it was confirmed that the higher-order mode (transverse mode) was suppressed and a laser beam with high beam quality having a single-peak shape was obtained. The following is a discussion on this point.

[0065] FIG. 7 is a diagram schematically showing the oscillation mode frequency with respect to the spatial position. The cases of Example 1 and Comparative Example 1 are shown for comparison. In the figure, the band-edge frequency FBE (photonic band-edge frequency) is shown by a broken line, and the photonic band gap (PBG) is shown by hatching. Also shown are the mesa region (Example 1) and the electrode region (Comparative Example 1) which are the current injection regions.

[0066] Here, the photonic band gap (PBG) means a forbidden band of light. That is, the photonic band gap indicates a frequency region where light cannot exist, and light having a frequency within this region cannot be transmitted.

[0067] Referring to FIG. 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, in the semiconductor layer inside the electrode region which is the current injection region, the carrier density increases and the refractive index decreases compared to the semiconductor layer outside the electrode region.

[0068] As a result, the band edge frequency of the semiconductor layer inside the electrode region becomes higher than that of the semiconductor layer outside the electrode region. That is, the leakage of light to the region outside the electrode region (i.e., the non-transparent absorption region) is reduced, and the horizontal light confinement is enhanced. As a result, the threshold gain difference between the fundamental mode and the higher-order mode is reduced, and it becomes easier to oscillate in the higher-order mode. Therefore, the transverse mode controllability deteriorates, and multimode oscillation is likely to occur.

[0069] On the other hand, in Example 1, in the outer semiconductor layer outside the mesa region (i.e., the region of groove 16G), the effective refractive index 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] And (B) when current is injected, 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 reduced. Note that FIG. 7 shows the case where the difference is zero. Also, particularly in the high current injection operation, a laser beam with high beam quality of stable fundamental mode oscillation can be obtained.

[0071] More specifically, as the difference in the band edge frequencies decreases, the leakage of light outside the electrode region (mesa region) becomes larger compared to Comparative Example 1. While the fundamental mode has a shape with an electric field intensity peak at the center of the electrode, the higher-order mode has a peak in the electric field intensity at a position shifted from the center of the electrode. Therefore, the influence of light leakage (i.e., loss) is higher for the higher-order mode > fundamental mode. As a result, the threshold gain difference between the fundamental mode and the higher-order mode increases, suppressing the higher-order mode.

[0072] 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.

Example

[0073] FIG. 8 is a cross-sectional view schematically showing an example of the structure of the PCSEL element 20 of Example 2. The configuration of the semiconductor layer of the PCSEL element 20 of Example 2 is the same as the configuration described in Example 1. In the following, the mesa structure of the PCSEL element 20 will be described in detail.

[0074] In the PCSEL element 20 of Example 2, similar to the case of Example 1, a groove 16G reaching 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 is formed in the second semiconductor layer 18. Further, a mesa 16M is formed in the second guide layer 16 by the groove 16G.

[0075] The mesa 16M has a cylindrical shape coaxial with the hole formation region 14R and is formed inside the hole formation region 14R when viewed from a direction perpendicular to the photonic crystal layer 14P. That is, 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).

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

[0077] As described with reference to FIG. 7, also in the PCSEL element 20 of Example 2, when current is injected, 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 reduced.

[0078] Further, in the PCSEL element 20 of Example 2, the diameter DG of the outer periphery of the groove 16G has a size equal to or larger than the diameter DP of the hole formation region 14R (DP ≤ DG). Therefore, 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, a decrease in the threshold gain difference between the fundamental mode and the higher-order mode can be suppressed, and a laser beam with high beam quality in which higher-order mode oscillation is suppressed can be obtained. Further, a laser beam with high beam quality of stable fundamental mode oscillation can be obtained particularly in high current injection operation.

[0080] Although the case where the hole formation region 14R of the photonic crystal layer 14P has a circular shape in a top view has been described, the present invention is not limited thereto. When the hole formation region 14R has a shape other than a circular shape, it is preferable that the outer periphery of the groove 16G is outside the hole formation region 14R, that is, the outer periphery of the groove 16G includes the hole formation region 14R in a top view.

Example

[0081] FIG. 9 is a cross-sectional view schematically showing an example of the structure of the PCSEL element 30 of Example 3. Compared with the PCSEL element 10 of Example 1, a mesa 16M is formed in the second guide layer 16 of the PCSEL element 30, but the region of the second guide layer 16 other than the columnar mesa 16M is formed flat (that is, with the same thickness).

[0082] In the PCSEL element 30, similar to the PCSEL element 10 of the first embodiment, the mesa 16M is formed inside the hole formation region 14R when viewed from the perpendicular direction with respect to the photonic crystal layer 14P. Further, the mesa 16M is a columnar mesa coaxial with the thin columnar 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, also in the PCSEL element 30 of the third embodiment, 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 reduced. Therefore, it is possible to suppress the 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, particularly in the high-current injection operation, a high-quality laser beam with stable fundamental mode oscillation can be obtained.

Embodiment

[0085] FIG. 10 is a cross-sectional view schematically showing an example of the structure of the PCSEL element 40 of the fourth embodiment. In the PCSEL element 40 of the present embodiment, 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] The mesa 20M is provided as a cylindrical layer coaxial with the hole formation region 14R, and is formed inside the hole formation region 14R when viewed from the vertical direction with respect to the photonic crystal layer 14P. That is, the diameter DM of the mesa 20M is smaller than the diameter DP of the hole formation region 14R of the photonic crystal layer 14P (DM < DP).

[0088] 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 upper surface of the second semiconductor layer 18, and the surfaces other than the upper surface of the mesa 20M are covered with an insulating film 21 such as SiO2.

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

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

[0091] The refractive index of the SiO2 insulating film 21 formed outside the mesa 20M (ITO layer) is n = 1.4 to 1.55, and the effective refractive index is such that (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] According to this structure, damage to the crystal due to dry etching during mesa formation can be avoided. Also, the manufacturing process can be simplified.

[0094] Furthermore, with the ITO / Ag electrode structure, the laser light emitted from the p - electrode side can be efficiently reflected to the substrate side, enabling high efficiency. Also, since the relatively high - resistance second p - side semiconductor (p - AlGaN) layer can be made thinner, high efficiency and high output are achievable.

[0095] Although the case where the mesa 20M is made of ITO has been described as an example, not limited to indium oxide - based materials, light - transmissive oxide conductors such as ZnO - based, ZrO - based, GaO - based, SnO - based, or alloy systems thereof can be used. Also, although the case of an ITO / Ag electrode structure has been described as an example, instead of Ag, metals with high reflectivity such as Al, Rh, Ru, Pt, Pd, Au, or alloys thereof can be used. Alternatively, a reflective film made of a dielectric multilayer film may be used instead of the metal.

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

[0097] In the above - described embodiments, the circular shape is not limited to a perfect - circle shape, and includes oval - shaped and oval - shaped including elliptical shapes, and the cylindrical shape includes elliptical - cylindrical and oval - shaped.

[0098] For example, in the above - described embodiments, the case where the hole - forming region 14R has a circular shape when viewed from a direction perpendicular to the photonic crystal layer (top view) has been described, but it is not limited thereto. For example, the hole - forming region may have an oval - shaped or the like. Alternatively, the hole - forming region may be an n - sided shape (n is an integer of 4 or more) including a square and a rectangle.

[0099] Also, the hole - forming region may be formed over the entire photonic crystal layer 14P.

[0100] Also, in the above - described embodiments, the case where the mesa has a cylindrical shape has been described as an example, but it is not limited thereto. 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] Further, the mesa structure preferably has a shape corresponding to the pore formation region and is formed to have a column shape corresponding to the formation region.

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

[0103] In the above-described embodiment, the photonic crystal laser made of the nitride semiconductor has been described. However, the present invention is not limited to this, and the present invention is also applicable to the photonic crystal laser made of the semiconductor of other crystal systems.

[0104] In addition, the numerical values in the above-described embodiment are merely examples and can be appropriately modified and applied.

Explanation of Reference Numerals

[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: Embedded 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 portion, 17: p-contact layer, 18: Second semiconductor layer, 20A: First electrode, 20B: Second electrode, 20L: Light emission region, 21: Insulating film, 23: Pad electrode, 90: PCSEL element (comparative example), DM: Diameter of mesa, DP: Diameter of hole formation region.

Claims

1. A light-transmitting substrate; an n-type Group III nitride semiconductor layer formed on the substrate and having an embedded photonic crystal layer having holes 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 a surface of the p-type Group III nitride semiconductor layer other than the mesa portion, the refractive index of the insulating film is smaller than the refractive index of the mesa portion, the mesa portion is formed inside the region in which the holes are formed when viewed from a direction perpendicular to the photonic crystal layer, in the III-nitride semiconductor photonic crystal surface-emitting laser element.

2. 2. The III-nitride semiconductor photonic crystal surface-emitting laser element according to claim 1, wherein the region in which the holes are formed has a circular shape, and the mesa portion has a cylindrical shape coaxial with the region in which the 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 Group 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 element 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 Group 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 Group III nitride semiconductor layer, the n-electrode having an opening for passing laser light therethrough.

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