Surface-emitting laser element and method for manufacturing the same

Abstract

To provide a surface light emission laser element having a larger coupling coefficient to a light wave transmitting in a photonic crystal layer, capable of causing oscillation operation with a low threshold current density, and provided with a photonic crystal, and a manufacturing method of the same.SOLUTION: A surface light emission laser element comprising a III nitride semiconductor includes: a first-conductivity type first guide layer having a first-conductivity type first clad layer, a photonic crystal layer formed on the first clad layer and having holes arranged with a two-dimensional periodicity in a plane parallel to the layer, and a first embedded layer formed on the photonic crystal layer and blocking the holes; a second embedded layer being crystal grown on the first embedded layer; an active layer formed on the second embedded layer; a second guide layer formed on the active layer; and a second-conductivity type, being an opposite conductivity-type to the first conductivity-type, second clad layer formed on the second guide layer. On the surface of the first embedded layer, pits are arranged at surface positions corresponding to the holes.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a surface emitting laser element and a method for manufacturing the surface emitting laser element.

Background Art

[0002] In recent years, the development of surface emitting lasers using photonic crystals has been underway. For example, Patent Document 1 discloses a semiconductor laser element aimed at manufacturing without performing fusion bonding.

[0003] Further, Patent Document 2 discloses a manufacturing method for fabricating a fine structure of a photonic crystal in a GaN-based semiconductor. Non-Patent Document 1 discloses that the growth rate in the lateral direction is increased by growth under reduced pressure to fabricate a photonic crystal.

[0004] Also, Non-Patent Document 2 discloses the in-plane diffraction effect and threshold gain difference of a photonic crystal laser, and Non-Patent Document 3 discloses a three-dimensional coupled-wave model of a square lattice photonic crystal laser.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Non-Patent Documents

[0006]

Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0007] In a surface emitting laser having a photonic crystal, it is necessary to reduce resonator loss in order to oscillate at a low threshold current density. In order to reduce resonator loss in a photonic crystal surface emitting laser, it is effective to increase the coupling coefficient (one-dimensional coupling coefficient: κ3) of light waves propagating in a direction parallel to the photonic crystal layer.

[0008] The present invention has been made in view of the above points, and an object thereof is to provide a surface emitting laser having a photonic crystal and a method for manufacturing the same, in which the coupling coefficient for light waves propagating through the photonic crystal layer is large and oscillation operation can be performed at a low threshold current density.

Means for Solving the Problems

[0009] A surface emitting laser element according to one embodiment of the present invention is a surface emitting laser element made of a group III nitride semiconductor, a first cladding layer of a first conductivity type, a photonic crystal layer formed on the first cladding layer and having holes arranged two-dimensionally periodically in a plane parallel to the layer, and a first embedding layer formed on the photonic crystal layer and closing the holes, a first guide layer of a first conductivity type having the same; a second embedding layer crystal-grown on the first embedding layer, an active layer formed on the second embedding layer, a second guide layer formed on the active layer, a second cladding layer of a second conductivity type opposite to the first conductivity type formed on the second guide layer layer, and the surface of the first embedding layer includes a pit arranged at a surface position corresponding to the hole.

[0010] A manufacturing method for manufacturing a surface-emitting laser device according to another embodiment of the present invention is as follows: (a) A step of growing a first cladding layer of a first conductivity type on a substrate; (b) A step of growing a first guide layer of a first conductivity type on the first cladding layer; (c) A step of forming holes arranged two-dimensionally periodically in a plane parallel to the first guide layer by etching in the first guide layer; (d) A step of supplying a gas containing a nitrogen source and forming a first embedded layer that closes the opening of the hole by mass transport; (e) After forming the first embedded layer, a step of supplying a group III raw material and forming a second embedded layer that fills and planarizes the first embedded layer. In the above step (d), the surface of the first embedded layer has pits derived from the holes.

Brief Description of the Drawings

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[0012] 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. [Threshold Gain of Photonic Crystal Surface-Emitting Laser] Generally, in a photonic crystal surface-emitting laser (hereinafter, also simply referred to as a photonic crystal laser), the diffraction efficiency of the diffraction grating inside the resonator is represented by the coupling coefficient κ, and the larger the coupling coefficient κ, the smaller the threshold gain.

[0013] In a photonic crystal surface emitting laser, light waves propagating in a plane parallel to the photonic crystal layer are diffracted not only in the directions of ±180° with respect to the propagation direction of the light waves, but also in the directions of ±90°. Therefore, in addition to the optical coupling coefficient κ3 (one-dimensional coupling coefficient) of the light waves propagating in the ±180° directions, there is an optical coupling coefficient κ 2D (two-dimensional coupling coefficient) for the light waves propagating in the ±90° directions.

[0014] That is, in a surface emitting laser having a photonic crystal, light waves propagating in the plane are diffracted by the diffraction effect of the photonic crystal to form a two-dimensional resonance mode. Here, if the component of the light that is not diffracted by the photonic crystal leaks in the in-plane direction, the resonator loss increases, leading to an increase in the threshold gain. Therefore, if this resonator loss can be suppressed, the threshold gain can be reduced, and oscillation operation can be performed at a low threshold current density.

[0015] In order to reduce the resonator loss in a photonic crystal surface emitting laser and perform oscillation operation at a low threshold current density, it is effective to increase the coupling coefficient (κ3) of the light waves propagating in the plane of the photonic crystal layer. [An example of the structure of a photonic crystal surface emitting laser] FIG. 1 is a cross-sectional view schematically showing an example of the structure of a surface emitting laser element (hereinafter, also simply referred to as a photonic crystal laser) 10 including a photonic crystal layer. As shown in FIG. 1, a semiconductor structure layer 11 is formed on a substrate 12. More specifically, an n-clad layer 13, an n-guide layer 14 including a first buried layer 14A, a second buried layer 21, an active layer 15, a guide layer 16, an electron blocking layer (EBL) 17, and a p-clad layer 18 are sequentially formed on the substrate 12 in this order. That is, the semiconductor structure layer 11 is composed of semiconductor layers 13, 14, 15, 16, 17, and 18. The n-guide layer 14 includes a photonic crystal layer 14P. The semiconductor structure layer 11 is made of a hexagonal nitride semiconductor. For example, it is made of a GaN-based semiconductor.

[0016] On the other hand, an n - electrode 19A is formed on the substrate 12 (the back surface), and a p - electrode 19B is formed on the p - clad layer 18 (the upper surface). For example, the p - electrode 19B can be formed in an appropriate shape such as an annular shape surrounding the photonic crystal region.

[0017] In this case, the light from the surface - emitting laser element 10 is extracted to the outside from the upper surface of the semiconductor structure layer 11 (that is, the surface of the p - clad layer 18) in a direction perpendicular to the active layer 15.

[0018] Note that the light from the surface - emitting laser element 10 can also be extracted to the outside from the lower surface side of the semiconductor structure layer 11. In that case, for example, the n - electrode 19A can be formed in an annular shape, and the p - electrode 19B can be provided at a position facing the center of the n - electrode 19A.

[0019] FIG. 2 is an enlarged cross - sectional view schematically showing the photonic crystal layer 14P in FIG. 1 and the holes (cavities) 14C arranged in the photonic crystal layer 14P. The holes 14C have a period PC, for example, in a square lattice pattern in a plane parallel to the crystal growth surface (semiconductor stacking surface), that is, the n - guide layer 14 (cross - section A - A in the figure), and are two - dimensionally arranged at the square lattice point positions and embedded in the n - guide layer 14. Note that the arrangement of the holes 14C is not limited to a square lattice pattern, and may be a periodic two - dimensional arrangement such as a triangular lattice pattern or a hexagonal lattice pattern.

[0020] The n - guide layer 14 is a semiconductor layer on the upper surface of the two - dimensionally arranged holes 14C (that is, a semiconductor layer having the upper surface of the holes 14C as its bottom surface), and is composed of a first embedded layer 14A that embeds the photonic crystal layer (PCL) 14P, the photonic crystal layer (PCL) 14P, and a base layer 14B that is a crystal layer on the substrate side of the photonic crystal layer (PCL) 14P. Also, a second embedded layer 21 is formed on the first embedded layer 14A. [Resonance effect of photonic crystal surface - emitting laser] In a surface-emitting laser equipped with a photonic crystal section (hereinafter sometimes simply referred to as a photonic crystal surface-emitting laser), in order to obtain a resonance effect, it is desirable that the diffraction effect in the photonic crystal section is high.

[0021] That is, in order to enhance the diffraction effect in a photonic crystal surface-emitting laser, (1) When the oscillation wavelength is λ and the effective refractive index of the photonic crystal section is n eff for a two-dimensional refractive index period P in the photonic crystal section, in the case of a square lattice two-dimensional photonic crystal, P = mλ / n eff (m is a natural number), and in the case of a triangular lattice two-dimensional photonic crystal, P = mλ×2 / (3 1 / 2 ×n eff )(m is a natural number) is satisfied, (2) The ratio (FF: filling factor) of the region with a different refractive index to the base material in the photonic crystal section is sufficiently large, (3) Among the light intensity distributions in the photonic crystal surface-emitting laser, the ratio (Γ PC : confinement factor) of the light intensity distributed in the photonic crystal section is sufficiently large. This is desirable.

[0022] In order to satisfy the above (1), it is necessary to appropriately set the lattice constant (period PC) of the photonic crystal according to the oscillation wavelength of the photonic crystal laser. Here, since the refractive index period P = the period PC, PC can be set based on P that satisfies the above (1). For example, when oscillating at a wavelength of 405 nm using a gallium nitride-based material, since n eff is about 2.5, when using a square lattice two-dimensional photonic crystal, the lattice constant may be set to about 163 nm.

[0023] In addition, in a square lattice two-dimensional photonic crystal, the direction in which the light wave is diffracted is the same as the lattice arrangement direction, but in a triangular lattice two-dimensional photonic crystal, the light wave is diffracted in a direction inclined 30° from the lattice arrangement direction. As a result, in the above (1), for P satisfied by the triangular lattice two-dimensional photonic crystal, 2 / 3 1 / 2 is multiplied.

Example

[0024] [Growth of Cladding Layer and Guide Layer] The manufacturing process of the semiconductor structure layer 11 will be described in detail below. Using the MOVPE (Metalorganic Vapor Phase Epitaxy) method as the crystal growth method, the semiconductor structure layer 11 was grown on the growth substrate 12 by atmospheric pressure growth.

[0025] As the growth substrate for the semiconductor structure layer 11, an n-type GaN substrate 12 with a +c plane growth surface and a c-axis inclined by 0.4° along the a-axis was used. The inclination (off angle) of the c-axis may be appropriately changed within the range where epitaxial growth is possible for GaN-based semiconductors. On the substrate 12, an n-type AlGaN with an Al (aluminum) composition of 4% (layer thickness: 2 μm) was grown as the n-cladding layer 13. III Trimethylgallium (TMG) and trimethylaluminum (TMA) were used as group III MO (metalorganic) materials, and ammonia (NH3) was used as a group V material. Also, silane (SiH4) was supplied as a doping material. The carrier density at room temperature was approximately 2×10 18 cm -3 -3.

[0026] Subsequently, TMG and NH3 were supplied, and an n-type GaN (layer thickness: 300 nm) was grown as the n-guide layer 14. Also, silane (SiH4) was supplied simultaneously with the growth for doping. The carrier density was approximately 2×10 18 cm -3 -3. [Formation of Holes in Guide Layer] The substrate after growing the n-guide layer 14, that is, the substrate with the n-guide layer 14 (hereinafter also referred to as the guide layer substrate), was taken out from the MOVPE apparatus, and fine pores (holes) were formed in the n-guide layer 14. With reference to FIGS. 3, 4A, and 4B, the formation of the pores will be described in detail below. Note that FIG. 3 is a cross-sectional view schematically showing the formation process of the pores CH. FIGS. 4A and 4B show images of the surface and cross-section of the guide layer substrate in the process after the formation of the pores CH by a scanning electron microscope (SEM), respectively. Note that FIG. 4B shows a cross-sectional SEM image along the broken line (white) shown in the surface SEM image of FIG. 4A.

[0027] The guide layer substrate on which the n-clad layer 13 and the n-guide layer 14 were grown on the substrate 12 was cleaned to obtain a clean surface (FIG. 3, (i)). Thereafter, a silicon nitride film (SiNx) SN was formed by plasma CVD (film thickness: 110 nm) (FIG. 3, (ii)).

[0028] Next, an electron beam (EB) lithography resist RZ was spin-coated on the SiNx film SN to a thickness of about 300 nm, and then placed in an electron beam lithography apparatus to form a pattern having a two-dimensional periodic structure on the surface of the guide layer substrate (FIG. 3, (iii)). More specifically, patterning was performed in which circular dots with a diameter of 100 nm were two-dimensionally arranged in a square lattice in the plane of the resist RZ with a period PC = 163 nm.

[0029] After developing the patterned resist RZ, the SiNx film SN was selectively dry-etched by an ICP-RIE (Inductive Coupled Plasma - Reactive Ion Etching) apparatus (FIG. 3, (iv)). As a result, cylindrical through-holes two-dimensionally arranged in a square lattice with an in-plane period PC were formed in the SiNx film SN.

[0030] Subsequently, the resist RZ was removed, and holes CH reaching from the surface to the interior of the patterned SiNx film SN were formed using the patterned SiNx film SN as a hard mask. More specifically, two-dimensionally arranged holes CH were formed in the n-guide layer 14 by dry etching using a chlorine-based gas in an ICP-RIE apparatus (FIG. 3, (v)).

[0031] The surface SEM image and the cross-sectional SEM image of the holes CH formed in the n-guide layer 14 at this time are shown in FIGS. 4A and 4B, respectively. As shown in the surface SEM image, a plurality of holes CH were formed two-dimensionally in a square lattice pattern (i.e., square lattice points) with a spacing (period) PC of 163 nm between the holes. Also, as shown in the cross-sectional SEM image of FIG. 4B, the depth of the holes CH formed in the n-guide layer 14 was approximately 150 nm, and the diameter of the holes CH was approximately 100 nm. That is, the holes CH were holes (holes) that opened on the upper surface and had a substantially cylindrical shape. [First Embedded Layer] The SiNx film SN of the guide layer substrate having two-dimensionally periodic holes CH formed in the n-guide layer 14 was removed using hydrofluoric acid (HF) (FIG. 3, (vi)), washed to obtain a clean surface, and introduced again into the MOVPE apparatus.

[0032] In the MOVPE apparatus, while heating the guide layer substrate to 1150° C. while supplying NH3 as a nitrogen source without supplying group III raw materials, and performing annealing for 1 minute (1 min) after the temperature increase, the holes CH were blocked by mass transport to form the first embedded layer 14A. Note that nitrogen (N2) was supplied as the ambient gas.

[0033] The cross-sectional SEM image of the portion where the holes CH in the n-guide layer 14 are filled is shown in FIG. 5A, and the schematic cross-sectional view thereof is shown in FIG. 5B. As shown in FIGS. 5A and 5B, a cavity 14C having a hexagonal column structure is formed, the inner surface of which consists of {10-11} facets (facets with a predetermined plane orientation), the width (WC) is 70 nm, and the depth (HC) is 118 nm. More specifically, the (000-1) plane appears on the surface (upper surface) of the cavity 14C on the active layer 15 side, the {10-10} plane appears on the side surface of the cavity 14C, and the {1-102} facet appears at the bottom on the substrate 12 side.

[0034] At this time, on the upper part of the n-guide layer 14, a first buried layer 14A is formed from the plane formed by the upper surface of the cavity 14C to the surface of the n-guide layer 14. The layer thickness (D1) of the first buried layer 14A was 26 nm.

[0035] FIG. 6A is an image of an atomic force microscope (AFM) showing the surface morphology of the first buried layer 14A, FIG. 6B is a graph showing the surface roughness of the cross-section along the line A-A in FIG. 6A, and FIG. 6C is a graph showing the surface roughness of the cross-section along the line B-B in FIG. 6A. It was confirmed that the surface of the first buried layer 14A is composed of the GaN(0001) plane and has a stepped terrace structure that is bunched with a width similar to the period PC (=163 nm).

[0036] Also, as shown in FIG. 6A, it was confirmed that pits PT appear in a square lattice pattern at the surface positions corresponding to the cavity 14C on the surface of the first buried layer 14A with the period of the cavity 14C. That is, the pits PT appear on the crystal orientation

[0001] with respect to the cavity 14C. In the present embodiment, when the first buried layer 14A is viewed from above, it was confirmed that the pits PT appear at positions overlapping the cavity 14C. That is, pits PT derived from or caused by the cavity 14C appear on the surface of the first buried layer 14A. When the off-angle of the crystal is large, when the first buried layer 14A is viewed from above, the pits PT may not overlap the cavity 14C, but they appear on the crystal orientation

[0001] with respect to the cavity 14C.

[0037] In forming the first embedded layer 14A, it is not necessary for the pits PT to be in a regular array state where they are two-dimensionally arrayed on the surface of the first embedded layer 14A. The first embedded layer 14A may be formed by mass transport such that the surface state is one where the pits PT remain on the surface of the first embedded layer 14A.

[0038] Also, the first embedded layer 14A is not limited to GaN. As the first embedded layer 14A, other crystals, for example, ternary or quaternary GaN-based semiconductor crystal layers may be formed, and it is preferable that the refractive index is higher than that of the n-clad layer 13. For example, AlGaN, InGaN, etc. with a lower Al composition than the n-clad layer 13 can be used. [Second Embedded Layer] After forming the first embedded layer 14A, while maintaining the temperature at 1150°C, the second embedded layer 21 was formed by supplying group III material gas (TMG) and group V material gas (NH3). In this example, hydrogen (H2) was supplied simultaneously.

[0039] A cross-sectional SEM image of the portion where the holes 14C in the n-guide layer 14 were filled at this time is shown in FIG. 7A, and a schematic cross-sectional view thereof is shown in FIG. 7B.

[0040] The second embedded layer 21 could be formed without changing the shape of the holes 14C. In this case, the total thickness of the first embedded layer 14A (D1 = 26 nm) and the second embedded layer 21 (D2 = 34 nm) was 60 nm.

[0041] FIGS. 8A and 8B are an AFM image showing the surface morphology of the second embedded layer 21 and a graph showing the surface roughness of the cross-section along the line A-A in FIG. 8A, respectively.

[0042] As shown in FIG. 8B, the surface of the second embedded layer 21 has a step-terrace structure with a step height equal to the height of one bilayer of GaN, and it was confirmed that an atomically flat (0001) surface of GaN was obtained.

[0043] As described above, in the formation of the first embedded layer 14A, the embedding by mass transport is completed so that the surface state is such that pits PT appear on the surface of the first embedded layer 14A, and the first embedded layer 14A is formed. Note that it is preferable that the first embedded layer 14A is formed in a state where pits PT are at least partially regularly arranged and remain on the surface of the first embedded layer 14A.

[0044] This is because when the first embedded layer 14A is formed, if the pits PT are completely embedded with migration atoms and / or molecules composed of Ga and / or GaN during mass transport and the surface state becomes equivalent to other parts, the migration atoms and / or molecules aggregate at the defective and transferred parts remaining in the first embedded layer 14A to form hillocks (protrusions), which inhibits the planarization of the second embedded layer 21.

[0045] Also, if the pits PT serve as a starting point during the growth (regrowth) of the second embedded layer 21, or even if the surface state where the pits PT remain is not necessarily regular, the second embedded layer 21 can be made uniform and planarized.

[0046] Note that in order to fill and planarize the pits PT, the layer thickness of the second embedded layer 21 is preferably larger than the layer thickness of the first embedded layer 14A.

[0047] Although the case where GaN is grown as the second embedded layer 21 has been described as an example, the crystal composition of the second embedded layer 21 may be different from that of the first embedded layer 14A. As the second embedded layer 21, other crystals, for example, ternary or quaternary GaN-based semiconductor crystal layers may be formed. For example, a crystal layer containing In in its composition such as an InGaN layer can be used as the second embedded layer 21. Also, by forming a crystal layer having a higher refractive index with respect to the emission wavelength than the first embedded layer as the second embedded layer 21, the coupling coefficient κ3 can be increased compared to the case of a GaN crystal layer. Also, the temperature for forming the second embedded layer 21 is preferably in the range of 750°C to 1150°C when the constituent material is GaN, and 750°C to 900°C when it is InGaN. [Growth of Active Layer and p-Side Semiconductor Layer] Subsequently, a multiple quantum well (MQW) layer was grown as the active layer 15. The barrier layer and well layer of the MQW were GaN and InGaN, respectively. For the growth of the barrier layer, after cooling the substrate to 800°C, triethylgallium (TEG) was supplied as a group III atom source and NH3 was supplied as a nitrogen source. 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. Note that the center wavelength of the PL (Photoluminescence) emission from the active layer 15 in this example was 410 nm.

[0048] After the growth of the active layer 15, the substrate was maintained at 800°C and the p-side guide layer 16 was grown with a layer thickness of 100 nm. The p-side guide layer 16 was grown as undoped GaN (u-GaN) by supplying TEG and NH3 without doping a dopant.

[0049] After the growth of the p-side guide layer 16, the substrate temperature was raised to 1050°C at a certain rate, and the electron blocking layer (EBL) 17 and p-clad layer 18 were grown. For the growth of the EBL 17, TMG and TMA were supplied as group III atom sources and NH3 was supplied as a nitrogen source. For the growth of the p-clad layer 18, TMG and TMA were supplied as group III atom sources and NH3 was supplied as a nitrogen source. The Al concentration of each layer of the EBL 17 and p-clad layer 18 was 18% and 6%, and the layer thicknesses were 17 nm and 600 nm, respectively. Also, during the growth of the EBL 17 and p-clad layer 18, Cp2Mg (Bis-cyclopentadienyl magnesium) was supplied as a dopant simultaneously with the group III atom source gas and nitrogen source. When activated in air at 700°C for 5 minutes, the carrier density of the p-clad layer (p-AlGaN clad layer) 18 was 4×10 17 cm -3 It was. [Coupling Coefficient κ3] As described above, in order to reduce the resonator loss and oscillate at a low threshold current density in the photonic crystal surface-emitting laser, it is effective to increase the coupling coefficient κ3 of the light wave propagating in the plane of the photonic crystal layer 14P.

[0050] To examine this point, for the GaN-based photonic crystal surface-emitting laser element having the structure shown in FIG. 9, the optical coupling coefficient κ3 was calculated by solving the coupled-wave equation.

[0051] Note that the photonic crystal surface-emitting laser used for calculating the optical coupling coefficient κ3 shown in FIG. 9 has an n-clad layer 13 (n-Al 0.04 Ga 0.96 N: layer thickness 2,000 nm), an n-guide layer 14 (n-GaN: layer thickness 120 nm), a photonic crystal layer 14P (n-GaN: layer thickness 120 nm), an embedded layer 14D (n-GaN: layer thickness DP = 0 to 120 nm), an active layer 15 (quantum well layer (u-In 0.08 Ga 0.92 N) with 2 layers of MQW), a p-side guide layer 16 (u-GaN: layer thickness 120 nm), an EBL 17 (p-Al 0.18 Ga 0.82 N: layer thickness 17 nm), and a p-clad layer 18 (p-Al 0.06 Ga 0..94 N: layer thickness 600 nm) stacked in this order.

[0052] FIGS. 10A and 10B show the changes in the coupling coefficient κ3 and the threshold gain with respect to the layer thickness DP of the embedded layer 14D, that is, the distance between the active layer 15 and the photonic crystal layer 14P. Note that the case where the filling factor (FF) is 6% is shown.

[0053] As shown in FIGS. 10A and 10B, in order to increase the coupling coefficient κ3 and oscillate at a low threshold current density, it can be seen that the total layer thickness of the first embedded layer 14A and the second embedded layer 21 (or the distance between the active layer 15 and the photonic crystal layer 14P) DP is preferably 100 nm or less. Further, the total layer thickness is preferably in the range of 20 nm to 100 nm.

[0054] As described above, in this embodiment, on a guide layer substrate in which holes CH having two-dimensional periodicity are formed in the n-guide layer 14, (i) By annealing while supplying a gas containing a nitrogen source, the openings of the holes are blocked by mass transport to form a first buried layer, (ii) A gas containing a group III raw material and a nitrogen source is supplied to form a first buried layer.

[0055] When the temperature is raised to the temperature at which mass transport occurs, the hole shape changes so that the surface energy becomes the smallest. That is, for example, the side surface of a hole in c-plane GaN is deformed into a {10-10} plane with a low dangling bond density. Further, the holes are closed by annealing, and a first buried layer whose surface is composed of the (0001) plane is formed. At this time, the surface of the first buried layer has a stepped terrace structure bunched with the same width as the periodic interval of the holes. Since the annealing temperature is the temperature at which mass transport occurs, the annealing temperature for closing the holes CH is preferably in the range of 1000°C to 1200°C when the n-guide layer is made of GaN.

[0056] Further, the first buried layer is composed of the same element as the n-guide layer in which the holes are formed.

[0057] The second buried layer supplies a group III raw material and grows a new layer. Therefore, it is not necessary to be composed of the same element as the n-guide layer in which the holes are formed like the first buried layer. However, from the viewpoint of optical confinement, it is preferably composed of a material having a refractive index equal to or higher than that of the first buried layer.

[0058] Due to the second buried layer, the bunched surface structure of the first buried layer is restored, and a surface having a stepped terrace structure with a step height of one bilayer is obtained. Therefore, the surface of the second buried layer becomes an atomically flat (0001) plane.

[0059] By the above method, in the GaN-based photonic crystal surface-emitting laser, the distance between the photonic crystal layer and the active layer can be made 100 nm or less. That is, in the GaN-based material photonic crystal surface-emitting laser, the optical coupling coefficient κ3 can be increased, and a photonic crystal surface-emitting laser capable of oscillating operation at a low threshold current density can be realized. [Hole filling method and structure of comparative example] FIG. 11 is a schematic diagram showing a hole filling method, which is a comparative example of the above embodiment. The SiNx film SN of the guide layer substrate in which two-dimensional periodic holes CH are formed in the n-guide layer 14 is removed using hydrofluoric acid (HF), washed to obtain a clean surface, and then introduced again into the MOVPE apparatus, which is the same as in the above embodiment (FIG. 11(a)).

[0060] In this comparative example, in the MOVPE apparatus, the guide layer substrate is heated to 1050° C., and group III material gas (TMG) and group V material gas (NH3) are supplied to perform crystal growth so as to form a recess having a {10-11} facet on the substrate surface, thereby closing the opening of the hole CH (FIG. 11(b)).

[0061] At this time, the distance E1 between the (000-1) plane, which is the upper surface of the embedded hole CH, and the (0001) plane, which is the outermost surface of the n-guide layer 14, was approximately 140 nm (FIG. 11(b)).

[0062] After the hole CH is closed with a {10-11} facet, the supply of the group III material gas is stopped, and while supplying the group V material gas (NH3), the temperature is raised to 1150° C. at a heating rate of 100° C. / min, and the temperature is held for 1 minute. As a result, the {10-11} facet formed on the surface of the n-guide layer 14 disappeared, and the surface became a flat (0001) plane. That is, the surface was planarized by mass transport.

[0063] At this time, the distance E2 between the surface (upper surface, (000-1) plane) on the active layer 15 side of the hole (cavity) 14C formed by embedding and the surface of the n-guide layer 14 (that is, the (0001) plane) was approximately 105 nm.

[0064] That is, according to the pore filling method of this comparative example, {10-11} facets preferentially grow to close the pores, and then the (0001) plane is formed on the surface by annealing. In this manufacturing method, the distance between the photonic crystal layer 14P and the active layer 15 can only be thinned to about 100 nm. That is, in this method, the optical coupling coefficient κ3 cannot be increased in the GaN-based photonic crystal surface-emitting laser, and it is difficult to perform the oscillation operation at a low threshold current density. [Crystallinity: SIMS analysis] When forming the pores CH in the n-guide layer 14 by etching, since oxygen is taken in, oxygen is also unintentionally taken into the first buried layer.

[0065] In the above-described embodiment, since the group III material gas (TMG), the group V material gas (NH3), and hydrogen (H2) are supplied to form the second buried layer 21, the oxygen unintentionally taken into the second buried layer 21 is drastically reduced. For example, the oxygen taken into the second buried layer 21 is 5×10 16 cm -3 as follows.

[0066] The results of SIMS (Secondary Ion Mass Spectrometry) analysis of the semiconductor structure layer 11 of this embodiment formed as described above are shown in FIG. 12. FIG. 12 shows the analysis results from a part of the p-clad layer 18 to a part of the n-clad layer 13. As shown in the oxygen (O) concentration profile, in the first buried layer 14A, the oxygen mixed in during the etching for forming the pores CH is 3×10 18 c m -3 present, while the oxygen concentration in the second buried layer 21 is confirmed to be 2×10 16 cm -3 or less (below the detection limit).

[0067] Thus, in the second buried layer 21, it was confirmed that the generation of group III atomic vacancies in the crystal due to high-concentration oxygen doping was suppressed, and a film with good crystallinity was obtained. Therefore, the crystal layer grown on the second buried layer 21 also has good crystallinity.

[0068] Note that the various numerical values and the like in the above embodiments are merely examples. They can be appropriately changed without departing from the scope of the present invention.

[0069] In the above embodiments, the case where the semiconductor structure layer 11 has the electron barrier layer 17 has been described as an example. However, the electron barrier layer 17 may not be provided. Alternatively, the semiconductor structure layer 11 may include a contact layer, a current diffusion layer, and other semiconductor layers.

[0070] In this specification, the case where the first conductivity type semiconductor (n-type semiconductor), the active layer, and the second conductivity type semiconductor (p-type semiconductor, which is the opposite conductivity type to the first conductivity type) are grown in this order has been described as an example. However, the first conductivity type may be p-type and the second conductivity type may be n-type.

[0071] As described in detail above, according to the present invention, it is possible to increase the coupling coefficient κ3 of the light wave propagating in the direction parallel to the photonic crystal layer, reduce the resonator loss, and provide a surface emitting laser capable of oscillating operation at a low threshold current density and a method for manufacturing the same.

Description of Reference Numerals

[0072] 10: Photonic Crystal Surface Emitting Laser 12: Substrate 13: n-Clad Layer 14: n-Guide Layer 14A: First Buried Layer 14P: Photonic Crystal Layer 14C: Hole 15: Active Layer 16: Guide Layer 18: p-Clad Layer 21: Second Buried Layer

Claims

1. A substrate; a first clad layer of a first conductivity type formed on the substrate; a first guide layer of a first conductivity type, the first guide layer including: a photonic crystal layer formed on the first cladding layer, the photonic crystal layer having a plurality of holes arranged with two-dimensional periodicity in a plane parallel to the layer; and a first burying layer closing the holes in the photonic crystal layer; a second buried layer formed on the first buried layer; an active layer formed on the second buried layer; a second guide layer formed on the active layer; a second cladding layer of a second conductivity type opposite to the first conductivity type formed on the second guide layer; A surface-emitting laser element made of a group III nitride semiconductor, wherein the oxygen concentration contained in the second buried layer is smaller than the oxygen concentration contained in the first buried layer, and the oxygen concentration contained in the second buried layer is 5×10 16 cm −3 or less.

2. the first buried layer and the second buried layer have the same crystal composition; 2. The surface emitting laser element according to claim 1, wherein the second buried layer is thicker than the first buried layer.

3. 3. The surface emitting laser element according to claim 1, wherein a surface roughness of the upper surface of the first buried layer is greater than a surface roughness of the upper surface of the second buried layer.

4. 4. The surface-emitting laser element according to claim 1, wherein an upper surface of the first buried layer has a step-and-terrace structure including pits formed at positions corresponding to the plurality of voids arranged with two-dimensional periodicity.

5. A method for manufacturing a surface-emitting laser element made of a Group III nitride semiconductor by MOVPE, comprising: (a) a step of growing a first cladding layer of a first conductivity type on a substrate; (b) a step of growing a first guide layer of the first conductivity type on the first cladding layer; (c) a step of forming holes in the first guide layer by etching, the holes being arranged with two-dimensional periodicity in a plane parallel to the first guide layer; (d) a step of supplying a gas containing a nitrogen source and forming a first burying layer by mass transport to close openings of the holes; and (e) a step of supplying a Group III source material after the formation of the first burying layer, to form a second burying layer that buryes and flattens the first burying layer, A method for manufacturing a surface emitting laser element, wherein the oxygen concentration in the second buried layer is lower than the oxygen concentration in the first buried layer, and the oxygen concentration in the second buried layer is 5×10 16 cm −3 or less.

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