Photonic crystal surface-emitting laser element

Abstract

To provide a photonic-crystal surface-emission laser element that has high beam quality, and that upon a CW operation, inhibits high-mode oscillation and side lobe, maintains a fundamental mode until high current injection, and has stable transverse and longitudinal modes.SOLUTION: A photonic-crystal surface emission laser element includes: a first semiconductor layer 12 in which a photonic-crystal layer 14P having holes arranged with two-dimensional periodicity in a face in parallel to the first semiconductor layer; an active layer 15 that is formed on the first semiconductor layer; a second semiconductor layer 18 that is formed on the active layer; a high refractive index layer 31 that is formed on the second semiconductor layer and that has an opening 31C exposing the second semiconductor at its central region; a transparent conductor layer 20B that covers the portion of the second semiconductor layer exposed through the opening, that is electrically connected to the second semiconductor layer, and has a refractive index lower than that of the high refractive index layer; and a light reflection film 32 that is provided on the transparent conductor layer and the high refractive index layer and that reflects light entered through the opening and the second semiconductor layer.SELECTED DRAWING: Figure 1A

Description

[Technical Field]

[0001] This invention relates to a photonic crystal surface-emitting laser element. [Background technology]

[0002] In recent years, the development of photonic crystal surface-emitting lasers (Photonic-Crystal 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] Furthermore, photonic crystal surface-emitting lasers are known that have a multi-lattice photonic crystal constructed by arranging multiple vacancies 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 large number of aggregates of heterorefractive-index regions, each consisting of multiple regions with different refractive indices from the base material and at least two of these regions having different thicknesses, are arranged periodically within a plate-shaped base material.

[0006] Furthermore, Non-Patent Document 3 discloses how to suppress multimode oscillations that lead to beam quality degradation by changing the vacancy size and lattice constant of a photonic crystal.

[0007] However, because photonic crystals have very small vacancies, it is difficult to manufacture them with high precision in terms of vacancy size and lattice constant.

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

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

[0010] [Non-Patent Document 1] Tanaka et al., Proceedings of the 2016 Autumn Meeting of the Japan Society of Applied Physics, 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.” [Overview of the project] [Problems that the invention aims to solve]

[0011] This invention is based on the finding that, in continuous wave operation or CW operation (CW: Continuous Wave Operation) of a photonic crystal surface-emitting laser element, the beam quality deteriorates due to a mechanism different from that of pulsed operation.

[0012] An object of the present invention is to provide a photonic crystal surface emitting laser device having a high beam quality with stable transverse and longitudinal modes, suppressing higher order mode oscillation and side lobes in CW operation, and maintaining the fundamental mode until high current injection is reached.

Means for Solving the Problems

[0013] A surface emitting laser device according to an embodiment of the present invention is a photonic crystal surface emitting laser device having a photonic crystal layer, comprising: a first semiconductor layer in which a photonic crystal layer having holes arranged with two-dimensional periodicity in a plane parallel to the layer is formed; an active layer formed on the first semiconductor layer; a second semiconductor layer formed on the active layer; a high refractive index layer formed on the second semiconductor layer and having an opening exposing the second semiconductor layer in a central region; [[ID=]17]a transparent conductor layer covering the second semiconductor layer exposed in the opening and electrically connected to the second semiconductor layer, and having a lower refractive index than the high refractive index layer; and a light reflecting film provided on the transparent conductor layer and the high refractive index layer, and reflecting light incident through the opening and the second semiconductor layer.

Brief Description of the Drawings

[0014] <所 [Figure 1A] FIG. 1 is a cross-sectional view schematically showing an example of the structure of a photonic crystal laser device (PCSEL device) 10 of Example 1. [Figure 1B] FIG. 2 is an enlarged cross-sectional view schematically showing a photonic crystal layer 14P and a pair of holes (air holes) 14K arranged in the photonic crystal layer 14P in FIG. 1A. [Figure 2A] FIG. 3 is a plan view schematically showing the upper surface of the PCSEL device 10. [Figure 2B] FIG. 4 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 14P (cross section A-A in FIGS. 1B). [Figure 2C]This is a schematic plan view showing the bottom surface of the PCSEL element 10. [Figure 3] This is a schematic plan view showing vacancy pairs 14K, consisting of main vacancies 14K1 and secondary vacancies 14K2, arranged in a square lattice position within the plane of the photonic crystal layer 14P. [Figure 4A] This figure shows the calculation results of the far-field image (FFP) when a constant current of 2A is applied to the PCSEL element 10 of Example 1. [Figure 4B] This figure shows the calculated oscillation spectrum when a constant current of 2A is applied to the PCSEL element 10 of Example 1. [Figure 5A] This figure shows the calculation results of the far-field image (FFP) when a constant current of 2A is applied to the PCSEL element of the comparative example. [Figure 5B] This figure shows the calculated oscillation spectrum when a constant current A is applied to the PCSEL element of the comparative example. [Figure 6] This figure shows the photonic bands for Example 1 (EMB1) and Comparative Example (CMP) with and without current injection. [Figure 7] This is a schematic cross-sectional view showing an example of the structure of the PCSEL element 40 in Example 2 (EMB2). [Figure 8] This is a schematic cross-sectional view showing an example of the structure of the PCSEL element 50 in Example 3 (EMB3). [Modes for carrying out the invention]

[0015] Preferred embodiments of the present invention will be described below, but these may be modified and combined as appropriate. In the following description and accompanying drawings, substantially identical or equivalent parts will be denoted by the same reference numerals. [Examples]

[0016] [Structure of a photonic crystal surface-emitting laser] A photonic crystal surface-emitting laser (hereinafter also referred to as a PCSEL) is a device that has a resonator layer parallel to the semiconductor light-emitting structure layer (consisting of an n-guide layer, a light-emitting layer, and a p-guide layer) that constitutes the light-emitting element, and emits coherent light in a direction perpendicular to the resonator layer.

[0017] On the other hand, 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 other words, 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 that parallel plane. That is, the direction of light extraction is perpendicular to the resonance direction (in the plane parallel to the photonic crystal layer).

[0018] Figure 1A is a schematic cross-sectional view showing an example of the structure of a photonic crystal laser element (PCSEL element) 10 of Example 1. The PCSEL element 10 is constructed by stacking a plurality of semiconductor layers on a substrate. These semiconductor layers are made of, for example, hexagonal nitride semiconductors such as GaN-based semiconductors.

[0019] Figure 1B is a schematic enlarged cross-sectional view showing the photonic crystal layer 14P and the pairs of air holes 14K arranged within the photonic crystal layer 14P in Figure 1A.

[0020] Figure 2A is a schematic plan view showing the top surface of the PCSEL element 10, Figure 2B is a schematic cross-sectional view showing the cross-section of the photonic crystal layer (PC layer) 14P in a plane parallel to the n-guide layer 14 (Figure 1B, AA cross-section), and Figure 2C is a schematic plan view showing the bottom surface of the PCSEL element 10.

[0021] The structure of the PCSEL element 10 will be described in detail below with reference to the drawings. In the PCSEL element 10, a first semiconductor layer 12, an active layer 15, and a second semiconductor layer 18 are stacked on a substrate 11.

[0022] The first semiconductor layer 12 consists of an n-clad layer (first clad layer) 13 and an n-guide layer (first guide layer) 14. The n-guide layer (first guide layer) 14 consists of a lower guide layer 14A, a photonic crystal layer (vacancy layer, or PC layer) 14P, and a filling layer 14B. The photonic crystal layer 14P has vacancies arranged with two-dimensional periodicity in a plane parallel to the layer.

[0023] The active layer 15 consists of a GaN barrier layer and In x Ga y It has a multiple quantum well structure (MQW structure) in which N(x+y=1) well layers are stacked alternately.

[0024] The second semiconductor layer 18 consists of a p-guide layer (second guide layer) 16A provided on the active layer 15, an electron barrier layer (EBL) 16B provided on the p-guide layer 16A, a p-cladding layer (second cladding layer) 16C provided on the electron barrier layer 16B, and a p-contact layer 17 formed on the p-cladding layer 16C. The p-guide layer 16A, electron barrier layer 16B, and p-cladding layer 16C constitute the p-side semiconductor layer 16.

[0025] The p-contact layer 17 is a semiconductor layer that improves ohmic contact with the p-electrode, and is formed of a semiconductor layer with a smaller energy band gap and / or a higher impurity concentration than the p-cladding layer 16C.

[0026] In other words, 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 the opposite conductivity type to the first conductivity type (e.g., p-type).

[0027] In this specification, we will describe 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. However, the first and second conductivity types may be p-type and n-type, respectively.

[0028] In this specification, "n-" and "p-" mean "n-side" and "p-side," respectively, and do not necessarily mean that they have n-type and p-type characteristics. For example, the n-guide layer refers to a guide layer located on the n-side of the active layer, and may be an undoped layer (or i-layer).

[0029] Furthermore, the n-clad layer 13 may consist of multiple layers rather than a single layer, in which case not all layers need to be n-layers (n-doped layers), and may include undoped layers (or i-layers). The same applies to the second semiconductor layer 18.

[0030] Furthermore, although the above describes the specific and detailed configuration of the semiconductor layers of the photonic crystal laser element 10, it only shows one example of an element structure. In short, it is sufficient to have a first semiconductor layer having a photonic crystal layer 14P, a second semiconductor layer, and an active layer (light-emitting layer) sandwiched between these layers, and to be configured to emit light when current is injected into the active layer.

[0031] For example, a photonic crystal laser element does not need to have all of the semiconductor layers described above. Alternatively, a photonic crystal laser element may have various semiconductor layers to improve its element characteristics (e.g., hole barrier layer, optical confinement layer, current confinement layer, tunnel junction layer, etc.).

[0032] Furthermore, an annular n-electrode (cathode) 20A (first electrode) is formed on the back surface of the substrate 11, and an anti-reflective coating 22 is provided inside the n-electrode 20A on the back surface of the substrate 11, which is the laser light emission surface.

[0033] A resonance frequency adjustment film (high refractive index layer) 31 is provided on the upper surface of the p-contact layer 17. As shown in Figure 2A, the resonance frequency adjustment film 31 has a circular (cylindrical) opening 31C with axis CX as its central axis, and the surface of the p-contact layer 17 is exposed through the opening 31C. The opening 31C may have an oval shape when viewed from above.

[0034] A p-electrode 20B (second electrode), which is a transparent electrode that fills the opening 31C, is formed in the opening 31C. That is, the p-electrode 20B covers the p-contact layer 17 exposed from the opening 31C and is connected to the p-contact layer 17 by ohmic contact.

[0035] The p electrode 20B is formed as a transparent conductive layer with a lower refractive index than the resonant frequency adjustment film (high refractive index layer) 31.

[0036] In this embodiment, the p electrode 20B is formed to cover the upper end surface of the resonant frequency adjustment film 31 and has a circular shape when viewed from above along the central axis CX (top view).

[0037] A light-reflecting film (hereinafter simply referred to as the reflective film) 32 is provided on the p electrode 20B. When viewed from above, the reflective film 32 is coaxial with the central axis CX of the opening 31C, has a similar shape to the opening 31C, and has a larger area than the opening 31C. That is, when viewed from above, the reflective film 32 is large enough to encompass the opening 31C, and in this embodiment, it has a circular shape.

[0038] The reflective film 32 only needs to be provided on the p electrode 20B, which is a transparent conductive layer, and on a part of the resonant frequency adjustment film (high refractive index layer) 31 (i.e., on a part of the periphery of the opening 31C). However, as shown in Figure 1A, if the p electrode 20B has a covering portion that covers the upper end surface of the resonant frequency adjustment film 31, the reflective film 32 may be formed with the covering portion of the p electrode 20B sandwiched between the reflective film 32 and the resonant frequency adjustment film 31.

[0039] Furthermore, the reflective film 32 is provided so as to cover at least the entire opening 31C when viewed from above, and it is preferable that it has a size greater than or equal to the effective diameter (beam diameter) of the laser beam. For example, the reflective film 32 has a circular shape coaxial with the central axis CX, and the Gaussian beam diameter (light intensity is 1 / e of the peak) is greater than or equal to the effective diameter (beam diameter) of the laser beam. 2 It is preferable that the diameter be greater than or equal to the diameter of the given value.

[0040] A pad electrode 23 is formed on the reflective film 32, electrically connected to the p electrode 20B. The pad electrode 23 is formed to embed the p electrode 20B and the reflective film 32. Note that the pad electrode 23 is not shown in Figure 2A.

[0041] The sides of the stacked semiconductor layers (i.e., the first semiconductor layer 12, the active layer 15, and the second semiconductor layer 18), the sides of the resonant frequency adjustment film 31, and the edges of the top surface are covered with a protective film 21 made of an insulator such as SiO2. For clarity in the figure, the protective film 21 is not hatched.

[0042] Light emitted directly from the photonic crystal layer 14P (directly emitted light Ld) and light emitted from the photonic crystal layer 14P, which passes through the p electrode 20B and the resonance frequency adjustment film 31 and is reflected by the reflective film 32 (reflected emitted light Lr) are emitted to the outside from the light emission region 20L on the back surface of the substrate 11.

[0043] As shown in Figure 1B, in this embodiment, the photonic crystal layer 14P has a double lattice structure. That is, vacancy pairs 14K (main vacancies 14K1 and sub-vacancies 14K2) are arranged in two dimensions at square lattice point positions with periodic PK on the crystal growth surface (semiconductor layer growth surface), i.e., the plane parallel to the n-guide layer 14 (cross section AA in the figure), and are embedded within the n-guide layer 14. Note that the photonic crystal layer 14P is not limited to a double lattice structure, but may also have a single lattice structure or a multi-lattice structure.

[0044] As shown in Figure 2B, in the photonic crystal layer 14P, vacancy pairs 14K are arranged periodically within, for example, a circular vacancy-forming region 14R. As shown in Figure 2C, the n-electrode (cathode) 20A is provided as an annular electrode outside the vacancy-forming region 14R so as not to overlap with the vacancy-forming region 14R when viewed perpendicular to the photonic crystal layer 14P (i.e., from above). The region inside the n-electrode 20A is the light-emitting region 20L.

[0045] Furthermore, as shown in Figures 2B and 2C, the annular n-electrode 20A is formed coaxially with the opening 31C and the void formation region 14R of the resonant frequency adjustment film 31, and the diameter DP of the void formation region 14R is smaller than the inner diameter DE of the n-electrode 20A (DP <DE)。 [Specific configuration of each layer of the PCSEL element 10] The details of each layer of the PCSEL element 10 are described below. The characteristic simulation of the element described later was performed based on the detailed configuration below. However, unless otherwise specified, the composition, layer thickness, etc. of each layer of the PCSEL element 10 are merely examples and can be modified and applied as appropriate.

[0046] (1) Substrate The substrate 11 is a hexagonal GaN single crystal substrate with high transmittance to light emitted from the active layer 15, and is a growth substrate. More specifically, the substrate 11 is a GaN single crystal whose main surface is the (0001) plane, which is the "+c" plane, where Ga atoms are arranged on the outermost surface.

[0047] Furthermore, the substrate can be either a perfectly aligned substrate with no offset on the main surface, or a substrate that is offset by, for example, about 1° in the m-axis direction. For example, a substrate offset by about 1° in the m-axis direction can achieve mirror-finish growth under a wide range of growth conditions.

[0048] The substrate surface (back side) where the light-emitting region 20L opposite the main surface is provided is the "-c" plane, which is the (000-1) plane where N atoms are arranged on the outermost surface. The -c plane is resistant to oxidation and is therefore suitable as a light extraction surface.

[0049] In this embodiment, the GaN substrate 11 is a substrate of n-type GaN single crystal and has the function of a contact layer with the n electrode.

[0050] The n-clad layer 13 is an n-type Al 0.04 Ga 0.96 N layer (layer thickness: 2 μm). The carrier concentration of the n-clad layer 13 at room temperature is 1×10 18 cm -3 . The n-clad layer may have an Al composition of 2% to 10% and a thickness of 1 to 3 μm.

[0051] (2) n-guide layer The lower guide layer 14A of the n-guide layer 14 is n-type GaN and has a layer thickness of 300 nm. The lower guide layer 14A may have a layer thickness of, for example, 20 to 500 nm. The carrier concentration of the lower guide layer 14A at room temperature is 1×10 18 cm -3 .

[0052] The photonic crystal layer 14P on the lower guide layer 14A includes regularly arranged air holes that resonate the light emitted from the active layer 15 in the horizontal plane.

[0053] The photonic crystal layer 14P is an undoped GaN layer and has a layer thickness of 100 nm. The photonic crystal layer 14P may have a layer thickness of, for example, 60 to 150 nm. Details of the air holes in the photonic crystal layer 14P will be described later in detail with reference to the drawings.

[0054] The embedded layer 14B on the photonic crystal layer 14P is an undoped GaN layer and has a layer thickness of 100 nm. The embedded layer 14B may be a layer with a thickness of 50 to 150 nm made of, for example, n-GaN or n-InGaN, or undoped GaN or undoped InGaN.

[0055] (2.1) Photonic crystal layer Figure 3 is a schematic plan view of a vacancy pair 14K, consisting of a main vacancy 14K1 and a secondary vacancy 14K2, arranged in a square lattice position within the plane of the photonic crystal layer 14P, as viewed from above. In other words, the photonic crystal layer 14P has a double lattice structure. For clarity in the drawing, the main vacancies 14K1 and secondary vacancies 14K2 are shown with hatching.

[0056] More specifically, the main vacancies 14K1 are arranged in a square lattice position with a period PK in two mutually orthogonal directions (x and y directions) with their centroids CD1. Similarly, the secondary vacancies 14K2 are arranged in a square lattice position with a period PK in the x and y directions with their centroids CD2. Specifically, the period PK was set to 179 nm. The oscillation wavelength of this embodiment is approximately 441 nm. Here, the period PK may be appropriately adjusted to match the gain wavelength emitted from the active layer.

[0057] The main void 14K1, when viewed from above (i.e., from a direction perpendicular to the photonic crystal layer 14P), has a long hexagonal shape surrounded by the m-plane, which is a {10-10} plane. The secondary void 14K2 also has a shape surrounded by the m-plane.

[0058] The x and y directions are inclined at 45° with respect to the long axis direction (<11-20> direction) and short axis direction (<1-100> direction) of the main void 14K1, respectively. In this specification, the xy coordinates are also referred to as void coordinates.

[0059] Furthermore, the center of gravity CD2 of the sub-vacuum 14K2 is spaced Δx in the x-direction and Δy in the y-direction relative to the center of gravity CD1 of the main vacuum 14K1. Here, Δx = Δy. That is, the center of gravity CD2 of the sub-vacuum 14K2 is spaced in the <1-100> direction from the center of gravity CD1 of the main vacuum 14K1. Specifically, the distance between the centers of gravity Δx in the x-direction and the distance between the centers of gravity Δy in the y-direction were set to 82.8 nm. Here, the distance between the centers of gravity Δx in the x-direction and the distance between the centers of gravity Δy in the y-direction may be appropriately adjusted according to the laser characteristics, such as the current value required for the device to which the laser element is applied, or the slope efficiency of the laser emission intensity.

[0060] (3)Active layer The active layer 15 consists of a GaN barrier layer with a thickness of 6 nm and an In layer with a thickness of 4.0 nm. 0.14 Ga 0.86 The structure has a multi-layer quantum well (MQW structure) in which N(x+y=1) well layers are stacked alternately. In this embodiment, a quantum well structure with two well layers was used.

[0061] The active layer 15 is positioned within 180 nm of the vacancy layer (photonic crystal layer 14P), a distance that allows for sufficient resonance effect from the vacancy layer.

[0062] (4) Second semiconductor layer The p-guide layer 16A of the second semiconductor layer 18 is an undoped InGaN layer with a thickness of 250 nm. The p-guide layer 16A may have a thickness of 100 to 500 nm, and may also be an undoped AlInGaN layer.

[0063] Furthermore, although the p-guide layer 16A was made an undoped layer considering light absorption by the dopant, it may be doped with magnesium (Mg), for example, to obtain good electrical conductivity.

[0064] The electron blocking layer (EBL) 16B provided on the p-guide layer 16A has a thickness of 15 nm, an Al composition of 20%, and is magnesium-doped p-type Al. 0.20 Ga 0.80 It is an N-layer structure.

[0065] The aluminum (Al) composition of EBL16B was chosen to create a barrier (energy difference) that prevents electron carriers from flowing into the p-clad layer 16C. Here, EBL16B may be a multilayer structure with varying Al concentrations, in addition to AlGaN with a constant Al concentration.

[0066] The p-clad layer 16C has a thickness of 100 nm and is made of magnesium-doped p-type Al. 0.06 Ga 0.94The material is N. The p-clad layer 16C may have an Al composition of 2% to 10% and a layer thickness of 100 to 350 nm.

[0067] In this embodiment, a p-contact layer 17 is provided on the p-clad layer 16C. The p-contact layer 17 has a thickness of 25 nm and is made of magnesium-doped p-type GaN. The carrier concentration of the p-contact layer 17 is set to a concentration that allows for ohmic bonding with the anode electrode provided on its surface.

[0068] Furthermore, an InGaN layer with a thickness of 2 nm or less, or a Si or O-doped p-type GaN layer with a thickness of 2 nm or less, may be provided on the outermost surface. By adding such layers, the contact resistance of the p-electrode can be reduced.

[0069] (5) Resonance frequency adjustment film (high refractive index layer) The resonant frequency adjustment film (high refractive index layer) 31 has a higher refractive index than the transparent p electrode 20B and is formed of a transparent and insulating material with respect to the oscillation wavelength of the active layer 15. The resonant frequency adjustment film 31 can be formed by depositing ZrO2 by sputtering.

[0070] The resonant frequency adjustment film 31 has a cylindrical opening (300 μm in diameter) with axis CX as its central axis. This opening 31C can be formed by well-known methods such as chemical etching or dry etching of the ZrO2 film.

[0071] Furthermore, the method for depositing the resonant frequency adjustment film 31 is not limited to sputtering; other methods such as CVD, electron beam evaporation, ALD, and ion plating can also be used.

[0072] Furthermore, as the resonant frequency adjustment film 31, ZrO2, TiO2, Ta2O5, Nb2O5, SiN x An insulating, light-transmitting dielectric layer such as the above can be used.

[0073] Alternatively, semiconductor layers such as GaN, AlGaN, InGaN, AlInGaN, and AlN can be used. In this case, the semiconductor layer may be of the opposite conductivity type (i.e., p-type) to the underlying semiconductor layer (n-type), or the semiconductor layer may be made insulating by ion implantation or the like.

[0074] The refractive index of the p electrode 20B (ITO) is approximately 2.0, and the refractive indices of the transparent dielectric layers are as follows: ZrO2 (2.26), TiO2 (2.26), Ta2O5 (2.21), Nb2O5 (2.32), SiN x (2.46) (the value in parentheses is the refractive index at a wavelength λ=440nm).

[0075] Furthermore, the semiconductor layer changes depending on its composition, but to give an example of its refractive index, consider GaN (2.46) and Al. 0. 1Ga 0.9 N(2.40), In 0. 1Ga 0.9 The refractive indices are N (2.60) and AlN (2.18) (the values ​​in parentheses are the refractive indices at a wavelength of λ = 440 nm).

[0076] As described above, these translucent dielectric layers or translucent semiconductor layers having a higher refractive index than the p electrode 20B can be used as the resonant frequency adjustment film 31.

[0077] The above-mentioned translucent dielectric layer or translucent semiconductor layer may be used as a single layer, but a layer formed by stacking multiple such layers can be used as the resonant frequency adjustment film 31. Furthermore, the above-mentioned layer may include at least one or more of these layers.

[0078] Furthermore, a further transparent dielectric layer such as SiO2 may be provided on the transparent dielectric layer and / or transparent semiconductor layer described above.

[0079] (6) p electrode (anode electrode) Indium tin oxide (ITO), a transparent conductor, was used as the p-electrode 20B. ITO can be formed by sputtering. The p-electrode 20B has a thickness of 135 nm and is formed to fill the opening 31C.

[0080] Furthermore, it is preferable to crystallize the deposited ITO film and heat it using a lamp heating method in a nitrogen gas atmosphere with added oxygen in order to reduce contact resistance and improve transparency.

[0081] In addition to ITO, transparent conductors such as ZnO and GZO (Ga-doped ZnO) can also be used for the p electrode 20B.

[0082] The p electrode 20B is in ohmic contact with the p contact layer 17. The thickness is preferably in the range of 50 nm to 300 nm. A thickness of 50 nm or more is preferable for it to function as a cladding (light confinement), because if it is too thick, the stress will increase, making it prone to delamination and increasing absorption loss.

[0083] (7) Reflective film The reflective film 32 on the p electrode 20B was made of Ag (silver) with a thickness of 300 μm. The reflective film 32 has a circular shape with a diameter of 350 μm centered on axis CX.

[0084] The material of the reflective film 32 is preferably one that has a high reflectivity to laser light. For example, metals such as Ag, Al, Rh, Pd, and Pt, or alloys based on these materials, can be used. Alternatively, a dielectric DBR made by stacking multiple layers of SiO2 / Ta2O5, for example, can be used. Furthermore, a combination of a dielectric DBR and a highly reflective metal film may also be used.

[0085] (8) Pad electrodes Pad electrodes 23 are formed on the reflective film 32 and are electrically connected to the p electrode 20B. The pad electrodes 23 are made of Ni / Pd / Au with thicknesses of 10 nm, 200 nm, and 500 nm respectively (Au is the outermost layer), and are formed to embed the p electrode 20B and the reflective film 32.

[0086] The pad electrode 23 has a rectangular shape measuring 700 μm × 700 μm when viewed from above. The PCSEL element 10 also has a rectangular prism shape measuring approximately 800 μm × 800 μm.

[0087] (9) n electrode (cathode electrode) The n electrode 20A is made of Ti / Pt / Au (Au is the outermost surface) and is in ohmic contact with the growth substrate 11. In addition to Ti / Pt / Au, the electrode material can be selected from Ti / Al, Ti / Rh, Ti / Al / Pt / Au, Ti / Au, V / Al / Au, V / Rh / Au, V / Al / Pt / Au, V / Pt / Au, etc. The outermost surface is preferably made of Au for wire bonding.

[0088] (10) Protective film The protective film 21 protects the aluminum (Al)-containing crystalline layer constituting the PCSEL from corrosive gases, etc. It also protects against deposits and short circuits caused by solder creep during mounting, improving reliability and yield. The material is not limited to SiO2; ZrO2, HfO2, TiO2, Al2O3, etc., can be selected.

[0089] [Characteristics of PCSEL element 10] The results of the characteristic simulation of the PCSEL element 10 of Example 1 are described below in comparison with the PCSEL element of the comparative example. The PCSEL element of the comparative example has the same structure as the PCSEL element 10 of Example 1, except that the material used as the resonant frequency adjustment film 31 is different.

[0090] In other words, in Example 1, a material with a higher refractive index than the p electrode 20B is used as the resonance frequency adjustment film 31, whereas in this comparative example, SiO2, a material with a lower refractive index than the p electrode 20B, is used.

[0091] In this simulation, heat generation is considered in the current injection region, and a refractive index distribution corresponding to this heat generation is established to set the effective refractive index. Outside the current injection region, the effective refractive index adjusted by the resonant frequency adjustment film 31 is set, and calculations are performed. The central peak temperature for heat generation was set to approximately 50°C.

[0092] Figures 4A and 4B show the calculated results of the far-field image (FFP) and oscillation spectrum, respectively, when a constant current of 2A (amperes) is applied to the PCSEL element 10 of Example 1. Figures 5A and 5B show the calculated results of the far-field image (FFP) and oscillation spectrum, respectively, when a constant current of 2A (amperes) is applied to the PCSEL element of the comparative example.

[0093] Figure 6 shows the photonic bands for Example 1 (EMB1) and Comparative Example (CMP) with and without current injection. The photonic band gap (PBG) is between the band edges BE1 and BE2. Region I (central region) corresponds to the opening 31C, i.e., the contact area between the p electrode 20B and the p-contact layer 17, and region II (peripheral region) corresponds to the formation area of ​​the resonant frequency adjustment film 31.

[0094] Referring to Figures 4A and 5A, we compare the far-field images, which represent the beam characteristics at a current of 2A, for Example 1 and the Comparative Example. In Example 1, a clean far-field image with a narrow divergence angle was observed. On the other hand, in the Comparative Example, cross-shaped side lobes appeared around the beam with a narrow divergence angle.

[0095] As shown in Figure 6, this is thought to be due to the significant decrease in the band edge frequency in region I, which is the current injection region, in the comparative example (CMP) due to heat generation in the current injection region.

[0096] On the other hand, during current injection, the semiconductor layer corresponding to region I, the current injection region, experiences an increase in carrier density and a decrease in refractive index compared to the semiconductor layer of the surrounding region II. This simultaneously causes a phenomenon where the band edge frequency of region I increases. However, the increase in refractive index due to heat generation during CW operation has a greater impact on the band edge frequency than the decrease in refractive index due to the increase in carrier density. As a result, the inventors concluded that the band edge frequency of region I decreased significantly.

[0097] More specifically, in the comparative example, since SiO2 with a lower refractive index than ITO is deposited around it, the photonic band in the current-injected region or the central region (region I) is reduced even before current is injected.

[0098] Furthermore, during current injection, the photonic band frequency decreases due to heat generation, exacerbating the effect, and deviates significantly from the photonic band frequency of the surrounding region (region II). Given this relative position of the photonic bands, it is thought that in-plane optical confinement is greatly weakened, the threshold gain of modes that oscillate with a narrow divergence angle increases, and modes that oscillate in a cross shape appear.

[0099] On the other hand, in the case of the embodiment, due to the relationship with an appropriate photonic band frequency, it is thought that oscillation in single mode can be achieved without degrading the threshold gain of a narrow beam divergence angle.

[0100] Referring to the oscillation spectra of Example 1 and Comparative Example shown in Figures 4B and 5B, respectively, it can be seen that Example 1 oscillates in a longitudinal single mode, while the Comparative Example oscillates in a longitudinal multimode. Note that the spectrum of the Comparative Example near 440.85 nm corresponds to the spectrum of the cross-shaped beam (Figure 5A).

[0101] Therefore, in CW operation, it is possible to provide a photonic crystal surface-emitting laser element with high beam quality that suppresses higher-order mode oscillation and side lobes, maintains the fundamental mode until high current injection occurs, and has stable transverse and longitudinal modes. [Examples]

[0102] Figure 7 is a schematic cross-sectional view showing an example of the structure of the PCSEL element 40 in Example 2 (EMB2).

[0103] The PCSEL element 40 differs from the PCSEL element 10 of Example 1 in that it has a resonance frequency adjustment film 31 that is thicker than the p electrode 20B. The layer configuration of the semiconductor light-emitting structure layer, consisting of the first semiconductor layer 12, the active layer 15, and the second semiconductor layer 18, is the same as that of the PCSEL element 10 of Example 1.

[0104] More specifically, the p-electrode 20B, which has a refractive index smaller than that of the resonant frequency adjustment film 31, has a thickness smaller than that of the resonant frequency adjustment film 31 and is formed on the p-contact layer 17. Furthermore, the p-electrode 20B is embedded in the opening 31C by the reflective film 32.

[0105] In other words, in this embodiment as well, a p electrode 20B, which is a transparent conductive layer, is provided in the opening 31C, which is the central region of the upper surface of the semiconductor light-emitting structure layer, and a resonance frequency adjustment film 31 having a higher refractive index than the transparent conductive layer is provided in the surrounding region.

[0106] According to the PCSEL element 40 of Example 2, the optical path length perpendicular to the semiconductor light-emitting structure layer in the region of the resonant frequency adjustment film 31 (region II or peripheral region) is longer than in region I (central region), allowing for a larger adjustment range. Therefore, the stability of the transverse and longitudinal modes during CW operation can be improved. [Examples]

[0107] Figure 8 is a schematic cross-sectional view showing an example of the structure of the PCSEL element 50 in Example 3 (EMB3).

[0108] The PCSEL element 50 differs from the PCSEL element 10 of Example 1 and the PCSEL element 40 of Example 2 in that it has a resonant frequency adjustment film 31 that reaches inside the p-clad layer 16C. The layer configuration of the semiconductor light-emitting structure layer, consisting of the first semiconductor layer 12, the active layer 15, and the second semiconductor layer 18, is the same as that of the PCSEL element 10 of Example 1.

[0109] More specifically, the resonant frequency adjustment film 31 reaches the interior of the p-clad layer 16C, and within the opening 31C, a portion of the p-clad layer 16C, the p-contact layer 17, and the p-electrode 20B are embedded and formed by the reflective film 32.

[0110] The structure of this embodiment can be realized by etching the p-contact layer 17 and p-cladding layer 16C in the region excluding the central region (region I) to form a resonant frequency adjustment film 31 in the region (region II).

[0111] In other words, in this embodiment as well, a p electrode 20B, which is a transparent conductive layer, is provided in the opening 31C, which is the central region of the upper surface of the semiconductor light-emitting structure layer, and a resonance frequency adjustment film 31 having a higher refractive index than the transparent conductive layer is provided in the surrounding region.

[0112] According to the PCSEL element 50 of Example 3, the optical path length perpendicular to the semiconductor light-emitting structure layer in the region of the resonant frequency adjustment film 31 (region II or peripheral region) is longer than in region I (central region), allowing for a larger adjustment range. Therefore, the stability of the transverse and longitudinal modes during CW operation can be improved.

[0113] In the above-described embodiment, the case of forming a resonant frequency adjustment film (high refractive index region: region II) having a circular (including oval) opening when viewed from above was explained as an example, but the invention is not limited to this. The region of the opening (low refractive index region: region I) may also be an n-sided polygon (n is an integer of 4 or more). In this case, it is preferable that it has a polygonal shape with 6 or more sides (6 ≤ n).

[0114] Furthermore, it is preferable that the opening (region I) of the resonant frequency adjustment film has a rotationally symmetric shape with respect to the central axis CX. For example, if the opening has a polygonal shape when viewed from above, it is preferable that the opening has a regular polygonal shape.

[0115] Furthermore, although the example given was that the opening has a cylindrical shape, the explanation is not limited to this, and it may also have a frustocone shape, a polygonal frustocone shape, or the like. In the case of a polygonal frustocone shape, it is preferable that the opening has a regular polygonal frustocone shape.

[0116] As described in detail above, the present invention provides a photonic crystal surface-emitting laser element with high beam quality that suppresses higher-order mode oscillation and side lobes in CW operation, maintains the fundamental mode until high current injection occurs, and has stable transverse and longitudinal modes. [Explanation of Symbols]

[0117] 10,40,50: 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: Embedding layer, 15: Active layer, 16A: Second guide layer, 16B: Electron barrier layer, 16C: Second cladding layer, 17: Contact layer, 18: Second semiconductor layer, 20A: First electrode, 20B: Translucent electrode (second electrode), 20L: Light emission region, 21: Protective film, 23: Pad electrode, 31C: Aperture, 31: Resonance frequency adjustment film (high refractive index layer)

Claims

1. A photonic crystal surface emitting laser element having a photonic crystal layer, A first semiconductor layer is formed in which a photonic crystal layer having vacancies arranged with two-dimensional periodicity in a plane parallel to the layer is formed, An active layer formed on the first semiconductor layer, A second semiconductor layer formed on the active layer, A high refractive index layer formed on the second semiconductor layer, having an opening in its central region through which the second semiconductor layer is exposed, A transparent conductive layer covering the second semiconductor layer exposed in the opening and electrically connected to the second semiconductor layer, having a refractive index lower than that of the high refractive index layer, A light-reflecting film is provided on the transparent conductive layer and the high refractive index layer, and reflects light incident through the opening and the second semiconductor layer, A photonic crystal surface-emitting laser element having a photonic crystal surface emission element.

2. The transparent conductive layer fills the opening, The photonic crystal surface emitting laser element according to claim 1, wherein the light-reflecting film is coaxial with the opening and, when viewed from a direction perpendicular to the photonic crystal layer, has a similar shape to the opening and a larger area than the opening.

3. The high refractive index layer has a greater thickness than the transparent conductive layer. The transparent conductive layer is embedded in the opening by the light-reflecting film. The photonic crystal surface emitting laser element according to claim 1, wherein the light-reflecting film is coaxial with the opening and, when viewed from a direction perpendicular to the photonic crystal layer, has a similar shape to the opening and a larger area than the opening.

4. The high refractive index layer is formed to a depth that extends from the surface side of the second semiconductor layer to the interior of the second semiconductor layer. Within the opening of the high refractive index layer, a portion of the surface side of the second semiconductor layer is embedded by the transparent conductive layer. The photonic crystal surface emitting laser element according to claim 1, wherein the light-reflecting film is coaxial with the opening and, when viewed from a direction perpendicular to the photonic crystal layer, has a similar shape to the opening and a larger area than the opening.

5. The photonic crystal surface emitting laser element according to any one of claims 1 to 4, wherein the opening has a circular shape when viewed from a direction perpendicular to the photonic crystal layer.

6. The aforementioned high refractive index layer is an insulating, light-transmitting dielectric layer, ZrO ２ layer, TiO ２ Layer, Ta ２ O ５ layer, Nb ２ O ５ Layer and SiN ｘ A photonic crystal surface-emitting laser element according to any one of claims 1 to 5, comprising at least one layer among the layers.

7. The photonic crystal surface emitting laser element according to any one of claims 1 to 5, wherein the high refractive index layer comprises at least one semiconductor layer selected from a GaN layer, an AlGaN layer, an InGaN layer, an AlInGaN layer, and an AlN layer, and the at least one semiconductor layer is insulating or has the opposite conductivity of the second semiconductor layer.

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