JP2011100934A - Semiconductor laser device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a semiconductor laser device that a group-III nitride substrate whose main surface is a nonpolarity surface and that includes an active layer with an In, wherein a resonance structure with less optical loss is achieved. <P>SOLUTION: The semiconductor laser device 1A includes: a group-III nitride substrate 11 whose main surface 11a is the nonpolarity surface; an active layer 27 with the In that is provided on the group-III nitride substrate 11; and a diffraction grating layer 17 that is provided along the active layer 27 and includes a periodic structure where a refractive index periodically varies in one- or two-dimension. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a semiconductor laser device.

  Non-Patent Document 1 describes an edge-emitting semiconductor laser element. This edge-emitting semiconductor laser element is formed on a GaN substrate whose main surface is a non-polar M-plane, and the resonance end face is formed by reactive ion etching (RIE).

  Non-Patent Document 2 describes a technique for obtaining a polarization characteristic desirable when a resonance end face is formed by cleaving by calculating a polarization characteristic of an AlInGaN substrate having a semipolar plane as a main surface.

Mathew C. SCHMIDT etal., “Demonstration of Nonpolar m-Plane InGaN / GaN Laser Diodes”, Japanese Journal of Applied Physics, Volume 46, No. 9, 2007, pp.L190-L191 A. Yamaguchi, “Theoreticalinvestigation on polarization control of semipolar-oriented InGaN quantum-wellemission using (Al) InGaN alloy substrates”, Applied Physics Letters, Volume 94, Article201104, 2009

  Conventionally, in a GaN-based semiconductor laser device, a resonance structure is generally formed on a GaN-based semiconductor substrate having a C-plane as a main surface. However, in some cases, the active layer contains In in a green-emitting semiconductor laser element or the like. When a group III nitride semiconductor containing In is grown on, for example, a GaN substrate, a piezoelectric field is generated by piezoelectric polarization due to distortion of the crystal structure. This piezo electric field becomes the largest when a group III nitride semiconductor containing In is grown on a GaN substrate having a C-plane as a main surface. When the piezo electric field of the active layer is increased, the electrons and holes injected into the active layer are separated, and the recombination probability contributing to light emission is reduced, so that the light emission efficiency is lowered. Therefore, when the active layer contains In, it is preferable to use a group III nitride substrate whose main surface is a nonpolar surface (semipolar surface or nonpolar surface).

  Further, as a method for forming a resonant structure when manufacturing a semiconductor laser element, a method of forming a reflective end face using cleavage or dry etching is generally used. However, since the group III nitride semiconductor crystal is a hexagonal crystal, even if an attempt is made to form a resonance structure including a reflective end face on a group III nitride substrate having a nonpolar surface as a main surface, the cleavage plane is the main surface. In some cases, it is difficult to form a smooth reflective end face by cleaving. Moreover, it is difficult to stably form a smooth reflection end face by the method of forming the reflection end face by dry etching. Thus, there is a problem that it is difficult to produce a resonant structure with little optical loss on a nonpolar surface.

  The present invention has been made in view of the above-described problems, and in a semiconductor laser device including a group III nitride substrate having a nonpolar plane as a main surface and an active layer containing In, resonance with less optical loss. The purpose is to realize the structure.

  In order to solve the above-described problems, a semiconductor laser device according to the present invention includes a group III nitride substrate having a semipolar plane or a nonpolar plane as a main surface, and an active containing In provided on the group III nitride substrate. And a diffraction grating layer including a periodic structure provided along the active layer and having a refractive index that periodically changes in one or two dimensions.

  In this semiconductor laser device, since an active layer containing In is provided on a group III nitride substrate having a nonpolar plane as a main surface, the piezoelectric field generated in the active layer is reduced, and the luminous efficiency is improved. be able to. In addition, since a diffraction grating layer including a periodic structure whose refractive index periodically changes in one or two dimensions is provided along the active layer, light generated in the active layer is confined by the periodic structure, and the semiconductor It can resonate inside. That is, according to this semiconductor laser element, since the light resonance structure is constituted by the periodic structure inside the semiconductor, not the reflection end face, there is little optical loss even when a good reflection end face cannot be formed by cleavage. A resonant structure can be realized.

  The semiconductor laser device may be characterized in that the periodic structure is a two-dimensional photonic crystal structure. Thereby, it is possible to suitably realize a periodic structure that efficiently reflects light generated in the active layer.

  In the semiconductor laser element, the principal surface of the group III nitride substrate is {10-10} plane, {11-20} plane, {10-11} plane, {10-13} plane, {11-22} In the range of 63 degrees or more and less than 80 degrees in the direction of the m-axis of the group III nitride substrate from the plane, the {20-21} plane, or the plane orthogonal to the reference axis extending along the c-axis of the group III nitride substrate It may be characterized by being a surface inclined at a corner. When the main surface of the group III nitride substrate is such a nonpolar surface, the piezoelectric field generated in the active layer containing In can be effectively reduced, and the light emission efficiency can be further improved. Further, by reducing the piezo electric field, the composition ratio of In in the active layer can be increased, and light having a longer wavelength can be emitted.

  According to the present invention, a resonant structure with little optical loss can be realized in a semiconductor laser device including a group III nitride substrate whose main surface is a nonpolar plane and an active layer containing In.

FIG. 1 is a side sectional view showing a configuration of a semiconductor laser device 1A as a first embodiment of the present invention. FIG. 2 is a diagram showing a triangular grating having a grating interval a as an example of a two-dimensional diffraction grating. FIG. 3 is a diagram showing a reciprocal lattice space which the triangular lattice shown in FIG. 2 has. FIG. 4A is a photonic band diagram showing the result of band calculation using a plane wave expansion method for a triangular lattice having a photonic crystal structure made of InP, and particularly the calculation result for the TE mode. FIG. 4B is an enlarged view in the vicinity of the point S in FIG. FIG. 5 is a perspective view showing an appearance of a group III nitride substrate 11 prepared for manufacturing the semiconductor laser device 1A as a main process of the method for manufacturing the semiconductor laser device 1A. 6A and 6B are side cross-sectional views showing the main steps of the method of manufacturing the semiconductor laser device 1A. FIGS. 7A and 7B are side cross-sectional views showing the main steps of the method of manufacturing the semiconductor laser device 1A. FIG. 8 is a sectional side view showing the main steps of the method of manufacturing the semiconductor laser device 1A. FIG. 9 is a sectional side view showing the main steps of the method of manufacturing the semiconductor laser device 1A. FIG. 10A is a graph showing the relationship between the inclination angle from the C-plane of the GaN substrate main surface and the piezoelectric field generated in the In _0.2 Ga _0.8 N layer grown on the GaN substrate. . FIG. 10B is a graph showing the relationship between the In composition X1 of the In _X1 Ga _{1 -X1} N layer grown on the GaN substrate and the piezoelectric field. FIG. 11 is a plan view showing a configuration of a diffraction grating layer 17a according to a modification of the embodiment.

  Embodiments of a semiconductor laser device according to the present invention will be described below in detail with reference to the accompanying drawings. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted.

(First embodiment)
A semiconductor laser device 1A according to an embodiment of the present invention shown in FIG. 1 includes a group III nitride substrate 11 having a main surface 11a. In one embodiment, the group III nitride substrate 11 is a Si-doped n-type GaN substrate. Alternatively, the group III nitride substrate 11 may be made of another group III nitride semiconductor such as InGaN. The main surface 11a is the other surface (nonpolar surface) excluding the C-plane of the group III nitride semiconductor crystal of the group III nitride substrate 11. For example, the main surface 11a is a nonpolar surface of a group III nitride semiconductor crystal, that is, a {10-10} plane (M plane) or a {11-20} plane (A plane). Alternatively, the main surface 11a is a semipolar plane of a group III nitride semiconductor crystal (for example, {10-11} plane, {10-13} plane, {11-22} plane, {20-21} plane, or group III) Or a surface inclined at an inclination angle ranging from 63 degrees to less than 80 degrees in the m-axis direction of the group III nitride substrate from the plane orthogonal to the reference axis extending along the c-axis of the nitride substrate. . The thickness of the group III nitride substrate 11 is, for example, 300 [μm].

  The semiconductor laser element 1 </ b> A includes an active layer 27 provided on the main surface 11 a of the group III nitride substrate 11. The active layer 27 is composed of a group III nitride semiconductor containing In. In one embodiment, the active layer 27 has a quantum well structure 29, but the structure of the active layer 27 is not limited thereto. The quantum well structure 29 includes well layers 29a and barrier layers 29b arranged alternately. The well layer 29a can be made of, for example, InGaN, InAlGaN, or the like, and the barrier layer 29b can be made of InGaN, InAlGaN, or the like having a larger band gap than the well layer 29a.

  Further, the semiconductor laser element 1 </ b> A includes a diffraction grating layer 17. In the present embodiment, the diffraction grating layer 17 is provided between the main surface 11 a of the group III nitride substrate 11 and the active layer 27, but the diffraction grating layer 17 may be provided on the active layer 27. The diffraction grating layer 17 is provided along the active layer 27 and includes a periodic structure in which the refractive index periodically changes in one or two dimensions. In one embodiment, the periodic structure is a two-dimensional diffraction grating structure made of a two-dimensional photonic crystal, and in another embodiment, the periodic structure is a one-dimensional diffraction grating structure.

  Specifically, the diffraction grating layer 17 includes a plurality of holes 21. The plurality of vacancies 21 are arranged one-dimensionally or two-dimensionally in the plane direction along the active layer 27, and a portion other than the vacancies 21 of the diffraction grating layer 17 is made of, for example, InGaN. The radius of the plurality of holes 21 is, for example, 70 [nm], and the pitch of the adjacent holes 21 is, for example, 166 [nm] (in the case of a triangular lattice). The layer thickness of the diffraction grating layer 17 is, for example, 100 [nm].

  The semiconductor laser element 1 </ b> A includes an n-type cladding layer 15. N-type cladding layer 15 is provided between main surface 11 a of group III nitride substrate 11 and diffraction grating layer 17. The n-type cladding layer 15 is made of a group III nitride semiconductor such as Si-doped n-type AlGaN. The layer thickness of the n-type cladding layer 15 is 2 [μm], for example.

  The semiconductor laser element 1 </ b> A includes a GaN-based semiconductor layer 23. The GaN-based semiconductor layer 23 is provided between the diffraction grating layer 17 and the active layer 27. The GaN-based semiconductor layer 23 is made of a GaN-based compound semiconductor, for example, Si-doped n-type GaN. The layer thickness of the GaN-based semiconductor layer 23 is, for example, 200 [nm].

  The semiconductor laser element 1 </ b> A includes a GaN-based semiconductor layer 31, an electron block layer 33, a p-type cladding layer 35, and a p-type contact layer 37. These layers are stacked on the active layer 27 in this order. The GaN-based semiconductor layer 31 is made of, for example, a GaN layer, and is preferably undoped in order to avoid light absorption by the dopant. The electron block layer 33 is made of, for example, an AlGaN layer having a larger band gap than that of the GaN-based semiconductor layer 31, and preferably contains a p-type dopant. The p-type cladding layer 35 is made of a GaN-based semiconductor that supplies holes to the active layer 27 and has a smaller refractive index than that of the active layer 27 for optical confinement. The p-type cladding layer 35 is made of, for example, an AlGaN layer. The p-type contact layer 37 preferably contains a high concentration of dopant in order to provide good electrical contact. The p-type contact layer 37 is made of, for example, a p-type GaN layer or a p-type AlGaN layer.

  The semiconductor laser device 1A includes an anode electrode 41a and a cathode electrode 41b. The anode electrode 41 a is provided on the p-type contact layer 37 and is in ohmic contact with the p-type contact layer 37. The cathode electrode 41 b is provided on the back surface 11 b of the group III nitride substrate 11 and is in ohmic contact with the group III nitride substrate 11.

  The operation of the semiconductor laser device 1A having the above configuration will be described. When a current is supplied between the anode electrode 41a and the cathode electrode 41b, holes are injected from the p-type contact layer 37 and the p-type cladding layer 35 into the active layer 27, and the group III nitride substrate 11 and the n-type cladding layer 15 are injected. From which electrons are injected. When holes and electrons (carriers) are injected into the active layer 27, carrier recombination occurs and light is generated. The wavelength of the generated light is determined by the band gap of the semiconductor (in this embodiment, InGaN or AlInGaN) constituting the active layer 27. When the active layer 27 has the quantum well structure 29, the thickness of the well layer 29a greatly affects the wavelength of light.

  Light generated in the active layer 27 is confined in and near the active layer 27 by the GaN-based semiconductor layers 23 and 31, but part of the light reaches the diffraction grating layer 17 as evanescent light. When the wavelength of the light reaching the diffraction grating layer 17 coincides with the period of the periodic structure formed by the plurality of holes 21 in the diffraction grating layer 17, the light is reflected in the direction along the periodic structure, and the active layer 27 will resonate.

  In particular, when the periodic structure of the diffraction grating layer 17 is a photonic crystal structure, part of the light of the active layer 27 reaches the diffraction grating layer 17 as evanescent light. When the wavelength of the evanescent light reaching the diffraction grating layer 17 coincides with the period of the photonic crystal structure, a standing wave is induced at the wavelength corresponding to the period. Laser oscillation can be caused by the feedback effect of the standing wave.

  Next, the case where the diffraction grating layer 17 has a two-dimensional diffraction grating will be described in more detail. Two-dimensional diffraction gratings have the property of overlapping when translated in the same period in at least two directions. Such a two-dimensional lattice is formed by arranging regular triangles, squares, or regular hexagons all over the surface and providing lattice points at each vertex. Here, a lattice formed using a regular triangle is a triangular lattice, a lattice formed using a square is a square lattice, and a lattice formed using a regular hexagon is a hexagonal lattice.

  FIG. 2 is a diagram showing a triangular grating having a grating interval a as an example of a two-dimensional diffraction grating. In FIG. 2, paying attention to the arbitrarily selected lattice point A, the direction from the lattice point A to the lattice point B is the Γ-X direction, and the direction from the lattice point A to the lattice point C is the Γ-J direction. To do. Here, a case where the wavelength of light generated in the active layer 27 corresponds to the lattice period in the Γ-X direction will be described.

The diffraction grating layer 17 can be considered to include three one-dimensional diffraction grating groups L, M, and N described below. The one-dimensional diffraction grating group L includes one-dimensional gratings L ₁ , L ₂ and L ₃ provided along the X-axis direction. The one-dimensional diffraction grating group M includes one-dimensional diffraction gratings M ₁ , M _2, and M ₃ provided along a direction that forms 120 ° with respect to the X-axis direction. The one-dimensional diffraction grating group N is composed of one-dimensional gratings N ₁ , N _2, and N ₃ provided along a direction that forms 60 ° with respect to the X-axis direction. An interval between the one-dimensional diffraction gratings included in each of the one-dimensional diffraction grating groups L, M, and N is set as d, and a grating interval in the one-dimensional diffraction grating is set as a.

  First, consider the one-dimensional diffraction grating group L. Light traveling in the direction from the lattice point A toward the lattice point B causes a diffraction phenomenon at the lattice point B. The diffraction direction is defined by the Bragg condition 2d · sin θ = mλ (m = 0, ± 1,...). Here, λ is the wavelength of light in the diffraction grating layer 17. When the diffraction grating is formed so as to satisfy the second-order Bragg reflection (m = ± 2), different grating points D, E, F, and G in the directions of θ = ± 60 ° and ± 120 °. Exists. In addition, lattice points A and K exist at angles θ = 0 ° and 180 ° corresponding to m = 0.

  For example, light diffracted toward the lattice point D at the lattice point B is diffracted according to the lattice group M at the lattice point D. This diffraction can be considered in the same manner as the diffraction phenomenon according to the grating group L. Next, the light diffracted toward the lattice point H at the lattice point D is diffracted according to the lattice group N. In this way, diffraction is performed in order of the lattice point H, the lattice point I, and the lattice point J. The light diffracted from the lattice point J toward the lattice point A is diffracted according to the lattice group N.

  As described above, the light traveling from the lattice point A to the lattice point B reaches the first lattice point A through a plurality of diffractions. For this reason, in the semiconductor laser element 1A, light traveling in a certain direction returns to the position of the original lattice point through a plurality of diffractions, so that a standing wave is generated between the lattice points. Therefore, the two-dimensional diffraction grating structure of the diffraction grating layer 17 acts as an optical resonance structure, that is, a wavelength selector and a reflector.

  The diffraction action described above for the lattice point A occurs at all lattice points arranged two-dimensionally. For this reason, it is considered that light propagating in the Γ-X direction is two-dimensionally coupled to each other by Bragg diffraction. In the diffraction grating layer 17, it is considered that a coherent state is formed by such two-dimensional coupling.

  FIG. 3 is a diagram showing a reciprocal lattice space which the triangular lattice shown in FIG. 2 has. FIG. 3 shows central points Γ of a plurality of brilliant zones in a reciprocal lattice space, and also shows a straight line connecting the central points Γ of two brilliant zones adjacent to each other and the two brilliant zones. A point X at which the boundary lines intersect with each other is shown, and further, a point J at which three adjacent brilliant zones contact each other is shown. The directions defined by the points Γ, X and J in FIG. 3 correspond to the Γ-X direction and the XJ direction described above.

  FIG. 4A is a photonic band diagram showing the result of band calculation using a plane wave expansion method for a triangular lattice having a photonic crystal structure made of InP, and particularly the calculation result for the TE mode. FIG. 4B is an enlarged view in the vicinity of the point S in FIG. The diffraction grating layer 17 shown in FIG. 1 can have the dispersion relationship shown in FIG. 4A, that is, the photonic band structure. The photonic band structure refers to a dispersion relationship defined for the energy of photons based on at least a two-dimensional periodic refractive index distribution provided in the medium.

As shown in FIG. 4A and FIG. 4B, band edges exist in the portions P and S in the figure in the point Γ and the wave number range in the vicinity thereof. Here, it is assumed that the portion S is a “first band end” and the portion P is a “second band end”. The normalized frequency ω ₁ at the first band end is about 0.35, and the normalized frequency ω ₂ at the second band end is about 0.61. Here, since the group velocity (dλ / dk) = 0 at the point Γ, when the normalized frequency ω of light is ω ₁ or ω ₂ , a standing wave is generated regardless of the crystal direction. When the same calculation is performed on a photonic crystal structure (triangular lattice) made of GaN, the normalized frequency ω ₁ at the first band end is about 0.47, and the normalized frequency ω ₂ at the second band end is about 0.82.

  Subsequently, a manufacturing method of the semiconductor laser device 1A of the present embodiment will be described. 5 to 9 are side sectional views showing main steps of the method of manufacturing the semiconductor laser device 1A of the present embodiment. In this embodiment, several group III nitride semiconductor layers for a semiconductor laser are formed by, for example, metal organic vapor phase epitaxy.

First, as shown in FIG. 5, a group III nitride substrate 11 is prepared. The group III nitride substrate 11 has a main surface 11a which is a semipolar surface, and the c-axis Cx of the group III nitride crystal is at an angle θ (θ> 0) with respect to the normal H of the main surface 11a. ) Only tilted. For example, the main surface 11a is a nonpolar surface of a group III nitride crystal, that is, a {10-10} plane (M plane) or a {11-20} plane (A plane). Alternatively, the main surface 11a is a semipolar surface of a group III nitride crystal (for example, {10-11} plane, {10-13} plane, {11-22} plane, {20-21} plane, or group III nitride). A surface inclined at an inclination angle ranging from 63 degrees to less than 80 degrees in the m-axis direction of the group III nitride substrate from a plane orthogonal to the reference axis extending along the c-axis of the physical substrate. The group III nitride substrate 11 is made of, for example, a hexagonal semiconductor In _S Al _T Ga _1- STN (0 ≦ S ≦ 1, 0 ≦ T ≦ 1, 0 ≦ S + T ≦ 1). It consists of type GaN.

  Next, the group III nitride substrate 11 is set in a growth furnace, and a process for a semiconductor laser is performed. First, after performing thermal cleaning of the main surface 11a of the group III nitride substrate 11, an n-type cladding layer 15 is grown as shown in FIG. The n-type cladding layer 15 can be made of a group III nitride semiconductor such as AlGaN. On this n-type cladding layer 15, an n-type GaN layer 43 to be the diffraction grating layer 17 (see FIG. 1) is grown. The n-type GaN layer 43 has a higher refractive index than the n-type cladding layer 15. Instead of the GaN layer 43, an InGaN layer or an AlGaN layer may be grown. After growing the GaN layer 43, the temperature of the growth furnace is lowered to a temperature close to room temperature, and the substrate W1 is taken out of the growth furnace.

  Next, a process of forming a plurality of recesses periodically arranged in one or two dimensions in the GaN layer 43 by removing a part of the GaN layer 43 will be described. As shown in FIG. 6B, a resist 45 for forming a plurality of recesses is uniformly applied on the GaN layer 43 of the substrate W1. The resist 45 is, for example, a resist for electron beam exposure. Then, by exposing the resist 45, a mask 19 is formed as shown in FIG. The mask 19 has an arrangement of openings for a plurality of recesses, and these openings are arranged in a lattice shape such as a triangular lattice or a square lattice.

  A plurality of recesses are formed in the GaN layer 43 using the mask 19. This formation is performed by processing such as etching. As shown in FIG. 7B, for example, the GaN layer 43 is etched using a mask 19 in a dry etching apparatus to form a diffraction grating layer 17 in which a pattern of periodic refractive index distribution is formed. In the diffraction grating layer 17, an array of a plurality of holes 21 that are periodically arranged in one or two dimensions corresponding to the openings of the mask 19 is formed. The plurality of holes 21 form a one-dimensional diffraction grating or a two-dimensional photonic crystal depending on the arrangement. After this etching, the mask 19 is removed to provide a substrate W2. The substrate W2 includes a patterned diffraction grating layer 17.

  The substrate W2 is set in the growth furnace, and the temperature of the growth furnace is raised to the growth temperature. Thereafter, as shown in FIG. 8, the GaN-based semiconductor layer 23 is formed on the diffraction grating layer 17. The GaN-based semiconductor layer 23 covers the plurality of holes 21 of the diffraction grating layer 17 while holding them.

  Subsequently, several Group III nitride semiconductor layers are grown. As shown in FIG. 8, an active layer 27 is grown on the GaN-based semiconductor layer 23. The active layer 27 is optically coupled to the plurality of holes 21. In one embodiment, the active layer 27 has a quantum well structure 29, and the quantum well structure 29 includes well layers 29a and barrier layers 29b arranged alternately.

  Next, a GaN-based semiconductor layer 31 is grown on the active layer 27. The GaN-based semiconductor layer 31 can be, for example, a GaN layer, and preferably comprises an undoped layer in order to avoid light absorption by the dopant. On the GaN-based semiconductor layer 31, an electron block layer 33, a p-type cladding layer 35, and a p-type contact layer 37 are grown in order. The electron block layer 33 can be made of, for example, an AlGaN layer having a larger band gap than that of the GaN-based semiconductor layer 31, and preferably includes a p-type dopant. The p-type cladding layer 35 is made of a group III nitride semiconductor that supplies holes to the active layer 27 and has a smaller refractive index than that of the active layer 27 for optical confinement. The p-type cladding layer 35 can be made of, for example, an AlGaN layer. The p-type contact layer 37 preferably contains a high concentration of dopant in order to provide good electrical contact. The p-type contact layer 37 can be composed of, for example, a p-type GaN layer or a p-type AlGaN layer. These growths provide the substrate W3.

  Thereafter, if necessary, the back surface of the group III nitride substrate 11 is ground. Then, as shown in FIG. 9, the anode electrode 41 a is formed on the p-type contact layer 37 and the cathode electrode 41 b is formed on the back surface 11 b of the group III nitride substrate 11. Finally, the substrate W3 on which the anode electrode 41a and the cathode electrode 41b are formed is divided into chips. Thus, the semiconductor laser device 1A is completed.

In the semiconductor laser device 1A of the present embodiment, an active layer 27 containing In is provided on a group III nitride substrate 11 whose main surface is a nonpolar surface. Here, FIG. 10A shows the relationship between the inclination angle of the main surface of the GaN substrate from the C plane and the piezoelectric field generated in the In _0.2 Ga _0.8 N layer grown on the GaN substrate. It is a graph. As shown in the figure, the piezo electric field in the active layer becomes the largest when the inclination angle of the main surface is 0 ° (that is, C-plane), and decreases as the inclination angle increases, and at an inclination angle of 45 °. 0 [V / cm]. When the inclination angle exceeds 45 °, the piezo electric field takes a negative value and becomes 90 [V / cm] again at 90 °.

FIG. 10B is a graph showing the relationship between the In composition X1 of the In _X1 Ga _{1 -X1} N layer grown on the GaN substrate and the piezoelectric field. In FIG. 10B, graphs G1 to G4 show cases where the inclination angles of the main surface of the substrate with respect to the C plane are 0 °, 30 °, 60 °, and 90 °, respectively. As shown in the figure, when the tilt angle is 0 ° or 30 °, the piezoelectric field tends to increase as the In composition increases. On the other hand, when the tilt angle is 60 °, the piezo electric field tends to decrease in negative value (increase absolute value) as the In composition increases.

  Thus, when growing a layer containing In on a group III nitride substrate, it is necessary to consider a piezoelectric field. That is, in a green laser diode or the like, the active layer may contain In. However, when the piezo electric field of the active layer is increased, electrons and holes injected into the active layer are separated, resulting in a decrease in luminous efficiency. End up. On the other hand, in the semiconductor laser device 1A of this embodiment, since the main surface of the group III nitride substrate 11 is a nonpolar surface, the piezoelectric field generated in the active layer 27 can be reduced and the light emission efficiency can be improved. .

  Conventionally, when the main surface of the substrate is different from the C-plane, it is difficult to create a resonance structure because the cleavage plane is not perpendicular to the main surface. In this respect, in the semiconductor laser device 1A of the present embodiment, the diffraction grating layer 17 including the periodic structure in which the refractive index periodically changes in one or two dimensions is provided along the active layer 27. The light generated in the layer 27 is reflected by this periodic structure and can resonate inside the semiconductor. That is, according to this semiconductor laser device 1A, since the light resonance structure is constituted by the periodic structure inside the semiconductor instead of the reflection end face, even if it is not possible to form a good reflection end face by cleavage, the optical loss is reduced. Fewer resonance structures can be realized.

  Further, as in the present embodiment, the periodic structure of the diffraction grating layer 17 is preferably a two-dimensional photonic crystal structure. Thereby, it is possible to suitably realize a periodic structure that efficiently reflects the light generated in the active layer 27. In addition, taking advantage of the characteristics of the photonic crystal structure, it becomes easy to control the beam and polarization, and the application can be expanded.

  Further, as in the present embodiment, the principal surface 11a of the group III nitride substrate 11 has {10-10} plane, {11-20} plane, {10-11} plane, {10-13} plane, {10-13} plane, 11-22} plane, {20-21} plane, or a plane perpendicular to the reference axis extending along the c-axis of the group III nitride substrate to 63 m or more and less than 80 degrees in the m-axis direction of the group III nitride substrate It is preferable that the surface be inclined at an inclination angle in the range of. Since the main surface 11a of the group III nitride substrate 11 is such a nonpolar surface, the piezoelectric field generated in the active layer 27 containing In can be effectively reduced, and the light emission efficiency can be further improved. Further, by reducing the piezoelectric field, the In composition ratio in the active layer 27 can be increased, and light having a longer wavelength can be emitted.

(Modification)
In the above embodiment, the triangular grating as shown in FIG. 2 has been described as an example of the two-dimensional diffraction grating of the diffraction grating layer 17. However, the periodic structure of the diffraction grating layer 17 may be as follows, for example.

  FIG. 11 is a plan view showing a configuration of a diffraction grating layer 17a according to a modification of the embodiment. As shown in FIG. 11, the diffraction grating layer 17a may be provided such that the plurality of holes 21 form a square lattice (grating interval d). In FIG. 11, paying attention to the arbitrarily selected lattice point W, the direction from the lattice point W to the lattice point P is the Γ-X direction, and the direction from the lattice point W to the lattice point Q is the Γ-M direction. To do. Here, a case where the wavelength of light generated in the active layer 27 corresponds to the grating period in the Γ-X direction will be described.

The diffraction grating layer 17a can be considered to include two one-dimensional diffraction grating groups U and V described below. The one-dimensional diffraction grating group U includes a plurality of one-dimensional gratings (for example, U _{1 to} U ₃ ) provided in the Y-axis direction. The one-dimensional diffraction grating group V includes one-dimensional gratings (for example, V _{1 to} V ₃ ) provided in the X-axis direction.

  First, consider the one-dimensional diffraction grating group U. Light reaching the lattice point P from the lattice point W causes a diffraction phenomenon at the lattice point P. The diffraction direction is defined by the Bragg condition 2d · sin θ = mλ (m = 0, ± 1,...) As in the case of the triangular lattice. When the diffraction grating is formed so as to satisfy the second-order Bragg reflection (m = ± 2), there are other grating points Q and R at an angle of θ = ± 90 °, and m = 0. There are also grid points W and S at corresponding angles θ = 0 ° and 180 °.

  The light diffracted toward the lattice point Q at the lattice point P is diffracted according to the one-dimensional diffraction grating group V at the lattice point Q. This diffraction can be considered in the same manner as the diffraction phenomenon according to the one-dimensional diffraction grating group U. Next, the light diffracted toward the lattice point T at the lattice point Q is diffracted according to the one-dimensional diffraction grating group U. In this way, the light is sequentially diffracted. The light diffracted from the lattice point T toward the lattice point W is diffracted according to the one-dimensional diffraction grating group V.

  Thus, the light traveling from the lattice point W to the lattice point P reaches the first lattice point W through a plurality of diffractions. That is, since light traveling in a certain direction returns to the original lattice point through a plurality of diffractions, a standing wave is generated between the lattice points. Therefore, the two-dimensional diffraction grating of the diffraction grating layer 17a functions as an optical resonator.

  In the two-dimensional diffraction grating of the diffraction grating layer 17a, the above phenomenon occurring at an arbitrary grating point W can occur at all the grating points. For this reason, it is considered that light propagating in the Γ-X direction is two-dimensionally coupled to each other by Bragg diffraction. In the two-dimensional diffraction grating of the diffraction grating layer 17a, it is considered that the three Γ-X directions are coupled by this two-dimensional coupling to form a coherent state.

Example 1
A blue-violet two-dimensional photonic crystal laser was fabricated using metal organic vapor phase epitaxy. As raw materials, trimethylgallium (TMG), trimethylindium (TMI), trimethylaluminum (TMA), ammonia (NH ₃ ), monosilane (SiH ₄ ), and cyclopentadienyl magnesium (Cp ₂ Mg) were used.

After an n-type GaN substrate having a {20-21} plane as a main surface is disposed on a susceptor, NH ₃ and H ₂ are supplied to the growth reactor, and the substrate is 1100 degrees Celsius at a pressure of 30 kPa. Thermal cleaning was performed at a temperature for 10 minutes. After the furnace pressure was set to atmospheric pressure, film formation was performed in the following order. First, TMG, TMA, NH ₃ and SiH ₄ were supplied to grow an n-type Al _0.03 Ga _0.97 N crystal having a thickness of 2 μm. Next, TMG, TMI, NH ₃ and SiH ₄ were supplied to grow an n-type GaN crystal having a thickness of 100 [nm] for a two-dimensional photonic crystal.

After lowering the substrate temperature, the epitaxial wafer was taken out of the growth furnace, and a mask for the photonic crystal was formed as follows. A photoresist for electron beam exposure was applied on the wafer, and a uniform resist film was formed using a spin coater. A square lattice pattern (radius 70 [nm], pitch 166 [nm]) for the photonic crystal was drawn in an area of 300 [μm] square of the resist film using an electron beam exposure apparatus. After developing the resist film to form a mask, an epitaxial wafer was placed in a reactive ion etching apparatus. Using the resist mask, the n-type GaN layer was partially removed and transferred by an etching gas Cl ₂ to form an n-type GaN layer (diffraction grating layer) having an array of holes corresponding to the pattern. Thereafter, the resist mask was removed from the epitaxial wafer for crystal growth.

After the epitaxial wafer was placed on the susceptor, the substrate temperature was raised to the growth temperature (1100 degrees Celsius). TMG, NH ₃ , and SiH ₄ were supplied at a furnace pressure of atmospheric pressure, and an n-type GaN layer of 200 [nm] was grown on the diffraction grating layer. And the temperature of the growth furnace was lowered | hung and the active layer which has a quantum well structure of 3 periods was formed. Specifically, TMG, TMI, and NH ₃ are supplied at a substrate temperature of 880 degrees Celsius to grow an undoped In _0.01 Ga _0.99 N barrier layer having a thickness of 15 [nm]. TMG, TMI, and NH ₃ were supplied at a substrate temperature of 800 ° _C. to grow an undoped In _0.07 Ga _0.93 N well layer having a thickness of 3 nm. By repeating this process, a multi-quantum well active layer having three periods was formed.

Subsequently, the temperature of the growth furnace was raised, TMG and NH ₃ were supplied at a growth temperature of 1100 degrees Celsius, and an undoped GaN layer having a thickness of 50 [nm] was grown. On top of this, TMG, TMA, NH ₃ and Cp ₂ Mg were supplied to grow a 20-nm-thick p-type Al _0.12 Ga _0.88 N electron blocking layer. Further, TMG, TMA, NH ₃ , and Cp ₂ Mg were supplied to grow a p-type Al _0.07 Ga _0.93 N cladding layer having a thickness of 600 [nm]. Further, TMG, NH ₃ , and Cp ₂ Mg were supplied to grow a p-type GaN contact layer having a thickness of 50 [nm]. After these growths, the epitaxial wafer was taken out of the growth furnace, and an anode electrode made of Ni / Au was formed on the p-type GaN layer. Further, after the GaN substrate was ground to a thickness of 100 [μm], a cathode electrode made of Ti / Au was formed on the back surface of the substrate.

  After going through the above steps, a 1 [mm] square chip was cut out so as to include a 300 [μm] square photonic crystal pattern. Then, this chip was mounted on a stem, and bonding was performed using a wire made of Au so that the chip could be energized. When a pulse current (repetition frequency 1 [kHz], pulse width 500 [nsec]) was applied to this blue-violet two-dimensional photonic crystal laser element at room temperature, oscillation as a photonic crystal laser was confirmed.

(Example 2)
A blue-violet distributed feedback laser was fabricated using metal organic vapor phase epitaxy. At that time, TMG, TMI, TMA, NH ₃ , SiH ₄ , and Cp ₂ Mg were used as raw materials.

First, after placing an n-type GaN substrate having a {20-21} plane as a main surface on a susceptor, NH ₃ and H ₂ are supplied to the growth reactor, and the pressure in the furnace is 1100 degrees Celsius at 30 [kPa]. Thermal cleaning was performed at a substrate temperature of 10 minutes. After the furnace pressure was set to atmospheric pressure, film formation was performed in the following order. First, TMG, TMA, NH ₃ and SiH ₄ were supplied to grow an n-type Al _0.03 Ga _0.97 N crystal having a thickness of 2 μm. Next, TMG, TMI, NH ₃ and SiH ₄ were supplied to grow an n-type GaN crystal having a thickness of 100 [nm] for the diffraction grating.

After lowering the substrate temperature, the epitaxial wafer was taken out from the growth furnace, and a mask for a one-dimensional diffraction grating was formed as follows. A photoresist for electron beam exposure was applied on the wafer, and a uniform resist film was formed using a spin coater. A one-dimensional diffraction grating pattern (pitch 80 [nm], region 50 [μm] × 600 [μm]) was drawn on a resist film using an electron beam exposure apparatus. After developing the resist film to form a mask, an epitaxial wafer was placed in a reactive ion etching apparatus. Using the resist mask, the n-type GaN layer was partially removed and transferred by etching gas Cl ₂ to form an n-type GaN layer (diffraction grating layer) having a one-dimensional diffraction grating structure corresponding to the pattern. Thereafter, the resist mask was removed from the epitaxial wafer for crystal growth.

Thereafter, in the same manner as in Example 1, an n-type GaN layer (200 [nm]), an InGaN active layer, an undoped GaN layer (50 [nm]), an Al _0.12 Ga _0.88 N electron blocking layer (20 [20] nm]), a p-type Al _0.07 Ga _0.93 N cladding layer (600 [nm]), and a p-type GaN contact layer (50 [nm]). After these growths, a Ni / Au anode electrode was formed on the p-type GaN layer, and after grinding the GaN substrate, a Ti / Au cathode electrode was formed on the back surface of the substrate.

  After going through the above steps, 1 [mm] square chips were cut out. Then, this chip was mounted on a stem, and bonding was performed using a wire made of Au so that the chip could be energized. When a pulse current (repetition frequency 1 [kHz], pulse width 500 [nsec]) was applied to this blue-violet distributed feedback laser element at room temperature, laser oscillation was confirmed.

  The semiconductor laser device according to the present invention is not limited to the above-described embodiments and modifications, and various other modifications are possible. For example, in the above-described embodiment, a two-dimensional diffraction grating having a two-dimensional photonic crystal structure and a one-dimensional diffraction grating are illustrated as an example of the periodic structure of the diffraction grating layer. However, the periodic structure in the present invention is not limited to these, and other It can be applied to various fine structures. In the above embodiment, the semiconductor laser device including the n-type GaN substrate is exemplified. However, the substrate used in the semiconductor laser device according to the present invention may be of any composition as long as it is a group III nitride substrate. Good.

  DESCRIPTION OF SYMBOLS 1A ... Semiconductor laser element, 11 ... Group III nitride board | substrate, 15 ... N-type clad layer, 17, 17a ... Diffraction grating layer, 19 ... Mask, 21 ... Hole, 23, 31 ... GaN-type semiconductor layer, 27 ... Active Layer 29, quantum well structure, 29a, well layer, 29b, barrier layer, 33, electron blocking layer, 35, p-type cladding layer, 37, p-type contact layer, 41a, anode electrode, 41b, cathode electrode.

Claims (3)

A group III nitride substrate having a nonpolar surface as a main surface;
An active layer containing In provided on the group III nitride substrate;
And a diffraction grating layer provided along the active layer and including a periodic structure whose refractive index periodically changes in one or two dimensions.   The semiconductor laser device according to claim 1, wherein the periodic structure is a two-dimensional photonic crystal structure.   The principal surfaces of the group III nitride substrate are {10-10} plane, {11-20} plane, {10-11} plane, {10-13} plane, {11-22} plane, {20-21] } Surface or a plane orthogonal to a reference axis extending along the c-axis of the group III nitride substrate is inclined at an inclination angle ranging from 63 degrees to less than 80 degrees in the m-axis direction of the group III nitride substrate. The semiconductor laser device according to claim 1, wherein the semiconductor laser device is a surface.

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