JP2010003746A - Semiconductor laser device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a surface light-emitting semiconductor laser device which is two-dimensionally arranged, and has few coupling loss and high light-emitting efficiency. <P>SOLUTION: The semiconductor laser device includes a laminated structure 31 of a nitride semiconductor, which is formed on a substrate 11 and includes a stripe-like optical waveguide 32 extending in parallel to a main face of the substrate 11. A reflection mirror 34 reflecting light transmitted through the optical waveguide 32 in a direction vertical to the main face of the substrate 11 is formed in at least one end face of the optical waveguide 32. A light transmission film 35 having an effective refractive index distribution whose effective refractive index is smaller in an outer edge part compared to a center part is formed in an optical outgoing region where light reflected by the reflection mirror 34 in the laminated structure 31 is output. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a semiconductor laser device, and more particularly to a surface emitting nitride semiconductor laser device.

  A semiconductor laser device (laser diode: LD) has excellent features such as small size, low cost, and high output. For this reason, it is used not only for applications such as optical communication and optical information recording, but also in a wide range of technical fields such as medicine and lighting. Development of a gallium nitride (GaN) -based semiconductor laser device that outputs pure blue laser light having a wavelength of 450 nm to 470 nm is also underway for use in laser display and liquid crystal backlights.

In display and backlight applications, in order to achieve high brightness, high output of GaN-based semiconductor laser devices is required. In order to increase the output of the semiconductor laser device, a method of arraying the semiconductor laser device has been devised. For example, FIG. 7 shows an example of an array of edge-emitting semiconductor laser devices 312 (see, for example, Patent Document 1), and FIG. 8 shows an array of surface-emitting (VCSEL) semiconductor laser devices 314. An example (see, for example, Patent Document 2) is shown.
JP 2003-158332 A JP 2007-13227 A

  However, the edge-emitting semiconductor laser device has a problem that it can be arrayed only in one dimension and cannot be highly integrated. Further, the surface emitting semiconductor laser device can be two-dimensionally arranged and can be highly integrated, but there is a problem that the VCSEL type semiconductor laser device has not been put into practical use in the GaN system.

  An object of the present invention is to solve the above-described conventional problems and to realize a surface emitting semiconductor laser device that can be arranged two-dimensionally, has a small light coupling loss, and has high light emission efficiency.

  In order to achieve the above object, according to the present invention, a semiconductor laser device has a light transmission film having an effective refractive index distribution formed in a light emitting region.

  Specifically, a semiconductor laser device according to the present invention includes a nitride semiconductor multilayer structure having a stripe-shaped optical waveguide formed on a substrate and extending in parallel with the main surface of the substrate, and at least one end surface of the optical waveguide. Formed in the light emitting region where the light reflected by the reflecting mirror in the laminated structure is emitted and reflected in the direction perpendicular to the main surface of the substrate. And a light transmission film formed so as to have an effective refractive index distribution having an effective refractive index smaller than that of the central portion.

  The semiconductor laser device according to the present invention includes a light transmission film formed so as to have an effective refractive index distribution whose effective refractive index is smaller at the outer edge portion than at the central portion. For this reason, it becomes possible to make the phase of the light which reached | attained the light emission area | region uniform. Therefore, a surface emitting semiconductor laser device with improved optical coupling efficiency and high luminous efficiency can be realized. As a result, a high-power nitride semiconductor laser device can be made into a two-dimensional array.

  In the semiconductor laser device of the present invention, the light transmission film has a plurality of alternately arranged convex portions and concave portions, and the period of the convex portions is equal to or less than the wavelength of light propagating through the optical waveguide and It is good also as a structure where the ratio with respect to a recessed part becomes small gradually toward an outer edge part from a center part.

  In this case, the ratio occupied by the convex portions may be formed so as to be point-symmetric with respect to the center of the light emission region.

  In this case, the light transmission film may be formed by a plurality of convex portions and concave portions formed in a planar concentric ellipse shape.

  In the semiconductor laser device of the present invention, the light transmission film may be composed of a nitride semiconductor.

  In the semiconductor laser device of the present invention, the reflection mirror may be formed on both end faces of the optical waveguide, and the light transmission film may be formed at a position corresponding to each reflection mirror.

  The present invention can realize a surface-emitting type semiconductor laser device that can be two-dimensionally arranged, has little light coupling loss, and has high emission efficiency.

  An embodiment of the present invention will be described with reference to the drawings. 1A and 1B show a semiconductor laser device according to an embodiment, FIG. 1A shows a planar configuration viewed from the <1000> direction, and FIG. 1B shows a cross section taken along line Ib-Ib in FIG. That is, it shows a cross-sectional configuration viewed from the <1-100> direction.

  As a material constituting the semiconductor laser according to the present embodiment, a material made of a hexagonal (wurtzite) crystal having a six-fold symmetry structure is used. The hexagonal GaN crystal is a polar crystal, and there are two types of planes with Ga atoms arranged and N atoms arranged with the same (0001) plane. In the present specification, a surface on which Ga atoms are arranged is denoted as a (0001) plane, and a surface on which N atoms are arranged is denoted as a (000-1) surface. The (0001) plane is expressed as + c plane, and the (000-1) plane is expressed as -c plane. Similarly, the (1-100) plane is denoted as + m plane, and the (-1100) plane is denoted as -m plane. In addition, when it only describes with c surface etc., it is the meaning including + c surface and -c surface.

  In the present specification, when the main surface is referred to as the (0001) plane, it means that the main surface includes a plane inclined by about ± 5 ° from the (0001) plane.

  As shown in FIG. 1, in the semiconductor laser device of the present embodiment, a nitride semiconductor multilayer structure 31 is formed on the + c plane of a substrate 11 made of n-type GaN whose main surface is a c-plane. The laminated structure 31 includes an n-type cladding layer 12, an n-side light guide layer 13, an active layer 14, a p-side light guide layer 15, and a carrier overflow suppression (OFS) layer 16, which are sequentially formed from the substrate side. The p-type cladding layer 17 and the contact layer 18 may be used.

The n-type cladding layer 12 may be n-type Al _0.08 GaN having a thickness of 2 μm, and the n-side light guide layer 13 may be n-GaN having a thickness of 0.1 μm. The active layer 14 may be a quantum well active layer in which a barrier layer made of In _0.02 GaN and a quantum well layer made of In _0.15 GaN are stacked in three cycles. The p-side light guide layer 15 may be p-GaN having a thickness of 0.1 μm, and the OFS layer may be Al _0.16 GaN having a thickness of 10 nm.

The p-type cladding layer 17 may be a strained superlattice having a thickness of 0.48 μm in which p-Al _0.16 GaN having a thickness of 1.5 nm and GaN having a thickness of 1.5 nm are repeatedly laminated 160 times. The contact layer 18 may be p-AlGaN having a film thickness of 0.05 μm.

  A silicon oxide film 20 is formed on the contact layer 18. The silicon oxide film 20 has a stripe-shaped opening that exposes the contact layer 18. A p-side electrode 21 is formed so as to fill the opening except in the vicinity of one end. The p-side electrode 21 may be a laminate of, for example, palladium (Pd) having a thickness of 40 nm and platinum (Pt) having a thickness of 35 nm. As a result, selective current injection becomes possible, and a striped optical waveguide 32 is formed inside the laminated structure 31.

  A wiring electrode 22 is formed on the p-side electrode 21 and the silicon oxide film 20. For example, the wiring electrode 22 may be a laminate of titanium (Ti) having a thickness of 50 nm, platinum (Pt) having a thickness of 200 nm, and gold (Au) having a thickness of 10 μm.

  By forming the wiring electrode 22 having a large film thickness, it is possible to mount a chip by wire bonding and to effectively dissipate heat generated in the active layer 14, thereby improving the reliability of the semiconductor laser device. Is possible.

  The wiring electrode 22 is preferably formed so as to cover as wide an area as possible. However, when the wiring electrode 22 is cut during cleavage and chip separation, the p-side electrode 21 in close contact with the wiring electrode 22 is peeled off from the contact layer 18. There is a fear. For this reason, it is preferable to form the wiring electrode 22 at a distance from the end of the chip.

  An n-side electrode 23 is formed on the surface of the substrate 11 opposite to the surface on which the laminated structure 31 is formed. The n-side electrode 23 may be a laminated film in which, for example, Ti, Pt, and Au are laminated to 5 nm, 10 nm, and 1000 nm, respectively.

A highly reflective coating 33 made of, for example, a three-period structure of SiO ₂ / Nb ₂ O ₅ is formed on one side surface (resonator end face) in the direction intersecting the optical waveguide 32 in the laminated structure 31.

  FIG. 1 shows an example in which the resonator end face is formed to be an m-plane. In this case, by forming the highly reflective coating 33, it is possible to reduce the ratio of the laser light emitted from the + m plane, which has a relatively low chemical stability. As a result, the ratio of laser light emitted from the + c plane, which has a relatively high chemical stability, increases, so that it is possible to realize a highly reliable semiconductor laser device with little degradation of the end face even during high output operation. it can.

  The end surface opposite to the end surface on which the highly reflective coating 33 is formed is processed so that the angle θ with the main surface of the substrate 11 is 45 degrees, and the light propagating in the optical waveguide 32 is the main surface of the substrate 11. A reflection mirror 34 that reflects in a direction perpendicular to the surface is formed.

  The light reflected by the reflection mirror 34 reaches the upper surface of the laminated structure 31 and is emitted. A light transmission film 35 is formed in the light emission region where the light reflected by the reflection mirror 34 is emitted. In the semiconductor laser device of the present embodiment, the light transmission film 35 is formed by a concave portion and a convex portion formed by selectively removing the upper portion of the p-type cladding layer 17. The light transmission film 35 has an effective refractive index distribution whose effective refractive index is smaller at the outer edge portion than at the central portion.

  The reason why the light transmission film 35 is formed in the light emission region will be described below. By forming the reflection mirror 34, it is possible to emit light propagating through the optical waveguide 32 in a direction perpendicular to the main surface of the substrate 11. As a result, the nitride semiconductor laser device can be made into a two-dimensional array. However, no optical waveguide structure is formed between the reflection mirror 34 and the upper surface of the laminated structure 31. For this reason, as shown in FIG. 2, the light reflected by the reflection mirror 34 spreads until reaching the upper surface of the laminated structure 31. When such spread of light occurs, the phase of light does not become constant on the surface of the laminated structure 31, resulting in light coupling loss. Further, since the light spread in the laminated structure 31 has anisotropy, the spread angle between the vertical far field pattern (FFP) and the horizontal FFP is greatly different.

  In the semiconductor laser device of this embodiment, a light transmission film 35 having an effective refractive index distribution whose effective refractive index is smaller at the outer edge portion than at the central portion is provided in a light emitting region that is a region where light reaches the upper surface of the laminated structure 31. Forming. For this reason, phase compensation of light can be performed in the light emission region. Therefore, it is possible to reduce the coupling loss.

  FIG. 3 shows the calculation result of the ratio of the light propagating in the optical waveguide reflected by the reflecting mirror 34 and then reflected on the surface of the laminated structure 31 to follow the reverse path to be coupled to the optical waveguide again. Yes. As shown in FIG. 3, when the light transmission film 35 is not formed, the coupling efficiency is only 66% even when the angle of the reflection mirror 34 is 45 degrees. On the other hand, when the light transmission film 35 is formed, the optical coupling efficiency is 100% when the angle of the reflection mirror 34 is 45 degrees.

  Further, in the semiconductor laser device of this embodiment, the light transmission film 35 has a planar elliptical shape in which the resonator length direction that is parallel to the optical waveguide 32 is longer than the resonator width direction that is the direction intersecting the optical waveguide 32. Forming. Since the light transmission film 35 has a lens effect based on the effective refractive index distribution, the difference in the spread angle between the vertical FFP and the horizontal FFP can be reduced.

  Hereinafter, a specific structure of the light transmission film 35 having such an effective refractive index distribution will be described. As shown in FIG. 4A, consider the structure of a light transmission film in which first materials 51A and second materials 51B are alternately arranged. The refractive index nh of the first material 51A is larger than the refractive index nl of the second material 51B. Light is felt at the position where the width of the adjacent first material 51A and the width of the second material 51B, that is, the period of the first material 51A is P and the width of the first material 51A is W. The effective refractive index neff can be expressed as shown in Equation (1).

neff = {W · nh + (P−W) · nl} / P (1)
However, it is assumed that the value of P is substantially equal to or smaller than the wavelength of light incident on the light transmission film.

  As is clear from the equation (1), the effective refractive index neff is a function of P and W. Therefore, if the value of P is gradually increased from the central portion toward the outer edge portion and the value of W is decreased, the effective refractive index is smaller at the outer edge portion as shown in FIG. 4B than at the central portion. A light transmission film having an effective refractive index distribution can be formed. Further, only one of P or W may be changed.

  The first material 51A and the second material 51B may be any material as long as there is a difference in refractive index. However, if the second material 51B is air, the refractive index difference is increased and a light transmission film is formed. It can be formed easily. Specifically, a light transmission film having an effective refractive index distribution can be formed by forming a recess in the first material 51A.

  In the present embodiment, the light transmission film 35 is a plurality of planar concentric elliptical concave and convex portions formed on the p-type cladding layer 17. In addition, the width of the concave portion gradually increases from the central portion toward the outer edge portion, and the width of the convex portion gradually decreases. In order to change the effective refractive index, the ratio of the convex portion to the concave portion may be gradually decreased from the central portion toward the outer edge portion. Therefore, the width of the convex portion may be constant and the width of the concave portion may be gradually increased from the central portion toward the outer edge portion, and the width of the concave portion may be constant and the width of the convex portion may be gradually decreased from the central portion toward the outer edge portion. May be. The amount of change in the ratio between the concave portion and the convex portion may be calculated as an optimum value in accordance with the spread of light in the light emission region.

  A method for manufacturing the semiconductor laser device according to this embodiment will be described below. First, the laminated structure 31 is formed on the <0001> plane of the n-type GaN substrate 11 using a metal organic chemical vapor deposition (MOCVD method). The raw material in the MOCVD method is, for example, trimethylgallium as a Ga raw material, trimethylindium as an In raw material, trimethylaluminum as an Al raw material, ammonia as an N raw material, silane gas as an Si raw material for n-type impurities, and biscyclopentadidi as an Mg raw material for p-type impurities Enilmagnesium may be used. Instead of MOCVD, other growth techniques that can grow nitride semiconductor laser structures such as molecular beam epitaxy (MBE) and chemical beam epitaxy (CBE) are used. May be.

After forming the laminated structure 31, a silicon oxide film (SiO ₂ ) is formed to 0.3 μm on the contact layer 18 by using a plasma CVD method or the like. Thereafter, a striped opening for forming a current injection portion and a planar concentric elliptical pattern for forming the light transmission film 35 are formed on the SiO ₂ film using photolithography and etching techniques. A resist pattern having is formed. Using the formed resist pattern as a mask, the SiO ₂ film is etched by, for example, inductively coupled plasma (ICP) etching using CF ₄ gas. As a result, an opening from which the contact layer 18 is exposed and a planar concentric elliptical SiO ₂ film pattern are formed. Thereafter, a mask exposing a region where the planar concentric elliptical SiO ₂ film pattern is formed is formed, and inductively coupled plasma (ICP) etching using, for example, Cl ₂ gas is performed. As a result, the contact layer 18 and a part of the p-type cladding layer 17 are removed from the opening with the mask. In the region where the planar concentric elliptical SiO ₂ film pattern is formed, since the p-type cladding layer 17 remains, a light transmission film 35 constituted by a plurality of planar concentric elliptical convex portions and concave portions is formed. The

  Next, the p-side electrode 21 is formed on the exposed portion of the contact layer 18 using photolithography and EB vapor deposition. Subsequently, the wiring electrode 22 is formed using photolithography and EB vapor deposition. The wiring electrode 22 is preferably formed by first forming a laminate of Ti, Pt, and Au so as to have film thicknesses of 50 nm, 200 nm, and 200 nm, respectively, and further increasing the film thickness of Au to 10 μm.

  Next, after the thickness of the substrate 11 is thinned from the back surface to about 100 μm using diamond slurry, Ti, Pt, and Au are respectively deposited on the back surface of the substrate 11 to 5 nm, 10 nm using, for example, EB vapor deposition. The n-side electrode 23 is formed by stacking 1000 nm.

  Next, the substrate on which the laminated structure 31 is formed is primarily cleaved along the m-plane so that the length in the <1-100> direction is 600 μm.

Next, a highly reflective coating 33 in which, for example, SiO ₂ and Nb ₂ O ₅ are laminated for three periods is formed on the + m plane on the side surface of the multilayer structure 31 using, for example, an ECR sputtering apparatus. The highly reflective coating 33 may be formed using alumina (Al ₂ O ₃ ).

  Next, by using a focused ion beam (FIB), an end surface (−m surface in FIG. 1) opposite to the + m surface on which the highly reflective coating 33 is applied is processed, so that the laser beam is emitted from the laminated structure 31. A reflection mirror 34 for reflecting on the upper surface (+ c surface) side is formed.

  FIGS. 5A and 5B are electron micrographs of the end face processed with FIB, and FIG. 5B is an enlarged view of the dotted line part of FIG. As shown in FIG. 5, a very smooth surface is formed, and a reflection mirror 34 having excellent reflection efficiency is formed.

  Any method may be used as long as it can form the reflection mirror 34 having an angle at which the laser beam is totally reflected. For example, instead of FBI, dry etching or wet etching using an alkaline solution under irradiation with hot phosphoric acid or UV light may be used.

  Next, a semiconductor laser device that outputs blue (wavelength 460 nm) laser light is obtained by performing secondary cleavage along the a-plane so that the length in the <11-20> direction is 200 μm.

  In the present embodiment, the example in which the reflection mirror 34 is provided on one side of the resonator end face has been described. However, as shown in FIG. 6, the reflection mirror 34 may be formed on both of the resonator end faces. When the angle of the reflection mirror 34 is 45 degrees, light can be emitted perpendicular to the main surface of the substrate, and the coupling efficiency is the highest. However, it is also possible to emit light obliquely with respect to the main surface of the substrate by changing the angle of the reflecting mirror.

In the present embodiment, the concave portion and the convex portion are formed by selectively removing a part of the p-type cladding layer 17, and the light transmission film 35 is formed. However, the light transmission film 35 may be formed by stacking another layer on the laminated structure 31 to form a convex portion and a concave portion. In this case, the material of the convex portion is not limited to the nitride semiconductor, and may be SiO ₂ or the like. By making the convex portion SiO ₂ , the difference in refractive index between the convex portion and the concave portion is reduced, but there is an advantage that the processing becomes easy.

  Although the example in which the main surface of the substrate 11 is the c-plane has been shown, another surface may be used. Further, a sapphire substrate, a silicon carbide substrate, a silicon substrate, or the like may be used instead of the GaN substrate. The material and film thickness of each layer formed on the substrate are examples, and may be changed as appropriate. For example, the contact layer 18 may be AlGaN instead of GaN. The light absorption edge of each of the active layer 14, the p-type cladding layer 17, and the contact layer 18 is greater than the photon energy of light reflected by the reflection mirror 34 and traveling through the laminated structure 31 in a direction perpendicular to the main surface of the substrate 11. If the energy is increased, the absorption of light propagating in the vertical direction in the laminated structure 31 is reduced. Therefore, output and reliability can be improved.

  In the present embodiment, an electrode stripe type semiconductor laser device is shown, but the same effect can be obtained in a ridge stripe type or buried type semiconductor laser device.

  The semiconductor laser device according to the present invention can realize a surface-emitting semiconductor laser device that can be arranged two-dimensionally, has a small coupling loss of light, and has high emission efficiency, and can be used as a light source for laser display and illumination. It is useful as a surface emitting nitride semiconductor laser device or the like.

(A) And (b) shows the semiconductor laser device which concerns on one Embodiment of this invention, (a) is a top view, (b) is sectional drawing in the Ib-Ib line | wire of (a). It is drawing for demonstrating the optical path of the light reflected by the reflective mirror. It is a graph which shows the relationship between the angle of a reflective mirror, and optical coupling efficiency. (A) is sectional drawing which shows the structure which produces | generates an effective refractive index distribution, (b) is a graph which shows the effective refractive index distribution produced | generated by the structure of (a). (A) And (b) is an electron micrograph which shows the end surface in which the reflective mirror was formed in the semiconductor laser apparatus concerning one Embodiment of this invention, (b) is an enlarged view of the dotted-line part of (a). . It is sectional drawing which shows the modification of the semiconductor laser apparatus which concerns on one Embodiment of this invention. It is a perspective view which shows the arrayed edge-emitting type semiconductor laser apparatus concerning a prior art example. It is a top view which shows the surface-emitting type semiconductor laser device by which the array was based on a prior art example.

Explanation of symbols

11 substrate 12 n-type cladding layer 13 n-side light guide layer 14 active layer 15 p-side light guide layer 16 carrier overflow suppression OFS layer 17 p-type cladding layer 18 contact layer 20 silicon oxide film 21 p-side electrode 22 wiring electrode 23 n-side Electrode 31 Laminated structure 32 Optical waveguide 33 High reflection coating 34 Reflection mirror 35 Light transmission film 51A First material 51B Second material

Claims (6)

A nitride semiconductor multilayer structure having a stripe-shaped optical waveguide formed on the substrate and extending in parallel with the main surface of the substrate;
A reflection mirror that is formed on at least one end surface of the optical waveguide and reflects light propagating through the optical waveguide in a direction perpendicular to the main surface of the substrate;
A light-transmitting film that is formed in a light emitting region where the light reflected by the reflecting mirror in the laminated structure is emitted and has an effective refractive index distribution having an effective refractive index smaller than that of the central portion at the outer edge portion. A semiconductor laser device. The light transmission film has a plurality of convex portions and concave portions arranged alternately,
The period of the said convex part is below the wavelength of the light which propagates the said optical waveguide, and the ratio with respect to the said recessed part of the said convex part becomes small gradually toward an outer edge part from the center part. Semiconductor laser device.   3. The semiconductor laser device according to claim 2, wherein a ratio occupied by the convex portion is formed to be point-symmetric with respect to a center of the light emitting region.   4. The semiconductor laser device according to claim 3, wherein the light transmission film is formed by the plurality of convex portions and concave portions formed in a planar concentric ellipse shape.   The semiconductor laser device according to claim 1, wherein the light transmission film is made of a nitride semiconductor. The reflection mirror is formed on both end faces of the optical waveguide,
The semiconductor laser device according to claim 1, wherein the light transmission film is formed at a position corresponding to each of the reflection mirrors.