JP2010258183A - Semiconductor laser device, and method for manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To attain a semiconductor laser device capable of obtaining an emission beam shape excellent in symmetry in the horizontal direction while achieving an end-face window structure to prevent deterioration of an end face in a semiconductor laser device including a GaN-based semiconductor. <P>SOLUTION: The semiconductor laser device includes: a semiconductor layer laminate 12 formed on a principal plane of a substrate 11 and having a hollow section 12a; and a refractive index adjusting layer 31 burying the hollow section 12a. The semiconductor layer laminate 12 includes: a flat section 12b; and a forbidden bandwidth increase portion 16a formed between the flat section 12b and the hollow section 12a. A forbidden bandwidth of an active layer 16 of the forbidden bandwidth increase portion 16a is larger than that of the flat section 12b. The semiconductor layer laminate 12 also includes a striped optical waveguide 28 avoiding the hollow section 12a, including the flat section 12b and the forbidden bandwidth increased portion 16a, and extending in a direction crossing a front end face 29. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a semiconductor laser device and a manufacturing method thereof, and more particularly to a semiconductor laser device using a nitride semiconductor and a manufacturing method thereof.

  Semiconductor laser devices are used in a wide range of technical fields such as medical care and partial lighting, as well as communication and optical disks, because of their excellent features such as small size, low cost, and high output. In recent years, gallium nitride (GaN) semiconductor laser devices having a light emission wavelength of 405 nm, particularly for high-density optical discs, have been vigorously developed. In addition, a pure blue laser having an emission wavelength of 450 nm to 470 nm using a GaN-based semiconductor laser device is being developed as a backlight for laser displays and liquid crystals.

  GaN-based semiconductor laser devices are required to have high output for high-speed and multilayer recording in optical disk applications. Further, in order to precisely control the writing and reading positions on the disc, higher density is required. In display applications and backlight applications, higher output is required for higher brightness. In order to increase the output of the semiconductor laser device, it is important to suppress a deterioration mode called end face deterioration in which the front end face that emits the laser light is deteriorated by the laser light.

  End-face degradation is caused by the temperature of the end face rising due to light absorption, atoms forming the front end face of the semiconductor laser device, atoms forming the coating material deposited on the resonator end face, or atoms present as the atmosphere of the laser apparatus. This is caused by a chemical reaction between and. The reaction increases crystal imperfections that cause laser light absorption at the front end face, and further increases light absorption at the front end face. Much of the absorbed light becomes heat, raising the temperature of the front end face and shrinking the semiconductor band gap. When the band gap of the semiconductor contracts, light absorption at the front end face further increases, and positive feedback occurs in which the temperature rises. As a result of this positive feedback, a problem called catastrophic optical damage (COD), in which the crystals on the front end face melt, will eventually occur.

  In order to overcome end face deterioration of the semiconductor laser device, an end face window structure using ion implantation or impurity diffusion is formed in a gallium arsenide (GaAs) semiconductor laser device (see, for example, Patent Documents 1 and 2). reference.). The end face window structure is a structure in which the forbidden band width in the vicinity of the front end face in the active layer is made larger than the forbidden band width in other regions of the resonator by impurity implantation or the like. By providing the end face window structure, the amount of light absorption in the vicinity of the light exit end face is reduced, and the amount of heat generated at the light exit end face is reduced, so that end face deterioration can be suppressed.

JP 63-164288 A JP-A 63-196088

  However, the GaN-based semiconductor laser device has strong bonding of GaN crystals and a small diffusion constant of implanted ions or impurities. For this reason, there is a problem that it is difficult to form an end face window structure by an ion implantation method or an impurity diffusion method. For this reason, in a GaN-based semiconductor laser device, a laser structure having an end face window structure has not yet been put into practical use.

  Furthermore, the GaN-based semiconductor laser device is required to have an outgoing beam shape with excellent horizontal symmetry.

  The present invention solves the above-described problems, realizes an end face window structure in a semiconductor laser device made of a GaN-based semiconductor, prevents end face deterioration, and obtains an outgoing beam shape having excellent horizontal symmetry. It aims to be able to realize.

  In order to achieve the above-mentioned object, the present invention forms a semiconductor laser device having an end window structure having a large forbidden band width at least at the front end face, and exhibits symmetry of the effective refractive index distribution around the optical waveguide at the front end face. An improved configuration is adopted.

  A first semiconductor laser device according to the present invention is a semiconductor which is formed by laminating a plurality of nitride semiconductor layers including an active layer on a main surface of a substrate, and has a hollow portion whose height is lower than other portions. A layer stack and a refractive index adjustment layer filling the recess, the semiconductor layer stack is formed between the flat portion in which the active layer is formed parallel to the main surface, and the flat portion and the recess. The active layer includes a forbidden band width increasing portion whose forbidden band width is larger than that of the flat portion, a front end surface that emits light, a flat portion and a forbidden band width increasing portion that avoids the depression and intersects the front end surface. And a striped optical waveguide extending.

  In the first semiconductor laser device, the optical waveguide includes a flat portion and a forbidden band width increasing portion while avoiding the recessed portion. By using the forbidden band width increasing portion as the end face window structure, it is possible to reduce light absorption at the front end face and prevent the generation of COD. Further, a refractive index adjusting layer is embedded in the recess. For this reason, the symmetry of the distribution around the optical waveguide of the effective refractive index distribution on the front end face can be improved. Therefore, it is possible to realize a semiconductor laser device that can obtain an output beam shape with high output and excellent symmetry.

  In the first semiconductor laser device, the refractive index adjustment layer may be an amorphous nitride semiconductor, silicon oxide, titanium oxide, or zinc oxide.

  A second semiconductor laser device according to the present invention includes a semiconductor layer stack formed by stacking a plurality of nitride semiconductor layers including an active layer on a main surface of a substrate. Is formed between the flat portion and the hollow portion, and the forbidden band width of the active layer is larger than that of the flat portion. A large forbidden band width increasing portion, a front end face that emits light, a striped optical waveguide that includes a flat portion and a forbidden band width increasing section that avoids a recess and extends in a direction intersecting the front end face, and an optical waveguide. It has a refractive index adjustment concave portion formed on the opposite side to the hollow portion with the nitride semiconductor layer partially removed.

  The second semiconductor laser device has a refractive index adjusting recess formed on the side opposite to the recess with the optical waveguide interposed therebetween, from which a part of the nitride semiconductor layer has been removed. Therefore, not only the end face window structure can be formed, but also the symmetry of the effective refractive index distribution at the front end face can be improved. Therefore, it is possible to realize a semiconductor laser device that can obtain an output beam shape with high output and excellent symmetry.

  In the first and second semiconductor laser devices, the substrate may have a recess for forming a recess and the recess may be formed on the recess for forming the recess.

  In this case, the depth of the recess for forming the recess may be 0.1 μm or more and 10 μm or less. In addition, the width in the direction along the front end surface of the recess for forming the recess is 2 μm or more and 200 μm or less, and the depth in the direction along the optical waveguide of the recess for forming the recess is 5 μm or more and 200 μm or less. The recess for forming the dent portion may be exposed to the front end surface or may be configured to have a distance of 50 μm or less from the front end surface.

  In addition, the distance from the end on the optical waveguide side to the optical waveguide in the recess for forming the recess may be 2 μm or more and 10 μm or less.

  In the first and second semiconductor laser devices, the semiconductor layer stack includes a recess forming recess from which a part of the nitride semiconductor layer below the active layer is removed, and the recess is formed as a recess. It is good also as a structure currently formed on the recessed part for use. In this case, the depth of the recess for forming the recess may be 0.01 μm or more and 10 μm or less.

  In the first and second semiconductor laser devices, the forbidden band width of the forbidden band width increasing portion may be 50 meV or more larger than the forbidden band width of the active layer in the flat portion.

  In the first and second semiconductor laser devices, the effective refractive index distribution in the direction parallel to the main surface at the front end surface may be symmetric about the optical waveguide.

  A method for manufacturing a semiconductor laser device according to the present invention is formed by laminating a plurality of nitride semiconductor layers including an active layer on a main surface of a substrate, and a recess having a height lower than that of other portions, A semiconductor layer stack including a flat portion in which an active layer is formed in parallel with the main surface, and a forbidden band width increasing portion formed between the flat portion and the recessed portion, wherein the forbidden band width of the active layer is larger than the flat portion. A step (a) of forming a body, and a step (b) of forming a refractive index adjustment recess by selectively removing the semiconductor layer stack, and in step (a), avoiding the depression and flat portion And a stripe-shaped optical waveguide including a forbidden band width increasing portion is formed, and in the step (b), the refractive index adjusting concave portion is formed on the opposite side of the concave portion with the optical waveguide interposed therebetween.

  The semiconductor laser device manufacturing method according to the present invention forms a striped optical waveguide that includes a flat portion and a forbidden band width increasing portion while avoiding a hollow portion, and thus forming an end surface window portion with reduced light absorption on the front end surface. can do. In addition, since the refractive index adjustment concave portion is formed on the opposite side of the hollow portion across the optical waveguide, a semiconductor laser device that improves the effective refractive index distribution in the horizontal direction on the front end face and emits a light beam with excellent symmetry Can be manufactured.

  The method for manufacturing a semiconductor laser device of the present invention further includes a step (c) of selectively removing a region including the recess in the semiconductor layer stack to form a first recess after the step (a). It may be. By adopting such a configuration, it becomes easy to equalize the sizes of the first concave portion and the refractive index adjusting concave portion, and the symmetry of the effective refractive index can be further improved.

  In the manufacturing method of the semiconductor laser device of the present invention, in the step (b), the depth of the refractive index adjusting recess may be made equal to the depth of the first recess.

  In the method for manufacturing a semiconductor laser device of the present invention, in step (a), a semiconductor layer stack may be formed on a substrate having a recess for forming a recess. In addition, the step (a) includes a step of forming a lower semiconductor layer having a recess for forming a recess on a flat substrate, and a step of forming an upper semiconductor layer including an active layer on the lower semiconductor layer. It is good also as a structure.

  According to the semiconductor laser device and the manufacturing method thereof according to the present invention, an end face window structure is realized in a semiconductor laser device made of a GaN-based semiconductor to prevent end face deterioration, and an emission beam shape having excellent horizontal symmetry is obtained. A semiconductor laser device can be realized.

(A)-(c) shows the semiconductor laser device which concerns on 1st Embodiment, (a) is a top view, (b) is sectional drawing in the Ib-Ib line | wire of (a), (c ) Is a cross-sectional view of the front end surface of (a). (A) And (b) is a measurement result of cathodoluminescence, (a) is a graph which shows the result of the scan of a-axis direction, (b) is a graph which shows the result of the scan of m-axis direction. (A) And (b) shows the effective refractive index distribution in a front end surface, (a) shows the profile at the time of forming a refractive index adjustment layer, (b) shows the case where the refractive index adjustment layer is not formed. Show profile. It is sectional drawing which shows the modification of the semiconductor laser apparatus which concerns on 1st Embodiment. (A)-(c) shows the semiconductor laser device which concerns on 2nd Embodiment, (a) is a top view, (b) is sectional drawing in the Vb-Vb line | wire of (a), (c ) Is a cross-sectional view of the front end surface of (a). It is a profile of the effective refractive index distribution in the semiconductor laser apparatus which concerns on 2nd Embodiment. (A)-(c) shows the modification of the semiconductor laser apparatus which concerns on 2nd Embodiment, (a) is a top view, (b) is sectional drawing in the VIIb-VIIb line | wire of (a). (C) is sectional drawing in the front end surface of (a). It is the profile of the effective refractive index distribution in the modification of the semiconductor laser apparatus which concerns on 2nd Embodiment.

  First, the principle of forming the end face window structure in the GaN-based semiconductor laser device will be described. In order to form the end face window structure, the forbidden band width of the active layer may be made larger than that of other portions on the front end face from which light is emitted. In the present specification, the front end face is an end face having a large light output among the two resonator end faces, and the rear end face is an end face having a smaller light output than the front end face on the opposite side to the front end face.

  When a semiconductor layer is formed on a substrate having a recess, the step coverage of the semiconductor layer is not 100%. Therefore, a semiconductor layer inclined with respect to the main surface of the substrate is formed around the recess. The inclination of the semiconductor layer is several degrees or less depending on the formation conditions. In the case of an active layer made of a nitride semiconductor, the inventor of the present application shows that the portion formed around the recess and slightly inclined with respect to the main surface of the substrate is compared with the portion formed parallel to the main surface of the substrate It has been found that the forbidden bandwidth (band gap energy) increases and the light absorption wavelength decreases. The reason why the band gap energy of the active layer formed slightly around the recess and slightly inclined is assumed as follows.

  When an indium gallium nitride (InGaN) -based active layer is formed on the c-plane (0001 plane) of a flat substrate, the In composition decreases and the emission peak energy decreases as the off angle with respect to the c-plane of the substrate increases. Although the cause of this phenomenon is not clear, it is a phenomenon that is recognized with good reproducibility. The active layer formed to be inclined on the c-plane of the substrate is the same as the active layer formed on the substrate having a large off angle. For this reason, the band gap energy of the slightly inclined portion formed around the recess in the active layer is larger than the portion formed in parallel to the main surface of the substrate, and the forbidden band width increasing portion is formed. The However, in the case of a substrate having an off angle, the inclination with respect to the c-plane becomes small depending on the direction of the inclination, so that the band gap energy may be smaller than the portion formed parallel to the main surface of the substrate. . However, at any position around the recess, the inclination with respect to the c-plane increases and the band gap energy increases.

  In a region including the front end face, if the active layer is slightly inclined with respect to the main surface of the substrate to form a forbidden band width increasing portion having a large band gap energy, the end face window structure can be easily formed even in the GaN-based semiconductor laser device. Can be formed.

  In order to use it as an end face window structure, the band gap energy in the forbidden band increasing portion only needs to be at least about 50 meV larger than the band gap energy of the other part of the active layer (part contributing to light emission), It is preferably about 100 meV or more, more preferably about 150 meV or more. However, it is considered that the increase in the band gap energy in the forbidden bandwidth increasing portion is caused by the decrease in the In composition due to the increase in the off angle. For this reason, the increase amount of the band gap energy in the forbidden band width increasing portion does not exceed the increase amount of the band gap energy when all of In contained in the portion contributing to light emission of the active layer is replaced with Ga. Therefore, in the case of a blue-violet laser device having an emission wavelength of about 405 nm, the maximum value of the difference in band gap energy between the forbidden band width increasing portion and the other portion of the active layer is smaller than about 330 meV. The theoretical maximum value of the increase amount of the band gap energy in the forbidden band width increasing portion varies depending on the elemental composition ratio of the active layer. For this reason, when the emission wavelength is different, a different value is obtained. For example, in the case of a green laser device having an emission wavelength of 540 nm, the maximum value of the band gap energy difference between the forbidden band width increasing portion and the other portion of the active layer is about 1100 meV.

  On the other hand, in an optical disc application, a display, and a backlight application, in order to simplify the optical system, it is required to make the outgoing beam of the GaN-based semiconductor laser device symmetrical. When the shape of the emitted beam is asymmetric, in a disk application, there is a possibility that a portion that originally needs to be irradiated with laser light is not irradiated or writing to an unintended portion occurs. In display and backlight applications, the optical system for shaping the emitted laser light becomes complicated, which causes an increase in cost. In order to make the outgoing beam shape symmetric, it is necessary to make the effective refractive index distribution of the structure that confines the laser light symmetrical about the optical waveguide.

  When a semiconductor layer is formed on a substrate having a recess, a recess having a low semiconductor layer height is formed above the recess. In the hollow portion, the effective refractive index of the semiconductor layer is smaller than that in the flat portion. For this reason, when the end face window region is formed using a substrate having a recess, the effective refractive index distribution is not symmetric about the optical waveguide, and the shape of the outgoing beam is asymmetric.

  In the following, an embodiment is used for a semiconductor laser device in which the active layer is inclined with respect to the main surface of the substrate at the front end face and the band gap energy is increased to form the end face window structure and the output beam shape is symmetric. I will explain.

(First embodiment)
1A to 1C show the semiconductor laser device according to the first embodiment. FIG. 1A shows a planar configuration, and FIG. 1B shows a cross-sectional configuration taken along line Ib-Ib in FIG. (C) has shown the cross-sectional structure in a front end surface. In FIG. 1, the plane orientation of the hexagonal GaN-based crystal is also shown. In FIG. 1, c represents the normal vector of the (0001) plane, that is, the c axis, a represents the normal vector of the (11-20) plane and its equivalent plane, that is, the a axis, and m represents (1-100). The normal vector of the surface and its equivalent surface, that is, the m-axis is shown. The negative sign “−” attached to the Miller index in the plane orientation represents the inversion of one index following the negative sign for convenience. Moreover, in FIG.1 (c), it does not describe about the end surface coat layer.

  As shown in FIG. 1, in the semiconductor laser device of this embodiment, a semiconductor layer stack 12 is formed on a substrate 11 having a recess 11a for forming a recess. A portion formed on the concave portion 11a of the substrate 11 is a recessed portion 12a whose height is lower than other portions. The recess 12a is different from the recess formed by removing a part of the semiconductor layer stack 12a, and each semiconductor layer is formed continuously. The portion formed on the flat region of the substrate 11 in the semiconductor layer stack 12 is a flat portion 12 b in which each layer including the active layer 16 is parallel to the main surface of the substrate 11. Between the hollow portion 12a and the flat portion 12b, an inclined portion 12c in which each layer including the active layer 16 is inclined with respect to the main surface of the substrate 11 is formed. The portion formed in the inclined portion 12c in the active layer 16 is a forbidden band width increasing portion 16a having a larger forbidden band width than the portion formed in the flat portion 12b. The optical waveguide 28 is formed so as to avoid a portion formed in the recess 12a of the active layer 16 and includes a portion formed in the flat portion 12b and a forbidden band width increasing portion 16a and extending in a direction parallel to the m-axis. ing. The forbidden band width increasing portion 16a is exposed at the front end face, and an end face window structure is formed.

The substrate 11 is a nitride semiconductor substrate whose main surface is a (0001) plane, such as a GaN substrate. The recess 11a may be formed by dry etching the substrate 11 using a mask made of silicon oxide (SiO ₂ ). Specifically, after a SiO ₂ film having a thickness of 600 nm is formed by a thermal CVD method using SiH ₄ , the depth in the m-axis direction (direction along the optical waveguide) is 100 μm using photolithography, A planar rectangular opening having a width in the a-axis direction (a direction along the front end face) of 50 μm is formed. Subsequently, using an inductively coupled plasma (ICP plasma) etching apparatus, a portion exposed from the opening of the substrate 11 is etched to a depth of 2 μm by carbon tetrafluoride (CF ₄ ) plasma. Subsequently, the SiO ₂ film is removed using hydrofluoric acid (HF).

Next, the semiconductor layer stack 12 is formed on the substrate 11 on which the recesses 11a are formed by using metal organic chemical vapor deposition (MOCVD). Specifically, for example, an n-type clad layer 13 made of n-type Al _0.03 Ga _0.07 N having a thickness of 2 μm and an n-type light guide layer 14 made of n-type GaN having a thickness of 0.1 μm are successively grown. Next, an active layer 16 having a quantum well (MQW) structure in which a barrier layer made of In _0.02 Ga _0.98 N and a well layer made of In _0.06 Ga _0.94 N are stacked three periods is formed. Further, a p-type light guide layer 17 made of p-type GaN having a thickness of 0.1 μm, a carrier overflow suppression layer (OFS layer) 18 made of Al _0.20 Ga _0.80 N having a thickness of 10 nm, and a thickness on the active layer 16. A p-type cladding layer 19 made of a strained superlattice having a thickness of 0.48 μm and a p-type contact layer 20 made of p-type GaN having a thickness of 0.05 μm are successively grown. The p-type cladding layer 19 may be formed by stacking 160 periods of p-type Al _0.16 Ga _0.84 N having a thickness of 1.5 nm and GaN.

  The portion of the semiconductor layer stack 12 formed above the recess 11a has a height lower than that of the other portions and becomes a recess 12a. Between the flat part 12b in which each semiconductor layer is formed in parallel with the main surface of the substrate 11 and the recess 12a, an inclined part 12c in which each semiconductor layer is inclined with respect to the main surface is formed.

  In the inclined portion 12c, the inclination of the active layer 16 with respect to the (0001) plane is larger than that in the flat portion 12b. For this reason, the part formed in the inclined part 12c in the active layer 16 becomes the forbidden band width increasing part 16a having a larger band gap energy than the part formed in the flat part 12b. The forbidden band width increasing portion 16 a has a light absorption wavelength shorter than other portions of the active layer 16. Therefore, the end face window structure can be formed by allowing the forbidden band width increasing portion 16 a to be the end of the optical waveguide 28.

Next, by selectively removing the p-type contact layer 20 and the p-type cladding layer 19 using a mask made of a SiO ₂ film, a ridge stripe portion with a width of 1.5 μm extending in the m-axis direction is formed. The ridge stripe portion may be formed by etching using, for example, a mask made of SiO ₂ having a thickness of 0.3 μm formed by a thermal CVD method.

After forming the ridge stripe portion, the refractive index adjustment layer 31 made of an amorphous group III nitride semiconductor is formed so as to fill the recess 12a. For example, Al _x Ga _y In _1-xy N (0 ≦ x ≦ 1, 0 ≦ y ≦ 1, x + y ≦ 1) may be used for the refractive index adjustment layer 31. What is necessary is just to adjust the composition ratio of Al, Ga, and In according to the structure of the semiconductor layer laminated body 12, the depth of the hollow part 12a, etc. FIG. In the present embodiment, for example, by using Ga _0.99 In _0.01 N, an effective refractive index distribution with high symmetry can be realized.

The refractive index adjustment layer 31 is formed, for example, by first forming an amorphous group III nitride semiconductor having a thickness of 2 μm on the entire surface of the substrate 11 by using an electron beam (EB) vapor deposition method. Next, a 0.2 μm thick SiO ₂ film is deposited on the amorphous group III nitride semiconductor by thermal CVD. Further, patterning is performed using photolithography and hydrofluoric acid so that the SiO ₂ film remains on the recess 12a. Thereafter, the amorphous group III nitride semiconductor is removed except for the recess 12a covered with the SiO ₂ film using, for example, hot phosphoric acid. As a method for removing the amorphous group III nitride semiconductor, hot concentrated phosphoric acid or aqua regia may be used in addition to hot phosphoric acid. Further, the SiO ₂ film is removed using hydrofluoric acid.

Thereafter, the insulating film 22, the p-side electrode 23, the wiring electrode 24, and the pad electrode 25 made of SiO _{2 are} formed by a known method. Further, after polishing the substrate 11 to about 100 μm, the n-side electrode 26 is formed. Thereafter, primary cleavage is performed along the m-plane so that the length in the m-axis direction is 600 μm. However, at the time of the primary cleavage, the forbidden band width increasing portion 16a is exposed at the end face to form the end face window structure. Thereafter, the substrate 11 that has been primarily cleaved is cleaved along the a-plane so that the width in the a-axis direction is 200 μm.

  FIGS. 2A and 2B show CL peak energies when the cathode luminescence (Cathode Luminescence: CL) spectrum is line-scanned in the a-axis direction and the m-axis direction of the semiconductor layer stack 12, respectively. One side of the recess 11a formed in the substrate 11 in the m-axis direction and the a-axis direction was 30 μm. 2A and 2B, since the center of the recess 11a is taken as the origin, the position of 15 μm on the horizontal axis in the figure becomes the edge of the recess 11a.

  In this case, as shown in FIG. 2A, the CL peak energy changes in the range of about 14 μm from the edge of the recess 11a in the a-axis direction, and the forbidden band width increasing portion 16a is formed. Further, as shown in FIG. 2B, it can be seen that the CL peak energy changes in the range of about 2 μm in the m-axis direction, and the forbidden band width increasing portion 16a is formed. Further, in the a-axis direction, the CL peak energy is greatest in a region about 5 μm away from the edge of the recess 11a. Therefore, it is desirable to form the optical waveguide 28 parallel to the m-axis direction at a distance of about 5 μm from the recess 11a in the a-axis direction. In a hexagonal GaN-based material, the m-plane is a natural cleavage plane. With such a configuration, the CL peak energy at the front end face is the largest, that is, the cleaving so as to expose the m-plane in the vicinity of the concave portion 11a and including the forbidden band width increasing portion 16a, that is, the laser light at the front end face It is possible to obtain a structure with the least amount of absorption. The optical waveguide 28 needs to be formed avoiding the recessed portion 12a. For this reason, it is only necessary to form an interval of about 1 μm or more from the end of the recess 11a in the a-axis direction, and it is preferable to provide an interval of about 2 μm or more. However, since the optical waveguide 28 cannot be formed on the inclined portion 12c if the interval is too long, it may be formed in a range of about 15 μm or less in this example, and is preferably formed in a range of about 10 μm or less.

  However, when the surface orientation of the main surface of the substrate 11 is the (0001) plane and there is no off-angle, the forbidden band width increasing portion 16a is formed on both sides of the recess 11a in the a-axis direction. However, when the surface orientation of the main surface of the substrate 11 is inclined from the (0001) plane and has an off angle, the forbidden band width increasing portion 16a is formed on either side of the recess 11a depending on the direction of the inclination. Is formed. Therefore, in this case, it is necessary to form the optical waveguide 28 on the side where the forbidden band width increasing portion 16a is formed. In addition, a portion with a large forbidden band width may be formed even if it is not inclined with respect to the main surface of the substrate depending on the state of the substrate, etc. Can be.

  In the present embodiment, each side of the recess 11a provided in the substrate 11 to form the recess 12a is parallel to the a axis and the m axis and the depth is 2 μm. Depending on the planar shape and depth, the position at which the amount of change in the CL peak energy in the laminated structure becomes the largest may be different. For example, as shown in FIGS. 2A and 2B, in this measurement example, the region in which the CL peak energy changes was in the range of 2 μm to 14 μm from the edge of the recess 11a. Depending on the shape of 11a and the film formation conditions of the semiconductor layer stack 12, the region where the CL peak energy changes may be in the range of 1 μm to 100 μm from the edge of the recess 11a. In such a case, the end face window structure with less light absorption is formed by forming the optical waveguide 28 so as to include the region where the change amount of the CL peak energy is the largest, and cleaving in the region where the change amount is the largest. can do.

  However, the effective refractive index is lowered in the depression 12a only by providing the depression 12a and forming the end face window structure. For this reason, the effective refractive index distribution in the horizontal direction (a-axis direction) on the front end face 29 is asymmetrical as shown in FIG. As described above, when the effective refractive index at the front end face 29 is left-right asymmetric, the horizontal beam shape of the laser light emitted from the front end face 29 becomes asymmetric, and the pattern of the far field image (FFP) becomes left-right asymmetric.

  On the other hand, the semiconductor laser device of this embodiment has a refractive index adjustment layer 31 embedded in the recess 12a. For this reason, the effective refractive index distribution in the horizontal direction on the front end face 29 is substantially bilaterally symmetric about the optical waveguide 28 as shown in FIG. Therefore, a semiconductor laser device in which the FFP pattern is nearly symmetrical can be obtained.

For the refractive index adjusting layer 31, it is preferable to select a material having a refractive index value in which the horizontal effective refractive index distribution on the front end face 29 is symmetric about the optical waveguide 28. However, if the material has an absolute refractive index larger than 1.0, the effect of improving the effective refractive index distribution can be obtained. In particular, if a group III nitride, which is the same material as the semiconductor layer stack, is embedded, the effect of improving the effective refractive index distribution can be enhanced. However, crystalline group III nitride is difficult to wet etch, and it is difficult to remove unnecessary portions after embedding. For this reason, it is preferable to use an amorphous group III nitride that can be easily removed. In addition, since zinc oxide (ZnO) has a refractive index close to that of GaN, it is preferable because the effect of improving the effective refractive index distribution is high. Furthermore, SiO _2, titanium oxide (TiO ₂ ), etc. can be easily embedded in the recess 12 a, and there is an advantage that the effective refractive index distribution can be improved while simplifying the formation process.

The insulating film 22, the p-side electrode 23, the wiring electrode 24, the pad electrode 25, and the n-side electrode 26 may be formed using a known method. For example, the following method may be used. After the refractive index adjustment layer 31 is formed, an insulating film (passivation film) 22 made of SiO ₂ having a thickness of 200 nm is formed on the entire surface of the substrate 11 by using a thermal CVD method. Next, a resist pattern (not shown) having an opening having a width of 1.3 μm along the ridge stripe portion is formed on the upper surface of the ridge stripe portion in the insulating film 22 by lithography. Subsequently, the insulating film 22 is etched using the resist pattern as a mask by, for example, reactive ion etching (RIE) using trifluoromethane (CHF ₃ ) gas, and the upper surface portion of the ridge stripe portion of the p-type contact layer 20 To expose. Next, a metal comprising palladium (Pd) having a thickness of 40 nm and platinum (Pt) having a thickness of 35 nm on the p-type contact layer 20 exposed at least from the upper surface of the ridge stripe portion by, for example, EB vapor deposition. A laminated film is formed. Thereafter, a region excluding the ridge stripe portion of the metal laminated film is removed by a lift-off method for removing the resist pattern, and the p-side electrode 23 is formed.

  Next, the planar dimension in the direction parallel to the ridge stripe portion is 500 μm, for example, so as to cover the p-side electrode 23 on the top of the ridge stripe portion on the insulating film 22 by the lithography method and the lift-off method, and the ridge stripe portion. A wiring electrode 24 having a plane dimension of 150 μm in the direction perpendicular to is selectively formed. The wiring electrode 24 is formed of a metal laminated film of titanium (Ti) / platinum (Pt) / gold (Au) having a thickness of 50 nm, 200 nm, and 100 nm, respectively.

  In general, the substrate 11 is in a wafer state, and a plurality of laser devices are formed in a matrix on the main surface of the substrate 11. When the wiring electrode 24 is cut when the substrate 11 in the wafer state is divided into individual laser chips, the p-side electrode 23 in close contact with the wiring electrode 24 may be peeled off from the p-type contact layer 20. Therefore, the wiring electrode 24 is preferably formed so as not to be connected between adjacent chips. After forming the wiring electrode 24, the thickness of the upper Au layer of the wiring electrode 24 is increased to about 10 μm by electrolytic plating to form the pad electrode 25. This makes it possible to mount the laser chip using wire bonding and to effectively dissipate the heat generated in the MQW active layer 5, thereby improving the reliability of the semiconductor laser device.

  Next, the back surface of the wafer-like substrate 1 formed up to the pad electrode 25 is polished using a diamond slurry, and the substrate 11 is thinned until the thickness of the substrate 11 becomes about 100 μm. Thereafter, the n-side electrode 26 made of Ti having a thickness of 5 nm, platinum having a thickness of 10 nm, and Au having a thickness of 1000 nm is formed on the back surface of the substrate 11 by, for example, EB vapor deposition.

  In the present embodiment, the semiconductor layer stack 12 is formed on the substrate 11 having the recess 11a in order to form the recess 12a. However, the semiconductor layer stack 12 is divided into a lower semiconductor layer 33 and an upper semiconductor layer 34 including an active layer, and a lower semiconductor layer having a recess 32 for forming a recess on a flat substrate as shown in FIG. After forming 33, the upper semiconductor layer 34 may be formed thereon. Specifically, for example, only the n-type cladding layer 13 is first grown and then selectively etched to form the recess 32 for forming the recess. Next, a layer above the n-type light guide layer 14 may be regrown on the n-type cladding layer 13 in which the recess 32 is formed. However, if the active layer 16 is included in the upper semiconductor layer, the upper semiconductor layer and the lower semiconductor layer may be divided by any layer. Further, a buffer layer may be formed between the substrate 11 and the n-type clad layer 13, and a recess 32 for forming a recess may be formed in the buffer layer.

(Second Embodiment)
5A to 5C show a semiconductor laser device according to the second embodiment. FIG. 5A shows a planar configuration, and FIG. 5B shows a cross-sectional configuration taken along the line Vb-Vb in FIG. (C) has shown the cross-sectional structure in a front end surface. In FIG. 5, the same components as those of FIG.

  In the semiconductor laser device of this embodiment, the refractive index adjustment layer that fills the recess 12a is not formed. Instead, the refractive index adjusting recess 12e is provided on the opposite side of the recess 12a with the optical waveguide 28 in the semiconductor layer stack 12 interposed therebetween. By forming the refractive index adjusting recess 12e, the effective refractive index distribution on the front end face 29 becomes substantially symmetrical about the optical waveguide 28 as shown in FIG.

  The refractive index adjusting recess 12e may be formed by selectively etching the semiconductor layer stack 12 using a mask having an opening at a position where the refractive index adjusting recess 12e is formed after forming the ridge stripe portion. If the size of the refractive index adjusting recess 12e is substantially the same as that of the recess 12a, the effective refractive index distribution can be made substantially symmetric. For example, when the depth in the m-axis direction of the recess 11 a formed in the substrate 11 is 100 μm, the width in the a-axis direction is 50 μm, and the depth is 2 μm, the recess 12 a formed in the semiconductor layer stack 12. The depth in the m-axis direction is about 95.5 μm, the width in the a-axis direction is about 48 μm, and the depth is about 2 μm. Accordingly, the refractive index adjusting recess 12e may have a depth in the m-axis direction of 95.5 μm, a width in the a-axis direction of 48 μm, and a depth of 2 μm. Further, the position of the refractive index adjusting recess 12e is preferably symmetric with respect to the recessed portion 12a with the ridge stripe portion as the center. However, even if the size of the refractive index adjusting recess 12e is not the same as that of the recess 12a, the effect of improving the FFP pattern can be obtained. Also, the formation position need not be completely symmetrical.

  Further, as shown in FIG. 7, the refractive index adjusting recess is not formed in accordance with the recess, but the first recess 12f is formed at the position of the recess, and the first recess 12f is sandwiched between the optical waveguides 28. A refractive index adjusting recess 12g substantially equal to the first recess 12f may be formed at a position opposite to the first recess 12f. In this case, the size of the first recess 12f and the size of the refractive index adjustment recess 12g can be easily aligned. Therefore, the symmetry of the effective refractive index distribution can be further improved.

  After forming the ridge stripe portion, the first recess 12f selectively etches the semiconductor layer stack 12 using a mask having openings at positions where the first recess 12f and the refractive index adjustment recess 12g are formed. To be formed. At this time, the first recess 12f may be formed in a region including the recess.

  In FIG. 7, the first recess 12 f is formed by removing the semiconductor layer 12 until the recess for forming the recess formed in the substrate 11 is exposed, and the refractive index adjusting recess 12 g also removes part of the substrate 11. It is formed to the depth to be. However, the depth of the first recess 12f may be further reduced so that a part of the semiconductor layer stack 12 remains. In this case, the depth of the refractive index adjusting recess 12g may be made shallower in accordance with the depth of the first recess 12f. In addition, the symmetry is improved when the depth of the first recess 12f and the depth of the refractive index adjustment recess 12g are the same, but a certain effect can be obtained even if they are different.

  When the deep first concave portion 12f and the refractive index adjusting concave portion 12g are formed so that a part of the substrate 11 is removed, the effective refractive index in the concave portion is significantly lowered as shown in FIG. However, there is no problem if the effective refractive index distribution is symmetric about the optical waveguide 28.

  Also in the second embodiment, the recess 11a is not formed in the substrate 11, but the semiconductor layer stack 12 is divided into the lower semiconductor layer and the upper semiconductor layer including the active layer, and the depression is formed in the lower semiconductor layer. For example, an upper semiconductor layer including an active layer may be regrown on the lower semiconductor layer in which the concave portion is formed.

  In each embodiment, the recess 11a for forming the recess has a depth in the m-axis direction of 100 μm and a width in the a-axis direction of 50 μm. The depth of the recess 11a in the m-axis direction defines the depth of the end face window. Accordingly, it may be about 5 μm or more, and preferably about 50 μm or more. However, if it is too large, the portion that contributes to light emission becomes small, and therefore it is preferably about 200 μm or less. Similarly, the width of the recess 11a in the a-axis direction may be about 2 μm or more, and preferably about 20 μm or more. The upper limit of the width varies depending on the resonator width, but it is meaningless to make it too large, so it should be about 200 μm or less, and preferably about 100 μm or less. The depth of the recess 11a may be at least about 0.1 μm, and preferably about 1 μm or more. Further, if it is too deep, formation becomes difficult, so it should be about 10 μm or less, and preferably about 5 μm or less.

  Moreover, what is necessary is just to form the recessed part 11a so that it may be exposed to a front end surface. However, if the forbidden band width increasing portion 16a can be exposed at the front end face, the recess 11a itself does not need to be exposed at the front end face. For this reason, it may be formed between about 50 μm away from the front end surface, and it is preferably formed within a range of about 20 μm or less.

  The same applies to the case where the recess is formed in the lower semiconductor layer instead of forming the recess in the substrate. However, when the recess is formed in the lower semiconductor layer, it is possible to form an inclined region in the active layer and form the end face window even if the depth is about 0.01 μm.

In each embodiment, description has been made assuming that the plane orientation of the main surface of the substrate is the (0001) plane. However, it may be a substrate having an off angle. However, in this case, it is necessary to form the optical waveguide so as to include an inclined portion on the side where the forbidden band width of the active layer becomes large, that is, a forbidden band width increasing portion. Further, an example is shown in which the front end face is parallel to the a-axis and the ridge stripe portion is formed parallel to the m-axis. In this way, cleavage is facilitated, but it is not always necessary to match these crystal axes. The growth substrate of the laminated structure is not limited to a GaN-based substrate (GaN substrate, AlGaN substrate, etc.) belonging to a hexagonal system, but other substrates capable of growing a GaN-based material, such as silicon carbide (SiC), silicon (Si ), Sapphire (single crystal Al ₂ O ₃ ), zinc oxide (ZnO), or the like can be used.

  In each embodiment, the resonator length of the semiconductor laser device is not particularly limited. However, if it is about 200 μm or more, it is easy to form the end face window structure, and if it is about 400 μm or more, it is more preferable. Although it is possible to cope with a case where the resonator length is long, formation is easy if it is about 2000 μm or less, and it is more preferable if it is about 1000 μm or less. Also, the resonator width is not particularly limited, but it can be easily formed if it is about 200 μm or more. Although it is possible to cope with the case where the resonator width is large, it is preferably about 400 μm or less.

  In each of the embodiments, the ridge stripe type semiconductor laser device has been described. However, the embedded semiconductor laser device can have the same configuration. Further, the semiconductor laser device in which the end face window structure is formed only on the front end face has been described, but the same end face window structure may be provided not only on the front end face but also on the rear end face. In this case, similarly to the front end face, a process for symmetrizing the refractive index distribution can be performed on the rear end face.

  The semiconductor laser device and the manufacturing method thereof according to the present invention are semiconductors that realize an end face window structure in a semiconductor laser device made of a GaN-based semiconductor to prevent end face deterioration and obtain an output beam shape with excellent horizontal symmetry. The laser device can be realized, and is useful as a nitride semiconductor laser device that requires high output and symmetry in the horizontal direction of emitted light, and a method for manufacturing the same.

DESCRIPTION OF SYMBOLS 11 Substrate 11a Concave part 12 Semiconductor layer laminated body 12a Depression part 12b Flat part 12c Inclination part 12e Refractive index adjustment recessed part 12f 1st recessed part 12g Refractive index adjustment recessed part 13 n-type clad layer 14 n-type light guide layer 16 Active layer 16a Forbidden band Width increasing portion 17 p-type light guide layer 19 p-type cladding layer 20 p-type contact layer 22 insulating film 23 p-side electrode 24 wiring electrode 25 pad electrode 26 n-side electrode 28 optical waveguide 29 front end face 31 refractive index adjustment layer 32 recess 33 Lower semiconductor layer 34 Upper semiconductor layer

Claims (18)

A plurality of nitride semiconductor layers including an active layer formed on the main surface of the substrate, the semiconductor layer stack having a recess having a height lower than that of other portions;
A refractive index adjustment layer filling the hollow portion,
The semiconductor layer stack is
A flat portion in which the active layer is formed parallel to the main surface;
A forbidden band width increasing portion formed between the flat portion and the indented portion, wherein the forbidden band width of the active layer is larger than the flat portion;
A front end surface that emits light;
A semiconductor laser device comprising: a striped optical waveguide that avoids the depression and includes the flat portion and the forbidden band width increasing portion and extends in a direction intersecting the front end face.   The semiconductor laser device according to claim 1, wherein the refractive index adjustment layer is made of an amorphous nitride semiconductor.   2. The semiconductor laser device according to claim 1, wherein the refractive index adjustment layer is made of silicon oxide, titanium oxide, or zinc oxide. A semiconductor layer stack formed by stacking a plurality of nitride semiconductor layers including an active layer on a main surface of a substrate,
The semiconductor layer stack is
A dent that is lower in height than other parts,
A flat portion in which the active layer is formed parallel to the main surface;
A forbidden band width increasing portion formed between the flat portion and the indented portion, wherein the forbidden band width of the active layer is larger than the flat portion;
A front end surface that emits light;
A striped optical waveguide that avoids the depression and includes the flat portion and the forbidden band width increasing portion and extends in a direction intersecting the front end surface;
A semiconductor laser device, comprising: a refractive index adjusting recess formed on a side opposite to the recess with the optical waveguide interposed therebetween, from which a part of the nitride semiconductor layer is removed.   5. The semiconductor laser device according to claim 1, wherein the substrate has a recess for forming a recess, and the recess is formed on the recess for forming the recess. .   The semiconductor laser device according to claim 5, wherein a depth of the recess for forming the recess is not less than 0.1 μm and not more than 10 μm.   The width in the direction along the front end surface of the recess for forming the recess is 2 μm or more and 200 μm or less, and the depth in the direction along the optical waveguide of the recess for forming the recess is 5 μm or more and 200 μm or less. The semiconductor laser device according to claim 5 or 6.   8. The semiconductor laser device according to claim 5, wherein the recess for forming the dent is exposed on the front end face or has a distance of 50 μm or less from the front end face. 9.   9. The semiconductor laser device according to claim 5, wherein a distance from an end on the optical waveguide side to the optical waveguide in the recess for forming the recess is 2 μm or more and 10 μm or less. 9. .   The semiconductor layer stack includes a recess forming recess from which a part of the nitride semiconductor layer below the active layer is removed, and the recess is formed on the recess forming recess. The semiconductor laser device according to claim 1, wherein the semiconductor laser device is provided.   11. The semiconductor laser device according to claim 10, wherein a depth of the recess for forming the depression is not less than 0.01 μm and not more than 10 μm.   12. The semiconductor laser device according to claim 1, wherein a forbidden band width of the forbidden band width increasing portion is 50 meV or more larger than a forbidden band width of the active layer in the flat portion.   13. The semiconductor laser device according to claim 1, wherein an effective refractive index distribution in a direction parallel to the main surface at the front end surface is symmetric about the optical waveguide. . A plurality of nitride semiconductor layers including an active layer are stacked and formed on the main surface of the substrate, and the active layer is formed in parallel to the main surface and a recess having a height lower than that of other portions. Forming a semiconductor layer stack including a flat portion, and a forbidden band width increasing portion formed between the flat portion and the recess portion, wherein the forbidden band width of the active layer is larger than the flat portion ( a) and
And (b) forming a refractive index adjusting recess by selectively removing the semiconductor layer stack.
In the step (a), a striped optical waveguide is formed that avoids the depression and includes the flat portion and the forbidden band width increasing portion,
In the step (b), the refractive index adjusting recess is formed on the opposite side of the recess with the optical waveguide interposed therebetween.   After the step (a), the method further comprises a step (c) of selectively removing a region including the recess in the semiconductor layer stack to form a first recess. Item 15. A method for manufacturing a semiconductor laser device according to Item 14.   16. The method of manufacturing a semiconductor laser device according to claim 15, wherein, in the step (b), the depth of the refractive index adjusting recess is made equal to the depth of the first recess.   17. The method of manufacturing a semiconductor laser device according to claim 14, wherein in the step (a), the semiconductor layer stack is formed on a substrate having a recess for forming a recess. . The step (a) includes forming a lower semiconductor layer having a recess for forming a recess on a flat substrate;
The method of manufacturing a semiconductor laser device according to claim 14, further comprising: forming an upper semiconductor layer including the active layer on the lower semiconductor layer.

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