WO2018105015A1 - Method for manufacturing semiconductor laser - Google Patents

Abstract

The semiconductor laser according to the present invention is formed by sequentially laminating a lower cladding layer (2), an active layer (4), and an upper cladding layer (6) on a semiconductor substrate (1). At least two window regions (13, 14) having different mixed crystal ratios in the active layer (4) are formed by performing an impurity diffusion step at least two times with respect to an end surface portion of the semiconductor laser.

Description

Manufacturing method of semiconductor laser

The present invention relates to a method of manufacturing a semiconductor laser used for a visible light source such as a projector.

Semiconductor lasers have the advantages of small size, good color reproducibility, low power consumption and high brightness compared to other light sources. In particular, a transverse single mode laser in which a transverse mode is a single mode by forming a ridge called a ridge on the upper surface of the laser has a small emission point size and a small change in the shape of a far-field image. For this reason, it is expected as a sweep-type display light source used for a small projector or the like that creates an image by sweeping laser light with a MEMS (Micro Electro Mechanical Systems) mirror or the like. In such an application, since it is necessary to irradiate the target position with the M condensed laser light, the direction of the beam emitted from the laser light is required to be stable.

An actual semiconductor laser may adopt a structure called a window region. This is to suppress the COD (catastrophic-optical-damage) destruction at the end face by diffusing impurities such as Zn in the vicinity of the end face of the laser, thereby mixing the quantum well structure of the active layer and widening the band gap. Yes (see, for example, Patent Document 1).

Japanese Unexamined Patent Publication No. 2014-3229

A difference in refractive index occurs between the window region and other areas. A Fabry-Perot etalon resonator is formed by two reflections, reflection by the difference in refractive index and reflection at the end face. This Fabry-Perot etalon resonator functions as a reflector having wavelength selectivity. However, when the light output changes, the wavelength at which the reflectance increases due to the window region or other heat generation. Therefore, there is a problem that the wavelength changes discontinuously when the light output is changed.

Furthermore, this wavelength change causes a problem that the refractive index of the material in the laser felt by the laser beam changes, and the laser emission direction fluctuates when the light output is changed. Furthermore, light absorption occurs at the interface between the window region and the other region because the band gap is not sufficiently widened. However, the sharper the interface is at one point, the more the light absorption is concentrated at one point, causing deterioration of characteristics and reliability. This wavelength change occurs both in the case of a DFB (Distributed Feedback) laser using a diffraction grating and in the case of a Fabry-Perot laser not using this.

Further, in the fourth embodiment of Patent Document 1, the Zn concentration in the window region is graded by providing the slope of the contact layer and performing Zn diffusion. However, it is not easy to etch with a very thin contact layer thickness. Further, in order to increase the width of the inclined region, it is necessary to change the etching amount gradually, but it is considered very difficult to manufacture such a structure.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a semiconductor laser manufacturing method capable of preventing fluctuations in the emission direction of laser light and improving reliability by a simple process. Is what you get.

A method of manufacturing a semiconductor laser according to the present invention includes: forming a semiconductor laser by sequentially laminating a lower clad layer, an active layer, and an upper clad layer on a semiconductor substrate; and diffusing impurities in an end surface portion of the semiconductor laser. Forming at least two window regions having different mixed crystallization ratios of the active layer by performing the step at least twice.

In the present invention, it is possible to form at least two window regions having different mixed crystal ratios of the active layer by a simple process of performing the impurity diffusion process at least twice on the end face portion of the semiconductor laser. As a result, the steepness of the band gap change at the interface between the window region and the others is reduced, so that fluctuations in the emission direction of the laser light can be prevented and the reliability can be improved.

FIG. 3 is a cross-sectional view of the semiconductor laser element according to the first embodiment of the present invention in a direction perpendicular to the resonator direction. FIG. 2 is a cross-sectional view taken along the line II of FIG. It is sectional drawing which shows the manufacturing process of the semiconductor laser which concerns on Embodiment 1 of this invention. It is sectional drawing which shows the manufacturing process of the semiconductor laser which concerns on Embodiment 1 of this invention. It is sectional drawing which shows the manufacturing process of the semiconductor laser which concerns on Embodiment 1 of this invention. It is sectional drawing which shows the manufacturing process of the semiconductor laser which concerns on Embodiment 1 of this invention. It is a figure which shows the window area | region by the side of the front end surface of the conventional semiconductor laser. It is a figure which shows the optical output dependence of FFP of the conventional semiconductor laser. It is a figure which shows the window area | region by the side of the front end surface of the semiconductor laser element which concerns on Embodiment 1 of this invention. It is sectional drawing which shows the manufacturing process of the semiconductor laser which concerns on Embodiment 2 of this invention. It is sectional drawing which shows the manufacturing process of the semiconductor laser which concerns on Embodiment 2 of this invention. It is sectional drawing which shows the manufacturing process of the semiconductor laser which concerns on Embodiment 3 of this invention. It is sectional drawing which shows the manufacturing process of the semiconductor laser which concerns on Embodiment 3 of this invention. It is a top view which shows the manufacturing process of the semiconductor laser which concerns on Embodiment 4 of this invention. It is a top view which shows the modification of the manufacturing process of the semiconductor laser which concerns on Embodiment 4 of this invention.

A method for manufacturing a semiconductor laser according to an embodiment of the present invention will be described with reference to the drawings. The same or corresponding components are denoted by the same reference numerals, and repeated description may be omitted.

Embodiment 1 FIG.
FIG. 1 is a cross-sectional view of the semiconductor laser device according to the first embodiment of the present invention in a direction perpendicular to the cavity direction. FIG. 2 is a cross-sectional view taken along the line II of FIG. On a semiconductor substrate 1 made of n-type GaAs, a lower cladding layer 2 made of n-type AlInP, a lower light guide layer 3 made of undoped AlInP, an active layer 4 made of GaInP, an upper light guide layer 5 made of undoped AlGaInP, p An upper cladding layer 6 made of p-type AlInP and a contact layer 7 made of p-type GaAs are laminated in this order.

The thickness of the semiconductor substrate 1 is 50 to 150 μm. The lower cladding layer 2 has a thickness of 0.5 to 4.0 μm and a carrier concentration of 0.5 to 1.5 × 10 ¹⁸ cm ⁻³ . The thickness of the lower light guide layer 3 and the upper light guide layer 5 is 0.02 to 0.4 μm. The thickness of the active layer 4 is 3.0 to 20 nm. The thickness of the upper cladding layer 6 is 0.5 to 4.0 μm, and the carrier concentration is 0.5 to 2.0 × 10 ¹⁸ cm ⁻³ . The contact layer 7 has a thickness of 0.05 to 0.5 μm and a carrier concentration of 1.0 to 4.0 × 10 ¹⁹ cm ⁻³ .

The upper cladding layer 6 and the contact layer 7 are etched to form a ridge. The width of this ridge is about 1.0 to 3.0 μm. A lateral refractive index difference is formed by the ridge, and the lateral mode can be made a single mode.

An insulating film 8 such as a SiN film is formed on both sides of the ridge. In the ridge top into which current is injected, the insulating film 8 is etched to form an opening. A p-side electrode 9 is formed on the contact layer 7 and the insulating film 8. The p-side electrode 9 is bonded to the contact layer 7 through the opening of the insulating film 8 with a low resistance. A gold plating layer 10 is formed on the p-side electrode 9. The p-side electrode 9 is formed by laminating thin films such as Ti, Pt, and Au, and has a total thickness of 0.05 to 1.0 μm. The thickness of the gold plating layer 10 is 1.0 to 6.0 μm.

An n-side electrode 11 is bonded to the lower surface of the semiconductor substrate 1, and a gold plating layer 12 is formed thereunder. The n-side electrode 11 is formed by laminating thin films such as Ti, Pt, and Au, and has a total thickness of 0.05 to 1.0 μm. The thickness of the gold plating layer 12 is 1.0 to 6.0 μm. The n-side electrode 11 and the gold plating layer 12 are formed only on the lower surface side of the semiconductor substrate 1.

Window regions 13 and 14 are formed by Zn diffusion on the front end surface and the rear end surface of the semiconductor laser. In the conventional window region, the Zn diffusion depth is substantially constant in the window region, but in this embodiment, the Zn diffusion depth is divided into two window regions 13 and 14 having different Zn diffusion depths. The rate of mixed crystallization of the active layer 4b in the window region 14 is larger than the rate of mixed crystallization in the active layer 4a in the window region 13.

In the window regions 13 and 14, the active layer 4 is mixed by Zn diffusion, and the band gap of the active layer 4 becomes larger than other regions. Therefore, the absorption of laser light is reduced and COD destruction can be suppressed. In the present embodiment, the active layer 4b in the window region 14 is more mixed and the band gap is larger than the active layer 4a in the window region 13.

In the end face portion of the semiconductor laser, the contact layer 7 is removed by etching, and the p-side electrode 9 and the gold plating layer 10 are not formed. This is because the window region does not act as a gain region of the laser, so there is no need to inject current, but rather current injection must be prevented to prevent heat generation due to Joule heat or the like.

Next, a method for manufacturing a semiconductor laser according to the present embodiment will be described. 3 to 6 are cross-sectional views showing the manufacturing steps of the semiconductor laser according to the first embodiment of the present invention. First, as shown in FIG. 3, a lower cladding layer 2, a lower light guide layer 3, an active layer 4, an upper light guide layer 5, and an upper cladding layer 6 are formed on a semiconductor substrate 1 by a crystal growth method such as MOCVD. , And the contact layer 7 are sequentially stacked to form a semiconductor laser. Next, a Zn diffusion prevention film 15 is formed on the entire upper surface of the wafer, and the Zn diffusion prevention film 15 is removed by etching only at the end face portion of the semiconductor laser. The contact layer 7 is etched using the Zn diffusion prevention film 15 as a mask. An insulating film such as SiO ₂ or SiN can be used as the Zn diffusion preventing film 15. Here, the contact layer 7 in the window region is removed because the Zn diffusion rate in the contact layer 7 is very slow, so that it is difficult to form the window region.

Next, as shown in FIG. 4, a ZnO film 16 serving as a Zn diffusion source is formed on the entire upper surface of the wafer. Thereafter, Zn is diffused to the active layer 4 by thermal annealing to mix the active layer 4 and form a window region 13. The thermal annealing condition is, for example, 620 ° C. for 30 minutes.

Next, as shown in FIG. 5, the ZnO film 16 and the Zn diffusion preventing film 15 are all removed by etching, and a Zn diffusion preventing film 17 is formed on the entire upper surface of the wafer. A part of the Zn diffusion preventing film 17 is removed by etching at the end face portion of the semiconductor laser. Next, as shown in FIG. 6, a ZnO film 18 is formed on the entire upper surface of the wafer, and a window region 14 is formed by thermal annealing.

At this time, since the window region 14 is formed after the window region 13 is formed, the amount of Zn passing through the active layer 4b in the window region 14 is larger than the amount of Zn passing through the active layer 4a in the window region 13. As a result, the active layer 4b in the window region 14 is more mixed and the band gap becomes larger than the active layer 4a in the window region 13.

Next, the Zn diffusion preventing film 17 and the ZnO film 18 are all removed by etching. Further, the contact layer 7 and the upper cladding layer 6 are etched using the insulating film mask to form the ridge shown in FIG. After forming the insulating film 8, the insulating film 8 on the ridge is removed, and the p-side electrode 9 and the gold plating layer 10 are formed. Thereafter, the back surface of the semiconductor substrate 1 is polished to a desired thickness, and the n-side electrode 11 and the gold plating layer 10 are formed to obtain the structure shown in FIG.

This semiconductor laser is cleaved so that the resonator length is 1.5 mm. A coating having a reflectance of 10% is applied to the front end face, and a coating having a reflectance of 90% is applied to the rear end face. Although the window region 14 is formed after the window region 13 is formed first, the structure and effects obtained are the same even if the window region 13 is formed after the window region 14 is formed first.

FIG. 7 is a view showing a window region on the front end face side of a conventional semiconductor laser. Since the transition from the Zn diffusion region to the Zn non-diffusion region is steep, the change in the band gap is also steep. The refractive index change of the active layer also becomes steep, and the laser beam is reflected at this transition portion, that is, the window interface. Light is reflected at the transition portion and the front end surface, and the reflection at these two points forms a Fabry-Perot etalon reflector.

Here, consider the case where the light output of a conventional semiconductor laser is changed. When the light output changes, the light absorption amount of the window region changes, and the refractive index changes due to the temperature change of the window region. Then, the wavelength dependence of the Fabry-Perot etalon reflector changes, and the oscillation wavelength changes discontinuously. As a result, the refractive index balance of the waveguide structure in the vertical direction of the element in the laser changes, and the emission direction of the vertical FFP (Far Field Pattern) is greatly fluctuated. FIG. 8 is a diagram showing the optical output dependency of FFP of a conventional semiconductor laser.

FIG. 9 is a diagram showing a window region on the front end face side of the semiconductor laser device according to the first embodiment of the present invention. It is divided into two window regions 13 and 14 having different Zn diffusion depths. In this case, the band gap of the active layer 4 has two changing points. Accordingly, since the Fabry-Perot etalon reflector has a plurality of reflection points, the wavelength dependency of the reflectance is dispersed and the reflectance is also lowered. As a result, the occurrence of wavelength change and vertical FFP fluctuations are greatly suppressed.

As the Zn diffusion depth increases and the amount of Zn passing through the active layer 4 increases, the band gap of the active layer in the window region increases and the COD suppressing effect of the window region increases. On the other hand, since the reflectance of the Fabry-Perot etalon reflector is increased, the problem of FFP is exacerbated. However, by forming the two window regions 13 and 14 having different Zn contents passing through the active layer, the effect of the window region can be increased without causing the problem of FFP. Here, the larger the amount of Zn that passes through the active layer 4, the larger the ratio of crystal mixing, that is, the composition ratio of the original elements becomes uniform. As a result, the light absorption energy becomes longer and the amount of light absorption is reduced.

In addition, although the band gap is not sufficiently large at the interface between the window region and other regions, light absorption is increased due to acceptor absorption by Zn. In the prior art, since the band gap changes sharply at the window region interface, light absorption is concentrated on the window region interface. Then, heat generation due to light absorption is concentrated at one point, so that the temperature rises greatly and internal deterioration at the window region interface is likely to occur. In the present embodiment, since the band gap change positions are dispersed in two places, heat generation is dispersed, and such a problem of deterioration is greatly relieved.

As described above, in the present embodiment, the two window regions 13 and 14 having different mixed crystal ratios of the active layer 4 are obtained by a simple process of performing the impurity diffusion process twice on the end face portion of the semiconductor laser. Can be formed. As a result, the steepness of the band gap change at the interface between the window region and the others is reduced, so that fluctuations in the emission direction of the laser light can be prevented and the reliability can be improved. Further, the widths of the window regions 13 and 14 can be arbitrarily set.

In the present embodiment, the case where there are two Zn diffusion regions in the window region has been described, but the same effect can be obtained even when there are three or more Zn diffusion regions. Although the window region using Zn diffusion has been described, the same effect can be obtained even in a window region using impurities such as Si.

Embodiment 2. FIG.
10 and 11 are cross-sectional views showing the manufacturing steps of the semiconductor laser according to the second embodiment of the present invention. The processes up to the formation of the Zn diffusion preventing film 15 and the subsequent etching of the contact layer 7 are the same as those in the first embodiment. Next, as shown in FIG. 12, a part of the upper clad layer 6 is removed by photolithography and etching at the end face portion of the semiconductor laser, and a step 19 is formed in the upper clad layer 6.

Next, as shown in FIG. 11, a ZnO film 16 is formed on the entire upper surface of the wafer. Next, thermal annealing is performed to diffuse Zn from the upper clad layer 6 side where the step 19 is formed, thereby forming two window regions 13 and 14. At this time, in the step 19 portion, the distance between the ZnO film and the active layer is shorter than in other regions. For this reason, the amount of impurities passing through the active layer 4 differs in the window regions 13 and 14 even if the Zn diffusion depth is the same. The subsequent manufacturing process is the same as that of the first embodiment.

According to the present embodiment, the same effect as in the first embodiment can be obtained only by adding the step of forming the step 19 in the upper cladding layer 6. Further, by changing the width and depth of the step 19, the step position and size of the band gap of the window regions 13 and 14 can be arbitrarily set.

Embodiment 3 FIG.
12 and 13 are cross-sectional views showing the manufacturing steps of the semiconductor laser according to the third embodiment of the present invention. As shown in FIG. 12, an SiO ₂ insulating film 20 that is an impurity transpiration prevention film is formed on a part of the ZnO film 16 that is an impurity diffusion source. This formation is performed by photolithography and etching, or lift-off using a resist film or the like.

Next, thermal annealing is performed to diffuse Zn from the ZnO film 16. In this case, if the SiO ₂ insulating film 20 is present, the transpiration of Zn or a compound thereof from the ZnO film 16 is suppressed, and Zn diffuses efficiently in the crystal. Therefore, the Zn diffusion speed in the portion with the SiO ₂ insulating film 20 is faster than the Zn diffusion speed in the portion without the SiO ₂ insulating film 20. For this reason, as shown in FIG. 13, two window regions 13 and 14 having different mixing ratios of the active layer 4 can be formed. The subsequent manufacturing process is the same as that of the first embodiment. The SiO ₂ insulating film 20 may be another insulating film or a metal film such as Ti.

According to the present embodiment, the same effect as in the first embodiment can be obtained by a simple process of adding the SiO ₂ insulating film 20. Further, by changing the width of the SiO ₂ insulating film 20, the step position of the band gap of the window regions 13 and 14 can be arbitrarily set.

Embodiment 4 FIG.
FIG. 14 is a top view showing the manufacturing process of the semiconductor laser according to the fourth embodiment of the present invention. An opening 21 is formed in the Zn diffusion preventing film 15 by etching. Although the opening is rectangular in the prior art, in the present embodiment, the opening 21 has a region 21a having a narrow opening width in a direction perpendicular to the resonator of the semiconductor laser and a region 21b having a wide opening width. Next, the contact layer 7 is etched using the Zn diffusion preventing film 15 as a mask. Next, a ZnO film 16 is formed on the entire upper surface of the wafer.

The contact region between the ZnO film 16 which is an impurity diffusion source and the end face portion of the semiconductor laser has a width in a direction perpendicular to the resonator of the semiconductor laser. Since the opening 21 has the above shape, the width of the contact region can be changed in two steps.

Next, thermal annealing is performed to diffuse Zn from the ZnO film 16. At this time, in the region 21a having a narrow opening, Zn diffuses in a region where the ZnO film 16 is not in contact with the semiconductor crystal, that is, in a direction perpendicular to the resonator. As a result, the amount of Zn diffused into the active layer 4 is reduced. Accordingly, two window regions 13 and 14 having different amounts of impurities passing through the active layer 4 can be formed below the region 21a and below the region 21b. The subsequent manufacturing process is the same as that of the first embodiment. Note that the width of the contact region between the impurity diffusion source and the end face portion is not limited to two steps, and may be changed to three or more steps.

FIG. 15 is a top view showing a modification of the manufacturing process of the semiconductor laser according to the fourth embodiment of the present invention. By continuously changing the opening width of the opening 21, the width of the contact region between the impurity diffusion source and the end face portion is continuously changed. Thereby, the amount of impurities passing through the active layer 4 in the window region is continuously changed.

According to the present embodiment, the same effect as that of the first embodiment can be obtained by a simple process of changing the etching shape of the Zn diffusion preventing film 15. Further, by changing the etching shape of the Zn diffusion preventing film 15, the step position and size of the band gap of the window regions 13 and 14 can be arbitrarily set. In addition, the band gap can be changed continuously.

1 semiconductor substrate, 2 lower cladding layer, 4 active layer, 6 upper cladding layer, 13, 14 window region, 16 ZnO film (impurity diffusion source), 20 SiO ₂ insulating film (impurity evaporation prevention film)

Claims (5)

Forming a semiconductor laser by sequentially laminating a lower clad layer, an active layer, and an upper clad layer on a semiconductor substrate;
And a step of forming at least two window regions having different mixed crystallization ratios of the active layer by performing an impurity diffusion step at least twice on an end face portion of the semiconductor laser. Method. Forming a semiconductor laser by sequentially laminating a lower clad layer, an active layer, and an upper clad layer on a semiconductor substrate;
Forming a step in the upper cladding layer at an end surface portion of the semiconductor laser;
And a step of forming at least two window regions having different mixing ratios of the active layer by diffusing impurities from the upper clad layer side where the step is formed. Method. Forming a semiconductor laser by sequentially laminating a lower clad layer, an active layer, and an upper clad layer on a semiconductor substrate;
Forming an impurity diffusion source on an end face portion of the semiconductor laser;
Forming an impurity evaporation prevention film on a part of the impurity diffusion source; and
And a step of forming at least two window regions having different mixed crystal ratios of the active layer by diffusing impurities from the impurity diffusion source after forming the impurity transpiration prevention film. Laser manufacturing method. Forming a semiconductor laser by sequentially laminating a lower clad layer, an active layer, and an upper clad layer on a semiconductor substrate;
Forming an impurity diffusion source on an end face portion of the semiconductor laser;
Forming at least two window regions having different rates of mixed crystallization of the active layer by diffusing impurities from the impurity diffusion source,
The contact region between the impurity diffusion source and the end surface portion has a width in a direction perpendicular to the resonator of the semiconductor laser,
A method of manufacturing a semiconductor laser, wherein the width is changed in at least two stages. 5. The method of manufacturing a semiconductor laser according to claim 4, wherein a ratio of mixed crystals of the active layer is continuously changed in the window region by continuously changing the width.

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