JP2004031522A - Semiconductor laser and manufacturing method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a semiconductor laser in which a lateral spread of guided light is uniformed while an effective reflectivity of the guided light from a diffraction grating is controlled within a constant range. <P>SOLUTION: A first clad layer 7 is formed on a semiconductor substrate 6. An active layer 8 is formed on the first clad layer 7. A second lad layer 9 is formed on the active layer 8. A current block layer 13 is formed on the second clad layer 9 to constrict a current. A third clad layer 11 is formed on the second clad layer 9 and has a ridge structure. A gain region for acquiring a gain required for laser oscillation, a phase control region for controlling oscillation wavelength, and a DBR region for adjusting the oscillation wavelength are constituted for laser beam oscillation in single vertical mode. The current block layer 13 is formed along both side surfaces of the third clad layer 11, and in the DBR region, a diffraction grating 17 is formed on the second clad layer 9, falling on the side part of the third clad layer 11. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a semiconductor laser and a method for manufacturing the same.
[0002]
[Prior art]
For high-density optical disks, there is a demand for blue laser light that emits light in a short wavelength range where the focused spot diameter is small on the optical disk and improves the reproduction / recording density of the optical disk.
[0003]
As an effective means for obtaining a blue laser beam, there is a method of converting an infrared laser beam into a short wavelength in a blue region by a second harmonic generation (SHG) technique. At present, nonlinear optical materials represented by LiNbO ₃ are widely used for semiconductor devices based on SHG (hereinafter, referred to as SHG devices). The LiNbO ₃ material has a distribution inversion structure formed by an ion exchange technique in accordance with the wavelength of the input infrared laser light. This distribution inversion structure is configured such that the wavelength of the infrared light in the waveguide, the wavelength of the blue laser light in the waveguide, and the period of the distribution inversion structure are integer ratios.
[0004]
Since the effective wavelength of the infrared laser light of the excitation source is limited to a narrow region by the SHG element, the infrared laser light source usually has high selectivity of the oscillation wavelength, oscillates in a single longitudinal mode, and A DBR (Distributed Bragg Reflector) laser capable of adjusting an oscillation wavelength that changes with temperature to a constant value is used.
[0005]
FIG. 4 shows an example of the structure of a conventional DBR laser. on the n-type GaAs substrate 101, the n-type _Al 0.5 _{Ga 0.5} As first cladding layer _102, Al _{0.12 Ga 0.88} As active layer 103, p-type _{Al 0.5 Ga} 0.5 As The two cladding layers 104 and the p-type Ga _0.7 Al _0.3 As first optical guide layer 105 are formed in this order.
[0006]
A p-type Al _0.5 Ga _0.5 As third cladding layer 106 is formed on the p-type Ga _0.7 Al _0.3 As first light guide layer 105 and the diffraction grating 105a. Further, on the p-type Al _0.5 Ga _0.5 As third cladding layer 106, a p-type Al _0.2 Ga _0.8 As etching stop layer 107 and a p-type Al _0.5 Ga _0.5 As fourth A clad layer 108 is formed, and a ridge structure 109 having a stripe shape is formed by these.
[0007]
On the p-type Ga _0.7 Al _0.3 As first optical guide layer 105, a diffraction grating 105 a is formed in a region other than a region below the region where the ridge structure 109 is formed, and in a region corresponding to the DBR region. Have been.
[0008]
An n-type Al _0.6 Ga _0.4 As current blocking layer 110 is formed in a region other than the region where the ridge structure 109 is formed on the p-type Al _0.5 Ga _0.5 As first cladding layer 106. A p-type GaAs contact layer 111 is formed on the n-type Al _0.6 Ga _0.4 As current blocking layer 110 and the p-type Al _0.5 Ga _0.5 As fourth cladding layer 108.
[0009]
In this DBR laser, the light distribution of the guided light in a direction parallel to the pn junction and perpendicular to the cavity direction (hereinafter, referred to as a lateral direction) shows a distribution spread by several to several tens of μm around the ridge structure. . Then, when the guided light spread in the horizontal direction passes through the diffraction grating 105a in the DBR region 200, it is reflected by the diffraction grating 105a and oscillates at a single wavelength. At this time, when the lateral spread of the guided light changes, the amount of the guided light passing through the diffraction grating 105a changes, and the effective reflectivity of the guided light received from the diffraction grating 105a changes.
[0010]
Generally, in the manufacturing process of a DBR laser, the lateral spread of guided light becomes non-uniform due to fluctuations in the lateral width of the ridge structure due to irregularities in the etching rate and fluctuations in the refractive index difference Δn due to non-uniformity in the composition of the crystal layer, As a result, in the obtained DBR laser, the effective reflectance that the guided light receives from the diffraction grating 105a is likely to be unstable.
[0011]
When the effective reflectivity of the guided light received from the diffraction grating 105a decreases, device characteristics such as an increase in the threshold value of an injection current for laser oscillation may be deteriorated, and conversely, the effective reflectivity may decrease. As the size increases, the guided light is reflected by a small number of diffraction gratings, and the single wavelength property of the oscillation wavelength decreases.
[0012]
[Problems to be solved by the invention]
As described above, in a semiconductor laser, in particular, a DBR laser, it is important to make the lateral spread of the guided light uniform and to control the effective reflectivity of the guided light received from the diffraction grating within a certain range.
[0013]
An object of the present invention is to provide a semiconductor laser in which the lateral spread of guided light becomes uniform and the effective reflectivity of the guided light received from the diffraction grating can be controlled within a certain range.
[0014]
[Means for Solving the Problems]
In order to achieve the above object, in a semiconductor laser according to the present invention, a first cladding layer formed on a semiconductor substrate, an active layer formed on the first cladding layer, and a first cladding layer formed on the active layer are formed. A second cladding layer, a current blocking layer formed on the second cladding layer for confining a current, and a third cladding layer having a ridge structure formed on the second cladding layer; The gain region, the phase control region for controlling the oscillation wavelength, and the DBR region for adjusting the oscillation wavelength are configured to perform single longitudinal mode laser light oscillation. The current blocking layer is formed along both side surfaces of the third cladding layer, and a diffraction grating is formed on the second cladding layer and on the side of the third cladding layer in the DBR region.
[0015]
With this configuration, the lateral spread of the guided light becomes uniform, and the effective reflectivity of the guided light received from the diffraction grating can be controlled within a certain range.
[0016]
In the semiconductor laser of the present invention, it is preferable that a fourth cladding layer having an equivalent effective refractive index smaller than that of the current blocking layer is formed on the current blocking layer and the third cladding layer.
[0017]
With this configuration, the light distribution in the lateral direction of the laser light can be widened so as to reach the diffraction grating layer sufficiently.
[0018]
In the semiconductor laser of the present invention, the effective refractive index in the direction parallel to the pn junction and perpendicular to the resonator direction is maximum in the third cladding layer, and gradually decreases as going outward from the third cladding layer. Is preferred.
[0019]
With this configuration, the light distribution in the horizontal direction of the laser light can be further expanded.
[0020]
In order to achieve the above object, in a method for manufacturing a semiconductor laser according to the present invention, a first clad layer, an active layer, a second clad layer, an optical waveguide layer, and a third clad layer are formed on a semiconductor substrate. Forming a ridge structure composed of the optical waveguide layer and the third cladding layer by etching the optical waveguide layer and the third cladding layer on the second cladding layer, respectively; Forming a diffraction grating in the optical waveguide layer by etching the optical waveguide layer in the DBR region, forming a current blocking layer along both side surfaces of the ridge structure, and further forming a fourth cladding layer on the current blocking layer. And forming a contact layer in this order.
[0021]
With this configuration, the fluctuation of the refractive index difference Δn due to the non-uniformity of the composition of the crystal layer is improved, and a semiconductor laser with small light loss can be stably manufactured.
[0022]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[0023]
FIG. 1 shows a DBR laser according to the present embodiment. The DBR laser according to the present embodiment has three regions: a gain region for obtaining a gain necessary for laser light oscillation, a phase control region for controlling the oscillation wavelength, and a DBR region for adjusting the oscillation wavelength. 6 is an n-type GaAs substrate, 7 is an n-type Al _0.5 Ga _0.5 As first cladding layer, 8 is a p-type Al _0.12 Ga _0.88 As active layer for generating laser light, and 9 is a p-type Al _0.5 Ga _0.5 As second cladding layer, p-type _Al 0.2 _{Ga 0.8} As optical guide layer 10, 11 is p-type _Al 0.5 _{Ga 0.5} As third cladding layer, 12 Is a ridge structure composed of a p-type Al _0.2 Ga _0.8 As optical waveguide layer 10 and a p-type Al _0.5 Ga _0.5 As third cladding layer 11, and 13 is an n-type Al _0.5 Ga _{0 0.5} As current blocking layer, 14 is a p-type Al _0.7 Ga _0.3 As fourth cladding layer having an equivalent effective refractive index smaller than that of the current blocking layer 13, and 15 is a p-type GaAs contact layer. The p-type GaAs contact layer 15 is electrically separated into a p-type GaAs contact layer 15a, a p-type GaAs contact layer 15b, and a p-type GaAs contact layer 15c corresponding to the gain region, the phase control region, and the DBR region, respectively. I have. Further, a p-type electrode 1, a p-type electrode 2, and a p-type electrode 3 are provided on each contact layer. _Reference numeral 17 denotes a diffraction grating made of p-type Al _0.2 Ga _0.8 As, and an n-type electrode 4 is provided below the n-type GaAs substrate 6. Reference numeral 5 denotes a laser emission end face from which laser light is emitted.
[0024]
In the DBR laser of the present embodiment, the band gap of the active layer in the phase control region and the DBR region and the band gap of the active layer near the laser emission end face 5 are made larger than the energy of the laser oscillation light generated in the gain region. Is preferred. Thereby, absorption of laser oscillation light in the phase control region and the DBR region is reduced, and the threshold value of the injection current for laser oscillation can be reduced. In addition, absorption of laser oscillation light in the vicinity of the laser emission end face 5 is reduced, and COD (Catastrophic Optical Damage), which is a phenomenon in which the laser emission end face 5 is melted and destroyed by laser light, is less likely to occur.
[0025]
In the DBR laser according to the present embodiment, the ridge structure 12 is formed in a stripe shape parallel to the resonator direction in the gain region, the phase control region, and the DBR region. It is preferable that the ridge structure 12 is inclined by 5 ° with respect to the resonator direction. Thereby, the ratio of the laser light reflected on the laser light reflecting surface facing the laser emission end face 5 to be fed back to the waveguide is reduced, and the wavelength selectivity of the diffraction grating 16 in the DBR region by distributed Bragg reflection is enhanced. In addition, the single wavelength property of the oscillation wavelength is further improved.
[0026]
FIG. 2A is a cross-sectional view of the gain region and the phase control region in a direction perpendicular to the resonator direction in the DBR laser according to the present embodiment. on the n-type GaAs substrate 6, n-type _{Al 0.5 Ga} 0.5 As first cladding layer _7, Al _{0.12 Ga 0.88} As active layer 8, p-type _{Al 0.5 Ga} 0.5 As first Two clad layers 9 are formed in this order. Further, on the p-type Al _0.5 Ga _0.5 As second cladding layer 9, the p-type Al _0.2 Ga _0.8 As optical waveguide layer 10 and the p-type Al _0.5 Ga _0.5 As third A ridge structure 12 composed of a cladding layer 11 is formed. On the p-type Al _0.5 Ga _0.5 As second cladding layer 9, an n-type Al _0.5 Ga _0.5 As current blocking layer 13 is formed along both side surfaces of the ridge structure 12. On the n-type Al _0.5 Ga _0.5 As current blocking layer 13 and the ridge structure 12, a p-type Al _0.7 Ga _0.3 As fourth cladding layer 14 having an equivalent effective refractive index smaller than that of the current blocking layer 13. And a p-type GaAs contact layer 15 are formed in this order. The surface of the p-type GaAs contact layer 15 is polished and flat.
[0027]
FIG. 2C is a cross-sectional view of the DBR laser according to the present embodiment in a direction perpendicular to the cavity direction in the DBR region. On the p-type Al _0.5 Ga _0.5 As second cladding layer 9, a p-type Al _0.2 Ga _0.8 As diffraction grating layer 16 a is formed on the side of the ridge structure 12, and the p-type Al _{0. An} n-type Al _0.5 Ga _0.5 As current blocking layer 13 is formed on the ₂ Ga _0.8 As diffraction grating layer 16 a along both side surfaces of the ridge structure 12. Thereby, the light distribution in the horizontal direction of the laser light can be expanded so as to reach the diffraction grating layer 16a sufficiently.
[0028]
As shown in FIG. 2B, the DBR laser of the present embodiment has two equivalent effective refractive index steps in the lateral direction. As described above, the effective refractive index becomes maximum in the third cladding layer 11 and gradually decreases as going outward from the third cladding layer 11. Thereby, the light distribution in the horizontal direction of the laser light can be further expanded.
[0029]
These two equivalent effective refractive index steps will be referred to as a first equivalent effective refractive index step and a second equivalent effective refractive index step in the order closer to the center of the laser emission. Is equal to the width of the p-type Al _0.2 Ga _0.8 As optical waveguide layer 10, and the second equivalent effective refractive index step is equal to the end of the p-type Al _0.2 Ga _0.8 As optical waveguide layer 10. The distance corresponds to the distance between the p-type Al _0.7 Ga _0.3 As fourth clad layer 14 and the bent portion. In this way, by having a plurality of equivalent effective refractive index steps, irregularities in the transverse mode light confinement effect due to fluctuations in the lateral width of the ridge structure 12 due to instability of the etching rate are reduced, and guided light The spread in the lateral direction becomes uniform. Further, as shown in FIG. 2C, a p-type Al _0.2 Ga _0.8 As diffraction grating layer 16 a is formed on the side of the ridge structure 12 in the DBR region. Thus, the amount of guided light passing through the diffraction grating formed in the diffraction grating layer 16a becomes constant, and the effective reflectance of the guided light received from the diffraction grating can be controlled within a certain range.
[0030]
According to the DBR laser of the present embodiment, a single wavelength characteristic of the oscillation wavelength can be stably realized. Further, since the spread of the guided light in the horizontal direction is stabilized, variation in the emission angle of the emitted laser light is reduced, and a good far-field image is obtained. By combining the DBR laser with the SHG element, the reliability is improved. A high second harmonic conversion element can be configured.
[0031]
Hereinafter, an example of a method of manufacturing a DBR laser according to the present embodiment will be described with reference to FIG.
[0032]
As shown in FIG. 3A, in the first crystal growth step, an n-type Al _0.5 Ga _0.5 As first is formed on the n-type GaAs substrate 6 by MOCVD (metal organic chemical vapor deposition). A clad layer 7, an Al _0.12 Ga _0.88 As active layer 8, a p-type Al _0.5 Ga _0.5 As second clad layer 9, a p-type Al _0.2 Ga _0.8 As optical waveguide layer 10, And a p-type Al _0.5 Ga _0.5 As third cladding layer 11 is formed in this order.
[0033]
Next, as shown in FIG. 3B, a ridge mask 17 which is a stripe-shaped insulating film is formed on the p-type Al _0.5 Ga _0.5 As third cladding layer 11. As the ridge mask 17, SiO ₂ or Si ₃ N ₄ can be used.
[0034]
Next, as shown in FIG. 3C, the ridge mask 17 is used as a mask, the p-type Al _0.2 Ga _0.8 As optical waveguide layer 10 is used as an etching stop layer, and the p-type Al _0.5 Ga _{0. 5} As third cladding layer 11 is etched to form a ridge shape. At this time, as an etchant, a selective etchant that etches the p-type Al _0.5 Ga _0.5 As third cladding layer 11 and does not etch the p-type Al _0.2 Ga _0.8 As optical waveguide layer 10, for example, It is necessary to use a hydrochloric acid-based etchant.
[0035]
Subsequently, a resist pattern for a diffraction grating is formed on the p-type Al _0.2 Ga _0.8 As optical waveguide layer 10 in the DBR region by a two-beam interference exposure method or an EB exposure method. Note that resist patterning is not formed in other regions, that is, in the gain region and the phase control region.
[0036]
Then, using the p-type Al _0.5 Ga _0.5 As second cladding layer 9 as an etching stop layer, the p-type Al _0.2 Ga _0.8 As optical waveguide layer 10 is etched in a region other than the DBR region. Remove. At this time, as an etchant, a selective etchant that etches the p-type Al _0.2 Ga _0.8 As optical waveguide layer 10 and does not etch the p-type Al _0.5 Ga _0.5 As second cladding layer 9, for example, It is necessary to use an ammonia peroxide etchant.
[0037]
As a result, in the DBR region, as shown in FIG. 3D, on the p-type Al _0.5 Ga _0.5 As second cladding layer 9 and on the side of the ridge structure 12, the waveguide light is To form a p-type Al _0.2 Ga _0.8 As diffraction grating layer 16a (selective wavelength: 820 nm) having a distributed Bragg reflection function, and in the other region, a p-type Al _0.5 Ga _0.5 As second clad The layer 9 is exposed.
[0038]
Next, in the second crystal growth step, in the DBR region by MOCVD, as shown in FIG. 3E, on the p-type Al _0.2 Ga _0.8 As diffraction grating layer 16a, In the region, an n-type Al _0.5 Ga _0.5 As current blocking layer 13 is formed on the p-type Al _0.5 Ga _0.5 As second cladding layer 9 so as to extend along both side surfaces of the ridge structure 12. I do.
[0039]
Next, the ridge mask 17 is removed by wet etching, and in the third crystal growth step, as shown in FIG. 3F, p-type Al _0.7 Ga _0.3 As Four cladding layers 14 and a p-type GaAs contact layer 15 are formed in this order.
[0040]
Then, the surface of the p-type GaAs contact layer 15 is polished and flattened. Here, in order to make ohmic contact with the p-type electrode, the carrier density of the p-type GaAs contact layer 15 is 1 × 10 ¹⁹ cm ⁻³ .
[0041]
Thereafter, in the p-type GaAs contact layer 15, separation grooves are formed between the gain region, the phase control region, and the DBR region by wet etching, and the regions are electrically separated from each other. Then, a p-type electrode is formed.
[0042]
According to the DBR laser manufacturing method of the present embodiment, since the number of crystal growth steps is a small number of three times in total, the variation in the refractive index difference Δn due to the non-uniformity of the crystal layer composition is improved, and further, the optical loss And a DBR laser with less noise can be stably manufactured.
[0043]
【The invention's effect】
According to the semiconductor laser of the present invention, since the semiconductor laser has a plurality of equivalent effective refractive index steps parallel to the pn junction and perpendicular to the cavity direction, the lateral spread of the guided light becomes uniform, Since the diffraction grating for performing distributed Bragg reflection is formed on the side of the structure, the effective reflectance of the guided light received from the diffraction grating can be controlled within a certain range. As a result, a single wavelength characteristic of the oscillation wavelength can be stably realized. Also, since the spread of the guided light in the horizontal direction is stabilized, the variation in the emission angle of the emitted laser light is reduced, and a good far-field image can be obtained. A harmonic conversion element can be configured.
[0044]
Further, according to the method of manufacturing a semiconductor laser of the present invention, the number of crystal growth steps is a small number of three times in total, so that the variation of the refractive index difference Δn due to the non-uniformity of the composition of the crystal layer is improved, and the optical loss is further reduced. Semiconductor laser with less noise can be stably manufactured.
[Brief description of the drawings]
1 is a perspective cross-sectional view of a DBR laser of the present invention; FIG. 2 is a cross-sectional view of each region of the DBR laser of the present invention; FIG. 3 is a process cross-sectional view showing a method of manufacturing the DBR laser of the present invention; Perspective view showing a technical DBR laser.
1 p-type electrode (gain area)
2 p-type electrode (phase control area)
3 p-type electrode (DBR region)
_Reference Signs _List 4 n-type electrode 5 laser emitting end face 6 n-type GaAs substrate 7 n-type Al _0.5 Ga _0.5 As first cladding layer 8 p-type Al _0.12 Ga _0.88 As active layer 9 p-type Al _0.5 Ga _0.5 As second cladding layer 10 p-type Al _0.2 Ga _0.8 As optical waveguide layer 11 p-type Al _0.5 Ga _0.5 As third cladding layer 12, 109 ridge structure 13 n-type Al _{0 .5} Ga _0.5 As current blocking layer 14 p-type _Al 0.7 _{Ga 0.3} As fourth cladding layer 15 p-type GaAs contact layer 15a p-type GaAs contact layer (gain region)
15bp p-type GaAs contact layer (phase control region)
15cp p-type GaAs contact layer (DBR region)
16 diffraction grating 16a p-type Al _0.2 Ga _0.8 As diffraction grating layer 17 ridge mask 101 n-type GaAs substrate 102 n-type Al _0.5 Ga _0.5 As first cladding layer 103 Al _0.12 Ga _{0. 88} As active layer 104 p-type Al _0.5 Ga _0.5 As second cladding layer 105 p-type Ga _0.7 Al _0.3 As first light guide layer 105 a p-type Ga _0.8 Al _0.2 As diffraction Lattice layer 106 p-type Al _0.5 Ga _0.5 As third cladding layer 107 p-type Al _0.2 Ga _0.8 As etching stop layer 108 p-type Al _0.5 Ga _0.5 As fourth cladding layer 110 n-type Al _0.6 Ga _0.4 As current blocking layer 111 p-type GaAs contact layer 200 DBR region

Claims (5)

A first cladding layer formed on a semiconductor substrate, an active layer formed on the first cladding layer, a second cladding layer formed on the active layer, and a second cladding layer formed on the second cladding layer; A current blocking layer for confining the applied current, a third cladding layer having a ridge structure formed on the second cladding layer, a gain region for obtaining a gain required for laser oscillation, and a phase control for controlling an oscillation wavelength. Region, and a DBR region for adjusting the oscillation wavelength is configured, a semiconductor laser performing single longitudinal mode laser light oscillation,
The current blocking layer is formed along both side surfaces of the third cladding layer,
A semiconductor laser, wherein a diffraction grating is formed on the second cladding layer and on a side portion of the third cladding layer in the DBR region. The semiconductor laser according to claim 1, wherein a fourth cladding layer having an equivalent effective refractive index smaller than that of the current blocking layer is formed on the current blocking layer and the third cladding layer. 3. The effective refractive index in the direction parallel to the pn junction and perpendicular to the resonator direction is maximized in the third cladding layer and gradually decreases outward from the third cladding layer. Semiconductor laser. 4. The semiconductor laser according to claim 3, having a plurality of equivalent effective refractive index steps. This is a method for manufacturing a semiconductor laser that includes a gain region for obtaining a gain required for laser oscillation, a phase control region for controlling the oscillation wavelength, and a DBR region for adjusting the oscillation wavelength, and performs single longitudinal mode laser light oscillation. hand,
Forming a first cladding layer, an active layer, a second cladding layer, an optical waveguide layer, and a third cladding layer in this order on a semiconductor substrate;
Etching the optical waveguide layer and the third cladding layer on the second cladding layer to form a ridge structure including the optical waveguide layer and the third cladding layer;
Etching the optical waveguide layer in the DBR region on the second cladding layer to form a diffraction grating in the optical waveguide layer;
A method for manufacturing a semiconductor laser, comprising: forming a current blocking layer along both side surfaces of the ridge structure, and further forming a fourth cladding layer and a contact layer in this order from above.

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