JPS6351556B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to improvement of a buried structure type semiconductor laser device, and more particularly to a semiconductor laser device with longitudinal mode multimode.

[Technical background of the invention and its problems]

In recent years, semiconductor laser devices have been used as light sources for video disks or digital audio disks, but semiconductor laser devices used for these types of applications emit beams into microscopic beams with a diameter of about 1 [μm]. In order to efficiently converge on a spot, it is desirable that the transverse mode be controlled to the lowest order, that the radiation angle of the beam be isotropic, and that there be no astigmatism. In order to meet these demands, transverse mode control semiconductor laser devices having a built-in waveguide structure with a buried structure or various types of substrate processing have been developed.

In a semiconductor laser device having a built-in waveguide mechanism, not only the transverse mode is stably controlled to the fundamental mode, but also the longitudinal mode generally oscillates as a single mode. It is known that when such a semiconductor laser device with good coherency is used for the above-mentioned purpose, return light due to reflection from the disk surface interferes inside the semiconductor laser device, causing noise. Therefore, attempts have been made to make the longitudinal mode multi-mode in order to prevent the generation of the above noise, but this multi-mode longitudinal mode is incompatible with transverse mode control to converge the laser beam to a minute spot. This is a major problem when using semiconductor laser devices as light sources for various types of disks.

Further, recently, the development of high-output semiconductor laser devices for the purpose of writing on optical disks has been progressing. In such a device, an optical output of 20 [mW] or more is required, so it is necessary to prevent the end face from deteriorating due to light. In order to prevent this end face deterioration, the end face of the active region is placed in the forbidden band rather than the active layer. There is a semiconductor laser device of a so-called end face buried structure type in which the semiconductor laser device is buried with a wide crystal layer. This method has the advantage that the active region is not in contact with air and the cavity end face is transparent to the laser beam, so deterioration of the cavity end face is unlikely to occur. It has an extremely superior structure. However, even in this type of semiconductor laser device, it is a common problem to realize multi-mode longitudinal mode in order to prevent the generation of noise due to returned light.

[Purpose of the invention]

SUMMARY OF THE INVENTION An object of the present invention is to provide a semiconductor laser device that can stably control the transverse mode to the fundamental mode and can make the longitudinal mode multi-mode, and is useful for use as a light source for optical disks, for example. be.

[Summary of the invention]

Generally, in a semiconductor laser device, it is known that when the reflectance of the laser resonator end face is reduced, the longitudinal mode becomes multimode. Therefore, in a semiconductor laser device with an end-face buried structure having a striped active region, by lowering the reflectance of the cavity end face, only the longitudinal mode can be made into a multi-mode while keeping the transverse mode as the fundamental mode. It is presumed that it can be done. The inventors of the present invention have carried out extensive research in consideration of this point, and have discovered that by tilting the end face of the embedded active region at a predetermined angle from the vertical, rather than perpendicular to the horizontal plane, it is possible to make the longitudinal mode multi-mode. Ta. In other words, by tilting the end face of the active region, the optical axis of the reflected light from the resonator end face is aligned with the active region due to the difference in effective refractive index between the buried region containing the active region and the buried region. A deviation occurs in both the incident position and the incident angle with respect to the optical axis. Due to this deviation, the coupling efficiency between the reflected light from the cavity facet and the oscillation mode in the active region decreases, and the apparent reflectance at the laser cavity facet decreases enough to cause the longitudinal mode to become multimode. It's for a reason. At this time, since the transverse mode is determined by the optical waveguide layer in the active region, it goes without saying that the transverse mode becomes a stable fundamental mode if the device has the striped active region described above.

According to experiments conducted by the present inventors, if the angle θ of inclination of the end face of the active region from the vertical is less than 10°, the reflectance will not be sufficiently reduced;
When the angle is 60° or more, the decrease in reflectance becomes significant, resulting in a significant decrease in optical output, which is not preferable.
In addition, the inclination angle of the above degree (10° to 60°) is
This could be easily achieved by anisotropic etching such as reactive ion etching or anisotropic etching that utilizes crystal plane orientation.

The present invention focuses on these points and provides a buried structure type in which the side and end surfaces of a striped heterojunction structure consisting of an active layer and a cladding layer are embedded with a crystal layer having a wider forbidden band width than the active layer. In the semiconductor laser device, the end face of the heterojunction structure is inclined with respect to a plane perpendicular to the long side direction of the active layer surface.

〔Effect of the invention〕

According to the present invention, it is possible to make only the longitudinal mode multimode while the transverse mode is stably controlled to the fundamental mode. Therefore, when used as a light source for an optical disk, it is possible to prevent the generation of noise due to interference of return light from the disk surface, and the effect is tremendous. Furthermore, this makes it possible to put a semiconductor laser device into practical use in an optical disk device, contributing to miniaturization and higher performance of the optical disk device.

[Embodiments of the invention]

FIG. 1 is a perspective view showing a schematic structure of a semiconductor laser device according to an embodiment of the present invention, and FIG. 2 is a cross-sectional view taken along the line A--A in FIG. 1 in the figure is N-
This is a GaAs substrate (semiconductor substrate), and on this substrate 1 there is an N-GaAlAs layer (cladding layer) 2, a P-
A double heterojunction structure consisting of a GaAs layer (active layer) 3 and a P-GaAlAs layer (cladding layer) 4 is formed in a mesa stripe shape. A P-GaAs layer (ohmic contact layer) 5 is formed on this double heterojunction structure, that is, on the cladding layer 4. The side and end surfaces of the double heterojunction structure and the ohmic contact layer 5 are filled with a GaAlAs layer (high resistance layer) 6. Here, the end faces of the double heterojunction structure and the ohmic contact layer 5 are not perpendicular to the horizontal plane but are provided at an angle of θ'=54°. That is, the end face of the double heterojunction structure is formed at an angle of θ=36° with respect to a plane perpendicular to the long side direction of the active layer 3. Further, the distance L from the end surface of the active layer 3 to the end surface of the high resistance layer 6 is set in a range of 20 to 30 [μm]. Note that 7 and 8 in the figure each indicate an electrode.

With such a configuration, the light emitted from the end face of the active layer 3 is directed downward due to the difference in effective refractive index between the active layer 3 and the high resistance layer 6, as shown by the dashed line in FIG. The beam is bent to a position and reflected at the resonator end face.
Therefore, the optical axis of the reflected light from the resonator end face deviates from the optical axis at the active layer 3 in both the incident position and the incident angle, and this deviation reduces the coupling efficiency with the reflected light from the resonator end face. The apparent reflectance of the laser resonator end face decreases. Therefore,
It is possible to create multiple vertical modes.

Next, a method of manufacturing the above-described embodiment device will be explained. Figures 3a to 3c are perspective views showing the manufacturing process of the device. First, as shown in FIG. 3a, an N-Ga _0.6 Al _0.4 As layer 2, a P-GaAs layer 3, and a P-Ga _0.6 Al _0.4 As layer 4 and P-GaAs layer 5
are successively grown, and then a Si ₃ N ₄ film 9 is formed in stripes on the P-GaAs layer 5 as a selective crystal growth mask. Then _CH3OH : _{H3PO4 :}
Using an etching solution of H ₂ O ₂ (volume ratio 3:1:1), selective etching is performed to a depth reaching the substrate 1 using the Si ₃ N ₄ film 9 as a mask, as shown in FIG. 3B. As a result of this etching, a 111 plane with a slow etching rate appears on the end face of the heterojunction structure.
This 111 plane forms an angle of 54° with respect to the 100 plane of the substrate 1. Next, as shown in FIG. 3c, a Ga _0.6 Al _0.4 As layer 6 (buried layer) is grown. Then said
After removing the Si ₃ N ₄ film 9, the electrodes 7 and 8 are deposited to form the semiconductor laser device of the end face buried structure type shown in FIG. 1.

Note that the present invention is not limited to the embodiments described above. For example, the semiconductor substrate is not limited to N type, but may be P type. Furthermore, instead of GaAs substrate
It is also possible to apply to an InP-InGaAsP laser device using an InP substrate. Further, values such as the inclination angle θ of the heterojunction structure and the distance L from the active layer end face to the resonator end face may be determined as appropriate according to specifications. Furthermore, in addition to being used as a light source for optical disk devices, it can also be used as a light source for various devices. In short, the present invention can be implemented with various modifications without departing from the gist thereof.

[Brief explanation of the drawing]

FIG. 1 is a perspective view showing a schematic structure of a semiconductor laser device with an edge-embedded structure according to an embodiment of the present invention, FIG. 2 is a sectional view taken along arrow A-A in FIG. 1, and FIGS. It is a perspective view showing the manufacturing process of the above-mentioned example device. 1...N-GaAs substrate (semiconductor substrate), 2...
N-Ga _0.6 Al _0.4 As layer (clad layer), 3...P-
GaAs layer (active layer), 4...N-Ga _0.6 Al _0.4 As layer (cladding layer), 5... P-GaAs layer (ohmic contact layer), 6... Ga _0.6 Al _0.4 As layer (high resistance buried layer), 7, 8... electrode, 9... Si ₃ N ₄ film (mask).

Claims (1)

[Claims] 1. The side and end surfaces of a striped heterojunction structure consisting of an active layer and a cladding layer are embedded with a crystal layer having a wider forbidden band width than the active layer,
In a buried structure type semiconductor laser device having no periodic irregularities in the optical waveguide, the end face of the heterojunction structure is tilted with respect to a plane perpendicular to the long side direction of the active layer surface, and the longitudinal mode is made into a multimode. A semiconductor laser device characterized by: 2 The inclination angle of the end face of the heterojunction structure is 10~
2. The semiconductor laser device according to claim 1, wherein the angle is set within a range of 60°.