1. Field of the Invention
This invention relates to an inner stripe semiconductor laser device and a method for the production of the semiconductor laser device. This invention further relates to a semiconductor wafer and a method for the production of the semiconductor wafer. As used herein, the term "semiconductor wafer" is referring to a semiconductor wafer with a plurality of semiconductor layers formed thereon, including an optical wave guide layer such as an active layer, which is then cleaved to produce semiconductor devices, for example, semiconductor laser devices or optical devices with an optical waveguide, such as optical branching filters, optical multiplexers, and optical switches.
2. Description of the Prior Art
In recent years, semiconductor laser devices have been extensively used as a light source for reading out the data from compact discs or video discs in optical disc driving units.
The transverse mode of laser oscillation is an important factor that affects not only the horizontal broadening angle of laser beams but also the threshold current, the current vs. optical output power characteristics, and the longitudinal mode characteristics. For the purpose of controlling the transverse mode of laser oscillation, semiconductor laser devices have a striped channel in the substrate. One example of such a semiconductor laser device is the channeled substrate planar (CSP) laser with an index waveguide structure. The V-channeled substrate inner stripe (VSIS) laser in which a striped channel is formed in the substrate through the current blocking layer is used for many applications.
One example of a VSIS laser is shown in FIG. 5. In this VSIS laser, on a p-GaAs substrate 1, an n-GaAs current blocking layer 2, a p-GaAlAs first cladding layer 3, a GaAlAs active layer 4, an n-GaAlAs second cladding layer 5, and an n-GaAs contact layer 6 are successively formed, the current blocking layer 2 having a striped channel 10 which reaches the semiconductor substrate 1. In this semiconductor laser device, the current for laser oscillation is confined to the striped channel 10. Of the light generated in the active layer 4, the light other than that above the striped channel 10 is absorbed by the current blocking layer 2 on either side of the striped channel 10, and the difference .DELTA.n in the effective refractive index is will appear between the area of the active layer 4 above the striped channel 10 and the area outside of this area. In the VSIS semiconductor laser device, the difference in the effective refractive index makes the fundamental transverse mode stabilized. Because the striped channel 10 forms an optical waveguide and a current path, VSIS laser devices can readily be produced. However, they have the disadvantage of having a relatively high oscillation threshold current of 40 to 60 mA.
In the production of VSIS laser devices, after the current blocking layer 2 is etched to form the striped channel 10, the first cladding layer 3 is grown by liquid phase epitaxy (LPE) so that the striped channel 10 is buried therewith. In this LPE growth, the rate of crystal growth on the side surfaces of the channel 10 is greater than that on the flat portion of the current blocking layer 2. As a result, the inside of the channel 10 is selectively buried and the surface of the first cladding layer 3 grown thereon becomes flat. Here, this planarization utilizes the strong dependence of the rate of crystal growth in LPE on the surface orientation of the substrate crystals, which can also be used to flatten substrates with a ridge portion.
The threshold current in VSIS lasers can effectively be reduced by making the first cladding layer 3 as thin as possible to prevent the transverse broadening of current within the first cladding layer 3, resulting in a reduction in ineffective current. However, when the growth time for the first cladding layer 3 is set short to make this layer thinner, the striped channel 10 may not be sufficiently buried, resulting in a striped concave portion in the first cladding layer 3, as shown in FIG. 6. When the active layer 4 is grown on the first cladding layer 3 with such a striped concave portion, the active layer 4 has a curved portion 11. In a semiconductor laser device with such a curved active layer, the transverse refractive index differs from that in normal devices, so that the far-field pattern of the laser beam will not be stabilized and the maximum optical output power may be lowered.
To prevent the occurrence of a curved portion in the active layer 4 when the first cladding layer 3 is made thin, the crystal growth on the flat portion of the current blocking layer 2 should be allowed to slow down sufficiently. For the purpose of accomplishing this slowdown of crystal growth, the following configurations can be considered:
(1) Ridge portion provided in the current blocking layer and the stripped channel formed in the ridge portion; and PA1 (2) Grooves (dummy grooves) similar to the striped channel (main channel), formed on each side of the striped channel.
Examples of the configuration (1) above include the buried twin-ridge substrate (BTRS) structure shown in FIG. 7. In this structure, two parallel shown in FIG. 7. In this structure, two parallel ridge 12a and 12b are formed on a terrace 13 in the substrate 1, and the striped channel 10 is formed between the ridges. In the growth of the first cladding layer 3, the growth on the side surfaces of the ridges 12a and 12b is accelerated due to the anisotropic property of crystal growth, so that the crystal growth on the flat portions of the ridges 12a and 12b slows down. This causes the striped channel 10 to become completely buried and the cladding layer 3 on both ridges ca be made thin.
Semiconductor laser devices with the respective structures shown in FIG. 8 and FIG. 9 are examples of the configuration (2). In these structures, dummy grooves 20 with the same depth as that of the striped channel 10 are formed on each side of the striped channel 10. In the semiconductor laser device shown in FIG. 8, only the striped channel 10 is formed on top of a terrace 13 in the substrate 1 to reach the substrate 1. Therefore, only the striped channel 10 serves as a current path. Furthermore, in the semiconductor laser device shown in FIG. 13, the striped channel 10 and the dummy grooves 20 all reach the substrate 1. To confine the current at the striped channel 10, grooves 14 are formed on each side of the striped channel 10 to prevent the current from leaking to the dummy grooves 20.
In the structures shown in FIG. 8 and FIG. 9, when the first cladding layer 3 is grown on the current blocking layer 2 by LPE, the crystal growth on the dummy grooves 20 is accelerated due to the orientation-dependent anisotropic property of crystal growth, so that the crystal growth on the flat portions of the current blocking layer 2 between channels slows down. Therefore, the striped channel 10 can be completely buried and the first cladding layer 3 on each side of the striped channel 10 can be made thin.
However, in both cases of the BTRS structure shown in FIG. 7 and the structure shown in FIG. 8, the substrate 1 must be etched to form the terrace 9. Furthermore, in the structure shown in FIG. 9, the grooves 14 for current confinement must be formed after the crystal growth step. As can be seen, each improvement has the disadvantage of an increase in the number of production steps for forming a current confining structure.
In the actual mass production of the VSIS laser device shown in FIG. 5, the great variation in the oscillation threshold current I.sub.th in the range of from 40 to 70 mA may be observed. Moreover, the measurements of current vs. optical output power characteristics (I-L characteristics) show that the optical output power may saturate at 5 to 15 mW. This phenomenon is known as the I-L kink, which is one of the causes of the production yield of semiconductor laser devices being decreased.
The following can be considered as the causes of the above-mentioned failure in device characteristics. First, the layer thickness of the active layer 4 above the striped channel 10 is not uniform, so that there will occur light loss, and, as shown in the upper part of FIG. 5, the effective refractive index difference .DELTA.n becomes asymmetric on the right and left sides above the striped channel 10. Also, when LPE growth is performed on the striped channel 10, the shape of the striped channel 10 becomes asymmetric on the right and left sides due to the occurrence of meltback, resulting in an asymmetrical difference .DELTA.n in the effective refractive index on the right and left sides.
In order to investigate the uniformity in the thickness of the active layer 4, a wafer was produced in which the Ga solution was removed from the growth surface after the LPE growth of the first cladding layer 3 and the active layer 4 shown in FIG. 5. FIGS. 10 and 11 are schematically exaggerated diagrams of the wafer. FIG. 10 shows the case where a semiconductor substrate was used with a surface which has a substantially uninclined orientation (i.e., orientation inclined by an angle of less than .+-.0.1 degrees, for example, from the [100] direction to the [011] direction). As shown in FIG. 10, striped concave portions are observed in the active layer 4 along the striped channels 10. These striped concave portions in the active layer 4 arise from the depressions in the vicinity of the striped channel 10 in the first cladding layer 3. The depressions in the first cladding layer 3 cause lack of uniformity in the thickness of the active layer 4 formed thereon.
FIG. 11 shows the case where a semiconductor substrate was used with a surface which has an orientation inclined from the [100] direction to the [011] direction by an angle of .+-.0.1 degrees or more. In this case, a stepped growth pattern is observed on the surface of the grown layer in the vicinity of the striped channels 10.
As can be seen, regardless of the degree of inclination with respect to the surface orientation of the substrate, the surface of the first cladding layer 3 does not become flat in the vicinity of the striped channels 10. This lack of flatness on the surface of the first cladding layer 3 shows variations in thickness of several hundred angstroms over a distance of several microns, so that it cannot be observed even by means of a scanning electron microscope (SEM). This lack of flatness occurs within the area 250 .mu.m.times.300 .mu.m which is the general size of semiconductor laser chips, and it not only causes the above-mentioned I-L kink but also induces light intensity noise and particularly noise due to returned light. It not only causes variations in thickness over the entire surface of a wafer which is several centimeters square, but also causes variations in the threshold current, far-field pattern, and other fundamental characteristics of semiconductor laser devices obtained from the same wafer.