JPH0430198B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a buried structure distributed reflection type semiconductor laser and a manufacturing method thereof. Distributed reflection semiconductor lasers and distributed feedback semiconductor lasers are
It is seen as a promising light source for long-distance broadband optical fiber communications because it oscillates in a single-axis configuration even during high-speed modulation.

A distributed reflection semiconductor laser has two regions: an active region that has a gain and a distributed reflection region that determines an oscillation wavelength.

FIG. 1 is a cross-sectional view of a wafer showing the structure and manufacturing process of a conventional buried structure distributed reflection type semiconductor laser divided into the above two regions. An active layer 102 is formed on top of the active layer 102, and a layer of 1
03, a cladding layer 103 is formed. 1st
After removing the cladding layer 103 and the active layer 102 in the portion that will become the distributed reflection region 120 as shown in FIG. 1b from the wafer shown in FIG.
4 and the cladding layer 1 in the distributed reflection region 120
05 is laminated as shown in Fig. 1c. active layer 1
02 and the remaining portion of the cladding layer 103 becomes an active region 110. FIGS. 1a, b, and c are cross-sectional views in the direction of the resonator. The wafer shown in FIG. 1c is mesa-processed, and the active region 110 is formed as shown in FIG.
1, e2, the active layer 102 and the waveguide layer 1
04 is completely buried inside the crystal by the first buried layer 106 and the second buried layer 107. Further, in the distributed reflection region 120, as shown in FIG. 1F, after etching is performed so that the upper part of the waveguide layer 104 is exposed, a grating is formed on the surface thereof. The above is the waveguide shape and manufacturing method of the conventional buried structure distributed reflection semiconductor laser.
In the steps e2 to f in FIG. 1, the waveguide layer 10
4 and the crystal in which the waveguide is embedded, using an etching solution that has selectivity in chemical etching characteristics to obtain a surface for the grating fabrication. Therefore, the waveguide layer 104 may protrude from the crystal. happen.

Such variations in the waveguide structure in the distributed reflection region 120 including the waveguide layer 104 result in variations in the wavelength within the waveguide, and determine the desired wavelength that should be set as the peak wavelength of the gain of the active region. This poses a major problem in determining the pitch of the grating to be used. Further, deterioration of the flatness of the surface on which the grating is manufactured due to the protrusions of the waveguide layer 104 causes problems in uniform application of resist, etc., and deteriorates the yield of manufacturing the grating by optical interference exposure. Needless to say, when using an etching solution that does not have selectivity in etching characteristics, highly accurate etching control is required, which affects the yield. Furthermore, it is difficult to flatten the upper surface of the surface shown in FIG. 1 e2 through crystal growth, which also affects the surface of f.

SUMMARY OF THE INVENTION An object of the present invention is to provide a distributed reflection type semiconductor laser that is easy to manufacture and a method for manufacturing the same, which eliminates the above-mentioned problems.

The semiconductor laser of the present invention includes a first waveguide layer having a refractive index higher than that of the semiconductor crystal, which partially fills the groove flatly in the longitudinal direction of the groove on the semiconductor crystal in which the groove is formed. and an active layer comprising a first cladding layer and an end surface in contact with an end surface of the first waveguide layer, at least from the inside to the top of the groove in a portion not buried by the first waveguide layer. , a multilayer structure including a second cladding layer having a refractive index smaller than that of the active layer, and a third cladding layer having a refractive index equal to or smaller than the second cladding layer, and the first waveguide layer or The at least one semiconductor layer stacked on the first waveguide layer has a periodic structure of projections and depressions. Further, in the manufacturing method of the present invention, a first waveguide layer having a refractive index higher than that of the semiconductor crystal is laminated on the entire surface of the semiconductor crystal in which the groove is formed so that the surface including the groove is flat. Reproducing the first groove by removing at least a portion of the first waveguide layer in the longitudinal direction of the groove using an etching solution that selectively removes only one waveguide layer; The structure includes the step of laminating at least a first cladding layer, an active layer, a second cladding layer, and a third cladding layer in this order.

According to this configuration, the first waveguide layer filling the groove becomes a waveguide in the distributed reflection region, and light is guided only in the first waveguide layer in the groove portion, and the light does not spread in the distributed reflection region and becomes active. It is possible to effectively return light to the area. Since the top surface of the first waveguide layer is flat and the first waveguide layer exists in the entire distributed reflection region, the first waveguide layer can be etched with good reproducibility using an etching solution with different etching characteristics depending on the semiconductor composition. It is easy to expose the top surface of the layer, and a good grating can be formed with good reproducibility. Furthermore, according to the manufacturing method of the present invention, the following effects can be obtained. As can be easily inferred from the conventional manufacturing method shown in FIG. The axis must match the optical axis of the distributed reflection area. As a method to avoid this drawback, there is a method of forming it in a pre-formed groove. However, even this method has the following drawbacks when it is a normal method. FIG. 2 shows an example of a cross-sectional view of an active region showing a buried active layer 201. FIG.

The width of the buried active layer is about 2 μm or less, and it is difficult to form grooves that determine the optical waveguide of the distributed reflection region accordingly.
Furthermore, since the active layer is located inside the crystal, the position of the active layer must first be detected, which complicates the process. Since the active layer is curved and partially broken, the process of removing the active layer is also difficult. However, according to the manufacturing method of the present invention,
The above problem can be easily solved. In the active region, the optical axis of the active region and the optical axis of the distributed reflection region are automatically aligned in order to make the groove that embeds the active layer in a crystal whose refractive index is smaller than that of the active region and the groove that determines the waveguide of light in the distributed reflection region the same groove. matches. Therefore, coupling of the distributed reflection region and the active region becomes easy.

Hereinafter, the present invention will be explained in detail using examples.

FIG. 3 is a sectional view when the semiconductor laser of the present invention is cut inside the groove in the cavity direction, and FIG. 5 is a sectional view including the manufacturing process of the active region and the distributed reflection region. 2 in the figure is the first waveguide layer, and its cross-sectional view perpendicular to the resonator direction is FIG. 5c, which forms a waveguide in the distributed reflection region 40. Also, 3 in Figure 3
is the active layer, whose sectional view perpendicular to the resonator direction is shown in FIG. 5b. The active layer 3 is completely embedded in a semiconductor crystal having a refractive index smaller than that of the active layer, and its end face is in contact with the end face of the first waveguide layer at a boundary 50 between the distributed reflection region 40 and the active region 30. The manufacturing process will be sequentially explained below.

4a, b, and c show a part of the manufacturing process of the semiconductor laser of the present invention.

In FIG. 4a, reference numeral 1 indicates a semiconductor crystal made of InP. 10 is a groove. The width of the groove was approximately 2 μm, and it was V-shaped. Then, as shown in Figure 4b,
On top of this, a quaternary crystal of InGaAsP with a composition of 1.3 μm was laminated so that the surface was flat. Furthermore, an etching mask is formed using ordinary photolithography, and a portion of the InGaAsP layer in the longitudinal direction of the trench 10 is removed using a mixed solution of H ₂ SO ₄ , H ₂ O, and H ₂ O _2, as shown in FIG. 4c. did. 30 in c is an active region, and 40 is a distributed reflection region in which grading 20 is formed. On top of that is the first cladding layer 3.
InP type, InGaAsP4 with a composition of 1.5 μm to become active layer 4
The original crystal has a smaller refractive index and a larger band gap (1.3μ
An InGaAsP layer (having a composition of m), an InP layer that will become the third cladding layer 6, and an InGaAsP layer that will become the cap layer 7 for making ohmic contact were laminated. 5a and 5b are cross-sectional views of the active region 30 before the first cladding layer 3 is laminated and after the cap layer 7 is laminated, respectively.

The active layer 4 is completely buried inside the trench 10 by selecting liquid phase growth conditions. FIGS. 5c and 5d are cross sections of the distributed reflection region 40 before the first cladding layer 3 is laminated and after the cap layer 7 is laminated. Since the surface of the distributed reflection region 40 is flat as shown in c, it is simply layered one after another in a flat manner.
For the InGaAsP layer, the wafer shown in d is
A mixture of H ₂ SO ₄ , H ₂ O ₂ and H ₂ O, for the InP layer
The surface of the first waveguide layer 2 was etched using a mixed solution of HCl and H ₂ O, and the distributed reflection region was returned to the cross-sectional structure shown in c. At this time, the first cladding layer 3 is
When etching with a mixture of HCl and H _{2 O}
Since the mixed solution hardly etches InGaAsP, etching automatically stops at the surface of the first waveguide layer, leaving a flat surface of the waveguide layer 3. Therefore, a grating could be easily fabricated thereon. In the distributed reflection region 40, the light passes through the groove 1 of FIG. 5c.
Most of the light is guided inside the first waveguide layer 2 inside the groove 10, and furthermore, in the active region 30, since the active layer 4 is located in the same groove 10, the light is sufficiently coupled to the active layer 4. Furthermore, if the composition and layer thickness of the second cladding layer 5 are optimized and the electromagnetic field distributions of both regions at the boundary 50 in FIG. 2 are matched, the coupling will be further increased. Finally, electrodes were formed on the P side and the n side to form a semiconductor laser. Here, the shape of the groove is V-shaped, but it does not need to be V-shaped.

As described above, the buried structure distributed reflection semiconductor laser of the present invention can be manufactured as easily as a normal buried structure semiconductor laser, and the buried structure distributed reflection semiconductor laser has a flat grating production surface and a high grating production yield. It's a laser.

[Brief explanation of drawings]

Figure 1 shows the structure and manufacturing process of a conventional buried structure distributed reflection type semiconductor laser.
01 is a substrate, 102 is an active layer, 103 is a cladding layer, 104 is a waveguide layer, 105 is a cladding layer of a distributed reflection region, 106 is a first buried layer, 107
is a second buried layer, 110 is an active region, 120
is the distributed reflection region. FIG. 2 is a diagram showing the shape of a typical type of active layer in which the active layer is grown in a groove, and 201 is the active layer. FIG. 3 is a sectional view in the cavity direction of the buried structure distributed reflection type semiconductor laser of the present invention, and FIGS. 4 and 5 are schematic diagrams showing a part of the manufacturing process. In the figure, 1 is a semiconductor crystal, 2
is the first waveguide layer, 3 is the second slat layer, 4 is the active layer, 5 is the second clad layer, 6 is the third clad layer, 7 is the cap layer, 10 is the groove, and 20 is the grating. , 30 is an active region, 40 is a distributed reflection region, 50 is a boundary between the active region and the distributed reflection region, 10
0 is the active layer, 200 is the waveguide layer of the distributed reflection region,
300 is a semiconductor crystal.

Claims (1)

[Scope of Claims] 1. A first waveguide layer having a refractive index higher than that of the semiconductor crystal that partially fills the groove flatly in the longitudinal direction of the groove on a semiconductor crystal in which a groove is formed,
A first cladding layer, an active layer whose end surface is in contact with an end surface of the first waveguide layer, and the active layer from at least the inside to the top of the groove in the portion not buried by the first waveguide layer. It has a multilayer structure including a second cladding layer having a smaller refractive index, a third cladding layer having a refractive index equal to or smaller than the second cladding layer, and the first waveguide layer or the first cladding layer What is claimed is: 1. A buried structure distributed reflection type semiconductor laser having a periodic structure of concave and convex portions on at least one semiconductor layer laminated on a waveguide layer. 2. A first waveguide layer having a higher refractive index than the semiconductor crystal is laminated on the entire surface of the semiconductor crystal in which the groove is formed so that the surface including the groove is flat, and only the first waveguide layer is selected. reproducing the first waveguide layer by removing at least a longitudinal portion of the first waveguide layer using an etching solution that removes at least one of the active waveguide layers; 1. A method of manufacturing a semiconductor laser, comprising the steps of sequentially laminating a second cladding layer, a second cladding layer, and a third cladding layer.