JPS6249758B2 - - Google Patents

Description

[Detailed description of the invention] The present invention relates to a semiconductor laser.

Long-wavelength semiconductor lasers using crystalline materials such as InP/InGaAsP are attracting attention as light sources with low optical fiber transmission loss, and their practical use is progressing. For practical use, a semiconductor laser is required that exhibits stable single transverse mode oscillation over a wide operating current and also exhibits excellent dynamic characteristics with suppressed relaxation oscillations. In order to meet these requirements, various striped structures have been proposed and prototyped.

Among them, the Journal of Applied Physics (Journal of Applied Physics) by Hirao et al.
The InP/InGaAsP BH semiconductor laser reported in Physics), Vol. 51, pp. 4539-4540 has a structure in which an InGaAsP (λ = 1.3 μm) layer, which serves as the active region, is surrounded by InP, which has a low refractive index and serves as a cladding layer. rice cake,
A current injection region is provided in a part of the cladding region adjacent to the active layer, and both ends of the region are buried with cladding layers of opposite conductivity type.

Since the BH semiconductor laser has a cladding layer not only in the vertical direction of the active region but also in the horizontal direction,
It has a rectangular refractive index distribution based on the refractive index difference, and not only does it continue to maintain stable transverse mode oscillation, but also the active region and current confinement region coincide, resulting in a small threshold current and suppressed relaxation oscillation. You can expect excellent properties such as:

However, if the active region is buried in the cladding layer, current leaks from the cladding layer adjacent to the active region and serves as a current injection region to the cladding layers adjacent to both ends, so that current cannot be effectively injected into the active region. However, it has disadvantages such as an increase in threshold current and poor temperature characteristics. To overcome this drawback, cladding layers of the same conductivity type as the current injection region are usually provided on both sides of the active region and used as blocking layers against leakage current. However, when the block layer becomes thick and comes into contact with the cladding layer that serves as the current injection region, current flows from the current injection region to the block layer of the same conductivity type, and the leakage current increases rapidly.On the other hand, when the block layer is thin, The current flowed over the blocking layer and there was no blocking effect. Therefore, in order to make the blocking effect work effectively, it is necessary to grow a blocking layer with the same thickness as the active region and along both ends of the active layer, and it is extremely difficult to control this growth and it is difficult to make it with good reproducibility. was impossible.

Furthermore, in order to obtain fundamental transverse mode oscillation in a BH semiconductor laser, it is necessary to create an active region with a width of 2 μm or less by etching, and in order to facilitate the growth of the block layer, buried cladding layer, etc., the depth and shape of the etching must be adjusted. It was an extremely complicated process that needed to be controlled. Furthermore, the thickness of the blocking layer was approximately 0.2 .mu.m, which is equivalent to the thickness of the active region, and leakage current could not be completely blocked.

A V-groove substrate is proposed by Imai et al. in IEICE technical research report Optical and Quantum Electronics OEQ81-14, pp. 79-84, as a buried type semiconductor laser which is relatively easy to manufacture and has reproducibility, except for the drawbacks of the BH semiconductor laser mentioned above. Internally implanted (VSB) lasers have been proposed. This involves growing a p-InP layer as a blocking layer on an n-InP substrate, forming a V-shaped groove, and forming an n-InP cladding layer, an active layer, and an active layer inside the V-shaped groove.
Furthermore, a p-InP cladding layer is continuously grown. The active layer grown inside the V-shaped groove in this way is
A buried semiconductor laser is fabricated in a state where it is surrounded by an InP layer. However, with this VSB laser, the growth rate on the V-groove wall surface is fast when growing inside the V-groove, so n
-The growth surface of the InP cladding layer is concavely curved at the center of the V-groove, and as a result, the active layer grown adjacent to this n-InP cladding layer has a thick and curved shape at the center and is located at both ends of the V-groove. It grows only in the form of dots in the block layer. Therefore, the refractive index of the central part of the active layer is high, and not only does first-order transverse mode oscillation easily occur, but the blocking layer and the p-InP cladding layer grown adjacent to the active layer are connected, making it easy for current to leak. It had drawbacks such as poor temperature characteristics.

The purpose of the present invention is to eliminate the above-mentioned drawbacks, to maintain fundamental transverse mode oscillation over a wide range of injection current, to oscillate at a low threshold with low leakage current, to perform concentric oscillation, and to improve coupling efficiency with optical fibers. Our goal is to provide high-quality semiconductor lasers.

The semiconductor laser of the present invention has a semiconductor layer having a multilayer structure including a first cladding layer, a blocking layer adjacent to the first cladding layer, and a current confining cladding layer adjacent to the blocking layer, and a current confining cladding layer. In a structure in which a groove having a trapezoidal cross section is formed from the side to the block layer side and the tip of the groove reaches the first clad layer, the first clad layer has a convex portion at the center of the tip of the groove, and the first clad layer and the block layer have a convex portion at the center of the tip of the groove. A guide layer having a refractive index higher than that of the cladding layer is formed in the groove adjacent to the layer, and an active region is formed adjacent to the guide layer so that its growth surface coincides with the growth surface of the blocking layer, and this active region and the current confinement cladding layer It has a structure in which a single or multilayer semiconductor layer is formed adjacent to the semiconductor layer.

The principle of the present invention is to apply the effective refractive index difference formed in the horizontal and lateral directions of the active layer due to the difference in the seepage of light into the guide layer and the principle of leaky mode.

Adjacent to the first cladding layer and the first cladding layer as in the present invention.
When a guide layer is grown in a groove having a bottom surface having a convex portion of the cladding layer, the guide layer grows thickly in the concave portions on both sides of the convex portion and thinly in the convex portion.

That is, since the side surfaces of the concave portion are composed of the side surfaces of the groove and the side surfaces of the convex portion, the growth rate of that portion is relatively faster than that of the flat portion. Therefore, by adjusting the height of the convex portion, the growth surface of the guide layer can be made flat. According to experiments conducted by the inventors, the height of the convex portion
If the thickness is set to 0.2 μm, the growth plane will become flat if it grows by about 0.5 μm from the bottom of the recess. If an active layer is grown adjacent to the guide layer in this state, a flat active layer can be obtained. Moreover, the thickness of the active layer is 0.2 μm ~ 0.3
Since the thickness may be around .mu.m or less, there is no risk that the groove surface will grow particularly thick during active layer growth, and the position of the active layer growth surface can be determined with good reproducibility and can be made to coincide with the block layer growth surface.

Since a flat active layer can be obtained as described above,
Unlike VSB lasers, which are curved and thick at the center, they do not easily allow primary transverse modes, and it is also easy to align the block growth surface with the active layer growth surface, which prevents leakage current and has a low threshold. Oscillation becomes possible.

On the other hand, in this structure, light from the active layer leaks out to the guide layer, which has a higher refractive index than the cladding layer and a lower refractive index than the active layer. At this time, the guide layer is thin at the convex portion of the groove bottom surface and thick at the concave portions at both ends thereof, so that the light seeping out from the active layer also has fewer convex portions and more concave portions. Therefore, the active layer has an effective refractive index distribution in which the central portion of the active layer above the convex portion has a low refractive index and the opposite ends thereof have high refractive indexes, resulting in a leaky guide mechanism inside the active layer. However, since the entire active layer has a much larger refractive index than both ends of the active layer, light within the active layer is guided by the refractive index guiding mechanism and produces stable transverse mode oscillation. FIG. 3 shows a refractive index distribution formed by one implementation of the present invention. Furthermore, as mentioned above, since the active layer has a leaky guide mechanism, light tends to spread throughout the active layer, and as a result, fundamental transverse mode oscillation can be maintained even if the active layer width is about 3 to 4 μm wide. . Moreover, even if the inflow current is increased, the light oscillates uniformly over the entire width of the active layer, so spatial hole burning in the carrier distribution is less likely to occur, and stable fundamental transverse mode oscillation can be maintained. Furthermore, since the oscillation region is wide, the external differential quantum efficiency is good and high optical output oscillation is possible.

Furthermore, the radiation angles of light in the horizontal direction and the vertical direction of the active layer can be made closer to each other due to the light leakage to the guide layer. Therefore, it becomes a point light source close to a concentric circle, and the coupling efficiency with the optical fiber can be increased.

In addition, in general, when the V-shaped groove is grown to fill it, the growth rate on the sides of the groove is fast, which causes stress in the center of the V-groove during growth, which causes slip dislocations and shortens the life of the laser element, reducing its reliability. I was afraid. On the other hand, with this structure, there is no such fear and the reliability of the semiconductor laser can be improved.

As explained above, the semiconductor laser according to the present invention has the following effects. The structure according to the present invention has a leaky guide mechanism in the horizontal direction within the active layer and has a mode broadening element, so that not only is spatial hole burning of the carrier distribution less likely to occur, but both ends of the active layer have a low refractive index. Since it is surrounded by low blocking layers, it has a refractive index guiding mechanism and can maintain extremely stable fundamental transverse mode oscillation over a wide current injection region.
In the structure of the present invention, the active layer is flat and it is relatively easy to align the active layer growth surface with the block layer growth surface, making it possible to oscillate at a low threshold without leakage current. Furthermore, the fact that fundamental transverse mode oscillation can be achieved even if the active layer is made as wide as several micrometers increases the tolerance of the manufacturing method. The spread angle of light in both the horizontal and vertical directions of the active layer can be made to be approximately the same, resulting in a point light source close to equicentric circles, and the coupling efficiency with the optical fiber can be increased. The inside of the active layer becomes a leaky guide, which spreads the mode, and since the active layer has an adjacent guide layer, the oscillation region is widened, the external differential quantum efficiency is high, and large optical output oscillation is possible. In this structure, the conventional V
The reliability of the semiconductor laser can be improved because the stress seen in trench-buried structures does not occur.

An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows a cross-sectional view of the semiconductor laser of the present invention during the manufacturing process. First, (100) plane n-InP
A p-InP block layer 11 with a thickness of 0.5 μm is placed on the substrate 10.
An n-InP current confinement cladding layer 12 is continuously grown to a thickness of 0.5 μm. SiO ₂ film 1 on cladding layer 12
3 and cut out two stripes with a width of 2 μm perpendicular to the (011) plane in parallel with an interval of 3 μm using the photoresist method to a depth of 0.2 μm with a mixed solution of 4HBr:1H ₃ PO ₄ .
m etching. Next, SiO ₂ in the center of the stripe
The film is removed and the n-InP substrate is etched until a convex surface is formed on the n-InP substrate. The (111) plane is etched to form a groove 100 having a trapezoidal cross section with a convex surface at the etched tip. The structure obtained through the steps up to now is shown in FIG. Next, the SiO ₂ film is removed and n-InGaAsP is placed inside the trench.
(λ=1.1μm) Guide layer 14 is inserted from the groove bottom recess.
Undoped InGaAsP grown to 0.5μm
(λ=1.3 μm) The active layer 15 is grown to a thickness of 0.2 μm.
At this time, the guide layer growth surface becomes flat, and the active layer growth surface and the block layer growth surface coincide. Then p-
An InP clad layer 16 is grown to fill the entire trench, and an n-InGaAsP (λ=1.1 μm) cap layer 17 is grown.
is continuously grown in liquid phase to a thickness of 0.5 μm. In general, the growth rate inside a groove having a (111) plane is about three times faster than in a flat area, so the guide layer 14 and active layer 15 grow on the current confining cladding layer 12 to a thickness of about 0.2 μm to 0.1 μm, respectively. Also plays the role of current confinement. The p-InP cladding layer 16 is made to have a thickness of 2 μm on this current confinement layer. A SiO ₂ film is deposited on the growth surface of the cap layer 17, and a striped window with a width of 3 μm is opened on the groove using the photoresist method, and Cd is diffused to form the p-InP cladding layer 1.
6. Control so that the depth is about 0.2 μm (Cd
Diffusion region 18). Next, the SiO ₂ film is removed and a p-type ohmic contact 19 and an n-type ohmic contact 20 are attached to the substrate side to obtain a semiconductor laser having the structure shown in FIG. 2.

In the semiconductor laser thus obtained, light in the active layer leaks to the guide layer, creating an effective refractive index difference in the central recess inside the active layer. On the other hand, since both ends of the active layer are sandwiched between block layers having a small refractive index, the entire active layer has a refractive index guiding mechanism.
FIG. 3 shows the effective refractive index difference obtained by this structure.

In the above embodiments, the p-type and n-type may be reversed, and the composition of each layer thickness, InGaAsP, diffusion material, etc. are not limited to the above. In addition, the above embodiment is InP-InGaAsP
Although the double heterojunction crystal material has been described, it can be applied to many other crystal materials such as GaAsSb-AlGaAsSb.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view showing the structure of a semiconductor laser according to an embodiment of the present invention in the process of being manufactured, that is, a groove is formed after successively growing a blocking layer and a cladding layer for current confinement on an InP substrate; FIG. 2 is a cross-sectional view of one embodiment of a semiconductor laser formed according to the present invention, and FIG. 3 is a diagram showing an effective refractive index formed in the semiconductor laser formed according to the present invention. . In the figure, 10... n-type InP substrate, 11...
p-type InP block layer, 12... n-type InP cladding layer, 13... SiO ₂ film, 14... n-type InGaAsP
(λ=1.1μm) Guide layer, 15...undoped
InGaAsP (λ=1.3μm) active layer, 16...p type
InP cladding layer, 17... n-type InGaAsP (λ=1.1
μm) Cap layer, 18...Cd diffusion region, 19
. . . p-type ohmic contact, 20 . . . n-type ohmic contact, 100 . . . groove.

Claims (1)

[Claims] 1. A semiconductor layer having a multilayer structure comprising a first cladding layer, a blocking layer adjacent to the first cladding layer, and a current confining cladding layer adjacent to the blocking layer, from the current confining cladding layer side to the blocking layer side. In a structure in which a groove having a trapezoidal cross section is formed and the tip of the groove reaches the first cladding layer, the first cladding layer has a convex portion at the center of the tip of the groove, and the first cladding layer has a convex portion adjacent to the first cladding layer and the block layer. Inside the groove, an active region is formed adjacent to a guide layer having a refractive index higher than that of the clad layer so that its growth surface coincides with the growth surface of the blocking layer, and a layer or a guide layer is formed adjacent to the active region and the current confining clad layer. A semiconductor laser characterized by having a multilayer semiconductor layer.