JPS621277B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a semiconductor laser having a structure effective for controlling oscillation mode.

Converting semiconductor lasers into fundamental modes is necessary for the practical application of broadband, long-distance transmission and low-distortion analog modulation in optical fiber communications. Control of the oscillation mode was first achieved with so-called electrode stripe lasers, a structure that confines optical energy and injected current to specific regions that minimize optical loss and wasteful recombination. Since then, various striped lasers have been developed and are still available today, but all of them have their own drawbacks and are unsatisfactory in terms of characteristics.

For example, when only the current distribution is confined in the lateral direction, such as in an electrode stripe laser, the laser light is mainly guided in the stripe direction by the gain distribution, but the waveguide effect due to this gain is unstable, and the current increases. As time goes on, higher-order transverse mode oscillations easily occur, and furthermore, the current-optical output characteristics often become distorted. This is because the electrode stripe laser does not have a structure that confines carriers and light in the lateral direction of the active layer. Therefore, in order to compensate for the above-mentioned drawbacks, a so-called rib guide or stripe laser in which a waveguide mechanism is structurally built inside a semiconductor laser has been proposed. This structure attempts to obtain stable fundamental mode oscillation by forming a light guide and a rib-structured waveguide in the light emitting region.

This rib guide stripe laser should be mentioned as the prior art prior to the present invention, and first, the structure, mechanism, etc. of this type of GaAs-AlGaAs semiconductor laser will be briefly explained. 1st
The figure is a schematic perspective view thereof. 1 is a belt-shaped groove 1
0 is formed on the n-type GaAs substrate, and the following layers are laminated thereon. A light confining layer 2 made of n-type Al _0.3 Ga _0.7 As, a light guide and carrier confining _layer 3 (hereinafter referred to _{as the} light guide layer) made of n-type Al _{0.1 Ga 0.9} As thick in the center. ), a GaAs active layer 4 , a p-type Al _0.3 Ga _0.7 As layer ₅ for confining light and carriers, and the central part of _the layer 2 for confining light is formed in the semiconductor substrate 1 . Because of the groove 10, it has a concave, dog-like shape. Electrodes 7 and 9 are in contact with each other via a SiO ₂ film 8 formed on the semiconductor substrate 1 and the p-type GaAs 6 of the electrode facilitation layer.
With such a structure, part of the light generated in the active layer 4 leaks into the light guide layer 3. Since the light guide layer 3 is sufficiently transparent to the light generated in the active layer 4, there is no loss of oscillated light within this layer.
There, the light spreads and propagates between the guide layer 3 and the active layer 4. Furthermore, these semiconductors 3 and 4 are sandwiched between a layer 2 that confines light and a layer 5 that confines light and carriers, each having a low refractive index, so that light is guided in the vertical direction by the layers 2 and 5 that have a low refractive index. Confined to areas 3 and 4. Also, in the lateral direction, the light guide layer 2
In the groove 9 region, the layer thickness is maximized at the center and becomes thinner toward the outside of the groove 10, so that the effective refractive index is increased at the center of the light guide layer 2. be confined in the department. Therefore, this structure is equivalent to providing an optical waveguide mechanism with a refractive index distribution in the lateral direction of the light emitting region, and is therefore characterized by stable fundamental mode oscillation even at high optical output. However, this rib-guided stripe laser has the following drawbacks due to insufficient lateral carrier confinement.

That is, the carriers injected into the active layer 4 spread in the lateral direction of the active layer 4 due to the diffusion effect, and the effective light emitting area becomes considerably wider than the actual electrode width. For example, in a striped laser with an electrode width of 4 .mu.m, the light emitting region of the active layer spreads from just below the electrode to a width of about 20 to 30 .mu.m on one side, with its intensity attenuating exponentially in the lateral direction. Carriers other than directly below the electrode do not directly contribute to oscillation, so
This lateral current component causes an increase in the operating current. The lateral spread current component is several times larger than the effective oscillation current component as the electrode width becomes less than 10 μm and becomes a wedge-stripe laser, so lateral confinement of this carrier becomes an important design issue.

In addition to increasing the operating current, the temperature rise in the active layer becomes large and the differential quantum efficiency decreases, making high-output operation impossible.

The purpose of this invention is to provide a semiconductor laser structure that eliminates the drawbacks of conventional semiconductor lasers, allows easy fundamental mode oscillation, has a low oscillation threshold current, has high differential quantum efficiency, and can achieve high output. It is to be.

The present invention is solved by the structure of a semiconductor laser as described below.

The main structure of the semiconductor laser of the present invention is as follows.

A light confinement layer, a light guide and carrier confinement layer (hereinafter abbreviated as light guide layer), an active layer, and a light and carrier confinement layer are successively grown and formed on a semiconductor substrate using a common epitaxial growth method. has been done. The light and carrier confinement layer is formed in the shape of a bowl with a striped central portion on one side, and is completely buried in the light guide layer, thereby providing the light guide layer with a bowl-shaped guide region.

Furthermore, the active layer is etched to remove the part formed on the circular guide region other than the central part away from the edge of the optical guide layer, and then a semiconductor layer is formed around the active layer by epitaxial growth again. It is embedded in.

The basic principle of the present invention will be explained with reference to FIGS. 2 and 3. Figure 2 shows the present invention in GaAs-
A typical example of a semiconductor laser implemented in an AlGaAs semiconductor is a schematic perspective view thereof, and FIGS. 3A, 3B, and 3C are schematic cross-sectional views of the main parts.

Semiconductor substrate 11 is n-type GaAs, optical confinement layer 1
2 is an n-type Al _0.3 Ga _{0.7 As} layer, and the optical guide layer 13 is also n-type _.
The active _layer 14 is GaAs, and _the light and carrier confinement layer 15 is _a p- _type Al _0.3 Ga _{0.7 As} layer.
_{The buried layer 16 is an n-type Al 0.2 Ga 0.7 As layer} .

Further, the optical guide layer 13 in the groove 19 portion provided in the semiconductor substrate 11 is in contact with the optical confinement layer 12 arched inwardly toward the semiconductor substrate 11, and has a convex center and a flat surface, that is, a plano-convex layer. It has a cross section of the shape. The active layer 14 is present only in a portion of the plano-convex guide region and is surrounded by a buried layer 16.

The n-type electrode 17 _{of the electrodes 17 and 18 is provided in contact with the semiconductor substrate, and the p-type electrode is provided in contact with the surface of the p-type Al 0.3 Ga 0.7 As layer .}

Typical thicknesses of each layer are 0.8 μm for the light confinement layer 12 and 0.4 μm for the light guide layer 13 at the center of the groove 19.
μm active layer 14 is 0.1 μm, light and carrier confinement layer 15 is 1.5 μm, and buried layer 16 is approximately 1.6 μm.
It is μm.

The operation is to apply a positive voltage to the electrode 18 and a negative voltage to the electrode 17,
Carriers are injected into the active layer 14 and emit light by recombination. The carriers injected into the active layer 14 are vertically adjacent to each of the adjacent semiconductors 13 and 15.
Laterally, each of the semiconductor layers 16 is completely confined within its active layer 14 . When the loss is overcome by the gain due to sufficient injection current, laser light is generated from the active layer 14. This light seeps into the light guide layer 13, and the light confinement layer 1 with a low refractive index
2 and a light and carrier confinement layer 15. The thickness of the central portion of the light guide layer 13 is different from that of the outside. Therefore, the lateral refractive index distribution of the light guide layer 13 is larger at the center than at the outside. In other words, an optical waveguide mechanism is built into the light emitting region, and stable fundamental mode oscillation is achieved due to the lateral confinement of light.

If the active layer thickness is sufficiently thin, that is, 1000 Å
When the amount is below, the proportion of light in the active layer is 15%.
Most of the light exists within the light guide layer. Therefore, the laser light propagates within the light guide layer,
The light is emitted from the light guide layer in the excited region (-' cross section) where there is no active layer. Furthermore, since the light guide layer in this region has an optical waveguide mechanism in which the refractive index changes both in the vertical and horizontal directions, the laser light can propagate without changing its fundamental mode.

Since the active layer is surrounded by the buried layer 16, the diffusion and spread of carriers is completely prevented, and most of the injected carriers contribute to oscillation.

Therefore, since there is no need to flow extra current, the operating current has the advantage of being small, and if the area of the injection region is constant, even if the dimensions (length) of the semiconductor laser are increased, it can be assembled without increasing the operating current. It becomes possible to design a semiconductor laser of an appropriate size. The increase in carrier injection efficiency also has the advantage of increasing luminous efficiency.

Another feature is that the end face of the active layer is not exposed, and deterioration of the end face of the active layer is completely eliminated.
This structure improves reliability by several times compared to conventional semiconductor lasers.

In the above embodiment, _the active region is GaAs and the region surrounding it is _{AlGaAs .}
Needless to say, it may be a quaternary crystal such as P _1-y . In this case, the InP substrate has a wider forbidden band width and a smaller refractive index than the crystal in the light emitting region, so the InP substrate has a function similar to the optical confinement layer of a GaAs-AlGaAs semiconductor laser. Therefore
In a semiconductor laser using an InP substrate, a groove is formed in the InP substrate, and a light guide layer is provided directly on the groove. That is, a layered structure in which the optical confinement layer of this embodiment is removed and InP is used for the buried layer has the same functions and effects as the present invention.

[Brief explanation of the drawing]

FIG. 1 is a schematic perspective view of a conventional semiconductor laser;
FIG. 2 is a schematic perspective view of a semiconductor laser according to an embodiment of the present invention, and FIG. 3 is a schematic cross-sectional view of its main parts. In the figure, 1, 11...semiconductor substrate, 2, 1
2...Light confinement layer, 3,13...Light guide layer,
4,14...active layer, 5,15...light and carrier confinement layer, 8...SiO ₂ film, 7,17...n
Type electrode, 9, 18...p type electrode, 10, 19...
Grooves, 16...buried layers are shown, respectively.

Claims (1)

[Claims] 1. A light guide and carrier confinement layer (1) having a strip-shaped convex portion on one surface and having a longitudinal cross-sectional shape that is thickest at the center and thinner toward both ends, and the other surface is flat. On the flat side of the light guide layer (hereinafter abbreviated as the light guide layer) and in the area facing the convex portion and away from the end of the light guide layer, the refractive index is larger than that of the light guide layer and the forbidden band width is narrower. an active layer is provided, a light and carrier confinement layer having a smaller refractive index than the active layer is provided on the active layer so as not to protrude from the active layer, and the active layer is provided on the flat surface of the light guide layer. A buried layer having a smaller refractive index and a wider forbidden band width than the active layer is provided in a region not provided with the active layer, and the periphery of the two-layer structure consisting of the active layer and the light and carrier confinement layer is buried. 1. A semiconductor laser comprising an optical confinement layer having a refractive index lower than that of the active layer, which is in contact with a surface of the guide layer having a convex portion.