JPS6174384A - Semiconductor laser - Google Patents

Abstract

PURPOSE:To prevent degradation of performance characteristics due to useless entrappment or reflection of light in a bent activation layer by a method wherein an activation layer is provided with a flat structure in a laser device of a hybrid structure wherein a refractive index guide and gain guide are combined. CONSTITUTION:The surface of a semiconductor substrate 21 is formed flat, whereon a first clad layer 22, activation layer 23 are formed, also flat. On the activation layer 23, a light-absorbing resion 35 is selectively formed, for the construction of a refractive index guide. A second clad layer 24 is formed to fill the light-absorbing region 35, whereafter a current-constricting region 27 is formed, by ion implantation, to serve as a gain guide structure. In this way, by building activation layers flat and free of curvature across the entire region along the length of the hybrid structure, degradation of performance characteristics is prevented that may otherwise be produced due to useless light entrapment.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a semiconductor laser suitable for use as a light source for recording or reading in a recording and reproducing apparatus for, for example, an optical video disc or a digital audio disc. In particular, the present invention relates to a semiconductor laser formed by a hybrid of a refractive index-guided structure and a gain-guided structure.

[Conventional technology]

Conventional semiconductor lasers are broadly classified into index guide type and gain guide type depending on their carrier and light confinement mechanisms.
By combining the two, we have created a semiconductor laser that takes advantage of the advantages of both and compensates for the disadvantages of both.
A hybrid semiconductor laser of a refractive index guide type and a gain guide type was disclosed in Japanese Patent Application Laid-open No. 1983-61982 filed by the present applicant.

That is, as explained in the above-mentioned Japanese Unexamined Patent Publication No. 59-61982, the conventional refractive index guided semiconductor laser is as shown in FIG. 5 or 6, for example.
There are some that show an expanded cross-section of each route. For example, an N-type GaAs substrate (11) is provided, and a striped recess (2) or a convex part ( 3
) is provided in advance, and this concave part (2) or convex part (3)
On the substrate (1) including the top, for example, N-type AlGaAs
a first cladding layer (4) consisting of P;
an active layer (5) of type or N type, for example of GaAs;
A second cladding layer (6) made of P-type AlGaAs is placed on top of this, and an electrode (not shown) is placed on top of this.
A P-type cap layer (7
) are formed by continuous epitaxial process.

The first and second cladding layers (4) and (6) have a band gap larger than that of the active layer (5), so that the active layer (5) and the first and second cladding layers ( 4) and (
6) A heterojunction is formed between them, and striped light emitting regions (8) extending oppositely in one direction due to the presence of recesses (2) or protrusions (3) of the substrate (1) are bent. formed between the parts. In other words, the bent portion confines light due to the difference in refractive index.

On the other hand, in a gain-guided semiconductor laser, for example, as shown in an enlarged cross-sectional view in FIG.
, the active layer 15), the second cladding layer (6), and the cap layer (7) are sequentially epitaxially grown to form a striped current path αψ.A current having a current blocking effect is formed by a PN junction or a high resistance layer. Narrowed regions (9) are formed on both sides of the current path α. This current narrowing region (9) can be formed, for example, by selectively implanting impurity ions, protons, etc. from above the cap 1i (71).In this way, the current is concentrated and the active layer (5
), a large portion of the carrier injection is formed in a limited manner in a stripe shape, and an effective refractive index difference is generated due to the carrier distribution difference, thereby forming a stripe-shaped resonant region.

The index-guided semiconductor laser and the gain-guided semiconductor laser described above each have advantages, but each has drawbacks. In other words, in the refractive index guided type, the longitudinal mode is a single mode, and therefore, when used as a light source for writing or reading in, for example, an optical video disc, there is a drawback that it is susceptible to noise due to returned light. . However, on the other hand, the so-called beam waist position
) exists in the vicinity of the tip surface of the light emitting region, which has the advantage that the focal position can be easily set in actual use. Furthermore, speed ratio #1 @ in the cross section parallel to the welding direction! The so-called far field pattern (far
Similarly, there is an advantage that the field pattern) is symmetrical and that it is easy to obtain a spot shape with small distortion as a writing light or a reading light in actual use, for example. In contrast, in a gain-guided semiconductor laser, the beam waist position is located approximately 20 μm inside the optical end face of the light emitting region, and furthermore, the far field pattern is asymmetrical, resulting in large astigmatism. There is a disadvantage that the distortion at the slot 7 becomes relatively large. However, this gain-guided semiconductor laser has the advantage that its longitudinal mode is multi-mode and is resistant to noise caused by the above-mentioned return light.

The semiconductor laser disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 59-61982 is a hybrid of a refractive index type semiconductor laser and a gain guided type semiconductor laser, and is designed to take advantage of the advantages of both and compensate for the disadvantages of both. For example, it is a semiconductor laser that has an excellent spot shape as a light source for writing or reading optical video discs or digital audio discs, etc., making it easy to design an optical lens system and easily obtaining an even better beam spot shape. be. In this semiconductor laser, a striped light-emitting region, that is, a resonator, is formed in the active layer, and a bent part of the active layer is provided in a part of the active layer within a range of about the width of the light-emitting region, and the active layer is provided with a bent part in the other part. The width of the layer is made sufficiently wider than the width of the light emitting region. That is, in this case, the light extraction end faces at both ends of the light emitting region are of a refractive index guide type, and at least a portion inside thereof is of a gain guide type.

An example will be explained with reference to FIGS. 8 to 11.

Fig. 8 is an enlarged plan view thereof, and Figs. 9 and 10 are enlarged cross-sectional views on line A-A and line B-B of Fig. 8, respectively.
FIG. 1 is an enlarged perspective view of the substrate.

In this case, for example, an N-type GaAs single crystal substrate (
11), and on this, a first cladding layer (12) which also serves as a buffer layer made of N-type AlGaAs having the same conductivity type as the base &(11) is epitaxially grown, and on this, N-type or P-type AlGaAs is formed. An active layer (13) made of, for example, GaAs is epitaxially grown, and a second cladding JN (14) made of, for example, P-type AlGaAs is epitaxially grown on this. A camp layer made of GaAs etc. with a high concentration (
15) is epitaxially grown.

These semiconductor layers (12) to (15) are grown by vapor phase epitaxial growth using, for example, the molecular beam epitaxial growth method (MBE method) or the thermal decomposition method of metal oxides (MOCVD method), that is, for example, the thermal decomposition of trimethylaluminum, trimethylgallium, and arsine. It can be formed continuously by a series of operations using a method or the like. For example, a semiconductor layer (
Protons are selectively implanted from the surface of 15), leaving a stripe-like current path portion (16), and the current path that has become resistive due to proton implantation in other parts is limited to 1 ull. A current constriction region (17) is formed. This current confinement region (17), ie, the depth of proton implantation, is selected to a depth that reaches the second cladding M (14) but does not reach the active layer (13). In this way, a stripe structure extending in one direction is formed, and the active layer (13) is formed by the so-called gain guide type structure explained in FIG.
In this method, a stripe-like light emitting region extending in one direction is formed on the substrate (1) in advance, as shown in FIG.
In 1), for example, four parts (18) are formed which are narrow at both ends of the stripe-shaped light emitting region, that is, at both end surfaces for light extraction and wide at the center part, and the four parts (18) are formed in the concave part (18). The above-mentioned cladding layer (12), active layer (13), and cladding layer (14) are sequentially formed so that the light-extracting end surfaces of the light emitting region are activated due to the presence of the recessed portions (18). (13) Bends are present on both sides of the light emitting region, so that a refractive index guide type structure is adopted.

Although not shown, one electrode is deposited on the cap layer (15), spanning, for example, the insulated current confinement region (17), and the other electrode is deposited on the back side of the substrate (11). It will be worn.

According to such a configuration, active Ji (
13), a stripe-like light emitting region is formed, but in this case, a refractive index guide type structure is adopted by forming bent portions on both sides at both ends of the light extraction surface. In the inner part, a flat active layer without these bent parts is formed wider than the striped light emitting region, and the current and light are confined by the carrier concentration distribution due to so-called current concentration, resulting in a so-called gain. A hybrid semiconductor laser having a guided configuration is constructed.

According to the semiconductor laser having this structure, for example, the refractive index guide operation section is provided at both ends of the light emitting region, and the gain guide operation section is provided at least in part inside the refractive index guide operation section. Since the beam waist position can be obtained and the far-field pattern has excellent symmetry, it is possible to construct a semiconductor laser with low spot distortion, easy focusing, and easy optical design. Since a small semiconductor laser can be obtained, it can be used as a recording or continuous light source for optical video discs, etc., and can be used to accurately record and read data.

[Problem that the invention seeks to solve]

In the case of using the above-mentioned hybrid semiconductor laser, a concave or convex portion is provided in the substrate in advance, and each semiconductor layer such as the first cladding layer and the active layer is formed on this by, for example, MOCVD,
Although epitaxial growth is performed by MBE, the thickness of the first cladding layer grown thereon is different between a wide concave or convex portion and a narrow concave portion. That is, for example, as shown in FIGS. 9 and 10, the cladding in a cross section that crosses the narrow portion of the recess (18) along line A-A in FIG. The thickness dl of (12) is larger than the thickness d2 of the cladding layer (12) in a cross section that crosses the wide part of the recess (18) on line B-B, and the surface of this cladding layer (12) has this A stepped portion due to the difference in thickness occurs near the boundary between the narrow portion and the wide portion of the recess (18). Therefore, this second cladding layer (
12) The active layer (13) formed thereon is bent across the stripe direction of the light emitting region, which has the effect of unnecessary light confinement and deteriorates the characteristics.

The present invention eliminates these drawbacks.

[Means for solving problems]

The present invention comprises a first cladding layer, an active layer, and a second cladding layer that are sequentially deposited on a semiconductor substrate as described above, and includes a refractive index guided structure section and a gain guided structure section. In a semiconductor laser having a hybrid cavity, particularly in the present invention, a first cladding layer and an active layer are formed flat on a flat main surface of a semiconductor substrate, and the refractive index guide type structure is formed in a light absorption region. The gain-guided structure is constructed by providing a current confinement region.

(Function) As described above, in the present invention, the active layer is configured to be flat, and the stripe-shaped resonator, which is a hybrid of the refractive index guide type structure and the gain guide type structure, is active over the entire length in the longitudinal direction. Since the layer is not bent, it is possible to avoid the above-mentioned deterioration of characteristics due to the presence of the bent portion.

〔Example〕

Next, an example of a semiconductor laser according to the present invention will be described, but in order to facilitate its understanding, the description will be made with reference to FIGS. 1 to 4 along with an example of its manufacturing method.

First, as shown in FIG.
A single crystal substrate (21) made of aAs and having one main surface (21a) formed as a flat surface is prepared, and its upper surface (21a) is
The substrate (21) is sequentially coated on top by MOCVD or MBE.
A semiconductor layer (22) constituting a part of the first cladding layer consisting of AI)<Gat-x^S of the same conductivity type as
an active ffji (23) made of Gal-yAs; a semiconductor layer (24A) of another conductivity type, p-type, made of A1)<Ga1-xAs and constituting the do layer portion of the second cladding layer;
Compared to the active layer (23), this active I! (23) Semiconductor layer (25) constituting a P or n type light absorption region made of A12Gat-zz^S and having the effect of absorbing light emitted from 18
and are sequentially epitaxyed.

Next, as shown in FIG. 2, the light-absorbing semiconductor layer (25) is selectively etched to form a light-absorbing region (35) from a portion of this semiconductor layer (25). This light absorption region (35) covers both longitudinal ends of the stripe portion of the resonator component to be finally formed in the active layer (23), that is, the portion facing the tip surface (resonance surface). They are sandwiched and placed on both sides with a distance d corresponding to the resonator width.

As shown in FIG. 3, the light absorption layer (25) is connected to the lower semiconductor layer (24A) of the second cladding layer from which it has been removed, so as to embed the light absorption layer (25). An upper semiconductor layer (24B) of the second cladding layer made of p-type For example, Ga with r11i concentration of the same conductivity type as
Each cap layer (26) made of As is ocvo.

Continuous epitaxy is performed by MBE or the like.

As shown in FIG. 4, along the longitudinal direction of the resonator to be sandwiched between the components of the resonator to be finally formed at a depth from the cap layer (26) to the second cladding layer (24). Ions such as B, He, and protons are implanted onto both sides of the cladding to create a high resistance or second cladding r=<z
4) and a current confinement region (27) of a conductivity type different from that of the cap layer (26).

The cap layer (26
) and the back surface of the substrate (25), electrodes (28) and (2
9) is ohmically deposited to obtain a semiconductor laser (30) according to the present invention.

In the above configuration, the composition of each layer is selected such that x>y>z≧0, and the distance between the active layer (23) and the light absorption layer (24) is such that the light emitted from the active layer (23) reaches For example, a distance of 0.3 μm is selected. Also, the first
The thickness of the cladding layer (22) is, for example, 1 μm.
The thickness of (23) is, for example, 0.05 μm, and the light absorption layer (2
The thickness of 5) can be chosen to be 1 μm.

In this configuration, when a voltage is applied in the forward direction between the electrodes (28) and (29) to energize, the current constriction region (27
) is prevented from forming a current path, the current path is restricted to the area sandwiched by the regions (27), the current path is constricted, and the current is concentrated there.

Therefore, carriers are mainly injected into the stripe portion under the portion of the active layer (23) sandwiched by the current confinement regions (27), and light is generated. At this time, light absorption N (25) is provided on both ends of this stripe section, so that it becomes active here! The light oscillated and seeped out from (23) is absorbed by the light absorption layer (25), so that at both ends of this stripe part, the part facing this light absorption layer (25) and the part not facing this light absorption layer (25) are separated. A difference in effective refractive index occurs between the ends, and a refractive index guide type structure is formed at both ends. However, in the center of the stripe section, since there is no light absorption 1a on both sides, only the current narrowing effect by the current narrowing region (27) exists, resulting in a gain-guided structure.

Thus, according to the semiconductor laser (30) according to the present invention, a hybrid resonator of a refractive index guide type and a gain guide type is configured.

In the above example, both ends of the resonator are of the refractive index guide type and the center part is of the gain guide type. Various arrangement configurations can be adopted, such as having both ends of the gain guide type. Further, the conductivity type of each part can be selected to be the same conductivity type opposite to that shown in the drawings, and various other changes can be made.

〔Effect of the invention〕

As described above, according to the present invention, a hybrid semiconductor laser is constructed that includes both a refractive index guide type and a gain guide type structure, so as described above, spot distortion is small.
Although it is possible to obtain a laser with low noise due to returned light, in particular in the present invention, since the active layer has a flat structure, characteristics due to unnecessary light confinement and reflection that occur when the active layer is bent are reduced. Deterioration can be avoided.

[Brief explanation of the drawing]

1 to 4 are enlarged perspective views of each process diagram of a manufacturing method for obtaining an example of a semiconductor laser according to the present invention, and FIGS. 5, 6, and 7 are respectively different views of conventional semiconductor lasers. Similarly, FIG. 8 is an enlarged plan view of an example of a conventional semiconductor laser, and FIGS. 9 and 10 are enlarged linear cross-sections on line A-A and line B-B in FIG. 8, respectively. 11 is an enlarged perspective view of FIG. 8. (21) is the substrate, (22) is the first cladding layer, (23
) is the active layer, (24) is the second Kura Nodo layer, (35)
is the light absorption region.

Claims (1)

[Claims] A semiconductor comprising a first cladding layer, an active layer, and a second cladding layer sequentially deposited on a semiconductor substrate, and having a hybrid resonator of a gain-guided structure and a refractive index-guided structure. In the laser, one principal surface of the semiconductor substrate is a flat surface, and the first cladding layer and the active layer are each formed flat on the flat principal surface, forming the gain-guided structure. A semiconductor laser comprising a current constriction region and a light absorption region constituting the refractive index guide type structure.