JPS607788A - Semiconductor laser - Google Patents

Abstract

PURPOSE:To obtain a semiconductor laser capable of laser oscillation of basic lateral mode in a low threshold current by a method wherein the P type current narrowing layer having a stripe groove part in its center is laminated on the N type semiconductor substrate and the same parts of an N type clad layer and further of a P type active layer are arranged on said current narrowing layer so as to be close to the substrate by utilizing the groove part. CONSTITUTION:The P type CaAs current narrowing layer 22 having a stripe groove part 21 in its center is formed on the N type GaAs substrate 20 and an N type Al0.45Ga0.55As first clad layer 23 is laminated on said layer 22 to fill the groove part 21 with arching inversely. Next, a P type Al0.15Ga0.85As active layer 24 similarly curving downward is grown on the groove part 21 and this curved part is made to be brought to the sustrate 20. After that, a P type Al0.45Ga0.55As second clad layer 25 and a P type GaAs ohmic contact layer 26 are deposited with laminating and the first electrode 27 and the second electrode 28 having the predetermined shape respectively are formed on the front surface and the back surface of the substrate 20.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical field to which the invention pertains] The present invention relates to a semiconductor laser, and more particularly to a buried type semiconductor laser whose oscillation mode is controlled.

[Prior art and its problems]

Generally speaking, in order to achieve continuous oscillation under high temperatures in a semiconductor laser, the original energy and injected current are confined to a specific region characterized by optical loss and wasteful recombination. It is necessary to make it a structure. Therefore, a so-called electrode stripe type semiconductor laser has appeared in which the electrode of the semiconductor laser is used as a stripe electrode to partially concentrate the current flowing through the active layer 3 and simultaneously concentrate the optical energy.

This electrode striped semiconductor laser reduces heat generation and enables continuous oscillation at room temperature.

The major drawbacks in terms of characteristics were the instability of the electromagnetic wave mode parallel to the active layer, that is, the transverse mode, and the change of the transverse mode in response to changes in the injection current.

This is because the electrode stripe type does not have a sufficient confinement structure for carriers and light in the lateral direction of the active layer. In other words, laser oscillation in a current range well above the threshold current is caused by zero-order or low-order mode oscillation because the gain necessary for oscillation exceeds the loss only in the active region directly under the stripe YG electrode. It is.

However, as the injection current is further increased, the region in which carriers are injected into the active layer expands, and the high gain region expands, so that higher-order modes occur and the transverse mode becomes unstable.

The instability of the transverse mode and the dependence on the flow of the injection section cause errors in tracking and the like when such a laser with unstable four negative modes is used in a pickup for an original disk. This is why a buried semiconductor laser is needed to compensate for the instability of the transverse mode.

For example, it was devised in Japanese Patent Publication No. 56-53237.

The outline of this embedded type semiconductor laser will be explained with reference to FIG.

This embedded semiconductor laser consists of a semiconductor substrate (1), a current confinement layer (2) consisting of a diffusion layer, a first cladding layer (3) that confines light and carriers, and a concave central part that is cup-shaped. The structure includes an active layer (4) that confines light and carriers, a second cladding layer (5) that confines light and carriers, and an ohmic contact layer (6).

A central portion of the active layer (4) is located within a groove (7) formed by the semiconductor substrate (1) and the current confinement layer (2).

The first electrode (8) and the second electrode (9) are provided on the ohmic contact layer (6) and the semiconductor substrate (1), respectively, and are provided on the first cladding layer (3) and the active layer (4). A forward biased first rectifying match 00) is formed at the interface.

The semiconductor substrate (1) and the cladding layer (3) of the frame 1 are of a conductivity type different from that of the Kamelyu narrow layer (2), and the semiconductor substrate (
1) and the second rectifying matching side at the interface of the current confinement layer (2).

A third rectifying junction α is generated at the interface between the current confinement N(2) and the first cladding layer (3). When the first laminar junction (10) is forward biased, the second rectifying junction (11) is forward biased and the third rectifying junction σ2) is reverse biased.

For example, if the semiconductor substrate (1) is N-type GaAs, the current confinement layer (2) is P-type GaAs, m 1 cladding layer (
3) is N-type GaAJAs, active layer (4) is P-type GaA
s, the second cladding layer (5) is P-type GaAgAs,
The ohmic contact layer (6) can be made of P-type GaAs. The center of the active layer (4) is a groove (7)
It is shaped like a bowl adjacent to the upper end of the .

When the first rectifying junction 00) is forward biased, the dead bodies generated as a result of carrier recombination are sandwiched between the first cladding layer (3) and the second cladding layer (5) with a high refractive index. is guided to the active layer (4). Therefore, the laser oscillation region is limited to ~the center of the active layer (4).

However, in this buried type semiconductor laser, the width of the groove (7) is 4 to 6 μm, which makes the process of forming the groove (7) and the crystal growth process difficult. There was a problem in that the width of the shape could not be narrowed, and laser oscillation could not be sufficiently set to the fundamental transverse mode.

[Purpose of the invention]

The present invention has been made in consideration of the above-mentioned problems, and it is an object of the present invention to provide a semiconductor laser capable of laser oscillation in a fundamental transverse mode with a low threshold current.

[Summary of the invention]

In the present invention, a current confinement layer of a second conductivity type is partially formed on a semiconductor substrate of a first conductivity type to form a first cladding layer of the first conductivity type, an ohmic contact layer of the first conductivity type, or a second conductivity type ohmic contact layer. In a semiconductor laser, an active layer is formed in a bowl shape through a first cladding layer in a groove portion that reaches a semiconductor substrate in which a current confinement layer is not formed.

A semiconductor laser characterized in that the active layer is closest to the semiconductor substrate at the center of the bowl shape, and the first cladding layer has a specific resistance of 5×10 −2 to 1 Ω.

〔Effect of the invention〕

The central part of the active layer's bowl shape is closest to the semiconductor substrate,
In addition, since the first cladding layer has a relatively high specific resistance of 0.05 to 1 Ω, the level of current injection into the active layer changes depending on the distance difference between each part of the active layer and the semiconductor substrate.

Therefore, the current is concentrated at the center of the groove, and since the active layer has a circular shape, light is sufficiently confined in the lateral direction, resulting in a stable laser with a low threshold value and a fundamental transverse mode.

[Embodiments of the invention]

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

FIG. 2 is a sectional view showing a typical example of a GaMAs semiconductor laser, and its structure will be explained below along with its manufacturing method.

A P-type GaAs layer is formed by liquid phase growth on a semiconductor substrate C20) with an impurity concentration of 1 to 3×10 18 /about 3 N-type GaAs.
It is formed to a thickness of 1.5 μm by vapor phase growth, diffusion, etc.

The P-type impurity concentration may be slightly higher than the basic concentration.

Next, a photoresist film is attached and exposed to a width of 2 μm.
A stripe-shaped window is provided and used as a mask during selective etching to form a stripe-shaped groove (2I) and a current confinement layer (22). The depth of the groove (21) is such that it comfortably reaches the semiconductor substrate (2).

In this case, it is 2 μm. The groove (21) contains a conventional chemical etching solution, namely 1 part phosphoric acid, 1 part hydrogen peroxide, and 5 parts methanol.
The desired depth can be obtained by etching at 20"O for about 3 minutes. At this time, the striped windows are opened parallel to the (100) direction.

This groove C! The side wall of 1) is slightly drum-shaped. This groove (
The width of 21) is 6μ at the bottom of the groove, 4.5μ at the constriction above it, and 5μ at the top. After forming the groove Qυ.

Remove the photoresist film. After that, it is continuously grown by liquid phase epitaxial growth. Specific resistance is 5×1
0 to 1 Ωm, N type 1'J o, 45 Ga o,
The first cladding layer (23) made of 56As is placed in the groove 01.
), the growth ends in an arcuate state of sinking, and on top of that an active layer consisting of P-type A13o-+aGao, 55As (
24j is grown in a similar arcuate shape to form a dog-like shape, with the center thereof being closest to the semiconductor substrate c20).

Next is P type M. A second cladding Jci C2ω made of Gao,..., As, and an ohmic contact layer (26) made of P-type GaAs are grown. The thickness of each layer is as follows:
is approximately 1 μn1, and the active layer (cladding layer G!5 of 2 squares i, o, x μm, i2) is 1.3 μm. Also, groove C2
1) The first layer between the active layer (24) at the edge and the semiconductor substrate (20)
The thickness of the cladding layer (2) is 2.1 μm or more. Finally, the first electrode Qη is connected to the ohmic contact layer (3
A second electrode (marked 2) is placed on the upper surface of the semiconductor substrate (fi).
20).

GaAA! The As semiconductor laser has a 4F+) structure as described above, and during operation, the first electrode (2η is negative, the second electrode is 08
) is positive, the active layer H and the first cladding layer (
The first rectifying junction 09) consisting of 231 is forward biased.

The second rectifying junction example consisting of the first cladding layer (23) and the current confinement layer <22+ is reverse biased, and therefore the current flows concentrated in the center of the groove 01J with the lowest resistance.

The light generated by recombination becomes laser oscillation when the gain exceeds the loss, but in this case, since the active i (24) is curved, the light is also confined in the 4-yellow direction and oscillates in the fundamental Yang mode. Note that when the specific resistance of the first cladding m (23) is less than 5 × 10 Ω·box, it is difficult to concentrate the current at the center of the active B′ (2, D), and when the resistivity exceeds 1Ω·box. It seems difficult to oscillate at a low threshold current.

The current concentration and the active layer's circular shape as described above result in a semiconductor laser that oscillates in the fundamental transverse mode with a low threshold current. Further, in this example, an N-type semiconductor substrate was used as the semiconductor substrate (2o), but a P-type semiconductor substrate was used as the current confinement layer (221).
is N type, the first cladding layer (23) is P type, and the second cladding layer G! It is clear that a semiconductor laser in which ω is of N type can also be applied.

[Brief explanation of drawings]

FIG. 1 is a sectional view showing a conventional example, and FIG. 2 is a sectional view not showing an embodiment of the present invention. 20... Semiconductor substrate, 21... Groove portion, 22... Current confinement layer 23... First cladding layer, 24... Active layer 25... Second cladding layer, 26... Ohmic contact layer 27.28. ,,electrode

Claims (1)

[Claims] A current confinement layer of a second conductivity type is partially formed on a semiconductor substrate of a first conductivity type, and a first cladding layer of a first conductivity type is formed on a semiconductor substrate of a first conductivity type.
Or a second conductivity type active layer. A second cladding layer of a second conductivity type and a second conductive ohmic contact layer are sequentially formed, and the first In a semiconductor laser in which an active layer is formed in a bowl shape through a cladding layer, the active layer is closest to the semiconductor substrate at the center of the bowl shape of the active layer, and The specific resistance of the cladding layer is 5X10”-
A semiconductor laser whose %, g represents a 1Ω tooth.