JPH01100985A - Semiconductor laser - Google Patents

Abstract

PURPOSE:To make the effective refractive index change gently in the direction of a junction to expand a region where a fundamental mode stably operates, and enable the high output operation of the fundamental mode, by forming an active layer flatly, and changing the the thickness of crystal layer in a multilayer type or an inclination type. CONSTITUTION:By processing twice the trench of a substrate 1, the thickness of a N-AlyGa1-yAs clad layer 2 is gently changed. The trench processing is performed, for example, as follows. Firstly, a 6mum-wide stripe type window is made in photo resist formed on the substrate 1, and etched slightly to a depth of about 0.5mum by using ammonia system etching liquid. By a second lithography process, a 4mum-wide stripe type window is made in the photo resist. By using phosphoric acid system etching liquid, a trench about 2mum-deep is formed, which has a shape like an inverted trapezoid. Thus a trench having a two-step inclined surface of different inclination angle is formed. The effective refractive index in the vicinity of the active layer 3 has a distribution wherein the trench form made on the substrate 1 is turned upside down.

Description

DETAILED DESCRIPTION OF THE INVENTION Summary of the Invention In a semiconductor laser with a flat active layer, the change in the effective refractive index in the direction of the junction surface is made gentle by changing the crystal layer thickness in multiple stages or in a sloped manner. This expands the basic mode stable operation region and also enables high output operation.

BACKGROUND OF THE INVENTION TECHNICAL FIELD This invention produces an effective refractive index change near an active layer by changing the crystal layer thickness depending on the location, and achieves confinement of a transverse mode of laser light by changing the refractive index. This invention relates to a semiconductor laser capable of fundamental mode oscillation.

Prior art and its problems V SIS (V-Channeled 5ubs) is an index-guided laser that changes the effective refractive index depending on the location by changing the crystal layer thickness.
trateInner 5tripe), CS
P (Channeled 5ubstratePla
nar), ridge waveguide laser, etc. In these semiconductor lasers, since the change in crystal layer thickness is step function-like when viewed in the direction of the junction plane, the effective refractive index also changes rapidly accordingly.

When the drive current is increased in order to increase the light emission output of the semiconductor laser described above, a phenomenon (plasma effect) occurs in which the effective refractive index is lowered by injected carriers, and this is the limit of fundamental mode operation. The fundamental mode operating limit is determined by the magnitude relationship between the amount of reduction in the effective refractive index due to the injected carriers and the effective refractive index difference that is structurally determined by the change in the crystal layer thickness. Therefore, in the conventional semiconductor laser structure, since the effective refractive index changes in the form of a step function as described above, there is only one critical point of the fundamental mode operation limit, and the upper limit of the output is determined by this critical point. Higher output operation was not possible.

These problems will be explained with reference to the drawings.

A C8P laser and a ridge waveguide laser are shown in FIGS. 6 and 7, respectively, as examples of realizing a refractive index waveguide laser by changing the crystal layer thickness.

In the C8P laser shown in Fig. 6, the n-
An n-A,gGay 1-yAs cladding layer 2 is grown on the GaAs substrate 1 so as to completely fill the recess, and then an AJGaAs active layer 3 is grown to be xL-x flat, and then pAlGaAsy is grown to be flat.
1-y The cladding layer 4 and the n-GaAs cap layer 5 are successively grown. In the device manufacturing process, a p-Zn diffusion region 6 for current confinement is formed by Zn diffusion, and a p-electrode 7 and an n-electrode 8 are formed as ohmic electrodes on both sides of the chip.

Is C8P laser n-Aj? GaAs crack)'
This is because the thickness of the ty layer 2 is different between the recess and both sides thereof.

A,g The amount of guided light generated in the GaAs active layer 3 seeping into the x1-xrl-GaAs substrate 1 differs depending on the location. The greater the amount of light seeping out, the greater the decrease in the refractive index, so the distribution of the effective refractive index in the bonding direction reflects the processed shape of the substrate 1 and becomes convex, which is the opposite of this shape.
The effective refractive index changes rapidly in a step-like manner.

In the ridge waveguide laser shown in Figure 7, the flat n-GaAs
An n-Al GaAs layer-3' rad layer 2 is formed on the substrate 1. Ajj Ga As active layer 3. p-
x 1-x AJ Ga As cladding layer 4 and p-Gay
ty As cap layer lO is continuously grown. After this, in a device formation step, a ridge portion consisting of a part of the cladding layer 4 and the cap layer IO is formed by etching using an RIE apparatus or the like. And an insulating film 9 (for example, S io 2
) is formed on the surface other than the top surface of the ridge portion, and a p-electrode 7 and an n-electrode 8 are formed as ohmic electrodes on both sides of the chip.

In the ridge waveguide laser, the amount of reduction in the refractive index is determined by how thick the p-AJI GaAs cladding layer 4 y ty is left during ridge processing. The amount of decrease in the refractive index increases as the thickness of the remaining p-AJGaAs Y1-y rad layer 4 becomes thinner, so the distribution of the effective refractive index in the junction direction reflects the shape of the ridge processing and It also has a step-like convex shape.

The decrease in refractive index due to injected carriers is shown in Figure 6.
This occurs in the substrate processing area in the laser, and in the so-called current concentration region in the ridge processing area in the ridge waveguide laser shown in FIG. operates in basic mode. Furthermore, the smaller the effective refractive index difference (or the difference between the effective refractive index difference and the amount of decrease in the refractive index), the easier the fundamental mode operation becomes.

Conversely, if the effective refractive index difference is large, higher-order modes are likely to occur.

In the conventional structure shown in FIGS. 6 and 7, the change in layer thickness during substrate processing or ridge processing is rapid and large, so as mentioned above, the change in the effective refractive index in the bonding direction is even greater. It is a step function. Therefore, the upper limit of the current injection amount for basic mode operation is uniquely determined,
The upper limit of the light output was also determined accordingly. Furthermore, since the difference between the effective refractive index difference due to a change in crystal layer thickness and the amount of decrease in refractive index due to injected carriers becomes close near the upper limit of current injection, the fundamental mode stable operation region is also limited to this narrow region.

SUMMARY OF THE INVENTION OBJECTS OF THE INVENTION An object of the present invention is to provide a semiconductor laser capable of fundamental mode operation over a wide range and at high output.

Structure and Effects of the Invention The present invention provides an index-guided semiconductor laser in which the effective refractive index is changed by varying the crystal layer thickness depending on the location, and the effective refraction is improved by making the crystal layer thickness change multi-stage or inclined. It is characterized in that the change in the ratio in the bonding direction is gradual and the active layer is formed flat.

According to this invention, the active layer is formed flat and the crystal layer thickness changes in multiple stages or in a sloped manner.

The effective refractive index can be changed gradually in the bonding direction. Therefore, the critical points of the fundamental mode operating limit come to exist continuously, the fundamental mode stable operating region widens, and high output fundamental mode operation becomes possible.

The flatness of the active layer greatly contributes to the ease of fundamental mode oscillation and to the elevation of the mountain.

DESCRIPTION OF THE EMBODIMENT FIGS. 1 and 2 show an example of a C8P laser, and the same parts as shown in FIG. 6 are given the same reference numerals.

In the structure shown in FIG. 1, the thickness of the n-Al GaAs cladding yt-y layer 2 is made gentler by forming grooves on the substrate 1 twice. Grooving of the substrate 1 is performed, for example, as follows. A photoresist formed on a substrate 1 by a photolithography process is first coated with 6
A striped window with a width of .mu.m is opened and lightly etched to a depth of about 0.5 .mu.m using an ammonia-based etching solution at 50.degree. Thereafter, a second photolithography process is performed to create a striped window in the photoresist with a width of 4 μm, the center line of which coincides with the center line of the first stripe pattern. Then, a groove with a depth of about 2 μm, which looks like an inverted trapezoid, is formed using a phosphoric acid etching solution.

By doing so, a groove with two slopes having different inclination angles is formed. The subsequent crystal growth process and device fabrication process are similar to those shown in FIG.

The effective refractive index near the active layer 3 has a distribution in which the shape of the groove formed in the substrate 1 is upside down.

FIG. 2 shows an example in which the grooves on the substrate 1 are processed three times. In this case as well, the thickness of the n-A Ga1-7As cladding layer 2 changes gradually. For example, the groove machining is performed as follows.

A window in the form of a stripe with a width of 8 μm in the first step and a stripe with a width of 6 μm in the second step is formed in the photoresist using a photolithography process, and the windows are lightly etched to a depth of about 0.2 μm using a phosphoric acid-based etching solution. Etch twice. After that, the third photolithography process made the film 4 μm thick.
Windows are opened in the photoresist in the form of stripes of width, and grooves resembling inverted trapezoids are formed with a depth of approximately 2 μm using a phosphoric acid etching solution. In this case as well, the crystal growth process and device fabrication process are the same as those shown in FIG. The effective refractive index distribution has a cross-sectional shape similar to that of the processed groove.

3 to 5 show examples of ridge waveguide lasers, and the same components as those shown in FIG. 7 are given the same reference numerals. Further, the crystal growth process is the same as that shown in FIG.

In FIG. 3, as an example of the ridge processing method, by etching with a sulfuric acid-based etching solution, the thickness of the p-A J Ga 1□As layer 4 is gradually changed only at the ridge portion, as shown in the figure. can do.

In the structure shown in FIG. 4, the ridge portion is manufactured as follows. Referring to FIG. 5, the first (first) photolithography process leaves a resist with a width of 6 μm and the second resist with a width of 8 μm, and the resist is lightly etched with a phosphoric acid-based etching solution to a width of about 0.2 μm. Etch twice in total depth. After that, the third photolithography process leaves resist in the form of 4 μm wide stripes and RI
The entire surface may be uniformly etched using an E device.

Although an AjtGaAs semiconductor laser is used in the above embodiment, it goes without saying that the present invention can be applied to semiconductor lasers using other materials. Further, the change in crystal film thickness is not limited to the above embodiment, and may be changed in one step or three or more steps. Furthermore, the crystal layer thickness may be changed by a crystal growth process (meltback).

[Brief explanation of the drawing]

FIGS. 1 and 2 show an example in which the present invention is applied to a CSP laser, and are a cross-sectional view and a refractive index distribution diagram. 3 to 5 show an example in which the present invention is applied to a ridge waveguide laser, and FIGS. 3 and 4 are cross-sectional views,
FIG. 5 shows the beam machining process. 6 and 7 are cross-sectional views showing conventional examples, with FIG. 6 showing a C8P laser and FIG. 7 showing a ridge waveguide laser, respectively. 1-n-GaAs substrate. 2...n-Aβ GaAs cladding layer. y1-y3-AI GaAs active layer. x 1-x 4 p-Al1 Ga As cladding layer. y1-y5...n-GaAs cap layer. 6... p''-Zn diffusion region. 7... p-electrode, 8... n-electrode. 9... insulating film. 10... p-GaAs cap layer.

Claims (1)

[Claims] In a refractive index waveguide semiconductor laser in which the effective refractive index is changed by varying the crystal layer thickness depending on the location, the change in the effective refractive index in the direction of the junction surface is made gentler by changing the crystal layer thickness in multiple stages or in a tilted manner. 1. A semiconductor laser characterized in that the active layer is flat and has a flat active layer.