JPS5949718B2 - Semiconductor laser device and its manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a semiconductor laser device and a method for manufacturing the same.

Various proposals have been made regarding the structure and manufacturing method of semiconductor lasers that operate in a stable fundamental transverse mode.

Stabilization of the oscillation mode is achieved by creating a refractive index difference in the direction of the junction surface of the active layer. One of the structures is the first
A semiconductor laser having an active layer 3 formed into a rib-type optical waveguide as shown in the figure has been proposed. This structure has a mechanism in which the active layer is partially thickened to create a difference in the equivalent refractive index of light propagating within the active layer, thereby confining the light to the thick portion. In this way, by providing the active layer with a built-in refractive index change, stable transverse mode oscillation can be obtained. The manufacturing method of this device is as follows: First, an n-type Ga7-xAlxAs cladding layer 2 and a GaA
s The active layer 3 is grown, a part of the active layer 3 is masked in a stripe shape, and the surface of the other active layer is slightly etched to make a difference in the thickness of the active layer 3 between the stripe part and the stripe part.

After that, a p-type Gal-xAlxAs cladding layer 4 and a p-type GaAs ohmic contact formation layer 5 are formed on the active layer 3.
to grow again. A 5102 film 6 is attached to the surface thereof, and an ohmic contact electrode 7 for current injection is formed directly above the thickened portion of the active layer. Ohmic electrode 7' also on the n side
Create and complete. In manufacturing this laser, two crystal growth steps are required to once etch the surface of the active layer, and in addition, the etching of the active layer 3 must be delicately controlled, which is a difficult process problem. I have it too.

Furthermore, since the surface of the active layer 3 is exposed during etching, it is thought that a large number of non-radiative recombination centers are introduced into the interface between the active layer 3 and the cladding layer 4. The present invention provides a new structure of a laser device and its manufacturing method, which provides a rib-type optical waveguide structure in the active layer through a single crystal growth process and realizes stable transverse mode oscillation, unlike the structure formed by the above-mentioned manufacturing method. The present invention will be described below based on embodiments with reference to the drawings.

First, as shown in FIG. 2a, there is a step-like step T on the surface of the substrate 8.
form.

Next, a first cladding layer 9, a second active layer 10, a third cladding layer 11, and a fourth ohmic electrode forming layer 12 are successively grown on the substrate 8 (FIG. 2b). An insulating film 13 is attached to the surface of the substrate, and a striped window 16 for current injection is formed directly above the step T on the substrate. After performing the diffusion for ohmic contact, the electrode metal film 14
Add. Also, an ohmic electrode 15 is provided on the substrate side to fabricate a laser element piece (FIG. 2c). When crystal growth is performed on a substrate with a step T, in the early stage of growth, the step T
It has a characteristic that the growth rate of the flat part is faster than that of the flat part.

This characteristic is particularly remarkable regarding the growth of the first layer 9. Therefore, under appropriate growth conditions, the thickness of the first cladding layer 9 is sufficient to prevent absorption of light immediately above the stepped portion T, and is thick enough to prevent absorption of light just above the stepped portion T. In the portion (hereinafter referred to as flat portion), all the light emitted from the active layer 10 can be absorbed by the substrate 8 by leaking to the substrate 8. GaAs-G
In the aAlAs-based laser, the first layer of n-GaO. 7A
lO. The thickness of the 3As layer 9 is 0.5 μM in the flat part, O. 3
μM, O. As the thickness changes from 2 μm, the light absorption coefficient changes from 100CffL−1,30 axis [1,1000 to 4, respectively. Moreover, when the thickness of the first layer 9 is 1 μm or more,
Almost no light is absorbed by the substrate 8, and the absorption coefficient is 10 (17
7! It is about -1. Therefore, in order for the substrate 8 to absorb the light emitted from the flat portion, the thickness of the first layer 9 should be 0.5 μm or less, and the thickness of the first layer directly above the step should be 1 μm or more. desirable. The active layer 10 has two bends 17 and 18 as shown in FIG. When there are bends 1T and 18, the active layer 10 between the bends 17 and 18 and the bends 17 and 18
Since the oscillation modes at the outer flat portions are different, a unique laser mode will inevitably stand between the two bends 17 and 18. Moreover, as mentioned above, the thickness of the active region 10 between the bends 17 and 18 is 10 to 20 times thicker than that of the flat part.
% thicker, a rib-type optical waveguide structure is naturally formed, and light is confined between the two bends 17 and 18. By the way, if the height of the step on the substrate is 2 μm, the interval between the two bends 17 and 18 will be about 5 μm, and the oscillation transverse mode may be TEO.

In order to oscillate a stable TEOO mode, ΔF10-3
Therefore, the width of the oscillation region needs to be 3 μm or less. This requirement is automatically satisfied in the structure of FIG. 2c. The reason for this is that although the distance between the bends 17 and 18 is approximately 5 μm, the thickness of the first layer 9 continuously decreases from the center toward the two bends 17 and 18, and therefore almost no absorption occurs at the center. However, as it approaches the bend, it is absorbed by the substrate, and the crystal oscillation mode becomes single near the center. Therefore, the laser of the present invention can stably perform single transverse mode oscillation. Current injection is performed by a striped electrode 14 provided directly above the stepped portion T.

Although FIG. 2C shows an example of an oxide stripe structure, any stripe structure other than this structure is possible. The stripe width reduces the spread of current, so
The thickness is preferably 10 μm or less. Since a laser having this structure does not require etching of the active layer, it can be easily manufactured with only one crystal growth step, and various problems that arise with lasers having a rib type optical waveguide structure are also solved.

A method for manufacturing a semiconductor laser having this new structure will be described below using a specific example. Example GaAs-Gal-XA! on an n-type GaAs substrate! XAs
A semiconductor laser constructed by is shown below.

As shown in FIG.
A step T of m is formed.

On the substrate 8 with this step T, the growth starting temperature is 845°C.
The first layer is n-type Ga, -XAIxAs with a thickness of 0.2 μm (thickness in the flat part), and the second layer is non-doped Ga, -, Al, by a liquid phase epitaxial method at a cooling rate of 0.58 C/min.
AslO is 0.1 μm (thickness in the flat part), third layer p-type Ga, -XIAlx7Asll is 1.5 μm 1 fourth layer
A layer of p-type GaAsl2 is continuously grown to a thickness of 1 μm. At this time,
The thicknesses of the first layer and second layer immediately above the step portion are 1 μm and 0.12 μm, respectively. Subsequently, a SlO2 film 1 is formed on the growth surface.
3, and a striped window 16 with a width of 5 μm is formed directly above the position of the step T on the substrate. Zinc for ohmic contact is diffused into the fourth p-type GaAsl2 layer exposed in a striped pattern, and then metal for the p-type electrode is vacuum deposited and alloyed to form the p-side ohmic electrode 14. Further, a metal for an n-type electrode is vacuum-deposited on the substrate 8 side, and an alloying process is performed to form an n-side ohmic electrode 15.
GaAs-Ga,-XAIxA produced in this way
The s semiconductor laser operated in a continuous oscillation mode in a stable transverse fundamental mode at room temperature, and the transverse mode remained stable even at a current value five times the threshold value.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view of a conventional laser device having a rib-type optical waveguide structure, and FIGS. 2a-2c are cross-sectional views of a manufacturing method according to an embodiment of the present invention. 8... N-type GaAs substrate, 9...G
al-XAlxAs layer, 10...n-type Ga, -
YAlyAs layer, 11... p-type Gal-FA
lx'As layer, 12...p-type GaAs layer, 13
... SiO2 film, 14 ... Metal film for p-side electrode, 15 ... Metal film for n-side electrode.

Claims (1)

[Scope of Claims] 1. A cladding layer is formed on a semiconductor substrate having a step to a thickness that prevents absorption of light in the step portion and a thickness that allows light to be absorbed by the substrate in portions other than the step portion. An active layer is formed on the cladding layer so as to be inclined at the stepped portion, and a current limiting layer for limiting the injection current is formed at a position facing the stepped portion, and the thickness of the active layer is is thicker above the stepped portion than at other portions. 2. A step of forming a step on the surface of the semiconductor substrate, and forming a cladding layer on the semiconductor substrate to a thickness that prevents absorption of light in the step portion and a thickness that allows light to be absorbed by the substrate in portions other than the step portion. and forming an active layer on the cladding layer so that it is inclined at the stepped portion, and the thickness of the active layer is thicker above the stepped portion than at other parts. A method for manufacturing a semiconductor laser device, comprising the steps of: forming a current limiting layer for limiting the injection current at a position facing the step portion;