JPS60145691A - Semiconductor laser - Google Patents

Abstract

PURPOSE:To obtain the titled device which stably oscillates on single lateral mode at a low threshold value and is excellent in frequency characteristic by a method wherein the side surface of a stepwise section containing the first- third semiconductor layers is provided with a double hetero structure where an active layer is sandwiched by the first and second clad layers having forbidden widths larger than that of the active layer. CONSTITUTION:An insulation semiconductor substrate 10 is provided with step 40 containing a double hetero structure formed of the first semiconductor layer 50, second semiconductor layer 60, and third semiconductor layers 70a and 70b made of two layers. The forbidden band widths of the semiconductor layers 50, 70a, and 70b are not larger than that of the active layer 15, and the forbidden band width of the semiconductor layer 60 is larger than that of the active layer 15. The side surface of this step 40 is provided with the double hetero structure of a semiconductor laser consisting of the first clad layer 11, the active layer 15, and the second clad layer 12. The light emitting region of this laser turns into the active layer 15 on the side surface of the step 40.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a semiconductor laser that oscillates in a stable single transverse mode with a low threshold value and has excellent frequency characteristics.

As a semiconductor laser, stable single-transverse mode oscillation with a low threshold is an important condition when used as a light source for optical communication and original information processing, and the frequency characteristics also depend on the amount of information transmitted, information processing, etc. It is an important element related to Conventional semiconductor lasers employ a buried structure, and effectively inject current into the light emitting region by reverse biasing the pn junction to lower the threshold voltage. It is important to reduce the capacitance of the junction.

However, in order to reduce the capacitance of this pn reverse bias junction, it is necessary to reduce the impurity concentration (doping concentration) of the p-type or n-type semiconductor layers that form the pn reverse bias capacitance. The impurity concentration cannot be easily reduced because it affects the static characteristics of the laser, the yield rate, and the temperature characteristics. For example, autodoping of impurities during crystal growth causes problems such as thinning of the n-current blocking layer and inversion to p-type.

Also, although it is effective to introduce a low concentration layer into the pn reverse bias junction, the thickness of the two layers needs to be thin in order not to significantly change the BH tank structure, and it is difficult to widen the depletion layer significantly. I could not do it.

FIG. 1 is a vertical cross-sectional view of a conventional semiconductor laser of this type, showing a semiconductor laser having a buried structure formed on an insulating substrate 10. As shown in FIG. In order to efficiently inject current into the active layer 15, an n-type current blocking layer 17 is provided between the p-type buried layers 16 and 18, and the reverse bias of the pn junction formed there allows the current to be injected into the active layer 15 except on both sides of the central mesa 80. The current is blocked. Therefore, it has a pn reverse bias junction capacitance, and this capacitance deteriorates the high frequency characteristics of the laser.

In order to reduce this pn reverse bias junction capacitance, it is necessary to reduce the doping concentration of the p layer or n layer that forms this junction capacitance, but if the doping of the n-type current blocking layer F is reduced, the current blocking Layer discontinuities occur due to autodoping. Furthermore, if an attempt is made to increase the thickness of the n-type block layer 170 to avoid autodoping, the n-type block layer 17 is likely to be stacked on the mesa 80 in the center, causing a problem. In addition, p-type buried layer 1
If the doping amount of 6.18 is reduced below a certain level, the oscillation threshold will increase by one level.

As described above, the layer thickness and doping amount of the layer forming the pn reverse bias cannot be changed significantly. Therefore, it was difficult to significantly reduce the pn reverse bias junction capacitance, and the high frequency characteristics were not good. Further, the insulating substrate 10 was only provided with insulation in order to form electrodes thereon.

SUMMARY OF THE INVENTION An object of the present invention is to solve these problems and to provide a semiconductor laser that oscillates in a stable single-transverse mode with a low threshold and has excellent frequency characteristics.

The structure of the semiconductor laser of the present invention includes, on an insulating semiconductor substrate, a first semiconductor layer having a smaller band gap than the active layer, a second semiconductor layer having a band gap larger than the active layer, and a second semiconductor layer having a band gap larger than the active layer. A step-like step portion including a third semiconductor layer consisting of several layers having a forbidden band width smaller than that of the active layer; It is characterized by having a double heterostructure in which the active layer is sandwiched between second cladding layers.

According to the configuration of the present invention, the first semiconductor layer, the second semiconductor layer, and the third semiconductor layer form a refractive index difference in the active layer, and the refractive index waveguide can cause single-transverse mode oscillation. . Furthermore, the refractive index waveguide width is approximately the value obtained by multiplying the second semiconductor layer thickness by the reciprocal of the sine of the inclination angle of the stepped step side surface, and is determined by the second semiconductor layer thickness. Furthermore, current is efficiently injected into the active layer on the side surface of the stepped step, which is the light emitting region, due to the current blocking effect of the insulating substrate and the third semiconductor layer. In addition, this current blocking is performed in the insulating semiconductor substrate and also in the pn reverse bias junction. Since the layer thickness is almost independent of the static characteristics of the laser, it is possible to increase the thickness of this third semiconductor layer and change the impurity concentration in this layer. A thick low concentration impurity layer can be formed. Therefore, the pn reverse bias junction capacitance can be significantly reduced and the high frequency characteristics can be improved.

The present invention will be explained in detail below with reference to the drawings.

FIG. 2 is a cross-sectional configuration diagram of an embodiment of the present invention.

In this embodiment, a first semiconductor layer 50. a second semiconductor layer 60 and a third semiconductor layer made of two layers;
It has a stepped step 40 including a double heterostructure formed of semiconductor layers 70a and 70b.

The first semiconductor layer 5 forming this double heterostructure
The forbidden band widths of the 0 and third semiconductor layers 70a and 70b are not larger than the forbidden band width of the active layer 15, and the forbidden band width of the second semiconductor layer 60 is larger than the forbidden band width of the active layer 15.

A first cladding layer 1 is formed on the side surface of this stepped step 40.
1. Active layer 15. It has a double heterostructure of a semiconductor laser consisting of a second cladding layer 12.

This laser emitting region becomes the active layer 15 on the side surface of the stepped step 40. In addition, 13 is a cap layer, 20.30
is an electrode.

No current is injected into the active layer 15 on the bottom surface of the stepped step 40 by the insulating semiconductor substrate 10, and the second semiconductor layer 60 is injected into the active layer 15 on the top surface of the stepped step 40. and third semiconductor layers 70a and 70b
No current is injected due to the pn junction reverse bias formed by the pn junction, and therefore current is effectively injected into the active layer 15 on the side surface of the stepped step 40.

FIG. 3 is a schematic diagram illustrating the oscillation transverse mode near the light emitting region of FIG. 2. FIG. Active layer 1 on the side of stepped step 40
5, the first semiconductor layer 50 or the third semiconductor layer 70a, which has a smaller forbidden band width than the active layer 15, is located near the active layer in parts A and d in the figure.

70b exists, and there is no semiconductor layer with a small forbidden band width nearby in portion B, and the spacing in the first semiconductor layer 50 or the third semiconductor layer 701 is larger than in portions A and C. The active layer 15 has a refractive index difference on the side surfaces of the stepped steps 40, and oscillates in a single transverse mode.

In addition, since the impurity concentration and layer thickness of the third semiconductor layers 70a and 70b do not affect the oscillation transverse mode, the layer thickness is increased to form a third semiconductor layer 70a with a small impurity concentration and a third semiconductor layer 70a with a normal impurity concentration. Since the semiconductor layer 70b can be divided into two layers and each layer can have sufficient thickness, the junction capacitance can be reduced, and the lower surface of the stepped step 40 is formed on the insulating substrate 1.
Since it is 0, the capacity becomes small.

This embodiment will be explained below using a specific example.

A p-type Q as layer that will become the first semiconductor layer 50 is formed on an insulating Ga As substrate 10 using a metal organic decomposition method (hereinafter referred to as MOCVD method), and a second semiconductor layer 6 is formed on the insulating Ga As substrate 10 to a thickness of 1 μm.
The p-type klo, s Qa o, y A s layer which becomes 0 is l μm.

Non-doped Ga A that becomes the third semiconductor layer 70a
3 layers with a thickness of 1 μm, n W which becomes the third semiconductor layer 70b
(ja Ass layer μm was continuously grown. The growth rate of the second semiconductor layer 60, which determines the refractive index waveguide width, was 200 people/
min, and has excellent controllability, so the width of the refractive index waveguide for stable single-transverse mode oscillation can be easily controlled. The thus produced wafer is subjected to conventional photolithography and chemical etching to partially remove part of the first semiconductor layer 50, or all or more of the first semiconductor layer 50 within the wafer. Stair step 40
Create.

The second MOC is applied to the wafer that has completed the above steps.
Alo, GaO, 7A S layers, which will become the first cladding layer 11 and the second cladding layer 12, have a thickness of 2 μm and a cap layer 13 is continuously grown by the VD method.

In the MOCV and IJ methods, as shown in FIG. 3, the film grows uniformly on the bottom, top and side surfaces of the steps. Next, the layers laminated on the upper surface of the steps, including the third semiconductor layers 7Qa and 70b, are partially removed by ordinary photolithography and chemical etching, and the p-side and N-side electrodes 20 and 30 were formed, and wirings 21 and 31 were connected to form a device. At this time, a normal insulating film is used to protect the exposed stepped side surfaces for electrode formation.

The laser oscillation in this example is a stable single-transverse mode in the step-side active layer, and the oscillation threshold current is l Q Q mhg degrees, and p, n
The capacitance of the reverse bias junction was less than 10 pF.

In this example, GaAs K is approximately lattice matched to A I
Although it is constructed using a GaAs mixed crystal system, it can also be manufactured using various mixed crystal systems. For example, an A/GaInP mixed crystal system lattice-matched to a GaAs substrate can be used as a real mixed crystal, or an InP substrate can be formed using In Qa As P.

Furthermore, in a crystal system in which the forbidden band width of the semiconductor layer of the active layer is larger than that of the semiconductor substrate, for example, the A lGa As system on the Q a As substrate of this example,
As shown in the second embodiment of FIG. 4, the structure of the kit Ga I n P yarn on the GaAs substrate does not include the first semiconductor layer 50 in FIG. 2, and can be fabricated by crystal growth. Although the number of semiconductor layers can be reduced by one, similar characteristics can be obtained in this case as well.

[Brief explanation of the drawing]

Figure 1 is a sectional view of a conventional semiconductor laser using an insulating substrate, Figure 2 is a sectional view of a first embodiment of the present invention, and Figure 3 shows the oscillation mode of the embodiment of Figure 2. The schematic diagram, FIG. 4, is a sectional view of a second embodiment of the present invention. In the figure, 10... insulating semiconductor substrate, 11...
First cladding layer, 12... Second cladding layer, 13... Cap layer, 15... Active layer, 16... Current confinement N117・・・・・・
Current blocking layer, 18...Buried layer, 19...
... Electrode forming layer, 20.30 ... Electrode, 2
1.31...Wiring, 40...Staircase step, 50...First semiconductor layer, 60...
...Second semiconductor JWI% 70a, 70b...
...Third semiconductor layer, 80...Mesa graduate once Jθ Mine 4koku

Claims (1)

[Scope of Claims] 1) On an insulating semiconductor substrate, a first semiconductor layer having a narrower band gap than the active layer, a second semiconductor layer having a band gap larger than the active layer, and the active layer. A step-like step portion including a third semiconductor layer having a smaller forbidden band width and consisting of several layers is provided, and first and second claddings having a larger forbidden band width than the active layer are provided on at least a side surface of the step portion. A semiconductor laser characterized in that it has a double heterostructure in which the active layer is sandwiched between layers. Illusion A patent characterized in that when the insulating semiconductor substrate is made of a material smaller than the forbidden band width of the active layer, the insulating semiconductor substrate also serves as the first semiconductor layer, thereby eliminating the need for the first semiconductor layer. A semiconductor laser according to claim 1.