JPH0449791B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a semiconductor laser used for optical communication, etc., and in particular improves current confinement in an active layer.
This invention relates to a semiconductor laser that enables high-output operation.
A buried heterostructure semiconductor laser (hereinafter referred to as BH LD), in which a band-shaped active layer is surrounded by a semiconductor layer with a smaller refractive index and a larger forbidden band width, has features such as a low lasing threshold and stable oscillation transverse mode. for,
It is about to be put into practical use as a light source for optical communications and optical information processing. In order to achieve good characteristics in this BH LD, it is necessary to effectively concentrate and flow current to the active layer, and various current blocking structures have been devised. As an example, Figure 1 shows
1 is a cross-sectional view of a dual channel planar BH LD (hereinafter abbreviated as DC-PBH LD) devised by the inventors of the present application. This is n-InP on a flat InP substrate 1.
Two parallel grooves 10 and 11 are formed in a double heterostructure (DH) substrate on which a buffer layer 2, an InGaAsP active layer 3, and a p-InP cladding layer 4 are grown, thereby forming a band-shaped active layer 20. After that, crystal growth is performed again to form a p-InP current confinement layer 5,
A p-InP current confinement layer 6, a p-InP buried layer 7, and a p-InGaAsP contact layer 8 are sequentially formed,
P-side and n-side electrodes 30 and 31 are formed. In this DC-PBH LD, a current blocking structure having a pn-pn layer structure is formed outside the two grooves 10 and 11. In this structure, the InGaAsP active layer 3 of the original DH wafer remains as current blocking layers 21 and 22.
It has been confirmed that this works effectively for current concentration. That is, in DC-PBH LD, p-n-p
Of the two types of transistors that make up the -n thyristor structure, an InGaAsP current blocking layer with a narrower bandgap is placed between the p-InP base and n-InP emitter of the higher gain n-p-n transistor. This significantly reduces the carrier transfer efficiency of this transistor and thus reduces the gain of this npn transistor. As a result, the p-n-p-n thyristor structure
It becomes difficult to turn on, and current is effectively concentrated in the active region. Generally, the thickness of the active layer of a semiconductor laser is
It is made fairly thin to achieve low thresholds, low vertical radiation angles, etc. For example, conventional DC-PBH
In LD, it is set to about 0.1 μm. Conventional DC−
In PBH LD, a current blocking layer with the same composition and same thickness as the active layer is formed in the current blocking structure, so the thickness of the current blocking layer is insufficient to realize effective current confinement, and therefore high This p-n-p-n thyristor often turns on at the injection excitation level, making high output operation somewhat difficult.

The semiconductor laser of the present invention includes a band-shaped active layer, a semiconductor layer having a smaller refractive index and a larger forbidden band width than the active layer, and surrounding at least both longitudinal sides and upper and lower surfaces of the active layer; a multilayer semiconductor layer having a pnpn current blocking structure formed in contact with the outer side in the longitudinal direction of the surrounding semiconductor layer, and having a pn junction portion of the multilayer semiconductor layer having the pnpn current blocking structure that is the same as the active layer. The present invention is characterized in that a current blocking layer is formed in the process of step 1 and is thicker than the active layer.

Alternatively, in the above semiconductor laser, the distance between the bottom surface of the substrate on which each of the semiconductor layers is formed and the active layer is larger than the distance between the bottom surface and the current blocking layer.

The present invention will be described in detail below with reference to the drawings.
FIG. 2 is a sectional view of a preferred embodiment of the present invention;
Figures a, b and c are cross-sectional views showing three stages in the fabrication process of the embodiment. In this embodiment, the current blocking layers 21 and 22 are thicker than the band-shaped active layer 20, and the p
It was confirmed that the -npn thyristor did not turn on, effective current confinement was performed, and higher output operation was possible than the conventional DC-PBH LD. The manufacturing method of this embodiment is shown in Figure 3a,
The explanation will be based on b and c.

First, a (001) InP substrate 1 was etched by photolithography and a chemical etching method using a hydrochloric acid-based etchant so that the upper part had a width of about 5 μm and a height of 1 μm.
A mesa stripe 40 with an orientation of 110> is formed. Next, n- is deposited on this substrate 1 by a liquid phase growth method.
InP buffer layer 2, InGaAsP active layer 3, p-InP
The cladding layers 4 are sequentially grown to form a double heterostructure crystal 41. n-InP buffer layer 2 and p-
The InP cladding layer 4 was grown by a supercooling method with a supersaturation degree of about 8° C., and the active layer 3 was grown by a so-called two-phase solution method in which InP crystals were not completely dissolved but floated in the solution. The thicknesses of layers 2, 3, and 4 are respectively different in areas other than mesa stripe 40.
The thicknesses were set to 2 μm, 0.3 μm, and 1 μm. At this time, on the upper surface of the mesa stripe 40, layers 2, 3,
The thickness of 4 is 1.5 μm, 0.1 μm, and 0.7 μm, respectively.
It became. That is, when growing using a two-phase solution method with a relatively low degree of supersaturation, the growth rate on the mesa stripe is significantly lower than the growth rate on the other flat areas. In this example, this property was utilized to fabricate InGaAsP layers with different thicknesses in a single crystal growth process. At this time, since the portion that will become the band-shaped active layer 20 is formed on the mesa stripe 40, it is formed at a position farther from the substrate 1 than the portions that will become the current blocking layers 21 and 22.

Next, the above-mentioned double heterostructure crystal 4 was etched using a photolithography method and a chemical etching method using a bromine-methanol etchant.
Two parallel grooves 10 and 11 are formed in 1. At this time, the shape of the surface of the crystal 41 before etching shown by the dotted line in FIG.
Determine the positional relationship with 1. That is, two grooves 1
The active layer mesa stripe 23 including the band-shaped active layer 20 sandwiched between
The two grooves 10 and 11 are arranged so as to be located approximately at the center of the 0 portion, and the two grooves 10 and 11 reach to a portion of the double heterostructure crystal 41 other than the mesa stripe 40.
Thereafter, it is placed in the crystal growth furnace again, and p-InP and n-InP current confinement layers 5, 6, p-InP buried layer 7, and p-InGaAsP contact layer 8 are grown in sequence by a two-phase solution method. At this time, by appropriately setting the growth time of each layer, the former two
does not grow on the upper surface of the active layer mesa stripe 23, and the latter two 7 and 8 can grow over the entire crystal surface. The thicknesses of layers 5, 6, 7, and 8 are 0.5 μm, 0.5 μm, and 1.5 μm at the flat portion, respectively.
It was set to 0.7 μm. Subsequently, a p-side electrode 30 is placed on the growth surface, and after thinning the substrate 1 by polishing, an n-shaped electrode 31 is deposited and alloyed, and the pellet is cut out by cleaving, scribing, etc., and the cross section is shown in FIG. The fabrication of the element is completed. FIG. 4 shows the pulse current-light output characteristics of the embodiment of the present invention and the conventional DC-light output characteristics.
An example of the pulse current vs. optical output characteristics of a PBH LD is shown. As shown by the curve 50, the optical output of the embodiment of the present invention is 150 mW or more from one side, whereas the characteristics of the conventional element in FIG. 1 are as shown by the curve 51. , output saturation becomes severe at about 100 mW, and higher output operation is impossible. This is because, in the embodiment of the present invention, the current blocking layers 21 and 22 in the current blocking structure are thicker than the conventional one.
This is because the thyristor structure is no longer turned on to a higher excitation level, so that the leakage current flowing to areas other than the band-shaped active layer 20 is significantly reduced even in a highly implanted excitation state. In the present invention, by forming a double heterostructure on a mesa stripe substrate, the strip-shaped active layer 20 and the current blocking layers 21, 2
2 at the same time, and the former is made thin enough to provide good transverse mode stability and low threshold;
Moreover, since the latter can be formed thickly, a BH LD with significantly improved high-output characteristics can be realized with approximately the same number of man-hours as a conventional BH LD.

As detailed above, according to the present invention, the current confinement function works effectively even at high injection excitation levels,
A semiconductor laser capable of high output operation can be provided.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view of a conventional buried structure semiconductor laser, FIG. 2 is a cross-sectional view of a preferred embodiment of the present invention, and FIGS. The respective cross-sectional views and FIG. 4 are pulse current-light output characteristic diagrams. DESCRIPTION OF SYMBOLS 1...Substrate, 2...Buffer layer, 3...Active layer, 4...Clad layer, 5, 6...Current confinement layer, 7...Buried layer, 8...Contact layer, 1
0,11...Groove, 20...Striped active layer, 21,2
2... Current blocking layer, 23... Active layer mesa stripe, 30, 31... Electrode, 40... Mesa stripe, 41... Double heterostructure crystal.

Claims (1)

[Scope of Claims] 1. A band-shaped active layer, a semiconductor layer having a smaller refractive index and a larger forbidden band width than this active layer, surrounding at least both longitudinal sides and upper and lower surfaces of this active layer, a multilayer semiconductor layer having a pnpn current blocking structure formed in contact with the outer side in the longitudinal direction of the surrounding semiconductor layer, and having a pn junction portion of the multilayer semiconductor layer having the pnpn current blocking structure that is the same as the active layer. 1. A semiconductor laser characterized in that a current blocking layer is formed in the process described above and is thicker than the active layer. 2. In the semiconductor laser described in the preceding paragraph, the distance between the bottom surface of the substrate on which each of the semiconductor layers is formed and the active layer is greater than the distance between the bottom surface and the current blocking layer. The semiconductor laser according to item 1.