JPS59184585A - Semiconductor laser of single axial mode - Google Patents

Abstract

PURPOSE:To enhance the productivity by simplifying the process of forming an electrode by a method wherein a region including a photo guide layer provided with diffraction grating is put in a p-n-p-n structure. CONSTITUTION:This multilayer film structural substrate has a p type InP leakage blocking layer 11 introduced under the photo guide layer 10. By the introduction of the leakage blocking layer 11, the multilayer films is of p-n-p-n junction in conductivity constitution at the part of a terrace region on the left side of a stepwise difference 40, and only of p-n junction sandwiching an active layer 3 at the region on the right side. Therefore, when a forward directional bias whose positive is on the side of a cap layer 8 and negative is the side of the substrate 1 is impressed, the current is all injected to the active layer 3 on the right side of the stepwise different 40 in the state of normal operation at the voltage of the TURN-ON of the p-n-p-n junction (approx. 10V) or less, resulting in light emitting recombination with good efficiency. Besides, the process of forming the electrode can be largely simplified, since the p-side metallic electrode 32 can be formed by vapor-depositing Ti/Pt/Au over the entire surface of the cap layer 8 as the electrode material.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a single-axis mode semiconductor laser suitable as a light source for fiber communication systems.

The transmission loss of optical fiber is 1.3μm wavelength band and 1.5μm.
Due to the ultra-low loss of 0.3 to 0.5 dB/1 m or less in the m-band, ultra-long distance transmission experiments with repeating intervals of 100 b or more are beginning to be conducted in various places. Such ultra-long distance optical fiber communication systems are expected to be applied to submarine communication systems, etc., where it is advantageous to lengthen the relay interval. In ultra-long distance transmission, the repeating interval that can be transmitted is not only limited by the transmission loss of optical fibers, but also by chromatic dispersion.

Fabry-Perot cavity semiconductor lasers, which have been developed as light sources for optical fiber communications, usually have multiple oscillation axis modes.
Transmissible repeater spacing or transmission capacity is limited by chromatic dispersion rather than transmission loss. Therefore, in order to realize a long-distance, high-speed original fiber transmission communication system, a semiconductor laser that oscillates in the C single-axis mode during high-speed modulation is required. Such semiconductor lasers include distributed feedback semiconductor lasers and distributed reflection semiconductor lasers in which a diffraction grating having a periodic structure is built inside without using a Fapley-Perot resonator. The author applied for a special application in 1986-1.
78824, invented a distributed reflection type semiconductor laser having the structure shown in FIG. 1(a). The feature of this distributed reflection type semiconductor laser is that the optical power layer 10 on which the diffraction grating 20 is formed and the active gI3 are connected in a butt manner by effectively utilizing the crystal orientation dependence of epitaxial growth. active layer 3
This is because the coupling efficiency of propagating light between the optical power layer 10 and the optical power layer 10 is as high as about 90%, and because a buried heterostructure is employed.

The generation threshold is as low as 20 to 30 mA at room temperature, and the differential quantum efficiency is as low as 30 to 50%. However, in this distributed reflection semiconductor Hayabusa laser, the p-type InP current blocking layer 5 and the type 11 find'@flow confinement ff! Regarding the area of mesa stripe 15 where j6 is not formed,
Looking at the cross section along ABC in the center, a pn junction is formed in the active layer 3, as shown in FIG. 1(b). Therefore, without the 8 i 02 insulating film 30 that controls the current injection region, current is also injected into the active layer 3 on the optical kite layer 5 which is unnecessary for laser operation, generating a reactive current.

Moreover, even if the insulating film 30 is formed on S r O, p-type I
Several meters of current flows around the nP buried layer 7 and the p-type InP cladding layer 4 and flows into this unnecessary active layer 3. As mentioned above, the conventional structure shown in Figure 1 (a) -
Axial mode semiconductor lasers require a long electrode formation process including the formation of a 5iO-insulating film to limit the current injection region. A drawback was the presence of some reactive current.

The present invention improves the structure of the distributed reflection type semiconductor laser to reduce the tactile current and improve the operating characteristics.
The present invention also provides a single-axis mode semi-coated laser with a simplified electrode formation process.

The light guide layer of the present invention includes a light guide layer that serves as a distributed reflection region with a diffraction grating formed on at least one of its upper or lower surfaces, and an active layer that performs light emission recombination, and the light guide layer and the active layer are connected to each other. is a uniaxial mode semiconductor having a structure in which the respective end surfaces are butted and connected, and the optical power layer and the active layer are surrounded at least on the upper and lower surfaces by a cylindrical layer having a low refractive index. In the laser, the semiconductor multilayer film in the vertical direction of the optical power layer has pnpMj and npn
A single-axis mode semiconductor laser is obtained which is characterized by a junction including at least one of the junctions.

Next, the present invention will be explained in detail using the drawings.

FIG. 2 (al is a perspective view showing the first embodiment of the present invention.

First, the manufacturing process will be described (! - In the first liquid phase epitaxial growth process, a p-type InP il leakage prevention layer (Zn-doped, IX
10111m1 thickness o, 3μWL) and n-type InGa
After producing a substrate laminated with an AsP light guide layer (composition of 1.3 μm in terms of C emission wavelength, thickness of 05 μm), He
- By interference exposure method using a Cd gas laser and Fol-IJ lithography method, a diffraction grating 20 with a period of '235QA with a pitch direction in the <110> direction is formed on the original kite layer 10.
form on top of. Next, use a Br-methanol-based etching solution to remove the right side of the step 40 parallel to the <ITO> direction.
．． Etching is performed to a depth of 0 μm 5− to produce a terraced substrate.

In the second liquid phase epitaxial growth process, n-type In
P buffer y/12 ('Sn doped, ×1oil
m-thickness 0.7 μm) and undoped InGaAsP
Active skin 3 (oscillation wavelength 1.ssμm composition, thickness 0.15μ
m) is grown from a growth solution having a relatively low degree of supersaturation of 2 to 3 degrees, so that it is grown in a shape that is interrupted on both sides of the stepped portion 40. Also, the next p-type InP cladding layer 4
(Zn doped, l x 1918m", thickness approx. 1.5μ
s) is grown from a growth solution having a relatively high degree of supersaturation of about 15 degrees so as to cover the entire surface including the area above the one-step difference. There are two grooves 1 parallel to the <110> direction on this substrate.
6.17 (width about 7 μm, depth about 3 μm), with a width of about 2
It is formed in the form of sandwiching a μm mesa stripe 15. Next, in the third liquid phase epitaxial growth process, p-type InP
Current blocking layer 5 (Zn doped, l
m s, thickness at flat part 0.5 μ”) e n type I
nP current confinement layer 6 (Te doped, 2X10”ff
i'', thickness 0.5 μm at the flat part] except for the upper part of the mesa stripe 15, and then p-type Inp
6- Buried layer 7 (Zn doped I
”, thickness 1.5 μm at flat part) and p-type InGa
AsP cap layer 8 (Zn doped, 5XIQ”cr
n", thickness 07 μm at the flat part) is deposited to cover the entire surface. This completes the entire epitaxial growth process. This multilayer structure substrate differs from the conventional example shown in FIG. 1 by p-type InP. This is because the leakage prevention layer 11 is introduced under the optical power layer 10. Looking at the cross-sectional view at the center of the mesa stripe 15 shown in FIG. As a result, the conductivity type configuration of the multilayer film is pnpn in the terrace-like region on the left side of the one-step difference portion 40.
It can be seen that they are joined. Further, the region on the right side of the stepped portion 40 is only a pn junction sandwiching the active layer 3. Therefore, when a forward bias is applied such that the cap layer 8 side is positive and the substrate 1 side is negative.

Under normal operating conditions below the voltage at which the pn pn junction turns on (approximately 10 V), all current is injected into the active layer 3 on the right side of the stepped portion 40 and is efficiently recombined in light. Since the embodiment shown in FIG. 2(a) has such a current limiting structure inside, the SiO* insulating film 30 is used to limit the current injection region, which was necessary in the conventional example shown in FIG. 1(a). does not require. Therefore, Ti/P can be used as an electrode material.
Since the p-side metal electrode 32 can be formed by depositing t/Au on the entire surface of the cap layer 8, the electrode formation process can be greatly simplified.

As for the operating characteristics, as can be seen from FIG. 2(b), when a forward bias voltage is applied, there is almost no current path flowing into the active layer 3 above the light guide layer 10, so the reactive current is reduced. We were able to lower the oscillation threshold and increase the differential quantum efficiency compared to conventional elements.

- To give an example, in a device in which the length of the active layer 3 on the right side of the step part 40 is approximately 200 μm, and the length of the light guide layer on the left side of the step part 40 is 400 μm, the oscillation threshold is 15 mA and the differential quantum efficiency is 55 cm. Got the value. The oscillation wavelength was 1.55 μm and a single axis mode.

Next, a second embodiment of the present invention will be shown using FIGS. 3(a) and 3(b). The structure of the second embodiment shown in the perspective view of FIG. 3 (al) differs from the structure of the first embodiment shown in FIG. ]
XIQ 6n, thickness approximately 0.5 μtx) 11 is formed on the optical power layer IO. In this case, the leakage prevention layer 11 is formed to a thickness of 0.3 μm on the original kite layer 10 in the first liquid phase epitaxial growth process. In this structure as well, the mesa stripe 15 in Fig. 3 (bl)
Since the multilayer film forms a p-npn junction in the terrace-like region on the left side of the two-step difference portion 40 shown in the cross-sectional view at the center of the figure, no current flows in this region. Therefore, as in the first embodiment, the p-side metal electrode 32 can be formed over the entire surface of the cap layer 8, which simplifies the electrode formation process, and also reduces reactive current, which can improve operating characteristics. Ta.

A third embodiment of the present invention is shown in FIG.

The difference from the first and second embodiments is that the epitaxial growth process is two times shorter. That is, first n-type InP
After Zn diffusion is performed on the surface of the substrate 1 to form a leakage prevention layer 1 with a thickness of about 08 μm, a diffraction grating 20 is formed on the surface.

Next, the same etching as in the first example was performed.

91 A step of 13 μm is provided on both sides of the step portion 40. In the first liquid phase epitaxial growth process, the n-type InGaAsP optical guide layer 10. n-type InP buffer layer 2. Non-dope I
After the nGaAsP active layer 3 is grown separately on both sides of the stepped portion 40, the p-type 1nP cladding layer 4 is continuously grown over the entire surface. The film thickness of each layer is 0.5 μm, 1 μm, 0.
When grown to a thickness of 15 .mu.m, ] .mu.m, the optical power layer IO on the diffraction grating and the active layer 3 are connected at the stepped portion 40 with their end surfaces butted against each other. The steps after producing this terraced substrate are the same as those in the first embodiment. This structure is
As shown in the cross-sectional view of FIG. 4(b), the left side of the stepped portion 40 has exactly the same laminated shape as in the first embodiment. In addition, the light guide layer 10 is additionally included on the right side of the stepped portion 40;
Between this hermitage and the active layer 3, there is an n-type InP buffer layer 2.
Since it is formed with a thickness of 1 μm, carrier injection into the active layer 3 and the original waveguide in the active layer 3 are not affected. Therefore, the third embodiment has the advantage that performance equivalent to that of the first embodiment can be achieved and the manufacturing process is shorter than that of the first embodiment.

10- In the above three embodiments, the Ir+GaAsP active layer 3 was sandwiched between InP layers on the upper and lower sides, but on one side or both sides was InGaAsP, which has a lower refractive index than the InGaAsP active layer 3! Large Qptical Cavity with 4 wave layers
y) structure. Also, the material (J
a A AlGaAs or InGaA on a substrate
It is also possible to use sP or the like.

Finally, to summarize the characteristics of the present invention, the electrode formation process can be simplified and productivity can be increased by making the region including the optical power layer provided with the diffraction grating into a pnpn multilayer structure compared to the conventional method. It is.

[Brief explanation of drawings]

FIG. 2 is a sectional view of the first embodiment of the invention; FIG. 3A is a perspective view of the second embodiment of the invention; FIG. , FIG. 4 (a) is a perspective view of the third embodiment of the present invention, and FIG. is an InGaAsP active layer, 4 is a p-type InP cladding layer, 5 is a p-type InP current blocking layer, 6 is an n-type InP current confinement layer, 7 is a p-type InP buried layer, 8 is a p-type InP layer
GaAsP cap layer, 10 is p-type or n-type I
nGaAsP light guide/e*, 11 is a p-type InPi leakage prevention layer, 15 is a mesa stripe, 16 and 17 are two parallel grooves, 20 is a diffraction grating, 30 is a 5ins insulating film, 3
2 is a p-side metal electrode, 33 is an n-side metal electrode, and 40 is a stepped portion. 1st mouth (b) 82 茬 31fl 范 〃 fig. (b)

Claims (1)

[Scope of Claims] 1. An optical power layer serving as a distributed reflection region having a diffraction grating formed on at least one of the upper or lower surfaces. an active layer that performs luminescent recombination, the optical guide layer and the active layer are connected such that their ends are butted, and the optical guide layer and the active layer are connected at least to the upper In a single-axis mode semiconductor laser having a structure in which the bottom surface is surrounded by a semiconductor layer with a low refractive index, the semiconductor multilayer film in the vertical direction of the optical guide layer has a pnp junction and an npn junction.
A single-axis mode semiconductor laser characterized by forming a junction including at least one of the junctions.