JPH01173774A - Distributed reflection semiconductor laser and manufacture thereof - Google Patents

Abstract

PURPOSE:To form a distributed reflection semiconductor laser for contriving a reduction in the oscillation threshold current of a semiconductor laser and the high luminous efficiency and the high output oscillation of the laser by a method wherein the end faces of a protective layer are made to protrude more in the lateral direction than the end faces of an active waveguide layer within the extent of a prescribed distance. CONSTITUTION:A GaInAsP active waveguide layer 3, a GaInAsP antimeltback AMB layer 5 and an n-type InP protective layer 4 are epitaxially grown in order on an InP buffer layer 2 on an InP substrate 1. Then, when parts in the lateral direction of the layers 3, 5 and 4 are removed by etching, overhang parts X are formed on the end faces of the layer 4 using a selective etching method. According to a Kossel model, a growth rate in a lateral direction is larger by one digit compared to a growth rate in a thickness direction in a normal epitaxial growth and in a distributed reflection DBR type semiconductor laser of a waveguide structure BIG, a distance l0 between the end face U of the layer 4 and the start of a diffraction grating is set in 0.2-1.0mum. Thereby, the diffraction grating is formed within an extent, wherein its growth to the lateral direction is generating, and a break of an external waveguide layer 7 on the boundaries between external waveguide regions 14 and an active waveguide region 15 is eliminated.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a single mode distributed reflection semiconductor laser used as a light source for optical fiber communications and a method for manufacturing the same.

[Prior art] Distributed reflection semiconductor laser (hereinafter abbreviated as DBR laser)
is a promising light source for coherent optical communication, which is a long-distance, high-capacity optical communication system.

The basic structure of this DBR type laser, as shown in FIG.
) area 14. One of the points in the structural design of this DBR type laser is that the active region 15 and the DBR region 1
4 and efficiently combine them. Therefore, conventionally, an ITG structure DBR type laser with an integrated double waveguide structure has been used.

BJB structure DBR type laser with direct coupling structure.

A BIG structure DBR type laser has been proposed. Of these,
A DBR laser with a BIG structure has a waveguide structure in which the active waveguide is wrapped by an external waveguide, and depending on the composition and thickness of the waveguide, the coupling efficiency between the active region and the DBR region can exceed 95%. To become a special application 1986-12
No. 181, Japanese Patent Application Laid-open No. 17190/1985, and pages 399 to 401 of Volume 24, No. 6 of the Japanese Journal of Applied Physics in 1985 (written by Suematsu, Arai, and Tomori, Tokyo Institute of Technology).

[Problems to be solved by the invention] However, when manufacturing a BIG structure DBR type laser, the ninth
As shown in the figure, a phenomenon was often observed in which the external waveguide layer 21 was interrupted at the boundary between the active region 15 and the DBR region 14, and the device did not operate as a DBR type laser at all. This discontinuity phenomenon is thought to be due to the step at the end face of the active waveguide layer 18. When the crystal cross section in the case where the external waveguide layer 21 is discontinued was deeply observed using an electron microscope (SEM), it was found that the discontinuity in the external waveguide layer 21 was caused by: ■ Shape of the stepped portion of the active waveguide layer 18 ■ Diffraction It has become clear that this is closely related to the distance between the grating 16 and the end face of the active waveguide layer 18.

In order to solve the problem of ■, the applicant of the present application filed the patent application
No. 2-218461 proposes a ``distributed reflection semiconductor laser and its manufacturing method'', and an outline of its contents will be explained here.

When forming the active waveguide layer 3 of the distributed reflection semiconductor laser, the (l11)A plane, which is difficult to epitaxially grow, is likely to be formed at the step portion of the end face, so crystal growth is difficult to occur at this step portion. Therefore, the outer waveguide layer 21 is interrupted at this portion. To avoid this,
This stepped portion is designed to form an overhanging eave on the end face of the InP protective layer 19, and this eave serves as a growth nucleus to promote lateral growth.

However, as pointed out in the problem (2), if the distance between the active waveguide layer 18 and the diffraction grating 16 is too far, the portion that grows laterally from the eaves cannot reach the diffraction grating 16. However, as shown in FIG. 1O, there was a problem in that it was interrupted at the boundary with the diffraction grating 16.

This invention was made to eliminate the above-mentioned problems, and aims to provide a highly efficient distributed reflection semiconductor laser by eliminating discontinuities in the external waveguide layer at the boundary between the active region and the DBR region. shall be.

[Summary of the Invention] At the boundary between the diffraction grating 16 and the active waveguide layer 18, complex growth occurs due to a combination of lateral growth from the eaves and thickness direction growth in the normal flat area. Abnormal growth is particularly likely to occur at the border. To avoid this, within the area where lateral growth is occurring,
A diffraction grating is formed, thereby eliminating discontinuities in the outer waveguide layer 21 at the boundaries.

[Means for Solving the Problems] The distributed reflection type semiconductor laser of the present invention includes a P-type or N-type semiconductor single crystal substrate 1 and an upper surface of the substrate 1 via a buffer layer 2. A non-doped active waveguide layer 3 formed in a stripe shape by an epitaxial growth method in the center, and a protective layer 4 having electrical characteristics opposite to those of the substrate 1, formed on the active waveguide layer 3 by an epitaxial growth method.
, a distributed Bragg reflector 6 formed on the top surface of the buffer layer 2 or the top surface of the substrate l, a distributed Bragg reflector on the top surface and end surface U of the protective layer 4, the active waveguide R3, and the end surface W1 of the anti-meltback layer 5. An external waveguide layer 7 having electrical characteristics opposite to that of the substrate 1 is formed by epitaxial growth on the top surface of the buffer layer 2 or substrate 1 provided with 6, and an external waveguide layer 7 is formed by epitaxial growth on the external waveguide layer 7. a cladding layer 8 and a contact layer 9 having electrical characteristics opposite to those of the substrate 1, an electrode 13 provided on the upper surface of the contact layer 9, and an electrode 12 provided on the lower surface of the substrate 1, and a protective layer The end surface U of 4 is the end surface W of the active waveguide layer 3
The distance between the end surfaces W and U is 0.
It is characterized by a range of 2 μm to 1.0 μm.

Further, the method for manufacturing a distributed reflection type semiconductor laser of the present invention includes P
A non-doped active waveguide layer 3 and a non-doped anti-melt back layer are formed on a buffer layer 2 having the same electrical characteristics as the substrate 1, which is formed by epitaxial growth on a type or N-type semiconductor single crystal substrate 1 or on the substrate 1 by epitaxial growth. After epitaxially growing the layer 5 and further epitaxially growing a protective layer 4 having electrical characteristics opposite to the substrate 1,
A striped film 10 is provided at the center of the upper surface of the protective layer 4, and the protective layer 4, the active waveguide layer 3, and the anti-meltback layer 5 are etched, leaving the central portion covered with the film 10. Using the protective layer 4 as a mask, the end surfaces of the active waveguide layer 3 and anti-meltback layer 5 are etched in the horizontal direction so that the end surface W of the active waveguide layer 3 and anti-meltback layer 5 is 0.000000000000000000000000000000000000000 than the end surface U of the protective layer 4. .2 μm to 1.0 μm, and the remaining protective layer 4. When forming the distributed Bragg reflector 6, which is a diffraction grating, by using the interference exposure method on the upper surface of the buffer layer 2 or the substrate 1 exposed on both sides of the active waveguide layer 3 and the anti-meltback layer 5, the distributed Bragg reflector 6 to 10 μm from the protective layer end surface U
Further, an external waveguide layer 7 having electrical characteristics opposite to that of the substrate 1 is epitaxially grown on the buffer layer 2 on which the distributed Bragg reflector 6 is formed or on the substrate 1 and on the protective layer 4. , on the outer waveguide layer 7 a cladding layer 8. having electrical properties opposite to those of the substrate 1. A contact layer 9 is epitaxially grown, and electrodes +2. It is characterized in that an electrode 13 is provided.

[Principle of the invention] The epitaxial growth mechanism is generally based on the Kotsu cell (
Kossel) model, which shows that growth progresses two-dimensionally. Liquid phase epitaxial growth (LP)
E) There is no exception in law. When viewed from an extremely microscopic perspective, the component elements (one atom molecule or cluster) that are undergoing crystal growth move around two-dimensionally on the surface of the base IJ2 crystal, and grow while entering the area with the lowest free energy.

Therefore, in order for this crystal growth to proceed smoothly, conditions must be created for the elements to move freely in the horizontal direction in two dimensions, and the places where these elements should enter, that is, the places that will become growth nuclei such as steps and kinks, must be created. It is important to create.

As already mentioned, when forming the active waveguide layer 3 of the distributed reflection type semiconductor laser, an overhang-shaped eave was formed on the end face as a growth nucleus to ensure smooth growth. Therefore, the present invention focuses on two-dimensional lateral movement of elements, that is, surface movement of elements.
migration).

As shown in FIG. 8, the DBR type laser is composed of an active region 15 that emits light and a DBR region 14 that has a diffraction grating. In particular, in the DBR region 14, grooves (unevenness) called diffraction gratings are formed with a pitch of about 2300 and a depth of 300 to 800. When crystal growth is to be performed, this surface unevenness makes growth difficult.

Furthermore, it has been found that the growth behavior is different between the case of growth on a flat semiconductor crystal surface and the case of growth on the diffraction grating 16 having an uneven surface. If the active region 15 and the DBR region 14 are too far apart, growth will begin separately in each region, and the outer waveguide layer 21 will be interrupted at the boundary, as described above.

To solve this problem, we utilize the fact that two-dimensional lateral growth occurs using the oval-hung eaves as a growth nucleus, and define the range Q in which this lateral growth occurs. (-10μm
), and these two regions are smoothly coupled by the GaInAsP external waveguide layer 7.

Next, the active region 15 and the DBR region 14 are made of GaInAs.
A method for achieving smooth coupling using the P external waveguide layer 7 will be explained below using the principle diagram of the present invention.

As shown in FIG. 1, on an InP buffer layer 2 on an InP substrate 1, a GaInAsP active waveguide layer 3. Garn
AsP anti-meltback (AMB) layer5. InP protective layer 4 is epitaxially grown in sequence.

Next, when removing the sides of the active waveguide layer 3, AMB layer 5, and protective layer 4 by etching, selective etching is used to form an overhang portion X on the end surface of the protective layer 4, as shown in FIG. form.

Usually, the epitaxial growth mechanism is the Ko
According to the SSEL model, it is known that the growth rate in the lateral direction is more than an order of magnitude higher than the growth rate in the thickness direction.
In the BIG-DrJR type semiconductor laser, the range that can be reached in the lateral direction using the overhang part X as a growth nucleus is 10 μm.
Therefore, the distance Q from the end surface U of the protective layer 4 to the beginning of the diffraction grating. By making the range within 10μ peak, the active region 15 and D)3R region 14 are
The external waveguide layer provides seamless coupling.

[Example] Hereinafter, an example of an actual manufacturing procedure will be described based on the method for manufacturing a distributed reflection type semiconductor laser according to the present invention.

As shown in FIG. 3, a P-type InP/< buffer layer 2. 3. Striped non-doped GaInAsP active waveguide layer. Non-doped GaInAsP anti-meltback layer5. An N-type InP protective layer 4 (thickness: 0.1 μm) is grown in order. The thickness of the P-type InP layer 2 is 20 μm, and the thickness of the active waveguide layer 3 is 0.1 μm to 0.4 μm.
Here, it was set to 0.15 μm. Moreover, the thickness of the protective layer 4 is
0.05 μm to 0°3 μm, here 0.1 μm
And so. Furthermore, the forbidden band width of the active waveguide layer 3 is 0.8 eV.
It is. The GaInAsP anti-meltback layer 5 is made of I
During the growth of the nP protective layer 4, the GaInAsP active waveguide layer 3
This is the first grain that prevents dissolution into the InP solution and smooths the interface.

Next, a striped S is formed on a predetermined portion of the N-type InP protective layer 4
An iN film 10 is formed. Stripe width is 200-40
The thickness was approximately 0 μm, and it was formed parallel to the (0 bottom) plane. Then, the portion of the N-type InP protective layer 4 that is not covered with the SiN film 10 is etched using an HCC-based etching solution. Then, using a l12SO4-based etching solution,
The non-doped GaInAsP anti-meltback layer 5 and the non-doped GaInAsP active waveguide layer 3 are etched.

At this time, as shown in FIG. 4, using the InP protective layer 4 as a mask, the GaInAsP anti-meltback layer 5 and the GaInAsP active waveguide layer 3 are exposed from the end surface of the InP protective layer 4 as shown in the figure. Then, selective etching is further performed by about 0.2 to 1.0 μm to form an overhang portion X on the end face of the protective layer 4.

Next, from the end surface U of the protective layer 4 including the overhang part X, point Q in the figure. As shown in , Si is spread about 3 to 10 μm wide.
The N film 11 is formed again. This SiN film! 1, when forming the next diffraction grating, 1) protection of the InP protective layer 4 in the overhang part X from the bromine water edgeant, and 11) reduction of the distance t2o between the diffraction grating and the overhang part X of the protective layer 4. Make a decision. This Q. control is important.

Next, as shown in FIG. 6, a distributed Bragg reflector 6 is placed on the P-type InP buffer layer 2 exposed using the interference exposure method.
A diffraction grating is formed. This diffraction grating is formed using a bromine water etching solution, and the pitch of the diffraction grating is
It is determined from the oscillation wavelength of the semiconductor laser. In addition, the depth of the diffraction grating is set at 30 mm to ensure sufficient reflection from the diffraction grating.
Approximately 0 to 800 people will be selected.

Next, as shown in FIG. 7, after removing the SiN film 11,
An N-type GaInAsP external waveguide layer 7 (thickness 0.3
5 μm). The thickness and composition of this GaInAsP external waveguide layer 7 are selected in consideration of the following points.

a) The coupling efficiency between the active region 15 and the DI3R region I4 is made sufficiently large.

b) Reduce current leakage from active region 15 to DBR region 14.

C) Ensuring sufficient light confinement in the external waveguide layer 7.

d) The growth rate in the lateral direction from the overhang portion X can be sufficiently high.

The thickness is 0°1 to 0.4 as a condition for oscillation in the fundamental mode.
The 0 μm9 composition is selected to be 1.20 to 1.35 μm at the photoluminescence wavelength λ9.

The composition has a forbidden band width of 0.918 eV=1.03 eV.

Further, on the upper surface of this external waveguide layer 7, a NIMInP cladding layer 8 (thickness: 1.5 μm) and an N-type GaInAsP contact layer 9 (thickness: 0.5 μm) are grown in this order by liquid phase or vapor phase epitaxial growth.

Regarding the composition of the quaternary GaInAsP mixed crystal, GaInA
sP external waveguide layer 7 λy=I, 28 μm GaInAs
P active waveguide layer 3 λy=1,52HnGaInAsP
Anti-meltback layer λy=t, ao μm GaInAsP contact layer 9 λy=1.10 μm.

Then, on the lower surface of the InP substrate 1 there is a P (lI11 electrode 12) and on the upper surface of the GaInAsP contact layer 9 there is a N0111 electrode.
An electrode 13 is provided. In order to limit the current distribution and efficiently flow current to the active region 15, the N-side electrode is formed into a striped electrode. Thereafter, the wafer is scribed into individual device chips and mounted in a package.

In the distributed reflection semiconductor laser manufactured as described above, the active region 15 and the DBR region 14 are smoothly coupled by the external waveguide layer 7, thereby enabling highly efficient coupling. Improves consistency as an optical waveguide.

What is described here is a P-type InP substrate, an N-type protective layer, an N-type external waveguide layer, and an N-type cladding layer.

The N-type contact layer was epitaxially grown, but these electrical characteristics were reversed and a P-type protective layer, P-type external waveguide layer, P-type cladding layer, and P-type protective layer, P-type external waveguide layer, P-type cladding layer, and
The mold contact layer may be grown epitaxially. Furthermore, the above explanation is based on GaInA formed on an InP substrate.
Although the sP-based distributed reflection laser was described above, the present invention can also be applied to a GaAs-based laser formed on a GaAs substrate. The structure of the distributed reflection type semiconductor laser of the present invention can be used in combination with a commonly used buried structure.

[Effects of the Invention] As explained above, the present invention makes the end face of the protective layer protrude from the end face of the active waveguide layer and the anti-meltback layer to form an overhang portion as a growth nucleus, and also Set the starting position of the grid 10 minutes from the overhang part.
When the external waveguide layer is epitaxially grown within μm, the discontinuity of the external waveguide layer at the end face of the active waveguide is eliminated, so a highly efficient coupling between the active region and the DBR region can be obtained, and the semiconductor laser We can expect a reduction in the oscillation threshold current, high luminous efficiency, and high output oscillation.

[Brief explanation of the drawing]

1 to 2 are diagrams for explaining the method of manufacturing a distributed reflection type semiconductor laser according to the present invention, and FIGS. 3 to 7 are cross-sectional views showing the manufacturing process of the semiconductor laser according to the present invention. FIG. 8 is a principle diagram of a distributed reflection type semiconductor laser, and FIGS. 9 and 10 are cross-sectional views of conventional distributed reflection type semiconductor lasers. 1...InP substrate, 2...InP buffer layer, 3...
- Active waveguide layer, 4... Protective layer, 5... Anti-melt buzz layer, 6... Distributed Bragg reflector, 7... External waveguide layer, 8- Cladding layer, 9... Contact layer, 12
...P(llI?li pole, 13...N side electrode, 14
...External waveguide region, 15...Active waveguide region, 1
6... Diffraction grating, 1... Active waveguide layer, 19...
Protective layer, 20... Anti-meltback layer, 21...
Outer waveguide layer. Patent issuer Sumitomo Electric Industries Co., Ltd. agent Patent attorney Maeda Ao and 1 other personwE1 Business 2 Figure 6 Figure 7

Claims (14)

[Claims] (1) A P-type or N-type semiconductor single crystal substrate 1 and a non-doped active waveguide layer formed in a stripe shape by epitaxial growth at the center of the upper surface of the substrate 1 or the upper surface of the substrate 1 via the buffer layer 2. 3, and a protective layer 4 having electrical properties opposite to those of the substrate 1, which is formed by epitaxial growth on the active waveguide layer 3.
, a distributed Bragg reflector 6 formed on the upper surface of the buffer layer 2 or the upper surface of the substrate 1, the upper surface and end surface U of the protective layer 4, the end surface W of the active waveguide layer 3 and the antimeldback layer 5, and the distributed Bragg reflector 6 formed on the upper surface of the buffer layer 2 or the upper surface of the substrate 1; An external waveguide layer 7 having electrical characteristics opposite to that of the substrate 1 is formed by epitaxial growth on the upper surface of the buffer layer 2 or substrate 1 on which the reflector 6 is provided, and an external waveguide layer 7 is formed by epitaxial growth on the external waveguide layer 7. A cladding layer 8 and a contact layer 9 having electrical characteristics opposite to those of the substrate 1, which are formed by the above, an electrode 13 provided on the upper surface of the contact layer 9, and an electrode 12 provided on the lower surface of the substrate 1, The end surface U of the protective layer 4 is the end surface W of the active waveguide layer 3
The distance between the end surfaces W and U is 0.
A distributed reflection semiconductor laser characterized in that the wavelength is in the range of 2 μm to 1.0 μm. (2) Substrate 1 is P-type InP single crystal, buffer layer 2 is P-type InP single crystal, and active waveguide layer 3 is non-doped GaInA.
sP mixed crystal, anti-meldback layer 5 is non-doped GaI
nAsP mixed crystal, protective layer 4 is N-type InP single crystal, external waveguide layer 7 is N-type GaInAsP mixed crystal, cladding layer 8 is N-type InP single crystal, and contact layer 9 is N-type GaInAsP mixed crystal. A distributed reflection semiconductor laser according to claim 1. (3) The distributed reflection semiconductor laser according to claim 2, wherein the forbidden band width of the active waveguide layer 3 is smaller than the forbidden band width of the external waveguide layer 7. (4) The forbidden band width of the active waveguide layer 3 is 0.8 eV,
The forbidden band width of the external waveguide layer 7 is from 0.92 eV to 1.03
Claim 3 characterized in that it is in the range of eV.
Distributed reflection type semiconductor laser as described in . (5) The distributed reflection semiconductor laser according to claim 2, wherein the thickness of the non-doped active waveguide 3 is in the range of 0.1 μm to 0.4 μm. (6) The distributed reflection semiconductor laser according to claim 5, wherein the thickness of the protective layer 4 is in the range of 0.05 μm to 0.3 μm. (7) The thickness of the outer waveguide layer 7 at the upper surface of the protective layer 4 is 0.
7. The distributed reflection semiconductor laser according to claim 6, characterized in that the diameter is in the range of 1 μm to 0.4 μm. (8) On a P-type or N-type semiconductor single crystal substrate 1,
Alternatively, provided on the substrate 1 by epitaxial growth,
A non-doped active waveguide layer 3 and a non-doped anti-meltback layer 5 are epitaxially grown on a buffer layer 2 having the same electrical characteristics as the substrate 1, and a protective layer 4 having the opposite electrical characteristics as the substrate 1 is further epitaxially grown. Then, a striped film 1 is formed at the center of the upper surface of the protective layer 4.
0, the protective layer 4, the active waveguide layer 3, and the anti-meltback layer 5 are etched leaving the central part covered with the film 10, and then the active waveguide layer 3 is etched using the protective layer 4 as a mask.
Then, the end faces of the anti-meltback layer 5 are etched in the horizontal direction so that the end face W of the active waveguide layer 3 and the anti-meltback layer 5 is 0.2 μm to 1.0 μm larger than the end face U of the protective layer 4.
Furthermore, the upper surface of the buffer layer 2 or the substrate 1 exposed on both sides of the remaining protective layer 4, active waveguide layer 3, and anti-meltback layer 5 is exposed using an interference exposure method. When forming the distributed Bragg reflector 6 which is a diffraction grating, the distributed Bragg reflector 6 is formed within 10 μm from the end surface U of the protective layer, and an external waveguide layer 7 having electrical characteristics opposite to that of the substrate 1 is further distributed. A cladding layer 8 having electrical properties opposite to those of the substrate 1 is epitaxially grown on the buffer layer 2 or substrate 1 on which the Bragg reflector 6 is formed and on the protective layer 4 . A method of manufacturing a distributed reflection type semiconductor laser, characterized in that a contact layer 9 is grown epitaxially, and an electrode 12 and an electrode 13 are provided on a substrate 1 and a cladding layer 8. (9) The method for manufacturing a distributed reflection semiconductor laser according to claim 8, wherein the epitaxial growth method is a liquid phase epitaxial growth method. (10) The method for manufacturing a distributed reflection semiconductor laser according to claim 8, wherein the epitaxial growth method is a vapor phase epitaxial growth method. (11) Substrate 1 is P-type InP single crystal, buffer layer 2 is P
type InP single crystal, active waveguide layer 3 is non-doped GaIn
AsP mixed crystal, anti-meltback layer 5 is non-doped Ga
InAsP mixed crystal, the protective layer 4 is N-type InP single crystal, the external waveguide layer 7 is N-type GaInAsP mixed crystal, the cladding layer is N-type InP single crystal, and the contact layer 9 is N-type GaInAsP mixed crystal. Claims 9 to 1
A method for manufacturing a distributed reflection semiconductor laser according to any one of item 0. (12) The protective layer 4 was formed using an HCl-based etching solution, and the active waveguide layer 3 and anti-meltback layer 5 were formed using an H_2S etching solution.
12. The method of manufacturing a distributed reflection type semiconductor laser according to claim 11, wherein selective etching is performed using an O_4-based etching solution. (13) Claim 1 states that the etching rate when etching the end surface W of the active waveguide layer 3 in the lateral direction is 0.02 μm/min to 0.2 μm/min.
2. A method for manufacturing a distributed reflection semiconductor laser according to item 2. (14) When forming the distributed Bragg reflector 6 using the interference exposure method, the end face W of the active waveguide layer 3 and the anti-meltback layer 5, which is formed in an overhang shape from bromine water as an etching solution, and the protective 14. The method of manufacturing a distributed reflection type semiconductor laser according to claim 9, wherein the end surface U of the layer 4 is covered with a SiN film or a SiO_2 film in order to protect the end surface U of the layer 4.