JPH0744312B2 - Method for manufacturing embedded semiconductor laser - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for manufacturing a semiconductor laser for optical communication capable of oscillating with high efficiency and modulating at high speed.

(Prior Art and Problems Thereof) Semiconductor lasers are being put to practical use as a light source for optical fiber communication. A semiconductor laser used as a light source for optical fiber communication needs to oscillate with high efficiency and be capable of high-speed modulation. Further, with the progress of practical use, there is a strong demand for a highly reproducible manufacturing method capable of producing a large number of semiconductor lasers with a high yield. However, the conventional semiconductor laser cannot satisfy the above three conditions at the same time. The following describes typical semiconductor lasers that have been conventionally manufactured, and semiconductor lasers that have recently been manufactured in an attempt to improve these laser characteristics and are closest to the above requirements, and explain why the above three requirements cannot be satisfied at the same time. To do.

A typical semiconductor laser that has been manufactured in the past is a double channel plane buried semiconductor laser (Double Channel Planar Buri).
ed Heterostructure Laser Diode: DC-PB HLD for short)
And are described in Journal of Lightwave Technology, LT-1, Volume 1983, March 1983, pp. 195-202. This semiconductor laser forms a p-n-p-n junction in a region other than the active region and blocks the current by an n-p reverse junction in order to selectively allow a current to flow in the stripe-shaped active region. There is. N-, which is represented by this structure
Since the current blocking structure by the p-reverse junction exerts a very good current blocking effect, it oscillates at a high efficiency exceeding 50%. Further, since the manufacturing process is a combination of highly reproducible processes, it can be manufactured with a high yield. However, there is a problem that the leakage current, which causes a decrease in efficiency, increases at high output due to the operation of the npn transistor formed in the block layer. In addition, np reverse junction is 10pF
Due to the above capacitance, it was difficult to modulate at a high speed exceeding 4 Gb / s.

Next, two semiconductor lasers designed to overcome the drawbacks of the above semiconductor lasers will be described. The first semiconductor laser is described in Proceedings of the Japan Society of Applied Physics (Proceedings of the 46th Annual Meeting of the Japan Society of Applied Physics, 2p-N-11, p. 206, autumn 1985). In this semiconductor laser, the capacitance of the diode is reduced by forming a groove in the flat block layer on both sides of the active layer until reaching the substrate so that high-speed modulation is possible. As a result, 5G at small signal modulation
It was possible to modulate at a relatively high frequency of Hz. However, even with this semiconductor laser, the parasitic capacitance was not sufficiently reduced, and modulation over 10 GHz was difficult. This is because, in this semiconductor laser, the current blocking layer due to the np reverse junction remains on both sides of the active layer, which causes residual capacity.

The second semiconductor laser is Electronics Letters, Vol. 22, No. 23, pages 1214 to 1215.
Page 1986, it is a semiconductor laser formed by vapor phase embedding. This semiconductor laser was designed to reduce the capacitance of the diode so that high-speed modulation is possible, and it was possible to modulate at a high frequency of 15 GHz during small signal modulation. Further, the structure is such that the leakage current that causes a decrease in efficiency is reduced so that the oscillation is performed with high efficiency, and the current is supplied only to the active layer portion. However, this semiconductor laser has a big problem in productivity. It is due to the structure of this semiconductor laser. In order to oscillate the semiconductor laser in the single transverse mode, the width of the active layer must be about 1 to 2 μm. This semiconductor laser has a mesa direction [01
1] The shape is an inverted trapezoid, which is narrowed downward. The groove to be formed is manufactured by precisely controlling the etching time using an etching solution having a known etching rate. However, since the etching rate of this etching solution largely depends on the temperature and concentration of the solution, there are problems in controllability and reproducibility. For this reason, there were many cases where etching did not reach the active layer due to breakage of the bank including the active region or shallow groove during etching. Therefore, the semiconductor laser structure is not suitable for mass production.

As described above, in the conventional semiconductor laser, it was not possible to simultaneously satisfy the three conditions of oscillating with high efficiency, capable of high-speed modulation, and being able to be manufactured with high yield and high reproducibility. The object of the present invention is to oscillate with high efficiency,
An object of the present invention is to provide a method for manufacturing a semiconductor laser having a structure capable of high-speed modulation and having high productivity.

(Means for Solving the Problems) According to the present invention, an active region has a refractive index lower than that of the active region on a semiconductor substrate whose surface is a (100) plane,
It has a double hetero structure sandwiched by clad layers having a forbidden band width larger than the forbidden band width of the active region, the active region is located in a bank between two grooves, and the inside of the groove is half In a method of manufacturing a buried semiconductor laser in which a current blocking layer made of an insulating semiconductor is formed, and a light guiding direction is parallel to a (011) direction, a side surface of the groove is (111) A.
And a second step of etching so that the bottom surface of the groove does not reach the active region, and a second step of etching to the upper surface of the active region while keeping the side surface of the groove as the (111) A surface. The third step of etching so that the bottom surface of the groove is flush with the upper surface of the clad layer below the active region, and the groove side surface of the lower portion of the active region is a single crystal surface and a forward mesa surface, and the bottom surface of the groove is A method for manufacturing a buried semiconductor laser, which comprises at least a fourth step of etching so as to reach the lower cladding layer, is obtained.

(Operation) In the semiconductor laser manufactured by the method of the present invention, the leakage current on both sides of the active region is blocked by the current blocking layer made of a semi-insulating semiconductor. Therefore, the leakage current in this portion can be remarkably reduced as compared with the conventional one, and the oscillation can be performed with high efficiency. Further, since the current blocking layer has no np reverse junction, it is possible to sufficiently reduce the parasitic capacitance generated in this portion unlike the DC-PBH LD. Therefore, high speed modulation becomes possible. On the other hand, in the semiconductor laser of the present invention, the bank containing the active region has an inverted trapezoidal shape in the upper part of the active region and the trapezoidal shape in the lower part of the active region, so that the bank containing the active region does not break. Furthermore, since the active layer width of the semiconductor laser is constant in any part of the wafer, the structure is suitable for mass production.

In the method of manufacturing a semiconductor laser according to the present invention, in order to control the width of the active region to 1 to 2 μm, etching is performed so that the upper part of the active region of the bank including the active region has an inverted trapezoidal shape.
By the first step, etching is performed so that the side surfaces of the groove on both sides of the clad layer above the active region become the (111) A plane. In the second step, etching is performed so that the bottom surface of the groove is flush with the upper surface of the active region, and the entire side surfaces of the groove on both sides of the cladding layer on the active region are (111) A surfaces. Here, the above etching process is made into two processes for the following reason. That is, the clad layer above the active region is etched, but the active region is not etched, and
1) Etching solution such as A is not found yet. That is, such an example is a case of etching a double-heterostructure crystal in which the active layer is InGaAsP and the upper and lower clad layers are InP.

Etchant solutions that etch InP but not InGaAsP include hydrobromic acid solution (HBr) and hydrogen chloride solution (HCl). However, when a stripe-shaped groove in the [011] direction is formed by using these liquids or a mixed liquid containing these liquids, all or part of the groove side surface becomes the (111) B surface. That is, it is difficult to make the bank including the active region into an inverted trapezoidal shape.

In the manufacturing method of the present invention, in the first step, a part of the groove side surfaces on both sides of the clad layer above the active region is previously set as the (111) A surface. If HBr is used as the etching solution in the subsequent second step, the pre-formed (111) A surface triggers the etching to proceed, so that it is easy to make the entire side surface of the groove a (111) A surface. You can Here, even if the depth of the groove formed in the first step varies within the wafer surface, in the second step the etching stops at the bottom surface of the groove at the upper surface of the active region, so the groove depth is the entire wafer. Will be uniform. Third that follows
In the step (2), an etching solution for etching only the active layer may be used so that the bottom surface of the groove is the upper surface of the clad layer below the active layer. Finally, if HBr or the like is used as the etching solution in the fourth step, the bank at the lower part of the active region becomes a trapezoidal shape, and the bottom of the groove reaches the lower clad layer. All of the above steps have good reproducibility and excellent mass productivity.

(Example) The present invention will be described in detail with reference to the drawings. FIG. 1 is a diagram showing an embodiment of a semiconductor laser manufactured by using the method of the present invention. The active region 15 has a forbidden band width of 0.95 eV
It is GaAsP. This active region 15 is surrounded by an n-type InP clad layer 16 and a p-type InP clad layer 14 from above and below on an n-type InP substrate 17, and from the left and right by a current blocking layer 19 of a semi-insulating semiconductor of Fe-doped InP. There is. That is, this semiconductor laser is of a refractive index guided type. A p-type InGaAsP cap layer 13 is provided on the p-type InP clad layer 14 so as to obtain a good electrical contact with the p-type electrode 11. Further, the SiO ₂ layer 12 is provided in a region other than the upper surface of the bank including the active region 15 so that the injected current selectively flows into the active region 15 sandwiched by the current blocking layers 19.
The n-type electrode 18 is formed on the entire surface of the n-type InP substrate 17.

An embodiment of the method of manufacturing the semiconductor laser shown in FIG. 1 will be described next. FIG. 2 shows cross-sectional views at each stage of the manufacturing process.

In this embodiment, the n-type InP clad layer 25 is formed on the n-type InP substrate 26.
(Thickness 3 μm, n = 1 × 10 ¹⁸ cm ^-3 , undoped InGaAsP
Active region 24 (thickness 0.1 μm, forbidden band width 0.95 eV), p-type InP
Cladding layer 23 (thickness 2.3 μm, p = 1 × 10 ¹⁸ cm ^-3 ), p-type In
GaAsP cap layer 22 (thickness 0.2 μm, p = 1 × 10 ¹⁹ cm ⁻³ ,
Forbidden band width 1.1eV) is used to stack double hetero structure semiconductor crystal. First, SiO ₂ layer is formed on this semiconductor crystal, and then by normal photolithography method [011]
A striped window 27 was formed in the direction (FIG. 2 (a)). At this time, the width of the window was 10 μm, and the width of the SiO ₂ layer on the upper surface where the bank was formed was 5 μm.

Next, as a first step, a semiconductor solution was etched for 30 seconds using a methanol solution containing 0.1% of bromine as an etching solution. At this time, the semiconductor crystal had a structure as shown in FIG. The depth of the groove is about 1 μm, the bottom of the groove does not reach the active region, and the side surface of the groove is (111) A.
An inverted trapezoidal dike was formed so that the surface appears.

Then, as a second step, the semiconductor crystal was etched using an HBr aqueous solution as an etching solution. At this time, the semiconductor crystal had a structure as shown in FIG. The bottom surface of the groove 28 becomes the top surface of the active region 24, and the p-type InP clad layer 2
The groove side surface of 3 continued to be the (111) A surface.

Then, as a third step, sulfuric acid 3, hydrogen peroxide 1, water 1
The semiconductor crystal was etched by using the mixed solution of (25 ° C.) as an etching solution. At this time, the semiconductor crystal had a structure as shown in FIG. Etching is n-type In
It stopped on the upper surface of the P clad layer 25.

Subsequently, as a fourth step, the semiconductor was etched using a HBr aqueous solution. At this time, the semiconductor crystal had a structure as shown in FIG. The bottom of the groove 28 is about 0.5 μm
The (111) B plane appeared on the side surface of the groove below the active region 24, reaching the type InP clad layer 25. The bottom of the groove was the (100) plane.

This semiconductor crystal was set in a vapor phase growth apparatus (in this example, a halide transport method vapor phase growth apparatus was used), and the groove 28 was selectively filled with a Fe-doped InP layer. Subsequently, a semiconductor laser as shown in FIG. 1 was manufactured by a commonly used semiconductor laser process.

The characteristics of the manufactured semiconductor laser have an oscillation threshold current of 14 mA,
The external differential quantum efficiency is 60%, and the oscillation threshold current is 1.
It was confirmed that a modulation band of 7 GHz or more was realized with the small signal modulation characteristics when biased at 5 times. On the other hand, as a result of randomly selecting the semiconductor lasers obtained from the wafer and examining the characteristics of the semiconductor lasers, it was found that the variation in the characteristics was extremely small and the lasers could be obtained with a high yield.

In the above embodiment, the forbidden band width of InGaAsP in the active region 15 is 0.9.
It was 5 eV (oscillation wavelength of 1.3 μm), but it is obvious that the composition is not limited to this.

In the above embodiment, the upper layer of the active region is p-type and the lower layer is n-type, but in the present invention, the upper layer may be n-type and the lower layer may be p-type.

In the upper embodiment, the side surface of the groove below the active region is the (111) B surface, but the invention is not limited to this, and may be the (21) surface, the (12) surface, or the (01) surface. Also, for the crystalline material used for the laser
Not limited to InGaAsP, it can be applied to many luminescent crystal materials such as AlGaAs, AlGaAsP, and GaAs.

(Effect of the Invention) The semiconductor laser manufactured by the method of the present invention has a structure that oscillates with high efficiency, is capable of high-speed modulation, and has high productivity. Further, according to the manufacturing method of the present invention, the semiconductor laser can be manufactured with high reproducibility and high yield.

[Brief description of drawings]

FIG. 1 is a sectional view of an embodiment of a semiconductor laser manufactured by using the method of the present invention, and FIGS. 2 (a) to 2 (e) are manufacturing process diagrams of an embedded semiconductor laser of the present invention. 11 …… p type electrode, 12,21 …… SiO ₂ layer, 13 and 22 …… p
-Type InGaAsP cap layer, 14 and 23 ... p-type InP clad layer, 15 and 24 ... Active region, 16 and 25 ... n-type InP
Cladding layer, 17 and 26 ... n-type InP substrate, 18 ... n-type electrode, 19 ... current blocking layer, 27 ... window, 28 ... groove

Claims (2)

[Claims] 1. A double hetero structure in which an active region is sandwiched between (100) planes of a semiconductor substrate and a clad layer having a refractive index lower than that of the active region and a large forbidden band width, and the active region is 2 It is located in a bank sandwiched between two grooves, and the inside of the groove is a current blocking layer made of a semi-insulating semiconductor, and an embedded semiconductor laser whose light guiding direction is parallel to the (011) direction is provided. In the method of manufacturing, the first step of etching so that the side surface of the groove becomes the (111) A plane and the bottom surface of the groove does not reach the active region, and the side surface of the groove is kept at the (111) A surface. A second step of etching to the upper surface of the active region, a third step of etching so that the bottom surface of the groove is flush with the upper surface of the cladding layer below the active region, and It is a crystalline surface and a forward mesa surface, and A fourth method of manufacturing a buried semiconductor laser, characterized in that the process comprises at least the etching so that the bottom surface of reaching the cladding layer of the bottom. 2. The one crystal plane of the groove side surface under the active region is
2. The method for manufacturing an embedded semiconductor laser according to claim 1, wherein the (111) B plane, the (21) plane, the (12) plane, or the (01) plane is one of them.