JPH0744311B2 - Method of manufacturing embedded semiconductor laser - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a semiconductor laser for optical communication that can oscillate with high efficiency and can be modulated at high speed, and a manufacturing method thereof.

(Prior Art and Problems Thereof) Semiconductor lasers have been put into practical use as a light source for optical fiber communication. A semiconductor laser used as a light source for optical fiber communication is required to oscillate with high efficiency and 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 the laser characteristics and that are closest to the above requirements, and the reasons why the above three requirements cannot be satisfied at the same time. Will be explained.

A typical semiconductor laser that has been manufactured in the past is a double channel plane buried semiconductor laser (Double Channel Planar Buried).
Heterostructure Laser Diode: DC-PBH LD for short) and is described in detail in Journal of Lightwave Technology, LT-1, Vol. 1983, March 1983, p. 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 flow the current in the stripe-shaped active region. ing. This structure is representative. n-
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 JSAP Academic Lecture Meeting, 2p-N-11, p.206, Autumn 1985). In this semiconductor laser, the capacitance of the diode is reduced by forming grooves in the flat block layers on both sides of the active layer until reaching the substrate so that high speed modulation is possible. As a result, it was possible to modulate at a relatively high frequency of 5 GHz during small signal modulation.
However, even with this semiconductor laser, the parasitic capacitance was not sufficiently reduced, and it was difficult to perform modulation over 10 GHz. 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 capacitance.

The second semiconductor laser is Electronics Letters, Vol. 22, No. 23, page 1214-121.
Pp. 5, 1986), it is a semiconductor laser formed by gas phase embedding. This semiconductor laser reduces the capacitance of the diode so that high-speed modulation is possible,
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 the efficiency decrease is oscillated with high efficiency, and the current is applied 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 a single transverse mode, the width of the active layer must be about 1 to 2 μm. This semiconductor laser has a mesa direction <
[0111] The trapezoid has an inverted trapezoidal shape that is oriented in the direction 011> and narrows 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. Therefore, there were many cases in which the etching did not reach the active layer due to the breakage of the bank including the active region or the shallow groove during the etching. Therefore, the semiconductor laser structure is not suitable for mass production.

As described above, the conventional semiconductor laser cannot simultaneously satisfy the three conditions that it oscillates with high efficiency, can perform high-speed modulation, and can be manufactured with high yield and high reproducibility. It is an object of the present invention to provide a method for manufacturing a semiconductor laser having a structure that oscillates with high efficiency, can perform high-speed modulation, and has high productivity.

(Means for Solving Problems) According to the invention of the present application, an active region has a refractive index lower than that of the active region on a semiconductor substrate whose surface is a (100) plane, and the active region is prohibited. It has a double hetero structure sandwiched by cladding layers having a forbidden band width larger than the band width, the active layer region is located in a bank sandwiched by two grooves, and the inside of the groove is made of a semi-insulating semiconductor. In the method of constructing an embedded semiconductor laser in which the light guiding direction is parallel to the (011) direction, the side surface of the groove portion is the (111) A surface and the bottom surface of the groove portion is A first step of etching to a depth that does not reach the active region; a second step of etching the bottom surface of the groove to be flush with the top surface of the active region; and a bottom surface of the groove portion below the active region. Until it is flush with the upper surface of the clad layer A buried semiconductor laser manufacturing method having at least a third step of etching is obtained.

(Operation) In the semiconductor laser manufactured by using 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 part can be significantly reduced compared to the conventional one.
Oscillation with high efficiency is possible. 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, since the low part of the high resistance layer on both sides of the active region is at the same height as the low part of the active region, the bank containing the active region is not broken. 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, etching is performed so that the bank including the active region has an inverted trapezoidal shape in order to control the width of the active region to 1 to 2 μm. By the first step, the side surfaces of the groove on both sides of the clad layer above the active region are (11
1) Etching is performed so that the surface A is obtained. In the second step, etching is performed so that the lower surface of the groove is flush with the upper surface of the active region and the entire side surface (111) A of the groove on both sides of the clad layer on the active region is the (111) A surface. Here, the above etching process is made into two processes for the following reason. That is, the clad layer on the active region is etched, but the active region is not etched, and there is very little etching liquid that makes the side surface of the groove the (111) A plane. That is,
An example of this is the case of etching a double-heterostructure crystal in which the active layer is InGaAsP and the upper and lower cladding layers are InP.

Etching solutions that etch InP but not InGaAsP include hydrobromic acid solution (HBr) and hydrogen chloride solution (HLl). However, when a stripe-shaped groove in the <011> direction is formed 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, a part of the groove side surfaces on both sides of the clad layer above the active region is previously set to the (111) A surface in the first step. If HBr is used as the etching solution in the subsequent second step, the (111) A surface that has been formed in advance triggers the etching, so that the entire side surface of the groove is (111) A.
It can easily be a surface. Here, even if the depth of the groove formed in the first step varies within the wafer surface,
Since the etching in the step (2) is stopped when the bottom surface of the groove reaches the upper surface of the active region, the groove depth becomes uniform over the entire wafer. In the subsequent third step, 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. All of the above steps have good reproducibility and excellent mass productivity.

(Example) The present invention will be described in more detail with reference to the drawings.
FIG. 1 is a sectional view showing an example of a semiconductor laser manufactured by using the present invention. Active region 3 has a forbidden band width of 0.95 eV
InGaAsP emits light at 1.3 μm. This active region 3 is n-type
On the InP substrate 1, n-type InP clad layer 2 and p-type In
It is surrounded by the P-clad layer 4 and from the left and right by a current blocking layer 6 of a semi-insulating semiconductor of Fe-doped InP composition. That is, this semiconductor laser is of a refractive index guided type. A p-type InGaAsP cap layer 5 is provided on the p-type InP clad layer 4 so as to obtain a good electrical contact with the p-type electrode 8. Further, in order to selectively flow the injected current into the active region 3 sandwiched by the current block layers 6, in the region other than the upper surface of the bank including the active region 3,
A SiO ₂ layer 7 is provided. The n-type electrode 9 is formed on the entire surface of the n-type InP substrate 1.

Next, an embodiment of the present invention for manufacturing the semiconductor laser shown in FIG. 1 will be described. FIG. 2 is a sectional view of an intermediate product formed in each step of the manufacturing method of the embodiment.

In this embodiment, the n-type InP clad layer 2 is formed on the n-type InP substrate 1.
(Thickness 3 μm, n = 1 × 10 ¹⁸ cm ^-3 ), undoped InGaAsP active region 3 (thickness 0.1 μm, forbidden band width 0.95 eV), p-type InP clad layer 4 (thickness 2.3 μm, p = 1 ×) 10 ¹⁸ cm ^-3 ), p-type InGaAsP cap layer 5 (thickness 0.2 μm, p = 1 × 10 ¹⁹ cm ^-3 , forbidden band width 1.1
eV) stacked double heterostructure semiconductor crystal was used. First, a SiO ₂ layer was formed on this semiconductor crystal, and then a stripe-shaped window 27 was formed in the (011) direction by a normal photolithography method (FIG. 2A). At this time, window 27
The width of the SiO ₂ layer 21 on the upper surface where the bank is formed is 5 μm
m.

Next, as a first step, a semiconductor solution was etched for 1 minute 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 3, and (11
1) An inverted trapezoidal embankment was formed so that the A side could appear.

Then, as a second step, the semiconductor crystal was etched using an HBr 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 became the upper surface of the active region 3, and the groove side surface of the p-type InP cladding layer 4 continued to be the (111) A surface.

Then, as a third step, a semiconductor crystal was etched using a mixed solution of sulfuric acid 3, hydrogen peroxide and water 1 (25 ° C.) as an etching solution. At this time, the semiconductor crystal had a structure as shown in FIG. The etching stopped on the upper surface of the n-type InP clad layer 2, and this surface became the groove bottom surface.

This semiconductor crystal was installed in a vapor phase growth furnace (in this example, a halide transport method vapor phase growth apparatus was used), and the groove 28 was Fe-doped.
It was selectively embedded by the 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 efficiency is 60%, and the oscillation threshold current is 1.5
It was confirmed that a modulation band of 6.5 GHz or more was realized with the small signal modulation characteristics when biased at double.

On the other hand, semiconductor lasers obtained from the wafer were randomly selected and their semiconductor laser characteristics were investigated. As a result, it was found that the variation in the characteristics was extremely small and the laser could be obtained with a high yield.

In the above embodiment, the band gap width of the InGaAsP crystal in the active region 3 was 0.95 eV, so that a laser oscillating at 1.3 μm was obtained, but it is clear that the composition is not limited to this mixed crystal composition.

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

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

[Brief description of drawings]

FIG. 1 is a sectional view of an embodiment of a buried type semiconductor laser manufactured by the method of the present invention, and FIG. 2 is a method of manufacturing the buried type semiconductor laser of FIG. 1 according to a second embodiment of the present invention. The process is shown, and this figure (a)-(d) is sectional drawing of the intermediate product formed in each process of manufacture. 1 ... n-type InP substrate, 2 ... n-type InP clad layer, 3 ...
Active region, 4 ... p-type InP clad layer, 5 ... p-type InGaA
sP cap layer, 6 ... Current blocking layer, 7,21 ... SiO
₂ layers, 8 ... p-type electrode, 9 ... n-type electrode, 27 ... window, 28
……groove.

Claims (1)

[Claims] 1. A clad layer having an active region on a semiconductor substrate whose surface is a (100) plane, having a refractive index lower than that of the active region, and having a forbidden band width larger than the forbidden band width of the active region. Having a double hetero structure sandwiched between them, the active layer region is located in a bank sandwiched by two grooves, and the inside of the groove is a current blocking layer made of a semi-insulating semiconductor,
In the method of constructing an embedded semiconductor laser in which the light guiding direction is parallel to the (011) direction, the side surface of the groove is (11
1) A first step of etching to a depth that does not reach the active region, and a second step of etching the bottom face of the groove to be flush with the top face of the active region. A method of manufacturing an embedded semiconductor laser, comprising at least a step and a third step of etching until a bottom surface of the groove is flush with an upper surface of a clad layer below the active region.