JPH05275797A - Manufacture of semiconductor laser - Google Patents

Abstract

PURPOSE:To improve controllability of a width of an active layer when a buried semiconductor laser using an InP substrate is manufactured and to satisfactorily form a current blocking layer. CONSTITUTION:A grating 33 is formed on an n-type InP substrate 31, and an n-type GaInAsP layer 35, a GaInAsP layer 37, a p-type InP layer 39 are formed thereon. An SiO2 mask 41 having a stripe direction of <011> and a width W of about 2mum is formed on the layer 39. The layer 39 is selectively etched with hydrochloric acid to form a first upper clad layer 39a, and hangs 41a are formed at the mask 41. The layers 37, 35 are etched by RIE with mixture gas of argon and chlorine to obtain an active layer 37a, an optical guide layer 35a. These layers 35a, 37a are etched by a sulfuric acid. Current blocking layers are formed at both sides of a stripelike mesa 43 by an MOVPE method.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing an embedded semiconductor laser (including one having an optical guide layer) using an InP substrate.

[0002]

2. Description of the Related Art A buried semiconductor laser is known as a practical semiconductor laser because it can oscillate in a single transverse mode and has a low oscillation threshold current. Further, an embedded semiconductor laser having a distributed feedback structure can further oscillate in a single axis mode, and is therefore known as a more suitable semiconductor laser.

An example of a method of manufacturing the former type of such embedded semiconductor lasers using an InP substrate is described in, for example, the document (Applied Physic Letters) relating to the applicant of the present application.
s Letters), Vol. 44. No. 10, p975-
977 (1984)). Below, FIG.
This conventional manufacturing method will be described with reference to (A) to (C) and FIGS. 5 (A) and (B). It should be noted that each of these figures shows a cross-sectional view of the state of the sample in the main steps of the manufacturing process.

First, on the p-type InP substrate 11, a p-type InP layer 13 as a semiconductor layer for forming a buffer layer, a GaInAsP layer 15 as a semiconductor layer for forming an active layer, and an n-type as a semiconductor layer for forming an upper clad layer are formed. The InP upper clad layer 17 is formed by the liquid phase epitaxial method. Next, a stripe-shaped etching mask 19 made of a SiO ₂ film on the n-type InP layer 17 has a stripe direction of [011] and a stripe width W 0 of about 20 μm.
The etching mask of 1 is formed by known photolithography technology and etching technology (FIG. 4A).

Next, a portion of the n-type InP layer 17 which is not covered with the etching mask 19 is etched by an etchant containing a mixture of hydrochloric acid and water (FIG. 4B).
Since this etchant has etching selectivity between InP and GaInAsP, when it reaches the GaInAsP layer 15, the etching in the depth direction automatically stops.

Next, potassium ferricyanide (K ₃ Fe (CN) ₆ ) and potassium hydroxide (KO) are used as etchants for selectively etching only GaInAsP.
H) and water are mixed together to form GaInA
The sP layer 15 is etched until its width W1 becomes about 2 .mu.m. Thereby, the GaInAsP active layer 15a
Are formed (FIG. 4 (C)). Although the width W1 of the active layer 15a is set to about 2 .mu.m, the width W0 of the etching mask 19 is set to about 20 .mu.m, which is ten times as large as the photolithography process.

Next, the p-type InP layer 13 and the n-type InP layer 17 are again formed by using an etchant in which hydrochloric acid and water are mixed.
Are etched. Note that this etching is performed until a part of the back surface of the etching mask 19 is exposed. When this etching is completed, a stripe-shaped double heterojunction structure portion 21 is obtained (FIG. 5A).

Next, the n-type InP current blocking layer 23 and the p-type InP current blocking layer 25 are formed on this sample by the liquid phase epitaxial method (FIG. 5 (B)).

After that, the etching mask 19 is removed. And, although not shown, a p-type electrode is formed on the back surface of the substrate 11.
Further, an n-type electrode is formed on the n-type InP upper cladding layer 17a side, and a buried type semiconductor laser is obtained.

In this semiconductor laser, when an appropriate voltage is applied between the p-type electrode and the n-type electrode to pass a current through the element,
Since the current blocking layers 23 and 25 are reverse biased, current is intensively injected into the active layer 15a which is forward biased. Therefore, this semiconductor laser oscillates efficiently with a low threshold. Further, the width W1 of the active layer 15a
Since it has been optimized, it oscillates in a single transverse mode.

[0011]

However, in the above-described method for manufacturing a semiconductor laser, the width W1 of the active layer 15a is reduced.
Since the etching mask 19 having a width of about 20 μm is used when the thickness is about 2 μm, the controllability of the width of the active layer 15a is poor. Further, since the etching means for forming the active layer 15a is wet etching, the active layer width is likely to vary among the semiconductor lasers manufactured in large numbers in the wafer, so that semiconductor lasers having uniform characteristics can be obtained. Was difficult.

Considering the case of manufacturing a semiconductor laser capable of controlling a single longitudinal mode as well, in such a semiconductor laser, a GaAsInP optical guide layer having a band gap larger than that of the active layer is formed above or below the active layer. It Even though this optical guide layer is the same GaAsInP layer as the active layer, its composition ratio is different from that of the active layer because the band gap is different, so the etching rates of both are different. Therefore, in the conventional manufacturing method described above, K ₃ Fe (CN) ₆
If the widths of the active layer and the light guide layer are adjusted with an etchant that is a mixture of KOH and H ₂ O, the widths of the two do not become the same.

In addition, in the InP-based semiconductor laser, the laser stripe direction is set to <011>, and a predetermined amount of eaves is provided on the etching mask (for example, the above-mentioned etching mask 19) used when the double heterojunction structure is formed. In the formed state, In on both sides of the double heterojunction structure
The growth of the P current blocking layer is performed by the metal organic vapor phase epitaxy (MOV
It is known that when the PE (Metal Organic Vapor Phase Epitaxy) method is used, the current block layer can be formed in a state where the thickness is well controlled and abnormal growth is small.
In this case, it is important to control the size of the eaves portion of the etching mask. However, in the above-described conventional manufacturing method, since the width W0 of the etching mask 19 and the width of the active layer 15a are greatly different, it is difficult to control the dimension of the eaves portion of the etching mask to a desired dimension.

The present invention has been made in view of the above circumstances. Therefore, an object of the present invention is a method of manufacturing a buried type semiconductor laser using an InP substrate and obtaining an active layer having a desired width. It is to provide a method capable of forming a current blocking layer by a MOVPE method.

[0015]

To achieve this object, according to the present invention, at the time of manufacturing a buried type semiconductor laser using an InP substrate, at least a semiconductor layer for forming an active layer and an active layer forming semiconductor layer are formed on the InP substrate. A step of forming a first upper clad layer forming semiconductor layer and, if necessary, a light guide layer forming semiconductor layer, and a stripe-shaped etching mask on the first upper clad layer forming semiconductor layer Forming an etching mask having a <011> direction and a stripe width substantially the same as an active layer width capable of oscillating the semiconductor laser in a single transverse mode;
A step of selectively etching the first upper clad layer forming semiconductor layer to form the first upper clad layer and forming an eaves portion on the etching mask; and a step of forming the active layer forming semiconductor layer. When a semiconductor layer for forming a light guide layer is formed, a step of etching each of the semiconductor layer for forming an active layer and the semiconductor layer for forming a light guide layer by the reactive ion etching method, and the above-mentioned reactive ion etching. In the active layer obtained in, also when forming a semiconductor layer for forming a light guide layer, the active layer and the light guide layer obtained by the reactive ion etching described above, the layer is etched and the etching mask described above, The step of etching the first upper clad layer and the InP substrate by a predetermined amount with an etchant that does not etch, and etching with the etchant The step of forming a current block layer on both sides of the striped mesa portion of the sample by a metal organic chemical vapor deposition method, the step of removing the etching mask from the sample on which the current block layer has been formed, and the step of removing the etching mask Forming a second upper cladding layer on the sample.

When the optical guide layer is provided, the semiconductor layer for forming the active layer may be formed on the element after forming the semiconductor layer for forming the optical guide layer, or vice versa, depending on the design of the semiconductor laser. But good. Further, a buffer layer may be provided on the InP substrate to improve the characteristics.

[0017]

According to the structure of the present invention, unnecessary portions of the semiconductor layer for forming an active layer (the semiconductor layers for forming the active layer and for forming the light guide layer when the light guide layer is provided) are removed by the reactive ion etching method. Since the active layer is obtained by removing, the controllability of the width of the active layer (width of each of the active layer and the light guide layer) is
It is improved as compared with the conventional method using the wet etching method.

Further, since a step of wet-etching the active layer (active layer and light guide layer) by a predetermined amount is provided after the reactive ion etching, the active layer (active layer and light guide layer) is formed during the reactive ion etching. Even if damage occurs, this can be resolved.

Before the active layer is formed, only the first upper clad layer forming semiconductor layer is selectively processed to form the first upper clad layer, and the eaves portion of the etching mask is formed in this step. Therefore, the eaves portion of the etching mask can be formed without being restricted by the width of the active layer, which facilitates the dimensional control of the eaves portion. Further, since the first upper clad layer and the second upper clad layer are separated, the first upper clad layer need not be so thick (it is enough to protect the active layer). Since the thickness can be made thin enough to be formed, the dimensional control of the eaves is facilitated also in this respect. Therefore, M on both sides of the double heterojunction structure
A desired InP current blocking layer can be formed by the OVPE method.

Further, since the thickness of the first upper clad layer does not have to be so large as described above, the height of the mesa portion filled with the current blocking layer can be lowered. Therefore, it is possible to satisfactorily embed the current blocking layer in the mesa portion.

[0021]

Embodiments of the method for manufacturing a semiconductor laser according to the present invention will be described below with reference to the drawings. However, the drawings used in the description merely schematically show the dimensions, shapes, and arrangement relationships of the respective constituents to the extent that the present invention can be understood. Numerical conditions such as dimensions and composition ratio of the etchant in the following description are only suitable examples within the scope of the present invention. Therefore, the present invention is not limited to these numerical conditions. In addition, the following examples,
This is performed by an example of manufacturing an embedded semiconductor laser having a distributed feedback structure.

FIGS. 1A and 1B and FIGS.
3 (C) and FIGS. 3 (A) to 3 (C) are process drawings for explaining the manufacturing method of the example. In each of the drawings, the state of the element in the main process of the manufacturing process is cut along the direction orthogonal to the laser stripe in the perspective view in FIG. 1A and in the other drawings. It is shown by an element cross-sectional view.

First, in this case, an n-type I as an InP substrate
A corrugated grating 33 is formed on the nP substrate 31 by a normal interference exposure method (FIG. 1A).

Next, on the substrate 31 on which the grating has been formed, an n-type GaI as a semiconductor layer for forming a light guide layer is formed.
nAsP layer 35, GaI as active layer forming semiconductor layer
The nAsP layer 37 and the p-type InP layer 39 as the first upper clad layer forming semiconductor layer are formed in this order by, for example, the MOVPE method (FIG. 1B). However, the film thickness of each layer should be an appropriate film thickness according to the design. Also, the layers 35, 37 differ from each other in composition as desired.

Next, a silicon oxide film (SiO 2 in this case) is formed on the p-type InP layer 39 as an etching mask forming thin film.
_{The two} thin films are formed by, for example, chemical vapor deposition (CVD (Chemic
al Vapor Deposition) method). Next, this silicon oxide film is processed by known photolithography technology and etching technology to form on the p-type InP layer 39,
The stripe direction is the <011> direction, and the stripe width W is substantially the same as the width of the active layer capable of oscillating the semiconductor laser in the single transverse mode (including, of course, the same case. For example, 1.5
A striped etching mask 41 having a thickness of 2 μm) is formed (FIG. 2A). The etching mask 41 may be made of another suitable dielectric film such as a silicon nitride film.

Next, the p-type InP layer 39, which is the semiconductor layer for forming the first upper cladding layer, is selectively etched to form the first
Etching mask 4 for forming upper clad layer 39a
The eaves portion 41a is formed on the substrate 1 (FIG. 2 (B)). In this embodiment, this selective etching is performed using a hydrochloric acid-based etchant (in this case, a mixture of hydrochloric acid and water at a volume ratio of 4: 1) and an etching temperature of 5 ° C. or less. Under these etching conditions, the etching proceeds isotropically, but GaInA which is the active layer forming semiconductor layer is formed.
Since the selection ratio between the sP layer 37 and the InP layer 39 that is the first upper cladding layer forming semiconductor layer is large, when the etchant reaches the GaInAsP layer 39, the etching in the depth direction is automatically stopped. Therefore, when the etching in the depth direction is stopped, an overhang (overhanging portion) 41a having the same thickness as the InP layer 39 can be formed on the etching mask 41.

Next, GaI which is a semiconductor layer for forming an active layer is formed.
nAsP layer 37 and n which is a semiconductor layer for forming a light guide layer
The type GaInAsP layer 35 is etched by reactive ion etching to obtain an active layer 37a and a light guide layer 35a having desired widths, respectively. As a result, the stripe-shaped mesa portion 43 is obtained (FIG. 2C). At the time of this etching, the etching time is controlled so that the height of the stripe-shaped mesa portion 43 becomes a height that can be favorably embedded when the mesa portion is buried later by the current blocking layer. In this embodiment, the etching time is controlled so that a mesa portion having a height of about 2 μm can be obtained. The etching as described above is performed, for example, in the document (198) relating to the applicant of this application.
Spring 5th, Proceedings of JSAP Academic Lecture, p. 173
This can be performed by the reactive ion etching method using a mixed gas of argon and chlorine as the reactive gas as disclosed in Lecture No. 29a-T-4). In this method, the GaInAsP layer 37 and the n-type GaInAsP layer 35 are used.
Since the InP substrate 31 and the InP substrate 31 can be etched in the thickness direction without selectivity, the GaInAsP layer 37 is formed.
The n-type GaInAsP layer 35 can be processed to have the same width as the width W of the etching mask 41.

Next, in order to remove the damage of the active layer 37a and the light guide layer 35a obtained by the above reactive ion etching during the reactive ion etching, these layers 35a and 37a are etched and the etching mask 4 is used.
1, etchant that does not etch the upper first cladding layer 39a and the InP substrate 31. In this embodiment, these layers 37a and 35a are formed by using a sulfuric acid-based etchant (sulfuric acid: hydrogen peroxide: water = 4: 1: 1 (volume ratio)). Each side surface is etched by a predetermined amount (FIG. 3 (A)) In this etching, the active layer 37a and the light guide layer 35a have different etching rates, and the active layer 37a has a higher etching rate. This etching is performed so that the side surface of 37a is etched by about 0.2 μm, instead of the sulfuric acid type etchant, potassium ferricyanide type etchant (for example, K ₃ Fe (CN) ₆ : KOH:
The same effect can be obtained by using H ₂ O = 1: 10: 100 (weight ratio)).

Next, a p-type InP layer 45a and an n-type InP layer 45b are formed in this order on both sides of the stripe-shaped mesa portion 43 of this sample (FIG. 3A) by the MOVPE method to form a current block. A layer 45 is obtained (FIG. 3 (B)). In this crystal growth, the width of the etching mask 41 is about 2 μm.
Since the thickness is m, the growth of these InP layers 45a and 45b does not occur on the etching mask 41, but occurs only on both sides of the mesa portion 43. In addition, the eaves portion 41a is formed on the etching mask 41.
Since abnormal growth does not occur in the growth of the InP layers 45a and 45b, the stripe-shaped mesa portion 43 can be filled with the relatively flat InP layers 45a and 45b. The thickness of each of the p-type InP current blocking layer 45a and the n-type InP current blocking layer 45b must be such that a sufficient breakdown voltage can be obtained when a voltage required for its operation is applied to this semiconductor laser. However, on the other hand, in buried growth, the controllability of the growth is better when the thickness of the current block layer is thinner, so these points are taken into consideration when making the decision. In this embodiment, when the carrier concentration of each of the layers 45a and 45b is about 5 × 10 ¹⁷ / cm ³ , the p-type InP layer 45 is formed.
The film thickness of a is 1.5 μm, and the film thickness of the n-type InP layer 45b is 0.5 μm.

Next, the etching mask 41 is removed with hydrofluoric acid. Next, a p-type InP layer as the second upper clad layer 47 is formed on this sample by, for example, the MOVPE method,
A p-type InGaAs layer 49 as the contact layer 49 is sequentially formed (FIG. 3C).

After that, although not shown, an n-type electrode is formed on the back surface of the substrate 31 and a p-type electrode is formed on the contact layer 49 by known methods to obtain an embedded semiconductor laser having a distributed feedback structure.

Although the embodiment of the method for manufacturing a semiconductor laser according to the present invention has been described above, the present invention is not limited to the above embodiment.

For example, the above-described embodiment is an example of manufacturing a buried feedback type semiconductor laser using an InP substrate and a distributed feedback type having a grating. However, the manufacturing method of the present invention can also be applied to manufacture of a normal Fabry-Perot type semiconductor laser. Further, of course, it can be applied to the case of manufacturing a semiconductor laser in which the substrate is a p-type InP substrate and the conductivity type of each layer is the opposite conductivity type.

[0034]

As is apparent from the above description, according to the method of manufacturing a semiconductor laser of the present invention, unnecessary portions of the semiconductor layer for forming the active layer and the semiconductor layer for forming the optical guide layer are formed by the reactive ion etching method. Is removed to obtain the active layer and the light guide layer, the controllability of the width of the active layer and the light guide layer is
It is improved as compared with the conventional method using the wet etching method. In addition, because a predetermined wet etching is performed,
Damage caused by reactive ion etching can be eliminated by this.

Before forming the active layer, only the first upper clad layer forming semiconductor layer is selectively processed to form the first upper clad layer, and in this step, the eaves portion of the etching mask is formed. Therefore, the eaves portion of the etching mask can be formed without being restricted by the width of the active layer. Furthermore, since the first upper clad layer and the second upper clad layer are separated, the thickness of the first upper clad layer can be reduced. From these points, the size of the eaves portion of the etching mask can be set to a desired size. Therefore, a desired current blocking layer can be formed on both sides of the double heterojunction structure by the MOVPE method.

Further, since the thickness of the first upper clad layer does not have to be so large as described above, the height of the mesa portion filled with the current blocking layer can be lowered, so that the filling of the mesa portion with the current blocking layer is favorable. Can be done.

[Brief description of drawings]

FIG. 1A and FIG. 1B are process drawings for explaining an example.

2A to 2C are diagrams for explaining an embodiment. FIG.
FIG.

3A to 3C are diagrams for explaining the embodiment.
FIG.

FIG. 4A to FIG. 4C are process diagrams for explaining the conventional technique.

5A and 5B are process diagrams following FIG. 4 for explaining the conventional technique.

[Explanation of symbols]

31: n-type InP substrate 33: grating 35: optical guide layer forming semiconductor layer (n-type GaInAsP
Layer) 35a: optical guide layer 37: active layer forming semiconductor layer (GaInAsP layer) 37a: active layer 39: first upper cladding layer forming semiconductor layer (p-type InP)
Layer) 39a: first upper clad layer 41: etching mask (SiO ₂ film) 43: stripe mesa portion 45: current blocking layer 45a: p-type InP layer 45b: n-type InP layer 47: second upper clad layer ( p-type InP layer) 49: contact layer (p-type InGaAs layer)

Claims (2)

[Claims] 1. When manufacturing a buried type semiconductor laser using an InP substrate, at least a semiconductor layer for forming an active layer and a semiconductor layer for forming a first upper clad layer and, if necessary, for forming an optical guide layer on the InP substrate. Forming a semiconductor layer, and forming a stripe-shaped etching mask on the first upper clad layer forming semiconductor layer, wherein the stripe direction is <
Forming an etching mask in which the width of the stripe in the 011> direction is substantially the same as the width of an active layer capable of oscillating the semiconductor laser in a single transverse mode; and selectively forming the first upper clad layer forming semiconductor layer. Etching to form the first upper clad layer and form an eaves portion on the etching mask; the active layer forming semiconductor layer; and the optical guide layer forming semiconductor layer when the active layer is formed. Forming a semiconductor layer for forming a light guide layer and a semiconductor layer for forming a light guide layer, a step of etching each of the semiconductor layers for forming a light guide layer, and an active layer obtained by the reactive ion etching. If the active layer and the light guide layer obtained by the reactive ion etching,
Said layer is etched and said etching mask, first
The upper clad layer and the InP substrate are etched by a predetermined amount with an etchant that is not etched, and a step of forming a current block layer by metalorganic vapor phase epitaxy on both sides of the stripe-shaped mesa portion of the sample etched by the etchant. A method of manufacturing a semiconductor laser, comprising: a step of removing the etching mask from the sample on which the current block layer has been formed; and a step of forming a second upper cladding layer on the sample on which the etching mask has been removed. 2. The method for manufacturing a semiconductor laser according to claim 1, wherein the selective etching of the semiconductor layer for forming the first cladding layer is performed with a hydrochloric acid-based etchant, and the etchant is used after reactive ion etching. Is a sulfuric acid-based etchant or potassium ferricyanide-based etchant.