JPH0824210B2 - Optical bistable semiconductor laser and manufacturing method thereof - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Outline] A structure of an optical bistable semiconductor laser exhibiting a bistable optical output with respect to an optical input and a manufacturing method thereof, in which a width of a separation groove for separating a plurality of electrodes is narrowed, and
An active layer, upper and lower cladding layers forming a heterojunction with the active layer, and electrodes for injecting a current into the active layer for the purpose of increasing the isolation resistance, and
In a semiconductor laser having a laser layer formed in a stripe shape, the electrode is divided into a plurality of pieces by a separation groove that is orthogonal to the stripe direction and reaches at least the upper cladding layer, and in addition, Both side surfaces of the laser layer and the separation groove are buried by a semi-insulating semiconductor layer.

TECHNICAL FIELD The present invention relates to a structure of an optical bistable semiconductor laser and a manufacturing method thereof.

An optical bistable semiconductor laser is a laser that can independently control the currents flowing through the divided electrodes and have two stable optical outputs. Such an optical bistable element is used for communication using optical signals. In the field of information processing, a functional element that enables a switch, a memory, an operation, and an amplification, and is small in size, easy to operate, and highly integrated, so that the research and development thereof is strongly promoted.

In such an optical bistable semiconductor laser, increasing the isolation resistance between the separated electrodes and narrowing the width of the isolation groove is the key to improving the characteristics, and a laser device having such a structure is desired. There is.

[Prior Art] The structure of an optical bistable semiconductor laser was written by GJ Lasher.
In 1964, an electrode (tandem electrode) structure was announced in which the electrode was divided into two in the cavity direction, one of which was used as a light amplification region and the other as a light absorption region (tandem electrode). Solid state Electronics Volume 7 (1964) .PP.7
07-716). This structure is the following double heterojunction,
The application of the embedded structure has almost reached the practical level.

Previously, the inventor proposed an optical bistable semiconductor laser in which the confinement layers (embedded layers) on both sides of the mesa stripe laser layer were embedded with semi-insulating high-resistance InP using the chemical vapor deposition (CVD) method. (See Japanese Patent Application No. 61-304518). With this structure, the resistance between the divided electrodes can be easily set to 1 KΩ or more, and the sneak of the injection current can be prevented, so that a uniform and highly reproducible element can be obtained.

FIG. 6 is a diagram showing the proposed conventional optical bistable semiconductor laser, in which FIG. 6 (a) is a perspective view and FIG. 6 (b) is its perspective view.
It is a XX 'sectional view. In the figure, 1 is an n-InP substrate, 2 is an n-InP substrate
Cladding layer (also called buffer layer), 3 is InGaAsP active layer (undoped), 4 is p-InP cladding layer, 5 is p ⁺ -InGaAsP
Cap layer, 8 is an insulating film made of SiO ₂ film, 9A and 9B are separate p-type electrodes, 10 is a backside n-type electrode, and 11 is a semi-insulating layer of the confinement layer.
InP layers (SI-InP layers), 12 are isolation trenches, and the striped laser layer 6 is composed of the InGaAsP active layers 3, p-I from the n-InP cladding layer 2.
It is formed in a portion reaching the nP cladding layer 4 and the p ⁺ -InGaAsP cap layer 5. The resistivity of the SI-InP layer 11 is 10 to 10
It is about Ωcm, and the dimensions of this laser device are as follows.

A = B = 300 μm, C = 120 μm, confinement layer depth d = 2.5-3 μm, stripe width W = 1-2 μm, separation width X = 15 μm (10-40 μm). The isolation groove 12 of this conventional structure is a groove having a depth removed up to the InGaAsP cap layer.

[Problems to be Solved by the Invention] In the above-mentioned optical bistable semiconductor laser, the relationship between the separation groove width and the resistance value between the electrodes, in which the resistance value between the electrodes is measured by changing the width of the separation groove 12 FIG. 7 shows a chart, and FIG. 8 shows a relational chart between the separation groove width and the oscillation threshold value which are obtained by measuring the oscillation threshold value by changing the separation groove width.

It is clear from the chart shown in FIG. 7 that the separation groove width can be widened in order to increase the separation resistance. On the other hand, it can be seen from FIG. 8 that the oscillation threshold value sharply rises when the separation groove width is widened, and when the groove width exceeds 50 μm, the oscillation threshold value becomes extremely large at several hundred mA or more. This is because there is no electrode in the separation groove and no current is injected into the active layer directly below the separation groove, which causes a large loss in the resonator and raises the oscillation threshold. However, this rise of the threshold value is a basic matter of characteristic deterioration, and it is desirable to maintain the threshold value as low as possible.

By the way, in the relationship diagram between the separation groove width and the electrode-to-electrode resistance value shown in FIG. 7 above, the ○ mark is the measured value, and the solid line is the resistance value of the InP clad layer obtained from the carrier transfer concentration. It is in agreement, but it is considered that the removal of the InP clad layer will significantly increase the isolation resistance, and by doing so, it becomes possible to further increase the isolation resistance without widening the isolation width. In addition, both InPs including the active layer 3
The clad layers 2 and 4 (lower part of the active layer) are removed, and the resistivity is 1
If the SI-InP layer 11 of 0 Ωcm is embedded, a separation resistance of 10 Ω or more can be expected when the separation groove width is 2 μm.

From this point of view, the present invention proposes a novel structure of an optical bistable semiconductor laser for the purpose of narrowing the width of the separation groove and increasing the separation resistance.

[Means for Solving the Problems] The purpose is to divide the electrode into a plurality of parts by a separation groove which is orthogonal to the stripe direction and reaches at least the upper cladding layer, and in addition, the stripe-shaped laser This is achieved by the structure of an optical bistable semiconductor laser in which both sides of the layer and the separating groove are buried by a semi-insulating semiconductor layer.

In addition, in order to form it, a step of forming a stripe-shaped laser layer including an active layer and upper and lower clad layers forming a heterojunction with the active layer on a semiconductor substrate, and both sides of the stripe-shaped laser layer. A step of forming a semi-insulating semiconductor layer for burying the surface, a step of forming an isolation groove orthogonal to the stripe direction and reaching the upper clad layer, and a step of burying the semi-insulating semiconductor layer in the isolation groove portion. The manufacturing method including the step of

Further, a step of forming an active layer and upper and lower clad layers forming a heterojunction with the active layer on a semiconductor substrate, and selectively etching the active layer and the upper and lower clad layers to form a stripe shape and orthogonal to the stripe direction. The manufacturing method includes a step of forming a laser layer divided into a plurality of parts by the separation groove, and a step of burying both side surfaces of the laser layer and the separation groove with a semi-insulating semiconductor layer.

[Operation] That is, according to the present invention, the isolation trench is formed to a depth reaching at least the upper cladding layer of the active layer, and the isolation trench is buried with the semi-insulating semiconductor layer. Then, the isolation resistance can be increased and the oscillation threshold can be lowered.

[Examples] Hereinafter, examples will be described in detail with reference to the drawings.

FIG. 1 is a diagram showing a structure (I) of a laser according to the present invention, in which FIG. 1 (a) is a perspective view and FIG. 1 (b) is a YY 'cross section thereof, showing the conventional structure shown in FIG. 1 is n-
InP substrate, 2 n-InP clad layer (buffer layer), 3 InGa
AsP active layer, 4 is p-InP clad layer, 5 is p ⁺ -InGaAsP cap layer, 6 is laser layer, 8 is SiO ₂ film (insulating film), 9A and 9B are separate p-type electrodes, 10 is n-type The electrode, 11 is an SI-InP layer, and 21 is an SI-InP layer in which the separation groove is buried. In this example, the separation groove is p
In the embodiment in which the -InP clad layer 4 (upper part of the active layer) is formed and the SI-InP layer 21 is buried in that portion, the laser structure is provided, the separation groove width X = 2 μm, and the stripe width W.
A separation resistance value of 56 KΩ can be obtained from a laser device having a dimension of = 1.5 μm. In addition, the rise in the oscillation threshold is only a few percent as compared with the usual semiconductor laser having a single p-type electrode. This is because the separation groove width is narrow and the InGaAsP active layer 3
It is considered that this is because the non-current injection width of is reduced and the loss in the resonator is reduced.

2 is a diagram showing a structure (II) according to the present invention. FIG. 2 (a) is a perspective view and FIG. 2 (b) is a ZZ 'cross section thereof, showing the structure shown in FIG. Although the same symbols are given to the same portions, 22 is an SI-InP layer in which the isolation groove is buried. Clad layer 2 (lower part of active layer)
In this embodiment, the SI-InP layer 22 is buried in the groove. Even if the active layer 3 is partially removed in the resonance direction and the SI-InP layer 22 is buried in the active layer portion in this way, the width of the SI-InP layer 22 is very narrow, about 2 μm.
Further, since the difference in optical refractive index between the InGaAsP active layer 3 and the iron-doped SI-InP layer 22 is not so large, a large propagation loss does not occur.

Beyond this InGaAsP active layer 3, n-InP clad layer 2
The structure (II) according to the present invention in which the SI-InP layer 22 is buried up to (the lower part of the active layer) is a structure in which the isolation layer having a higher resistance than that of the structure (I) is provided. According to the implementation results,
A separation resistance value of 10 Ω or more was obtained from a laser element having a separation groove width X = 2 μm and a stripe width W = 1.5 μm, and the mutual diffusion of injection current between tandem electrodes became negligible.

On the other hand, the p-InP on the upper side without cutting the InGaAsP active layer 3
The above-mentioned structure (I) according to the present invention in which the SI-InP layer 21 is buried up to the clad layer 4 (upper part of the active layer) is a structure in which the light propagation loss of the active layer is relatively small. ), (II is used depending on whether the isolation resistance or the oscillation threshold is prioritized.

The above example is an example of the structure of an optical bistable semiconductor laser having one separation groove. However, even if a structure in which two or more separation grooves are formed and three or more tandem electrodes are provided, the separation groove is similarly formed. When the SI-InP layer is buried in, the isolation resistance can be similarly increased and the oscillation threshold can be reduced. Fig. 3 (a), (b)
Shows another structural example in which two tandem electrodes are provided by forming two isolation trenches. In the figure (a), the isolation trenches are formed up to the p-InP cladding layer 4 as in the embodiment shown in FIG. And there S
In the example in which the I-InP layers 21A and 21B are buried, 9A, 9B and 9C are p electrodes. In FIG. 3 (b), similar to the embodiment shown in FIG. 2, isolation trenches are formed beyond the active layer 3 up to the n-InP cladding layer 2, and SI-InP layers 22A and 22B are buried therein, and p Electrodes 9A, 9B,
This is an example in which 9C is provided.

Next, a method of forming the above structure according to the present invention will be described. 4 (a) to 4 (e) are structures (I) according to the present invention.
FIG. 4A is a perspective view showing the order of forming steps of the n-InP substrate 1 by a known liquid phase epitaxial growth (LPE) method. Cladding layer 2, InGaAsP active layer 3, p-InP cladding layer 4, p ⁺ -InGaAsP
The cap layer 5 is grown to have a double heterojunction structure.

4 (b); Next, a stripe-shaped SiO ₂ film mask 31 (thickness of about 3000Å) is formed, and this is used as a protective film to etch the n-InP clad layer 2 to a width of 1 to 1.5 μm.
A stripe-shaped laser layer 6 including an active layer 3 having a height of about 2.5 μm is formed. The etching method is wet etching using nitric acid or dilute hydrochloric acid.

See FIG. 4 (c); then, the striped SiO ₂ film mask 31 is left as it is, and the portion removed by etching is used as a mask to dope iron with metal organic thermal decomposition vapor deposition (MOCDV) method. The SI-InP layer 11 is grown and buried until the surface becomes flat. At that time, so-called selective epitaxial growth is performed without forming a growth layer on the SiO ₂ film.

See FIG. 4 (d); next, the SiO ₂ film mask 31 is removed to form a new SiO ₂ film mask 32 in the direction perpendicular to the mask, and the p-InP clad layer 4 is etched to a width by using it as a protective film. Two
A stripe-shaped separation groove 13 having a size of μm and a height of 1.7 μm is formed.

See FIG. 4 (e); then, the SiO ₂ film mask 32 is left as it is, and the SI-InP layer doped with iron by the vapor phase epitaxial growth (VPE) method is used in the separation groove 13 portion using this as a mask.
21 is grown and buried until the surface becomes flat. At that time, selective epitaxial growth is performed without growing on the SiO ₂ film. The VPE method is used here because the VPE method is suitable for forming the growth layer in the narrow groove portion.

In the following, the SiO ₂ film mask 31 is removed, the p electrodes 9A and 9B and the n electrode 10 are formed, and the process is completed as shown in FIG.

Next, FIGS. 5 (a) to 5 (c) are perspective views in order of the forming process of the structure (II) according to the present invention, which will be sequentially described with reference to FIG. 5 (a); Similar to the above method, the n-InP clad layer 2 and InGaAsP are formed on the n-InP substrate 1 by the LPE method.
Active layer 3, p-InP clad layer 4, p ⁺ -InGaAsP cap layer 5
To grow.

See FIG. 5 (b); then, the SiO ₂ film mask 33 (film thickness 3000Å) for simultaneously etching the confinement layer and the separation groove.
Is formed and is used as a protective film to etch to the n-InP clad layer 2 to form a striped laser layer 6 including an active layer 3 having a width of 1 to 1.5 μm and a height of 2.5 μm.
To form. The etching method is wet etching using nitric acid or dilute hydrochloric acid.

See FIG. 5 (c); next, the SiO ₂ film mask 33 is left as it is, and this is used as a mask in the etching removed portion,
The SI-InP layers 11 and 22 doped with iron are grown and buried by the MOCDV method until the surface becomes flat. At that time, a growth layer is not formed on the SiO ₂ film, and selective epitaxial growth is performed.

After that, the SiO ₂ film mask 33 is removed and the p-electrodes 9A and 9B and the n-electrode 10 are formed to complete the process as shown in FIG.

This structure (II) is easy to manufacture because the SI-InP layers 11 and 22 are grown at the same time and only one burying step is required.

[Effects of the Invention] As is clear from the above description, according to the present invention, a bistable semiconductor laser having a high separation resistance and a low oscillation threshold value can be obtained, which significantly contributes to the development of optical communication such as OEIC. Is.

[Brief description of drawings]

FIG. 1 is a diagram showing a structure (I) according to the present invention, FIG. 2 is a diagram showing a structure (II) according to the present invention, and FIGS. 3 (a) and 3 (b) are other structures according to the present invention. The figure which shows an example, FIGS. 4 (a)-(e) are perspective views in order of the formation process of the structure (I) concerning this invention, FIG. 5 (a)-(c) are the structures (II) concerning this invention. FIG. 6 is a diagram showing a conventional structure, FIG. 7 is a diagram showing a relationship between a conventional isolation groove width and an isolation resistance, and FIG. 8 is a conventional isolation groove width and an oscillation threshold value. It is a relationship chart. In the figure, 1 is an n-InP substrate, 2 is an n-InP clad layer (buffer layer), 3 is an InGaAsP active layer, 4 is a p-InP clad layer, 5
Is a p ⁺ -InGaAsP cap layer, 6 is a laser layer, 8 is a SiO ₂ film (insulating film), 9A, 9B, 9C are separate p-type electrodes, 10 is an n-type electrode, 11, 21, 22, 21A, 21B , 22A and 22B are semi-insulating InP layers (SI-I
nP layer), 12, 13 are isolation trenches, and 31, 32, 33 are SiO ₂ film masks.

Claims (3)

[Claims] 1. A semiconductor laser comprising an active layer, upper and lower clad layers forming a heterojunction with the active layer, electrodes for injecting current into the active layer, and a laser layer formed in a stripe shape. , The electrode is divided into a plurality of parts by a separation groove which is orthogonal to the stripe direction and reaches at least the upper cladding layer, and in addition, both side surfaces of the stripe-shaped laser layer and the separation groove are semi-insulating. An optical bistable semiconductor laser characterized by being buried by a semiconductor layer. 2. A step of forming a stripe-shaped laser layer comprising an active layer and upper and lower cladding layers forming a heterojunction with the active layer on a semiconductor substrate, and a semi-burying method for burying both side surfaces of the stripe-shaped laser layer. The method includes a step of forming an insulating semiconductor layer, a step of forming an isolation groove orthogonal to the stripe direction and reaching the upper cladding layer, and a step of burying a semi-insulating semiconductor layer in the isolation groove portion. A method for manufacturing an optical bistable semiconductor laser, characterized by comprising: 3. A step of forming an active layer and upper and lower clad layers forming a heterojunction with the active layer on a semiconductor substrate, and the active layer and the upper and lower clad layers are selectively etched to form stripes and the stripes. The method includes a step of forming a laser layer divided into a plurality of pieces by a separation groove orthogonal to the direction, and a step of burying both side surfaces of the laser layer and the separation groove with a semi-insulating semiconductor layer. A method for manufacturing an optical bistable semiconductor laser, comprising: