JPS6272191A - Semiconductor laser - Google Patents

Abstract

PURPOSE:To obtain a semiconductor laser of a BH structure with a flat epitaxial surface by a method wherein the 2nd cladding layer on an active layer and the active layer are formed into a normal mesa shape by mesa etching and the active layer is scraped to a required stripe width by side etching and buried layers are successively formed on the mesa surface. CONSTITUTION:An N-type InP buffer layer 2, an undoped active layer 3 and a P-type InP cladding layer 4 are formed on an N-type InP substrate 1. The P-type InP cladding layer 4 is formed into a normal mesa shape. The stripe width of the active layer 3 is controlled by side etching. After such etching processes, a P-type InP buried layer 5, an N-type InP buried layer 6 and a P-type InGaAsP contact layer 7 are successively made to grow. The width W of the top surface of the mesa of the P-type Inp cladding layer 4 is made to be less than about 4mum. With this constitution, the buried layers 5 and 6 are not made to grow on the top surface of the mesa of the P-type InP cladding layer 4 or do not climb up along the side surface of the cladding layer 4 nd grow flat so that the P-type InGaAsP contact layer 7 can be sufficiently flat.

Description

[Detailed description of the invention] Industrial applications The present invention relates to an embedded semiconductor laser.

2. Description of the Related Art Conventionally, various structures have been proposed for semiconductor lasers capable of low threshold current and single mode oscillation, and among them, a buried type (BH) structure semiconductor laser is superior.

Hereinafter, a BH groove structure raw material laser made of InGaAsP/InP-based materials will be described without reference to the drawings.

FIG. 3 is a sectional view of a conventional BH groove structure semiconductor laser. In FIG. 3, 21 is a (100) n-type InP substrate;
22 is an n-type InP cladding layer, 23 is an InGaAsP active layer, 24 is a P-type InP cladding layer, and 25 is a P-type InP layer.
This is a GaAs P contact layer. Striped S is formed on the epitaxial wafer surface in the (OTI) direction.
A t02 film is formed and reverse mesa etching is performed using a Br2-methanol solution to form a KP type InP layer 26. on the reverse mesa surface. n
The buried layers of the InP layer 27 and the n-type InGaAsp layer 28 are sequentially formed by liquid phase growth to provide single current confinement and lateral light confinement effects from the active layer region.

However, in the above structure, the semiconductor surface is not flat but convex, and when mounted on the P side down,
Wetting between the mount metal and the P-type electrode 29 is poor, and thermal resistance increases, resulting in an adverse effect on the temperature characteristics of the semiconductor laser. Further, during wire bonding to the laser chip, pressure due to bonding concentrates on the surface of the mesa portion, damaging the active layer 23 and causing deterioration of characteristics. On the other hand, the total thickness of the buried layer 26.2728 is adjusted in consideration of the height of the mesa part in order to flatten the semiconductor surface with regard to controllability of liquid phase growth as well as the problem of poor assembly as described above. There is a need to. In addition, since the reverse mesa structure is adopted, the crystallinity between the reverse mesa side surface and the buried layer 26, 27, 28 is poor, and the metal of the electrode 29 invades the active layer 23 along the reverse mesa side surface, resulting in deterioration of the device. It is the cause of.

Therefore, the BH groove structure PBH with a flattened semiconductor surface
) has been proposed, and an example is shown in FIG. In Figure 4,
31 is a (100) n-type InP substrate, 32 is an n-type InP cladding layer, 33 is an InGaAsP active layer, and 34 is a P-type I
It is an nP cladding layer. After mesa etching the epitaxial wafer made of the above semiconductor layer, a P-type InP buried layer 36. is formed by liquid phase growth. n-type InP buried layer 3
7. A P-type InP layer 3 and a P-type InGaAsP contact layer are successively formed.

Here, a buried layer 36°3 is placed on the P-type InP cladding layer 34.
It is thought that the reason why 7 is not formed is that when the stripe width of the mesa is as narrow as 6 μm or less, the growth solution on the mesa surface does not reach supersaturation. P produced in this way
BH lasers have a flat surface as shown in Figure 4, so they suffer from the problems of stress on the mesa part that occurs during mounting, poor immersion with the mount metal, and crystallinity on the side of the reverse mesa. This solves problems such as deterioration of the active layer due to metal intrusion due to poor quality. However, a drawback of the conventional PBH laser is that the n-type InP buried layer 3 near both sides of the mesa part
7 is thin, the PnPn thyristors formed in the buried layers on both sides of the mesa part are turned on during high injection operation, increasing the reactive current flowing to areas other than the active layer 33, resulting in a problem that the optical output characteristics deteriorate. It is occurring.

Problems to be Solved by the Invention In such conventional BH groove structure semiconductor lasers, the surface is not flat, so heat dissipation is poor, many assembly defects occur, and crystal growth reproducibility is also problematic.

In addition, regarding the PBH structure semiconductor laser proposed to solve these problems, there is a problem in terms of current confinement, in which reactive current is generated during high injection operation because there is a thin region in a part of the buried layer. The present invention has been made in view of the above points, and an object thereof is to provide a BH groove structure semiconductor laser having a flat semiconductor strip surface that can be manufactured by a simple process.

Means for Solving the Problems In order to solve the above-mentioned problems, the present invention performs mesa etching on the second cladding layer and the active layer on the active layer in a sequential mesa shape, and the active layer is etched to a desired stripe width. Side etching is performed, and then a buried layer is sequentially and flatly formed on the mesa surface.

Operation According to the present invention, the semiconductor surface is flattened, so that heat dissipation of the semiconductor laser is improved, and the problem of assembly defects caused by wire bonding and the like is also reduced. The reproducibility of embedded growth is also excellent. In addition, because it has a forward mesa structure, the pressure on the chip that occurs during bonding is not applied directly to the active layer, and even if the metal of the surface electrode invades along the mesa side, it does not directly enter the active layer. Therefore, it is expected that the lifespan will be extended. Furthermore, since the buried layer has a uniform and flat thickness, current narrowing characteristics are also excellent.

Embodiment FIG. 1 is a cross-sectional structural diagram showing a first embodiment of the semiconductor laser of the present invention. In FIG. 1, 1 is a (1oo) n-type InP substrate, 2 is an n-type InP buffer layer (thickness d~3μ
m), 3 is an undoped active layer ((1-o, 2 μm),
4 is a P-type InP cladding layer (d~1.6 μm).

The P-type InP cladding layer 4 has a forward mesa shape as shown in FIG. 1, and the stripe orientation is in the (011) direction. In addition, the method of forward mesa etching is CvDS
By etching with a hydrochloric acid-based aqueous solution using the 102 or sputtered 5tO2 film as a mask, it can be easily etched into a mesa shape. P-type I using this hydrochloric acid aqueous solution
The nP layer 4 is selectively etched until the surface of the active layer 3 is exposed. Then, the selectively exposed active layer 3 is selectively etched using a sulfuric acid/hydrogen peroxide aqueous solution. On the other hand, regarding the stripe width of the active layer 3,
Side etching can be performed with good controllability by using a hydrogen peroxide aqueous solution.

After performing the above etching process, the embedded LPE
Do growth. As for the LPE growth conditions, for example, the melting time is 620"030 minutes.

By cooling at 0.7°C for 1 minute, the p-type InP buried layer 5 (d ~ 0.5 pm) and n-type In
P buried layer 6 (d~1.s μm) and P type InGaA
sP=+7 tact layer 7 (d~1 μm) was grown. Here, in order to prevent heat damage to the substrate, measures were taken by covering the substrate with an InP wafer, introducing PH3 gas, or using Sn melt in which InP was dissolved.

By performing buried LPE growth in this way, the first
The shrimp structure shown in the figure was obtained. Here, the width W of the mesa upper surface of the P-type InP cladding layer 4 was set to 4 μm or less. This is because the W is 4 μm or less and the P-type InP cladding layer 4 is
By passing the above-described sequential mesa etching process, a buried layer 6 is formed on the mesa upper surface of the P-type InP cladding layer 4.
．． 6 is a P-type InP cladding layer 4 without growth.
grows flat without climbing up the sides of P.
The type InGaAs P contact layer 7 has become sufficiently flat.

Then, P-type and n-type electrodes were formed and assembled with the P side down relative to the laser chip, and the laser characteristics were investigated. First, regarding assembly defects, because the surface is flat, the number of defects caused by pressure on the chip due to wire bonding, etc., has been drastically reduced. In addition, the threshold current oscillates at a low threshold current of 16 to 20 mA in room temperature CW operation compared to an active layer width of 2 μm, and the semiconductor laser has good current-optical output characteristics, particularly in high-temperature operation. An improvement in the characteristics of shi was observed.

Next, a second embodiment will be described. FIG. 2 is a cross-sectional structural diagram showing a second embodiment. The structure and shape of semiconductor layers 1, 2, 3 and 4 are the same as in the first embodiment. The difference is that the buried structure is n-type InP as shown in Figure 2.
Buried layer 6 (d~0.5pm), P-type InP buried layer 9 (d~1..5 μm), n-type InGaAa
P:y 7 tact layers 1o (d~1 μm) are sequentially formed, and an n-type 1nGaAs layer immediately above the P-type InP cladding layer 4 is formed.
The point is that a part of the P contact layer 10 is P-type diffused to narrow the current. Although a P-type InP buried layer 9 is embedded on the mesa upper surface of the P-type InP cladding layer 4, it is not necessarily necessary due to the structure of the laser, and the P-type I
n-type InGaAs directly on the mesa upper surface of the nP cladding layer 4.
The P type diffusion region 11 may be provided by forming the P contact layer 1o.

The characteristics of the semiconductor laser having the structure of the second embodiment will be described. In the conventional BH tank structure laser (see FIG. 3), the interface between the P-type InP buried layer 26 and the n-type InP buffer layer 22 is poor.

This is thought to be because the n-type InP buffer layer is once exposed to air after mesa etching, and an oxide film or the like is generated on the surface. Therefore, the injection current causes so-called leakage current flowing from the P-type InP cladding layer 24 to the substrate 21 through the P-type InP buried layer 26, which adversely affects the increase in threshold current and temperature characteristics. The same can be said for the structure of the first embodiment.

However, in the structure of the second embodiment, since the PN buried layer interface is between the n-type 1nP buried layer 8 and the P-type InP buried layer 9, there is almost no leakage current flowing from the injection current to the buried layer, which is good. characteristics were obtained.

Although the embodiments have been described using InGaAsP/InP-based materials, the same can be said of AlGaAs-based materials and other materials.

Effects of the Invention As described above, according to the present invention, the surface of a buried semiconductor laser can be easily flattened through a simple process, and is extremely effective in terms of semiconductor laser characteristics, especially temperature characteristics. In addition, the number of defects that occur during assembly has also decreased. Moreover, since the conventional buried semiconductor laser shown in Figure 3 uses a (111)A reverse mesa surface, there is a problem with the crystallinity between the buried layer and the reverse mesa surface, which affects reliability, life, etc. However, in the present invention, since the A side of such a polyhedral index is not used, reliability is also greatly improved. In addition, since the forward mesa structure is used, pressure on the laser chip may be applied directly to the active layer during assembly processes such as wire bonding, and even if the electrode metal on the surface penetrates along the side of the mesa, it will not reach the active layer. Since there is no need to do this, reliability can be expected to improve.

Furthermore, since the buried layer is flat and has a uniform thickness, current narrowing characteristics are also excellent.

From the above points of view, the structure is much more suitable for practical use than the various semiconductor lasers currently being devised, and has high industrial value.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view of a buried semiconductor laser according to a first embodiment of the present invention, FIG. 2 is a cross-sectional view of a buried semiconductor laser according to a second embodiment of the present invention, and FIGS. FIG. 2 is a cross-sectional view of an embedded semiconductor laser. 3... Active layer, 6... P-type buried layer,
6...N type buried layer, 8...N type buried layer, 9...P type buried layer. Name of agent: Patent attorney Toshio Nakao and 1 other person No. 1
Figure 3 Figure 20 Riverside Electrode Figure 4

Claims (4)

[Claims] (1) a first conductivity type cladding layer on the first conductivity type substrate;
an active layer and a second conductivity type cladding layer;
1. A semiconductor laser device characterized in that the conductive type cladding layer and the active layer are formed in a mesa shape in one direction, and both sides of the mesa portion are buried flatly. (2) The semiconductor laser device according to claim 1, wherein the active layer formed in a mesa shape is side-etched to form a desired stripe width. (3) A second conductivity type buried layer, a first conductivity type buried layer, and a second conductivity type buried layer are sequentially formed on both sides of the mesa portion, and the surface of the first conductivity type buried layer is The second conductive type buried layer is located below the surface of the second conductive type cladding layer, and the second conductive type buried layer is formed to cover the surface of the second conductive type cladding layer. 1
3. The semiconductor laser device according to item 1 or 2. (4) A first conductive type buried layer, a second conductive type buried layer, and a first conductive type buried layer are sequentially and flatly formed on both sides of the mesa portion, and the second conductive type buried layer is the same as the first conductive type buried layer. A diffusion region of the second conductivity type is formed so as to cover the surface of the second conductivity type cladding layer, and a part of the first conductivity type buried layer is formed in a stripe shape in the same direction as the normal mesa. 3. The semiconductor laser device according to claim 1, wherein a mold cladding layer is also formed.