JPH04307780A - semiconductor laser - Google Patents

Abstract

PURPOSE:To contrive a reduction in the threshold value of a semiconductor laser and an increase in the efficiency of the laser by a method wherein the laser is formed of a first clad layer, a first optical waveguide layer, an active layer, a second optical waveguide layer and a second clad layer and the active layer is formed into a single quantum well structure wherein the number of a well layer is 1. CONSTITUTION:An N-type buffer layer 12, an N-type lower clad layer 13, an N-type lower optical waveguide layer 14, a quantum well active layer 15, a P-type upper optical waveguide layer 16, a p-type upper clad layer 17 and a p-type contact layer 18 are laminated in order on an N-type GaAs substrate 11. The layer 18 and the layer 17 are processed into a rib form to form an optical waveguide and the optical waveguide is filled with ZnSe which is a II-VI compound semiconductor. A laser chip has a refractive index waveguide region in the vicinity of each resonator end face and has a gain waveguide region at its center part. The layer 15 is a single quantum well using a non- doped GaAs layer as a well layer. Thereby, a reduction in the threshold value of a semiconductor laser and an increase in the efficiency of the laser can be realized.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a compound semiconductor laser diode used for picking up optical discs and the like.

0002

2. Description of the Related Art A semiconductor laser used for picking up an optical disk is required to have high output characteristics when writing data and low noise characteristics when reading data. FIG. 6 shows a low-noise semiconductor laser published by Iwano et al. in the Proceedings of the Japan Society of Applied Physics (Spring 1988, 28P-ZH-9). The above semiconductor laser includes a cladding layer (61) having a refractive index smaller than that of the active layer (61) on both sides of the active layer (61).
It has a double heterostructure sandwiched between layers 13 and 17), and a rib-shaped optical waveguide is formed by etching halfway through the upper cladding layer (17). The semiconductor laser has a refractive index waveguide structure near the cavity end face and a gain waveguide structure at the center of the chip. By creating a refractive index waveguide region and a gain waveguide region in one chip in this way, the advantages of the refractive index waveguide structure can be achieved while maintaining the multi-longitudinal mode oscillation feature of the gain waveguide structure. That is, low threshold value, high efficiency, low astigmatism, etc. can be satisfied at the same time.

0003

[Problems to be Solved by the Invention] However, since conventional semiconductor lasers have a gain waveguide region in the center of the chip, they inevitably have a large reactive current component compared to a normal index-guided semiconductor laser. This results in an increase in the threshold current value or a decrease in oscillation efficiency. As a result, a large current needs to be applied to obtain a constant optical output, which increases the temperature of the chip and causes the optical output to thermally saturate before the optical density at the end face reaches the optical damage level. . For this reason, in order to achieve high output with conventional low-noise semiconductor lasers, it is necessary to reduce the reactive current to achieve a low threshold and high efficiency, and to keep the heat generation of the chip low. It is necessary.

0004

[Means for Solving the Problems] The semiconductor laser of the present invention has (1) a refractive index waveguide structure in which the refractive index waveguide width and the current injection region width are the same in the vicinity of at least one end face;
In other regions, the refractive index waveguide width is sufficiently wider than the current injection region width to form a gain waveguide structure, and the optical waveguide II- In a semiconductor laser having a structure embedded in a group VI compound semiconductor layer, an optical waveguide of the semiconductor laser is
At least a first cladding layer, a first optical waveguide layer, an active layer, a second optical waveguide layer, and a second optical waveguide layer are laminated on a semiconductor substrate.
The active layer has a single quantum well structure with one well layer.

(2) The semiconductor laser described in (1) above is characterized in that the number of quantum well layers is two or more.

[0006]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of a semiconductor laser according to the present invention is shown in FIGS. 1 and 2.

[0007] On an n-type GaAs substrate (11), an n-type buffer layer (12), an n-type lower cladding layer (13), an n-type lower optical waveguide layer (14), a quantum well active layer (15), p
mold upper optical waveguide layer (16), p-type upper cladding layer (17)
) and a p-type contact layer (18) are sequentially laminated. The contact layer and the upper cladding layer are processed into a rib shape to form an optical waveguide, and the optical waveguide is II-VI
It is filled with ZnSe, which is a group compound semiconductor. Further, the laser chip has a refractive index waveguide region near the cavity end face and a gain waveguide region at the center of the chip. The active layer is a single quantum well with non-doped GaAs as a well layer, and the thickness of the well layer is 50 Å (0.005 μm).
It is.

Next, a method for manufacturing a high-power semiconductor laser according to the present invention will be explained using FIGS. 3 and 4. Note that FIG. 3 shows a cross-sectional view of the refractive index waveguide region, and FIG. 4 shows a cross-sectional view of the gain waveguide region during the manufacturing process.

First, a laminated structure as shown in Table 1 was prepared using n-type Ga.
Sequential epitaxial growth is performed on an As substrate. Growth is performed by metal organic chemical vapor deposition (MOCVD) using organometallic compounds such as trimethyl gallium ((CH3)3Ga:TMG) and hydrides such as arsine (AsH3) as raw materials, and the growth temperature is 730°C. ℃.

0010

[Table 1]

Next, an insulating film (19) made of silicon dioxide (SiO2) or the like is deposited on the contact layer by thermal CVD (FIGS. 3 and 4(a)), and patterned by a photolithography process. Furthermore, ribs are etched using the insulating film as a mask. A sulfuric acid-based etchant is used to etch the ribs, and etching is performed to the boundary between the upper optical waveguide layer and the cladding layer (Figures 3 and 4).
(b)).

After etching the ribs, the ribs are filled with ZnSe with the insulator mask left in place. The buried growth performed here is also performed by organometallic chemical vapor deposition. As raw materials, dimethylzinc=dimethylselenium adduct (DMZn-DMSe) and hydrogen selenide were used (FIGS. 3 and 4(c)).

The polycrystalline ZnSe deposited on the ribs is removed by reactive ion beam etching (RIBE). Here, the polycrystalline ZnSe on the ribs is completely removed, and at the same time, by taking advantage of the difference in etching rates between ZnSe and SiO2, the etching time is increased so that the top surface of the rib and the top surface of the ZnSe layer are at the same height. Adjust and planarize the chip (Figures 3 and 4(d)).

After removing the polycrystalline ZnSe layered on the ribs, a gain waveguide region is created. In the gain waveguide region,
It is necessary to make the current injection width smaller than the refractive index waveguide width. Therefore, the contact layer other than the central current injection region is removed by etching. Since the upper cladding layer has a high Al composition of 0.45, contact cannot be established even if an electrode is directly formed on it, and current is not injected from outside the current injection region (Fig. 4(e)). ).

In the manner described above, the refractive index waveguide region and the gain waveguide region were respectively formed. Finally, attach the board to 100μ
After polishing to a thickness of m and depositing p-side and n-side electrodes, a ZnSe-embedded separated confinement single quantum well laser (SCH-SQW-LD) is completed (Figs. 3 and 4(f)
)).

The active layer of the semiconductor laser of the present invention has a quantum well structure whose film thickness is less than the de Broglie wavelength of electrons, and the level of electrons or holes in the active layer is quantized. There is. Compared to a double heterostructure semiconductor laser in which the active layer is made of a bulk crystal with a film thickness of about 0.1 μm, by quantizing the energy level of the active layer,
The luminous efficiency is high and the threshold current value is low.

In a semiconductor laser whose noise is reduced by introducing a gain waveguide region into a part of the chip,
A certain amount of reactive current flows in the gain waveguide region. Therefore, compared to a semiconductor laser consisting only of a refractive index waveguide region (hereinafter referred to as a simple stripe laser),
In order to obtain a constant light output, it becomes necessary to flow more current. In particular, when a large current is applied to obtain high output, the amount of heat generated by the laser chip increases, causing optical damage (CO) to the end face.
D: Catastropic Optical Dam
(hereinafter abbreviated as COD) level, the optical output becomes thermally saturated. However, in the case of the semiconductor laser of the present invention in which the active layer has a quantum well structure, the optical output was not thermally saturated and the end face was destroyed at a maximum output of about 75 mW. The effect of introducing a quantum well structure into the active layer in order to suppress heat generation of the chip, in other words, to reduce the drive current, is more effective when the reactive current component is large.

By the way, when the thickness of the well layer is changed, the energy level of carriers in the active layer changes. This means that the oscillation wavelength can be changed depending on the thickness of the active layer. Conventionally, in a semiconductor laser having a bulk active layer, the oscillation wavelength has been controlled by changing the Al composition of the active layer. However, in quantum well semiconductor lasers, the oscillation wavelength can be controlled by changing the thickness of the well layer, so GaAs can be used as the material for the active layer. GaAs is a binary compound semiconductor, and compared to AlGaAs, which is a ternary compound semiconductor, it has excellent controllability and can easily produce a high-quality film. Therefore, improvement in luminous efficiency can be expected.

If the active layer is an AlGaAs-based bulk crystal, if the Al composition exceeds 0.45, the transition changes from direct to indirect, making laser oscillation impossible. That is, it is impossible to create a short wavelength laser of 620 nm or less. However, in quantum well lasers, the well layer,
By appropriately selecting the Al composition and film thickness of the barrier layer, it is also possible to create a semiconductor laser that oscillates at a short wavelength of 620 nm or less.

[0020] The active layer is not a single quantum well structure but a Ga
Well layers of As and Al0.3Ga0.7As, respectively.
By using a double quantum well structure as a barrier layer, even higher output can be achieved. In a single quantum well structure with only one well layer, carriers overflow from the well layer when the well layer is thin or when the injection current is large. As a result, the current that is not involved in light emission increases, the threshold current value increases, and the luminous efficiency decreases. However, when a double quantum well structure is introduced into the active layer, carriers are more strongly confined in the active layer, and carrier overflow can be suppressed to a low level. In other words, the luminous efficiency of the quantum well semiconductor laser can be further increased.

FIG. 5 shows a third embodiment of the present invention, in which (a) shows a cross section of the refractive index waveguide region and (B) shows a cross section of the gain waveguide region. The difference from the two examples mentioned above is that the ribs are etched until they reach the substrate, and the ribs are filled with Zn that lattice matches the GaAs.
These are the two points made by S0.06Se0.94 (51).

In this embodiment, since the ribs are filled with ZnSSe mixed crystal whose lattice constant matches that of the active layer, stress due to lattice mismatch is less likely to be applied to the active layer. Therefore, lattice defects are less likely to occur in the active layer, and as a result, the reliability and performance of the semiconductor laser can be improved. Also, from the side of the ZnSSe that forms the buried layer, no stress is generated at the interface with the rib side surface during the buried growth, making it possible to form a high-quality buried layer and at the same time improving adhesion to the rib side surface. This improves the confinement effect of light waves and carriers.

In the description of the embodiments of the semiconductor laser of the present invention, the case where ZnSe or ZnSSe, which is a II-VI group compound semiconductor, is used as the buried layer has been explained. Similar effects can be obtained even when a group compound semiconductor is used. That is, selenium, sulfur, tellurium, etc. can be used as group VI raw materials, and zinc, cadmium, etc. can be used as group II raw materials, and mixed crystals of binary, ternary, and quaternary systems that combine these Also, good characteristics can be obtained. In addition, in any case, forming the active layer
It goes without saying that better results can be obtained by lattice matching the III-V compound semiconductor and the buried layer.

In addition, the semiconductor laser of the present invention is made of AlGa
Laser materials other than As-based, such as InGaAsP-based, I
The present invention can be similarly applied to nGaP-based materials. Also,
In the example, the structure in which the conductivity type of each layer is all reversed (
Similar effects can be expected for structures in which p is replaced by n and n is replaced by p.

[0025]

Effects of the Invention The semiconductor laser of the present invention, in which a quantum well active layer is introduced into a low-noise semiconductor laser, has the following effects.

(1) By using a quantum well active layer, the luminous efficiency was dramatically improved, and at the same time, the oscillation threshold current value was lowered. This is because the energy levels of carriers in the active layer are quantized. In a semiconductor laser having a structure like the present invention in which the drive current is relatively large and heat is easily generated, lowering the threshold value and increasing efficiency are particularly important, and the introduction of a quantum well structure is very effective.

(2) Conventionally, the oscillation wavelength was controlled by changing the Al composition of the active layer. However, if the active layer has a quantum well structure, the oscillation wavelength can be changed by changing the thickness of the well layer. As a result, GaAs, which produces high-quality crystals, can be used in the active layer instead of AlGaAs, which is a ternary compound semiconductor, making it possible to obtain a laser chip with high luminous efficiency and excellent reliability. Become.

(3) In the quantum well active layer, the oscillated light easily leaks out from the active layer into the upper and lower optical waveguides, and the optical density of the active layer decreases, so high output can be achieved. Furthermore, the light density that causes optical damage to the end facets is about 1.5 to 2 times higher than that of a semiconductor laser with a bulk active layer, which is also advantageous in terms of high output.

(4) By forming the active layer into a multiple quantum well, overflow of carriers from the active region is suppressed. That is, the reactive current is reduced and the heat generation of the chip is suppressed.

[Brief explanation of drawings]

FIG. 1 is a perspective view showing a first embodiment of the present invention.

FIG. 2 is a cross-sectional view showing the first embodiment of the present invention, (a)
FIG. 4B is a cross-sectional view showing a refractive index waveguide region and FIG. 3B a gain waveguide region.

FIGS. 3(a) to 3(f) are cross-sectional views showing the manufacturing process of the refractive index waveguide region of the semiconductor laser of the present invention.

FIGS. 4(a) to 4(f) are cross-sectional views showing the manufacturing process of the gain waveguide region of the semiconductor laser of the present invention.

FIG. 5 is a sectional view showing a second embodiment of the present invention, (a)
FIG. 4B is a cross-sectional view showing a refractive index waveguide region and FIG. 3B a gain waveguide region.

FIG. 6 is a perspective view showing a prior art.

[Explanation of symbols]

11 N-type GaAs substrate 12 N-type GaAs buffer layer 13 N-type AlGaAs cladding layer 14 N-type AlGaAs optical waveguide layer 15 Quantum well active layer 16 p-type AlGaAs optical waveguide layer 17 p-type AlGaAs cladding layer 18 p-type GaAs contact layer 19 SiO2 mask 20 Single crystal ZnSe 21 Polycrystalline ZnSe 22 p-type ohmic electrode 23 N-type ohmic electrode 51 Single crystal ZnSSe 61 Active layer

Claims (2)

[Claims] Claim 1: A refractive index waveguide structure in which the refractive index waveguide width and the current injection region width are the same in the vicinity of at least one end face, and in other regions, the refractive index waveguide width is sufficiently wider than the current injection region width. III-V with a gain waveguide structure
In a semiconductor laser having a rib-shaped optical waveguide made of a group compound semiconductor layer and having a structure in which the optical waveguide is embedded with a group II-VI compound semiconductor layer, the optical waveguide of the semiconductor laser is laminated at least on a semiconductor substrate. The active layer is formed of a first cladding layer, a first optical waveguide layer, an active layer, a second optical waveguide layer, and a second cladding layer, and the active layer has a single quantum well layer of 1. A semiconductor laser characterized by a well structure. 2. The semiconductor laser according to claim 1, wherein the number of quantum well layers is two or more.