JPH08204282A - Semiconductor laser - Google Patents

Abstract

PURPOSE: To prevent the active layer of a semiconductor laser which is provided with a saturable absorption layer for self-exciting vibrations from being deteriorated in light confining factor and to prevent the light beam emitted from the laser from being deformed in shape due to the introduction of the saturable absorption layer. CONSTITUTION: Saturable absorption layers 105 and 109 are respectively provided at positions within 200nm from active layers in clad layers on both sides of an active layer 107. Namely, an n-type AlGaInP clad layer 110, n-type AlGaInP saturable absorption layer 109, n-type AlGaInP clad layer 108 having a film thickness of 100nm, AlGaInP active layer 107, p-type AlGaInP clad layer 106 having a film thickness of 100nm, p-type AlGaInP saturable absorption layer 105, p-type AlGaInP clad layer 104 are successively grown on an n-type GaAs substrate 1 and the clad layer 104 is machined to the shape of Chinese character 'projecting portion'. Then an n-type block layer 103 is formed so as to surround the projecting portion-shaped projecting section and a p-type contact layer 102 is formed on the layer 103.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser,
In particular, the present invention relates to a semiconductor laser used as a light source of information processing equipment.

[0002]

2. Description of the Related Art In recent years, red AlGaInP semiconductor lasers have been put into practical use, and are expected as a light source for high density optical disk devices. When the light emitted from the semiconductor laser is focused on the optical disk to reproduce information, a part of the reflected light returns to the inside of the resonator of the semiconductor laser, so-called return light noise occurs in the laser light, and C of the reproduction signal is generated. It is known that the / N ratio (carrier / noise ratio) deteriorates. This C
As a method for suppressing the deterioration of the / N ratio,
The high frequency superposition method and the self-excited vibration (pulsation) method have been used.

The former is a method of superposing a high frequency current on a drive current of a semiconductor laser, which is to reduce the coherency of laser light and suppress laser noise due to return light by superposing a high frequency current. However, in this method, in order to perform high frequency superimposition, it is necessary to generate a drive current of a relatively high output of about +10 dBm and a relatively high frequency of 200 to 700 MHz. The volume is a major impediment to cost reduction and size reduction of the optical disk device. Further, the high-frequency current generation circuit radiates high-frequency electromagnetic waves to the outside of the device, which causes a problem of electromagnetic interference.

On the other hand, the pulsation laser has a saturable absorption region inside, and carriers in the saturable absorption region, carriers in the active layer, and oscillated light cooperate to cause self-excited oscillation (pulsation). . Since the coherency of the oscillated light in the self-excited oscillation state is low, laser noise due to the returned light is less likely to occur. In this method, it is not necessary to perform high frequency superimposition using the high frequency current generating circuit described above, and very few high frequency electromagnetic waves are emitted to the outside. The conventional technology of the pulsation laser will be described below.

Several saturable absorption structures have been reported to realize pulsation lasers. Adachi et al.
We have reported a structure in which the region adjacent to the current injection region of the active layer acts as a saturable absorption region (Proceedings of the 55th SPSJ Annual Meeting, No.3, p.939, 20p-s-15, 1994). 9 years
(Month) [The reported structure corresponds to that of FIG. 1 (d) with the saturable active layer 5 removed]. In this laser structure, the remaining thickness of the cladding layer needs to be strictly controlled in the etching process. Also, the equivalent refractive index difference (Δn) in the lateral direction
Needs to be small. In Adachi et al.'S report, the value of Δn is small, about 3 × 10 ⁻³ .

On the other hand, Goto et al. And Matsumoto et al. Reported the pulsation operation of a semiconductor laser having a saturable absorption layer inside the cladding layer (Proceedings of the 41st Joint Lecture on Applied Physics, No. 3, p.990, 28p-K-9, March 1994, and Proceedings of the 55th Annual Meeting of the Japan Society of Applied Physics No.3, p.933,
20a-S-6, September 1994). As shown in FIG. 1D, this structure has a structure in which a cladding layer 8, an active layer 7, a cladding layer 6, a saturable absorption layer 5, a ridge-shaped cladding layer 4 and a current block surrounding the cladding layer 8, an active layer 7, a cladding layer 6, and Layer 3, contact layer 2
Are laminated, and the first electrode 1 is provided on the contact layer 2, and the second electrode 12 is provided under the semiconductor substrate 11.

In this laser structure, when the clad layer 4 is processed into a ridge shape, the saturable absorption layer 5 serves as an etching stopper, so that strict control is not required in the etching process, and the main structure is eliminated. The composition and layer thickness of the saturable absorption layer, which are parameters, can be controlled by a crystal growth apparatus having excellent controllability. Moreover, it is not necessary to reduce the difference in the horizontal equivalent refractive index. Therefore, it is possible to obtain a higher manufacturing yield, higher fundamental transverse mode controllability, and better laser light beam quality than those of the laser structure of Adachi et al.

[0008]

However, in the laser structure of Goto et al., The saturable absorbing layer is provided only on one side of the active layer, and the distance from the active layer is large (at least 250 μm) is set.
Here, the reason why the distance from the active layer to the saturable absorption layer must be increased is that the current blocking layer 3 is pnpn.
This is because the cladding layer 6 must be thicker than a certain thickness in order to block the current depending on the structure. In the conventional example, since the distance from the active layer to the saturable absorption layer is large, there are problems that the confinement coefficient of the active layer is greatly reduced with the introduction of the saturable absorption layer, and the focused beam shape is distorted. There were issues to be improved.

The present invention has been made in view of this point, and an object thereof is to make the optical confinement coefficient of the active layer less likely to be influenced by the saturable absorption layer, thereby facilitating the design,
Another object is to make the shape of the focused beam good so that the reading from the optical disk can be made with less noise.

[0010]

In order to achieve the above object, according to the present invention, a semiconductor substrate (11) of the first conductivity type is provided.
1) a first conductivity type first clad layer (110), a first conductivity type saturable absorption layer (109), a first conductivity type second
Clad layer (108), active layer (107), second conductivity type first clad layer (106), second conductivity type saturable absorption layer (105), second conductivity type second clad layer (1
04), and a semiconductor laser having a contact layer (102) of the second conductivity type formed in this order.

According to the present invention, the first conductivity type clad layer (20) is formed on the first conductivity type semiconductor substrate (211).
8), the active layer (207), the second conductivity type first cladding layer (206), the second conductivity type saturable absorption layer (205),
Second conductivity type second clad layer (204) having a cross-sectional shape of a “convex” shape, second conductivity type contact layer (202)
Is formed in this order, and the raised portion of the second cladding layer of the second conductivity type is surrounded by the current blocking layer (203) of the first conductivity type.

[0012] Preferably, the first conductivity type second cladding layer (108) and the second conductivity type first cladding layer (108).
The thickness of the clad layer (106) or the second clad layer (206) of the second conductivity type is 200 nm or less.

[0013]

1 (a) to 1 (c) are sectional views of a semiconductor laser for explaining the operation of the present invention, and the structure of a conventional example is shown in FIG. 1 (d) for comparison with these. Has been done.
The difference from the conventional example of the semiconductor laser of the present invention shown in FIGS. 1 (a) to 1 (c) is that the clad layer on the substrate side, that is, the clad layer 1 is different.
0 and the clad layer 8 also have a saturable absorption layer 9 formed therein. The clad layer sandwiched between the active layer and the saturable absorption layer has a thickness of 200 nm or less [FIG. ), (B) is 100 nm, FIG. 1 (c) is 200 nm, whereas in the conventional example of FIG. 1 (d) is 250 nm or more], and as a result, the thickness of the saturable absorbing layer is reduced. [However, in FIGS. 2 and 3 below, the distance L _SA between the saturable absorbing layer and the active layer is 200 in the conventional example of FIG. 1 (d).
The data is shown in the case of nm.].

First, the design of the thickness of the saturable absorbing layer will be described. In order for the saturable absorption layer to induce pulsation, the saturable absorption layer needs to confine light above a certain level. In the semiconductor laser having the structure shown in the figure, the optical confinement coefficient Γ _SA of the saturable absorption layer required to induce pulsation is Γ _SA = 0.05. FIG. 2 shows the optical confinement coefficient (Γ _SA ) of the saturable absorption layer and the thickness (d
_SA )). If the distance (L _SA ) between the saturable absorbing layer and the active layer is as long as 250 nm or more and the saturable absorbing layer is only on one side of the cladding layer as reported by Goto et al. Is required. For example, L _SA
Is close to 200 nm [Fig. 1 (d)], 40
A saturable absorption layer having a considerably large thickness of nm is required.

On the other hand, when the saturable absorbing layer is provided on both sides of the cladding layer [FIG. 1 (c)], the required thickness of the saturable absorbing layer is reduced to 30 nm. Furthermore, if the saturable absorption layer is brought close to the active layer and L _SA = 100 nm is set [Fig.
(A), (b)], the required thickness of the saturable absorber layer is 20 n
to m.

In the laser structure of Goto et al., The saturable absorption layer has a thickness of 250 nm in order to exert the function of the current block.
Although it is necessary to set the above, according to the example of FIG. 1A of the present invention [or the second embodiment], this limitation is eliminated, so that L _SA ≦ 200 nm, for example, L _SA = 100 nm. It is possible to reduce the thickness (d _SA ) of the saturable absorption layer.

FIG. 3 shows the confinement coefficient Γ _SA of the saturable absorption layer and the confinement coefficient Γ of the active layer with L _SA as a parameter.
It is a graph which shows the relationship with _ACT . As shown in the figure, the smaller the L _SA , the confinement factor Γ _{ACT of the} active layer.
Is less susceptible to the confinement factor Γ _SA of the saturable absorber layer. Therefore, the smaller the L _SA , the easier the design of the semiconductor laser.

FIG. 4 is a graph showing the relationship between the confinement coefficient Γ _SA of the saturable absorber layer and the spot size (d _ACT / Γ _ACT ) in the direction orthogonal to the active layer, with L _SA as a parameter (however, , D _ACT is the thickness of the active layer). As can be seen from the figure, the spot size in the direction orthogonal to the active layer is also
The smaller L _SA , the less likely it is to be affected by the provision of the saturable absorption layer.

FIG. 5 shows a near-field image and a refractive index profile of the semiconductor laser of the present invention shown in FIGS. 1 (a) and 1 (b), and FIG. 6 shows the near-field image of the present invention shown in FIG. 1 (c). It is a near-field image and a refractive index profile of a semiconductor laser. Also, FIG.
2A is a near-field image and a refractive index profile of the conventional semiconductor laser shown in FIG. The near-field image of the semiconductor laser of the present invention has less distortion than that of the conventional example. This is because the saturable absorption layer is symmetrically provided on both sides of the active layer and the saturable absorption layer is close to the active layer.

[0020]

Embodiments of the present invention will now be described with reference to the drawings. [First Embodiment] FIG. 8 is a sectional view showing a first embodiment of the present invention. This semiconductor laser was manufactured as follows. First, on an n-type GaAs substrate 111, an n-type A made of Si-doped (Al _0.7 Ga _0.3 ) _0.5 In _0.5 P
The lGaInP clad layer 110 has a film thickness of 1.0 μm,
The n-type AlGaInP saturable absorption layer 109 made of Si-doped (Al _0.1 Ga _0.9 ) _0.5 In _0.5 P is set to 20 nm.
Of Si-doped (Al _0.7 Ga _0.3 ) _0.5 In
_An n-type AlGaInP clad layer 108 made of _0.5 P having a film thickness of 100 nm and having an undoped (Al _0.1 Ga
_0.9 ) _0.5 In _0.5 P AlGaInP active layer 1
07 with a thickness of 40 nm is Zn-doped (Al _0.7 Ga
_0.3 ) _0.5 In _0.5 P p-type AlGaInP cladding layer 106 having a film thickness of 100 nm and Zn-doped (A
l _0.1 Ga _0.9 ) _0.5 In _0.5 P composed of p-type AlGa
The InP saturable absorption layer 105 has a thickness of 20 nm and is made of Zn-doped (Al _0.7 Ga _0.3 ) _0.5 In _0.5 P.
-Type AlGaInP clad layer 104 with a thickness of 1.0 μm is formed by metalorganic vapor phase epitaxy (Metalorganic Vapor Deposition).
Phase Epitaxy: MOVPE method). Crystal growth conditions are: growth temperature: 700 to 850 ° C., V
/ III gas flow rate ratio: 30 to 1500.

After crystal growth, a silicon oxide film is deposited by the CVD method and processed into stripes. And
Using this silicon oxide film as a mask, a part of the clad layer 104 is selectively removed by etching to form a striped ridge having a bottom width of about 5 μm. Using the silicon oxide film as a mask again, the n-type GaAs current blocking layer 103 was selectively grown. After the selective growth, the silicon oxide film used as a mask is removed, and p-type GaAs is formed on the surfaces of the current block layer 103 and the cladding layer 104.
The contact layer 102 was grown.

Further, the p-type GaAs contact layer 102
P-side electrode 101 in ohmic contact with n-type GaAs
An n-side electrode 112 that makes ohmic contact with the substrate 111 is formed to form a laser end face. Finally, the laser facet is coated with a dielectric film to complete the fabrication of the semiconductor laser of this embodiment.

[Second Embodiment] FIG. 9 is a sectional view showing a second embodiment of the present invention. This semiconductor laser was manufactured as follows. First, on the n-type GaAs substrate 211, Si-doped (Al _0.7 Ga _0.3 ) _0.5 In _0.5 P
The n-type AlGaInP cladding layer 208 made of
Undoped (Al _0.1 Ga _0.9 ) _{0.5 with a} thickness of μm
The AlGaInP active layer 207 made of In _0.5 P is 40
Zn-doped (Al _0.7 Ga _0.3 ) _0.5
P-type AlGaInP cladding layer 20 made of In _0.5 P
6 to a film thickness of 100 nm and Zn-doped (Al _0.1 Ga
_0.9 ) _0.5 In _0.5 P p-type AlGaInP saturable absorption layer 205 having a thickness of 20 nm is Zn-doped (A
l _0.7 Ga _0.3 ) _0.5 In _0.5 P composed of p-type AlGa
The InP clad layer 204 was sequentially grown to a thickness of 1.0 μm by the metal organic chemical vapor deposition method. The crystal growth conditions are the same as in the case of the first embodiment.

After crystal growth, a silicon oxide film is deposited by the CVD method and processed into a stripe shape. And
Using this silicon oxide film as a mask, a part of the clad layer 204 is selectively removed by etching to form a striped ridge having a bottom width of about 5 μm. Using the silicon oxide film as a mask again, the n-type GaAs current blocking layer 203 was selectively grown. After the selective growth, the silicon oxide film used as a mask is removed, and p-type GaAs is formed on the surfaces of the current block layer 203 and the clad layer 204.
The contact layer 202 was grown.

Further, the p-type GaAs contact layer 202
P-side electrode 201 that makes ohmic contact with the n-type GaAs
An n-side electrode 212 that makes ohmic contact with the substrate 211 is formed to form a laser end face. Finally, the laser facet is coated with a dielectric film (not shown) to complete the fabrication of the semiconductor laser of this embodiment.

[Modifications of Embodiments] The preferred embodiments of the present invention have been described above, but the present invention is not limited to these embodiments, and various modifications can be made within the scope of the claims. Is. For example, although the case where the n-type semiconductor substrate is used has been described in the embodiment, the same effect can be obtained by using the p-type semiconductor substrate. In this case, all the conductivity types in the above-mentioned embodiments may be reversed. Also, as the substrate, a GaAsP substrate may be used instead of GaAs.

Further, AlGaInPAs may be used instead of AlGaInP as the semiconductor layer for crystal growth. In the example, Si is used as the n-type dopant.
Although the example of using Zn as the p-type dopant has been described above, other dopants such as Se as the n-type dopant and Mg, Be or the like as the p-type dopant may be used instead of these dopants.

The crystal growth method is not limited to the MOVPE method, but the molecular beam growth method (MBE method) or the metal organic molecular beam growth method (M
OMBE method), gas source molecular beam growth method (GSMBE
Method) and the like can also be used.

Strain QW of tensile strain or compressive strain may be adopted for the active layer, and in this case, improvement of laser characteristics such as oscillation threshold can be expected. Alternatively, the strain QW may be adopted for the saturable absorber layer. The differential gain of the saturable absorber layer can be increased by introducing an appropriate lattice strain. If the differential gain of the saturable absorber layer is large, the light absorption amount of the saturable absorber layer necessary for generating pulsation is reduced. Therefore, the above change is desirable for improving the laser characteristics.

[0030]

As described above, in the semiconductor laser of the present invention, the distance between the active layer and the saturable absorption layer is shortened, and the saturable absorption layers are provided on both sides of the active layer. Therefore, the confinement coefficient and spot size of the active layer are less likely to be affected by the design of the saturable absorption layer, and the laser characteristic analysis and optimization design are facilitated. Further, since the distortion of the laser beam shape can be reduced, it becomes possible to form a good-quality focused spot on the optical disk.

Furthermore, since the required thickness of the saturable absorber layer can be reduced, a large lattice strain similar to that of the strained QW active layer can be introduced. Thereby, the band gap energy of the saturable absorption layer, the absorption spectrum shape,
It is possible to freely control the differential gain, etc.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of a semiconductor laser for explaining the operation of the present invention [(a) to (c) show the structure of the semiconductor laser of the present invention, and (d) shows the structure of a conventional example].

FIG. 2 is a graph showing the relationship between the confinement coefficient of the saturable absorber layer and the film thickness of the saturable absorber layer with the distance from the active layer to the saturable absorber layer as a parameter.

FIG. 3 is a graph showing the relationship between the confinement coefficient of the saturable absorption layer and the confinement coefficient of the active layer, with the distance from the active layer to the saturable absorption layer as a parameter.

FIG. 4 is a graph showing the relationship between the confinement coefficient of the saturable absorber layer and the spot size, with the distance from the active layer to the saturable absorber layer as a parameter.

5 is a refractive index profile and a light intensity profile of a semiconductor layer of the semiconductor laser shown in FIGS. 1 (a) and 1 (b).

6 is a refractive index profile and a light intensity profile of a semiconductor layer of the semiconductor laser shown in FIG. 1 (c).

7 is a refractive index profile and a light intensity profile of a semiconductor layer of the semiconductor laser shown in FIG. 1 (d).

FIG. 8 is a sectional view of the semiconductor laser according to the first embodiment of the present invention.

FIG. 9 is a sectional view of a semiconductor laser according to a second embodiment of the present invention.

[Explanation of symbols]

1 1st electrode 2 Contact layer 3 Current block layer 4, 6, 8, 10 Clad layer 5, 9 Saturable absorption layer 7 Active layer 11 Semiconductor substrate 12 2nd electrode 101, 201 p-side electrode 102, 202 p-type GaAs contact Layer 103, 203 n-type GaAs current blocking layer 104, 204, 106, 206 p-type AlGaInP
Cladding layer 105, 205 p-type AlGaInP saturable absorption layer 107, 207 AlGaInP active layer 108, 208, 110 n-type AlGaInP clad layer 109 n-type AlGaInP saturable absorption layer 111, 211 n-type GaAs substrate 112, 212 n-side electrode

Claims (5)

[Claims] 1. A first conductivity type first clad layer, a first conductivity type saturable absorption layer, and a first conductivity type semiconductor substrate on a first conductivity type semiconductor substrate.
Conductive type second clad layer, active layer, second conductive type first
The semiconductor laser, wherein the clad layer, the second-conductivity-type saturable absorption layer, the second-conductivity-type second clad layer, and the second-conductivity-type contact layer are formed in this order. 2. The second clad layer of the second conductivity type,
2. The semiconductor laser according to claim 1, wherein the stripe shape or the sectional shape is a "convex" shape. 3. A clad layer of a first conductivity type, an active layer, a first clad layer of a second conductivity type, a saturable absorption layer of a second conductivity type, and a cross-sectional shape “ A second conductive type second clad layer having a convex shape and a second conductive type contact layer are formed in this order, and the second conductive type second clad layer is formed.
The semiconductor laser, wherein the raised portion of the clad layer is sandwiched between current blocking layers of the first conductivity type. 4. The film thickness of the second clad layer of the first conductivity type and the first clad layer of the second conductivity type, or the first clad layer of the second conductivity type is 200 nm or less. The semiconductor laser according to claim 1 or 3, characterized in that. 5. The semiconductor substrate of the first conductivity type is GaAs
Alternatively, the active layer is made of GaAsP and the active layer is AlG.
4. The semiconductor laser according to claim 1, wherein the semiconductor laser is made of aInP or AlGaInPAs.