JP2954358B2 - Semiconductor laser and cleavage method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a low-noise semiconductor laser used for a light source of an optical disk system or the like.

BACKGROUND ART In recent years, demand for semiconductor lasers has been increasing in the fields of optical communication, laser printers, optical disks, and the like, and research and development have been actively promoted, mainly for GaAs and InP systems. Of these, in the optical information processing field, in particular, AlG with a wavelength of about 780 nm
A method of recording / reproducing information using an aAs-based semiconductor laser beam has been put to practical use, and has become widespread in compact discs and the like.

During reproduction of an optical disk using a semiconductor laser as a light source, intensity noise occurs due to feedback of reflected light from the disk surface and changes in temperature. Such intensity noise is
Induces a signal reading error. Therefore, a semiconductor laser with low intensity noise is indispensable as a light source for an optical disk.

Conventionally, low-power AlGaAs-based semiconductor lasers used as light sources for read-only optical disks have adopted a structure in which saturable absorbers are intentionally formed on both sides of the ridge stripe to reduce noise. By,
Low noise has been achieved. With such a configuration, multiplication of the vertical mode is achieved. When a semiconductor laser oscillates in a single longitudinal mode and there is disturbance due to feedback of light or a change in temperature, a nearby longitudinal mode starts to oscillate due to a slight change in a gain peak. The mode in which the oscillation is started in this way causes a conflict with the original oscillation mode, which causes noise. On the other hand, when the longitudinal mode is multiplied by the above-described method, the intensity change of each mode is averaged and the intensity change due to disturbance does not occur, so that it is possible to obtain a stable low noise characteristic. .

As another method, an attempt to obtain more stable self-sustained pulsation characteristics is disclosed in Japanese Patent Application Laid-Open No. 63-202083. Specifically, a self-pulsation type semiconductor laser is realized by providing a layer capable of absorbing output light.

Further, Japanese Patent Application Laid-Open No. 6-260716 reports that the operating characteristics of a semiconductor laser are improved by making the band gap of the active layer substantially equal to the band gap of the absorbing layer. In the above-mentioned publication, a red semiconductor laser is particularly mentioned as an example.

FIG. 16 is a sectional view schematically showing the configuration of the self-pulsation type semiconductor laser disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 6-260716.

Specifically, an n-type GaInP buffer layer 1302 and an n-type AlGaInP cladding layer 1303 are formed on an n-type GaAs substrate 1301, and a GaInP strained quantum well (MQW) is formed in the cladding layer 1303.
An active layer 1304 is formed. In the cladding layer 1303, a strained quantum well saturable absorption layer 1305 is further formed. On top of cladding layer 1303, cladding layer 1306 and p
A GaInP contact layer 1307 is formed in a ridge shape. Both sides of the ridge composed of the cladding layer 1306 and the contact layer 1307 are buried with an n-type GsAs current blocking layer 1308. Further, a p-type GsAs cap layer 1309 is formed on the contact layer 1307 and the current blocking layer 1308, and a p-electrode 1310 is formed on the cap layer 1309, while an n-electrode is formed on the back surface of the substrate 1301. 1311 is formed.

The above-mentioned Japanese Patent Application Laid-Open No. 6-260716 attempts to obtain good self-sustained pulsation characteristics with the above configuration.

DISCLOSURE OF THE INVENTION According to one aspect of the present invention, a semiconductor laser includes an active layer and a clad structure sandwiching the active layer, the clad structure including a saturable absorbing layer, wherein the saturable absorbing layer is formed of In. _x G
a _1-x As _y P _1-y (0 <x <1, 0 ≦ y ≦ 1)

Preferably, the distance between the saturable absorbing layer and the active layer is about 200 Å or more.

In one embodiment, the cladding structure further includes a light guide layer, and the saturable absorber layer is adjacent to the light guide layer. Alternatively, the cladding structure further includes a light guide layer, and the saturable absorption layer is disposed near the light guide layer.

In one embodiment, the saturable absorption layer has a strained quantum well structure, and an energy gap between ground levels of the saturable absorption layer is about 30 meV to about 200 meV higher than an energy gap of the active layer. small.

Preferably, the cladding structure excluding the active layer and the saturable absorbing layer is made of (Al _x Ga _{1 -x} ) _y In _{1 -y} P.

According to another aspect of the present invention, a semiconductor laser includes an active layer and a clad structure sandwiching the active layer, wherein the clad structure includes a saturable absorbing layer, and the saturable absorbing layer is formed of InGa
The saturable absorption layer is made of AsP and has a strained quantum well structure.

Preferably, the distance between the saturable absorbing layer and the active layer is about 200 Å or more.

In one embodiment, the cladding structure further includes a light guide layer, and the saturable absorber layer is adjacent to the light guide layer. Alternatively, the cladding structure further includes a light guide layer, and the saturable absorption layer is disposed near the light guide layer.

In one embodiment, an energy gap between ground levels of the saturable absorbing layer is smaller than an energy gap of the active layer by about 30 meV to about 200 meV.

Preferably, the cladding structure excluding the active layer and the saturable absorbing layer is made of (Al _x Ga _{1 -x} ) _y In _{1 -y} P.

According to yet another aspect of the present invention, a semiconductor laser comprises:
An active layer and a clad structure sandwiching the active layer are provided. The clad structure includes a saturable absorbing layer, and the saturable absorbing layer is made of InGaAs.

Preferably, the distance between the saturable absorbing layer and the active layer is about 200 Å or more.

In one embodiment, the cladding structure further includes a light guide layer, and the saturable absorber layer is adjacent to the light guide layer. Alternatively, the cladding structure further includes a light guide layer, and the saturable absorption layer is disposed near the light guide layer.

In one embodiment, an energy gap between ground levels of the saturable absorption layer is smaller than an energy gap of the active layer by about 30 meV to about 200 meV.

Preferably, it said clad structure excluding the saturable absorbing layer and the active layer is made of Al _z Ga _1-z As.

According to yet another aspect of the present invention, a semiconductor laser comprises:
An active layer and a cladding structure sandwiching the active layer, the cladding structure including a saturable absorbing layer, wherein the thickness of the saturable absorbing layer is about 100 angstroms or less.

Preferably, the thickness of the saturable absorber layer is less than about 80 Å.

In one embodiment, the cladding structure further includes a light guide layer, and the saturable absorber layer is adjacent to the light guide layer. Alternatively, the cladding structure further includes a light guide layer, and the saturable absorption layer is disposed near the light guide layer.

In one embodiment, the saturable absorption layer has a strained quantum well structure, and an energy gap between ground levels of the saturable absorption layer is about 30 meV to about 200 meV higher than an energy gap of the active layer. small.

According to yet another aspect of the present invention, a semiconductor laser comprises:
An active layer; an n-type cladding layer sandwiching the active layer;
A cladding layer, a current blocking layer having an opening formed on the first p-type cladding layer, and a second p-type cladding layer formed in the opening. The second mold cladding layer includes a saturable absorbing layer.

In one embodiment, the saturable absorption layer is made of InGaAsP or InGaAs.

In one embodiment, a mode control layer is formed in the current blocking layer.

Preferably, the first p-type cladding layer is made of a material having a band gap larger than that of the n-type cladding layer. For example, the first p-type cladding layer is an AlGaInPN layer.

According to still another aspect of the present invention, there is provided a cleavage method. The cleavage method is a method for cleaving a substrate having a plane orientation inclined in a predetermined direction with respect to the plane orientation of the just plane, and cleaves the substrate by applying a stress in the direction of the plane orientation of the substrate. .

Still another cleavage method of the present invention is a method for cleaving a crystal whose plane orientation is inclined from the [100] direction to the [0-1-1] direction. To cleave the crystal.

Accordingly, the present invention provides (1) a semiconductor laser having stable self-pulsation characteristics and high reliability by selecting a specific material as a constituent material of the saturable absorption layer included in the semiconductor laser. And (2) to provide a cleavage method that can be used in the above-described semiconductor laser manufacturing process.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1A and 1B are a cross-sectional view and a perspective view showing a configuration of a semiconductor laser according to a first embodiment of the present invention.

FIG. 2 is a diagram showing changes in the Al composition in the active layer of the semiconductor laser shown in FIGS. 1A and 1B and in the vicinity thereof.

FIG. 3 is a diagram showing gain characteristic curves of GaInP and InGaAsP.

FIG. 4 is a diagram showing changes over time in optical output and carrier density in the semiconductor lasers shown in FIGS. 1A and 1B.

FIG. 5 is a diagram showing the relationship between the light output and the current in the semiconductor laser shown in FIGS. 1A and 1B.

FIG. 6 is a diagram showing an example of an actually measured waveform of a temporal change of the optical output in the semiconductor laser shown in FIGS. 1A and 1B.

FIG. 7 is a diagram for explaining the relationship between the tilt direction of the substrate and the preferred cleavage direction.

FIG. 8 is a cross-sectional view illustrating a configuration of a semiconductor laser according to the second embodiment of the present invention.

FIG. 9 is a diagram showing a change in the Al composition in the active layer of the semiconductor laser shown in FIG. 8 and in the vicinity thereof.

FIG. 10 is a diagram showing the relationship between the light confinement ratio (light confinement coefficient) and the thickness of the light guide layer in the semiconductor laser shown in FIG.

FIG. 11 is a cross-sectional view illustrating a configuration of a semiconductor laser according to the third embodiment of the present invention.

FIG. 12 is a diagram showing gain characteristic curves of GsAs and InGaAs.

FIG. 13 is a cross-sectional view illustrating a configuration of a semiconductor laser according to the fourth embodiment of the present invention.

FIG. 14 is a sectional view showing the configuration of the semiconductor laser according to the fifth embodiment of the present invention.

FIG. 15 is a diagram showing gain characteristic curves of GsAs and GaInP.

FIG. 16 is a cross-sectional view illustrating an example of a configuration of a self-pulsation type semiconductor laser according to the related art.

BEST MODE FOR CARRYING OUT THE INVENTION Prior to the description of a specific embodiment of the present invention, first, a result of a study performed by the inventors of the present application in a process leading to the present invention will be described.

As described above, in the related art, various reports have been made on the use of the self-excited oscillation phenomenon in a semiconductor laser. However, according to the study by the present inventors,
It has been found that it is difficult to obtain self-sustained pulsation characteristics in the AlGaInP semiconductor laser because the gain characteristic curve of the material is significantly different from that in the AlGaAs semiconductor laser. In this regard,
This will be described with reference to FIG.

FIG. 15 is a diagram illustrating gain characteristics of GsAs and GaInP.
These materials (GsAs and GaInP) are mainly used as constituent materials of the active layers of the AlGsAs-based semiconductor laser and the AlGaInP-based semiconductor laser, respectively.

According to the study by the present inventors, it has become clear that the larger the slope of the gain characteristic curve with respect to the carrier density, the easier the self-sustained pulsation characteristics can be obtained. The reason is related to the fact that strong oscillation of carriers is required inside the saturable absorption layer in order to obtain self-sustained pulsation characteristics, and the smaller the slope of the gain characteristic curve with respect to the carrier density, the less light is emitted. This is because the carrier density can be changed by absorption, and as a result, carrier vibration is likely to occur.

However, in the case of GaInP, it has been found that it is relatively difficult to obtain self-excited oscillation characteristics because the slope of the gain characteristic curve is smaller than that of GsAs.

According to the experimental results of the present inventors, in the case of a red semiconductor laser, it is difficult to obtain a stable self-sustained pulsation characteristic by simply adding a saturable absorption layer as performed in the related art. Was revealed. In particular, it is difficult to obtain stable self-sustained pulsation characteristics at high temperatures. Also,
In a configuration like the related art, light consumption in the saturable absorption layer is large, and as a result, the threshold current tends to increase. Therefore, the reliability at high temperatures, for example, about 80 ° C. may be affected.

Here, another parameter when causing self-sustained pulsation in the semiconductor laser is the carrier density inside the saturable absorption layer. Specifically, a method is conceivable in which the volume of the saturable absorbing layer is made sufficiently small to relatively increase the carrier density therein. Considering this point, there is an optimum value for the deposition of the saturable absorbing layer (that is, its length), and it is difficult to obtain a stable self-excited oscillation characteristic only by adding the saturable absorbing layer.

Further, in order to generate a stable self-excited oscillation phenomenon, it is necessary to secure appropriate light confinement in the saturable absorption layer. For this purpose, it is easy to simply think that the saturable absorbing layer should be close to the active layer, but if the distance between them is too close, the adverse effect on operating characteristics due to the overflow of minority carriers from the active layer occurs. I do. Therefore, the distance between the active layer and the saturable absorbing layer must be about 200 Å or more, preferably about 500 Å or more. Therefore, it is necessary to secure light confinement while securing a sufficient distance between the active layer and the saturable absorption layer. If these conditions are not satisfied at the same time, the desired self-excitation will result. It becomes difficult to obtain a semiconductor laser having oscillation characteristics.

The above-mentioned examination results are mainly related to red semiconductor lasers composed of AlGaInP-based materials.
Even for AlGsAs-based materials whose oscillation wavelength is in the infrared region, ensuring stable self-excited oscillation with a low threshold current is important.
The prior art cannot achieve this sufficiently. In particular, in a state where high output is desired, this problem becomes remarkable.

Hereinafter, some of the various embodiments of the present invention achieved based on the above-described study results will be described with reference to the accompanying drawings.

First Embodiment FIGS. 1A and 1B are a cross-sectional view and a perspective view showing a structure of a semiconductor laser 100 having self-pulsation characteristics according to a first embodiment of the present invention.

In the semiconductor laser 100, on the n-type GsAs substrate 101,
A layered structure of semiconductor material is formed. In particular,
The substrate 101 moves from the (100) plane in the (0-1-1) direction.
It is inclined about 8 degrees. In the specification of the present application, the notation “−1” included in the notation of the plane orientation is indicated by a bar above the number “1”. It means the same content as "" with. For example, the above description of the “(0-1-1) direction”
It means “(0) direction”.

On a substrate 101, an n-type GsAs buffer layer 102, an n-type Al
A GaInP cladding layer 103, a GaInP active layer 104, a first p-type cladding layer 105a made of AlGaInP, and an InGaAsP saturable absorption layer 106 are sequentially formed. On the saturable absorption layer 106, a second p-type cladding layer 105b made of AlGaInP and a p-type GaInP contact layer 110 are formed. The second p
The mold cladding layer 105b and the p-type contact layer 110 are formed in a ridge shape, and n-type GsAs current blocking layers 111 are formed on both sides of the ridge. Further, on the p-type contact layer 110 and the n-type current block layer 111, p-type GsAs
A cap layer 112 is formed. And cap layer 1
A p-electrode 113 is formed on 12, while an n-electrode 114 is formed on the back surface of the substrate 101.

The typical thickness of each layer described above is as follows.

Table 1 Name Reference number Thickness Cap layer 112 3 μm Contact layer 110 500Å Second p-type cladding layer 105b 0.9 μm Saturable absorption layer 106 60Å First p-type cladding layer 105a 0.2 μm Active layer 104 500Å N-type cladding layer 103 1.0 μm Buffer layer 102 0.3 μm FIG. 2 shows a change in the Al composition x of (Al _x Ga _{1 -x} ) _0.5 In _0.5 P in the active layer 104 of the semiconductor laser 100 and its vicinity. As shown, in the semiconductor laser 100, n
Mold cladding layer 103, first and second p-type cladding layers 105a
And the Al composition x in the case 105b is set to 0.7.

Further, as shown in the above table, in the semiconductor laser 100, the thickness of the saturable absorption layer 106 is set to about 60 Å. If the saturable absorption layer 106 is too thick, its volume becomes too large, so that it is difficult to increase the carrier density. Therefore, a change in the carrier density inside the saturable absorption layer 106, that is, strong vibration of the carrier is less likely to occur, and as a result, it is difficult to obtain the self-excited oscillation characteristics. For this reason, it is desirable that the saturable absorbing layer be thin.

The composition of the InGaAsP saturable absorption layer 106 is, for example, In _0.5 Ga
_0.5 As _0.1 P _0.9 . The lattice constant of InGaAsP having this composition is larger than the lattice constant of GsAs forming the substrate 101, and as a result, a compressive stress is applied to the saturable absorption layer. Thus, the saturable absorption layer 106 in the semiconductor laser 100 has a strained quantum well structure.

FIG. 3 shows the gain characteristics (the relationship between carrier density and gain) of GaInP and InGaAsP. This shows that InGaAsP has a larger slope of the gain characteristic curve.

According to the study by the present inventors, in a semiconductor laser having a saturable absorption layer, the larger the slope of the gain characteristic of the material used for the saturable absorption layer (the ratio of the change in gain to the change in carrier density), the more the self-sustained pulsation. It became clear that characteristics were easily obtained. In other words, the greater the slope of the gain characteristic curve, the more advantageous for self-sustained pulsation. This is explained as follows. In order to obtain self-sustained pulsation characteristics, strong vibration of carriers inside the saturable absorption layer is required. Here, as the slope of the gain characteristic curve is larger, the carrier density can be changed with less light absorption. As a result, the larger the slope of the gain characteristic curve, the more easily the carrier is oscillated, and the more easily the self-excited oscillation phenomenon occurs.

The data in Fig. 3 is based on InGaAsP lattice-matched to the substrate.
Is obtained with respect to When compressive stress is applied to InGaAsP (for example, InGaAsP formed on a GsAs substrate)
_{In the case of 0.5} Ga _0.5 As _0.1 P _0.9 ), the slope of the gain characteristic curve becomes even larger than in the case shown in FIG. 3, which is more effective for realizing self-pulsation.

FIG. 4 is a simulation result of a temporal change of an optical output waveform when the optical output is about 5 mW. In addition,
The change of carrier density in the active layer and the saturable absorption layer is also illustrated. From this, it is clear that the oscillation phenomenon of the light output continues.

FIG. 5 is a measurement example of the current-light output characteristic of the self-pulsation type semiconductor laser 100 of the present embodiment. In this case, it can be seen that the threshold current is about 60 mA. As shown in FIG. 5, the point that the current-light output characteristic of the self-pulsation type semiconductor laser is different from that of a normal semiconductor laser is that a sharp rise of the characteristic curve is recognized near the threshold current. Is a point. This is because the saturable absorption layer is present in the self-pulsation type semiconductor laser, and the optical output is not emitted to the outside until a certain amount of carriers is injected. When the carrier injection amount exceeds a certain value, laser oscillation occurs, and the light output increases in proportion to the injection current amount.

FIG. 6 is a measurement example of an optical output waveform in the self-pulsation type semiconductor laser 100 of the present embodiment. Similar to the simulation result shown in FIG. 4, the light output greatly fluctuates as time passes, and it can be confirmed that self-excited oscillation occurs. In this semiconductor laser, about 20 ° C. to about 80 ° C.
A stable relative intensity noise (RIN) specification of about -135 dB / Hz or less has been obtained over a wide temperature range up to ° C.

As described above, according to the experiment performed by the inventors of the present application, in the case of a red semiconductor laser, stable addition of a saturable absorption layer as is generally performed in the related art is stable. It is difficult to obtain the self-excited oscillation characteristics described above. For example, even if a Ga _0.5 In _0.5 P layer having a normal doping level is used as a saturable absorbing layer, it is difficult to obtain a stable self-excited oscillation characteristic. In particular, it is difficult to obtain self-excited oscillation characteristics at high temperatures.

Further, in the configuration of the related art, light consumption in the saturable absorption layer is large, and as a result, the threshold current tends to increase. As a result, reliability at high temperatures (for example, 80 ° C.) may be affected.

On the other hand, if a saturable absorption layer is formed using a material having a large slope of the gain characteristic curve (for example, InGaAsP) as in the present invention, a stable operating characteristic can be obtained even at a high temperature of about 80 ° C. It is possible to obtain a semiconductor laser having a small size.

The use of the saturable absorption layer 106 made of an InGaAsP layer has a great advantage in manufacturing a semiconductor laser. That is,
The InGaAsP layer has a structure of the semiconductor laser 100 shown in FIG.
In addition to the function as the saturable absorption layer 106, it has a function as an etching stop layer in an etching process for forming a ridge. Specifically, when the second p-type cladding layer 105b made of AlGaInP is etched using an etching solution such as sulfuric acid to be processed into a ridge shape,
Etching can be stopped at the InGaAsP layer.

On the other hand, Ga having a normal structure is used as an etching stop layer.
When InP is used, the etching selectivity between the second p-type cladding layer 105b and the GaInP layer can be as small as about 10, although it depends on the etching conditions. For this reason, etching cannot be reliably stopped at the GaInP layer, and over-etching occurs, which may cause variations in the operating characteristics of the laser. Further, if the etching proceeds to the vicinity of the active layer, the reliability may be reduced. On the other hand, InGa has both a function as an etching stop layer and a function as a saturable absorption layer.
When an AsP layer is used, a large value of about 20 can be obtained as an etching selectivity between the second p-type cladding layer and InGaAsP. Thereby, the manufacturing process of the semiconductor laser can be easily performed.

As described above, the use of the InGaAsP layer as the saturable absorption layer has a significant effect in terms of both the operating characteristics of the semiconductor laser and the manufacturing process.

By the way, in producing a semiconductor laser, after forming a laser structure by growing a laminated structure on a substrate, a cavity end face is formed by cleavage. At this time, in the present embodiment, since the inclined substrate is used as the substrate, it is necessary to pay attention to the direction in which the stress is applied in the cleavage step. This will be described below.

FIG. 7 is a diagram schematically illustrating a crystal structure of the cavity facet of the semiconductor laser, that is, the (0-1-1) plane of the GsAs substrate. In the figure, a dotted line indicated as “AB” is a surface of the substrate, and a crystal is grown on this surface. Primitively, the substrate surface has a step-like shape, which corresponds to an atomic step.

Here, as described above, the substrate is inclined by about 8 degrees in the [0-1-1] direction from the plane orientation of the (100) plane.
Taking this point into account, when cleaving the crystal (substrate) to obtain individual semiconductor laser 100 chips, stress is applied from B to A in FIG. This is because when a stress is applied in a direction opposite to the above (ie, from A to B), a crystal crack to be generated from A to B is formed along the (100) plane during the cleavage process. This is because a certain cleavage is not realized. Such a situation results in a defect due to cleavage and lowers the production yield.
Therefore, cleavage is performed from B to A in the figure.

Such a feature relating to the directionality of cleavage is not limited to the GsAs substrate, but also applies to a substrate made of another material as long as it is an inclined substrate. More generally, the inclination is from a direction (B in the example of FIG. 7) in which a smaller angle can be obtained as an angle between the just surface (the (100) plane in the example of FIG. 7) and the inclined substrate surface. The substrate is cleaved by applying a stress in the direction.

After forming an end face by cleaving the substrate, the obtained end face (resonator end face) is subjected to a coating process. Specifically, for example, a predetermined number of pairs of the SiO ₂ film and the SiN film are stacked from the side close to the end face by the ECR sputtering method. Here, the thicknesses of the SiO ₂ film and the SiN film are respectively λ / 4
(Where λ is the oscillation wavelength).

Second Embodiment FIG. 8 is a cross-sectional view illustrating a structure of a semiconductor laser 200 having self-pulsation characteristics according to a second embodiment of the present invention.

In the semiconductor laser 200, a buffer layer 702, an n-type AlGaInP cladding layer 703, and a GaInP
Multiple quantum well active layer 704 including a well layer, first p-type cladding layer 705a made of AlGaInP, p-type light guide layer 707, AlGaI
A second p-type cladding layer 705b made of nP and an InGaAsP quantum well saturable absorption layer 706 are sequentially formed. On the saturable absorption layer 706, a third p-type cladding layer 705c made of AlGaInP and a p-type GaInP contact layer 710 are formed. Third p-type cladding layer 705c and p-type contact layer
710 is formed in a ridge shape, and both sides of the ridge are embedded with an n-type GsAs current blocking layer 711.
Further, the p-type contact layer 710 and the n-type current block layer 7
A p-type GsAs cap layer 712 is formed on 11. Then, a p-electrode 713 is formed on the cap layer 712, while an n-electrode 714 is formed on the back surface of the substrate 701.

FIG. 9 shows a change in the Al composition x of (Al _x Ga _{1 -x} ) _0.5 In _0.5 P in the active layer 704 of the semiconductor laser 200 and the vicinity thereof. As shown, in the semiconductor laser 200, n
The Al composition x in the mold cladding layer 703 and the first, second, and third p-type cladding layers 105a, 105b, and 105c is set to 0.7.

In the semiconductor laser 200 of the present embodiment, in addition to using the multiple quantum well active layer 704 as the active layer 704,
The quantum well layer is also used as the saturable absorption layer 706.
Note that the constituent material of the saturable absorption layer 706 is InGaAsP.

Further, in the semiconductor laser 200 of the present embodiment, the light guide layer 707 is provided near the saturable absorption layer 706. Here, the reason for providing the light guide layer 707 is as follows.

When the saturable absorption layer 706 is a quantum well layer, the overall thickness of the layer is reduced, so that the light confinement rate (light confinement coefficient) of the light into the saturable absorption layer 706 is extremely reduced.
As a result, light is not sufficiently absorbed by the saturable absorption layer 706,
Saturation of the gain does not occur sufficiently. Therefore, no self-excited oscillation occurs as it is. On the other hand, if the light guide layer 707 as described above is provided, the light confinement rate (light confinement coefficient) of the light in the saturable absorption layer 706 can be increased. Specifically, by providing the light guide layer 707, if the light confinement rate (light confinement coefficient) to the saturable absorption layer 706 is set to a value of at least about 1.2% or more, self-excited oscillation occurs. It becomes possible.

When the saturable absorption layer 706 is a quantum well layer, its thickness becomes thin. Therefore, the light confinement ratio (light confinement coefficient) of light in the saturable absorption layer 706 cannot be set to a size necessary for self-pulsation by the saturable absorption layer 706 alone without providing the light guide layer 707. . On the other hand, when the number of quantum well layers constituting the saturable absorption layer 706 is increased for the purpose of increasing the optical confinement ratio (optical confinement coefficient), the saturable absorption layer
The overall volume of 706 substantially increases, the carrier density decreases, and self-sustained pulsation no longer occurs.

The inventors of the present application have revealed that self-excited oscillation can be obtained even if the number of layers of the saturable absorption layer 706 is increased to increase the light confinement ratio (light confinement coefficient) to the saturable absorption layer 706. Did not. On the other hand, according to the present invention, (1)
Self-sustained pulsation was realized for the first time by a combination of the two conditions of not increasing the volume of the saturable absorption layer 706 and (2) providing the light guide layer 707.

In the semiconductor laser 200 of the present embodiment, the multiple quantum well active layer 704 has a quantum well structure with a total thickness of about 50 Å, and includes three well layers. AlGaInP having an Al composition x = 0.5 is used as a barrier layer included in the quantum well structure.

On the other hand, for the light guide layer 707 related to the quantum well saturable absorption layer 706, AlGaInP having an Al composition x = 0.5 is used, and its thickness is about 1000 Å. In order to achieve the intended purpose of the light guide layer 707, its thickness may be about 200 mm or more.

Here, FIG. 10 shows the thickness of the light guide layer 707 and the active layer 704.
6 shows the relationship between the saturable absorption layer 706 and the light confinement ratio (light confinement coefficient). Thus, if the thickness of the light guide layer 707 is about 300 Å or more, the light confinement rate (light confinement coefficient) of the light into the saturable absorption layer 706 is about 1.2.
% Or more. This makes it possible to reliably generate self-excited oscillation as described above. On the other hand, if the light guide layer 707 is too thick, the light confinement ratio (light confinement coefficient) of the light into the active layer 704 becomes small, which may increase the threshold current. It is desirable that the light confinement rate (light confinement coefficient) in the active layer 704 is about 5% or more. Considering this point, FIG. 10 shows that the light guide layer
Preferably, the thickness of 707 is less than about 1200 °.

Therefore, it is desirable that the thickness of the light guide layer 707 is not less than about 300 Å and not more than about 1200 Å.

In the actual sample of semiconductor laser 200, about 1
Self-excited oscillation occurs in a wide temperature range from 0 ° C to about 80 ° C. In the above temperature range, stable relative intensity noise (RIN) characteristics of about -135 dB / Hz or less are obtained.

As described above, in the present embodiment, the introduction of the quantum well structure into the active layer 704 of the semiconductor laser 200 increases the maximum optical output by about 20%. Further, a reduction in the threshold current value and an increase in output can be realized, and operation at a high temperature becomes possible. These are realized only by using InGaAsP as a constituent material of the saturable absorption layer 706 and adopting a novel structure in which the light guide layer 707 is provided as described above.

Further, the active layer 704 is used for further improving the operation characteristics.
Needless to say, even when a strained quantum well structure is used for the saturable absorption layer 706, a semiconductor laser having self-pulsation characteristics can be obtained according to the present invention.

Third Embodiment FIG. 11 is a cross-sectional view illustrating a structure of a semiconductor laser 300 having self-pulsation characteristics according to a third embodiment of the present invention.

In the semiconductor laser 300, on an n-type GsAs substrate 1001, an n-type AlGsAs cladding layer 1002, a multiple quantum well active layer 1003 including a GsAs well layer, a first p-type cladding layer 1004 made of AlGaInP, and an InGaAs quantum well Saturable absorption layer 1005,
They are formed sequentially. On the saturable absorption layer 1005, an n-type AlGsAs current blocking layer 10 having a stripe-shaped opening is provided.
06 is formed. Further, a second p-type cladding layer 1007 made of AlGsAs is formed on the saturable absorption layer 1005 and the current blocking layer 1006, and further a p-type GsAs
A cap layer 1008 is formed. And the cap layer
A p-electrode 1009 is formed on 1008, while an n-electrode 1010 is formed on the back surface of the substrate 1001.

The typical thickness of each layer described above is as follows.

Table 2 Name Reference number Thickness Cap layer 1008 3 μm Second p-type cladding layer 1007 0.9 μm Current blocking layer 1006 0.6 μm Saturable absorption layer 1005 50Å First p-type cladding layer 1004 0.15 μm Multiple quantum well active layer 1003 300Å n-type cladding layer 1002 1.0 μm In the above, for the multiple quantum well active layer 1003,
The total thickness is shown.

As shown in Table 2, in the semiconductor laser 300, the thickness of the saturable absorption layer 1005 is set to about 50 °. Saturable absorption layer
If 1005 is too thick, its volume becomes too large, so that it is difficult to increase the carrier density. For this reason, a change in the carrier density inside the saturable absorption layer 1005, that is, strong vibration of the carrier is unlikely to occur, and as a result, it is difficult to obtain self-excited oscillation characteristics. Therefore, the saturable absorption layer 1005
Should be thinner.

The composition of the InGaAs saturable absorption layer 1005 is, for example, In _0.2 Ga
_0.8 As The lattice constant of InGaAs having this composition is
The lattice constant is larger than the lattice constant of GsAs forming the substrate 1001, and as a result, a compressive stress is applied to the saturable absorption layer 1005. Thus, the saturable absorption layer 1 in the semiconductor laser 300 is
005 has a strained quantum well structure.

FIG. 12 shows gain characteristics (relationship between carrier density and gain) of GsAs and InGaAs. This shows that the slope of the gain characteristic curve is larger in InGaAs.

As mentioned earlier, in the study by the present inventors,
In a semiconductor laser having a saturable absorption layer, the larger the slope of the gain characteristic of the material used for the saturable absorption layer (the ratio of the change in gain to the change in carrier density), the easier it is to obtain self-pulsation characteristics. Was revealed. In other words, the greater the slope of the gain characteristic curve, the more advantageous for self-sustained pulsation. This is explained as follows.
In order to obtain self-sustained pulsation characteristics, strong vibration of carriers inside the saturable absorption layer is required. Here, as the slope of the gain characteristic curve is larger, the carrier density can be changed with less light absorption. As a result, the larger the slope of the gain characteristic curve, the more easily the carrier is oscillated, and the more easily the self-excited oscillation phenomenon occurs.

In the sample of the actually manufactured semiconductor laser 300, about 1
Self-excited oscillation occurs in a wide temperature range from 0 ° C to about 100 ° C. In the above temperature range, stable relative intensity noise (RIN) characteristics of about -135 dB / Hz or less are obtained.

As described above, in the present embodiment, by using a new material called InGaAs as a constituent material of the saturable absorption layer 1005, compared with the case where the saturable absorption layer is formed of another material such as GsAs, the threshold current is increased. And operation at high temperatures becomes possible. In the above description, the saturable absorption layer made of InGaAs is described as an example.
Even when the saturable absorption layer is formed using P, the same effect as described above can be obtained.

Fourth Embodiment FIG. 13 is a cross-sectional view illustrating a structure of a semiconductor laser 400 having self-pulsation characteristics according to a fourth embodiment of the present invention.

The structure of the semiconductor laser 400 is similar to the structure of the semiconductor laser 300 described with reference to FIG. Corresponding components have the same reference numbers allotted, and description thereof will not be repeated here.

The configuration of the semiconductor laser 400 is different from the configuration of the semiconductor laser 300 in that the saturable absorption layer 1000 is provided in the second p-type cladding layer 1007,
06, a mode control layer 1012 is provided, and
The point is that an etching stop layer 1011 for controlling the etching for forming the opening in the current blocking layer 1006 is provided.

The etching stop layer 1011 is a thin layer, and the layer 1011 hardly absorbs light. Accordingly, the light generated in the active layer 1003 is hardly absorbed by the etching stop layer 1011 and spreads into the second p-type cladding layer 1007, and the light generated in the second p-type cladding layer 1007 can be formed. Saturated absorption layer 1000
Is absorbed by The light absorption by the saturable absorption layer 1000 is saturated, and thereafter has a gain. As a result, the carrier vibrates to have self-excited oscillation characteristics.

In the structure of the semiconductor laser 400 as described above, for example,
By setting the Al composition of AlGsAs constituting the current blocking layer 1006 to 0.6 and the Al composition of AlGsAs constituting the first cladding layer 1004 and the second cladding layer 1007 to 0.4, the current blocking layer in the active layer 1003 is formed. The effective refractive index in a region immediately below the opening of 1006 is higher than the effective refractive index in the region on both sides thereof (that is, the region of the active layer 1003 covered by the current blocking layer 1006). Thus, a real refractive index guided laser structure can be obtained, and a semiconductor laser having high output and low threshold current characteristics can be realized.

Further, in the semiconductor laser 400 of the present embodiment, the mode control layer 1012 is provided in the current blocking layer 1006. This mode control layer 1012 is for the purpose of preventing the electric field of light from being present only under the current blocking layer 1006 and not under the opening when the light absorption by the saturable absorption layer 1000 is large. Is provided. When the mode control layer 1012 is provided, light is also absorbed in this layer 1012. As a result, light can also exist below the opening of the current blocking layer 1006, and consequently, transverse mode oscillation is realized.

As described above, in the present embodiment, the threshold current of the semiconductor laser is reduced by using the real refractive index guided laser structure. Further, in the low output state, high output can be achieved while obtaining self-excited oscillation characteristics. Further, by providing the mode control layer 1012 in the current block layer 1006,
Light modes can be prevented from being separated just below the opening of the current blocking layer 1006.

Fifth Embodiment FIG. 14 is a cross-sectional view showing a structure of an AlGaInP-based red semiconductor laser 500 as a fifth embodiment of the present invention.

The structure and function of the semiconductor laser 500 are almost the same as those of the semiconductor laser 400 according to the fourth embodiment described with reference to FIG. The difference is that the semiconductor laser 500 of the present embodiment uses AlGaInP as a constituent material.
In the configuration of the semiconductor laser 500 (FIG. 14), components corresponding to each component (substrate and layer) included in the configuration of the semiconductor laser 400 (FIG. 13) include the semiconductor laser 400 (FIG. 1).
The value obtained by adding “400” to the reference number in 3) is given as the reference number. For example, a substrate 1401 included in the semiconductor laser 500 illustrated in FIG. 14 corresponds to the substrate 1001 included in the semiconductor laser 400 illustrated in FIG. From this, the semiconductor laser 500
A detailed description of the configuration is omitted.

In the multiple quantum well structure of the active layer 1403 of the semiconductor laser 500, the composition of the well layer is Ga _0.5 In _0.5 P, and the composition of the barrier layer is (Al _0.7 Ga _0.3 ) _0.5 In _0.5 P. The compositions of the n-type cladding layer 1402, the first p-type cladding layer 1404, and the second p-type cladding layer 1407 are each (Al _0.7 Ga _0.3 )
_0.5 In _0.5 P. Further, the compositions of the current blocking layer 1406, the saturable absorption layer 1400, and the mode control layer 1412 are (Al
_0.5 Ga _0.5 ) _0.5 In _0.5 P. However, saturable absorption layer 1400
The mode control layer 1412 may be made of In _0.2 Ga _0.8 As.

Semiconductor laser of the present embodiment having the above configuration
500 has the same effect as the semiconductor laser 400 in the fourth embodiment. The oscillation wavelength is in a red region of about 660 nm.

In the above description, the composition of the first p-type cladding layer 1404 is (Al _0.7 Ga _0.3 ) _0.5 In _0.5 P, but from the viewpoint of preventing electron overflow during high-temperature operation, the above (Al _0.7 Ga _0.3 ) An (AlGa) InPN layer having a band gap larger than the _0.5 In _0.5 P layer may be used. This (Al
The Ga) InPN layer can be grown by flowing NH ₃ gas at a temperature of about 900 ° C. Also, in growing the (AlGa) InPN layer, the amount of the In (indium) element having a large atomic radius was increased by the amount of the N (nitrogen) element having a small atomic radius to match the lattice constant of the GsAs substrate. It can be grown as a layer having a lattice constant. The specific composition is, for example, (Al _0.7 Ga _0.3 ) _0.4 In _0.6 P _0.9
N is assumed to be _0.1 . Thus, the (AlGa) InNP layer is formed by the first p
The mold cladding layer 1404 can prevent the overflow of electrons even during high-temperature operation, and can improve the temperature characteristics of the semiconductor laser.

Furthermore, when the (AlGa) InNP layer is used, it is possible to realize selective etching between the AlGaInP layer and the (AlGa) InNP layer, so that there is no need to provide a separate etching stop layer, and the manufacturing process is simplified. Is done.

INDUSTRIAL APPLICABILITY As described above, in the semiconductor laser of the present invention,
InGaAsP (however, its composition is In _x Ga _1-x As _y P _1-y (0 <x <
By providing a saturable absorbing layer made of 1, 0 ≦ y ≦ 1), the slope of the gain characteristic curve (gain with respect to carrier density) can be improved compared to the case where a saturable absorbing layer made of GaInP is used. Change rate) can be increased. This makes it possible to change the carrier density with a small amount of light absorption, and the carrier is likely to vibrate. At the same time, the energy gap of the saturable absorbing layer can be made smaller than the energy gap of the active layer by about 30 meV to about 200 meV. As a result, stable self-excited oscillation characteristics can be easily obtained, and relative noise can be reduced.

Further, in the semiconductor laser of the present invention, by setting the thickness of the saturable absorption layer to about 100 angstroms or less, it is possible to easily increase the carrier density due to light absorption. As a result, carrier oscillation is likely to occur, and stable self-excited oscillation characteristics can be easily obtained.

In addition, in the semiconductor laser of the present invention, a low-power output is achieved by providing a real refractive index waveguide type laser structure and providing a saturable absorption layer in the cladding layer located above the opening of the current blocking layer. In the region, it is possible to provide a semiconductor laser having a self-excited oscillation characteristic and capable of simultaneously increasing the output. Further, by providing a mode control layer in the current block layer, it is possible to prevent laser light mode separation at the opening of the current block layer.

Continuation of the front page (56) References JP-A-9-83064 (JP, A) JP-A-9-214058 (JP, A) JP-A-9-205253 (JP, A) JP-A-8-18160 (JP) , A) IEEE Photon. Tech. Lett. 7 [12] (1995) p. 1406-1408 Appl. Phys. Lett. 67 [10] (1995) p. 134-1345 (58) Field surveyed (Int.Cl. ⁶ , DB name) H01S 3/18

Claims (22)

(57) [Claims] 1. A semiconductor laser comprising an active layer and a clad structure sandwiching the active layer, wherein the clad structure includes a saturable absorption layer, and the saturable absorption layer is formed of In _x Ga _{1. -x} As _y P _1-y (0 <x <1, 0 <y ≦ 1)
Consisting of a semiconductor laser. 2. The semiconductor laser according to claim 1, wherein a distance between said saturable absorption layer and said active layer is about 200 Å or more. 3. The cladding structure further includes a light guide layer, wherein the saturable absorber layer is adjacent to the light guide layer.
The semiconductor laser according to claim 1. 4. The semiconductor laser according to claim 1, wherein said cladding structure further includes a light guide layer, and said saturable absorption layer is disposed near said light guide layer. 5. A semiconductor laser having an active layer and a clad structure sandwiching the active layer, wherein the clad structure includes a saturable absorbing layer, and the saturable absorbing layer is formed of In _x Ga _{1−. x} As _y P
_1-y (0 <x <1, 0 <y ≦ 1), wherein the saturable absorption layer has a strained quantum well structure, and the saturable absorption layer is located between ground orders of the saturable absorption layer. The energy gap is about 30 meV more than the energy gap of the active layer.
Semiconductor laser smaller by about 200 meV 6. The semiconductor laser according to claim 1, wherein said cladding structure excluding said active layer and said saturable absorption layer is made of (Al _x Ga _{1 -x} ) _y In _{1 -y} P. 7. A active layer, and a cladding structure sandwiching the active layer, a semiconductor laser with a said cladding structure includes a saturable absorbing layer, the saturable absorption layer is an In _x Ga _{1 -x} As _y P _1-y (0 <x <1, 0 <y ≦ 1)
And a saturable absorption layer having a strained quantum well structure. 8. The semiconductor laser according to claim 7, wherein a distance between said saturable absorption layer and said active layer is about 200 Å or more. 9. The optical fiber according to claim 9, wherein the cladding structure further includes a light guide layer, wherein the saturable absorbing layer is adjacent to the light guide layer.
A semiconductor laser according to claim 7. 10. The semiconductor laser according to claim 7, wherein said cladding structure further includes a light guide layer, and said saturable absorption layer is disposed near said light guide layer. 11. The semiconductor laser according to claim 7, wherein an energy gap between ground orders of the saturable absorbing layer is smaller than an energy gap of the active layer by about 30 meV to about 200 meV. 12. The semiconductor laser according to claim 7, wherein said cladding structure excluding said active layer and said saturable absorbing layer is made of (Al _x Ga _{1 -x} ) _y In _{1 -y} P. 13. The semiconductor laser according to claim 1, wherein said cladding structure excluding said active layer and said saturable absorption layer is made of Al _z Ga _{1 -z} As. 14. The semiconductor laser according to claim 1, wherein said saturable absorption layer has a thickness of about 100 angstroms or less. 15. The semiconductor laser according to claim 14, wherein said saturable absorption layer has a thickness of about 80 Å or less. 16. The semiconductor laser according to claim 14, wherein said cladding structure further includes a light guide layer, and said saturable absorption layer is adjacent to said light guide layer. 17. The semiconductor laser according to claim 14, wherein said cladding structure further includes a light guide layer, and said saturable absorption layer is disposed near said light guide layer. 18. The saturable absorption layer has a strained quantum well structure, and the energy gap between ground orders of the saturable absorption layer is about 30 times smaller than the energy gap of the active layer.
15. The semiconductor laser of claim 14, wherein the semiconductor laser is about 200 meV less than meV. 19. A current block having an active layer, an n-type cladding layer and a first p-type cladding layer sandwiching the active layer, and an opening formed on the first p-type cladding layer. And a second p-type cladding layer formed in the opening, wherein the second p-type cladding layer includes a saturable absorption layer.
A semiconductor laser, wherein the saturable absorber layer is In _x Ga _1-x As _y P
_1-y (0 <x <1, 0 <y ≦ 1), a semiconductor laser. 20. An active layer, an n-type cladding layer sandwiching the active layer, a first p-type cladding layer, and a current block having an opening formed on the first p-type cladding layer. And a second p-type cladding layer formed in the opening, wherein the second p-type cladding layer includes a saturable absorption layer.
A semiconductor laser, wherein a mode control layer is formed in the current blocking layer. 21. An active layer, an n-type cladding layer and a first p-type cladding layer sandwiching the active layer, and a current block having an opening formed on the first p-type cladding layer. And a second p-type cladding layer formed in the opening, wherein the second p-type cladding layer includes a saturable absorption layer.
A semiconductor laser, wherein the first p-type cladding layer comprises:
A semiconductor laser made of a material having a band gap larger than a band gap of the n-type cladding layer. 22. The semiconductor laser according to claim 21, wherein said first P-type cladding layer is an AlGaInPN layer.