JPH09246662A - Semiconductor laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a semiconductor laser having a stable self-excited oscillation characteristic and high reliability. SOLUTION: This laser comprises an n-type GaAs substrate 101, active layer 104, and pair of clad layers 103, 105 disposed at both sides of the active layer. It has an emission region and saturable absorptive region in a resonator direction, and absorptive region has a saturable absorptive layer 104s grown at the same time as the active layer. This layer 104s is doped with a p-type dopant at high concn. to reduce the carrier life due to the light absorption, thus obtaining a stable self-excited oscillation characteristic. Thus it is possible to realize a semiconductor laser having a low relative noise intensity over a wide temp. range.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low noise self-excited oscillation type semiconductor laser used as a light source of an optical disk system.

[0002]

2. Description of the Related Art In recent years, the demand for semiconductor lasers has increased in the fields of optical communication, laser printers, optical disks, and the like.
Research and development have been actively pursued centering on the system and the InP system. In the field of optical information processing, the wavelength is 7
A method of recording / reproducing information by the light of an 80 nm AlGaAs semiconductor laser has been put into practical use and has come into widespread use in compact discs and the like.

However, these optical disk devices have recently been required to have an increased storage capacity, and the demand for short wavelength lasers has been increased accordingly. Al
The GaInP-based semiconductor laser has a wavelength of 630 to 690 nm.
It is possible to oscillate in the red region, and it is possible to obtain the shortest wavelength light among the semiconductor lasers currently in practical use. Therefore, it is promising as a next-generation large-capacity optical information recording light source that replaces the conventional AlGaAs semiconductor laser. By the way, the semiconductor laser generates intensity noise due to feedback of the reflected light from the disk surface or a change in temperature when reproducing the optical disk, and induces a signal reading error. Therefore, a laser with little intensity noise is indispensable for the light source of the optical disk.

Conventionally, in a read-only low-power AlGaAs semiconductor laser, noise reduction is achieved by adopting a structure in which saturable absorbers are intentionally formed on both sides of the ridge stripe in order to reduce noise. I have been trying. This makes it possible to make the vertical mode multi. When the laser oscillates in the vertical single mode, if a disturbance such as optical feedback or temperature change enters, the adjacent longitudinal mode starts oscillating due to a slight change in the gain peak. Cause a conflict. This is a cause of noise, and when the longitudinal modes are multi-modulated, the intensity change of each mode is averaged, and moreover, it does not change due to disturbance, so that stable low noise characteristics can be obtained.

Japanese Unexamined Patent Publication No. 7-263799 discloses that FIG.
As described above, a semiconductor laser in which a saturable absorption layer is formed in the cavity direction to reduce the noise level has been reported. A sectional view of this laser is shown in FIG. This laser
A p-type electrode is not formed on the cavity end face, and a region where no current is injected is provided in the active layer, and this region is used as a saturable absorption region.

[0006]

However, as in the conventional laser, it is difficult to obtain self-sustained pulsation in order to lower the noise level by merely making the region where current is not injected into the saturable absorption layer. . According to the experiments by the inventors,
It has been revealed that it is difficult to obtain self-sustained pulsation characteristics in the AlGaInP-based semiconductor laser because the gain characteristics of the materials are significantly different from those in the AlGaAs-based semiconductor laser. It is shown in FIG. FIG. 10 shows the gain characteristics of GaAs and GaInP. Here, GaAs and GaInP are AlGaAs semiconductor laser and AlGaIn, respectively.
It is a material mainly used for the active layer of a P-based semiconductor laser.

In order to obtain the self-sustained pulsation characteristic, it is required that the slope of the gain characteristic with respect to the carrier density is large. However, in the case of GaInP, it has been found that it is relatively difficult to obtain self-sustained pulsation characteristics because the gradient is smaller than that of GaAs.

According to the experimental results of the inventors of the present application, in the case of the red semiconductor laser, due to the above-mentioned difference in gain characteristics, stable self-excited oscillation characteristics can be obtained only by providing a saturable absorption region as in the conventional example. It turned out to be difficult to obtain. That is, the self-sustained pulsation phenomenon is more likely to occur as the carrier lifetime of the saturable absorption layer is shorter, and the carrier lifetime is not shortened and self-sustained pulsation is less likely to occur unless the saturable absorption region is devised. The reason is that if the carrier lifetime is long, the contribution of spontaneous emission light to the rate of change of the carrier density of the saturable absorption layer with time becomes small, and the carrier vibration is less likely to occur.

The present invention has been made in view of the above circumstances, and in particular, it is possible to provide a semiconductor laser having stable self-excited oscillation characteristics by optimally setting a saturable absorption region constituting the semiconductor laser. To aim.

[0010]

In order to solve the above problems, the semiconductor laser of the present invention forms a light emitting region and a saturable absorption region in the cavity direction. The saturable absorption layer in the saturable absorption region has a function of absorbing the laser light from the light emitting region and immediately extinguishing carriers generated by the absorption. That is, the life of the carrier is shortened, and as a result, the contribution of spontaneous emission to the time change rate of the carrier is increased,
Self-oscillation can be easily generated and relative noise can be reduced. It has been found that the carrier lifetime is considerably effective if it is about 6 ns or less.

Specifically, the active layer in the saturable absorption region absorbs laser light and saturates the absorption. The active layer is doped with impurities in order to shorten the life of carriers generated by absorption of laser light. As a result, the life of carriers generated by absorption of laser light can be shortened.

In the semiconductor laser of the present invention, the doping level of this saturable absorption layer is preferably 1 × 10 18 (c).
By setting m-3) or more, the life time of the carrier is reduced. As a result, the contribution of spontaneous emission to the time rate of change of carriers increases, self-sustained pulsation can easily occur, and relative noise can be reduced.

Further, the saturable absorption region (saturable absorption layer)
Is formed only on a part of the laser resonator, and the light emitting region is not formed with a region that actively absorbs laser light. Therefore, since this laser has little absorption, it can be used also as a laser for high output.

When this semiconductor laser is used as a light source of an optical information processing apparatus, the laser can be used to record an optical disk because it has good noise characteristics during reproduction of the optical disk and can be used at high output. Since reproduction and recording can be performed with one laser, it is possible to realize an optical information processing device with a simple configuration and high performance.

[0015]

Embodiments of the present invention will be described below with reference to the drawings.

(Embodiment 1) First, FIG. 1 shows a change in the life time of electrons with respect to the doping concentration of a p-type doped saturable absorber layer. From this, it is understood that the lifetime is greatly influenced by the doping level.

In the conventional laser, it is difficult to obtain the short life time required for self-sustained pulsation. The self-excited oscillation phenomenon is more likely to occur as the life time of the saturable absorption layer is shorter. This is because the shorter the life time, the larger the time change of the carriers in the saturable absorption layer necessary for causing the self-excited oscillation phenomenon. According to the experiments conducted by the inventors, it has been found that the life time is preferably about 6 nanoseconds or less. The relationship between the life time and the doping level is such that when the doping level is low, the life time becomes long, for example, 1 × 10 ¹⁸ (c
m ^-3 ) or less, exceeding 6 nanoseconds. On the other hand, by increasing the doping level to about 2 × 10 ¹⁸ (cm ⁻³ ), it becomes possible to reduce the doping level to about 3 nanoseconds.

In order to utilize this effect, a semiconductor laser having the following structure in which the doping level of the saturable absorbing layer is increased was manufactured. FIG. 2 shows a sectional structural view and a constitutional perspective view of the first embodiment.

101 is [01] from the plane orientation of the (100) plane.
1] is an n-type GaAs substrate tilted by 8 degrees, and a buffer layer 1 made of n-type GaInP is formed on the substrate 101.
02, an n-type cladding layer 103 made of AlGaInP,
GaInP active layer 104, p-type AlGaIn
The first cladding layer 105a made of P, p-type GaIn
The etching stopper layer 106 made of P is sequentially formed.

A ridge-shaped second p-type cladding layer 105b made of AlGaInP and a p-type GaIn are formed on the upper portion thereof.
A contact layer 110 made of P is formed. A current blocking layer 111 made of an n-type GaAs layer is formed on both sides of the ridge-shaped second p-type cladding layer 105b and the contact layer 110. Further contact layer 11
0 and the current blocking layer 111, a cap layer 112 made of p-type GaAs is formed.
2 has a p-electrode 113 and a substrate 101 has an n-electrode 114.
Are formed respectively. The doping level and film thickness are as follows. Further, FIG. 11 shows a bandgap energy diagram from the n-type cladding layer to the second p-type cladding layer.

[0021]

[Table 1]

FIG. 2B is a perspective view showing the structure of this embodiment. The laser resonator end face in the front of the figure is the (0-11) face, the rear end face is the (01-1) face, and the laser light is emitted from the front (0-11) end face. FIG.
[Fig. 6] is a configuration side view of the (0-1-1) plane (actually, it is inclined by 8 degrees in the [100] direction).

As shown in FIG. 12, the laser has a light emitting region and a saturable absorption region. Furthermore, in the saturable absorption region, there is a p-type diffusion region. No electrode for current injection is formed on the upper surface of the saturable absorption region, but a SiO 2 layer for protecting the upper surface is formed. The saturable absorption region has a p-type diffusion region, and the p-type dopant is diffused from the etching stopper layer 106 to the middle of the n-type cladding layer 103.

A saturable absorber layer is formed in the saturable absorber region. The saturable absorption layer 104s is formed simultaneously with the active layer 104 of the light emitting layer. Zn as the p-type dopant is 2 × 10 ¹⁸ c in the saturable absorber layer 104 s.
It is doped with about m ⁻³ . As a result, the highly doped saturable absorption layer 104s for the laser light in the light emitting region is
Is absorbed and saturated, and the life of the carrier can be shortened, so that stable self-oscillation characteristics can be realized.

This laser exhibits a self-sustained pulsation characteristic in a low output region up to about 10 to 15 mW, but does not exhibit a self-sustained pulsation characteristic in a higher output region than this. However, since the saturable absorption layer is not formed on the entire surface of the laser resonator but only on a part of the resonator, absorption of laser light is achieved even in a high output region of 10 to 15 mW or more. It is suitable for high power lasers, as it has a small amount. If the absorption region is formed over the entire surface, a kink due to COD will occur before the output becomes high due to current injection. Therefore, as in this embodiment,
By forming a saturable absorption region in a part of the resonator and providing a saturable absorption layer in that region, a self-excited oscillation is shown in the low output region, and as a high output laser of single mode oscillation in the high output region. Also works.

An application of this semiconductor laser to an optical disk device is shown in FIG. From semiconductor laser 80 wavelength 66
The 0 nm laser beam 802 is collimated by a collimator lens 803, divided into three beams (not shown) by a diffraction grating 804, passes through a half prism 805, and is converged by a condenser lens 806. Tie a spot with a diameter of 1 μm on top. The reflected light on the disk 807 passes through the condenser lens 806 again, and the half prism 80
The light is reflected by the light receiving lens 5 and is passed through the cylindrical lens 809 narrowed down by the light receiving lens 808 to enter the photodiode 810 and converted into an electric signal. At this time, the shift of the optical disc 807 in the radial direction is detected by the three divided beams, and the positional shift of the focus is detected by the cylindrical lens 809. This deviation is corrected by fine adjustment of the optical system by the drive system 811.

In this way, if the semiconductor laser is applied to an optical disk device equipped with a condensing optical system for guiding the laser light from the semiconductor laser to the optical disk and a light receiving section for reading the light reflected by the optical disk, it is possible to obtain low output power by itself. Due to the excited oscillation characteristic, the disc can be read (reproduced) without being much affected by the return light to the laser, and can be used even at high output, so writing (recording) to the disc can also be performed and one laser can The optical disk device has excellent characteristics with a simple structure capable of reading and writing.

Next, a method of manufacturing this laser will be described. On the n-type GaAs substrate 101, the n-type GaInP buffer layer 202 and AlGaIn are formed by MOVPE method.
P-type n-type clad layer 203, GaInP active layer 204, p-type AlGaInP first clad layer 105, p-type GaInP etching stopper layer 206, and AlGaInP second p-type clad layer 105b. Are sequentially epitaxially grown. The active layer was non-doped, Zn of trimethyl Zn was used as the p-type dopant, and Si was used as the n-type dopant.

Next, Zn is vapor-phase-diffused from the upper surface of the portion serving as the saturable absorption region, using the SiO 2 film as a mask on the second p-type cladding layer serving as the light emitting region. This Zn
To the middle of the n-type cladding layer 103, and the active layer 1
04 is p-type due to the diffusion of Zn to be a saturable absorption layer.

The impurity concentration of the saturable absorption layer is
Although it is set to 2 × 10 ¹⁸ cm ⁻³ , it is preferably 1 × 10 ¹⁸ or more in order to reduce the life of carriers generated by optical absorption.

After that, the SiO 2 film mask is removed, and a stripe pattern of 5 μm is etched by dry etching in the cavity direction to form a second p-type cladding layer 10.
5b is processed into a ridge shape.

The dry etching conditions at this time are that the flow rate of chlorine is 11 SCCM and the flow rate of nitrogen is 3.5 SCCM, so that the chlorine flow rate / nitrogen flow rate ratio is 3.1. Accelerating voltage of 650
When dry etching was performed at V, an internal pressure of 2.5 mTorr, and a sample temperature of 100 ° C., the ridge had a symmetrical trapezoidal shape. Usually, the etching using the off-substrate does not become symmetrical as shown in FIG. Since the substrate is inclined from the (100) plane, usually, in wet etching using a sulfuric acid-based etching solution, the inclination of the side surface at the bottom of the ridge is gentle and the ridge is asymmetric. As a result, the injection of carriers into the active layer and the confinement of light become non-uniform, and the transverse mode becomes unstable. This is especially noticeable when operating the laser at high temperatures.

Further, in the AlGaInP type semiconductor laser, the inclination angle is further increased in order to shorten the wavelength of the laser light, and the larger this angle, the greater the asymmetry of the ridge.

However, in the dry etching method of the present embodiment, as shown in FIG. 7A, in the dry etching under these conditions, the symmetrical ridge 702 can be formed even though the off substrate is used.

The dry etching for forming the ridge is stopped at the etching stopper layer 106. p-type AlGaIn
Although the P second cladding layer 105b is etched by dry etching, the thickness of the p-type AlGaInP second cladding layer is 1.5 μm. Therefore, first, 1.3 μm is etched by the first dry etching having a strong sputter component,
The remaining 0.2 μm may be etched by the second dry etching having a strong chemical reaction component. In this way, the layers below the etching stopper layer will not be damaged during etching.

After forming the ridge of the second p-type cladding layer, the n-type GaAs current blocking layer 111, which is an n-type buried layer, is selectively grown using the SiO 2 film mask as it is.

Then, the SiO2 mask is removed and the p-type G
After growing the aAs cap layer 112, n-type GaAs
The substrate 101 has an n-type electrode 114, and the cap layer 112 has a p layer.
The mold electrode 113 is formed to complete the laser structure. The p-type electrode is not formed on the p-type diffusion region as shown in FIG. This is because if the p-type electrode is too close to the p-type diffusion region, the leak current will increase.

In the method of manufacturing the semiconductor laser, the flow rate ratio of the halogen gas and the nitrogen gas is particularly great in forming the ridge,
The angle θ from the bottom surface of the ridge to the side surface of the ridge is set by using a flow rate of 1.4 ≦ chlorine flow rate / nitrogen flow rate ≦ 4.0.
Is 60 to 90 degrees, and a symmetrical ridge structure laser can be manufactured despite using an off-substrate.

As a result, the confinement of carriers and the confinement of light in the active layer do not become non-uniform, and a laser having a stable transverse mode and high reliability can be obtained. Further, in this etching, the etching rate did not decrease despite the etching of the compound semiconductor containing Al.

The laser structure grown on the substrate forms the cavity facets by cleavage. At this time, since the substrate is an inclined substrate, it is necessary to pay attention to the direction in which stress is applied.

In FIG. 3, the resonator end face, that is, GaA
The crystal structure of the (0-11) plane of the s substrate is schematically shown. AB is the surface of the substrate, and the crystal is grown on this surface. Atomically, the substrate surface has a stepped shape, which is an atomic step. It can be seen that this substrate is inclined by 8 degrees in the [011] direction from the plane orientation of the (100) plane. FIG. 2B is a perspective view showing the structure of this laser, in which the substrate is tilted in the [011] direction by 8 degrees from the (100) plane. That is, this substrate is [011] from the plane orientation of the (100) plane.
Since it is tilted by 8 degrees in the direction A from B (B ') direction
Stress is applied in the (A ′) direction to break the crystal. The reason for this is that when stress is applied from A, the cracks that were heading from A to B as shown in FIG. 3 progressed in the direction along the (100) plane from the middle, resulting in defects due to cleavage and a decrease in yield. is there. Therefore, the crawling is performed from the direction B to the direction A. This is Ga
The present invention is not limited to As substrates, but applies to inclined substrates such as semiconductor substrates. Cleavage is performed by applying stress in a direction in which the angle between the just plane (here, the (100) plane) and the substrate plane becomes smaller (B instead of A).

After the end face is formed by cleaving, the end face of the resonator is coated. From the end face side, S with a film thickness of λ / 4
The iO2 film and the SiN film having a thickness of λ / 4 were set as one cycle. S
The ECR sputtering method was used to stack the iO2 and SiN films.

Next, FIG. 4 shows the current-light output characteristics of this semiconductor laser. The threshold current is 50 mA. What differs from the characteristics of a normal semiconductor laser is that a sharp rise is seen near the threshold current. This is because the saturable absorption layer exists, so that the optical output is not emitted to the outside until the injection amount of carriers is reached to some extent.

When it exceeds a certain value, laser oscillation occurs and the optical output starts to increase in proportion to the injection current. FIG. 5 shows the simulation result of the optical output waveform with respect to time in P in the figure
Shown in In FIG. 5, it can be seen that the oscillation phenomenon of the light output continues. FIG. 6 shows an output waveform of the self-excited oscillation type semiconductor laser. The optical output vibrates greatly with time and self-oscillates.

Still, the amount of current injection is increased, and the self-excited oscillation is stopped when the optical output is about 10 mW, and normal single mode oscillation is achieved. However, as described above, in the laser of this embodiment, since the absorption region is not formed over the entire laser cavity direction, the light absorption is small, so 40 mW.
Laser light can be output up to a high output range. That is, a self-excited oscillation is realized at a low output, and a single mode oscillation is realized at a high output, and it becomes an essential laser for an optical disk device.

(Second Embodiment) A second embodiment of the present invention will be described. The semiconductor laser of this embodiment has a quantum well structure in the active layer, so that the efficiency is high and a higher optical output can be obtained. The multiple quantum well active layer has three wells with a film thickness of 50 Å.

In this example, the maximum optical output could be increased by about 20% by introducing the quantum well structure into the multiple quantum well active layer. In addition, low threshold current, high temperature operation and high output are possible. Further, according to the semiconductor laser of the present embodiment, the self-sustained pulsation phenomenon similar to that of FIG.
A relative noise intensity (RIN) of 0 dB / Hz or less is also obtained.

[0048]

As described above, the semiconductor laser of the present invention has the following features.
A saturable absorption region is provided in a part of the laser cavity direction, and by controlling the carrier lifetime generated by optical absorption in the saturable absorption layer in this region, stable self-sustained pulsation characteristics are obtained. It is a thing.

Further, since the saturable absorption layer is formed only in a part of the laser resonator, it absorbs less laser light and can also realize a function as a high power laser.

Due to these functions, this semiconductor laser 1
This makes it possible to read and write optical discs on a stand, which is essential for optical disc devices.

[Brief description of drawings]

FIG. 1 is a correlation diagram of a doping concentration of a saturable absorption layer and a carrier lifetime in a first embodiment of the present invention.

FIG. 2 is an AlGaInP according to the first embodiment of the present invention.
Sectional view and perspective view of the semiconductor laser diode

FIG. 3 is a diagram for explaining a slope of a tilted substrate.

FIG. 4 is a current-light output characteristic diagram of this semiconductor laser.

FIG. 5 is a diagram of time waveforms of optical output and carrier density in the first embodiment of the present invention.

FIG. 6 is a diagram of measured time waveforms of optical output and carrier density in the first embodiment of the present invention.

FIG. 7 is a diagram comparing a symmetrical ridge according to the present invention with a conventional asymmetrical ridge.

FIG. 8 is a block diagram of an optical information processing device of the present invention.

FIG. 9 is a sectional view showing the structure of a conventional semiconductor laser.

FIG. 10 is a gain characteristic diagram of GaAs and GaInP.

FIG. 11 is a bandgap energy diagram of the semiconductor laser according to the first embodiment of the present invention.

FIG. 12 is a side view and a plan view of the configuration of the semiconductor laser according to the first embodiment of the present invention.

[Explanation of symbols]

 101 n-type GaAs substrate 102 buffer layer 103 n-type cladding layer 104 active layer 104s saturable absorption layer 105a first p-type cladding layer 105b second p-type cladding layer 106 etching stopper layer 111 current blocking layer 110 contact layer 112 cap Layer 113 p electrode 114 n electrode 117 light emitting region 118 saturable absorption region

Claims (18)

[Claims] 1. A semiconductor laser having an active layer, a clad layer sandwiching the active layer, and a saturable absorption layer formed in a part in the cavity direction, and having a self-pulsation characteristic. 2. The saturable absorption layer comprises Al _x Ga _y In
The semiconductor laser according to claim 1, which is a _1-x-y P-based (0≤x≤1, 0≤y≤1) material. 3. An active layer, a clad layer sandwiching the active layer, and a saturable absorption region formed in a part in the cavity direction, and the carrier lifetime in the saturable absorption region is about 6 ns. A semiconductor laser. 4. An active layer, a clad layer sandwiching the active layer, and a saturable absorption region formed in a part in the cavity direction, wherein a portion of the saturable absorption region that absorbs laser light has impurities. And a semiconductor laser having a concentration of 1 × 10 ¹⁸ (cm ⁻³ ) or more. 5. The semiconductor laser according to claim 1, wherein the saturable absorption region saturable absorption layer is doped with impurities. 6. The semiconductor laser according to claim 1, wherein the saturable absorption region is formed in the vicinity of the end face of the laser, and the laser light is extracted from the end face opposite to the end face. 7. The semiconductor laser according to claim 1, 2 or 4, wherein the saturable absorption layer is p-type. 8. A semiconductor laser exhibiting self-sustained pulsation characteristics at low output and single-mode characteristics at high output. 9. A semiconductor laser showing a self-excited oscillation characteristic at a low output of 10 mW or less and a single mode oscillation characteristic at an output of more than 10 mW. 10. The semiconductor laser according to claim 8, a condensing optical system that condenses laser light emitted from the semiconductor laser on an information carrier, and a light receiving unit that receives reflected light from the information carrier. Information processing device equipped with. 11. The information processing apparatus according to claim 10, wherein a self-excited oscillation characteristic is used for reading the information carrier, and a single-mode oscillation characteristic is used for writing the information carrier. 12. A method of cleaving a tilted substrate, which comprises cleaving a substrate by applying stress in a direction in which the plane orientation of the substrate is tilted from the plane orientation of the just plane. 13. A method of cleaving a crystal having a plane orientation inclined from the [100] direction to the [011] direction, by stressing the crystal from the [0-1-1] direction to the [011] direction. Cleavage method for crystals. 14. The cleavage method according to claim 1, wherein the crystal is GaAs. 15. A dry etching method of AlGaInP, wherein the relationship between the flow rate of a gas containing chlorine element and the flow rate of a nitrogen gas in the AlGaInP layer is controlled by a mask formed on AlGaInP. Flow rate / nitrogen gas flow rate ≧ 1 and etching with an internal pressure of 1 mTorr or more to obtain an almost symmetrical AlGaInP ridge structure. 16. AlGaInP formed on a tilted substrate
The etching method according to claim 15, wherein the etching is performed. 17. A MOVP on an n-type GaAs tilted substrate.
By the E method, an n-type AlGaInP clad layer, an active layer,
a step of epitaxially growing a p-type AlGaInP clad layer, and forming a mask on the p-type AlGaInP clad layer,
A step of etching the p-type AlGaInP layer at a gas flow rate containing a halogen element / a nitrogen gas flow rate ≧ 1 at an internal pressure of 1 mTorr or more to form a ridge; and a step of filling both sides of the ridge with a current block layer. A method for manufacturing a semiconductor laser having the following. 18. The p-type AlGaInP cladding layer comprises p-type
-Type AlGaInP first cladding layer and p-type AlGaI
The method of manufacturing a semiconductor laser according to claim 17, further comprising an nP second clad layer, wherein an etching stopper layer is provided between the first clad layer and the second clad layer.