JP3792434B2 - Self-oscillation type semiconductor laser - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a structure of a self-pulsation type semiconductor laser, and more particularly to a structure of a self-pulsation type semiconductor laser having a low threshold current capable of high-output and low-noise operation under a high-temperature operating environment.
[0002]
[Prior art]
Semiconductor lasers are widely used in information equipment and systems as light sources for optical disk devices or light sources for optical communication equipment. For example, a semiconductor laser has been widely used as a light source for an optical pickup in an optical disk medium recording / reproducing apparatus such as a DVD (Digital Versatile Disk) and a magneto-optical disk, which has recently attracted attention as a high-density storage device. .
By the way, in a semiconductor laser used as a light source of an optical pickup of a recording / reproducing apparatus for an optical disk medium, noise is generated by reflected light from the optical disk board surface. Suppressing the generation of this noise is important in improving the characteristics of the semiconductor laser.
[0003]
One way to reduce noise is to drive the semiconductor laser at high frequencies. This is because the oscillation spectrum becomes multimode by high-frequency driving, and the influence of return light can be reduced.
However, in this case, a high-frequency superposition module is required, and there is a problem (EMC problem) that component costs increase and electromagnetic noise is radiated.
[0004]
Accordingly, self-pulsation type semiconductor lasers have attracted attention. The self-excited oscillation laser has low noise characteristics similar to the above-described high-frequency drive semiconductor laser, and has excellent advantages of low cost and no generation of electromagnetic noise, with low threshold current and low drive current, Self-oscillation operation at high temperature and high output, and long-term reliability.
[0005]
By the way, the self-excited oscillation operation can be obtained by introducing a saturable absorber in the laser resonator and controlling the saturable absorption amount.
Such a self-oscillation operation and its laser structure are described in, for example, Extended Abstract of Conference on Solid State Device and Material published in 1986 (Extended Abstract of 18th Conference on). (SolidState Devices and materials) page 153, paper number D-1-2, and page 21 of the 11th semiconductor laser symposium proceedings held in 1994.
In the above self-oscillation type semiconductor laser, the active layer beside the mesa stripe is a saturable absorbing layer.
[0006]
Hereinafter, as a first conventional example, referring to FIG. 6, an AlGa InP having a ridge waveguide structure, which is a typical self-oscillation laser structure, and having an active layer on both sides of the waveguide as a saturable absorber. A red self-pulsation type semiconductor laser will be described.
As shown in FIG. 6, the first conventional AlGaInP-based self-pulsation type semiconductor laser has an n-GaAs substrate 1, an n-GaAs buffer layer 2, an n-AlGaInP cladding layer 3, a GaInP / AlGaInP multiple quantum as shown in FIG. Well active layer 4, p-AlGaInP cladding layer 5, p-GaInP heterobuffer layer 6, p-GaAs cap layers 7 and 8, n-GaAs current blocking layer 9, p-side electrode 11, and n-side electrode on the back surface of the substrate 10 is provided.
[0007]
The stacked structure of the upper part of the p-AlGaInP cladding layer 5, the p-GaInP heterobuffer layer 6, and the p-GaAs cap layer 7 has a mesa structure, and its side part is buried by the n-GaAs current blocking layer 9. ing.
The mesa side portion 41 of the GaInP / AlGaInP multiple quantum well active layer 4 serves as a saturable absorber. The mesa width is 4 μm at the bottom. By controlling the size of the mesa side portion 41, self-excited oscillation can be obtained.
[0008]
In this laser, light spreads around the portion directly below the convex upper portion of the p-AlGaInP cladding layer 5 in the active layer 4.
On the other hand, the current injected through the convex portion of the p-AlGaInP cladding layer 5 does not spread sufficiently in the lateral direction in the active layer 4. Therefore, the gain region and the mesa side portion 41 function as a saturable absorber immediately below the convex portion of the p-AlGaInP cladding layer 5 in the active layer 4.
[0009]
By changing the remaining thickness h of the p-AlGaInP cladding layer located under the n-GaAs current blocking layer 9, the volume of the saturable absorber 41 can be controlled, and h can be set to 0.30 μm to 0.45 μm. By doing so, a self-excited oscillation operation is obtained. As a result, low-noise operation of the semiconductor laser becomes possible.
[0010]
Incidentally, as shown in the first conventional example, in an AlGaInP-based self-excited oscillation type semiconductor laser, a GaAs layer is usually used as a buried layer.
GaAs has a large absorption capacity for the oscillation light of an AlGaInP-based self-excited oscillation type semiconductor laser having a wavelength in the range of 620 nm to 690 nm. For this reason, the laser threshold current increases due to waveguide loss, and This causes a decrease in external differential quantum efficiency.
[0011]
In order to solve this problem, a low threshold and high-efficiency operation of an AlGaInP laser by using AlInP having no light absorption coefficient for oscillation light as a buried layer has been reported as a second conventional example. (1994 Semiconductor Laser Conference Proceedings, Th3.5, page 243).
[0012]
When an AlInP buried layer is used, light absorption by the buried layer is reduced, so that the active layer light can be easily spread in the lateral direction. For this reason, even if the p-AlGaInP cladding layer remaining thickness h is reduced, the volume of the saturable absorber 41 can be increased.
The inventor made the active layer well layer unstrained GaInP with a thickness of 5 nm, the number of well layers was 7, the remaining p-AlGaInP cladding layer thickness was 0.25 μm, and the AlInP buried layer had a flat layer thickness. When a laser having a mesa width of 0.2 μm and a mesa width of 5 μm at the mesa bottom was prototyped, a laser that self-oscillated to 15 mW was obtained. The maximum temperature at which self-oscillation was obtained was 80 ° C. The oscillation wavelength at 25 ° C. was 645 nm.
[0013]
One of the factors that cause the self-excited oscillation to stop at a high light output is that the volume of the saturable absorber decreases during high-power operation. This is because the carrier density of the active layer 4 immediately below the convex portion of the p-AlGaInP clad layer 5 decreases due to high output, the refractive index difference between the inside and outside of the mesa increases, and the spread in the optical lateral direction decreases. It is.
In a laser using an AlInP buried layer, the volume of the saturable absorber can be increased, so that the influence of the decrease in the volume of the saturable absorber due to higher output can be reduced, and self-pulsation can be obtained up to a high optical output. I can do it.
[0014]
[Problems to be solved by the invention]
However, there is a problem that the threshold current of the above self-oscillation laser to which the embedding by the AlInP layer is applied is 90 mA, which is larger than the threshold value of 70 mA of the laser having the GaAs layer as the buried layer. .
The present invention has been made in view of these problems, and an object of the present invention is to provide a laser that has a low threshold current and that oscillates at a high temperature and a high output.
[0015]
[Means for Solving the Problems]
The present invention has been made to solve the above-described problems, and includes a first conductivity type cladding layer, an active layer, and a second conductivity type cladding layer, which are sequentially stacked on a first conductivity type GaAs substrate. In the self-pulsation type semiconductor laser in which the active layer on both sides of the waveguide functions as a saturable absorber,
The second conductivity type cladding layer has a mesa structure, and at least a part of both sides of the mesa structure is a first current block layer and a second current layer formed on the first current block layer. Embedded in the current blocking layer, and the material of the first current blocking layer is AlInP or AlGaAs having a light absorption coefficient smaller than that of the second current blocking layer with respect to the oscillation light of the laser,
The thickness outside the mesa of the second conductivity type cladding layer is 0.2 μm or less, the active layer is configured as a multiple quantum well structure, and the number of well layers in the multiple quantum well structure is 5 or more and 7 or less, The active layer is characterized by being provided with in-plane compressive strain having a strain amount of 0.1% or more. Even when the number of well layers is reduced, the same effect as that of applying compressive strain to the active layer is obtained. Note that the outside of the mesa means a region of the second cladding layer excluding the upper portion of the second cladding layer that is a part of the mesa structure.
[0017]
Preferably, the in-plane compressive strain amount of the active layer is 0.3% or more, and more preferably, the in-plane compressive strain amount of the active layer is 0.5% or more. By applying in-plane compressive strain to the active layer, the differential gain of the active layer is reduced, the differential gain of the saturable absorption layer is increased, and the maximum self-oscillation temperature is increased.
[0018]
Preferably, a portion of the current blocking layer that is in contact with the mesa-shaped second cladding layer is of the first conductivity type. Thereby, the injection current flowing through the current blocking layer can be reduced, and the threshold current can be lowered.
[0019]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described specifically and in detail with reference to the accompanying drawings.
Embodiment 1
This embodiment is an example of an embodiment of a self-pulsation semiconductor laser according to the present invention, and FIG. 1 is a perspective view showing a configuration of the self-pulsation semiconductor laser of this embodiment.
As shown in FIG. 1, a self-pulsation type semiconductor laser (hereinafter simply referred to as a semiconductor laser) of this embodiment is formed on a GaAs substrate 1 with a GaAs buffer layer 2, an n-AlGaInP cladding layer 3, a GaInP / The stacked structure includes an AlGaInP multiple quantum well active layer 4, a p-AlGaInP cladding layer 5, a GaInP heterobuffer layer 6, and a p-GaAs cap layer 7.
[0020]
The upper part of the p-AlGaInP cladding layer 5, the GaInP heterobuffer layer 6, and the p-GaAs cap layer 7 have a mesa structure, and both sides of the mesa, that is, both sides thereof, are n-AlInP current blocking layers 21 and The n-GaAs current blocking layer 9 is buried.
A p-GaAs cap layer 8 and a p-side electrode 11 are further provided on the stacked structure and the current blocking layer. An n-side electrode 10 is provided on the back surface of the GaAs metal 1.
[0021]
Next, with reference to FIG. 1, a method for fabricating the semiconductor laser of the present embodiment will be described. First, a GaAs buffer layer 2, an n-AlGaInP cladding layer 3, an active layer 4, a p-AlGaInP cladding layer 5, a GaInP heterobuffer layer 6, and a p-GaAs cap layer are formed on the n-GaAs substrate 1 by a reduced pressure MOVPE method. 7 are sequentially laminated.
Next, the lower part of the p-AlGaInP clad layer 5 is exposed by photolithography using a SiO ₂ mask (not shown) and etching, and the upper part of the p-AlGaInP clad layer 5, the GaInP heterobuffer layer 6, The GaAs cap layer 7 is formed into a mesa stripe.
Subsequently, using the same SiO ₂ mask as a mask, the AlInP current blocking layer 21 and the n-GaAs current blocking layer 9 are selectively grown in this order to fill the mesa.
[0022]
Next, after removing the SiO ₂ mask, the p-GaAs cap layer 8 is formed by the reduced pressure MOVPE method.
A p-side electrode 11 is formed on the p-GaAs cap layer 8, the back surface of the n-GaAs substrate 1 is polished to adjust the substrate to an appropriate thickness, and then an n-side electrode 10 is formed. The laser structure shown can be obtained.
[0023]
As a raw material for the reduced pressure MOVPE method, trimethylaluminum, triethylgallium, trimethylindium, phosphine, arsine, disilane as an n-type impurity, and diethyl zinc as a p-type impurity are used.
Further, the growth temperature of film formation by the reduced pressure MOVPE method is 660 ° C., the growth pressure is 70 Torr, and the ratio of the Group V material supply amount / Group III material supply amount is 1: 500.
[0024]
Here, the composition profiles of the n-AlGaInP cladding layer 3, the active layer 4, and the p-AlGaInP cladding layer 5 will be described with reference to FIG.
As shown in FIG. 2, the active layer 4 has a multiple quantum well structure in which well layers 31 and barrier layers 32 are alternately stacked, and has light guide layers 33 on both sides of the multiple quantum well layer. The active layer well layer 31 is made of GaInP having a thickness of 5 nm and an in-plane compressive strain of 0.3%, and the number of well layers is seven.
The remaining thickness h of the p-AlGaInP cladding layer is 0.15 μm, and the flat layer thickness of the AlInP buried layer is 0.2 μm. The mesa width is 5 μm at the bottom of the mesa.
[0025]
A semiconductor laser sample having the above-described configuration was prototyped and the threshold current value was measured and found to be 55 mA. In addition, self-oscillation was obtained at an optical output of at least 10 mW in an operating environment of 90 ° C. The oscillation wavelength was 655 nm at 25 ° C.
[0026]
Next, the effect of the present invention will be described with reference to FIG. FIG. 3 shows the dependence of the threshold current value on the flat layer thickness d of the AlInP buried layer for a laser with an active layer well having no strain and a number of well layers of 7.
When the remaining thickness h of the p-AlGaInP cladding layer was 0.25 μm, the threshold current value increased as the AlInP layer thickness d increased. This is a tendency opposite to the reduction in threshold current by AlInP embedding that has been obtained conventionally.
[0027]
As a result of investigation by the present inventor on the cause, the volume of the saturable absorber 41 is significantly increased as a result of weak light confinement in the lateral direction, resulting in an excessive increase in laser loss. It turned out to be.
This is because the number of wells in the active layer is increased to 7 in order to increase the volume of the saturable absorber.
[0028]
In FIG. 3, the region where the threshold current decreases as the AlInP layer thickness d increases is a region where the light absorption of the GaAs current blocking layer 9 is significantly reduced. On the other hand, the region where the threshold current increases as the AlInP layer thickness d increases is a region where the increase in light absorption of the saturable absorber 41 is significant.
As the p-AlGaInP remaining thickness h increases, the magnitude of the light absorption of the saturable absorber 41 relative to the light absorption of the GaAs current blocking layer 9 increases relatively, and the threshold increases as the AlInP layer thickness d increases. The region where the value current increases is enlarged.
Note that the number of well layers of a normal laser in which self-pulsation cannot be obtained is 3 or 4. In this laser, the threshold current reduction effect by AlInP embedding is observed.
[0029]
By reducing the remaining thickness h of the p-AlGaInP cladding layer, light confinement in the lateral direction can be appropriately suppressed, and the threshold current can be reduced.
The threshold current value of the laser with p-AlGaInP remaining thickness h of 0.15 μm and AlInP layer thickness d of 0.2 μm was as low as 60 mA. However, at this time, the maximum self-oscillation temperature was as low as 30 ° C.
[0030]
The result of the study by the present inventor will be described with reference to FIG. FIG. 4 shows the dependence of the gain on the carrier number density, that is, the gain curve, including the active region and the saturable absorption region.
The differential gain, which is the slope of the gain curve, was compared between GaAs buried and AlInP buried. The reason why the temperature at which self-oscillation occurs is reduced is that light absorption by the n-GaAs current blocking layer 9 is reduced by AlInP embedding, and the carrier number density at the operating point of the active region is reduced. As a result, the differential gain of the active layer increases and the differential gain of the saturable absorption layer decreases.
[0031]
In order to obtain self-excited oscillation, it is described in JP-A-61-160988 that the differential gain in the active region must be small and the differential gain in the saturable absorption region must be large, and this condition cannot be satisfied. This is the reason why the maximum self-oscillation temperature is lowered in the AlInP embedded laser.
[0032]
When compressive strain of 0.3% is applied to the active layer well 31 as in the first embodiment, the threshold current value is approximately the same p-AlGaInP remaining thickness h and AlInP layer thickness d as in the case of no strain. Indicates dependency.
This is because even when compressive strain is applied to the active layer well 31, the light distribution does not change greatly, and therefore the volume of the saturable absorber 41 is almost the same as in the case of no strain.
[0033]
On the other hand, as described above, the maximum self-oscillation temperature was significantly increased by applying compressive strain to the active layer. A high self-pulsation maximum temperature of 90 μm was obtained with a laser having a p-AlGaInP remaining thickness h of 0.15 μm and an AlInP layer thickness d of 0.2 μm.
The reason why the maximum self-oscillation maximum temperature is increased is that as a result of adding 0.3% compressive strain to the active layer well 31, the differential gain of the active layer is reduced and the differential gain of the saturable absorption layer is increased. .
Note that the effect of reducing the threshold current by embedding AlInP is obtained when the remaining thickness h of p-AlGaInP is 0.2 μm, as shown in FIG. In particular, when h is set to 0.15 μm or less, this effect becomes more remarkable.
[0034]
The conductivity type of the AlInP current blocking layer 21 is undoped in the present embodiment, but it may be n-type by doping silicon as an impurity.
Furthermore, by increasing the amount of compressive strain applied to the active layer well 31, it is possible to enhance the differential gain reduction effect in the active region and the differential gain increase effect in the saturable absorption region.
[0035]
Embodiment 2
The present embodiment is another example of the self-oscillation semiconductor laser according to the present invention.
In this embodiment, the amount of compressive strain applied to the active layer well 31 is 0.5%, and the layer thickness of the active layer well layer is 4.5 nm. Everything else is the same as the first embodiment.
The maximum self-oscillation temperature of this laser was as high as 100 ° C. The threshold current value was 55 mA, and the oscillation wavelength at 25 ° C. was 657 nm. The reason why the thickness of the active layer well layer 31 is 4.5 nm and is smaller than 5 nm in the case of the unstrained active layer is to keep the oscillation wavelength substantially the same, but the essential effect of the invention is not changed. .
[0036]
Embodiment 3
The present embodiment is still another example of the self-pulsation type semiconductor laser according to the present invention.
The self-pulsation type semiconductor laser of this embodiment is an example in which the threshold current is reduced by reducing the number of well layers of the active layer, and the number of active layer wells 31 is five. Everything else is the same as the first embodiment.
With this laser, a threshold current value of 45 mA was obtained. The maximum self-oscillation temperature was 80 ° C. In the conventional example, when the number of active layer well layers was reduced to 5 wells while keeping the active layer well unstrained, the threshold current could be reduced to 50 mA, but the maximum temperature at which self-oscillation occurred was 30 ° C. .
[0037]
When the number of well layers is reduced, the confinement of light in the vertical direction is lowered and the light distribution is widened, so that the influence of light absorption by the GaAs current blocking layer 9 is increased. Therefore, with respect to the effect of the semiconductor laser of the first embodiment, reducing the number of well layers and reducing the p-AlGaInP remaining thickness h are equivalent. That is, even when the number of wells is reduced, there is an effect of applying compressive strain to the active layer.
[0038]
Embodiment 4
By the way, in Embodiment 1, the typical value of the threshold current was 55 mA, but the yield at which the threshold current value was 60 mA or less was as low as 40%.
When the current-voltage characteristics of the semiconductor laser of Example 1 were examined, it was found that a laser with a high threshold current had a low rise voltage and a current of at least 1 mA flowing at 1.5 V bias.
Most lasers with a low threshold current have a current of 0.1 mA or less at 1.5 V bias. That is, it has been found that the leakage current passing through the current blocking layers 9 and 21 is the cause of the decrease in yield.
[0039]
Therefore, Embodiment 4 is still another example of the self-oscillation type semiconductor laser according to the present invention and is an example in which the yield is improved. FIG. 5 is a perspective view showing the configuration of the semiconductor laser of the fourth embodiment.
The semiconductor laser of this embodiment example has the same configuration as that of the semiconductor laser of Example 1 except that the configuration of the current blocking layer is different.
As shown in FIG. 5, the semiconductor laser of this embodiment example has a p-type conductivity type in contact with the p-AlGaInP cladding layer 5 as a current blocking layer, and a p-AlInP current blocking layer 22; The AlInP current blocking layer 21 and the n-GaAs current blocking layer 9 are stacked.
[0040]
The layer thickness of the p-AlInP current blocking layer 22 at the flat portion is 0.1 μm, and the layer thickness of the AlInP current blocking layer 21 at the flat portion is 0.1 μm. The layer thickness of the flat portion including the p-AlInP current blocking layer 22 and the AlInP current blocking layer 21 is 0.2 μm, which is the same as the layer thickness 0.2 μm of the AlInP current blocking layer 21 of the first embodiment. is there.
The carrier concentration of the p-AlInP current blocking layer 22 is 4 × 10 ¹⁷ cm ⁻³ .
Magnesium is used as the p-type impurity, and cyclopentadium magnesium is used as the magnesium raw material.
[0041]
The typical value of the threshold current of this laser was 55 mA, but the yield at which the threshold current value was 60 mA or less was 90%, which was greatly improved.
The reason why the leakage current passing through the current blocking layers 9 and 21 in the first embodiment is generated is that the p-AlGaInP remaining thickness h is as small as 0.15 μm and the pnpn type current blocking structure is easily destroyed.
In the fourth embodiment, the total p-type layer thickness of the p-AlGaInP cladding layer 5 and the p-AlInP layer 22 is set to 0.25 μm, thereby preventing the pnpn-type current block structure from being broken and improving the yield.
[0042]
It is possible that self-oscillation cannot be obtained due to the volume of the saturable absorber being reduced by the current flowing along the p-AlInP current blocking layer 22, but the maximum self-oscillation temperature is 90 ° C. was obtained.
This is because the specific resistance of the p-AlInP layer 22 was higher than that of the p-AlGaInP layer 5, and the amount of current flowing along the p-AlInP layer 22 was sufficiently small.
[0043]
The above-described exemplary embodiments are all typical values, and different layer thicknesses and compositions can be used as long as the effects of the invention are not changed.
Further, all the embodiments have been described in the case where the n-GaAs current blocking layer 9 is provided on the AlInP current blocking layer 21, but the n-GaAs current blocking layer 9 may not be provided if the current block is sufficient. .
Although the present invention has been described by taking the case of an AlGaInP laser as an example, the present invention can be applied to an AlGaAs laser, an AlGaInN laser, a laser using a II-VI group compound semiconductor crystal, or the like.
[0044]
【The invention's effect】
As described above, according to the present invention, at least part of the current blocking layer provided on both sides of the mesa structure is formed of a material having a small light absorption coefficient with respect to the oscillation light of the laser, and the active layer is formed on the active layer. By giving in-plane compressive strain with a strain amount of 0.1% or more, the differential gain of the active region can be reduced and the differential gain of the saturable absorber can be increased even if the mode loss is reduced. A semiconductor laser that self-oscillates to a high temperature with a drive current can be obtained.
[Brief description of the drawings]
FIG. 1 is a perspective view showing a configuration of a self-excited oscillation type semiconductor laser according to Embodiment 1;
FIG. 2 is a composition profile diagram for explaining a configuration of an active layer of the self-pulsation type semiconductor laser of Example 1;
FIG. 3 is a diagram of a threshold current value and an AlInP buried layer thickness for explaining a conventional problem.
FIG. 4 is a graph of a gain curve for explaining a conventional problem.
5 is a perspective view showing a configuration of a self-pulsation type semiconductor laser according to Embodiment 4. FIG.
FIG. 6 is a perspective view showing a configuration of a conventional self-pulsation type semiconductor laser.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 GaAs substrate 2 GaAs buffer layer 3 n-AlGaInP clad layer 4 GaInP / AlGaInP multiple quantum well active layer 5 p-AlGaInP clad layer 6 GaInP heterobuffer layer 7, 8 GaAs cap layer 9 n-GaAs current block layer 10 n-electrode 11 p-electrode 21 AlInP current blocking layer 22 p-AlInP current blocking layer 31 GaInP well layer 32 AlGaInP barrier layer 33 AlGaInP light guide layer 41 saturable absorption region

Claims (4)

Are sequentially laminated on the first conductive type GaAs substrate, a first conductivity type cladding layer, an active layer, a second conductivity type cladding layer, the self waveguide both sides of the active layer functions as a saturable absorber In an excitation oscillation type semiconductor laser,
The second conductivity type cladding layer has a mesa structure, and at least a part of both sides of the mesa structure is a first current block layer and a second current layer formed on the first current block layer. Embedded in the current blocking layer, and the material of the first current blocking layer is AlInP or AlGaAs having a light absorption coefficient smaller than that of the second current blocking layer with respect to the oscillation light of the laser,
The thickness outside the mesa of the second conductivity type cladding layer is 0.2 μm or less, the active layer is configured as a multiple quantum well structure, and the number of well layers in the multiple quantum well structure is 5 or more and 7 or less, A self-pulsation type semiconductor laser, wherein the active layer is given an in-plane compressive strain having a strain amount of 0.1% or more. 2. The self-pulsation type semiconductor laser according to claim 1 , wherein an in-plane compressive strain applied to the active layer is 0.3% or more. 2. The self-pulsation type semiconductor laser according to claim 1 , wherein an in-plane compressive strain applied to the active layer is 0.5% or more.   4. The self-pulsation type semiconductor laser according to claim 1, wherein a portion of the current blocking layer that is in contact with the mesa-shaped second cladding layer is of a first conductivity type. 5.

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