JP3235778B2 - Semiconductor laser - Google Patents

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-oscillation type semiconductor laser used for a light source of an optical disk system or the like.

[0002]

2. Description of the Related Art In recent years, the demand for semiconductor lasers in the fields of optical communication, laser printers, optical discs, etc.
Research and development have been actively promoted mainly on s-systems and InP-systems. In the optical information processing field, the wavelength
A system for recording / reproducing information with the light of an 80 nm AlGaAs-based semiconductor laser has been put into practical use, and has been widely used in compact discs and the like.

However, recently, an increase in the storage capacity of these optical disk apparatuses has been required,
Accordingly, there is an increasing demand for realizing a short wavelength laser. An AlGaInP-based semiconductor laser can oscillate in a red region at a wavelength of 630 to 690 nm.
This is the one that can obtain the shortest wavelength light among the semiconductor lasers currently in practical use. Therefore, the conventional AlGaAs
It is promising as a next-generation large-capacity optical information recording light source that replaces semiconductor lasers.

Incidentally, a semiconductor laser generates intensity noise due to feedback of reflected light from the disk surface or a change in temperature during reproduction of an optical disk, thereby inducing a signal reading error.
Therefore, a laser with low intensity noise is indispensable for the light source of the optical disk.

Conventionally, in a read-only, low-output AlGaAs semiconductor laser, noise has been reduced by adopting a structure in which a saturable absorber is intentionally formed on both sides of a ridge stripe. This makes it possible to multiply the vertical mode. In other words, if a disturbance such as feedback of light or temperature change enters while the laser oscillates in the longitudinal single mode, the neighboring longitudinal mode starts to oscillate due to a small change in the gain peak, and the laser oscillates with the original oscillation mode. And this causes noise. When the longitudinal mode is multiplied, the intensity change of each mode is averaged and does not change due to disturbance, so that stable low noise characteristics can be obtained.

As another method, a method for obtaining a more stable self-excited oscillation characteristic is disclosed in Japanese Patent Application Laid-Open No. 63-20883. Here, a self-pulsation type semiconductor laser is realized by providing a layer capable of absorbing output light.

Further, Japanese Patent Application Laid-Open No. Hei 6-260716 reports that the characteristics were improved by making the energy gap of the active layer substantially equal to the energy gap of the absorbing layer. In particular, the energy gap of the strained quantum well active layer is substantially equal to that of the strained quantum well saturable absorption layer. With this configuration, an attempt is made to obtain good self-excited oscillation characteristics. A similar configuration is also described in JP-A-7-22695.

[0008]

However, according to the study of the present inventors, it is found that good self-pulsation characteristics cannot be obtained only by making the energy gap between the saturable absorbing layer and the active layer substantially equal. It became clear.

Further, it can be said that the laser having the self-excited oscillation characteristic has a slightly higher threshold current than the laser which does not self-oscillate at the same wavelength and the same output.

Therefore, the present invention considers the energy gap difference between the saturable absorbing layer and the active layer, the conditions of the end face reflection, and the like, so that a stable threshold current with a small threshold current and effective for low noise characteristics can be obtained. An object of the present invention is to provide a semiconductor laser having excitation oscillation characteristics.

[0011]

A semiconductor laser according to the present invention is a semiconductor laser having at least an active layer having a quantum well layer and a clad structure sandwiching the active layer. A saturated absorbing layer and the saturable absorbing layer;
An etching stopper layer provided on the converging layer , wherein an energy gap of the saturable absorbing layer is about 30 meV to about 200 meV higher than an energy gap between ground levels of the quantum well layer of the active layer. And the reflectivity of each end face is about 40% to about 90% ,
Ground level of saturable absorption layer and layer adjacent to saturable absorption layer
Energy gap difference with the bottom of the conduction band of
0 meV to about 210 meV, thereby achieving the above objective.

According to another aspect of the present invention, there is provided a semiconductor laser including at least an active layer having a quantum well layer, and a clad structure sandwiching the active layer, wherein the clad structure has a saturable absorption layer. And provided on the saturable absorption layer
And includes an etching stopper layer, the energy gap of the saturable absorption layer, than the energy gap between the ground level of the quantum well layer of the active layer, less about 30meV by about 200 meV, the slope efficiency of about 0.1
mW / mA to about 0.4 mW / mA.
Ground level of the absorption layer and conduction band of the layer adjacent to the saturable absorption layer
Energy gap difference with the bottom of
~ 210 meV, thereby achieving the above objectives. Preferably, the slope efficiency is about 0.5.
15 mW / mA to about 0.35 mW / mA. In addition,
The term “slope efficiency” as used herein refers to “the efficiency in a portion excluding a light absorption region (see, for example, FIG. 13) found in the light output-current characteristics of a semiconductor laser having self-sustained pulsation characteristics”. .

In one embodiment, the cladding structure further includes a light guide layer.

Preferably, the resonator length L is about 350 μm ≦
L ≦ about 700 μm.

Preferably, the stripe width W is about 2.0 μm ≦ W ≦ about 6.0 μm.

The active layer and the saturable absorption layer are made of Al
It is a layer composed of _{x Ga y In 1-y P} (0 ≦ x ≦ 1,0 ≦ y ≦ 1).

[0017]

[0018]

[0019]

[0020]

[0021]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have studied the relationship between the "energy gap difference (ΔE) between an active layer and a saturable absorbing layer" and self-excited oscillation. In the specification of the present application, the energy gap difference (ΔE) between the active layer and the saturable absorption layer means “the active layer and the saturable absorption layer have a quantum well structure” when “the active layer and the saturable absorption layer have a quantum well structure. From the energy gap between the ground levels of the quantum well layer (E'ga), the energy gap between the ground levels of the saturable absorption layer (E'gs)
(E′ga−E′gs) ”. FIG. 1 schematically shows the relationship between these energy gaps (E′ga, E′gs) and band gaps (Ega, Egs). In general, in a semiconductor layer having a quantum well structure, the energy gap between the ground levels is not between the bottom of the conduction band and the bottom of the valence band, but the energy difference (E'g) between the quantum levels. Therefore, the energy gap is about 70 meV larger than the normal band gap (Eg).

When the active layer has a quantum well structure and the saturable absorption layer has a bulk structure, the difference in energy gap between the active layer and the saturable absorption layer is determined as follows: From the energy gap between the ground levels of the layer (E'ga), the band gap (E
gs) minus “value”.

In the present invention, the saturable absorbing layer may have a quantum well structure or a bulk structure. Therefore, in the present specification, for convenience, the “energy gap of the saturable absorbing layer” is defined as follows. That is, when the saturable absorption layer has a quantum well structure, the energy gap between ground levels (E′g
s), and when the saturable absorption layer has a bulk structure, it means its “band gap (Egs)”. By using the term “energy gap of the saturable absorption layer”, “the energy gap difference between the active layer and the saturable absorption layer” is defined as “the ground level of the active layer of the quantum well layer of the active layer before laser oscillation. Value obtained by subtracting the energy gap of the saturable absorber from the energy gap between
Is expressed as

As a result of a study by the inventors of the present invention, the energy gap difference (ΔE) between the active layer and the saturable absorbing layer was set to about 3
It has been clarified that when the voltage is changed from 0 meV to about 200 meV, the saturable absorbing layer efficiently absorbs the laser light and also saturates the light absorption, so that stable self-pulsation can be obtained. If the energy gap difference (ΔE) between the active layer and the saturable absorbing layer is smaller than about 30 meV, self-excited oscillation cannot be obtained. This is probably because the energy gap difference is small and the saturable absorbing layer does not absorb much laser light. In addition, the energy gap difference (△
If E) exceeds about 200 meV, light absorption in the saturable absorption layer becomes too large, and the saturable absorption layer does not exhibit saturation characteristics, so that self-pulsation does not occur. Therefore, the energy gap difference (ΔE) is about 30 meV to about 200 m.
It turned out that eV was good.

According to the current crystal growth technology, the energy gap and the energy gap difference (△
E) can be controlled with an accuracy of several meV or less. Therefore, if the energy gap difference (ΔE) between the formed active layer and the saturable absorbing layer is about 10 meV, the active layer and the saturable absorbing layer must have the energy gap difference (ΔE). It is recognized that it was formed with the intention of. Therefore, if the energy gap difference (ΔE) between the active layer and the saturable absorbing layer is about 10 meV or more, the energy gap between the ground level of the quantum well layer of the active layer and the energy gap of the saturable absorbing layer becomes “ Almost the same. "

In particular, when the energy gap difference (ΔE) is in the range of about 50 meV to about 100 meV, the saturation condition of the saturable absorbing layer is optimal, and stable self-sustained pulsation is achieved even at a high operating temperature. When the energy gap difference (ΔE) exceeds about 100 meV, the light absorption in the saturable absorbing layer gradually increases, and the operating current slightly increases. Therefore, it can be said that the energy gap difference is preferably about 100 meV or less. Thus, when the energy gap difference is in the range of about 50 meV to about 100 meV, the operating current of the semiconductor laser does not increase, and
Very good self-excited oscillation characteristics can be obtained. In particular, when there is a possibility that the semiconductor laser may be operated in a relatively high temperature environment such as when the semiconductor laser is mounted on an automobile, it is preferable to set such an energy gap difference.

When the volume of the saturable absorbing layer is reduced, the carrier density in the saturable absorbing layer can be easily increased. The saturable absorption layer absorbs the laser light emitted by the active layer, and generates a pair of electrons and holes. If the volume of the saturable absorption layer is small, the amount of light absorbed per unit volume increases, and the carrier density increases. Can be easily raised. Then, the state easily becomes saturated, and the effect of saturable absorption becomes remarkable. Therefore,
The experiments by the present inventors have clarified that the thinner the saturable absorption layer, the stronger and more stable self-pulsation characteristics can be obtained. In order to cause such strong and stable self-sustained pulsation, it is preferable that the thickness of the saturable absorbing layer be in the range of about 10 Å to about 100 Å. However, even if the thickness of the saturable absorption layer exceeds about 100 Å and has a bulk structure, there is no problem if the energy gap difference is set within a preferable range. Further, the saturable absorbing layer may be provided separately in a plurality.

Further, the semiconductor laser of the present invention may have a configuration in which such a small-volume saturable absorption layer and a light guide layer are combined. In the semiconductor laser of the present invention, if the light guide layer is provided in the cladding structure, the thickness of the saturable absorption layer is reduced as in the quantum well layer in order to reduce the volume of the saturable absorption layer. In addition, it is possible to prevent a situation in which stable self-sustained pulsation cannot be obtained as a result of an extremely reduced light confinement rate in the saturable absorption layer. If a light guide layer is used, for example,
At least 1.2% of the confinement rate in the saturable absorber layer while maintaining the light confinement rate in the active layer at about 5.0% or more.
At about the same level or more, stable self-sustained pulsation can be generated.

As described above, the light guide layer in the present invention is for increasing the light confinement rate of the saturable absorption layer, and is disposed at a position away from the active layer. In this regard,
It is significantly different from a conventional light guide layer arranged at a position adjacent to the active layer in order to increase the light confinement rate of the active layer.

The positional relationship between the saturable absorbing layer and the light guide layer is optimally determined in consideration of the volume of the saturable absorbing layer and light confinement. The saturable absorption layer may be formed in the light guide layer, or may be formed in the p-type cladding layer near the light guide layer.

Further, by providing the etching stopper layer in the laminated structure of the semiconductor laser, the saturable absorption layer is not etched even in the etching step for forming the ridge stripe. As a result, variations in the wafer during manufacturing can be suppressed, and a large number of semiconductor lasers with small variations in characteristics can be manufactured in the wafer.

Regarding the reflectivity of the end face of the semiconductor laser, if the end face reflectivity is small, the absorption region in the light output-current characteristic becomes large, and it becomes difficult to control the light output by the current. On the other hand, if the end face reflectance is too large, the slope efficiency becomes small and the drive current becomes large, and as a result,
A decrease in the maximum light output occurs due to a decrease in the kink. Therefore, by controlling the reflectivity of the end face, the light absorption region can be reduced and the threshold current can be reduced, and the light output of the laser can be controlled in a stable light output region appearing next to the light absorption region. Thus, a self-pulsation type semiconductor laser having stable controllability is realized.

Similarly, by controlling the resonator length and stripe width of the laser, a self-pulsation type laser having stable controllability can be obtained.

In a self-pulsation type semiconductor laser using a saturable absorption layer, it is known that a jump occurs immediately before laser oscillation in light output-current characteristics.

Generally, in a semiconductor laser, it is desirable to reduce a threshold current for laser oscillation. Here, the threshold current of the semiconductor laser is related to the loss of the laser. That is, the threshold gain is gth, the light propagation loss is αp, the free carrier loss inside the laser is αf,
When the mirror loss at the end face is αM, gth = αp + αf
+ ΑM. From this equation, it can be seen that the loss can be reduced to reduce the threshold current.

Of the three loss terms appearing in the above equation, the free carrier loss αf is caused by injection of carriers into the active layer and the like, and its control is difficult. Therefore, in order to reduce the threshold current, it is conceivable to control the remaining propagation loss αp and mirror loss αM. Therefore, the present inventors have paid particular attention to the mirror loss αM among the above, and have examined the relevance to the reduction of the threshold current.

In the following, specific configurations of various embodiments of the present invention will be described with reference to the accompanying drawings, including the results of studies on the points described above.

(First Embodiment) FIG. 2 is a sectional view of an embodiment of a semiconductor laser according to the present invention.

This semiconductor laser has an n-type GaAs substrate 1
01 and a semiconductor laminated structure formed on a GaAs substrate 101. This semiconductor laminated structure has an n-type GaAs
s buffer layer 102, n-type AlGaInP cladding layer 1
03, a multiple quantum well active layer 104 composed of AlGaInP and GaInP, a first p-type AlGaInP cladding layer 105a, and a saturable absorption layer 10 composed of p-type GaInP.
6. The second p-type AlGaInP cladding layer 105b is included.

Second p-type AlGaInP cladding layer 10
5b, a third p-type cladding layer 105c
Stripe-shaped ridge portions (width: about 2.0 μm to about 6.0 μm) extending in the resonator length direction are formed. A contact layer 110 is formed at a position corresponding to the upper surface of the ridge of the third p-type cladding layer 105c. Also, the third
An n-type GaAs current blocking layer 111 is formed on both sides of the p-type cladding layer 105c and the contact layer 110. The n-type GaAs layer current block layer 111 is formed to extend further laterally from the side surface of the ridge.

Contact layer 110 and current block layer 11
1, a p-type GaAs cap layer 112 is formed. A p-electrode 113 is formed on the upper surface of the cap layer 112, and an n-electrode 114 is formed on the back surface of the substrate 101.

The active layer 104 has a multiple quantum well structure including three well layers and barrier layers.

Further, an etching stopper layer 100 is provided on the saturable absorption layer 106 and below the ridge stripe and the current block layer 111 of the third p-type cladding layer 105c. This etching stopper layer 1
The composition ratio of 00 is Ga _0.51 In _0.49 P, and a selection ratio in the etching step can be obtained between the third p-type cladding layer 105c and AlGaInP.

In the present specification, the remaining portion of the semiconductor laminated structure excluding the buffer layer, the active layer, the etching stopper layer, the contact layer, the cap layer, and the current blocking layer is referred to as a “cladding structure” as a whole. To
In the case of this embodiment, the n-type AlGaInP cladding layer 1
03, the first p-type AlGaInP cladding layer 105a,
The saturable absorption layer 106, the second p-type AlGaInP cladding layer 105b, and the third p-type AlGaInP cladding layer 105c form a cladding structure.

The doping levels and film thicknesses of the respective semiconductor layers constituting the laminated structure of this embodiment are as shown in Table 1 below.

[0046]

[Table 1]

FIG. 3 shows the semiconductor laser of this embodiment from the vicinity of the active layer 104 to the etching stopper layer 10.
The distribution of the Al composition x of (Al _x Ga _{1 -x} ) _0.5 In _0.5 P up to around 0 is shown. In the embodiment described above, n
The Al composition of the mold cladding layer 103, the first p-type cladding layer 105a, the second p-type cladding layer 105b, and the third p-type cladding layer 105c is 0.7. Active layer 104
_Are formed of Ga _0.45 In _0.55 P and Ga _0.40 In _0.60 P, respectively, and therefore have a larger lattice constant than the surrounding layers.
In both cases, compression strain is applied.

An important point for stably causing self-pulsation is the difference in energy gap between the quantum well layer of the active layer 104 and the saturable absorption layer. In the first embodiment, the energy gap difference was about 57 meV, and stable self-pulsation was obtained.

The inventor of the present application examined the function of the saturable absorbing layer and the energy gap difference. The results are described below.

First, reference is made to FIGS. 4A to 4C.
As can be seen from FIG. 4A, laser oscillation (self-oscillation) occurs when the injection current reaches about 40 mA, and when the injection current further increases, self-oscillation occurs at point A in FIG. Stops and normal laser oscillation occurs. The maximum light output obtained by self-pulsation will be referred to as Pmax. In the example of FIG. 4A, Pmax is 4.0 mW. With a current smaller than the current that gives Pmax, as shown in FIG. 4C, the light output greatly oscillates with the passage of time, and self-pulsation having a stable amplitude is obtained. However, with a current larger than the current that gives Pmax, as shown in FIG. 4B, the amplitude of the light output gradually decreases over time,
Normal laser oscillation occurs.

Similarly to the injection current, even when the operating temperature T exceeds a certain level, there is a tendency that self-sustained pulsation does not occur. Here, the maximum temperature at which self-excited oscillation is observed is defined as Tmax.
In other words, Tmax can be said to be the temperature at which the self-sustained pulsation stops.

In FIG. 5, the horizontal axis is the energy gap difference (meV), and the vertical axis is Tmax (temperature at which self-excited oscillation stops).
And Pmax (maximum light output of self-sustained pulsation at room temperature),
It is a graph which shows each relationship. As a result of the experiment, when the energy gap difference is about 10 meV and about 20 meV
In the case of V, self-excited oscillation was not observed, and self-excited oscillation was observed at about 30 meV. At 30 meV, self-sustained pulsation was observed up to 51 ° C. At that time, self-sustained pulsation was achieved up to an optical output of 5 mW.

As shown in FIG. 5, when the energy gap difference was about 30 meV or more, self-pulsation started to occur, and self-pulsation was confirmed up to about 200 meV. Especially, the energy gap difference is about 50 meV to about 100 me.
In the range of V, Tmax is high and Pmax is also large, which is a practically preferable range.

If the energy gap difference exceeds about 100 meV, the absorption of laser light in the saturable absorption layer becomes large due to the difference in energy gap between the active layer and the saturable absorption layer. As a result, the operating current becomes slightly large. Tend to be. This will be described with reference to FIG.

In FIG. 6, the horizontal axis represents the energy gap difference (m
eV), and the vertical axis is a graph of operating current (mA). Thus, when the energy gap difference exceeds about 100 meV, the operating current becomes larger than about 130 mA.

FIG. 7 is a graph showing the correlation between the operating current and the life of the semiconductor laser of this embodiment. This graph is based on the results obtained when the optical output of the semiconductor laser is maintained at about 5 mW and the operating temperature is about 60 ° C. From this, to make the life more than about 5000 hours,
FIG. 13 shows that the operating current may be reduced to about 130 mA or less.
Understand from.

As can be seen from FIGS. 6 and 7, it is desirable that the energy gap difference is about 100 meV or less from the viewpoint of life.

FIG. 8 is a graph showing current-light output characteristics of this semiconductor laser. The horizontal axis of the graph indicates the injection current (mA) to the laser, and the vertical axis indicates the optical output (mW). The threshold current is about 50 mA. The characteristic of the self-pulsation type semiconductor laser is different from that of a normal semiconductor laser in that a sharp rise in optical output is observed near a threshold current as shown in FIG. This is because light output is not emitted to the outside until a certain amount of carriers is injected due to the presence of the saturable absorbing layer. When the injection current exceeds a certain value, laser oscillation occurs, and the light output starts to increase in proportion to the injection current.

FIG. 9 shows the semiconductor laser of this embodiment shown in FIG.
7 shows an output waveform at point P of FIG. As shown in FIG. 9, the light output greatly oscillates in only 2 ns,
Self-excited oscillation was confirmed.

In the semiconductor laser of the present invention, the doping level of the saturable absorbing layer is set to about 2 × 10 ¹⁸ (cm ⁻³ ).
The lifetime of the carrier has been reduced. As a result, the contribution of spontaneous emission to the time change rate of carriers increases, and self-sustained pulsation can be easily generated. Doping has an effect of reducing the carrier lifetime if the dopant concentration is about 1 × 10 ¹⁸ (cm ⁻³ ) or more.

In the above structure, the saturable absorbing layer having a thickness of about 50 Å is used, but the thickness of the saturable absorbing layer is not limited to this. The saturable absorption layer may have a multiple quantum well structure or a bulk structure.

FIG. 10 shows a manufacturing process of this semiconductor laser.
This will be described with reference to (a) to (g).

First, as shown in FIG.
On an aAs substrate 101, a buffer layer 102 made of n-type GaAs and an n-type clad layer 10 made of AlGaInP
3, an active layer 104 having a multiple quantum well structure composed of AlGaInP and GaInP, a first p-type cladding layer 105a composed of p-type AlGaInP, a saturable absorption (SA) layer 106 composed of p-type GaInP, and a second layer composed of AlGaInP. P-type cladding layer 105b, etching stopper (ES) layer 100, and third p-type cladding layer 105c
Are sequentially formed. For forming each layer, for example, a method such as a metal organic chemical vapor deposition method (MOVPE method) or a molecular beam epitaxy method (MBE method) can be used, and typically, the MOVPE method is used.

On the third p-type cladding layer 105c, a stripe-shaped SiO ₂ film is patterned as shown in FIG. Then, using the SiO ₂ film as a mask, the third p-type cladding layer 105c is wet-etched with an etching solution to form a ridge stripe as shown in FIG.

The etching is stopped by the p-type cladding layer 10.
The etching is performed with the etching stopper layer 100 formed in 5a, 105b and 105c. Etching stopper layer 10
Numeral 0 is made of GaInP, and has a selectivity with respect to an etching solution with respect to a p-type cladding layer made of AlGaInP. Therefore, the etching stops at the etching stopper layer 100.

Thereafter, as shown in FIG. 10D, the n-type GaAs current blocking layer 11 is formed on both sides of the ridge stripe.
Form one. Then, as shown in FIG.
After removing the O ₂ film, as shown in FIG.
The buried growth is performed by using the GaInP cap layer 112. Finally, as shown in FIG. 10G, a p-type electrode 113 and an n-type GaAs substrate 1 are formed on the cap layer 112.
The n-type electrode 114 is formed on the back surface of the semiconductor device 01, and the semiconductor laser is completed.

As described above, in this embodiment, the etching stopper layer 100 is provided in the p-type cladding layers 105b and 105c on the saturable absorption layer 106. Thus, the saturable absorption layer 106 is not affected by the etching process. On the other hand, when the etching stopper layer 100 is not formed as in this embodiment, FIG.
Problems described with reference to (a) to (c) may occur.

The etching for forming the ridge stripe is wet etching, has excellent controllability such as etching depth, and is optimal for accurately forming a desired shape. However, since the etching is wet etching, the amount of etching may vary within the wafer surface. As a result, specifically, the shape of the ridge stripe differs in the wafer plane.

FIG. 11A is a cross-sectional view when the etching of the p-type cladding layer has been completely stopped at the saturable absorbing layer. In this case, the thickness of the saturable absorption layer is uniform both inside and outside the ridge stripe, and the thickness does not change. However, in the wafer surface, not only the good etching process is realized as shown in FIG. 11A, but also the etching progresses too much as shown in FIG. In some cases, the absorption layer has a reduced thickness. If the etching proceeds further excessively, there may be a case where the saturable absorbing layer outside the stripe is completely lost as shown in FIG. As described above, when there is a variation in the processing shape by etching for each semiconductor laser chip in the wafer surface, the operating characteristics of each laser chip vary. In particular, in the self-pulsation type laser, the self-pulsation characteristics are determined by the saturable absorption layer. Therefore, the thickness of the saturable absorption layer is constant inside each semiconductor laser chip, and between different chips. The fact that the variation in the thickness of the saturable absorption layer is suppressed is a point where a laser having good operation characteristics can be mass-produced.

FIGS. 12A and 12B show variations in optical output among a plurality of semiconductor laser chips manufactured on the same wafer surface according to the present embodiment and the prior art.
This will be described with reference to FIG.

As shown in FIG. 12A, in the structure using the etching stopper layer according to the present embodiment, even if the threshold current is measured for each different sample, the measured value is almost constant at about 60 mA. It is. However, in the prior art configuration having no etching stopper layer, FIG.
As shown in (b), the threshold current varies between samples. This is because in a certain sample, the etching for forming the ridge stripe progressed too much, and the saturable absorption layer outside the ridge stripe became thin.

[0072]

When the etching stopper layer is provided as a layer separate from the saturable absorbing layer as in the present embodiment, the carriers in the saturable absorbing layer are separated from the layer adjacent to the saturable absorbing layer (specifically, In general, p that sandwiches the saturable absorption layer
It is necessary to suppress diffusion to the mold cladding layer). This is because when the carrier overflows from the saturable absorbing layer to the adjacent layer, the saturable characteristic of the semiconductor laser changes. This point will be described again with reference to FIG.

When laser light is absorbed by the saturable absorption layer, carriers are generated in the saturable absorption layer. If the carrier overflows from the saturable absorption layer due to diffusion, the operation of the semiconductor laser is adversely affected. Therefore, it is necessary to prevent such carrier diffusion. Here, the energy gap difference at the bottom of the conduction band between the saturable absorption layer and the layer adjacent to it (p-type cladding layer) was examined.

Since the saturable absorption layer has a quantum well structure, the bottom of the discrete level (base level) was used as the origin of the energy level. As a result, the energy difference at which the semiconductor laser exhibits good operation characteristics is about 100 meV to about 21 meV.
It was found to be 0 meV. Energy difference is about 10
If it is smaller than 0 meV, carriers generated by light absorption in the saturable absorption layer will diffuse into the adjacent layer. If the energy difference is larger than about 210 meV, problems such as an increase in electric resistance in the p-type cladding layer and deterioration of crystallinity occur. That is, in order to increase the energy difference from the ground level of the saturable absorption layer as the origin, it is necessary to increase y of the p-cladding layer (Al _x Ga _{1 -y} ) _y In _{1 -y} P. However, when y is increased, doping of the p-type cladding layer becomes difficult, and the electric resistance increases. further,
Since the lattice constant becomes smaller than that of the GaAs substrate as y increases, the strain inside the p-type cladding layer increases due to the increase in lattice mismatch, and defects occur. Therefore, the energy difference between the ground level of the saturable absorption layer and the adjacent p-type cladding layer is preferably about 100 meV to about 210 meV.

P-type cladding layer (Al _x Ga _{1 -x} ) _y In _{1 -y}
In order to match the composition of P with the GaAs substrate, y may be about 0.51. G with increasing y
The lattice constant is smaller than that of the aAs substrate, and tensile strain is applied inside the cladding layer. However, if y> 0.51, a necessary energy gap can be secured. On the other hand, the Al composition x is 0.5 or more (the product of x and y is larger than 0.5), and more preferably 0.6 or more (the product of x and y is 0.1 or more).
3 is better). To prevent the diffusion of carriers, it is necessary to increase the energy difference at the bottom of the conduction band. This can be realized by increasing the energy difference.

Next, reduction of the threshold current by reducing the mirror loss αM at the end face was examined. The result will be described below.

The mirror loss αM is given by αM = 1 / (2L) × l
og (1 / (Rf * Rr)). Here, L is the resonator length, Rf is the reflectance of the laser emission end face, and Rr is the reflectance of the end face that does not emit laser.

FIG. 13 is a graph showing the photocurrent output characteristics measured using the reflectance of the end face as a parameter. Here, the resonator length L = 500 μm. The light output (m
W), the horizontal axis represents the injection current (mA). In order to control the mirror loss by changing the reflectivity of the end face, the end face was coated with a multilayer structure of an SiO ₂ / TiO ₂ insulating film.

As shown in FIG. 13, the threshold current can be reduced by increasing the reflectance at the end face. However, since the slope efficiency also decreases as the threshold current decreases, if the end face reflectance is too large, the injection current value at which kink occurs in the high light output region gradually decreases, and the semiconductor laser in the high light output region The usable range has narrowed,
It cannot be put to practical use. On the other hand, when the reflectance is reduced, the injection current value at which the kink is generated increases. For this reason, the usable range of the semiconductor laser in the high light output region becomes large (it can be used up to a higher output level), but the threshold current increases and the light absorption region becomes large. It becomes difficult to control the light output.

FIG. 14 shows a line indicating that the kink level is 5 mW or more and a line indicating that the light absorption region is 5 mW or less in the relationship between the end face reflectivity (horizontal axis) and the light output (vertical axis). Are respectively added. Thus, if the reflectivity of each end face of the semiconductor laser is set to about 40% or more and about 90% or less, a practical semiconductor laser can be realized even in consideration of the kink level and the light absorption region. In the above measurement, the temperature is set to about 25 ° C., but as the temperature increases, the threshold current increases (temperature characteristic). Considering that the practical operating temperature range of the semiconductor laser is about 0 ° C. to about 80 ° C., the end face reflectance is preferably about 50%.
It is good to set to about 70%.

Here, the influence of the slope efficiency will be further discussed below.

The slope efficiency refers to the slope of a linear portion in the light output-current characteristics of a semiconductor laser. However, if the semiconductor laser has the self-sustained pulsation characteristic, the characteristic has a “jump” corresponding to the light absorption region. Therefore, the “slope efficiency” when the semiconductor laser has the self-excited oscillation characteristic is defined as a slope of a portion not including the “skip (light absorption region)” in the graph showing the light output-current characteristic.

FIGS. 15A and 15B are graphs showing the relationship between temperature and relative intensity noise (RIN). Specifically, (a) shows that the slope efficiency (Se) is 0.42 mW.
/ B and (b) are measurement data when the slope efficiency (Se) is 0.15 mW / mA. Thus, in the graph of (b) in which the slope efficiency is small, good RI is obtained in the temperature range of -10 ° C to 70 ° C.
The N characteristic is shown, but the slope characteristic is large (a)
In the graph, the RIN characteristic deteriorates rapidly in a temperature range of about 30 ° C. or less. As described above, according to the study by the inventors, it has become clear that the value of the slope efficiency is related to the RIN characteristic particularly in a low temperature region.

Therefore, in order to examine the effect of the slope efficiency on the deterioration of the RIN characteristic in a low temperature region, the following experiment was further performed. That is, for samples having various slope characteristics, temperature-RIN as shown in FIG.
The characteristic data was measured, and a temperature (critical temperature) at which the noise intensity level increased by 3 dB from the baseline in the data was obtained, and a graph as shown in FIG. 16 was created. In FIG. 16, the horizontal axis is the slope efficiency, and the vertical axis is the critical temperature determined as described above.

In FIG. 16, a white circle plot is a good product (OK sample) in which the deterioration of the RIN characteristic is within an allowable range, and a black circle plot is a defective product (NG sample) in which the deterioration of the RIN characteristic is remarkable. From this, the slope efficiency Se
For samples greater than about 0.4 mW / mA, the critical temperature is about 30 ° C. or higher. This is about 30
A large amount of noise is generated in a temperature range of less than ° C, which means that it is not suitable for practical use. On the other hand, if the slope efficiency is about 0.4 mW / mA, the critical temperature is about 20 ° C. Further, when the slope efficiency is about 0.35 mW / mA, the critical temperature on the graph is 10 ° C. or lower (data at the critical temperature of 10 ° C. or lower is omitted in FIG. 16). From this, it was confirmed that a semiconductor laser having excellent RIN characteristics over a wide temperature range can be obtained by reducing the slope efficiency.

As described above, the upper limit of the preferable setting range of the slope efficiency is determined from the viewpoint of reducing noise at a low temperature.

On the other hand, the lower limit of the preferable setting range of the slope efficiency is determined from the viewpoint of the operating current. That is, if the slope efficiency is greater than about 0.1 mW / mA, no increase in operating current is observed even if the environmental temperature increases to about 60 ° C. Furthermore, the slope efficiency is about 0.15 mW /
If it is larger than mA, no increase in operating current is observed even if the environmental temperature increases to about 80 ° C. Generally, as the operating current increases, the laser life is shortened. Therefore, in practice, it is preferable that the operating current does not increase over the widest possible temperature range. Therefore, this point is the lower limit of the preferable setting range of the slope efficiency. Influence decisions.

FIG. 17 is a graph showing the relationship between the slope efficiency of the semiconductor laser and the usable temperature range, which was created in consideration of the above matters. From the perspective of operating current stability, FIG. 1 is drawn based on the above relationship between slope efficiency and ambient temperature associated with operating current variations.
The region below line 7 (a) is a preferred region. on the other hand,
From the viewpoint of the RIN characteristic in the low temperature region, a region above the line (b) in FIG. 17 drawn based on the data in FIG. 16 is a preferable region.

In consideration of the above points, the slope efficiency becomes
Preferably from about 0.1 mW / mA to about 0.4 mW / mA,
More preferably, about 0.15 mW / mA to about 0.35 m
Set in the range of W / mA.

Next, the resonator length of this semiconductor laser was examined. FIG. 18 shows the relationship between the resonator length and the drive current.

When the resonator length is about 700 μm or more, the required driving current becomes too large. On the other hand, when the cavity length is about 300 μm or less, the level of the necessary driving current is small, but the volume of the active region is small, so that the density of the current injected with the operation increases, which adversely affects the reliability. . Therefore, considering these points, the resonator length L
Is preferably set in the range of about 300 μm ≦ L ≦ about 700 μm.

Next, the width W of the ridge stripe of this semiconductor laser was examined. FIG. 19 shows the relationship between the stripe width and the drive current.

When the stripe width is small, the threshold current increases due to an increase in waveguide loss, and a high drive current is required. On the other hand, if the stripe width is large, the kink level decreases and the maximum light output decreases. As shown in FIG. 19, the stripe width W is set to about 2.0 μm ≦ W ≦ about 6.0.
It is preferable to set it in the range of μm because a low drive current and a high kink level can be realized at the same time.

As described above, by controlling the end face reflectivity, the cavity length, and the stripe width, the light absorption region can be reduced and the threshold current can be reduced. As a result, the light output can be controlled in a region following the light absorption region, so that a semiconductor laser having stable controllability can be obtained.

Furthermore, by setting the slope efficiency in an appropriate range, it is possible to suppress the noise level in a low-temperature region while securing a stable operating current, and to realize good operating characteristics of the semiconductor laser.

(Second Embodiment) Next, a second embodiment of the semiconductor laser according to the present invention will be described with reference to FIG.

This semiconductor laser has an n-type GaAs substrate 701 and a semiconductor laminated structure formed on the GaAs substrate 701. This semiconductor laminated structure has n-type Ga
As buffer layer 702, n-type AlGaInP cladding layer 703, multiple quantum well active layer 704 made of AlGaInP and GaInP, first p-type AlGaInP cladding layer 705a, light guide layer 707, second p-type AlGa
It includes an InP cladding layer 705b, a saturable absorption layer 706 made of p-type GaInP, and a third p-type AlGaInP cladding layer 705c. Third p-type AlGaInP
On the upper surface of the cladding layer 705c, a fourth p-type AlGaI
A stripe-shaped ridge portion (width: about 2.0 μm to about 6.0 μm) formed of the nP cladding layer 705d and extending in the resonator length direction is formed. A contact layer 710 is formed on the upper surface of the ridge portion of the fourth p-type cladding layer 705d. An n-type GaAs current blocking layer 711 is formed on both sides of the fourth p-type cladding layer 705d and the contact layer 710. The n-type GaAs layer current block layer 711 is formed to extend further laterally from the side surface of the ridge.

Contact layer 710 and current block layer 7
On top of this, a p-type GaAs cap layer 712 is formed. Further, a p-electrode 713 is formed on the upper surface of the cap layer 712, and an n-electrode 714 is formed on the back surface of the substrate 701.
Are formed.

In the configuration of the present embodiment as described above,
The active layer 704 has a multiple quantum well structure including three well layers and barrier layers.

Further, an etching stopper layer 700 is provided on the saturable absorption layer 706 (more precisely, the third p-type cladding layer 705c) and below the ridge stripe of the fourth p-type cladding layer 705d. Have been. The composition ratio of the etching stopper layer 700 is Ga _0.51 In _0.49 P, and the selectivity in the etching step should be high between the third p-type cladding layer 705c and the AlGaInP of the fourth p-type cladding layer 705d. Can be.

In this embodiment, the n-type AlGaInP
Cladding layer 703, first p-type AlGaInP cladding layer 705a, light guide layer 707, second p-type AlGaI
nP cladding layer 705b, saturable absorption layer 706, third p-type AlGaInP cladding layer 705c, and fourth p-type AlGaInP cladding layer 705c.
The type AlGaInP cladding layer 705d forms a cladding structure. The difference from the configuration of the above embodiment is that
That is, the light guide layer 707 is provided in the clad structure.

FIG. 21 shows a band diagram of the conduction band of this structure. Also in the case of this embodiment, it is necessary to suppress the carrier in the saturable absorption layer from diffusing into the layer adjacent to the saturable absorption layer. Again, the energy gap difference is about 100 meV to about 210 meV, the energy difference between the ground level of the saturable absorption layer and the bottom of the conduction band of the adjacent cladding layer.
Set to.

In this embodiment, the volume of the saturable absorption layer is reduced, and the light guide layer is provided in the clad structure. The smaller the volume of the saturable absorbing layer, the more easily the carrier density can be increased. As the carrier density is higher, the light absorption is more likely to be saturated, so that the saturable absorption effect becomes more remarkable. Therefore, as the volume of the saturable absorption layer is reduced, stronger self-pulsation is obtained. However, there is a problem that the smaller the volume of the saturable absorbing layer, the smaller the light confinement rate in the saturable absorbing layer. In the present embodiment, the light guide layer is provided between the active layer and the saturable absorbing layer, so that the distribution of the laser light from the active layer toward the saturable absorbing layer is widened, thereby confining the light in the saturable absorbing layer. The saturable absorption layer and the interaction of light are strengthened.
As described above, the light guide layer of the present embodiment increases the light confinement rate of the saturable absorption layer, and has a significantly different function from the conventional light guide layer that increases the light confinement rate of the active layer.

Here, the optimum range of the end face reflectivity does not change by providing the light guide layer, and may be the same range as in the first embodiment. The same applies to the appropriate range of the resonator length L, the stripe width W, and the slope efficiency.

[0106]

As described above, according to the present invention, the energy gap between the saturable absorbing layer and the active layer is controlled, and
By controlling the reflectivity at the laser end face, it is possible to realize a laser having a self-excited oscillation characteristic in which the light absorption region is small, the threshold current is small, and the light output can be controlled stably. Alternatively, by setting the slope efficiency in an appropriate range, it is possible to suppress the noise level in a low-temperature region while securing a stable operation current, and to realize good operation characteristics of the semiconductor laser.

Further, by providing an etching stopper layer on the saturable absorption layer, semiconductor lasers with little variation in characteristics can be stably mass-produced.

Further, by adjusting the energy gap difference between the saturable absorbing layer and the cladding layer adjacent to the saturable absorbing layer, the diffusion of carriers from the saturable absorbing layer to the adjacent layer is suppressed, and the self-sustained pulsation characteristics with good characteristics are improved. A laser having the above characteristics.

[Brief description of the drawings]

FIG. 1 is a schematic diagram for explaining an energy gap.

FIG. 2 is a structural sectional view of the semiconductor laser according to the first embodiment of the present invention.

FIG. 3 is a band gap energy diagram of the semiconductor laser of FIG. 2;

FIGS. 4A to 4C are diagrams for explaining light output characteristics of the semiconductor laser of FIG. 2;

FIG. 5 is a diagram showing a relationship between an energy gap difference and Tmax (temperature at which self-sustained pulsation stops) and Pmax (maximum optical output of self-sustained pulsation at room temperature).

FIG. 6 is a diagram showing a relationship between an energy gap difference and an operation current.

FIG. 7 is a diagram showing a relationship between an operating current and a lifetime.

FIG. 8 is a diagram showing light output-current characteristics.

FIG. 9 is a diagram showing output characteristics.

FIGS. 10A to 10G are cross-sectional views illustrating the steps of manufacturing the semiconductor laser of FIG. 2;

FIGS. 11A to 11C are views for explaining a problem caused by etching when forming a ridge stripe. FIGS.

FIGS. 12A and 12B are diagrams schematically showing variations in optical output between samples formed on the same wafer surface.

FIG. 13 is a diagram showing an optical output with respect to an injection current when the reflectance of the end face is changed.

FIG. 14 is a diagram for explaining an optimum range of the reflectance of the end face.

FIGS. 15A and 15B are temperature-relative noise intensity characteristic diagrams for different slope efficiency values.

FIG. 16 is a diagram showing a relationship between a slope efficiency and a critical temperature in temperature-relative noise intensity characteristics.

FIG. 17 is a diagram for explaining an optimum range of slope efficiency.

FIG. 18 is a diagram illustrating a relationship between a resonator length and a drive current.

FIG. 19 is a diagram showing a relationship between a stripe width and a drive current.

FIG. 20 is a structural sectional view of a semiconductor laser according to a second embodiment of the present invention.

21 is a band gap energy diagram of the semiconductor laser of FIG.

[Explanation of symbols]

 REFERENCE SIGNS LIST 100 etching stopper layer 101 substrate 102 buffer layer 103 n-type cladding layer 104 active layer 105 a, 105 b, 105 c p-type cladding layer 106 saturable absorption layer 110 contact layer 111 current blocking layer 112 cap layer 113 p electrode 114 n electrode 700 etching stopper Layer 707 Light guide layer

Continuing from the front page (72) Inventor Toshiya Fukuhisa 1006 Kadoma Kadoma, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (72) Inventor Masaya Manno 1006 Odaka Kadoma Kadoma City, Osaka Matsushita Electric Industrial Co. (72) Inventor Yasuhito Kubuchi 1006 Kadoma, Kazuma, Osaka Prefecture Inside Matsushita Electric Industrial Co., Ltd. (56) References JP-A-7-106695 (JP, A) JP-A-4-206984 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) H01S 5/50-5/50

Claims (7)

(57) [Claims] 1. A semiconductor laser comprising: an active layer having at least a quantum well layer; and a cladding structure sandwiching the active layer, wherein the cladding structure includes a saturable absorbing layer and the saturable absorbing layer.
Includes an etching stopper layer provided on the energy gap of the saturable absorption layer, than the energy gap between the ground level of the quantum well layer of the active layer,
About 30 meV to about 200 meV, each end face having a reflectivity of about 40% to about 90% , and having a ground level of the saturable absorbing layer and an area adjacent to the saturable absorbing layer.
The energy gap difference between the bottom of the conduction band of
A semiconductor laser that is between 100 meV and about 210 meV . 2. A semiconductor laser comprising: an active layer having at least a quantum well layer; and a clad structure sandwiching the active layer, wherein the clad structure includes a saturable absorption layer and the saturable absorption layer.
Includes an etching stopper layer provided on the energy gap of the saturable absorption layer, than the energy gap between the ground level of the quantum well layer of the active layer,
Slope efficiency from about 0.1 mW / mA to about 0.4 mW / m
A , the ground level of the saturable absorption layer and the adjacent saturable absorption layer
The energy gap difference between the bottom of the conduction band of
A semiconductor laser that is between 100 meV and about 210 meV . 3. The slope efficiency is about 0.15 mW / m.
3. The semiconductor laser according to claim 2, wherein the power is from A to about 0.35 mW / mA. 4. The semiconductor laser according to claim 1, wherein said cladding structure further includes a light guide layer. 5. The resonator length L is about 350 μm ≦ L ≦ about 70
5. The semiconductor laser according to claim 1, which has a thickness of 0 μm. 6. The semiconductor laser according to claim 1, wherein a stripe width W satisfies about 2.0 μm ≦ W ≦ about 6.0 μm. 7. The method according to claim 1, wherein the active layer and the saturable absorption layer are made of Al.
_{x Ga y In 1-y P} (0 ≦ x ≦ 1,0 ≦ y ≦ 1) is a layer composed of a semiconductor laser according to any one of claims 1 to 6.