JPH09298335A - Semiconductor laser and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a semiconductor laser having stable self-oscillation characteristics up to a high temperature region. SOLUTION: An N-type buffer layer 102, an N-type clad layer 103, a strained multiquantum well active layer 104, a first P-type clad layer 105, and an etching stop layer 106 composed of P-type AlGaAs are formed in order on an N-GaAs board 101. A ridge type second P-type clad layer 107, a strained quantum well saturable absorbing layer 108 constituted of a P-type GaInP, a third P-type clad layer 109 and a P-type contact layer 110 are formed in order on the etching stop layer 106. After that, both sides of the ridge are filled with a P-type carrier confinement layer 111 and a current block layer 112 constituted of N-type GaAs. A P-type cap layer 113 is formed on the contact layer 110 and the current block layer 112. By confining carriers pumped in the saturable absorbing layer, in the vertical and the horizontal directions, with a hetero barrier, stable self- oscillation characteristics can be shown up to a high temperature region.

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 for a light source of an optical disk system and a method for manufacturing the same.

[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.
System and InP system are being actively researched and developed. In the field of optical information processing, in particular, a method of recording / reproducing information by an AlGaAs semiconductor laser having a wavelength of 780 nm has been put into practical use and has come into widespread use in compact discs and the like.

However, recently, an increase in recording capacity of these optical disk devices has been required,
Along with this, there is an increasing demand for short wavelength lasers.
The wavelength of the AlGaInP based semiconductor laser is 630 to 69.
It is possible to oscillate in the red region of 0 nm, and it is possible to obtain the light of the shortest wavelength 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 reflected light from the disc surface and a change in temperature during reproduction of the optical disc, and induces a signal reading error. Therefore, a laser with little intensity noise is indispensable for the light source of the optical disk. Conventionally, reproduction only, low output AlG
In the aAs semiconductor laser, a structure in which saturable absorbers are intentionally formed on both sides of the ridge stripe has been adopted to self-oscillate the laser to reduce noise. This makes it possible to make the vertical mode multi. When the laser is oscillating in the vertical single mode, if a disturbance such as optical feedback or temperature change enters, the adjacent longitudinal mode will start to oscillate due to the subtle change in the gain peak. Cause a conflict. This is a cause of noise. When the longitudinal modes are multi-modulated, the intensity change of each mode is averaged and reduced, so that stable low noise characteristics can be obtained.

Further, as another method, a method for obtaining more stable self-excited oscillation is disclosed in JP-A-63-202083. Here, a self-pulsation type semiconductor laser is realized by providing a layer capable of absorbing output light.

Further, in Japanese Patent Laid-Open No. 6-260716, it is reported that the characteristics are improved by making the bandgap of the active layer and the bandgap of the absorption layer substantially equal. A red semiconductor laser is described here.

FIG. 8 is a schematic sectional view showing a conventional self-excited oscillation type semiconductor laser disclosed in Japanese Patent Laid-Open No. 6-260716. Reference numeral 801 denotes a substrate made of n-type GaAs, and a buffer layer 802 made of n-type GaInP, a cladding layer 803 made of n-type AlGaInP, and a strained multiple quantum well active layer 804 are sequentially formed on the substrate 801.

A strained quantum well saturable absorption layer 805 is formed in the cladding layer 803. On the strained multiple quantum well active layer 804, a clad layer 806 made of p-type AlGaInP and a strained quantum well saturable absorption layer 805 are formed. A ridge-shaped clad layer 80 is formed on top of it.
6 and a contact layer 807 made of p-type GaInP. Both sides of the ridge-shaped cladding layer 806 and the contact layer 807 are filled with a current blocking layer 808 made of n-type GaAs.

Further, the contact layer 807 and the block layer 8
A cap layer 809 made of p-type GaAs is formed on the 08 layer, and a p-side electrode 81 is formed on the cap layer 809.
0, an n-side electrode 811 is formed on the substrate 801 side.

Further, FIG. 9 shows a strained quantum well saturable absorption layer 80.
5 shows the compositional structure diagram of No. 5, and (Al0.7Ga0.5) 0.5I
A barrier layer 901 made of n0.5P and a well layer 902 made of GaxIn1-xP (film thickness 100Å, strain +0.5 to 1.0%) are alternately laminated. In this conventional example, three well layers 902 are formed. ing. Here, the band gap of the strained multiple quantum well active layer 804 and that of the strained quantum well saturable absorption layer 805 are substantially equal. This structure is intended to obtain good self-sustained pulsation characteristics.

[0011]

[Problems to be Solved by the Invention] However, AlGaI
It has been clarified that it is difficult to obtain self-sustained pulsation characteristics of the nP-based semiconductor laser because the gain characteristics of the material are significantly different from those of the AlGaAs-based semiconductor laser. In Figure 10, GaAs
And the gain characteristics of GaInP are shown. Here, GaAs and GaInP are materials mainly used for the active layers of the AlGaAs semiconductor laser and the AlGaInP semiconductor laser, respectively.

Generally, the self-excited oscillation phenomenon is more likely to occur as the rate of change of carrier density in the saturable absorber layer is larger, and the rate of time change is given by the sum of contributions of stimulated emission and spontaneous emission. In order to increase the contribution of stimulated emission, it is required that the gain variation with the carrier density variation is large.
However, as shown in FIG. 10, in the case of GaInP, GaA
The carrier density dependency of the gain is lower than that of s, and the variation of the gain coefficient is small.

According to the experimental results of the inventors of the present application, in the case of the red semiconductor laser, the contribution of stimulated emission is small due to the difference in the above-mentioned gain characteristics. Therefore, the band gap between the active layer and the saturable absorption layer is reduced as in the conventional example. It has been found that it is difficult to obtain stable self-sustained pulsation characteristics just by making them equal.
That is, in the case of a red semiconductor laser, in order to obtain stable self-sustained pulsation characteristics, the contribution of spontaneous emission to the rate of change of the carrier density of the saturable absorbing layer is made larger, that is, the carrier lifetime of the saturable absorbing layer is reduced. There is a need to.

Further, the optical disk system is generally 80 ° C.
There is a demand for operation guarantee up to a high temperature range, and a semiconductor laser is also required to have stable self-excited oscillation characteristics up to the same temperature. However, in the structure of the conventional example, the carriers excited in the saturable absorption layer by absorbing the output light are confined in the same layer by the hetero barrier in the crystal growth direction. Spread freely inside. Therefore, the confinement of the carriers excited in the saturable absorption layer varies depending on the ambient temperature, which in turn changes the self-sustained pulsation characteristic.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a semiconductor laser having a self-sustained pulsation characteristic which is stable up to a high temperature region, and a method for manufacturing the same.

[0016]

In order to solve the above problems, the semiconductor laser of the present invention is a layer in which the upper surface, the lower surface and the side surface of the saturable absorption layer have a band gap larger than the band gap of the saturable absorption layer. It is characterized by being surrounded by.

The semiconductor laser of the present invention is characterized in that carriers excited in the saturable absorption layer by absorbing output light are confined in the vertical and lateral directions by the hetero barrier.

The semiconductor laser manufacturing method of the present invention comprises the steps of forming a semiconductor laminated structure including an active layer and a saturable absorbing layer on a substrate, etching a part of the saturable absorbing layer, and Forming a layer having a bandgap larger than that of the saturable absorption layer on the etching surface.

A method of manufacturing a semiconductor laser according to the present invention comprises a step of forming a semiconductor laminated structure including an active layer and a clad layer on a substrate, and a part of the semiconductor laminated structure having an inverted mesa shape halfway up the clad layer. And a step of forming a saturable absorbing layer inside the inverted mesa-shaped groove.

[0020]

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

(First Embodiment) FIG. 1 is a schematic sectional view showing a semiconductor laser according to the first embodiment of the present invention. Reference numeral 101 denotes a substrate made of n-type GaAs, and a buffer layer 1 made of n-type GaInP on the substrate 101.
02, an n-type cladding layer 103 made of (Al0.7Ga0.3) 0.5In0.5P, a strained multiple quantum well active layer 104, (Al0.7
First p-type cladding layer 1 made of Ga0.3) 0.5In0.5P
05, an etching stop layer 1 made of p-type AlGaAs
06 are sequentially formed. A ridge-shaped (Al
A second p-type cladding layer 107 made of 0.7Ga0.3) 0.5In0.5P, a strained quantum well saturable absorption layer 108 made of p-type GaInP, and a third layer made of (Al0.7Ga0.3) 0.5In0.5P. The p-type cladding layer 109 and the contact layer 110 made of p-type GaInP are sequentially formed.

This ridge-shaped second p-type cladding layer 10
7, both sides of the strained quantum well saturable absorption layer 108, the third p-type cladding layer 109, and the contact layer 110 are p-type.
Carrier confinement layer 111 made of (Al0.5Ga0.5) 0.5In0.5P and current blocking layer 1 made of n-type GaAs.
It is embedded by 12.

Further, on the contact layer 110 and the current blocking layer 112, a cap layer 11 made of p-type GaAs is formed.
3, a p-side electrode 114 is formed on the cap layer 113, and an n-side electrode 115 is formed on the substrate 101 side.

2 and 3 are explanatory views showing a method of manufacturing the semiconductor laser shown in FIG. A buffer layer 102 (film thickness 1.0 μm), an n-type clad layer 103 (film thickness 1.0 μm), and a strained multiple quantum well active layer 104 are sequentially formed on a substrate 101 by a metal organic chemical vapor deposition method (hereinafter referred to as MOVPE method). Form. The composition structure diagram of the strained multiple quantum well active layer 104 is shown in FIG. 4, in which (Al0.5Ga0.5) 0.5In0.5P is formed on the guide layer 401 (film thickness 100 Å).
0.5) 0.5In0.5P barrier layer 402 (film thickness 50Å)
And GaInP well layer 403 (film thickness 50Å strain +0.5
%) Are laminated alternately, and (Al0.5Ga0.5) 0.5
A guide layer 401 (film thickness 100Å) made of In0.5P is formed.

On the strained multiple quantum well active layer 104, a first p-type cladding layer 105 (thickness 1500Å), an etching stop layer 106, and a second p-type cladding layer 107 (thickness 300).
Å), Strained quantum well saturable absorption layer 108 (film thickness 100A strain +
0.5%), the third p-type cladding layer 109 (film thickness 0.85μ
m), and the contact layer 110 (film thickness 500Å) is sequentially formed by the MOVPE method (FIG. 2A). Etching stop layer 1
The Al composition and film thickness of 06 are set in a range that does not absorb output light.

Next, a SiO 2 film is formed on the contact layer 110 by the CVD method, and patterning is performed on the stripes having a width of about 3 μm by a photolithography process to form a mask 116 (FIG. 2B). Then, the contact layer 110, the third p-type cladding layer 109, the strained quantum well saturable absorption layer 108, and the second p-type cladding layer 107, which are not covered with the mask 116, are etched by the hydrochloric acid-based etchant to etch the etching stop layer 106. It is etched up to a ridge shape (Fig. 2 (c)). After that, by selective growth, the carrier confinement layer 111 (film thickness 500Å) and the current blocking layer 1
12 (film thickness 0.75 μm) is embedded and grown on both sides of the ridge (FIG. 3 (d)). Then, the mask 1 is made with the buffered hydrofluoric acid solution.
After removing 16, the cap layer 113 is formed by the MOVPE method (FIG. 3E), and the p-side electrode 114 is formed on the cap layer 113 and the n-side electrode 115 is formed on the substrate 101 side (FIG. 3). (f)).

The structure is different from the conventional structure in that the strained quantum well saturable absorption layer 108 is partially removed by etching,
That is, the carrier confinement layer 111 having a large band gap is arranged on the side surface thereof. As a result, the strained quantum well saturable absorption layer 108 and the carrier confinement layer 111
Since a hetero barrier is formed at the interface of, the carriers excited in the saturable absorption layer by absorbing the output light are
It can be trapped not only vertically but also laterally. Therefore, as compared with the conventional structure in which the carriers are confined only in the vertical direction, the carrier recombination rate is significantly increased, and the carrier life is shortened. Therefore, the rate of change in carrier density of the saturable absorption layer with time becomes larger than that in the conventional example, and the self-sustained pulsation characteristic can be easily obtained.

Since the carriers excited in the saturable absorption layer are also confined in the lateral direction by the hetero barrier, lateral diffusion of the carriers is suppressed. Therefore,
Due to the lateral diffusion as in the conventional example, the distribution state of carriers excited in the saturable absorption layer does not change with the ambient temperature. Therefore, stable self-sustained pulsation characteristics can be obtained even if the environmental temperature changes.

The conduction type of the carrier confinement layer 111 is preferably the same p type as that of the strained quantum well saturable absorption layer 108. If it is n-type, a reverse bias is applied between it and the strained quantum well saturable absorption layer 108, so that the hetero barrier for carriers excited in the saturable absorption layer is reduced and the lateral confinement effect is reduced. Is. Further, in order to suppress an increase in operating current due to the hole current flowing into the carrier confinement layer 111, the p-type carrier concentration of the carrier confinement layer 111 is as low as possible, and the film thickness is excited in the saturable absorption layer. It is desirable to reduce the carrier confinement effect within the effective range.

FIG. 5 shows the relative noise intensity (RIN) characteristics of the semiconductor laser of the present invention. The semiconductor laser of the present invention has a low noise characteristic of -130 dB or less, which is stable in a wide temperature range of 20 to 80 degrees required for a semiconductor laser for an optical disk system. It can be seen that the characteristics are exhibited.

As the carrier confinement layer 111,
(Al0.5Ga0.5) 0.5In0.5P was used, but the material and composition are not limited as long as the band gap difference from the strained quantum well saturable absorption layer 108 is sufficient for confining carriers. Further, the second p-type cladding layer 107 may be omitted. In that case, the effect of confining carriers excited in the saturable absorption layer is reduced, but the Al composition of the etching stop layer 106 is set to about 0.5 and the film thickness is set to 100 Å or less to increase the energy of the conduction band. can do.

(Second Embodiment) FIG. 6 is a schematic sectional view showing a semiconductor laser according to a second embodiment of the present invention. A substrate 601 is made of n-type GaAs, and a buffer layer 6 made of n-type GaInP is formed on the substrate 601.
02, an n-type cladding layer 603 made of (Al0.7Ga0.3) 0.5In0.5P, a strained multiple quantum well active layer 604, (Al0.7
First p-type cladding layer 6 made of Ga0.3) 0.5In0.5P
05, a strained quantum well saturable absorption layer 606 made of p-type GaInP is sequentially formed. A second p-type cladding layer 607 made of ridge-shaped (Al0.7Ga0.3) 0.5In0.5P and a contact layer 608 made of p-type GaInP are sequentially formed on the upper portion thereof. Both sides of the ridge-shaped second p-type cladding layer 607 and the contact layer 608 are filled with a current blocking layer 609 made of n-type GaAs. Further, a cap layer 610 made of p-type GaAs is formed on the contact layer 608 and the current block layer 609. The p-side electrode 611 is on the cap layer 610 and the n-side electrode 612 is on the substrate 601 side.
Are formed respectively.

The difference between this embodiment and the conventional example is shown in FIG.
That is, the portion outside the ridge of the strained quantum well saturable absorption layer 606 is more highly doped than the portion inside the ridge as shown in FIG. As a result, (1) the Fermi level is different between the portion outside the ridge and the portion inside the ridge of the strained quantum well saturable absorption layer 606, and a hetero barrier is generated at the boundary between the inside and outside. (2) The strained quantum well saturable absorption layer 60
Since the order of Ga and In occupying the group III sublattice is different between the portion outside the ridge and the portion inside the ridge of 6 and the portion outside the ridge is more disordered, the band gap becomes larger than that inside the ridge. . Due to these two effects, the carriers excited in the saturable absorption layer are confined not only in the vertical direction but also in the lateral direction. Therefore, as in the first embodiment, even if the environmental temperature changes, the range of 20 to 80 degrees is achieved. Thus, stable self-sustained pulsation characteristics can be obtained.

(Third Embodiment) FIG. 7 is a schematic view showing a semiconductor laser and a method for manufacturing the same according to a third embodiment of the present invention. A substrate 701 is made of n-type GaAs, and a buffer layer 702 made of n-type GaInP, an n-type cladding layer 703 made of AlGaInP, a strained multiple quantum well active layer 704, and AlGaI are formed on the substrate 701.
First p-type cladding layer 705 made of nP, p-type Al
Etching stop layer 706 made of GaAs, p-type Al
A carrier confinement layer 707 made of GaInP and a current block layer 708 made of n-type GaAs are sequentially formed by MOVP.
It is formed by the E method (FIG. 7 (a)).

Next, a SiO2 film is formed on the current blocking layer 708 by a CVD method and patterned by a photolithography process to form a mask 716 (FIG. 7).
(b)). Then, the current blocking layer 708 and the carrier confinement layer 707 which are not covered with the mask 716 are etched up to the etching stop layer 706 to form an inverted mesa shape (FIG. 7C). Next, the second p-type cladding layer 70 made of AlGaInP is formed in and outside the inverted mesa-shaped groove.
9, strained quantum well saturable absorption layer 710, third p-type cladding layer 711 made of AlGaInP, p-type GaInP
A contact layer 712 made of p-type GaAs and a cap layer 713 made of p-type GaAs are sequentially formed by MOVPE, and a p-side electrode 714 is formed on the cap layer 613 and an n-side electrode 715 is formed on the substrate 701 side (FIG. 7 (d)).

Part of the carrier confinement layer 707 and the current blocking layer 708 are etched in an inverted mesa shape,
Since the growth rate on the side surface of the reverse mesa groove is extremely small, the second p-type cladding layer 709, the strained quantum well saturable absorption layer 710, the third p-type cladding layer 711, and the contact layer are formed inside and outside the groove. 712 is divided. Therefore, both sides of the strained quantum well saturable absorption layer 710 in the groove are covered with a carrier confinement layer having a larger band gap, so that the carriers excited in the saturable absorption layer are not only vertically but laterally. Is also trapped. Therefore, the first embodiment
Similarly, the carrier life is shortened and carrier diffusion is suppressed, so that stable self-excited oscillation characteristics can be obtained even when the environmental temperature changes.

As a result of manufacturing the semiconductor laser shown in FIG.
A self-excited oscillation phenomenon has been confirmed, and a low noise characteristic of -130 dB or less was obtained in a wide temperature range of 20 to 60 degrees.

[0038]

As described above, in the present invention, the carriers excited in the saturable absorption layer are confined not only in the vertical direction but also in the lateral direction by the hetero barrier, so that the life of the carrier is shortened and Carrier diffusion is suppressed. Therefore, there is a remarkable effect that it is possible to realize a low-noise semiconductor laser that exhibits stable self-sustained pulsation characteristics even when the environmental temperature changes.

[Brief description of drawings]

FIG. 1 is a schematic sectional view of a semiconductor laser according to a first embodiment of the present invention.

FIG. 2 is an explanatory view showing a method for manufacturing the semiconductor laser shown in FIG.

FIG. 3 is an explanatory diagram showing a method for manufacturing the semiconductor laser shown in FIG.

4 is a compositional structure diagram of a strained multiple quantum well active layer in the semiconductor laser shown in FIG.

5 is a diagram showing noise characteristics of the semiconductor laser shown in FIG.

FIG. 6 is a schematic sectional view of a semiconductor laser according to a second embodiment of the present invention.

FIG. 7 is a schematic diagram showing a semiconductor laser and a method for manufacturing the same according to a third embodiment of the present invention.

FIG. 8 is a schematic sectional view showing a conventional semiconductor laser.

FIG. 9 is a composition structure diagram of a strained quantum well saturable absorption layer in a conventional semiconductor laser.

FIG. 10 is a diagram showing gain characteristics with respect to carrier density of GaAs and GaInP.

[Explanation of symbols]

 Reference Signs List 101 substrate 102 buffer layer 103 n-type cladding layer 104 strained multiple quantum well active layer 105 first p-type cladding layer 106 etching stop layer 107 second p-type cladding layer 108 strained quantum well saturable absorption layer 109 third p Type cladding layer 110 contact layer 111 carrier confinement layer 112 current blocking layer 113 cap layer 114 p-side electrode 115 n-side electrode 116 mask 401 guide layer 402 barrier layer 403 well layer 601 substrate 602 buffer layer 603 n-type cladding layer 604 strained multiple quantum Well active layer 605 First p-type cladding layer 606 Strained quantum well saturable absorption layer 607 Second p-type cladding layer 608 Current blocking layer 609 Contact layer 610 Cap layer 611 P-side electrode 612 N-side electrode 701 Substrate 702 Buffer layer 70 n-type cladding layer 704 strained multiple quantum well active layer 705 first p-type cladding layer 706 etching stop layer 707 carrier confinement layer 708 current blocking layer 709 second p-type cladding layer 710 strained quantum well saturable absorption layer 711 third P-type clad layer 712 contact layer 713 cap layer 714 p-side electrode 715 n-side electrode 801 substrate 802 buffer layer 803 clad layer 804 strained multiple quantum well active layer 805 strained quantum well saturable absorption layer 806 clad layer 807 contact layer 808 current Block layer 809 Cap layer 810 P-side electrode 811 N-side electrode 901 Barrier layer 902 Well layer

(72) Inventor Akira Takamori 1006 Kadoma, Kadoma, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (72) Hideto Adachi, 1006 Kadoma, Kadoma City, Osaka Matsushita Electric Industrial Co., Ltd.

Claims (9)

[Claims] 1. At least an active layer and a saturable absorption layer, wherein the upper surface, the lower surface and the side surface of the saturable absorption layer are surrounded by a layer having a band gap larger than the band gap of the saturable absorption layer. Semiconductor laser. 2. The semiconductor laser according to claim 1, wherein a conductivity type of the saturable absorption layer and a conductivity type of a layer having a bandgap larger than that of the saturable absorption layer are the same. 3. The semiconductor laser according to claim 1, wherein a portion of the saturable absorption layer outside the ridge is more disordered than a portion inside the ridge. 4. A portion of the saturable absorber layer outside the ridge is more heavily doped than a portion inside the ridge.
4. The semiconductor laser according to claim 1. 5. A carrier having at least an active layer and a saturable absorption layer, wherein carriers excited in the saturable absorption layer by absorbing output light are confined in a vertical direction and a lateral direction by a hetero barrier. Semiconductor laser. 6. A step of forming a semiconductor laminated structure including an active layer and a saturable absorber layer on a substrate, a step of etching a part of the saturable absorber layer, and the saturable absorber on a surface to be etched. Forming a layer having a bandgap larger than the bandgap of the layer. 7. A step of forming a semiconductor laminated structure including an active layer and a clad layer on a substrate, a step of etching a part of the semiconductor laminated structure into a mesa shape up to the middle of the clad layer, and the mesa shape. And a step of forming a saturable absorption layer inside the groove. 8. The method for manufacturing a semiconductor laser according to claim 7, wherein the mesa is an inverted mesa. 9. A step of forming at least an active layer and a saturable absorption layer, and a layer having a band gap larger than a band gap of the saturable absorption layer on an upper surface, a lower surface and a side surface of the saturable absorption layer. And a method of manufacturing a semiconductor laser, the method including: