JPH0632324B2 - Optical bistable semiconductor laser - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention relates to an optical bistable semiconductor exhibiting bistable operation, which is one of the most important components as an optical functional element used for optical switching / optical information processing. Regarding laser.

(Prior art and its problems) Among optical functional devices, optical bistable semiconductor lasers that have a wide range of applications such as optical logic, optical switches, optical storage, optical signal waveforms, and shaping amplification utilize the essential features of light. It is expected as an element and its basic study is under way. Regarding the development of optical bistable semiconductor lasers, 20 years ago, 196 of SOLID STATE ELECTRO-NLCS magazine by GJ LASHER.
The first paper was published on page 707 in Vol. 7 (4th year), and in later years, the one with a low oscillation threshold at room temperature and close to a practical level was published by Applied Physics Letters by Mr. KYLAU. 1982 Volume 40 3
A paper described on page 69 reports in the form of a tandem electrode-embedded heterostructure. In this semiconductor laser, the electrode on the side close to the active layer is divided into two parts by the groove, and the gain in the active layer exceeds the loss in the optical amplification region, and since no current is injected, the loss exceeds the gain in saturable absorption. It is divided into areas. In this case, the groove is merely for electrically blocking the two regions, and a current is injected into the active layer directly below the groove by a diffusion current from the optical amplification region. For this reason, in this semiconductor laser, if the bisected electrodes are shared,
Optical output-current characteristics similar to ordinary semiconductor lasers (hereinafter L
-I characteristic) and no optical bistable operation occurs. In order to perform the optical bistable operation, it is necessary to reduce the injection current into the saturable absorption region to about −100 μA (in this case, the injection current into the optical amplification region is about +30 mA). However, if the injection current to the saturable absorption region fluctuates by several μA,
The result that the hysteresis shape in the L-I characteristic and the current width showing the optical bistable operation are significantly changed is reported in a paper by Rho et al. For this reason, it was important to improve the controllability of this injection current as an optical functional element that requires stable operation. As a measure against this, an optical bistable semiconductor laser (Japanese Patent Application No. 58-
142922). This is a semiconductor laser having a structure in which a semiconductor layer that blocks current injection is stacked on a part of an active layer and a groove is formed immediately above the semiconductor layer. By adopting this semiconductor layer, a saturable absorption region is formed in the active layer portion directly under the groove, and the injection current into the divided electrode is about several mA. In this case, it can be almost ignored that the L-I characteristic of the optical bistable operation changes even if the injection current changes by several hundred μA due to external factors. However, when the groove is not etched close to the active layer, the resistance between the two divided electrodes is, for example, only about 61Ω when the groove width is 30 μm, and therefore a part of the injection current is caused by the potential difference between the divided electrodes. There is a problem that independence of the injection current itself cannot be maintained, for example, the characteristics are stable in the static characteristics but become slightly unstable in the dynamic characteristics because the current flows in the order of milliamperes. In the method of etching the groove, the thickness of the semiconductor layer after crystal growth may vary depending on the location, and it is not very desirable to determine the depth of the groove simply by etching time, and there are some problems in reliability. there were. Therefore, the authors proposed the formation of a semiconductor layer that stops etching above the active layer as a method of determining the etching depth of the groove, and filed a patent application under the name of "bistable semiconductor laser" (Japanese Patent Application No. 58-167798). ). In the case of this invention, 2
The resistance between the divided electrodes can be increased to 1 KΩ, and the characteristics can be made uniform as compared with the conventional one.

However, the yield of the optical bistable operation in which the region directly below the groove functions as the saturable absorption region can be improved only to about half only by the method of forming only the depth of the groove with good reproducibility.
This is because the injection current circulates in the active layer directly below the groove due to the diffusion current from the divided electrodes. Therefore, there has been a demand for an optical bistable semiconductor laser having a structure that enables more uniform optical bistable operation.

(Object of the Invention) An object of the present invention is to provide an optical bistable semiconductor laser having uniform characteristics by further improving the yield of the optical bistable operation.

(Structure of the Invention) According to the present invention, in a semiconductor laser in which an electrode is divided into a plurality of electrodes in the resonance axis direction by a groove, a first semiconductor is provided above an active layer, which interrupts current injection into a region immediately below the groove of the active layer. A second semiconductor layer having a conductivity type different from that of the first semiconductor layer and a material composition different from that of the first semiconductor layer, the second semiconductor layer further comprising:
In the etching process for forming the groove, the second
The semiconductor bistable semiconductor laser is a semiconductor layer necessary for completely removing the semiconductor layer of 1) and leaving the first semiconductor layer.

(Detailed Description of Configuration) The present invention has solved the problems of the prior art by adopting the above configuration. First, the first semiconductor layer is crystal-grown so that current injection is blocked so that a saturable absorption region is formed in a part of the active layer, and a second semiconductor layer having a different material composition is further formed thereon. Grow crystals. After that, for example, when the groove is formed by chemical etching, since the material composition of the first semiconductor layer is different from that of the second semiconductor layer, the difference in etching rate due to the difference in etchant is used to increase the depth of the groove. It is suppressed from the electrode to the place where the first semiconductor layer remains. In this way, the resistance between the two or more electrodes divided in the resonator axial direction can be 1 KΩ or more. Since the resistance of the ordinary semiconductor laser itself is about 5 to 7Ω, the current between the divided electrodes can be suppressed to several tens of μA or less. With this size, the size is several tens of meters.
The injection current of A into each electrode is almost unaffected and can maintain its independence. Regarding the L-I characteristic, the saturable absorption region is directly below the groove, so that an optical bistable operation with uniform characteristics can be realized. As described above, the optical bistable semiconductor laser having uniform characteristics can be realized by further improving the yield of the optical bistable operation.

(Example) Hereinafter, the present invention will be described in detail with reference to the drawings. FIG. 1 is a plan sectional view showing the shape of an active layer in the case where an electrode separation groove according to an embodiment of the present invention is not formed, and FIG.
A sectional view taken along the line A-A ', (b) a sectional view taken along the line BB', and FIG. 3 are perspective views of this embodiment. In this example,
In the stripe-shaped buried heterostructure, the two grooves forming the mesa stripe including the active layer are formed so that the side surface on the opposite side of the mesa stripe is partially narrowed and the groove dividing the electrode into two has a desired depth. It was made to become. A planar stripe type buried heterostructure semiconductor laser is a semiconductor laser in which a mesa stripe including an active layer is buried with P-type and n-type semiconductor layers. This is disclosed in the patent application by Mr. Kitamura et al. Details on “-166666”.

This embodiment is manufactured as follows. The n-InP buffer layer 11 and the non-doped InGaAsP active layer are formed on the n-InP substrate 10 by P-InP by liquid phase or vapor phase growth method.
A photoresist is applied to the DH substrate on which the clad layer 13 is laminated, and a wafer having the shape shown in FIG. 1 is manufactured by ordinary photolithography and etching. next,
This wafer was subjected to liquid phase epitaxy by a P-InP first current blocking layer 14 and an n-InP second current blocking layer 1.
5, the P-InGaAsP etching layer 16, the P-InP burying layer 17, and the P-InGaAsP cap layer 18 are sequentially formed. As shown in FIG. 1, two second and third grooves 22 for forming the mesa stripe 20 including the active layer 12,
23 is a straight line on the side of the mesa stripe 20, the mesa stripe 2
The side away from 0 has a wide portion 24 and a narrow portion 25. In the second crystal growth, in the wide portions 24 of the second and third grooves 22 and 23 forming the mesa stripe 20, as shown in FIG. 2, the first and second current blocking layers 14 and 15 does not grow on Mesastlive 20. On the other hand, in the narrow portion 25 of the second and third grooves 22 and 23, the first current block layer 14 fills the inside of the second and third grooves 22 and 23. The shape near the mesa stripe just before growing 15 becomes flat.
Therefore, the second current blocking layer 15 is the mesa stripe 2
It covers the whole area without interruption at the top of 0. Similarly, the etching layer 16 also entirely covers. After crystal growth, AuZn is vapor-deposited on the cap layer 18 in order to make ohmic contact on the P side. Further, a photoresist is applied, and AuZn, the cap layer 18, the burying layer 17, and the etching layer 16 immediately above the narrow portion 25 of the second and third grooves 22 and 23 are sequentially removed by ordinary photolithography and etching to resonate. The first groove 21 is formed so that the P-side electrode is divided into two in the device axis 30 direction. For etching AuZn, use a mixture of KI + I ₂ , for etching InGaAsP cap layer 18, etching layer 16 using a mixture of H ₂ SO ₄ + H ₂ O ₂ , and for etching LnP buried layer 17, use HC 1 + H. A dilute solution of ₂ O was used. Since H ₂ SO ₄ + H ₂ O ₂ and HC 1 + H ₂ O hardly etch other materials, the semiconductor layer directly below the first groove becomes the second current blocking layer 15. Then Au
Alloy Zn. Next, the n-InP substrate 10 is polished to 100-
After setting the thickness within the range of 150 μm, AuGeNi is vapor-deposited and alloyed for the n-side ohmic contact, and the wafer fabrication is completed. This wafer is first cleaved by the usual cleaving method.
A device is manufactured by forming a resonator surface at a right angle to the mesa stripe 20 so that the first and second P-side electrodes 26 and 27 are divided by the groove 21. When the first and second P-side electrodes 26, 27 of this element are positively biased and the n-side electrode 28 is negatively biased, this element may exhibit stable bistable operation in LI characteristics and optical input / output characteristics. it can. The first P-side electrode 26 and the second
The P-InP clad layer 13 electrically bridges the P-side electrode 27, but since it is adjacent to the active layer, most of the injection current flows to the active layer. Therefore, when the width of the first groove 21 was 25 μm, the inter-electrode resistance on the P side could exceed 1 KΩ. Because of this level of resistance, the first and second
The injection currents to the P-side electrodes 26 and 27 of (1) did not affect each other, and a uniform optical bistable operation could be realized with a high yield in the range of 20 to 40 mA. The size of the optical bistable semiconductor laser of this embodiment is such that the width of the mesa stripe 20 is 1.5 μm,
The width of the wide groove portion 24 is 7 μm and the width of the narrow portion 25 is 3 μm.
The length of the narrow groove portion 25 is 20 μm, the length of the first P-side electrode 26 is 100 μm, and the length of the second P-side electrode 27 is 150 μm. Since the state of crystal growth greatly varies depending on the growth method, growth conditions, etc., it goes without saying that appropriate dimensions should be adopted together with them. Also the first P
The ratio of the lengths of the side electrode 26 and the second P-side electrode 27 in the resonator axis 30 direction is not limited.

In the above embodiment, the second groove immediately below the first groove 21 is used.
The second and third methods for forming the current blocking layer 15 over the entire surface including the upper portion of the mesa stripe 20 are as follows.
The grooves 22 and 23 of the above have a straight line on the side of the mesa stripe 20 and a wide portion 24 and a narrow portion 25 on the side away from the mesa stripe 20, but a straight line on the side away from the mesa stripe 20 Wide part 24 on the side of
And a narrow portion 25 may be provided. Also in this case, if the portion directly under the first groove 21 corresponds to the narrow portion 25, the same crystal growth as in the above-described embodiment can be performed, and there is no difference in the characteristic surface. In the embodiment, the P-side electrode has a AuZn full-surface electrode structure, but it may have an oxide stripe structure or P-
Instead of the InGaAsP cap layer 18, an n-InGaAsP cap layer may be converted into the P layer by, for example, Zn diffusion only near the upper surface of the mesa stripe 20. Although InP / InGaAsP-based semiconductor materials are used in the above embodiments, GaAlAs
Other semiconductor materials such as / GaAs may be used.

Further, although the embedded stripe structure is adopted in the embodiment, another stripe structure may be used.

[Brief description of drawings]

FIG. 1 is a plan sectional view of an embodiment of the present invention, and FIG.
(a) and (b) are partial sectional views of FIG. 1, and FIG. 3 is a perspective view of an embodiment of the present invention. In the figure, 10 ... n-InP substrate, 11 ... n-InP buffer layer, 12 ...
InGaAsP active layer, 13 ... P-InP clad layer, 14 ... P-
InP first current blocking layer, 15 ... n-InP second current blocking layer, 16 ... P-InGaAsP etching layer, 17 ...
... P-InP buried layer, 18 ... P-InGaAsP cap layer, 20 ... Mesa stripe, 21 ... First groove, 22 ... Second groove, 23 ... Third groove, 24 ... Wide part, 25 ……
Narrow part, 26 ... first P-side electrode, 27 ... second P-side
Side electrode, 28 ... N-side electrode, 30 ... Resonator axis, respectively.

Claims (1)

[Claims] 1. A semiconductor laser in which an electrode is divided into a plurality of electrodes in the axial direction of a cavity by a groove, and a first laser is provided above an active layer to interrupt current injection into a region of the active layer immediately below the groove.
And a second semiconductor layer having a conductivity type different from that of the first semiconductor layer and having a different material composition from the first semiconductor layer except at least the groove portion. The second semiconductor layer is a semiconductor layer necessary for completely removing the second semiconductor layer in the groove forming portion and leaving the first semiconductor layer in the etching step at the time of forming the groove. An optical bistable semiconductor laser characterized by: