JPH0422034B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a bistable semiconductor laser exhibiting bistable operation, which is one of the most important components as an optical functional element used for optical exchange and optical information processing.

Among optical functional devices, bistable devices have a wide range of applications such as optical logic, optical switches, optical storage, optical amplification, and shaping of optical signal waveforms, and are expected to be used as devices that take advantage of the essential characteristics of light. Fundamental studies are currently underway.

The operation of bistable devices has been confirmed using various materials such as electro-optic crystals and liquid crystals, but devices using semiconductor materials are attracting particular attention as they can best take advantage of their high speed and integration potential. ing. Regarding this, for example, Mr. Lassiar (GJ
SOLID STATE ELECTRONICS (SOLID・STATE・
The possibility of bistable operation in semiconductor lasers was theoretically estimated in a paper published in 1964, Vol. Mr. (KYLAU) et al. applied physics
APPLIED PHYSICS LETTERS
A tandem-type buried heterostructure bistable semiconductor laser was reported in a paper published in 1982, Vol. 40, p. 369 of the Japanese Journal. In this semiconductor laser, the electrode near the active layer is divided into two parts by a groove: an optical amplification region where the gain exceeds the loss in the active layer, and a saturable absorption region where the loss exceeds the gain because no current is injected. It is divided into two areas. The groove merely separates the optical amplification region from the saturable absorption region. In this semiconductor laser, when the two electrodes are made common, the current-optical output characteristics of a normal semiconductor laser are exhibited, and therefore no bistable characteristic occurs. In order to obtain bistable characteristics, the current injected into the saturable absorption region is reduced to around -100 μA (in this case, the current injected into the optical amplification region is
The injection current must be approximately 300mA). This is considered to mean that the saturable absorption region behaves like a photodiode. However, if the current injected into the saturable absorption region fluctuates by several microamperes, the shape of hysteresis in the current-optical output characteristic and the current width indicating bistable operation will change significantly. Therefore, in order to utilize this bistable operation in optical functional devices, it was necessary to extremely improve the controllability of the injection current. As a countermeasure to this problem, the inventors of the present invention presented the 1985 National Conference of the Institute of Electronics and Communication Engineers, and the tandem electrode structure DC-BBH (DC- PBH) There is a semiconductor laser. In this bistable semiconductor laser, the groove not only separates the electrodes, but also serves as a current non-injection region where the injection current is prevented from going around directly under the groove. Therefore, even if the two divided electrodes are shared, bistable operation is observed in the current-light output characteristics due to the presence of the groove. In addition, by combining the currents injected into the separated electrodes, the shape of hysteresis in the current-optical output characteristic, the shape of the hysteresis in the current-optical output characteristic that shows bistable operation, and the current width that shows bistable operation can be adjusted to a desired size. Furthermore, the injection current can be controlled within a fluctuation range of about 0.5 mA. However, the yield of bistable semiconductor lasers having such characteristics is not necessarily good, and recent experimental results have revealed that the cause of this is the etching depth when forming the grooves. If the etching is insufficient, there is almost no difference from the characteristics shown by Low et al. in the conventional example, and if the etching becomes excessive and reaches the active layer, laser oscillation will no longer occur. Therefore, in order to increase the yield, it was necessary to stop the formation of grooves by etching at a certain appropriate depth.

An object of the present invention is to improve the yield of bistable operation and provide a bistable semiconductor laser having uniform characteristics.

According to this invention, at least the mesa stripe including the active layer formed by two substantially parallel first and second grooves reaching the active layer is separated from at least the semiconductor layer above the mesa stripe. In a semiconductor laser embedded with semiconductor layers of different conductivity types, a semiconductor etching stop layer of the same conductivity type but with a different material composition is laminated on the upper layer of the semiconductor cladding layer directly above the active layer, and an electrode near the active layer is laminated. A bistable semiconductor laser is obtained, characterized in that a third groove dividing the semiconductor layer into two or more in the direction of the cavity axis is etched up to just before the semiconductor etching stop layer.

In this invention, in a semiconductor laser having two or more electrodes divided in the direction of the cavity axis, it is possible to form a groove, which is important for bistable operation, or to make the width and depth of the groove to a desired size with good reproducibility. It is in. The width of the groove can currently be formed with an accuracy of ±1 μm using conventional photolithography technology. However, the depth of the groove varies from wafer to wafer after crystal growth.
Furthermore, even within the same wafer, the reproducibility of the depth from the active layer to the electrodes is somewhat poor due to variations from location to location.
To improve this, a semiconductor etching stop layer with a material composition different from that of the surrounding semiconductor layer is provided between the active layer and the electrode, and when forming the third groove, the semiconductor etching stop layer is etched before the semiconductor etching stop layer. It is sufficient to completely remove the semiconductor layers up to that point. If such processing is performed, the depth from the groove to the active layer will be approximately constant, and variations in characteristics can be reduced. Since the depth from the active layer to the groove is less than 1 μm, the leakage current from the electrode to the active layer directly under the groove, which is the non-current injection region, is significantly reduced, and the reproducibility of bistable operation is almost the same as the groove size. It's decided. Since the groove width can be easily controlled, bistable semiconductor lasers with uniform characteristics can be obtained with high yield.

The present invention will be described in detail below with reference to the drawings.
FIG. 1 shows a perspective view of a bistable semiconductor laser which is a first embodiment of the present invention. A planar type buried heterostructure semiconductor laser is one in which a mesa stripe including an active layer is buried with p and n type semiconductor layers. I am familiar with 1st
The embodiment is fabricated as follows. First, an n-InP substrate 10 is grown by liquid phase or vapor phase growth method.
On top, an n-InP buffer layer 11 and a non-doped
A photoresist is applied to a DH substrate on which an InGaAsP active layer 12 and a P-InP cladding layer 13 are laminated, and a wafer having first and second grooves 14 and 15 is manufactured by ordinary photolithography technology. Next, this wafer is grown using liquid phase growth technology.
A first current blocking layer 16 of InP and a second current blocking layer 17 of n-InP are sequentially formed. In this case, since the width of the mesa stripe 18 is usually narrow, 1 to 2 μm, the first and second current blocking layers 16 and 17 are not grown on the mesa stripe 18. After the second current blocking layer 17 is grown, The first and second grooves 14 and 15 are completely filled and the mesa stripe 18
The area around the top is a depression that is almost flat. Next, p- which is important for controlling the depth of the third groove 18 by etching.
InGaAsP etch stop layer 19 followed by p
-InP buried layer 20 and p-InGaAsP cap layer 21 are formed. After crystal growth, AuZn is deposited on the cap layer to establish ohmic contact on the p side. Further, a photoresist is applied and a third groove 30 is formed by ordinary photolithography and etching so as to divide the p-side electrode 22 into two in the direction of the resonator axis. in this case
AuZn is removed with a mixed solution of KI+I ₃ , the cap layer 21 is removed with a sulfuric acid+hydrogen peroxide based etching solution, and the buried layer 20 is further removed with a nitric acid based etching solution. Since the nitric acid-based etching solution etches InP but not InGaAsP, etching stops when the p-InGaAsP etching stop layer appears on the surface, and the third groove 30 is formed. Next, AuZn is alloyed. Next, polish the n-InP substrate 10 to 120 μm.
After achieving a certain thickness, Au-Ge-Ni is vapor-deposited for the n-side ohmic contact and alloyed to complete the wafer fabrication. By using this wafer conventional cleavage method, a device is manufactured by forming a resonator at right angles to the mesa stripe 18 so that the p-side electrode 22 is divided by the third groove 30. When this element is biased with the p-side electrode 22 divided into two parts as positive and the n-side electrode 23 as negative, this element functions as a bistable semiconductor laser having two levels that are stable with respect to current input or optical input. This is due to the following reason. In this bistable semiconductor laser, the third groove 30 serves as a current non-injection region, so the portion directly below the third groove 30 is considered to be a saturable absorption region. The saturable absorption region has the property that when the photon density or carrier density is sufficiently small, it acts as a loss having a certain finite magnitude, and when it becomes large to a certain extent, the loss nonlinearly becomes zero. Because this bistable semiconductor laser is a buried heterostructure, it can easily be operated at room temperature and with low operating currents. Also the third
Since the groove 30 is a saturable absorption region, by appropriately combining the currents injected into the p-side electrode 22 divided into two and changing the size of the saturable absorption region, the width of the bistable operation, for example, the current width can be adjusted. can be changed. In the first embodiment, when the currents injected into the two divided p-side electrodes 22 are equal, the threshold for starting oscillation is 40 mA at room temperature, and the current width indicating bistable operation at this time is 10 mA. In addition, the p-side electrode 22
By combining the injection currents, the current width exhibiting bistable operation could be varied in the range of 1 to 30 mA. These characteristics apply from the third groove 30 to the active layer 12.
Since the depth has been improved so that each element has the same depth, it is now possible to obtain relatively uniform characteristics instead of the uneven characteristics as in the past. The size of the bistable semiconductor laser in this example is two p-side electrodes 22 with a mesa stripe width of 1.5 μm.
The lengths of the grooves are 100 μm and 150 μm, respectively, and the width of the third groove 30 is 25 μm. Since the appearance of crystal growth varies greatly depending on the growth method, growth conditions, etc., it goes without saying that appropriate dimensions should be adopted in conjunction with these factors.

Next, FIG. 2 shows a sectional view of a bistable semiconductor laser which is a second embodiment of the present invention. This second
In this example, the p-
Instead of growing the InGaAsP etch stop layer 19 after the n-InP current blocking layer 17, the final step of forming the DH crystal is the p-InP current blocking layer 17.
This is performed subsequent to the growth of the InP cladding layer 13. Therefore, when forming the first and second grooves 14 and 15, the etching stop layer 19 is removed as shown in FIG. Other steps are the same as in the first embodiment. In this example as well, as in the first example, the yield of bistable operation was improved and uniform characteristics were obtained.

In the above embodiment, the p-side electrode has a full-surface electrode structure of AuZn, but in order to increase the resistance between the two p-side electrodes 22, an oxide strip structure may be used, or an n-InGaAsP cap layer may be used instead of the p-InGaAsP cap layer 21. The layer may be grown and converted into a P layer by, for example, Zn diffusion only near the top surface of the mesa stripe 18. Furthermore, although InP/InGaAsP-based semiconductor materials were used in the above embodiments, other semiconductor materials such as GaAlAs/GaAs-based materials may also be used. Further, in the above embodiment, the third groove 30
The p-side electrode was divided into two parts in the above, but by creating two or more grooves corresponding to the third groove 30 and dividing the p-side electrode into three or more parts, it is possible to make the p-side electrode bistable or even multistable. It may also have characteristics.

[Brief explanation of drawings]

FIG. 1 is a perspective view of the first embodiment of the present invention;
The figure is a diagram showing a cross-sectional view of the second embodiment. In the figure, 10... n-InP substrate, 11
... p-InP cladding layer, 12 ... InGaAsP active layer, 13 ... p-InP cladding layer, 14 ... first
groove, 15...second groove, 16...p-InP first
current blocking layer, 17... n-InP second current blocking layer, 18... mesa stripe, 19...
p-InGaAsP etching stop layer, 20...
p-InP buried layer, 21... p-InGaAsP cap layer, 22... p-side electrode, 23... n-side electrode,
30...represents the third groove, respectively.

Claims (1)

[Claims] 1. Two almost parallel first wires that reach the active layer,
In a semiconductor laser in which a multilayered mesa stripe including the active layer sandwiched between second grooves is embedded with at least a semiconductor layer of a conductivity type different from the semiconductor layer above the mesa stripe, a semiconductor cladding layer directly above the active layer is provided. A semiconductor etching stop layer of the same conductivity type but different material composition is laminated on the upper layer, and a third groove is formed to divide the electrode near the active layer into two or more in the resonator axis direction. A bistable semiconductor laser characterized by being etched to the front of the top layer.