JPH0738195A - Bistable semiconductor laser and manufacture thereof - Google Patents

Abstract

PURPOSE:To make it possible to perform high-speed setting and resetting and to make it possible to perform the repeating operation at several tens of gigahertz by using a multiple-quantum well layer, which is grown at the specified temperature lower than crystal growing temperature in an ordinary method, as saturable absorbing region. CONSTITUTION:A bistable laser part 10 is constituted of a p-side electrode 1 in a gain region, a p-side electrode 2 in a saturable absorbing region, a p- InGaAs cap layer 3, a p-type InP clad layer 4, an MQM waveguide layer 5 comprising an InGaAs well layer, or InGaAsP or InP barrier layer, an n-type InP clad layer 6, an n-type InP substrate 7, an n-side electrode 8 and a saturable absorbing region 9. At this time, the crystal growth of the laser part is performed at the ordinary substrate temperature. The growth of the saturable absorbing region 9 is performed at the considerably lower temperature in comparison with the laser part, i.e., 150-400 deg.C. Thus, the repeating operation of several tens of gigahertz can be realized.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a bistable semiconductor laser which performs optical logic / optical switching operations and can be used in optical switching / optical repeaters expected to constitute optical communication / optical information systems, and a method of manufacturing the same. It is about.

[0002]

Bistable semiconductor lasers having hysteresis in current-to-optical output characteristics and optical input / output characteristics have features such as a high-speed optical switch, optical logic operation, and optical memory, and therefore function to configure optical communication / optical information system. Expected as a device.

FIG. 1 shows a conventional structural view (cross-sectional view) of a bistable semiconductor laser (Japanese Patent Laid-Open No. 62-296573) having a multiple quantum well structure as an active layer. In the figure, 12 is a p-side electrode in the gain region, 13 is a p-side electrode in the saturable absorption region, 1
4 is p ⁺ -InGaAs cap layer, 15 is p-type InP
Clad layer, 16 MQW waveguide layer (composed of InGaAs well layer, InGaAsP or InP barrier layer), 17 n-type InP clad layer, 18 n-type InP
The substrate 19 is an n-side electrode. Also, 20 is a lens, 2
Reference numeral 1 is external injection light. The groove between the gain region 12 and the saturable absorption region 13 is formed by the InGaAs cap layer 14 and the p-type I.
A part of the nP clad layer 15 is removed by means such as chemical etching, or proton implantation or Ga FIB is performed.
(Focused Ion Beam) is formed by increasing the resistance by implantation.

FIG. 2 shows the relationship between the light output Lout and the injection current (hereinafter referred to as gain current) Ig to the gain region of the bistable laser portion of the laser. The oscillation wavelength of the bistable laser is located on the longer wavelength side than the two-dimensional exciton absorption peak wavelength due to the bandgap reduction effect. Here, when the applied voltage V _C to the saturable absorption region or the gain quenching region is changed from the forward bias to the reverse bias, the quantum confined Stark effect causes 2
The absorption peak wavelength of the three-dimensional exciton shifts to the long wavelength side, and the absorption amount increases. Therefore, when the applied voltage to the saturable absorption region or the gain quenching region is decreased, the oscillation threshold current and the hysteresis width both increase as shown in the figure. At this time, the two-dimensional excitons play a role as a saturable absorber. Here, the applied voltage V _C of the saturable absorption region 13 is set to a value at which the current-optical output characteristic of the bistable laser exhibits hysteresis as shown in (3) of FIG.
Set. In the figure, for the purpose of performing a memory operation, the saturable absorption region,
The applied voltage in the gain quenching region is set.

FIG. 3 shows the relationship between the externally injected light intensity and the output light intensity when light is externally injected into the device shown in FIG.
Shown in. Since the bias current in the gain region is set to I _g ′ shown in FIG. 2, when an optical pulse having a peak intensity _{equal to} or higher than the threshold input light L _th is input, the bistable laser oscillates even after the input light disappears. Represents a memory characteristic that continues.

Here, in order to reset the bistable semiconductor laser that has once entered the oscillating state, the applied voltage to the saturable absorption region is reduced, and the carriers excited in the conduction band in that region are extracted to saturate. It is necessary to restore the absorption that was being done. However, the carrier lifetime of a semiconductor takes several nanoseconds in the non-biased state, and therefore, it is several tens of nanoseconds.
Repetitive operation of gigahertz is difficult. Even when a reverse bias voltage is applied and the carrier extraction effect by the electric field is used, there is a limit because the oscillation threshold current of the bistable laser increases and driving itself becomes difficult. Therefore, it is essential to use a material that has a short carrier life in the non-biased state.

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to provide a bistable semiconductor laser which performs a memory operation, which can be set and reset at high speed, and which realizes repetitive operation of several tens of GHz, and a bistable semiconductor laser thereof. It is to provide a manufacturing method.

[0008]

In order to solve the above-mentioned problems, the bistable semiconductor laser of the present invention is first of all:
In a bistable semiconductor laser having a multi-quantum well structure as an active layer, the saturable absorption region is a multi-quantum well layer grown at 150 ° C. to 400 ° C. which is lower than the crystal growth temperature usually performed. It has a feature.

Further, in the laser according to the present invention, the saturable absorption region is grown to a level of a normal growth temperature (for example,
It is characterized by being annealed at 500 ° C.

Furthermore, the present invention is characterized in that the saturable absorption region is doped with a p-type element or Be during its growth.

On the other hand, the method of manufacturing a bistable semiconductor laser of the present invention is characterized by first growing a multiple quantum well layer formed in a saturable absorption region at 150 ° C. to 400 ° C.

Further, the manufacturing method of the present invention is characterized in that a p-type element or Be is added as a dopant to the multiple quantum well layer.

Further, the manufacturing method of the present invention is characterized by including a step of annealing the quantum well layer after forming the multiple quantum well layer.

[0014]

Conventionally, as is well known, the growth of the quantum well layer forming the saturable absorption region is performed at about 500 ° C. by a gas source molecular beam epitaxy (MBE) apparatus. In the quantum well layer thus formed, the excited carriers are dominated by the radiative recombination process. The carrier lifetime due to this radiative recombination process is extremely long and is several to several tens of nanoseconds. Therefore, as described above, it is difficult for the bistable semiconductor laser using such a quantum well layer as a saturable absorption region to repeatedly operate at several tens of gigahertz.

On the other hand, as in the present invention, when the multiquantum well layer forming the saturable absorption region is grown at a temperature of 150 ° C. to 400 ° C. at a lower growth temperature, it is recombined to a deep level. It is considered that the center is formed, and therefore the carrier lifetime is accelerated to about 100 picoseconds. If the growth of this quantum well layer is performed at less than 150 ° C., it is considered that the wavelength change of absorption due to excitons does not occur. Therefore, it is difficult to use it as a growth temperature. Further, when the growth temperature exceeds 400 ° C., the carrier life begins to become long, so that the growth temperature exceeding 400 ° C. cannot be used. Further, if a p-type dopant or Be is introduced during growth, the carrier life can be reduced to about 1 picosecond. When the grown quantum well layer is annealed at a temperature of about 500 ° C., the doped acceptor is activated and the carriers generated during the low temperature growth are compensated, so that the quantum well layer has an extremely high resistance. be able to. Therefore, if this quantum well layer is used as an optical nonlinear material in the saturable region of the bistable semiconductor laser, it is possible to make a bistable semiconductor laser capable of extremely high-speed set / reset and capable of performing repetitive operation of several 20 GHz. It will be possible.

[0016]

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

The sectional structure of the bistable laser portion of this structure is shown in FIG.
Shown in. 1 is a p-side electrode in the gain region, 2 is a p-side electrode in the saturable absorption region, 3 is a p ⁺ -InGaAs cap layer, 4 is a p-type InP clad layer, and 5 is an MQW waveguide layer (InGa).
As well layer, InGaAsP or InP barrier layer), 6 is an n-type InP clad layer, and 7 is an n-type In
A P substrate, 8 is an n-side electrode, and 9 is a saturable absorption region. Further, 10 denotes a lens and 11 denotes externally injected light.

The saturable absorption region of this structure is not formed during the growth of the semiconductor multilayer film forming the laser portion as shown in the figure, and the portion corresponding to the saturable absorption region 9 is once chemically etched or made reactive. Ion etching method (RI
E: Reactive ion etching, reactive ion beam etching method (RIBE: React)
After removal by iv ion beam etching, etc., both regions are grown respectively by a molecular beam epitaxy (MBE) device, and then unnecessary portions of crystals grown on the gain region are removed. Is. At that time, the crystal growth of the laser portion is performed at a normal substrate temperature, and the saturable absorption region 9 is grown at a temperature considerably lower than that of the laser portion, that is, 150 ° C. to 400 ° C. .

The method of growing the saturable absorption region 9 and the characteristics of the resulting saturable absorption region will be described below.

The apparatus used was a well-known molecular beam epitaxy apparatus, and the growth was carried out under the following growth conditions. In addition, Be was doped as an acceptor.

(I) Group III source: In, Ga, Al (metal) (ii) Group V source: AsH ₃ gas (flow rate 2 cc
m) (iii) Degree of vacuum during growth: 1.3 × 10 ⁻⁵ Torr (iv) Substrate rotation speed: 20 rpm (v) Growth speed: 2.6 μm / h (vi) Growth temperature: 200 ° C. FIG. 5 shows the wavelength dependence of the absorption coefficient of an InGaAs / InAlAs quantum well layer grown at 200 ° C. by a gas source molecular beam epitaxy apparatus and doped with Be as an acceptor. The broken line shows the absorption spectrum at the normal growth temperature (non-dope), and the solid line shows the absorption spectrum at the low temperature growth. The substrate grown at a low temperature has a two-dimensional exciton which is slightly blurred as compared with a substrate grown at a normal growth temperature, but exhibits a characteristic step-like shape.

FIG. 6 shows an example in which the change in transmittance of the quantum well layer is measured by the pump probe method. This absorption recovery time reflects the carrier lifetime, meaning that the quantum well layer recovers to the initial state within a few picoseconds.

Under the same conditions as above, only the growth temperature is set to 15
The quantum well layer was grown by changing the temperature to 0 ° C, 200 ° C, 300 ° C, 400 ° C, and 600 ° C. When the carrier lifetime of the quantum well layer grown at each temperature was measured, the results shown in FIG. 7 were obtained. As is clear from the figure, the growth temperature is 4
When the temperature exceeds 00 ° C, the carrier life begins to increase, so the growth temperature is preferably 400 ° C or lower. Also, 150
There is no problem in the value of carrier life near ℃, but 150
If the temperature is lower than 0 ° C, there is a high possibility that the wavelength change of the absorption due to excitons will not occur, so that the growth below 150 ° C should be avoided.

Next, in order to investigate the influence of the change in the Be doping amount on the carrier lifetime, the Be doping amount alone is changed from 0 cm ^-3 to about 8 c under the same conditions as described above.
The quantum well layer was grown by changing the pattern to m ^{-3 in} four ways, and the carrier lifetime of each was measured. The result is shown in FIG. For comparison, a change in carrier lifetime with respect to a change in doping amount of the quantum well layer grown at a growth temperature of 500 ° C. is also shown in FIG. As is clear from the figure, in the growth at 200 ° C., the carrier life is drastically shortened by Be doping.

As described above, when the quantum well layer (saturable absorption region) is grown at a low temperature and then annealed at a normal growth temperature, the effect of reducing the carrier lifetime is enhanced. When a p-type dopant such as Be is introduced during low temperature growth of the saturable absorption region, the effect of reducing the carrier life is further enhanced. Although the absorption recovery time depends on the number of wells in the MQW structure, the barrier layer pressure, the potential height of the barrier layer, and the difference in materials, the laser part and the saturable absorption region are formed by separate growth. Therefore, independent structural design is possible, and the degree of freedom of creation is large.

Setting by external injection light of bistable laser,
FIG. 9 shows the reset operation by applying a reverse bias voltage to the saturable absorption region, and the change over time in the carrier density of the gain region and saturable absorption region at that time. Since the recovery of absorption saturation in the saturable absorption region is faster than 10 picoseconds, even if the applied voltage for resetting is shortened to that extent, the carrier density reaches a stable state, absorption is increased, and complete resetting is possible. It will be possible. Also, if the voltage applied to the saturable absorption region is set to be low so that the fluctuation of the carrier density is as small as possible, the interval between the reset voltage signal and the set light can be made extremely short, and as a whole, several tens of tens.
Repetitive operation with high gigahertz can be realized.

InGaAsP / InP as a crystal material
I have described the system, but InGaAs / In (Ga)
AlAs system, AlGaAs / GaAs system, InGaAs
/ GaAs strained superlattice system, InGaAs / InGaAsP
It goes without saying that the same effect can be realized even in the strained superlattice system.

Further, in the above embodiment, the gas source molecular beam epitaxy method was used for manufacturing, but the present invention can be applied to a normal molecular beam epitaxy method.

[0029]

As described above, according to the present invention, in a bistable semiconductor laser having a multiple quantum well structure as an active layer, the saturable absorption region is lower than the crystal growth temperature (150 ° C. It is formed by growth at 400 ° C. and is annealed at about the normal growth temperature after the growth of the saturable absorption region, or the absorption saturation is achieved by introducing a p-type element or Be during the growth of the saturable absorption region. By setting the recovery time of 1 to less than 10 picoseconds, it is possible to realize the repeated operation of several tens of GHz.

[Brief description of drawings]

FIG. 1 is a sectional view showing a configuration of a conventional bistable semiconductor laser.

FIG. 2 is a graph showing optical output characteristics with respect to gain current in a conventional bistable semiconductor laser.

FIG. 3 is a graph showing light input / output characteristics of a conventional bistable semiconductor laser.

FIG. 4 is a sectional view showing a structure of a bistable semiconductor laser of the present invention.

FIG. 5 is a graph showing the wavelength dependence of the absorption coefficient of the saturable absorption region of the bistable semiconductor laser of the present invention.

FIG. 6 is a graph for explaining an example of the present invention, and is a graph showing a measurement result of a saturable absorption recovery time of a Be-doped low temperature grown quantum well layer by a pump probe method.

FIG. 7 is a graph for explaining an example of the present invention and is a graph showing a relationship between a growth temperature of a quantum well layer used in a saturable absorption region of a bistable semiconductor laser of the present invention and its excited carriers. .

FIG. 8 is a graph for explaining the present invention, and is used for the saturable absorption region of the bistable semiconductor laser of the present invention and the Be doping amount in the quantum well layer and the relaxation to the initial state after excitation of the quantum well layer. It is a graph which shows the relationship with time (carrier life).

FIG. 9 is a graph showing an operating waveform of a bistable laser of the present invention and a change over time in carrier density.

[Explanation of symbols]

1 electrode in gain region 2 electrode in saturable absorption region 3 p ⁺ -InGaAs cap layer 4 p-type InP clad layer 5 InGaAs / InP or InGaAs / I
nGaAsP MQW layer 6 n-type InP clad layer 7 InP substrate 8 n-side electrode 9 saturable absorption region 10 lens 11 external injection light

Front page continuation (72) Inventor Hidetoshi Iwamura 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nihon Telegraph and Telephone Corporation

Claims (6)

[Claims] 1. An active layer having a multiple quantum well structure,
A bistable semiconductor laser in which a saturable absorption region is formed from the multiple quantum well layer, wherein the multiple quantum well layer is grown at 150 ° C to 400 ° C. 2. The bistable semiconductor laser according to claim 1, wherein a p-type element or Be is added as a dopant to the multiple quantum well layer. 3. The bistable semiconductor laser according to claim 1, wherein the multiple quantum well layer is annealed after growth. 4. A method of manufacturing a bistable semiconductor laser, comprising stacking a multiple quantum well structure as an active layer on a semiconductor substrate and forming the multiple quantum well layer in a saturable absorption region,
A method of manufacturing a bistable semiconductor laser, wherein the multi-quantum well layer is grown at 150 ° C to 400 ° C. 5. The method of manufacturing a bistable semiconductor laser according to claim 4, wherein a p-type element or Be is added as a dopant to the multiple quantum well layer. 6. The method of manufacturing a bistable semiconductor laser according to claim 4, further comprising a step of performing an annealing treatment after forming the multiple quantum well layer.