JPH08248363A - Waveguide type multiple quantum well optical control element - Google Patents

Abstract

PURPOSE: To obtain an optical control element which operates on a low voltage, has a broad band width, is low in insertion loss, has high performance and is small in size by specifying the thickness of well layers and introducing a specific tensile stress into these well layers. CONSTITUTION: A non-doped InP clad layer 2 is grown at 0.5μm, MQW optical waveguide layers 3 consisting of 20 layers of quantum well structures consisting of InGaAsP layers of a thickness of 11 to 14mn introduced with 0.3 to 0.5% tensile stress as the well layers and InP layers of a thickness of 5nm as barrier layers and a non-doped InP clad layer 4 at 1 to 2μm and an InGaAs cap layer 5 at 0.5μm by an org. metal vapor growth method, etc., on an insulatable InP substrate 1. Next, parts 6 added with a P type impurity and parts 7 added with an (n) type impurity are so formed that voltage can be impressed on the MQW layers 3 in parallel therewith from outside by growing the crystals added with the impurities in such a manner that the one electrical conductivity shape varies from the other across a width of 1 to 4μm within the plane perpendicular to the respective layers with an SiN film as a mask.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-performance compact waveguide type multiple quantum well optical control device which performs optical modulation, optical switching, etc., and has a high optical coupling efficiency that enables particularly high speed and low voltage driving. Is.

[0002]

2. Description of the Related Art Optical devices that perform optical modulation, optical switching, etc. are important in terms of high speed, low voltage driving, and low insertion loss. These are not independent of each other but depend on each other and are designed according to the application of the device. Due to the progress of crystal growth technology in recent years, a semiconductor multiple quantum well (MQW) having good characteristics
By making use of the quantum size effect, it has been reported that optical modulators and optical switches that are more efficient and smaller than conventional devices using bulk (for example, the Institute of Electronics, Information and Communication Engineers) Journal C-1, J74-C
-1, Volume pp. 414-420) does not satisfy the above three items at the same time, and the 3 dB band is as wide as 40 GHz or more, but the voltage required to obtain an extinction ratio of 20 dB is as high as 7 V and the insertion loss is as large as 13 dB. Has become. Normally, in this type of device, the bandwidth is limited by the element capacitance. To reduce the voltage, the MQW layer may be thinned, but as a result, the element capacitance increases and the bandwidth becomes narrower (FIG. 5). ). This is as shown in FIG.
This is because the quantum confined Stark effect is utilized by sandwiching the P and N highly-doped layers 12 and 14 on both sides of and applying an electric field in a direction perpendicular to the MQW layer 13. Also, M
If the QW layer is made thinner, the light confinement becomes weaker, and light leaks to the layer heavily doped with P and N, where it is absorbed by free carriers and the propagation loss increases as shown in FIG.

On the other hand, an attempt to apply an electric field in a direction parallel to the MQW layer has been made by D, C, and Chemmla et al., Applied Physics Letters. 4
Volume 2, pp. Although it was reported in 864-866 (1983) (FIG. 4), it required a voltage of 500 V or more, and was not practically usable at all. This is because the structure shown in FIG. 4 is adopted to apply the electric field in the direction parallel to the MQW layer.
This is because the electric field was applied to the clad layer and the electric field strength was nonuniformly applied to the MQW layer.

To solve this, we first
, And a second semiconductor layer having a second band gap, respectively, and a so-called multi-quantum well (MQ
W) on both sides of the multiple quantum well layer in the semiconductor device,
As shown in FIG. 7, a so-called waveguide structure is formed by sandwiching a third semiconductor having a refractive index equal to or smaller than that of the first semiconductor, and the waveguide structure is formed in a plane perpendicular to the above layers. We proposed a waveguide-type multi-quantum well optical control device in which impurities are added so that one of them has a different conductivity type from the other, and a voltage can be applied in parallel to the multi-quantum well layer from the outside. (Japanese Patent Laid-Open No. 05-259567). this is,
As expected, the device capacitance could be reduced, high-speed operation and high coupling efficiency could be achieved, but the exciton absorption oscillator strength was small,
Further, since the impurities are added by diffusion or ion implantation, the width of the waveguide is not constant, the applied electric field is non-uniform, and low voltage operation cannot be performed.

[0005]

FIG. 2 shows how the absorption coefficient changes when an electric field is applied to the MQW structure. FIG. 2 (a) is perpendicular to the layer, and FIG. 2 (b) is The cases parallel to the layers are shown. In the MQW structure, a sharp absorption line called an exciton exists even at room temperature, and it shifts to the long wavelength side while slightly widening its half-value width in FIG. 2 (a) by applying an electric field, whereas in FIG. 2 (b). In, the peak position does not change and the half-value width is greatly widened at a low electric field. The solid line, the broken line, and the dotted line in FIG. 2 indicate the degree of the applied electric field. The strength of this electric field is smaller than that in the case of (a) by one digit or more, and so far, as described in the above-mentioned conventional technique, the mechanism of FIG. 2 (a) has been widely used. In the above mechanism, in order to apply an electric field perpendicularly to the MQW layer, a structure (so-called PIN structure) in which the non-doped MQW layer is sandwiched by layers doped with P-type and N-type impurities is adopted. Therefore, as shown in FIG. 5, the thickness of the MQW layer limits the magnitude of the element capacitance and the voltage, and in order to obtain a constant electric field strength, the thickness of the MQW layer must be reduced. Brings about an increase in device capacity and free carrier absorption,
There is a problem that the frequency response characteristic does not extend and the propagation loss increases. Further, in order to maintain the frequency response characteristic to some extent, the thickness of the MQW layer is set to a certain thickness or more, so that the spot diameter of the light propagating in the waveguide becomes small and the coupling loss with the optical fiber is large. There was (Fig. 6). Furthermore, the carriers generated by light absorption must cross many hetero interfaces, and the operating speed may be limited by the sweep time of the carriers. In particular, in the case of a combination of materials having a high hetero barrier (for example, InP and InGaAs), pile-up of holes occurs at the hetero interface, and the deterioration of the response speed under high light input has been a problem. In order to prevent this, a material having a low hetero barrier must be used, which results in reduction of the quantum size effect.

In the present invention, an electric field parallel to the MQW layer can be applied, and the element capacitance and the spot diameter specified by the MQW layer thickness peculiar to the conventional modulators and switches of this type are optimized independently. In addition, it is an object of the present invention to realize a high-performance and small-sized light control element which has a high exciter absorption oscillator strength, operates at a low voltage, has a wide bandwidth and has a low insertion loss.

[0007]

[Means for Solving the Problems] The above-mentioned objects are the first and the second.
The first and second semiconductor layers having the respective band gaps, and the both sides of the multiple quantum well layer in the multiple quantum well semiconductor device having a multilayer heterostructure having a thickness smaller than the Bohr radius. Forming a sandwiched waveguide structure with a third semiconductor having a refractive index equal to or smaller than that of the semiconductor, and sandwiching the waveguide structure in a plane perpendicular to each of the layers, one of the other being the other. In a waveguide type multi-quantum well optical control device in which impurities are added so as to have different conductivity types and a voltage can be externally applied in parallel to the multi-quantum well layer, the well layer has a thickness of 11 nm to 14 nm.
And by introducing a tensile stress of 0.3% to 0.5% into this well layer.

In the above waveguide structure, the width not doped with impurities is 1 to 4 μm, and the thickness is 0.05 to 0.5.
6 μm is achieved by making the lateral spot diameter and the vertical spot diameter in the optical waveguide structure of the light wave propagating through the waveguide structure substantially equal.

That is, the refractive index and the energy band in the waveguide structure doped with impurities are made smaller or larger than those in the undoped region, respectively, to confine light in the lateral direction well and in the longitudinal direction. By controlling the thickness of the MQW layer for confining light, the spot diameter in the lateral direction of the optical waveguide structure and the spot diameter in the vertical direction of the light wave propagating in the waveguide structure are made substantially equal. further,
A tensile stress of 0.3% to 0.5% is introduced into the well layer, and the thickness of the well layer is set to 11 nm to 14 nm to increase the exciton absorption oscillator strength. Further, a material having a high hetero barrier is introduced into the barrier layer to increase the exciton absorption oscillator strength.

[0010]

In the present invention, the prior application, Japanese Patent Laid-Open No. 05-25, is used.
The purpose of No. 9567, namely, a large change in absorption coefficient at a low electric field, and the device capacitance does not depend on the MQW layer thickness, and therefore the spot diameter of light propagating in the waveguide can be increased by reducing the MQW layer thickness. , The point that the coupling loss with the optical fiber can be reduced (see FIG. 6) and that the highly doped layer is not above and below the MQW layer even if the light confinement is weakened by reducing the MQW layer thickness, The structure shown in FIG. 2B is adopted, which naturally includes the fact that the absorption does not increase, and also pays attention to the low voltage operation by increasing the exciton absorption. The result of a detailed logic study is shown in FIG. 8. This shows a change in oscillator strength of exciton absorption when stress is introduced into the quantum well layer using the well thickness as a parameter. By introducing the tensile stress, the oscillator strength of exciton absorption is significantly improved. At this time, the well thickness is 11
It is understood that it is only necessary to set the distance between nm and 14 nm and to introduce the tensile stress into this well layer from 0.3% to 0.5%.

FIG. 6 shows a single mode optical fiber and MQW.
The MQW layer thickness dependence of the coupling loss of light with the waveguide is shown. The spot diameter of the light propagating in the MQW waveguide is MQW.
As the layer thickness becomes thinner, it becomes larger and becomes closer to the spot diameter of the light propagating in the single mode optical fiber.
It can be seen that the coupling loss can be reduced.

In the structure of the present invention, the MQW layer is used for the waveguide structure, and the direction in which the electric field is applied is parallel to the MQW layer. Therefore, a large change in absorption coefficient and a large change in refractive index can be obtained in a low electric field. In addition, since the element capacitance is determined by the width of the MQW layer, not by the MQW layer thickness, the element capacitance which is the limiting factor of the response speed is small and a high speed response is possible. Further, since the MQW layer can be made thicker or thinner, the mode spot diameter of light guided therethrough can be widened, the coupling loss with the single mode fiber can be reduced, and the insertion loss of the element is small. Further, the P and N impurity-added layers provided to enable the voltage to be applied in parallel to the MQW layer have the same or smaller refractive index and bandgap energy than those of the undoped portion, or The light confinement in the lateral direction is good because it can be the same or larger. Furthermore, a tensile stress of 0.3% to 0.5% is introduced into the well layer, and the thickness of the well layer is set to 11 nm to 14 nm to increase the exciton absorption oscillator strength and a material having a high hetero barrier. Is introduced into the barrier layer to increase the exciton absorption oscillator strength, a large absorption coefficient change and refractive index change can be obtained in a low electric field, and the response speed under high light input does not deteriorate. A material having a high hetero barrier can be used, and the quantum size effect can be effectively used.

[0013]

Embodiments of the present invention will now be described with reference to the drawings. FIG. 1 is a diagram showing an embodiment of a waveguide type multiple quantum well light control device according to the present invention. On the insulating InP substrate 1, a non-doped InP clad layer 2 having a tensile stress of 0.5 μm and a thickness of 12 nm is formed by metalorganic vapor phase epitaxy (MOVPE) or molecular beam epitaxy (MBE).
The InGaAsP layer containing 4% is used as a well layer and has a thickness of 5 nm.
The MQW optical waveguide layer 3 and the non-doped InP clad layer 4 each consisting of 20 quantum well structure layers using the InP layer as a barrier layer
˜2 μm, InGaAs cap layer 5 was grown to 0.5 μm. In addition, each of the above InP clad layers 2 and 4
Corresponds to the third semiconductor layer. Next, using the SiN film as a mask, a width of 1 to 4 μm is sandwiched in a plane perpendicular to each layer to grow crystals doped with impurities such that one has a conductivity type different from the other, and the MQW layer is externally applied. A portion 6 to which a p-type impurity has been added and a portion 7 to which an n-type impurity has been added are formed so that a voltage can be applied in parallel with. At this time, the refractive index and the band gap energy of the portion to which the impurities are added are set to be the same, smaller, or the same or larger than those of the portions to which the impurities are not added. Next, the cap layer 5 on the impurity-free region is selectively removed, and the p-electrode 18 and the n-electrode 19 are formed in the impurity-added regions 6 and 7, respectively.

The above embodiment is an InGaAsP / InP MQW.
Although described with respect to structures, the present invention is not limited to other MQW structures, such as InGaAs / InAlAs, InGaAsP / InGaAs.
P, InGaAs / InGaAsP, InGaAlAs / InAlA
It goes without saying that the present invention can be applied to MQW structures such as s, GaAs / AlGaAs and the like.

[0015]

As described above, the waveguide type multi-quantum well optical control device according to the present invention comprises the first and second semiconductor layers having the first and second band gaps, respectively, and has a thickness of Bohr. A waveguide structure in which both sides of the multi-quantum well layer in a multi-quantum well semiconductor device composed of multi-layered heterostructures thinner than the radius are sandwiched by a third semiconductor having a refractive index equal to or smaller than that of the first semiconductor. And sandwiching the waveguide structure in a plane perpendicular to the layers, adding impurities so that one of them has a different conductivity type from the other, and a voltage is applied in parallel to the multiple quantum well layer from the outside. In the waveguide-type multi-quantum well optical control device capable of applying a voltage, the thickness of the well layer is set to 11 nm to 14 nm, and the tensile stress is introduced to the well layer from 0.3% to 0.5%. , The width of the waveguide If set to about 1 to 4 μm, M
The strength of the electric field applied to the QW layer can be made appropriate, and the drive voltage operates at a maximum of several volts. At this time, since the element capacitance can be defined by the width of the waveguide, a high speed response of several times to one digit or more is observed as compared with the conventional element in which an electric field is applied perpendicularly to the MQW layer. Since the thickness of the MQW layer can be made independent of the device capacitance, the MQW layer thickness is set to 0.05 to 0.6.
By making the thickness as thin or thick as μm, the coupling efficiency with the optical fiber can be made several times better than that of an element in which an electric field was perpendicularly applied to the MQW layer. Furthermore, a tensile stress of 0.3% to 0.5% is introduced into the well layer, and the thickness of the well layer is set to 11 nm to 14 nm to increase the exciton absorption oscillator strength and a material having a high hetero barrier. Is introduced into the barrier layer to increase the oscillator strength of exciton absorption, a large absorption coefficient change and refractive index change can be obtained in a low electric field, and there is no deterioration in response speed under high light input. A material having a high hetero barrier can be used, and the quantum size effect can be effectively used.

Furthermore, although the present invention has been described for the intensity modulator using the change of the absorption coefficient, the change of the absorption coefficient is linked by the relationship between the change of the refractive index and the Kramers-Krenig, and the change of the refractive index is used. Phase modulator,
It goes without saying that the present invention can also be applied to an intensity modulator using interference caused by a change in refractive index.

[Brief description of drawings]

FIG. 1 is a diagram showing an embodiment of a waveguide type multiple quantum well light control device according to the present invention.

2A and 2B are diagrams showing a change in absorption coefficient when an electric field is applied to a multiple quantum well structure, wherein FIG. 2A is a vertical application and FIG. 2B is a parallel application. .

FIG. 3 is a diagram showing a configuration of a waveguide-type multi-quantum well optical control device that applies an electric field in a direction perpendicular to a conventional multi-quantum well structure.

FIG. 4 is a diagram showing a configuration of a waveguide-type multi-quantum well optical control element that applies an electric field in a direction parallel to a conventional multi-quantum well structure.

FIG. 5 is a diagram showing device performance of a waveguide type multiple quantum well optical control device when an electric field is applied in the vertical direction of a conventional multiple quantum well structure.

FIG. 6 is a diagram showing changes in coupling loss of a waveguide type multiple quantum well optical control device when an electric field is applied in the vertical direction of a conventional multiple quantum well structure.

FIG. 7 is a diagram showing a configuration example of a waveguide type multiple quantum well optical control device in which a waveguide structure is formed by sandwiching both sides of a multiple quantum well layer with semiconductor layers having different conductivity types.

FIG. 8 is a diagram for explaining the principle of the present invention.

[Explanation of symbols]

 2, 4 ... Third semiconductor (InP cladding layer) 3 ... Multiple quantum well layer 6 ... P-type impurity region 7 ... N-type impurity region

Claims (2)

[Claims] 1. A multi-quantum well semiconductor device comprising a first and second semiconductor layers having first and second band gaps, respectively, and having a multi-layered heterostructure having a thickness smaller than a Bohr radius. A third semiconductor layer having a refractive index equal to or smaller than that of the first semiconductor is formed on both sides of the multiple quantum well layer.
Form a sandwiched waveguide structure, sandwich the waveguide structure in a plane perpendicular to each layer, and add impurities so that one of them has a different conductivity type from the other, In a waveguide type multi-quantum well optical control device capable of applying a voltage in parallel to the multi-quantum well layer, the well layer has a thickness of 11 nm to 14 nm and a tensile stress of 0.3%. A waveguide type multi-quantum well optical control device characterized by being introduced from 0.5% to 0.5%. 2. The waveguide structure has a width not doped with impurities of 1 to 4 μm and a thickness of 0.05 to 0.6 μm.
2. The waveguide type multiple quantum well control device according to claim 1, wherein the spot diameter in the lateral direction and the spot diameter in the vertical direction of the light wave propagating in the waveguide structure are made substantially equal to each other. .