JPH05259567A - Waveguide type multiple quantum well light control element - Google Patents

Abstract

PURPOSE:To realize high speed response and improve coupling efficiency to an optical fiber, by forming a waveguide structure by sandwiching both sides of a multiple quantum well layer between third semiconductor layers whose defractive index is equal to or smaller than that of a first semiconductor layer, and applying a voltage parallel with the multiple quantum well layer from the outside. CONSTITUTION:On an insulating InP substrate 1, the following are grown; a non-doped InAlAs clad layer 2, an MQW optical waveguide layer 3 composed of 5 periodes of quantum well structure wherein an InGaAs layer is used as a well layer and an InAlAs layer is used as a barrier layer, a non-doped InAl clad layer 4, and an InGaAs cap layer 5. By using an SiN film as a mask, impurities are added to each of the layers in the manner in which one is different from the other in conductivity type on the mutual vertical planes, and a part 6 to which P-type impurities are added and a part 7 to which N-type impurities are added are formed in order to be able to apply a voltage parallel with the MQW layer 3 from the outside. A P electrode 8 is formed in the region 6 to which P-type impurities are added. An N electrode is formed in the region 7 to which N-type impurities are added.

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 capable of driving at a very high speed and a low voltage. Is.

[0002]

2. Description of the Related Art Optical devices for performing optical modulation and optical switching are required to have 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, semiconductor multiple quantum wells (hereinafter referred to as MQW) having good characteristics
By making use of the quantum size effect, it has been reported that optical modulators and 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 important points 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 7 V.
And the insertion loss is as large as 13 dB.
Usually, 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 narrow. As shown in FIG. 3, both sides of the MQW layer 13 are sandwiched by P and N highly doped layers 12 and 14, and an electric field is applied in a direction perpendicular to the MQW layer 13 by a P electrode 18 and an N electrode 19. This is because the quantum confined Stark effect is used. Further, if the MQW layer is made thin, light confinement becomes weak, and light leaks to the layer heavily doped with P and N, where it is absorbed by free carriers and propagation loss increases. On the other hand, an attempt to apply an electric field in a direction parallel to the MQW layer has been reported by D. S. Chemilla et al., Applied Physics Letters 42
Vol. (1983), p. 864 to p. 866, but required a voltage of 500 V or higher, which was not practical at all. This is because the structure shown in FIG. 4 was adopted to apply the electric field in the direction parallel to the MQW layer, so that the electric field was applied to the clad layer and the electric field strength was nonuniformly applied to the MQW layer. Is.

[0003]

FIG. 2 shows how the absorption coefficient changes when an electric field is applied to the MQW structure. In FIG. 2, (a) shows the case perpendicular to the layer, and (b) shows the case parallel to the layer. In the MQW structure, sharp absorption lines called excitons exist even at room temperature, and this is due to the application of an electric field.
In FIG. 2A, the half width is shifted to the long wavelength side while being slightly widened, whereas in FIG. 2B, the peak position is not changed and the half width is greatly widened in a low electric field. The strength of the electric field is smaller than that in the case of (a) by one digit or more, and so far, the mechanism shown in FIG. 2 (a) has been often used as described in the conventional art. 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 P-type and N-type impurity doped layers was 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.
As a result, there is a problem that the element capacitance and free carrier absorption are increased, the frequency response characteristics are not extended, and the propagation loss is increased. Further, in order to maintain the frequency response characteristics to some extent, the thickness of the MQW layer is set to a certain thickness or more, so that the spot diameter of light propagating in the waveguide becomes small and the coupling loss with the optical fiber becomes large. There was a problem.

According to the present invention, an electric field parallel to the MQW layer can be applied to independently optimize the element capacitance and spot diameter defined by the MQW layer thickness peculiar to the conventional modulators and switches of this type. The present invention aims to obtain a high-performance and compact waveguide-type multiple quantum well optical control device that operates at a low voltage, has a wide band, has a low insertion loss, and has high performance.

[0005]

The above object is to provide a first semiconductor layer having a first band gap and forming a barrier layer, and a second semiconductor layer having a second band gap and forming a well layer, In a waveguide type multi-quantum well optical control device having a multi-quantum well layer formed by alternately laminating, a third semiconductor layer having a refractive index on both sides of the multi-quantum well layer which is equal to or smaller than that of the first semiconductor layer. To form a waveguide structure, sandwich the waveguide structure in a plane perpendicular to each layer, and add impurities so that one of them is different in conductivity type from the other, thereby forming a multi-quantum well layer. This is achieved by applying an external voltage in parallel.

[0006]

In the structure of the present invention, the MQW layer is used as a waveguide structure, and the direction in which the electric field is applied is parallel to the MQW layer. Therefore, a large change in the absorption coefficient and the change in the refractive index can be obtained in a low electric field. Moreover, since the element capacitance is determined by the width of the MQW layer regardless of the thickness thereof, the element capacitance which is a limiting factor of the response speed is small and a high speed response is possible. Also, the above MQ
Since the W layer can be thinned, the mode spot diameter of light guided through the W layer can be widened, the coupling loss with the single mode fiber can be reduced (FIG. 6), 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 are formed by ion implantation or impurity diffusion to cause mixed crystal, and the refractive index and the band thereof are increased. Gap energy is made smaller and larger than that in the non-impurity-doped portion to improve lateral light confinement and longitudinal light confinement to control the thickness of the MQW layer. The spot diameter of the light wave propagating through the waveguide structure in the lateral direction and the spot diameter in the vertical direction are made substantially equal to each other to reduce the coupling loss.

[0007]

Embodiments of the present invention will now be described with reference to the drawings. FIG. 1 is a configuration diagram showing an embodiment of a waveguide type multiple quantum well optical control device according to the present invention, and FIG. 2 is a diagram showing a change in absorption coefficient when an electric field is applied to the multiple quantum well structure.
3B is a diagram showing a case of applying in the vertical direction, and FIG. 6B is a diagram showing a case of applying in the parallel direction.

In FIG. 1, non-doped InAlAs is formed on an insulating InP substrate 1 by metalorganic vapor phase epitaxy (MOVPE) or molecular beam epitaxy (MBE).
The cladding layer 2 is 0.3 μm thick and has a thickness of 7.5 nm.
Layer is a well layer and an InAlAs layer having a thickness of 5 nm is a barrier layer. The MQW optical waveguide layer 3 is composed of 5 periods, the undoped InAlAs clad layer 4 is 1 to 2 μm, and the InGaAs layer is InGaAs.
The cap layer 5 was grown to 0.5 μm. Next, using the SiN film as a mask, an impurity is added so as to sandwich a width of 1 to 4 μm in a plane perpendicular to each layer so that one of them has a conductivity type different from the other, and a voltage is applied from the outside in parallel to the MOW layer 3. So as to be applied, a portion 6 doped with a P-type impurity and a portion 7 doped with an N-type impurity are formed. At this time, the impurity-added portion is formed by ion implantation or impurity diffusion to cause mixed crystal, and its refractive index and band gap energy are made smaller and larger than those of the undoped portion, respectively. Keep it. The cap layer 5 on the region having no impurities is selectively removed to form the P-type impurity-added region 6 and the N-type impurity-added region 7,
The P electrode 8 and the N electrode 9 are formed respectively.

In the waveguide structure, the width not doped with impurities is set to 1 to 4 μm, and the thickness thereof is set to 0.05 to 0.5.
With a thickness of 3 μm, 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 can be made substantially equal. Further, it is desirable that the product of the length of the element which interacts with the light of the waveguide structure and the absorption coefficient of the light propagating through the waveguide structure exceeds 2.3.

The above embodiment is based on the InGaAs / InAlAs system MQ.
Although embodiments have been described for W structures, the present invention is not limited to MQW structures such as InGaAs / InP, InGaAs /.
InGaAsP, InGaAlAs / InAlAs, GaAs / Al
It goes without saying that it can also be applied to MQW structures such as GaAs.

[0011]

As described above, the waveguide type multiple quantum well optical control device according to the present invention comprises the first semiconductor layer having the first band gap and forming the barrier layer and the well layer having the second band gap. In a waveguide-type multi-quantum well optical control device having a multi-quantum well layer formed by alternately laminating second semiconductor layers to be formed, both sides of the multi-quantum well layer are equal to or more than the first semiconductor layer. Third with a small refractive index
The multiple quantum well is formed by sandwiching a semiconductor layer to form a waveguide structure, sandwiching the waveguide structure in a plane perpendicular to each layer, and adding impurities so that one of them has a conductivity type different from that of the other. By applying a voltage from the outside in parallel to the layers, the width of the waveguide is set to about 1 to 4 μm.
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 can be observed as compared with a conventional element in which an electric field is vertically applied to the MQW layer. Moreover, since the thickness of the MQW layer can be made independent of the element capacitance, the MQW layer thickness is made as thin as 0.05 to 0.3 μm, and the coupling efficiency with the optical fiber is applied with an electric field perpendicular to the conventional MQW layer. It can be made several times better than the device.

The above is intended for the intensity modulator using the change in absorption coefficient, but the change in the absorption coefficient has a relationship between the change in the refractive index and Kramers-Krenig, and the phase modulator using the change in the refractive index and the change in the refractive index are used. It is also possible to apply to an intensity modulator that utilizes the interference caused by.

[Brief description of drawings]

FIG. 1 is a configuration 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 changes in absorption coefficient when an electric field is applied to a multiple quantum well structure, where FIG. 2A shows a case where an electric field is applied in a vertical direction, and FIG. 2B shows a case where an electric field is applied in a parallel direction. It is a figure.

FIG. 3 is a schematic configuration diagram of a conventional guided multi-quantum well optical control device in which an electric field is applied to the multi-quantum well structure in a vertical direction.

FIG. 4 is a schematic configuration diagram of a conventional waveguide type multiple quantum well light control device in which an electric field is applied in a parallel direction to the multiple quantum well structure.

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

FIG. 6 is a diagram showing device performance of a conventional waveguide type multi-quantum well optical control device in which an electric field is applied vertically to the multi-quantum well structure.

[Explanation of symbols]

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

Claims (1)

[Claims] 1. A multi-layer structure in which a first semiconductor layer having a first band gap and forming a barrier layer and a second semiconductor layer having a second band gap and forming a well layer are alternately laminated. In a waveguide type multiple quantum well optical control device having a quantum well layer, a waveguide structure is formed by sandwiching both sides of the multiple quantum well layer with a third semiconductor layer having a refractive index equal to or smaller than that of the first semiconductor layer. Then, the waveguide structure is sandwiched in a plane perpendicular to each layer, impurities are added so that one of them has a different conductivity type from the other, and a voltage is applied from the outside in parallel to the multiple quantum well layer. A waveguide type multi-quantum well optical control device characterized by the above.