JPH07113991A - Optical modulation element - Google Patents

Abstract

PURPOSE:To provide the optical modulation element having a large coefft. of absorption and wide operation range by continuously changing the lattice constants of semiconductor thin films to constitute quantum wells in their lamination direction. CONSTITUTION:An (n) type semiconductor layer 11, an undoped strain quantum well structure 12, a (p) type semiconductor layer 13 and a (p) type semiconductor layer 14 for ohmic contact with electrodes are laminated on an (n) type semiconductor substrate 10. The cathode electrode 21 is formed on the rear surface of the substrate 10 and the anode electrode 22 is formed on the 4 (p) type semiconductor layer 14. The anode and cathode electrodes are opened with windows for passing light. The substrate 10 is formed of InP, the (n) type semiconductor layer 11 and the (p) type semiconductor layer 13 are formed of semiconductors of an InGaAlAs system or InGaAsP system having the operation wavelength of the strain quantum well structure, i.e., the energy gap larger than the energy gap of the material of the quantum wells. The lattice constants of the semiconductor thin films of the quantum well layers are continuously changed in their lamination direction in such a manner, by which the strain quantum wells changed in the quantity of the strain are formed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light modulator.

[0002]

2. Description of the Related Art In order to realize high-density information processing and information transmission utilizing the incoherence of light, an optical modulator or an optical switch that can be integrated two-dimensionally with high density is required.

FIG. 5 shows an example of an optical modulator utilizing the change in the amount of light absorption when an electric field is applied to the semiconductor quantum well structure. About this optical modulator, the physical review B magazine “Physical” published on July 15, 1985
Review B ", pages 1043-1060," Electric field depth "
Ndence of optical absorpt
ion near the band gap of
"quantum-well structure". As a specific structure, gallium arsenide (GaAs) is used for the quantum well and aluminum gallium arsenide (AlGaAs) is used for the barrier layer to form a multiple quantum well structure in which a large number of them are integrated. With AlGaAs, the structure is such that an electric field can be applied to the quantum well by applying scissors and applying a voltage. Similarly, by using indium gallium arsenide (InGaAs) for the quantum well and indium phosphide (InP) or indium aluminum arsenide (InAlAs) for the barrier layer, it operates at a wavelength of 1.55 μm used in optical communication. A light modulation element that operates is obtained. The phenomenon in which the absorption spectrum of the quantum well structure moves to the long wavelength side due to the electric field is known as the quantum confined Stark effect, and shows the relationship as shown in FIG. In the figure, among the quantum levels formed in the quantum well, light absorption (hereinafter abbreviated as heavy hole absorption) and electron due to the transition between the quantum first level and the heavy hole first level. The absorption associated with the transition between the first level and the first level of light holes (hereinafter, abbreviated as light proton absorption) is shown. When the wavelength of the incident light is set to a wavelength side slightly longer than the absorption peak wavelength when no electric field is applied, the absorption is small when the electric field is not applied and the light passes through the modulator, but the absorption peak is incident when the electric field is applied. Since the light coincides with the light wavelength, a large amount of light is absorbed, and it becomes difficult for light to pass therethrough, the light can be modulated.

However, when the absorption peak is moved to the long wavelength side by applying an electric field, the absorption function is also reduced due to the spatial separation of electrons and holes.
In the example of FIG. 6, since the absorption of heavy holes is always on the longer wavelength side than that of light holes, only the absorption by heavy holes is involved in the light modulation operation, and the absorption function depends on the electric field. To decrease. Therefore, in the example shown in FIG. 7, a semiconductor having a lattice constant different from that of the barrier layer is used for the quantum well layer, and tensile absorption is applied to the quantum well layer so that the absorption edge of the light hole is previously set to a longer wavelength side than the heavy hole. In this state, light holes and heavy holes are absorbed at the same wavelength under an electric field. As a result, the absorption function, which has been uniformly reduced by the application of an electric field, is to be increased by using the absorption of both heavy holes and light holes. The principle of operation of this optical modulator is described in "IE Journal of Q," published by June 1992, IEEE Journal of Quantum Electronics.
1496-1 of "quantum Electronics"
Article "Electroabsorbt" on page 507
Ion Enhancement in Tensil
e Strained Quantum Wells
via AbsorptionEdge Merge
g ”. As a specific structure, the quantum well layer has an indium composition of 0.49 and a gallium composition of 0.5.
When InP is used for the InGaAs layer and the barrier layer of No. 1 and the width of the quantum well is 12.1 nm, the absorption peaks of the heavy holes and the light holes coincide with each other when an electric field of 52 kV / cm is applied.

[0005]

However, when tensile strain is uniformly applied, there is only one point where the transition wavelengths of heavy holes and light holes overlap, and the absorption function is large in the heavy state. However, strict settings are required for both the operating wavelength and the applied electric field, and the range of operating conditions is narrow.

The object of the present invention is to eliminate the above-mentioned drawbacks,
An optical modulator having a large absorption coefficient and a wide operating range is provided.

[0007]

The optical modulator of the present invention comprises:
A semiconductor quantum well structure, a pn junction structure for applying an electric field to the quantum well structure, and an electrode, and a semiconductor thin film to be a quantum well is characterized in that the lattice constant continuously changes in the stacking direction. There is.

[0008]

In the present invention, the strained quantum well is formed by continuously changing the lattice constant of the semiconductor thin film of the quantum well layer in the stacking direction to change the strain amount. In the valence band, the band edge energies of light holes and heavy holes are the same in the unstrained state, but the two are separated according to the amount of strain applied, and in the case of tensile strain, light The band edge of the hole is on the lower energy side than that of the heavy hole, that is, on the long wavelength side,
On the contrary, in compressive strain, heavy holes are on the low energy side. As a result, if the amount of strain is continuously changed, the energy at the band edge can be made to have a gradient, and at this time, the gradient is opposite between heavy holes and light holes.

This strained quantum well is formed in a pn junction,
An electric field is applied to the strained quantum well by applying a reverse voltage. If the structure is such that the amount of tensile strain in the quantum well is continuously reduced from the side of the p-type semiconductor, the band edge of light holes tilts in the same direction as the electric field due to the effect of the strain described above, and the The band edge of the hole tilts in the opposite direction. The amount of movement of the absorption peak due to the electric field effect based on the quantum confined Stark effect is larger for heavy holes than for light holes, but the effect of the electric field is weakened for heavy holes due to the band tilt due to strain, and The amount of movement is small, but conversely, the light holes enhance the electric field effect and the amount of movement of the absorption peak is large. Therefore, by selecting the amount of change in strain and the thickness of the quantum well layer, the absorption peak wavelengths of heavy holes and light holes when an electric field is applied are made to coincide, and the movement amount of both absorption peaks in a wide range of the electric field is adjusted. It is possible to keep them almost equal.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the present invention will be described with reference to the drawings.

In actual light absorption, light absorption via excitons occurs at an energy of a few meV lower than the energy between the levels of electrons and holes. Absorption of.

FIG. 1 shows an embodiment of the optical modulator of the present invention.

An n-type semiconductor layer 1 is formed on an n-type semiconductor substrate 10.
1, undoped strained quantum well structure 12, p-type semiconductor layer 1
3 and the p-type semiconductor layer 14 for ohmic contact with the electrode are laminated, and the cathode electrode 21,
The anode electrode 22 is formed on the p-type semiconductor layer 14. Windows are opened in the anode and cathode electrodes to allow light to pass therethrough. As the semiconductor used for each layer in this embodiment, the substrate 10 is InP, and the n-type semiconductor layer 11 and the p-type semiconductor layer 13 are InGaA having an operating wavelength of a strained quantum well structure, that is, an energy gap larger than the quantum well material.
The semiconductor is an lAs-based or InGaAsP-based semiconductor.

FIG. 2 is an embodiment showing the structure of the strained quantum well structure portion 12 of FIG. 1, which is shown as an energy band diagram in a state where no electric field is applied. The barrier layer is InP30
In the quantum well layer 31, the In composition of InGaAs is 0.5.
The amount of strain is continuously changed from 3 to 0.40. The well width is 8.4 nm. FIG. 3 shows an energy band diagram of the strained quantum well structure of FIG. 2 at 80 kV / cm.
The electric field is applied. FIG. 4 shows the change in absorption peak wavelength due to heavy holes and light holes when an electric field is applied to the quantum well. Both of them show almost a wide range of electric field from 60 kV / cm to 100 kV / cm. The same electric field dependence is shown.

In the embodiment shown in FIG. 1, the material of the barrier layer of the strained quantum well structure is not only InP but also a material having a larger energy gap than InGaAs, such as InAl.
It is also possible to use GaAs or InGaAsP. Further, when the strained quantum well structure portion is made of InGaAsp or InGaAlAs having the same change of the lattice constant, it is possible to operate at a wavelength of 1.3 μm.

[0016]

According to the optical modulator of the present invention, an optical modulator having a large absorption amount and a wide operating range can be obtained.

[Brief description of drawings]

FIG. 1 is a diagram for explaining an embodiment of a light modulation element of the present invention.

2 is an energy band diagram of the strained quantum well structure of the embodiment of FIG. 1 of the present invention.

FIG. 3 is an energy band diagram when an electric field is applied to the strained quantum well structure.

FIG. 4 is a diagram showing light absorption of the light modulation element of FIG.

FIG. 5 is a diagram for explaining a conventional example of a light modulation element.

FIG. 6 is a diagram for explaining a conventional example of a light modulation element.

FIG. 7 is a diagram for explaining a conventional example of a light modulation element.

[Explanation of symbols]

 Reference Signs List 10 semiconductor substrate 11 n-type semiconductor layer 12 strained quantum well structure 13 p-type semiconductor layer 14 p-type semiconductor contact layer 21 cathode electrode 22 anode electrode 30 InP barrier layer 31 InGaAs strained quantum well layer

Claims (2)

[Claims] 1. A quantum well structure in which an undoped semiconductor thin film is sandwiched between undoped semiconductors having a larger energy gap than the semiconductor thin film, and a pn formed by a p-type semiconductor layer and an n-type semiconductor layer for applying an electric field to the quantum well structure. A junction structure is laminated on a semiconductor substrate, the pn junction structure has an electrode for applying a voltage, and the semiconductor thin film serving as the quantum well of the quantum well structure has a lattice constant continuous in the laminating direction. An optical modulator that is changing. 2. The lattice constant of the semiconductor thin film gradually changes from a smaller lattice constant to a larger or larger lattice constant of a semiconductor serving as a substrate, and the direction of the change is
2. The light modulation element according to claim 1, wherein the p-type semiconductor layer side has the smallest lattice constant, and the lattice constant increases toward the n-type semiconductor layer.