JPH0711656B2 - Optical bistable element - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an optical bistable element that realizes functions such as storage and calculation at high speed with respect to a low power optical input.

[Conventional technology]

An optical information processing system that realizes an information processing function by using light as a carrier of information effectively utilizes the characteristics of light such as wide band property, non-inductive property, and massively parallel information processing capability, and thus is used in the current electronic information processing system. It is expected to enable high-capacity, ultra-high-speed information processing that exceeds the limit. But,
At present, elements having sufficient characteristics as constituent parts such as optical arithmetic elements and optical storage elements necessary for realizing an optical information processing system have not been realized, and various materials and optical functions using the same have been realized. The device is being researched.

One of the most promising materials for optical functional devices is to sandwich an ultra-thin semiconductor layer (hereinafter referred to as a well layer) with a semiconductor layer having a larger energy gap (hereinafter referred to as a barrier layer). Composed of well structure semiconductors,
Alternatively, there is a multi-quantum well structure semiconductor formed by sequentially stacking a plurality of quantum well structures. Since the quantum well structure exhibits various interesting characteristics different from those of the semiconductor thick film, it has been attempted to realize an optical functional device by utilizing this. One of the interesting properties of a quantum well structure semiconductor is that it has a larger electric field effect than a thick film. That is, by applying an electric field in the direction perpendicular to the layer interface to the quantum well structure, there is an effect that a large change in the light absorption coefficient and the refractive index appears as compared with the thick film.

As a typical example of an optical bistable device that has been conventionally realized by utilizing the field effect of a multi-quantum well structure semiconductor, a self-electro-optic device (self-electro-optic device) is used.
vice) and the Applied Physics Letter (Ap
plied Physics Letter) Vol. 45, No. 1, pp. 13-15. In this device, electrodes are placed on the upper and lower surfaces of a p-i-n multi-layer structure in which a p-type semiconductor layer and an n-type semiconductor layer sandwich a low impurity concentration multiple quantum well structure semiconductor from above and below, respectively, and a resistor is provided between the electrodes. A direct-current power supply is connected to apply a reverse voltage. When not irradiated with light, the current flowing by the reverse voltage application to the pin multilayer structure is extremely small, and almost all the voltage of the DC power source is applied to the multiple quantum well structure semiconductor layer. When this p-i-n multilayer structure is irradiated with light of an appropriate wavelength, part of it is absorbed by the multiple quantum well layer to generate photocurrent, and the other part is transmitted. When the photocurrent flows, a voltage drop occurs due to the resistance interposed between the DC power supply and the pin multilayer structure, and the voltage applied to the multiple quantum well structure decreases. If the wavelength of the irradiation light is appropriately selected, a significant increase in the absorption coefficient is realized due to the electric field effect of the multiple quantum well layer as the voltage decreases. Since the increase of the absorption coefficient brings about the further increase of the photocurrent and the decrease of the applied voltage, a strong non-linearity with hysteresis appears in the relationship between the irradiation light and the transmitted light, and the self-electro-optic-device is an optical bistable device. Functions as an element.

[Problems to be solved by the invention]

An object of the present invention is to solve the following problems of the above-mentioned conventional technology. That is, the self
In order to operate an electro-optic device with light irradiation of relatively low power, it is necessary to use a resistor with a large resistance value of about 10 MΩ to generate a sufficient voltage drop with a small photocurrent. Value R and p-i-
The operating speed of the device, which is limited by the product (CR) of the electric capacitance C of the n multi-layer structure, is relatively low, about several hundred ns. That is,
The optical bistable device according to this conventional technique needs to use a large resistance value because it is necessary to generate a remarkable voltage change due to a weak photocurrent, and it is difficult to obtain a high-speed operation.

It is an object of the present invention to provide an optical bistable element that does not require the use of a large resistance value and can operate at high speed.

[Means for solving problems]

The optical bistable element of the present invention comprises a second semiconductor layer of a second conductivity type, a third semiconductor layer of a first conductivity type, and a second semiconductor layer of a second conductivity type on a first semiconductor layer of a first conductivity type. Has a Shockley diode structure in which electrodes are provided on the upper surface and the lower surface of a semiconductor multi-layer structure formed by sequentially stacking a fourth semiconductor layer of a mold,
In the vicinity of the bonding surface of the second semiconductor layer and the third semiconductor layer,
At least a part of at least one of these semiconductor layers has a single or multiple quantum well structure, and the light absorption edge of this quantum well structure is on the longer wavelength side compared to the light absorption edge of the external region. It is characterized by being located.

[Action]

According to the structure of the present invention, a single or multiple quantum well structure is installed near the center np junction of a pnpn type Shockley diode structure. As is well known to those skilled in the art, when a voltage is applied to a Shockley diode in the direction in which the electrode placed on the p-layer outside the Shockley diode is positive, this element is in an off state where almost no current flows in some cases. In other cases, it is in an on state in which a current flows. In the off state, most of the voltage applied between the electrodes of the Shockley diode is applied to the central np junction, but in the on state, almost no voltage is applied to this junction. According to the present invention, since a single or multiple quantum well structure is installed near this central np junction, the voltage applied to this quantum well structure differs greatly between the on state and the off state. The light absorption coefficient of the well structure portion greatly differs between the two states, reflecting the large electric field effect. As described in detail below, according to the configuration of the present invention, the Shockley diode of
Switching from the f state to the on state is also realized by irradiation with light having a wavelength absorbed in the quantum well structure, and if a proper wavelength is selected, the light transmittance of the electron well structure is changed by the voltage change due to this switching. In the on state after switching, it will be significantly larger than in the off state.
The switching of the voltage applied to this quantum well structure occurs due to the electrical nonlinearity of the Shockley diode, and it does not require the connection of a resistor with a large resistance value as in the conventional example. Can be made sufficiently small, and thus high speed operation can be realized.

Next, the operating principle of the optical bistable element according to the present invention will be described in more detail. The optical bistable element according to the present invention electrically functions as a Shockley diode. As is well known, from the off state of the Shockley diode o
Switching to the n-state occurs by increasing the forward applied voltage to this diode from zero to above a certain threshold voltage V ₂ . Further, the applied voltage of the diode in the on state is reduced to a constant threshold voltage V ₁ different from V ₂ to return from the on state to the off state. In general, V ₁ <V ₂ , so this current-voltage characteristic shows electrical bistability.

This switching from the off state to the on state can also be realized by light irradiation. Absorbed in the quantum well structure under a constant bias voltage Vb lower than the threshold voltage V ₂ (and for efficient operation of this optical bistable device, it is desirable to use the quantum well structure part). Irradiation with light of a wavelength (which is not absorbed in other regions) produces a photocurrent, but when this photocurrent reaches a sufficiently large value, the Shockley diode switches to the on state. Once you have very close set the bias voltage Vb to the threshold voltage V _2, very little photocurrent, thus Suichingu is realized by a very slight light radiation. Once switching to the on state occurs, as long as the bias voltage Vb is higher than V ₁ , the on state is maintained even if the light irradiation is stopped.

The operating principle of the optical bistable element according to the present invention is that bistable optical transmission characteristics of the element occur due to the switching to the on state by the light irradiation and the holding thereof. That is, in the off state, as described above, most of the voltage applied to the Shockley diode is applied to the np junction at the center thereof. Therefore, a high electric field is applied to the quantum well structure part installed near this junction,
As a result, the light absorption edge λ ₂ of the quantum well structure is moved to the longer wavelength side than the absorption edge λ ₁ when there is no electric field. Therefore, λ ₁
For signal light of wavelength λ such that <λ <λ ₂ , the quantum well structure portion has a strong optical absorption coefficient, and the light transmittance of this element is small. That is, the optical bistable element exhibits a low transmission state. However, when the power of the irradiation light increases above a certain value Pc, the shock current is switched to the on state by the photocurrent generated by this absorption. As a result, the electric field applied to the quantum well structure is significantly reduced, the light absorption coefficient for the irradiation light of wavelength λ (> λ ₁ ) is significantly reduced, and the device exhibits large light transmittance. That is, the light transmission characteristics of this optical bistable element cause switching to a high transmission state.

The reverse switching from the high transmission state to the low transmission state is achieved as follows. The bias voltage Vb applied to the Shockley diode is greater than V ₁ (Vb>
In the case of V ₁ ), the recovery is realized by reducing the voltage Vb to V ₁ or less. That is, in this case, the effect of light irradiation is stored until the voltage is reduced. Whether or not the transmission state has changed due to the signal light can be determined as the magnitude of the light transmittance by irradiating the reading light with a power smaller than Pc before the restoration by the voltage reduction. In this mode of operation, the optical bistable element can be used as an optical storage element that recovers by voltage switching.

On when the bias voltage Vb is lower than the threshold voltage V _1.
The optical bistable element in the state returns to the off state by reducing the optical power applied to the element to a certain threshold value P ₀ (P ₀ <Pc) different from Pc. An example of how to use the optical bistable element in this operation mode is as follows. Constant power Pb (P ₀ <Pb <Pc) for the optical bistable element
When the signal light Ps is radiated in the state of irradiating the bias light of Pb + Ps> Pc, the optical bistable element switches to the high transmission state. In this case, the high transmission state is maintained even after the signal light is stopped, but when the bias light is also stopped, the low transmission state is restored. That is, in this operation mode, the optical bistable element can be used as an optical storage element that is restored by stopping the bias light.

To summarize the above, this optical bistable device has a bistable optical transmission characteristic of the quantum well structure, which is accompanied by the electrical bistable state of the Shockley diode due to light irradiation. It is based on the operating principle.

〔Example〕

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 shows a sectional view of an optical bistable element according to a first embodiment of the present invention. In this embodiment, an n-type GaAs substrate 10 is used, and an n-type Al x Ga _1- x As layer 11 (impurity concentration 5 × 1
0 ¹⁷ cm ^-3 ), p-type AlxGa _1- xAs layer 12 (impurity concentration 5 × 10
¹⁷ cm ^-3 ), p-type multiple quantum well layer 13 (impurity concentration 5 x 10
¹⁵ cm ^-3 or less), n-type AlxGa _1- xAs layer 14 (impurity concentration 2 ×)
10 ¹⁶ cm ⁻³ ) and a p-type Al x Ga _1- x As layer 15 were sequentially laminated to form a semiconductor multilayer structure. The multi-quantum well layer 13 consists of a 150 Å thick GaAs layer 131 and a 20 Å thick Al x Ga _1- x As layer.
The layers 132 were formed by stacking 25 periods on each other. Al component ratio x
Was set to 0.3. The thickness of layers 11 and 15 was 2 to 3 μm. The layer 12 has a thickness of 0.2 μm, and the layer 14 has a thickness of 0.5 μm.
And

In order to prevent light absorption by the GaAs substrate 10, the substrate 10 near the center of the device is removed by selective etching to reduce the radius
A circular hole portion 18 of 25 μm was formed. Further, in order to reduce the area of the junction and reduce the junction capacitance, etching is performed from the upper surface to reach the n-type layer 11 to form a cylindrical portion 19 having a radius of about 40 μm whose center substantially coincides with the circular hole 18 of the substrate 10. Except for layers 12-15
And part of the surface of layer 11 was removed. On the underside of layer 10,
An electrode 20 was formed on the entire surface except for the circular hole portion 18. Further, the electrode 21 was formed on the upper surface of the layer 15 remaining in a cylindrical shape, outside the circle having a radius of about 25 μm at the center.

Between the electrodes 20 and 21 of the optical bistable device having the above configuration, the upper electrode 2
After applying a voltage in the forward direction in which 1 is positive, a laser beam with a wavelength of about 8500Å concentrated on a spot diameter slightly smaller than the diameter of this circular hole is passed through the circular hole 18 on the bottom surface of the element. ,
Irradiation was perpendicular to the layer interface. When the applied voltage between the electrodes was 3 V, the device switched from the low transmission state to the high transmission state when the irradiation light power was increased to about 10 μW. After that, even if the light irradiation was stopped, the high transmission state was maintained, and the device exhibited optical bistability. This device returned to the low transmission state by reducing the applied voltage to 0V. Switching speed is
With 1 ns or less, a high speed operation of two digits or more was obtained as compared with the conventional example.

Next, a second embodiment of the present invention will be described with reference to FIG.
FIG. 2 is a perspective view showing a second embodiment of the present invention. The layer structure (layers 10 to 15) of this embodiment is the same as that shown in FIG.
Is exactly the same as the embodiment of After forming a stripe-shaped SiO ₂ protective film on the upper surface of this layer structure, etching is performed from the upper surface to form a layer outside the stripe-shaped portion having a width of 2 μm.
12 to 15 and part of the surface of the layer 11 were removed to form a stripe-shaped mesa portion 29. Furthermore, this mesa part 29
High resistance AlxGa by MOCVD method with the SiO ₂ film left on top of
_{A 1-} xAs layer region 30 was grown and the above-mentioned mesa portion 29 was embedded. Also at this time, the Al composition x was selected to be 0.3. Next, after removing the SiO ₂ film on the upper part of the mesa, electrodes 21 and 20 were formed on the entire upper and lower surfaces of this multilayer structure.

The end faces of the multi-layer structure crystal in contact with the ends of the embedded stripe-shaped mesas are formed by cleavage that is perpendicular to the layer interface, and are a pair of parallel flat faces. A non-reflective coating was applied to this flat surface.

In this embodiment, the multiple quantum well layer 13 is the AlxGa _1- xAs layer 1
2, 14 and the high resistance Al x Ga _1- x As layer region 30 are embedded in the stripe region, but the refractive index thereof is
It functions as an optical waveguide because it is several percent larger than the AlxGa _1- xAs region. Therefore, a wavelength of about 8500 is applied to one end face of this optical waveguide.
The Å laser beam was focused and made incident, and the power of the transmitted light emitted from the other end face was measured. When the applied voltage between the electrodes 20 and 21 was about 3 V, when the incident optical power was gradually increased, the incident power was about 1 μW, and switching from the low transmission state to the high transmission state occurred. This high transmission state was maintained even when light incidence was stopped, and the element of this example also exhibited optical bistability. The switching speed was 0.5ns or less, and high-speed operation was obtained.

In the above examples, the materials are GaAs and AlGaAs.
However, it is clear that the same effect can be obtained by combining InP and InGaAsP.

As described above, in the optical bistable element according to the present invention, the change in the voltage applied to the quantum well structure portion contained therein is caused by the nonlinear current-voltage characteristics of the Shockley diode structure, The high resistance connection required by the prior art is not required, and thus high speed operation is possible.

In the optical bistable element according to the present invention, a current flows in the high transmission state, that is, in the ON state of the Shockley diode, but in order to prevent the current from becoming excessively large, a power source and the optical bistable element are provided. A resistance element may be interposed therebetween. Further, for the same purpose, a high resistance is previously provided between the layer 15 and the electrode 21 or between the layer 11 and the layer 10.
An AlGaAs layer may be introduced. The size of the resistance introduced outside or inside the device by these methods is at most 1 kΩ or less, which is sufficiently smaller than about 1 MΩ required by the conventional self-electro-optic device. The high speed, which is a feature of the optical bistable device according to the invention, is not significantly impaired.

〔The invention's effect〕

As described in detail above, according to the present invention, low-speed light irradiation causes a high-speed change in the voltage applied to the quantum well structure incorporated in the device. Since the light transmission characteristics of the part can be changed, an optical bistable element that can operate at low optical power and can operate at high speed can be obtained.

[Brief description of drawings]

FIG. 1 is a sectional view of an optical bistable element for explaining a first embodiment of the present invention, and FIG. 2 is a perspective view of an optical bistable element for explaining a second embodiment of the present invention. 10 …… n-type GaAs substrate 11 …… n-type AlxGa _1- xAs layer 12 …… p-type AlxGa _1- xAs layer 13 …… p-type multiple quantum well layer 14 …… n-type AlxGa _1- xAs layer 15 …… p Type AlxGa _1- xAs layer 20 …… Lower electrode 21 …… Upper electrode

Claims (1)

[Claims] 1. A second semiconductor layer of a second conductivity type, a third semiconductor layer of a first conductivity type and a fourth semiconductor layer of a second conductivity type on a first semiconductor layer of the first conductivity type. A semiconductor multi-layer structure in which the semiconductor layers are sequentially laminated, and a Shockley diode structure in which electrodes are provided on the upper surface and the lower surface of the semiconductor multilayer structure, and these semiconductors near the bonding surface of the second semiconductor layer and the third semiconductor layer are formed. At least a part of at least one of the layers has a single or multiple quantum well structure, and the light absorption edge of this quantum well structure is located on the longer wavelength side compared to the light absorption edge of the external region. An optical bistable device characterized by.