KR0141343B1 - Impedance mismatched non-biased optical bistable logic device - Google Patents

Abstract

The present invention is bi-stable characteristics when using the conventional structure in the fabrication of voltage-free optical bistable logic devices such as ESQW S-SEED, ACQW S-SEED, SMQW S-SEED without the application of external voltage The present invention relates to a method of improving the voltage-free optical bistable characteristics by using an optical input resistance-mismatched AFP structure in overcoming the insufficient points using an asymmetric Fabry-Faro (AFP) resonance structure.

In the commonly used optical-impedance-matched AFP structure, the ON / OFF intensity ratio has a great advantage, but the difference in reflectance (Δ1R) and optical bistable width (Δ1), which are more important in optical systems, are significant. There was a disadvantage of being relatively small.

As a solution to this problem, reducing the thickness of the intrinsic region of the MQW by appropriately reducing the number of cycles of the MQW may result in a resistance mismatch that can greatly increase Δ1R and Δ1 while maintaining the proper ON / OFF intensity ratio required by the optical system. AFP structure is presented.

In addition, in the structure of the present invention, since the thickness of the intrinsic region is further reduced than that of the resist-matched AFP structure, the intrinsic electric field due to the given intrinsic voltage determined by the material of the PIN diode is increased, thereby making the optical bistable characteristic larger. It is also a structure.

Description

IMPEDANCE MISMATCHED NONBIASED OPTICAL BISTABLE LOGIC DEVICE

Figure 1a is a cross-sectional view showing a typical reflective self-optic effect device (SEED) structure.

Figure 1b is a cross-sectional view showing an Asymmetric Fabry-Perot (SFP) -SEED structure

FIG. 2a is a curve showing reflectance characteristics of the product of the light absorption coefficient of the reflective SEED and the thickness of the light absorption layer at an exciton wavelength, which is an operating wavelength.

2b is a change in light absorption coefficient with respect to the vertical field at the exciton wavelength of a multi-quantum well having nonlinear light absorption characteristics.

Figure 3a is an example of a curve showing the characteristic of the product of the light absorption coefficient of the ON / OFF intensity ratio and the light absorption layer thickness in the optical bistable characteristics of the AFP-SEED, the point A1 is the resistance match AFP-SEED, the point B1 is Example of resistance mismatch AFP-SEED.

Figure 3b is an example of a curve showing the characteristic of the product of the light absorption layer thickness of the ON / OFF reflectance difference in the optical bistable characteristics of the AFP-SEED, the point A1 is the resistance match AFP-SEED, the point B1 is the resistance mismatch AFP- An example of SEED.

Figure 4a is a circuit diagram of a symmetric (S) -SEED to enable voltage-free optical bistable.

4b is a load curve of S-SEED that enables voltageless photo bistable.

5 is an example of optical bistable characteristics of resistance mismatch AFP-NOBS (Nonbiased Optical Bistable S-SEED) and resistance mismatch AFP-NOBS according to the present invention.

Explanation of symbols on the main parts of the drawings

1: semiconductor substrate

2: undoped, high refractive index λ / 4n layer

3: low λ / 4n layer of undoped low refractive index

4: cathode contact layer of N ⁺ (or P ⁺ anode contact layer)

5: undoped buffer layer

6: barrier layer forming an undoped multiple quantum well

7: well layer forming an undoped multiple quantum well

8: P ⁺ anode contact layer (or N ⁺ cathode contact layer)

SEED: self electro-optic effect device

S-SEED: Symmetrical SEED with two PIN diode SEEDs connected in series

NOBS: nonbiased optical bistable S-SEED

Rb: reflectance of the lower mirror

Rf: reflectance of the upper mirror

MQW: multiple quantum wells

ESQW: extremely shallow quantum well

ACQW: asymmetric coupled quantum well

SMQW: strained MQW

CR: ON / OFF contrast ratio

ΔR: Difference between ON / OFF reflectance

△: light bistable width

α _on : light absorption coefficient in ON-state

α _off : light absorption coefficient in OFF state

D: total thickness of the light absorbing layer (well layer)

± V _op : Operating voltage at no-voltage optical bistable

V _bi : Built-in voltage of PIN diode SEED

I _ph : Photocurrent of PIN Diode SEED

D1: the first diode that makes up S-SEED

D2: second diode constituting S-SEED

P _in : incident light intensity

P _out : Output light intensity

The present invention is a non-voltage optical bistable symmetric seed (Nonbiased Optical Bistable) using an asymmetric Fabry-Perot (hereinafter referred to as 'AFP') resonance structure that can greatly increase the optical bistable characteristics without applying an external voltage Symmetric self electro-optic effect device (hereinafter referred to as 'NOBS'), and more specifically, relatively small ON / OFF reflectance difference (ΔR) in an existing matched structure of optical input resistance. ) And the optical bistable width (△).

Optical bistability is a very important property in optical signal processing.

A seed using a multiple quantum well (hereinafter referred to as 'MQW') (Self Electro-optic Effect Device (hereinafter referred to as 'SEED')) is a representative device exhibiting such optical bistable characteristics.

In particular, SEED is known to be a device suitable for parallel optical signal processing because it is easy to implement a two-dimensional optical bistable device using a general semiconductor process.

SEED generally has a PIN diode structure in which MQW is an intrinsic region. In order to show the optical bistable characteristic, a symmetrical SEED (hereinafter referred to as 'S-SEED') in which two PIN diode SEEDs are connected in series must be configured to apply reverse voltage.

Therefore, when manufacturing a semiconductor, a metal wire for applying reverse voltage, a metal wire for connecting between S-SEED devices, a wire by wire bonding, and the like are required.

Such an arrangement of wires and external wires on the substrate may cause inductive coupling between the wires, electromagnetic interference by electromagnetic fields, crosstalk, and the like.

In particular, when forming a two-dimensional S-SEED array, all the cathodes of each S-SEED connected to the power supply must be connected (as is the anode connected to the power supply). They cannot be separated electrically.

Therefore, as the number of S-SEEDs forming a two-dimensional array increases, the above problems become more serious, resulting in a lower yield of devices and lower integration.

In addition, wires by wire bonding with metal wires on a substrate are not suitable for high-speed switching because they act as parasitic passive element components when forming SEED modulators and S-SEED circuits. There are also disadvantages.

The above problems with S-SEED and S-SEED arrays, which require a general reverse voltage, are known as voltage-free optical bistable S-SEEDs (hereinafter referred to as 'NOBS') that can function as optical bistable logic without voltage application. ) Can be removed if possible.

NOBS that can have optical bistable without conventional voltage includes MQW as a shallow quantum well (Extremely Shallow Quantum Well: 'ESQW'), Asymmetric Coupled Quantum Well (hereinafter, 'ACQW') It is known that a structure using strained MQW (hereinafter referred to as SMQW) is possible. However, when they are applied to the conventional reflective SEED structure, that is, when the total thickness of the MQW constituting the malleable region is increased to about 1 μm for a large amount of light absorption, the voltage-free optical bistable characteristics are very small. There is a flaw.

This is because the intrinsic electric field due to the intrinsic voltage of the PIN diode which enables NOBS is small in a SEED structure having a typical intrinsic region thickness of 1 mu m.

In order to solve this problem of NOBS and in particular to increase the ON / OFF Contrast Ratio (hereinafter referred to as 'CR'), the AFP resonance of the optical input impedance matched (impedance matched) The use of structures has been suggested.

Considering the degree of light absorption of the MQW, the total thickness of the light-absorbing quantum wells required to construct an AFP resonance structure with a matched optical input resistance is reduced, which results in an intrinsic electric field for a given built-in voltage rather than the general structure. Because it can increase greatly.

However, there is an advantage in that the CR is very large (theoretical infinity) in the case of using an AFP whose optical input resistance is set to zero (0) in the OFF-state, but the reflectance in the ON-state and the OFF-state is advantageous. There is a disadvantage that the difference ΔR and the light bistable width Δ are relatively small.

It is an object of the present invention to provide a structure that maximizes ΔR and Δ while properly maintaining CR in the fabrication of NOBS using an AFP resonance structure capable of greatly increasing optical bistable characteristics without applying an external voltage.

Figure 1a shows an example of a typical reflective SEED structure.

Periodically, a first QWL (Quarter-Wave Layer) 2 having a large refractive index and a second QWL 3 having a small refractive index are formed on the substrate 1 with an optical thickness of 1/4 of an MQW exciton operating wavelength. A PIN diode having an MQW as an intrinsic region is formed on the lower mirror layer grown by using the anti-reflection treatment to increase the light absorption efficiency of the device.

In the formation of a PIN diode, the layer 4 may be formed of an n ⁺ layer, the layer 8 may be formed of a p ⁺ layer, on the contrary, the layer 4 may be a p ⁺ layer, and the layer 8 may be an n ⁺ layer. It may be formed as.

Except for the quantum well layer, the p layer, the n layer, the barrier layer, the buffer layer, and the mirror layer should all be light absorbing materials for the operating wavelength.

Meanwhile, as shown in FIG. 1B, when the QWL is stacked on top to form an upper mirror layer, the optical length, which is the thickness L from the lower mirror to the upper mirror, is adjusted to be an integer multiple of 1/4 of the operating wavelength. The mode of the AFP and the resonant wavelength of the excitons form a phase-matched AFP structure.

At this time, it is possible to obtain an AFP structure in which the reflectance Rf of the upper mirror by the interface of the air / semiconductor is 0.32 even without stacking the QWL for the upper mirror layer.

Figure 2a shows an example of the reflectance according to the product of the light absorption coefficient (α) of the MQW of the reflective SEED described above and the total thickness (D) of the light absorbing layer, a general reflective SEED structure with an antireflective layer, upper mirror Is related to the structure of AFP-SEED whose reflectance Rf is 0.32 and 0.5, respectively.

In FIG. 2A, A1 and A2 denote values of αD satisfying the optical input resistance-matching conditions of AFP-SEED with Rf = 0.32 and AFP-SEED with Rf = 0.5, respectively.

Since the light absorption coefficient? Of the MQW is determined by the structure of the MQW, it generally decreases with the increase of the electric field at the exciton resonance wavelength of the MQW, as shown in FIG.

Therefore, the decrease of α as the electric field increases leads to an increase in the reflectance R as shown in FIG. 2A, and satisfies the condition that the reflectivity increases as the electric field increases normally-off. Thus, the optical bistable characteristics will be obtained. Such a normally-off condition is possible in the left region of the points A1 and A2 satisfying the optical input resistance match in FIG. 2A for AFP-SEED.

As an example, the ratio (CR) of the reflectance of the ON-state and the OFF-state to the product of α _off (α of OFF-state) and D of an AFP-SEED having an anti-reflective layer and Rf = 3.2 ( ΔR) is shown in FIGS. 3A and 3B, respectively, considering the case where the value of α _on / α _off is 0.35.

In AFP-SEED, at the point A1 where the optical input resistance satisfies the matched condition, the value of CR may be very large as shown in FIG. 3a, but the ΔR value of FIG. 3b is less than 0.2.

On the other hand, at the point B1 that satisfies the optical input resistance mismatch condition, the value of CR can be maintained at about 0.3 while maintaining a sufficient value of about 10.

In particular, in NOBS, since the value of α is changed only by the intrinsic voltage without externally applied voltage, the value of α _on / α _off may be somewhat smaller than 0.35, so that the value of ΔR is relatively small and the minimum required CR is required. It is more necessary to maintain and maximize the value of ΔR. Here, AFP-SEED with mismatched optical input resistance maintains phase-matching, that is, the distance between the lower mirror and the upper mirror (L) that satisfies the condition that the AFP mode matches the resonance wavelength of the excitons. It can be easily implemented by reducing the number of cycles.

Figure 4a shows a circuit diagram forming a NOBS.

Since there is no applied voltage, the voltage between points V1 and V2 can be considered as the voltages V _op and -V _op at stable operating points as shown in the load curve of NOBS in FIG. 4b.

This means that the electric field across each diode is (V _bi -V _op ) / ti and (V _bi + V _op ) / ti when NOBS shows optical bistable.

ti is the total thickness of the intrinsic region.

Therefore, it is advantageous to reduce ti as much as possible to increase the electric field difference between two diodes in the absence of applied voltage.

AFP-SEED with impedance-mismatching optical input resistance is made up of the total thickness (D) of the light absorption layer smaller than AFP-SEED with impedance-matching optical input resistance, which may further reduce ti. will be.

An example of the optical bistable properties of NOBS based on the principle of NOBS described above is shown in FIG.

The difference between AFP-NOBS and resistance match is obvious. The resistance matched AFP-NOBS has a larger CR because the OFF-state value is closer to zero than the resistance mismatched AFP-NOBS, but ΔR and optical bistable width (△) are relatively small. .

In particular, in the case of NOBS, the value of the reflectance in the ON-state is very small when the resistance is matched, which is somewhat unsuitable for the application in the optical system.

Therefore, designing the structure of the AFP-SEED using the concept of optical input resistance mismatch proposed in the present invention maintains the appropriate CR required for the optical system, and the degree of ΔR and optical bistable required for the optical system application. There is an effect that the realization of a NOBS having a width Δ is possible.

Claims (1)

A multi-quantum well (MQW) that enables a voltageless bistable characteristic as an intrinsic region, and an upper mirror layer and a lower mirror layer of different reflectance at the exciton resonance wavelength of the multi-quantum well; In a seed (SEED; Self Electro-optic Effect Device) using an asymmetric Fabry-Perot (AFP) structure in which the resonance wavelength coincides with the resonance wavelength of the excitons; The total thickness of the light absorbing layer by having multiple quantum wells having a period smaller than that of the multiple quantum wells (MQW) of the optical input resistance matching asymmetric Fabry-Perot Seed (AFP-SEED) ( D) Voltage-free optical bistable logic element with mismatched optical input resistance.