JPS6232667A - Optical detector for superconductive tunnel junction - Google Patents

Abstract

PURPOSE:To convert weak signal to an electric signal up to a high frequency in a broad wavelength region of infrared rays, by providing a tunnel barrier vertically with respect to an intermediate layer in an upper electrode, on which light is projected, confining optically excited quasi-particles, utilizing high sensitivity of a superconductive optical detector, and implementing a high speed characteristic. CONSTITUTION:A low superconductive electrode 2 is provided on a substrate 1. An intermediate layer 3 of an insulator or a semiconductor, which forms a tunnel barrier, is formed on the electrode 2. An upper electrode 5 of a superconductor, which includes a tunnel barrier in the film, is provided. The upper and lower electrodes of the superconductor are isolated by an insulating layer 6. Each layer of these four layers 2, 3, 5 and 6 can be manufactured by a thin film forming technology utilizing a vacuum evaporation method or a sputtering method using a suitable target and a pattern forming technology utilizing a lithography method.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a photodetector that is high speed and highly sensitive in a wide infrared wavelength band.

2. Description of the Related Art Conventionally, photodetectors using semiconductor materials have been developed. However, since the energy gap of the half 41 is wide, infrared light with low photon energy does not excite the photon across the gap, making detection impossible.

To be more specific, in order to increase the speed of photodetectors that use photoconduction, it is necessary to shorten the carrier lifetime by introducing deep levels or using amorphous materials. , on the contrary, the quantum efficiency decreases significantly,
A decrease in sensitivity occurs.

On the other hand, in photodiodes, the reaction speed is determined by the CR time constant, so attempts have been made to increase the speed by lowering the resistance of the semiconductor and reducing the capacitance of the junction, but on the other hand, the light-receiving area becomes smaller. A decrease in sensitivity occurs.

As described above, it is difficult for photodetectors using semiconductor materials to simultaneously satisfy high sensitivity and high speed.

Furthermore, in a photodetector using a superconductor that exhibits an excitation phenomenon even for low-energy photons in the infrared region, the response speed depends on the relaxation time of the quasiparticle to the Cooper pair and the escape of phonons from the superconductor. It is determined by the time of Q, 1nsec
It was difficult to achieve the following high-speed characteristics.

Problems to be Solved by the Invention The present invention aims to eliminate these drawbacks and provide a high-speed and highly sensitive photodetector in the infrared region.

Means for Solving the Problems According to the present invention, in a photodetector having a three-layer structure of a lower electrode of a superconductor thin film, an intermediate layer forming a tunnel barrier, and an upper electrode of a superconductor thin film, A tunnel barrier is provided perpendicular to the intermediate layer in the upper electrode that is irradiated with light to confine the photoexcited quasiparticles, thereby making use of the high sensitivity of superconductor photodetection and high speed detection. Realize the characteristics.

FIG. 1 shows a structural diagram of a photodetector using a superconductor according to the present invention, in which a lower superconducting electrode 2 is placed on a substrate 1.
An insulating or semiconductor intermediate layer 3 serving as a tunnel barrier is formed thereon, and an upper electrode 5 made of a superconductor including a tunnel barrier 4 in the film is further provided. The upper and lower electrodes 2 and 5 of the superconductor are formed by an insulating layer 6.
Separated by These four layers 2.3.5.
Each of the layers 6 can be manufactured by a vacuum evaporation method, a thin film forming technique using a sputtering method using an appropriate target, and a pattern forming technique using a phosphor graph method.

Geizuki Next, the operation associated with the light irradiation 7 will be explained.

Generally, in a superconducting state, electrons form a Cooper pair, and the energy is at a level Δ lower than the Fermi level.

On the other hand, the Cooper pair is destroyed by external energy and becomes a single electron without forming a pair. This electron is called a quasiparticle and has an energy six or more higher than the Fermi level. There is an energy gap of 2△ between the lowest energy electronic state of this quasiparticle and the level of the Cooper pair.

FIG. 2 shows current (I)-voltage (V) characteristics in a tunnel junction using the same type of superconducting material as an electrode.

(a) shows the superconducting state when no voltage is applied to the tunnel junction, (a) shows the case when the bias voltage is lower than twice the superconducting energy gap width △, and (C) shows the case when the bias voltage exceeds 2Δ. This is the case. In (a), tunneling between the pair of Coopers is possible and a superconducting current flows, but the quasi-particles are unlikely to flow due to the tunnel barrier even if there is a density difference between them. In (b) and (C), the Cooper pair tunnels through the tunnel barrier at a frequency proportional to the bias voltage (AC Josephson effect), but does not contribute as a DC current component. Therefore, as shown in Figure 2, the I-V characteristics are as follows: In case (a), a small current accompanying the tunneling of quasiparticles excited by heat during superconductivity occurs, and in case (c), a large current caused by voltage excitation occurs. It will flow.

By the way, in the state shown in (a) in FIG. 2, the particles that tunnel are different, so the Josephson junction functions as a filter that identifies and separates the Cooper pair and the quasiparticle. That is, in state (a), the Cooper pair passes through the barrier, but the quasiparticles cannot move and accumulate. On the other hand,
et al.), the quasiparticle passes through the barrier and contributes to the direct current, but the Cooper pair does not. Focusing on this characteristic,
By connecting junctions with different critical current values in series and appropriately placing each junction in a voltage state or a superconducting state using a bias current, it is possible to improve the efficiency of an element using a pair of quasiparticles and a Cooper.

In the photodetector shown in Figure 1, when the superconductor 5 constituting the tunnel junction is irradiated with light with energy exceeding the gap width of 2Δ, the Cooper pair is destroyed, and the number of photons absorbed by the superconductor is Quasiparticles proportional to (the amount of light) are generated in the upper electrode 5.

The tunnel barrier or tunnel junction 3 is shown in FIG. 2(b).
When biased in this state, the quasiparticles in the upper electrode 5 excited by light tunnel through the junction barrier 3 and flow as a current. By measuring this current value, optical detection (photon detection) is possible.

The broken line in the IV characteristics in FIG. 2 represents the characteristics when light is irradiated.

FIG. 3 shows a typical detection circuit. The detection circuit shown in FIG. 3 includes a constant voltage source 8, a superconducting photodetection junction 9, and a load resistor R, and in the case of high-speed operation, 50Ω is often selected as the resistance R from the viewpoint of impedance matching with the measurement system. . In this circuit, an example of the load straight line when the output voltage of the constant voltage source is set to {circle around (2)} is shown in the [-V characteristic in FIG. 2]. By light irradiation, the operating point moves on this load straight line, and light detection is possible.

In a photodetector that uses this superconducting tunneling phenomenon, junction resistance is kept low because electrons are tunneled, and since the electrodes are also superconducting, the resistance is zero. Therefore, the resistances connected in series and in parallel to the equivalent circuit of the detector are extremely small, and the RC constant that causes delays in the electric circuit becomes small. In fact, a switching circuit using a Josephson junction with the same type of structure already has 6 x 10-"
A response speed of sec is reported.

Next, we will discuss the relaxation properties of quasiparticles. The excited quasiparticle releases a phonon over a certain period of time and relaxes into a pair of Coopers. This relaxation time is related to the superconducting mechanism because it includes interaction with phonons during the process, and is about 10-'' sec for strongly coupled superconductors with strong electron-phonon interactions, and for weakly coupled superconductors with strong electron-phonon interactions. In coupled superconductors, it is estimated to be longer; on the other hand, the velocity of quasiparticles is approximately the same as the velocity of electrons at the Fermi surface, so it is usually around LO8 cm/sec.
It is c. Therefore, due to the law of conservation of motion, the time required for the quasiparticles excited by light irradiation to move linearly across the upper electrode 5 (film thickness approximately 0.1 μm) from the surface to the bottom barrier is 10 − ” sec. Therefore, the quasiparticle can tunnel through the barrier without relaxing into the Cooper pair. Note that the quasiparticle excitation by phonons caused by the relaxation of the quasiparticle into the Cooper pair is caused by the lower electrode 2. It does not contribute to tunnel current because it occurs in

In addition to the above, the excited quasiparticles are scattered by phonons or defects in the electrode 5. This process is the same as electrical resistance, and occurs in 10-" to 10-15 seconds at room temperature. However, at extremely low temperatures, the phonon density decreases, so
The probability of scattering by phonons decreases.

Therefore, quasiparticles penetrate and pass through the tunnel layer without collisions in the electrodes.

Next, let us consider the process by which quasiparticles tunnel through the barrier layer. The thickness of the barrier layer is as thin as several nanometers, and the passage time is short due to the l-son tunnel effect. Incidentally, the transmission of quasiparticles through the barrier layer is a stochastic process, and some quasiparticles are reflected by the barrier and return to the upper electrode 5, causing a delay in the relaxation time. This probability depends on the bias voltage at the junction; the higher the voltage, the higher the transmission probability and the shorter the relaxation time.

From the above, the response speed of the detection element takes into account the electric circuit and the quasi-particle relaxation process, which is 10-''see or less.

In the case of a finite temperature, a tunnel current flows due to quasiparticles generated by thermal excitation, resulting in a dark current. This dark current acts as noise and reduces the S/N ratio. This dark current value is proportional to the thermally excited quasiparticle density, so it depends on the temperature T, and exp (
−Δ/kT). Therefore, it decreases as the temperature decreases, and becomes zero at absolute zero. Therefore, by lowering the detection element temperature, noise is reduced, and highly sensitive detection of weak light becomes possible. Note that the structure does not change even if the temperature decreases, so the high-speed response characteristics are maintained.

By the way, the energy gap Δ of a superconductor is meV
And narrow. Therefore, quasiparticles are excited even by far infrared light with a wavelength of up to 1 mm, and a tunnel current flows. Therefore,
High-speed optical signals can be detected over a wide wavelength range. On the other hand, when infrared rays with high energy and short wavelengths are incident, a time delay occurs due to quasiparticle relaxation, resulting in a decrease in response speed. That is, a quasiparticle excited to high energy interacts with a Cooper pair or a phonon via Coulomb force or a magnetic field, and relaxes to a low energy state. In this process, the pair of Coopers is broken again, new quasiparticles are generated, and as illustrated in FIG. 4(a), they diffuse into the upper electrode in the absence of the tunnel barrier 4. For example, a quasiparticle excited by light with a wavelength of 1 μm has an energy comparable to the Fermi energy (the increase in velocity at #10' is 105 cm/5 ec), so the probability of electron-electron interaction decreases due to the Pauli exclusion law. does not occur, but collides with a pair of Coopers in a short time (10-" sec), resulting in quasiparticle excitation. Also, the destruction of a pair of Coopers occurs in a similar amount of time due to the magnetic field generated by the movement of this quasiparticle. As this quasiparticle spreads spatially, it eventually emits phonons and contributes to the tunneling current until it returns to the Cooper pair.Therefore, the fall of the current is equal to the relaxation time of the quasiparticle to the Cooper pair (= IQ -" 5ec), and the tail will be drawn.

On the other hand, as illustrated in FIG. 4 (0:l), if a tunnel barrier 4 is formed in the upper electrode 5 and brought into a superconducting state, tunneling of quasiparticles is unlikely to occur as described above. Therefore, the diffusion movement of quasiparticles generated by photoexcitation is stopped by the barrier, and diffusion into a wide spatial area within the upper electrode 5 does not occur. Moreover, even if it diffuses out of the electrode, the probability that it will re-enter the area surrounded by the barrier is small. In this way, the quasiparticles excited by light are confined in a narrow region surrounded by a barrier, so there is no fall delay caused by spatial spread, and high-speed response is possible. Note that, as can be seen from FIG. 4, the current must flow from the lower electrode to the upper electrode.

EXAMPLES The present invention will be described in detail below. However, the present invention is not limited to these examples in any way.

Example I Ba (Pbo, 7BI0.3) A porcelain with a composition of 1.504 was used as a target, and a gas mixture of 50% each of argon and oxygen was heated at a gas pressure of 10-'Torr by high-frequency sputtering.
Below, a single crystal thin film having a composition of BaPbo, 7BIO, 303 with a thickness of about 1500 was formed on a 5rTi03 single crystal substrate (110) heated to 500° C. at a plate voltage of 1.4V. The thin film thus obtained was 8~
It has a superconducting temperature of 9. Next, the single crystal thin film is formed into a lower electrode by photolithography and chemical etching. Next, A1°03 was formed to a thickness of about 30 mm as a barrier layer. In this case, deposition was performed by RF sputtering using Al 203 porcelain as a target at a substrate temperature of 300°C. On this Al2O3 barrier layer, BaPbo, tBlo, and
A polycrystalline thin film of 303 was deposited to a thickness of 2000. In this polycrystalline thin film, a tunnel barrier is formed perpendicularly to the substrate along the grain boundaries, and the Josephson critical current value is larger than that of the Al2O3 barrier. Therefore, it was possible to bring the Al2O3 barrier into a voltage state while keeping the tunnel junction in the electrode in a superconducting state, and optical detection was possible by observing changes in the quasiparticle tunneling current.

Example 2 A lower electrode is formed by depositing about 3,000 Nb metal thin films on a sapphire substrate heated to 500° C. by high-frequency sputtering. Next, A1°03 as a barrier layer is deposited on the lower electrode to a thickness of about 30 nm by electron beam evaporation. A pb overcrystal metal thin film is immediately formed on this Al2O3 barrier layer. pb is pure oxygen 10-'T
It was deposited by evaporation method in a pelger at orr. The polycrystalline film of the upper electrode formed as a result becomes a tunnel barrier by forming a thin oxide film during vapor deposition along the grain boundaries.

This tunnel barrier has a larger Josephson critical current value than the Al2O3 barrier, so the above-described operation is possible and high-speed photodetection is possible.

As described in detail, the photodetector of the present invention is capable of converting weak light in a wide infrared wavelength range up to high frequencies into electrical signals. Since this electrical signal is a current change in a superconducting tunnel junction, it can be connected in the same cooling bath as a circuit system using a Josephson junction with high-speed switching characteristics, and can be used for signal processing. From this point of view, C) for high-speed optical communication in the infrared region is useful as a detector for high-speed physical phenomenon analysis.

[Brief explanation of the drawing]

Fig. 1 is a cross-sectional view of the photodetector of the present invention, Fig. 2 is a graph showing the current-voltage characteristics and operating characteristics of the detector, Fig. 3 is a circuit diagram of an example of a measurement circuit system, and Fig. 4 is a photoexcitation This explains the behavior of quasiparticles within the electrode, where (a) is the case where there is no tunnel barrier in the upper electrode, and (a) is the case where there is a superconducting tunnel barrier in the upper electrode. (Main reference numbers) 1. Substrate, 2. Lower electrode, 3. Intermediate layer forming a tunnel barrier, 4. Tunnel barrier, 5. Upper electrode, 6. Insulating layer, 7. Incident light. , 8... Constant voltage source, 9... Photodetector, R... Load resistance

Claims (3)

[Claims] (1) A lower electrode made of a superconductor thin film, an intermediate layer provided on the lower electrode to form a tunnel barrier, and a superconducting thin film provided on the intermediate layer with a tunnel barrier layer formed vertically. What is claimed is: 1. A superconducting tunnel junction photodetector comprising a three-layer structure and an upper electrode. (2) The upper electrode is an oxide superconductor BaPb_1_
The superconducting tunnel junction photodetector according to claim 1, characterized in that it is formed of a polycrystalline thin film -_xBi_xO (0.05≦x≦0.35). (3) The superconducting tunnel junction photodetector according to claim 1, wherein the upper electrode is formed of a Pb polycrystalline metal thin film.