rus
Issue #1/2021
N. A. Kulchitsky, A. V. Naumov, V. V. Startsev, M. A. Demyanenko
Detection in the Terahertz Range. Part 1
Detection in the Terahertz Range. Part 1
DOI: 10.22184/1993-7296.FRos.2021.15.1.52.68
The paper discusses the problems associated with the development of technology for detectors of terahertz radiation. The main physical phenomena and recent progress in various methods of detecting terahertz radiation (direct detection and heterodyne detection) are considered. The advantages and disadvantages of direct detection sensors and sensors with heterodyne detection are discussed. In the first part, a number of features of direct detection are considered and a description of some types of terahertz detectors of direct detection is given.
The paper discusses the problems associated with the development of technology for detectors of terahertz radiation. The main physical phenomena and recent progress in various methods of detecting terahertz radiation (direct detection and heterodyne detection) are considered. The advantages and disadvantages of direct detection sensors and sensors with heterodyne detection are discussed. In the first part, a number of features of direct detection are considered and a description of some types of terahertz detectors of direct detection is given.
Теги: direct frequency band heterodyne detection sensitivity terahertz radiation гетеродинное детектирование полоса частот прямое терагерцевое излучение чувствительность
Detection in the Terahertz Range
Part 1
N. A. Kulchitsky 1, 2, A. V. Naumov 3, V. V. Startsev 3, M. A. Demyanenko 4
State Scientific Center of the Russian Federation, JSC Scientific and Production Association “Orion”, Moscow, Russia.
MIREA – Russian Technological University, (RTU MIREA), Moscow, Russia.
JSC “Optical and Mechanical Design Bureau Astrohn”, Lytkarino, Moscow region, Russia.
A. V. Rzhanov Institute of Semiconductor Physics, SB of RAS, Novosibirsk, Russia.
The paper discusses the problems associated with the development of technology for detectors of terahertz radiation. The main physical phenomena and recent progress in various methods of detecting terahertz radiation (direct detection and heterodyne detection) are considered. The advantages and disadvantages of direct detection sensors and sensors with heterodyne detection are discussed. In the first part, a number of features of direct detection are considered and a description of some types of terahertz detectors of direct detection is given.
Key words: terahertz radiation, direct, heterodyne detection, frequency band, sensitivity
Received on: 15.12.2020
Accepted on: 10.01.2021
Introduction
Terahertz (THz) radiation is electromagnetic radiation, the frequency spectrum of which is located between the infrared and millimeter ranges. The boundaries between these types of radiation are defined differently in different sources. In this work, for definiteness, it is assumed that the THz range is within 0.1–10 THz (30 µm – 3 mm), partially overlapping with the mid-infrared (2.5–50 µm) and millimeter (30–300 GHz, 1–10 mm) ranges, as well as including narrower submillimeter and subterahertz ranges. Devices operating in the terahertz range are becoming increasingly important in various applications (for example, in security, medical, for imaging) [1, 2]. THz waves are effective in detecting the presence of water, and thus, they can effectively distinguish various objects on human bodies (the water content in the human body is about 60%), since the garments are transparent. A brief description of the history of the development of terahertz research is given, for example, in [3]. At the moment, the market for terahertz applications has just left the initial phase of development. Therefore, the forecast for the development of the terahertz technology market (Fig. 1) is quite evaluative. However, we can say with confidence that the market has great prospects.
It is known that radiation detectors can be divided into a class of photonic (quantum) photodetectors, in which the photon energy is converted into a certain primary reaction of the electronic system of the photodetector, and a class of thermal ones, in which the photon energy is converted into heat, and the reaction of the photodetector arises as a result of an increase in the temperature of the sensitive element. The critical difference between detection in the THz range and detection in the infrared range is the low energy of THz photons, which complicates the development of photon detectors of THz radiation. Currently, there is a wide variety of terahertz radiation sensors, both relatively traditional (for example, bolometers) and based on various principles and materials that have appeared recently.
All THz detection systems can also be subdivided into two groups, coherent (heterodyne) detection systems and incoherent (direct detection) detection systems [4]. The former make it possible to determine not only the amplitudes of the signals, but also their phases, which is important for increasing the amount of information received about the object. This also makes it possible to realize the highest characteristics of the detector’s sensitivity and spectral resolution.
Coherent signal detection systems use the principle of heterodyne circuits, since up to now, for high frequencies of radiation, there are no own amplifiers. The detected signals are converted into significantly lower frequencies (ν ≈ 1–30 GHz), which are then amplified by low-noise amplifiers. Basically, these systems are selective (narrowband) detection systems. Incoherent detection systems only detect signal amplitudes and are generally broadband systems. The detectors used in these two detection systems are similar in many cases, but some of them, for example, uncooled thermal detectors, prefer not to be used in coherent systems due to their relatively long response time (τ ≈ 10–7 s).
From the point of view of use and applications, it is important to divide THz radiation receivers into two classes: cooled and uncooled. The advantage of cooled (deeply cooled) detectors is their extremely high sensitivity, which is characterized by noise-equivalent power (NEP) ~10–18–10–20 W / √ Hz at an operating temperature of T = 100–200 mK [5]. Due to their high sensitivity, such superconducting detectors are preferable in conditions of low background photon flux and have found applications, in particular, in astronomy. The advantages of uncooled detectors, in addition to their low cost and ease of use, are their suitability for the manufacture of large-format array detectors. The NEP of uncooled array microbolometric receivers of the THz range with a format of 320 × 240 can reach values of 40 pW or 2 · 10–13 W / √ Hz at a frequency of 3 THz [13].
It should be noted that NEP is defined as the rms input signal power WS required to provide a rms output signal (S) that is equal to the rms noise (N) measured in a 1 Hz bandwidth. However, sometimes noise in the full bandwidth (Δν) defined by a specific measurement circuit is used to determine NEP. In this case, NEP is in watts. This often leads to ambiguity and confusion in the literature. For example, in [11], uncooled THz bolometers developed by the Canadian company INO are presented, characterized by NEP = 24,7 24.7 pW (at a radiation frequency of 4.25 THz), measured in the frequency band Δν = 160 kHz. While in the review [12], the same NEP values are given with the indication of the unit W / √ Hz. To avoid such confusion, we will use the term minimum detectable power (MDP) here if the noise is not scaled to a 1 Hz bandwidth.
The aim of this work is to review modern terahertz detectors, both quantum and thermal, which are most widely used for either direct or heterodyne detection.
Direct detection detectors
Direct detection sensors are suitable for applications that do not require the ultra-high spectral resolution (ν / Δν ≈ 106) provided by heterodyne spectroscopic systems. Unlike heterodyne detection systems, for them there are no technical problems in the formation of multielement matrices due to the need to use a local oscillator (reference radiation source) of high power and detectors with a short response time (τ ≈ 10–10–10–11 s). Therefore, detectors operating at room temperature with a relatively long response time (τ ≈ 10–2–10–3 s) and moderate sensitivity can be used in direct detection systems. Among such detectors for forming THz images, for example, Golay cells and pyroelectric detectors, bolometers and microbolometers are used, which use antennas to couple radiation with small absorbing regions. The NEP value for uncooled detectors usually ranges from 10–12 to 10–9 W / √ Hz (Table 1) [5].
Various types of cooled semiconductor detectors are also used (for example, bolometers with hot electrons based on InSb and bolometers based on impurity Si and Ge) [4,5] with a response time (τ ≈ 10–6–10–8 s) and NEP ≈ 10–13–5 · 10–17 W / √ Hz at an operating temperature T < 4 К. Bolometers cooled to T ≈ 100–300 mK have the highest sensitivity among other direct-action detectors in the sub-mm and mm spectral ranges, reaching NEP limited by fluctuations in cosmic background radiation. Doped photoresistors with direct detection (for example, based on Ge: Ga) are sensitive up to a wavelength of about 400 μm and can be combined into matrices. Their threshold power can reach NEP ≈ 5 · 10–17 W / √ Hz at λ = 150 µm and operating temperature T = 2 К.
A schematic diagram of direct detection is shown in Fig. 2. Both the signal radiation with the power Ws and the background radiation with the power WB are incident on the detector. Focusing optics (lenses, mirrors, etc.) are used to collect radiation from a large area and focus it on a detector. Often, an optical filter is located upstream of the detector to remove background radiation in the spectral range of wavelengths other than the signal wavelength. The relatively small electrical signal of the detector is amplified and further processed.
The ability to detect small signals for detectors with direct detection is limited by the insurmountable background photon noise that does not become small even for the cosmic background. The performance of these detectors is limited by background noise compared to heterodyne detectors, which are limited by quantum noise. As a rule, the threshold power recorded by detectors with direct detection is higher than for heterodyne ones, which is due to the contribution of other noise present in the detector itself, in circuit elements and amplifiers.
The advantage of systems with direct detection is the relative simplicity and the possibility of developing large-format matrices. Most imaging systems use direct detection.
Detector types
The complexity of the development of terahertz devices lies in the fact that when terahertz radiation is detected, some principles of operation of photonic and electronic devices cease to operate. Terahertz radiation is characterized by a low photon energy (4 meV for radiation with a frequency of 1 THz) and therefore photonic terahertz devices with quantum transitions can operate only at low temperatures. The limiting frequency of operation of electronic devices is determined by the time of flight of an electron in the active region of the device, which in turn depends on the speed of the carriers. For heterostructures, the maximum speed of flight of electrons in the active region is of the order of several units of 107 cm / s, while the speed of plasma waves in the gate channel of the transistor is two orders of magnitude higher, which made it possible to develop THz radiation detectors based on field effect transistors.
Golay cell
In the class of thermal detectors, special attention should be paid to the Golay detector [6], which is 5–15 times superior in limiting detectivity to pyroelectric and thermocouple detectors (also operating without cryogenic cooling), and is one of the most broadband. The Golay receiver is a kind of volumetric gas thermometers, in which the change in the volume of gas with a change in temperature is measured, and is based on the gas law of J. Charles. In the domestic literature, there are several names for the same device at the same time: pneumatic radiation receiver, optical-acoustic receiver, optical-pneumatic radiation measuring transducer. The optical-acoustic receiver (a synonym for a pneumatic receiver and a Golay cell) of infrared radiation is based on the optical-acoustic effect discovered in 1880 by Alexander Bell and investigated by Tyndall and Roentgen. This effect consists in the fact that if a gas capable of absorbing infrared radiation is irradiated with a stream of modulated infrared radiation, then the result is a fluctuation in the temperature of the gas and its pressure, as well as acoustic vibrations. The frequency of oscillations depends on the frequency of modulation of the flow, and the intensity of oscillations depends on the ability of a given gas to absorb infrared radiation and on the intensity of radiation.
A modern pneumatic receiver consists of an expansion chamber filled with gas, one of the ends of which is hermetically sealed by a thin membrane, the surface of which is covered with a layer of a substance that strongly absorbs the received radiation (Fig. 3). The second end is closed by a thin, elastic membrane, on the outer surface of which a metallic mirror coating is applied.
Radiation entering the chamber heats up the gas, which expands and bends the mirror membrane, causing a signal from the readout optical system, for example, by deflecting a focused beam of visible light. The sensitivity of the Golay cell is limited only by the thermal noise of heat transfer between the absorbing film and the gas filling the receiver, which makes it possible to obtain very high detectivity (D* > 3 · 109 cm Hz1/2 W–1) and volt-watt sensitivity (105–106 V / W). The sensitivity of the device substantially depends on the modulation frequency of the input radiation flux and has a pronounced maximum. At low frequencies, the decrease in sensitivity is explained by the fact that the pressure in the expansion chamber has time to equalize with the pressure in the compensation chamber, and at high frequencies, the gas does not have time to heat up and cool down.
Schottky Barrier Diodes
Structures based on Schottky barriers are one of the main elements of THz technologies. Detectors with SBD (Schottky Barrier Diode) of the THz range are used both for direct detection and as nonlinear elements in heterodyne mixers in a wide temperature range T = 4–300 K. Unlike conventional diodes based on pn-junction, Schottky diodes have a significantly high speed, which makes it possible to use them at frequencies up to several terahertz [7]. Schottky diodes have this property due to the fact that charge transport in them is mainly due to thermal emission of electrons through the energy barrier arising in the metal-semiconductor contact. As a rule, such receivers are constructed on the basis of δ-doped Schottky diodes with beam terminals built into the antennas. Historically, the first structures on Schottky barriers had point contacts in the form of tapered metal wires (whiskers). For example, p-Si / W contacts have been widely used. At room temperature, they had a threshold power of NEP ≈ 4 · 10–10 W / √ Hz. We also used point contacts made of tungsten or beryllium bronze to n-Ge, n-GaAs, n-InSb. GaAs SBDs are still used today as mixers in low noise heterodyne receivers.
A cross-section of an SBD with an equivalent junction circuit is shown in Fig. 4. It consists of a junction (less than a few μm2) between the platinum anode and the n-GaAs epitaxial layer. The tip of the metal bar provides electrical contact to the anode and also serves as a long wire antenna to communicate with external radiation. Mixing of waves occurs at the nonlinear resistance of the transition Rj. The series resistance Rs of the diode and the voltage-dependent junction capacitance Cj are parasitic elements that degrade performance.
However, there are some limitations to this Schottky diode technology with whisker contacts. Since the 1980s, development efforts have focused on the production of planar Schottky diodes.
With the aim of using planar technology in the range from 300 GHz to several THz, a “no substrates” technology has been developed. With this approach, the diodes are integrated with the matching network, most of the GaAs substrate is removed from the crystal, and the circuit is created on the remaining epitaxially grown GaAs membrane (Fig. 5). Epitaxial GaAs is the most commonly used semiconductor for planar Schottky diode mixers, although other III–V materials are also used in some applications.
SBD mixers can operate in room temperature conditions up to ν ≈ 25 THz, but in reality with relatively low SBD noise, they are usually used in the frequency range <5 THz. Receivers based on mixers based on Schottky barriers operating at room temperature usually have a radiometric sensitivity of ΔT ≈ 0.05 К at n = 500 GHz and ΔT ≈ 0.5 К at 2.5 THz for an integration time of 1 s and a 1 GHz preliminary detection bandwidth [5]. Parasitic parameters Rs and Cj (Fig. 4) determine the critical frequency of the diode, which is equal to 1 / 2 π Rs Cj. With a decrease in the transition area, the capacitances of the transitions decrease, which increases the operating frequency. But at the same time, it increases the series resistance. Existing devices have an anode diameter of about 0.25 µm and a capacitance Cj of about 0.25 fF. For high-frequency action, GaAs layers are doped to a concentration of n ≈ (5–10) · 1017 cm–3.
Junction capacitance is voltage dependent since the size of the depletion region depends on the applied bias.
In the low frequency range (ν ∼< 0,1 THz), the action of diodes based on Schottky barriers can be described by the mixer theory, which takes into account the parasitic parameters of the Schottky diode (variable capacitance of the diode, series resistance of the diode). However, at high frequencies, several parasitic mechanisms appear, for example, the skin effect (surface effect), and it is also necessary to take into account high-frequency processes in a semiconductor material, such as carrier scattering, carrier transfer time through the barrier (it is about 1 ps), and See also dielectric relaxation time.
At room temperature, SBDs with direct detection realize NEP of about ~3 · 10–10–10–8 W / √ Hz at n = 891 GHz [7,8].
Bolometers with electromagnetic coupling
In cases where high sensitivity is not required (for example, in systems with active illumination using THz emitters, such as quantum cascade lasers and free electron lasers), high spatial resolution, image rendering speed, and ease of use of receivers become relevant. To solve these problems, matrix microbolometric receivers of large format, sensitive to the THz range, can be used. High sensitivity of uncooled microbolometric receivers to terahertz radiation is provided in two ways. The first is the use of antennas connected to the microbolometer in a resistive and / or capacitive way (Fig. 6). The second consists in the use of thin metal absorbers applied to the thermally insulated membrane of the bolometer (Fig. 7). The first is mainly developed by LETI (France) [1, 19], and the second – by NEC (Japan) [2, 17]. In both cases, additional optical cavities can be used. In the long-wavelength part of the THz range, it is preferable to use an antenna, since it allows the electromagnetic power to be supplied to the sensitive element, the size of which can be much smaller than the wavelength.
At present, in both versions at wavelengths of ≈100 µm, the threshold power MDP ≈ 30–40 pW / pixel at wavelengths of 100–200 µm is achieved (NEP < 4 · 10–13 W / √ Hz, since the characteristic measuring frequency band matrix receivers of 320 × 240 format is usually more than 10 kHz). The speed of the receivers is about 10–15 ms, so the frame rate does not exceed 60–100 Hz. It should be noted here that the reported NEP values (>10–10 W / √ Hz) for uncooled THz array microbolometers in a number of reviews [1,2,18,19] are outdated, since back in 2008 in [17] the Japanese uncooled array microbolometric receivers of the THz range were presented by NEC. with MDP ≈ 40 pW / pixel. For many applications, the wavelengths of the received radiation should reach the order of 1000 µm, since in this spectral region the transparency of many materials is noticeably higher than at wavelengths of 100–200 µm. However, at wavelengths of the order of 1000 microns, the threshold power of the above microbolometers increases to 1000 pW / pixel and more [14,15] due to a violation of the consistency of the impedance of the resonator (optical cavity) and vacuum.
In Russia, uncooled microbolometric detectors of THz radiation with a thin metal absorber in 160 × 120 and 320 × 240 formats, with a threshold power of ≈3 nW / pixel at a wavelength of 130 μm, were developed and demonstrated at the Institute of Semiconductor Physics, SB of RAS.
The increased experimental value of the threshold power is due to the use of germanium windows with high absorption of THz radiation and a two-fold lower bias voltage of the bolometer, which is used to provide a large dynamic range required for the operation of a receiver with powerful THz radiation from the Novosibirsk free electron laser [16]. Replacing the germanium window with a silicon one and using an increased bias voltage of the bolometer allows one to lower the threshold power, MDP, to ≈250 pW / pixel. A further decrease in the MDP requires an increase in the thickness of the optical resonator (the height of the bolometer suspension above the multiplexer) and a decrease in the thermal conductivity of the microbolometers. Also, microbolometric receivers of THz radiation of the antenna type were developed and demonstrated in the format 53 × 40 and 32 × 24 with antenna sizes of 150 × 150 and 250 × 250 μm, respectively, intended for operation in the submillimeter range. The experimental MDP value measured at a wavelength of 130 μm was ≈30 nW / pixel. Unlike superconducting and metal bolometers, uncooled bolometers based on vanadium oxides have a resistance of about 100 kΩ, which presents a certain difficulty for matching the antenna to the load. In the design of the IPP SB RAS, the load for the antenna is not the thermosensitive element of the microbolometer itself, but a narrow metal strip applied to the upper layer of silicon nitride between the contacts to the vanadium oxide layer and, therefore, having good thermal contact with the microbolometer, but electrically isolated from the thermosensitive layer. The strip is 70 μm long, 2 μm wide and 200 nm thick, and the resistance is about 100 ohms. The antenna is made of a highly conductive metal and is suspended above the silicon readout circuit at a height of 2.5 µm using silicon nitride braces. These developments of Institute of Semiconductor Physics, SB of RAS allowed ODB Astrohn LLC for the first time in Russia to manufacture several pilot semi-industrial batches of microbolometric arrays sensitive in the terahertz region with a pixel pitch of 25 µm.
The technology for manufacturing antennas and absorbers, being, in fact, a planar technology using photolithography, allows the formation of fragments of absorbers with different spectral and polarization sensitivities on one structure. The working spectral range of absorbers is limited only by their manufacturing technologies, which currently cover the wavelength range from 1.6 to 10000 µm [15].
Pyroelectric detectors
The active element in these devices is a pyroelectric material, in which an electric field is induced when the temperature changes. Thus, a pyroelectric detector, unlike a bolometer, does not require a constant voltage source for operation, and at the same time gives a direct response to incident radiation in the form of a voltage at its terminals. The characteristics of pyroelectric detectors, which are also used in arrays of terahertz chambers, are significantly inferior to microbolometers in sensitivity and are similar in terms of the relaxation time [1].
Field effect transistor detectors
Another widely used class of terahertz radiation detectors is field effect transistor (FET) detectors with high electron mobility. The source and drain of such a transistor are connected by a flat channel filled with a two-dimensional electron gas in which plasma waves of terahertz frequency can propagate (Fig. 8). The nonlinear properties of plasma excitations (electron density waves) in nanoscale field-effect transistors make their response possible at frequencies significantly higher than the cutoff frequency of the device, which is due to the ballistic transport of electrons. The results obtained with field-effect transistors used as terahertz detectors show that the FET can be used for resonant and non-resonant (broadband) detection (see, e. g., [2]). The resonant frequency is tuned by changing the gate voltage, which can be used to create selective tunable solid-state detectors.
These receivers can operate over a wide temperature range up to room temperature. FET detection was observed in HEMTs based on GaAs / AlGaAs, InGaP / InGaAs / GaAs, GaN / AlGaN, and in silicon MOSFETs [1, 2, 4, 5]. Plasma oscillations can also be observed in a two-dimensional electron channel with a back-biased Schottky junction and a double quantum well FET with a periodic lattice gate. Physical mechanism, supporting the creation of stable oscillations lies in the reflection of plasma waves at the boundaries of the transistor with the subsequent amplification of the amplitude of the waves. Plasma excitations in an FET based on a material with a sufficiently high electron mobility can be used for both generation and detection of terahertz radiation.
The domestic company “MWAVE” (with an international representation represented by TeraSense Group Inc.) [20] currently produces several modifications of GaAs / AlGaAs plasmon detectors. The main feature of the technology is the ability to create wide-format cameras up to 128 × 128 pixels with a total sensor size of 40 × 40 cm. Typical array receivers designed for detecting radiation at 100 GHz and 300 GHz consist of pixels 1.5 × 1.5 mm and 0.5 × 0.5 mm in size.
When studying silicon MOSFETs with a gate length of 20–300 nm at room temperature and an emission frequency of 0.7 THz, it was found that the response depends on the gate length and the gate voltage. A volt-watt sensitivity of 200 V / W and NEP > 10–10 W / √ Hz was implemented, which demonstrates the potential of Si MOSFETs as sensitive detectors of terahertz radiation. Also, a 3×5 focal plane array based on Si MOSFET was created, fabricated using 0.25 μm CMOS technology. Each pixel of the array consists of a 645 GHz antenna coupled to an FET detector and a 43 dB voltage amplifier with a 1.6 MHz bandwidth. The NEP value of 3 · 10–10 W / √ Hz was achieved, which paves the way for the implementation of wideband THz detectors and focal plane arrays with a high frame rate of image formation based on CMOS technology. The performance of these fast detectors at room temperature is similar to that of other uncooled detectors in the THz frequency range.
Conclusion
Real progress in terahertz detector technology is driven by the solution of technological problems, the application of new physical concepts and phenomena, and promising applications. The characteristics of several types of discrete detectors and small-format arrays operating at low or sub-Kelvin temperatures and covering the entire THz range are close to their limiting characteristics. However, the future improvement of the technical characteristics of THz radiation detection systems and the growth of their market will be ensured by the use of large-format matrices, and first of all, uncooled or poorly cooled ones. Similar to how it happened in the infrared range.
Uncooled and cooled heterodyne SBD detectors can provide relatively high sensitivity and are suitable for many applications in the THz spectral range, but they are difficult to combine into arrays with a large number of pixels due to the lack of powerful compact solid-state local oscillators. Today, systems are available both with single-pixel coherent SBD detectors and with a small number of pixels, but their effective application at ν > 1 THz remains an important problem.
Most terahertz spectrometers with medium resolution often use uncooled detectors operating over a wide frequency band. The advantages of uncooled detectors are the relative simplicity of the circuit, as well as their ability to operate at room temperature over a wide frequency band. Their NEP is in the range of 10–9–10–11 W / √ Hz.
Uncooled detectors based on microbolometers coupled with a thin metal absorber or antenna are promising for creating large-format arrays used in low-cost systems. Such developed or under development uncooled THz receivers of direct detection with NEP ≈ 10–12 W / √ Hz can be used in many low-resolution spectroscopic applications and active observation systems.
Studies aimed at creating new terahertz detectors, for example, based on low-dimensional structures made of HgCdTe, based on quantum rings and quantum dots Ge / Si, graphene, etc. will be presented in the continuation of the review.
ABOUT AUTHORS
Kulchitsky Nikolai Alexandrovich, Doctor of Scien. (Engineering); e-mail: n.kulchitsky@gmail.com; Prof., Moscow Technological University (MIREA), Chief Specialist, SSC RF, JSC Orion Scientific-Production Association, Moscow, Russia.
ORCID: 0000-0003-4664-4891
Naumov Arkady Valerievich, Head of research and production direction, ASTROHN Technology Ltd, https://astrohn.ru, Lytkarino, Moscow region, Russia.
ORCID: 0000-0001-6081-8304
Startsev Vadim Valerievich, Cand. of Scien. (Engineering), ASTROHN Technology Ltd, https://astrohn.ru, Lytkarino, Moscow region, Russia.
ORCID:0000-0002-2800-544X
M. A. Demyanenko, Cand. of Scien.(Phys.&Math), Senior Researcher, A. V. Rzhanov Institute of Semiconductor Physics, SB of RAS, Novosibirsk, Russia.
ORCID: 0000-0002-8840-9446:
CONTRIBUTION BY THE MEMBERS OF THE TEAM OF AUTHORS
The article was prepared on the basis of work by all members of the team of authors.
CONFLICT OF INTEREST
The authors claim that they have no conflict of interest. All authors took part in writing the article and supplemented the manuscript in part of their work.
Part 1
N. A. Kulchitsky 1, 2, A. V. Naumov 3, V. V. Startsev 3, M. A. Demyanenko 4
State Scientific Center of the Russian Federation, JSC Scientific and Production Association “Orion”, Moscow, Russia.
MIREA – Russian Technological University, (RTU MIREA), Moscow, Russia.
JSC “Optical and Mechanical Design Bureau Astrohn”, Lytkarino, Moscow region, Russia.
A. V. Rzhanov Institute of Semiconductor Physics, SB of RAS, Novosibirsk, Russia.
The paper discusses the problems associated with the development of technology for detectors of terahertz radiation. The main physical phenomena and recent progress in various methods of detecting terahertz radiation (direct detection and heterodyne detection) are considered. The advantages and disadvantages of direct detection sensors and sensors with heterodyne detection are discussed. In the first part, a number of features of direct detection are considered and a description of some types of terahertz detectors of direct detection is given.
Key words: terahertz radiation, direct, heterodyne detection, frequency band, sensitivity
Received on: 15.12.2020
Accepted on: 10.01.2021
Introduction
Terahertz (THz) radiation is electromagnetic radiation, the frequency spectrum of which is located between the infrared and millimeter ranges. The boundaries between these types of radiation are defined differently in different sources. In this work, for definiteness, it is assumed that the THz range is within 0.1–10 THz (30 µm – 3 mm), partially overlapping with the mid-infrared (2.5–50 µm) and millimeter (30–300 GHz, 1–10 mm) ranges, as well as including narrower submillimeter and subterahertz ranges. Devices operating in the terahertz range are becoming increasingly important in various applications (for example, in security, medical, for imaging) [1, 2]. THz waves are effective in detecting the presence of water, and thus, they can effectively distinguish various objects on human bodies (the water content in the human body is about 60%), since the garments are transparent. A brief description of the history of the development of terahertz research is given, for example, in [3]. At the moment, the market for terahertz applications has just left the initial phase of development. Therefore, the forecast for the development of the terahertz technology market (Fig. 1) is quite evaluative. However, we can say with confidence that the market has great prospects.
It is known that radiation detectors can be divided into a class of photonic (quantum) photodetectors, in which the photon energy is converted into a certain primary reaction of the electronic system of the photodetector, and a class of thermal ones, in which the photon energy is converted into heat, and the reaction of the photodetector arises as a result of an increase in the temperature of the sensitive element. The critical difference between detection in the THz range and detection in the infrared range is the low energy of THz photons, which complicates the development of photon detectors of THz radiation. Currently, there is a wide variety of terahertz radiation sensors, both relatively traditional (for example, bolometers) and based on various principles and materials that have appeared recently.
All THz detection systems can also be subdivided into two groups, coherent (heterodyne) detection systems and incoherent (direct detection) detection systems [4]. The former make it possible to determine not only the amplitudes of the signals, but also their phases, which is important for increasing the amount of information received about the object. This also makes it possible to realize the highest characteristics of the detector’s sensitivity and spectral resolution.
Coherent signal detection systems use the principle of heterodyne circuits, since up to now, for high frequencies of radiation, there are no own amplifiers. The detected signals are converted into significantly lower frequencies (ν ≈ 1–30 GHz), which are then amplified by low-noise amplifiers. Basically, these systems are selective (narrowband) detection systems. Incoherent detection systems only detect signal amplitudes and are generally broadband systems. The detectors used in these two detection systems are similar in many cases, but some of them, for example, uncooled thermal detectors, prefer not to be used in coherent systems due to their relatively long response time (τ ≈ 10–7 s).
From the point of view of use and applications, it is important to divide THz radiation receivers into two classes: cooled and uncooled. The advantage of cooled (deeply cooled) detectors is their extremely high sensitivity, which is characterized by noise-equivalent power (NEP) ~10–18–10–20 W / √ Hz at an operating temperature of T = 100–200 mK [5]. Due to their high sensitivity, such superconducting detectors are preferable in conditions of low background photon flux and have found applications, in particular, in astronomy. The advantages of uncooled detectors, in addition to their low cost and ease of use, are their suitability for the manufacture of large-format array detectors. The NEP of uncooled array microbolometric receivers of the THz range with a format of 320 × 240 can reach values of 40 pW or 2 · 10–13 W / √ Hz at a frequency of 3 THz [13].
It should be noted that NEP is defined as the rms input signal power WS required to provide a rms output signal (S) that is equal to the rms noise (N) measured in a 1 Hz bandwidth. However, sometimes noise in the full bandwidth (Δν) defined by a specific measurement circuit is used to determine NEP. In this case, NEP is in watts. This often leads to ambiguity and confusion in the literature. For example, in [11], uncooled THz bolometers developed by the Canadian company INO are presented, characterized by NEP = 24,7 24.7 pW (at a radiation frequency of 4.25 THz), measured in the frequency band Δν = 160 kHz. While in the review [12], the same NEP values are given with the indication of the unit W / √ Hz. To avoid such confusion, we will use the term minimum detectable power (MDP) here if the noise is not scaled to a 1 Hz bandwidth.
The aim of this work is to review modern terahertz detectors, both quantum and thermal, which are most widely used for either direct or heterodyne detection.
Direct detection detectors
Direct detection sensors are suitable for applications that do not require the ultra-high spectral resolution (ν / Δν ≈ 106) provided by heterodyne spectroscopic systems. Unlike heterodyne detection systems, for them there are no technical problems in the formation of multielement matrices due to the need to use a local oscillator (reference radiation source) of high power and detectors with a short response time (τ ≈ 10–10–10–11 s). Therefore, detectors operating at room temperature with a relatively long response time (τ ≈ 10–2–10–3 s) and moderate sensitivity can be used in direct detection systems. Among such detectors for forming THz images, for example, Golay cells and pyroelectric detectors, bolometers and microbolometers are used, which use antennas to couple radiation with small absorbing regions. The NEP value for uncooled detectors usually ranges from 10–12 to 10–9 W / √ Hz (Table 1) [5].
Various types of cooled semiconductor detectors are also used (for example, bolometers with hot electrons based on InSb and bolometers based on impurity Si and Ge) [4,5] with a response time (τ ≈ 10–6–10–8 s) and NEP ≈ 10–13–5 · 10–17 W / √ Hz at an operating temperature T < 4 К. Bolometers cooled to T ≈ 100–300 mK have the highest sensitivity among other direct-action detectors in the sub-mm and mm spectral ranges, reaching NEP limited by fluctuations in cosmic background radiation. Doped photoresistors with direct detection (for example, based on Ge: Ga) are sensitive up to a wavelength of about 400 μm and can be combined into matrices. Their threshold power can reach NEP ≈ 5 · 10–17 W / √ Hz at λ = 150 µm and operating temperature T = 2 К.
A schematic diagram of direct detection is shown in Fig. 2. Both the signal radiation with the power Ws and the background radiation with the power WB are incident on the detector. Focusing optics (lenses, mirrors, etc.) are used to collect radiation from a large area and focus it on a detector. Often, an optical filter is located upstream of the detector to remove background radiation in the spectral range of wavelengths other than the signal wavelength. The relatively small electrical signal of the detector is amplified and further processed.
The ability to detect small signals for detectors with direct detection is limited by the insurmountable background photon noise that does not become small even for the cosmic background. The performance of these detectors is limited by background noise compared to heterodyne detectors, which are limited by quantum noise. As a rule, the threshold power recorded by detectors with direct detection is higher than for heterodyne ones, which is due to the contribution of other noise present in the detector itself, in circuit elements and amplifiers.
The advantage of systems with direct detection is the relative simplicity and the possibility of developing large-format matrices. Most imaging systems use direct detection.
Detector types
The complexity of the development of terahertz devices lies in the fact that when terahertz radiation is detected, some principles of operation of photonic and electronic devices cease to operate. Terahertz radiation is characterized by a low photon energy (4 meV for radiation with a frequency of 1 THz) and therefore photonic terahertz devices with quantum transitions can operate only at low temperatures. The limiting frequency of operation of electronic devices is determined by the time of flight of an electron in the active region of the device, which in turn depends on the speed of the carriers. For heterostructures, the maximum speed of flight of electrons in the active region is of the order of several units of 107 cm / s, while the speed of plasma waves in the gate channel of the transistor is two orders of magnitude higher, which made it possible to develop THz radiation detectors based on field effect transistors.
Golay cell
In the class of thermal detectors, special attention should be paid to the Golay detector [6], which is 5–15 times superior in limiting detectivity to pyroelectric and thermocouple detectors (also operating without cryogenic cooling), and is one of the most broadband. The Golay receiver is a kind of volumetric gas thermometers, in which the change in the volume of gas with a change in temperature is measured, and is based on the gas law of J. Charles. In the domestic literature, there are several names for the same device at the same time: pneumatic radiation receiver, optical-acoustic receiver, optical-pneumatic radiation measuring transducer. The optical-acoustic receiver (a synonym for a pneumatic receiver and a Golay cell) of infrared radiation is based on the optical-acoustic effect discovered in 1880 by Alexander Bell and investigated by Tyndall and Roentgen. This effect consists in the fact that if a gas capable of absorbing infrared radiation is irradiated with a stream of modulated infrared radiation, then the result is a fluctuation in the temperature of the gas and its pressure, as well as acoustic vibrations. The frequency of oscillations depends on the frequency of modulation of the flow, and the intensity of oscillations depends on the ability of a given gas to absorb infrared radiation and on the intensity of radiation.
A modern pneumatic receiver consists of an expansion chamber filled with gas, one of the ends of which is hermetically sealed by a thin membrane, the surface of which is covered with a layer of a substance that strongly absorbs the received radiation (Fig. 3). The second end is closed by a thin, elastic membrane, on the outer surface of which a metallic mirror coating is applied.
Radiation entering the chamber heats up the gas, which expands and bends the mirror membrane, causing a signal from the readout optical system, for example, by deflecting a focused beam of visible light. The sensitivity of the Golay cell is limited only by the thermal noise of heat transfer between the absorbing film and the gas filling the receiver, which makes it possible to obtain very high detectivity (D* > 3 · 109 cm Hz1/2 W–1) and volt-watt sensitivity (105–106 V / W). The sensitivity of the device substantially depends on the modulation frequency of the input radiation flux and has a pronounced maximum. At low frequencies, the decrease in sensitivity is explained by the fact that the pressure in the expansion chamber has time to equalize with the pressure in the compensation chamber, and at high frequencies, the gas does not have time to heat up and cool down.
Schottky Barrier Diodes
Structures based on Schottky barriers are one of the main elements of THz technologies. Detectors with SBD (Schottky Barrier Diode) of the THz range are used both for direct detection and as nonlinear elements in heterodyne mixers in a wide temperature range T = 4–300 K. Unlike conventional diodes based on pn-junction, Schottky diodes have a significantly high speed, which makes it possible to use them at frequencies up to several terahertz [7]. Schottky diodes have this property due to the fact that charge transport in them is mainly due to thermal emission of electrons through the energy barrier arising in the metal-semiconductor contact. As a rule, such receivers are constructed on the basis of δ-doped Schottky diodes with beam terminals built into the antennas. Historically, the first structures on Schottky barriers had point contacts in the form of tapered metal wires (whiskers). For example, p-Si / W contacts have been widely used. At room temperature, they had a threshold power of NEP ≈ 4 · 10–10 W / √ Hz. We also used point contacts made of tungsten or beryllium bronze to n-Ge, n-GaAs, n-InSb. GaAs SBDs are still used today as mixers in low noise heterodyne receivers.
A cross-section of an SBD with an equivalent junction circuit is shown in Fig. 4. It consists of a junction (less than a few μm2) between the platinum anode and the n-GaAs epitaxial layer. The tip of the metal bar provides electrical contact to the anode and also serves as a long wire antenna to communicate with external radiation. Mixing of waves occurs at the nonlinear resistance of the transition Rj. The series resistance Rs of the diode and the voltage-dependent junction capacitance Cj are parasitic elements that degrade performance.
However, there are some limitations to this Schottky diode technology with whisker contacts. Since the 1980s, development efforts have focused on the production of planar Schottky diodes.
With the aim of using planar technology in the range from 300 GHz to several THz, a “no substrates” technology has been developed. With this approach, the diodes are integrated with the matching network, most of the GaAs substrate is removed from the crystal, and the circuit is created on the remaining epitaxially grown GaAs membrane (Fig. 5). Epitaxial GaAs is the most commonly used semiconductor for planar Schottky diode mixers, although other III–V materials are also used in some applications.
SBD mixers can operate in room temperature conditions up to ν ≈ 25 THz, but in reality with relatively low SBD noise, they are usually used in the frequency range <5 THz. Receivers based on mixers based on Schottky barriers operating at room temperature usually have a radiometric sensitivity of ΔT ≈ 0.05 К at n = 500 GHz and ΔT ≈ 0.5 К at 2.5 THz for an integration time of 1 s and a 1 GHz preliminary detection bandwidth [5]. Parasitic parameters Rs and Cj (Fig. 4) determine the critical frequency of the diode, which is equal to 1 / 2 π Rs Cj. With a decrease in the transition area, the capacitances of the transitions decrease, which increases the operating frequency. But at the same time, it increases the series resistance. Existing devices have an anode diameter of about 0.25 µm and a capacitance Cj of about 0.25 fF. For high-frequency action, GaAs layers are doped to a concentration of n ≈ (5–10) · 1017 cm–3.
Junction capacitance is voltage dependent since the size of the depletion region depends on the applied bias.
In the low frequency range (ν ∼< 0,1 THz), the action of diodes based on Schottky barriers can be described by the mixer theory, which takes into account the parasitic parameters of the Schottky diode (variable capacitance of the diode, series resistance of the diode). However, at high frequencies, several parasitic mechanisms appear, for example, the skin effect (surface effect), and it is also necessary to take into account high-frequency processes in a semiconductor material, such as carrier scattering, carrier transfer time through the barrier (it is about 1 ps), and See also dielectric relaxation time.
At room temperature, SBDs with direct detection realize NEP of about ~3 · 10–10–10–8 W / √ Hz at n = 891 GHz [7,8].
Bolometers with electromagnetic coupling
In cases where high sensitivity is not required (for example, in systems with active illumination using THz emitters, such as quantum cascade lasers and free electron lasers), high spatial resolution, image rendering speed, and ease of use of receivers become relevant. To solve these problems, matrix microbolometric receivers of large format, sensitive to the THz range, can be used. High sensitivity of uncooled microbolometric receivers to terahertz radiation is provided in two ways. The first is the use of antennas connected to the microbolometer in a resistive and / or capacitive way (Fig. 6). The second consists in the use of thin metal absorbers applied to the thermally insulated membrane of the bolometer (Fig. 7). The first is mainly developed by LETI (France) [1, 19], and the second – by NEC (Japan) [2, 17]. In both cases, additional optical cavities can be used. In the long-wavelength part of the THz range, it is preferable to use an antenna, since it allows the electromagnetic power to be supplied to the sensitive element, the size of which can be much smaller than the wavelength.
At present, in both versions at wavelengths of ≈100 µm, the threshold power MDP ≈ 30–40 pW / pixel at wavelengths of 100–200 µm is achieved (NEP < 4 · 10–13 W / √ Hz, since the characteristic measuring frequency band matrix receivers of 320 × 240 format is usually more than 10 kHz). The speed of the receivers is about 10–15 ms, so the frame rate does not exceed 60–100 Hz. It should be noted here that the reported NEP values (>10–10 W / √ Hz) for uncooled THz array microbolometers in a number of reviews [1,2,18,19] are outdated, since back in 2008 in [17] the Japanese uncooled array microbolometric receivers of the THz range were presented by NEC. with MDP ≈ 40 pW / pixel. For many applications, the wavelengths of the received radiation should reach the order of 1000 µm, since in this spectral region the transparency of many materials is noticeably higher than at wavelengths of 100–200 µm. However, at wavelengths of the order of 1000 microns, the threshold power of the above microbolometers increases to 1000 pW / pixel and more [14,15] due to a violation of the consistency of the impedance of the resonator (optical cavity) and vacuum.
In Russia, uncooled microbolometric detectors of THz radiation with a thin metal absorber in 160 × 120 and 320 × 240 formats, with a threshold power of ≈3 nW / pixel at a wavelength of 130 μm, were developed and demonstrated at the Institute of Semiconductor Physics, SB of RAS.
The increased experimental value of the threshold power is due to the use of germanium windows with high absorption of THz radiation and a two-fold lower bias voltage of the bolometer, which is used to provide a large dynamic range required for the operation of a receiver with powerful THz radiation from the Novosibirsk free electron laser [16]. Replacing the germanium window with a silicon one and using an increased bias voltage of the bolometer allows one to lower the threshold power, MDP, to ≈250 pW / pixel. A further decrease in the MDP requires an increase in the thickness of the optical resonator (the height of the bolometer suspension above the multiplexer) and a decrease in the thermal conductivity of the microbolometers. Also, microbolometric receivers of THz radiation of the antenna type were developed and demonstrated in the format 53 × 40 and 32 × 24 with antenna sizes of 150 × 150 and 250 × 250 μm, respectively, intended for operation in the submillimeter range. The experimental MDP value measured at a wavelength of 130 μm was ≈30 nW / pixel. Unlike superconducting and metal bolometers, uncooled bolometers based on vanadium oxides have a resistance of about 100 kΩ, which presents a certain difficulty for matching the antenna to the load. In the design of the IPP SB RAS, the load for the antenna is not the thermosensitive element of the microbolometer itself, but a narrow metal strip applied to the upper layer of silicon nitride between the contacts to the vanadium oxide layer and, therefore, having good thermal contact with the microbolometer, but electrically isolated from the thermosensitive layer. The strip is 70 μm long, 2 μm wide and 200 nm thick, and the resistance is about 100 ohms. The antenna is made of a highly conductive metal and is suspended above the silicon readout circuit at a height of 2.5 µm using silicon nitride braces. These developments of Institute of Semiconductor Physics, SB of RAS allowed ODB Astrohn LLC for the first time in Russia to manufacture several pilot semi-industrial batches of microbolometric arrays sensitive in the terahertz region with a pixel pitch of 25 µm.
The technology for manufacturing antennas and absorbers, being, in fact, a planar technology using photolithography, allows the formation of fragments of absorbers with different spectral and polarization sensitivities on one structure. The working spectral range of absorbers is limited only by their manufacturing technologies, which currently cover the wavelength range from 1.6 to 10000 µm [15].
Pyroelectric detectors
The active element in these devices is a pyroelectric material, in which an electric field is induced when the temperature changes. Thus, a pyroelectric detector, unlike a bolometer, does not require a constant voltage source for operation, and at the same time gives a direct response to incident radiation in the form of a voltage at its terminals. The characteristics of pyroelectric detectors, which are also used in arrays of terahertz chambers, are significantly inferior to microbolometers in sensitivity and are similar in terms of the relaxation time [1].
Field effect transistor detectors
Another widely used class of terahertz radiation detectors is field effect transistor (FET) detectors with high electron mobility. The source and drain of such a transistor are connected by a flat channel filled with a two-dimensional electron gas in which plasma waves of terahertz frequency can propagate (Fig. 8). The nonlinear properties of plasma excitations (electron density waves) in nanoscale field-effect transistors make their response possible at frequencies significantly higher than the cutoff frequency of the device, which is due to the ballistic transport of electrons. The results obtained with field-effect transistors used as terahertz detectors show that the FET can be used for resonant and non-resonant (broadband) detection (see, e. g., [2]). The resonant frequency is tuned by changing the gate voltage, which can be used to create selective tunable solid-state detectors.
These receivers can operate over a wide temperature range up to room temperature. FET detection was observed in HEMTs based on GaAs / AlGaAs, InGaP / InGaAs / GaAs, GaN / AlGaN, and in silicon MOSFETs [1, 2, 4, 5]. Plasma oscillations can also be observed in a two-dimensional electron channel with a back-biased Schottky junction and a double quantum well FET with a periodic lattice gate. Physical mechanism, supporting the creation of stable oscillations lies in the reflection of plasma waves at the boundaries of the transistor with the subsequent amplification of the amplitude of the waves. Plasma excitations in an FET based on a material with a sufficiently high electron mobility can be used for both generation and detection of terahertz radiation.
The domestic company “MWAVE” (with an international representation represented by TeraSense Group Inc.) [20] currently produces several modifications of GaAs / AlGaAs plasmon detectors. The main feature of the technology is the ability to create wide-format cameras up to 128 × 128 pixels with a total sensor size of 40 × 40 cm. Typical array receivers designed for detecting radiation at 100 GHz and 300 GHz consist of pixels 1.5 × 1.5 mm and 0.5 × 0.5 mm in size.
When studying silicon MOSFETs with a gate length of 20–300 nm at room temperature and an emission frequency of 0.7 THz, it was found that the response depends on the gate length and the gate voltage. A volt-watt sensitivity of 200 V / W and NEP > 10–10 W / √ Hz was implemented, which demonstrates the potential of Si MOSFETs as sensitive detectors of terahertz radiation. Also, a 3×5 focal plane array based on Si MOSFET was created, fabricated using 0.25 μm CMOS technology. Each pixel of the array consists of a 645 GHz antenna coupled to an FET detector and a 43 dB voltage amplifier with a 1.6 MHz bandwidth. The NEP value of 3 · 10–10 W / √ Hz was achieved, which paves the way for the implementation of wideband THz detectors and focal plane arrays with a high frame rate of image formation based on CMOS technology. The performance of these fast detectors at room temperature is similar to that of other uncooled detectors in the THz frequency range.
Conclusion
Real progress in terahertz detector technology is driven by the solution of technological problems, the application of new physical concepts and phenomena, and promising applications. The characteristics of several types of discrete detectors and small-format arrays operating at low or sub-Kelvin temperatures and covering the entire THz range are close to their limiting characteristics. However, the future improvement of the technical characteristics of THz radiation detection systems and the growth of their market will be ensured by the use of large-format matrices, and first of all, uncooled or poorly cooled ones. Similar to how it happened in the infrared range.
Uncooled and cooled heterodyne SBD detectors can provide relatively high sensitivity and are suitable for many applications in the THz spectral range, but they are difficult to combine into arrays with a large number of pixels due to the lack of powerful compact solid-state local oscillators. Today, systems are available both with single-pixel coherent SBD detectors and with a small number of pixels, but their effective application at ν > 1 THz remains an important problem.
Most terahertz spectrometers with medium resolution often use uncooled detectors operating over a wide frequency band. The advantages of uncooled detectors are the relative simplicity of the circuit, as well as their ability to operate at room temperature over a wide frequency band. Their NEP is in the range of 10–9–10–11 W / √ Hz.
Uncooled detectors based on microbolometers coupled with a thin metal absorber or antenna are promising for creating large-format arrays used in low-cost systems. Such developed or under development uncooled THz receivers of direct detection with NEP ≈ 10–12 W / √ Hz can be used in many low-resolution spectroscopic applications and active observation systems.
Studies aimed at creating new terahertz detectors, for example, based on low-dimensional structures made of HgCdTe, based on quantum rings and quantum dots Ge / Si, graphene, etc. will be presented in the continuation of the review.
ABOUT AUTHORS
Kulchitsky Nikolai Alexandrovich, Doctor of Scien. (Engineering); e-mail: n.kulchitsky@gmail.com; Prof., Moscow Technological University (MIREA), Chief Specialist, SSC RF, JSC Orion Scientific-Production Association, Moscow, Russia.
ORCID: 0000-0003-4664-4891
Naumov Arkady Valerievich, Head of research and production direction, ASTROHN Technology Ltd, https://astrohn.ru, Lytkarino, Moscow region, Russia.
ORCID: 0000-0001-6081-8304
Startsev Vadim Valerievich, Cand. of Scien. (Engineering), ASTROHN Technology Ltd, https://astrohn.ru, Lytkarino, Moscow region, Russia.
ORCID:0000-0002-2800-544X
M. A. Demyanenko, Cand. of Scien.(Phys.&Math), Senior Researcher, A. V. Rzhanov Institute of Semiconductor Physics, SB of RAS, Novosibirsk, Russia.
ORCID: 0000-0002-8840-9446:
CONTRIBUTION BY THE MEMBERS OF THE TEAM OF AUTHORS
The article was prepared on the basis of work by all members of the team of authors.
CONFLICT OF INTEREST
The authors claim that they have no conflict of interest. All authors took part in writing the article and supplemented the manuscript in part of their work.
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