With the objective of producing high-performance infrared radiation detectors, we have undertaken the study of devices based on the Y-Ba-Cu-O material produced in amorphous film (a-YBCO) by DC sputtering at low temperature (< 150 °C) on p-doped silicon substrates coated with a SiOx oxide film. Two types of structures have been considered: simple planar structures, where a-YBCO is connected to in-plane metal contacts, and trilayer structures where a-YBCO is sandwiched between the two contacts. The near-infrared response was recorded with a laser source at 850 nm wavelength, amplitude modulated up to 40 MHz. The main characteristics of the responses are: i) a 𝑓+2 behavior at very low frequency (resulting from two dipolar relaxations); ii) a typically pyroelectric behavior in 𝑓+1 up to a few tens of kHz; iii) a maximum response followed by a decrease in 𝑓−1/2, reflecting the heat diffusion through the substrate. All of those results could be interpreted with the help of a theoretical model with adjustable parameters. Small size trilayer devices exhibit a very fast response (time constant: 11 ns). Besides, with noise equivalent power levels as low as 8 pW/Hz1/2 and detectivity values of 8×108 cm⋅Hz1/2·W−1 at 1 MHz, our uncooled devices are standing at the state-of-the-art both in terms of sensitivity and bandwidth.
The well known Y-Ba-Cu-O superconductor (oxygen rich) material also exhibits a semiconducting phase (oxygen depleted), which offers in its amorphous form (a-YBCO), an attractive solution for the easy fabrication of room temperature thermal radiation detectors. The operating mechanism is related to a sensitive pyroelectric response that originates from the permanent dipolar moment of the Y-Ba-Cu-O unit cell. In the first part of this paper, we investigate material aspects of a-YBCO thin films (surface morphology, electrical transport and optical properties) for a better understanding of the microstructure vs. conductivity relationship. In the second part, we report on the near-infrared (NIR) characterization of planar and trilayer detector devices fabricated on silicon substrates. These detectors exhibit a very fast response (time constant tau = 1.9 µs for a planar device; tau = 0.12 µs for a trilayer device) as compared to commercially available pyroelectric sensors. The best noise equivalent power (NEP) and detectivity D*, which are at the state of the art, were observed in the NIR at 10 kHz modulation frequency: NEP = 2.0 pW/rootHz and D* = 6.6×10^9 cm•rootHz/W for planar device; NEP = 2.6 pW/rootHz and D* = 5.7×10^9 cm•rootHz/W for trilayer device. Evolution towards THz detection is finally considered
Hot electron bolometers (HEB) exploiting the properties of the high-TC superconductor (HTS) Y-Ba-Cu-O, are offering a competitive alternative to THz Schottky mixers, which require moderate cooling (e.g., 60 to 80 K). This arises from the HTS HEBs expected wide bandwidth (tens of GHz), and low local oscillator (LO) power requirements: tens of microwatts, whereas several milliwatts are necessary to pump Schottky diode LOs efficiently. In fact, the large instantaneous bandwidth is related to the extremely short electron to phonon relaxation time (1 to 2 ps, typically) in YBa- Cu-O. It is much longer in low-TCsuperconductors (LTS), e.g., about 20 ns in NbN. Besides, as required for LTS materials, it is mandatory to fabricate high quality and ultra thin HTS epitaxial films, so to process nano-bolometers exhibiting good mixing performances (i.e., double sideband noise temperature Tn and conversion gain G). Most of all, the challenge for Y-Ba-Cu-O remains the chemical reactivity and the aging effects, as reported previously. The main objectives of this paper are: i) to predict Y-Ba-Cu-O HEB heterodyne mixer performances, and ii) to exploit those predictions to simulate a stand-off passive detection system for, e.g., screening or security applications.
Since the observation of pyroelectric properties in oxygen depleted semiconducting Y-Ba-Cu-O, the interest of its amorphous phase (a-YBCO) obtained at low deposition temperature (150 °C) has been demonstrated for near-infrared (NIR) detection. At the core of the uncooled thermal detector development, there is the material choice for sensing the incoming radiation. Apart from its manufacturing compatibility with silicon technologies for further integration with readout electronics, a low noise level and a high value of the pyroelectric coefficient are highly desirable material properties. In the first part of this paper, we investigate room temperature noise performances of planar and trilayer detectors fabricated on silicon substrates. The best noise equivalent power (NEP) and detectivity D*, which are at the state of the art, were observed at 10 kHz modulation frequency: NEP = 2.0 pW/Hz1/2 and D* = 6.6×109 cm·Hz1/2/W for planar structures; NEP = 2.6 pW/Hz1/2 and D* = 5.7×109 cm·Hz1/2/W for trilayers. These detectors also exhibit a very fast response (time constant τ = 1.9 μs for planar, and τ = 0.12 μs for trilayer devices) as compared to commercially available pyroelectric sensors. In the second part, we examine the pyroelectric response of a-YBCO to extract the pyroelectric coefficient p. A first estimate of p gave a value as high as 600 μC·m−2·K−1 at 300 K. The pyroelectric figure of merit Fd which takes into account dielectric properties of the material (dielectric constant and dielectric losses) is also discussed with respect to results published for pyroelectric oxide thin films sputtered on silicon substrates.
Since the observation of pyroelectric properties in oxygen depleted semiconducting Y-Ba-Cu-O, the interest of its amorphous phase (a-YBCO) obtained at low deposition temperature (150°C) has been demonstrated for near-infrared (NIR) detection. In the first part of this paper, we investigate material aspects of a-YBCO thin films (surface morphology, electrical transport and optical properties) for a better understanding of the microstructure vs. conductivity relationship. In the second part, we report on the NIR characterization of planar and trilayer detector devices fabricated on silicon substrates. These detectors exhibit a very fast response (time constant τ = 1.9 μs for planar device; τ = 0.12 μs for trilayer device) as compared to commercially available pyroelectric sensors. The best noise equivalent power (NEP) and detectivity D* , which are at the state of art, were observed at 10 kHz modulation frequency: NEP = 2.0 pW/Hz1/2 and D* = 6.6×109 cm·Hz1/2/W for planar device; NEP = 2.6 pW/Hz1/2 and D* = 5.7×109 cm·Hz1/2/W for trilayer device. We have interpreted this fast response by means of an analytical model without adjustable parameters. In the third part, the potential of THz detection is examined, in the case of a-YBCO coupling to a planar antenna. The general coupling conditions of the THz incident radiation to a-YBCO are examined first, with relation to the film THz absorption coefficient and conductivity measured by time-resolved spectroscopy. The coupling conditions of the film to the readout circuitry are then examined, with relation to the Schottky nature of the metal/a-YBCO contacts.
In the THz range, high-TC superconductor (HTS) hot electron bolometers (HEB) are offering a competitive alternative to moderately cooled (e.g., 60 to 80 K) Schottky mixers. This is due to HTS HEBs large expected bandwidth (tens of GHz), and low local oscillator power requirements (tens of microwatts, as compared to several milliwatts required for Schottky diode pumping). Indeed, the large instantaneous bandwidth is driven by the very short electron to phonon relaxation time in Y-Ba-Cu-O HTS oxide − 1 to 2 ps, typically, whereas it is about 20 ns in NbN, a low- TC superconductor (LTS). Besides, as for the LTS counterparts, it is mandatory to grow ultra thin high quality HTS epitaxial films, in order to process micro or nano-bolometers (nano-constrictions) exhibiting good mixing performances. Early HEB models were based on the point bolometer approach, which describes the device in terms of thermal reservoirs only. We have extended the hot spot model (initially introduced for LTS HEBs) to Y-Ba-Cu-O HEBs, taking into account the spatial dependence of the electron and phonon temperatures along the nano-constriction. We have also introduced the THz frequency effects in the Y-Ba-Cu-O superconducting transition as well as the impedance matching between the nanoconstriction and the antenna. We have checked the feasibility of stand-off target detection operating in the passive mode with an Y-Ba-Cu-O HEB THz heterodyne mixer. For instance, detection at 5 m through cotton cloth in passive imaging mode could be readily achieved in standard humidity conditions with 10 K resolution at 2.5 THz.
High-TC superconducting (HTS) hot electron bolometers (HEB) are promising THz mixers due to their large expected
bandwidth and low local oscillator (LO) power requirements at 60-80 K operating temperature. To obtain HEB efficient
mixing, it is mandatory to grow very thin high quality HTS films leading to good micro or nano-bolometer
superconducting properties. The challenge for Y-Ba-Cu-O resides, however, in the chemical reactivity of the material
and the related aging effects. Early HEB models described the device in terms of thermal reservoirs only, namely the
electrons and the phonons of the superconductor. The electron-phonon interaction time, which drives the HEB mixer
ultimate response, is 1-2 ps for Y-Ba-Cu-O, with an expected bandwidth close to 100 GHz. Recently, we introduced the
hot spot model for Y-Ba-Cu-O HEBs, taking - more realistically - the spatial dependence of the electron temperature
along the nano-bolometer (or constriction) length into account. From DC analysis, the I-V characteristics could be
deduced. In this paper, we further consider a full description of the constriction impedance at THz frequencies, which
allows to work out the mixer performance in terms of double sideband noise temperature TDSB and conversion gain G.
For a constriction of technologically achievable dimensions, i.e., 400 nm long x 400 nm wide x 35 nm thick, minimum
TDSB = 1900 K at 9 μW LO power, with G = -9.5 dB, is obtained at 400 GHz, assuming impedance matching with a selfcomplementary
planar antenna.
The yttrium-barium-copper oxide cuprate (YBCO) is well-known to exhibit superconducting properties in its YBa2Cu3O(6+x) phase for x close to 1. Oxygen depletion (x ≈ 0.3–0.5) of this compound leads, however, to a semiconductor. An unusual although promising application of YBCO in this semiconducting form can be sought in the field of uncooled thermal detectors of the bolometer type due to its large temperature coefficient of resistance (TCR = 1/R (dR/dT) = –3 to –4 %/K). Besides, semiconducting YBCO films can be deposited without substrate heating in amorphous semiconducting form (a-YBCO), which makes the integration of this material compatible with already processed signal readout electronics (e.g. a CMOS chip). In the present work, we consider two a-YBCO bolometric geometries, i.e. planar (#Si-pla) or trilayer (#Si-tri), and compare their detection performance with reference to already reported semiconducting devices, mainly designed for room temperature operation. For both detector devices, the response was measured at 850 nm wavelength for experimental convenience. The response of device #Si-pla exhibited a low-frequency regular low-pass bolometric behavior (30 Hz cut-off), followed by a high-pass behavior (60 kHz cut-off / 3 μs time constant) that could be assigned to the pyroelectric state of a-YBCO. The response of device #Si-tri exhibited the high-pass behavior only. Detectivity values up to 3.5×108 cm⋅Hz1/2⋅W−1 have been measured. The high frequency sensitivity offers a promising solution for fast imaging applications, especially in the far IR / THz range where moderate cost systems should be considered.
YBa2Cu3O6+x compounds are well known to exhibit superconducting properties for x > 0.5 and semiconducting
properties for lower oxygen content. Superconducting YBCO was obtained commercially; the semiconducting material
was deposited by sputtering at room temperature. In order to migrate from superconducting to uncooled semiconducting
far-infrared bolometer technologies, we have first realized and compared the performance of 2 × 2 pixel arrays made
from both materials deposited on MgO substrates. Pixels were in the shape of meanders, embedded in an area of about 1
mm2. Pixel detectivity and thermal crosstalk were studied in the 1 Hz to 100 kHz modulation frequency range by using a
850 nm solid state laser. Secondly we have improved the geometry of semiconducting YBCO bolometers fabricated on
silicon substrates, in order to match their impedance with the impedance of the antenna required for working in the THz
range. First optical results are also presented, where both regular bolometric and pyroelectric responses are exhibited.
Superconducting hot electron bolometer (HEB) mixers are a competitive alternative to other technologies in the terahertz
frequency range because of their ultrawide bandwidth, high conversion gain, and low intrinsic noise level. A process to
fabricate stacked YBaCuO / PrBaCuO ultra-thin films (in the 15 to 40 nm range) etched to form 0.4 μm × 0.4 μm
constrictions, elaborated on MgO (100) substrates, has been previously described. HEB structures were fabricated on
such stacks, covered by log-periodic planar gold antennas, aiming at spanning the 0.9 to 7 THz range. Ageing effects
were observed, however, with the consequence of increased electrical resistance, significant degradation of the regular
bolometric response, so preventing HEB mixing action. Several measures have been attempted to address these
problems, mainly by considering the embedding technological issues related to the YBaCuO constriction electrical
coupling to the antenna and the intermediate frequency (IF) circuitry. For this purpose, the YBaCuO impedance was
analyzed, and mismatch to antenna and IF strip was considered. Besides, THz antenna simulations were performed and
validated against experiments on scaled models at GHz frequencies. Electromagnetic coupling to the incoming radiation
was also studied, including crosstalk between neighbor antennas forming a linear imaging array.
There is a strong need for wideband and sensitive THz receivers for radio astronomy and remote sensing applications, for which superconducting Hot Electron Bolometer (HEB) mixers are very competitive. Besides, new THz applications have arisen because of interesting interaction with various media, for which room temperature detectors are highly attractive. We have used YBa2Cu3O7-d (YBCO) oxides to fabricate bolometers, either of high-Tc superconducting HEB type (high oxygen content, &dgr; < 0.3) or semiconducting type (low oxygen content, &dgr; > 0.5). Firstly, we fabricated HEBs made from superconducting YBCO ultrathin films (15 to 40 nm thick) etched to form submicrometer constrictions. In order to investigate the feasibility of highly sensitive HEB linear arrays for passive THz imaging applications, extensive technological runs were performed to prevent ageing effects on both the pixel electrical and optical characteristics. Secondly, we designed YBCO semiconducting bolometric pixels for room temperature operation. Due to the reduced sensitivity and bandwidth with respect to superconducting HEBs, we considered the feasibility of 2D arrays for active THz imaging. As a first experimental step, pixel responsivity and thermal crosstalk between pixels were studied in the 1 Hz to 100 kHz modulation frequency range, so to evaluate the adequate frame refreshing rate.
There is a strong need for wideband and sensitive THz receivers for radio astronomy and remote sensing applications, for which superconducting Hot Electron Bolometer (HEB) mixers are very competitive. Besides, many new THz applications have arisen because of interesting interaction with various media. For practical and economical issues, room temperature detectors are highly attractive for these latter. We have used YBaCuO oxides to fabricate bolometric pixels, either of high-Tc superconducting HEB type (high oxygen content) or semiconducting type (low oxygen content). In the THz range, such materials without any antenna would be totally reflective to the electromagnetic radiation. Moreover, integrated planar antenna structures are recommended for optimal coupling to the small detection area. The aim of this work was to investigate broadband THz antennas coupled to YBaCuO pixels, and address the specific problems arising for each family. For HEBs, self-complementary log-periodic wideband antennas, exhibiting quasi-constant impedance, were chosen. After designing / simulating the THz antenna, we scaled it down to the microwave region for experimental validation purposes. A key issue arose because a thick (10 cm) and MgO electrically equivalent substrate (epsr = 10) was needed to fabricate the large-scale model. For room temperature semiconducting pixels, the main objective was to maintain a large bandwidth despite the difficult matching of the antenna impedance to the large bolometer resistance, namely in the kΩ range. Starting from a high-impedance single dipole-like structure, multi-dipole solutions were investigated to increase the bandwidth. Radiation pattern and polarization issues have also been considered.
Superconducting Hot Electron Bolometer (HEB) mixers are a competitive alternative to Schottky diode mixers or other conventional superconducting receiver technologies in the terahertz frequency range because of their ultrawide bandwidth (from millimeter waves to the visible), high conversion gain, and low intrinsic noise level, even at 77 K. A new technological process has been developed to realize HEB mixers based on high temperature superconducting materials, using 15 to 40 nm thick layers of YBa2Cu3O7-δ (YBCO), sputtered on MgO (100) substrates by hollow cathode magnetron sputtering. Critical temperature values of YBCO films were found in the 85 to 91 K range. Sub-micron HEB bridges (0.8 μm x 0.8 μm) were obtained by combining electronic and UV lithography followed by selective etching techniques. Realization of YBCO HEB coupling to planar integrated gold antennas was also considered.
In this work, we have characterized YBaCuO high Tc superconducting thin films deposited on (001) MgO substrates and compared their noise properties. The films were sputtered on substrates which were annealed at different temperatures prior to deposition. The noise measurements were performed under the same conditions:
1) Without bias, the films are at equilibrium and exhibit only thermal noise proportional to the real part of the impedance for the voltage fluctuations or proportional to the real part of the admittance for the current fluctuations.
2) With bias, the films exhibit 1/f noise due to the conductivity fluctuations.
The extra noise is compared with Hooge's empirical relation. The normalized noise spectral density (Sv / V2) measured at 300 K as a function of the substrate annealing temperature displays a bell-shaped dependence with a maximum at a critical temperature.
YBaCuO thin film surfaces have been studied with an original laboratory-made attachment associated with a commercial atomic force microscope. Using a doped silicon probe coated with doped diamond, we have obtained simultaneously topographical and local contact resistance surface images within a given area of the sample. YBaCuO films on various types of substrates were observed: polycrystalline yttria-doped zirconia (PYSZ), and MgO or SrTiO3 single crystals. For YBaCuO films grown on PYSZ, the electrical image has clearly revealed the presence of electrical disconnection zones between grains, which correspond to grain boundary areas observed on the topographical image. The presence of such defects can explain the modest critical current density (Jc approximately equals 3 X 104 A/cm2 at 77 K) measured on these granular films. On the opposite, for films grown on single-crystal substrates, the electrically connected areas between grains are visible on the electrical images, that can be correlated to better electrical transport properties of the films. Moreover, island-shaped grains exhibiting terraces of one unit cell vertical height could be seen, on close inspection of films grown on PYSZ and MgO substrates. For YBaCuO elaborated on SrTiO3 substrates, the observed grain structure rather exhibited a spiral shape.
Among the two categories of superconducting detectors that are actually operational for competitive performance in the submillimeter-wave/terahertz range, SIS devices used as heterodyne mixers offer very low noise level, but are frequency limited by the superconducting energy gap. Bolometric detectors on the other hand, although of lesser performance a priori, have no intrinsic frequency limitation due to their purely thermal sensing principle. Moreover, their inherent slow response can be overcome by developing hot- electron bolometer technologies based on superconducting nanostructures, that allow promising output frequencies of several GHz. In order to implement competitive submillimeter wave detectors, wideband planar antennas are preferred to improve the radiation to device coupling, rather than conventional bolometric structures (of either monolithic or composite type) that do not allow both sensitive and fast detector operation. All these aspects are commented and many realizations covering a large number of technologies, in view of both homodyne and heterodyne detection, are reviewed, commented and discussed.
The main phenomena that may be responsible of radiation detecting mechanisms in superconductors are described in a first part of this paper. Several examples are given, ranging from broadband and sensitive bolometric devices to ultrafast nonbolometric infrared detectors. The second part is devoted to the study of various transition edge bolometers designed to be built at low cost. Polycrystalline zirconia substrates have been used, to grow YBCO films by both in situ and ex situ oxygenation after radiofrequency sputtering. The voltage responsivity at 10.6 micrometers wavelength has been studied with respect to the modulation frequency of the incident radiation, both experimentally and theoretically. A 2D thermal model has been developed, allowing to interpret the complex (amplitude and phase) experimental frequency responses of various devices. In particular, the amplitude response can be described as a succession of f-1 and F-1/2 segments. Noise measurements show NEP and detectivity values reaching a very satisfactory level for granular films.
Since the discovery of high-Tc superconducting cuprates, the interest in envisioning various radiation detecting mechanisms has been considerably revived. If the implementation of Josephson micro-junctions is greatly inhibited by difficulties to setup multilayer processes, simpler devices - that involve a single superconducting film deposition step - are technologically more accessible.
High-Tc YBaCuO thin films have been prepared on polycrystalline zirconia substrates by low-temperature RF diode sputtering, followed by a rapid thermal annealing. A Tc(R
An overview of applications of YBaCuO thin films is presented emphasizing the RF properties of these high-Tc superconductors and microelectronics uses with silicon-based substrates. The substrates considered include the MOS dielectrics SiO2 and Si3N4, and a low-temperature deposition processing technique for these films followed by post-annealing is detailed. The time of the rapid thermal annealing following film processing is shown to dictate the electrical and structural characteristics of the films. Rapid thermal annealing is shown to be an effective method for manufacturing superconducting films of silicon substrates, although several cycles of the process can block all superconducting properties. The use of alternative buffer layers is considered as a means for protecting against this effect in superconductors composed of silica on silicon nitride.
In order to optimize the performances of YBaCuO thin films, elaborated by
a two step process on SiO2/SiN4/Si substrates, we have studi&1 the effect of
rapid thermal annealing (EA) conditions on low temperature (i.e. < 200°C)
sputter&i layers, in the thickness range 0.3 - 1.5 m. Both the superconducting
properties and the surface quality of the layers have been investigated.
One of the advantages of this process is to allow the use of a very thin
nitride layer (i.e. 45 A), to avoid the harmful diffusion of silicon from the
wafer. This fast technique is also efficient to prevent interdiffusion between
the silica and YBaCuO layers . Our first results show that an onset temperature
in the 87-91K range can been obtained, with T(R.o) 60 K and 'c 500 A. Clii (at
T(R.o /2) . It is also shown that these figures might be improved by repeated
RTA cycles.
Joseph Baixeras, Francois Carrie, Jean Chabrerie, Ferechteh Hosseini Teherani, Alain Kreisler, Tibor Pech, Gilles Poullain, Jean Hamet, Jacques Muniesa, Michel Rapeaux, Joan Aymami
We have sthdied the ability of high Te supereonduotors to operate as
electrical switches or current limiters . For this purpose, we have usi YBaCuO
films, deposit&I on polycrystalline zirconia slabs by various techniques:
conventional two-step processes as well as rapid thermal annealing have been
consider&1 . The main results are that with a Tc(onset) of 91 K, the variation
of the critical current density versus fruency shows that it will be
possible to use the films at 50-60 Hz, although we ne&i to improve our low
current density films to demonstrate this.
In order to optimize the performances of YBaCuO thin films, elaborated by
a two step process on Si02/Si3N4/Si substrates, we have studied the effect of
rapid thermal annealing (FI'A) conditions on low temperature (i.e. < 200°C)
sputtered layers, in the thickness range 0.3 - 1.5 gin. Both the superconduc-
ting properties and the surface quality of the layers have been investigated.
One of the advantages of this process is to allow the use of a very thin
nitride layer (i.e. 45 A), to avoid the harmful diffusion of silicon from the
wafer. This fast technique is also efficient to prevent interdiffusion between
the silica and YBaCuO layers. Our first results show that an onset temperaure
in the 87-91K range can been obtained, with 60 K and 'c 500 AOlfl (at
T(R..0)/2). It is also shown that these figures might be improved by repeated
RTA cycles.
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