Alain Kreisler

Amorphous semiconducting Y-Ba-Cu-O: a silicon-compatible material for IR uncooled sensitive detection with microsecond response time

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.

Broadband dielectric and IR pyroelectric response of amorphous Y-Ba-Cu-O oxygen depleted thin films

We report on the study of dielectric properties of amorphous semiconducting YBa2Cu3O6+x (x < 0.5, YBCO) thin films with two different thicknesses measured in a broad frequency range (from 40 Hz to 2 GHz) and in the 210 to 430 K temperature range, using a coaxial discontinuity technique. At all temperatures, dielectric permittivity and conductivity spectra exhibit typical features of dielectric relaxation processes. A first relaxation at low frequency (300 Hz - 2 kHz), observed only at high temperature, might be attributed to interfacial effects. Grain boundaries of the YBCO thin films could explain the second relaxation observed at higher frequency (800 Hz - 660 kHz). The higher relaxation frequencies increase with temperature and film thickness and follow a thermally activated behavior of the Arrhenius-type. Application to IR detection with a film deposited on silicon has been targeted: the observed pyroelectric behavior of the detector exhibits a fast response in the μs range.

Modeling Broadband Antennas for Hot Electron Bolometers at Terahertz Frequencies

There is a strong need for wideband and sensitive receivers for radio astronomy and remote sensing applications in the terahertz (THz) region. Superconducting hot electron bolometer (HEB) mixers are a competitive alternative to conventional mixer technologies in the THz range. The aim of this chapter is to model antennas coupled to HEBs belonging to an array working in the 1–7 THz range, typically. Several broadband antenna types were investigated before selecting the log-periodic structure. In order to perform experiments in microwave-devoted anechoic chambers, the antenna working frequency band has been scaled down to the 2.25–17.5 GHz range. We have so measured a front-to-side gain ratio of 18 dB at center frequency, which is quite satisfactory for HEB THz imaging arrays. Experimental results performed with scaled models, as compared with simulations performed with a commercially available finite element software, exhibited on the whole good matching within a 2–3 octave bandwidth.

Modelling of the electric characteristic of type II superconductors by means of Ligurian minimisation

The electrical behaviour of type II superconductors can be characterised-in a first approximation-by a strongly non-linear (even not univocal) e-j relationship. Numerical modelling is necessary to accede to the local characteristic e-j starting from experimental data: the global characteristic U-I of the sample. Classical methods such as Newton-Raphson might fail in dealing with materials having such a characteristic as superconductors have. We thus use an absolutely convergent method based on convex optimisation, with the Ligurian as objective function. Numerical modelling shows that the smooth U-I characteristic might be generated by a simple e-j characteristic with a well-defined critical current density. It is shown how this technique may be extended to solve complete Maxwell equations in the most general case.

High-Tc micro and nano-constrictions modeling: hot spot approach for DC characteristics and HEB THz mixer performance

High-TC hot electron bolometers (HEB) are promising THz mixers due to their expected wide bandwidth, large mixing gain, and low intrinsic noise. We have simulated the characteristics of YBaCuO HEBs with a hot spot model usually dedicated to low-TC phonon cooled devices. With respect to e.g., NbN, the high-TC specificities are mainly arising from the large values of YBaCuO phonon thermal conductivity, and the film to MgO substrate phonon escape time. The consequent effects on the electron temperature profiles along the YBaCuO constriction and the current voltage DC characteristics are considered. The conversion gain G and noise temperature TN were computed for two constriction dimensions. For a 100 nm long × 100 nm wide × 10 nm thick constriction at 9 microwatt local oscillator (LO) power, the expected double sideband (DSB) TN = 1520 K (G = −13.7 dB). For a larger (but more realistic according to YBaCuO aging effects) 400 nm long × 400 nm wide × 35 nm thick constriction, minimum TN = 1210 K DSB at 35 microwatt LO power (G = −13.1 dB).

Modeling of Broadband Antennas for Room Temperature Terahertz Detectors

The purpose of this chapter is to present modeling investigations dedicated to planar broadband antennas with nearly frequency-independent characteristics. These antennas are used to couple efficiently terahertz radiation to semi-conducting YBCO bolometers of high impedance (kΩ range). Both detector and antenna structures are attached on thin substrates to avoid losses due to substrate high-order modes. The antenna properties (radiation diagram, reflection coefficient, input impedance) have been investigated by software modeling with regard to the need of high impedance and high bandwidth. By modifications of conventional concepts (spiral antenna, log-periodic dipole array), an impedance increase of about 50% could be achieved while maintaining wideband properties at the same time. For extreme high antenna impedance we propose a novel design of a grounded multi-tail dipole antenna. The reached antenna impedance maximum for this structure is 2 kΩ for a bandwidth of 10% at the center frequency f = 2 THz. All concepts were proven by measurements of large-scale models in anechoic chambers (in the 1–4 GHz range, typically).

Influence of thermal annealing of MgO substrate on 1/f noise of YBaCuO superconducting thin films

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 Hooges 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.

Recent progress in HTSC bolometric and nonbolometric detectors

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.