Annick F. Dégardin

Semiconducting YBCO Thin Films for Uncooled Terahertz Imagers

The terahertz domain (500 GHz - 5 THz) has been object of unceasing research activities, due to the wide range of conceivable applications in these fields. This study focuses on the development of semiconducting YBa2Cu3O6+x (YBCO) thin films to be used as sensitive elements on future uncooled terahertz imagers working on a thermal principle. YBCO thin films have been hollow-cathode sputtered on MgO single-crystals under different conditions. Electrical and structural characterizations have then been carried out. The resistivity of the thin films and the temperature coefficient of resistance (TCR) have been determined. X-ray diffraction and atomic force microscopy analyses have then been performed. If compared with materials currently used as sensing element in commercial near-infrared imagers, electrical characterization shows values of the TCR comparable to amorphous silicon and almost two times better than VOx-compounds.

From Superconducting to Semiconducting YBCO Thin Film Bolometers: Sensitivity and Crosstalk Investigations for Future THz Imagers

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 close to room temperature (100degC, typically). In order to migrate from superconducting to uncooled semiconducting far-infrared bolometer technologies, we have compared the performance of 2 times 2 pixel arrays. 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. Specific THz antenna concepts were also considered.

Nanopatterning and Hot Spot Modeling of YBCO Ultrathin Film Constrictions for THz Mixers

High-TC hot electron bolometers (HEB) are promising THz mixers due to their expected wide bandwidth, large mixing gain, and low intrinsic noise. To achieve this goal, 0.6-μm-size constrictions were patterned on YBaCuO-based, 10-40-nm-thick films grown on (100) MgO substrates, which as previously reported, exhibited good DC superconducting properties. In this paper, we have simulated the DC and mixer characteristics of YBaCuO HEBs with a hot spot model usually dedicated to low-TC devices. For a 100 nm × 100 nm × 10 nm constriction, the expected double sideband noise temperature TN is 2000 K for 5 μW local oscillator (LO) power (G = -13.5 dB conversion gain). For a larger (but more realistic according to YBaCuO aging effects) 600 nm × 1000 nm × 35 nm constriction, TN = 1300 K at 200 μW LO power (G = -12 dB). This approach is expected to allow optimizing the operation of the HEB constriction coupled to a THz planar antenna.

Fabrication of high-Tc superconducting hot electron bolometers for terahertz mixer applications

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.

High critical current densities observed in PrBa2Cu3O7−δ∕YBa2Cu3O7−δ∕PrBa2Cu3O7−δ ultrathin film constrictions

An original process has been developed for fabricating 0.45×0.45μm2 constrictions with normal metal banks using stacked YBa2Cu3O7−δ and PrBa2Cu3O7−δ ultrathin films. High values of critical temperature Tc≈89K and critical current densities Jc≈2×107A∕cm2 at 77K were measured on 17nm thick YBa2Cu3O7−δ, after the complete fabrication on MgO substrate. The observed robust superconducting properties of the device under applied magnetic field are attributed to large edge barriers opposed to vortex entry inside the nanobridge section. As there is no apparent degradation of the superconducting properties after long time storage, the benefit of using ultrathin PrBa2Cu3O7−δ buffer and overlayer in YBa2Cu3O7−δ constrictions is confirmed. Such YBa2Cu3O7−δ nanowires are shown well adapted to phase-slip based devices and expected to fulfill the performances required in high frequency applications.

YBCO thin film sputtering: an efficient way to promote microwave properties

To obtain high quality YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) films by on-axis rf-magnetron sputtering, we have undertaken a twofold study: first, we have investigated the role of sputtering plasma parameters (dc self-bias voltage, rf power, oxygen flux and water vapor flux introduced into the plasma) in order to limit resputtering effect. In particular, we have shown the significance of mastering the dc self-bias voltage. Secondly, we have deposited YBCO films on [001] LaAlO/sub 3/ substrates taking into account the optimum flux conditions previously determined, and put into light the effect of the substrate temperature. Cross-optimization of the microstructural, electrical and microwave properties has been performed with respect to substrate temperature. A narrow optimal temperature window has been established to obtain films suitable for microwave applications.

MgO substrate surface optimization for YBaCuO thin film growth

To obtain high quality Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7-x/ (YBaCuO) films grown on [001] MgO substrates, we have investigated the influence of high temperature substrate annealing prior to film deposition. In particular, we discuss the effect of annealing conditions (atmosphere, temperature and time) on MgO substrate morphology observed with atomic force microscopy. Thermal treatment under specific conditions resulted in the formation of [001]-oriented terraces bounded by [100]-oriented steps on the substrate surface. An improvement in the crystalline quality, as well as better DC electrical properties and microwave surface resistance values, has been observed for films sputtered onto substrate surfaces exhibiting such regular nanoscale step structures.

Nonlinear microwave effects in superconducting microstrip resonators based on YBCO thin films

Linear microstrip resonators based on YBaCuO thin films made by RF magnetron sputtering on MgO substrates have been characterized with transmission microwave measurements as a function of temperature (from 10 K to T/sub c/) and the incident microwave power (from -30 to 10 dBm). The variation of the resonant frequency with temperature has been used to determine the intrinsic parameters (/spl lambda//sub 0/, T/sub c/) of the thin film using a fitting procedure and appropriate models. For low microwave powers (<-18 dBm), the variation of the quality factor, the insertion loss and the surface resistance do not exhibit any nonlinear effect. For higher microwave powers nonlinear effects appear and the variation of the quality factor has shown that they vary in a different way according to three temperature zones. Various theoretical models have been used to discuss the experimental results. These zones can be correlated to the temperature dependence of the surface resistance, in particular to grain boundaries resistance and to penetration of microwave Josephson vortices into the weak links. Moreover, the variation of the measured surface resistance as a function of the RF surface magnetic field is discussed considering different models.

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.

Semiconducting YBaCuO films grown on silicon substrates: IR room temperature sensing and fast pyroelectric response

YBa2Cu3O6+x (YBCO) oxides, well known as superconductors for x > 0. 5, are semiconductors (SC) for lower oxygen content. SC films DC sputtered at 150 °C in amorphous form (a-YBCO) exhibited a temperature coefficient of resistance competitive with other bolometric sensing materials. Such deposition conditions are highly beneficial to integrate a radiation detector on a silicon chip bearing already processed readout electronics. IR detector structures have been processed: simple planar or more advanced metal/a-YBCO/metal trilayers. IR response was investigated at 850 nm as a function of the modulation frequency. The planar structure exhibited a regular bolometric response below 100 Hz, then a pyroelectric high-pass response up to 100 kHz. The trilayer device only exhibited the pyroelectric behavior up to 100 kHz . In the pyroelectric regime, detectivity values peaked above 2×108 cm Hz½/W with time constants in the μs range, under both DC biased and unbiased conditions.