Vishal S. Jagtap

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.

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.

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.

Migrating from superconducting to semiconducting YBCO thin film bolometers as future far-infrared imaging pixels

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.

Ageing and embedding issues for high-Tc superconducting hot-electron bolometers for THz imaging

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.

Superconducting and semiconducting YBaCuO thin film bolometer investigations for future THz imaging arrays

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.