W. Kula

Investigation of semiconducting YBaCuO thin films: A new room temperature bolometer

We explore the application of the semiconducting phases of YBaCuO thin films as a bolometer for uncooled infrared detection. For this study, four different structures were built with different types of buffer layers: YBaCuO on a Si substrate with and without a MgO buffer layer, and on an oxidized Si substrate with and without a MgO buffer layer. These films were all amorphous without a detectable long range order. For comparison, crystalline tetragonal YBa 2 Cu 3 O 6.5 and YBa 2 Cu 3 O 6.3 thin films on a LaAlO 3 substrate were included into the study. All six films exhibited semiconducting resistance versus temperature characteristics. The bolometer figures of merit, responsivity, and detectivity were calculated from the measured temperature coefficient of resistance ~TCR! and the inherent noise characteristics of the temperature sensing element. The room temperature TCRs for all four amorphous films were greater than 2.5% K 21 . The highest TCR of 4.02% K 21 was observed on the amorphous YBaCuO thin film deposited on MgO/Si without a SiO2 layer. The TCR of the tetragonal films, on the other hand, remained 2% K 21 or less in the same temperature range. Noise measurements performed in the 1‐100 Hz frequency range revealed a quadratic dependence on the bias current as would be expected from ohmic electrical characteristics. The Johnson and 1/f regions were clearly identified in the noise spectrum. From TCR and noise measurements, we estimated the amorphous semiconducting YBaCuO bolometers would have a responsivity as high as 3.8310 5 V/W and a detectivity as high as 1.6310 9 cm Hz 1/2 /W for 1 mA bias current and frame frequency of 30 Hz if integrated with a typical air-gap thermal isolation structure.

Laser patterning of Y‐Ba‐Cu‐O thin‐film devices and circuits

We report our studies on electrical properties of Y‐Ba‐Cu‐O test devices and circuits fabricated using a laser‐writing patterning technique. The patterning procedure is noninvasive, does not require a patterning mask, and does not contaminate nor damage the surface of patterned films. Our laser‐written, oxygen‐rich lines (typically 4–100 μm wide) possess excellent superconducting properties with zero resistivity at 89.5 K and critical current densities of above 2 MA/cm2 at 77 K. On the other hand, oxygen‐poor regions are semiconducting and exhibit thermally activated transport, well described by a three‐dimensional, variable‐length hopping process. Their resistance below 100 K is above 10 MΩ/square. A number of test structures patterned by laser writing, such as a microbridge, coplanar transmission line, open‐ended microwave resonator, photoconductive switch, and Y‐Ba‐Cu‐O field‐effect transistor, have been presented.

Charge transport in amorphous and tetragonal semiconducting YBaCuO films

Abstract We have explored the charge transport mechanisms in six different YBaCuO semiconducting thin films in the temperature range of 70 K to room temperature. Two of the samples were deposited on LaAlO 3 substrate and were tetragonal with the composition of YBa 2 Cu 3 O 6.5 and YBa 2 Cu 3 O 6.3 . The other four were amorphous as-deposited on Si substrate with and without a MgO buffer layer, and on an oxidized Si substrate with and without a MgO buffer layer. All tested films exhibited semiconductor-type resistance vs. temperature characteristics with increasing resistance as the temperature was decreased. Around room temperature all six samples had thermally activated transport characteristics that was interpreted as activation of hole-like carriers from localized states around the Fermi level to extended states. As the temperature was decreased, two tetragonal samples went through a transition to a variable range hopping-like conduction. The amorphous ones remained within the thermally-activated transport regime in the temperature range of 253 K to 318 K, with E A ≈ 0.2 eV.

Simple patterning of spray-deposited Y-Ba-Cu-O films

A new patterning method suitable for spray-deposited Y-Ba-Cu-O films has been demonstrated. The method is a single-step process, based on the application of metal stencil masks to define a desired pattern. Lines less than 50 mu m wide, with superconducting properties identical to these of unpatterned films, have been successfully deposited.

Laser patterning of YBa/sub 2/Cu/sub 3/O/sub x/ thin films protected by in-situ grown SrTiO/sub 3/ cap layer

We report our studies on laser processing techniques suitable for patterning of YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) thin films, as well as YBCO/SrTiO/sub 3/ bilayers, consisting of in-situ grown YBCO films with 100-nm-thick single-crystalline SrTiO/sub 3/ cap layers. The patterning is achieved through both laser inhibition, in which an intense, focused laser beam locally melts YBCO transferring it into an insulating glass-like material, and laser writing, based on a laser-controlled diffusion of oxygen in or out of the YBCO film. We have found that oxygen easily migrates through the SrTiO/sub 3/ layer, allowing to reversibly convert the underlying YBCO film between the superconducting and semiconducting (virtually insulating at low temperatures) phases. Using laser inhibition and writing, we were able to form in a single YBCO/SrTiO/sub 3/ sample well-defined regions of various electrical properties, showing that laser processing can be successfully used in patterning and/or electrical trimming of multilayered YBCO circuits. Aging studies, performed on an almost 2-years-old laser-written YBCO film, showed a very good, both structural and electrical stability of our laser-processed structures.<<ETX>>

Patterning of thin-film high-T/sub c/ circuits by the laser-writing method

The authors report studies on a laser-based 2-D patterning technique that implements a focused beam from a continuous-wave (CW) Ar-ion laser to locally heat up an epitaxial Y-Ba-Cu-O film in a controlled (oxygen-rich or oxygen-poor) atmosphere. It is shown that the laser-writing method enables a reproducible fabrication in the same film of oxygen-depleted (semiconducting) regions next to enriched ones (superconducting), in a manner similar to n- and p-type diffusion regions in semiconductors. A number of test structures, laser-written in initially oxygen-rich or oxygen-poor films deposited on MgO, SrTiO/sub 3/, and LaAlO/sub 3/ single crystals, have been measured. The best, very homogeneous superconducting (oxygen-rich) circuits were patterned in oxygen-depleted YBCO deposited in the LaAlO/sub 3/ substrates. The structures exhibited a 0.5-K-wide superconducting transition, T/sub c0/=89.5 K, and J/sub c/ above 10/sup 5/ A/cm/sup 2/.<<ETX>>

Microwave noise emission from high Tc thin films

A pronounced X‐band microwave noise emission from Y‐Ba‐Cu‐O thin films has been observed. Emission depends strongly on temperture, bias current, and magnetic field. Two types of emission lines can be distinguished in the power versus dc bias spectra. The lines which remain at stable current positions against temperature we tentatively ascribe to the interaction of depinned flux flow with a two‐dimensional Josephson junction array. The lines that appear at fixed voltage positions may be attributed to the Josephson radiation of a distributed network of junctions. No significant emission was detected from Bi‐Ca‐Sr‐Cu‐O films.

Laser‐induced modification of transport properties of Y–Ba–Cu–O step‐edge weak links

We report on the laser‐induced permanent changes of the critical current (Ic) and normal resistance (Rn) of YBa2Cu3O7−x (YBCO) step‐edge Josephson junctions. The 2‐ to 20‐μm‐wide junctions were prepared from a 200‐nm‐thick YBCO film deposited by a pulsed KrF excimer laser onto 300‐nm‐high steps etched in the LaAlO3 substrate. The laser modification experiments were performed by illuminating the junctions at 50 K with a focused Ar‐ion laser beam of various intensities. Depending on the illumination power density, either increase or decrease of the junction Ic has been observed. In particular, after illumination at the 0.6×105 W/cm2 power level, a 75% enhancement of Ic and increase of the IcRn product up to 25% were obtained without a measurable change in the junction critical temperature. The laser‐induced modifications were very reproducible and remained unchanged even after a subsequent room‐temperature/helium thermal cycling of the sample. Photoassisted, thermally activated oxygen redistribution in the ...

Hall effect in semiconducting epitaxial and amorphous Y-Ba-Cu-O thin films

An experimental study of the Hall effect in nonmetallic Y-Ba-Cu-O thin films is reported. Both epitaxial crystalline YBa2Cu3O6+x (x⩽0.5) and multiphase/amorphous Y-Ba-Cu-O thin films were studied. The structure of the samples was measured by x-ray diffraction and Raman microprobe. The amorphous Y-Ba-Cu-O samples were found to have a grain size of about 100 A. The conduction properties were studied and analyzed for the two types of samples over a wide temperature range including room temperature. The Hall effect measurements showed positive charge carriers with a concentration ranging from 1017 to 1020 cm−3 at room temperature. The mobility was found to decrease with higher Hall carrier concentration. The empirical relationship for the mobility dependence on impurity concentration agreed with the relationship between mobility and the experimental Hall carrier concentration, suggesting that the same localized states were responsible for both providing the carriers and reducing the mobility through scatterin...

Superconducting fluctuations: paraconductivity, excess Hall effect and magnetoconductivity in 2223-BiSrCaCuO thin films

Abstract We report a detailed study of the influence of thermodynamic fluctuations of the superconducting order parameter on various normal-state transport properties in 2223-(Bi,Pb)SrCaCuO thin films at temperatures near the superconducting transition. Measurements of the electrical resistivity, the magnetoresistance and the Hall effect were analyzed with regard to fluctuation contributions, using theories for 2-dimensional, layered superconductors. We obtained a consistent set of parameters, which fit all magneto-transport measurements above 118 K, but observed a remarkable enhancement of both excess Hall effect and negative magnetoconductivity closer to Tc, whereas the zero-field fluctuation conductivity follows the theoretical predictions down to 110 K. No significant contributions from the indirect (Maki-Thompson) fluctuation process were found.