M. Gurvitch

VO2 films with strong semiconductor to metal phase transition prepared by the precursor oxidation process

We describe a relatively simple, reliable, and reproducible preparation technique, the precursor oxidation process, for making VO2 films with strong semiconductor-to-metal phase transition. Sputter-deposited metal precursor V films were oxidized in situ in the deposition chamber for 2.5–7h at 370–415°C in 0.2Torr O2 to form 22–220nm VO2. The strength [resistivity ratio, RR=ρS∕ρM] and sharpness (hysteresis width ΔTC) of T-dependent semiconductor-to-metal hysteretic phase transition in VO2 were our most immediate and relevant quality indicators. In 200-nm-range films, the process was optimized to yield RR=(1–2)×103, ΔTC∼11°C and absolute resistivity in a semiconducting phase ρS=0.4±0.2Ωm, close to resistivity in bulk single crystals of VO2. Films were characterized by scanning electron microscopy, atomic force microscopy, grazing-incidence x-ray diffraction, and Raman spectroscopy, and found to be polycrystalline single-phase VO2. We also measured optical reflectivity RT(λ) from 200to1100nm, and Rλ(T) from ...

All‐high‐Tc superconductor rapid‐single‐flux‐quantum circuit operating at ∼30 K

We have implemented a simple circuit of the rapid single‐flux‐quantum (RSFQ) logic family using a single‐layer YBa2Cu3O7−x thin‐film structure with 14 in‐plane Josephson junctions formed by direct electron beam writing. The circuit includes two dc/SFQ converters, two Josephson transmission lines, a complete RS SFQ flip‐flop, and an SFQ/dc converter (readout SQUID). Low‐frequency testing has shown that the dc‐current‐biased circuit operates correctly and reliably at T∼30 K, a few degrees below the effective critical temperature of the junctions. Prospects for a further increase of the operation temperature and implementation of more complex RSFQ circuits are discussed in brief.

High quality YBa2Cu3O7 Josephson junctions made by direct electron beam writing

High‐Tc Josephson junctions have been fabricated by direct electron beam writing over YBa2Cu3O7 thin‐film microbridges, using scanning transmission electron microscope (STEM) with an accelerating voltage of 80–120 kV. Annealing at 330–380 K increases Tc and Ic of the junctions and makes them more stable. In the operating range of a few degrees below Tc, the junctions show 100% magnetic field modulation of the critical current, microwave‐induced Shapiro steps oscillating according to the resistively shunted junction (RSJ) model, and RSJ current‐voltage characteristics with IcRn product up to 0.5–0.6 mV at 75 K and 0.3 mV at 77 K.

Nonhysteretic behavior inside the hysteresis loop of VO2 and its possible application in infrared imaging

In the resistive phase transition in VO 2 , temperature excursions taken from points on the major hysteresis loop produce minor loops. For sufficiently small excursions these minor loops degenerate into single-valued, nonhysteretic branches NHBs linear in log versus T and having essentially the same or even higher temperature coefficient of resistance TCR as the semiconducting phase at room temperature. We explain this behavior based on the microscopic picture of percolating phases. Similar short NHBs are found in otherwise hysteretic optical reflectivity. We discuss the opportunities NHBs present for infrared imaging technology based on resistive microbolometers. It is possible to choose a NHB with 10 2 ‐10 3 times smaller resistivity than in a pure semiconducting phase, thus providing a microbolometer operating without hysteresis, with low tunable resistivity, and high TCR. Unique features of the proposed method and projected figures of merit are discussed in the context of uncooled focal plane array IR visualization technology.

Tc enhancement by low energy electron irradiation and the influence of chain disorder on resistivity and Hall coefficient in YBa2Cu3O7 thin films

Abstract The effect of electron irradiation with incident energy ( E ≤ 40 keV) lower than the threshold for displacement of in-plane atoms on T c , resistivity and Hall coefficient has been studied on fully oxygenated YBa 2 Cu 3 O 7−δ thin. films. In contrast to the known T c suppression at high-energy particle irradiation, an increase in T c has been found as well as a factor of two increase in both the T -linear resistivity slope and the Hall coefficient as a function of radiation fluence. It has been shown that the changes observed in transport properties can be explained as being caused by disruption of the conductivity of CuO chains by radiation-induced chain oxygen defects.

Critical currents and Josephson penetration depth in planar thin‐film high‐Tc Josephson junctions

The temperature dependence of the critical current in planar high‐Tc Josephson junctions fabricated in YBa2Cu3O7 thin films by focused electron irradiation has been studied. It is shown that in the range of critical current densities spanning more than five orders of magnitude and temperature range 0.1≤T/Tc≤1, the critical current density jc varies as (1−T/Tc)2. The T dependence of the critical current, however, is affected by the transition from the narrow junction to the wide junction limit as jc increases. An expression for the Josephson penetration depth in thin‐film coplanar structures is derived, and magnetic field penetration depth in junction banks is extracted from the Ic(T) dependences.

Electron beam irradiation of Y1Ba2Cu3O7−x grain boundary Josephson junctions

The properties of the Y1Ba2Cu3O7−x biepitaxial Josephson junctions were reproducibly modified by a focused electron beam irradiation of the interface region. The junctions were fabricated by depositing Y1Ba2Cu3O7−x thin film by cylindrical magnetron sputtering technique on the (110) SrTiO3 substrate, partially covered by a pregrown MgO seed layer. The junction parameters can be adjusted controllably by applying an appropriate dose. Electron irradiation decreased the critical current of the junctions IC and increased the normal state resistance times area to values of the order of 1(μ Ω cm2). Some other effects, such as the disappearance of the excess current, were also observed. The original properties of the junctions could be partly restored by isothermal annealing. We also speculate that some aspects of the nature of the grain boundary barriers can be better understood from the study of the properties of irradiated junctions.

Electron beam writing in fabricating planar high-Tc Josephson junctions

Abstract Electron beam irradiation was utilized to fabricate planar Josephson junctions in Y 1 Ba 2 Cu 3 O 7 thin films. After the micron-size bridges had been fabricated by way of standard optical lithography, they were modified using a well focused nanometer STEM probe with beam energy within the range of 80 – 120 keV. Modified junctions exhibit a two-step normal/superconductor transition. We attribute the lower transition temperature, which is of the order of 77 K, to the transition of the damaged region. Shapiro steps under applied microwave radiation of the frequency 10–15 GHz, as well as oscillation of the critical current in a magnetic field were observed up to 70 K. A comparison with the RSJ model is made and a possible damage mechanism is discussed.

Nonhysteretic Phenomena in the Metal–Semiconductor Phase-Transition Loop of

Hysteresis observed in the resistive semiconductor-to-metal phase transition in VO2 causes problems in bolometric readout, and thus is an obstacle in utilizing this strong phase transition in bolometric sensor applications. It is possible to avoid the unwanted hysteresis when operating in limited temperature ranges within the hysteresis loop of VO2. Nonhysteretic branches (NHB-s) traced in such limited temperature intervals turned out to have much higher temperature coefficient of resistance (TCR) than VO2 at room temperature: while TCR at 25° C in VO2 is close to 3%, peak TCR values in NHB-s reach 6% in VO2 films on Si/SiO2 substrates and 21% in films on crystalline sapphire substrates. At the same time, the nanoscopic-scale mixture of semiconducting and metallic phases in VO2 within its hysteresis loop provides for partially shunted low resistivity, thus creating an unprecedented combination of record high semiconducting TCR and metal-like low resistance. This combination may benefit the uncooled focal plane array microbolometer IR visualization technology.

\hbox{VO}_{2}

The properties of the Y1Ba2Cu3O7−x biepitaxial Josephson junctions were reproducibly modified by a focused electron beam irradiation of the interface region. The junctions were fabricated by depositing Y1Ba2Cu3O7−x thin film by cylindrical magnetron sputtering technique on the (110) SrTiO3 substrate, partially covered by a pregrown MgO seed layer. The junction parameters can be adjusted controllably by applying an appropriate dose. Electron irradiation decreased the critical current of the junctions IC and increased the normal state resistance times area to values of the order of 1(μ Ω cm2). Some other effects, such as the disappearance of the excess current, were also observed. The original properties of the junctions could be partly restored by isothermal annealing. We also speculate that some aspects of the nature of the grain boundary barriers can be better understood from the study of the properties of irradiated junctions.