The hole-density dependence of the normal-state resistivity and transition temperature 7Y of YBa2Cu.iC>7-.v was studied using the electric field effect in ultrathin films of 1 to 8 unit cells thickness. The change in resistivity was found to be equal to the field-induced variation in the areal carrier density, leading to a relation that the conductivity is proportional to the hole density. A similar linear dependence was also found in T c . However, a saturation occurred in both cases but at different hole densities. The result reveals different effects of hole filling in influencing normal-state and superconducting transport of YBa 2 Cu30 7v .
A multilayer high Tc superconducting field-effect transistor-like structure was made from ultrathin YBa2Cu3O7−x films. An epitaxially grown dielectric SrTiO3 insulation layer, which had a forward bias breakdown voltage of about 20 V, allowed an electric field induced change in the channel layer of 1.25×1013 carrier/cm2 per volt of the gate voltage. A significant modulation of the normal state and superconducting properties was observed in samples with YBa2Cu3O7−x channel layers of a few unit cells thick. By applying gate voltage of different polarities, Tc was both suppressed and enhanced by ∼1 K. The resistance was modulated by as much as 20% in the normal state and by over 1500% near the zero resistance temperature.
Deposition of silicon nitride at low temperatures by plasma-enhanced chemical vapor deposition requires an efficient source of activated precursors and high-current, low-energy ion assist. We report the deposition of silicon nitride at substrate temperatures <100 °C using a permanent magnet electron cyclotron resonance plasma reactor capable of generating uniform plasmas over 300 mm diameters. The effects of gas mixture, silane flow, pressure, and microwave power on the film deposition rate, composition and bonding, index of refraction, stress, and etch rate in buffered oxide etch solution are reported. The N2/SiH4 flow ratio and microwave power both influence the film index and hydrogen content and bonding. For a SiH4 flow of 30 sccm and N2/SiH4 ∼0.75, hydrogen is equally distributed between Si–H and N–H sites and total hydrogen content is minimized. At a deposition rate of 500–600 Å/min, a threshold in microwave power of ∼1100 W exists, above which films with buffered oxide etch rates <150 Å/min result. Near the threshold microwave power compressive stress <400 MPa is observed, with increasing stress at higher microwave powers.
The dielectric constant ■ of thin epitaxial SrTiO3 films used as dielectric layers in high Tc superconducting field-effect devices has been studied. ■ in these thin films was found to have much weaker dependence on temperature and applied electric field than the bulk material. The breakdown characteristics of gate structures containing SrTiO3 dielectric layers are strongly influenced by the metal used as the gate electrode.
The effects of charge-carrier density modulation on critical currents and vortex dynamics in ultrathin YBaaCuaO?-^ films were investigated through transport measurements in field-effect devices. Two distinct dissipation regions in resistive transitions in applied magnetic fields are identified by the electric field effect. A Kosterlitz-Thouless-like transition is observed in zero magnetic field, which can be tuned by the electric field. The importance of thermal fluctuations for the dissipation in high-7c superconductors is stressed. PACS numbers: 74.4a+k, 74.60.Jg, 74.70.Vy, 74.75.+tRecent work has shown that the properties of ultrathin films of YBa2Cu307-5 (YBCO) can be significantly modulated by the application of electric fields [1-5]. The electric field changes the charge-carrier density in a film without changing the chemical structure or composition, thus providing a powerful tool for understanding hightemperature superconductors. Mannhart et ai [4] studied the electric field effect on the critical current density and explained it by field-induced depinning. However, the consistency of this model with details of the effects remains to be proven. In this Letter we report the measurement of resistive transitions and the current-voltage characteristics of ultrathin YBCO films which are subjected to external electric and magnetic fields. It turns out that the electric field effect reveals details of the dissipation processes which are hard to access by other techniques. A transition from a high-temperature, high-dissipation state to a low-temperature, low-dissipation state is observed and identified by a different dependence on the charge-carrier density. The results provide evidence for a Kosterlitz-Thouless-type transition in zero magnetic field, and a transition from a low-temperature (solid or hexatic) to a high-temperature (liquid) vortex phase in magnetic fields.The sample in this work consisted of a c-axis-oriented YBCO film prepared by pulsed laser deposition with a nominal thickness of 5 nm, i.e., four unit cells. The thickness fluctuation of such films is generally of the order of one unit cell [6]. On top of the YBCO layer, a 400-nmthick SrTiOs dielectric layer was deposited in situ with the same technique, and then a gold gate electrode was evaporated. Details of the sample preparation and SrTiOs properties have been published earlier [5,7]. The charge-carrier density in the YBCO layer was modulated by applying a gate voltage between the gate electrode and the YBCO film. The total applied charge was measured by an electrometer. The maximum applied gate voltage is limited by the breakdown behavior of the dielectric. To avoid interference of leakage currents with the test currents, the gate voltage was kept low enough for the leakage current to not exceed 10 nA, while the measurement currents ranged from /iA to mA. Current-voltage
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