Y. Koval

Interface screening and imprint in poly(vinylidene fluoride/trifluoroethylene) ferroelectric field effect transistors

We investigated the imprint effect in ferroelectric capacitors and field effect transistors (FETs) with a poly(vinylidene fluoride/trifluoroethylene) [P(VDF-TrFE)] ferroelectric insulator. The shift in switching voltages and the change in the ferroelectric FET (FeFET) channel conductance were measured as a function of time and the thickness of the ferroelectric layer. Analyzing our experimental data, we show that the imprint originates from interface-induced processes, which effectively screen polarization charges in P(VDF-TrFE). This phenomenon significantly influences the retention of FeFET channel conductance and the memory functionality of FeFET with P(VDF-TrFE).

Electrical investigations on metal/ferroelectric/insulator/semiconductor structures using poly[vinylidene fluoride trifluoroethylene] as ferroelectric layer for organic nonvolatile memory applications

Poly[vinylidene fluoride trifluoroethylene] (P[VDF/TrFE]) is a ferroelectric polymer and a candidate for the application in a ferroelectric field effect transistor, which is considered as a nonvolatile and nondestructive readout memory cell. In this contribution the authors focus on metal/ferroelectric/insulator/semiconductor capacitor structures with P[VDF/TrFE] as ferroelectric layer. Measuring the capacitance of Al/P[VDF/TrFE]/SiO2/Si stacks in dependence of the applied bias and analyzing the oxide capacitance in dependence of the thickness of the ferroelectric layer, they observe a formation of an interfacial nonferroelectric layer. This layer is responsible for reduced values of polarization profound for thinner P[VDF/TrFE] layers in this system. Capacitance-time measurements at such stacks show the possibility to distinguish between a higher and a lower capacitance state for more than 5 days. Long-time measurements revealed imprint and fatigue-like behavior.

Conductance enhancement of polymethylmethacrylate bombarded by low-energy ions

It has been found that films of polymethylmethacrylate (PMMA) show a substantial conductance after bombardment by Ar ions with energy of 250–1250eV. The appearance of the conductance is attributed to graphitization processes in the subsurface layer. As the energy of ions increases, the conductivity of PMMA is greatly enhanced. We have found that, at low electric fields, the conductance is provided by variable range hopping with a strong influence of Coulomb interactions. At high electric fields, the transformed PMMA reveals non-Ohmic behavior: the conductance is an exponential function of E∕T.

Graphitization of polymer surfaces by low-energy ion irradiation

The surface of several polymers was graphitized by low-energy ion irradiation. Their conducting properties were studied as a function of the energy of the ions and the irradiation temperature. It was found that at rather modest ion energies (∼1000eV) and irradiation temperatures (<400°C), polymer surfaces transform to a graphitized state. The graphitized layer consists of overlapping graphite islands with a diameter of 1–3nm and exhibits a semimetallic conductivity. Gradually reducing the energy of the ions and the irradiation temperature, the authors observed a transition from semimetallic to variable range hopping conductivity.

Tuning superconductivity by carrier injection

We have found that by extensive current injection along the c-axis, the superconducting properties of Bi2Sr2CaCu2O8+δ can be changed effectively. We show that critical temperature, c-axis resistivity, and critical current of intrinsic Josephson junctions can be tuned in a large range from underdoping to extreme overdoping. This effect is reversible and persistent. Our results can be explained by trapping charges in the insulating layers, which induce a change of carrier concentration in superconducting planes. This floating gate concept can be a general property of layered materials where the insulating charge reservoir layers are separated from the conducting planes.

Fabrication and characterization of glassy carbon membranes

In this work, the authors focus on a method to fabricate arbitrary shaped free standing membranes with a thickness less than 20 nm, produced from different polymers with the help of low-energy ion irradiation. The authors analyze the thickness of the membranes and its dependence on the details of the irradiation process. In order to tune the properties of the suspended membranes, an additional ion irradiation step has been used. This step is applied to already suspended membranes and leads to several effects, such as heating, shape transformation, etc. These effects were analyzed for irradiation with Ar+ and He+ ions. The authors have found that He+ irradiation has a significant advantage over Ar+ irradiation providing strained, smooth, and homogeneous membranes. In order to measure the electrical properties of the suspended membranes, the authors invented a new method to contact the membranes. These low resistance contacts can be achieved as the authors describe in detail. The membranes electrical properties after He+ ion irradiation at different temperatures are presented. Finally, the authors analyze Raman spectra, and thermal and electrical conductivity of the highly conducting membranes. The authors conclude that after high temperature He+ ion irradiation the membranes consist of material similar in properties to the glassy carbon obtained by pyrolysis. However, this method does not require high temperature pyrolysis step, which makes integration with on-chip electronics more feasible.

Superconductivity induced by carrier injection into non-superconducting Bi2Sr2CaCu2O8 + δ

Doping by carrier injection was investigated in strongly oxygen-depleted Bi2Sr2CaCu2O8 + δ. In order to perform four point c-axis measurements in non-superconducting samples, a new fabrication technique was developed. We were able to show that non-superconducting Bi2Sr2CaCu2O8 + δ can be converted into a superconducting, even overdoped state only by applying a large bias current along the c-axis. A critical temperature of 91 K and a c-axis critical current density of 2000 A cm−2 were achieved. Moreover, the doping effect is persistent and reversible resembling the resistive-memory switching effect observed earlier in various oxides. The mutual influence of oxygen doping and change of the charge carrier concentration by carrier injection is discussed.

Polymer surfaces graphitization by low-energy He+ ions irradiation

The electrical and optical properties of surfaces of polyimide and AZ5214e graphitized by low-energy (1 keV) He+ irradiation at different polymer temperatures were investigated. The conductivity of the graphitized layers can be controlled with the irradiation temperature within a broad range and can reach values up to ∼1000 S/cm. We show that the electrical transport in low-conducting samples is governed by thermally activated hopping, while the samples with a high conductivity show a typical semimetallic behavior. The transition from thermally activated to semimetallic conductance governed by the irradiation temperature could also be observed in optical measurements. The semimetallic samples show an unusually high for graphitic materials carrier concentration, which results in a high extinction coefficient in the visible light range. By analyzing the temperature dependence of the conductance of the semimetallic samples, we conclude that the scattering of charge carriers is dominated by Coulomb interactio...

Fabrication and characterization of sub-micron thin film intrinsic Josephson junction arrays

We have fabricated intrinsic Josephson junction arrays in thin films of Tl-Ba-Ca-Cu-O. Such arrays are candidates for applications in the sub-millimeter waveband both by virtue of the large gap energy and due to the existence of inductive and capacitive coupling mechanisms between junctions in the array. Characterization of such junctions is complicated by the fact that the transport properties are dominated by Josephson fluxon flow (for junctions whose dimensions exceed the Josephson penetration depth) and premature switching to the voltage state at bias currents less than the critical current (for junctions whose dimensions are less than the Josephson penetration depth). Here we show that the magnetic-field dependence of the switching current is not Fraunhofer-like, although there is clear minimum corresponding to the point at which a single flux quantum is inserted between the cuprate double-planes. Nevertheless a Fraunhofer-like dependence can be obtained if the critical current is experimentally defined by specifying a low-voltage criterion.

Interlayer tunneling spectroscopy of mixed-phase BSCCO superconducting whiskers

In this work, we present a study on the interlayer tunneling spectroscopy (ITS) of mixed-phase BiSrCaCuO (BSCCO) superconducting whiskers. The tunneling experiments were carried out on the artificial cross-whisker (twist angle of 90?) junctions. A multiple superconducting energy gap in the cross-whisker junctions was observed, which is attributed to the presence of different doping levels of two Bi2Sr2CaCu2O8+? phases (Bi-2212), rather than two different phases, in the BSCCO whiskers, namely Bi2Sr2CaCu2O8+? and Bi2Sr2Ca2Cu3O8+? (Bi-2212 and Bi-2223). The temperature dependence of the energy gaps was discussed in the framework of the BCS T-dependence. On the other hand, the carrier concentration of the cross-whisker junction was changed by the carrier injection process. The effects of the carrier injection on the critical current, I c, and the ITS of intrinsic Josephson junctions were investigated in details.