Luca Pellegrino

Multistate memory devices based on free-standing VO2/TiO2 microstructures driven by Joule self-heating.

Two-terminal multistate memory elements based on VO(2)/TiO(2) thin film microcantilevers are reported. Volatile and non-volatile multiple resistance states are programmed by current pulses at temperatures within the hysteretic region of the metal-insulator transition of VO(2). The memory mechanism is based on current-induced creation of metallic clusters by self-heating of micrometric suspended regions and resistive reading via percolation.

High-frequency surface acoustic wave device based on thin-film piezoelectric interdigital transducers

Using high-quality epitaxial c-axis Pb(Zr0.2Ti0.8)O3 films grown by off-axis magnetron sputtering onto metallic (001) Nb-doped SrTiO3 substrates, a nonconventional thin-film surface acoustic wave device based on periodic piezoelectric transducers was realized. The piezoelectric transducers consist of a series of ferroelectric domains with alternating polarization states. The artificial modification of the ferroelectric domain structure is performed by using an atomic force microscope tip as a source of electric field, allowing local switching of the polarization. Devices with 1.2 and 0.8μm wavelength, defined by the modulation period of the polarization, and corresponding to central frequencies in the range 1.50–3.50GHz have been realized and tested.Using high-quality epitaxial c-axis Pb(Zr0.2Ti0.8)O3 films grown by off-axis magnetron sputtering onto metallic (001) Nb-doped SrTiO3 substrates, a nonconventional thin-film surface acoustic wave device based on periodic piezoelectric transducers was realized. The piezoelectric transducers consist of a series of ferroelectric domains with alternating polarization states. The artificial modification of the ferroelectric domain structure is performed by using an atomic force microscope tip as a source of electric field, allowing local switching of the polarization. Devices with 1.2 and 0.8μm wavelength, defined by the modulation period of the polarization, and corresponding to central frequencies in the range 1.50–3.50GHz have been realized and tested.

Fabrication of submicron-scale SrTiO3−δ devices by an atomic force microscope

By applying a negative voltage to the conducting tip of an atomic force microscope, we modify on submicron-scale semiconducting oxygen deficient SrTiO3−δ thin films grown on LaAlO3 substrates. In comparison with the as-grown film, the modified regions present different electrical and structural properties, which can be exploited to realize submicrometer circuits. After a discussion on the mechanisms of the process, we report a prototype of a SrTiO3−δ-based sidegate field-effect transistor, showing a 4% modulation of channel resistivity with gate voltages up to 40 V.

SrTiO3-based metal–insulator–semiconductor heterostructures

We explored the feasibility of employing strontium titanate (SrTiO3) as semiconducting material in field-effect metal–insulator–semiconductor epitaxial heterostructures. This idea was suggested by the observation of a dramatic effect of the oxygen deficiency on SrTiO3−δ transport properties, which brings about metallic behavior with low-temperature mobility values comparable with those commonly found for silicon. By pulsed-laser deposition, we realized patterned field-effect devices, showing a resistance enhancement up to 90%. This promising result could open perspectives for crystalline-oxide electronics.

Digitalized magnetoresistance observed in (La,Pr,Ca)MnO3 nanochannel structures

We have examined the magnetoresistances of (La,Pr,Ca)MnO3 channel structures with different channel widths. The 500nm channel showed an extremely sharp metallic transition from charge ordered insulator to ferromagnetic metal at low temperature, while the 10 and 1μm channels showed gradual metallic transitions. The sharpness of the metallic transition was increased from 1.3×102T−1 in the 10μm channel to 3.5×104T−1 in the 500nm channel at 10K. Such extremely sharp metallic transition can be accounted for electronic phase separation phenomena.

Programmable Mechanical Resonances in MEMS by Localized Joule Heating of Phase Change Materials

A programmable micromechanical resonator based on a VO2 thin film is reported. Multiple mechanical eigenfrequency states are programmed using Joule heating as local power source, gradually driving the phase transition of VO2 around its Metal-Insulator transition temperature. Phase coexistence of domains is used to tune the stiffness of the device via local control of internal stresses and mechanical properties. This study opens perspectives for developing mechanically configurable nanostructure arrays.

All-SrTiO3 field effect devices made by anodic oxidation of epitaxial semiconducting thin films

We report a field effect device fully made of strontium titanate (STO). This perovskite-type material is very attractive for oxide electronics both for its notable dielectric properties as well as for its semiconducting properties in the doped state. We exploit both of these properties by developing a field effect device in which oxygen deficient STO acts as a conducting channel and stoichiometric STO as a dielectric barrier. Such a barrier is obtained by electrochemical oxidation of the surface of an oxygen deficient semiconducting STO film, deposited by pulsed laser ablation in ultrahigh vacuum conditions. The channel conductivity is varied by the application of an electric field between the channel itself and a metallic gate deposited onto the dielectric barrier. Modulation capability of more than 60% is achieved by applying potential lower than 1 V. Conductivity changes are due to electrostatic induced variations of the charge carrier density (n). This result is confirmed by Hall effect measurements during gate biasing. The very good agreement of the measured n with the value calculated from the device capacitance proves the electrostatic origin of the effect observed.We report a field effect device fully made of strontium titanate (STO). This perovskite-type material is very attractive for oxide electronics both for its notable dielectric properties as well as for its semiconducting properties in the doped state. We exploit both of these properties by developing a field effect device in which oxygen deficient STO acts as a conducting channel and stoichiometric STO as a dielectric barrier. Such a barrier is obtained by electrochemical oxidation of the surface of an oxygen deficient semiconducting STO film, deposited by pulsed laser ablation in ultrahigh vacuum conditions. The channel conductivity is varied by the application of an electric field between the channel itself and a metallic gate deposited onto the dielectric barrier. Modulation capability of more than 60% is achieved by applying potential lower than 1 V. Conductivity changes are due to electrostatic induced variations of the charge carrier density (n). This result is confirmed by Hall effect measurements d...

Epitaxial copper oxide thin films deposited on cubic oxide substrates

We study the growth conditions of Cu2O thin films deposited on MgO (0?0?1) and SrTiO3 (0?0?1) substrates by pulsed laser ablation, in order to explore the compatibility between semiconducting p-type Cu2O and other perovskite oxides in view of the fabrication of oxide electronics heterostructures. We find that in both cases perfect epitaxy, high crystalline quality and good out-of-plane orientation are achieved. In this context, epitaxy plays a major role in driving the phase formation. On the other hand, in films deposited at temperatures higher than 700??C transport is inhibited by poor grain connectivity, which is an inevitable consequence of the necessity for the crystal to release the lattice strain. Instead, better connectivity and bulk-like values of resistivity, as well as good crystallinity and orientation, are obtained for films deposited at 650??C. This should be kept in mind for the fabrication of stacked layer oxide heterostructures, where deep grooves between adjacent grains would be a serious drawback both for vertical and planar transport.

Current-controlled lithography on conducting SrTiO3−δ thin films by atomic force microscopy

The application of local anodic oxidation to conducting perovskite oxides such as SrTiO3−δ thin films causes a local decomposition of the films shown by the formation of mounds at the surface. The size of the patterns is limited by the dimension of the water meniscus which forms under the biased tip of the atomic force microscope (AFM). Due to the instabilities of the water meniscus, biasing with a constant voltage in contact mode does not guarantee a good uniformity of the patterns. In this work, we show and discuss how, by controlling the electrical current through the AFM tip, it is possible to realize lines with uniform widths down to 150 nm over a total length of hundreds of micrometers.

High mobility in ZnO thin films deposited on perovskite substrates with a low temperature nucleation layer

High electron mobility is measured down to low temperature in epitaxial ZnO thin films deposited on (110) oriented strontium titanate substrates. Electron mobility is evaluated by both magnetoresistance and resistivity-Hall effect data. Values up to 400cm2∕Vs are found below 50 K in epitaxial thin films grown by a two-step method: first a 100-nm-thick ZnO relaxing layer is deposited on the SrTiO3 (110) substrate at relatively low temperature (550–600 °C) and then the deposition temperature is raised up to 800 °C for the growth of a second ZnO layer. Reflection high energy electron diffraction analysis during the deposition, ex situ x-ray diffraction and AFM morphology studies performed separately on each layer reveal that the first layer grows in a quasi-two-dimensional mode while the increased temperature in the second step improves the crystalline quality of the film. The integration of ZnO transparent semiconductor with high-k dielectric perovskite substrates may lead to a wide variety of new electroni...