Novel concept of gas sensitivity characterization of materials suited for implementation in FET-based gas sensors

Abstract : We propose a novel technique to investigate the gas sensitivity of materials for implementation in field-effect transistor-based gas sensors. Our technique is based on the measurement of the surface charge induced by gas species adsorption, using an electrometer. Platinum sensitivity to hydrogen diluted in synthetic air has been evaluated with the proposed charge measurement technique in the operation temperature range from 80 to 190 °C at constant H2 concentration of 4 % and for different concentrations ranging from 0.5 to 4 % at 130 °C

Synthesis of BiFeO3 thin films on single-terminated Nb : SrTiO3 (111) substrates by intermittent microwave assisted hydrothermal method

We report on a simple and fast procedure to create arrays of atomically flat terraces on single crystal SrTiO3 (111) substrates and the deposition of ferroelectric BiFeO3 thin films on such single-terminated surfaces. A microwave-assisted hydrothermal method in deionized water and ammonia solution selectively removes either (SrO3)4− or Ti4+ layers to ensure the same chemical termination on all terraces. Measured step heights of 0.225 nm (d111) and uniform contrast in the phase image of the terraces confirm the single termination in pure and Nb doped SrTiO3 single crystal substrates. Multiferroic BiFeO3 thin films were then deposited by the same microwave assisted hydrothermal process on Nb : SrTiO3 (111) substrates. Bi(NO3)3 and Fe(NO3)3 along with KOH served as the precursors solution. Ferroelectric behavior of the BiFeO3 films on Nb : SrTiO3 (100) substrates was verified by piezoresponse force microscopy

Thermionic Emission Based Resistive Memory with Ultrathin Ferroelectric BiFe_1–<i>x</i>Cr_<i>x</i>O₃ Films Deposited by Mineralizer-Free Microwave-Assisted Hydrothermal Synthesis

We develop resistive switching memory devices employing ultrathin (∼2.5 nm) BiFe_1–<i>x</i>Cr_<i>x</i>O₃ ferroelectric films, deposited by microwave-assisted hydrothermal (MWHT) synthesis, as the potential barrier. BiFeO₃ is a multiferroic material highly suitable for nonvolatile semiconductor memories due to its polar and magnetic ordering at room temperature. Chromium is incorporated to enhance the material’s multiferroic properties. As compared to more commonly used physical and chemical vapor deposition tools, MWHT is an extremely cost-efficient tool that provides high reproducibility and stoichiometry control. Using no mineralizer and thereby reducing leakage currents and structural disorder, the ferroelectric phase is reached in our films after 2 cycles of microwave irradiation, and atomic step-terraces can be seen on the surface, attesting to a highly ordered film growth. Using platinum (Pt) top electrodes and conductive substrates (Nb:SrTiO₃ and Pt) as bottom electrodes, memory devices are fabricated. Resistive switching is confirmed through electrical characterizations, and resistance ratios of ∼20 and ∼10⁵ are obtained for the SrTiO₃/BiFe_1–<i>x</i>Cr_<i>x</i>O₃/Pt and Pt/BiFe_1–<i>x</i>Cr_<i>x</i>O₃/Pt designs, respectively. By correlating the band alignment of both designs to their current–voltage behavior, we demonstrate that thermionic emission is the dominant charge transport mechanism. Under quasi-static conditions, the SrTiO₃/BiFe_1–<i>x</i>Cr_<i>x</i>O₃/Pt devices are switched over 35 cycles on 10 different electrodes

Control of strong-field ionization in ferroelectric lithium niobate: Role of the spontaneous polarization

We report the control of tunnel ionization in lithium niobate (LiNbO$_3$) using phase-controlled two-color laser fields. Through a macroscopic observable of high contrast, we disclose the crucial contribution of the microscopic spontaneous polarization of the ferroelectric material to the ionization rate: as the relative two-color phase is varied, the ablated area of LiNbO$_3$ is modulated by 35% when the laser and crystal polarization directions are parallel. Rotating the sample by 180° around the laser propagation axis leads to an out-of-phase modulation. We use a two-band model to highlight the key contribution of the material's spontaneous polarization for the symmetry breaking of the ionization rate. Our results open new perspectives for the direct control of ionization dynamics in solids by tailoring the electric field of femtosecond laser pulses