National Institute Of Materials Physics - Romania

Phase-change memories have reached an advanced degree of maturity, although, to be able to meet the increasing storage demand, multilevel capability is needed. A GeTe memristor is obtained in an amorphous state and it is subjected to a specific thermal treatment which initiates the transition toward the crystalline state. It is found that this crystalline state initialization process is highly beneficial for subsequently obtaining a large number of intermediate resistive states between the high and low resistive states. Multiple resistance levels are achieved by operating the devices in both DC sweeps and rectangular pulse modes in the low-voltage subthreshold regime. The conduction is modeled using a space charge limited conduction model, showing three distinct conduction regions in the high resistive state which merge toward a single conduction region as the low resistive state is approached. The obtained memristors can be used as multilevel nonvolatile memories or as synapses in neuromorphic computing.

Thin film metal-ferroelectric-metal capacitors with an equal mixture of hafnium oxide and zirconium oxide as the ferroelectric material are fabricated using iridium oxide as the electrode material. The influence of the oxygen concentration in the electrodes during crystallization anneal on the ferroelectric properties is characterized by electrical, chemical, and structural methods. Forming gas, O-2, and N-2 annealing atmospheres significantly change the ferroelectric performance. The use of oxygen-deficient electrodes improves the stabilization of the ferroelectric orthorhombic phase and reduces the wake-up effect. It is found that oxygen-rich electrodes supply oxygen during anneal and reduce the amount of oxygen vacancies, but the nonferroelectric monoclinic phase is stabilized with a negative impact on the ferroelectric properties.

In this work, amorphous tin oxide thin films were deposited by non-reactive radio frequency magnetron sputtering. A ceramic SnO2 target was used, while different working pressures were employed. The target to substrate distance was fixed to 17 cm, and the substrate was not intentionally heated. The properties of SnO2 (thickness, refractive index dispersion, optical band gap, resistivity, free carriers concentration, carriers mobility, carriers majority type and their scattering time) have been inferred from spectroscopic ellipsometry, conventional UV-Vis spectroscopy and specific Hall electrical measurements. Thickness and refractive index are slightly dependent on the deposition conditions, while the optical band gap, free carriers concentration and their mobilities are changing from sample to sample. The evolution of the optical band gap and carriers concentration is correlated to the active defects concentration. Amorphous SnO2 films grown at 0.4 Pa have the lowest resistivity of 0.86 Omega cm, a carrier concentration of 1.05x10(18)cm(-3) Vs. The average optical transmittance in visible spectrum is 76%.