Jaan Aarik

Resistive switching of Hf _x Ti _{1− x} O _y (0.07 ≤ x ≤ 0.64) films was studied. Increase in x , enabling formation of orthorhombic Hf _x Ti _{1− x} O _y , caused an increase in the breakdown electric field strength, resistive switching memory window, and endurance.

This study investigated the characteristics of radiofrequency, middle-pressure argon plasma used in the atomic layer deposition (ALD) of Al₂O₃ films. Based on the electrical characteristics—the current, voltage, and phase shift between them—and the stability of the plasma plume, the optimum plasma power, allowing reliable switching on of the plasma for any step of an ALD cycle, was determined. Spectral measurements were performed to determine the gas temperature and reactive species that could be important in the ALD process. The density of metastable argon atoms was estimated using tunable laser absorption spectroscopy. It was concluded that plasma heating of substrates did not affect film growth. The crystallization-enhancing effect of plasma observed in these experiments was due to the action of OH radicals produced in the plasma.

This study investigated the characteristics of radio-frequency middle-pressure argon plasma used in the atomic layer deposition (ALD) of Al2O3 films. Based on the electrical characteristics, that is current, voltage, and phase shift between them, and stability of plasma plume, the optimum plasma power, allowing reliable switching on the plasma for any step of an ALD cycle, was determined. Spectral measurements were performed to determine the gas temperature and reactive species that could be important in the ALD process. The density of metastable argon atoms was estimated using tunable laser absorption spectroscopy. It was concluded that plasma heating of substrates did not affect film growth and proposed that the crystallization-enhancing effect of plasma was due to the action of OH radicals, produced in the plasma.

Thin films containing either multilayer ZrO₂:Al₂O₃ structures or ZrO₂ deposited on Zr_xAl_yO_z buffer layers were characterized. The films were grown by atomic layer deposition (ALD) at 300 °C from ZrCl₄, Al(CH₃)₃, and H₂O. The multilayer ZrO₂:Al₂O₃ structures were grown repeating different combinations of ZrO₂ and Al₂O₃ ALD cycles while the Zr_xAl_yO_z layers were obtained in a novel process using ALD cycles based on successive adsorption of ZrCl₄ and Al(CH₃)₃, followed by surface reaction with H₂O. The films were grown on TiN electrodes, and supplied with Ti top electrodes, whereby Zr_xAl_yO_z films were exploited as thin buffer layers between TiN and ZrO₂. The as-deposited ZrO₂ films and ZrO₂:Al₂O₃ structures with sufficiently low concentrations of Al₂O₃ were crystallized in the form of cubic or tetragonal ZrO₂ polymorph possessing relative permittivities reaching 35. Notably, multilayered ZrO₂:Al₂O₃ films could exhibit resistive switching behavior with ratios between low- and high-resistive-state current values, extending up to five orders of magnitude. Implications of multilevel switching were recorded. In the double-layered Zr_xAl_yO_z-ZrO₂ stacks, the ON/OFF current ratios remained below 40, but the endurance could become extended over 3000 cycles. Remarkably, instabilities, when detected in endurance behavior expressed by reduction in an ON/OFF current ratio could be compensated and the current values restored by real time readjustment of the programming voltage amplitude.

Crystal structure and electrical properties of hafnium-praseodymium oxide thin films grown by atomic layer deposition on ruthenium substrate electrodes were characterized and compared with those of undoped HfO₂ films. The HfO₂ reference films crystallized in the stable monoclinic phase of HfO₂. Mixing HfO₂ and PrO_x resulted in the growth of nanocrystalline metastable tetragonal HfO₂. The highest relative permittivities reaching 37–40 were measured for the films with tetragonal structures that were grown using HfO₂:PrO_x cycle ratio of 5:1 and possessed Pr/(Pr + Hf) atomic ratios of 0.09–0.10. All the HfO₂:PrO_x films exhibited resistive switching behavior. Lower commutation voltages and current values, promising in terms of reduced power consumption, were achieved for the films grown with HfO₂:PrO_x cycle ratios of 3:1 and 2:1 and showing Pr/(Pr + Hf) atomic ratios of 0.16–0.23. Differently from the undoped HfO₂ films, the Pr-doped films showed low variability of resistance state currents and stable endurance behavior, extending over 10⁴ switching cycles.

High-density phases of TiO₂, such as rutile and high-pressure TiO₂-II, have attracted interest as materials with high dielectric constant and refractive index values, while combinations of TiO₂-II with anatase and rutile have been considered promising materials for catalytic applications. In this work, the atomic layer deposition of TiO₂ on α-Al₂O₃ (0 0 0 1) (c-sapphire) was used to grow thin films containing different combinations of TiO₂-II, anatase, and rutile, and to investigate the properties of the films. The results obtained demonstrate that in a temperature range of 300–400 °C, where transition from anatase to TiO₂-II and rutile growth occurs in the films deposited on c-sapphire, the phase composition and other properties of a film depend significantly on the film thickness and ALD process time parameters. The changes in the phase composition, related to formation of the TiO₂-II phase, caused an increase in the density and refractive index, minor narrowing of the optical bandgap, and an increase in the hardness of the films deposited on c-sapphire at T_G ≥ 400 °C. These properties, together with high catalytic efficiency of mixed TiO₂-II and anatase phases, as reported earlier, make the films promising for application in various functional coatings.

Mixed films of a high-permittivity oxide, Er₂O₃, and a magnetic material, Fe₂O₃, were grown by atomic layer deposition on silicon and titanium nitride at 375 °C using erbium diketonate, ferrocene, and ozone as precursors. Crystalline phases of erbium and iron oxides were formed. Growth into three-dimensional trenched structures was demonstrated. A structure deposited using tens to hundreds subsequent cycles for both constituent metal oxide layers promoted both charge polarization and saturative magnetization compared to those in the more homogeneously mixed films.