Influence of composition and structure on resistive switching properties of hafnium–titanium-oxide thin films grown by atomic layer deposition

Abstract

Resistive switching devices based on Hf_xTi_1−xO_y thin films with Hf/(Hf + Ti) cation ratios (x) of 0.07–0.64 were studied. The Hf_xTi_1−xO_y films were grown by atomic layer deposition (ALD) using TiCl₄, HfCl₄ and H₂O as precursors in supercycles that included different numbers of TiO₂ ALD cycles and a single HfO₂ ALD cycle. The films were grown onto RuO₂ bottom electrodes at 350 °C. Pt top electrodes were deposited by electron beam evaporation. The Hf_xTi_1−xO_y films with the x values 0.07–0.19 predominantly contained the rutile phase, while a crystalline phase isomorphous with the orthorhombic HfTiO₄ was formed in the films with x of 0.30–0.64. The elemental and phase compositions had a marked effect on electrical characteristics of the films. Significant increase in the breakdown electric field strength (E_B) was observed when x increased from 0.07 to 0.19 resulting in E_B values of 4.7–6.3 MV cm⁻¹ for Hf_xTi_1−xO_y with x ranging from 0.19 to 0.64. Most importantly, the samples containing the orthorhombic phase demonstrated superior resistive switching performance, that is, low-resistive-state (LRS) to high-resistive-state (HRS) conductance ratios >100 and endurance exceeding 10⁴ resistive switching cycles at room temperature, excellent retention of LRS and HRS at 110 °C, and resistive switching at temperatures up to 140 °C. Measurements with various pulse widths of the applied voltage revealed that the reset process limited the operation speed of these devices.