The growth of oxygen vacancy controlled HfxTi1-xO2 layers deposited by gas injection magnetron sputtering technique

Abstract

Hf_xTi_1-xO₂ layers (0.3 ≤ x ≤ 0.86) were grown by the Gas Injection Magnetron Sputtering (GIMS) technique. Binary Hf-Ti targets with the Hf/Ti atomic ratios of 1/4, 2/3, 3/2, and 4/1, fabricated using plasma surface sintering, were used for reactive sputtering in the fixed Ar/O₂ gas mixture. The initial growth of the GIMS-deposited Hf_xTi_1-xO₂ layers was characterized by an amorphous structure, which crystallized along the oxide layer growth direction when the Hf content was sufficient (x > 0.74). The lattice structure of oxide layers with x  ≤ 0.86 exhibited monoclinic crystallization, which is characteristic of HfO₂. However, when x = 0.74, a highly non-equilibrium mixture of monoclinic and rhombohedral HfO₂ phases was formed. The XPS study showed that these layers’ Hf-O_x and Ti-O_x bonds were locally O-deficient, indicating that oxygen vacancies control the formation of Hf_xTi_1-xO₂. The optical properties of the fabricated layers also changed markedly when the chemical composition was modified. For example, an increase in the Hf fraction reduced the extinction coefficient from 1 to 0.2 at 250 nm and the refractive index from 2.45 to 2.3 in the visible range. We indicated that oxygen vacancies create the mid-band states, shifting the absorption towards lower energies. As the x value was increased from 0.30 to 0.86, the corresponding absorption shifted from 3.20 to 2.55 eV. In addition, Hf-rich layers revealed a significant increase in hardness and reduced Young’s modulus by 73 % and 30 %, respectively. All layers were also hydrophobic.