Thermal Stability of Amorphous Metal Oxides: The Interplay of Secondary Cations, Degree of Substitution, and Local Structure

Indium-based amorphous metal oxides (AMOs) are important materials that enable a wide range of electronic and optoelectronic applications. The metal cations that substitute for In are known to effectively tune the material properties as well as to retain the desirable amorphous state when subjected to high operating temperatures. While much attention focuses on property tuning, there is less focus on those fundamental factors that enhance thermal stability. Hence, in this paper, we employ in situ X-ray scattering and ab initio molecular dynamics (MD) to systematically study the effects of secondary cations on the crystallization process. A series of amorphous (In_1-xM_x)₂O₃ thin films (M = Sn, Zn, and Ga; x = 5, 10, 20, and 30%) are grown by pulsed laser deposition (PLD) under identical deposition conditions. The films are then annealed isochronally, and the degree of crystallinity, crystallization temperature (T_c), and crystallization time (τ_c) are determined by a quantitative analysis of the time-evolved X-ray scattering patterns. All doped films have a T_c higher than the corresponding undoped films, and significantly higher T_c values (>400 °C) are found at the higher degrees of Zn and Ga substitution (20 and 30%). For the same level of substitution, T_c increases in the order Sn > Zn > Ga, suggesting increasing crystallization barriers and local structural disorder. All crystallized films are in the common cubic phase except for IZO-20%, where an additional rhombohedral phase is observed throughout the crystallization process. The pole figure analysis reveals the detailed preferred orientations buried in postannealed crystalline films. Complementary ab initio molecular dynamics (MD) simulations of the as-deposited AMOs provide an informative theoretical perspective. The medium-range structures characterized by the value and variance of effective coordination numbers around metal and oxygen atoms are found to play an important role in explaining the observed crystallization dynamics in these oxides.