Structural, optical and mechanical properties of amorphous and crystalline alumina thin films

• Structure–property correlations in alumina films grown by electron-beam evaporation
• Amorphous films crystallize into γ and δ-alumina on annealing in air at 800 °C.
• δ and θ-alumina films are stable up to 1130 °C and do not transform to α-phase.
• Amorphous alumina films contain [5]Al and [4]Al structural units in the ratio of 1:2.
• [5]Al decreases whereas [6]Al concentration increases on crystallization.

Thin films of amorphous alumina of thickness 350 nm were deposited on fused silica substrates by electron beam evaporation. Amorphous films were annealed at several temperatures in the range: 400–1130 °C and changes in film crystallinity, short-range structure, optical and mechanical properties were studied. X-ray diffraction studies found that crystallization starts at 800 °C and produces γ and δ-alumina, the latter phase grows with heat treatment and the sample was mostly δ and θ-alumina after annealing at 1130 °C. The as-deposited amorphous alumina films have low hardness of 5 to 8 GPa, which increases to 11 to 12 GPa in crystalline sample. 27Al Magic Angle Spinning Nuclear Magnetic Resonance was used to study the short-range order of amorphous and crystalline alumina films and it was found that amorphous alumina film contains AlO5 and AlO4 structural units in the ratio of 1:2. The concentration of AlO5 was significantly suppressed in crystalline film, which contains 48% of Al3 + ions in AlO6, 7% in AlO5 and 45% in AlO4 units.