Incorporation of oxygen during oxidative annealing of thermally evaporated In films

The paper describes studies on compositional, morphological and optical characteristics of thin indium oxide films prepared by annealing thermally evaporated indium metal films in 423–723 K temperature range in air. The incorporation of oxygen in the films has been probed by depth profiling oxygen using 18O(p, α)15N nuclear reaction and 3.05 MeV 16O(α, α)16O resonant scattering. The morphology of the films has been examined by atomic force microscopy while their structure by glancing incidence X-ray diffraction. As grown In films are polycrystalline and consist of well-aligned (In) hillocks. The hillocks in thin films (∼12 nm) are nanosized and conical in shape while those in thicker films (∼130 nm) are micron-sized with rather flat tops. Nanosized hillocks impart films enhanced reactivity towards oxygen. Consequently thinner films contain high amount of adsorbed oxygen in as-deposited state and undergo facile oxidation. The hillocks are obliterated in the process. The enhanced reactivity is attributed to high surface energy, generally associated with nanoparticles, and residual stress. These films exhibit high transmission (>90%) on annealing beyond 473 K. Micro-sized hillocks, on the other hand lend pronounced roughness to the films. Roughness and lower surface free energy which manifests in the form of flatter hillock tops, make the films oxidation-resistant. Depth profile measurements in thicker films show that oxidation starts at the surface and proceeds into the interiors of the film with increase in the duration of annealing. The films are deficient in oxygen, even as X-ray diffraction shows the formation of polycrystalline indium oxide. These have low transmission (<30%) and their band gaps increase with the temperature and duration of annealing. The increase in band gap is attributed to the gradual oxidation of interior regions that are initially significantly deficient in oxygen and improvement in crystallinity.