Vacuum, Ar, and O₂ annealing effects on bandgap-tunable semiconducting amorphous Cd–Ga–O thin films

Abstract

Bandgap-tunable (2.5 ≤ E_g ≤ 4.3 eV) amorphous Cd–Ga–O films with mobilities of ∼10 cm² V⁻¹ s⁻¹ were annealed in vacuum (∼10⁻⁴ Pa), Ar, and O₂ atmospheres. The Cd concentrations ([Cd]/([Cd] + [Ga])) of 70, 50, and 20% films, with respective bandgap energies of ∼2.5, ∼3.0, and ∼4.0 eV, were employed for representative narrow, middle, and wide bandgap samples, respectively. The carrier concentrations of all of the annealed films, except those with a Cd concentration of 70% annealed in a vacuum and Ar, showed minimum values at annealing temperatures between 200 and 300°C. This was probably due to structural relaxation, which leads to low in-gap states. The threshold carrier concentrations for demonstrating high mobility (∼10 cm² V⁻¹ s⁻¹) were 10¹⁸ cm⁻³ for films with a Cd concentration of 70% and ≤10¹⁷ cm⁻³ for films with a Cd concentration of 50% films. O₂ annealing of films with a Cd concentration of 20% effectively decreased the carrier concentration to ∼ 10¹⁵ cm⁻³, with a Hall mobility of ∼3 cm² V⁻¹ s⁻¹, which is a distinctively high mobility for amorphous oxide with bandgap energy of ∼4 eV. The marked decreases in the carrier concentration and mobility of the Cd concentration of 20% film at 600°C with O₂ annealing were assumed to be due to the formation of microcrystalline Ga₂O₃.