2017 Volume 125 Issue 4 Pages 333-337
Bandgap-tunable (2.5 ≤ Eg ≤ 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.