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

Abstract

Bandgap-tunable (2.5 ≤ Eg ≤ 4.3 eV) amorphous CdGaO films with mobilities of ∼10cm2V-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 cm2V-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 cm2V-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.