Heavily doped metal oxide thin films combining high visible transmittance and low electrical resistance are used in a multitude of optoelectronic devices, where their performance is highly dependent on the structural defects and density of electronic states associated with doping. This study explores the structural, optical, and electronic properties of Sn-doped indium oxide (In2O3:Sn) and Al-doped zinc oxide (ZnO:Al) thin films, which were prepared by sputtering on unheated glass substrates and subsequently annealed in N2 at different temperatures between 250 °C and 450 °C. These samples reach free electron densities above 1020 cm−3 due to the presence of extrinsic donors (mainly substitutional defects of SnIn and AlZn) and also intrinsic donors (oxygen vacancies), which change with the annealing temperature due to oxygen desorption and/or cation migration processes. The volume of the crystal lattice expands (up to a maximum of 1.1%) and the band gap widens (up to a maximum of 17.9%) with respect to the undoped material, increasing with electron density. Additional absorption is due to band tail, at an energy ~10% below the undoped band gap, which varies slightly with the carrier concentration. The same general behavior is observed for both materials, with particularities in terms of crystal lattice and electronic states, which can be tuned by the heating temperature.
Cecilia Guillén (Sat,) studied this question.