Zr-doped indium oxide electrodes: Annealing and thickness effects on microstructure and carrier transport

Abstract

Zr-doped indium oxide (In2O3:Zr) has been shown to satisfy the requirements of low resistance, wide band gap, and high infrared transmittance for application as a front contact in broadband solar cells. However, the reduction of indium usage in front of transparent electrodes is still an unsatisfied requirement. With the goal of reducing the amount of indium while leveraging its properties, in this work, In2O3:Zr films with reduced thickness compared to those standardly used in solar cells are studied. 100 to 15-nm-thick films were sputtered at room temperature and annealed in distinct atmospheres to study the links between thickness, microstructure, and optoelectronic properties. As-deposited films exhibit an amorphous microstructure embedding bixbyite In2O3 nanocrystals. Annealing in neutral (N2) or reducing atmosphere (H2) allows a slight growth of these crystallites but the layers remain mostly amorphous. Whereas annealing in air results in polycrystalline films with an average grain lateral size ranging from 350 to 500 nm. The large crystalline grains formed during air annealing lead to increased electron mobility for all thickness: up to 100cm2V-1s-1 for 100-nm-thick films and up to 50cm2V-1s-1 for 15-nm-thick films, which is remarkable for such thin polycrystalline films. Conversely, H2 annealing ensures high free-carrier densities (>1×1020cm-3) but not high mobilities, still achieving conductivities between 1000 and 2000Scm-1, with the films less than 50-nm-thick keeping high broadband transmittance. The possibility of thinning down In2O3:Zr to a few tens of nanometers while keeping both high lateral conductivity and good transparency makes this material a promising candidate to reduce the amount of indium in optoelectronic applications, such as flexible touch screens and solar cells.