Semiconductor to metal transition in degenerate ZnO: Al films and the impact on its carrier scattering mechanisms and bandgap for OLED applications

Abstract

Temperature dependent Hall measurements revealed that ionized impurity scattering was the dominant mechanism in sputter deposited, degenerate, aluminum doped zinc oxide (AZO) films up to ~530 nm thickness, and a semiconductor to metal transition was observed when thickness was further increased. With the increase in film thickness, the mobility and conductivity also increased from 6.70 to 18.7 cm² V⁻¹ s⁻¹ and 1.83 × 10²–8.28 × 10² (Ω cm)⁻¹, respectively. However, this was accompanied by a larger than 0.2 eV Burstein–Moss blue-shift of the interband absorption edge determined from absorption spectra. The movement of the Fermi level further into the conduction band that accompanies the Burstein–Moss shift results in a corresponding workfunction decrease of the films. This means that the interface barrier for hole injection in anode applications such as organic light emitting diodes (OLEDs) becomes larger, which translates into higher turn-on voltages and lower current and power efficiencies compared to indium tin oxide anodes. It is suggested that improving conductivity through mobility increases, and increasing workfunction through surface functionalization may improve the prospects of AZO films in OLEDs and other applications where in addition to conductivity and transparency, workfunction is also critical.