Microstructure and optical properties of ultra-thin NiO films grown by atomic layer deposition

High-quality ultrathin films of the p-type wide bandgap semiconductor NiO are potentially interesting candidates for a number of frontier applications. Here we have fabricated a series of NiO thin films with a precise thickness control by atomic layer deposition from Ni-bis(2,2,6,6-tetramethyl-3,5-heptanedionate) and O₃ precursors. While randomly oriented polycrystalline films were obtained on glass and Si substrates, epitaxial growth was achieved on sapphire, SrTiO₃ and MgO single crystals. Optical energy gaps were determined from UV-vis absorption spectra for the films grown on borosilicate glass, and found to decrease with decreasing film thickness. This observation is in contrast to the expected bandgap increase through quantum confinement, but can be explained by an increased Coulomb interaction. Furthermore, NiO films doped with Co, Cu and Mn were deposited. Electrical measurements showed that the doping has a much higher impact on the two-probe resistance than on the resistivity. A likely explanation is a change of the contact resistance by a change of hole density.