Substrate reactivity as the origin of Fermi level pinning at the Cu2O/ALD-Al2O3interface

The reduction of a Cu2O layer on copper by exposure to TMA during atomic layer deposition of Al2O3 has recently been reported [Gharachorlou et al., ACS Appl. Mater. Interfaces 2015, 7, 16428-16439]. The here presented study analyzes a similar process, leading to the reduction of a homogeneous Cu2O thin film, which allows for additional observations. Angle-resolved in situ X-ray photoelectron spectroscopy confirms the localization of metallic copper at the interface. The evaluation of binding energy shifts reveals the formation of a Cu2O/Cu Schottky barrier, which gives rise to Fermi level pinning in Cu2O. An initial enhancement of the ALD growth per cycle (GPC) is observed only for bulk Cu2O samples and is thus related to lattice oxygen, originating from deeper-lying regions than just the first few layers from the surface. The oxygen out-take from the substrate is limited to the first few cycles, which is found to be rather due to a saturated copper reduction, than due to the oxygen diffusion barrier of Al2O3