Charge transfer at the interface between complex oxide thin films

The continuous downscaling of electronic components brings us closer to the limits of conventional semiconductors, driving a search for alternative materials such as transition metal oxides. For these small dimensions, interface effects such as charge transfer can play a major role in the device properties. Where these effects can be predicted by band-bending at the interface for conventional semiconductors, these principles cannot be applied to transition metal oxides due to a lacking reference. A proposed solution lies in using oxygen bands as reference, because of the continuous oxygen octahedral network across the interface. This theory was tested by fabricating various combinations of ultra-thin LaCoO3 and LaTiO3 layers using Pulsed Laser Deposition, in order to investigate the charge transfer at the interface between these materials. Several characterization techniques were used to determine the valence of the transition metal ions and the spatial distribution. In reference LaCoO3, the expected Co3+ valence was observed, but when LaCoO3 was sandwiched between LaTiO3, a strong Co2+ signal emerges (up to 100% Co2+ for a 2 unit cell thick LaCoO3 layer sandwich between LaTiO3). This Co2+ signal was determined to originate from a region of about 3 unit cells from the interface. Additional experiments using a LaAlO3 spacer proved successful in blocking the charge transfer, providing additional control on the valence. The charge transfer seems independent of temperature or substrate choice, making this a promising new way of doping materials without disrupting the structure, possibly enabling novel properties