Enhanced ionic conduction mechanisms at LiI/Al2O3 interfaces

Electrical conduction in LiI thin films has been measured in situ during growth as a function of film thickness and temperature. Films grown at 27 °C exhibited an enhancement of the ionic conduction of ∼40× the bulk value in a region extending ∼100 nm above the interface. The enhanced conduction was not stable and decreased with a log(time) dependence during annealing at the growth temperature. The activation energy for conduction was 0.47±0.03 eV, approximately equal to the value for extrinsic conduction in bulk LiI. Films grown at 100 °C, on the other hand, showed no enhancement near the interface, and a nearly linear increase in conductance with thickness after an initial nucleation stage. The conductivity of these films was stable at and above the growth temperature and exhibited an activation energy of 0.75±0.03 eV, which is comparable to the value for intrinsic LiI. X-ray diffraction measurements on the samples showed that all films were cubic LiI. Films grown near room temperature had a strong (002) orientation while those grown at elevated temperatures were (111) with a mosaic spread of <0.4°. The data show conclusively that there is no enhancement due to space-charge layer formation at the film/substrate interface. On the other hand, the data are all consistent with a model that assumes the conduction enhancement is caused by the overlapping strain fields of dislocations formed during growth.