Electrical Conductivity Studies in Sol-Gel-Derived Li-Doped NiO Epitaxial Thin Films

The electrical properties of transparent Li-doped nickel oxide (NiO) epitaxial thin films grown on MgO (100) substrates using a sol-gel spin-coating technique are investigated as to the Li concentration as determined by secondary ion mass spectroscopy. The epitaxial growth of the cubic Li-doped NiO films with a cube-on-cube relationship is confirmed by X-ray diffraction measurements. The electrical conductivity increases superlinearly with increasing Li concentration from 0.028 to 7.5 at%. The temperature dependence of the conductivity and the linear dependence of the activation energy on the mean distance between Li ions suggest that the conductivity can be explained within a hopping conduction framework. However, the significant Hall voltages observed in Hall effect measurements suggest the existence of nonhopping carriers in addition to hopping carriers. The Li concentration dependence of the conductivity cannot be well explained by the widely used model of small polaron hopping in which small polarons conduct through thermally activated hopping between nearest-neighbor Ni sites. Instead, the observed dependence is well explained quantitatively by a model combining polaronic interacceptor hopping and nonhopping conductions. Polaronic interacceptor hopping conduction dominates for Li concentrations below 1 at%, whereas nonhopping conduction dominates for higher Li concentrations.