Thickness-dependent dc conductivity of lithium borate glasses

Amorphous lithium borate films are prepared by ion-beam sputtering using a 0.20.3em0exLi₂O∙0.80.3em0exB₂O₃ glass as a target material. The films of a thickness between 7 and 7000.3em0exnm are deposited onto silicon substrates between two metallic AlLi electrodes serving as electrical contacts. dc conductivities of the glass layers of different thicknesses are determined by temperature-dependent impedance measurements. An increase of the specific dc conductivity of about 3 orders of magnitude is observed, when the thickness of the layers decreases from 120 down to 70.3em0exnm. This conductivity increase in the case of very thin films leads to an absolute conductance per unit area of 0.320.3em0exΩ^-10.3em0excm^-2 at 250.2em0ex^∘C, which is sufficient for technical application of the glass layers in solid-state ionic devices. Three alternative explanations are suggested to explain this nontrivial conductivity increase: structural modifications at the interfaces, the formation of space-charge regions, and the existence of randomly distributed ion-conducting channels.