Thickness-dependent dc conductivity of lithium borate glasses

Abstract

Amorphous lithium borate films are prepared by ion-beam sputtering using a $0.2\phantom{\rule{0.3em}{0ex}}{\mathrm{Li}}_{2}\mathrm{O}∙0.8\phantom{\rule{0.3em}{0ex}}{\mathrm{B}}_{2}{\mathrm{O}}_{3}$ glass as a target material. The films of a thickness between 7 and $700\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ 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 $7\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. This conductivity increase in the case of very thin films leads to an absolute conductance per unit area of $0.32\phantom{\rule{0.3em}{0ex}}{\ensuremath{\Omega}}^{\ensuremath{-}1}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$ at $25\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, 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.