Thin film electrolytes are promising materials for applications in microelectronics devices like microbatteries, microsupercapacitors, electrochromic displays and sensors. Lithium-containing oxide materials in particular can be developed in stable amorphous thin films with promising performance in microbattery devices. However, the physicochemical properties of thin films may differ substantially from those of the target bulk materials. Thus, a challenge in thin film science and technology concerns the control of properties of thin films in terms of chemical composition and conditions of film deposition.
New amorphous electrolytes are developed in thin film forms suitable for applications in electrochemical devices like the microbateries shown in the picture.
Thin films were developed in the Li-borate, xLi2O-(1-x)B2O3 (0<x<0.75), and Li-borosulphate, yLi2SO4-(1-y)LiBO2 (0<y<0.7), systems by rf-sputtering in argon and nitrogen atmospheres using appropriate target compositions (collaboration with ICMCB-CNRS and ENSCPB, University of Bordeaux 1, France). The structure of thin films was probed by infrared transmittance and reflectance spectroscopy, and the spectra were simulated using rigorous expressions for the infrared response of the film/substrate bilayer system. Comparison was made between the measured spectra of thin films and those simulated on the basis of the optical response functions of bulk glasses having the nominal composition of the target material employed for sputtering.
This procedure allows for the identification of compositional and structural differences between thin films and bulk glasses. A key result of the study was that sputtering leads to development of Li-borate thin films with lithium content lower than that of the target material. Additional variations in the short-range order structure were found to originate from differences in thermal history between thin films and bulk glasses. The infrared study has demonstrated also the incorporation of nitrogen into films sputtered under a nitrogen atmosphere. Such films were found to exhibit improved stability and enhanced ionic conductivity relative to all oxide films.
E.I. Kamitsos, M. Dussauze, C.P.E. Varsamis, P. Vinatier and Y. Hamon, ''Thin film glassy electrolytes: structure and composition by experimental and simulated infrared spectra'', J. Phys. Chem. C 111, 8111 (2007).
M. Dussauze, A. Giannoudakos, L. Velli, C.P.E. Varsamis, M. Kompitsas and E.I. Kamitsos, ''Structure and optical properties of amorphous lead-germanate films developed by pulsed laser deposition'', J. Chem. Phys. 127, 034704 (2007).
Y. Hamon, P. Vinatier, E.I. Kamitsos, M. Dussauze, C.P.E. Varsamis, D. Zielnok, C. Roesser and B. Roling, ''Nitrogen flow rate as a new key parameter for the nitridation of electrolyte thin films'', Solid State Ionics 179, 1223 (2008).
Simulations of Infrared Transmittance Spectra of Thin Films
Low-resolution experimental transmittance spectra (10 cm-1, red lines) of Li-borate thin films deposited from a Li-diborate (Li2O-2B2O3) target on silicon wafer under different radio frequency powers (P). The calculated transmittance spectra (blue lines) correspond to thin films having the Li2O-2B2O3 composition, and their indicated film thickness is the best value obtained by simulation of each experimental spectrum.