Dielectric and Li transport properties of electron conducting and non-conducting sputtered amorphous Ta2O5 films

Two types of sputtered thin film amorphous tantalum oxide (Ta2O5) were studied: one electron conducting Ta2O5 (ec-Ta2O5) and the other non-conducting Ta2O5 (nc-Ta2O5). The as-deposited films were characterized by impedance spectroscopy (IS) and isothermal transient ionic current (ITIC) measurements. From IS, the de conductivity 2 x 10 (-14) S/cm was obtained for the ec-Ta2O5 film at an applied ac potential of 50 mV whereas a Value less than or equal to 1 x 10(-17) S/cm was obtained for the nc-Ta2O5 film. Li conducting properties were studied using the galvanostatic intermittent titration technique and ITIC measurements on the intercalated samples. Despite the very dissimilar de conductivities of the as-deposited films, the two Ta2O5 samples showed surprisingly similar Li ion conducting properties for small Li/Ta2O5 ratios. The Li ion mobility was in the range 1.1 x 10(-9)-3.0 x 10(-9) cm(2)/V s for both films. However, the Li storage behaviour as well as the chemical diffusion coefficient differed. For the nc-Ta2O5 film a plateau was observed in the equilibrium potential vs, composition curve for Li;Ta2O5 ratios between 7 x 10(-5) and 2 x 10(-3). This plateau was likely to have been caused by attractive interactions between the intercalated ions, possibly large enough to cause phase separation. The attractive interactions were shown to suppress the chemical diffusion coefficient in this composition range. (C) 2001 Elsevier Science Ltd. All rights reserved.