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 stud ied: one electron conducting Ta2O5 (ec-Ta2O5) and the other non-conducting Ta2O5 (nc-Ta2O5). The as-deposited films were characterized by impedance sp ectroscopy (IS) and isothermal transient ionic current (ITIC) measurements. From IS, the de conductivity 2 x 10 (-14) S/cm was obtained for the ec-Ta2 O5 film at an applied ac potential of 50 mV whereas a Value less than or eq ual to 1 x 10(-17) S/cm was obtained for the nc-Ta2O5 film. Li conducting p roperties were studied using the galvanostatic intermittent titration techn ique and ITIC measurements on the intercalated samples. Despite the very di ssimilar de conductivities of the as-deposited films, the two Ta2O5 samples showed surprisingly similar Li ion conducting properties for small Li/Ta2O 5 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 ch emical diffusion coefficient differed. For the nc-Ta2O5 film a plateau was observed in the equilibrium potential vs, composition curve for Li;Ta2O5 ra tios between 7 x 10(-5) and 2 x 10(-3). This plateau was likely to have bee n caused by attractive interactions between the intercalated ions, possibly large enough to cause phase separation. The attractive interactions were s hown to suppress the chemical diffusion coefficient in this composition ran ge. (C) 2001 Elsevier Science Ltd. All rights reserved.