Substrate effect on mechanical characterizations of aluminum-doped zinc oxide transparent conducting films

The measurements of thin-film mechanical properties are generally influenced by the elastic and plastic responses of substrates and consequently may be inaccurate. Thus in this study, the effects of three different substrates, including sapphire, glass and polyimide, on the mechanical characterizations of aluminum-doped zinc oxide transparent conducting films are evaluated. From nanoindentation tests, it was found that a high film/substrate hardness ratio contributed an early-initiated substrate effect. For a hard film/soft substrate system (zinc oxide on polyimide), the measured hardness and elastic modulus markedly dropped at very small indentation depths due to the insufficient strength of the soft substrate to sustain the applied stress. A modification of the Bhattacharya model and the calibration of the King model were also made in this study; some important factors were established. Moreover, the interface adhesion energy between the film and soft substrate, measured by nanoscratch, was high because the compliant deformation of the soft substrate released accumulated stresses and then retarded interface delamination.