We demonstrated that a mass density and size effect are dominant factors to limit the transport properties of very thin amorphous Sn-doped In2O3 (a-ITO) films. a-ITO films with various thicknesses (t) ranging from 5 to 50 nm were deposited on non-alkali glass substrates without intentional heating of the substrates by reactive plasma deposition with direct-current arc discharge. a-ITO films with t of more than 10 nm showed a high Hall mobility (μH) of more than 50 cm2/V s. For 5-nm-thick a-ITO films, we found that μH was as high as more than 40 cm2/V s. X-ray reflectivity measurement results revealed that the mass density (dm) determined the carrier transport in a-ITO films. For a-ITO films with t of more than 10 nm, dm had a high value of 7.2 g/cm3, whereas a-ITO films with t of less than 10 nm had low dm ranging from 6.6 to 6.8 g/cm3. Quantitative new insight from a size effect on the carrier transport is given for a-ITO films with t of less than 10 nm. This study shows that the ratio of t to mean free path of carrier electrons governed μH.
Keywords: Amorphous film; Hall mobility; Mass density; Mean free path; Tin-doped indium oxide.