Thickness dependent structural, optical and electrical properties of Ti-doped ZnO films prepared by atomic layer deposition

Highly conductive and transparent titanium-doped zinc oxide (TZO) films with thicknesses ranging from 24 to 284 nm were deposited using atomic layer deposition. The TZO films change preferred orientation from c-axis orientation to a-axis orientation with increasing film thickness. Growth modeling suggests this may stem from hexagonal bipyramid grain shapes, or less likely a co-existence of aligned grains along either a- or c-axis. The surface grain size increases with film thickness from 15 to 30 nm for 24 and 284 nm thick films, respectively. Both carrier concentration and mobility increase with thickness, which is attributed to increased grain size and crystallinity. The lowest resistivity of 2.0 × 10−3 Ω cm was achieved for the largest film thickness. The transmittance and reflectance of the TZO films show thickness-dependent properties in the IR region caused by changes in carrier concentration, increasing carrier absorption and changing the plasma frequency to higher energy wavelengths.