Effect of rotational speed on physical properties of Cu doped SnO2 thin films

Abstract

The sol-gel spin-coating (SGSC) technique, with rotational speed varied from 2000 to 3500 rpm, was employed to prepare Cu-doped SnO₂ (CTO) films. XRD patterns of the CTO films show a polycrystalline tetragonal structure. The reduction in crystallite size from 36 to 21 nm with increasing rotational speed supports the increase in microstrain. The rise in rotational speed promotes the formation of smaller crystallites because of rapid film solidification, which in turn increases microstrain owing to enhanced lattice distortions and grain-boundary effects. The morphology of the CTO films shows homogeneous growth with a spherical shape, and agglomerated grains were observed at higher rotational speeds. Cu-doped SnO₂ prepared at 3500 rpm shows an optimum transmittance of 82% in the visible region. The optical band-gap energy of Cu-doped SnO₂ films increased from 3.20 to 3.65 eV because of the Burstein-Moss effect. This study provides a novel correlation between lattice distortion (via basal plane angle) and surface roughness evolution in Cu-doped SnO₂ thin films, offering deeper insights into the structural-morphological relationship influencing their physical properties.