Impact of annealing on structural and optical properties of CoPc thin films

Abstract

The structural and optical properties of four samples of cobalt phthalocyanine (CoPc) thin films having almost the same thickness deposited onto silicon substrates by organic molecular beam deposition (OMBD) were investigated. The intensity distributions of the X-ray diffraction (XRD) patterns confirm the crystalline nature of the films and presents a single sharp dominant peak at 2q=6.9o (d=12.72 ) of the α-CoPc phase unit cells, which indicates to a preferential orientation (200) direction, then undergoes a phase transition into β- form in a preferential orientation (001) direction after annealing at 250 and 350◦C. Furthermore it was shown that an increasing in the crystallite size (L) occurs upon annealing. The small values of the roughness (7.04 nm, 7.2 nm) obtained from atomic force microscopy (AFM) measurements show relatively smooth and flat surfaces. Despite the identity of the samples deposition conditions, the post growth annealed films show different morphological features of a mean grain size about of 11-26 nm. The optical energy gap was estimated from the absorption coefficient using Tauc's relation.