Optical, morphological and electrical properties of rapid thermally annealed CoPc/n-Ge heterostructures for photodiode applications

Effect of rapid thermal annealing (RTA) temperature on electrical, morphological and optical properties of cobalt phthalocyanine (CoPc)/n-Ge heterostructures is investigated. UV–Vis measurements of as-deposited and annealed CoPc thin films show the presence of Q band transition at 618 nm and 690 nm absorption wavelength suggesting π-π* transitions. A phase transition is evidenced for the 300 °C annealed CoPc sample. Further, no degradation of Q band intensities is noticed which signifies highly stable CoPc thin films even after annealing at 400 °C. AFM measurements of CoPc thin film on Ge substrate reveal an increased RMS roughness with increased annealing temperature until 200 °C and further roughness was found to be decreased for the sample annealed at 300 °C. A clear change in the morphology of CoPc thin films is observed for the 300 °C annealed sample than 200 °C annealed sample which shows an evidence for the phase transition of CoPc thin films from α-phase to β-phase. This phase transition of CoPc thin films was also confirmed using XRD measurements. FESEM with EDS measurements were carried out for the as-deposited and annealed heterostructures. The surface topography of all the samples reveals a spherical-grained morphology and the grains seem to be highly dispersed in 400 °C annealed CoPc/Ge layers. The electrical and current transport mechanisms of the Au/CoPc/n-Ge heterostructure have been explored at different annealing temperatures by using I–V and C–V characteristics. Notably, the results reveal that superior barrier parameters are observed for the heterostructure annealed at 300 °C than as-deposited heterostructure. Rapid thermal annealing of the Au/CoPc/Ge heterostructure shows its promising applications in the areas of optoelectronic or photoresponsive devices.