Thermal-annealing-induced effects in chemically deposited cobalt sulphide thin films

摘要

The thermal transformation of chemically deposited Co_3S_4 thin films to CoO films by air annealing has been achieved. The decomposition of a 500 nm thick Co_3S_4 thin film to a CoO film involves the creation of non-stoichiometric Co_3S_4, Co_3S_4 + CoS_2 mixed phase and non-stoichiometric CoO, as confirmed from x-ray diffraction patterns, energy-dispersive analysis of x-ray fluorescence data and optical and electrical characteristics. The 270 ℃ air-annealed films exhibited the lowest sheet resistance of about 10~7 Ω/□ and an electrical conductivity activation energy (E_σ) of 0.35 eV in the temperature range from room temperature to 150 ℃. A temperature of 310 ℃ was established as the transition temperature for the conversion of the Co_3S_4 + CoS_2 mixed phase to the non-stoichiometric CoO. Air annealing of the films for 1 h at temperatures in the 350-400 ℃ range converted them to nearly stoichiometric CoO films with an increase in the sheet resistance to about 10~10 Ω/□. The E_σ value also increased to 1.05 eV The absorption coefficient data on the films are interpreted in terms of direct and indirect band-to-band transitions.