Band gap shift, structural characterization and phase transformation of CdSe thin films from nanocrystalline cubic to nanorod hexagonal on air annealing

Abstract

Cadmium selenide (CdSe) thin films were deposited on a glass substrate using the chemical bath deposition method at room temperature. The films were deposited using cadmium acetate as a Cd2+ ion source and sodium selenosulfate as a Se2− ion source. The ‘as-deposited’ CdSe thin films are red in colour and specularly reflective. The ‘as-deposited’ CdSe layers grew with nanocrystalline cubic phase along with some amorphous phase present, with an optical band gap ‘Eg’ of 2.3 eV and electrical resistivity of the order of 105–106 Ω cm. The ‘as-deposited’ film is annealed in air at 673 K for 4 h and the effect of annealing on structural, morphological, optical and electrical properties is studied. It is worth noting that after annealing, metastable nanocrystallite cubic phase transforms into stable well crystalline hexagonal phase and films show a ‘redshift’ of ‘0.6 eV’ in their optical band gap ‘Eg’. After annealing, the crystallites' size increases from 45 Å to 180 Å, which results in a decrease in electrical resistivity. These changes have been attributed to the crystallite size dependent properties of CdSe semiconductor thin films.