Investigation of aluminium and indium in situ doping of chemical bath deposited CdS thin films

Abstract

Aluminum and indium in situ doping of CdS using chemical bath deposition (CBD) is investigated. The effects of Al and In-doping on optical properties as well as on electrical properties, crystal structure, chemistry and morphology of CdS films are studied. Al doping of CdS using CBD is shown to be successful where a resistivity as low as 4.6 × 10−2 Ω cm and a carrier density as high as 1.1 × 1019 cm−3 were achieved. The bandgap of Al-doped films decreases to a minimum of 2.26 eV, then slightly increases and finally saturates at 2.30 eV as the [Al]/[Cd] ratio in solution increases from 0.018 to 0.18. X-ray diffraction studies showed Al3+ ions entering the lattice substitutionally at low concentration and interstitially at high concentration. Phase transition, due to annealing, and induced lattice damage, due to doping, were detected by micro-Raman spectroscopy. Film stoichiometry was found to be sensitive to Al concentration, while film morphology was unaffected by Al doping. Indium doping using CBD, however, was found to be highly unlikely due to the low solubility of indium sulfide. Instead, the formation of InS/In2S3 dominated the deposition process over CdS.