On the yellow-band emission in CdS films

CdS polycrystalline thin films were prepared by the chemical bath depositio n (CBD) method on glass substrates. X-ray diffraction (XRD) studies show th at the films grow in the cubic zinc-blende crystalline phase. Upon thermal annealing (TA) in Ar + S-2 flux at normal pressure in the temperature range 240-510 degreesC, the evolution of the transformation into the hexagonal w urtzite phase is observed. This hexagonal crystalline structure is the stab le phase. From XRD diagrams the phase transition can be appreciated to occu r upon TA at approximately 300 degreesC. Photoluminescence (PL) data prove that the green-emission band is present for well-defined phases - cubic or hexagonal ones. A second band located at 2.2 eV appears for samples near th e transition region. This band at 2.2 eV, called the yellow band, has alrea dy been reported to be associated with interstitial Cd atoms. A model for t his yellow-band-mechanism formation, arising during the phase transformatio n, has been proposed based on Frenkel-pair creation.