The optical and structural properties of group II-VI (CdTe) and group IV-VI (TiO2) semiconductor nanocrystals grown in thin film matrix

This study is mainly concerned with the investigation of the optical and structural properties of CdTe (group II-VI) and anatase-phase TiO2 (group IV-VI) thin films. The experimental techniques used in this work are of three types: (i) optical transmission (ABS), (ii) X-ray diffraction (XRD), and (iii) Raman scattering (Rs). Physical Vapor Deposition (PVD) is employed to grow CdTe and TiO2 thin films of different thicknesses on glass substrates. As regard to CdTe samples, optical absorption spectroscopy shows that Urbach energy (EU), related to the width of long-wavelength tail, decreases from 827 to 585 meV with increasing thickness from 100 to 500 nm respectively. The thickness-dependent grain sizes for CdTe samples were calculated using the value of size-dependent optical absorption edge energy. A redshift of 710 meV in the absorption edge energy occurs with increasing thickness from 100 to 500 nm. Thickness-dependent shifting and widening of X-ray diffraction lines are probably due to grain size, reduced to nanometer scales, and strain effects. Relative to the phonon frequency of bulk CdTe crystal, size-dependent blueshift was observed in Raman spectra for the Longitudinal Optical (LO) phonon frequency, while redshift for the Transverse Optical (TO) and Longitudinal Acoustic (LA) phonons frequency is noticed. As regard to TiO2 thin films, optical absorption spectroscopy shows that EU decreases from 212 to 176 meV as the thickness increases from 143 to 221 nm respectively. The particle size, which depends on the film thickness, and particle size distribution were obtained by fitting the experimental data of the absorption coefficient (α) to a model of α as a function of the photon energy E. Absorption edge energy of the film (ER) was also calculated, it decreases by 165 meV as the thickness increases from 143 to 221 nm. Forbidden energy of bandgap transition was also determined from the data obtained by absorption. Two types of transitions are observed in TiO2 nanocrystals (direct and indirect). The phonon confinement model was employed to interpret the grain size effect in Raman scattering. Relative to the Raman active Eg mode of bulk TiO2 crystal, size-dependent blueshift was noticed in Raman lines for the first and second Eg modes (Eg(1):≈ 144 cm-1 and Eg(2):≈ 197 cm-1 respectively). The results of the different techniques employed in this work were combined together to study size effect for grains at nanometer scales, size-dependent strain, and structural disorder. The redshift observed in the bandgap energy is proposed to be related to the increase in the grain size. Urbach energy, related to the structural disorder, decreases with increasing grain size which means that the structural disorder decreases with increasing size.