Defect analysis and microstructural characterization of zinc-substituted cadmium oxide nanocrystallites by positron annihilation and supplementary methods

Abstract

Pure and zinc-doped cadmium oxide nanocrystallites of sizes in the range 25 nm to 16 nm are synthesized by adopting a chemical precipitation method and by varying the doping concentration from 0.0 to 0.25 at.%. The decrease in nanocrystallite sizes with increasing substitution is expected from the smaller ionic radii of Zn2+. But more revealing is the interfacial defects formation at higher concentration of doping, which is attributed to the dissimilar crystalline structure of ZnO and CdO. X-ray diffraction patterns show well defined peaks and additional characterisation is done through transmission electron microscopy. The optical band gap measurements indicate the dominance of substitution-induced disorder over the confinement of excitons, leading to a decrease in the band gap energies. The results of positron annihilation studies confirm the cancellation of the existing vacancy type defects in the initial stage, followed by the substitution. Photoluminescence spectra reveal the distinct peaks of optical plasmonic excitations and the defect population in the bandgap and the intensity variations agreed with that of the defect related positron annihilation intensity. The segregation of ZnO phase leading to the formation of interfacial boundaries is found as a strong deterrent against the success of continued substitution.

The Franz–Keldysh effect in the optical absorption spectrum of a TlGaSe2 layered semiconductor caused by charged native defects

“…A direct and unambiguous identification of native vacancy type defects and/or dipolar vacancy complexes in TlGaSe 2 layered crystal is still an open question. It has been demonstrated that the native isolated vacancies as well as vacancy complexes in some wide-bandgap semiconductor materials can be directly identified using a positron annihilation lifetime spectroscopy method [64,65]. To date, identification of isolated chalcogen vacancies (monovacancies), which could be an abundant defect in all two-dimensional TlMX 2 wide-bandgap dichalcogenides, or metal-chalcogen vacancy complexes at the atomic level is also possible by performing a combination of experiments, such as x-ray photoelectron spectroscopy, Auger electron spectroscopy, and electron paramagnetic resonance.…”