Showing papers by "Anindya Sarkar published in 2017"

Defect generation and recovery in polycrystalline ZnO during annealing below 300 °C as studied by in situ positron annihilation spectroscopy

Abstract: Annealing effect of high-purity granular ZnO up to 286 °C has been studied by in situ Doppler broadening of electron-positron annihilated γ-ray line shape measurement. Increase of S-parameter has been observed during annealing at 182 and 286 °C which saturates in a time scale of ~90 min. The increase of S-parameter during 182 °C annealing is related to the removal of carbon and loosely bound hydroxyl groups from ZnO, thereby increasing open-volume defects. During 286 °C annealing, such open volumes agglomerate and migrate to grain surface regions causing further increase of open volumes at the positron annihilation site. Ex situ photoluminescence measurement has been carried out with samples annealed at 182 and 286 °C. Room-temperature PL results are consistent with positron annihilation spectroscopic findings. Additionally, 10 K PL spectrum shows large increase of 3.311 eV emission in 286 °C-annealed sample. This particular emission is related to typical crystal defects in ZnO which is a matter of discussion till date. The present study provides understanding on the interplay of defects in relatively low-temperature-annealed ZnO and is important from theoretical perspective as well as for improving the performance of ZnO as photocatalytic and gas-sensing agent.

Clustered vacancies in ZnO: Chemical aspects and consequences on physical properties

Abstract: Chemical nature of point defects, their segregation, cluster or complex formation in ZnO is an important area of investigation. In this report, 1.2 MeV Ar ion beam is used to incorporate defects in granular ZnO. Evolution of defective state with irradiation fluence 1 x 10^14 and 1 x 10^16 ions/cm2 has been monitored using XPS, PL and Raman spectroscopic study. XPS study shows presence of oxygen vacancies (VO) in the Ar ion irradiated ZnO. Zn(LMM) Auger spectra clearly identifies transition involving metallic zinc in the irradiated samples. Intense PL emission from IZn related shallow donor bound excitons (DBX) is visible in the 10 K spectra for all samples. Although overall PL is largely reduced with irradiation disorder, DBX intensity is increased for the highest fluence irradiated sample. Raman study indicates damage in both zinc and oxygen sub-lattice by energetic ion beam. Representative Raman modes from defect complexes involving VO, IZn and IO are visible after irradiation with intermediate fluence. Further increase of fluence shows, to some extent, a homogenization of disorder. Huge reduction of resistance is also noted for this sample. Certainly, high irradiation fluence induces a qualitative modification of the conventional (and highly resistive) grain boundary (GB) structure of granular ZnO. Low resistive path, involving IZn related shallow donors, across the GB can be presumed to explain resistance reduction. Open volumes (VZn and VO) agglomerate more and more with increasing irradiation fluence and finally get transformed to voids. Results as a whole have been elucidated with a model which emphasizes possible evolution of new defect microstructure that is distinctively different from the GB related disorder. Based on the model, qualitative explanations of commonly observed radiation hardness, colouration and ferromagnetism in disordered ZnO have been put forward.