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

Abstract

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