Annealing effect on nano-ZnO powder studied from positron lifetime and optical absorption spectroscopy

Abstract

Mechanical milling and subsequent annealing in air at temperatures between 210 and 1200°C have been carried out on high purity ZnO powder to study the defect generation and recovery in the material. Lowering of average grain size (from 76±1to22±0.5nm) as a result of milling has been estimated from the broadening of x-ray lines. Substantial grain growth in the milled sample occurs above 425°C annealing temperature. Positron annihilation lifetime (PAL) analysis of the samples shows a distinct decrease of the average lifetime of positrons very near the same temperature zone. As indicated from both x-ray diffraction (XRD) and PAL results, high temperature (>700°C) annealed samples have a better crystallinity (or lower defect concentration) than even the nonmilled ZnO. In contrast, the measured optical band gap of the samples (from absorption spectroscopy) does not confirm lowering of defects with high temperature annealing. Thermally generated defects at oxygen sites cause significant modification of the optical absorption; however, they are not efficient traps for positrons. Different thermal stages of generation and recovery of cationic as well as anionic defects in granular ZnO are discussed in the light of XRD, PAL, and optical absorption studies.

Defects induced ferromagnetism in Mn doped ZnO

“…A recent report has also revealed [36] We get further insight of the dominant presence of V Zn in the samples from analyzing the optical absorption data. The details of the measurement technique, data analysis and results have been published earlier [26,38]. Here, in Fig.…”

“…This can be seen from the enlarged view of the (002) Mechanical milling induces structural disorder in the GB as well in the grain interiors. A fraction of defects at the grain interiors may migrate [26] to GB region due to annealing. A small fraction also gets recovered [26,27] with annealing at 500 o C. As a whole, the (002)…”

“…Positron annihilation lifetime (PAL) technique is one of the best techniques for probing vacancy defects in semiconductors [20,24,25]. For systems with nanometer size length scale, where positron diffusion length in the material is higher than the mean grain size, positrons populate near the disordered grain boundaries and annihilate there [26]. The resultant PAL parameters bear information regarding the nature and abundance of defects at the annihilation sites.…”

Thermal evolution of defects in as-grown and electron-irradiated ZnO studied by positron annihilation

“…Up to now, several works have been conducted on the study of defects in ZnO by PAS. [25][26][27][28][29][30][31][32][33] Except for defects in the as-grown state, they can also be introduced by many other ways, such as irradiation or plastic deformation. High energy electron irradiation is one of the effective ways to introduce simple and number controllable defects.…”

UV-luminescent MgZnO semiconductor alloys: nanostructure and optical properties

“…The bandgap energy can be ascertained via Tauc analysis, while the in-gap properties can be studied via Urbach analysis [21,22]. These two methods are wellestablished and commonly used for the investigation of semiconductor properties [23][24][25]. Figure 4 presents the transmission spectra of the as-grown and the two annealed samples; the inset is the Urbach analysis.…”