Growth kinetics of ZnO nanocrystallites: Structural, optical and photoluminescence properties tuned by thermal annealing R.G. Singh a, * , Fouran Singh b , Vinod Kumar c , R.M. Mehra a a Department of Electronic Science, University of Delhi South Campus, Benito Juarez Road, New Delhi 110021, India b Materials Science Group, Inter University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi 110067, India c Department of Physics, Gurukula Kangri University, Haridwar 249 404, India article info Article history: Received 20 May 2010 Accepted 20 October 2010 Available online 3 November 2010 Keywords: Photoluminescence Coalescence phenomena LSW theory Sol-gel ZnO abstract In this paper, we report temperature dependent studies of ZnO nanocrystallites deposited by the sol-gel spin coating process. The films are annealed at different temperature from 500 to 900  C to grow the size of crystallites in controlled environment. The crystallite size, lattice parameters, and strain in the nanocrystallites are calculated. A coarsening kinetics is understood by Lifshitz, Slyozov and Wagner (LSW) theory and the activation energy of the coarsening process is estimated. Optical absorption and Photoluminescence (PL) measurements were carried out to investigate the effect of coarsening on optical properties of the ZnO thin films. The origin of PL spectra is explained in terms of the stabilization of structural defects and size of the crystallites. Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction Zinc oxide (ZnO) is a multifunctional material that has been investigated from 1950s and utilized in many applications [1]. Zinc oxide an important wide band gap semiconductor material is an optoelectronic application [2,3]. ZnO could be a strong candidate for solid-state white light (SSWL) devices due to the versatile emission properties in the entire visible to UV emission with high efficiency [4]. The interest in ZnO is renewed by its prospects in optoelectronics applications owing to its direct wide band gap (E g w 3.34 eV at 300 K) and large exciton binding energy of w60 meV [5]. It ensures an intense near band edge exciton emis- sion at room temperature and above, as the value of exciton binding energy is about 2.4 times that of the thermal energy (k B T ¼ 25 meV) available at room temperature (RT). ZnO is a promising metal oxide semiconductor for optoelectronic, photovoltaic, data storage, biochemical and chemical sensors applications [6e10]. The optical properties of a semiconductor have their depen- dence on both intrinsic and extrinsic effects. Intrinsic optical transitions take place between the electrons in the conduction band and holes in the valence band, including excitonic effects due to the Coulomb interaction [11,12]. Extrinsic properties are related to dopants/impurities or point defects and complexes, which usually create electronic states in the band gap, and therefore influence both optical absorption and emission processes. The electronic states of the bound excitons, which may be bound to neutral or charged donors and acceptors, depend strongly on the semiconductor material, in particular the band structure [13]. For a shallow neutral donor bound exciton (DBE), for example, the two electrons in the bound exciton state are assumed to pair off into a two-electron state with zero spin. The additional hole is then weakly bound in the net hole-attractive Coulomb potential set up by this bound two-electron aggregate. Similarly, neutral shallow acceptor bound excitons (ABE) are expected to have a two-hole state derived from the top-most valence band and one electron interaction. On the other hand extrinsic transitions could be seen in optical spectra such as free-to- bound (electron-acceptor), bound-to-bound (donor-acceptor) and the so-called yellow/green luminescence [14]. In the present work sol-gel spin coating is used for depositing the nanocrystalline films, as Sol-gel approach offer many advantages, including low pro- cessing temperature and molecular level homogeneity, simple and low-cost [15]. Thermal annealing is an important post-growth treatment that has been applied in many fields such as metallur- gical and semiconductor industry. Typically, it results in softening of the metal through removal of defects and the internal stresses caused by the defects. The effects of annealing temperature on the nucleation and growth process of thin films have been well established. It generally involves the process of nuclei formation, growth, and coalescence. At a lower growth temperature, the * Corresponding author. E-mail address: rgsnsc@gmail.com (R.G. Singh). Contents lists available at ScienceDirect Current Applied Physics journal homepage: www.elsevier.com/locate/cap 1567-1739/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2010.10.013 Current Applied Physics 11 (2011) 624e630