250 keV Ar 2+ ion beam induced grain growth in tin oxide thin films T. Mohanty a, ⁎, S. Dhounsi a , P. Kumar b , A. Tripathi b , D. Kanjilal b a School of Physical Sciences, Jawaharlal Nehru University, New Delhi-110067, India b Inter University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi-110067, India abstract article info Available online 5 March 2009 PACS Codes: 68.55.A 61.46.Df 68.37.Ps 81.07.Bc Keywords: Ion bombardment XRD AFM Grain Growth UV/VIS optical absorption Nanocrystalline tin oxide (SnO 2 ) thin films of 200 nm thickness were deposited on quartz and sapphire substrates by e-beam evaporation method. The substrate temperature was kept at 200 °C to enhance the surface diffusion of the atoms. The films were characterized by atomic force microscopy (AFM), glancing angle X-ray diffraction (GAXRD) and UV-visible spectroscopy for morphological, structural and optical characterization respectively. The nanocrystalline grains are found to be 4±2 nm in radius. These nanocrystalline thin films were bombarded by 250 keV Ar + beam to study ion beam induced grain growth and surface modification. There occurs red shift in UV/visible absorption band edge of tin oxide thin films after ion bombardment, confirming quantum confinement effect. GAXRD and AFM studies show agglomeration of nanocrystalline grains after Argon ion bombardment in the keV energy range. Ion beam induced defects enhance the diffusion of atoms leading to uniformity in size. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Nanocrystalline semiconductors have electronic properties inter- mediate between those of macro-crystalline solids and molecular entities. The properties of such materials are fascinating and have formed the subject of intense research in recent years [1]. These materials behave differently from bulk semiconductors. With decreas- ing particle size, the band structure of semiconductor changes; the band gap increases and the edges of the bands splits into discrete energy levels. These quantum size effects have stimulated great interest in both basic and applied research. Tin dioxide is a transparent semiconductor material of high chemical and mechanical stability. Only one stable phase is known which has a tetragonal arrangement of atoms receiving the names of rutile or cassiterite. Pure tin oxide is an n-type wide band gap (~3.5 eV) semiconductor due to the presence of oxygen vacancies, which electrochemically act as electron donors. Because of its optical (transparent for visible light and reflective for IR) and electrical properties, it is used in many applications such as transparent conducting electrodes, photovoltaic devices, photosen- sors and catalysts [2]. However the most important use is the active layer in gas sensing devices. In such applications, reducing the size down to a few nanometers, is of interest since the sensitivity of the sensors can be greatly increased as a result of the associated increase of the active surface where oxygen adsorption/desorption can take place. Nanoparticles of SnO 2 are widely used in areas where surface properties are important such as oxidation catalysts and gas monitoring devices. Nanocrystalline grains of SnO 2 immobilized in a film find large application in sensors [3]. Nanocrystalline tin oxide thin films can be synthesized with various techniques such as sputtering, physical vapor depositions, e-beam evaporation and sol-gel techni- ques [4]. Films deposited by these techniques are generally poly- crystalline, retaining the crystal structure of the bulk material. The preferred orientation of the crystallites as well as the crystal size are dependent on the precursor, deposition techniques and conditions. Surface morphology of nanocrystalline thin films plays an important role when they are used for application. Ion bombardment technique can be used to improve many of the surface properties of the thin films [5]. These include enhanced hardness, wear resistance and resistance to chemical attack; and reduced friction. Merits of the ion implantation process include that it does not adversely affect component dimension. Surface modification depends on the ion species, energy, and flux. It is used for many applications such as modifying the electrical properties of semicon- ductors and improving the mechanical or chemical properties of alloys, metals, and dielectrics. Surface instabilities and resulting self- organization processes play an important role for large area array nano-structuring. The occurrence of such instabilities in thin film systems can be triggered by energetic ion bombardment and the subsequent self-assembly of the surface can be nicely controlled by fine-tuning of the ion bombardment conditions. Ion bombardment process directs beams of accelerated ions into the target materials in order to modify their near-surface region and generate radiation damage in a controllable manner. The energy of the ion is transferred to the solid almost instantaneously into a highly localized volume of Surface & Coatings Technology 203 (2009) 2410–2414 ⁎ Corresponding author. Tel.: +91 11 26748769; fax: +91 11 26717537. E-mail address: tmohanty@mail.jnu.ac.in (T. Mohanty). 0257-8972/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2009.02.108 Contents lists available at ScienceDirect Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat