RBS depth profiling and optical characterization of multilayers of TiO 2 (20 nm) and Ge (15 nm) Mirza Sajjad Hussain a , Mazhar Mehmood a, * , Jamil Ahmad a , M. Tauseef Tanvir a , A. Faheem Khan a , Turab Ali b , Arshad Mahmood c a National Centre for Nanotechnology & Department of Metallurgy and Materials Engineering, Pakistan Institute of Engineering and Applied Sciences, Islamabad 45650, Pakistan b National Center for Physics, Islamabad, Pakistan c NILOP, PO Nilore, Islamabad, Pakistan highlights < Multilayers of TiO 2 (20 nm) and Ge (15 nm). < RBS depth profiling. < Effects of annealing. < Inter-layer diffusion and various degrees of oxidation of Ge layers. < Enhancement of quantum confinement effects and changes in optical properties. article info Article history: Received 25 February 2012 Received in revised form 23 September 2012 Accepted 25 October 2012 Keywords: Multilayers Rutherford backscattering spectroscopy (RBS) Interfaces Diffusion abstract Rutherford Backscattering Spectrometry (RBS) has been employed for studying inter-layer diffusion and mixing of TiO 2 and germanium in the multi-layers grown on the soda lime glass substrate. TiO 2 layers were grown by electron-beam evaporation of titanium in an oxygen atmosphere and Ge layers were grown by resistive heating. UVevisible and Raman spectroscopy were carried out for optical charac- terization. RBS spectra were analyzed by SIMNRA which confirmed the successful formation of alternate layers of TiO 2 and Ge with a thickness of 20 nm and 15 nm, respectively. Annealing caused interface mixing. The approximate stoichiometry of interfaces was of GeTiO 2 . The thickness of these layers increased with annealing temperature. Two to five top Ge layers also underwent oxidation to form GeO and GeO 2 , depending on annealing temperature. Nuclear Reaction Analysis (NRA) signal of oxygen from top layers was also recorded to confirm the pick-up of oxygen from the annealing atmosphere. An increase in the transmission efficiency with blue shift of the absorption edge was also observed with annealing temperature, as associated with the decrease in the effective thickness of Ge layers. Shift of Raman peaks confirmed the stoichiometric changes as a result of annealing. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction Single crystal silicon wafer based solar cells are the most commonly available cells on commercial level, but their maximum efficiency (w15%) is very limited as compared to their costs [1]. There are two important reasons; (i) With the technology being matured, their costs are dominated by the costs of their starting materials [2] and (ii) the single crystal silicon is very much choosy in absorbing a certain part of the entire solar spectrum because of its unique energy band structure along with many loss mechanisms of absorbed solar energy [1]. They are also called first generation solar cells. The second generation solar cells based on thin films of polycrystalline silicon and other compound semiconductors have been reported to have relatively higher efficiencies and lesser costs but their efficiencies are still constrained by the choosy absorption of solar spectrum depending upon their unique energy band structures. Cadmium sulphide, amorphous silicon, copper indium diselenide, cadmium telluride are important semiconductors used in such solar cells [3]. Current focus of the technology is on developing such materials that are capable of absorbing most part of the solar spectrum and converting it into electron-hole pairs for conduction [4]. The advent of nanotechnology has made it possible * Corresponding author. E-mail address: mazhar@pieas.edu.pk (M. Mehmood). Contents lists available at SciVerse ScienceDirect Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys 0254-0584/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2012.10.037 Materials Chemistry and Physics 139 (2013) 17e26