Structural investigations of ITO-ZnO films grown by the combinatorial pulsed laser deposition technique Doina Craciun a , Gabriel Socol a , Nicolaie Stefan a , Marimona Miroiu a , Ion N. Mihailescu a , Aurelian-Catalin Galca b , Valentin Craciun a,c, * a National Institute for Lasers, Plasma and Radiation Physics, Bucharest, Romania b National Institute for Materials Physics, Bucharest, Romania c Major Analytical Instrumentation Center, Materials Science and Engineering, University of Florida, Gainesville, FL 32611, United States 1. Introduction One of the best-known and most used methods to obtain new and better properties or functionalities from a material is by doping or alloying it with other materials. Since it is not yet possible to predict, based on first principles, the right amount of doping or alloying element required to optimize a specific property of the new phase or material studied, one must design ways to find the right recipe with minimum effort and cost. The combinatorial method, where two or more sources of atomized materials are simultaneously or alternatively used for deposition on a single substrate is an great example of such optimization work [1–5]. By changing both the direction and the dose of the atomized incoming fluxes, one has control upon the lateral and vertical compositional gradients of the deposited thin film material. Depending on the resolution of the analytical techniques used for characterization, a single sample could now provide a rather large range of various compositions and/or thicknesses. We have used the combinatorial pulsed laser deposition (C-PLD) method to obtain transparent and conductive oxides films by mixing indium tin oxide (ITO) with pure zinc oxide. Such films are used for the top transparent electrode for solar cells, controlling also the amount of light that is captured and protecting the cell from the ambient. Pure ITO is the most used material for transparent and conducting electrodes [6,7]. Unfortu- nately, it is rather expensive and scarce, which limit its use in a large scale, industrial production of solar cells. There were many studies and investigation aiming at replacing or mixing ITO with inexpensive and abundant ZnO [8–10]. By mixing these two components we also wanted to tailor the mechanical, crystalline, electrical and optical properties of these films. 2. Experiment The mixed films were deposited using the C-PLD technique, where ITO and ZnO targets were located in two different positions 15 mm apart and ablated sequentially using a KrF laser. The deposition sequence was 20 pulses from the ITO target followed by 20 pulses from the ZnO target to ensure a good atomic mixing of the components. The total number of pulses was varied from 1000 to 5000, which resulted in films thicknesses from around 50 to 250 nm across a 50 mm long substrate. The material was collected on Si or quartz substrates heated from 300 up to 500 8C. Films grown separately from each target will have their maximum thickness positioned at the intersection between the perpendi- culars from the laser impact area on the targets with the substrate. Applied Surface Science 255 (2009) 5288–5291 ARTICLE INFO Article history: Available online 31 July 2008 Keywords: Pulsed laser deposition Combinatorial TCO ITO ZnO ABSTRACT Mixtures of transparent and conductive oxides such as ITO-ZnO have been grown by a combinatorial pulsed laser deposition technique from two targets that were located 15 mm apart. The films were deposited on (1 0 0)Si and quartz substrates that were heated at temperatures from 300 to 500 8C. Measurements of the In to Zn ratios along the transversal axis of the substrates, which passes through the maximum thickness points corresponding to each target position were performed using energy dispersive X-ray spectroscopy and spectroscopic ellipsometry. From simulations of the X-ray reflectivity spectra, collected with a 2 mm mask on different locations along the transversal axis of the samples, the density and thickness of the deposited films were calculated and then the In to Zn ratios. The crystalline structure and electrical properties of the deposited films were also investigated along the same axis. Changes in the ratio of In/Zn along this axis resulted in changes of the film lattice constant and texture. ß 2008 Elsevier B.V. All rights reserved. * Corresponding author at: National Institute for Lasers, Plasma and Radiation Physics, Bucharest, Romania. E-mail address: valentin.craciun@inflpr.ro (V. Craciun). Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.07.120