Development of smooth CuInGa precursor films for CuIn 1 - x Ga x Se 2 thin film solar cell applications Mohamed Samir Hanssen, Harry Efstathiadis, Pradeep Haldar ⁎ College of Nanoscale Science and Engineering, University at Albany, State University of New York, Albany, NY 12203, United States abstract article info Article history: Received 4 January 2010 Received in revised form 25 March 2011 Accepted 31 March 2011 Available online 7 April 2011 Keywords: Copper indium gallium selenide Radio-frequency magnetron sputtering Two-step process Surface morphology Selenization X-ray diffraction CuInGa precursor thin films were deposited using a CuGa (75–25 at.%) and an In 3″ diameter target material simultaneously by RF magnetron sputtering. The precursor films were deposited on Si and glass substrates at -80 °C and room temperature, and characterized by Rutherford backscattering spectroscopy, Auger electron spectroscopy, scanning electron microscopy, atomic force microscopy and X-ray diffraction. The effects of gun power density and substrate temperatures on resulting precursor film properties were investigated. Precursor films deposited at -80 °C have a smooth morphology with a 75% reduction in all roughness values and are more dense and homogeneous in structure compared to precursors deposited at room temperature. Therefore these precursors will result in better selenization process reproducibility. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Polycrystalline thin film material based on CuIn 1 - x Ga x Se 2 (CIGS) is a promising candidate as an absorber material for low-cost and high-efficiency photovoltaic devices. High efficiencies were achieved with CIGS thin films, with a world-record efficiency cell of 19.9% [1]. These CIGS thin films were deposited by co-evaporation through a three-step process. However, co-evaporation has difficulties in scaling-up for a large manufacturing system because of its complexity. In addition, large area development of high quality and uniform CIGS films is a challenging task. On the other hand, a two-step process, in which the selenization step follows the sputtering step of the metallic precursors, is easier to scale up and seems to be the most feasible approach for industrial production use. Co-sputtering of CuInGa precursor thin films, from a dual target approach was proposed to form homogeneous precursor material [2,3]. However, for deposition at room temperature and higher temperatures, In segregates, and forms In-rich islands on the film surface, while the CuInGa precursor film becomes rough and inhomo- geneous in structure. In order to obtain compositional uniformity of precursor films, stacked-metal or alloy layers, such as In/CuGa, CuGa/In, CuGa/In/CuGa or In/CuGa/In films, have been investigated [4,5]. Nevertheless, these precursors give rise to In-rich phases on the surface and turn out to have a rough surface morphology. In addition, the surface morphology has an impact on the CdS/CIGS interface properties [6]. Since smooth morphology of the precursor results in smooth morphology of the CIGS films, the preparation of CuInGa precursors with a smooth surface is important [3,6–9]. Also, the selenization kinetics and chalcopyrite syntheses depend strongly on the state of the metallic precursor film after deposition. Therefore the precursor film morphology and homogeneity also have a great influence on the reaction process. Because inhomogeneous precursors with In-rich phases on the surface result inhomogeneous precursor/Se interfaces, it is possible that, in this system, substantial fluctuations in the absorber synthesis kinetics yield problems of inhomogeneous absorber synthesis. Co-sputtered precursors deposited at room temperature yield smoother and more homogeneous thin films then sequentially sputtered pre- cursors at room temperature and the smooth precursor films presented in this work are even more homogeneous and have a better surface morphology then the co-sputtered precursors deposited at room temperature and, therefore, could result in more homogeneous absorber layers, and may result in better device efficiencies [10,11]. Because of the strong influence of the precursor sputter process on the surface morphology, homogeneity and the related absorber formation, it is of great importance to have smooth and homogeneous precursor thin films. It has been reported that evaporation of single elemental layers on a substrate cooled to -160 °C resulted in smooth Cu and Ga thin films [12]. However, the cooled-substrate approach was not further investigated in the form of multi-compound thin films obtained by co-sputtering. In this work, we report the deposition of CuInGa precursor films at substrate temperature below and at room temperature. The depen- dence of the chemical composition, sticking coefficient, morphology Thin Solid Films 519 (2011) 6297–6301 ⁎ Corresponding author at: College of Nanoscale Science and Engineering, University at Albany, State University of New York, 255 Fuller Road, Albany, NY 12203, United States. E-mail address: phaldar@uamail.albany.edu (P. Haldar). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.03.138 Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf