Formation of CuIn 1 - x Al x Se 2 thin films studied by Raman scattering J. Olejníček a,c, ⁎, C.A. Kamler b , S.A. Darveau a , C.L. Exstrom a , L.E. Slaymaker a , A.R. Vandeventer a , N.J. Ianno b , R.J. Soukup b a Department of Chemistry, University of Nebraska at Kearney, 905 W. 25th St. Kearney, NE 68849-1150, USA b Department of Electrical Engineering, University of Nebraska-Lincoln, 209N WSEC Lincoln, NE 68588-0511, USA c Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 182 21 Prague 8, Czech Republic abstract article info Article history: Received 17 February 2010 Received in revised form 7 February 2011 Accepted 9 February 2011 Available online 24 February 2011 Keywords: Copper Aluminium Indium Selenide Chalcopyrites Raman Spectroscopy Solar cells X-ray diffraction CuIn 1 -x Al x Se 2 (CIAS) thin films (x = 0.06, 0.18, 0.39, 0.64, 0.80 and 1) with thicknesses of approximately 1 μm were formed by the selenization of sputtered Cu―In―Al precursors and studied via X-ray diffraction, inductively coupled plasma mass spectrometry and micro-Raman spectroscopy at room temperature. Precursor films selenized at 300, 350, 400, 450, 500 and 550 °C were examined via Raman spectroscopy in the range 50–500 cm -1 with resolution of 0.3 cm -1 . Sequential formation of In x Se y , Cu 2 -x Se, CuInSe 2 (CIS) and CIAS phases was observed as the selenization temperature was increased. Conversion of CIS to CIAS was initiated at 500 °C. For all CuIn 1 -x Al x Se 2 products, the A 1 phonon frequency varied nonlinearly with respect to the aluminum composition parameter x in the range 172 cm -1 to 186 cm -1 . Published by Elsevier B.V. 1. Introduction In the last 20 years, the chalcopyrite semiconductors of the CuInSe 2 (CIS) family have been investigated as extremely promising materials for high-efficiency photovoltaic devices [1–3]. Currently the greatest energy conversion efficiency of 19.9% has been achieved with a CuIn 1 - x Ga x Se 2 (CIGS) (x ~ 0.3) absorber layer that has a band gap of 1.2 eV [4]. The band gap currently accepted as the ideal for terrestrial photovoltaic applications is 1.37 eV [5]. However, with band gaps greater than 1.3 eV the efficiency of CIGS devices is limited by degradation of the electronic properties of the CIGS layer, leading to losses in fill factor and open circuit voltage, and a decrease in the junction quality factor [6,7]. Moreover, gallium is a scarce and expensive material. Therefore, attention has been focused on identifying an alternate solar cell absorber material. One of these potential materials is another chalcopyrite, CuIn 1 -x Al x Se 2 (CIAS). The CIAS band gap can be varied between 1.04 eV for CuInSe 2 and 2.68 eV for CuAlSe 2 (CAS). Since the band gap energy of CAS is significantly higher than the similar chalcopyrite CuGaSe 2 (CGS) with E g only 1.69 eV, CIAS can reach the theoretical ideal bandgap value for terrestrial single-junction solar cells with only 27% Al substitution [8]. This was experimentally confirmed in Ref. [9], which shows that alloying with Al allows the band gap to increase with less variation in lattice spacing than with Ga. CuIn 1 - x Al x Se 2 thin films or nanocrystals have been prepared using a variety of methods such as sequential deposition of the constituents followed by selenization [10–12], co-evaporation [8,9,13], flash evaporation [14], rf magnetron sputtering of mixed binary selenides [15] and chemical bath deposition [16,17]. To date, the world record in energy conversion efficiency for CIAS thin films solar cells 16.9% was reported by Marscillac et al. in 2002 for a sample with 13% Al substitution with a band gap of 1.16 eV [9]. In order to further increase efficiency and overcome this record efficiency value it is necessary to determine the structural and electro-optical properties of CIAS thin films. Spectroscopic characterization and correlation to elemental composition and phase behavior is necessary in order to understand and develop this material. In this work CuIn 1 - x Al x Se 2 thin films were studied as a function of composition and selenization conditions in an attempt to investigate the effect of the aluminum addition on the structural and optical properties measured by Raman spectroscopy and X-ray diffraction (XRD). 2. Experimental details CuIn 1 -x Al x Se 2 thin films were deposited onto soda lime glass (SLG) by a two-step process consisting of DC magnetron sputtering of composite Cu―In―Al metallic precursors, followed by selenization in an Ar and Se atmosphere. All precursor layers were sputtered from composite targets with fixed stoichiometric ratios of Cu 0.45 In 0.55 -y Al y Thin Solid Films 519 (2011) 5329–5334 ⁎ Corresponding author at: Department of Chemistry, University of Nebraska at Kearney, 905 W. 25th St. Kearney, NE 68849-1150, USA. Tel.: +1 308 865 8565; fax: +1 308 865 8399. E-mail address: olejn@fzu.cz (J. Olejníček). 0040-6090/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.tsf.2011.02.030 Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf