Growth characteristics of Cu(In,Ga)Se 2 thin films using 3-stage deposition process with a NaF precursor R. Sakdanuphab a,b , C. Chityuttakan a,b , A. Pankiew c , N. Somwang c , K. Yoodee a,b , S. Chatraphorn a,b,n a Department of Physics, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand b Research Center in Thin Film Physics, Thailand Center of Excellence in Physics, CHE, 328 Si Ayutthaya Road, Bangkok 10400, Thailand c Thai Microelectronics Center, Suwintawong Road, Chachoengsao 24000, Thailand article info Article history: Received 7 December 2010 Accepted 26 January 2011 Communicated by K.W. Benz Available online 2 February 2011 Keywords: A3. Cu(In,Ga)Se 2 A3. Na precursor A3. MBD A3. 3-stage growth process B3. Solar cell abstract Cu(In,Ga)Se 2 or CIGS thin films with NaF precursor are grown on Mo coated soda-lime glass (SLG) substrates using a molecular beam deposition (MBD) technique. The growth characteristics of the CIGS films deposited using the 3-stage process are examined by interrupting the deposition process at the end of each stage and the transition temperature at the beginning of the second stage. The evolution of the CIGS films derived from the compounds, e.g. g-(In,Ga) 2 Se 3 , Cu(In,Ga)Se 2 +Cu x Se, is investigated by comparing the properties of the films to those without an NaF precursor. The XRD spectra show the dominant peak of (1 0 5) preferred orientation of the g-(In,Ga) 2 Se 3 enhanced by the NaF precursor which is seen at the end of the first stage. The decrease of (2 2 0)(2 0 4) intensity of the CIGS film is found at the end of the second stage followed by the increase of (1 1 2) intensity at the end point. The AFM images at the end of the first stage show a smooth surface with similar grain shape in the films with the NaF precursor. During the second stage, the grain size of the Cu-rich CIGS film increases with slightly sharper grain boundaries. However, at the end point, the CIGS film is fully obtained and shows small sharp grains corresponding to the increase of (1 1 2) orientation. The cross-section SEM images show small columnar grains with deep grain boundaries at the end point. The AES depth profiles show that most Na atoms are located near the bottom layer in the first stage and diffuse to the surface of the film after increasing the temperature. Then the Na atoms uniformly distribute into the Cu-rich CIGS film during the second stage. Significantly high Na content is found at the surface of the CIGS film at the end point. In addition, a gradient of Ga composition in the CIGS film is also observed in the AES measurement. A simple model of Na-enhanced CIGS thin film growth based on the experimental results is proposed to describe the growth and doping mechanisms. & 2011 Elsevier B.V. All rights reserved. 1. Introduction Over the past three decades, the technology of solar cell has been progressively developed to achieve higher conversion effi- ciency. Solar cells based on thin film technology have attracted researchers and commercials because of their high efficiency, low material usage and high yield processes. One of the thin film solar cells with promising high efficiency makes use of Cu(In,Ga)Se 2 (CIGS) as a photon absorber in a multi-layer structure: Ni-Al grid/ ZnO/CdS/CIGS/Mo/SLG, whose efficiency is nearly 20% as demon- strated by NREL [1]. CIGS is a p-type ternary compound semicon- ductor with a chalcopyrite structure. In principle, the p-type behavior of normal semiconductor can be generated by its native defects such as vacancies, interstitials, anti-site defects and clusters of defects. The hole concentration of the CIGS film can be enhanced by the growth process and by additional atoms. Na atoms from the SLG substrate or Na compounds (e.g. Na 2 Se or NaF) have been shown to give positive effects in the CIGS film in terms of conductivity and efficiency enhancement [2–4]. The roles of Na on the conductivity of the CIGS film have been investigated [2] together with the doping mechanisms [5] where the substitution of Na in the CIGS structure, i.e. Na (In,Ga) and Na Cu , is considered. It is believed that Na (In,Ga) increases p-type defects and the Na Cu formed as a neutral defect reduces n-type defects by decreasing the In or Ga atoms on the Cu sites, i.e. (In,Ga) Cu . However, the excess Na atoms occupying V Cu decrease the p-type defects (V x represents the vacancy at the x site). In addition, Na on the surface catalyzes O 2 dissociation by supplying oxygen on the Se vacancies (V Se ) in the CIGS film, which reduces an n-type doping [5]. As expected, Na affects the electronic property by enhancing the carrier concentra- tion and the film conductivity, and thus improves the solar cell efficiency. The influence of Na on the crystal structure has also been investigated [6]. There are still some arguable issues on the Na effects in structural properties; the presence of Na during the growth process induces an increase in grain size and crystal quality Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jcrysgro Journal of Crystal Growth 0022-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2011.01.077 n Corresponding author at: Department of Physics, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand. Tel.: +662 218 7560; fax: +662 253 1150. E-mail address: Sojiphong.C@chula.ac.th (S. Chatraphorn). Journal of Crystal Growth 319 (2011) 44–48