Sulfurization temperature effects on the growth of Cu 2 ZnSnS 4 thin film Hyesun Yoo a , JunHo Kim a, * , Lixin Zhang b a Department of Physics, University of Incheon, 12-1 Songdo-dong, Yeonsu-gu, Incheon 406-772, Republic of Korea b School of Physics, Nankai University, Tianjin 300071, PR China article info Article history: Received 4 August 2011 Received in revised form 2 January 2012 Accepted 7 January 2012 Available online 25 January 2012 Keywords: Cu 2 ZnSnS 4 Sulfurization Film growth Raman spectroscopy Secondary phase Band gap abstract We made Cu 2 ZnSnS 4 (CZTS) thin films by sulfurization of Cu/Sn/Cu/Zn metallic films. Sulfurizations were carried out under different thermal annealing conditions, where maximum temperatures were 440  C (LT-CZTS) and 550  C (HT-CZTS). For LT-CZTS films, secondary phases such as SnS 2 and Cu 2x S were observed, whereas for HT-CZTS films secondary impurities were not detected. Chemical composition of LT-CZTS film was observed to be very non-uniform. Highly Sn-rich and Zn-rich regions were found on the film surface of LT-CZTS. However, averaged chemical composition for larger area was close to stoichi- ometry. The HT-CZTS film showed homogeneous structural and chemical composition features. But, for HT-CZTS film, the Sn composition was observed to be decreased, which was due to the Sn-loss. By UV eVisible spectroscopy, optical band gaps of LT- and HT-CZTS films were measured to be w1.33 eV and w1.42 eV, respectively. The band gap of LT-CZTS film was also observed to be smaller by photo- luminescence measurement. The depressed band gap of LT-CZTS film may be ascribed to some defects and low band gap impurities such as Cu 2 SnS 3 and Cu 2-x S in the LT-CZTS film. Crown Copyright Ó 2012 Published by Elsevier B.V. All rights reserved. 1. Introduction CIGS solar cell has achieved w20% conversion efficiency in laboratory cell scale, which is one of the highest efficiency among various thin film solar cells. However, CIGS solar cells adopt rare earth materials, In and Ga, which are also used in the display industry. Thus, considering applications in large scale, alternative solar cell with earth abundant element is desirable. In terms of this view point, kesterite Cu 2 ZnSnS 4 (CZTS) is very promising absorber material [1e4]. It includes earth abundant materials Sn, Zn, and less toxic S. In additions, CZTS exhibits excellent material properties such as direct band gap of 1.4e1.5 eV and large absorption coeffi- cient of 10 4 cm 1 in visible spectrum range. Recently, the conversion efficiency of CZTS solar cell was dramatically increased. For the record efficiency of CZTS, vacuum- based process yielded 6.77% [5], and hydrazine-adopted solution process showed 9.6% [6], and nanoparticle-based process gave 7.2% [7] conversion efficiency. However, although device performance was greatly improved, basic researches on CZTS material itself are insufficient. For example, fabrication of compositionally uniform CZTS film is still hard task due to Sn-loss during annealing process [8]. Considering that high efficiency CZTS solar cell can be realized just with Cu-poor and Zn-rich CZTS [9], well-adjustment of chemical composition of CZTS is prerequisite. Hence, systematic sulfurization experiments under controlled temperature and surrounding atmo- sphere come to be significant. On the other hand, for Cu2ZnSnSe2 (CZTSe) film the determination of exact band gap was controversial. Some reported band gap of CZTSe was observed to be 0.8e1 eV [10e12], while others claimed that it was measured to be 1.4e1.5 eV [13e15]. In this paper, we report that on the CZTS film the band gap can be affected by the sulfurization temperature of stacked metallic film. In additions, the detection of secondary phases in CZTS films by using X-ray diffraction (XRD) is very hard because lattice constants of the secondary phases are similar to those of CZTS. Here, we show experimental results that Raman spectroscopy can reveal possible secondary phases with higher sensitivity. We report the impact of sulfurization condition on structural and optical properties of CZTS films. By sulfurization of Cu/Sn/Cu/ Zn/glass stacked films, we made CZTS films under different sulfu- rization conditions. The fabricated films were characterized by XRD, Raman spectroscopy, UVeVisible spectroscopy, photo- luminescence (PL), scanning electron microscopy (SEM) and energy-dispersive X-ray analysis (EDX). Based on these measure- ment results, growth properties of CZTS films will be discussed. 2. Experiments We made precursor metallic films by rf sputtering method. The metallic films were sequentially deposited onto glass so as to * Corresponding author. E-mail address: jhk@incheon.ac.kr (JunHo Kim). Contents lists available at SciVerse ScienceDirect Current Applied Physics journal homepage: www.elsevier.com/locate/cap 1567-1739/$ e see front matter Crown Copyright Ó 2012 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2012.01.006 Current Applied Physics 12 (2012) 1052e1057