Impact of thickness and heat treatment on some physical properties of thin Cu2SnS3 films

Copper tin sulfide (Cu2SnS3) thin films have been grown on glasssubstrate with different thicknesses (500, 750 and 1000) nm by flashthermal evaporation method after prepare its alloy from theirelements with high purity. The as-deposited films were annealed at473 K for 1h. Compositional analysis was done using Energydispersive spectroscopy (EDS). The microstructure of CTS powderexamined by SEM and found that the large crystal grains are shownclearly in images. XRD investigation revealed that the alloy waspolycrystalline nature and has cubic structure with preferredorientation along (111) plane, while as deposited films of differentthickness have amorphous structure and converted to polycrystallinewith annealing temperature for high thickness. AFM measurementsshowed that the grain size of the films was increasing by annealing.The ultraviolet- visible absorption spectrum measurement indicatedthat the films have a direct energy band gap. Eg decrease withthickness and increase with annealing.


Introduction
The energy crisis and environment problems becoming more and more serious, it is very urgent to explore new energy resource, Solar energy is a promising new energy resource which is clean and sustainable [1]. In recent years, there has been a great deal of interest in the study of nontoxic semiconductors from both the fundamental and technological points of view for solar cell materials [2]. The I-IV-VI ternary group of semiconductor was attracted much attention because of their interesting properties [3]. The multiple local coordination geometry around the metal ion center and the catenation ability of sulfur for I-IV-VI ternary chalcogenides, many kinds of Cu-Sn-E(S, Se) have been synthesized [4]. The ternary semiconductors Cu 2 SnX 3 (X = S, Se) are found frequently as secondary phases in synthesized Cu 2 ZnSnS 4 and Cu 2 ZnSnSe 4 samples ,as candidates for low-cost thinfilm solar-cell absorbers, quaternary semiconductors Cu 2 ZnSnS 4 (CZTS) and Cu 2 ZnSnSe 4 (CZTSe) have been studied intensively in the past five years [5].The research groups have been focusing their attention to this compound to increase the efficiency in the near future, despite the advantages that come from CZTS, it shows a complex structure and requires very controlled growth conditions, If this process starts with metallic precursors, intermediate binary and ternary sulfides are formed, these compounds will remain after the end of the growth process if the composition and the sulfurization conditions are carefully controlled, among these ternary sulfides, Cu 2 SnS 3 is the most relevant compound [6]. The ternary Cu-Sn-S (CTS) system belongs to group I-IV-VI semiconductors which have attracted great attention because it's a potential earth abundant absorber material for photovoltaic devices application, light emitting diode, nonlinear optical materials, photo catalytic activity and lithium-ion electrode materials due to suitable optoelectronic properties and attractive raw-material features depending on its crystal structure, and their outstanding optical-thermalmechanical properties [2, 7,8]. CTS consists of low-cost non-toxic and abundant elements [8], and it is a ptype [9], with narrow direct band-gap semiconductor [10,11] ranging from 0.87-1.77 eV and exhibits high optical absorption coefficient (>10 4 cm −1 ) [12]. Therefore Cu 2 SnS 3 used in the heart of a thin films solar cell as the absorber layer which efficiently harvests sunlight [13]. Cu 2 SnS 3 exists in a number of polymorphs, One of them, the cubic Cu 2 SnS 3 , is a high temperature polymorph (>775 ͦ C) and is not likely to appear in kesterite thin films due to the fact that typical growth temperatures are limited to 600 ͦ C [3]. This paper reports the impact of thickness and annealing temperatures on the structural, morphological, and optical properties of CTS thin films which deposited by flash thermal evaporation technique.

Experimental procedure
CTS alloy has been prepared with appropriate weight percentage (2:1:3) using high purity elemental Cu (Carloerba), Sn (England), and S (Ferac Berlin) which about 99.999 %. The elemental analysis of this alloy was carried out by using energy-dispersive X-ray spectroscopy (EDS) detector (Bruker nano GmbH, Germany) coupled with SEM model (Inspect S50 system) which give morphology of this alloy, also X-ray powder diffraction (XRD) investigated by (Miniflex II Rigaku company, Japan). Xray diffractometer using Cu K α radiation (λ=1.5418 A ͦ ). Cu 2 SnS 3 alloy used as a source material to prepare thin films which deposited by flash thermal evaporation technique with different thickness with rate of deposition equal to 3.808 nm/min at high vacuum (10 -5 mbar) using Edward Auto 360 vacuum coating unit.
The annealing process was carried out to prepare films in vacuum at 473 K for 1h. The structures of as deposited and annealed films were investigated. Also, topography of the films surface were studied by employing atomic force microscope (AFM) (AA3000 Scanning probe Microscope SPM, tip NSC35/AIBS from Angstrom Advanced Inc., USA). UV-visible absorption spectra were obtained using a Shimadzu (UV-1650 PC) spectrophotometer over the wavelength range (300-1100) nm and calculated the optical properties of as    It is obvious that the optical energy gap (E g ) decrease from 2.41 eV to 1.92 eV for 500 nm, and 1000 nm respectively, the last result is approach to Zhao and Cheng results [9]. This decrement of E g with increasing thickness may be related to increase the absorption coefficient. E g decreasing by increase thickness because it is well known that E g value of bulk material is lower than that of film sample, that means there is a reduction occur in E g values, since the structure will be approach from that of bulk material. Generally, E g influence by annealing temperature where it increases when films annealed.
In general, the the extinction coefficient (k) increase for as deposited and annealed films at 473 K with thickness increasing due to increase of the grain size.
For as deposited CTS films, the refractive index (n) increased from 2.34 to 2.52 when thickness increased from 500 nm to 1000 nm. n increased by annealing films at 473 K.
It is obvios from Table 5 that the behavior of the real part of dielectric constant (ɛ r ) and imaginary part (ɛ i ) have the same as that of n and k respectively with the variation of thickness.
It can be observed that the real dielectric constant (ɛ r ) for as deposited CTS films increased from 5.48 to 6.39 by increasing thickness from 500 nm to 1000 nm, while imaginary dielectric constant part (ɛ i ) increased from 0.158 to 0.26 for CTS films. Generally, ɛ r and ɛ i increase by increasing thickness.

Conclusions
Effect of film thickness and annealing temperature on the structural, morphological, and optical properties of Cu 2 SnS 3 films deposited by flash thermal evaporation technique were investigated. X-ray diffraction study of the CTS powder shows that this material crystallizes in cubic structure. The structures of as deposited films have amorphous structure and improve with annealing. AFM measurement showed that the average grain size increase with annealing. The results of the optical characterization revealed that the CTS films of thickness 1000 nm have low transmittance in the visible range. The results also revealed that the Cu 2 SnS 3 films have direct band gap of about 1.92 eV. This indicates that these films are active in the visible portion of the spectrum and could be employed as absorber layer in the solar cells.