Optical properties of thermally evaporated SnS thin films
Introduction
SnS is one of the tin chalcogenide layered semiconductors in group IV–VI. It may exhibit the p-type conductivity [4], [5], [6], [7], [9], [10], [16], n-type conductivity [1], [4] depending on the concentration of tin and it may also changes its type of conduction from p to n-type conduction in accordance with treatment temperature [11], [12].
Different values of energy gap have been obtained [1], [2], [4], [5], [6], [7] for SnS ranging from 1 to 2.33 eV depending on the resulting structure obtained by different techniques and the occurring type of electron transitions. The requirements imposed on films used as a light absorber are (i) they must have an energy gap of about 1.5 eV with indirect allowed transition and (ii) a high absorption coefficient >104 cm−1. Because SnS crystallizes in orthorhombic structure, it can be used in n–p homojunction [11] and n–p heterojunction [11], [12]. The nature is abundant in elements Sn and S which moreover are non-polluting during SnS growing process. SnS thin films can be prepared by a variety of methods [1], [2], [4], [5], [7], [9] with the purpose of manufacturing thin films suitable for use as a solar absorber in optoelectronic devices and photovoltaic applications [12]. Among these methods, the thermal evaporation technique received little interest in the literature [8], [13]. The most recently studies on thermally evaporated SnS films were probably made by Deraman et al. [13] who investigated the influence of substrate temperatures on optical band gap of SnS films. They obtained a value of 1.07 eV for the energy gap of treated SnS films. Due to the increasing interest of SnS, the current investigation attempts to find out ways of growing SnS thin films with better properties for photovoltaic and solar cell applications using thermal evaporation under vacuum and controlled deposition conditions with attention given to its structural and optical properties.
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Experimental procedures
Thin films of SnS were prepared by thermal evaporation process using molybdenum boat, on optical flat fused quartz substrates for optical measurements and on glass substrates for structural investigations at room temperature. The chamber was evacuated down to 2×10−3 Pa. The film thickness and deposition rate were controlled during deposition by means of a quartz balance (model FTM 4) and were also measured interferometrically [14].
The structural analysis of SnS films with thickness of 360 nm
Results and discussion
Fig. 1 shows the EDAX spectrum of evaporated SnS, only Sn and S peaks of nearly equal intensities were observed. The evaluated elemental composition was Sn––50.05 at.% and S––49.95 at.%. The XRD studies of the bulk SnS after fine powdering is shown in Fig. 2. Table 1 gives the calculated d-spacing for the above pattern, as well as those obtained from ICDD pattern No. 39-354 for comparison. The XRD studies indicated that the fine powder of the bulk material corresponds to a herbenzite SnS with
Conclusion
The main conclusion of the current investigation can be summarized as follows: Thermal evaporation of bulk crystalline SnS materials resulted in amorphous films with energy gap of 1.4 eV and absorption coefficient >105 cm−1. SnS is the only structural phase formed during annealing of SnS films in the temperature range 373–573 K. A significant growth of planes (1 0 1) and (1 1 1) is noticed upon annealing of amorphous SnS films at 473 K. Annealing of thermally evaporated SnS films at 473 K resulted
References (31)
- et al.
Thin Solid Films
(1998) - et al.
Thin Solid Films
(1987) - et al.
Solar Energy Mater. Solar Cells
(1996) - et al.
Solar Energy Mater. Solar Cells
(1998) - et al.
Thin Solid Films
(1989) - et al.
Thin Solid Films
(1997) - et al.
Surf. Sci.
(1966) Thin Solid Films
(1980)- et al.
J. Phys. D: Appl. Phys.
(2000) - et al.
J. Phys. D: Appl. Phys.
(1999)