Successive ionic layer adsorption and reaction (SILAR) method for the deposition of large area (∼10 cm2) tin disulfide (SnS2) thin films
Introduction
There has been increasing interest during the past few decades in semiconducting chalcogenide thin films because of their wide application in various fields of science and technology, leading to a drastic cut in the cost of production of semiconductor devices. Metal chalcogenide films have been extensively studied because of their potential application in electronic, optical, and superconductor devices [1], [2], [3]. Tin chalcogenides show electrical and optical properties that are useful in many devices, such as current controlled devices, switching devices, and photoconducting cells [4], [5].
Various methods have been employed for the deposition of SnS and SnS2 thin films. SnS films have been prepared by vapor transport [6], electrodeposition from nonaqueous solutions, and controlled precipitation [7], [8]. Single crystals of SnS2 have been grown by chemical and physical vapor transport [9], [10], [11]. Thin films of SnS2 have been grown by the vapor phase [12]. Lokhande [13] has reported on the chemical deposition of SnS2 (Eg = 2.35 eV) films from an acidic medium solution containing Sn+4 ions. The films exhibited photoactivity in polysulfide electrolyte.
Compared to other methods, the successive ionic layer adsorption and reaction (SILAR) method is a simple, less expensive, and less time-consuming method for the deposition of binary semiconducting thin films. It is also applicable in the deposition of large-area thin films. The SILAR method was introduced by Nicolau in the mid-1980s [14]. The method has been employed to grow selected II–VI compounds, especially, CdS, ZnS, and CdS/ZnS multilayers. In the SILAR method, the substrate is immersed separately into two precursor solutions, and washed in between with water to get rid of the loosely bound species. Thus, one SILAR cycle consists of adsorption of the cation precursor, rinsing with water, adsorption of anion precursor, followed by reaction and another rinsing.
The growth rates of thin films in the SILAR method have varied between a quarter and a half of a monolayer, depending on the experimental conditions [14], [15], [16], [17], [18]. This shows that aqua ligands remain at least partially intact during adsorption, thereby lowering the density of cations and anions in one layer. However, the growth of the thin film can be controlled at an accuracy of one SILAR cycle. Valkonen et al. [15] have deposited ZnS, CdS films on a variety of substrates, including glass, ITO-coated glass, and single crystals of GaAs(100), to study the effect of different types of substrates on the crystallinity of the film.
In this paper, it is our intention to promote interest in the SILAR method as applied to tin disulfide thin films. We used the SILAR method to prepare large-area (∼10 cm2) SnS2 thin films on glass substrate, under optimized deposition conditions. We also deposited SnS2 thin films on a single crystal wafer of Si(111) to study the effect of substrate on the crystallinity of the films.
Section snippets
Substrate cleaning
Substrate cleaning plays an important role in the deposition of thin films. Microslide glass of the dimensions of 26 × 76 × 1 mm and a single crystal wafer of Si(111) 20 × 20 × 0.5 mm were used as substrates. The glass substrate was cleaned in commercial detergent solution, rinsed well in deionized water, then dried, while the single crystal wafer of Si(111) was hydrogen terminated as follows: the Si wafer was dipped into 40% NH4F at 10°C for 15 min, then rinsed with deionized water before use.
Preparation of SnS2 films
Scheme of SILAR method
For the deposition of tin disulfide using the SILAR method, cleaned substrate is immersed in tin(II) chloride (0.2 mol·dm−3) precursor for 20 s. Tin(II) ions are adsorbed on the surface of the substrate. The unadsorbed ions are separated out by rinsing the substrate in highly purified water for 10 s. When such a substrate is immersed in sodium sulfide (0.1 mol·dm−3) precursor for 20 s, the following reaction takes place: Thus, one layer of SnS2 is formed. The unreacted
Conclusions
A simple method, namely, successive ionic adsorption and reaction (SILAR), was employed to deposit large-area SnS2 thin films onto glass substrates. Film quality depends upon the preparative parameters. XRD studies showed that the SnS2 films have a hexagonal crystal structure. The SnS2 films on Si(111) substrate exhibit nanocrystalline grain growth, whereas on glass substrate, they are amorphous or consisted of fine grains. The optical bandgap is 2.6 eV, and room temperature resistivity is of
Acknowledgements
The authors are thankful to U.G.C. New Delhi, India for financial support under project No. F-10-7/97(SR-I).
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