Solution precursor plasma deposition of nanostructured CdS thin films

https://doi.org/10.1016/j.materresbull.2011.12.018Get rights and content

Abstract

Cadmium sulfide (CdS) films are used in solar cells, sensors and microelectronics. A variety of techniques, such as vapor based techniques, wet chemical methods and spray pyrolysis are frequently employed to develop adherent CdS films. In the present study, rapid deposition of CdS thin films via plasma spray route using a solution precursor was investigated, for the first time. Solution precursor comprising cadmium chloride, thiourea and distilled water was fed into a DC plasma jet via an axial atomizer to create ultrafine droplets for instantaneous and accelerated thermal decomposition in the plasma plume. The resulting molten/semi-molten ultrafine/nanoparticles of CdS eventually propel toward the substrate to form continuous CdS films. The chemistry of the solution precursor was found to be critical in plasma pyrolysis to control the stoichiometry and composition of the films. X-ray diffraction studies confirmed hexagonal α-CdS structure. Surface morphology and microstructures were investigated to compare with other synthesis techniques in terms of process mechanism and structural features. Transmission electron microscopy studies revealed nanostructures in the atomized particulates. Optical measurements indicated a decreasing transmittance in the visible light with increasing the film thickness and band gap was calculated to be ∼2.5 eV. The electrical resistivity of the films (0.243 ± 0.188 × 105 Ω cm) was comparable with the literature values. These nanostructured polycrystalline CdS films could be useful in sensing and solar applications.

Highlights

► Inexpensive process with capability to produce large scale nanostructured coatings. ► Technique can be employed to spray the coatings on any kind of substrates including polymers. ► The CdS coatings developed have good electrical conductivity and optical properties. ► Coatings possess large amount of particulate boundaries and nanostructured grains.

Introduction

CdS, a metal chalcogenide semiconductor belonging to II–IV group, is one of the promising materials for use in various optoelectronic devices, light detectors, photo conductors, display panels, light emitting diodes and sensors [1], [2], [3], [4], [5], [6]. It could also be used for photovoltaic conversion. It is preferred over other wide band gap materials for optical window applications due to its compact crystallographic cell and electronic affinity with CuInSe2, InP, CdTe and other p-type semiconductors [7]. There is a vast literature available reporting high efficiency solar devices of CdS with Cu2S, Cu(InGa)Se2, and CdTe [8], [9], [10].

Usually, CdS exists in two different forms namely α-CdS (hexagonal Wurtzite) and β-CdS (cubic, zinc blend). β-CdS is a metastable phase and forms at low temperatures. α-CdS is a stable phase and forms at high temperatures, however, annealing of β-CdS can also result in the formation of α-CdS at high temperatures. For solar cell applications, hexagonal structured α-CdS films are preferred because of stability and band gap [11]. Microstructural features, such as grain size and shape of α-CdS are very important as they can affect the electrical properties of solar cells.

CdS films are usually deposited by techniques like molecular beam epitaxy (MBE) [12], metal organic chemical vapor deposition (MOCVD) [13], close-spaced sublimation (CSS) [14], electrodeposition [15], sputtering [16], successive ionic layer adsorption and reaction (SILAR) [17], pulsed laser deposition (PLD) [18], and spray pyrolysis [19]. Except, spray pyrolysis, all the mentioned techniques come under bottom–up approach where the deposition of films starts either from atoms or ionic species and growth kinetics are dependent on the material transport phenomena. Vapor phase reactions for CdS formation occur at high temperatures [20] and low pressure conditions, as a result hexagonal structured CdS was observed to form. Low temperature techniques, such as SILAR and CBD, could result in cubic phase or a combination of cubic and hexagonal structured CdS, which are not preferred for many applications. On the other hand, spray pyrolysis is relatively faster compared to the other methods, but requires heating of substrates above certain temperatures under regular or inert atmospheric conditions, which limits the substrate choices that can withstand high temperatures as well as chemical corrosion at high temperatures. Also, as the number of layers increases substrate temperature needs to be increased in order to achieve right decomposition of the precursor in the top layers of the films. Finally, less control over the microstructures and properties makes spray pyrolysis an unattractive process for many applications. In contrast to all the above discussed processing methods, here a technique with solution precursor, similar to the spray pyrolysis, however, with high temperature plasma processing conditions is being investigated, for the first time, to develop continuous CdS films. In this technique, the solution precursor is pumped through high temperature plasma plume via a liquid atomizer where it thermally decomposes into molten/semi-molten nanoclusters of desired compound before depositing on the substrate.

Section snippets

Plasma spray setup

Fig. 1 shows the plasma set-up (a modified 100HE gun, Progressive Technologies Inc., Grand Rapids, MI, USA) used for spraying CdS films from a solution precursor. The power of this gun can be varied between 35 kW and 100 kW. An axial liquid atomizer specially designed by Mohanty et al. [21], [22] was used in this gun to feed the solution precursor into DC plasma jet. The solution precursor is fed with help of a peristaltic pump through a polyurethane tube. Then the precursor flows through an

Spray process and chemical analysis

Different ratios of CdCl2 and thiourea, as shown in Table 1, were mixed in the solution precursor to deposit CdS films using plasma spray process. The concentration of CdCl2 was always kept below 1 M to avoid precipitation of complexes at room temperature in the solution precursor [25], [26]. For a ratio of 1:1 between the CdCl2 and thiourea, unconverted CdCl2 and few unidentified peaks with no traces of CdS phase were detected in the X-ray diffraction (XRD) pattern of the films, see Fig. 2. For

Conclusions

Nanostructured thin films of CdS were successfully deposited using solution precursor plasma spray route that is capable of high throughputs. The amount of thiourea was found to be critical in the solution precursor to obtain CdS phase as well as to reduce the amount of residual chlorine. The CdS films obtained for a ratio of 1:10 between CdCl2 and thiourea were continuous with fine grain size in the range of few tens of nanometers. Rough morphology of the CdS films was due to particulate

Acknowledgments

Authors would like to thank Jarret Whetstone (Filmetrics Inc., New York), Nagaswetha Pentyala and Elizabeth Shnerpunas (The University of Michigan, Dearborn) for thickness, electrical and AFM measurements of the CdS films, respectively.

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