Process and characterisation of chemical bath deposited manganese sulphide (MnS) thin films
Introduction
Metal chalcogenide thin film preparation by chemical bath deposition (CBD), is currently attracting considerable attention as it is relatively inexpensive, simple and convenient for large area deposition. A variety of substrates such as insulators, semiconductors or metals can be used, since it is a low temperature process which avoids oxidation or corrosion of metallic substrates. It is a slow process which facilitates better orientation of crystallites with improved grain structure. Depending upon the deposition conditions, film growth can take place by ion-by-ion condensation or adsorption of the colloidal particles (cluster-by-cluster) from the solution onto a substrate. Using this method, thin films of group II–VI, V–VI, I–III–VI etc. have been deposited [1], [2], [3], [4].
Recently it is found that the CBD method is most useful for the deposition of buffer layers for CuInS2/CuInSe2 (CIS/CISe) and CuInGaSe2 (CIGSe) solar cells. The high efficiency solar cells have been fabricated by using CdS buffer layers deposited by the CBD method [5], [6], [7]. Presently, CBD-CdS is found to be superior than the CdS films prepared by other physical or chemical methods. The CdS films of 100–200 Å thickness are found to be continuos and well covered over CIS/CISe or CIGSe film surfaces [8]. In addition to this, the surface modifications of CIS/CISe and CIGSe films during the deposition of buffer layers are found to improve the short circuit current, ISC, and/or open circuit voltage, Voc, magnitudes. The efforts to make CIS/CISe and CIGSe solar cells with Cd-free buffer layers are also concentrated on the other chemical bath deposited ZnS, ZnSe, SnS2, ZnO, In2S, etc. films [9], [10], [11].
During recent years, materials known as dilute magnetic semiconductors (DMS) have become a focus of intense research activity as they exhibit an interesting combination of magnetism and semiconductivity. MnS is such a DMS material (band gap energy, Eg, 3.1 eV) having potential use in solar cell applications as a window/buffer material. The cubic α-phase of MnS appears to be stable above room temperature. The β-phase and ν-phase of MnS can be prepared at low temperature, but they transform to the o-phase above 200°C. The α-phase is retained at all the temperatures [12], [13]. Recently, CBD-MnS thin films have been prepared by Pramanik et al. [14] and Lokhande and Gadave [15] using thioacetamide and thiosulphate as the sulphide ion sources, respectively. The films onto glass substrates were found to be amorphous or consisting of small grains. However, the detailed investigations regarding the deposition process and kinetics and film characterisation of CBD-MnS film have not been reported.
In the present investigation, we describe the deposition of MnS films from an aqueous medium using thioacetamide as a sulphide ion source. The preparative parameters are optimised in order to obtain good quality MnS films and a growth mechanism is proposed. The MnS film characterisation including structural, compositional, optical, etc. is presented.
Section snippets
Deposition of MnS films
Deposition of MnS is based on the slow release of Mn2+ and S2− ions in a solution which then condenses onto the substrate. For this, Mn salt with a suitable complexing agent as a Mn2+ and thioacetamide as S2− are used in an aqueous medium.
The depositions were carried out onto commercial glass slides, fluorine doped tin oxide (FTO) coated glass, and glassy carbon substrates. The glass and FTO coated glass substrates were cleaned with detergent, dried and degreased with ethanol in an ultrasonic
Growth mechanism of MnS film
The MnS film can be obtained from an aqueous bath containing Mn salt and a suitable complexing agent which allows us to control the Mn2+ concentration and to have a soluble species of Mn2+ in aqueous medium. The deposition process is based on the slow release of Mn2+ and S2− ions in the solution which then condense on an ion-by-ion basis on the substrate that are suitably mounted in the solution. The deposition of MnS occurs when the ionic product of Mn2+ and S2− exceeds the solubility product
Conclusions
We have found that the MnS film can be deposited from a bath containing Mn acetate, thioacetamide, hydrazine hydrate and triethanolamine in aqueous medium. The presence of oxygen in EDAX and RBS analyses showed that films deposited are of the composition Mnx(OH,S)y The film on glass substrate consists of nanocrystalline particles of about 30–40 Å in diameter. The optical absorption studies showed that the MnS is direct band gap material with band gap energy, Eg, equal to 3.02 eV. The TRMC
Acknowledgements
The authors are grateful to Dr. C. Colbeau-Justin for helpful discussion on TRMC studies. C.D.L. wishes to acknowledge the Alexander von Humboldt Foundation, Germany and P.S.P. wishes to acknowledge DAAD, Germany for the award of fellowships to carry out the research work.
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