Effect of annealing on chemically deposited polycrystalline CdTe thin films

doi:10.1016/j.jallcom.2009.10.146

Journal of Alloys and Compounds

Volume 491, Issues 1–2, 18 February 2010, Pages 77-80

https://doi.org/10.1016/j.jallcom.2009.10.146 Get rights and content

Abstract

The effect of annealing on structural, optical and surface properties of CdTe thin films grown by chemical bath deposition was investigated. CdTe thin films have been deposited by using triethanolamine as a complexing agent in an aqueous alkaline medium at 60 °C. Thin films of CdTe were characterized by UV–visible spectrophotometer, X-ray diffraction technique, and scanning electron microscopy. Optical studies showed that, it has a high absorption coefficient (10⁴–10⁵ cm⁻¹) and band gap values were found to be in the region of 1.45–1.38 eV. X-ray diffraction studies showed that all the films are polycrystalline in nature. The lattice parameter, grain size were calculated and correlated with annealing temperature.

Introduction

A variety of binary semiconductors especially from II to VI group of the periodic table have been studied extensively due to their potential use in photoconductive devices and solar cells [1], [2], [3]. Cadmium telluride is the member of II–VI family having band gap of 1.45 eV [4]; which matches the maximum of solar spectrum and also have high absorption coefficient of the most promising material for the fabrication of opto-electronic semiconductor devices, photovoltaic cells, laser windows, gamma ray detector and p–n diodes etc. [5]. The CdTe semiconductor material exists in either zinc–blende (cubic) or wurtzite (hexagonal) modification in solid state. The structural, optical and surface morphology of CdTe thin films are depends upon various preparation parameters such as method of preparation, kind of impurity and maintenance of the stoichiometry during the deposition. A variety of techniques have been reported in the literature [6], [7], [8], [9]. Very few reports are available in the literature, which describes the CdTe thin film by chemical bath deposition. The solar cell grade CdTe thin films can be prepared by employing annealing steps that improves both the structural and optical properties of the CdTe thin films.

Section snippets

Experimental details

All chemicals used were of analytical grade. All solutions were prepared in Millipore water (18.2 MΩ). A stable sodiumtellerosulphite (Na₂TeSO₃) prepared from tellurium metal powder in a sodium sulphite solution by a reflux action. Sodium tellerosulphite is a source of Te²⁻ ions and CdSO₄ being a source of Cd²⁺ ions. Commercially available glass slides (75 mm × 25 mm × 2 mm) washed repeatedly with chromic acid and water were used to deposit thin films of CdTe. For deposition of CdTe, 10 mL of (1 M) CdSO₄

Characterization of CdTe thin films

The layer thickness of the CdTe thin films was measured by weight difference method weight of sample was measured by very sensitive digital Metler balance with an accuracy of ±0.1 mg by considering bulk density of CdTe (6.20 g/cm³). Optical properties were studied by taking absorption spectra of films by using a UV–visible spectrophotometer (UV-1650 PC; Shimadzu) in the range from 600 to 900 nm at room temperature. X-ray diffraction of all the samples were recorded using an X-ray diffractometer

Growth mechanism

Sodium tellerosulphite solution was synthesized from tellurium metal powder in a sodium sulphite solution by a reflux action. The decomposition of sodium tellerosulphite was made possible in an aqueous alkaline medium containing CdSO₄ and complexed with triethanolamine that allowed controlling Cd²⁺ ions. The principle of decomposition process was followed to provide a slow generation of Te²⁻ ions in such a way that, the solubility product (8.70 × 10⁻³⁵) is just exceeded. The CdTe thin film growth

Conclusions

The simple Chemical Bath Deposition (CBD) technique was used to deposit the polycrystalline CdTe thin films on glass substrate. The novelty of the method is to avoid the clumsy, tedious and time-consuming process compared to deep method. The good quality of the samples mainly obtained at the optimum deposition parameters such as temperature 60 °C, time 90 min, speed of the substrate rotation 70 rpm and pH 10.5 This method is effectively feasible for large area deposition on any substrate.

Acknowledgement

One of the Authors (KMG) is grateful to University of Pune, Pune for providing the Financial Assistance for this research work.

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Effect of annealing on chemically deposited polycrystalline CdTe thin films

Abstract

Introduction

Section snippets

Experimental details

Characterization of CdTe thin films

Growth mechanism

Conclusions

Acknowledgement

Solar Energy Mater. Solar Cell

Thin Solid Films

J. Appl. Phys.

J. Appl. Phys.

Adv. Mater.

Vac. Sci. Technol. B

J. Phys. D: Appl. Phys.

Chalogenide Lett.