Growth and characterisation of n- and p-type ZnTe thin films for applications in electronic devices
Introduction
Group II-VI compound semiconductor materials have found a wide application in a variety of solid-state electronic devices such as electroluminescence devices (for example, light emitting diodes (LED)), photosensors, thin-film transistors, and solar cells. Zinc Telluride (ZnTe) happens to be one of the II-VI binary compound semiconductors which find numerous applications in optoelectronic devices, switching devices and macro-electronic devices such as solar panels [1], [2], [3], [4]. It is also a direct bandgap semiconductor with bandgap energy of 2.20–2.26 eV [5], [6]. Over the years, ZnTe semiconductors have found a useful application as a p-type window material in hetero-junction solar cells fabricated from chalcogenide semiconductors such as CdS [3], CdSe [4] and CdTe. p-ZnTe is also a promising candidate for ZnTe/CdTe heterojunction device structures [6] and for development of graded bandgap solar cells. Apart from being used as a window material, thin film ZnTe semiconductors have also found a useful application as a back contact material to CdTe-based solar cells [7]. John et al. doped ZnTe with Cu in order to achieve low resistivity electrical contacts thus making it more useful as a back contact to thin film solar cells [8].
The electrical conductivity type of ZnTe material grown by conventional methods has been reported to be p-type. According to Mandel [9], n-type electrical conduction is difficult to achieve due to self-compensation. However, some researchers have been able to achieve n-type electrical conduction in ZnTe by extrinsic doping. Extrinsic dopants such as Al, Cl and Sn have been used to achieve n-type ZnTe [10], [11], [12], [13], [14], [15]. Fischer et al. [10] and Chang et al. [11] have been able to prepare n-type ZnTe thin films by using Al as the dopant. Ogawa et al. [12] also obtained n-type ZnTe layers by doping with Al using triethylaluminium. DiNezza et al. [13] likewise reported the growth of n-type ZnTe films on GaSb substrates. These authors achieved the n-type electrical conductivity by thermally diffusing Al into the ZnTe film. Also, the authors reported the fabrication of ZnTe p-n homo-junction diodes with rectifying J–V characteristics and photo-voltaic (PV) behaviour to further confirm the successful growth of n-type ZnTe film. The uses of Cl and Sn as dopants to achieve n-ZnTe have also been demonstrated by Tao et al. [14] and Makhny et al. [15] respectively.
Several techniques have been used for the deposition of ZnTe thin films. Some of these methods are: closed space sublimation (CSS) [16], hydrothermal [17], molecular beam epitaxy [11], rf-magnetron sputtering [18], metallo-organic chemical vapour deposition (MOCVD) [19], metallo-organic vapour phase epitaxy (MOVPE) [20], thermal evaporation [21] and electrodeposition [22], [23], [24], [25]. According to Mahalingam et al. [24], electrodeposition (ED) technique provides a suitable method to prepare continuous and thin semiconductor films. ED technique has divers advantages among which are low capital cost, low temperature growth, ability of bandgap engineering and the ability to control the film thickness by varying the deposition time and potential.
In this work, ZnTe thin films have been electrodeposited on fluorine-doped tin oxide (FTO) coated conducting glass using a two-electrode system in aqueous solution. Zinc sulphate heptahydrate and TeO2 have been used as precursors for deposition of ZnTe thin films. Using the ED technique, we report the development of p-type and for the first time, n-type ZnTe thin films by intrinsic doping. The intrinsic doping was achieved by varying the composition of ZnTe; this was done by simply changing the deposition potential without the addition of extrinsic dopants like Al, Cl and Sn. Also, in this paper, we report for the first time the fabrication of p-n homo-junction diodes fabricated purely from intrinsically doped electroplated ZnTe layers.
Section snippets
Experimental details
ZnTe thin films were cathodically electroplated on glass/FTO substrates in a potentiostatic mode using GillAC ACM potentiostat. The glass/FTO substrates used in this work is TEC 15 having a sheet resistance of 13 Ω/square. To grow a uniform thin film semiconductor with good adherence to the substrate, the surface of the substrate must be thoroughly cleaned. To achieve this, the substrates cleaning were carried out in ultrasonic medium containing soap solution for 15 min. A further rinsing
Cyclic voltammetry
Cyclic voltammetry studies were performed in an aqueous solution that contains 0.015 M ZnSO4·7H2O and 2 ml of dissolved TeO2 solution at a pH of 3.50 ± 0.02. A FTO coated glass substrate was used as the working electrode to study the mechanism of deposition of ZnTe thin films. A computerised GillAC potentiostat was used to carry out this voltammetric study at a sweep rate of 180 mV min−1. In this technique, a range of cathodic potentials from 0 to 2000 mV was applied across the electrolyte
Conclusions
The growth of ZnTe thin films have been successfully achieved by electrodeposition technique using 2-electrode system. The electroplated ZnTe layers are polycrystalline with hexagonal crystal structures and preferred orientation along the (002) plane. The electrical conductivity types show both n- and p-type and electroplating provides a convenient intrinsic doping simply by changing the composition. As seen from the results of EDX analysis, variation in the atomic composition of Zn:Te was
Acknowledgement
The authors would like to acknowledge the contributions made by P. A. Bingham, H. I. Salim, O. Ayotunde, B. Kadem, F. Fauzi and O. K. Echendu. The principal author wishes to thank the Commonwealth Scholarship Commission and Sheffield Hallam University for financial support to undertake this research. The Federal University of Technology, Akure, Nigeria is also acknowledged for their financial support.
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