Electrodeposition of Tellurium Thin Films from Citric Acid Bath

Abstract

Tellurium is a very interesting element which exhibits intermediate properties of metals and non-metals. Semiconducting properties of tellurium allow for a number of its applications in many industry branches. Physicochemical properties of thin tellurium layers are more and more frequently exploited in many electronic devices like infrared photoconductive detectors, holographic recording materials, sensors, thermoelectric, piezoelectric and optoelectronic devices. Recently, high interest has been focused on gas sensors constructed on the basis of thin tellurium layers whose sensitivity and selectivity significantly depend on the thickness of semiconducting coatings. Moreover, this element, when combined with d-block metals, allows for obtaining advanced optical materials used in constructing solar cells, diodes and radiation emitters of different wave length. The leading methods of thin tellurium coatings production are physical and chemical vapour deposition methods, which enable production of coatings with different thickness. However, one of the alternative ways to obtain tellurium coatings can be the electrochemical method. The process of electrochemical tellurium deposition can be performed both from acidic and alkaline aqueous solutions. However, it is difficult to achieve it due to electrochemical properties of tellurium and its very low solubility in acidy aqueous solutions. It results in conducting deposition of tellurium under conditions of diffusion control, which means that the rate of the coating growth is very low and it results in amorphous or fine-crystalline structure. Moreover, there is a risk of obtaining coatings of dendritic structure or even powders. Additionally, due to the mechanism of tellurium reduction from aqueous solutions, a competitive reaction leading to the reduction of deposited tellurium to Te-2 may occur causing dissolution of the tellurium coating and a decrease of the electrolysis process efficiency. This work presents the description of the electrochemical process of formation thin tellurium layers from citrate acidic solution. The suggested methodology consists in the preparation of stable acidic baths with high content of tellurium, and with the addition of citrate acid. In order to analyse the mechanism of the process of tellurium deposition, the electroanalytical tests were conducted. The tests of cyclic voltammetry and hydrodynamic ones were performed with the use of polycrystalline gold disk electrode. The range of potentials in which deposition of tellurium in direct four-electron process is possible was determined as well as the reduction of deposited Te0 to Te2- and its re-deposition as a result of the comproportionation reaction. On the basis of the obtained results, the deposition of tellurium was conducted by the potentiostatic method. The influence of a deposition potential and a concentration of TeO2 in the solution on the rate of tellurium coatings deposition was examined. The presence of tellurium was confirmed by X-ray spectrofluorometry and electron probe microanalysis. In order to determine the phase composition and the morphology, the obtained coatings were analysed with the use of x-ray diffraction and scanning electron microscopy. This work was supported by the Polish National Center of Science under grant 2011/01/D/ST5/05743.