A phase modification of CdSe electrodeposits induced by substrate roughness

Cadmium chalcogenides are tetrahedrally bonded solids, preferentially crystallizing in either the wurtzite, hexagonal structure (CdS, CdSe) or the zinc blende, cubic one (CdTe). The wurtzite structure of CdSe, which is associated under certain conditions to better semiconductive properties and higher resistivity against photocorrosion, has been observed in thin films prepared by various electroless techniques [1, 2]. CdSe preparation by electrochemical means leads, mostly, to the obtaining of a metastable, kinetically controlled cubic phase, which can be transformed to the hexagonal one by an annealing process [3–5]. Cathodic electrodeposition from aqueous solutions is a simple preparation method successfully applied to obtain semiconductive materials, especially of the II–IV class compounds (eg. [6]). As reported in some of our previous works [5, 7–9], the controlled co-deposition of the CdSe constituent elements from a high temperature aqueous bath (85–90 ◦C), involving underpotential reduction of Cd, provides compact and coherent cubic structured semiconductive, polycrystalline layers with a more or less pronounced (111) preferred orientation. The aim of the present article is to report on the substrate-induced possibility of adopting a mixed hexagonal-cubic CdSe structure at certain electrolytic conditions. Thin CdSe films, of 2–3 μm in thickness were produced potentiostatically employing a rotating disc electrode setup [9] from excessive in Cd2+ (0.2 M) solutions containing small amounts of SeO2 (1× 10−4 to 1× 10−3 M) at a pH equal to 2.2, at 85 ◦C. The counterelectrode was a platinum grid and the potential of the working electrode was monitored against a Hg/HgSO4 saturated sulphate reference (SSE; ESSE vs. NHE ≈ 0.59 V at 85 ◦C, [10]). Cathodic polarization curves were recorded at a potential sweep rate of 2.5 mV/s. The crystallographic structure of CdSe films obtained on commercially pure Ti and Ni (s/ 10 mm) as well as SnO2/glass (TO/glass) (s/ 12 mm) disc electrodes was identified in terms of X-ray diffraction data by a Siemens D5000 X-ray diffractometer. The preparation of reproducible Ni, Ti and TO/glass electrode surfaces is described elsewhere [5, 9, 10]. Several Ni electrodes (denoted as Ni-R) were chemically etched by HNO3 in order to acquire a roughened surface. A partial removal of Ti passivating oxide layer, accom-