Michael Neumann-Spallart

Electrochemical deposition of MoS2 thin films by reduction of tetrathiomolybdate

Abstract Molybdenum sulphide was cathodically electrodeposited from aqueous solutions of sodium tetrathiomolybdate. The as-deposited films were X-ray amorphous with a composition, measured by microprobe analysis, close to MoS 2 . Annealing these films in Ar resulted in highly-textured films of MoS 2 with the van der Waals planes parallel to the substrate. A small expansion in the c spacing of the annealed films was explained by the presence of oxygen in the crystals. A direct bandgap of 1.78 eV was found for the annealed films.

Electrodeposition of zinc selenide

Abstract The synthesis of polycrystalline thin films of cubic ZnSe by electrochemical plating on conducting substrates is described. The influence of deposition parameters such as electrolyte composition, deposition potential and temperature on the crystallanity and on the chemical composition of the films is discussed. Of the various plating techniques (potentiostatic, potentiodynamic pulse and galvanostatic), potentiostatic plating offers the best control of film composition. An excess of Se with respect to perfect stoichiometry cannot be avoided by adjusting electrolyte composition and deposition potential, but can be decreased by vacuum annealing.

Chemical bath deposition of cubic copper (I) selenide and its room temperature transformation to the orthorhombic phase

The chemical bath deposition of cubic copper (I) selenide (Cu2-xSe), thin films has been achieved on an inert Pt substrate from a selenosulfite-containing bath at 75 °C. The electrochemical polarisation of this film at −0.78 V vs. SCE leads to the transformation of the compound into the orthorhombic phase. The lattice parameter of the face-centered cubic copper (I) selenide increases from 5.742 A to 5.761 A due to the decrease of the concentration of Cu vacancies upon electrochemical polarisation. The transformation of the cubic to an orthorhombic phase starts to occur when copper vacancies reach a critical value of x < 0.15. This transformation seems to be induced by a Cu3Se2 impurity in the cubic Cu2-xSe phase.

Electrodeposition of zinc telluride

Abstract The synthesis of polycry stalline thin films of cubic ZnTe by electrochemical plating on conducting substrates (titanium, nickel and F:SnO 2 glass) is described. Electrodeposition involves potentiostatic reduction from an acid aqueous bath. The influence of bath composition (concentrations of the precursors TeO 2 and Zn 2+ , and pH), deposition potential and temperature on the chemical composition of the films and on their crystallinity is discussed. The thin films were characterized by X-ray diffraction (XRD), electron probe microanalysis, scanning electron microscopy, and optical measurements. For optimized deposition parameters, the XRD pattern of a single-phase, cubic ZnTe was found. Otherwise, the patterns of either Zn or Te were found in addition in some samples. Best results (stoichiometric ZnTe films with a reddish aspect) were obtained from TeO 2 saturated solution at a pH of 4.5,98 °C, and a Zn 2+ concentration of 0.02 M, employing deposition potentials between −0.8 V and −0.9 V vs. Ag AgCl . The optical and crystalline properties of the films improved by annealing for 1 h at 450 °C in argon. By this treatment excess Te found in samples prepared under certain conditions could also be removed.

Electrodeposition of lead sulphide in acidic medium

Abstract Polycrystalline thin films of lead sulphide were electrodeposited on titanium, aluminium and stainless steel (SS) substrates at a constant potential of −0.7V vs. Ag|AgCl|(sat)KCl electrode using 1mM solutions of Pb(NO 3 ) 2 and Na 2 S 2 O 3 at pH 2.7, 2.8 and 2.9 Except at pH 2.9 (which also gave tetragonal PbS 2 ) all other pHs gave single phase PbS (fee). While deposition on Al gave a crystalline phase of PbS with very prominent (200) and (111) planes, that on SS substrate showed growth of the (200) plane only. Although the deposition of PbS on Ti was good. it showed (111) and (200) planes of low intensity compared to that on Al. Film thickness and grain size were found to be of the order of micrometres. The cyclic voltammetry of the film formation was studied in a potential range −1 to 0V (Ag|AgCl) followed by XRD, SEM and AFM analyses. The mechanism of growth is discussed.

Electroless deposition on copper substrates and characterization of thin films of copper (I) selenide

Abstract Cuprous selenide films are prepared by reacting oxide free copper metal with selenous acid at room temperature (22°C). The formation of Cu(I) oxide is avoided by degassing the bath with argon. From the X-ray diffractograms (XRD) and electron probe microanalysis (EPMA) of such films it can be concluded that copper deficient, orthorhombic Cu(I) selenide is formed.

Photoelectrochemical etching of silicon

Abstract The state of the art of electrochemical and photoelectrochemical etching of silicon in HF is reviewed, which leads to interesting and novel surface morphologies. New results are presented concerning the change of electronic properties engendered by photoelectrochemical etching and the use of such modified surfaces as rectifying interfaces. It is emphasized that PEC-etching is thus a major new area of application of photoelectrochemistry.

Characterization of II-VI compounds on porous substrates

We investigate the possibilities for electrodeposition of II–VI compounds on porous substrates. ZnTe and CdTe are deposited on porous TiO2 and characterised by scanning electron microscopy and electrical measurements. It is shown that CdTe can be deposited as a homogeneous film covering a highly structured substrate in conformal manner, while ZnTe is able to fill up the porous structure to a certain extent. However, the ZnTe deposition exhibits a strong tendency for irregular growth, and a smooth top surface has so far not been obtained. These morphological differences can be attributed to different deposition mechanisms. While CdTe growth is characterised by a chemical autoregulation mechanism that allows stoichiometric deposition, ZnTe deposition appears to show a higher dependence on potential and Te concentration. As a consequence, exposed regions of the substrate are more readily covered with ZnTe. The growth near the SnO2 contact is apparently zinc rich, indicating a potential drop over the TiO2 together with a diffusion limitation for Te. Despite inhomogeneous growth, efficient electron transfer from the ZnTe to the TiO2 substrate is observed.

Physical properties of electrochemically deposited cadmium mercury telluride films

Abstract Films of cadmium-rich cadmium mercury telluride (CdxHg1−xTe, CMT) 0.1–2 μm thick were formed on conducting glass (SnO2) by electrochemical (potentiostatic) deposition from an aqueous bath. 1−x ranged from 0 to 0.25. Dependingon the mercury content, a preferential orientation of the films was detected by X-ray diffraction studies. The homogeneous composition of the layers was confirmed by electron microprobe and secondary ion mass spectrometry measurements. The optical properties are in accordance with a semiconductor of variable band gap, 1 1−x > 0. CMT layers of n- and p-type were obtained by appropriate choices of deposition potential and annealing conditions, as shown by the photovoltaic effect of CMT/Au junctions.

Photoelectrochemical Characterization of Semitransparent WO3 Films

Tungsten trioxide films on transparent substrates (glass and F:SnO 2 or indium-tin oxide coated glass) were prepared by various methods like doctor-blading, dip-coating, layer-by-layer painting, and spin-coating. Films ofup to 10 X 10 cm were grown and characterized by X-ray diffraction, scanning electron microscopy, optical reflectance and transmittance, and photoelectrochemical measurements. Well-crystallized WO 3 with monoclinic structure prevails on all substrates after annealing at or above 550°C. Films grown via layer-by-layer painting and spin-coating using organic precursors consist of WO 3 particulates of around 50 nm. Such films are specular and highly transparent outside the bandgap. Their photoactivity extends up to 470 nm and junctions with 0.1 M HClO 4 yield incident photon to current conversion efficiencies values above 0.9 at 313 nm and 0.1 at 436 nm. Under solar illumination, photocurrents of up to 0.91 mA/cm 2 (front-side illumination) and 0.67 mA/cm 2 (back-side illumination) were drawn.