Kamal K. Mishra

A re-examination of the mechanisms of electrodeposition of CdX and ZnX (X = Se, Te) semiconductors by the cyclic photovoltammetric technique

Abstract The electrodeposition mechanisms in two families of compound semiconductors, namely CdX and ZnX (X = Se, Te), were re-examined by cyclic voltammetry in the dark and under illumination of the substrate electrode/electrolyte interphase. In situ diagnostic probes of the mechanistic pathway and the electrodeposition chemistry were furnished by the photoresponse of the forming semiconductor films under band-gap illumination. Thus, a new mechanism involving the metal ion catalyzed deposition of the chalcogen species, is proposed for CdSe and ZnSe. In the case of CdTe and ZnTe, Teo nucleation preceded subsequent semiconductor formation via the underpotential deposition of the metal ions. Two types of photoresponse, namely photoconductive and photoelectrochemical, were distinguished on the basis of their varying dependence on the applied potential. These cyclic photovoltammetric data are finally discussed within the framework of existing methods combining light excitation (or emission) and voltammetric scanning of the electrode/electrolyte interphase.

A voltammetric study of the electrodeposition chemistry in the Cu + In + Se system

Abstract The mechanism of formation of CuInSe2 thin films on a glassy carbon surface during voltammetric scanning was examined in detail for the first time using a combination of Pourbaix analyses, cyclic and hydrodynamic voltammetry of the binary In + Se and Cu + Se systems, along with cyclic photovoltammetry, i.e. cyclic voltammetry combined with periodic white light illumination of the electrode/electrolyte interphase, on the ternary Cu + In + Se system. A sulfuric acid matrix containing SeO2 and uncomplexed Cu2+ and In3+ ions was used in all cases. The data on the binary systems were consistent with the facile formation of a Cu2−xSe solid phase in the Cu+Se system and a kinetically sluggish interaction between In and Se in the In+Se case. An internally consistent mechanistic scheme is proposed for the ternary system involving the concurrent formation of the Cu2−xSe phase, its subsequent reduction coupled with the 6 e− reduction of H2SeO3 to H2Se, and finally the underpotential assimilation of In into the solid phase leading to the photoactive chalcopyrite semiconductor, CuInSe2. The cathodic decomposition of the initially formed Cu2−xSe is shown to be a key to subsequent assimilation of In and formation of CuInSe2. The photocathodic response observed for this thin-film formation was diagnostic of a Cu-rich ternary composition and consequent p-type behavior for the conditions pertaining to this study.

Electrodeposition and Characterization of SnS Thin Films

A new room‐temperature electrodeposition technique was devised to synthesize thin films on indium tin oxidecoated glass slides. This technique is based on a nonaqueous ethylene glycol bath containing anhydrous and elemental sulfur. Three types of electrosyntheses, namely, potentiostatic, galvanostatic, and pulse modes, are discussed and their relative merits compared. A wide variety of characterization techniques were employed to develop a self‐consistent and complementary picture of the morphology, composition, and photoactivity of the thin films. These included scanning electron microscopy, x‐ray diffractometry, electron probe microanalyses, Auger electron spectroscopy, x‐ray photoelectron spectroscopy, optical analyses, and voltammetry. The photoactivity of these films was evaluated using photoelectrochemical techniques. Finally, the dark and photocorrosion behavior of these films are discussed with the aid of Pourbaix diagrams.

Electrosynthesis of Thin Films of CdZnSe : Composition Modulation and Bandgap Engineering in the Ternary System

This paper concerns electrodeposition chemistry in the Cd‐Zn‐Se system. First, the thermodynamics for the cathodic electrosynthesis of and were analyzed with the aid of Pourbaix diagrams. Strategies for modulating the Cd content and thus the photoresponse of alloy thin films were developed. These thin films were analyzed by cyclic photovoltammetry and photocurrent spectroscopy. X‐ray photoelectron spectroscopy provided further confirmation of the alloy composition. Finally a ramp‐hold method was developed for modulating the composition of these thin films to generate superstructures. These experiments demonstrate the feasibility of securing modulation lengths spanning the submicron regime.

Anodic electrosynthesis of cadmium selenide thin films : characterization and comparison with the passive/transpassive behavior of the CdX (X = S, Te) counterparts

This paper reports on cadmium selenide thin films electrosynthesized by an anodic route employing alkaline selenide solutions. The thermodynamic aspects of the electrodeposition chemistry were first explored via Pourbaix diagrams; the kinetic aspects were studied by linear sweep voltammetry on a Cd anode. The photoaction spectra of these anodic thin films revealed an optical gap energy (1.7 eV) in good agreement with the value known for CdSe. Their luminescence response, however, suggested a high density of carrier recombination centers located {approximately}0.2 eV below the conduction band. X-ray photoelectron spectroscopy revealed that the anodic thin films were stoichiometric in composition. In situ examination of the material during thin film growth by cyclic photovoltammetry revealed anodic photoeffects at potentials just past the Cd corrosion wave. The growth kinetics in the passive region adhered to a direct logarithmic rate law, while a diffusion mechanism was seen to prevail in the transpassive regime.

Dissolution Windows for Wet Chemical Processing of Silicon and Silicon Dioxide: Potential‐pH Diagrams for the Si ‐ F ‐ H 2 O System

Potential-pH diagrams are presented for the systems Si-H 2 O and Si-F-H 2 O. It is shown that the stability field of elemental silicon lies well below the water stability region, indicating that silicon is highly unstable in water relative to oxidation to Si(IV). Thus, the ability to suppress oxide formation must be attributed to kinetic effects. It is shown further that the introduction of HF and F - into the aqueous phase results in a partial displacement by SiF 6 2- of the Si(OH) 4 (aq) and SiO 2 stability fields originally present in the Si-H 2 O system. Under some conditions, the SiF 5 2- stability domain is nested between two SiO 2 stability fields. It is suggested that the minimal etching rates observed at relatively low and relatively high pH solutions of HF may be related in part to the presence of the solubility walls in both pH regimes.

Anodic oxidation of telluride ions in aqueous base : a rotating ring-disk electrode study

The anodic oxidation of telluride ions was studied in using mainly hydrodynamic voltammetry. Thermodynamic (Pourbaix) analyses predicted stepwise oxidation of Te2− to , Te0, , and finally . Consistent with this expectation, was detected as a intermediate product at the ring, both in the forward , and in the reverse electron transfer directions. Studies with authentic samples of in revealed that this species could be collected at the ring both in the oxidative (−0.80V vs. SCE) and in the reduction (−1.20V) modes. Consequently, comparison of the ring response at −0.80V and −1.20V with the disk current profiles in provided further confirmation for the electrochemical generation of the intermediate. Finally, further oxidation of Te0 to and was detected via cyclic and hydrodynamic voltammetry.

Anodic growth of CdS thin films: an in situ Raman spectroelectrochemical study

There has been much recent interest in the fabrication of Group II-VI compound semiconductor thin films by anodic and cathodic electrodeposition methods [l-9]. These studies have been, in part, motivate by the use of these materials in solar and other optoelectronic device applications. A major problem with electrodeposition, however, is contamination of the target material with “impurity” phases; this is a particular difficulty with compound semiconductors (e.g., Te is usually deposited along with CeTe, cf. ref. 10). Unfortunately, monitoring of electrochemical parameters (e.g. charge, current, potential) alone, during the deposition process, provides only limited info~ation content in terms of molecular details. It is advantageous to combine an in situ spectroscopic probe with electrochemistry in such cases because of the extreme sensitivity and molecular selectivity of the former. We had used this strategy previously via cyclic photovolt~met~ to study electrodeposition mechanisms within the Group II-VI system 171. We now wish to report how Raman spectroscopy could be an especially useful probe for monitoring the chemistry associated with a semiconductor electrodeposition sequence. We have observed routinely via surface analysis, high oxygen levels (up to 13 atom %) in Group II-VI se~#nductor tbin films synthesized by the anodic route (cf. ref. 6). Analysis of Auger peak shapes and XRS binding energies, however, revealed the 0 to be not associated with the chalcogen component within the film. Further, depth profiles revealed this 0 to be uniformly distributed throughout the thin film, thus ruling out surface oxides as the origin. The present Raman spectro-