THIN-LAYER ELECTROCHEMICAL STUDIES OF THE OXIDATIVE UNDERPOTENTIAL DEPOSITION OF SULFUR AND ITS APPLICATION TO THE ELECTROCHEMICAL ATOMIC LAYER EPITAXY DEPOSITION OF CDS

The oxidative underpotential deposition (UPD) of S from a sulfide solution has been demonstrated and investigated using a polycrystalline Au electrode in a thin-layer electrochemical cell. UPD S appears to form spontaneously when a clean Au electrode is exposed to the sulfide solution. Subsequent oxidation of the sulfide to bulk sulfur is quantitative at potentials above -0.6 V vs. Ag/AgCl/1 M NaCl. This adsorbed S is quantitatively converted to sulfate in acidic electrolytes at potentials above 0.7 V. Oxidation of adsorbed S in a base solution also appears to form sulfates at potentials above 0.3 V; however, the process was considerably slower than that in an acid. The use of oxidative S UPD in the deposition of CdS using the method of electrochemical atomic layer epitaxy (ECALE) is also reported. An ECALE cycle was developed using a 2.5 mM Na2S solution alternated with a 10 mM CdSO4 solution. Both S and Cd were deposited at -0.6 V. Studies of deposits resulting from up to five ECALE cycles produced coverages of 0.45 monolayers of Cd and S per cycle, where 0.5 monolayers would have been ideal. In addition, the dependence of the deposit coverage on the Cd deposition potential was examined; this displayed the classic ''S'' shape expected for a surface-area-limited process.