Studies of the Field Dependent Photoluminescence of n ‐ GaAs Electrodes with and without Cobalt Films

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Abstract
The photoluminescence (PL) resulting from the radiative recombination of the photoexcited e − and h + in electrodes, with and without a 10 nm thick cobalt film, was studied in solutions potentiostatically as a function of applied voltage (band bending). The electrode was photoexcited at 633 nm using a CW He‐Ne laser beam chopped at 890 Hz and the PL spectrum recorded. PL due to bulk recombination was observed between 810–875 nm with a peak at 853 nm (1.45 eV) that was assigned to band‐to‐band transitions. The PL intensity at 853 nm peak was followed as a function of applied voltage at different sweep rates. As the bands are bent up gradually, the PL decays exponentially due to increased separation in the depletion layer and the PL quenching fits the “dead layer” model. After holding the electrode under photoanodic conditions, if the field is reversed to the forward direction and the bands are bent from upward to downward position, then a burst of PL occurs soon after crossing the FBP. A complex mechanism of hole trapping and photoanodic formation of oxide/hydroxide interface states is involved followed by cathodic reduction of the oxide and eventually formation of a hydride layer. The reduction process gives rise to interband, recombinations and PL having bandgap energy above the background level. This phenomenon of surface PL can be modulated using different rates of potential sweeps resulting in one or more PL peaks. at pH 9 has no effect on the bulk recombinations whereas a 10 nm thick cobalt film was found to suppress the PL burst at the FBP. The possible mechanisms have been considered.

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