Photoelectrochemical Properties of a GaP Electrode with an n/p Junction

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Abstract
The photoelectrochemical (PEC) properties of a GaP electrode containing p/n junction have been studied in 0.1M H2SO4 and 0.1M NaOH. This electrode material exemplifies the problem with semiconductor overlayers and how PEC techniques can be used to probe the interior properties of multijunction materials. This system showed both cathodic and anodic photoresponse. If illuminated at 500 nm and biased more positive than -0.95V vs. Ag/AgCl, the photocurrent was anodic, but it became cathodic if biased negative of this potential. If the wavelength was 430 nm the photoanodic response vanished and only photocathodic response was observed. Charge accumulation at the p/n and p/electrolyte junctions affects the magnitude of photocurrents and was studied with light modulated photocurrent voltammetry and impedance measurement. From these results we concluded that the magnitude of the photocurrents is determined by the hole transport through the n/p and p/electrolyte junctions. The quantum efficiency (QE) increased at 550 nm and extended to 570 nm which is not expected for GaP as the bandgap of GaP corresponds to 550 nm. SIMS analysis showed an unexpectedly high concentration of boron in the n-GaP substrate and the epitaxial n-layer and a boron enrichment at the n/substrate interphase. The formation of boron phosphide, which has a bandgap of ～620 nm, during the fabrication process was considered to be responsible for the increased QE at 550 nm

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