A Comprehensive Investigation of HCI- and Br₂/NH₃(aq)-Etched p-InP Interfaces

The physical and chemical composition of p-InP surfaces prepared with HCI and Br₂/NH₃(aq) etches have been investigated using SEM and XPS, and the interfacial energetics have been studied using capacitance and open-circuit photo-voltage measurements. The Br₂/NH₃(aq) etch produced a microscopically smooth surface with a surface layer containing less than a monolayer of impurities, composed primarily of InPO₄ with some In(OH)₃ and some adsorbed hydroxyl species. The HCI etch produced a rough, crystalline surface with a one to two monolayer indium-rich surface layer which was heavily hydrated and contained In₂O₃ or In(OH)₃ and adsorbed hydroxyl species. The HCI etch also resulted in greater surface hydrocarbon contamination than the Br₂/NH₃(aq) etch. Capacitance measurements of the p-InP/acetonitrile interface reveal that the interfacial energetics of Br₂/NH₃(aq) etched electrodes are controlled by filling of empty surface states by solution redox couples, while those of the HCl-etched electrodes are controlled by electrochemical growth of a surface film. The Br₂/NH₃(aq) etch yields a p-InP surface which allows efficient electron transfer to the solution resulting in a deep depletion energetic condition under extreme reverse bias conditions, while HCl-etched electrodes enter an inversion region where excess electrons accumulate in the space charge region. Empty surface states in the bandgap of Br₂/NH₃(aq)-etched electrodes were found to be evenly distributed at a density of ca. 1.7 × 10¹² cm⁻² V⁻¹. The open-circuit photovoltages of Br₂/NH₃(aq)-etched electrodes approached the bulk recombination limited value of 800 mV for redox couples with E(0′) near −1.0V, but for the most negative and the most positive redox couples, deviations in the expected behavior were observed. These are explained by bandedge movement which alters the barrier height. These studies reveal that the behavior of p-InP photoelectrochemical cells is determined largely by the method of surface pretreatment and only in part by bulk properties of the semiconductor material. Even when the surface films are only a few monolayers thick, their effect on the surface energetics are dramatic.