A COMPREHENSIVE INVESTIGATION OF HCL- 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. © 1990, The Electrochemical Society, Inc. All rights reserved.