Towards Passivation of Ge(100) Surfaces by Sulfur Adsorption from a (NH4)2S Solution: A Combined NEXAFS, STM and LEED Study

Abstract

Using x-ray absorption spectroscopy, scanning tunneling microscopy and low-energy electron diffraction we have studied the surface chemistry and atomic structure of the sulfur passivation layer formed on Ge(100) surfaces upon treatment in an aqueous (NH4)2S solution at room temperature. This treatment was shown to yield incomplete sulfur coverage (<1 monolayer) and residual Ge oxides regardless of the time the substrates are immersed in the solution. Scanning tunneling microscopy images of the surface structure of the passivation layer reveal the coexistence of locally ordered and large disordered areas, attributed to S-Ge and O-Ge bonds, respectively. The passivated surfaces exhibit a pronounced (1 × 1) electron diffraction pattern. The formation of the passivation layer appears to be dependent on the state of the Ge surface prior to sulfidation, i.e. on the presence of surface oxides, which hamper the formation of a long-range ordered sulfur monolayer.