In situ AFM and STM investigation of electrochemical hydride growth on Ge(110) and Ge(111) surfaces

In situ images of Ge(110) and Ge(111) surfaces under potential control have been obtained with an atomic force microscope (AFM) and a scanning tunneling microscope (STM). Atomic resolution images, using the AFM, of Ge(111) and Ge(110) revealed lattice structures close to that expected for bare Ge even in a potential region where the surface is hydroxylated. Long-range images of the Ge surface revealed the reversible, anisotropic formation and dendritic growth of spikes of a hydride species at the Ge surface upon cathodic polarization. These spikes were manifested as tip artifacts with both the AFM and the STM. The observed density of hydride features on the surface was about 1 × 10⁹/cm² which is an order of magnitude less than the number of surface states previously predicted from impedance measurements. This indicates that each hydride feature is composed of several surface states. The features appeared ca. 25 nm higher in the STM images relative to the AFM, indicating a high degree of electron density around or associated with each feature. The STM locally enhances the formation of the hydride features. Anodic polarization causes the hydride spikes to disappear and the surface to anneal. The hydride most likely forms at defects in the hydroxide overlayer after the hydroxide overlayer is formed on anodic polarization.