Reversible phase transformation on Ge(111) surface by potential

Abstract

Reversible phase transformation on atomically flat Ge(111) induced by potential in 40% NH4F solution has been observed with in situ scanning tunneling microscope. The abrupt change in Ge surface morphology as a function of the potential is attributed to the phase transformation, where small terraces are converted into large terraces as the potential is changed from -1.32 to -0.98 V. Cyclic voltammograms show that the charge under the cathodic peak is almost the same as that under the anodic peak (4.2×10-4 C/cm2). Combined with ex situ X-ray photoelectron spectroscopy and secondary-ion mass spectroscopy results, we conclude that Ge(111) surface is terminated by a monolayer hydroxide or hydride depending on the applied potential. The formation of hydroxide-terminated Ge(111) is likely to occur at -0.98 V, far away from the anodic peak in the cyclic voltammogram. This fact suggests that the reversible phase transformation between monolayer hydrogen- and hydroxyl-capped Ge(111) surface is induced by potential. An atomically flat Ge(111) surface prepared with wet cleaning provides an attractive substrate for epitaxial growth of compound semiconductors.