Surface structures of clean and oxidized Nb(100) by LEED, AES, and STM

The surface structures of a Nb(100) single crystal during thermal cleaning in ultrahigh vacuum (UHV) and oxidation in low-pressure oxygen at 300 and 900 K have been studied by combined auger electron spectroscopy, low-energy electron diffraction, and scanning tunneling microscopy. The oxygen-induced (3x1)-O, (4x1)-O, c(2x2)-O, and clean (1x1) structures are sequentially observed on the Nb(100) surface at atomic resolution during thermal cleaning in UHV at temperatures from 1970 to 2500 K. At 300 K, the clean Nb(100) surface is sequentially oxidized into the c(2x2)-O and (1x1)-O structures and the amorphous oxides of NbO and NbO2 in oxygen. At 900 K, the clean Nb(100) surface is sequentially oxidized into the c(2x2)-O, (4x1)-O, and (3x1)-O structures in oxygen. The c(2x2)-O and (1x1)-O structures result from oxygen chemisorption and the (3x1)-O and (4x1)-O structures result from the epitaxial growth of NbO nanocrystals on Nb(100). Atomic models for these oxygen-induced structures are proposed and the atomic-scale oxidation processes of the Nb(100) surface at 300 and 900 K are discussed.