Impact of O2 exposure on surface crystallinity of clean and Ba terminated Ge (100) surfaces

In analogy with the case of Sr on Si [Y. Liang, S. Gan, M. Engelhard, Appl. Phys. Lett. 79 (2001) 3591], we studied surface crystallinity and oxidation behaviour of clean and Ba terminated Ge(1 0 0) surfaces as a function of oxygen pressure and temperature. The structural and chemical changes in the Ge surface layer were monitored by LEED, XPS and real-time RHEED. In contrast to the oxidation retarding effect, observed for 1/2 monolayer of Sr on Si, the presence of a Ba termination layer leads to a pronounced increase in Ge oxidation rate with respect to clean Ge. In fact, while the Ge(1 0 0) surface terminated with 1/2 ML Ba amorphizes for a pO2 of 102 Torr, LEED indicates that clean Ge forms a thin (4.5 A ° ), 1x1 ordered oxide upon aggressive O2 exposure (150 Torr, 200 8C, 30 min).We briefly discuss the origins for the difference in behaviour between Ba on Ge and Sr on Si.