O2 K/GE(100)2X1 AND O2/CS/GE(100)2X1 - PUZZLING BEHAVIOR OF K AND CS IN THE OXIDATION OF GERMANIUM/

The effect of alkali-metal overlayers (Y, Cs) on the room-temperature oxidation of the Ge(100)2 x 1 surface in the presence of molecular oxy gen was investigated by core-level and valence-band photoemission spec troscopy using synchrotron radiation. In strong contrast to the behavi or observed for many other elemental or compound semiconductor surface s, the presence of a K or Cs layer does not enhance the oxidation rate as expected but rather poisoned it since the K (Cs) covered Ge(100)2 x 1 surface exhibits significantly smaller amount of oxides for the sa me oxygen exposures. This behavior indicates that K does not act as a catalyst as observed in the cases of many other semiconductor surfaces , but as an oxygen acceptor, in agreement with a classical model of el ectron affinity.