Helium Scattering Studies of the Germanium (111) and (100) Surfaces at High Temperatures.

The structures of the Ge(111) and Ge(100) surfaces at high temperatures (above 650 K) have been investigated using thermal energy helium atom scattering. Helium scattering from Ge(111) at 298 K leads to the expected c(2 x 8) diffraction pattern, although repeated cleaning with argon-ion sputtering and heating above 1100 K produces a (2 x 8)-like pattern which is interpreted as due to an increased defect density on the surface. Above 650 K, all non-specular diffraction peaks are thermally attenuated at exponential rates to the noise level, with the strongest first-order peak vanishing near 1000 K. Between 1020 K and 1060 K the specular peak doubles in height, and the diffuse background intensity increases by 40% at the specular position, with its angular distribution sharpening by approximately 10%. Above 1060 K the specular peak is attenuated at a higher rate than below 1020 K, and the diffuse background decreases from its maximum level. No broadening is observed in any of the diffraction peaks. In light of previous studies indicating a disordering phase transition on this surface near 1050 K, the helium scattering results are interpreted as indicating a reduced surface corrugation after the transition to a phase that is laterally disordered but strongly confined to the surface plane ("layered liquid"). Another possible explanation for the increase in specular intensity is that it is a central peak effect due to a second-order phase transition. Helium scattering from Ge(100) at 298 K leads to the expected (2 x 1) diffraction pattern. As the temperature is increased, all diffraction peaks including the specular are attenuated exponentially, until they vanish into the noise above 950 K; no peak broadening is observed. Because of this attenuation no definite conclusion can be reached about the existence of a previously reported (2 x 1) to (1 x 1) phase transition combined with adatom and vacancy proliferation occurring at 955 K; however, the increase in the diffuse background which would be expected from such a transition is not observed. An increase in the exponential attenuation of the specular peak is observed above 800 K, and this is attributed to a small adatom and vacancy density on the surface which increases with temperature.