Observation of phase transitions on the (111) and (100) surfaces of Si near 1000 K with He atom diffraction

He diffraction has been used to study structural phase transitions on the (111) and (100) surfaces of Si. The seventh-order peaks of the Si (111) -7 X 7 reconstructed surface observed at room temperature first decreased with increasing temperature and then disappeared near 1140 K. A sharp decrease in the specular intensity and a sharp increase in the diffuse scattering were also observed near 1140 K, but there was no broadening of the peaks as the temperature increased. We confirm earlier evidence that the 7 X 7 to "1X1" phase transition at 1138 ± 7 K on the Si( 111 ) surface is an order-disorder one. The He scattering from the Si(100)-2X1 reconstructed surface stable at room temperature was also measured as the temperature increased. There was a change in the Debye-Waller slope at 930 ± 20 K which is evidence for the structural phase transition expected from theoretical calculations and deduced from earlier experiments in this laboratory on the desorption kinetics of alkali atoms from this surface. These experiments show that He diffraction offers a useful way of studying phase transitions on semiconductor surfaces.