Date of Award
Doctor of Philosophy (PhD)
Electrical & Computer Engineering
Hani E. Elsayed-Ali
Ravindra P. Joshi
Karl H. Schoenbach
The phase transitions at the low-index surfaces of germanium were investigated using conventional and 100-ps time-resolved reflection high-energy electron diffraction. For the time-resolved studies, the surface is heated by 100-ps laser pulse while a synchronized electron beam probes the structure. When heated by 100-ps laser pulse, Ge(111)-c(2x8) and Ge(100)-(2x1) reconstruction is seen to overheat above the onset temperatures for the disordering under thermodynamic equilibrium. Slow heating shows that the adatoms in the Ge(111)-c(2x8) reconstruction state start to disorder at ∼510 K and are converted to a totally disordered adatom arrangement at 573 K. For heating with 100-ps laser pulses, time-resolved electron diffraction shows that the Ge(111)-c(2x8) reconstructed adatom arrangement starts to disorder at 584 ± 16 K, 74 ± 16 K above the onset temperature of 510 K for the disordering of Ge(111)-c(2x8) observed for slow heating. For slow heating, on a heated stage, the Ge(100)-(2x1) reconstruction is observed to lose its long range order between 900 K and 1000 K. For heating with 100-ps laser pulses, time-resolved electron diffraction shows that the Ge(100)-(2x1) reconstructed surface starts to disorder at 1027 ± 44 K, 127 ± 44 K above the onset temperature of 900 K for the disordering of Ge(100)-(2x1) observed for slow heating. The overheating of Ge(100)-(2x1) heated by ultrafast laser pulse is consistent with the domain wall proliferation during the Ge(100)-(2x1) − (1x1) phase transition. For Ge(111) incomplete melting, the phase transition spreads from 1020 K to 1070 K with slow heating, while for 100-ps laser heating, the incomplete melting occurs in the transient temperature range from 1083 ± 23 K to 1138 ± 32 K. Overheating of the 1–2 topmost bilayers on Ge(111) surface during incomplete melting phase transition is attributed to the layering effect which results in an energy barrier for the 1–2 topmost layers to melt.
On the melting of the low-index surfaces of germanium, the Ge(111) surface is observed to remain in its incomplete melting structure up to at least Tm + 134 ± 40 K when heated by a 100-ps laser pulse. Both the Ge(100) and Ge(110) surfaces are observed to melt near the bulk melting temperature when heated with 100-ps laser pulses, which favor the lack of surface superheating of Ge(100) and Ge(110). The overheating of the incomplete melting state of Ge(111) above the melting point is attributed to the strong layering effect of the topmost 1–2 germanium liquid layers in contact with the solid substrate underneath and the metallization of the topmost 1–2 liquid layers.
"Time-Resolved Electron Diffraction Studies of Phase Transitions at Low-Index Germanium Surfaces"
(2001). Doctor of Philosophy (PhD), Dissertation, Electrical & Computer Engineering, Old Dominion University, DOI: 10.25777/0fyg-qj86