Time-resolved electron diffraction study of the Ge( [formula omitted])-(2×1)–(1×1) phase transition

Abstract

The dynamics of the Ge(1 0 0)-(2×1)–(1×1) phase transition has been investigated by 100-ps time-resolved reflection high-energy electron diffraction (RHEED). A laser pulse heats the surface while a synchronized electron pulse is used to obtain a diffraction pattern of the surface. For slow heating, on a heated stage, the Ge(1 0 0)-(2×1) reconstruction is observed to lose its long-range order between 900 and 1000 K. For heating with 100-ps laser pulses, time-resolved electron diffraction shows that the Ge(1 0 0)-(2×1) reconstructed surface starts to disorder at 1027±44 K, 127±44 K above the onset temperature of 900 K for the disordering of Ge(1 0 0)-(2×1) observed for slow heating. The overheating of the Ge(1 0 0)-(2×1) structure by ultrafast laser pulses is indicative of the first-order nature of the transition, and provides support for the conclusion that the mechanism of the Ge(1 0 0)-(2×1)–(1×1) phase transition involves (2×1) domain wall proliferation instead of dimer break-up. RHEED intensity and profile analysis under slow heating shows increased surface vacancy density and roughness with temperature. The onset of the increase in surface roughness, as measured by RHEED profile width, correlates with the Ge(1 0 0)-(2×1)–(1×1) phase transition.