Kinetics of structural relaxation and equilibrium

The description of the equilibrium state in terms of formation energies does not consider the rate at which the structure equilibrates. The configurational coordinate diagram of Fig. 6.1 illustrates the energy barrier which must be overcome in order to reach equilibrium. The excitation over the barrier corresponds to the motion of atoms which, for example, allow a four-fold silicon atom to transform into a three-fold dangling bond. The energy barrier is manifested in the kinetics of equilibration and structural relaxation. 

Measurements of the equilibrium state make use of the observation that rapid cooling freezes the structure, allowing the characteristics of a high temperature state to be measured at a lower temperature. The freezing is incomplete, however, and the structure slowly relaxes to the equilibrium appropriate to the lower temperature. Figs. 6.4 and 6.9 give examples of the relaxation in doped and undoped a-Si H, and the data of Fig. 6.4 are replotted in a normalized form in Fig. 6.20. The relaxation time is about a year at room temperature, so that most experiments performed at or below this temperature can be considered to be completely in the frozen state. 

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