Dangling bonds equilibration

Thermal equilibration effects are also present in undoped a-Si H (Smith et al. 1986). The dangling bond density varies reversibly with temperature, as is shown in Fig. 6.8 for samples of different deposition conditions. The experiment is performed by rapidly cooling the material from different temperatures to freeze in the equilibrium configuration and measuring the ESR spin density of the g = 2.0055 resonance. The defect density increases four-fold between 200 C and 400 °C, with an activation energy of 0.15-0.2 eV. The relaxation times are slower than in the doped material at the same temperature, and the equilibration temperature of about 200 °C, for a cooling rate of... 

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. 

There is considerable evidence that hydrogen diffusion is the mechanism of thermal equilibration. The evidence is necessarily indirect because the hydrogen apparently does not participate directly in the defect or dopant states, but acts as a catalyst to the motion. For example, the movement of a hydrogen atom from one Si—H bond at site A to a neighboring dangling bond at site B is described by... 

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