Evolution of the Potential-Energy Surface of Amorphous Silicon

The link between the energy surface of bulk systems and their dynamical properties is generally difficult to establish. Using the activation-relaxation technique, we follow the change in the barrier distribution of a model of amorphous silicon as a function of the degree of global relaxation. We find that while the barrier-height distribution, calculated from the initial minimum, is a unique function that depends only on the level of relaxation, the reverse-barrier height distribution, calculated from the final state, is independent of global relaxation, following a different function. Moreover, the resulting gained or released energy distribution is a simple convolution of these two distributions indicating that the activation and relaxation parts of the elementary relaxation mechanism are completely independent. This characterized energy landscape can be used to explain nanocalorimetry measurements.

Figure 1

Fraction of coordination defects (mostly threefold and fivefold coordinated atoms) as a function of the configurational energy per atom (in eV) for the 4000-atom model of $a$-Si. The large circles indicate the location of the four configurations selected for further analysis. Inset: energy per atom as a function of event during the preparation of the configurations.

Figure 2

(a) Forward and (b) reverse energy-barrier distribution, and (c) asymmetry energy distribution for configurations $C1$ to $C4$.

Figure 3

Collapsed (a) forward and (b) reverse energy-barrier distributions, following the equations given in the text. (c) Asymmetry distribution for configuration $C2$ and the corresponding distribution generated by convoluting the respective forward and reverse-barrier distributions.

Figure 4

Heat flux as a function of temperature for Configurations $C1$ to $C4$ (lines) compared with the heat flux released during relaxation of ion-implanted $a$-Si samples (experimental data from Karmouch et al. [2]).