Energy landscape of relaxed amorphous silicon (original) (raw)
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Activated mechanisms in amorphous silicon: An activation-relaxation-technique study
2000
At low temperatures, dynamics in amorphous silicon occurs through a sequence of discrete activated events that locally reorganize the topological network. Using the activation-relaxation technique, a data base containing over 8000 such events is generated, and the events are analyzed with respect to their energy barrier and asymmetry, displacement and volume expansion/contraction. Special attention is paid to those events corresponding to diffusing coordination defects. The energetics is not clearly correlated with the displacement, nor with the defect density in well relaxed configurations. We find however some correlation with the local volume expansion: it tends to increase by about 4 eV/Å 3. The topological properties of these events are also studied; they show an unexpectedly rich diversity.
MRS Proceedings, 2001
The activation-relaxation technique (ART) is a method for finding saddle points in high-dimensional energy landscapes. ART has already been applied to a wide range of materials including amorphous semiconductors, Lennard-Jones glasses, and proteins. In spite of its successes, a number of fundamental questions remain to be answered regarding the biases associated with its sampling of the saddle points. We present here results of a detailed analysis of the biases in the simulation of amorphous silicon. We focus in particular on the biases of the method in sampling saddle points, the completeness of the sampling and the sensitivity of these quantities to variations of the different parameters. Ý Present address: Département de physique, Université de Montréal, C.P. 6128 succ. Centre-ville, Montréal
Basic mechanisms of structural relaxation and diffusion in amorphous silicon
MRS Proceedings, 2001
ABSTRACTThe low-temperature dynamics in amorphous silicon occurs through a sequence of discrete, activated events that reorganize the topology of the network. In this review, we present some recent work done to understand better the nature of these events and the associated dynamics ina-Si. Using the activation-relaxation technique (ART), we generated more than 8000 events in a 1000-atom model ofa-Si, providing an extensive database of relaxation and diffusion mechanisms. The generic properties of these events, such as the number of involved atoms and the activation energies, were investigated and foundto be in agreement with experimental data. As it turns out, the bond-transposition mechanism proposed by Wooten, Winer and Weaire (WWW) some time ago plays an important role in the events generated by ART. We have therefore turned to an optimized version of the WWW algorithm to generate the best overall configurations ofa-Si available today. We discuss the details of the optimization ...
Identification of Relaxation and Diffusion Mechanisms in Amorphous Silicon
Physical Review Letters, 1998
The dynamics of amorphous silicon at low temperatures can be characterized by a sequence of discrete activated events, through which the topological network is locally reorganized. Using the activation-relaxation technique, we create more than 8000 events, providing an extensive database of relaxation and diffusion mechanisms. The generic properties of these events-size, number of atoms involved, activation energy, etc.-are discussed and found to be compatible with experimental data. We introduce a topological classification of events and apply it to study those events involving only fourfold coordinated atoms. For these, we identify and present in detail three dominant mechanisms.
Evolution of the Potential-Energy Surface of Amorphous Silicon
Physical Review Letters, 2010
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.
Numerical Studies Of The Dynamics Of Silicon: Relaxation, Nucleation And Energy Landscape
2000
Using various simulation techniques, such as molecular dynamics and the activation- relaxation technique, we are slowly developing a consistent picture of the dynamical prop- erties of amorphous silicon. For example, results of an extensive search for the activated events surrounding a single minimum, in a well-relaxed model represented by a modied Stillinger-Weber potential, conrm that barrier height at the transition
Experimentally constrained molecular relaxation: The case of hydrogenated amorphous silicon
Physical Review B, 2007
We have extended our experimentally constrained molecular relaxation technique ͓P. Biswas et al., Phys. Rev. B 71, 54204 ͑2005͔͒ to hydrogenated amorphous silicon: a 540-atom model with 7.4% hydrogen and a 611-atom model with 22% hydrogen were constructed. Starting from a random configuration, using physically relevant constraints, ab initio interactions, and the experimental static structure factor, we construct realistic models of hydrogenated amorphous silicon. Our models confirm the presence of a high-frequency localized band in the vibrational density of states due to Si-H vibration that has been observed in recent vibrational transient grating measurements on plasma enhanced chemical vapor deposited films of hydrogenated amorphous silicon.
2018
We have extended our experimentally constrained molecular relaxation technique ͓P. Biswas et al., Phys. Rev. B 71, 54204 ͑2005͔͒ to hydrogenated amorphous silicon: a 540-atom model with 7.4% hydrogen and a 611-atom model with 22% hydrogen were constructed. Starting from a random configuration, using physically relevant constraints, ab initio interactions, and the experimental static structure factor, we construct realistic models of hydrogenated amorphous silicon. Our models confirm the presence of a high-frequency localized band in the vibrational density of states due to Si-H vibration that has been observed in recent vibrational transient grating measurements on plasma enhanced chemical vapor deposited films of hydrogenated amorphous silicon.
Physical Review B, 2013
The nature of structural relaxation in disordered systems such as amorphous silicon (a-Si) remains a fundamental issue in our attempts at understanding these materials. While a number of experiments suggest that mechanisms similar to those observed in crystals, such as vacancies, could dominate the relaxation, theoretical arguments point rather to the possibility of more diverse pathways. Using the kinetic activation-relaxation technique, an off-lattice kinetic Monte Carlo method with on-the-fly catalog construction, we resolve this question by following 1000 independent vacancies in a well-relaxed a-Si model at 300 K over a timescale of up to one second. Less than one percent of these survive over this period of time and none diffuse more than once, showing that relaxation and diffusion mechanisms in disordered systems are fundamentally different from those in the crystal.
Acta Materialia, 2009
The structural relaxation of amorphous materials is described as arising from the superposition of elementary processes with varying activation energies. We show that it is possible to obtain the kinetic parameters of these processes from differential scanning calorimetry experiments. The transformation rate is predicted for the transient decay when an isotherm is reached and for the relaxation threshold detected in partially relaxed samples.