Title: Structure of Surfaces and Voids in Amorphous Silicon  

Speaker: Martin Z. Bazant

Authors: Martin Z. Bazant (Dept. of Mathematics, MIT),
  Efthimios Kaxiras (Dept. of Physics, Harvard University),
  D. Papaconstantopoulos and M. Mehl
  (Complex Systems Branch, Naval Research Lab, Washington DC)

MRS Fall Meeting, Symposium J, December 3, 1998

Abstract: Little is known experimentally about the structure of amorphous
semiconductor surfaces and voids and their effect on mechanical and
electronic properties.  Reliable theoretical predicitions are hampered
by the fact that the system sizes (> 200 atoms)
and simulation times (> 1 ns) required are prohibitively large
for quantum-mechanical methods, while at the same time existing
empirical methods are incapable of describing amorphous structures
with sufficient realism. In the case of silicon, we address these
difficulties with a hybrid approach. Using our Environment-Dependent
Interatomic Potential (EDIP), which provides a remarkably realistic
description of a-Si, we create surface and void samples
by direct quench of the liquid through a first-order
phase transition. The smaller samples (216-atoms) are validated by
relaxing all atoms with forces from density functional theory in the
local density approximation (LDA); we find that there is no significant
relaxation and the structure does not change qualitatively. The
electronic band structure of the various samples is then
computed within the tight binding approximation (TBA).  We also consider
very large samples (up to 51,200 atoms) using EDIP to study
long-wavelength fluctuations.  In contrast to the surfaces of c-Si
which tend to be undercoordinated ($Z < 4$), we find that the a-Si
surface consists of a 5-7 Angstrom thick layer of 60--80\% $Z = 4$ and
20--40\% $Z = 5$ atoms, while the voids contain a few $Z = 3$ atoms.