Atomistic models of hydrogenated amorphous silicon nitride from first principles

Abstract

We present a theoretical study of hydrogenated amorphous silicon nitride ͑a-SiN x :H͒, with equal concentrations of Si and N atoms ͑x =1͒, for two considerably different densities ͑2.0 and 3.0 g / cm 3 ͒. Densities and hydrogen concentration were chosen according to experimental data. Using first-principles molecular-dynamics within density-functional theory the models were generated by cooling from the liquid. Where both models have a short-range order resembling that of crystalline Si 3 N 4 because of their different densities and hydrogen concentrations they show marked differences at longer length scales. The low-density nitride forms a percolating network of voids with the internal surfaces passivated by hydrogen. Although some voids are still present for the high-density nitride, this material has a much denser and uniform space filling. The structure factors reveal some tendency for the nonstoichiometric high-density nitride to phase separate into nitrogen rich and poor areas. For our slowest cooling rate ͑0.023 K/fs͒ we obtain models with a modest number of defect states, where the low ͑high͒ density nitride favors undercoordinated ͑overcoordinated͒ defects. Analysis of the structural defects and electronic density of states shows that there is no direct one-to-one correspondence between the structural defects and states in the gap. There are several structural defects that do not contribute to in-gap states and there are in-gap states that do only have little to no contributions from ͑atoms in͒ structural defects. Finally an estimation of the size and cooling rate effects on the amorphous network is reported.

Effect of topological disorder on structural, mechanical, and electronic properties of amorphous silicon nitride: An atomistic study

“…In addition to undercoordinated and over-coordinated defects, a number of ab initio derived models report the presence of wrong bonds (N-N or Si-Si bonds) 30,31,35 based on a geometric criterion, these are commonly reported in edge-sharing tetrahedron. Also, recent MD simulations with empirical potentials find shorter N-N bonds and postulate them to explain a small peak in the neutron diffraction experiments.…”

“…On the other hand, theoretical calculations based on classical and ab initio molecular dynamics (MD) and Monte Carlo methods have been used to model a-Si 3 N 4 and its variants. [27][28][29][30][31][32][33][34][35][36] While structural, electronic, and vibrational properties based on these models have been extensively reported, comprehensive models capturing the effect of inherent local variability in the atomic structure and topological disorder of a-Si 3 N 4 are still lacking. Consequently, theoretical predictions of trap depths based on these models report few distinct levels in the band-gap, as opposed to a range of values as expected from the atomic variability of the amorphous network 37 and reported in recent TSCIS measurements.…”

Quantum confinement and band offsets in amorphous silicon quantum wells

“…Fluctuations in the band edges (especially in the conduction band) of silicon nitride are present. They originate from the inhomogeneities that are intrinsic to the silicon nitride with this particular composition and density [52].…”

“…A model of the QWs was constructed by merging cells of a-Si:H and a-SiN:H. The cells were prepared previously with the "cooling from liquid" method [51,52]. This is a common method that simulates rapid cooling of a melt, a process similar to the formation of glasses.…”

“…Artificial Si-Si bonds inside "square structures" are discarded from the count (see Ref. [52] for details). The column denoted by D tot contains the total number of structural defects.…”