Finite-temperature properties of amorphous silicon

The Electronic Disorder Landscape of Mixed Halide Perovskites

Band gap tunability of lead mixed halide perovskites makes them promising candidates for various applications in optoelectronics. Here we use the localization landscape theory to reveal that the static disorder due to iodide:bromide compositional alloying contributes at most 3 meV to the Urbach energy. Our modeling reveals that the reason for this small contribution is due to the small effective masses in perovskites, resulting in a natural length scale of around 20 nm for the "effective confining potential" for electrons and holes, with short-range potential fluctuations smoothed out. The increase in Urbach energy across the compositional range agrees well with our optical absorption measurements. We model systems of sizes up to 80 nm in three dimensions, allowing us to accurately reproduce the experimentally observed absorption spectra of perovskites with halide segregation. Our results suggest that we should look beyond static contribution and focus on the dynamic temperature dependent contribution to the Urbach energy.

Next generation of the self-consistent and environment-dependent Hamiltonian: Applications to various boron allotropes from zero- to three-dimensional structures

An upgrade of the previous self-consistent and environment-dependent linear combination of atomic orbitals Hamiltonian (referred as SCED-LCAO) has been developed. This improved version of the semi-empirical SCED-LCAO Hamiltonian, in addition to the inclusion of self-consistent determination of charge redistribution, multi-center interactions, and modeling of electron-electron correlation, has taken into account the effect excited on the orbitals due to the atomic aggregation. This important upgrade has been subjected to a stringent test, the construction of the SCED-LCAO Hamiltonian for boron. It was shown that the Hamiltonian for boron has successfully characterized the electron deficiency of boron and captured the complex chemical bonding in various boron allotropes, including the planar and quasi-planar, the convex, the ring, the icosahedral, and the fullerene-like clusters, the two-dimensional monolayer sheets, and the bulk alpha boron, demonstrating its transferability, robustness, reliability, and predictive power. The molecular dynamics simulation scheme based on the Hamiltonian has been applied to explore the existence and the energetics of ∼230 compact boron clusters BN with N in the range from ∼100 to 768, including the random, the rhombohedral, and the spherical icosahedral structures. It was found that, energetically, clusters containing whole icosahedral B12 units are more stable for boron clusters of larger size (N > 200). The ease with which the simulations both at 0 K and finite temperatures were completed is a demonstration of the efficiency of the SCED-LCAO Hamiltonian.

First-principles calculations of the Urbach tail in the optical absorption spectra of silica glass

We present density-functional theory calculations of the optical absorption spectra of silica glass for temperatures up to 2400 K. The calculated spectra exhibit exponential tails near the fundamental absorption edge that follow the Urbach rule in good agreement with experiments. We discuss the accuracy of our results by comparing to hybrid exchange correlation functionals. We show that the Urbach rule holds in a frequency interval where optical absorption is Poisson distributed with very large statistical fluctuations. In this regime, a direct relation between the optical absorption coefficient and electronic density of states is derived, which provides a link between photoemission and absorption spectra and is used to determine the lower bound to the Urbach frequency regime.

Comparison of the Kubo formula, the microscopic response method, and the Greenwood formula

For a mechanical perturbation, the microscopic response method is equivalent to and more convenient to use than the Kubo formula. When the gradient of the carrier density is small, the current density reduces to that used by Greenwood.

Materials modeling by design: applications to amorphous solids

In this paper, we review a host of methods used to model amorphous materials. We particularly describe methods which impose constraints on the models to ensure that the final model meets a priori requirements (on structure, topology, chemical order, etc). In particular, we review work based on quench from the melt simulations, the 'decorate and relax' method, which is shown to be a reliable scheme for forming models of certain binary glasses. A 'building block' approach is also suggested and yields a pleading model for GeSe(1.5). We also report on the nature of vulcanization in an Se network cross-linked by As, and indicate how introducing H into an a-Si network develops into a-Si:H. We also discuss explicitly constrained methods including reverse Monte Carlo (RMC) and a novel method called 'Experimentally Constrained Molecular Relaxation'. The latter merges the power of ab initio simulation with the ability to impose external information associated with RMC.

The structure of electronic states in amorphous silicon

We illustrate the structure and dynamics of electron states in amorphous Si. The nature of the states near the gap at zero temperature is discussed and especially the way the structure of the states changes for energies ranging from midgap into either band tail (Anderson transition). We then study the effect of lattice vibrations on the eigenstates, and find that electronic states near the optical gap can be strongly influenced by thermal modulation of the atomic positions. Finally, we show the structure of generalized Wannier functions for amorphous Si, which are of particular interest for efficient ab initio calculation of electronic properties and forces for first principles dynamic simulation.