Atomistic Structure of Band-Tail States in Amorphous Silicon (original) (raw)

Band tail states and the Anderson transition in amorphous silicon

Journal of Non-Crystalline Solids, 1998

We compute approximations to the electronic states near the gap in a large and realistic model of a-Si. The spatial Ž. structure of the states are computed explicitly and discussed. The properties of the local to extended Anderson transition is described. A qualitative picture of the Anderson transition is described. Implications for conductivity and doping are briefly discussed.

Theoretical study on the nature of band-tail states in amorphous Si

Band-tail states are routinely invoked in models of a-Si:H, including defect pool models and models of light-induced defects. These models describe the band-tail states as being localized on a single stretched bond. However, to our knowledge, there is no theoretical or experimental work to justify these assumptions. In this work we use ab initio calculations to support earlier tight-binding calculations that show that the band-tail states are very delocalized-involving large numbers of atoms as the energy is varied from midgap into the tails. Our work also shows that valence-band-tail states are statistically associated with short bonds ͑not long bonds͒, and conduction-band states with long bonds. We have slightly modified a 512-atom model of a-Si due to Djordjevic, Thorpe, and Wooten ͓Phys. Rev. B 52, 5688 ͑1995͔͒ to produce a large model of a-Si:H with realistic band tails, radial distribution function, and vibrational spectrum. Above all, we created and used a model with no spectral or geometrical defects. ͓S0163-1829͑98͒03148-8͔ PHYSICAL REVIEW B 15 DECEMBER 1998-I VOLUME 58, NUMBER 23 PRB 58 0163-1829/98/58͑23͒/15624͑8͒/$15.00 15 624

The structure of electronic states in amorphous silicon

Journal of Molecular Graphics and Modelling, 1999

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.

Electronic structure and the nature of electronic states of amorphous silicon

Physics Letters A, 2001

In this Letter we present results of Monte Carlo simulation of a model of amorphous Si using an efficient tight-binding technique which gives high quality, reliable structure of amorphous Si. We present the structural and electronic properties of the model and study the nature of electronic states. The electronic states near the band edges have been found to be localized using participation numbers calculation.

Approximate ab initio calculations of electronic structure of amorphous silicon

Physical Review B, 2000

We report on ab initio calculations of electronic states of two large and realistic models of amorphous silicon generated using a modified version of the Wooten-Winer-Weaire algorithm and relaxed, in both cases, with a Keating and a modified Stillinger-Weber potentials. The models have no coordination defects and a very narrow bond-angle distribution. We compute the electronic density-of-states and pay particular attention to the nature of the band-tail states around the electronic gap. All models show a large and perfectly clean optical gap and realistic Urbach tails. Based on these results and the extended quasi-one-dimensional stringlike structures observed for certain eigenvalues in the band tails, we postulate that the generation of model a-Si without localized states might be achievable under certain circumstances.

Anderson transition and thermal effects on electron states in amorphous silicon

Journal of Non-Crystalline Solids, 2000

I discuss the properties of electron states in amorphous Si based on large scale calculations with realistic several thousand atom models. A relatively simple model for the localized to extended (Anderson) transition is reviewed. Then, the effect of thermal disorder on localized electron states is considered. It is found that under readily accessible conditions, localized (midgap or band tail) states and their conjugate energies may fluctuate dramatically. The possible importance of non-adiabatic atomic dynamics to doped or photo-excited systems is briefly discussed.

Role of defects in the electronic properties of amorphous/crystalline Si interface

Physical Review B, 2001

The mechanism determining the band alignment of the amorphous/crystalline Si heterostructures is addressed with direct atomistic simulations of the interface performed using a hierarchical combination of various computational schemes ranging from classical model-potential molecular dynamics to ab-initio methods. We found that in coordination defect-free samples the band alignment is almost vanishing and independent on interface details. In defect-rich samples, instead, the band alignment is sizeably different with respect to the defect-free case, but, remarkably, almost independent on the concentration of defects. We rationalize these findings within the theory of semiconductor interfaces.

Electronic properties of amorphous/crystalline Si interface from atomic-scale simulations

The band alignment of the amorphous/crystalline Si heterostructures is studied by means of a direct atomistic simulation of the interface morphology. To this aim we have adopted a hierarchical combination of various computational schemes, ranging from classical model-potential and tight-binding molecular dynamics to ab-initio methods. The different tools have been used to provide reliable atomistic modelling of interface structure and to evaluate electronic properties, respectively. Remarkably, the band alignment is found to be almost the same for all those samples with a non negligible concentration of coordination defects in the amorphous region, whereas it is sizeably different for a defect-free sample. The explanation of this result is ascribed to the semi-metallic behaviour of the defect-rich a-Si in the former case, and to the establishment of a ``genuine'' semiconductor/semiconductor type of interface in the latter. The role of global structural properties in the elec...

Defect transition energies and the density of electronic states in hydrogenated amorphous silicon

Journal of Non-Crystalline Solids, 2002

Using photoluminescence excitation (PLE) spectroscopy, we report detailed measurements of the fundamental absorption threshold below the optical gap in hydrogenated amorphous silicon (a-Si:H). These measurements suggest that the density of neutral defects is much greater than the densities of charged defects in intrinsic a-Si:H. The positions and widths of the corresponding transition energies are determined and agree with two models proposed to describe the density of states in a-Si:H.

Structure and physical properties of paracrystalline atomistic models of amorphous silicon

Journal of Applied Physics, 2001

We have examined the structure and physical properties of paracrystalline molecular dynamics models of amorphous silicon. Simulations from these models show qualitative agreement with the results of recent mesoscale fluctuation electron microscopy experiments on amorphous silicon and germanium. Such agreement is not found in simulations from continuous random network models. The paracrystalline models consist of topologically crystalline grains which are strongly strained and a disordered matrix between them. We present extensive structural and topological characterization of the medium range order present in the paracrystalline models and examine their physical properties, such as the vibrational density of states, Raman spectra, and electron density of states. We show by direct simulation that the ratio of the transverse acoustic mode to transverse optical mode intensities I TA /I TO in the vibrational density of states and the Raman spectrum can provide a measure of medium range order. In general, we conclude that the current paracrystalline models are a good qualitative representation of the paracrystalline structures observed in the experiment and thus provide guidelines toward understanding structure and properties of medium-range-ordered structures of amorphous semiconductors as well as other amorphous materials.