Date of Award

Summer 2019

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

School

Mathematics and Natural Sciences

Committee Chair

Parthpratim Biswas

Committee Chair School

Mathematics and Natural Sciences

Committee Member 2

Chris Winstead

Committee Member 2 School

Mathematics and Natural Sciences

Committee Member 3

Khin Maung Maung

Committee Member 3 School

Mathematics and Natural Sciences

Committee Member 4

Ras B. Pandey

Committee Member 4 School

Mathematics and Natural Sciences

Committee Member 5

Gopinath Subramanian

Committee Member 5 School

Polymer Science and Engineering

Abstract

The microstructure of voids in pure and hydrogen-rich amorphous silicon (a:Si) network was studied in ultra-large models of amorphous silicon, using classical and quantum- mechanical simulations, on the nanometer length scale. The nanostructure, particularly voids of device grade ultra-large models of a:Si was studied, in which observed three-dimensional realistic voids were extended using geometrical approach within the experimental limit of void-volume fractions. In device-grade simulated models, the effect of void morphology; size, shape, number density, and distribution on simulated scattering intensities in small- angle region were investigated. The evolution of voids on annealing below the crystallization temperature (≤ 800 K) was examined, where the extent of the void reconstruction was reported by using high-quality three-dimensional rendering software and calculating an average size and volume of the voids. Additionally, the role of bonded and non-bonded hydrogens near the vicinity of the void’s wall in a:Si network was observed.

Our simulated results suggested that, in extended void structures, X-ray scattering intensities in the small- angle region were sensitive to the number density, size, shape and the distribution of the voids in unequal strength. In both classical and local ab initio molecular dynamics models of a:Si, the reconstruction of the voids were observed but in later models, with and without present hydrogen reconstruction effect was observed greater. The distribution and dynamics of bonded and non-bonded hydrogen in heavily hydrogenated (≥ 14 at.%) ultra-large models of a:Si suggested that, void’s wall were decorated with more silicon dihydride (SiH2) bonds and 9-13% of the total H were realized as molecular hydrogen (H2) respectively from 300 K- 800 K annealing temperature. This work suggested that, a:Si sample with≥14 at.% H and ≤ 0.2% volume-fraction of voids, may be appropriate for interface hydrogenated amorphous silicon/crystalline silicon (a:Si:H/c-Si) material used in heterojunction silicon solar cell to obtain the better-passivated surface due to the presence of mobile non-bonded hydrogens.

ORCID ID

0000-0003-1677-4601

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