Morphological Study of Voids in Ultra-Large Models of Amorphous Silicon (original) (raw)

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 quantummechanical 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 smallangle region were investigated. The evolution of voids on annealing below the crystallization temperature (≤ 800 K) were examined, where the extent of the void reconstruction were 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 smallangle 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 (SiH 2) bonds and 9-13% of the total H were realized as molecular hydrogen (H 2) 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 presence of mobile non-bonded hydrogens. ii ACKNOWLEDGMENTS I would sincerely like to thank my advisor and committee chair Prof. Parthapratim Biswas for his continuous guidance, tireless support and excellent mentorship for successful completion of my P.hD. at the University of Southern Mississippi. I would especially like to thank Prof. Chris Winstead who also has remarkable influence on my academic as being my MS research project advisor and Ph.D. dissertation committee member. Also, I would like to thank other committee members Prof. Ras B. Pandey, Prof. Khin Maung Maung and Prof. Gopinath Subramanian, who have provided valuable suggestion and comments for completion of my P.hD. Most importantly, I would like to thank my collaborators Dr. Raymond Atta-Fynn for providing Texas supercomputing facility, Prof. David Drabold and Prof. Stephen Elliott for their continuous feedback and valuable correction of my work. Also, I would like to extend my acknowledgement to the School of Mathematics and Natural Sciences, all physics faculties and staffs for providing me the opportunity to be a student and pursue my graduate study in this institution. I would like to thank National Science Foundation for partially funding financial support during my P.hD. It will not be enough without thanking my beloved parents Lilawoti Paudel (Grandmother), Nanda Lal Paudel (Father) and Ranjti Paudel (Mother) for their moral and financial support, love and care and faith and wishes on me. And, I would also like to thank my brother Dr. Trilochan Paudel for his effort to motivate me in Physics and my spouse Sunita Subedi Paudel for family support, unconditional love and inspiration for working hard. I would like to thank my little son Denish Paudel for his understanding in every situation during P.hD. on his age. Finally, I would like to thank my friend Dil Limbu and his family for their great help during my stay. Finally, I would like to extend my gratitude to my other family members, sisters and brother in law who have always been on my side during my P.hD. journey. iii