Could Porosity Induce Gaps in the Vibrational Density of States of Nanoporous Silicon?
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Could Porosity Induce Gaps in the Vibrational Density of States of Nanoporous Silicon? Juan Carlos Noyola1, Alexander Valladares1, R. M. Valladares1 and Ariel A. Valladares2 ([email protected]) 1 Facultad de Ciencias, Universidad Nacional Autónoma de México. Apartado Postal 70-542, México D. F. 04510, MEXICO. 2 Instituto de Investigaciones en Materiales, Universidad Nacional Autónoma de México. Apartado Postal 70-360, México D. F. 04510, MEXICO. ABSTRACT As in our previous work [1] nanoporous silicon periodic supercells with 1000 atoms but now with 80 % porosity were constructed using the Tersoff potential and our novel approach [2]. The approach consists first in constructing a crystalline diamond-like supercell with a density (volume) close to the real value, and then lowering the density by increasing the volume, subjecting the resulting periodic supercell to Tersoff-based molecular dynamics processes at a temperature of 300 K, followed by geometry relaxation [1]. As in the ab initio approach [2] the resulting samples are also essentially amorphous and display pores along some of the crystallographic directions. We report the radial (pair) distribution function (RDF), g(r), the bond angle distribution, the pore structure where prominent and a computational prediction for the vibrational density of states for this structure. We then compare it to the 50 % porous sample presented in Ref [1]. The soft acoustic phonons are displaced towards lower energy in the 80 % porosity sample whereas the optical modes are displaced towards higher energies. The pseudo gap, existing in the 50 % porous sample, is depleted even more in the 80 % sample indicating a tendency towards the creation of a phonon gap for higher porosity materials. Some conjectures that point to the possible engineering of porous materials to produce predetermined phonon properties are discussed. INTRODUCTION Controlled defective silicon is of extreme importance in the development of electronic devices and therefore detailed knowledge of its microstructure is fundamental since this determines the electric and optical properties on which these various technical applications are based. Whereas crystalline silicon, both pure and doped, has been decisive in the development of the electronic industry, amorphous silicon (aSi) has been experimentally studied for only a few decades, whereas porous silicon (pSi) on the other hand has not received the same broad attention from researchers, except for its luminescent properties. Some of the experimental results available on the microstructure of aSi are limited to structure factors measured using Xrays [3] or neutron diffraction [4-6] experiments. The Fourier transform of the structure factor yields the radial distribution function (RDF) which contains only one-dimensional information, it is therefore necessary to construct models that will help distinguish the three-dimensional atomic structures consistent with experimental results. Molecular dynamics is a useful tool to generate computational models that
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