Structure and physical properties of paracrystalline atomistic models of amorphous silicon

Abstract

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.

Aluminum nanoscale order in amorphous Al92Sm8 measured by fluctuation electron microscopy

“…2,3,15 At a given annealing temperature, some of those clusters are supercritical and grow into stable crystals, which predicts the observed decrease in nucleation rate under isothermal annealing. 4 Figure 2 shows that the degree of MRO, indicated by the height of the peaks, 16 is reduced by the same isothermal anneal. If the sample is crystallizing, why do we measure less order?…”

Increasing medium-range order in amorphous silicon with low-energy ion bombardment

“…This effect has been predicted by simulations of the vibrational densities of state as a function of paracrystallite size and density in computer models. 2 We observe no evidence of height or width variations in the TO peak, indicating that the short range order is not changing. In general, the TA/TO ratio, often used to estimate short range order, varies because the width of the TO band varies with bond angle distribution, simultaneously changing the TO peak height; the TA peak remains constant.…”

“…In FEM the statistical variance, V, of dark-field hollowcone transmission electron microscopy ͑TEM͒ images taken at a MRO-scale ͑1.5 nm͒ microscope resolution is measured as a function of the diffraction vector magnitude k. All simulations performed to date 2,3 indicate that V(k) and MRO are monotonically related and that a fine-grained, highly strained structure termed ''paracrystalline'' provides the best match to FEM data, whereas continuous random networks fail to do so. 4 We hypothesize that the paracrystalline structure is produced during growth of a-Si via a frustrated polycrystalline growth surface: at low substrate temperatures (T sub ), crystalline nuclei are produced on the growth surface but are quickly buried by subsequent nucleation events.…”

“…5 Initial simulations performed to probe the effect of paracrystallite sizes on FEM also demonstrated changes in the vibrational and electronic densities of states. 2,3 Therefore, in addition to FEM, we probe the the vibrational and electronic densities of states via Raman scattering and spectroscopic ellipsometry ͑SE͒.…”

Thermal crystallization of sputter-deposited amorphous Ge films: Competition of diamond cubic and hexagonal phases

“…9 The MRO clusters introduced by vapor deposition are probably distorted, 26 and are thus named d-MRO. A noteworthy characteristic of the PC model is that large strains are stored both within the d-MRO clusters themselves and also between each cluster and the CRN matrix.…”