We propose to analyze ellipsometry data by using effective medium approximation (EMA) models. Thanks to EMA, having nanocrystalline reference dielectric functions and generalized critical point (GCP) model the physical parameters of two series of samples containing silicon nanocrystals, i.e. silicon rich oxide (SRO) superlattices and porous silicon layers (PSL), have been determined. The superlattices, consisting of ten SRO/SiO2 layer pairs, have been prepared using plasma enhanced chemical vapor deposition. The porous silicon layers have been prepared using short monopulses of anodization current in the transition regime between porous silicon formation and electropolishing, in a mixture of hydrofluoric acid and ethanol. The optical modeling of both structures is similar. The effective dielectric function of the layer is calculated by EMA using nanocrystalline components (nc-Si and GCP) in a dielectric matrix (SRO) or voids (PSL). We discuss the two major problems occurring when modeling such structures: (1) the modeling of the vertically non-uniform layer structures (including the interface properties like nanoroughness at the layer boundaries) and (2) the parameterization of the dielectric function of nanocrystals. We used several techniques to reduce the large number of fit parameters of the GCP models. The obtained results are in good agreement with those obtained by X-ray diffraction and electron microscopy. We investigated the correlation of the broadening parameter and characteristic EMA components with the nanocrystal size and the sample preparation conditions, such as the annealing temperatures of the SRO superlattices and the anodization current density of the porous silicon samples. We found that the broadening parameter is a sensitive measure of the nanocrystallinity of the samples, even in cases, where the nanocrystals are too small to be visible for X-ray scattering. Major processes like sintering, phase separation, and intermixing have been revealed as a function of annealing of the SRO superlattices. © 2010 Elsevier B.V. All rights reserved.

In this study we investigate the electronic transport, the optical properties, and photocurrent in two-dimensional arrays of silicon nanocrystals (Si NCs) embedded in silicon dioxide, grown on quartz and having sizes in the range between less than 2 and 20 nm. Electronic transport is determined by the collective effect of Coulomb blockade gaps in the Si NCs. Absorption spectra show the well-known upshift of the energy bandgap with decreasing NC size. Photocurrent follows the absorption spectra confirming that it is composed of photo-generated carriers within the Si NCs. In films containing Si NCs with sizes less than 2 nm, strong quantum confinement and exciton localization are observed, resulting in light emission and absence of photocurrent. Our results show that Si NCs are useful building blocks of photovoltaic devices for use as better absorbers than bulk Si in the visible and ultraviolet spectral range. However, when strong quantum confinement effects come into play, carrier transport is significantly reduced due to strong exciton localization and Coulomb blockade effects, thus leading to limited photocurrent. © 2011 Gardelis et al.

In this work, we study theoretically the resonant coupling between longitudinal optical surface vibrations of Si-OH and/or Si-O-Si and electron and hole states in the silicon nanocrystals (Si NCs) within light emitting porous Si (PSi) thin films in the framework of the Fröhlich interaction. The results of this analysis are compared with experimental results, which show considerable enhancement and a redshift of the photoluminescence (PL) spectrum of a fresh as-grown PSi thin film after prolonged laser irradiation or after aging in air. These effects coincide with the formation of Si-OH and Si-O-Si bonds on the surface of PSi. The redshift of the PL spectrum is due to the pinning of the bandgap of the light emitting Si NCs, as both oxidation via laser irradiation in air and aging in air introduce energy states in the Si NC band gaps. According to the theoretical analysis, the PL enhancement is assigned to inhibition of nonradiative channels rather than to an enhancement of radiative channels in the Si NCs within the PSi film, due to a strong coupling of the surface Si-OH and/or Si-O-Si vibrational modes to the electronic sublevels in the Si NCs within the PSi layer. © 2011 American Institute of Physics.

We report on the lateral transport in a single two-dimensional (2D) array of Si nanocrystals of different sizes grown by low pressure chemical vapor deposition (LPCVD) of silicon on a quartz substrate and subsequent oxidation at high temperature. The initial nanocrystal size in the z-direction was 5 nm, while it was reduced to ∼3 nm after oxidation. The nanocrystals in the x-y plane were connected by grain boundaries and/or by very thin silicon oxide barriers, while a thin oxide layer was formed on their surface. The electrical measurements showed that current in the film is mainly governed by thermionic emission over the barriers (grain boundaries or dielectric barriers) at high temperatures and by tunneling at lower temperatures. Charge traps at the interfaces of the silicon nanocrystals with the oxide and at the grain boundaries cause considerable hysteresis in the current-voltage characteristics. Hydrogen passivation of the charge traps reduces considerably the hysteresis effect and the activation energy of the thermionic emission, while revealing a clear Coulomb gap. © 2011 American Institute of Physics.