At the high temperatures during hot-wire assisted chemical vapor deposition, ther- mal emission of electrons from the filament occurs. We studied the effect of filament bias, and thus the filament-to-substrate current, on the structural, electronic and optical properties of amorphous and nanocrystalline silicon deposited by this method. The current drawn by the substrate can be varied by many orders of magnitude as thermally emitted electrons are increasingly collected with applied bias voltage. The crystallinity of the nanocrystalline samples is not affected by the bias voltage. The defect density in amorphous silicon is affected by the electron bombardment at high bias voltage only, for which we also find a significant reduction in the mobility-lifetime product from steady-state photoconductivity.
GeS2-CdSe superlattices and composite films are prepared by consecutive thermal evaporation of CdSe and GeS2 in vacuum. CdSe layer thickness varies between 1 and 10 nm, while the thickness of GeS2 layers is either equal (in superlattices) to or 20 times greater (in composite films) than that of CdSe layers. Standard spectral photocurrent measurements and various constant photocurrent methods are used to study optical absorption of all samples. An overall blueshift is observed with decreasing CdSe layer thickness of superlattices. This shift is related to a size-induced increase of the optical band gap of CdSe due to one-dimensional carrier confinement in the continuous nanocrystalline CdSe layers. A number of features are observed in the absorption spectra of composite films containing CdSe nanocrystals with average radii of approximately 2.5 and approximately 3.3 nm. They are discussed in terms of three-dimensional carrier confinement and are considered a manifestation of excited electron states in CdSe nanocrystals embedded in GeS2 thin film matrix. In addition to these discrete features, the exponential dependence of the optical absorption (Urbach) edge indicates a distribution of "valence band" tail states associated with disorder. Transient photoconductivity measurements made on similarly prepared SiOx-CdSe superlattices exhibit a rapid fall in photocurrent by a power law decay over several orders of magnitude of time, which is consistent with multi-pletrapping transport via an extensive distribution of deep defects.
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