Impact of AFM-induced nano-pits in a-Si:H films on silicon crystal growth

Conductive tips in atomic force microscopy (AFM) can be used to localize field-enhanced metal-induced solid-phase crystallization (FE-MISPC) of amorphous silicon (a-Si:H) at room temperature down to nanoscale dimensions. In this article, the authors show that such local modifications can be used to selectively induce further localized growth of silicon nanocrystals. First, a-Si:H films by plasma-enhanced chemical vapor deposition on nickel/glass substrates are prepared. After the FE-MISPC process, yielding both conductive and non-conductive nano-pits in the films, the second silicon layer at the boundary condition of amorphous and microcrystalline growth is deposited. Comparing AFM morphology and current-sensing AFM data on the first and second layers, it is observed that the second deposition changes the morphology and increases the local conductivity of FE-MISPC-induced pits by up to an order of magnitude irrespective of their prior conductivity. This is attributed to the silicon nanocrystals (<100 nm) that tend to nucleate and grow inside the pits. This is also supported by micro-Raman spectroscopy

Synthesis, structure, and opto-electronic properties of organic-based nanoscale heterojunctions

Enormous research effort has been put into optimizing organic-based opto-electronic systems for efficient generation of free charge carriers. This optimization is mainly due to typically high dissociation energy (0.1-1 eV) and short diffusion length (10 nm) of excitons in organic materials. Inherently, interplay of microscopic structural, chemical, and opto-electronic properties plays crucial role. We show that employing and combining advanced scanning probe techniques can provide us significant insight into the correlation of these properties. By adjusting parameters of contact- and tapping-mode atomic force microscopy (AFM), we perform morphologic and mechanical characterizations (nanoshaving) of organic layers, measure their electrical conductivity by current-sensing AFM, and deduce work functions and surface photovoltage (SPV) effects by Kelvin force microscopy using high spatial resolution. These data are further correlated with local material composition detected using micro-Raman spectroscopy and with other electronic transport data. We demonstrate benefits of this multi-dimensional characterizations on (i) bulk heterojunction of fully organic composite films, indicating differences in blend quality and component segregation leading to local shunts of photovoltaic cell, and (ii) thin-film heterojunction of polypyrrole (PPy) electropolymerized on hydrogen-terminated diamond, indicating covalent bonding and transfer of charge carriers from PPy to diamond

Hot Electrons in Amorphous Silicon

At extremely high electric fields (F≤0.55 MV/cm) and high temperatures (300Ta-Si:H) is obtained and therefore the shallow trapping is substantially reduced. New data clearly demonstrate that the free electron (band) mobility in a-Si:H decreases when the electric field increases, contrary to other disordered materials (e.g., amorphous selenium). In this sense the free carrier transport in a-Si:H is similar to the hot carriers in crystals when phonon scattering prevails

Optical absorption and light scattering in microcrystalline silicon thin films and solar cells

Optical characterization methods were applied to a series of microcrystalline silicon thin films and solar cells deposited by the very high frequency glow discharge technique. Bulk and surface light scattering effects were analyzed. A detailed theory for evaluation of the optical absorption coefficient α from transmittance, reflectance and absorptance (with the help of constant photocurrent method) measurements in a broad spectral region is presented for the case of surface and bulk light scattering. The spectral dependence of α is interpreted in terms of defect density, disorder, crystalline/amorphous fraction and material morphology. The enhanced light absorption in microcrystalline silicon films and solar cells is mainly due to a longer optical path as the result of an efficient diffuse light scattering at the textured film surface. This light scattering effect is a key characteristic of efficient thin-film-silicon solar cells

On the transport properties of microcrystalline silicon

To determine the charge collection mechanism in hydrogenated microcrystalline silicon (μc-Si:H) solar cells, we have measured the electronic transport properties of μc-Si:H by time-of-flight and by ac capacitance and conductance on a unique 5.6 μm thick sample. We found the electron drift mobility μD=2.8±0.2 cm2 V−1 s−1, thermally activated with EA=0.14±0.1 eV. Evidence for field inhomogeneity was observed as an initial maximum of the photocurrent transients and as an increase of capacitance over the geometrical value. The frequency dependence of the capacitance exhibits marked differences from a-Si:H and is proposed as a tool for studying the effects of microstructure on electronic properties. Changes of the sample capacitance with temperature and illumination were observed. As a consequence of the inhomogeneity of the material, several different activation energies were found: 0.14 eV for electron drift mobility, 0.29 eV for ac conductivity, 0.4 eV for steady state dark conductivity and finally ≥0.8 eV for the photocapacitance relaxation

Ultrafast Bimolecular Recombination in Nanocrystalline Hydrogenated Silicon

In the multilayers of hydrogenated nanocrystalline and amorphous silicon bimolecular recombination coefficient can be reduced in half, while in low-temperature hydrogenated nanocrystalline silicon samples it can be reduced by one order of magnitude. The similarity of the activation energies of both the bimolecular recombination (B) and the Langevin-type recombination (B$\text{}_{L}$) coefficients point to decisive role of tunneling in processes of meeting of electrons and holes, although the ratio B/B$\text{}_{L}$<0.01