Control of hydrogenation and modulation of the structural network in Si:H by interrupted growth and H-plasma treatment

Autogenic single p/n-junction solar cells from black-Si nano-grass structures of p-to-n type self-converted electronic configuration

Photovoltaic performance of solar cells automatically improves when the absorber layer itself simultaneously acts as the anti-reflection nanostructure with an enhanced active absorber area on the front surface. Combined physical and chemical etching of p-c-Si wafers by (Ar + H2) plasma in inductively coupled low-pressure plasma CVD produces various nanostructures with subsequent minimization of reflectance. At a reduced temperature, the rate constant of thermal diffusion of atomic-H in the Si-network becomes smaller, leading to enhanced chemical etching reactions that further increase at an elevated RF power. Regrowth of the SiHn precursors produced by etching and subsequent hydrogenation in the plasma develops a high density of elongated nano-grass structures, which further align with sharp tips via Ar+ ion bombardment and elimination of loosely bound amorphous over-layers, on application of negative dc substrate bias during real-time etching and regrowth. A significantly reduced reflectance (∼0.5%) via coherent light trapping within the uniformly distributed vertically aligned nano-grass surfaces evolves truly black-silicon (b-Si) nanostructures, which further self-convert from the p-type to n-type electronic configuration via etching-mediated modification of B-H bonds from BH1 to BH2 and/or BH3 states, producing autogenic p/n junctions. Using (Ar + H2) plasma etched b-Si nano-grass structures at low temperature (∼200 °C), one-step fabrication of autogenic single p/n-junction proof-of-concept solar cells is accomplished. There is plenty of room for further progress in device performance.

An investigation on the effect of high partial pressure of hydrogen on the nanocrystalline structure of silicon carbide thin films prepared by radio-frequency magnetron sputtering

The aim of the study reported in this paper is to investigate the role of the high partial pressure of hydrogen introduced during the growth of nanocrystalline silicon carbide thin films (nc-SiC:H). For this purpose, we report the preparation as well as spectroscopic studies of four series of nc-SiC:H obtained by radio-frequency magnetron sputtering at high partial pressure of hydrogen by varying the percentage of H2 in the gas mixture from 70% to 100% at common substrate temperature (TS=500°C). The effects of the dilution on the structural changes and the chemical bonding of the different series have been studied using Fourier transform infrared and Raman spectroscopy. For this range of hydrogen dilution, two groups of films were obtained. The first group is characterized by the dominance of the crystalline phase and the second by a dominance of the amorphous phase. This result confirms the multiphase structure of the grown nc-SiC:H thin films by the coexistence of the SiC network, carbon-like and silicon-like clusters. Furthermore, infrared results show that the SiC bond is the dominant absorption peak and the carbon atom is preferentially bonded to silicon. The maximum value obtained of the crystalline fraction is about 77%, which is relatively important compared to other results obtained by other techniques. In addition, the concentration of CHn bonds was found to be lower than that of SiHn for all series. Raman measurements revealed that the crystallization occurs in all series even at 100% H2 dilution suggesting that high partial pressure of hydrogen favors the formation of silicon nanocrystallites (nc-Si). The absence of both the longitudinal acoustic band and the transverse optical band indicate that the crystalline phase is dominant.

Low temperature plasma processing of nc-Si/a-SiNx:H QD thin films with high carrier mobility and preferred (220) crystal orientation: a promising material for third generation solar cells

The nc-Si-QDs/a-SiN_x:H (∼5.7–1.3 nm) thin-films grown by low-temperature Inductively-coupled plasma, possess high carrier-mobility, electrical-conductivity, photosensitivity and preferred (220) crystal orientation, suitable for third-generation solar cells.

Photoluminescent silicon quantum dots in core/shell configuration: synthesis by low temperature and spontaneous plasma processing

Quantum confinement in zero-dimensional silicon nanocrystals (nC) in the quantum dot (QD) configuration has triggered a tremendous interest in nanostructured device technology. However, the formation of Si-QDs eventually proceeds through multi-step routes and involves high temperature processing that impedes preferred device configuration. The present work demonstrates the formation of nC-Si QDs of controlled size, density and distribution through one-step and spontaneous plasma processing, at a low substrate temperature (300 °C) compatible for device fabrication. Direct growth of nC-Si/SiO(x) core/shell quantum dots embedded in the a-Si matrix, 6.4-3.7 nm in diameter and with number density in the range ∼ 6 × 10(9)-1 × 10(11) cm(-2) has been accomplished, following a novel route where He dilution to SiH(4) in RF plasma CVD has been found instrumental. On gradual reduction in the size of QDs, splitting of the energy bands widens the optical band gap and induces visible photoluminescence that appears controllable by tuning the size and density of the dots. This low temperature and spontaneous plasma processing of nC-Si/SiO(x) core/shell QDs that exhibit the quantum size effect in photoluminescence is being reported for the first time.

Hydrogenated Nanocrystalline Silicon Thin Films Prepared by Hot-Wire Method with Varied Process Pressure

Hydrogenated nanocrystalline silicon films were prepared by hot-wire method at low substrate temperature (200∘C) without hydrogen dilution of silane (SiH4). A variety of techniques, including Raman spectroscopy, low angle X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, atomic force microscopy (AFM), and UV-visible (UV-Vis) spectroscopy, were used to characterize these films for structural and optical properties. Films are grown at reasonably high deposition rates (>15 Å/s), which are very much appreciated for the fabrication of cost effective devices. Different crystalline fractions (from 2.5% to 63%) and crystallite size (3.6–6.0 nm) can be achieved by controlling the process pressure. It is observed that with increase in process pressure, the hydrogen bonding in the films shifts from Si–H to Si–H2and(Si–H2)ncomplexes. The band gaps of the films are found in the range 1.83–2.11 eV, whereas the hydrogen content remains <9 at.% over the entire range of process pressure studied. The ease of depositing films with tunable band gap is useful for fabrication of tandem solar cells. A correlation between structural and optical properties has been found and discussed in detail.