Confined high-pressure chemical deposition of hydrogenated amorphous silicon

Hydrogenated amorphous silicon (a-Si:H) is one of the most technologically important semiconductors. The challenge in producing it from SiH(4) precursor is to overcome a significant kinetic barrier to decomposition at a low enough temperature to allow for hydrogen incorporation into a deposited film. The use of high precursor concentrations is one possible means to increase reaction rates at low enough temperatures, but in conventional reactors such an approach produces large numbers of homogeneously nucleated particles in the gas phase, rather than the desired heterogeneous deposition on a surface. We report that deposition in confined micro-/nanoreactors overcomes this difficulty, allowing for the use of silane concentrations many orders of magnitude higher than conventionally employed while still realizing well-developed films. a-Si:H micro-/nanowires can be deposited in this way in extreme aspect ratio, small-diameter optical fiber capillary templates. The semiconductor materials deposited have ~0.5 atom% hydrogen with passivated dangling bonds and good electronic properties. They should be suitable for a wide range of photonic and electronic applications such as nonlinear optical fibers and solar cells.     

An EELS and EXELFS study of amorphous hydrogenated silicon carbide

A series of thin films of amorphous hydrogenated silicon carbide (a-SiCH) produced by RF plasma decomposition of propane and silane has been studied by electron energy-loss spectroscopy (EELS) and extended energy-loss fine structure (EXELFS) studies. The composition of the films has been determined by EELS and the nearest neighbour spacings have been determined by EXELFS. These results, along with the energy of the plasmon loss peaks, have been compared with the deposition conditions for each film. The results show that for a large gas ratio (C₃H₈/(C₃H₈+SiH₄)) the films have a high proportion of carbon and are similar to a-CH in structure, whereas those films prepared with Y = 0.4 or 0.5 have nearest neighbour spacings consistent with those for tetrahedrally bonded carbon. The films prepared with lowest Y have nearest neighbour spacings similar to those for amorphous silicon carbide. The results for a-SiCH have been compared with the results of EELS and EXELFS of CVD diamond films, amorphous carbon and amorphous silicon.     

Effect of charge on bond strength in hydrogenated amorphous silicon

We have studied the effect of excess charge on the bond strength in the silanes SiH₄ and Si₂H₆ to assess whether charge trapping in a solid-state lattice might promote the technologically important photodegradation of amorphous silicon alloys (the Staebler-Wronski effect). The calculations indicate that both positive and negative charges reduce the strength of Si? H and Si? Si bonds considerably, to the point where they may be broken easily by visible or even infrared light. © 1994 by John Wiley & Sons, Inc.     

Microspot-based ELISA in microfluidics: chemiluminescence and colorimetry detection using integrated thin-film hydrogenated amorphous silicon photodiodes

Microfluidic technology has the potential to decrease the time of analysis and the quantity of sample and reactants required in immunoassays, together with the potential of achieving high sensitivity, multiplexing, and portability. A lab-on-a-chip system was developed and optimized using optical and fluorescence microscopy. Primary antibodies are adsorbed onto the walls of a PDMS-based microchannel via microspotting. This probe antibody is then recognised using secondary FITC or HRP labelled antibodies responsible for providing fluorescence or chemiluminescent and colorimetric signals, respectively. The system incorporated a micron-sized thin-film hydrogenated amorphous silicon photodiode microfabricated on a glass substrate. The primary antibody spots in the PDMS-based microfluidic were precisely aligned with the photodiodes for the direct detection of the antibody-antigen molecular recognition reactions using chemiluminescence and colorimetry. The immunoassay takes ~30 min from assay to the integrated detection. The conditions for probe antibody microspotting and for the flow-through ELISA analysis in the microfluidic format with integrated detection were defined using antibody solutions with concentrations in the nM-μM range. Sequential colorimetric or chemiluminescence detection of specific antibody-antigen molecular recognition was quantitatively detected using the photodiode. Primary antibody surface densities down to 0.182 pmol cm(-2) were detected. Multiplex detection using different microspotted primary antibodies was demonstrated.     

SIMS depth profile analysis for investigations of the lithium-diffusion in hydrogenated amorphous silicon

Summary SIMS depth profiling, using an oxygen primary beam at close to normal incidence, was applied to study lithium diffusion in thin films of hydrogenated amorphous silicon (a-Si:H) as well as in layered structures of doped and undoped silicon-based alloys. A strongly increasing decay length of the lithium profiles was observed for increasing primary beam energies and is attributed to the Li accumulation at the SiO_x/Si interface. The stability of the lithium incorporation in a-Si:H is found to depend on the presence of charged acceptors or defects and of doping gradient related electrical fields.     

AFM and STM investigations of hydrogenated amorphous silicon: topography and barrier heights

As-grown films of hydrogenated amorphous silicon (a-Si?:?H, highly phosphorous-doped) were investigated by atomic force microscopy (AFM) and scanning tunneling microscopy (STM). Hills up to 10 nm in height and 10 to 20 nm in diameter have been observed by AFM. By using STM in a new high-sensitivity mode, (1) atomically smooth areas (roughness about 0.3 Å rms) which occur at the top of the hills, (2) subnanometer structures several Å in height which cover large parts of the surface have been identified. Simultaneous measurements of the local apparent barrier heights (LABH) show a clear correlation to the topography. Areas showing subnanometer structures have always low LABHs while the highest values of the LABH occur on the smooth areas.     

Investigation of the electronic structure and properties of cluster models of silica by the MNDQ method

Using the LCAO MO SCF method in the MNDO valence approximation, we have carried out a systematic study of the electronic and spatial structure of cluster models of the bulk phase and the hydroxylated and chemically modified surface of silica surface. We propose a technique for taking into account the crystallochemical environment of the clusters modeling the bulk phase of SiO₂, based on passivation of the abnormal valencies at the boundary of the clusters by hydrogen atoms, the geometrical location of which ensures homogeneity in the electron density distribution on the silicon atoms of the model fragment. We give a comparative analysis of the electronic characteristics of the studied cluster models. We consider the nature of the adsorption centers and the properties of the hydroxylated and modified silica surface.Translated from Teoreticheskaya i Éxperimental'naya.Khimiya, Vol. 22, No. 5, pp. 533–545, September–October, 1986.     

Novel alignment mechanism of liquid crystal on a hydrogenated amorphous silicon oxide

The mechanism of liquid crystal (LC) alignment has been investigated during the last few decades for inorganic materials as well as for organic materials; however, it has not been clearly confirmed for some alignment materials. Inorganic alignment materials such as amorphous silicon oxide (a-SiOx) and hydrogenated amorphous silicon oxide (a-SiOx:H) are deposited on indium tin oxide (ITO) films on glass by reactive sputtering deposition. After deposition, the inorganic alignment materials are irradiated using an Ar+ ion beam (IB) for LC alignment. On the basis of the experimental results, a-SiOx films deposited by the sputtering do not align the LC, but a-SiOx:H films treated with varying IB energies, IB incident angles, IB doses, and IB irradiation times have excellent alignment properties and electrooptical properties, identical to those of polyimide (PI). These results imply that inorganic alignment layers irradiated by IB can be adopted as an LC alignment layer instead of rubbed PI. Additionally, hydrogen plays an important role in LC alignment because of the difference in alignment properties between a-SiOx films and a-SiOx:H films. We investigate the mechanism of IB-treated inorganic alignment layers and suggest that LCs are aligned by chemical effects, such as van der Waals interaction, more than by physical effects, such as morphology effects, in the inorganic alignment layer irradiated by IB.     

Temperature dependency of the intrinsic carrier density of hydrogenated amorphous silicon in MOS structures

The admittance versus frequency of a hydrogenated amorphous silicon metal oxide semiconductor capacitor is measured at a fixed bias in inversion and for temperatures in the range of 20–50 °C. The data are fitted to theoretical capacitance and conductance curves where the time constant of inversion is the result of the fit. In turn, the time constant can be converted to the (minority) carrier lifetime so that a lifetime value for each measurement temperature is available. The conversion from the time constant to the minority carrier lifetime requires the knowledge of the temperature-dependent intrinsic carrier density or rather its activation energy. The criterion for the correct choice is a temperature-independent carrier lifetime. Three published room temperature values of the intrinsic carrier density have been tested. The carrier lifetime activation energy is E _a = 0.70 ± 0.03 eV. Received: 17 June 1998 / Accepted: 23 October 1998     

Simulation of the infrared spectra of amorphous silicon alloys

The infrared absorption spectra of hydrogenated amorphous silicon alloys have been simulated using MOPAC. The calculated spectra compare well with those obtained experimentally. Simulation studies have also been carried out on the effect of contamination of hydrogenated amorphous silicon by common atmospheric contaminants: oxygen and nitrogen. These studies provide a basis for the study of diffusion and reconstruction of amorphous silicon alloys during annealing and photodegradation. This article indicates that infrared spectroscopy, combined with molecular modeling, could be used to study the molecular mechanisms associated with the photodegradation of amorphous silicon alloys. © 1996 by John Wiley & Sons, Inc.     

Investigation of the electronic vibrational structure of furan by REMPI

Resonance-enhanced multiphoton ionization (REMPI) has been applied to detect the hot bands as well as the cold bands of the 1a2-->-->3dxy(1A1) Rydberg transition of furan (jet-cooled, mass-analyzed). Based on the unambiguous assignment of the hot bands, a complete vibronic analysis is given for the cold bands of this transition (up to 4600 cm(-1) above the origin). This analysis can be used for the interpretation of the vibrational structure in the X 2A2 photoelectron band. The energy ordering of the five 3d Rydberg states is 1A2 approximately 2A2相似文献   

Atomic and electronic structure of the surface of porous silicon layers

Atomic and electronic structure of the surface layers of porous silicon was studied by the methods of the near fine structure spectroscopy at the edge of X-ray absorption and ultrasoft X-ray emission spectroscopy. The thickness of the oxide layer and the degree of distortion of silicon-oxygen tetrahedra in this layer were estimated. The thickness of the surface oxide layer on the amorphous layer covering the nanocrystals of porous silicon that was kept during one year is several times greater than the thickness of the natural oxide in the single crystal silicon wafers. Distortion of the silicon-oxygen tetrahedron, the basic structural units of the silicon oxide, is accompanied by elongation of Si-O bonds and an increase in the Si-O-Si bond angles.     

Investigation of deep states in thin films of hydrogenated amorphous silicon by photo-induced current transient spectroscopy

Summary Photo-induced current transient spectroscopy (PICTS) was performed for thin films of undoped hydrogenated amorphous silicon (a-SiH). Existing theoretical models i.e. the standard model of PICTS (the transient currents are limited by thermal emission of carriers from localized to mobile states) and the recombination model due to Rose (the transient currents are limited by recombination) fail to describe the experimental data. It is concluded that the recombination model has to be extended to arbitrary distributions of the density of states in order to be applicable to the present case.

---

Untersuchung tiefer Zustände in dünnen Schichten von wasserstoffhaltigem amorphem Silicium mit Hilfe der Photostrom-Transienten-Spektroskopie

     

Fe-S簇合物的Fe-Fe相互作用及电子结构规律

本文对Fe-S原子簇化合物中的Fe-Fe相互作用作了详细的分子轨道分析。指出硫桥对形成原子簇的重要作用以及对Fe-Fe间成键的影响。在理论分析的基础上提出了Fe平均价态的计算公式。     

LDA electronic structure calculations on Au13 cluster

The electronic structure of the cubo-octahedral Au₁₃ cluster was investigated within a self-consistent molecular-orbital Slater-type-orbital framework. The scalar relativistic calculated density of states (DOS) for the gold cluster under consideration show relatively similar features to those obtained experimentally or theoretically earlier calculated by fully relativistic methods. © 1997 John Wiley & Sons, Inc.