We study the distribution of hydrogen and various hydride configurations in realistic models of a-Si:H for two different concentration generated via experimentally constrained molecular relaxation approach (ECMR) [1]. The microstructure corresponding to low (< 10%) and high (> 20%) concentration of H atoms are addressed and are compared to the experimental results with particular emphasis on the size of H clusters and local environment of H atoms.The linewidths of the nuclear magnetic resonance (NMR) spectrum for the model configurations are calculated in order to compare to the experimental NMR data. Our study shows the presence of isolated hydrogen atoms, small and relatively large clusters with average proton-proton neighbor distance in the clusters around 1.6 to 2.4 Angstrom that have been observed in multiple quantum NMR experiments.
Experimentally Constrained Molecular Relaxation: The case of hydrogenated amorphous silicon
我们已经将我们的实验约束分子弛豫技术（P. Biswas { \it et al}，Phys. Rev. B { \bf 71} 54204（2005））扩展到氢化非晶硅：构建了具有7.4%氢的540原子模型和具有22%氢的611原子模型。从一个随机构型出发，利用物理相关约束、{ \it ab initio}相互作用和实验静态结构因子，我们构建了氢化非晶硅的真实模型。我们的模型证实了存在一个高频局域带的振动态密度由于Si-H振动，已观察到在最近的振动瞬态光栅测量等离子体增强化学气相沉积薄膜的氢化非晶硅。
The thermally-induced structural evolution of silicon- and oxygen-containing hydrogenated amorphous carbon (a-C:H:Si:O) was investigated by X-ray photoelectron and absorption spectroscopy, as well as molecular dynamics (MD) simulations. The spectroscopic results indicate that the introduction of Si and O in hydrogenated amorphous carbon (a-C:H) increases the activation energy for the conversion of sp3- to sp2-bonded C. MD simulations indicate that the higher thermal stability of a-C:H:Si:O compared to a-C:H derives from the lower fraction of strained C-C sp3 bonds in a-C:H:Si:O.
The growth of thin amorphous hydrogenated carbon films (a-C:H) on diamond (111) surface from the bombardment of CH2 radicals is studied using molecular dynamics simulations. The structural analysis shows that the local structure (e.g., the first coordination number of C atoms) of a-C:H depends critically on the content of hydrogen. The increase of kinetic energy of incident radicals leads to the decrease of hydrogen content, which subsequently changes the ratio of sp3 bonded C atoms in a-C:H.
Surface resistivity of hydrogenated amorphous carbon films was measured as a function of the applied electrical field. The measured dependence shows a sharp ambipolar peak near zero gate voltage. Furthermore, we found that in some samples sheet resistance at the peak is as low as 7.5 kΩ/sq. This value is the same order of magnitude as the sheet resistance of a defect free graphene monolayer. Therefore a conclusion is made that an intrinsic graphene with dimensions of at least millimeters exist on the surface of amorphous carbon films. These results can open new perspectives not only for graphe…
The evolution is presented of infrared transmission spectra of hydrogenated and chlorinated amorphous silicon films versus the chlorine concentration and the annealing temperature. As the chlorine content increases, an increase of the 2100 cm-1 band intensity is observed. We associate this effect with the existence of SiH2CI2, mixed configurations. The Sic1 stretching band, located at 545 cm-1 in unannealed samples, shifts continuously up to 590 cm-1 as the annealing temperature increases. This displacement is attributed to a predominance of SiCI2, species around 500°C and their transformation…
The dynamics of charge carriers in amorphous semiconductors fundamentally differ from those in crystalline semiconductors due to the lack of long-range order and the high defect density. Despite intensive technology-driven research interests and the existence of well-established experimental techniques, such as photoconductivity time-of-flight and ultrafast optical measurements, many aspects of the dynamics of photo-excited charge carriers in amorphous semiconductors remain poorly understood. Here we demonstrate direct imaging of carrier dynamics in space and time after photo-excitation in hyd…
We have studied the effects of oxygen on hydrogenated amorphous/crystalline silicon films in terms of their structural and optical properties. Different hydrogenated silicon oxide (SiO:H) and silicon (Si:H) films are fabricated between microcrystalline and amorphous transition region. X-ray diffraction, Raman, FTIR and UV-Vis emission spectrometry have been used to characterize different films. A comparison of the results with those of different types of films like hydrogenated amorphous silicon oxide (a-SiO:H), hydrogenated amorphous silicon (a-Si:H) and microcrystalline silicon ($μ$c-Si:H) f…
Specific heat measurements from 2 to 300 K of hydrogenated amorphous silicon prepared by hot-wire chemical vapor deposition show a large excess specific heat at low temperature, significantly larger than the Debye specific heat calculated from the sound velocity. The as-prepared films have a Schottky anomaly that is associated with metastable hydrogen in the amorphous network, as well as large linear and excess cubic term commonly associated with tunneling two-level systems in amorphous solids. Annealing at 200 °C, a temperature that enables hydrogen mobility but not evaporation, irreversibly …
Raman micro-spectroscopy as a tool to measure the absorption coefficient and the erosion rate of hydrogenated amorphous carbon films heat-treated under hydrogen bombardment
We deposit intrinsic hydrogenated amorphous silicon (a-Si:H) thin films by reactive pulsed laser deposition, for various hydrogen pressures in the 0-20 Pa range, at a low deposition temperature of 120C, and investigate the hydrogen incorporation, structural, optical, and electrical properties of the films, as a function of the ambient hydrogen pressure. The film thickness decreases linearly as the hydrogen pressure increases. The hydrogen content of the films is determined by infrared spectroscopy and the optical bandgap from UV-Vis-NIR transmittance and reflectance measurements. Electric meas…
Ultrananocrystalline diamond/hydrogenated amorphous carbon composite thin films synthesized via coaxial arc plasma possess a marked structural feature of diamond grains embedded in an amorphous carbon and a hydrogenated amorphous carbon matrix which are the largest constituents of the films. Since the amorphous nature yields much larger light absorption coefficients as well as a generation source of photo-induced carriers with UV rays, these films can be potential candidates for deep-UV photodetector applications. From some previous studies p-type conduction of the films has been realized by d…
Specific heat measurements of hydrogenated amorphous silicon prepared by hot-wire chemical vapor deposition show a large density of two-level systems at low temperature. Annealing at 200 °C, well below the growth temperature, does not significantly affect the already-low internal friction or the sound velocity, but irreversibly reduces the non-Debye specific heat by an order of magnitude at 2 K, indicating a large reduction of the density of two-level systems. Comparison of the specific heat to the internal friction suggests that the two-level systems are uncharacteristically decoupled from ac…
Superconducting resonators used in millimeter-submillimeter astronomy would greatly benefit from deposited dielectrics with a small dielectric loss. We deposited hydrogenated amorphous silicon films using plasma-enhanced chemical vapor deposition, at substrate temperatures of 100°C, 250°C and 350°C. The measured void volume fraction, hydrogen content, microstructure parameter, and bond-angle disorder are negatively correlated with the substrate temperature. All three films have a loss tangent below $10^{-5}$ for a resonator energy of $10^5$ photons, at 120 mK and 4-7 GHz. This makes these film…
Electronic states of trans-polyacetylene, poly(p-phenylene vinylene) and sp-hybridised carbon species in amorphous hydrogenated carbon probed by resonant Raman scattering
We have studied the effect of nitriding on the humidity sensing properties of hydrogenated amorphous carbon (a-C:H) films. The films were prepared in two stages combining the techniques of physical deposition in vapor phase evaporation (PAPVD) and plasma pulsed nitriding. By deconvolution of the Raman spectrum we identified two peaks corresponding to the D and G modes characteristic of a-C:H. After the N$_2$-H$_2$ plasma treating, the peaks narrowed and shifted to the right, which we associated with the incorporation of N into the structure. We compared the sensitivity to the relative humidity…
The nanostructure of hydrogenated amorphous silicon (a Si:H) is studied by a combination of small-angle X-ray (SAXS) and neutron scattering (SANS) with a spatial resolution of 0.8 nm. The a-Si:H materials were deposited using a range of widely varied conditions and are representative for this class of materials. We identify two different phases which are embedded in the a-Si:H matrix and quantified both according to their scattering cross-sections. First, 1.2 nm sized voids (multivacancies with more than 10 missing atoms) which form a superlattice with 1.6 nm void-to-void distance are detected…
Ultranonocrystalline diamond/hydrogenated amorphous carbon composite thin films consist of three different components; ultrananocrystalline diamond crystallites, hydrogenated amorphous carbon, and grain boundaries between them. Since grain boundaries contain a lot of dangling bonds and unsaturated bonds, they would be a cause of carrier trap center degrading device performance in possible applications such as UV photo-detectors. We experimentally demonstrate hydrogen atoms preferentially incorporate at grain boundaries and terminate dangling bonds by means of several spectroscopic techniques. …
We present a computational study of the void-induced microstructure in amorphous silicon ($\it a$-Si) by generating ultra-large models of $\it a$-Si with a void-volume fraction of 0.3$\%$, as observed in small-angle x-ray scattering (SAXS) experiments. The relationship between the morphology of voids and the intensity of scattering in SAXS has been studied by computing the latter from the Fourier transform of the reduced pair-correlation function and the atomic-form factor of amorphous silicon. The effect of low-temperature ($\le$ 600 K) annealing on the scattering intensities and the microstr…
The Effect of Hybrid Photovoltaic Thermal Device Operating Conditions on Intrinsic Layer Thickness Optimization of Hydrogenated Amorphous Silicon Solar Cells
Structural and superconducting transitions of layered van der Waals (vdW) hydrogenated germanene (GeH) were observed under high-pressure compression and decompression processes. GeH possesses a superconducting transition at critical temperature (Tc) of 5.41 K at 8.39 GPa. A crystalline to amorphous transition occurs at 16.80 GPa while superconductivity remains. An abnormally increased Tc up to 6.1 K has been observed in the decompression process while the GeH remained amorphous. Thorough in-situ high-pressure synchrotron X-ray diffraction and in-situ high-pressure Raman spectroscopy with the d…
Low-loss deposited dielectrics will benefit superconducting devices such as integrated superconducting spectrometers, superconducting qubits and kinetic inductance parametric amplifiers. Compared with planar structures, multi-layer structures such as microstrips are more compact and eliminate radiation loss at high frequencies. Multi-layer structures are most easily fabricated with deposited dielectrics, which typically exhibit higher dielectric loss than crystalline dielectrics. We measured the sub-kelvin and low-power microwave and mm-submm wave dielectric loss of hydrogenated amorphous sili…
In order to get homogeneous nanostructured Aluminum Nitride deposits, thin films were grown at room temperature on [001] Si substrates by radio frequency magnetron reactive sputtering. The deposits were analysed by Transmission Electron Microscopy, energy dispersive X-ray spectroscopy and Auger electron spectroscopy. Their microstructure and chemical composition were studied versus the plasma working pressure and the radio frequency power. Systematic analysis of cross views of the films allowed the authors to draw a microstructure/process parameters map. Four microstructural types were disting…
In this work, we studied amorphous carbon ($a$-C) thin films deposited using direct current (dc) and high power impulse magnetron sputtering (HiPIMS) techniques. The microstructure and electronic properties reveal subtle differences in $a$-C thin films deposited by two techniques. While, films deposited with dcMS have a smooth texture typically found in $a$-C thin films, those deposited with HiPIMS consist of dense hillocks surrounded by a porous microstructure. The density of $a$-C thin films is a decisive parameter to judge their quality. Often, x-ray reflectivity (XRR) has been used to meas…
The effect of temperature modes and heating rates (Vh) on the shrinkage kinetics of submicron and fine aluminum oxide powders has been studied. The objects of research comprised (i) submicron alfa-Al2O3 powder, (ii) submicron alfa-Al2O3 powder with an amorphous layer on particle surface, (iii) fine alfa-Al2O3 powder. The alumina ceramic specimens were produced by Spark Plasma Sintering (SPS). Equally fine powders (i) and (ii) were used to analyze the effect of an amorphous layer on sintering kinetics. Powders (i) and (iii) were used to analyze the effect of the initial particle size on shrinka…
Hydrogenation of amorphous silicon (a-Si:H) is critical for reducing defect densities, passivating mid-gap states and surfaces, and improving photoconductivity in silicon-based electro-optical devices. Modelling the atomic scale structure of this material is critical to understanding these processes, which in turn is needed to describe c-Si/a-Si:H heterjunctions that are at the heart of the modern solar cells with world record efficiency. Density functional theory (DFT) studies achieve the required high accuracy but are limited to moderate system sizes a hundred atoms or so by their high compu…
We report on the correlation between the boson-peak and structural changes associated with thermally-treating amorphous indium-oxide films. In this process, the resistance of a given sample may decrease by a considerable margin while its amorphous structure is preserved. In the present study, we focus on the changes that result from the heat-treatment by employing electron-microscopy, X-ray, and Raman spectroscopy. These techniques were used on films with different stoichiometry and thus different carrier-concentration. The main effect of heat-treatment is material densification, which presuma…
This study provides a new insight into the processes which occur when thin film hydrogenated amorphous silicon is subjected to UV laser irradiation in the presence of oxygen. It achieves this by observing the effects of subjecting the films to progressively increasing laser radiation doses. This reveals that an array of nuclei is first created, leading to the formation of a well-defined crystalline network, consistent with the structure of silicon oxide. Further irradiation results in the formation of cone-like structures on the crystalline network, due to silicon-oxygen bond breakage and migr…
In this paper, we explore the effect of H and its bonding configurations on the defect state density and orbital localization of hydrogenated amorphous Si (a-Si:H)/crystalline Si (c-Si) heterostructures using density functional theory (DFT) studies of model interfaces between amorphous silicon (a- Si)/a-Si:H and c-Si. To model the atomic configuration of a-Si on c-Si, melting and quenching simulations were performed using classical molecular dynamics (MD). Different hydrogen contents were inserted into the a-Si in different bonding configurations followed by DFT relaxation to create the stable…
On the basis of density functional calculations and using Bader's atom in molecule theory, this article presents quantitative microscopic analyses on the bonding properties of amorphous silicon (a-Si) which could reflect in the observable mechanical and electronic behaviors of this material. In addition, the occurrence and strength of quantum confinement of charge carriers in a composition of silicon crystal nano slabs (SiNSs) embedded in hydrogenated a-Si (a-Si:H) semiconductor are studied. It is shown that the strongest confinement effect happens for Si slabs limited in [100] direction. The …
Here we describe a new high frequency/high field continuous wave and pulsed electrically detected magnetic resonance (CW EDMR and pEDMR) setup, operating at 263 GHz and resonance fields between 0 and 12 T. Spin dependent transport in illuminated hydrogenated amorphous silicon p-i-n solar cells at 5 K and 90 K was studied by in operando 263 GHz CW and pEDMR alongside with complementary X-band CW EDMR. Benefiting from the superior resolution at 263 GHz, we were able to better resolve EDMR signals originating from spin dependent hopping and recombination processes. 5 K EDMR spectra were found to …
Strain in a material induces shifts in vibrational frequencies, which is a probe of the nature of the vibrations and interatomic potentials, and can be used to map local stress/strain distributions via Raman microscopy. This method is standard for crystalline silicon devices, but due to lack of calibration relations, it has not been applied to amorphous materials such as hydrogenated amorphous silicon (a-Si:H), a widely studied material for thin-film photovoltaic and electronic devices. We calculated the Raman spectrum of a-Si:H \ab initio under different strains $ε$ and found peak shifts $Δω=…
