Further study on structural and electronic properties of silicon phosphide compounds with 3:4 stoichiometry

Si₃N₄ has been extensively studied, due to its potential applications in electronic devices. Substitution of N by P results in Si₃P₄ which is a relatively unknown material. In this study, we carried out further investigation on the structural and electronic properties of Si₃P₄ using first principles total energy method based on the density functional theory and the generalized gradient approximation. Our calculations show that pseudocubic-Si₃P₄ is energetically favored. However, the present study based on the generalized gradient approximation predicts that the phase is more stable than the γ phase which was predicted to be more energetically stable in an earlier study based on the local density approximation. Other properties such as bulk modulus, band structure, of Si₃P₄ calculated using GGA are consistent with the LDA results.     

Electronic properties of low temperature epitaxial silicon thin film photovoltaic devices grown by HWCVD

This study addresses the correlation of the electrical, surface, and structural evolution of HWCVD crystalline Si thin films with temperature, thickness, and hydrogen dilution. Scanning electron microscopy and atomic force microscopy reveal an increase with surface roughness with hydrogen dilution, as expected, while showing increasing surface roughness with substrate temperature, in contrast to previous studies of crystalline Si growth. This suggests that H desorption enables more contaminant absorption of the growing surface with increasing temperature, in turn increasing roughness. The open-circuit voltage of these films is shown to increase significantly over time, ∼ 50 mV over one week, due to the decrease in surface recombination velocity associated with the growth of a native oxide layer. This indicates the importance of post-deposition treatments for surface passivation.     

化学溶液法外延组装ZnO分级纳米结构

采用气相法、液相法相结合的方法外延组装了一种形貌新颖的复杂ZnO分级纳米结构--"纳米毛刷".首先用热蒸发的方法制备了宽面为极性面的ZnO纳米带,然后采用化学溶液法,在强碱溶液中在ZnO纳米带的极性面上外延生长Zno纳米棒阵列,实现了ZnO分级纳米结构"由下而上"地外延组装.采用负离子配位多面体生长基元模型讨论了ZnO分级纳米结构的外延组装机理.这种ZnO分级结构的实现,可望作为ZnO纳米器件的原型材料构建新型光电器件.     

Stressed solid-phase epitaxial growth of ion-implanted amorphous silicon

The kinetics of stressed solid-phase epitaxial growth (SPEG), also referred to as solid-phase epitaxy, solid-phase epitaxial regrowth, solid-phase epitaxial recrystallization, and solid-phase epitaxial crystallization, of amorphous () silicon (Si) created via ion-implantation are reviewed. The effects of hydrostatic, in-plane uniaxial, and normal uniaxial compressive stress on SPEG kinetics are examined in intrinsic (0 0 1)Si. Particular emphasis is placed on unifying the results of different experiments in a single-stress-dependent SPEG model. SPEG kinetics are observed to suffer similar exponentially enhanced growth rates in hydrostatic and normal uniaxial compressive stress. However, there are discrepancies between researchers in terms of the influence of in-plane stress on growth rates. Two different stress-dependent SPEG models are thus advanced, each with different physical bases. The model advanced by Aziz et al. proposes SPEG can be modeled as a single-atomistic process while the model advanced by Rudawski et al. suggests that stress influences the nucleation and migration processes of growth differently and that SPEG cannot be modeled as a single step. The basis for the Rudawski et al. model is based on the crystal island and ledge migration model of SPEG advanced by others. Morphological instabilities of the growing /crystalline interface with in-plane compression are also addressed within the context of both the Aziz et al. and Rudawski et al. models. Finally, using the Rudawski et al. model, it is possible to examine, characterize, and isolate the different atomistic processes during growth. Calculation of the activation energies for nucleation and migrations processes suggests that the activation energy of 2.7 eV observed for the growth rate in stress-free SPEG by Olsen and Roth is representative of the activation energy for the single-atomistic process of crystal island nucleation. Thus, the study of stressed SPEG provides a new atomistic picture of the nature of growth.     

The effect of sintering additive on fracture behavior of carbon-whisker-reinforced silicon carbide composites

Hot-pressed silicon carbide composites reinforced with carbon fiber were prepared. Aluminum and yttrium oxides served as sintering additives and low-cost phase SiC was used as starting powder, instead of the more expensive β-SiC. In the sintering process, the SiC-matrix grains grew larger via solution reprecipitation. Reaction of Al₂O₃/Y₂O₃ additives with SiO₂ on the surface of SiC or its oxidation products caused formation and distribution of a low-eutectic-point phase around the SiC grains and carbon whiskers. Such amorphous films can be found in triple-junctions and boundaries of SiC grains. Excess sintering additives improve the room-temperature flexural strength, but reduce the fracture toughness. Coupled with a higher sintering temperature, they contribute to the diffusion of yttrium ions into carbon fiber, and make the reaction layer thicker. Non-homogeneous amorphous inclusions between grains and whiskers are harmful for mechanical properties. A combination of grain bridging, crack deflection and whisker debonding can improve fracture toughness.     

Incorporation of InAs nanostructures in a silicon matrix: growth, structure and optical properties

MBE growth and properties of InAs nanoscale islands formed on silicon are reported. Islands capped with Si emit a photoluminescence band in the 1.3 μm region. Upon annealing at increased substrate temperature, extensive interdiffusion leads to the formation of an InAs solid solution in the Si cap layer. Additionally, InAs-enriched regions with extensions of 6 nm, exhibiting two kinds of ordering, are observed. The ordering of InAs molecules occurs, respectively, in (101) and planes inclined to (110) and planes parallel to the [001] growth direction.     

Bismuth telluride nanostructures: preparation,thermoelectric properties and topological insulating effect

Bismuth telluride is known to wield unique properties for a wide range of device applications. However, as devices migrate to the nanometer scale, significant amount of studies are being conducted to keep up with the rapidly growing nanotechnological field. Bi₂Te₃ possesses distinctive properties at the nanometer level from its bulk material. Therefore, varying synthesis and characterization techniques are being employed for the realization of various Bi₂Te₃ nanostructures in the past years. A considerable number of these works have aimed at improving the thermoelectric (TE) figure-of-merit (ZT) of the Bi₂Te₃ nanostructures and drawing from their topological insulating properties. This paper reviews the various Bi₂Te₃ and Bi₂Te₃-based nanostructures realized via theoretical and experimental procedures. The study probes the preparation techniques, TE properties and the topological insulating effects of 0D, 1D, 2D and Bi₂Te₃ nanocomposites. With several applications as a topological insulator (TI), the topological insulating effect of the Bi₂Te₃ is reviewed in detail with the time reversal symmetry (TRS) and surface state spins which characterize TIs. Schematics and preparation methods for the various nanostructural dimensions are accordingly categorized.     

Structural,elastic, electronic and optical properties of new layered semiconductor BaGa2P2

We report the results of a detailed first-principles based density functional theory study of the structural, elastic, electronic and optical properties of a recently synthesized layered semiconductor BaGa₂P₂. The optimized structural parameters are in excellent agreement with the experimental structural findings, which validates the used theoretical method. The single crystal and polycrystalline elastic constants are numerically estimated using the strain–stress method and Voigt–Reuss–Hill approximations. Predicted values of the elastic constants suggest that the considered material is mechanically stable, brittle and very soft material. The three-dimensional surface and its planar projections of Young’s modulus are visualized to illustrate the elastic anisotropy. It is found that Young’s modulus of BaGa₂P₂ show strong dependence on the crystallographic directions. Band structure calculation reveals that BaGa₂P₂ is a direct energy band gap semiconductor. The effective masses of electrons and holes at the minimum of the conduction band and maximum of the valence band are numerically estimated. The density of state, charge density distribution and charge transfers are calculated and analyzed to determine the chemical bonding nature. Dielectric function, refractive index, extinction coefficient, absorption coefficient, reflectivity and electron-loss energy function spectra are computed for a wide photon energy range up to 20 eV. Calculated optical spectra exhibit a noticeable anisotropy.     

Dendritic and epitaxial growths of ZnO particle-rod and rod-rod nanostructures with quasi-one-dimensional and two-dimensional configurations

Quasi-one-dimensional and two-dimensional ZnO nanostructures have been fabricated through thermal evaporation approach. The microstructures of the ZnO nanostructures have been studied using scanning electron microscopy and high-resolution electron microscopy. Quasi-one-dimensional ZnO nanostructures are formed by dendritic growths of ZnO nanoparticles from the stem nanorods surfaces, forming particle-rod nanostructures. While epitaxial growths of branch nanorods from the stem nanorods configure two-dimensional ZnO nanostructures. The epitaxial growth orientation relationship can be described as [2? 110]_R1 || [2? 110]_R2 and (0001) _R1 || (011?0)_R2. The growth mechanism of the quasi-one-dimensional and two-dimensional ZnO nanostructures has been discussed.     

First principles study of surface properties for silicon carbide-derived carbon

The surface properties for silicon carbide-derived carbon (i.e. Si-CDC) are investigated by employing the density functional theory (DFT) plane-wave pseudopotential method. The calculated results show that, the carbon atoms on the Si-CDC first layer, except for the ones linking to sub-layer Si atoms, fall into the second atomic layer, exhibiting a nanoporous surface feature. The Si removal leads to stronger covalent bonding among the C-Si and C-C atoms, implying more stable C atoms on the Si-CDC surface. Different from the case for the SiC, the second atomic layer has the most important contribution on the Si-CDC electrons of surface state. Moreover, the narrower band gap for the Si-CDC surface indicates a superior conductivity.     

Characterization and analysis of epitaxial silicon phosphorus alloys for use in n-channel transistors

In this work, we demonstrate substitutional phosphorus concentration as high as 12 at.% in epitaxial silicon. It is observed that 10 at.% substitutional phosphorus doping is equivalent in tensile strain to incorporating 2.1 at.% substitutional carbon into the silicon lattice. Phosphorus doping of this order produces tensile strain levels suitable for n-channel metal-oxide semiconductor field-effect transistor uniaxial stressor applications. This work focuses on the experimental and theoretical analyses of phosphorus doped silicon based on high resolution X-ray rocking curves, secondary ion mass spectroscopy, Rutherford backscattering spectroscopy, and molecular dynamic modeling.     

Semiconducting and structural properties of CrSi2 A-type epitaxial films on Si(111)

Chromium disilicide (CrSi₂) films 1 000 Å thick have been prepared by molecular beam epitaxy on CrSi₂ templates grown on Si(111) substrate. The effect of the substrate temperature on the structural, electrical and optical properties of CrSi₂ films has been studied by transmission and scanning electron microscopies, optical microscopy, electrical resistivity and Hall effect measurements and infrared optical spectrometry. The optimal temperature for the formation of the epitaxial A-type CrSi₂ film have been found to be about 750°C. The electrical measurement have shown that the epitaxial A-type CrSi₂ film is p-type semiconductor having a hole concentration of 1 × 10¹⁷cm⁻³ and Hall mobility of 2 980 cm² V⁻¹ s⁻¹ at room temperature. Optical absorption coefficient data have indicated a minimum, direct energy gap of 0.34 eV. The temperature dependence of the Hall mobility (μ) in the temperature range of T = 180–500 K can be expressed as μ = 7.8 × 10¹⁰T⁻³cm²V⁻¹s⁻¹.     

Microstructure and electronic properties of microcrystalline silicon carbide thin films prepared by hot-wire CVD

An overview on microstructural and electronic properties of stoichiometric microcrystalline silicon carbide (μc-SiC) prepared by Hot-Wire Chemical Vapor Deposition (HWCVD) at low substrate temperatures will be given. The electronic properties are strongly dependent on crystalline phase, local bonding, strain, defects, impurities, etc. Therefore these quantities need to be carefully investigated in order to evaluate their influence and to develop strategies for material improvement. We will particularly address the validity of different experimental methods like Raman spectroscopy and IR spectroscopy to provide information on the crystalline volume fraction by comparing the results with Transmission Electron Microscopy (TEM) and X-Ray diffraction data. Finally the electronic properties as derived from optical absorption and transport measurements will be related to the microstructure.     

Role of microstructure in electronic transport behavior of highly crystallized undoped microcrystalline Si Films

Significant correlation is established between the observed electrical properties (room temperature value of dark conductivity and its activation energy) of the plasma deposited highly crystallized undoped hydrogenated microcrystalline silicon (μc-Si:H) films with their microstrucural properties (various aspects like fractional composition of constituent grains, morphology, and crystalline orientation). The conductivity shows three distinct trends with increasing film thickness irrespective of the different deposition parameters or conditions, and these three zones are applicable to all the samples, indicating that all the samples fundamentally belong to three different microstructural classes (with distinct microstructural attributes like thickness and features of grains and conglomerates) corroborative with these three zones of electrical behavior.     

Determination of the optical properties of non-uniformly thick non-hydrogenated sputtered silicon thin films on glass

An improved envelope method (EM) is presented in this paper that allows the determination of the refractive index (n_f) and absorption coefficient (_f) of non-uniformly thick, absorbing films on a slightly absorbing substrate from a single transmission measurement. The limitation of the previous version of the EM [R. Swanepoel, J. Phys. E: Sci. Inst. 17 (1984) 896] only permitted the evaluation of samples that exhibited a transparent region in the near infrared (NIR). As an initial test of the improved EM, n_f and _f of a 0.5-μm thick epitaxially grown silicon-on-sapphire (SOS) film were determined over the range of 1.1–3.2 eV, with increased absorption being observed at the silicon: sapphire interface. Subsequently, sputtered amorphous silicon (a-Si) films, which exhibit absorption throughout the visible–NIR spectrum, were successfully characterised and a definite trend towards lower absorption coefficients for films deposited at higher temperatures was observed. After the a-Si films were subjected to solid phase crystallisation (SPC), increased sub-bandgap absorption was attributed to higher defect levels in the films, which also resulted in amorphous features remaining in the Raman spectra.     

Conductivity and dielectric properties of silicon nitride thin films prepared by RF magnetron sputtering using nitrogen gas

Silicon nitride is an important material in very-large-scale integration fabrication and processing. Recent work on films prepared by radio frequency magnetron sputtering using nitrogen gas have shown that the relative permittivity is typically 6.3 and that aluminium forms an ohmic contact to this material. Under direct current (DC) bias the films exhibited space-charge-limited conductivity with a bulk trap density of the order of 2×10²⁴ m⁻³. In the present work alternating current electrical measurements were made on identical samples as a function of frequency and temperature. Conductivity appeared to be by hopping at lower temperatures, giving way to a free-band conduction process with activation energy of typically 0.44 eV at higher temperatures. Over a limited range of frequency and temperature the model of Elliott was applicable, and yielded a value of 2.87×10²³ m⁻³ for the density of localised states, in reasonable agreement with our estimate of the trap density from DC measurements. As in the DC measurements capacitance followed a geometric relationship with relative permittivity 6.3, and showed a moderate decrease with increasing frequency and an increase with increasing temperature, tending towards a constant value at high frequencies and low temperatures. The loss tangent showed a minimum in its frequency dependence, which appeared to shift to higher frequencies with increasing temperature. The measurements are consistent with the model of Goswami and Goswami for samples having ohmic contacts, and are typical of results obtained on other insulating thin film structures.     

The effect of aluminum additive on structure evolution of silicon oxycarbide derived from polysiloxane

Silicon oxycarbide (SiOC) and aluminum-containing silicon oxycarbide (SiAlOC) glasses were obtained through pyrolysis in argon atmosphere at 1000 °C of a polymethyl(phenyl)siloxane resin and aluminum tri-sec-butoxide-derived siloxane networks. These glasses were further annealed at 1200, 1300, and 1400 °C in vacuum atmosphere to investigate their high-temperature behavior. The two types of glasses were characterized by X-ray diffraction, ²⁹SiMASNMR, ²⁷AlMASNMR, and chemical element analysis. The aluminum incorporated into structure plays a major role on the thermal stability of SiOC by hindering carbothermal reductions. It can be found that introducing aluminum into structure should be an effective way to enhance the thermal stability of SiOC glasses.     

Molecular dynamics investigation of temperature dependent structural and fracture properties of amorphous silicon nitride

Abstract

Silicon nitride presents good mechanical properties and thermal stability at high temperature. As the experiments have limitations in the micro-/nanoscale characterisation of structural and fracture properties at high temperatures, atomistic simulation is the proper way to investigate the mechanism of this unique feature. In the present paper, the structural and fracture properties of amorphous silicon nitride (a-Si₃N₄) were studied at temperatures up to 1500 K. The simulation results consist of experiments on radial distribution function, temperature dependent yield stress and Young’s modulus. Based on the structural and mechanical results of α-Si₃N₄ at different temperatures, the structure–property correlations were discussed.     

Optical properties of non-stoichiometric SiO2 as a function of excess silicon content and thermal treatments

The infrared (IR) absorption spectra and the behavior of the refraction index of a two-phase non-stoichiometric SiO₂ film with excess Si have been studied as a function of the excess of Si and post-deposition thermal treatment. The oxides were deposited by low-pressure chemical vapor deposition using SiH₄ and N₂O as reactant gases at a substrate temperature in the range of 650 to 750 °C. Some of the films were given a final annealing treatment at temperatures ranging from 700 to 1100 °C in N₂ for 30 min. Both annealed and as-deposited oxides have IR absorption peaks associated with the bending, rocking and stretching modes of the Si-O-Si bonds in SiO₂, although the exact location of these peaks is different for different contents of excess silicon and it also depend on the post-deposition thermal treatment given to the oxides. Unannealed samples present a shift of the stretching peak towards low wavenumbers as the excess of Si is increased. The samples annealed at 1000 °C on the other hand do not present this shift. Unannealed samples with large content of Si also present an absorption peak at 890 cm⁻¹ that could be associated with partially oxidized Si. It is suggested that at least part of the excess Si in the as-deposited samples is present in the form of an SiO_x phase while in the annealed samples a clear separation occurs between a Si and a SiO₂ phase. The behavior of the refraction index is similar for both types of sample, increasing as the excess silicon is increased.     

Predictions of novel nanostructures of silicon by metal encapsulation

Recent studies using ab initio total energy calculations have shown exciting possibilities of developing novel metal encapsulated caged clusters of silicon with fullerene-like, Frank–Kasper and other polyhedral structures. In contrast to carbon for which empty cage fullerene structures are stable with 20 or more atoms, 10–16 atom silicon cage structures are stabilized by a guest metal atom. These nanoclusters are predicted to exhibit luminescence in the visible range and could find applications in biological systems, optoelectronics, and as tagging material. The Raman and infrared spectra have been calculated and they could help in the experimental identification of the structures. Interaction of these clusters with metal as well as oxygen or hydrogen atoms show that the fullerene structure is stable. Also the interaction between clusters themselves is weak and the ionization potentials, large. These properties make them attractive for cluster assembled materials such as nanowires, nanotubes, and other 2 and 3D structures. Studies on hydrogen interaction have led to the predictions of empty center hydrogenated silicon fullerenes Si_nH_n with large HOMO–LUMO gaps. These could further be doped endohedrally or exohedrally to produce novel silicon fullerenes with a variety of properties opening new ways of using silicon for diverse applications.