Si Pairs Research Articles

Carbon-Isovalent Dopant Pairs in Silicon: A Density Functional Theory Study

Carbon (C) is an important isovalent impurity in silicon (Si) that is inadvertently added in the lattice during growth. Germanium (Ge), tin (Sn), and lead (Pb) are isovalent atoms that are added in Si to improve its radiation hardness, which is important for microelectronics in space or radiation environments and near reactors or medical devices. In this work, we have employed density functional theory (DFT) calculations to study the structure and energetics of carbon substitutional-isovalent dopant substitutional CsDs (i.e., CsGes, CsSns and CsPbs) and carbon interstitial-isovalent dopant substitutional CiDs (i.e., CiGes, CiSns and CiPbs) defect pairs in Si. All these defect pairs are predicted to be bound with the larger isovalent atoms, forming stronger pairs with the carbon atoms. It is calculated that the larger the dopant, the more stable the defect pair, whereas the CsDs defects are more bound than the CiDs defects.

A numerical study attitude of compatibility in scintillator detectors through matching pair scintillator-photodiode materials

In a group of scintillator detectors where photodiodes have replaced photomultiplier tubes, one way to increase the efficiency of the device is proper matching between the radiation spectrum of the scintillator and the spectral sensitivity of the photodiode. This study seeks to find the most compatible pair of scintillators with photodiodes using the simulation of photodiodes in COMSOL Multiphysics. In this research, three scintillators NaI(Tl), CsI(Tl), and Bi4Ge3O12 (BGO), along with three photodiodes silicon (Si) and gallium arsenide (GaAs) and indium phosphide (InP), have been studied to find the best compatible pair. Among the selected scintillators and photodiodes, the pair of Si with BGO produced the highest output current. Generating more current in a detector for constant input radiation means more sensitivity of that detector to the incident beam. In another part of this research, using the simulated photodiode in COMSOL Multiphysics, the current generated in the photodiode has been obtained for a fixed value of the ratio of the length of the photodiode to the junction depth of doping for various lengths. Then, using the Origin Pro software, the best curve corresponding to the data obtained from the simulation was determined. The results showed that these points coincide with an exponential curve with an excellent approximation. Using these types of detectors in the International Linear Collider (ILC) and in the Positron Emission Tomography (PET) field shows the unique applications of detectors.

Why Te Doping Can Break the Traditional n-Type Doping Limit of Silicon.

We report a theoretical investigation of the impact of hyperdoping with chalcogens (Se and Te) and pnictogens (P and As) on free-carrier concentrations of Si, employing density functional theory calculations. Our results illustrate that isolated substitutional chalcogens in moderately doped Si function as deep donors that are difficult to ionize at room temperature, unlike isolated substitutional pnictogens. The pairing of substitutional defects is found to be energetically favorable for every dopant element, implying that the concentration of substitutional pairs can be significant in hyperdoped Si. However, chalcogen-substitutional pairs have the capability to increase the carrier concentration, whereas pnictogen-substitutional pairs serve only as compensators for n-type doping. By evaluating the carrier concentrations for Te- and P-hyperdoped Si, we demonstrate the importance of substitutional Te pairs of Te-hyperdoped Si in breaking the traditional n-type doping limit observed in pnictogen-hyperdoped Si. Our work elucidates the underlying microscopic mechanisms that give rise to substantial carrier densities in chalcogen-hyperdoped Si, which will pave the way for the development of high-performance silicon-based devices.

Effects of Mn and Si on the ferrite decarburization of spring steel

Using a combination of experiments and first-principles calculations, we systematically investigated the influence of Mn and Si on the ferrite decarburization process in spring steel. The experimental findings demonstrated that the presence of Mn inhibited the ferrite decarburization of spring steel, and this inhibition effect was further enhanced when Mn and Si were mixed. The calculation results revealed the following insights: (1) In α-Fe, the interactions between Mn and neighboring carbon atoms were primarily weakly mutually attractive, while the interaction between Si and neighboring carbon atoms changed from repulsion to weak attraction with increasing distance. (2) The introduction of Mn raised the energy barrier for the outward diffusion of the carbon in α-Fe, thereby impeding its diffusion. (3) The coexistence of Mn and Si in α-Fe to formed an Mn–Si pair, resulting in a more pronounced inhibition of the outward diffusion of the carbon. These results significantly contribute to a deeper understanding of how alloying elements influence the decarburization process in steels and shed light on potential strategies for optimizing this aspect in steel production.

Nonreciprocal terahertz wavefront manipulation based on a magneto-optical metasurface with an orthogonal meta-atom pair

Active wavefront manipulation and nonreciprocal transmission give key performances in beam steering, unidirectional isolation, and multichannel multiplexing. In this work, we propose a magneto-optical (MO) Pancharatnam–Berry (PB) metasurface, which consists of a pair of orthogonal anisotropic Si and InSb columns. This orthogonal meta-atom pair (OMP) structure leads to destructive interference in the geometric phase so that the wavefront manipulation is in the OFF state without a biased magnetic field (BMF). When the BMF is applied, the nonreciprocal transmission effect for different spin states in InSb breaks the original destructive interference, so the devices provide different geometric phase distribution for different magnetic field directions and spin states, achieving the ON state of the nonreciprocal wavefront manipulation, where cross-polarization circular dichroism is higher than 0.98 and the polarization conversion rate is over 97%. Furthermore, based on this mechanism, a beam deflector and a vortex beam generator are designed by arranging the OMP with different azimuthal angles. Compared with ordinary reciprocal wavefront manipulation, this work shows that this MO PB metasurface with OMP structure has irreplaceable functions in the isolation, selection, and conversion of the two conjugate spin beams, providing a unique path for active THz wavefront manipulation.

Formula omitted] spectrometer of [formula omitted]-decay electrons with Si(Li)-detectors

We present a description of the originally developed β-spectrometer consisting of two Si(Li)-detectors with sensitive area thickness above 8 mm and 4π-geometry. The full absorption spectrometer allows for direct measurements of β-spectra, disregarding the corrections to response function induced by the electron backscattering from the crystal surface. In case of β-spectra of transitions to the excited state of the daughter isotope additional 3″ BGO-detector is used in order to detect the γ-quanta in coincidence with the pair of Si(Li)-spectrometers.

Implementation of the electron track-structure mode for silicon into PHITS for investigating the radiation effects in semiconductor devices

In order to elucidate the mechanism of radiation effects in silicon (Si) devices, such as pulse-height defects and semiconductor soft errors, we developed an electron track-structure model dedicated to Si and implemented it into particle and heavy ion transport code system (PHITS). Then, we verified the accuracy of our developed model by comparing the ranges and depth-dose distributions of electrons in Si obtained from this study with corresponding experimental values and other simulated results. As an application of the model, we calculated the mean energies required to create an electron–hole pair in crystalline Si. Our calculated result agreed with the experimental data when the threshold energy for generating secondary electrons was set to 2.75 eV, consistent with the corresponding data deduced from past studies. This result suggested that the improved PHITS can contribute to the precise understanding of the mechanisms of radiation effects in Si devices.

Ab Initio Study of Chiral Phonons in Ternary YAlSi Compound

Ternary compound YAlSi crystallizes with orthorhombic Cmcm symmetry. This structure contains Y–Si pairs of atoms, creating honeycomb-like sublattices. In this paper, we investigate the dynamical properties of the system, focusing on the chiral modes. These modes are associated with the circular motion of the atoms. We show that the chiral modes can be realized in the YAlSi compound, and it makes this compound suitable for further experimental study of the chiral phonons.

Terahertz scattering-type near-field microscopy quantitatively determines the conductivity and charge carrier density of optically doped and impurity-doped silicon

A terahertz scattering-type scanning near-field optical microscope is used for nano-scale non-invasive conductivity measurements on bulk silicon samples. We first investigate the case where the density of charge carriers is determined by optical interband excitation. We show that the amplitude and phase of the near-field signal are reproduced by simulations based on an established near-field interaction model, which takes the Drude conductivity, ambipolar carrier diffusion, and known recombination properties of photo-excited carrier pairs in Si into account. This study is then extended to impurity-doped Si. We demonstrate that the phase of the near-field signal, which can easily be measured in absolute terms, allows us to quantitatively determine the conductivity of the specimens, from which the carrier density is derived based on the known carrier momentum relaxation time. A measurement at a single properly chosen terahertz frequency is sufficient. The technique proposed here holds promise for the spatially resolved quantitative characterization of micro- and nanoelectronic materials and devices.

Dependency of a localized phonon mode intensity on compositional cluster size in SiGe alloys

Using molecular dynamics, we found that the localized phonon-mode spectrum in SiGe alloys, which was recently discovered by an inelastic x-ray scattering experiment, changes according to the size distribution of compositional clusters in alloys. By varying the spatial distributions of Si and Ge, alloy models with differing compositions of Si and Ge clusters were able to be produced. For a range of alloys comprising 20%–80% Ge, a mixture of small and intermediate sized clusters will give the highest intensities of the local mode. The Si–Ge optical mode intensity increases with the local mode intensity, but the Si–Ge bond alone is not sufficient to produce the local mode. Si rich alloys with small Ge clusters produce the highest local mode intensities, suggesting that this mode is caused by small Ge clusters surrounded by Si pairs.

Molecular dynamics simulation of the effect of cooling rate on the structure and properties of lithium disilicate glass

The effects of cooling rate on the structure and properties of lithium disilicate (LS2) glass are investigated using molecular dynamics (MD) computer simulations. The evolution of structural features such as pair distribution function, bond angle distribution, and Li coordination distribution are determined, and correlated with dynamic and static properties to elucidate the effects of cooling rate. The density, elastic moduli, and diffusion coefficient are found to be highly sensitive to cooling rate, whereas the Si pair distribution function and bond angle distribution are weakly affected by the cooling rate. Additionally, the changes of Si-O-Si bond angle and Li coordination number suggest the formation of Li cluster at lower cooling rates. Furthermore, by comparing results from other simulations and reported experiments, we confirm that the increase of cooling rate leads to an increase of conductivity and a decrease of density. At very high cooling rates, we find that all the atoms in LS2-glass do not relax simultaneously, but do so in two distinct configurations.

On generic complexity of the problem of searching of isomorphism for finite semigroups

This article is devoted to the investigation of generic complexity of the problem of searching of isomorphism for finite semigroups. That problem can be formulated as follows. For any pair of isomorphic finite semigroup Si and S2 with elements {1, 2,…, n}, represented by its multiplication tables, we need to find a bijection (permutation) of the set {1, 2,…, n}, which implements an isomorphism between S1 and S2. As for the classical isomorphism problem, any polynomial (even probabilistic) algorithms are unknown for the problem of searching of isomorphism for finite semigroups. We construct natural subproblems of this problem, for which there is no polynomial generic algorithm, provided that this problem is hard in the classical case. The study was carried out within the framework of the state contract of the Sobolev Institute of Mathematics (project no. 0314-2019-0004).

Correlation between mixing enthalpy and structural order in liquid Mg−Si system

The mixing enthalpies and structural order in liquid Mg−Si system were investigated via ab-initio molecular dynamics at 1773 K. By calculating the transferred charges and electron density differences, the dominance of Si−Si interactions in the chemical environments around Si was demonstrated, which determined that the mixing enthalpy reached the minimum on Mg-rich side. In terms of Honeycutt and Anderson (HA) bond pairs based on the partial pair correlation functions, the attraction between Si−Si pairs and Mg atoms was revealed, and the evolution of structural order with Si content was characterized as a process of constituting frame structures by Si−Si pairs that dispersed Mg atoms. Focusing on tetrahedral order of local Si-configurations, a correlation between the mixing enthalpy and structural order was uncovered ultimately, which provided a new perspective combining the energetics with geometry to understand the liquid Mg−Si binary system.

A Silicon 4π Spectrometer of β-Decay Electrons with Energies of up to 3 MeV

A 4π β spectrometer consists of two Si(Li) detectors with a sensitive region that is more than 8-mm thick. Using this total-absorption spectrometer it is possible to make a direct measurement of β spectra without correcting the response function for the electron backscattering from the crystal surface. The β spectra of transitions to the excited states of daughter nuclei have been measured using an additional 3'' BGO detector of γ rays, which is connected in coincidence with the pair of Si(Li) detectors.

Langmuir-Type Mechanism for In-Situ Doping in CVD Si and Si1−xGex Epitaxial Growth

In-situ doping process of B and P in CVD Si and Si1−xGex (100) epitaxial growth is investigated using SiH4–GeH4–dopant gas (B2H6 or PH3)–H2 gas mixture. For lower dopant gas partial pressure, the in situ doping is explained quantitatively by the Langmuir-type adsorption and reaction at Si–Si, Si–Ge or Ge–Ge pair sites on the (100) surface. For higher B2H6 gas partial pressure (P B2H6), it is proposed that SiH4, GeH4 and B2H6 molecules are adsorbed and react partially at the B-occupied sites where B2H6 molecules have been adsorbed on the (100) surface. For higher PH3 gas partial pressure (P PH3), it is proposed that the in situ doping and Ge incorporation are enhanced by the self-limited adsorption of PH3 molecules and the decomposition of GeH4 at the P-occupied sites where PH3 molecules have been adsorbed on the (100) surface, respectively, and that the decrease of P concentration with increasing P PH3 in high P PH3 region is caused by lower segregation coefficients of P at Si–Ge and Ge–Ge pair sites than that at Si–Si pair sites according to the Henry’s law. Fairly good agreement is obtained between all the experimental data and the modified Langmuir-type mechanism for in situ doping.

Understanding thermal expansion of pressurized silica glass using topological pruning of ring structures

AbstractWe investigate the structural origin of the large thermal expansion coefficient of hot‐compressed silica glass upon heating to a threshold temperature using molecular dynamics (MD) simulations. While the simulated thermal expansion at low temperature correlates well with the elongation of the average interatomic separation distance of the Si–O or Si–Si pair, the excess thermal expansion due to hot compression mainly results from change in medium range order, including a decrease in the population of large rings upon heating, without significant modification of the small ring population and without changing the short range ordering, such as the Si‐O coordination number. The reduction in the characteristic ring size can be connected to the high thermal expansion through the topological pruning mechanism. Such large thermal expansion is suppressed when the silica glasses are held at their respective quench pressures. The suppression of this excess thermal expansion is consistent with the pressure stabilization of the large ring structures.

X-ray scattering study of water confined in bioactive glasses: experimental and simulated pair distribution function.

Temperature-dependent total X-ray scattering measurements for water confined in bioactive glass samples with 5.9 nm pore diameter have been performed. Based on these experimental data, simulations were carried out using the Empirical Potential Structure Refinement (EPSR) code, in order to study the structural organization of the confined water in detail. The results indicate a non-homogeneous structure for water inside the pore, with three different structural organizations of water, depending on the distance from the pore surface: (i) a first layer (4 Å) of interfacial pore water that forms a strong chemical bond with the substrate, (ii) intermediate pore water forming a second layer (4-11 Å) on top of the interfacial pore water, (iii) bulk-like pore water in the centre of the pores. Analysis of the simulated site-site partial pair distribution function shows that the water-silica (Ow-Si) pair correlations occur at ∼3.75 Å. The tetrahedral network of bulk water with oxygen-oxygen (Ow-Ow) hydrogen-bonded pair correlations at ∼2.8, ∼4.1 and ∼4.5 Å is strongly distorted for the interfacial pore water while the second neighbour pair correlations are observed at ∼4.0 and ∼4.9 Å. For the interfacial pore water, an additional Ow-Ow pair correlation appears at ∼3.3 Å, which is likely caused by distortions due to the interactions of the water molecules with the silica at the pore surface.

Betavoltaic battery based on reduced-Graphene-Oxide/Si heterojunction

Abstract This paper presents a new beta converter cell based on reduced graphene oxide (rGO)/Si heterojunction suitable for betavoltaic batteries. The potential barrier created in the rGO/Si interface induces an internal electric field in the Si substrate. This internal electric field can be used for separating the beta-generated electron-hole pairs in Si. Ni63, with the activity of 5mci is used as the beta radioisotope source. The current-voltage characteristic of the cell is measured with and without radioisotope radiations. The circuit model of the junction is also presented. The cell shows an open circuit voltage of 34 mV, a short circuit current of 0.41 μA/cm2, and high conversion efficiency of 3.9% under exposure of beta radioisotope Ni63. The presented structure, additionally, benefits from the capability of graphene in the absorption of beta particles and carrier separation via ununiformed finger shape of front electrodes.

Identification of the donor and acceptor states of the bond-centered hydrogen–carbon pair in Si and diluted SiGe alloys

The electrical and structural properties of two levels (E90 and H180) in diluted n- and p-type Si1 − xGex alloys (0 ≤ x ≤ 0.070) are investigated by high-resolution Laplace deep level transient spectroscopy measurements and first-principles calculations. By exploiting the presence of Ge atoms close to a substitutional C atom, we show that E90 and H180 belong to the same C–H pair (labeled CH1BC) with H in a bond-centered configuration (C—HBC—Si). The relative energies of the various configurations of the CH pair are calculated, and the complete vibrational spectra in the lowest-energy structures for each charge state are predicted.

Bonding and electronics of the silicene/MoTe2 interface under strain

Abstract Density functional theory calculations have been performed to investigate the structural and electronic properties of silicene/MoTe2 heterostructure under strain. The results show three different interaction cases depending on the interface configuration. The interactions can be weak and limited to van der Waals forces, or strong and covalent producing hybrid states between pz–pz and px/py–pz orbital pairs of Si and Te. Covalent bonding gives rise to well-defined, energetically favorable configurations with properties distinguishably different from those observed in vdW-only configurations. The new hybrid states shared between the layers allow for more effective charge transfer between them, which is critical for the performance of electronic and optoelectronic devices.