Si Vacancies Research Articles

Preparation, thermoelectric properties, and crystal structure of boron-doped Mg2Si single crystals

Mg2Si is a potential thermoelectric (TE) material that can directly convert waste energy into electricity. In expectation of improving its TE performance by increasing electron carrier concentration, the element boron (B) is doped in Mg2Si single crystals (SCs). Their detailed crystal structures are definitely determined by using white neutron holography and single-crystal x-ray diffraction (SC-XRD) measurements. The white neutron holography measurement proves that the doped B atom successfully substitutes for the Mg site. The SC-XRD measurement confirms the B-doping site and also reveals the presence of the defect of Si vacancy (VSi) in the B-doped Mg2Si SCs. The fraction of VSi increases with increasing B-doping concentration. In the case of B-doped Mg2Si polycrystals (PCs), VSi is absent; this difference between the SCs and PCs can be attributed to different preparation temperatures. Regarding TE properties, the electrical conductivity, σ, and the Seebeck coefficient, S, decreases and increases, respectively, due to the decrease in the electron carrier concentration, contrary to the expectation. The power factor of the B-doped Mg2Si SCs evaluated from σ and S does not increase but rather decreases by the B-doping. The tendencies of these TE properties can be explained by considering that the donor effect of the B atom is canceled by the acceptor effect of VSi for the B-doped Mg2Si SCs. This study demonstrates that the preparation condition of Mg2Si should be optimized to prevent the emergence of an unexpected point defect.

First-principles calculations of Stark shifts of electronic transitions for defects in semiconductors: the Si vacancy in 4H-SiC

Point defects in solids are promising single-photon sources with application in quantum sensing, computing and communication. Herein, we describe a theoretical framework for studying electric field effects on defect-related electronic transitions, based on density functional theory calculations with periodic boundary conditions. Sawtooth-shaped electric fields are applied perpendicular to the surface of a two-dimensional defective slab, with induced charge singularities being placed in the vacuum layer. The silicon vacancy (V Si) in 4H-SiC is employed as a benchmark system, having three zero-phonon lines in the near-infrared (V1, V1′ and V2) and exhibiting Stark tunability via fabrication of Schottky barrier or p-i-n diodes. In agreement with experimental observations, we find an approximately linear field response for the zero-phonon transitions of V Si involving the decay from the first excited state (named V1 and V2). However, the magnitude of the Stark shifts are overestimated by nearly a factor of 10 when comparing to experimental findings. We discuss several theoretical and experimental aspects which could affect the agreement.

The mechanism of elastic and electronic properties of Tungsten Silicide (5/3) with vacancy defect from the first-principles calculations

The development prospects of transition metal silicon-based materials have received considerable attention. Tungsten-based silicides are one of the most important silicon-based high-temperature materials. Here, the mechanism of elastic and electronic properties of Tungsten Silicide (5/3) with vacancy defects was further studied using the first-principles approach. The vacancy formation enthalpy, elastic constants, elastic moduli, brittle/ductile behavior, hardness and electronic structure of the perfect W5Si3 and W5Si3 with vacancies were calculated and discussed, respectively. The result demonstrates that W5Si3 with W vacancies exhibit the better stability than W5Si3 with Si vacancies. Vacancy defects weaken the mechanical strength for the perfect W5Si3. The removal of the Si atoms makes W5Si3 with vacancies exhibit the better ductility than the parent W5Si3. The electronic structures explain the mechanism for structural stability and mechanics properties of W5Si3 by vacancy defects.

Dislocation sink annihilating threading dislocations in strain-relaxed Si1−xGex layer

We proposed a dislocation sink technology for achieving Si1−xGex multi-bridge-channel field-effect-transistor beyond 5 nm transistor design-rule that essentially needs an almost crystalline-defect-free Si1−xGex channel. A generation of a dislocation sink via H+ implantations in a strain-relaxed Si0.7Ge0.3 layer grown on a Si substrate and a following annealing almost annihilate completely misfit and threading dislocations located near the interface between a relaxed Si0.7Ge0.3 layer and a Si substrate. A real-time (continuous heating from room temperature to 600 °C) in situ high-resolution-transmission-electron-microscopy and inverse-fast-Fourier-transform image observation at 1.25 MV acceleration voltage obviously demonstrated the annihilation process between dislocation sinks and remaining misfit and threading dislocations during a thermal annealing, called the [SiI or GeI + VSi or VGe → Si1−xGex] annihilation process, where SiI, GeI, VSi, and VGe are interstitial Si, interstitial Ge, Si vacancy, and Ge vacancy, respectively. In particular, the annihilation process efficiency greatly depended on the dose of H+ implantation and annealing temperature; i.e. a maximum annihilation process efficiency achieved at 5 × 1015 atoms cm−2 and 800 °C.

A first-principles study on the influences of metal species Al, Zr, Mo and Tc on the mechanical properties of U3Si2.

A first-principles approach is employed to study the influences of the metal species Al, Zr, Mo and Tc on the mechanical properties of U3Si2. When the Al, Zr, Mo and Tc atoms diffuse into the vacancy sites, they dissolve into the lattice, as confirmed by the solution energies. It is found that the compounds of U3Si2 with low amounts of Al, Zr, Mo, and Tc in the Si vacancies or Al, Zr, and Mo in the U vacancies can behave in the manner of ductility. However, in the cases where Al, Zr, Mo and Tc occupy the interstitial sites, all the compounds are demonstrated to be brittle. Furthermore, the stress-strain relationship for the U3Si1.9375Mo0.0625 system was calculated, which illustrates the enhanced ductility. The current results indicate that the substitution of Si by carefully selected metal atoms can enhance the performance of U3Si2.

Effects of vacancy on the electronic and optical properties of β-Si3N4 by first-principles

We use a first-principles calculation to explore the effects of vacancies on structural, electronic and optical properties of [Formula: see text]-[Formula: see text] based on density functional theory (DFT). The results show that after optimization, the Si vacancy system of [Formula: see text]-[Formula: see text] is more difficult to produce than N vacancy system under the same thermodynamic conditions. The band gaps including N vacancy and Si vacancy systems are smaller than that of the perfect crystal. The charge density difference and population analysis show that the bonding near Si vacancy has stronger covalent property, whereas those near nitrogen vacancy have stronger ionic property. For Si vacancy system, the materials have much higher values of the imaginary part of the dielectric constant than those of N vacancies and perfect [Formula: see text]-[Formula: see text]. The maximum value of the Si vacancy system in absorption and reflection spectra is lower than those in different nitrogen vacancy systems.

Semiconducting SiGeSn high-entropy alloy: A density functional theory study

High-entropy alloys (HEAs), which have been intensely studied due to their excellent mechanical properties, generally refer to alloys with multiple equimolar or nearly equimolar elements. According to this definition, Si-Ge-Sn alloys with equal or comparable concentrations of the three group IV elements belong to the category of HEAs. As a result, the equimolar elements of Si-Ge-Sn alloys likely cause their atomic structures to exhibit the same core effects of metallic HEAs such as lattice distortion. Here, we apply density functional theory (DFT) calculations to show that the SiGeSn HEA indeed exhibits a large local distortion effect. Unlike metallic HEAs, our Monte Carlo and DFT calculations show that the SiGeSn HEA exhibits no chemical short-range order due to the similar electronegativity of the constituent elements, thereby increasing the configurational entropy of the SiGeSn HEA. Hybrid density functional calculations show that the SiGeSn HEA remains semiconducting with a bandgap of 0.38 eV, promising for economical and compatible midinfrared optoelectronic applications. We then study the energetics of neutral single Si, Ge, and Sn vacancies and (expectedly) find wide distributions of vacancy formation energies, similar to those found in metallic HEAs. However, we also find anomalously small lower bounds (e.g., 0.04 eV for a Si vacancy) in the energy distributions, which arise from the bond reformation near the vacancy. Such small vacancy formation energies and their associated bond reformations retain the semiconducting behavior of the SiGeSn HEA, which may be a signature feature of a semiconducting HEA that differentiates from metallic HEAs.

Study on magnetic properties of Fe-doped 3C-SiC nanowires

With the development of semiconductor spintronics, dilute magnetic semiconductors have become a research hotspot recently. Fe-doped 3C-SiC nanowires (C/Si = 1.4) with ferromagnetism at 300 K were fabricated by LPCVD. The electronic structures and magnetic properties of Fe-doped 3C-SiC nanowires without or with vacancies were studied by first-principle calculation. The experimental results reveal that the samples mainly grew along the [1 1 1] direction and possessed the obvious ferromagnetism at 100 K, 200 K, and 300 K. The theoretical calculations show that Fe-doped nanowires without vacancy are metallic property and non-magnetic, Fe-doped nanowires with C vacancy are metallic property and stable antiferromagnetism, and Fe-doped nanowires with Si vacancy are half-metal materials and stable ferromagnetism. The results of the density of states indicate that the magnetism originates from the hybridization of Fe-3d, C-2p and Si-3p states. The room temperature ferromagnetism of the 3C-SiC nanowires should be attributed to the coexistence of Fe impurity and Si vacancy.

Correlation of native defects between epitaxial films and polycrystalline BaSi2 bulks based on photoluminescence spectra

We conducted photoluminescence (PL) measurements at 8 K on BaSi2 epitaxial films and polycrystalline bulks. PL intensities were enhanced for films grown at 650 °C compared to those at 580 °C due to the reduction of carrier recombination centers. The PL spectra were fitted well by four Gaussian curves peaking at almost the same energies regardless of the film and bulk forms. Their contribution changed depending on the Ba-to-Si atomic ratio in BaSi2. Based on the PL spectra and defect levels acquired by deep-level transient spectroscopy, we established a model for radiative defects which stem from intrinsic defects such as Si vacancies.

Density functional theory capture of electronic structures and optical properties of vacancy doped 3C-SiC systems

Using a first principles calculation we investigate the electronic structures and optical properties of vacancy doped 3C-SiC systems. It is found that the C vacancy doping 3C-SiC has most considerable stability compared to that doped by Si vacancy when the Schottky defect occurs. The energy band density increases after doping and some impurity levels appear near the Fermi level, which may increase the carrier concentration and electronic intraband or interband transition. The charge-difference density maps show that the covalent order of bonding is VSi-VSi > VSi > pristine > VC > VC-VC. For the VSi doped system, the static dielectric constant of increases to 21.581 and the imaginary part raises remarkably, demonstrating its potential applications in the fields of aerospace applications. Simultaneously, a ‘blue shift’ phenomenon can be observed for the doping systems. The systems exhibit strong absorption and reflection capacities in the range of 5–10 eV before and after doping, showing the ‘Barrier Type’ feature. The host peak values of the L(ω) in the doped systems are lower, presenting the feature of the semiconductors.

Relationship between electrical properties and interface structures of SiO2/4H-SiC prepared by dry and wet oxidation

We have investigated the relationship between the electrical properties and interfacial atomic structure of SiO2/4H-SiC interfaces, prepared by dry and wet thermal oxidation procedures with 4H-SiC (0001) and 4H-SiC (000-1) substrates, using extended x-ray absorption fine structure (EXAFS) spectroscopy and electrical methods. From the current–voltage (I–V) and capacitance–voltage (C–V) measurements, the gate leakage current onset and density of interface states were shown to depend on the thermal oxidation procedure used for creating SiO2/4H-SiC (000-1) interfaces. This dependence was not observed for SiO2/4H-SiC (0001) interfaces. From EXAFS oscillations, we found that C and Si vacancies formed at the SiC side of SiO2/4H-SiC (0001) and SiO2/4H-SiC (000-1) interfaces, respectively. Compressive stress at the SiC sides of SiO2/4H-SiC (0001) and SiO2/4H-SiC (000-1) interfaces prepared with the dry thermal oxidation procedure caused decreases in bond lengths. Stress at the interface was smaller in the case of wet oxidation. Wet oxidation released the interface stress more effectively for 4H-SiC (000-1) substrates than for 4H-SiC (0001) substrates. A high gate leakage current onset related to a high interface charge, while a high density of interface states related to a high interface stress and a high interface roughness.

Building and exploring libraries of atomic defects in graphene: Scanning transmission electron and scanning tunneling microscopy study.

The presence and configurations of defects are primary components determining materials functionality. Their population and distribution are often nonergodic and dependent on synthesis history, and therefore rarely amenable to direct theoretical prediction. Here, dynamic electron beam-induced transformations in Si deposited on a graphene monolayer are used to create libraries of possible Si and carbon vacancy defects. Deep learning networks are developed for automated image analysis and recognition of the defects, creating a library of (meta) stable defect configurations. Density functional theory is used to estimate atomically resolved scanning tunneling microscopy (STM) signatures of the classified defects from the created library, allowing identification of several defect types across imaging platforms. This approach allows automatic creation of defect libraries in solids, exploring the metastable configurations always present in real materials, and correlative studies with other atomically resolved techniques, providing comprehensive insight into defect functionalities.

Determining the Jahn-Teller stabilization energy of adatom vacancies on a Si(111)- 3×3 :B surface

A charged vacancy on the Si surface, which generates deep levels within the band gap to accommodate multiple charge states, becomes increasingly important with the device scaling. Yet its characteristics have not been addressed as comprehensively as the bulk vacancy in Si. Here we generate the adatom vacancies on a heavily B doped Si(111) surface and measure their structural and electronic properties by using scanning tunneling microscopy. We find that they are Jahn-Teller distorted in the ground state but undergo the symmetry-restoring transition when gated by the external bias. We also determine the stabilization energy of the Jahn-Teller transition as well as the hopping energy between the dangling bonds of the adatom vacancy quantitatively.

(Invited) Mie-like Scattering of Phonons by a Single Vacancy in IV-VI Compounds

Defects engineering has been a widely used approach for developing low thermal conductivity materials. Also, for some materials (e.g., IV-VI compound), considerably large amount of vacancies was often observed in single crystalline samples. The atomic vacancy in dilute case was assumed as a point defect that can cause the Rayleigh scattering with its rate proportional to the fourth power of phonon frequency. Due to the large dependence of scattering rate on phonon frequency, a vacancy was considered to only weakly suppress the mode thermal conductivity of low frequency acoustic phonons. We show that this does not hold for IV-VI materials. We compare the phonon scattering by a single vacancy in group IV (diamond and Si) and IV-VI (PbTe and GeTe) crystals using an exact ab initio Green’s function approach. The phonon-vacancy scattering rates in diamond and Si follow the well-known fourth-power dependence on phonon frequency. However, PbTe and GeTe exhibit phonon scattering rates that have much weaker frequency dependence. As a result, a vacancy scatters low frequency acoustic phonons more strongly in PbTe and GeTe than in Si and diamond. This unusual behavior in PbTe and GeTe originates from the significant changes of interatomic force constants that extend up to the distance of 10 angstrom from the vacancy site. Therefore, a vacancy in IV-VI materials cannot be assumed as a point defect but its finite size should be considered for phonon scattering. In contrast, a vacancy in Si and diamond changes interatomic force constants only up to 3 angstrom range. Our study provides new insights on the phonon spectral contribution to thermal transport when vacancies exist in IV-VI materials and will help to develop low thermal conductivity materials by engineering vacancies.

Coupled inter-site reaction and diffusion: Rapid dehydrogenation of silicon vacancies in natural olivine

Abstract Natural olivine from Zermatt (Switzerland) and Almklovdalen (Norway) were dehydrogenated at 1 bar, 517–1009 °C at various oxygen fugacity conditions. Following experiments, H contents (either bulk, or core-rim profiles) were measured using Fourier transform infrared spectroscopy, and spectra were resolved into Gaussian peaks. The starting olivine contained ∼150 and ∼10 wt. ppm H2O for the Zermatt and Almklovdalen samples respectively, but importantly the initial H distribution in both corresponded to defects with 4H+ occupying Si vacancies. Experiments in pure forsterite Padron-Navarta and Hermann, (2017) showed that this defect diffuses very slowly relative to other H diffusion mechanisms in olivine. Conversely, we show that, in the Almklovdalen samples, H loss from the Si-vacancy defect can be extremely rapid, and approaches the fastest known mechanism of H diffusion (proton-polaron diffusion) in olivine. The rate of dehydrogenation from the Zermatt olivine is slightly slower, more consistent with the diffusivity of hydrogenated M-site vacancies. The sum of all defects, along with the integrated area of the main Si vacancy peak (3612 cm−1) generally shows simple diffusive behaviour, whereas profiles (or maps) resolved into individual peaks reveal complex profile shapes not consistent with a simple out-diffusion mechanism. This behaviour can be modeled as a combination of inter-site reaction and diffusion, whereby H leaves the tetrahedral site, moves into a faster diffusion pathway, then rapidly exits the crystal. The rate of H loss from, or gain into, olivine can therefore be either diffusion limited, reaction limited, or a combination of the two. This may explain current discrepancies between experiments conducted under different conditions and between experimental and natural data, including the recent observation that H stored in Si vacancies in metamorphic olivine may be retentive over millions of years, despite the capability to diffuse rapidly via a different mechanism.

Marked enhancement of the photoresponsivity and minority-carrier lifetime of BaSi2 passivated with atomic hydrogen

Passivation of barium disilicide ($\mathrm{BaS}{\mathrm{i}}_{2}$) films is very important for their use in solar cell applications. In this paper, we demonstrated the effect of hydrogen (H) passivation on both the photoresponsivity and minority-carrier lifetime of $\mathrm{BaS}{\mathrm{i}}_{2}$ epitaxial films grown by molecular beam epitaxy. First, we examined the growth conditions of a 3-nm-thick hydrogenated amorphous silicon (a-Si) capping layer formed on a 500-nm-thick $\mathrm{BaS}{\mathrm{i}}_{2}$ film and found that an H supply duration (${t}_{\mathrm{a}\text{\ensuremath{-}}\mathrm{Si}:\mathrm{H}}$) of 15 min at a substrate temperature of 180 \ifmmode^\circ\else\textdegree\fi{}C sizably enhanced the photoresponsivity of the $\mathrm{BaS}{\mathrm{i}}_{2}$ film. We next supplied atomic H to $\mathrm{BaS}{\mathrm{i}}_{2}$ epitaxial films at 580 \ifmmode^\circ\else\textdegree\fi{}C and changed supply duration (${t}_{\mathrm{BaSi};\mathrm{H}}$) in the range of 1--30 min, followed by capping with an a-Si layer. The photoresponsivity of the films changed considerably depending on ${t}_{\mathrm{BaSi};\mathrm{H}}$ and reached a maximum of 2.5 A/W at a wavelength of 800 nm for the sample passivated for ${t}_{\mathrm{BaSi};\mathrm{H}}=15$ min under a bias voltage of 0.3 V applied to the front-surface indium-tin-oxide electrode with respect to the back-surface aluminum electrode. This photoresponsivity is approximately one order of magnitude higher than the highest value previously reported for $\mathrm{BaS}{\mathrm{i}}_{2}$. Microwave photoconductivity decay measurements revealed that the minority-carrier lifetime of the $\mathrm{BaS}{\mathrm{i}}_{2}$ film with the highest photoresponsivity was 14 \ensuremath{\mu}s, equivalent to its bulk carrier lifetime ever reported. We performed theoretical analyses based on a rate equation including several recombination mechanisms and reproduced the experimentally obtained decay curves. We also calculated the total density of states of $\mathrm{BaS}{\mathrm{i}}_{2}$ by ab initio studies when one Si vacancy existed in a unit cell and one, two, and three H atoms occupied Si vacancy or interstitial sites. A Si vacancy caused a localized state with two energy bands to appear close to the middle of the band gap. In certain cases, H passivation of the Si dangling bonds can markedly decrease trap concentration. From both experimental and theoretical viewpoints, we conclude that an atomic H supply is beneficial for $\mathrm{BaS}{\mathrm{i}}_{2}$ solar cells.

Point defect populations of forsterite revealed by two-stage metastable hydroxylation experiments

Hydroxylation is a method that allows “decoration” of the pre-existing point defect structure of nominally anhydrous minerals, such as olivine. We tested this method on synthetic forsterite (Fo: $${\text{Mg}}_{2} {\text{SiO}}_{4}$$ ) crystals. To control starting point defect structures, Fo crystals were pre-annealed at different temperatures ( $$1100{-}1500\,{^\circ }{\text{C}}$$ ), silica activity conditions (forsterite–enstatite Fo–En and forsterite–periclase Fo–Per) and oxygen fugacity (0.21 and $$10^{-6}$$ bars). Then low-temperature hydroxylation (900 °C, 1.5 GPa) of the crystals successfully allowed the decoration with protons of pre-existing point defect structures, as subsequently revealed by infrared spectroscopy. Protons are arranged in three different point defect stoichiometries in Fo, related to Mg and Si vacancies ([Mg] and [Si], respectively) as well as to a trivalent cation-associated substitution mechanism ([triv]). Over the timescale and equilibrium conditions studied, hydroxylation does not reset the point defect structure inherited from pre-anneal. The data further show that the concentrations of [Mg]-, [Si]- and [triv]-hydrated defects are function of pre-anneal silica activity and temperature. Laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis of the crystals revealed diffusion of Al and Fe into the crystals during the pre-annealing, a phenomenon clearly promoted at high $${\text{a}}_{{{\text{SiO}}_{2} }}$$ . The data confirm a very fast mechanism of Al diffusion in Fo during pre-annealing, and suggest a strong coupling between $${\text{H}}^{+}$$ and $${\text{Al}}^{3+}$$ during hydroxylation. Overall, they show the strong importance of $${\text{a}}_{{{\text{SiO}}_{2} }}$$ and temperature in the incorporation of trace cations in forsterite, and the subsequent effects of incorporation of trace cations on Mg- and Si-related point defects in Fo. The dry point defect population of Fo is determined by interactions between the trace trivalent cations and dry Si and Mg vacancies. Without trace elements, T only has a limited effect on Mg- and Si-related point defect populations. Finally, approaching or potentially slightly exceeding the Fo–En solidus leads to strong changes in the trace element concentration and point defect population in Fo, which may be related to either partial melting or pre-melting effects.

Defect and temperature effects on the mechanical properties of kaolinite: a molecular dynamics study

AbstractMolecular dynamics simulations of different defective kaolinites under tension were performed to reveal the effects of defect location, type, density and temperature on their mechanical properties. Four types of defective kaolinite with Si vacancies were constructed. Based on the atomic-scale deformation and failure processes of defective kaolinite and its stress–strain curves, the Young's moduli and tensile strengths in three crystal directions were obtained and compared with the existing theoretical values from the literature. The defect location at each layer does not affect the mechanical properties of kaolinite and the cracks initiated at the defective sites. The atom density of each model was calculated in order to investigate the defect-type effect on the mechanical properties of kaolinite. The simulation results also showed that kaolinite exhibits brittle failure behaviour and the mechanical properties degrade significantly with increasing defect density and temperature. The influence of temperature on the mechanical properties of defective kaolinite is discussed in detail.

Effect of point defects on electronic and magnetic properties of single-layer SiO

ABSTRACTBased on spin-polarised density functional theory calculations, we investigated the effect of point defects on electronic and magnetic properties of the single-layer (SL) asymmetric washboard silicon oxide (aw-SiO). The SL-aw-SiO is a counterpart of black phosphorene, and a new candidate of two-dimensional material family. This structure is dynamically and thermally stable and is a nonmagnetic semiconductor with a direct band gap. We found that single vacancy and divacancy give rise to significant change in the electronic and magnetic properties of SL-aw-SiO. The band gap of aw-SiO can be tuned by the substitution of Si atom instead of O atom, the antisite defect, the O atom vacancy and two atom vacancies. In addition, impurity states due to the defects can occur in the band continua and hence the band gap of aw-SiO is reduced. Having an integer magnetic moment, SL-aw-SiO upon Si vacancy and by substitution of O atom instead of Si atom may display half-metallic features.

Spin and charge state dependent electrical and magnetic properties of diamond with defects of vacancy and substituted silicon by first-principles calculation

In this work, the electrical and magnetic properties of diamond crystals with defects of substituted silicon (SiC), vacancy (VC), Si-vacancy (SiV), and Si-divacancy (SiV2) complexes are investigated by first-principles calculations considering the spin and charge state. For the cases of VC, SiV, and SiV2, the typical spin-splitting related defect levels are presented in the bandgap, mainly stemming from the dangling bonds surrounding the vacancies. The magnetic moments appear for the cases of neutral (VC0 and SiV0) and negatively charged (VC−, SiV−, and SiV2−) defects, determined by the occupied spin states and defect configurations. It reveals that the charge state plays an important role in modulating the magnetic moments and energy splitting of defect levels in diamond. This work provides theoretical guidance for achieving a class of spin-related diamond semiconductor devices.