Si Vacancies Research Articles

Defect structure transformation after thermal annealing in a surface layer of Zn-implanted Si(001) substrates

A combination of high-resolution X-ray diffractometry, Rutherford back scattering spectroscopy and secondary-ion mass spectrometry (SIMS) methods were used to characterize structural transformations of the damaged layer in Si(001) substrates heavily doped by Zn ions after a multistage thermal treatment. The shape of the SIMS profiles for Zn atoms correlates with the crystal structure of the damaged layer and depends on the presence of the following factors influencing the mobility of Zn atoms: (i) an amorphous/crystalline (a/c) interface, (ii) end-of-range defects, which are located slightly deeper than the a/c interface; (iii) a surface area enriched by Si vacancies; and (iv) the chemical interaction of Zn with Si atoms, which leads to the formation of Zn-containing phases in the surface layer.

First-principles study of OH defects in zircon

The infrared spectroscopic properties of selected OH defects in zircon are investigated by first-principles calculations. The explicit treatment of the coupled nature of OH motions in the stretching modes, together with the calculation of the intensity and polarization of absorption bands, makes it possible to directly compare theoretical and experimental data. The bands observed at 3,420 cm−1 (polarization parallel to c axis) and 3,385 cm−1 (polarization perpendicular to c axis) in natural and synthetic samples correspond to the IR-active vibrational modes of the hydrozircon defect, that is, fully protonated Si vacancy. The broad band observed at 3,515 cm−1 in the spectrum of zircon crystals grown in F-rich environments is consistent with the occurrence of composite (OH,F) tetrahedral defects. Calculations also show that the band observed at 3,200 cm−1 in the spectrum of synthetic undoped samples can be ascribed to fully protonated Zr vacancies. The theoretical values of integrated absorption coefficients indicate that general correlations can be reasonably used to determine the concentration of OH groups in zircon.

Novel half-metal and spin gapless semiconductor properties in N-doped silicene nanoribbons

We carry out a spin polarized first-principles study on the energetic and electronic properties of zigzag silicene nanoribbons (ZSiNRs) doped with N atoms, as well as N and Si vacancy (VSi) complexes. The formation energy analysis shows that the doped N atom and N-VSi complex prefer the edge sites in ZSiNRs. Due to breaking the degeneracy of the spin-polarization in ZSiNR, the substitution of N for Si atom exhibits a spin gapless semiconductor (SGS) property. When the N-VSi complex is introduced forming so called pyridine- and pyrrole-like structure in ZSiNR, they also exhibit half-metal or SGS behaviors with 100% spin-polarized currents in the Fermi level. These interesting properties may further stimulate potential applications of silicene-based nanostructures in nanoelectronics.

Impact of isovalent doping on the trapping of vacancy and interstitial related defects in Si

We investigate the impact of isovalent (in particular lead (Pb)) doping on the production and thermal stability of the vacancy-related (VO) and the interstitial-related (CiOi and CiCs) pairs in 2 MeV electron irradiated Si samples. We compare the Cz-Si samples with high and low carbon concentration, as well as with Pb-C and Ge-C codoped samples. Using Fourier Transform Infrared Spectroscopy (FTIR), we first determine that under the examined conditions the production of VO decreases with the increase of the covalent radius of the prevalent dopant. Moreover, the production of the VO, CiOi, and CiCs pairs is quite suppressed in Pb-doped Si. In addition, we conclude to an enhanced trapping of both Ci and Cs by Pb impurity under irradiation. The results are further discussed in view of density functional theory calculations. The relative thermodynamic stability of carbon and interstitial related complexes was estimated through the calculations of binding energies of possible defect pairs. This allows to investigate the preferred trapping of vacancies in Pb-doped samples and interstitials in the Ge-doped samples. The different behavior is revealed by considering the analysis of the ratio of vacancy-related to interstitial-related clusters derived from the FTIR measurements. The presence of PbV complexes is confirmed due to the mentioned analysis.

First Principle Study on the Electric Structure of β-FeSi<sub>2</sub> with Native Point Defects

The projector-augmented plane wave potentials method under the density functional theory (DFT ) was used to calcu-late the electronic structure of perfect and native point defective β-FeSi2 crystal. The calculated band structure shows that the band gap of perfect crystal is about 0.74eV, which is a little smaller than the experimental of about 0.9eV. The density of states results predicts that β-FeSi2 with Fe vacancies behaves n-type, and that with Si vacancies will shows p-type, which is in accordant with the experimental results.

EPR and emission study of silicon suboxide nanopillars

ABSTRACTThe results of correlated electron paramagnetic resonance (EPR) and photoluminescence (PL) study of obliquely deposited porous SiOxfilms after step-by-step 15 min annealing within 105 min in vacuum at 950°C are presented. The low intensity symmetrical and featureless EPR line with a g-value g=2.0044 and a linewidth of 0.77 mT has been detected in as-sputtered films and attributed to dangling bonds (DB) of silicon atoms in amorphous SiOxdomains withx=0.8. Successive annealing results in decreasing this line and the appearance of an intense EPR line with g=2.0025, linewidth of 0.11 mT and a hyperfine doublet with 1.6 mT splitting. According to the parameters this spectrum has been attributed to theEXcenter, a hole delocalized over four non-bridging oxygen atoms grouped around a Si vacancy in SiO2. The impact of chemical treatment before annealing and duration of anneals on the defect system, and a correlation of the PL intensity with decreasing of theDBEPR signal are discussed.

Role of Nanolaminated Crystal Structure on the Radiation Damage Tolerance of Ti3SiC2: Theoretical Investigation of Native Point Defects

Nanolaminated Ti3SiC2, a representative MAX phase, shows excellent tolerance to radiation damage. In this paper, first‐principles calculations were used to investigate the mechanism of intrinsic point defects in order to explain this outstanding property. Formation energies of intrinsic point defects are calculated and compared; and the results establish a low‐energy disorder mechanism in Ti3SiC2. In addition, the migration energy barriers of Si vacancy, Si interstitial, and TiSi antisite yield very low values: 0.9, 0.6, and 0.3 eV, respectively. The intercalation of Si atomic plane between Ti3C2 nanotwinning structures dominates the formation and migration of intrinsic native point defects in Ti3SiC2. The present study also highlights a novel method to improve radiation damage tolerance by developing nanoscale‐layered structure which can serve as a sink or rapid recovery channel for point defects.

The Magnetic Ordering of SiCN Films Prepared by Ion Implantation

Room-temperature ferromagnetism was observed in the SiCN films prepared by ion implantation. The result indicates that N ion implantation dosage in the film has great effect on the observed room-temperature ferromagnetism of the films. Along with the increase of ion implantation dosage, the N ions increase and the magnetism enhances. Because of the ion implantation will cause a lot of defects on the surface of SiC films, which will induce a lot of vacancies. The C atoms are replaced by the N ions doped, the concentration of the N ions decides the charges states and spin polarizations of Si vacancy defects. Local magnetic moment is induced because of the spin polarization of the Si vacancy defects, and the films show ferromagnetic properties.Charge states and spin polarizations of silicon vacancy defects can be manipulated by N atoms which induces the ferromagnetism.

DFT investigation of the influence of ordered vacancies on elastic and magnetic properties of graphene and graphene‐like SiC and BN structures

AbstractInfluence of ordered monovacancies on elastic properties of graphene is theoretically investigated by density functional theory (DFT) calculations. Inverse linear dependence of the graphene Young's modulus on the concentration of vacancies has been revealed and migration rate of the vacancies has been calculated as a function of applied strain. It is shown that the migration rate can be controlled by applying various strains or temperatures. The influence of ordered monovacancies on magnetic properties of graphene as well as graphene‐like hexagonal carbon silicide (2D‐SiC) and the boron nitride (h‐BN) structures is investigated. It is established that the presence of vacancies in all systems yields the appearance of local magnetic moment. However, in 2D‐SiC structure the magnetic moment occurs only in the case of a Si vacancy. Influence of the distance between vacancies on the ferromagnetic or anti‐ferromagnetic ordering for all structures is established.

Milling-induced polymorphic transformation in MoSi2

Abstract The mechanism of phase selection during mechanical milling has been investigated first with a large perspective using an exhaustive number of transformations reported in the literature and next with focusing on the specific system of MoSi2. The former shows that the phases exhibiting smaller values of the volume per one mole of atoms and the number of atoms in a reduced unit cell are preferred during mechanical milling. The latter involves both experimental and theoretical investigations. The dynamic equilibrium between the stable α and metastable β phases is reached with negligible Fe contamination, which is explained by a transformation mechanism based on synchroshear. The β phase formation is promoted in the presence of Fe, which is attributed to the formation of Fe-substituted MoSi2 with Si vacancies according to first-principles calculation results.

Improvement in Film Quality of Epitaxial Graphene on SiC(111)/Si(111) by SiH4 Pretreatment

The epitaxy of graphene on 3C-SiC/Si (GOS) has attracted much attention owing to its viability to fuse graphene with Si-based technologies. It is known that the surface condition of the 3C-SiC thin film before graphitization plays a decisive role in determining the quality of the GOS film. We have investigated the effect of the pretreatment of the 3C-SiC thin film in vacuo at a SiH4 partial pressure of 6.7 ×10-4 Pa on the subsequent formation of graphene. As a result, it is revealed that the SiH4 pretreatment restores the defects on the SiC surface, such as the Si vacancy and point defects formed by the presence of native oxides, and improves the quality of graphene. The effect is found to be highest when the substrate temperature is 1173 K.

Oxygen-excess-related point defects in glassy/amorphous SiO2 and related materials

An insight is given into recent experimental advances in the spectroscopic studies of oxygen-excess intrinsic defects, in glassy SiO2 and α-quartz. By controlling excess oxygen in a-SiO2, and the conditions of F2-laser irradiation, SiO2 glass samples can be obtained with optical absorption almost exclusively dominated by single defect, oxygen dangling bonds (“non-bridging oxygen hole centers” or NBOHCs), without the presence of complementary Si dangling bonds (generic “E′-centers”). This allows for a more accurate determination of the spectral shape of NBOHC optical absorption in UV and vacuum UV spectral regions. The temperature dependence of NBOHC electron paramagnetic resonance (EPR) signal intensity is stronger than predicted by Curie’s law (1/T) even at temperatures at and below 77K. Dangling bonds are characteristic of an amorphous state and do not exist in a regular crystal lattice. However, site-selective luminescence shows that highly ordered NBOHCs exist in particle-irradiated α-quartz, evidently either on the border between the damage tracks and the crystalline phase or as a part of Si vacancies. They are different from the common “glass-like” NBOHCs in a-SiO2 by giving distinct sharp zero-phonon lines with characteristic energies in luminescence spectra instead of a continuous distribution of lines. Two distinct types of luminescent NBOHCs, associated with the long and short Si–O bonds in α-quartz are suggested. EPR data corroborate the presence of oriented NBOHCs in neutron-irradiated α-quartz and confirm distinct NBOHCs at the sites of “long” and “short” Si–O bonds in α-quartz.

First-principles study of the SiNx/TiN(001) interface

The structure of the SiN${}_{x}$ tissue phase in superhard TiN/SiN${}_{x}$ nanocomposites has been debated in the literature. We present a theoretical investigation of the possibility of crystalline and coherent (001) interfaces that satisfies the two necessary criteria, stability with respect to lattice vibrations as well as to variations in stoichiometry. It is found that one monolayer of Si tetrahedrally coordinated by N in a B3-like geometry embedded between B1-TiN(001) surfaces is both dynamically stable and thermodynamically stable with respect to vacancy formation. However, with increasing layer thickness the B3-type structure becomes unstable with respect to Si vacancy formation. Instead we suggest that a tetragonal D${0}_{22}$-like order of Si vacancies can stabilize the interface. These structures are in line with the experimental findings of the crystalline tissue phase which has coherent interfaces with TiN.

Coupled (Li+, Al3+) substitutions in hydrous forsterite

Atomistic computer simulations methods are used to examine the influence of Li and Al impurities on the uptake of hydrogen in forsterite. We find that Li′ Mg +OH • O is more stable at the Mg1 site than at the Mg2 site and that Li + increases the ability of forsterite to incorporate hydrogen associated with magnesium sites. When both Al and Li are present, then a complex comprising a bound Al • Mg2 –Li′ Mg1 defect is highly stable. When all three impurity components are mixed together, then hydrogen will strongly partition to Si vacancies forming the hydrogarnet defect. Thus the ability of forsterite to incorporate water is likely to be intimately linked to the nuances of defect chemistry, and to concentrations of impurity elements such as Li + and Al 3+ .

Thermoelectric properties of silicon carbide nanowires with nitride dopants and vacancies

The thermoelectric properties of cubic zincblend silicon carbide nanowires (SiCNWs) with nitrogen impurities and vacancies along [111] direction are theoretically studied by means of atomistic simulations. It is found that the thermoelectric figure of merit ZT of SiCNWs can be significantly enhanced by doping N impurities together with making Si vacancies. Aiming at obtaining a large ZT, we study possible energetically stable configurations, and disclose that, when N dopants locate at the center, a small number of Si vacancies at corners are most favored for n-type nanowires, while a large number of Si vacancies spreading into the flat edge sites are most favored for p-type nanowires. For the SiCNW with a diameter of 1.1 nm and a length of 4.6 nm, the ZT value for the n-type is shown capable of reaching 1.78 at 900K. The conditions to get higher ZT values for longer SiCNWs are also addressed.

Improved photovoltaic properties of a-Si/β-FeSi 2/c-Si double heterojunction by Al-doping

Al-doped β-FeSi 2 thin film was sputtered on Si substrate and then applied to the amorphous-Si/β-FeSi 2/crystalline-Si (a-Si/β-FeSi 2/c-Si) double heterojunction. The X-ray diffraction result confirmed the formation of β-FeSi 2 crystallization. The result of carrier lifetime measurement implied that Al-doping could improve the carrier lifetime and infrared response property of β-FeSi 2 thin film. Such improvements were ascribed to the reduction of Si vacancy density by Al atom occupation. Based on the improved Al-doped β-FeSi 2 thin film, the prepared a-Si/β-FeSi 2/c-Si double heterojunction exhibited prominent enhancements in open-circuit voltage, short-circuit current density, fill factor, and energy conversion efficiency than that of the un-doped β-FeSi 2 double heterojunction. These results reveal an attractive way to improve the photovoltaic property of a-Si/β-FeSi 2/c-Si double heterojunction using Al-doped β-FeSi 2 thin film.

Cs diffusion in cubic silicon carbide

Undesired release of Cs through a silicon carbide coating of nuclear fuel is a significant concern for the design of the Very High Temperature Reactor (VHTR). However, mechanisms of Cs transport are currently unclear. To better understand the possible mechanisms of Cs release here we use density functional theory to study diffusion of Cs in crystalline bulk SiC. Cs point defects and Cs – vacancy clusters have been investigated for stability and structure. The most stable state for the Cs impurity in SiC, under n-type doping conditions, is found to be a negatively charged Cs atom substituting for a C atom and bound to two Si vacancies ( Cs C – 2 V Si 3 - ). Bulk diffusion coefficients are estimated for several Cs impurity states. The Cs C – 2 V Si 3 - defect structure is found to have the lowest overall activation energy for diffusion with a value of approximately 5.14 eV. This activation energy agrees well with diffusion activation energies estimated for Cs in SiC based on high temperature integral release experiments.

Physics of Schottky-barrier change by segregation and structural disorder at metal/Si interfaces: First-principles study

Schottky-barrier changes by the segregation and structural disorder are studied using the first-principles calculations and adopting Au/Si interface. The Schottky barrier for electrons simply decreases as increasing the valency of segregated atoms from II to VI families, which variation is shown closely related to how the Si atoms are terminated at the interface. On the other hand, the structural disorders (defects) prefer to locate near the interface and the Schottky barrier for hole carriers does not change in cases of Si vacancy and Au substitution, while it increases in cases of Si and Au interstitials reflecting the appearance of Si dangling bonds.

A first-principles investigation of hydrous defects and IR frequencies in forsterite: The case for Si vacancies

We investigate charge-balanced hydrous magnesium and silicon defects [(2H) X Mg, (4H) X Si] by first principles. Two new lowest-energy hydrogen configurations are proposed for (4H) X Si. With these new configurations, the distribution of O-H stretching phonon frequencies in Group I (>3450 cm –1 ) are better reproduced. Substitution of silicon with four hydrogen atoms gives rise to significant elongation of distances between O atoms at the tetrahedron of the silicon vacancy. Our calculations indicate that the correlation between O-O distances and O-H stretching phonon frequencies, which has been well established for hydrous minerals, does not apply directly to nominally anhydrous minerals and should not be used to determine the identity of hydrous defects responsible for infrared absorption peaks.

Ferromagnetism Induced by Intrinsic Defects and Boron Substitution in Single-Wall SiC Nanotubes

On the basis of density functional theory (DFT) methods, we study the magnetic properties and electronic structures of the armchair (4, 4) and zigzag (8, 0) single-wall SiC nanotubes with various vacancies and boron substitution. The calculation results indicate that a Si vacancy could induce the magnetic moments in both armchair (4, 4) and zigzag (8, 0) single-wall SiC nanotubes, which mainly arise from the p orbital of C atoms surrounding Si vacancy, leading to the ferromagnetic coupling. However, a C vacancy could only bring about the magnetic moment in armchair (4, 4) single-wall SiC nanotube, which mainly originates from the polarization of Si p electrons, leading to the antiferromagnetic coupling. In addition, for both kinds of single-wall SiC nanotubes, magnetic moments can be induced by a boron atom substituting for C atom. When two boron atoms locate nearest neighbored, both kinds of single-wall Si(C, B) nanotubes exhibit antiferromagnetic coupling.