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Synthetic two-dimensional (2D) mono-elemental crystals, namely X-enes, have recently emerged as a new frontier for atomically thin nanomaterials with on-demand properties. Among X-enes, antimonene, the β-phase allotrope of antimony, is formed by atoms arranged in buckled hexagonal rings bearing a comparatively higher environmental stability with respect to other players of this kind. However, the exploitation of monolayer or few-layer antimonene and other 2D materials in novel opto-electronic devices is still hurdled by the lack of scalable processes. Here, we demonstrated the viability of a bottom-up process for the epitaxial growth of antimonene-like nanocrystals (ANCs), based on a Metal-Organic Chemical Vapor Deposition (MOCVD) process, assisted by gold nanoparticles (Au NPs) on commensurate (1 1 1)-terminated Ge surfaces. The growth mechanism was investigated by large- and local-area microstructural analysis, revealing that the etching of germanium, catalyzed by the Au NPs, led to the ANCs growth on the exposed Ge (1 1 1) planes. As a supportive picture, ab-initio calculations rationalized this epitaxial relationship in terms of compressively strained β-phase ANCs. Our process could pave the way to the realization of large-area antimonene layers by a deposition process compatible with the current semiconductor manufacturing technology.

N-type doping in Si by shallow impurities, such as P, As and Sb, exhibits an intrinsic limit due to the Fermi-level pinning via defect complexes at high doping concentrations. Here we demonstrate that doping Si with the chalcogen Te by non-equilibrium processing, a deep double donor, can exceed this limit and yield higher electron concentrations. In contrast to shallow impurities, both the interstitial Te fraction decreases with increasing doping concentration and substitutional Te dimers become the dominant configuration as effective donors, leading to a non-saturating carrier concentration as well as to an insulator-to-metal transition. First-principle calculations reveal that the Te dimers possess the lowest formation energy and donate two electrons per dimer to the conduction band. These results provide novel insight into physics of deep impurities and lead to a possible solution for the ultra-high electron concentration needed in today's Si-based nanoelectronics., 26 pages, including the suppl information

The localization of the donor electron wave function can be of key importance in various silicon applications, since for example it determines the interactions between neighboring donors. Interestingly, the physical confinement of the electrons in quasi-one-dimensional nanostructures, like silicon nanowires, noticeably affects this property. Using fully ab initio calculations, we show that the delocalization of the donor electron wave function along the axis of a nanowire is much greater in [011] oriented nanowires for phosphorus and selenium donors. We also demonstrate that its value can be controlled by applying a compressive or tensile uniaxial strain. Finally, we discuss the implications of these features from both an experimental and a theoretical point of view. © 2013 American Chemical Society.

On the basis of ab initio simulations, the formation of an epitaxial phase that has the anatase structure has been proposed as the microscopic mechanism responsible for the preferential orientation of monoclinic HfO2 films on the high-mobility (0 0 1) oriented Ge and GaAs substrates. In fact, the oriented monoclinic structure follows the in-plane axis of the anatase phase as proved by X-ray scattering measurements. The fact that epitaxial HfO2 anatase has no bulk counterpart is explained by our calculations as due to the unfavorable Helmholtz free energy of anatase phase when the condition of epitaxy is released.

Due to the confinement effect, the donor wave functions in nanostructures are highly localized on the defect and can even be deformed by the local geometry of the system. This can have relevant consequences on the hyperfine structure of the defect that can be exploited for advanced electronic applications. In this work we employ ab initio density functional calculations to explore the hyperfine structure of S and Se substitutional defects in silicon and germanium nanowires. We show that, if the tetrahedral symmetry is preserved, the hyperfine contact term is only marginally dependent on the nanowire orientation, while it can undergo drastic changes if the symmetry is lost. In addition, we provide an analysis of the strain dependence of the hyperfine structure for the different orientations of the nanowires.

We study the physical properties of MnAs under strain by using accurate first-principles pseudopotential calculations. Our results provide new insight on the physics of strained multilayer that are grown epitaxially on different lattice mismatched substrates and which are presently of interest for spintronic applications. We compute the strain dependence of the structural parameters, electronic bands, density of states and magnetization. In the region of strain/stress that is easily directly accessible to measurements, the effects on these physical quantities are linear. We also address the case of uniaxial stress inducing sizeable and strongly non linear effects on electronic and magnetic properties.

(a) Institut de Physique et Chimie desMate´riaux de Strasbourg (IPCMS), UMR 7504duCNRS,Universite´LouisPasteur,23rueduLoess,F-67037StrasbourgCedex,France(b)Max-Planck-Institutfu¨rFestko¨rperforschung,Heisenbergstr.1,D-70569Stuttgart,Germany(ReceivedAugust28,2000)Thepressuredependencesofthephononlifetimesofzone-centeropticalphononsofvariousgroupIVand III–V semiconductors are calculated from first principles using third-order density-func-tionalperturbationtheory.Themicroscopicmechanismresponsible,i.e.theanharmonicdecayofaphonon into phonons of different energy, is revealed and the sensitive dependence on pressure isstudiedindetail.Theresults arecomparedwithrecentmeasurements of thepressure dependenceofthelinewidthsofthefirstorderRamanlinesofSi,Ge,andSiC.

We present an ab initio calculation of the real part of the phonon self energy for longitudinal and transverse optical Raman phonons in GaAs, AlAs, GaP and InP. The calculated temperature dependences of transverse and longitudinal modes at the Γ-point are in good agreement with available experimental data. This calculation permits us to analyze the different contributions to the real part of the phonon self-energy. In particular we are able to reproduce the observed asymmetry of the Raman line of the transverse optical mode in GaP. Furthermore, we explain why the frequency of this mode changes less with temperature than that of the transverse mode in the other materials investigated. The pressure dependence of the real and the imaginary parts of the phonon self-energy are also discussed.

We describe, within an ab initio approach, the stabilization of the tetragonal phase versus the monoclinic one in yttrium-doped zirconia. The process is believed to be influenced from different mechanisms. Indeed, we show that there is a delicate balance between the change in electrostatic and kinetic energy and exchange-correlation effects. In the tetragonal phase, the perturbation induced by doping is better screened at the price of sacrificing correlation energy. Our work opens the opportunity to use the same approach to predict the tetragonal phase stabilization of materials such as zirconia or hafnia, with different and less characterized dopants.

We report a density functional study of the electronic properties and hyperfine structure of substitutional selenium in silicon nanowires using plane-wave pseudopotential techniques. We simulated hydrogen passivated [001] oriented nanowires with a diameter up to 2 nm, analyzing the effect of quantum confinement on the defect formation energy and on the hyperfine parameters as a function of the diameter and of the defect position. We show that substitutional Se in silicon has favorable configurations for positions near the surface with possible formation of chalcogen - hydrogen complexes. We also show that hyperfine interactions increase at small diameters, as long as the nanowire is large enough to prevent surface distortion which modifies the symmetry of the donor wave function. Moreover, surface effects lead to strong differences in the hyperfine parameters depending on the Se location inside the nanowire, allowing the identification of an impurity site on the basis of electron paramagnetic resonance spectra.

By means of the effective mass approximation we have computed without any adjustable parameters the energy levels and hyperfine splitting of a P impurity in silicon spherical nanocrystals as a function of the nanocrystal radius. Confinement effects produce an enhancement of the energy levels and of the hyperfine splitting as the crystal radius is decreased. Our results are in good agreement with available experimental data.

Within the framework of the envelope function approximation we have computed – without adjustable parameters and with a reduced computational effort due to analytical expression of relevant Hamiltonian terms – the energy levels of the shallow P impurity in silicon and the hyperfine and superhyperfine splitting of the ground state. We have studied the dependence of these quantities on the applied external electric field along the [001] direction. Our results reproduce correctly the experimental splitting of the impurity ground states detected at zero electric field and provide reliable predictions for values of the field where experimental data are lacking. Further, we have studied the effect of confinement of a shallow state of a P atom at the center of a spherical Si-nanocrystal embedded in a SiO2 matrix. In our simulations the valley–orbit interaction of a realistically screened Coulomb potential and of the core potential are included exactly, within the numerical accuracy due to the use of a finite basis set, while band-anisotropy effects are taken into account within the effective-mass approximation.

We propose a mechanism leading to the local epitaxy observed in $\mathrm{Hf}{\mathrm{O}}_{2}$ films grown on Ge(001). Our model is based on state-of-the-art ab initio calculations compared with available experimental data. According to the proposed model, the observed preferential orientation of the monoclinic structure is related to the relaxation of the epitaxially stabilized anatase phase when a critical thickness is reached. In fact, the preferential orientation of the monoclinic structure follows the in-plane axis of the anatase phase, as proven by accurate x-ray scattering data. We predict that the anatase phase, which has no bulk counterpart and has a calculated dielectric constant comparable to the bulk monoclinic one, is almost lattice matched with the Ge(001) substrate. Although the observation of the anatase phase is still missing, its stabilization would allow a control at the atomic level of the $\mathrm{Hf}{\mathrm{O}}_{2}∕\mathrm{Ge}(001)$ interface, possibly providing better performances in the next generation complementary-metal-oxide-semiconductor devices.

Within the framework of density functional theory, we have computed structural and vibrational properties of high-K dielectric oxides, HfO2 and TiO2. We have considered different polytypes of these oxides and computed the corresponding static dielectric constants in order to investigate what are the more promising materials suitable for technological application. We found the flourite phase of hafnium dioxide, that is a promising candidate as gate oxide in ultra-scaled complementary metal-oxide-semiconductor (CMOS) technology, is structurally unstable due to the presence of soft phonon modes that have wave-vectors in the region of the Brillouin zone containing the K- and X-points. According to our results, the fluorite TiO2 is structurally unstable as well but, contrary to HfO2, the soft phonon modes have wave-vectors in the whole Brilouin zone. Calculations of the thermodynamical properties of the baddeleyite HfO2, the stable phase at ambient condition, complete the work. (c) 2006 Elsevier Ltd. All rights reserved.

The half-metallic NiMnSb compound is recently attracting renewed interest for its application in devices making use of spin polarized charges injected into conventional semiconductor. For technological purposes it is desirable to design interfaces where the peculiar half-metallic character of NiMnSb is not degraded. We focus here on the NiMnSb/GaAs(0 0 1) heterojunction that we investigate by accurate pseudopotential density functional—local spin density calculations. We find in general that the half-metallicity is lost when NiMnSb is joined with the semiconductor, even at the interface with a mixed (Mn,As) plane which is one of the most promising for maintaining the desired half-metallicity. In this case, however, the effect is localized just on the interfacial atoms. Electronic and magnetic bulk properties are recovered within a couple of atomic planes far from the interface, so that the band alignments are well defined.

We propose ${\mathrm{Si}}_{x}{\mathrm{Ge}}_{1\ensuremath{-}x}$ alloys as suitable candidates for spintronic applications. Remarkably, our first-principles investigation explains, within the local spin density approximation, the microscopic mechanism that stabilizes the half metallicity in Si matrices under Ge alloying and shows that it can spontaneously occur: in fact, half metallicity is determined by $p\text{\ensuremath{-}}d$ hybridization in the presence of Ge atoms surrounding Mn impurities, and this particular environment is energetically favored over other possible local composition fluctuations, such as excess of Si atoms around Mn. A detailed discussion of the trends of defect formation energy and of magnetization as a function of alloy composition and configuration completes the present work.

We have measured the phonon dispersion of wurtzite CdS by inelastic neutron scattering in a single crystal made from the nonabsorbing isotope 114Cd. One of the two silent B1-modes occurs at 3.96 THz ( k = 0 ). It is significantly lower and less dispersive than so far assumed. Previous semiempirical lattice dynamical models need to be reanalyzed. However, the observed dispersion branches compare favorably with an ab-initio calculation.

We present first principles calculations of the magnetic properties and the magneto-optical Kerr effect of iron thin films epitaxially grown on the [0 0 1] surface of two paramagnetic noble metals: Cu and Ag. We investigate the role of hybridization with the substrate and we show how the relaxation effects influence the complex Kerr angle. Our results are obtained using the relativistic full-potential linear muffin-tin method. The film is modeled using a slab geometry within a supercell technique.

We present an ab initio calculation of the infrared vibrational spectrum of amorphous silicon. The electron polarization, which is the main ingredient to obtain the infrared spectra, is evaluated using the recent formulation in terms of Berry's phase, while the time evolution of the system is studied using Car-Parrinello molecular dynamics. Our results are in quantitative agreement with experimental data and also with the results of empirical calculations based on a bond-dipole model. © 1997 American Institute of Physics.

A theoretical investigation of the X-ray magnetic circular dichroism (XMCD) and the magnetic properties of a Fe, Co and Ni monolayer on a W(1 1 0) substrate is presented. The Ni monolayer shows very weak ferromagnetism, whereas Fe- and Co-based systems show an in-plane magnetization in agreement with experiments. The Fe and Co spin magnetic moments are reduced from their bulk value due to the surface relaxation and the hybridization with the d valence electrons of W. The orbital magnetic moments are found to be larger for the in-plane magnetization axis for all atoms. The calculated XMCD spectra reflect the anisotropy of the orbital magnetic moments.

We have measured Raman spectra of several SiC polytypes ( $3C$, $6H$, $15R$) made from natural silicon $({\ensuremath{\approx}}^{28}\mathrm{Si})$ and ${}^{30}\mathrm{Si}$. The isotope shifts of the phonon frequencies show characteristic variations with their effective wave vector in the zinc blende $\left(3C\right)$ modification which arises from Brillouin zone backfolding. This allows us to determine the phonon eigenvectors of $3C$ SiC for the dispersion branches along the [111] direction. The observed magnitudes of the Si and C ion displacements, as well as their relative phase, confirm bond charge model and ab initio calculations.