Local Chemical Bonds Research Articles

Electronic structure of the alkaline-earthsilicon nitrides M2Si5N8 (M = Ca and Sr)obtained from first-principles calculations and optical reflectancespectra

Results of first-principles band-structure calculations for theternary alkaline-earth silicon nitrides M2Si5N8 (M = Ca and Sr) are presented. In the structures ofM2Si5N8 (M = Ca, Sr and Ba), the N atoms showconnections to two (N[2]) and three (N[3]) neighbouringsilicon tetrahedral centres. Calculations show that the localelectronic structure is strongly dependent on the local chemicalbonding. The valence band is dominated by N 2p hybridized with thes, p states of the alkaline-earth-metal and silicon atoms. The upper partof the valence band is dominated by the 2p states of N[2]atoms, while the N[3] 2p states lie about 2 eV below the Fermilevel. The bottom of the conduction band consists of the N 3scharacters hybridized with s orbitals of the alkaline-earth metals,while the s character of Si atoms is higher in energy.Sr2Si5N8 is a semiconductor with a direct energy gapat Γ, while Ca2Si5N8 is an indirectsemiconductor. Optical diffuse reflectance spectra show an energygap of 4.9 eV for Ca2Si5N8, 4.5 eV forSr2Si5N8, as well as 4.1 eV forBa2Si5N8, in fair agreement with the calculatedvalues.

Local aspects of the adsorbate-substrate chemical bond in N/Cu(100) and O/Cu(100)

Both the occupied and unoccupied adsorbate valence states have been probed by x-ray emission and x-ray absorption spectroscopies for N/Cu(100) and the low coverage phase of O/Cu(100). By carrying out experiments using two different measurement geometries, the adsorbate out-of-plane states ${(2p}_{z})$ have been separated from the in-plane states ${(2p}_{\mathrm{xy}}).$ The experimental results are compared to the partial density of states (DOS) obtained from full-potential linear muffin-tin orbital calculations, with good agreement in the case of N/Cu(100). This agreement confirms that it is possible to interpret the experimental data in terms of the adsorbate partial DOS. Our combined experimental and theoretical study also provides a detailed understanding of the local chemical bond for N and O adsorbed on Cu(100).

Lone Pair Interactions in Zintl Phases and Intermetallic Compounds: Influence on Electronic Properties in Stannides and Plumbides

AbstractThe phases K6Sn23Bi2, K6Sn25, NaSn5, BaSn3, BaSn5, and K5Pb24 depict the structural transition from Zintl phases with localized chemical bonds to typical intermetallic compounds which may even have superconducting properties. The question of the nature of the chemical bond in these compounds is studied with the help of quantum mechanical calculations. Tight binding band structure calculations and real space representations using the Electron Localization Function (ELF) show that free electron pairs play a crucial role for the description of the chemical bond in polar intermetallic compounds. Interactions between lone pairs have a dominant influence on the electronic structures. The coincident appearance of quasi-molecular localized states in form of lone pairs and disperse delocalized bands at the Fermi level EF is discussed with respect to a ‘chemical view’ of the superconductivity observed for BaSn3, BaSn5, and K5Pb24.

Local Chemical Bonding around Rare‐Earth Ions in α‐ and ß‐Si3N4

First‐principles molecular‐orbital calculations of α‐ and ß‐Si3N4 with a trivalent lanthanide (Ln3+) ion at the interstitial site are conducted using model clusters that are composed of 41‐43 atoms, neglecting lattice relaxation effects. When an interstitial Ln3+ ion is present, strong antibonding between the Ln3+ ion and Si3N4 is found. The magnitude of the antibonding is almost the same between the alpha‐ and ß‐Si3N4 matrices. On the other hand, the Si‐N bond around the Ln3+ ion is notably reinforced in alpha‐Si3N4 but not so much in ß‐Si3N4. The different electronic response to the presence of the Ln3+ ion for the Si‐N bond is concluded to be the origin of the different solubilities of interstitial Ln3+ ions between alpha‐ and ß‐SiAlONs that are reported experimentally. The contribution of the electric field that is induced by the presence of a trivalent charge at the interstitial site is examined in detail; we have found that the Si‐N bond strength is not simply determined by the electric field but rather in a more complex manner.

Auger electron spectroscopy study of the electronic structure of porous silicon–metal interfaces

Abstract The use of Auger electron spectroscopy (AES) for the study of local chemical bonds and electronic structure has been applied to investigations of the electronic structure close to the surface layer of porous silicon and of the electronic structure of porous silicon–titanium and silicon–gold interfaces. The results obtained show that close to the surface layer of porous silicon the electron concentration in the conduction band increases under Ar + ion sputtering. This result provides confirmation of the model of an increasing energy band gap under porous silicon formation from single crystal silicon. Titanium and gold deposition leads to an increase in the high-frequency dielectric constant and in the electron concentration in the conduction band.

Electron-energy-loss core-edge structures in manganese oxides.

Oxygen K and manganese ${\mathit{L}}_{2,3}$ edges electron-energy-loss spectra have been measured for a series of manganese oxides with different nominal oxidation states for the cation (MnO, ${\mathrm{Mn}}_{3}$${\mathrm{O}}_{4}$, ${\mathrm{Mn}}_{2}$${\mathrm{O}}_{3}$, ${\mathrm{MnO}}_{2}$, and ${\mathrm{BaMnO}}_{4}$). Spectra have been processed for quantitative elemental analysis and for the evaluation of manganese ${\mathit{L}}_{2,3}$ white-line intensity ratios and normalized total intensities. Prepeaks on the oxygen K edge are sensitive to the nature of local chemical bonds and their detailed substructures are interpreted with the support of a molecular-orbital picture. As for the cation point of view, the analysis of the white-line intensities provides a satisfactory measurement of the effective 3d orbital occupancy as well as a hint of the local magnetic properties. The present study also confirms that the hybridization between oxygen 2p and manganese 3d orbitals plays an important role when considering the electronic structures of these oxides.

Contribution of the LZS model to the positive ion emission out of metallic samples in the presence of chemically active elements

When a metallic target is bombarded by an ion beam of a few keV, atoms are ejected out of the sample, giving rise to the phenomenon of sputtering. Some of these atoms spontaneously ionize during their ejection. The fraction of sputtered particles being ionized depends on the nature of the target. For instance, the emission intensity of the metal ions is drastically enhanced by the presence of oxygen at the very surface of the sample [1,2]. In this last situation, and more generally when chemically active elements are present in the first superficial layers of the sample, a strong localization of the electronic states occurs for the atoms belonging to these layers, giving rise to localized chemical bonds. The observed enhancement could result from the breaking of such bonds involving the metal atom under the effect of the collisions occurring during the sputtering process. According to the final state of an external electron of the metal atom, the separation would lead either to two neutral partners, or to two ions. In that last situation, a positive metal ion is emitted and the electron remains in a localized acceptor state associated with a negative ion. In atomic beam collisions, the Landau-Zener-Stueckelberg (LZS) model was introduced in order to describe the dissociation of chemical bonds of ionic character [3]. It was applied to the emission of positive metal ions out of solid samples by Yu and Mann [4,5]. However, it may be wondered whether this model is suitable to describe the possible breaking of bonds at the surface of metallic samples, and what can be its contribution to a better understanding of such events. This paper completes and explicits two other papers [6,7] aiming to provide elements of answer to these questions.

Franck-Condon transition component distributions in chemical bond formation processes in condensed systems

Chemical species conversion dynamics in condensed systems or at interfaces has been treated in terms of “chemical” Franck-Condon (FC) transitions, taking into account local chemical bond formation connected with electronic partial charge transfer. In this paper we continue to study energy distribution patterns of transition components in the quantum regime up to classical limits of subsystems or of the total system, allowing for sudden (canonical) equilibration of initial state as well as for a simple case of solvent friction. Conditions for including multidimensional potential coupling are outlined.

Chemical bonding at metal-semiconductor interfaces

With the aid of cluster calculations and bandstructure calculations, we show that an interpretation in terms of local chemical bonds can also explain the structures of extended systems, such as the CoSi2/Si(111) and NiSi2/Si(111) metal-semiconductor interfaces.

Local Electronic Structure and Chemical Bonding around Fe-Vacancy Pair in Aluminium

Local Electronic Structure and Chemical Bonding around Fe-Vacancy Pair in Aluminium

Mössbauer isomer shifts and the electronic structure of covalent solids

The observed trends in Mössbauer isomer shifts of 119Sn, 121Sb, 125Te and 67Zn as constituents in covalent solids reflect the nature of the local chemical bond. A detailed analysis of these measurements is presented based on the crystalline electron densities which are calculated self-consistently and scalar relativistically with the first principles linear-muffin-tin-orbital method in the atomic-sphere-approximation. Quantitative comparisons with experimental data yield accurate and parameter-free calibrations of the Mössbauer transitions considered. The pressure variation of the 119Sn isomer shift in metallic tin is investigated through the 95 kbar structural phase transition. Finally, the contribution from the core electrons to the isomer shift is discussed.

Chemical trends in lattice location of implanted impurities in A III B V compounds

The lattice location and the electronic configuration of various groups of nontransition elements of the Periodic Table implanted in A III B V semiconducting compounds are discussed on the assumption that the local chemical bond plays the dominant role. Using this scheme, a simple explanation is proposed for the discrepancy in the electrical activity of high-dose n- and p-type implants in GaAs.

Electric structures and121Sb isomer shifts of covalent crystals containing Sb as a component

The electronic structures of crystalline AlSb, GaSb, InSb, SnSb, and Sb metal are calculated self-consistently using the linear muffin-tin-orbital method in the scalar-relativistic and atomic-sphere approximations. The valence electron density on the nuclear site is calculated and compared with experimental values for the isomer shifts. The trends observed are well understood in terms of the nature of the local chemical bond in these materials. The value for the relative change in nuclear radius for the 121Sb Mossbauer transition of Delta R/R=(-10.4+or-1.0)*10-4 is derived.

On the mixed group III and group V implantations in silicon and germanium

A simple explanation of the mixed group III and group V implantation in silicon and germanium, based on the local chemical bond, is proposed. In particular, the charge states of the Frenkel defects and their role in the reaction kinetics are discussed in detail.

Chemical states study of Si in SiO x films grown by PECVD

Thin films of SiO x have been grown by low temperature (350°C) plasma enhanced chemical vapor deposition (PECVD), and the local chemical bond of Si and O has been investigated by infrared (IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and Auger electron spectroscopy (AES). Comparisons between the IR spectra of Si-rich oxides (SiO x , x < 2) and the corresponding spectra of stoichiometric SiO 2 and low oxygen content a-Si:H:O alloys demonstrates a continuous variation in the character of the Si-O stretching, bending and rocking vibrations as a function of the oxygen concentration. In particular, the frequency of the dominant stretching vibration displays a linear frequency shift with the relative oxygen concentration. A similar behavior prevails in the XPS spectra where the position of the Si2p core states shifts monotonically with increasing oxygen concentration from a value of about 98.8 eV in a-Si to 103.1 eV in a-SiO 2. The IR and XPS then reflect the average chemical bonding in these films. In contrast, the AES spectra of suboxides display two strong features in the SiL 23VV spectrum. We show that one of these features at approximately 78 to 80 eV is associated with ejection of electrons from Si-O valence bonding states, and a second at about 90 eV is associated with ejection of electrons from Si-Si bonding states. Therefore, the AES spectra reflects the details of the local bonding, i.e., the relative numbers of Si-O and Si-Si bonds, while the IR and XPS spectra reflect the average chemical bonding. In this context, the combination of the three techniques confirms that the suboxide fims produced by this low temperature PECVD process are homogeneous, in contrast to being two phases with regions of a-Si and a-SiO 2.

Bond breaking and the ionization of sputtered atoms.

We have studied the static-mode ion-beam sputtering of ${\mathrm{Si}}^{+}$ from a Si (100) surface during oxidation and nitridation. The data are consistent with the ionization of sputtered atoms resulting from the breaking of the local chemical bond during sputtering. A model is proposed to explain the dependences of the ionization probability of the ionization potential, emission energy, and isotopic mass.

Chemisorption of Si on Al(111) surfaces: A local-chemical-bond analysis from Auger transition density of states

Auger and electron loss spectroscopies have been used to study the local chemical bond between Si and Al, in the first stages of growth of Si deposited at room temperature on Al(111) surfaces. Si follows a layer-by-layer mechanism up to 2 monolayers with the formation of an Al(111)-3 × 3-Si structure at about 0.44 monolayers. A detailed analysis of the L 2,3VV Auger spectra for this structure allows to interpret the Si and Al Auger transition density of states (TDOS) in terms of the actual p-like partial DOS centered on the Si and Al sites. The experimental results indicate a strong SiAl interaction with the formation of a p-type local covalent bond between the Si and Al surface atoms.

Mechanisms of atomic ion emission during sputtering

Several major experimental and theoretical findings made in the last few years on the mechanisms of secondary ion emission are summarized. There is a strong indication that the phenomena can be divided into two categories: Ion emission from the surfaces of metals and semiconductors tends to have a strong correlation with the work function and can be described quite well with an electron tunneling model. Ion emission from systems which show large chemical enhancement has only a weak correlation with global surface properties like the work function or the bandgap. Instead it seems to be related more to local chemical bonds and coordination numbers. A localized bond breaking picture may be more appropriate in these circumstances.

On the lattice location of implanted impurities in silicon

As implanted ions are brought to rest at relatively low energies, one can anticipate that their lattice sites are predominantly determined by the local chemical bond depending on the band structure of the host crystal and the electronic configuration of the implanted impurities. Consequently, one can expect that for a given pure semiconductor the occupancy of various impurity sites will vary with the valency of the impurity atoms. As an example, impurity sites of several groups of nontransition elements of the Periodic Table implanted in silicon are discussed and compared with recent experimental data. Further, a few simple rules concerning the behaviour of implanted impurities in silicon are formulated giving some insight into the nature of the electronic structure of implanted semiconductors.

Chemical trends in lattice location of implanted impurities in silicon

Abstract On the assumption that the lattice location of implanted impurities is determined by the local chemical bond, the impurity sites of various groups of (non-transition) elements of the Periodic Table implanted in pure silicon are discussed in terms of their valency. The theoretical results are in reasonable agreement with available experimental data.