About the existence of an antiferromagnetic ground state for the (1× 1) structure of Si(111)

A recent proposal by Duke and Ford (Surface Sci. 111 (1981) L685) of strong correlations at the metastable (1× 1) Si(111) surface is shown to be equivalent to our earlier theory of magnetic or charge instabilities at this surface. It is recalled how this can be discussed from a perturbation analysis. Estimates of the Coulomb parameter U are given using recent theoretical results for dangling bond states in silicon. They support the conclusion of an antiferromagnetic 2 × 1 ground state for this surface.     

Giant core-exciton effects on Si(111) 7 × 7 surfaces

We present the first direct experimental evidence for a large surface influenced core-exciton effect on silicon. The Si(111) 7 × 7 L_2,3 absorption edge has been studied with core-level electron energy loss (ELS) and X-ray photoemission spectroscopy (XPS). An excitonic shift of ～1–2 eV have been found for transitions from Si(2p) to empty surface states.     

Location of direct transitions on the Λ line of the band structure of the noble metals using angle resolved photoelectron spectroscopy (ARUPS)

The triangulation method of angle-resolved photoemission for the absolute localization of direct transitions in k-space has been used on (110) and (111) faces in the ΓKLUX or (1$\text{1}$0) mirror plane in order to determine some points of the electronic bulk band structure of Cu, Ag and Au along the Λ line. It is found that the experimental points for the occupied bands and the unoccupied bands are in good agreement with recent ground state band structure calculations based on the local density functional theory.     

Generalized susceptibilities and phonon anomalies in Pd and Pt metals

The generalized susceptibility, χ(q), in Pd and Pt for q along the [100], [110], [111], and [120] directions was determined from their APW and RAPW energy band structures, respectively, using the analytic tetrahedron linear energy scheme of Rath and Freeman. The band structures were previously found to yield Fermi surface radii, temperature dependencies of the static magnetic susceptibility, χ(T), resistivity, and a spin lattice relaxation, T₁T, in very good agreement with experiment. In the χ(q) calculations, we used 2048 tetrahedra in 1/48th irreducible BZ and the energy eigenvalues for bands 4, 5, and 6 which cross the Fermi energy as fitted to a Fourier series representation. The intraband parts of χ(q) at q = 0 for both metals are found to agree with the density of states at the Fermi energy to without 0.5%. Our results show that the dominant contribution to χ_intra arises from the dominant band 5 whose “jungle-gym” FS has strong nesting features; the main peak for Pd occurs at the same q value (= 0.65π/a) for q along the [0q0], [q, q, 0], and [q, q, q] directions. The locus of this main peak is a square in the (0, 0, 1) plane. The maximum of χ_intra for q along the [110] and [111] directions are 23% and 13%, respectively, higher than the value of χ(q) at q = 0. For q along the [010] and [120] directions, the peak is, however, lower than the value of χ_intra at q = 0. Hence, while phonon anomalies are predicted for the [110] and [111] directions, no anomaly is predicted for either the [100] or [120] direction. The predicted q value for the [110] anomaly, $\text{q}\text{= 0.65π/a}$ is close to the experimental value of ～0.7 π/a. Although there may be a hint of an anomaly at 0.56 [111] in the measurements, a more detailed investigation of this region is called for. For platinum, χ_intra for q along the [010], [110] and [111] directions has main peaks which occur at q = 0.68 π/a, 0.75 π/a, and 0.85 π/a, respectively. Here too, this main peak comes from the nesting of the jungle-gym Fermi surface which is not, however, as flat as that of palladium. Anomalies are predicted (although weaker in Pt than in Pd) along [110] and [111] but not along [100] and [120]. The [110] anomaly is close to the measured q value (～0.7–0.8 π/a). Also in agreement with experiment, we predict a weaker [110] anomaly for Pt than for Pd. In both Pd and Pt, weaker anomalies are predicted for the [111] direction than for the [110] direction.     

Ab-initio calculations of luminescence and optical gain properties in silicon nanostructures

Density-functional and many body perturbation theory calculations have been carried out in order to study the optical properties both in the ground and excited state configurations, of silicon nanocrystals in different conditions of surface passivation. Starting from hydrogenated clusters, we have considered different Si/O bonding geometries at the interface. We provide strong evidence that not only the quantum confinement effect but also the chemistry at the interface has to be taken into account in order to understand the physical properties of these systems. In particular, we show that only the presence of a surface Si–O–Si bridge bond induces an excitonic peak in the emission-related spectra, redshifted with respect to the absorption onset, able to provide an explanation for both the observed Stokes shift and the near-visible PL experimentally observed in Si-nc. For the silicon nanocrystals embedded in a SiO₂ matrix, the optical properties are discussed in detail. The strong interplay between the nanocrystal and the surrounding host environment and the active role of the interface region between them is pointed out, in very good agreement with the experimental results. For each system considered, optical gain calculations have been carried out giving some insights on the system characteristics necessary to optimize the gain performance of Si-nc. To cite this article: E. Degoli et al., C. R. Physique 10 (2009).     

Electronic structure of SiO2(111) thin film

The electronic structure of (111) surface of β-crystobalite is investigat ed using the empirical tight binding method. Our calculations identify surface states in the conduction band, band gap and valence band. The surface state formed from silicon-s and p_z orbitals, which is believed to account for the structure in the O K excitation spectra, lies in the band gap. It is seen that oxygen adsorption on the surface removes surface states and gives rise to a sharp peak at about — 3.8 eV below the valence band edge.     

Electronic states of (2 × 1) and (1 × 1) (111) surfaces of Ge,Si, diamond,GaAs and Ge on Si

The “dangling-bond” surface state dispersion curves, E(k), have been calculated for the (2 × 1) and (1× 1) (111) surfaces of Ge, Si, and diamond, for (1 × 1) GaAs, and for (2 × 1) Ge on Si. The calculations employ the sp³s1 empirical tight-binding model of Vogl et al. and the atomic relaxation of Feder et al. The surface state band gaps are in good agreement with optical-absorption and electron-energy-loss measurements for Ge and Si. For the assumed epitaxial geometry, Ge on Si is predicted to shift the dangling-bond states downward by ≈0.1 to 0.4 eV.     

Progressive changes in atomic structure and gap states on Si(0 0 1) by Bi adsorption

From ab initio studies employing the pseudopotential method and the density functional scheme, we report on progressive changes in geometry, electronic states, and atomic orbitals on Si(0 0 1) by adsorption of different amounts of Bi coverage. For the 1/4 ML coverage, uncovered Si dimers retain the characteristic asymmetric (tilted) geometry of the clean Si(0 0 1) surface and the Si dimers underneath the Bi dimer have become symmetric (untilted) and elongated. For this geometry, occupied as well as unoccupied surface states are found to lie in the silicon band gap, both sets originating mainly from the uncovered and tilted silicon dimers. For the 1/2 ML coverage, there are still both occupied and unoccupied surface states in the band gap. The highest occupied state originates from an elaborate mixture of the p_z orbital at the Si and Bi dimer atoms, and the lowest unoccupied state has a ppσ* antibonding character derived from the Bi dimer atoms. For 1 ML coverage, there are no surface states in the fundamental bulk band gap. The highest occupied and the lowest unoccupied states, lying close to band edges, show a linear combination of the p_z orbitals and ppσ* antibonding orbital characters, respectively, derived from the Bi dimer atoms.     

Electron states on the (111) surface of copper

A calculation of electronic states at the (111)-surface of Cu in the energy region around E_F is presented. The calculation is based on the empirical pseudopotential method. A proper position of an abrupt potential barrier ensures overall charge neutrality. A surface state is found at 0.13 eV below E_F, being localized midway between atomic layers. The effective mass of the associate bands is 0.39m_e. Our results are in good agreement with directional photoemission data.     

Intracavity laser absorption spectroscopy of HDO between 11 645 and 12 330 cm

The weak absorption spectrum of monodeuterated water, HDO, has been recorded by intracavity laser absorption spectroscopy (ICLAS) between 11 645 and 12 330 cm⁻¹. This spectrum is dominated by the ν₂ + 3ν₃ band of HDO at 11969.76 cm⁻¹. A total of 497 energy levels belonging to 12 vibrational states were determined while only 140 levels were previously reported from a recent investigation by Fourier transform spectroscopy in the same spectral region. The rovibrational identification process of the 1378 lines assigned to the HDO isotopologue was mostly based on the results of the accurate variational calculations of Schwenke and Partridge. The overall agreement between these calculations and the observed spectrum is very good. However, strong discrepancies in the calculated line intensities were evidenced in a few cases corresponding to an intensity transfer to a dark state through local resonance interaction.     

Si(111)面上氮原子薄膜的电子态密度第一性原理计算及分析

由于氮原子在Si(1 1 1)表面成键的失配度最小,因此考虑Si(1 1 1)取向上用不同百分比的氮原子钝化硅表面悬挂键.由第一性原理计算结果显示,当Si(1 1 1)表面层中的氮原子含量为75%—100%时,带隙展宽并且有局域陷阱态产生. 我们提出相应的局域电子态模型,从而解释了Si基氮膜光致荧光(PL)发光增强实验的物理机理. 关键词： 第一性原理计算 氮化Si薄膜 PL发光增强 局域陷阱态     

Influence of surface recombination and interface states on the performance of β-FeSi2/c-Si heterojunction solar cells

To explore the origin of low conversion efficiency for novel β-FeSi₂/c-Si heterojunction solar cells, the effect of surface recombination and interface states on the cell performance has been investigated by numerical simulation. The present results show that surface recombination of β-FeSi₂ film plays an important role in limiting the cell property since the photovoltaic behavior of β-FeSi₂ is quite sensitive to surface recombination due to its especial characteristic of very high optical absorption coefficient. Surface quality of β-FeSi₂ film should be much improved for better cell performance. In addition, it is shown that interface states between β-FeSi₂ film and crystalline silicon are critical to device characterization. Interface states should be minimized to obtain higher conversion efficiency. If surface recombination and interface states can be best suppressed, potential conversion efficiency for the cell may be up to 28.12% at 300 K under illumination of AM 1.5, 100 mW/cm².     

Wet-chemical passivation of atomically flat and structured silicon substrates for solar cell application

Special sequences of wet-chemical oxidation and etching steps were optimised with respect to the etching behaviour of differently oriented silicon to prepare very smooth silicon interfaces with excellent electronic properties on mono- and poly-crystalline substrates. Surface photovoltage (SPV) and photoluminescence (PL) measurements, atomic force microscopy (AFM) and scanning electron microscopy (SEM) investigations were utilised to develop wet-chemical smoothing procedures for atomically flat and structured surfaces, respectively. Hydrogen-termination as well as passivation by wet-chemical oxides were used to inhibit surface contamination and native oxidation during the technological processing. Compared to conventional pre-treatments, significantly lower micro-roughness and densities of surface states were achieved on mono-crystalline Si(100), on evenly distributed atomic steps, such as on vicinal Si(111), on silicon wafers with randomly distributed upside pyramids, and on poly-crystalline EFG (Edge-defined Film-fed-Growth) silicon substrates.The recombination loss at a-Si:H/c-Si interfaces prepared on c-Si substrates with randomly distributed upside pyramids was markedly reduced by an optimised wet-chemical smoothing procedure, as determined by PL measurements. For amorphous-crystalline hetero-junction solar cells (ZnO/a-Si:H(n)/c-Si(p)/Al) with textured c-Si substrates the smoothening procedure results in a significant increase of short circuit current I_sc, fill factor and efficiency η. The scatter in the cell parameters for measurements on different cells is much narrower, as compared to conventional pre-treatments, indicating more well-defined and reproducible surface conditions prior to a-Si:H emitter deposition and/or a higher stability of the c-Si surface against variations in the a-Si:H deposition conditions.     

Transition energies, wavelengths, oscillator strengths and transition probabilities between 1s, 1sns and 1snp (2?n?9) states for He-like Silicon

Multiconfiguration Hartree-Fock calculations with Breit-Pauli relativistic corrections are reported for electric dipole transition (E1) energies, wavelengths, weighted oscillator strengths and transition probabilities between all of the 1s², 1sns and 1snp (2?n?9) states for He-like silicon. The results are found to compare very well with available other results except Δn=0 due to differences in the calculated excitation energies.     

A model pseudopotential calculation of the electronic structure of Si (111)-1 × 1 surface

We report a model pseudopotential calculation of the electronic structure of Si (111)-1 × 1 unrelaxed, relaxed and expanded surfaces. It is shown that our results for the surface states on these faces are in good agreement with results from other self-consistent pseudopotential and semi-empirical tight-binding calculations.     

Study of Ag/Si(111) submonolayer interface: I. Electronic structure by angle-resolved UPS

Angle-resolved ultraviolet photoelectron spectra have been measured for well defined Ag/Si(111) submonolayer interfaces of (1) $\text{Si(111)(}\text{3}\text{×}\text{3}\text{)R30°-Ag}$, (2) “Si(111)(6 × 1)-Ag”, and (3) Ag/Si(111) as deposited at room temperature. Non-dispersive and very narrow (FWHM ～ 0.4–0.5 eV) Ag 4d derived peaks are found at 5.6 and 6.5 eV below the Fermi level for surface (1) and at 5.3 and 6.0 eV for surface (2). Dispersions of sp “binding” states in the energy range between E_F and Ag 4d states have been precisely determined for surface (1). Electronic structures similar to those of the Ag(111) surface, including the surface state near E_F, have been observed for surface (3).     

Rovibrational analysis of the absorption spectrum of H2O around 1.02 μm by ICLAS-VECSEL

The very weak water vapor absorption spectrum has been investigated between 9520 and 10 010 cm⁻¹ by intracavity laser absorption spectroscopy based on a vertical external cavity surface emitting laser. The rovibrational assignment performed on the basis of the ab initio calculations of Schwenke and Partridge has allowed for determining 156 new energy levels belonging to a total of 13 vibrational states. The assignment process is detailed. The results are compared with the available databases and discussed in regard of recent investigations by Fourier transform spectroscopy.     

Mössbauer absorption by soft ferromagnets in radio-frequency magnetic field

The simultaneous influence of periodical magnetic field reversals at the nucleus between the values ±h ₀ and of magnetostrictive vibrations on the shape of the Mössbauer absorption spectrum is analyzed. The effect of a constant external magnetic field is taken into account by assuming unequal durations of the states ?h ₀ and +h ₀. It is shown that such asymmetric reversals of the magnetic field lead to splitting of the absorption lines into Zeeman patterns corresponding to the time-averaged magnetic field h ₀ R, where R is the asymmetry parameter of the reversals. The calculations agree well with experiment.     

UV-photoemission at constant wavevector component k6: Application to copper

Angle-resolved energy distribution curves of photoelectrons obtained at constant wavevector component k₆ have been presented for the surface of Cu(111). The main observed transitions can be interpreted as due to direct interband transitions between the APW initial states and free-electron-like final states which are calculated simply in one-dimension along k_⊥ lines parallel to the surface normal.