Si Valence Band Research Articles

Atomic and electronic structures of heat treated 6H–SiC surface

We have studied heat-treated (950–1300°C) 6H–SiC(0001)Si and (0001̄)C face with photoemission spectroscopy using synchrotron radiation (SR-PES) and low energy electron diffraction (LEED). We observed LEED patterns of SiC 1×1, 3 × 3 , 3 × 3 +6 3 ×6 3 and graphite 1×1 sequentially with increasing heating temperature for (0001)Si face and SiC 1×1 for (0001̄)C face, respectively. We have measured Si(2p) spectra and valence band energy distribution curves (VB-EDCs). The trend of sublimation of Si atoms from surface is different between Si- and C-face. 3 × 3 superstructure must be Si-derived. The 6 3 ×6 3 structure could be explained as a moiré pattern caused by monolayer-graphite sitting on SiC surface. Si 3s-derived state of SiC 1×1 is different between SiC 1×1 for Si- and C-face. It is suggested that a single crystal graphite layer grows on Si-face and a polycrystalline graphite is formed on C-face for heated specimens above 1150°C.

Amorphous and crystalline IrSi Schottky barriers on silicon

The electro-optical properties of the films are studied by measuring the infrared optical absorption and the yield of hole photoemission across the Schottky barrier to the p-Si substrate. The Schottky barrier height of the amorphous a-IrSi to the valence band of Si is 0.17–0.18 eV. The barrier height of the polycrystalline c-IrSi depends on the degree of texturing of the silicide; the Schottky barrier height increases from 0.14 eV to 0.18 eV when the substrate temperature is increased from room temperature to 400 °C during the Ir metal deposition prior to the 500 °C silicidation. The non-linearity of the photoemission yield observed for the amorphous a-IrSi in the Fowler plot is due to an energy-dependent photon absorption of these amorphous films. An evaluation of the scattering events in the metal films shows that the mean free path for inelastic scattering is λin=500 nm in the amorphous a-IrSi which is one order of magnitude larger than that in the crystalline c-IrSi. The mean free path for quasi-elastic scattering λqe=0.3 nm, of the amorphous a-IrSi is in the order of the interatomic distance, indicating a localization of the electronic states.

Metal–oxide–semiconductor capacitance–voltage characteristics and band offsets for Si1−yCy/Si heterostructures

Metal–oxide–semiconductor (MOS) capacitors were fabricated on in situ doped n- and p-type Si1−yCy/Si heterostructures grown on Si substrates by chemical vapor deposition. Strained Si1−yCy epitaxial layers with substitutional carbon contents up to 1.6% were studied. High frequency and quasistatic capacitance–voltage (C–V) measurements exhibit well-behaved MOS characteristics, indicating high electronic material quality. Band alignments were extracted from MOS C–V measurements and one-dimensional device simulations performed over a range of temperatures. The conduction band energy of strained Si1−yCy is lower than that of Si by approximately 65 meV for 1 at. % carbon, while the valence band shows negligible offset to Si valence band.

Investigation of Ge on Si(100) quantum wells by photoelectron spectroscopies

In the present work we report on the evolution of the electronic states as a function of Ge overlayer thickness ranging from a fraction of a monolayer to few tens of monolayers. The Ge states above the Si valence band top could be clearly detected at every overlayer thickness d. For d<∼6 Å, the Ge state density exhibits a double linear edge. Upon increasing the overlayer thickness the split between the two edges decreases and the double edge structure disappears for d>∼6 Å, i.e., at the critical thickness for the transition from strained to unstrained overlayers. For these larger thicknesses, the Ge edge becomes broad and featureless. The energy separation between the Ge onset and the Si valence top increases up to 0.70 eV at larger overlayer thicknesses.

Tight-binding sp 3d 5 Hamiltonian for Si

A new nearest-neighbor empirical tight-binding Hamiltonian is obtained for the homopolar cubic elemental semiconductor Si. The Hamiltonian features an sp 3d 5 basis centered on each site, is an 18 × 18 matrix at each wavevector in the Brillouin zone, and has 13 adjusted parameters. This Hamiltonian provides a good description of both the valence and conduction bands of Si.

Electronic states of thin epitaxial layers of Ge on Si(100)

The valence density of states of Ge grown epitaxially on Si(100) is investigated as a function of thickness by yield spectroscopy and photoemission techniques. A double edge is present in the yield data for thicknesses smaller than the lattice relaxation critical thickness. Furthermore, the line-up of the Ge states to the Si valence band varies with overlayer thickness. Photoemission techniques fail to detect this behavior. The causes for the discrepancy are analyzed.

Theoretical investigation of a current in the direction of B in p-type silicon in fields

In this article we present a theoretical study on Hall transport of hot holes in the anisotropic valence band of Si at low temperatures (T = 20 K) in crossed E- and B-fields. A current in the direction of B is found, which may lead to a Hall field in the direction of B of the order of 5% of the applied electric field. We used a valence band model including non-parabolicity and anisotropy. The scattering models employed in the Monte Carlo simulation are acoustic and optical phonon scattering.

Internal photoemission at the Si/SiO2 and Si/metal interface

We present experimental results from infrared photocurrent measurements at the Si/metal and Si/SiO2/metal interface. We observe internal photoemission from the Si valence band into metal states or into empty interface states. For the p-Si/SiO2/metal interface the photocurrent threshold occurs at a photon energy of 0.138 eV, indicating that the Fermi level is pinned close to the valence band. We argue that this is the case only for discrete locations of the interface and suggest that the pinning is caused by a defect-related acceptor level.

First-principles study onB12clusters in Si

The ab initio calculation is reported on the ${\mathrm{B}}_{12}$ clusters in crystalline Si, whose existence has been recently suggested experimentally. We investigated the atomic and the electronic structures for two possible configurations of ${\mathrm{B}}_{12}$ clusters; icosahedron and cubo-octahedron. We found that both of the clusters capture the valence electrons, which results in the generation of an unoccupied level in the valence band of crystalline Si. It was also found that the icosahedral ${\mathrm{B}}_{12}$ in Si is more stable than the cubo-octahedral ${\mathrm{B}}_{12}$, although the symmetry of a cubo-octahedron (${\mathrm{O}}_{\mathrm{h}}$) is more favorable to crystalline Si than that of an icosahedron (${\mathrm{I}}_{\mathrm{h}}$). The calculated results suggest that the experimentally observed ${\mathrm{B}}_{12}$ clusters take the icosahedral configuration.

Behavior of Si Photoelectrodes under High Level Injection Conditions. 1. Steady-State Current−Voltage Properties and Quasi-Fermi Level Positions under Illumination

The behavior of the quasi-Fermi levels of electrons and holes at various semiconductor/liquid interfaces has been probed through the use of thin, high purity, low dopant density single crystal Si photoelectrodes. Since standard Air Mass 1.5 illumination is sufficient to produce high level injection conditions in such samples, minimal electric fields can be present near the solid/liquid interface. Under these conditions, efficient charge separation relies on establishment of kinetic asymmetries at the back contacts while effectively sustaining photogenerated carrier concentration gradients in the photoelectrode. These conditions were achieved for Si/CH3OH interfaces in contact with the 1,1‘-dimethylferrocene+/0, cobaltocene+/0, methyl viologen2+/+, and decamethylferrocene+/0 redox couples. For redox couples having energies near the top of the Si valence band, such as 1,1‘-dimethylferrocene+/0, the sample acted like an n-type photoelectrode, yielding large photovoltages for collection of electrons at the ba...

A New Model Silicon/Silicon Oxide Interface Synthesized from H10Si10O15 and Si(100)- 2×1

A model silicon/silicon oxide interface, synthesized from the spherosiloxane H10Si10O15 and Si(100)-2×1, has been characterized by study of the Si 2p core-levels and valence band region using soft X-ray photoemission. In addition, the intact H10Si10O15 cluster was condensed at -160° C onto Si(111)-H and characterized. The measured photoemission features are in good agreement with the results of previous model studies.

Spectroscopy of intersubband transitions in Si–Si 1− xGe x quantum wells

We investigated intersubband transitions in the valence band of Si 1− x Ge x quantum wells. After a brief review of the recent achievements related to intersubband transitions, we present an original technique, photo-induced IR absorption spectroscopy, to study intersubband transitions in both doped and undoped quantum wells. A comparison with photo-induced intersubband absorption in GaAs quantum wells as well as the variation of the intersubband absorption with Ge content, layer thickness and doping and the temperature dependence of photo-induced absorption are reported. The normal incidence IR modulation is presented in a second part. It is shown that the index variations associated with free carrier and intersubband absorptions significantly affect the IR modulation. The thermal stability of narrow Si–Si 1− x Ge x quantum wells is investigated in a third section. The interdiffusion leads to a red shift of the intersubband transitions. It is shown that the local variation of the effective mass can enhance the normal incidence intersubband absorption. Finally, intersubband relaxation in doped quantum wells is investigated using a picosecond free electron laser. The subband lifetime is first deduced from the saturation of intersubband absorption vs. pump intensity. In a second step, the subband lifetime is measured directly by time-resolved pump and probe experiments. Both techniques demonstrate that the subband lifetime T 1 is shorter than 1 ps.

Structures and Properties of Semiconductor Microclusters. Structures and Properties of B12 Clusters in Si Crystals.

A high concentration of hole is generated without annealing, by high dose ion-implantation of boron into silicon substrate. A model has been proposed that B12 clusters are generated in crystalline Si and act as double acceptors.Recently, we investigated the atomic and electronic structures for B12 clusters by the ab initio calculation. It was found that the clusters capture the valence electrons, which results in the generation of unoccupied level in valence band of crystalline Si and the generation of holes. It was also found thatthe icosahedral B12 in Si is more stable than the cubo-octahedral B12.

Intrinsic valence and conduction bands of Si(111)-1 x 1.

The valence bands of the unreconstructed H-Si(111)-1\ifmmode\times\else\texttimes\fi{}1 are investigated using angle-resolved ultraviolet photoelectron spectroscopy. The high quality of the surface and the absence of reconstruction allow us to observe bulk bands comparable to theoretical calculations. The asymmetric dispersion of the valence bands along the ${\overline{M}}^{\ensuremath{'}}\overline{\ensuremath{\Gamma}}\overline{M}$ direction of the surface Brillouin zone confirms the asymmetry observed for the conduction bands. Such an asymmetry, stemming from the fact that the family of ($11\overline{2}$) planes are not mirror planes in the bulk of Si, provides a supplementary means of disentangling bulk states from surface states.

Intersubband absorption in Si/Si1−x−yGexCy quantum wells

Intersubband absorption in the valence band of Si–Si1−x−yGexCy multiquantum wells is reported. The quantum wells with x≊17% and y≊1% and thicknesses around 3 nm are grown pseudomorphically by rapid thermal chemical vapor deposition on Si(001). The carbon is incorporated in substitutional site using methylsilane as the gas precursor. The quantum wells exhibit near-band-edge photoluminescence with no-phonon and phonon-assisted replica. The energy position of the no-phonon and phonon-assisted replica are shifted to high energy with respect to bulk alloys due to the quantum confinement of the first heavy hole subband. The intersubband absorption between confined subbands is measured in a multipass waveguide geometry under optical pumping. Absorptions for light with an electric field polarized either parallel or perpendicular to the growth axis are observed in both SiGe and SiGeC quantum wells. The absorptions peak at 100 and 130 meV and involve bound-to-bound and bound-to-continuum transitions. The spectral positions of the intersubband absorptions are discussed from numerical calculations accounting for the band gap variation induced by carbon.

Longitudinal conductivity of Ge/Si heterostructures with quantum dots

The conductance along an island layer of Ge quantum dots buried in silicon was investigated. The sizes of the islands varied in the range D ≈ 12−19 nm. It was found that the charge transport is characterized by two activation energies. The first one is associated with the thermal emission of holes from Ge quantum wells into the valence band of Si. The second one is due to the tunneling of holes between islands under Coulomb blockade conditions and is determined by the electrostatic charging energy of a quantum dot.

Hole confinement in boron δ-doped Si quantum wells studied by admittance spectroscopy

The admittance spectroscopy technique has been used to study the hole confinement in boron δ-doped Si quantum wells. A carrier thermal emission model is proposed to derive the activation energies of holes confined in the quantum wells from the measured conductance spectra. For the same peak doping concentration, the conductance peak shifts towards higher temperatures as the thickness of the δ-doped layer increases. The activation energies obtained from the measurements coincide well with the results of a self-consistent calculation of the subbands in the quantum wells. It verifies that the conductance peaks correspond to the hole emissions from the hole ground states in the δ-quantum wells to the Si valence band.

Admittance spectroscopy studies of boron delta -doped Si quantum wells.

The admittance spectroscopy technique has been used to study the hole confinement in boron \ensuremath{\delta}-doped Si quantum wells. Based on the carrier thermal emission model, the activation energies of holes confined in the quantum wells are obtained from the measured conductance spectra. For different well widths, i.e., different boron doping amounts, the well depths and the subband positions are different. We observe the shifts of the conductance peaks in the spectra for \ensuremath{\delta}-doped samples with the peak doping concentration of 2\ifmmode\times\else\texttimes\fi{}${10}^{20}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ and doped thicknesses of 1.2, 3.0, and 5.0 nm. The activation energies derived from the measurements are 0.11, 0.30, and 0.34 eV, respectively. A self-consistent calculation of the subbands in the quantum wells verifies that these activation processes correspond to the hole emissions from the hole ground states in the \ensuremath{\delta}-doped quantum wells to the Si valence band.

Adsorption of water on Si(001)-2 × 1 and Si(111)-7 × 7 surfaces at 90 and 300 K: A Si 2p core-level and valence band study with synchrotron radiation

We have studied by soft X-ray photoemission ( hv = 145 eV) the modifications of the Si 2p core-level and of the valence band during the adsorption of water on Si(001)-2 × 1 and Si(111)-7 × 7 surfaces, at two substrate temperatures (300 and 90 K). In all cases, water reacts with silicon. We have followed the decay of Si 2p and valence-band surface states, as well as the growth of oxidation states, with increasing exposures to H 2O. The breaking of H 2O into -H and -OH fragments and their attachment to the triply coordinated surface atoms (silicon dimers on Si(001), adatoms and rest-atoms on Si(111)) is not the only reactional mechanism. Oxygen atoms are also inserted into SiSi bonds, so that oxidation offers an alternative channel to H 2O dissociation and chemisorption. For both orientations, surface oxidation is facilitated at low temperature. The relative reactivity of the dangling-bonds of Si(111)-7 × 7 has been investigated: both valence band and Si 2p spectra indicate clearly that the rest-atoms are less reactive than the adatoms.

Deep-level impurities in edge-defined film-fed-growth silicon

Edge-defined film-fed-growth (EFG) Si is investigated using deep-level transient spectroscopy and surface photovoltage. An impurity energy level of CrB was found at 0.27 eV above the valence band in EFG Si contaminated with Cr. The Cr diffusion coefficient in EFG Si was obtained as 2×10−17 cm2/s at room temperature using association and dissociation of CrB pairs after a 210 °C dissociation anneal. Most of the deep-level transient spectroscopy (DLTS) spectra are not analyzable with conventional methods due to abnormally broad peaks. DLTS spectra of as-grown EFG Si are modeled using a Gaussian distribution of impurity energy states. The simulated DLTS peaks agree well with measured data explaining the origin of the deep-level impurities of EFG Si.