Properties of an Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/Si interface

The phase chemical composition of an Al₂O₃/Si interface formed upon molecular deposition of a 100-nm-thick Al₂O₃ layer on the Si(100) (c-Si) surface is investigated by depth-resolved ultrasoft x-ray emission spectroscopy. Analysis is performed using Al and Si L_{2, 3} emission bands. It is found that the thickness of the interface separating the c-Si substrate and the Al₂O₃ layer is approximately equal to 60 nm and the interface has a complex structure. The upper layer of the interface contains Al₂O₃ molecules and Al atoms, whose coordination is characteristic of metallic aluminum (most likely, these atoms form sufficiently large-sized Al clusters). The shape of the Si bands indicates that the interface layer (no more than 10-nm thick) adjacent to the substrate involves Si atoms in an unusual chemical state. This state is not typical of amorphous Si, c-Si, SiO₂, or SiO_x (it is assumed that these Si atoms form small-sized Si clusters). It is revealed that SiO₂ is contained in the vicinity of the substrate. The properties of thicker coatings are similar to those of the 100-nm-thick Al₂O₃ layer and differ significantly from the properties of the interfaces of Al₂O₃ thin layers.     

Linear and nonlinear optical spectroscopy of gadolinium iron borate GdFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript>

The optical spectra and the second-harmonic generation (SHG) are studied in a noncentrosymmetric GdFe₃(BO₃)₄ magnet. In the region of weak absorption (α～20–400 cm^?1) below ～3 eV, three absorption bands are distinguished, which can be unambiguously assigned to forbidden electronic transitions from the ground ⁶A₁ state of the Fe³⁺ ion to its excited states ⁴T₁(～1.4 eV), ⁴T₂(～2 eV), and ⁴A₁, ⁴E(～2.8 eV). Intense absorption begins in the region above 3 eV (α～2–4×10⁵ cm^?1), where two bands at ～4.0 and 4.8 eV are observed, which are caused by allowed electric dipole charge-transfer transitions. The spectral features of SHG in the 1.2–3.0-eV region are explained by a change in the SHG efficiency caused by a change in the phase mismatch. It is shown that in the weak absorption region, phase matching can be achieved for SHG.     

Luminescence characteristics of the Pr<Superscript>3+</Superscript> ion in SrB<Subscript>4</Subscript>O<Subscript>7</Subscript> and SrB<Subscript>6</Subscript>O<Subscript>10</Subscript>

Spectral and kinetic characteristics of the luminescence and luminescence excitation spectra of polycrystalline SrB₄O₇:Pr (1%) and SrB₆O₁₀:Pr (1%) samples are studied at 150–170 K. The samples show an intense luminescence band in the vicinity of 405 nm (¹ S ₀→¹ I ₆ transitions of Pr³⁺) and shorter wavelength bands also assigned to transitions from the ¹ S ₀ level. The main luminescence decay constant is ～2×10^?7 s. The excitation spectra of the ¹ S ₀ luminescence in these crystals are significantly different. The SrB₄O₇:Pr crystal shows three well-resolved bands at 6.14, 6.55, and 6.91 eV in the region of the 4f ²→4f ¹5d transitions and a complex structure in the region of interband transitions (7.1–20 eV), whereas the SrB₆O₁₀:Pr crystal shows a weakly structured band at 6.31 eV and no excitation in the region of the interband transitions. The physical mechanisms that may be responsible for the observed features of the spectra are discussed.     

Stability and band offsets between Si and LaAlO<Subscript>3</Subscript>

The replacement of traditional SiO₂ with high-k oxides allows the physical thickness of the gate dielectric to be thinner without the tunneling problem in Si-based metal-oxide-semiconductor field-effect transistors. LaAlO₃ appears to be a promising high-k material for use in future ultra large scale integrated devices. In the present paper, the electronic properties of Si/LaAlO₃ (001) heterojunctions are investigated by first-principles calculations. We studied the initial adsorption of Si atoms on the LaAlO₃ (001) surface, and found that Si atoms preferentially adsorb on top of oxygen atoms at higher coverage. The surface phase diagrams indicate that Si atoms may substitute oxygen atoms at the LaO-terminated surface. The band offsets, electronic density of states, and atomic charges are analyzed for the various Si/LaAlO₃ heterojunctions. Our results suggest that the Si/AlO₂ interface is suitable for the design of metal oxide semiconductor devices because the valence and conduction band offsets are both larger than 1 eV.     

A computer study of the Raman spectra of the (GaN)<Subscript>129</Subscript>, (SiO<Subscript>2</Subscript>)<Subscript>86</Subscript>, and (GaN)<Subscript>54</Subscript>(SiO<Subscript>2</Subscript>)<Subscript>50</Subscript> nanoparticles

The Raman spectra of the (GaN)₁₂₉, (SiO₂)₈₆, and (GaN)₅₄(SiO₂)₅₀ nanoparticles were calculated using the molecular dynamics method. The spectrum of (SiO₂)₈₆ had three broad bands only, whereas the Raman spectrum of (GaN)₁₂₉ contained a large number of overlapping bands. The form of the Raman spectrum of (GaN)₅₄(SiO₂)₅₀ was determined by the arrangement of the GaN and SiO₂ components in it. The nanoparticle with a GaN nucleus had a continuous fairly smooth spectrum over the frequency range 0 ≤ ω ≤ 600 cm⁻¹, whereas the spectrum of the nanoparticle with a SiO₂ nucleus contained well-defined bands caused by vibrations of groups of atoms of different kinds and atoms of the same kind.     

Investigation of HfO<Subscript>2</Subscript>/SiO<Subscript>2</Subscript>/<Emphasis Type="Italic">n</Emphasis>-Si(001)-based MOS structures via ballistic electron emission microscopy

(Au, Pt)/HfO₂/SiO₂/n-Si(001) metal-oxide-semiconductor structures with a thin (≈0.5 nm) SiO₂ layer, which is formed between HfO₂ and Si during atomic layer deposition of oxide layers, have been investigated via ballistic electron emission spectroscopy. The potential barrier heights at the (Au, Pt)/HfO₂ interfaces have been determined experimentally. The peculiarities observed in the curves of dependence of the collector current on the voltage between a scanning tunneling microscope probe and a metallic electrode are related to electron transport through the vacancy defect region of HfO₂ and the quantum-mechanical interference of electron waves arising from multiple reflections at the interfaces of the two-layer dielectric and at the interfaces of dielectric with a substrate and a metallic electrode.     

Preparation and investigation of composite SiO<Subscript>2</Subscript>-TiO<Subscript>2</Subscript> films

SiO₂-TiO₂ films [Si:Ti = 1:(0.06–2.3)] are obtained by the sol-gel method. The structural and photoluminescent properties of the films and powders heat-treated at different temperatures are studied. It is shown that after 700°C the composite consists of TiO₂ crystallites that are structurally similar to anatase and distributed in an amorphous SiO₂ matrix. The photoluminescence spectra have maxima at 450–500 nm. The photoluminescence intensity depends on the treatment temperature and TiO₂ content. __________ Translated from Zhurnal Prikladnoi Spektroskopii Vol. 74, No. 3, pp. 357–361, May–June, 2007.     

Polarized optical absorption of MnGa<Subscript>2</Subscript>S<Subscript>4</Subscript> single crystals

Optical absorption of MnGa₂S₄ single crystals is studied at two light polarizations (E ||C and E⊥ C). The polarization splitting of the absorption edge points to a splitting of the valence band of MnGa₂S₄. A contribution to the crystal-field splitting is made by two factors, namely, by a difference in the pseudopotential of cationic sublattice atoms and by tetragonal compression of the lattice along the C axis. A scheme of optical transitions in MnGa₂S₄ in the Brillouin zone center is suggested, according to which the optical transitions Г₃ + Г₄ → Г₁ occur in the polarization E ⊥ C, and the Г₂ → Г₁ transitions occur in the polarization E || C.     

Fundamental band edge optical absorption in Bi<Subscript>0.5</Subscript>Sb<Subscript>1.5</Subscript>Te<Subscript>3</Subscript>

Spectra of optical absorption in Bi_0.5Sb_1.5Te₃ films grown on mica and KBr substrates have been investigated for T = 145 and 300 K. The data obtained have been analyzed together with the data of investigations on the fundamental absorption edge for Bi₂Te₃ available in the scientific literature. It has been revealed that the interband absorption spectra for both Bi_0.5Sb_1.5Te₃ and Bi₂Te₃ represent a superposition of two components corresponding to direct and indirect allowed optical transitions. In this case, the least energy gap separating the valence band and the conduction band is direct for Bi_2−xSb_xTe₃ (x ≤ 1.5, T = 300 K). For Bi_0.5Sb_1.5Te₃ the temperature variation rates have been estimated for the thresholds of direct and indirect interband transitions. It has been shown that this solid solution is direct gap solution at T ≥ 145 K. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 50–52, July, 2008.     

Density functional study of low-lying isomers of SiO<Subscript>4</Subscript>, GeO<Subscript>4</Subscript> and CO<Subscript>4</Subscript>, and their relation to tetrahedral solid phases

In silica (SiO₂) and in most silicates, atomic associations exist with composition SiO₄ and a structure with four O atoms in tetrahedral coordination around the Si atom. A similar feature is observed in germania (GeO₂) and some solids containing Ge instead of Si, although the number of phases containing GeO₄ tetrahedra is smaller. In contrast, and in spite of the fact that C is in the same column of the periodic table as Si and Ge, CO₂ is a molecular solid, and crystalline and amorphous phases of CO₂ showing CO₄ tetrahedra are only obtained under extremely high pressures. We have investigated the relation between free SiO₄, GeO₄ and CO₄ clusters and the tetrahedral associations found in the solids mentioned above. The lowest energy structure of those three free clusters is planar, but they have near-tetrahedral and distorted-tetrahedral isomers. The promotion energy from the planar structure to the distorted tetrahedral is low in SiO₄, large in CO₄, and intermediate in GeO₄. This correlates with the facility to form tetrahedral associations in the solids.     

First comparison of electric field induced second harmonic of near-infrared femtosecond laser pulses in reflection and transmission generated from Si/SiO<Subscript>2</Subscript> interfaces of a silicon membrane

For the first time electric field induced second harmonic (EFISH) generation of femtosecond (fs) laser pulses (λ=800 nm, τ=75±5 fs, rep. rate=80 MHz, E _pulse≤10 nJ) is observed in transmission through a thin free-standing silicon (Si) membrane of 10-μm thickness and compared to the well-known EFISH results in reflection by use of the z-scan technique. EFISH in reflection and transmission unequivocally originate from the front and rear Si/SiO₂ interfaces, respectively, with SiO₂ being the natural oxide on the Si surfaces. Frequency conversion is enhanced by photoinduced electric fields across the Si/SiO₂ interfaces caused by charge-carrier injection from Si into the oxide. The z-scan results and time-dependent measurements allow comparison of the EFISH signal amplitudes and time constants detected in transmission and reflection, demonstrating the need for further investigation.     

Microscopic bonding and macroscopic strain relaxations at Si-SiO<Subscript>2</Subscript> interfaces

Optical second harmonic generation studies of the temperature dependence of the phase angle differences from contributions of terrace and edge bonding at silicon–silicon dioxide interfaces prepared on vicinal Si(111) wafers have revealed an interface relaxation process at an annealing temperature between 850 and 900 °C. Complementary studies by synchrotron soft X-ray photoelectron spectroscopy have established that this relaxation is associated with changes in the concentration and composition of local suboxide bonding environments in an ultrathin, interfacial transition region that is approximately one molecular layer thick. This relaxation occurs at a significantly lower temperature than an approximately 990±10 °C relaxation of macroscopic compressive strain in the bulk of the SiO₂ film. This paper establishes an analogy between i) the Si–SiO₂ interface in which there is a transition from a rigid substrate, Si, to an ideal continuous random covalent network, SiO₂, in which the average number of bonding constraints/atom matches the network dimensionality, and ii) a concentration dependent transition between under- and over-constrained local bonding in non-crystalline glass alloys such as Ge_xSe_1-x. The interfacial suboxide transition region is demonstrated to have properties in common with a regime of alloy compositions in which self-organization reduces bond constraint induced strain, thereby stabilizing these compositions against aging as for example in time-dependent changes in the glass transition temperature. These comparisons provide important new insights into defect formation at Si–SiO₂ interfaces, as well as interfaces between Si and alternative high-k dielectrics being considered for advanced Si devices including Al₂O₃ and transition metal silicate alloys, e.g., (ZrO₂)_x(SiO₂)_1-x. This new perspective is also extended to interfaces between GaN and SiO₂, where the interfacial transition region is a suboxide of Ga, GaO_x, with x<1.5. PACS 85.30.Tv; 42.65.-k; 79.40.+z; 73.20.-r; 77.55.+f     

Anomalous temperature dependence of photoluminescence in GeO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> films and GeO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>/SiO<Subscript>2</Subscript> nano-heterostructures

The optical properties of GeO _x film and GeO _x /SiO₂ multilayer heterostructures (with thickness of GeO _x layers down to 1 nm) were studied with the use of Raman scattering and infrared spectroscopy, ellipsometry and photoluminescence spectroscopy including temperature dependence of photoluminescence. The observed photoluminescence is related to defect (dangling bonds) in GeO _x and interface defects for the case of GeO _x /SiO₂ multilayer heterostructures. From analysis of temperature dependence of photoluminescence intensity, it was found that rate of nonradiative transitions in GeO _x film has Berthelot type, but anomalous deviations from Berthelot type temperature dependence were observed in temperature dependences of photoluminescence intensities for GeO _x /SiO₂ multilayer heterostructures.     

Effects of LaNiO<Subscript>3</Subscript> on the structures and properties of SrBi<Subscript>2</Subscript>Ta<Subscript>2</Subscript>O<Subscript>9</Subscript> thin films

SrBi₂Ta₂O₉(SBT)/LaNiO₃(LNO)/Si and SBT/Pt/TiO₂/SiO₂/Si multilayers were fabricated by pulsed laser deposition. With Pt top electrodes, the measured remanent polarization (2P_r) of Pt/SBT/LNO/Si and Pt/SBT/Pt/TiO₂/SiO₂/Si capacitors was 6.5 C/cm² and 5.2 C/cm², respectively. Using LNO as both bottom electrodes and buffer layers, enhanced non-c-axis crystalline SBT films were induced, which resulted in a 2P_r greater than that of the Pt/SBT/Pt/TiO₂/SiO₂/Si capacitor. The hysteresis loop of the Pt/SBT/LNO/Si capacitor showed a great external-field-dependent horizontal shift. Using an electron-injection model, this dependence was addressed. The fatigue-free property of the Pt/SBT/LNO/Si capacitor was experimentally established, in that the non-volatile polarization decreased by less than 5% of the initial value after 1.44×10⁹ switching cycles . PACS 77.84.Dy; 68.65.+g     

An Ellipsometric Investigation of the Optical Properties of Ru<Subscript>2</Subscript>Ge<Subscript>3</Subscript> and Ru<Subscript>2</Subscript>Sn<Subscript>3</Subscript> Compounds

Ellipsometric investigations of the optical properties of Ru₂Ge₃ and Ru₂Sn₃ intermetallic compounds are carried out in the wavelength range from 0.22 to 15 μm. The nature of interband light absorption is analyzed based on a comparative analysis of the experimental and theoretical frequency dependences of an optical conductivity. The obtained results confirm the existence of energy gaps at the Fermi level in the electronic spectra of these materials predicted earlier by the band-structure calculations.     

Inelastic neutron scattering in Zr<Subscript>2</Subscript>NiH<Subscript>1.9</Subscript> and Zr<Subscript>2</Subscript>NiH<Subscript>4.6</Subscript>

In this paper we report the results obtained from inelastic neutron scattering measurements on Zr₂NiH_1.9 and Zr₂NiH_4.6 using triple-axis spectrometer at Dhruva reactor, Trombay. The spectrum up to 35 meV represents largely the lattice modes of Zr and Ni atoms. The vibrational frequencies of hydrogen atoms are expected predominantly at higher energies. The phonon spectra from 35–180 meV were recorded using a Be filter as analyser. In order to analyse the observed neutron spectra, we assume a set of Ein-stein modes due to the hydrogen atoms which are delta functions in energy. These delta functions are broadened by the resolution of the instrument. The vibrational frequencies obtained from the fitting of the observed phonon spectra have been assigned to various tetrahedral sites in both the compounds.     

Energy band structure and X-ray spectra of phenakite Be<Subscript>2</Subscript>SiO<Subscript>4</Subscript>

The electronic structure of crystalline phenakite Be₂SiO₄ is investigated using x-ray emission spectroscopy (XES) (Be K _α XES, Si L _{2, 3} XES, O K _α XES) and x-ray absorption spectroscopy (XAS) (Be 1s XAS, Si 2p XAS, O 1s XAS). The energy band structure is calculated by the ab initio full-potential linearized augmented-plane-wave (FLAPW) method. The total and partial densities of states and the dispersion curves for the Be₂SiO₄ compound are presented. It is shown that the top of the valence band and the bottom of the conduction band of the Be₂SiO₄ compound are predominantly formed by the oxygen 2p states. According to the results obtained, the electron transition with the lowest energy supposedly can occur at the center of the Brillouin zone. The effective masses of electrons (0.5m _e ) and holes (3.0m _e ) for the Be₂SiO₄) compound are estimated.     

Nonradiative relaxation of photoexcited O<Stack><Subscript>1</Subscript><Superscript>0</Superscript></Stack> centers in glassy SiO<Subscript>2</Subscript>

The processes involved in the excited-state relaxation of hole O ₁ ⁰ centers at nonbridging oxygen atoms in glassy SiO₂ were studied using luminescence, optical absorption, and photoelectron emission spectroscopy. An additional nonradiative relaxation channel, in addition to the intracenter quenching of the 1.9-eV luminescence band, was established to become operative at temperatures above 370 K. This effect manifests itself in experiments as a negative deviation of the temperature-dependent luminescence intensity from the well-known Mott law and is identified as thermally activated external quenching with an energy barrier of 0.46 eV. Nonradiative transitions initiate, within the external quenching temperature interval, the migration of excitation energy, followed by the creation of free electrons. In the final stages, this relaxation process becomes manifest in the form of spectral sensitization of electron photoemission, which is excited in the hole O ₁ ⁰ -center absorption band.     

Scanning tunneling microscopy observation of ultrathin epitaxial CoSi<Subscript>2</Subscript>(111) films grown at a high temperature

Scanning tunneling microscopy (STM) is used to study the basic laws of growth of ultrathin epitaxial CoSi₂(111) films with Co coverages up to 4 ML formed upon sequential deposition of Co and Si atoms taken in a stoichiometric ratio onto the Co–Si(111) surface at room temperature and subsequent annealing at 600–700°C. When the coverage of Co atoms is lower than ~2.7 ML, flat CoSi₂ islands up to ~3 nm high with surface structure 2 × 2 or 1 × 1 grow. It is shown that continuous epitaxial CoSi₂ films containing 3–4 triple Si–Co–Si layers grow provided precise control of deposition. CoSi₂ films can contain inclusions of the local regions with (2 × 1)Si reconstruction. At a temperature above 700°C, a multilevel CoSi₂ film with pinholes grows because of vertical growth caused by the difference between the free energies of the CoSi₂(111) and Si(111) surfaces. According to theoretical calculations, structures of A or B type with a coordination number of 8 of Co atoms are most favorable for the CoSi₂(111)2 × 2 interface.     

Optical spectra and excited state relaxation dynamics of Sm<Superscript>3+</Superscript> in Gd<Subscript>2</Subscript>SiO<Subscript>5</Subscript> single crystal

Single crystals of gadolinium orthosilicate Gd₂SiO₅ containing 0.5 at% and 5 at% of Sm³⁺ were grown by the Czochralski method. Optical absorption spectra, luminescence spectra and luminescence decay curves were recorded for these systems at 10 K and at room temperature. Comparison of optical spectra recorded in polarized light revealed that the anisotropy of this optically biaxial host affects the intensity distribution within absorption and emission bands related to transitions between multiplets rather than the overall band intensity. It has been found that among four bands of luminescence related to the ⁴G_5/2→⁶H_J (J=5/2–11/2) transitions of Sm³⁺ in the visible and near infrared region the ⁴G_5/2 →⁶H_7/2 one has the highest intensity with a peak emission cross section of 3.54×10⁻²¹ cm² at 601 nm for light polarized parallel to the crystallographic axis c of the crystal. The luminescence decay curve recorded for Gd₂SiO₅:0.5 at% Sm³⁺ follows a single exponential time dependence with a lifetime 1.74 ms, in good agreement with the ⁴G_5/2 radiative lifetime τ _rad=1.78 ms calculated in the framework of Judd-Ofelt theory. Considerably faster and non-exponential luminescence decay recorded for Gd₂SiO₅:5 at% Sm³⁺ sample was fitted to that predicted by the Inokuti-Hirayama theory yielding the microparameter of Sm³⁺–Sm³⁺ energy transfer C _da=1.264×10⁻⁵² cm⁶×s⁻¹.