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.     

Influence of Cr<Superscript>+</Superscript> ion implantation and pulsed ion-beam annealing on the formation and optical properties of Si/CrSi<Subscript>2</Subscript>/Si(111) heterostructures

The effect of pulsed ion-beam annealing on the surface morphology, structure, and composition of single-crystal Si(111) wafers implanted by chromium ions with a dose varying from 6 × 10¹⁵ to 6 × 10¹⁶ cm⁻² and on subsequent growth of silicon is investigated for the first time. It is found that pulsed ion-beam annealing causes chromium atom redistribution in the surface layer of the silicon and precipitation of the polycrystalline chromium disilicide (CrSi₂) phase. It is shown that the ultrahigh-vacuum cleaning of the silicon wafers at 850°C upon implantation and pulsed ion-beam annealing provides an atomically clean surface with a developed relief. The growth of silicon by molecular beam epitaxy generates oriented 3D silicon islands, which coalesce at a layer thickness of 100 nm and an implantation dose of 10¹⁶ cm⁻². At higher implantation doses, the silicon layer grows polycrystalline. As follows from Raman scattering data and optical reflectance spectroscopy data, semiconducting CrSi₂ precipitates arise inside the silicon substrate, which diffuse toward its surface during growth.     

A high efficient antireflective down-conversion Y<Subscript>2</Subscript>O<Subscript>3</Subscript>:Bi<Superscript>3+</Superscript>, Yb<Superscript>3+</Superscript> thin films

The high efficient antireflective down-conversion Y₂O₃:Bi, Yb films have been prepared successfully on Si(100) substrates by pulsed laser deposition (PLD) method, Upon excitation of ultraviolet photon varying from 300 to 400 nm, near-infrared emission of Yb³⁺ was observed for the film, can be efficiently absorbed by silicon (Si) solar cell. Most interestingly, there is a very low average reflectivity 1.46% for the incident light from 300 to 1100 nm. To the best of our knowledge, this is the lowest reflectance for the down-conversion thin films prepared by cost efficient method. The surface topography of the high efficient antireflective films can be controllably tuned through the substrate template regulation by optimizing process parameters. Besides, the results showed that there is a close relationship between luminescent property and morphology of the film. With the change of the surface morphology, the intensity of Bi³⁺ and Yb³⁺ emission peaks increase first and then decrease. The obtained results demonstrate that this film can enhance the Si solar cell efficiency through light trapping and spectrum shifting.     

Distribution of <Superscript>28</Superscript>Si, <Superscript>29</Superscript>Si,and <Superscript>30</Superscript>Si isotopes under plastic deformation in subsurface layers of Si: B crystals

The redistribution of ²⁸Si, ²⁹Si, and ³⁰Si isotopes in subsurface layers of Si: B single crystals after their plastic deformation has been revealed. It has been found that the distribution profile of ²⁸Si and ²⁹Si isotopes becomes smoother after deformation, whereas the ³⁰Si isotope distribution remains unchanged. A change in the subsurface profile of the ²⁹SiO oxide is observed, which indicates the migration of the ²⁹Si isotope in the composition of oxygen complexes during plastic deformation.     

<Superscript>29</Superscript>Si, <Superscript>27</Superscript>Al, <Superscript>1</Superscript>H and <Superscript>23</Superscript>Na MAS NMR Study of the Bonding Character in Aluminosilicate Inorganic Polymers

²⁹Si, ²⁷Al, ¹H and ²³Na solid-state magic-angle spinning (MAS) nuclear magnetic resonance (NMR) has been used to relate nominal composition, bonding character and compressive strength properties in aluminosilicate inorganic polymers (AIPs). The ²⁹Si chemical shift varies systematically with Si-to-Al ratio, indicating that the immediate structural environment of Si is altering with nominal composition. Fast ¹H MAS and ²⁹Si T _SiH/T _1ρ relaxation measurements demonstrated that occluded pore H₂O mobility within the disordered cavities is slow in comparison with H₂O mobility characteristics observed within the ordered channel structures of zeolites. The ²⁷Al MAS NMR data show that the Al coordination remains predominantly 4-coordinate. In comparison with the ²⁹Si MAS data, the corresponding ²⁷Al MAS line shapes are relatively narrow, suggesting that the AlO₄ tetrahedral geometry is largely unperturbed and the dominant source of structural disorder is propagated by large distributions of Si–O bond angles and bond lengths. Corresponding ²³Na MAS and multiple-quantum MAS NMR data indicate that Na speciation is dominated by distributions of hydration states; however, more highly resolved ²³Na resonances observed in some preparations supported the existence of short-range order. New structural elements are proposed to account for the existence of these Na resonances and an improved model for the structure of AIPs has also been proposed. Authors' address: John V. Hanna, NMR Facility, Institute of Materials and Engineering Science, Lucas Heights Research Laboratories, Australian Nuclear Science and Technology Organisation, Private Mail Bag 1, Menai, NSW 2234, Australia     

Effect of the atomic composition of the surface on the electron surface states in topological insulators A<Stack><Subscript>2</Subscript><Superscript>V</Superscript></Stack>B<Stack><Subscript>3</Subscript><Superscript>VI</Superscript></Stack>

The results of the theoretical investigation of the surface electronic structure of A₂^VB₃^VI compounds containing topologically protected surface states are reported. The ideal Bi₂Te₃, Bi₂Se₃, and Sb₂Te₃ surfaces and surfaces with an absent external layer of chalcogen atoms, which were observed experimentally as monolayer terraces, have been considered. It has been shown that the discrepancy between the calculated Fermi level and the value measured in the photoemission experiments can be attributed to the presence of the “dangling bond” states on the surface of the terraces formed by semimetal atoms. The fraction of such terraces on the surface has been estimated.     

Intense White Luminescence from Combustion Synthesized Ca<Subscript>12</Subscript>Al<Subscript>14</Subscript>O<Subscript>33</Subscript>: Yb<Superscript>3+</Superscript>/Yb<Superscript>2+</Superscript> Single Phase Phosphor

The Ca₁₂Al₁₄O₃₃: Yb³⁺/Yb²⁺ single phase nano-phosphor has been synthesized through combustion route and its luminescence and lifetime studies have been carried out up to 20 K using 976 and 266 nm excitations. The samples heated in open atmosphere have shown the presence of Yb in Yb³⁺ and Yb²⁺ states. The 976 nm excitation results a cooperative upconversion emission at 486 nm due to the Yb³⁺ state and a broad band in the blue region and has been assigned to arise from the defect centers. The 266 nm excitation on the other hand results a broad emission band even from as-synthesized phosphor without doping of Yb, the width of which increases in presence of Yb due to the emission from Yb²⁺ ions formed in heated samples. The white emission covers almost whole visible region with bandwidth 190 nm. The ions in Yb²⁺ state has been found to increase with the increase in heating temperature up to 1,273 K. A back conversion of Yb²⁺ to Yb³⁺ has been observed for higher temperatures. Effect of boric and phosphoric acids as flux on the emission properties of Yb³⁺ and Yb²⁺ states have been examined and discussed. Quantum yield of emission has also been determined for different samples.     

Silicon isotope separation utilizing infrared multiphoton dissociation of Si<Subscript>2</Subscript>F<Subscript>6</Subscript> irradiated with two-frequency CO<Subscript>2</Subscript> laser lights

Silicon isotope separation has been performed utilizing the infrared multiphoton dissociation (IRMPD) of Si₂F₆ irradiated with two-frequency CO₂ laser lights. The two-frequency excitation method improved the separation efficiency by keeping the high enrichment factors. For example, Si₂F₆ with the ²⁸Si fraction of 99.4% was obtained at 40.0% dissociation of Si₂F₆ after the simultaneous irradiation of 100 pulses with 966.23 cm^-1 photons (0.089 J/cm²) and 954.55 cm^-1 photons (0.92 J/cm²), while 1000 pulses were needed to obtain 99.0% of ²⁸Si at 27.2% dissociation in the case of single frequency irradiation at 954.55 cm^-1 (0.92 J/cm²). The single-step enrichment factors of ²⁹Si and ³⁰Si increased with increasing Si₂F₆ pressure. The reason for this enhancement has been discussed in terms of the rotational and vibrational relaxations by collisions with ambient gases. PACS 42.62.Cf; 82.30.Lp; 82.50.Bc     

Composition,morphology, and electronic structure of the nanophases created on the SiO<Subscript>2</Subscript> Surface by Ar<Superscript>+</Superscript> ion bombardment

The influence of Ar⁺ bombardment on the composition and the structure of the SiO₂/Si surface is studied. A thin Si film is found to form on the SiO₂ surface subjected to high-dose ion bombardment.     

A miniature tunable TEA CO<Subscript>2</Subscript> laser using isotope <Superscript>13</Superscript>C<Superscript>16</Superscript>O<Subscript>2</Subscript> and <Superscript>12</Superscript>C<Superscript>16</Superscript>O<Subscript>2</Subscript>

A miniature tunable TEA CO₂ laser using isotope ¹³C¹⁶O₂ as the active medium is developed to extend the spectral range of CO₂ lasers for further application. The optimization of the energy parameters of the tunable TEA ¹³C¹⁶O₂ laser and the same laser using ¹²C¹⁶O₂ are studied. When a gas mixture (¹³C¹⁶O₂: N₂: He = 1: 1: 3) at a total pressure of 6.4 × 10⁴ Pa is used, the TEA ¹³C¹⁶O₂ laser of a 45-cm³ active volume obtains 51 emission lines in the [00⁰1–10⁰0] and [00⁰1–02⁰0] bands. The maximum pulse energy of the TEA ¹³C¹⁶O₂ laser is about 357 mJ. The same laser using the conventional gas mixture (¹²C¹⁶O₂: N₂: He = 1: 1: 3) at a pressure of 6.66 × 10⁴ Pa is measured to obtain 69 laser emission lines and the maximum pulse energy of laser radiation is about 409 mJ.     

I. Electron-impact ionization and dissociation of C<Subscript>2</Subscript>H<Stack><Subscript>2</Subscript><Superscript>+</Superscript></Stack> and C<Subscript>2</Subscript>D<Stack><Subscript>2</Subscript><Superscript>+</Superscript></Stack>

Absolute cross-sections for electron-impact ionization and dissociation of C₂H₂⁺ and C₂D₂⁺ have been measured for electron energies ranging from the corresponding thresholds up to 2.5 keV. The animated crossed beams experiment has been used. Light as well as heavy fragment ions that are produced from the ionization and the dissociation of the target have been detected for the first time. The maximum of the cross-section for single ionization is found to be (5.56 ± 0.03)× 10^-17 cm² around 140 eV. Cross-sections for dissociation of C₂ H₂⁺ (C₂D₂⁺) to ionic products are seen to decrease for two orders of magnitude, from C₂D⁺ (12.6 ± 0.3) × 10^-17 cm² over CH⁺(9.55 ± 0.06) × 10^-17 cm², C⁺ (6.66 ± 0.05) × 10^-17 cm², C₂⁺ (5.36 ± 0.27) × 10^-17 cm², H⁺ (4.73 ± 0.29) × 10^-17 cm² and CH₂⁺ (4.56 ± 0.27) × 10^-18 cm² to H₂⁺ (5.68 ± 0.49) × 10^-19 cm². Absolute cross-sections and threshold energies have been compared with the scarce data available in the literature.     

<Superscript>29</Superscript>Si MAS-NMR study of oxide conductors derived from the apatite-type La<Subscript>9.33</Subscript>Si<Subscript>6</Subscript>O<Subscript>26</Subscript> compound

The preparation of (La_9.33−2x/3Sr _x □_0.67−x/3)Si₆O₂₄O₂ (0 ≤ x ≤ 2) samples with different amounts of cation vacancies is reported. Structure and unit-cell parameters were deduced by Rietveld analysis of XRD patterns. Structural features that enhance oxygen conductivity in Sr-doped apatites are discussed. Up to three components were detected in ²⁹Si MAS-NMR spectra which change with the amount and distribution of cation vacancies. In general, oxygen conductivity increases with the amount of vacancies at La1 (6h) sites, passing through a maximum for x = 0.4. In the case of activation energy, a minimum is detected near x = 1.2, indicating that entropic and enthalpic change in different ways. The presence of cation vacancies should enhance oxygen hopping along c-axis; however, the analysis of the frequency dependence of conductivity suggests that oxygen motions are produced along three axes.     

Polymerization of solid C<Subscript>60</Subscript> under C<Stack><Subscript>60</Subscript><Superscript>+</Superscript></Stack> cluster ion bombardment

The structure transformation occurring in fullerene film under bombardment by 50 keV C₆₀⁺ cluster ions is reported. The Raman spectra of the irradiated C₆₀ films reveal a new peak rising at 1458 cm⁻¹ with an increase in the ion fluence. This feature of the Raman spectra suggests linear polymerization of solid C₆₀ induced by the cluster ion impacts. The aligned C₆₀ polymeric chains composing about 5–10 fullerene molecules have been distinguished on the film surface after the high-fluence irradiation using atomic force microscopy (AFM). The surface profiling analysis of the irradiated films has revealed pronounced sputtering during the treatment. The obtained results indicate that the C₆₀ polymerization occurs in a deep layer situated more than 40 nm below the film surface. The deep location of the C₆₀ polymeric phase indirectly confirms the dominant role of shock waves in the detected C₆₀ phase transformation.     

Electronic structure of FeSi<Subscript>2</Subscript>

The electronic structure of volume and film iron disilicides with crystal structure of the type of α leboit and fluorite was calculated using the linearized augmented plane-wave formalism. Joint and local partial densities of electronic states, x-ray emission spectra in different series of all inequivalent atoms of these phases, and photoelectron spectra for different excitation energies were obtained. γ-FeSi₂ was found to be, unlike α-FeSi₂, an unstable phase in both the volume and film realizations. X-ray L _2,3 emission spectra of silicon in the iron group disilicides NiSi₂, CoSi₂, and FeSi₂ were compared. NiSi₂, CoSi₂, and α-FeSi₂ exhibit transformation of the maximum in the near-Fermi region of the Si L _2,3 spectra as one crosses over from a bulk to a film sample. This transformation is closely connected with phase stability and may serve as a criterion of thermodynamic stability of the iron-group transition-metal disilicides.     

Photodesorption from ultra-thin metal films – a comparison of SO<Subscript>2</Subscript> and NO<Subscript>2</Subscript> on Ag/Si(100)

The photochemistry of SO₂ on thin epitaxial Ag films (5–60 nm) deposited on Si(100) has been studied using laser light with the wavelengths of 266, 355, and 532 nm. SO₂ desorbs with cross sections of 1.7×10^-19,1.7×10^-20 and 2.9×10^-21 cm², respectively. The average translation energy, 〈E_trans/2k〉, is 440 K for 266 and 355 nm light, and 270 K for 532 nm light. Cross sections for a 60 nm thick Ag film are practically identical to the ones for Ag(111) as the substrate. An increase by a factor of ∼3.5 is observed when the film thickness is reduced to 5 nm for 266 and 355 nm light. No significant change is observed for 532 nm excitation. The film thickness has no significant influence on the translational energy of the photodesorbed molecules. The data are discussed in connection with the change of absorptivity of the metal film–semiconductor system. A model is put forward which takes into account the light absorption in the Si substrate and the reduced relaxation of excited electrons in Si. Modelling indicates that electrons excited in the Si substrate with energies and parallel momenta not allowed in Ag contribute to the surface chemistry after crossing the gap in the projected band structure of Ag(111). PACS 82.45.MP; 73.63.-b; 82.50.Bc     

On new two-dimensional structures produced on the Si (111) and Si (100) surface upon molecular-beam epitaxy of cobalt and silicon

Highly perfect epitaxial heterostructure CoSi₂ films have been grown on the surface of Si (111) and Si (100) single crystals by the method of molecular-beam epitaxy. The optimal regimes of the film growth with different thicknesses have been determined. It has been shown that short-term annealing of epitaxial films at T = 900–950 K leads to the formation of new CoSi₂/Si(111)-2 × 2 and CoSi₂/Si(100)−2 × 4 superstructures.     

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.     

Luminescence and electronic excitations in Li<Subscript>6</Subscript>Gd(BO<Subscript>3</Subscript>)<Subscript>3</Subscript>: Ce<Superscript>3+</Superscript> crystals

This paper reports on a study of the luminescence emitted by Li₆Gd(BO₃)₃: Ce³⁺ crystals under selective photoexcitation to lower excited states of the host ion Gd³⁺ and impurity ion Ce³⁺ within the 100–500-K temperature interval, where the mechanisms of migration and relaxation of electronic excitation energy have been shown to undergo noticeable changes. The monotonic 10–15-fold increase in intensity of the luminescence band at 3.97 eV has been explained within a model describing two competing processes, namely, migration of electronic excitation energy over chains of Gd³⁺ ions and vibrational energy relaxation between the ⁶ I _j and ⁶ P _j levels. It has been shown that radiative transitions in Ce³⁺ ions from the lower excited state 5d ¹ to ² F _5/2 and ² F _7/2 levels of the ground state produce two photoluminescence bands, at 2.08 and 2.38 eV (Ce1 center) and 2.88 and 3.13 eV (Ce2 center). Possible models of the Ce1 and Ce2 luminescence centers have been discussed.     

Examination of the <Superscript>16</Superscript>O + <Superscript>28</Superscript>Si system with microscopic and phenomenological potentials

The ¹⁶O + ²⁸Si reaction has been widely studied both experimentally and theoretically and has been claimed to show indications of chaotic scattering. In order to examine this claim and to address whether reaction models such as the optical one could explain the experimental data, we have analyzed the ¹⁶O + ²⁸Si system within the framework of the optical model for ten energies from 29.0 to 45.0 MeV, by using microscopic folded potentials, which are based on M3Y nucleon-nucleon, alpha-alpha effective interactions and a phenomenological shallow potential. All potentials describe the individual angular distributions very well at forward angles. However, they fail to describe the individual angular distributions over the whole angular range up to 180°. Nevertheless, we have been able to explain the experimental data by modifying the surface region of the microscopic real potentials by adding two surface potentials. With these correction potentials, we have obtained very good agreement for the individual angular distributions over the whole angular range for the given energies as well as for the experimental data near the Coulomb barrier. The failure of these optical potentials in explaining the scattering observables of this reaction without corrections puts a question mark on the model and supports the idea of a chaotic behavior. The text was submitted by the authors in English.     

Spectroscopic Properties and Energy Transfer Analysis of Tm<Superscript>3+</Superscript>-Doped BaF<Subscript>2</Subscript>-Ga<Subscript>2</Subscript>O<Subscript>3</Subscript>-GeO<Subscript>2</Subscript>-La<Subscript>2</Subscript>O<Subscript>3</Subscript> Glass

This paper reports on the spectroscopic properties and energy transfer analysis of Tm³⁺-doped BaF₂-Ga₂O₃-GeO₂-La₂O₃ glasses with different Tm₂O₃ doping concentrations (0.2, 0.5, 2.0, 2.5, 3.0, 3.5, 3.5, 4.0 wt%). Mid-IR fluorescence intensities in the range of 1,300 nm−2,200 nm have been measured when excited under an 808 nm LD for all the samples with the same pump power. Energy level structure and Judd-Ofelt parameters have been calculated based on the absorption spectra of Tm³⁺, cross-relaxation rates and multi-phonon relaxation rates have been estimated with different Tm₂O₃ doping concentrations. The maximum fluorescence intensity at around 1.8 μm has been obtained in Tm₂O₃-3 wt% sample and the maximum value of calculated stimulated emission cross-section of Tm³⁺ in this sample is about 0.48 × 10⁻²⁰ cm² at 1,793 nm, and there is not any crystallization peak in the DSC curve of this sample, which indicate the potential utility of Tm³⁺-doped BaF₂-Ga₂O₃-GeO₂- La₂O₃ glass for 2.0-μm optical fiber laser.