Theory for the ultrafast dynamics of excited clusters: interplay between elementary excitations and atomic structure

We present a theoretical study of the short-time relaxation of clusters in response to ultrafast excitations using femtosecond laser pulses. We analyze the excitation of different types of clusters (Hg_n, Ag_n, Si_n, C₆₀ and Xe_n) and classify the relaxation dynamics in three different regimes, depending on the intensity of the exciting laser pulse. For low-intensity pulses (I<10¹² W/cm²) we determine the time-dependent structural changes of clusters upon ultrashort ionization and photodetachment. We also study the laser-induced non-equilibrium fragmentation and melting of Si_n and C₆₀ clusters, which occurs for moderate laser intensities, as a function of the pulse duration and energy. As an example for the case of high intensities (I>10¹⁵ W/cm²), the explosion of clusters under the action of very intense ultrashort laser fields is described. Received: 26 November 1999 / Published online: 2 August 2000     

Electronic structure,mechanical and optical properties of ternary semiconductors Si1-xGexC (X = 0, 0.25, 0.50, 0.75, 1)

The electronic structure of silicon carbide with increasing germanium content have been examined using first principles calculations based on density functional theory. The structural stability is analysed between two different phases, namely, cubic zinc blende and hexagonal phases. The zinc blende structure is found to be the stable one for all the Si_1-xGe_xC semiconducting carbides at normal pressure. Effect of substitution of Ge for Si in SiC on electronic and mechanical properties is studied. It is observed that cubic SiC is a semiconductor with the band gap value 1.243?eV. The band gap value of SiC is increased due to the substitution of Ge and the band gap values of Si _0.75 Ge _0.25 C, Si _0.50 Ge _0.50 C, Si _0.25 Ge _0.75 C and GeC are 1.322 eV, 1.413 eV, 1.574 eV and 1.657?eV respectively. As the pressure is increased, it is found that the energy gap gets decreased for Si_1-x Ge_xC (X?=?0, 0.25, 0.50, 0.75, 1). The elastic constants satisfy the Born – Huang elastic stability criteria. The bulk modulus, shear modulus, Young’s modulus and Poisson’s ratio are also calculated and compared with the other available results.     

Synthesis and properties of In2(Se1-xTex)3 thin films: a new semiconductor compound

In₂(Se_1-xTe_x)₃ polycrystalline films were prepared by a dual-source thermal evaporation technique. The depositions onto glass and SnO₂-coated glass substrates were carried out in a vacuum chamber and followed by an annealing in neutral ambient (Ar or N₂). The structural, morphological and compositional studies of the films were made by X-ray diffraction, energy-dispersive X-ray analysis, X-ray photoelectron spectroscopy, scanning electron microscopy, Raman scattering and optical transmission. Optimum conditions are investigated for the formation of the ternary compound In₂(Se_1-xTe_x)₃ in order to tune the band gap by changing the Te concentration. The film properties as a function of Te amount are discussed. It is shown that single-phase, textured and homogeneous layers of In₂(Se_1-xTe_x)₃ can be grown with x≤0.2 at optimal deposition and heat treatment conditions. For x≅0.17 these films showed an energy band gap of about 1.45 eV and an electrical conductivity at room temperature six orders of magnitude higher than that of the binary γ-In₂Se₃ thin films. Received: 9 July 1999 / Accepted: 25 November 1999 / Published online: 13 July 2000     

In-gap density of states in hole- and electron-doped CuO2 planes probed by scanning tunneling spectroscopy

The low-energy electronic structure of a c-axis Sr_xA_yCuO₂ (A is alkaline earth cation, x+y≦1, hole- and electron-doped infinite layer) thin film, grown by laser-molecular-beam epitaxy on a SrTiO₃ (001) substrate, has been studied using ultrahigh-vacuum scanning tunneling microscopy/spectroscopy. Images have been obtained for co-deposited Sr_xA_yCuO₂ thin films, which show the surface consisting of flat terraces separated by steps that are unit cell high. Tunneling spectra of undoped Sr_0.3Ca_0.7CuO₂ indicate a wide band gap of 1.8 eV which is consistent with the charge transfer gap. Hole-doped Sr_0.85CuO₂ shows in-gap states appearing at both the valence and conduction band edges. In contrast, for the electron-doped Sr_0.9La_0.1CuO₂, in-gap states appear predominantly above the Fermi level, and the spectral shape becomes asymmetric around the Fermi level. When these two systems are compared, barrier-height measurements reveal that there is no apparent shift of the Fermi level measured from the vacuum level. This suggests that the framework of the rigid-band picture might break down implying a strongly correlated electron system. Received: 20 August 1998 / Accepted: 15 February 1999 / Published online: 5 May 1999     

The early oxynitridation stages of hydrogen-terminated (100) silicon after exposure to N2:N2O. Nitrogen bonding states

Nitridation of hydrogen-terminated silicon in a diluted N₂:N₂O atmosphere was studied by X-ray photoemission spectroscopy and high-resolution electron microscopy. Our analysis showed that the broad N(1s) peak of width 1.5 eV at 398–399 eV, usually reported in the literature, is preceded by the formation of a narrow peak of width around 1.0 eV at 397.5 eV, attributed to the moiety Si₃N in which silicon is only marginally oxidized, and two other peaks at 400.0 eV and 401.5 eV, attributed to the moieties Si₂NOSi and SiNO, respectively. Received: 11 July 2001 / Accepted: 19 September 2001 / Published online: 20 December 2001     

The effect of defects on the electronic structure and magnetic map of the Fe<Subscript>2</Subscript>CrSi Heusler alloy: ab-initio calculations

Density functional theory (DFT) calculations are performed using the full-potential linearized augmented plane wave (FP-LAPW) and generalized gradient approximation (GGA) to study the electronic and magnetic properties of perfect and defected Fe₂CrSi Heusler alloy. The perfect structure was found to be a half-metallic ferromagnet with a total magnetic moment of 2 μ _B and a band gap 0.6 eV. The Fermi level is found to be in the middle of this gap, which is promising for fabricating tunneling magnetoresistance (TMR) devices. Among the studied defected structures Fe_Si and Cr_Si antisite defects as well as Fe-Si and Cr-Si defects destroyed the half metallicity. However the remaining antisite, swap and vacancy defects retained the half metallicity with band gaps lower than the perfect case.     

Optical diffuse reflectance spectra of GaSb nanocrystals embedded in SiO2 matrix by radio-frequency magnetron co-sputtering

Nanocrystalline GaSb embedded in SiO₂ films was grown by radio-frequency magnetron co-sputtering. X-ray diffraction pattern and transmission electron microscopy (TEM) confirm the existence of GaSb nanocrystals in the SiO₂ matrix. The average size of GaSb nanoparticles is in the range of 3 to 11 nm. Diffuse reflectance spectra were used to characterize the small change of the band gap of the semiconductor. The diffuse reflectance spectra shows that the absorption peaks have a large blueshift of about 4.0 eV of the absorption relative to that of bulk GaSb. It has been explained by quantum confinement effects. Room temperature optical transmission spectra show that the absorption edge exhibits a very large blueshift of about 2.1 eV with respect to that of bulk GaSb in agreement with quantum confinement. Received: 28 July 1999 / Accepted: 27 October 1999 / Published online: 1 March 2000     

Electronic structure of solid C48N12 aza-fullerene

I report electronic structures and the cohesive energy for face-centered-cubic (fcc) solid C₄₈N₁₂ using generalized-gradient density-functional theory. The full vibrational spectrum of the C₄₈N₁₂ cluster is calculated within the harmonic approximation at the B3LYP/6-31G* level of theory. The results show that fcc is energetically preferred and a more stable crystal form than body-centered-cubic (bcc). C₄₈N₁₂ clusters are found to condense by a weak (0.29 eV) van der Waals force. The band gap of fcc C₄₈N₁₂ is calculated to be 1.3 eV at the GGA-PW91 level, whereas the HOMO-LUMO gap is calculated to be 2.74 eV using B3LYP/6-31G*.     

Observation of strain relaxation in Si1-xGex layers by optical and electrical characterisation of a Schottky junction

We have studied the effect of the strain relaxation on the band-edge alignments in a Pt/p-Si_1-xGe_x Schottky junction with x=0.14 by internal photoemission spectroscopy and current–voltage measurements. We have shown that the variations in the band-edge alignments can be observed directly by measuring the optical and electrical properties of a simple Schottky junction. The strain in the Si_1-xGe_x layer has been partially relaxed by thermal treatments at two different temperatures. The degree of relaxation and other structural changes have been determined by a high-resolution X-ray diffractometer. Both optical and electrical techniques have shown that the barrier height of the Pt/Si_0.86Ge_0.14 junction increases with the amount of relaxation in the Si_1-xGe_x layer. This shows that the valence-band edge of the Si_1-xGe_x layer moves away from the Fermi level of the Pt/Si_1-xGe_x junction. The band-edge movement results from the increase in the band gap of the Si_1-xGe_x layer after the strain relaxation. This result agrees with the theoretical predictions for the strain-induced effects on the Si_1-xGe_x band structure. Received: 18 October 2000 / Accepted: 19 December 2000 / Published online: 23 March 2001     

SiC bonding in ( C 60 ) n Si m clusters

This paper deals with a new type of SiC bonding where silicon atom seems to bridge C₆₀ molecules. We have studied films obtained by deposition of (C₆₀)_nSi_m clusters prepared in a laser vaporization source. Prior deposition, free ionized clusters were studied in a time-of-flight mass spectrometer. Mixed clusters (C₆₀)_nSi_m were clearly observed. Abundance and photofragmentation mass spectroscopies revealed the relatively high stability of the (C₆₀)_nSi _n ⁺ , (C₆₀)_nSi _{n - 1} ⁺ and (C₆₀)_nSi _{n - 2} ⁺ species. This observation is in favor of the arrangement of these complexes as polymers where the C₆₀ cages may be bridged by a silicon atom. Free neutral clusters are then deposited onto substrate making up a nanogranular thin film (≃ 100 nm). The film is probed by Auger and X-ray photoemission spectroscopies, but above all by surface enhanced Raman scattering. The results suggest an unusual chemical bonding between silicon and carbon and the environment of the silicon atom is expected to be totally different from the sp³ lattice: ten or twelve carbon neighbors might surround silicon atom. The bonding is discussed to the light of the so-called fullerene polymerization as observed for pure fullerite upon laser irradiation. This opens a new route for bridging C₆₀ molecules together with an appreciable energy bonding, since the usual van der Waals bonding in fullerite could be replaced by an ionocovalent bond. Such an assumption must be checked in the future by XAS and EXAFS experiments. Received 15 November 2000     

a-Si:H中本征缺陷引起的赝隙态

本文取原子集团模型Si₈H₁₈,Si₁₇及从连续无规网络中挖取的集团模型Si₂₉和Si₄₇,用CNDO LCAO-MO方法计算其电子结构,探讨了a-Si:H中由弱键、弯键和带电组态等本征缺陷引起的赝隙态分布。结果表明,当弱键拉伸时,两个弱键态移动并收缩至带隙中央;过剩电荷使弱键能级移至价带顶或导带底附近;弯键态主要出现在价带顶附近。当弯键曲率较大时,弯键态上移至带隙中央以下的区域。结构拓扑无序导致 关键词：     

Nano-star formation in Al-doped ZnO thin film deposited by dip-dry method and its characterization using atomic force microscopy, electron probe microscopy, photoluminescence and laser Raman spectroscopy

Zinc oxide doped with Al (AZO) thin films were prepared on borosilicate glass substrates by dip and dry technique using sodium zincate bath. Effects of doping on the structural and optical properties of ZnO film were investigated by XRD, EPMA, AFM, optical transmittance, PL and Raman spectroscopy. The band gap for ZnO:Al (5.0 at. wt.%) film was found to be 3.29 eV compared with 3.25 eV band gap for pure ZnO film. Doping with Al introduces aggregation of crystallites to form micro-size clusters affecting the smoothness of the film surface. Al³⁺ ion was found to promote chemisorption of oxygen into the film, which in turn affects the roughness of the sample. Six photoluminescence bands were observed at 390, 419, 449, 480, 525 and 574 nm in the emission spectra. Excitation spectra of ZnO film showed bands at 200, 217, 232 and 328 nm, whereas bands at 200, 235, 257 and 267 nm were observed for ZnO:Al film. On the basis of transitions from conduction band or deep donors (CB, Zn_i or V_OZn_i) to valence band and/or deep acceptor states (VB, V_Zn or O_i or O_Zn), a tentative model has been proposed to explain the PL spectra. Doping with Al³⁺ ions reduced the polar character of the film. This has been confirmed from laser Raman studies.     

Buried (Fe1-xCox)Si2 layers with variable band gap formed by ion beam synthesis

Buried layers of (Fe_{1 –x} Co _x )Si₂ were prepared by sequent implantation of iron and cobalt into (100) silicon. The depth distributions of iron and cobalt and the atomic concentration ratio silicon/metal were determined by Auger Electron Spectroscopy (AES) and Rutherford Backscattering Spectrometry (RBS). The phase composition and the microstructure of the silicide layer were studied by X-ray diffraction and electron microscopy. The band gap energy was evaluated from Infrared (IR) reflection and transmission experiments. The semiconducting-FeSi₂ structure remains stable up to a cobalt fraction ofx = 0.2 if the iron silicide is stabilized by an intermediate annealing between iron and cobalt implantation. With increasing cobalt content, the electrical resistivity as well as the energy of the direct band gap of the (Fe_{1 –x} Co _x )Si₂ layer decrease. In this way, a band gap tuning between 0.84 and 0.70 eV is possible. The gap energy is found to vary quadratically with the Co content in the composition range 0 <x < 0.15.     

Deposition of mass-selected cluster ions using a pulsed arc cluster-ion source

A PACIS (pulsed arc cluster-ion source) developed for high average cluster-ion currents is presented. The performance of the PACIS at different operational modes is described, and the suitability for cluster-deposition experiments is discussed in comparison with other cluster-ion sources. Maximum currents of mass-selected cluster ions of 3–6 nA of small Si_n ^- (n=4–10) clusters and 0.3–0.5 nA of large Al_n ^+/- (n=20–70) clusters are achieved. The mass-selected cluster ions are soft-landed on a substrate at residual kinetic energies lower than 1 eV/atom, and the samples are characterized by X-ray photoelectron spectroscopy and scanning tunneling microscopy. First results on the soft landing of “magic” Si₄ ^- clusters on graphite are presented. Received: 30 May 2001 / Accepted: 14 June 2001 / Published online: 2 October 2001     

Femtosecond ionization and fragmentation of free silicon nanoparticles

Femtosecond-laser spectroscopy is used to study the photoionization and photofragmentation of large neutral silicon clusters in a beam. Silicon clusters Si_n with sizes up to n≈6000, corresponding to nanoparticles with diameters up to 6 nm, are generated in a laser vaporization source. Nanosecond- and femtosecond-laser ionization are employed to characterize the free silicon nanoparticles. Excitation with intense femtosecond-laser pulses leads to prompt formation of doubly and triply charged Si_n clusters. Additionally, strong fragmentation of charged clusters occurs by Coulomb explosion, resulting in high released kinetic energies. Multiply charged atoms up to Si⁴⁺ with initial kinetic energies in the range of 500 eV are observed for laser intensities of about 10¹³ W/cm². Pump–probe spectroscopy yields decay times of the intermediate resonances of the nanoparticles. Received: 22 January 2000 / Published online: 7 August 2000     

Raman spectroscopy of carbon nitride films deposited using the filtered cathodic vacuum-arc technique combined with a radio-frequency nitrogen-ion beam

TiO₂–Fe₂O₃ semiconducting films were prepared by the resistive evaporation method on a glass substrate. Impedance and optical absorption measurements were made on these samples. From the impedance analysis (assuming an equivalent RC circuit), it was observed that the admittance and impedance functions are frequency dependent. From the optical absorption spectrum, the energy band-gap value was found to be 1.15 eV. The charge-carrier concentration (N) is calculated by using Mott–Schottky relations. I-V measurements were also made on these samples. Received: 2 November 1999 / Accepted: 24 August 2000 / Published online: 9 November 2000     

Electronic band gap of SrSe at high pressure

The electronic band gap of SrSe, in the CsCl-stuctured phase, was measured to 42 GPa via optical absorption studies. The indirect electronic band gap was found to close monotonically with pressure for the range of pressures studied. The change in band gap with respect to pressure, dE_gap/dP, was determined to be −6.1(5)×10⁻³ eV/GPa. By extrapolation of our line fit, we estimate band gap closure to occur at 180(20) GPa.     

X-ray photoemission and X-ray absorption studies of Hf-silicate dielectric layers

Photoelectron spectroscopy and X-ray absorption spectroscopy (XAS) measurements have been performed on HfSi_xO_y and HfSi_xO_yN_z dielectric layers, which are potential candidates as high-k transistor gate dielectrics. The hafnium silicate layers, 3-4 nm thick, were formed by codepositing HfO₂ and SiO₂ (50%:50%) by MOCVD at 485 °C on a silicon substrate following an IMEC clean. Annealing the HfSi_xO_y layer in a nitrogen atmosphere at 1000 °C resulted in an increase in the Si⁴⁺ chemical shift from 3.5 to 3.9 eV with respect to the Si⁰ peak. Annealing the hafnium silicate layer in a NH₃ atmosphere at 800 °C resulted in the incorporation of 10% nitrogen and the decrease in the chemical shift between the Si⁴⁺ and the Si⁰ to 3.3 eV. The results suggest that the inclusion of nitrogen in the silicate layer restricts the tendency of the HfO₂ and the SiO₂ to segregate into separate phases during the annealing step. Synchrotron radiation valence band photoemission studies determined that the valence band offsets were of the order of 3 eV. X-ray absorption measurements show that the band gap of these layers is 4.6 eV and that the magnitude of the conduction band offset is as little as 0.5 eV.     

Vibrational and Light-Emitting Properties of Si/Si1−xSnx Heterostructures

Multilayer heterostructures Si/Si_1−xSn_x grown by molecular beam epitaxy on Si (001) substrates have been studied by Raman and photoluminescence spectroscopy. Raman spectra exhibit peaks corresponding to the vibrations of Si-Sn and Sn-Sn bonds; the vibrations of Sn-Sn bonds imply the presence of tin nanocrystals in heterostructures. Two photoluminescence bands at 0.75 eV (1650 nm) and 0.65 eV (1900 nm) have been observed in heterostructures at low temperatures. The former band can be attributed to optical transitions in quantum wells in the type II heterostructure Si/Si_1−xSn_x. The latter band can be associated with excitons localized in tin nanocrystals.

     

X-ray diffraction and photoelectron spectroscopy study of swift heavy ion irradiated Mn/p-Si structure

The electronic structure and interfacial chemistry of thin manganese films on p-Si (1 0 0) have been studied by photoelectron spectroscopy measurements using synchrotron radiation of 134 eV and from X-ray diffraction data. The Mn/p-Si structures have been irradiated from swift heavy ions (∼100 MeV) of Fe⁷⁺ with a fluence of 1 × 10¹⁴ ions/cm². Evolution of valence band spectrum with a sharp Fermi edge has been obtained. The observed Mn 3d peak has been related to the bonding of Mn 3d-Si 3sp states. Mn 3p (46.4 eV), Mn 3s (81.4 eV) and Si 2p (99.5 eV) core levels have also been observed which show a binding energy shift towards lower side as compared to their corresponding elemental values. From the photoelectron spectroscopic and X-ray diffraction results, Mn₅Si₃ metallic phase of manganese silicide has been found. The silicide phase has been found to grow on the irradiation.