1,721,040 research outputs found
2D vs. 3D behaviour of free-carriers gas in delta-doped p-type GaAs(100).
No full textThe free-carrier-induced plasma excitation in delta-doped p-type GaAs(001) has been studied by means of high-resolution electron-energy-loss spectroscopy (HREELS). Several samples, with different values of doping and depth of the dopant layer, have been investigated at various primary-beam energies. The HREEL spectra show a strong dependence on the doping level. We were able to reproduce satisfactorily all the measured spectra using a suitable dielectric model of a classically confined free-carrier gas, pointing out the two-dimensional character of the free-carrier gas in the samples having the two lowest dopings. On the contrary, a characteristic three-dimensional behavior of the plasma excitation is exhibited by the most doped sample
HOLE-PLASMON DAMPING ON HEAVILY DOPED P-TYPE GAAS(110)
No full textThe free-carrier-induced plasma excitation in heavily doped p-type GaAs(110) is studied by means of high-resolution electron-energy-loss spectroscopy. This surface hole-plasmon feature is very broad and approximately centered at 60 meV. The large broadening is due to the low hole mobility and its value depends on the energy of the incident-electron beam. Two causes can concur to determine this behavior: the momentum dependence of the Landau damping of the plasmon (due to the nonlocal nature of the free-carrier response) and the dependence of the hole mobility on the distance from the surface in the crystal. Measurements performed after increasing hydrogen exposures, which modify the subsurface charge region, allowed us to conclude that the momentum dependence of the Landau damping is the cause of the observed variation of the plasmon damping with the primary beam energy
Vibrational and collective excitations on heavily doped p-type GaAs(110) surface
No full textWe investigated the space-charge layer at the surface of heavily doped GaAs(110) by means of HREELS, looking at the surface coupled collective excitations (phonon-plasmon, i.e. plasmarons). We could reproduce the general behaviour of the HREEL spectra based on a dielectric three-layer model, finding a strong value for the value of the plasmon damping
QUASI-2-DIMENSIONAL ELECTRON-GAS AT SUBMONOLAYER COVERAGES OF CS ON INSB(110)
No full textWe present a complementary study of the space charge layer formation and band bending determination of caesium adsorption on InSb(110), by means of photoemission and high-resolution electron energy loss spectroscopy. At extremely low Cs coverage (below 0.01 monolayer), the Fermi level gets pinned into the InSb conduction band and an accumulation layer is formed. The quasi-two-dimensional (2D) behaviour of the accumulated carriers is experimentally deduced from the dispersion of the free-carrier plasmon. Eventually, further Cs deposition causes a reversed Fermi level pinning, with the formation of a depletion layer at saturation coverage
Multiple traumatic injury to maxillary incisors in an adolescent female: treatment outcome with two years follow-up
Full text linkNumber, type and severity of dental injuries per patient differ according to the patient’s age and the cause of accident. The trauma group resulting from pedestrian-, bicycle-, and car-related injuries is usually dominated by multiple dental injuries, injuries to the supporting bone and soft-tissue injuries. This report describes a case of a 16.2-year-old female who suffered traumatic injuries to her permanent maxillary incisors after a car accident. Concussion of tooth 12, extrusive luxation of tooth 11,
avulsion of tooth 21 and subluxation with complicated crown fracture of tooth 22 were observed at the emergency visit 75 minutes after the trauma. Tooth 21 was dry stored for 15 minutes, then in milk for 60 minutes. The treatment plan according to IADT guidelines was performed with the satisfaction of the dentists and the patient. After 1 year follow-up a replacement root resorption of tooth 21
was diagnosed; it was then considered severe at the time of the 2 year control visit. Educational programs are essential to optimize the treatment outcome both at the accident site and also at the dental office
Space charge layer, metallization and collective excitations of the Bi/GaAs interface.
No full textThe collective excitations and the metallization of the Bi/GaAs(I 10) interface grown at room temperature, up to the completion of a few tens of a monolayer, have been studied by means of the high-resolution electron-energy-loss spectroscopy (HREELS). Through analysis of the HREELS data, also by means of an appropriate semiclassical dielectric model, the modifications experienced by the substrate-related loss structures (Fuchs-Kliewer phonon and dopant-induced free-carrier plasmon) and by the quasielastic peak are related to changes in the dielectric response of the overlayer and in the semiconductor space-charge region. The influence of bismuth is effective in enlarging the depletion layer thickness, leaving the interface semiconducting at the monolayer-coverage scale. A band bending value of 0.56 eV is obtained at the coverage of one monolayer on the highly n-type doped sample (n approximately 2.7 X 10(18) cm-3); a determination free from any possible surface photovoltaic effect. At coverages greater than two monolayers, which corresponds to a structural transition, the intermediate structural phase becomes metallic, thus marking a clear semiconductor-metal transition. This intermediate metallic stage further develops towards the formation of actual semimetallic crystalline bismuth layers oriented with the basal plane parallel to the substrate surface
Temperature dependence of acetylene adsorption and reaction on Si(111)-(7 x 7)
No full textThe adsorption and reaction of acetylene dosed on the Si(111)-(7 x 7) surface has been studied by means of X-ray photoemission, Auger, and HREELS spectroscopies for different substrate temperatures. For exposure at temperature of 600 degreesC, only an extremely thin reacted layer can be obtained, which hinders further molecular reaction at the surface, acting as a barrier layer. The increase in surface diffusion occurring between 600 and 750 degreesC overrides the barrier layer effect, allowing the growth of a thicker SiC. (C) 2001 Elsevier Science B.V. All rights reserved
Acetylene adsorption on the Si(111)-(7X7) surface: Ultraviolet photoemission and high-resolution electron-energy-loss spectroscopies
No full textThe room temperature adsorption and the thermal reaction of acetylene on the Si(111)-(7 x 7) surface has been investigated by means of ultraviolet-photoemission spectroscopy and high-resolution electron-energy-loss spectroscopy (HREELS). In the first stage of the adsorption process, the evolution of the restatom- and adatom-related electronic states as a function of acetylene exposure is well described by a modified di-sigma -bonding model, in which the metallic character of the (7 x 7) surface plays a fundamental role. For higher exposure, all the restatom. dangling bonds are saturated and the adsorption process continues through saturation of the remaining adatoms. In this phase, a molecular tilting is observed, possibly accompanied by a surface atom rearrangement. Upon annealing to 600 degreesC, acetylene dehydrogenation and partial desorption occur, as demonstrated by the disappearance of the C-H modes in HREELS and the reappearance of the adatom. and restatom dangling-bond states in valence-band spectra. At 700 degreesC the formation of a Si-C compound is observed. Atomic-force-microscopy topographic images taken ex situ after annealing to 800 degreesC show that the Si-C interface is constituted by islands with lateral dimension of similar to 300 Angstrom, while the overall surface roughness is 10-20 Angstrom.The room temperature adsorption and the thermal reaction of acetylene on the (formula presented) surface has been investigated by means of ultraviolet-photoemission spectroscopy and high-resolution electron-energy-loss spectroscopy (HREELS). In the first stage of the adsorption process, the evolution of the restatom- and adatom-related electronic states as a function of acetylene exposure is well described by a modified di-σ-bonding model, in which the metallic character of the (formula presented) surface plays a fundamental role. For higher exposure, all the restatom dangling bonds are saturated and the adsorption process continues through saturation of the remaining adatoms. In this phase, a molecular tilting is observed, possibly accompanied by a surface atom rearrangement. Upon annealing to (formula presented) acetylene dehydrogenation and partial desorption occur, as demonstrated by the disappearance of the C-H modes in HREELS and the reappearance of the adatom and restatom dangling-bond states in valence-band spectra. At (formula presented) the formation of a Si-C compound is observed. Atomic-force-microscopy topographic images taken ex situ after annealing to (formula presented) show that the Si-C interface is constituted by islands with lateral dimension of (formula presented) while the overall surface roughness is (formula presented) © 2001 The American Physical Society
Study of the transition from the ideal Si(111)-H(1 x 1) surface to the (7 x 7) reconstruction by HREELS, UPS and LEED
No full textThe temperature-induced evolution of the Si(1 1 1)-H(1 x 1) surface towards the (7 x 7)-reconstruction has been studied by means of UPS, HREELS and LEED techniques. We found that H atom desorption occurs at about 500 degreesC, and the full development of the (7 x 7)-phase, checked by both UPS and LEED, occurs around 700 degreesC. In the valence band spectra, the (7 x 7)-related restatom peak is present already for annealing temperature of 465 degreesC, while the adatom-related peak starts to appear only around 600 degreesC and fully develops at 700 degreesC. A band bending is detected and its behavior discussed in relation to the surface state evolution. In the first annealing steps, the desorption-induced defects cause the shift of the surface Fermi level and the consequent upwards band bending, which reaches the maximum value of 0.47 eV for annealing to 465 degreesC. The subsequent evolution of the dangling-bond states into the (7 x 7) surface states determines the band bending decrease till 0.1 eV. (C) 2001 Elsevier Science B.V. All rights reserved
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