LABORATORY
ELECTRON MICROSCOPY AND ELECTRON DIFFRACTION

HEAD: Assoc.Prof. Maria Kalitzova, Ph.D.
tel: 7144-340; e-mail: markaliz@issp.bas.bg

TOTAL STAFF: 3
RESEARCH SCIENTISTS: 2
A. Peeva, Ph.D.
M. Georgieva

RESEARCH ACTIVITIES:

    The Laboratory of Electron Microscopy and Electron Diffraction (LEME) is actively involved in fundamental studies of the structure and electrical properties of elemental (Ge, Si) and compound semiconductors (GaAs, InP, GaN), as well as of some dielectric layers (Ta₂O₅, ZnO) serving as an active dielectric for submicron electronic applications. The technique of ion implantation is used for near surface modification and processing. High Frequency Electromagnetic Field is applied as post-implantation treatment. A variety of complementary analytical techniques is used: notably Transmission Electron Microscopy in diffraction contrast (CTEM) and High Resolution Transmission Electron Microscopy in phase contrast imaging mode (HRTEM), Rutherford Backscattering Spectroscopy (RBS), four-point probe and AC conductivity electrical measurements. The experimental results obtained are compared with data from computer simulations of the process of radiation defect formation. Moreover, the HRTEM images are digitally processed. The investigations carried out within the LEME provide a new information on the processes of defect production and damage removal in implanted materials. The complex study of high-dose implanted silicon gives new insights into the effect of ion-assisted nanocluster precipitation.
    (100)-oriented silicon substrates were implanted with 50 keV Te⁺ or Pb⁺ ions at room temperature with doses varied in the range 1x10¹⁵ - 1x10¹⁸ cm^-2. Formation of ion beam-assisted Te and Pb nanocrystals in the layer amorphized by the implantation has been established. The accumulation of impurities, sputtering and formation of high-dose implanted profile were modelled by a dynamic computer simulation code. The calculated concentration profiles at various ion fluences were compared with the measured RBS profiles and the depth distribution of ion beam-induced nanoclusters obtained by cross-sectional TEM, in an attempt to clarify the role of these effects in the formation of precipitates at high-dose implantation.
    Nanosized precipitation in high dose Zn⁺ and Bi⁺ implanted Si is investigated by High Resolution TEM in cross sectional specimens (XHRTEM). In spite of the different diffusivities of Zn and Bi in Si, their low solubility results in the precipitation of nanosized metallic inclusions revealed as superlattices composed of the host Si matrix and the implanted species.
    RF sputtered Ta₂O₅ films (23-26 nm) on Si, before and after high temperature (873, 1123 K) annealing in oxygen atmosphere have been investigated with respect to their dielectric and microstructural properties. Both High Resolution TEM analyses and electrical measurements indicate the presence of extremely thin SiO₂ layer at the interface with Si. The as-deposited and annealed at 873 K films are amorphous, while after 1123 K annealing the film crystallizes in orthorhombic phase.
    XHRTEM demonstrates the formation of Te nanoclusters (NCs) embedded in the amorphized by the implantation Si layer at fluences ≥ 1x10¹⁶ cm^-2. Post-implantation treatment with 0.45 MHz High Frequency Electromagnetic Field leads to enlargement of Te NCs, their diffusion and accumulation at the a-Si surface and formation of laterally connected extended Te structures above the percolation threshold, appearing at ion fluence of 1x10¹⁷ cm^-2. The results obtained are discussed in terms of two-phase isotropic spinodal structure.

1. P. Bankov, M. Kalitzova, D. Karpuzov, G. Zollo, G. Vitali, C. Pizzuto, Ch. Angelov,
   J. Faure, L. Killian, Dose-dependent dynamics of nanoclusters distribution in silicon implanted with Te⁺ and Pb⁺ ions: computer simulation and TEM study, Vacuum 69: 455-460 (2003)
2. G. Zollo, G. Vitali, M. Kalitzova, D. Manno, High-resolution electron microscopy of Zn- and Bi- related superlattices in ion implanted (100) Si, J. Mat. Sci: Mat. In Electronics 14: 783-786 (2003)
3. E. Atanassova, M. Kalitzova, G. Zollo, A. Paskaleva, A. Peeva, M. Georgieva, G. Vitali, High temperature-induced crystallization in tantalum pentoxide layers and its influence on the electrical properties, Thin Solid Films 426: 191-199 (2003)
4. M. Kalitzova, E. Vlakhov, Y. Marinov, K. Gesheva, V. Ignatova, O. Lebedev, C. Muntele and R. Gijbels, Effects of High-Frequency Electromagnetic Fields on Te⁺ - implanted (001) Si, Vacuum, accepted for publication

Financed by the National Foundation for Scientific Research at the Ministry of Science and Education:
F 1310: Ion-activated crystal nucleation in amorphous media: effects of high-dose implantation with heavy ions

1. BAS - Bulgaria, CNR - Italy, "Structural and Electrical Characterisation of New Dielectrics and High Band-gap Semiconductors"
2. BAS - Bulgaria, FZ Rossendorf - Germany, "Nanocluster structures in high dose implanted Si and microstructures Ta₂O₅-Si with high dielectric constant"
3. ISSP - Bulgaria - University of Reims, France, "Radiation Defects induced in diamond-type materials by heavy ions implantation"

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