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Electronic structure of point defects in amorphous materials

The electrical and optical processes of Si-based amorphous materials, which are of special importance for their use in optical fiber technologies and optoelectronic devices, are determined mainly by local structural defects. Thus, considerable efforts have been made in the past decades to understand the structural, optical and electronic properties of point defects in amorphous SiO2.

Recently, quantum-chemical calculations have proved increasingly useful for the study of the structure and properties associated with point defects in amorphous insulators and semiconductors. In particular, a cluster approach based on the ab initio molecular orbital method has provided several new insights into the microstructure of local defects not only in oxide glasses but also in chaologenide glasses. It is often claimed that such cluster calculations may bridge the gap between the properties of isolated molecules and the condensed phase.

According to the cluster approach, the structure of a defect center in amorphous solids is described by a finite number of atoms comprising the local defect and its surrounding environment. Model clusters are composed of 10-100 atoms depending on the aim of the study and the available computer facilities. Geometries of the model clusters are optimized at the Hartree-Fock (HF), post-HF, and/or density functional theory (DFT) levels to search their local minimum configurations, and their structure, optical and vibrational properties, and excited states are investigated at the respective levels of theory.

This approach may not fully take into account the actual effect of condensed environments around the defect of interest. However, it is reasonable to expect that the electronic states associated with the defect are rather localized and do not extend through the corresponding solid, implying that the electronic structure of the defect in amorphous solids is reasonably modeled by the cluster calculations. The aim of this project is get a new insight into point defects in pure amorphous SiO2 (a-SiO2) and other related amorphous materials.

Related papers

  • T. Uchino, M. Takahashi and T. Yoko
    E' Centers in Amorphous SiO2 Revisited: A New Look at an Old Problem
    Phys. Rev. Lett. 86 5522-5525 (2001).
  • T. Uchino, M. Takahashi and T. Yoko
    Structure and Generation Mechanism of the Peroxy-Radical Defect in Amorphous Silica
    Phys. Rev. Lett. 86 4560-4563 (2001).
  • T. Uchino, M. Takahashi and T. Yoko
    Mechanism of Interconversion among Radiation-Induced Defects in Amorphous Silicon Dioxide >
    Phys. Rev. Lett. 86 1777-1780 (2001).
  • T. Uchino, D. C. Clary, and S. R. Elliott
    Mechanism of Photoinduced Changes in the Structure and Optical Properties of Amorphous As2S3
    Phys. Rev. Lett. 85 3305-3308 (2000).
  • Electron excitation processes in SiO2-based materials

    Radiation-induced point defects in amorphous SiO2 (a-SiO2) and related metal-oxide-semiconductor (MOS) devices have been extensively studied during the past decades. The electronic excitation during ionizing radiation will produce primarily electrons, holes, and excitons, and the defect formation will be closely related to the subsequent self-trapping and/or decay processes of these radiation-induced particles.

    The principal tool for the identification of the microscopic structure of these defects is electron paramagnetic resonance (EPR) spectroscopy. It is curious to note, however, that with the exception of a few impurity related species, there has been no report of the EPR signature of such electron trapping centers in a-SiO2. That is, all the defects monitored by EPR so far in a-SiO2 are either of the trapped-hole type or else result from self-trapping of excitons.

    We have recently presented a possible model of the electron trapping process in a-SiO2. Using quantum chemical calcultions, we have demonstrated that an edge-sharing tetrahedral dimer, which has attracted renewed interests as a model of a strained silica surface, is one of the probable candidates for an electron trapping center at least near the surface of a-SiO2.

    The electron trapping process associated with the edge-sharing structure leads to the generation of the neutral E'-like center, whereas the trapped electron is localized at a non-bridging oxygen site. The trapping of electrons at the edge-sharing SiO4 sites will account for the underlying experimental features of electron excitation processes peculiar to a-SiO2 .

    Related papers

  • T. Uchino and T. Yoko
    Mechanism of Electron Localization at Edge-Sharing Units in Amorphous SiO2
    Phys. Rev. B 68 041201(R) (2003)
  • Structure and properties of nanometer-sized silica particles

    Amorphous small fine particles have attracted considerable attention in recent years. In particular, silica-based nanophase materials have been the subject of intensive studies because of their potential applications in nanoscale silicon optoelectronic devices.

    One example of such extremely small silica particles is fumed silica, which is produced at high temperature (1400-1800 oC) by the hydrolysis of silicon tetrachloride vapor in a flame of hydrogen and oxygen. Since fumed silica has very high specific surface area (100-400 m2/g), its surface reactivity and the related surface properties have been studied by many researchers during the past decades. As compared with its surface properties, the structural information of fumed silica itself is still lacking.

    We have recently reported that fumed silica exhibits a unique structural modification under pressure. That is, a pressure-induced structural transition, accompanied by densification, occurs at lower pressures (2-8 GPa) than would normally be expected for bulk silica glass (over 10G Pa). This suggests that nanometer-sized silica particles have a lower threshold for irreversible compaction than bulk silica glass. This unprecedented behavior of fumed silica most likely results from the intrinsic structural characteristics of the material, and a detailed knowledge of the structure will be useful for a better understanding of the properties of the fine-particle oxides. We are, therefore, carrying out a series of structural analysis of fumed silica using infrared and Raman spectroscopies and a high-energy X-ray diffraction method.

    Related papers

  • T. Uchino, A. Aboshi, S. Kohara, Y. Ohishi, M. Sakashita, and K. Aoki
    Microscopic structure of nanometer-sized silica particles
    Phys. Rev. B 69 155409 (2004)
  • T. Uchino, A. Sakoh, M. Azuma, S. Kohara, M. Takahashi, M. Takano, and T. Yoko
    Anelastic Compression of Nanometer-Sized Silica Particles under High Pressure: A High Energy X-ray Diffraction Measurement
    Phys. Rev. B 67 092202 (2003)
  • T. Uchino, A. Sakoh, M. Azuma, M. Takano, M. Takahashi, and T. Yoko,
    A Novel Pressure-Induced Polymorphic Transition from Fumed Silica to Transparent Amorphous SiO2 at Room Temperature
    J. Phys.: Condens. Matter 14 11111-11114 (2002)
  • Preparation of luminescent silica glass

    The strong photoluminescence (PL) observed at room temperature in the visible range from porous silicon and its related silicon nanostructures has stimulated numerous research work in the development of the silicon-based materials for optoelectronic and display applications. In these samples, the visible PL is generally attributed to the presence of nanoscale silicon aggregates, although the precise mechanism of such an efficient PL emission is still a matter of considerable debate.

    We have recently found that transparent gpureh bulk SiO2 glass is prepared from nanometer-sized silicon-dioxide particles by heating at 980 oC in air for more than 100 h, exhibiting intense unprecedented visible light emission by 266-nm irradiation. Intense PL, which appeared white to the naked eye, was observed in the wavelength range from 300 to 800 nm. The resultant PL is quite stable after prolonged exposure to the atmosphere and shows no appreciable light-induced degradation. The present bulk SiO2 glass hence gives a new class of silicon-oxide-based luminescent materials that do not contain any silicon nanostructures.

    Related papers

  • T. Uchino and T. Yamada
    White Light Emission from Transparent SiO2 Glass Prepared from Nanometer-Sized Silica Particles
    Appl. Phys. Lett. 85 1164-1166 (2004)
  • T. Yamada and T. Uchino
    Photoluminescence Decay Dynamics of Transparent Silica Glass Prepared from Nanometer-Sized Silica Particles
    Appl. Phys. Lett. 87 081904 (2005)
  • N. Sagawa and T. Uchino
    Visible Luminescence from Octadecylsilane Monolayers on Silica Surfaces: Time-Resolved Photoluminescence Characterization
    Appl. Phys. Lett. 87 251923 (2005)