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
E' Centers in Amorphous SiO2 Revisited: A New Look at an Old Problem
Phys. Rev. Lett. 86 5522-5525 (2001).
Structure and Generation Mechanism of the Peroxy-Radical Defect in Amorphous Silica
Phys. Rev. Lett. 86 4560-4563 (2001).
Mechanism of Interconversion among Radiation-Induced Defects in Amorphous Silicon Dioxide >
Phys. Rev. Lett. 86 1777-1780 (2001).
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
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
Microscopic structure of nanometer-sized silica particles
Phys. Rev. B 69 155409 (2004)
Anelastic Compression of Nanometer-Sized Silica Particles under High Pressure: A High Energy X-ray Diffraction Measurement
Phys. Rev. B 67 092202 (2003)
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
White Light Emission from Transparent SiO2 Glass Prepared from Nanometer-Sized Silica Particles
Appl. Phys. Lett. 85 1164-1166 (2004)
Photoluminescence Decay Dynamics of Transparent Silica Glass Prepared from Nanometer-Sized Silica Particles
Appl. Phys. Lett. 87 081904 (2005)
Visible Luminescence from Octadecylsilane Monolayers on Silica Surfaces: Time-Resolved Photoluminescence Characterization
Appl. Phys. Lett. 87 251923 (2005)