When a dielectric is irradiated by electrons with energy E
of several kiloelectron volts, a large number of processes take place:
backscattering of incident electrons, excitation and ionization of the
electrons in the dielectric with binding energies lower than
E, creation of excitons, radiative and nonradiative decays of
the excited and ionized states, slowing down of the primary and
secondary electrons, and thermalization in the conduction band. The
thermalized electrons can move freely in the unoccupied conduction
states of the material. If electric connection exists between the
dielectric and the apparatus, then the charges normally flow out.
Thermalized electrons can also be trapped in excited levels localized
in the band gap of the dielectric and nonradiative and radiative
recombinations from these levels can be observed. The number of the
trapped electrons varies with the structural characteristics of the
dielectric. In a monocrystal, this number is weak because the number of
the defect states in the band gap is small, making the localization of
the charges restricted. In contrast, in a polycrystal or amorphous
material, the number of the trapped electrons can be large and
increases with the disorder. Information on the charge effects suffered
by the sample during its irradiation can be deduced by studying the
trapping of electrons in localized states and, consequently, by
analyzing radiations emitted from these states in the visible and X-ray
ranges. In the case of oxides, F+ centers
(oxygen–ion vacancy having trapped one electron) and F
centers (F+ center having trapped a second
electron) are generally present. We will show that the
F+ [harr ] F conversion can be used to
study the dynamic of the trapping in the oxides. Application to various
samples of crystallized and amorphous alumina will be presented.
Raman Silicon Laser Using a Photonic Crystal Nanocavity