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Positron annihilation gamma energy distribution, lifetime spectroscopy and time-of-flight method were used to study surfactant-templated mesoporous silica films deposited on glass. The lifetime depth profiling was correlated to Doppler broadening and 3γ annihilation fraction measurements to determine the annihilation characteristics inside the films. A set of consistent fingerprints for positronium annihilation, o-Ps reemission into vacuum, and pore size was directly determined. The lifetime measurements were performed in reflection mode with a specially designed lifetime spectrometer mounted on a slow positron beam system. The intensity of the 142 ns vacuum lifetime component was recorded as a function of the energy of the positron beam. In a film with high porosity a reemission efficiency of as high as 40 % was found at low positron energy. Positron lifetime in samples capped by a thin silica layer was used to determine the pore size. The energy of the reemitted o-Ps fraction was measured by a time-of-flight detector, mounted on the same system, allowing determination of both o-Ps re-emission efficiency and energy in the same sample. We demonstrate the potential of the simultaneous use of different positron annihilation techniques in the study of thin porous films.

Positron-lifetime experiments have been performed to investigate the metastability of the point defects produced in the electron irradiation of semi-insulating GaAs. The measurements in darkness indicate the presence of Ga vacancies and Ga antisite defects in a negative charge state. Illumination at 25 K reveals another type of a defect, which has a vacancy in its metastable state. The metastable vacancies can be observed most effectively after illumination with 1.1-eV photons and they are persistent up to the annealing temperature of 80--100 K. The introduction rate of the metastable defects is about 0.3 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, which is close to the values reported earlier for the As antisite. The metastable properties of the defects resemble those of the well-known EL2 center in as-grown GaAs. We associate these defects to As antisites, which exhibit the metastability predicted by the theory: in the metastable configuration the As antisite atom relaxes away from the lattice position, leaving a Ga site vacant.

We have performed positron lifetime experiments under monochromatic illumination in undoped semi-insulating GaAs. A negative vacancy, identified as the Ga vacancy, is observed in darkness. Illumination with 1.42 eV photons below 150 K reveals another type of vacancy, identified as the As vacancy. The As vacancy has a negative charge state above the ionization level at 50±5 meV below the conduction band. This level offers a microscopic explanation to the optical near-band-edge absorption. The concentrations of both Ga and As vacancies are between 10 15 and 10 16 cm -3