Dynamics of excitonic states in GaAs/AlGaAs quantum wells

The influence of photoexcited carriers on the dynamics of the absorption spectra of GaAs/Al_xGa_1−2x As multilayer quantum wells is investigated experimentally. It is found that at quasiparticle densities all the way up to 10¹¹ cm⁻² the saturation of the excitonic absorption is due to both a decrease of oscillator strength and broadening of the excitonic lines. It is shown that in the case of femtosecond resonance laser exci-tation the decrease of oscillator strength is due to free electron-hole pairs, while the broadening and energy shift of the excitonic lines are due to the exciton-exciton interaction. The lifetimes of free electron-hole pairs and excitons (≈65 ps and ≈410 ps, respectively) are determined from the exponential decrease of the change in the oscillator strength and in the width and energy position of the excitonic lines. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 3, 139–144 (10 August 1997)     

Reversal of the laminar-turbulent transition of a boundary layer on a cooled surface

A new series of experiments has investigated the influence of hoarfrost on the surface of a model on the position of the boundary layer transition for the same water vapor concentration in the working part of the wind tunnel and the same Mach number and unit Reynolds number but different amounts (masses) of hoarfrost on the investigated section of the model.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 2, pp. 170–172, March–April, 1984.     

Mechanisms of interactions between atomic hydrogen and vacancies in the silicon crystal lattice

SCF-MO-LCAO calculations in the MINDO/3 approximation were used to determine some mechanisms of interactions between atomic hydrogen and silicon lattice vacancies and between interstitial silicon atoms and hydrogen-charged lattice vacancies. In a completely hydrogen-charged vacancy, hydrogen atoms are localized on the Si−Si bond of the second coordination sphere with respect to the vacancy, so that the crystal lattice is ordered around the vacancy. The capture of atomic hydrogen in any charge state by a vacancy significantly decreases the potential barriers of the annihilation of the Frenkel pairs. After an interstitial atom has been captured by a hydrogen-charged vacancy, it is energetically profitable for the hydrogen atom to transfer to a neighboring vacancy. The interaction mechanisms revealed are consistent with the model of the accelerated annealing of lattice vacancy defects by high-frequency plasma treatment. In addition, the calculation results suggest that materials with hydrogen-charged silicon should be more stable to ionizing radiation than materials with hydrogen-free silicon, since the probability that the interstitial silicon involved in the Frenkel pair will be recaptured by the lattice point is rather high. Kiev Polytechnical Institute. Translated fromZhurnal Strukturmoi Khimii, Vol. 37, No. 1, pp. 22–28, January–Feburary, 1996. Translated by I. Izvekova