Influence Of Screening And Polaronic Effect On The Binding Energies Of Donors In Quantum Wells With Finite Barriers Under Hydrostatic Pressure

In this thesis, the impurity states in quantum wells with finite barriers are investigated by considering the influence of hydrostatic pressure and screening effect from the quasi-2D electronic gas on the coulombic potential of an impurity. A variational method is adopted to discuss donor binding energies by considering the variations of isotropic effective band masses of an electron, dielectric constants and band gaps with hydrostatic pressure.The numerical calculation is performed for the binding energies of impurity states in GaAs/Al_xGa_1-xAs quantum wells. The relations between the binding energies of donors and Al concentration, well width and hydrostatic pressure are given. The difference between the cases with and without screening is also discussed. The result indicates that the screening increases with pressure and decreases the binding energies of impurity states obviously.The numerical results show that the binding energies of impurity states increase with pressure. Not only does the screening decrease the binding energies but also decrease their sentibility to pressure. The binding energy first increases, then reaches a maximum value at the middle well width, finally decreases slowly with increasing the well width for both with and without screening. The screening effect sharpens the maximum peak. Its influence on the binding energy becomes more obvious when the well width is either small or large.The results indicate that the binding energies increase nonlinearly and monotonously with increasing Al concentration from 0 to 0.4. Screening decreases the binding energies and its property is similar to that without screening.