Resonant states in modulation-doped heterostructures
Abstract: This thesis deals with the properties of donors placed inside or outside a heterostructure quantum well (QW). The focus of the investigation has been on the formation of resonant states, which are a hybridization of the discrete localized impurity levels and the continuous two-dimensional QW subbands. The impact of such states on the optical properties and the noise spectrum have been investigated, and possible applications, in particular related to emission of far-infrared radiation, have been considered. After a summary in Swedish for the general public and an introduction, four papers are presented. In the first two, the formalism of resonant states is developed, and the two final ones deal with applications. In Paper 1 the donor is placed outside an GaAs/AlGaAs quantum well. A model for the resonant coupling of the localized donor state and the QW subbands is developed. Two representations of the impurity potential are considered: a zero-range potential and the Coulomb potential. We calculate the width and position of the resonant state as a function of the distance of the donor to the well, and also the influence of the resonant state on the density of states. Paper 2 presents a more general method for calculating the energy levels of donors placed both inside and outside strained Si/SiGe quantum wells. From this non-variational method we the binding energies of all localized states and the position and width of the resonant states. We are also able to evaluate the wave functions, which is used to calculate the absorption spectrum. The influence on the donor ground state from the central cell effect is also considered. In Paper 3 we perform a self-consistent calculation of the potential profile of a Si/SiGe QW structure which in recent experiments was found to generate strong THz-emission. It is shown that the position of the resonant states support the conclusion that the mechanism of generation of the radiation can be the same as in bulk p-Ge, viz. that the carriers in the heavy-hole QW subband are captured into an excited resonant state attached to the light-hole subband. They may then make a radiative transition to the impurity ground state. Finally, in Paper 4 we consider one possible way to treat the influence of the resonant states on the QW current noise. A fully self-consistent calculation yields the temperature dependence of the generation-recombination noise due to a shallow donor placed outside a Si/SiGe well.
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