Ge/Si quantum dots in external electric and magnetic fields

Electric field-induced splitting of the lines of exciton optical transitions into two peaks is observed for Ge/Si structures with quantum dots (QDs). With increasing field, one of the peaks is displaced to higher optical transition energies (blue shift), whereas the other peack is shifted to lower energies (red shift). The results are explained in terms of the formation of electron-hole dipoles of two types differing in the direction of the dipole moment; these dipoles arise due to the localization of one electron at the apex of the Ge pyramid and of the other electron under the base of the pyramid. By using the tight-binding method, the principal values of the g factor for the hole states in Ge/Si quantum dots are determined. It is shown that the g factor is strongly anisotropic, with the anisotropy becoming smaller with decreasing QD size. The physical reason for the dependence of the g factor on quantum-dot size is the fact that the contributions from the states with different angular-momentum projections to the total wave function change with the QD size. Calculations show that, with decreasing QD size, the contribution from heavy-hole states with the angular-momentum projections ±3/2 decreases, while the contributions from light-hole states and from states of the spin-split-off band with the angular-momentum projections ±1/2 increase.