Impact of the valley degree of freedom on the control of donor electrons near a Si/SiO_2 interface

摘要

We analyze the valley composition of one electron bound to a shallow donor close to a Si/barrier interface as a function of an applied electric field. A full six-valley effective mass model Hamiltonian is adopted. For low fields, the electron ground state is essentially confined at the donor. At high fields the ground state is such that the electron is drawn to the interface, leaving the donor practically ionized. Valley splitting at the interface occurs due to the valley-orbit coupling, V_(VO)~I = |V_(VO)~Ie~(iθ). At intermediate electric fields, close to a characteristic shuttling field, the electron states may constitute hybridized states with valley compositions differing from the donor and the interface ground states. The full spectrum of energy levels shows crossings and anticrossings as the field varies. The degree of level repulsion, thus, the width of the anticrossing gap, depends on the relative valley compositions, which vary with |V_(VO)~I|, 6 and the interface-donor distance. We focus on the valley configurations of the states involved in the donor-interface tunneling process, given by the anticrossing of the three lowest eigenstates. A sequence of two anticrossings takes place and the complex phase 6 affects the symmetries of the eigenstates and level anticrossing gaps. We discuss the implications of our results on the practical manipulation of donor electrons in Si nanostructures.