The Effect of Electronic Correlations on the Double Photoionization Spectra of Two-Electron Impurity Centers in Multiwell Quantum Structures

Abstract

Impurities of different chemical elements inevitably emerge during the preparation of semiconductor nanostructures or are intentionally added to change their transport and optical properties. Most impurities in a bound state can have two or more electrons, rather than one electron. In this case, interelectronic correlations enabling the double photoionization of an impurity atom, one of fundamental reactions, start to play an important role. The aim of the present work was to calculate the first ionization potential of a two-electron impurity center in a semiconductor quantum well by the variation method, as well as to theoretically study the effect of correlations on the double photoionization spectra of two-electron impurity centers in a multiwell quantum structure. The bond energy and first ionization potential of a two-electron atom were calculated by the variation method where the second ionization potential was taken as an empirical parameter. The expression for the impurity light absorption coefficient was obtained in the dipole approximation taking into account the quantum-well width variance. The zero-radius potential method was generalized to the case of two-electron impurities with zero effective nuclear charge in semiconductor quantum wells. An analytical expression for the first ionization potential of a two-electron impurity center was obtained by the variation method in terms of the semiempirical model. The impurity light absorption coefficient upon the one-photon photoionization of a two-electron impurity in a multiwell quantum structure was calculated in the dipole approximation. It is shown that, due to the spatial restriction in a quantum well in one direction, electronic correlations are enhanced to result in higher threshold values of the second ionization potential than those in quantum dots and, as a consequence, more rigid existence conditions of two-electron impurity states. Also, a decrease in the quantum size effect and an increase in the electronic correlation in multiwell quantum structures compared to quasi-zero-dimensional structures were shown to result in transformation of the absorption curve, which is expressed in an increase in the dip between peaks in the double-humped spectral curve.