Excitons in type-II quantum-dot systems: a comparison of the GaAs/AlAs and InAs/GaSb systems

calculation of the exciton binding energy (Ex) and oscillator strength for quantum dots in type-II semiconductor systems is presented. These structures consist of a spherical dot of one semiconductor embedded in a second semiconductor. As in the type-II exciton systems in quantum wells the electron is confined in one semiconductor and the hole is confined in the other due to band lineups in the two materials which make this arrangement energetically favorable. The authors have considered the systems (i) GaAs/AlAs where the electron is confined in the X state in the AlAs while the hole is confined in the InAs dot while the hole is confined in the GaSb (for a dot radius of less than 56 Å) and (ii) InAs/GaSb where the electron is confined in the InAs dot while the hole is confined in the GaSb (for a dot of radius less than 87 Å). While both of these systems are indirect in real space the GaAs/AlAs system is also indirect in k space. They compare Ex in the type-II dot to Er, the binding energy of a bulk hydrogenic impurity in the bulk barrier. The authors find that for the case of infinite barriers Ex