Exciton states and oscillator strength in two vertically coupled InP/InGaP quantumdiscs

Abstract

Quantum mechanical coupling and strain in two vertically arranged InP/InGaP quantumdots is studied as a function of the size of the dots and the spacer thickness. Thestrain distribution is determined by the continuum mechanical model, while thesingle-band effective-mass equation and the multiband theory are employed to compute the conduction and valence band energy levels,respectively. The exciton states are obtained from an exact diagonalization approach, andwe also compute the oscillator strength for recombination. We found that the light holesare confined by strain to the spacer, which is the reason that the hole statesexhibit coupling at much larger distances as compared with the electrons. At smalld,the doublet structure of the hole energy levels arises as a consequence of the relocation of the lighthole from the matrix to the regions located outside the stack, close to the dot–matrix interface. Whend varies, the exciton ground state exhibits numerous anticrossings with other states,which are related to the changing spatial localization of the hole as a function ofd. The oscillator strength of the exciton recombination is strongly reduced in a certain rangeof spacer thicknesses, which effectively turns a bright exciton state into a darkone. This effect is associated with anticrossings between exciton energy levels.