Multi-excitons in self-assembled InAs/GaAs quantum dots: A pseudopotential, many-body approach

– We use a many-body, atomistic empirical pseudopotential approach to predict the multi-exciton emission spectrum of a lens shaped InAs/GaAs self-assembled quantum dot. We discuss the effects of (i) The direct Coulomb energies, including the differences of electron and hole wavefunctions, (ii) the exchange Coulomb energies and (iii) correlation energies given by a configuration interaction calculation. Emission from the groundstate of the N exciton system to the N − 1 exciton system involving e0 → h0 and e1 → h1 recombinations are discussed. A comparison with a simpler single-band, effective mass approach is presented. High-resolution single-dot spectroscopy [1–5] of InAs/GaAs self-assembled quantum dots shows that as the excitation intensity is increased, thus loading more excitons into the dots, new emission lines appear both to the red and to the blue of the fundamental emission line observed at low excitation power. “State filling” effects, leading to the recombination of high energy electron-hole pairs, cannot explain the red-shifted emission lines, nor the fact that the number of lines exceeds the number of allowed single-particle transitions. In this letter we present a theory of self-assembled semiconductor quantum-dots, based on a pseudopotential many-body expansion that demonstrates that it is multi-exciton transitions that are responsible for this complex observed spectral structure. We isolate and clarify three distinct physical effects; (i) electron-hole wavefunction asymmetry, leading to a blue shift of the fundamental exciton transition as the number of spectator excitons loaded into the dot increases, (ii) electron-electron and hole-hole exchange interactions which red shift all even multiexciton decays (biexciton, four-exciton) and split the odd multiexciton decays (tri-exciton, five-exciton) into multiple-lines and (iii) correlation effects which red shift the biexciton leading to its binding with respect to the monoexciton. We will first describe the qualitative picture of multi-excitons and then describe a quantitative model. The essential physics of such transitions can be understood by considering what happens to the ground-state recombination of the lowest electron level, e0, and the lowest hole level, h0, if other electrons and holes are present in the dot as “spectators”. The schematic figures in the center of Fig. 1 depict the fundamental e0 − h0 recombination in the presence of 0 to 5 “spectator” electron-hole pairs (we assume here that all levels are spatially non-degenerate [6]). We distinguish here two exciton series; (i) when the initial number of excitons, N is even, the initial electron configuration is “closed shell”, e.g. (e20)(h 2 0) for N = 2, whereas (ii) when N is odd, the initial configuration has an open shell both in the electron and in the hole manifold, e.g. (e20)e 1 1(h 2 0)h 1 1 for N = 3, (parentheses are used for the closed shell orbitals). The distinction between the “closed shell” and “open shell” multiexciton is important, since in the initial state of the N=even, “closed shell” series the spectator levels