No abstract

Single-dot spectroscopy is now able to resolve the energies of excitons, multi-excitons, and charging of semiconductor quantum dots with 1 meV resolution. We discuss the physical content of these energies and show how they can be calculated via Quantum Monte Carlo (QMC) and Configuration Interaction (CI) methods. The spectroscopic energies have three pieces: (i) a "perturbative part" reflecting carrier-carrier direct and exchange Coulomb energies obtained from fixed single-particle orbitals, (ii) a "self-consistency correction" when the single particle orbitals are allowed to adjust to the presence of carrier-carrier interaction, and (iii) a "correlation correction." We first apply the QMC and CI methods to a modle single-particle Hamiltonian: a spherical dot with a finite barrier and single-band effective mass. This allows us to test the convergence of the CI and to establish the relative importance of the three terms (i) -(iii) above. Next, we apply the CI method to a realistic single-particle Hamiltonian for a CdSe dot, including via a pseudopotential description the atomistic features, multi-band coupling, spin-orbit effects, and surface passivation. We include all bound states (up to 40,000 Slater determinants) in the CI expansion. Our study shows that: (1) typical exciton transition energies, which are ∼ 1 eV, can be calculated to better than 95% by perturbation theory, with only a ∼ 2 meV correlation correction; (2) typical electron addition energies are ∼ 40 meV, of which correlation contributes very little (∼ 1 meV); (3) typical biexciton binding energies are positive and ∼ 10 meV and almost entirely due to correlation energy, and exciton addition energies are ∼ 30 meV with nearly all contribution due to correlation; (4) while QMC is currently limited to a single-band effective mass Hamiltonian, CI may be used with much more realistic models, which capture the correct symmetries and electronic structure of the dots, leading to qualitatively different predictions than effective mass models; and (5) and CI gives excited state energies necessary to identify some of the peaks that appear in single-dot photoluminescence spectra.

The radiative recombination rates of interacting electron-hole pairs in a quantum dot are strongly affected by quantum correlations among electrons and holes in the dot. Recent measurements of the biexciton recombination rate in single self-assembled quantum dots have found values spanning from two times the single exciton recombination rate to values well below the exciton decay rate. In this paper, a Feynman path-integral formulation is developed to calculate recombination rates including thermal and many-body effects. Using real-space Monte Carlo integration, the path-integral expressions for realistic three-dimensional models of InGaAs/GaAs, CdSe/ZnSe, and InP/InGaP dots are evaluated, including anisotropic effective masses. Depending on size, radiative rates of typical dots lie in the regime between strong and intermediate confinement. The results compare favorably to recent experiments and calculations on related dot systems. Configuration interaction calculations using uncorrelated basis sets are found to be severely limited in calculating decay rates.

We report on the observation of photoluminescence from positive, neutral and negative charge states of single semiconductor quantum dots. For this purpose we designed a structure enabling optical injection of a controlled unequal number of negative electrons and positive holes into an isolated InGaAs quantum dot embedded in a GaAs matrix. Thereby, we optically produced the charge states -3, -2, -1, 0, +1 and +2. The injected carriers form confined collective 'artificial atoms and molecules' states in the quantum dot. We resolve spectrally and temporally the photoluminescence from an optically excited quantum dot and use it to identify collective states, which contain charge of one type, coupled to few charges of the other type.These states can be viewed as the artificial analog of charged atoms such as H − , H −2 , H −3 , and charged molecules such as H + 2 and H +2 3 . Unlike higher dimensionality systems, where negative or positive charging always results in reduction of the emission energy due to electron-hole pair recombination, in our dots, negative charging reduces the emission energy, relative to the charge-neutral case, while positive charging increases it. Pseudopotential model calculations reveal that the enhanced spatial localization of the hole-wavefunction, relative to that of the electron in these dots, is the reason for this effect.

We present results of correlated pseudopotential calculations of an exciton in a pair of vertically stacked InGaAs/GaAs dots. Competing effects of strain, geometry, and band mixing lead to many unexpected features missing in contemporary models. The first four excitonic states are all optically active at small interdot separation, due to the broken symmetry of the single-particle states. We quantify the degree of entanglement of the exciton wave functions and show its sensitivity to interdot separation. We suggest ways to spectroscopically identify and maximize the entanglement of exciton states.