SESSION EC09: OPTICS OF SEMICONDUCTOR DOTS I: THEORY AND SPECTROSCOPY
Monday morning, 22 March 1999; Room 363W GWCC at 10:30; P. Sercel, Univerisity of Oregon, presiding.

10:30
EC09 1 Electronic Structure of semiconductor quantum dots
Alex Zunger (National Renewable Energy Laboratory)

The electronic structure of nanostructures has previously been almost universally addressed by the ``standard model'' of the effective-mass, k.p envelope function approach. While eminently successful for quantum wells, this model breaks down for small structures, in particular, for small dots and wires. Until recently, it was impractical to test the ``standard model'' against more general approaches that allow many-band (\Gamma-X-L) coupling. However, it is now possible to apply the all-band pseudopotential method to 10^3-10^6 atom nanostructures. This talk will outline the method that makes this possible and present recent results.

Method: The shape (e.g. pyramids, spheres, rings) is assumed, and the atomic positions are determined by minimizing the atomistic (VFF) strain energy. The electronic potential is expressed as a superposition of nonlocal, screened atomic pseudopotentials obtained via systematically correcting the LDA potentials to fit measured band gaps, effective masses and deformation potentials. The Hamiltonian is diagonalized in a plane-wave basis via the ``Folded Spectrum Method''. After obtaining the single-particle states we compute the electron-hole Coulomb and exchange integrals, setting up and solving the excitonic problem via Configuration Interaction.

Systems: The method is applied both to ``free-standing'' surface-passivated dots (InP, CdSe, GaAs, InAs) as well as to ``semiconductor embedded'' self-assembled dots (InAs/GaAs, InP/GaP).

Results: (i) The electron-hole Coulomb interaction does not scale with size R as previously thought (R^-1); also the exchange does not scale as R^-3. (ii) The electron-hole exchange interaction has a large and previously neglected long-range piece. (iii) The single particle levels of tetrahedral dots do not have pure s, p... envelope function symmetry as suggested by k.p methods, but exhibit mixed-parity. (iv) Sufficiently small GaAs dots have an indirect band gap (v) Concentrically-nested GaAs/AlAs ``Russian doll'' structures show charge separation, (vi) Under pressure, InP dots exhibit \Gamma-X anti crossing, while the bulk solid shows level crossing under pressure (vii) A square base InAs pyramid shows a split conduction p level and an in-plane anisotropy for the lowest exciton transitions both in conflict with effective mass predictions.