Excitons in CdSe quantum dots
U.E.H. Laheld ,
Institutt for fysikk, Norges teknisk-naturvitenskapelige universitet,
N-7034 Trondheim, Norway
G.T. Einevoll ,
Institutt for matematiske realfag, Nordlysobservatoriet,
Universitetet i Tromsø, N-9037 Tromsø, Norway
Physical Review B 55, 5184-5204 (1997)
Abstract
Recent advances in material synthesis have provided samples with
CdSe quantum dots with a degree of monodispersity high enough to allow
for observation of excited exciton states and their size
dependence [Norris et al., Phys. Rev. Lett. 72, 2612 (1994)].
Here we report theoretical results for these exciton states
using the effective bond-orbital model
(EBOM) for the hole and single-band effective-mass theory (EMT) for the
electron in an iterative Hartree scheme including the Coulomb interaction
and finite offsets. We present results for hole energies, exciton energies,
and exciton oscillator strengths and compare with experiments and other
theoretical results.
Our results are found to account for most of the
important features of the experimental absorption spectra by Norris et al. .
In particular, experimental states corresponding to the exciton ground state
(1 \Gamma_8 - 1S_e), as well as the
2 \Gamma_8 - 1S_e and 3 \Gamma_8 - 1S_e excited states,
have been identified. Also, a set of experimental exciton states
observed lifted with an energy close to the spin-orbit splitting
\lambda \approx 420 meV above the exciton ground state, have been identified as
\Gamma_7 - 1S_e spin-orbit split-off states with large oscillator strengths.
A non-perturbative study of the effects of the crystal-field splitting,
which is inherent in hexagonal
CdSe quantum dots, revealed patterns of avoided crossings,
accompanied with redistribution of oscillator strengths, between different exciton
states for increasing values of the crystal-field splittings. In
CdSe where the crystal-field splitting is \approx 25 meV, the splitting is not
expected to have a significant effect on the present quantum dot absorption spectra.