Temperature dependence of radiative recombination in CdSe quantum dots with enhanced confinement
S. V. Zaitsev, T. Kümmell*, G. Bacher*, D. Hommel+
Institute of Solid State Physics RAS, 142432 Chernogolovka, Russia
*Universität Duisburg-Essen, 47057 Duisburg, Germany
+Institut für Festkörperphysik, Halbleiterepitaxie, Universität Bremen, 28359 Bremen, Germany
Abstract
We studied in details the recombination dynamics and
its temperature dependence in epitaxially grown neutral CdSe/ZnSSe
quantum dots with additional wide-band gap MgS barriers. Such
design allows to preserve a very high quantum yield and track the
radiative recombination dynamics up to room temperature. A fast
initial decay of ns followed by a slow decay with a
time constant ns is observed at low temperature T <
50 K. The fast decay gradually disappears with increasing
temperature while the slow decay shortens and above 100 K
predominantly a single-exponential decay is observed with a time
constant ns, which is weekly temperature dependent up
to 300 K. To explain the experimental findings, a two-level model
which includes bright and dark exciton states and a temperature
dependent spin-flip between them is considered. According to the
model, it is a thermal activation of the dark exciton to the
bright state and its consequent radiative recombination that
results in the long decay tail at low temperature. The doubling of
the decay time at high temperatures manifests a thermal
equilibrium between the dark and bright excitons.