14.4 Quantum Dots
425
Fig. 14.43 Optical
emission spectra
(T = 2.3 K) of a single
InGaAs/GaAs quantum dot
at different laser excitation
levels P as labeled. The
single exciton (X) and
biexciton (XX) lines are
indicated. Adapted
from [1411]
14.4.4 Optical Properties
The optical properties of QDs are related to their electronic density of states. In particular, optical
transitions are allowed only at discrete energies due to the zero-dimensional density of states.
Photoluminescence from a single QD is shown in Fig. 14.43. The δ-like sharp transition is strictly true
only in the limit of small carrier numbers ( 1 exciton per dot on average) since otherwise many-body
effects come into play that can encompass recombination from charged excitons or multiexcitons. At
very low excitation density the recombination spectrum consists only of the one-exciton (X) line. With
increasing excitation density small satellites on either side of the X-line develop that are attributed to
charged excitons (trions) X
+ and X
− . On the low-energy side, the biexciton (XX) appears. Eventually,
the excited states are populated and a multitude of states contribute with rich fine structure. In bulk
material the biexciton (Sect. 9.7.10) is typically a bound state, i.e. its recombination energy E XX is lower
than that of the exciton E X . A similar situation is present in Fig. 14.43. It was pointed out in [1412] that
in QDs the biexciton recombination energy can also be larger than the exciton recombination energy.
In [1413] the modification of the QD confinement potential of InAs/GaAs QDs by annealing was
reported. The exciton binding energy (E X -E XX ) is tuned from positive (‘normal’) to negative values
upon annealing (Fig. 14.44).
The charging state of the exciton can be controlled in a field-effect structure. The recombination
energy is modified due to Coulomb and exchange effects with the additional carriers. In charge-tunable
quantum dots [1414] and rings [1415] exciton emission has been observed in dependence of the number
of additional electrons. The electron population can be controlled in a Schottky-diode-like structure
through the manipulation of the Fermi level with the bias voltage. At high negative bias all charge
carriers tunnel out of the ring and no exciton emission is observed. A variation of the bias then leads to
an average population with N = 1, 2, 3, . . . electrons. The recombination of additional laser-excited
425
Fig. 14.43 Optical
emission spectra
(T = 2.3 K) of a single
InGaAs/GaAs quantum dot
at different laser excitation
levels P as labeled. The
single exciton (X) and
biexciton (XX) lines are
indicated. Adapted
from [1411]
14.4.4 Optical Properties
The optical properties of QDs are related to their electronic density of states. In particular, optical
transitions are allowed only at discrete energies due to the zero-dimensional density of states.
Photoluminescence from a single QD is shown in Fig. 14.43. The δ-like sharp transition is strictly true
only in the limit of small carrier numbers ( 1 exciton per dot on average) since otherwise many-body
effects come into play that can encompass recombination from charged excitons or multiexcitons. At
very low excitation density the recombination spectrum consists only of the one-exciton (X) line. With
increasing excitation density small satellites on either side of the X-line develop that are attributed to
charged excitons (trions) X
+ and X
− . On the low-energy side, the biexciton (XX) appears. Eventually,
the excited states are populated and a multitude of states contribute with rich fine structure. In bulk
material the biexciton (Sect. 9.7.10) is typically a bound state, i.e. its recombination energy E XX is lower
than that of the exciton E X . A similar situation is present in Fig. 14.43. It was pointed out in [1412] that
in QDs the biexciton recombination energy can also be larger than the exciton recombination energy.
In [1413] the modification of the QD confinement potential of InAs/GaAs QDs by annealing was
reported. The exciton binding energy (E X -E XX ) is tuned from positive (‘normal’) to negative values
upon annealing (Fig. 14.44).
The charging state of the exciton can be controlled in a field-effect structure. The recombination
energy is modified due to Coulomb and exchange effects with the additional carriers. In charge-tunable
quantum dots [1414] and rings [1415] exciton emission has been observed in dependence of the number
of additional electrons. The electron population can be controlled in a Schottky-diode-like structure
through the manipulation of the Fermi level with the bias voltage. At high negative bias all charge
carriers tunnel out of the ring and no exciton emission is observed. A variation of the bias then leads to
an average population with N = 1, 2, 3, . . . electrons. The recombination of additional laser-excited