Charge Carriers In Quantum Dots Research Articles

Coherent control of a high-orbital hole in a semiconductor quantum dot.

Coherently driven semiconductor quantum dots are one of the most promising platforms for non-classical light sources and quantum logic gates which form the foundation of photonic quantum technologies. However, to date, coherent manipulation of single charge carriers in quantum dots is limited mainly to their lowest orbital states. Ultrafast coherent control of high-orbital states is obstructed by the demand for tunable terahertz pulses. To break this constraint, we demonstrate an all-optical method to control high-orbital states of a hole via a stimulated Auger process. The coherent nature of the Auger process is proved by Rabi oscillation and Ramsey interference. Harnessing this coherence further enables the investigation of the single-hole relaxation mechanism. A hole relaxation time of 161 ps is observed and attributed to the phonon bottleneck effect. Our work opens new possibilities for understanding the fundamental properties of high-orbital states in quantum emitters and for developing new types of orbital-based quantum photonic devices.

Hot Charge Carriers in Quantum Dots: Generation, Relaxation, Extraction, and Applications

AbstractThis article discusses the hot charge carriers in semiconductor quantum dots (QDs): their generation, relaxation, extraction and applications in different technologically relevant areas. It has been reported that the most common ways to generate hot charge carriers are photo‐excitation by energy more than the band gap energy. However, recently the other means to generate hot charge carriers such as doping, and semiconductor plasmon interaction have also been evolved and their advantages over the conventional method have been discussed. Relaxation dynamics of hot charge carriers and different processes related to this such as multiple exciton generation, Auger recombination, phonon vibrations and ligand assisted charge carrier relaxation are mentioned. It was shown that the relaxation mechanism of hot charge carriers (both hot electron and hot hole) follow different pathways and either of the mechanism is playing a role depending on the QDs structure and the surrounding conditions. Studies pertaining to interaction of QDs with other semiconductors, metal nanoparticles or with different molecular adsorbates suggest that the hot charge carrier extraction is possible in these heterojunctions with extraction time in sub‐ps scale. Application of hot charge carriers in different fields such as photovoltaics, photo‐catalysis, infrared detectors, and H2 detection have been shown and it was observed that the efficiency of the above‐mentioned processes is much higher when the hot charge carriers are involved, suggesting their importance in these areas.

Effect of the deposition of cobalt on the optoelectronic properties of quantum-confined In(Ga)As/GaAs heteronanostructures

The systematic features of the inf luence of defect formation during the deposition of a cobalt contact on the optoelectronic characteristics of structures containing InAs/GaAs quantum dots and InxGa1–xAs/GaAs quantum wells are studied. From analysis of the temperature dependences of the photosensitivity of the InAs/GaAs quantum-dot structures, the values of the resultant recombination lifetime of photoexcited charge carriers in quantum dots at different conditions of Co deposition and at different structural parameters are determined.

On the lifetime of charge carriers in quantum dots at low temperatures

The nonequilibrium lifetime of charge carriers in a quantum dot has been experimentally and theoretically investigated. It has been shown that, at low temperatures when the ground state is fully occupied, the lifetime is almost independent of the excitation density and determined only by radiative recombination. Such behavior is theoretically explained and it is shown that, under the condition of the fully occupied ground state, the Auger recombination process can be suppressed by spin effects. The suppression of Auger recombination in such a system is microscopically described.

A Molecule to Detect and Perturb the Confinement of Charge Carriers in Quantum Dots

This paper describes unprecedented bathochromic shifts (up to 970 meV) of the optical band gaps of CdS, CdSe, and PbS quantum dots (QDs) upon adsorption of an organic ligand, phenyldithiocarbamate (PTC), and the use of PTC to map the quantum confinement of specific charge carriers within the QDs as a function of their radius. For a given QD material and physical radius, R, the magnitude of the increase in apparent excitonic radius (ΔR) upon delocalization by PTC directly reflects the degree of quantum confinement of one or both charge carriers. The plots of ΔR vs R for CdSe and CdS show that exciton delocalization by PTC occurs specifically through the excitonic hole. Furthermore, the plot for CdSe, which spans a range of R over multiple confinement regimes for the hole, identifies the radius (R∼1.9 nm) at which the hole transitions between regimes of strong and intermediate confinement. This demonstration of ligand-induced delocalization of a specific charge carrier is a first step toward eliminating current-limiting resistive interfaces at organic-inorganic junctions within solid-state hybrid devices. Facilitating carrier-specific electronic coupling across heterogeneous interfaces is especially important for nanostructured devices, which comprise a high density of such interfaces.

Non-Markovian dynamics of charge carriers in quantum dots

We have investigated the dynamics of bound particles in multilevel current-carrying quantum dots. We look specifically in the regime of resonant tunnelling transport, where several channels are available for transport. Through a non-Markovian formalism under the Born approximation, we investigate the real-time evolution of the confined particles including transport-induced decoherence and relaxation. In the case of a coherent superposition between states with different particle number, we find that a Fock-space coherence may be preserved even in the presence of tunneling into and out of the dot. Real-time results are presented for various asymmetries of tunneling rates into different orbitals.

Laudatio for Gerhard Abstreiter

Laudatio for Gerhard Abstreiter

Laudatio for Gerhard Abstreiter

Laudatio for Gerhard Abstreiter

Relaxation of charge carriers in quantum dots with the involvement of plasmon-phonon modes

A new mechanism of intraband relaxation of charge carriers in quantum dots (QDs) incorporated in a heterostructure at a relatively large distance from its doped elements is considered. Relaxation is caused by coupling of the electron subsystem of the QD to plasmon-phonon excitations of doped components of the heterostructure via the electric potential induced by these excitations. It is shown that this interaction with bulk plasmon-LO-phonon modes is possible only due to their spatial dispersion. The performed estimations of relaxation rates have shown that the mechanism under consideration is quite efficient even if the QDs are at a distance as large as 100 nm from the doped regions of the heterostructure. If this distance is about several tens of nanometers, this mechanism can become dominant.

Phononless hopping conduction in two-dimensional layers of quantum dots

Regularities are studied in charge transport due to the hopping conduction of holes along two-dimensional layers of Ge quantum dots in Si. It is shown that the temperature dependence of the conductivity obeys the Efros-Shklovskii law. It is found that the effective localization radius of charge carriers in quantum dots varies nonmonotonically upon filling quantum dots with holes, which is explained by the successive filling of electron shells. The preexponential factor of the hopping conductivity ceases to depend on temperature at low temperatures (T<10 K) and oscillates as the degree of filling quantum dots with holes varies, assuming values divisible by the conductance quantum e 2/h. The results obtained indicate that a transition from phonon-assisted hopping conduction to phononless charge transfer occurs as the temperature decreases. The Coulomb interaction of localized charge carriers has a dominant role in these phononless processes.