Phonon-assisted auger recombination in quantum well semiconductors

Phonon-assisted Auger recombination (AR) is shown to be an important loss mechanism in a quantum well semiconductor in addition to the direct AR. Theoretical investigations demonstrate that it is of the same order of magnitude and has the same temperature dependence as in bulk material, just as direct AR, provided that the material parameters and the carrier concentrations are the same as in the bulk.     

Phonon-assisted recombination of free electron-hole pair in HgI2

The emission spectrum due to the indirect recombination of free electrons and holes has been observed in the layer-type red-HgI₂ in coexistence with the spectrum caused by the recombination of free direct exciton. The characteristics in emission spectrum is caused by the presence of weak interlayer van der Waals forces.     

Temperature dependence of radiative recombination in CdSe quantum dots with enhanced confinement

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 ∼0.6 ns followed by a slow decay with a time constant ∼30–50 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 ∼1.3 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.     

Phonon-assisted damping of Rabi oscillations in semiconductor quantum dots

Electron-phonon interaction is a major source of optical dephasing in semiconductor quantum dots. Within a density matrix theory the electron-phonon interaction is considered up to the second order of a correlation expansion, allowing the calculation of the quantum kinetic dephasing dynamics of optically induced nonlinearities in GaAs quantum dots for arbitrary pulse strengths and shapes. We find Rabi oscillations renormalized and a damping that depends on the input pulse strength, a behavior not known from exponential dephasing mechanisms.     

Strong electron-hole exchange in coherently coupled quantum dots

We have investigated few-body states in vertically stacked quantum dots. Because of a small interdot tunneling rate, the coupling in our system is in a previously unexplored regime where electron-hole exchange plays a prominent role. By tuning the gate bias, we are able to turn this coupling off and study a complementary regime where total electron spin is a good quantum number. The use of differential transmission allows us to obtain unambiguous signatures of the interplay between electron and hole-spin interactions. Small tunnel coupling also enables us to demonstrate all-optical charge sensing, where a conditional exciton energy shift in one dot identifies the charging state of the coupled partner.     

Size-dependent electron-hole exchange interaction in CdSe quantum dots

Il Nuovo Cimento D - We have investigated the fine structure of luminecence of CdSe nanocrystals observed with size-selective excitation. We show that the luminescence line closest to the laser...     

Size-dependent electron-hole exchange interaction in CdSe quantum dots

Il Nuovo Cimento D - We have investigated the fine structure of luminecence of CdSe nanocrystals observed with size-selective excitation. We show that the luminescence line closest to the laser...     

Inverted electron-hole alignment in InAs-GaAs self-assembled quantum dots

New information on the electron-hole wave functions in InAs-GaAs self-assembled quantum dots is deduced from Stark effect spectroscopy. Most unexpectedly it is shown that the hole is localized towards the top of the dot, above the electron, an alignment that is inverted relative to the predictions of all recent calculations. We are able to obtain new information on the structure and composition of buried quantum dots from modeling of the data. We also demonstrate that the excited state transitions arise from lateral quantization and that tuning through the inhomogeneous distribution of dot energies can be achieved by variation of electric field.     

Phonon-assisted Kondo effect in single-molecule quantum dots coupled to ferromagnetic leads

Based on the infinite-U Anderson model spin-polarized transport through the tunnel magnetoresistance (TMR) system of single-molecule quantum dot is investigated under the interplay of strong electron correlation and electron-phonon (e-ph) coupling. The spectral density and the nonlinear differential conductance are studied using the extended non-equilibrium Green's function method through calculating the dot-level splitting self-consistently. The results exhibit that a serial of peaks emerge on the two sides of the main Kondo peak for the antiparallel magnetic configuration of electrodes, while for the parallel case both the main and phonon-assisted satellite Kondo peaks all split up into two asymmetric peaks even at zero-bias. Correspondingly, the nonlinear differential conductance displays a set of satellite-peaks around the Kondo-peak in the presence of the e-ph interaction. Furthermore, extra maxima and minima appear in the TMR curve. The TMR alternates between the positive and the negative values along with the variation of bias voltage.     

Evidences for Auger recombination in electron-hole droplets (silicon)

The microwave absorption technique was used to measure the transient free carrier density following an electric injection in silicon at 4°K. The experiment agrees with the idea that droplets are obtained by free carrier condensation directly, without a preliminary formation of free excitons.¹ An Auger recombination process in droplets explains the observed kinetics; parameters are measured.     

Long-range radiative interaction between semiconductor quantum dots

We model the resonant excitation transfer between semiconductor quantum dots, accounting for the radiative nature of the electromagnetic field. The model based on Maxwell equations and on a non-local linear susceptibility accounts both for the instantaneous dipole–dipole coupling, decaying as R⁻³, and for retardation effects, decaying as R⁻¹. The coupling is strongly resonant and its spatial range is of the order of the wavelength, due to the radiative nature of the retarded contribution.     

Transport in split-gate silicon quantum dots

We report on the transport properties of novel Si quantum dot structures with controllable electron number through both top and side gates. Quantum dots were fabricated by a split-gate technique within a standard MOSFET process. Four-terminal dc electrical measurements were performed at 4.2 K in a liquid helium cryostat. Strong oscillations in the conductance through the dot are observed as a function of both the top gate bias and of the plunger bias. An overall monotonic and quasi-periodic movement of the peak conductance is observed which is believed to be associated with the bare level structure of the electronic states in the dot coupled with the Coulomb charging energy. Crossing behavior is observed as well, suggestive of either many-body effects or symmetry breaking of the dot states by the applied bias.     

Photoactivation of silicon quantum dots

We show that free-standing silicon quantum dots (QDs) can be photoactivated by blue or UV optical irradiation. The luminescence intensity increases by an order of magnitude for irradiation times of several minutes under moderate optical power. The cut-off energy for photoactivation is between 2.1 and 2.4 eV, not very different from the activation energy for hydrogen dissociation from bulk silicon surfaces. We propose the mechanism for this effect is associated with silicon-hydride bond breaking and the subsequent oxidation of dangling bonds. This phenomenon could be used to “write” luminescent quantum dots into pre-determined arrays.     

Nano-laser on silicon quantum dots

A new conception of nano-laser is proposed in which depending on the size of nano-clusters (silicon quantum dots (QD)), the pumping level of laser can be tuned by the quantum confinement (QC) effect, and the population inversion can be formed between the valence band and the localized states in gap produced from the surface bonds of nano-clusters. Here we report the experimental demonstration of nano-laser on silicon quantum dots fabricated by nanosecond pulse laser. The peaks of stimulated emission are observed at 605 nm and 693 nm. Through the micro-cavity of nano-laser, a full width at half maximum of the peak at 693 nm can reach to 0.5 nm. The theoretical model and the experimental results indicate that it is a necessary condition for setting up nano-laser that the smaller size of QD (d < 3 nm) can make the localized states into band gap. The emission energy of nano-laser will be limited in the range of 1.7-2.3 eV generally due to the position of the localized states in gap, which is in good agreement between the experiments and the theory.     

Nonradiative electron-hole recombination by a low-barrier pathway in hydrogenated silicon semiconductors

A microscopic pathway for nonradiative electron-hole recombination by large structural reconfiguration in hydrogenated Si is found with first-principles calculations. Trapped-biexciton formation leads to a low-barrier reconfiguration of the H atom, accompanied by crossing of doubly occupied electron and hole levels in the band gap. This crossing represents the nonradiative recombination of the carriers, without multiphonon emission. The proposal provides a mechanism for carrier-induced H emission during metastable degradation of hydrogenated amorphous silicon.     

Phonon-assisted polar exciton–transitions in self-organized InAs/GaAs quantum dots

Phonon-assisted exciton transitions are investigated for self-organized InAs/GaAs quantum dots (QDs) using selectively excited photoluminescence (PL) and PL excitation spectroscopy. The results unambiguously demonstrate intrinsic recombination in the coherent InAs/GaAs QDs and the absence of a Stokes shift between ground state absorption and emission. Phonon-sidebands corresponding to a phonon energy of 34 meV are resolved and Huang–Rhys parameters of 0.015 and 0.08 are found for phonon-assisted emission and absorption, respectively, which are about one order of magnitude larger than in bulk InAs. Calculations of the exciton–LO–phonon interaction based on an adiabatic approximation and realistic wave functions for ideal pyramidal InAs/GaAs QDs show this enhanced polar coupling to result from the particular confinement and the strain-induced piezoelectric potential in such strained low-symmetry QDs.     

Phonon-assisted auger recombination in degenerate semiconductors

Phonon-assisted Auger recombination is studied in strongly degenerate semiconductors since normal Auger recombination is negligible in this case. The results are discussed in connection with an electron-hole-plasma. There are some indications that phonon-assisted Auger recombination is an important process in this case.