A. V. Rodina

Absorption and intensity-dependent photoluminescence measurements on CdSe quantum dots: assignment of the first electronic transitions

CdSe is used as a prototype to show the implications of valence-band degeneracy for the optical properties of strongly quantum-confined nanocrystals. Absorption spectra and photoluminescence spectra obtained under intermediate and strong pulsed excitation show the presence of new structures. The energy levels for the electron and the hole are calculated with the spherical confinement, the nonparabolicity of the conduction band, and the valence band degeneracy taken into account. The oscillator strengths of the dipole-allowed transitions are also calculated. This model is found to be in good agreement with the experimental observations, which originate mainly from the quantization of the energy spectrum of holes with due account given to valence-band degeneracy.

General boundary conditions for the envelope function in the multiband k.p model

We have derived general boundary conditions (BCs) for the multiband envelope functions (which do not contain spurious solutions) in semiconductor heterostructures with abrupt heterointerfaces. These BC require the conservation of the probability flux density normal to the interface and guarantee that the multiband Hamiltonian be self-adjoint. The BC are energy independent and are characteristic properties of the interface. Calculations have been performed of the effect of the general BC on the electron energy levels in a potential well with infinite potential barriers using a coupled two band model. The connection with other approaches to determining BC for the envelope function and to the spurious solution problem in the multiband k.p model are discussed.

The role of polarization fields in Auger-induced efficiency droop in nitride-based light-emitting diodes

The rates of non-radiative Auger recombination (AR) and radiative recombination (RR) in polar GaN/AlN quantum wells (QWs) are calculated. It is shown that in these QWs the polarization field not only suppresses the RR but also strongly enhances the rate of AR. As a result, the polarization field triggers the Auger-induced efficiency droop, which, according to the calculations, does not exist in non-polar GaN/AlN QWs. We demonstrate that in polar QWs the droop can be overcome by suppression of AR using a gradual variation of the QW layer composition, which compensates the effect of the electric field acting on holes.

Nonradiative Auger Recombination in Semiconductor Nanocrystals

We calculate the rate of nonradiative Auger recombination in negatively charged CdSe nanocrystals (NCs). The rate is nonmonotonic, strongly oscillating with NC size, and sensitive to the NC surface. The oscillations result in nonexponential decay of carriers in NC ensembles. Using a standard single-exponential approximation of the decay dynamics, we determine the apparent size dependence of the Auger rate in an ensemble and derive CdSe surface parameters consistent with the experimental dependence on size.

Field-enhanced ionization of deep-level centers as a triggering mechanism for superfast impact ionization fronts in Si structures

We investigate the origin of free carriers that initiate impact ionization in depleted high-voltage p-n junctions under dynamic breakdown conditions and deterministically trigger superfast ionization fronts that propagate several times faster than the saturated drift velocity. We argue that in Si structures triggering occurs due to the field-enhanced ionization of process-induced deep-level centers identified as sulfur impurities. This impurity is a double-level electron trap with low recombination activity. It is present in high-voltage Si structures due to the side effect of widely used fabrication technology. We calculate the field and temperature dependences of the ionization probability for the upper midgap level (0.28eV) and midgap level (0.54eV) in electric fields up to 5×105V∕cm as well as the occupation of these levels at different temperatures. The emission of free electrons is sufficient to trigger the ionization front from zero temperature to ∼400K, in agreement with experiments. At room tempe...

Theory of the Zeeman effect in semiconductor nanocrystals

Abstract We study quantum confinement and surface effects on the linear Zeeman splitting of the electron quantum size energy levels in spherical semiconductor nanocrystals and spherical layered semiconductor heterostructures. We find that heterostructure interfaces and nanocrystal surfaces directly affect the values of the electron Lande factors. We calculate the size dependence of the electron effective g-values in bare CdSe and ZnO nanocrystals using different choices of boundary conditions (BC) for the envelope functions at the surface using 8×8 and 14×14 Kane models, respectively.

Dynamics of Intraband and Interband Auger Processes in Colloidal Core–Shell Quantum Dots

Conventional colloidal quantum dots (QDs) suffer from rapid energy losses by nonradiative (Auger) processes, leading to sub-ns lifetimes in all excited states but the lowest-energy single exciton. Suppression of interband Auger decay, such as biexciton Auger recombination, has been achieved with the design of heterostructured core-shell QDs. Auger-like processes are also believed to be responsible for rapid intraband hot-electron cooling in QDs. However, the simultaneous effect of shell growth on interband Auger recombination and intraband hot-electron cooling has not been addressed. Here we investigate how the growth of a CdS shell affects these two relaxation processes in CdSe/CdS core-shell QDs. Using a combination of ultrafast pump-push-probe spectroscopy on the QD ensemble and analysis of the photon statistics from single QDs, we find that Auger losses in the biexciton state are suppressed with increasing shell thickness, while hot-electron cooling remains unaffected. Calculations conducted within an eight-band k·p model confirm the experimental dependence of the biexciton Auger decay on the shell thickness, and provide insights into the factors determining the cooling rate of hot carriers.

Effects of strain on the valence band structure and exciton-polariton energies in ZnO

The uniaxial stress dependence of the band structure and the exciton-polariton transitions in wurtzite ZnO is thoroughly studied using modern first-principles calculations based on the

Second-harmonic generation spectroscopy of excitons in ZnO

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Ground state of the holes localized in II-VI quantum dots with Gaussian potential profiles

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