M. Tadić

Nonsymmetrized hamiltonian for semiconducting nanostructures in a magnetic field

In the framework of the Burt-Foreman theory a nonsymmetrized eight-band effective-mass Hamiltonian is derived for nanostructures in the presence of a magnetic field. The Hamiltonian is tested for the case of a cylindrical quantum dot with parabolic in-plane confinement potential in a perpendicular magnetic field. We compare the results of our nonsymmetrized model to the singleband and conventional multiband calculations, where ad hoc symmetrization is used. The model is tested on GaAs/ Al0.3Ga0.7As, GaAs/AlAs, and InAs/GaAs quantum dots, where strain is not included in the model in Order to resolve the influence of the boundary on the electronic structure. In structures with a large difference of Luttinger parameters between the constituent materials, such as InAs/GaAs quantum dots, the conventional multiband models lead to unphysical high magneticfield solutions that are substantially different from those obtained from the nonsymmetrized Hamiltonian and single-band model for the ground state. A similar behavior is observed for the case of InAs/GaAs quantum wells, where energy levels as a function of k(t) are analyzed. This discrepancy is attributed to an overestimation of band mixing in conventional models because of the inappropriate treatment of the boundary.

The optimization of optical gain in the intersubband quantum well laser

A systematic procedure is described for the maximization of gain in optically pumped intersubband lasers, via optimal tailoring of the quantum well profile. The procedure relies on using the inverse spectral theory, allowing one to shift the bound states as desired, and additionally to make the isospectral reshaping of the well, with the eventual aim of finding the best combination of those optical dipole and electron–phonon scattering matrix elements which are relevant for the laser gain. Example design is presented for a laser based on the AlxGa1−xAs system, and the band nonparabolicity is accounted for in the final design.A systematic procedure is described for the maximization of gain in optically pumped intersubband lasers, via optimal tailoring of the quantum well profile. The procedure relies on using the inverse spectral theory, allowing one to shift the bound states as desired, and additionally to make the isospectral reshaping of the well, with the eventual aim of finding the best combination of those optical dipole and electron–phonon scattering matrix elements which are relevant for the laser gain. Example design is presented for a laser based on the AlxGa1−xAs system, and the band nonparabolicity is accounted for in the final design.

Hole subbands in freestanding nanowires: six-band versus eight-band k·p modelling.

The electronic structure of GaAs, InAs and InSb nanowires is studied using the six-band and the eight-band k·p models. The effect of the different Luttinger-like parameters (in the eight-band model) on the hole band structure is investigated. Although GaAs nanostructures are often treated within a six-band model because of the large bandgap, it is shown that an eight-band model is necessary for a correct description of its hole spectrum. The camel-back structure usually found in the six-band model is not always present in the eight-band model. This camel-back structure depends on the interaction between light and heavy holes, especially the ones with opposite spin. The latter effect is less pronounced in an eight-band model, but could be very sensitive to the Kane inter-band energy (E(P)) value.

Exciton states and oscillator strength in two vertically coupled InP/InGaP quantum discs

Quantum mechanical coupling and strain in two vertically arranged InP/InGaP quantum dots is studied as a function of the size of the dots and the spacer thickness. The strain distribution is determined by the continuum mechanical model, while the single-band effective-mass equation and the multiband theory are employed to compute the conduction and valence band energy levels, respectively. The exciton states are obtained from an exact diagonalization approach, and we also compute the oscillator strength for recombination. We found that the light holes are confined by strain to the spacer, which is the reason that the hole states exhibit coupling at much larger distances as compared with the electrons. At small d, the doublet structure of the hole energy levels arises as a consequence of the relocation of the light hole from the matrix to the regions located outside the stack, close to the dot–matrix interface. When d varies, the exciton ground state exhibits numerous anticrossings with other states, which are related to the changing spatial localization of the hole as a function of d. The oscillator strength of the exciton recombination is strongly reduced in a certain range of spacer thicknesses, which effectively turns a bright exciton state into a dark one. This effect is associated with anticrossings between exciton energy levels.

Tunable skewed edges in puckered structures

We propose a type of edges arising due to the anisotropy inherent in the puckered structure of a honeycomb system such as in phosphorene. Skewed-zigzag and skewed-armchair nanoribbons are semiconducting and metallic, respectively, in contrast to their normal edge counterparts. Their band structures are tunable, and a metal-insulator transition is induced by an electric field. We predict a field-effect transistor based on the edge states in skewed-armchair nanoribbons, where the edge state is gapped by applying arbitrary small electric field

Interband optical absorption in a circular graphene quantum dot

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Exciton states in a nanocup in the presence of a perpendicular magnetic field

. A topological argument is presented, revealing the condition for the emergence of such edge states.

The multiband effective-mass model of the electronic structure and intersubband absorption in p-type-doped twinning superlattices

We investigate the energy levels and optical properties of a circular graphene quantum dot in the presence of an external magnetic field perpendicular to the dot. Based on the Dirac–Weyl equation and assuming zero outward current at the edge of the dot we present the results for two different types of boundary conditions, i.e. infinite-mass (IMBC) and zigzag boundary conditions. We found that the dot with zigzag edges displays a zero-energy state in the energy spectra while this is not the case for the IMBCs. For both boundary conditions, the confinement becomes dominated by the magnetic field, where the energy levels converge to the Landau levels as the magnetic field increases. The effect of boundary conditions on the electron- and hole-energy states is found to affect the interband absorption spectra, where we found larger absorption in the case of IMBCs. The selection rules for interband optical transitions are determined and discussed for both boundary conditions.

Intersubband optical transition matrix elements for hole states in semiconductor quantum wells

The exciton states in a strained (In,Ga)As/GaAs nanocup are theoretically determined. We explore how the nanocup bottom thickness (t) affects the magnetic field dependence of the exciton energy. Strain distribution is computed by the continuum mechanical model under the approximation of isotropic elasticity. The exciton wave functions are expanded into products of the electron and hole envelope functions. For small t, the exciton ground state has zero orbital momentum and exhibits small oscillations of the second derivative when the magnetic field increases. When t approaches the value of the cup height, however, the exciton levels exhibit angular momentum transitions, whose behavior is similar to that for type-II quantum dots. Small oscillations of the oscillator strength for exciton recombination are found when the magnetic field increases. An increase in thickness of the nanocup bottom has only a small effect on those oscillations for the optically active exciton states, but the exciton ground state becomes dark when the magnetic field increases. Hence, the results of our calculations show that an increase in thickness of the nanocup bottom transforms the exciton ground energy level dependence on magnetic field from the one characteristic of type-I rings to the one characteristic of type-II dots.

Normal and skewed phosphorene nanoribbons in combined magnetic and electric fields

The electronic structure and the infrared light absorption in p-doped twinning superlattices are calculated. The standard 6 × 6 k p Hamiltonian is employed, with the split-off band taken into account. The effective scattering potential at the twinning interface is modelled by appropriate -potentials. The calculation of the intersubband absorption is also performed. The origin of each absorption peak is identified, and the polarization dependence is explained in terms of the structural parameters. The magnitude of the absorption coefficient indicates considerable benefits offered solely by the change of the atomic stacking sequence, i.e. the enhanced scattering related to it. Good agreement between our results and those computed by the pseudopotential theory is found.