Shallow Donor Impurity States with Excitonic Contribution in GaAs/AlGaAs and CdTe/CdSe Truncated Conical Quantum Dots under Applied Magnetic Field

Using the effective mass approximation in a parabolic two-band model, we studied the effects of the geometrical parameters, on the electron and hole states, in two truncated conical quantum dots: (i) GaAs-(Ga,Al)As in the presence of a shallow donor impurity and under an applied magnetic field and (ii) CdSe–CdTe core–shell type-II quantum dot. For the first system, the impurity position and the applied magnetic field direction were chosen to preserve the system’s azimuthal symmetry. The finite element method obtains the solution of the Schrödinger equations for electron or hole with or without impurity with an adaptive discretization of a triangular mesh. The interaction of the electron and hole states is calculated in a first-order perturbative approximation. This study shows that the magnetic field and donor impurities are relevant factors in the optoelectronic properties of conical quantum dots. Additionally, for the CdSe–CdTe quantum dot, where, again, the axial symmetry is preserved, a switch between direct and indirect exciton is possible to be controlled through geometry.

Electronic structure of vertically coupled quantum dot-ring heterostructures under applied electromagnetic probes. A finite-element approach

We theoretically investigate the electron and hole states in a semiconductor quantum dot-quantum ring coupled structure, inspired by the recent experimental report by Elborg and collaborators (2017). The finite element method constitutes the numerical technique used to solve the three-dimensional effective mass equation within the parabolic band approximation, including the effects of externally applied electric and magnetic fields. Initially, the features of conduction electron states in the proposed system appear discussed in detail, under different geometrical configurations and values of the intensity of the aforementioned electromagnetic probes. In the second part, the properties of an electron-hole pair confined within the very kind of structure reported in the reference above are investigated via a model that tries to reproduce as close as possible the developed profile. In accordance, we report on the energies of confined electron and hole, affected by the influence of an external electric field, revealing the possibility of field-induced separate spatial localization, which may result in an indirect exciton configuration. In relation with this fact, we present a preliminary analysis of such phenomenon via the calculation of the Coulomb integral.

Low-frequency lattice phonons in halide perovskites explain high defect tolerance toward electron-hole recombination

Low-cost solution-based synthesis of metal halide perovskites (MHPs) invariably introduces defects in the system, which could form Shockley-Read-Hall (SRH) electron-hole recombination centers detrimental to solar conversion efficiency. Here, we investigate the nonradiative recombination processes due to native point defects in methylammonium lead halide (MAPbI3) perovskites using ab initio nonadiabatic molecular dynamics within surface-hopping framework. Regardless of whether the defects introduce a shallow or deep band state, we find that charge recombination in MAPbI3 is not enhanced, contrary to predictions from SRH theory. We demonstrate that this strong tolerance against defects, and hence the breakdown of SRH, arises because the photogenerated carriers are only coupled with low-frequency phonons and electron and hole states overlap weakly. Both factors appreciably decrease the nonadiabatic coupling. We argue that the soft nature of the inorganic lattice with small bulk modulus is key for defect tolerance, and hence, the findings are general to other MHPs.

Магнетотранспортная спектроскопия интерфейсных, квантово-ямных и гибридных состояний в структурах с многочисленными слоями HgTe толщиной 16 нм

The longitudinal and Hall components of the resistivity tensor are measured in structures with multiple HgTe layers 16 nm thick in magnetic fields to 12 T at temperatures from 1.5 to 300 K. The slope of the magnetic-field dependence of the Hall resistance is found to change its sign at a certain critical temperature T _ c = 5 and 10 K in the two studied samples, which indicates the presence of two types of charge carriers and a change in the relation between their contributions to the Hall resistance with temperature. The low critical temperature and manifestation of the “two-component” nature of the Hall curves only at T > T _ c prove that the ground state of the system at T = T _ c is gapless. At higher temperatures (20 K < T < 200 K), the Hall concentration is proportional to the temperature with good accuracy. The description of the charge-carrier dispersion laws by the 8-band kp model taking into account Γ_8-band-edge splitting caused by mechanical stresses, which forms both types of state in HgTe, makes it possible to quantitatively describe the observed magnetotransport features. It is shown that they are associated with the simultaneous filling of electron and hole states formed as a result of mixing interface states responsible for the topological-insulator phase and the quantum-confined states in the Γ_8 band.