Exciton

About: Exciton is a research topic. Over the lifetime, 31603 publications have been published within this topic receiving 810642 citations.

Polarized excitons in nanorings and the optical Aharonov-Bohm effect

Abstract: The quantum nature of matter lies in the wave function phases that accumulate while particles move along their trajectories. A prominent example is the Aharonov-Bohm phase, which has been studied in connection with the conductance of nanostructures. However, optical response in solids is determined by neutral excitations, for which no sensitivity to magnetic flux would be expected. We propose a mechanism for the topological phase of a neutral particle, a polarized exciton confined to a semiconductor quantum ring. We predict that this magnetic-field induced phase may strongly affect excitons in a system with cylindrical symmetry, resulting in switching between ``bright'' exciton ground states and novel ``dark'' states with nearly infinite lifetimes. Since excitons determine the optical response of semiconductors, the predicted phase can be used to tailor photon emission from quantum nanostructures.

Optical properties of InGaN quantum wells

Abstract: The emission mechanisms of strained InGaN quantum wells (QWs) were shown to vary depending on the well thickness L and InN molar fraction x . The QW resonance energy was shifted to lower energy by the quantum confined Stark effect (QCSE) due to the internal piezoelectric field, F PZ . The absorption spectrum was modulated by QCSE and quantum-confined Franz–Keldysh effect (QCFK) for the wells, in which, for the first approximation, the product of F PZ and L (potential drop across the well) exceeds the valence band discontinuity, Δ E V . In this case, dressed holes are confined in the triangular potential well formed at one side of the well. This produces apparent Stokes-like shift (vertical component). The QCFK further modulated the absorption energy for the wells with L greater than the three dimensional free exciton Bohr radius, a B . For the wells having high InN content ( F PZ × L >Δ E V , Δ E C ), electron and hole confined levels drop into the triangular potential wells formed at opposite sides of the wells, which reduces the wavefunction overlap. Doping of Si in the barriers partially screens F PZ resulting in a smaller Stokes-like shift, shorter recombination decay time, and higher emission efficiency. Si-doping was found to improve the interface quality and surface morphology, resulting in an efficient carrier transfer from high to low bandgap energy portions of the well. Effective in-plane localization of carriers in quantum disk size potential minima, which are produced by nonrandom alloy potential fluctuations enhanced by the large bowing parameter and F PZ , produces confined e–h pair whose wavefunctions are still overlapped. Their excitonic features are pronounced provided that L a B and F PZ × L E V (quantized exciton). Several cw laser wafers exhibit stimulated emission from these energy tail states even at room temperature.

Electronic structures of zero-dimensional quantum wells

Abstract: The electronic structures of zero-dimensional quantum wells are studied with a spherical model in the framework of the effective-mass theory. The mixing effect of the heavy and light holes is taken into account, and the symmetry classification and the energy levels of hole states are obtained. The energies of the donor and acceptor states are calculated. The difference between the shallow-impurity states and the eigenstates for the small semiconductor sphere disappears. The selection rules for the optical transition between the conduction- and valence-band states are obtained. The An =0 selection rule is not followed strictly because of the mixing of the L- and (L +2)-orbital wave functions in the wave functions of the hole states. The exciton binding energies are calculated for the small GaAs spheres. The energy levels of the ZnSe spheres are given as functions of the radius and compared with the experiments.

Dielectric functions (1 to 5 eV) of wurtzite MgXZn1 -XO (x≤0.29) thin films

Abstract: The optical dielectric functions for polarization perpendicular and parallel to the c-axis (optical axis) of pulsed-laser-deposition grown wurtzite MgxZn1−xO (0⩽x⩽0.29) thin films have been determined at room temperature using ellipsometry for photon energies from 1 to 5 eV. The dielectric functions reveal strong excitonic contributions for all Mg concentrations x. The band gap energies (E0A=3.369 eV for ZnO to 4.101 eV for x=0.29) show a remarkable blueshift. The exciton binding energy (61 meV for ZnO) decreases to approximately 50 meV for x≈0.17 and increases to approximately 58 meV for x=0.29. In contrast to ZnO, the MgxZn1−xO alloys are found uniaxial negative below the band gap energy, opposite to previously reported results.

Bound excitons in ZnO: Structural defect complexes versus shallow impurity centers

Abstract: ZnO single crystals, epilayers, and nanostructures often exhibit a variety of narrow emission lines in the spectral range between 3.33 and 3.35 eV which are commonly attributed to deeply bound excitons ($Y$ lines). In this work, we present a comprehensive study of the properties of the deeply bound excitons with particular focus on the ${Y}_{0}$ transition at 3.333 eV. The electronic and optical properties of these centers are compared to those of the shallow impurity related exciton binding centers ($I$ lines). In contrast to the shallow donors in ZnO, the deeply bound exciton complexes exhibit a large discrepancy between the thermal activation energy and localization energy of the excitons and cannot be described by an effective mass approach. The different properties between the shallow and deeply bound excitons are also reflected by an exceptionally small coupling of the deep centers to the lattice phonons and a small splitting between their two electron satellite transitions. Based on a multitude of different experimental results including magnetophotoluminescence, magnetoabsorption, excitation spectroscopy (PLE), time resolved photoluminescence (TRPL), and uniaxial pressure measurements, a qualitative defect model is developed which explains all $Y$ lines as radiative recombinations of excitons bound to extended structural defect complexes. These defect complexes introduce additional donor states in ZnO. Furthermore, the spatially localized character of the defect centers is visualized in contrast to the homogeneous distribution of shallow impurity centers by monochromatic cathodoluminescence imaging. A possible relation between the defect bound excitons and the green luminescence band in ZnO is discussed. The optical properties of the defect transitions are compared to similar luminescence lines related to defect and dislocation bound excitons in other II--VI and III--V semiconductors.