Properties of Excitons Bound to Neutral Donors in GaAs-AlxGa1-xAs Quantum-Well Wires

In the effective-mass approximation, using a simple two-parameter wave function and a one-dimensional （ID） equivalent potential model, we calculate variationally the binding energy of an exciton bound to a neutral donor （D^0, X） in finite GaAs-AIxGa1-xAs quantum well wires （QWWs）. At the wire width of 25 A, the binding energy has a peak value, which is also at the position of the peak of the exciton binding energy, and the center-of-mass wave functions of excitons reaches the most centralized distribution. In addition, the changing tendency of the average interparticle distance as the wire width is reverse to that of the binding energy.

Ionized Acceptor Bound Exciton States in Wurtzite GaN/Al_xGa(1-x)N Cylindrical Quantum Dot

Based on the framework of the effective-mass approximation, the ionized acceptor bound exciton （A- X） binding energy and the emission wavelength are investigated for a cylindrical wurtzite （WZ） GaN/A1x Ga1-xN quantum dot （QD） with finite potential barriers by means of a variational method. Numerical results show that the binding energy and the emission wavelength highly depend on the QD size, the position of the ionized acceptor and the Al composition x of the barrier material AIxGal-xN. The binding energy and the emission wavelength are larger when the acceptor is located in the vicinity of the left interface of the QD. In particular, the binding energy of （ A-, X） complex is insensitive to the dot height when the acceptor is located at the left boundary of the QD. The ionized acceptor bound exciton binding energy and the emission wavelength are both increased if Al composition x is increased.

Grain size control in quasi-two-dimensional perovskite thin film via intermediate phase engineering for efficient bound exciton generation

Quasi-two dimensional(2D)perovskites have emerged as a promising class of materials due to their remarkable photoluminescence efficiency,which stems from their exceptionally high exciton binding energies.The spatial confinement of excitons within smaller grain sizes could enhance the formation of biexcitons leading to higher radiative recombination efficiency.However,the synthesis of high-quality quasi-2D perovskite thin films with controllable grain sizes remains a challenging task.In this study,we present a facile method for achieving quasi-2D perovskite thin films with controllable grain sizes ranging from 500 to 900 nm.This is accomplished by intermediate phase engineering during the film fabrication process.Our results demonstrate that quasi-2D perovskite films with smaller grain sizes exhibit more efficient bound exciton generation and a reduced stimulated emission threshold down to 15.89µJ cm^(−2).Furthermore,femtosecond transient absorption measurements reveal that the decay time of bound excitons is shorter in quasi-2D perovskites with smaller grain sizes compared to that of those with larger grains at the same pump density,which is 230.5 ps.This observation suggests a more efficient exciton recombination process in the smaller grain size regime.Our findings would offer a promising approach for the development of efficient bound exciton lasers.

Radiative life time of an exciton confined in a strained GaN/Ga_(1-x)Al_xN cylindrical dot:built-in electric field effects

The binding energy of an exciton in a wurtzite GaN/GaAlN strained cylindrical quantum dot is investigated theoretically. The strong built-in electric field due to the spontaneous and piezoelectric polarizations of a GaN/GaAlN quantum dot is included. Numerical calculations are performed using a variational procedure within the single band effective mass approximation. Valence-band anisotropy is included in our theoretical model by using different hole masses in different spatial directions. The exciton oscillator strength and the exciton lifetime for radiative recombination each as a function of dot radius have been computed. The result elucidates that the strong built-in electric field influences the oscillator strength and the recombination life time of the exciton. It is observed that the ground state exciton binding energy and the interband emission energy increase when the cylindrical quantum dot height or radius is decreased, and that the exciton binding energy, the oscillator strength and the radiative lifetime each as a function of structural parameters （height and radius） sensitively depend on the strong built-in electric field. The obtained results are useful for the design of some opto-photoelectronic devices.

Electronic structures of impurities and point defects in semiconductors

A brief history of the impurity theories in semiconductors is provided. A bound exciton model is proposed for both donor-and acceptor-like impurities and point defects, which offers a unified understanding for ＂shallow＂ and ＂deep＂impurities and point defects. The underlying physics of computational results using different density-functional theorybased approaches are discussed and interpreted in the framework of the bound exciton model.