Bo E. Sernelius

Defect related issues in the “current roll-off” in InGaN based light emitting diodes

Defect related contributions to the reduction of the internal quantum efficiency of InGaN-based multiple quantum well light emitting diodes under high forward bias conditions are discussed. Screening of localization potentials for electrons is an important process to reduce the localization at high injection. The possible role of threading dislocations in inducing a parasitic tunneling current in the device is discussed. Phonon-assisted transport of holes via tunneling at defect sites along dislocations is suggested to be involved, leading to a nonradiative parasitic process enhanced by a local temperature rise at high injection.

Surface recombination in ZnO nanorods grown by chemical bath deposition

Verticallywell-aligned ZnO nanorods on Si substrates were prepared by atwo-step chemical bath deposition (CBD) method. The optical properties ofthe grown ZnO nanorods were investigated by time reso ...

Reactively sputtered ZnO: Al films for energy-efficient windows

Abstract ZnO: Al films were produced by dual-target reactive magnetron sputtering. The effects of different deposition conditions were investigated. Under optimized conditions, 0.3 μm thick films had about 1% luminous absorptance, about 85% thermal IR reflectance, and a d.c. resistivity of about 5 × 10 −4 Ω cm. Band gap widening could be quantitatively explained from an effective mass model for n-doped semiconductors, provided that the polar character of ZnO was accounted for.

OPTICAL PROPERTIES OF DOPED INGAN/GAN MULTIQUANTUM-WELL STRUCTURES

A systematic investigation of the photoluminescence spectra from InGaN/GaN multiquantum-well structures with different levels of Si doping in the quantum well has been carried out, in order to study the screening of the strain induced piezoelectric field and potential fluctuations. It is found that the emission energy strongly depends on the carrier concentration, originating from the doping or photogeneration. The observed strong shift with Si doping can only partly be explained by the screening of the piezoelectric field. The main shift is suggested to be related to the screening of the localization potentials.

Polarization propagator calculations of the polarizability tensor at imaginary frequencies and long-range interactions for the noble gases and n-alkanes

A study was performed to demonstrate that the complex polarization propagator method can be used to calculate the electric dipole polarizability on the imaginary frequency axis. The propagator was ...

Radiative recombination in In 0.15 Ga 0.85 N/GaN multiple quantum well structures

We present a study of the radiative recombination in In 0.15 Ga 0.85 N/GaN multiple quantum well samples, where the conditions of growth of the InGaN quantum layers were varied in terms of growth temperature ( 18 cm −3 in the well), with excitation intensity and with delay time after pulsed excitation (also shifts up to 0.2 eV). We suggest a two-dimensional model for electron- and donor screening in this case, which is in reasonable agreement with the observed data, if rather strong localization potentials of short range (of the order 100 A) are present. The possibility that excitons as well as shallow donors are impact ionized by electrons in the rather strong lateral potential fluctuations present at this In composition is discussed

Retarded interactions in graphene systems

We first demonstrate how two-dimensional sheets are incorporated in the formalism for planar structures. Then we derive the interaction in the geometry of two freestanding graphene sheets and of on ...

Free electron behavior in InN: On the role of dislocations and surface electron accumulation

The free electron behavior in InN is studied on the basis of decoupled bulk and surface accumulation electron densities in InN films measured by contactless optical Hall effect. It is shown that the variation in the bulk electron density with film thickness does not follow the models of free electrons generated by dislocation-associated nitrogen vacancies. This finding, further supported by transmission electron microscopy results, indicates the existence of a different thickness-dependent doping mechanism. Furthermore, we observe a noticeable dependence of the surface electron density on the bulk density, which can be exploited for tuning the surface charge in future InN based devices.

Electric dipole polarizabilities and C6 dipole-dipole dispersion coefficients for sodium clusters and C60

The frequency-dependent polarizabilities of closed-shell sodium clusters containing up to 20 atoms have been calculated using the linear complex polarization propagator approach in conjunction with Hartree-Fock and Kohn-Sham density functional theories. In combination with polarizabilities for C(60) from a previous work [J. Chem. Phys. 123, 124312 (2005)], the C(6) dipole-dipole dispersion coefficients for the metal-cluster-to-cluster and cluster-to-buckminster-fullerene interactions are obtained via the Casimir-Polder relation [Phys. Rev. 73, 360 (1948)]. The B3PW91 results for the polarizability of the sodium dimer and tetramer are benchmarked against coupled cluster calculations. The error bars of the reported theoretical results for the C(6) coefficients are estimated to be 5%, and the results are well within the error bars of the experiment.

Casimir interactions in graphene systems

The non-retarded Casimir interaction (van der Waals interaction) between two freestanding graphene sheets as well as between a graphene sheet and a substrate is determined. We present several diffe ...