Envelope-function formalism for valence bands in wurtzite quantum wells

Polarization and magneto-optical properties of excitonic emission from wurtzite CdTe/(Cd,Mg)Te core/shell nanowires

Wurtzite CdTe and (Cd,Mn)Te nanowires embedded in (Cd,Mg)Te shells are grown by employing vapour-liquid-solid growth mechanism in a system for molecular beam epitaxy. A combined study involving cathodoluminescence, transmission electron microscopy and micro-photoluminescence is used to correlate optical and structural properties in these structures. Typical features of excitonic emission from individual wurtzite nanowires are highlighted including the emission energy of 1.65 eV, polarization properties and the appearance B-exciton related emission at high excitation densities. Angle dependent magneto-optical study performed on individual (Cd,Mn)Te nanowires reveals heavy-hole-like character of A-excitons typical for wurtzite structure and allows to determine the crystal field splitting, Δ_CR. The impact of the strain originating from the lattice mismatched shell is discussed and supported by theoretical calculations.

Multivalence-band calculation of the excitonic dielectric function for hexagonal GaN

The complex dielectric function of hexagonal gallium nitride (α-GaN) is obtained from a numerical solution of the excitonic Schrödinger equation taking into account the full 6 × 6 valence-band structure. The valence-band parametrization includes anisotropy, nonparabolicity, and the coupling of angular-momentum eigenstates. Spectra of excitonic eigenfunctions are obtained from a time-dependent initial-value problem, which is solved via an exponential split-operator method. In particular, we calculate the dielectric function and the reflectivity of a-plane GaN with polarization vectors parallel and perpendicular to the c-axis of the crystal. The simulated reflection spectra are in excellent agreement with recent experimental data and allow the unambiguous identification of the experimentally observed excitonic resonances. The binding energies of the FXA, FXB, and FXC excitons found in our calculation differ by up to 27%, depending on the chosen parameter set. An important consequence of this observation is that the experimentally observed splittings of the excitons cannot be used for the parametrization of the valence band near the Γ-point, but need to be corrected by the differences of the binding energies. This is of general relevance for all spectroscopic measurements in semiconductors with a wide bandgap.

Electronic Structure of Biaxially-Strained Wurtzite Crystals GaN and AlN

We present first-principles studies of the effect of biaxial (OOOl)-strain on the electronic structure of wurtzite GaN, and A1N. We provide accurate predictions of the valence band splittings as a function of strain, which may facilitate the interpretation of data from strained samples. The conduction and valence band effective mass tensors for A1N and GaN are also presented. The computed crystal-field and spin-orbit splittings in unstrained materials as well as the computed deformation potentials are in accord with available experimental data. We show that the numerically computed band energies can be excellently represented in terms of a 6-band k · p model. The present calculations are based on the first-principles pseudopotential method within the local-density formalism and include the spin-orbit interactions non-perturbatively.

Precipitative growth templated by a fluid jet

Tubular growth by chemical precipitation at the interface between two fluids, a jet and its surroundings, underlies the development of such important structures as chimneys at hydrothermal vents. This growth is associated with strong thermal and/or solute gradients localized at those interfaces, and these gradients, in turn, often produce radial compositional stratification of the resulting tube wall. A fundamental question underlying these processes is how the interplay between diffusion, advection, and precipitation determines the elongation rate of the tubes. Here we report experimental and theoretical results that reveal a regime in which there exists a new scaling law for tube growth. The model system studied consists of a jet of aqueous ammonia injected into a ferrous sulfate solution, precipitating iron hydroxides with varying oxidation states at the jet boundary. Despite the complex chemistry and dynamics underlying the precipitation, the tube growth exhibits a strikingly simple scaling form, with characteristic lengths and times increasing linearly with the mean velocity of the jet. These observations follow from a kinetic model of advection-dominated flows.