Wetting layer

Abstract: The deposition of InAs on GaAs proceeds first by two‐dimensional (2D) growth and above a 1.75‐monolayer coverage by the formation of single‐crystal dots on a residual 2D wetting layer. By atomic force microscopy measurements, we show that the first dots formed are in the quantum size range (height 30 A, half‐base 120 A), that the dispersion on their sizes is remarkably low (±10%), and that they are located fairly regularly (interdot distance 600 A). Upon further growth, density and shapes do not change but sizes increase up to double values before coalescence occurs. Self‐organized growth in strained structures is then shown to be a simple and efficient way of building regular quantum dots.

Abstract: We have experimentally determined a phase diagram for cylinder-forming polystyrene-block-polybutadien-block-polystyrene triblock copolymer in thin films. The phase behavior can be modeled in great detail by dynamic density functional theory. Deviations from the bulk structure, such as wetting layer, perforated lamella, and lamella, are identified as surface reconstructions. Their stability regions are determined by an interplay between surface fields and confinement effects.

Abstract: Spin-coating is a very widely used technique for making uniform thin polymer films. For example, the active layers in most experimental semiconducting polymer-based devices, such as light-emitting diodes and photovoltaics, are made this way. The efficiency of such devices can be improved by using blends of polymers; these phase separate during the spin-coating process, creating the complex morphology that leads to performance improvements. We have used time-resolved small-angle light scattering and light reflectivity during the spin-coating process to study the development of structure directly. Our results provide evidence that a blend of two polymers first undergoes vertical stratification; the interface between the stratified layers then becomes unstable, leading to the final phase-separated thin film. This has given us the basis for establishing a full mechanistic understanding of the development of morphology in thin mixed polymer films, allowing a route to the rational design of processing conditions so as to achieve desirable morphologies by self-assembly.

Abstract: We present a theory of the electronic structure of GaN/AlN quantum dots (QD's), including built-in strain and electric-field effects. A Green's function technique is developed to calculate the three-dimensional (3D) strain distribution in semiconductor QD structures of arbitrary shape and of wurtzite (hexagonal) crystal symmetry. We derive an analytical expression for the Fourier transform of the QD strain tensor, valid for the case when the elastic constants of the QD and matrix materials are equal. A simple iteration procedure is described, which can treat differences in the elastic constants. An analytical formula is also derived for the Fourier transform of the built-in electrostatic potential, including the strain-induced piezoelectric contribution and a term associated with spontaneous polarization. The QD carrier spectra and wave functions are calculated using a plane-wave expansion method we have developed, and a multiband $\mathbf{k}\ensuremath{\cdot}\mathbf{P}$ model. The method used is very efficient, because the strain and built-in electric fields can be included analytically through their Fourier transforms. We consider in detail the case of GaN/AlN QD's in the shape of truncated hexagonal pyramids. We present the calculated 3D strain and electrostatic potential distributions, the carrier spectra, and wave functions in the QD's. Due to the strong built-in electric field, the holes are localized in the wetting layer just below the QD bottom, while electrons are pushed up to the pyramid top. Both also experience an additional lateral confinement due to the built-in field. We examine the influence of several key factors on the calculated confined state energies. Use of a one-band, effective-mass Hamiltonian overestimates the electron confinement energies by \ensuremath{\sim}100 meV, because of conduction-band nonparabolicity effects. By contrast, a one-band valence Hamiltonian provides good agreement with the calculated multiband ground-state energy. Varying the QD shape has comparatively little effect on the calculated levels, because of the strong lateral built-in electric field. Overall, the transition energies depend most strongly on the assumed built-in electric field. The calculated variation of transition energy with quantum dot size is in good agreement with the available experimental data.