Self-organized formation and evolution of quantum dot (QD) ensembles with a multimodal size distribution is reported. Such ensembles form after fast deposition near the critical thickness during a growth interruption (GRI) prior to cap layer growth and consist of pure InAs truncated pyramids with heights varying in steps of complete InAs monolayers, thereby creating well-distinguishable subensembles. Ripening during GRI manifests itself by an increase of sub-ensembles of larger QDs at the expense of sub-ensembles of smaller ones, leaving the wetting layer unchanged. The dynamics of the multimodal QD size distribution is theoretically described using a kinetic approach. Starting from a broad distribution of flat QDs, a predominantly vertical growth is found due to strain-induced barriers for nucleation of a next atomic layer on different facets. QDs having initially a shorter base length attain a smaller height, accounting for the experimentally observed sub-ensemble structure. The evolution of the distribution is described by a master equation, which accounts for growth or dissolution of the QDs by mass exchange between the QDs and the adatom sea. The numerical solution is in good agreement with the measured dynamics. (c) 2006 Elsevier B.V. All rights reserved.

High concentration of optical power in a narrow exit angle is extremely important for numerous applications of laser diodes, for example, for low-cost fiber pumping and coupling, material processing, direct frequency conversion, etc. Lasers based on the longitudinal photonic band crystal (PBC) concept allow a robust and controllable extension of the fundamental mode over a thick multi-layer waveguide region to achieve a very large vertical optical mode spot size and, consequently, a very narrow vertical beam divergence. Many undesirable effects like beam filamentation, lateral multimode operation and catastrophic optical mirror damage (COMD) are strongly reduced. 650 nm GaInP/GaAlInP PBC lasers show narrow far field pattern (FWHM similar to 7 degrees) stable up to the highest output powers. Differential efficiency up to 85% is demonstrated. Total single mode output power as high as 150 mW is achieved in 4 mu m-wide stripes in continuous wave operation, being limited by COMD due to not passivated facets. The lateral far field FWHM is 4 degrees. 840 nm GaAs/GaAlAs PBC lasers show a vertical beam divergence of 80 (FWHM) and a high differential efficiency up to 95% (L=500 mu m). A total single mode CW power approaches 500 mW for 1 mm-long 4 mu m-wide stripes devices at similar to 500 mA current, being COMD-limited. The lateral far field FWHM is 5 degrees. Another realization of a longitudinal PBC laser allows lasing in a single high-order vertical mode, a so-called tilted mode, which provides wavelength selectivity and substantially extends the possibility to control the thermal shift of the lasing wavelength. In a multilayer laser structure, where the refractive index of each layer increases upon temperature, it is possible to reach both a red shift of the lasing wavelength for some realizations of the structures, and a blue shift for some others. Most important, the absolute thermal stabilization of the lasing wavelength of a semiconductor laser can be realized.

Formation of a multimodal quantum dot (QD) ensemble by strained layer epitaxy of InAs on GaAs near the critical value for the onset of the 2D-3D transition is studied. Reflection anisotropy spectroscopy is employed to confirm that a smooth surface is maintained during strained layer growth prior to QD formation. Instantaneous capping after deposition leads to InAs quantum wells with some thickness flucuations. Multimodal QD InAs ensembles form after an at least short growth interruption prior to cap layer deposition. The QDs consist of pure InAs with heights varying in steps of complete InAs monolayers. Related exciton energies indicate a simultaneous increase of both height and lateral extension, i.e. a shell-like increase of sizes. The formation of the multimodal QD ensemble is described by a kinetic approach. A growth scenario is presented where QDs having initially shorter base length stop vertical growth at a smaller height, accounting for the experimentally observed shell-like sub-ensemble structure. (c) 2006 Elsevier B.V. All rights reserved.

Formation and evolution of a multimodal InAs/GaAs quantum dot (QD) ensemble during a growth interruption prior to cap layer deposition is studied. These particular kinds of QDs form self-organized after deposition of an InAs layer close to the critical thickness for elastic relaxation and after a short growth interruption. The QDs consist of pure InAs with heights varying in steps of complete InAs monolayers, have well-defined, flat, top and bottom interfaces, and show indications for steep side facets in transmission electron micrographs. QDs with a common height represent a subensemble within the QD ensemble, showing an emission peak with small inhomogeneous broadening. The evolution occurs by an increased appearance of subensembles with higher QDs and disappearance of subensembles related to smaller QDs, which accordingly dissolve. Dissolution proceeds essentially by a decrease of height, and only to a small amount by lateral shrinking. Thickness and composition of the wetting layer do not change during this process; growth and dissolution originate solely from material exchange between different QD subensembles. The evolution slows down for prolonged growth interruption, but the QD ensemble does not attain equilibrium within a time scale of minutes being eventually limited by the onset of plastic relaxation. Formation and dynamics of the observed evolution of the multimodal QD size distribution is theoretically well described by a kinetic approach, which implies strain-controlled adatom kinetics in the mass exchange between the QDs mediated by the adatom sea.

We have applied the semiconductor alloy decomposition theory to find the effect of the tetragonal intrinsic strain in the surface overlayer on the critical temperature of spinodal decomposition. It is shown that the critical temperature is reduced due to the outward surface relaxation. The inward surface relaxation increases the critical temperature.