We report on AlGaN multiple-quantum-well separate confinement laser heterostructures grown by plasma-assisted molecular-beam epitaxy directly on c-sapphire at low temperatures (<800 °C). Threading dislocation density was reduced down to 109–1010 cm−2 owing to both intentionally introduced strained AlGaN/AlN superlattices and self-organized blocking structures in the AlGaN step-graded buffer layers. The quantum wells were fabricated by a submonolayer digital alloying technique. Calculations of the optical gain and confinement in the optically pumped laser structures yielded its optimum design comprising an asymmetric waveguide. Lasing at 303 nm with the relatively low threshold excitation density of 0.8 MW/cm2 at 295K has been achieved.

A procedure for the synthesis of monodisperse spherical mesoporous silica particles (MSMSPs) via the controlled coagulation of silica/surfactant clusters into spherical aggregates with mean diameters of 250-1500 nm has been developed. The synthesis is fast (taking less than 1 h) because identical clusters are simultaneously formed in the reaction mixture. The results of microscopic, x-ray diffraction, adsorption and optical measurements allowed us to conclude that the clusters are ∼15 nm in size and have hexagonally packed cylindrical pore channels. The channel diameters in MSMSPs obtained with cethyltrimethylammonium bromide and decyltrimethylammonium bromide as structure-directing agents were 3.1 ± 0.15 and 2.3 ± 0.12 nm, respectively. The specific surface area and the pore volume of MSMSP were, depending on synthesis conditions, 480-1095 m(2) g(-1) and 0.50-0.65 cm(3) g(-1). The MSMSP were used to grow opal-like photonic-crystal films possessing a hierarchical macro-mesoporous structure, with pores within and between the particles. A selective filling of mesopore channels with glycerol, based on the difference between the capillary pressures in macro- and mesopores, was demonstrated. It is shown that this approach makes it possible to control the photonic bandgap position in mesoporous opal films by varying the degree of mesopore filling with glycerol.

This letter reports on the self-organized growth of nanoscale dot-like CdSe-based islands during molecular beam epitaxy of CdSe/ZnSe nanostructures with a CdSe thickness between 0.75 and 3.0 monolayers. An increase in the nominal CdSe thickness results in a higher density of islands (up to 2×1010 cm−2) and is accompanied by dramatic enhancement of the photoluminescence efficiency. The density of large relaxed islands appears to saturate at a value of (3–4)×109 cm−2. Room temperature (Zn, Mg)(S, Se)-based optically pumped lasers with an extremely low threshold (less than 4 kW/cm2), as well as (Be, Mg, Zn)Se-based injection laser diodes using a single (2.5–2.8) monolayer thick CdSe active region, both demonstrating significantly enhanced degradation stability, have been fabricated and studied.

This paper reports on novel approaches developed for plasma-assisted molecular beam epitaxy of Al-rich AlGaN epilayers and quantum well heterostructures on c-sapphire, which allowed us to fabricate low-threshold optically-pumped separate confinement heterostructure lasers emitting in the mid-UV spectral range (258-290 nm) with the threshold power density below 600 kW cm −2 . The optimum buffer structure has been developed which provides lowering the near-surface threading dislocation density down to 1.5 × 10 8 and 3 × 10 9 cm −2 for screw and edge types, respectively, and improving the surface morphology (rms < 0.7 nm at the area of 3 × 3 μm −2 ). It comprises the high-temperature (780 °C) migration enhanced epitaxy growth of a (30-70) nm thick AlN nucleation layer on c-Al 2 O 3 , followed by a 2 μm thick AlN buffer grown under the metal-rich conditions in the Al-flux modulation mode and containing several (up to 6) ultra-thin (∼3 nm) GaN interlayers grown at N-rich conditions. Proper strain engineering in AlGaN single quantum well heterostructure grown atop of the AlN buffer layer enables one to preserve dominant TE polarization of both spontaneous and stimulated emission even at shortest obtained wavelength (258 nm). The threshold power density of stimulated emission as low as 150 kW cm −2 at 289 nm for a single quantum well laser structure has been demonstrated.

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