It is demonstrated that the emission of InAs quantum dots ͑QDs͒ capped with GaAsSb can be extended from 1.28 to 1.6 m by increasing the Sb composition of the capping layer from 14% to 26%. Photoluminescence excitation spectroscopy is applied to investigate the nature of this large redshift. The dominant mechanism is shown to be the formation of a type-II transition between an electron state in the InAs QDs and a hole state in the GaAsSb capping layer. The prospects for using these structures to fabricate 1.55 m injection lasers are discussed. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2173188͔ Self-assembled InAs/ GaAs quantum dots ͑QDs͒ are of considerable interest due to their physical properties and potential applications, for example, long wavelength GaAsbased QD lasers operating in the 1.3-1.6 m telecommunications wavelength range. 1 High-performance InAs/ GaAs QD lasers, with emission close to 1.3 m, have been demonstrated using InGaAs, InAl͑Ga͒As, and GaAsSb capping layers ͑CLs͒ to directly cover the InAs QDs. [2][3][4][5][6] In addition, there have been a number of attempts to extend the emission wavelength beyond 1.5 m, with room-temperature ͑RT͒ photoluminescence ͑PL͒ above 1.5 m having been demonstrated for large InAs/ GaAs QDs, 7 GaInNAs/ GaAs QDs, 8 InAs QDs with an In 0.45 Ga 0.55 As CL, 9 InAs QDs with InGaNAs CL, 10 and InAs QDs grown on InGaAs or GaAsSb metamorphic buffer layers. 11,12 In a previous letter, we reported ϳ1.3 m emission from InAs QDs with a GaAsSb CL. 6 Evidence for a type-II system for Sb compositions Ͼ14% was obtained, and a 1.3 m laser was fabricated. In the present letter we show that RT emission at 1.6 m may be obtained by increasing the Sb composition to 26%. The compositional dependence of the electronic band structure is probed using a combination of PL and PL excitation ͑PLE͒.The samples were grown in a V80H molecular beam epitaxy system equipped with conventional solid sources for group-III elements and EPI cracker sources for As and Sb. The QDs were formed by depositing 2.8 monolayers ͑MLs͒ of InAs at a rate of ϳ0

Stacked layers of In(Ga)As on GaAs(001) self-assembled quantum rings (QR) for laser application have been studied. Several samples with three stacked QR layers have been grown by molecular beam epitaxy with GaAs spacers from 1.5 to 14 nm. The optical and structural properties have been characterized by photoluminescence spectroscopy and by atomic force microscopy, respectively. For GaAs spacers larger that 6 nm, the stacked QR layers present similar properties to single QR layers. A semiconductor laser structure with three stacked layers of QR separated 10 nm in the active region has been grown. This spacer ensures well-developed rings with optical emission like that of a single layer. Laser diodes have been processed with 1-2 mm cavity lengths. The stimulated emission is multimodal, centred at 930 nm (77 K), with a threshold current density per QR layer of 69 A cm −2. In this work, it is demonstrated that stacking rings is possible, and that a broad area laser with three QR layers can be fabricated successfully.

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We demonstrate high efficiency triple junction solar cells with submillimeter dimensions in an all-back-contact architecture. 550 × 550 μm2 cells flash at 41.3% efficiency under the air mass 1.5 direct normal spectrum at 50 W/cm2 at 25 °C. Compared to standard size production cells, the micro cells have reduced performance at 1-sun due to perimeter recombination, but the performance gap closes at higher concentrations. Micro cells integrated with lens arrays were tested on-sun with an efficiency of 34.7%. All-back-contact architecture and submillimeter dimensions are advantageous for module integration and heat dissipation, allowing for high-performance, compact, lightweight, and cost-effective concentrated photovoltaic modules.

High quality dilute nitride subcells for multijunction solar cells are achieved using GaInNAsSb. The effects on device performance of Sb composition, strain and purity of the GaInNAsSb material are discussed. New world records in efficiency have been set with lattice-matched InGaP/GaAs/GaInNAsSb triple junction solar cells and a roadmap to 50% efficiency with lattice-matched multijunction solar cells using GaInNAsSb is shown.