Semiconductor lasers emitting at 1.55 μm with external differential efficiencies >1 have been created by monolithically connecting several active regions in series within a single optical waveguide. This is accomplished by epitaxially stacking a number of p–i–n multiquantum well active regions with intermediate n++–p++ back diodes, which enable the entire terminal current to flow through each active region stages in series. Such lasers should also improve the impedance match as well as provide for low-noise, high-efficiency microwave links.

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P-type interface charge control layers for enabling GaN/SiC separate absorption and multiplication avalanche photodiodes Appl. Phys. Lett. 101, 093506 (2012) Recent improvements of the JET lithium beam diagnostic Rev. Sci. Instrum. 83, 10D533 (2012) Bulk and interfacial deep levels observed in In0.53Ga0.47As/GaAs0.5Sb0.5 multiple quantum well photodiodeWe have studied the influence of structural defects on the spatial variation of radiative and nonradiative recombination in an InGaAs/AlGaAs/GaAs resonant cavity enhanced npn heterojunction phototransistor ͑HPT͒ structure using cathodoluminescence ͑CL͒ and electron beam-induced current ͑EBIC͒ imaging. Absorber layers of InGaAs/GaAs multiple quantum wells ͑MQWs͒ are used to provide the photosensitivity for light with wavelengths that are transparent to the GaAs substrate. The current gain of the HPT under an applied bias voltage depends on the hole accumulation efficiency in the base and therefore on the hole lifetime. Strain relaxation-induced misfit dislocations in the MQWs are found to create regions of enhanced nonradiative recombination thereby reducing locally the hole accumulation efficiency and current gain. The reduction in the local EBIC signal caused by the dark line defects is less than ϳ20%, suggesting that misfit dislocations in this sample have a relatively small impact on overall device performance. EBIC and CL are found to be excellent complementary probes of the hole accumulation efficiency and relative recombination rates, owing to the high spatial resolution ͑ϳ1 m͒ of excitation of the electron beam used in these techniques. The temperature dependence of the EBIC and spectrally integrated CL images is examined for 85рTр300 K, and reveals that nonradiative recombination in the vicinity of misfit dislocations is predominantly thermally activated with observed spatial variations in lifetime and activation energy.

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The molecular beam epitaxy and the characterization of single-step grown long wavelength vertical cavity surface emitting lasers (LW–VCSEL) are reported. The devices were fabricated using highly reflective AlGaAsSb/AlAsSb distributed Bragg reflectors (DBRs), and an AlGaInAs-based active region embedding a tunnel (or Esaki) junction. The VCSELs operate at 1.55 μm at room temperature with a threshold current density of 1.4 kA/cm2. Characterization of the VCSELs and DBRs’ thermal and electrical properties is presented. Major improvement on the voltage drop in an InGaAs capped DBR is demonstrated, resulting in a value as low as 40 mV/pair.

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