Letter pubs.acs.org/NanoLett
Monolithically Integrated High‑β Nanowire Lasers on Silicon B. Mayer,† L. Janker,† B. Loitsch,† J. Treu,† T. Kostenbader,† S. Lichtmannecker,† T. Reichert,† S. Morkötter,† M. Kaniber,† G. Abstreiter,‡ C. Gies,§ G. Koblmüller,† and J. J. Finley*,† †
Walter Schottky Institut and Physik Department, Technische Universität München, Am Coulombwall 4, Garching 85748, Germany Institute of Advanced Study, Technische Universität München, Lichtenbergstraße 2a, 85748 Garching, Germany § Institute for Theoretical Physics, University of Bremen, 28334 Bremen, Germany ‡
S Supporting Information *
ABSTRACT: Reliable technologies for the monolithic integration of lasers onto silicon represent the holy grail for chip-level optical interconnects. In this context, nanowires (NWs) fabricated using III−V semiconductors are of strong interest since they can be grown site-selectively on silicon using conventional epitaxial approaches. Their unique onedimensional structure and high refractive index naturally facilitate low loss optical waveguiding and optical recirculation in the active NW-core region. However, lasing from NWs on silicon has not been achieved to date, due to the poor modal reflectivity at the NW-silicon interface. We demonstrate how, by inserting a tailored dielectric interlayer at the NW-Si interface, low-threshold single mode lasing can be achieved in vertical-cavity GaAs−AlGaAs core−shell NW lasers on silicon as measured at low temperature. By exploring the output characteristics along a detection direction parallel to the NW-axis, we measure very high spontaneous emission factors comparable to nanocavity lasers (β = 0.2) and achieve ultralow threshold pump energies ≤11 pJ/pulse. Analysis of the input−output characteristics of the NW lasers and the power dependence of the lasing emission line width demonstrate the potential for high pulsation rates ≥250 GHz. Such highly efficient nanolasers grown monolithically on silicon are highly promising for the realization of chip-level optical interconnects. KEYWORDS: Nanowire lasers, GaAs-AlGaAs, monolithic integration, optical pumping
U
Figure 1a shows a schematic representation of the GaAs− AlGaAs core−shell NWs employed in the present study. Initially, a sacrificial NW growth technique is used to prepare ∼80 nm diameter pinholes in a ∼250 nm thick SiO2 interlayer that is deposited on silicon (see Methods and Supporting Information, SI). An 80 nm thick GaAs inner core is then grown using solid source molecular beam epitaxy (MBE) with a length of ∼19 ± 1 μm (Figure 1a). Since this inner-core diameter is much too small to support low loss optical waveguiding, the diameter of the GaAs NW is then selectively widened only in the region above the SiO2 interlayer by switching to MBE conditions favoring lateral growth. In a final growth step, the NW surface is passivated,19 by using 10 nm of AlGaAs and completed with a 10 nm GaAs cap to produce core−shell GaAs−AlGaAs NWs connected to the silicon substrate via the inner core that extends throughout the SiO2 interlayer. Finite difference time domain (FDTD) simulations indicate that the NW with a diameter of ∼470 nm is sufficient to support low loss guided modes with confinement factors >0.9.9,10 A typical SEM image recorded from a single NW
ltracompact all-optical interconnects on silicon are required for enhanced data processing speeds while minimizing the energy cost per bit.1 However, integrating materials providing significant gain onto silicon chips and realizing lasers remains a major challenge. In this respect, III−V semiconductor nanowires (NWs) are of particular interest2−5 since their small footprint facilitates direct growth on silicon.6,7 However, the low refractive index contrast between the NW and the silicon substrate provides poor modal reflectivity (
