Letter pubs.acs.org/NanoLett
An Electrically-Driven GaAs Nanowire Surface Plasmon Source Pengyu Fan,†,∥ Carlo Colombo,‡,∥ Kevin C. Y. Huang,†,∥ Peter Krogstrup,§ Jesper Nygård,§ Anna Fontcuberta i Morral,‡ and Mark L. Brongersma*,† †
Geballe Laboratory for Advanced Materials, Stanford University, Stanford, California 94305, United States Laboratoire des Matériaux Semiconducteurs, Institut des Matériaux, Ecole Polytechnique Fédérale de Lausanne, Switzerland § Nano-Science Center, Niels Bohr Institute, University of Copenhagen, Denmark ‡
S Supporting Information *
ABSTRACT: Over the past decade, the properties of plasmonic waveguides have extensively been studied as key elements in important applications that include biosensors, optical communication systems, quantum plasmonics, plasmonic logic, and quantum-cascade lasers. Whereas their guiding properties are by now fairly well-understood, practical implementation in chipscale systems is hampered by the lack of convenient electrical excitation schemes. Recently, a variety of surface plasmon lasers have been realized, but they have not yet been waveguide-coupled. Planar incoherent plasmonic sources have recently been coupled to plasmonic guides but routing of plasmonic signals requires coupling to linear waveguides. Here, we present an experimental demonstration of electrically driven GaAs nanowire light sources integrated with plasmonic nanostrip waveguides with a physical cross-section of 0.08λ2. The excitation and waveguiding of surface plasmon-polaritons (SPPs) is experimentally demonstrated and analyzed with the help of full-field electromagnetic simulations. Splitting and routing of the electrically generated SPP signals around 90° bends are also shown. The realization of integrated plasmon sources greatly increases the applicability range of plasmonic waveguides and routing elements. KEYWORDS: Surface plasmon light source, nanowire light emitting diode, subwavelength light source, plasmonic circuitry
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junctions. Metallic strip waveguides with Y-splitters and bends offer great flexibility in routing optical signals.12−17 In this study, the electrically driven emission is provided by 280 nm diameter core−shell GaAs NWs with a radial p-i-n junction. These NWs were grown by molecular beam epitaxy (MBE) as described in the Methods section and the p-type core is exposed on one end of the wire by a time-calibrated wet etch of the n-type shell. The electrical contacts on both ends of the wire were generated by electron beam lithography (EBL). Figure 1a shows a typical I−V characteristic of a fabricated nanowire device, which clearly indicates a diode-like behavior. The inset shows a scanning electron microscope (SEM) image of the device that highlights the junction area near the electrode contacting the p-type core. Figure 1b shows optical images in the “on” and “off” state of the device. A distinct bright spot of emission can easily be identified in the junction area when an electrical bias is applied across the nanowire. Electroluminescence (EL) spectrum of the emission peaks around 860 nm, which is characteristic of GaAs-based emitters.18 Based on the core−shell geometry of the NW, one would perhaps expect a uniform EL signal along the entire length of the wire. However, by choosing a thin p-type GaAs core (80 nm) to
n the quest to develop increasingly compact optical sources, plasmonics is playing an increasingly prominent role. It has facilitated the realization of coherent nanolasers,1−5 electrically driven plasmon diodes,6,7 and single plasmon sources.8 After these successful proof-of-concept experiments, the natural next step is to realize compact, electrically driven sources that can be integrated with optoelectronic circuitry. Pioneering work on waveguide-coupled surface plasmon-polariton (SPP) sources has demonstrated coupling to planar metal−insulator6 and metal−insulator−metal (MIM) waveguides.7 However, routing of optical signals between functional devices on a chip requires the realization of power-efficient nanoscale sources to be coupled to waveguides with a finite lateral cross section.9−11 To accomplish these goals, one needs to leverage the advances in nanoscale light sources and fabrication of complex nanoscale plasmonic circuitry. In this work, we present an experimental demonstration of an integrated SPP source and a linear plasmonic waveguide consisting of an electrically driven GaAs nanowire (NW) light-emitting diode (LED) coupled to metallic plasmonic strip waveguides. As the source produces incoherent radiation, it can be termed a surface plasmonemitting diode (SPED) in analogy to the coherent counterpart known as a surface plasmon laser (SPASER).3 The NW geometry offers the possibility to realize a compact emission volume while facilitating easy electrical injection from both © 2012 American Chemical Society
Received: July 8, 2012 Revised: August 16, 2012 Published: August 27, 2012 4943
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of the polarizer is rotated by 90°. Figure 2d shows a polar plot that captures the polarization behavior of the collected emission from the top waveguide. This data is consistent with the notion that SPPs are propagating along the strip waveguides. SPPs are transverse magnetic (TM) waves with a significant longitudinal electric field, that is, in the SPP propagation direction. When this wave is decoupled at the flat waveguide termination, it results in a free space wave polarized along the waveguide direction. It is also worth noting that the sides of the strip waveguide near its termination appear dark, consistent with the fact that we are observing scattering from a guided wave as opposed to “randomly” scattered light originating from the source that happens to hit the end of the guide. A two-dimensional (2D) full-field finite-difference timedomain (FDTD) simulation can visualize the emission from a NW LED and the coupling to SPPs supported by the Ag/SiO2 interface. Figure 2e shows the time-averaged Poynting vector distribution from the NW and along the bottom of the Ag film overcoating the wire (see Methods for details). From an analysis of the power flow, an estimated 10% of the total emitted power from the NW LED is coupled to SPPs supported by the strip waveguides and most of the emission is directed into the substrate. The emission from smaller diameter wires can more strongly be coupled to SPPs and may enable the development of more power-efficient sources in the future. The free space emission could also be removed completely by placing the emitter inside a metal−insulator− metal waveguide.19 Previously, it was shown that the bound SPP mode supported by the interface between the Ag and high index silica substrate exhibits no cutoff waveguide width.20 For this reason, SPPs can be guided along deep subwavelength waveguides. This is in stark contrast to the waveguide modes supported by the top Ag surface, which exhibit cutoff when the waveguide width is approximately equal to the free space wavelength of the guided optical signal.21 Guiding of NW LED emission was examined for four different waveguide widths in the range from 300 to 700 nm. The narrowest guide has a width which is less than half of the emission wavelength of NW LED (∼860 nm). Figure 3a shows the characteristic scattering of guided SPPs from the end of these waveguides, approximately 15 μm from the source region. The emission looks qualitatively similar to what was seen in Figure 2b,c, although some scattered light can also be observed from the sides of one of the waveguides (400 nm wide guide). This scattered light is attributed to imperfections in the waveguide fabrication. Although several defects (roughness) are visible in the SEM image of the waveguides, the scattered light along the length is minimal. This interesting observation may be explained by the fact that the guided SPPs propagate along a very smooth Ag/SiO2 interface. The mode profiles for the four different waveguides generated with an electromagnetic mode solver are shown in Figure 3b. Next, we explored the possibility of electrically injecting SPPs into more complex routing elements, including a 90° bend and a Y-shaped splitter as illustrated in Figure 1d. These types of plasmonic elements have been shown before and serve as critical elements for point-to-point guiding of SPP signals and splitting SPP signals into multiple optical channels.9 Figure 4a shows a colored SEM image of a NW source coupled to a strip waveguide featuring a 90° bend with a 2 μm bending radius. This source is also linked to a Y-shaped plasmonic splitter. When the NW SPED is turned on, scattered light can be
Figure 1. GaAs nanowire LED characteristics and device concept. (a) I−V curve of a fabricated GaAs nanowire device with radial p-i-n junction demonstrating a characteristic diode-like behavior. Inset shows an SEM image of the junction area near the p-contact with falsecolors highlighting the structure: green is for the wire with n-GaAs shell intact, red is for the part of the wire with p-GaAs core exposed by etching, and yellow is the p-contact region. Scale bar is 1 μm. (b) CCD image of the NW LED device under white light illumination in the “off” state (top) under zero bias, and the “on” state when a forward bias is applied across the NW (bottom). An emission spot is observed at the junction when the NW source is turned on. Scale bar is 1 μm. (c) Schematic of the proposed electrically driven SPP source coupled to a variety of passive plasmonic routing elements, including waveguides, bends, and splitters.
reduce the hole conductance, the electron−hole pairs can be made to preferentially recombine at the junction between the exposed p-GaAs core and the unetched n-GaAs shell. The localization of carrier recombination allows for confinement of the emission volume to a deep subwavelength region. Such a compact source region allows for easy coupling to linear plasmonic waveguides and routing elements as pictorially shown in Figure 1c. To demonstrate the coupling of our NW source to the SPP modes of metallic strip waveguides, we fabricated two identical straight Ag strips (200 nm thick, 1 μm wide, and 7 μm long) extending from the point of emission on the NW LED (see Figure 2a). The two strips are deliberately chosen to be normal to each other and symmetrically placed relative to the NW axis. When a bias is applied across the NW, light emission is observed from both waveguides' terminations. However, when the light is collected through a linear polarization filter, the relative intensities at the two terminations vary with the orientation of the filter. When the transmission axis of the polarizer is aligned along the length of one of the waveguides, light can only be detected from the end of that particular waveguide and the other waveguide termination appears dark (Figure 2b,c). The situation reverses when the transmission axis 4944
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Figure 2. Emission from GaAs nanowire LED coupled to surface plasmons guided by Ag strip waveguides. (a) SEM image of the structure collected by CCD camera under lamp illumination with false-color highlighting the different components: yellow is for large electrical contacts, cyan is for the GaAs nanowire, and green indicates the location of the Ag strip waveguides. Scale bar is 2 um. The dashed boxes indicate the areas shown (with matching boundary colors) in (b,c), detailing the distribution of the scattered light intensity near the terminations of both strip waveguides when only collecting light with the electric field component polarized along the top (red box) and the bottom (blue box) waveguides, respectively. The red arrows depict the transmission axis of the polarizer and gray lines show the outline of the waveguides. Scale bars are 1 μm. (d) Polar plot of the light emission intensity from the top guide (red box) at different polarization angles of collection (blue curve). The polarization angles (degrees) are noted by numbers around the circle, and the light intensity (a.u.) is noted by numbers along the radial axis. Polarization directions that are parallel (45°) and normal (135°) to the top strip are highlighted by the red dashed lines. (e) Time-averaged Poynting vector map calculated with a full-field 2D finite-difference time-domain algorithm for the Ag-coated NW geometry excited by a horizontally oriented electric dipole placed at the center of the NW. The colormap is on a logarithmic scale and the black lines are streamlines of the time-averaged Poynting vector showing the direction of the local power flow. It can be seen that a fraction of the light is coupled to SPP that propagate along at the Ag/SiO2 interface.
aimed to minimize splitting losses by making the splitting junction as “sharp” as possible. We have demonstrated the design of an extremely compact and functional integrated GaAs NW SPED coupled to a plasmonic strip waveguide. The subwavelength footprint of both emission volume and SPPs mode profile allows for significant flexibility in routing the flow of light on a chip. A similar design concept could be implemented with NWs emitting at different frequencies and to other plasmonic waveguiding structures such as the MIM slot waveguides that facilitate deep subwavelength routing. Furthermore, other elements such as modulators24 and photodetectors25,26 with similar dimensions could be added to ultimately achieve highspeed, compact, and electrically driven plasmonic nanocircuits. Methods. Growth of Coreshell GaAs Nanowires. The NW core is grown using a self-catalyzed vapor−liquid−solid (VLS) mechanism27 with a Ga deposition rate corresponding to a nominal growth of 0.27 μm/h for 45 min at 630 °C and a V/III ratio of 60. The p-doping is achieved using a flux of beryllium which would lead to a nominal doping of 3.5 × 1019 at./cm3 in the case of thin film growth.28,29 The shell is grown around the wire core by changing the growth conditions to those typical for planar thin film growth. This is accomplished by lowering the temperature to 465 °C, switching the As source from As4 to
observed coming off the very end of the bend (Figure 4b). The signal is predominantly polarized along the direction of the guide near its termination, indicating that NW emission was indeed guided as SPPs and has turned 90° following the waveguide bend. The Y-shaped splitter consists of a 1 μm wide trunk and two identical perpendicularly oriented branches of 500 nm width. Scattered light was observed from the termination of both branches (Figure 4c). The scattered light emerging from each branch termination was shown to be predominantly polarized along the direction of the branch. This is consistent with the notion that SPPs are injected into the trunk and subsequently split in two distinct directions in the Ysplitter. It is worth noting that some light is leaked out of waveguide bend and emerges from the Y-splitter junction. Detailed studies have been performed on the losses that can occur in plasmonic waveguide bends17 and these can be reduced by increasing the radius of curvature or by using other higher confinement waveguides.22 Studies on Y-splitters have also indicated the importance of avoiding rounding of metal waveguides in the junction region where the main waveguide is branched into two directions.23 Such rounding prevents adiabatic separation of the waveguides by inducing a mismatch between the incoming and branched mode field distributions. For this reason, we have 4945
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Device Fabrication. After growth, GaAs NWs were harvested and dispersed onto a Si substrate cover by a 1 μm thermally grown SiO2 layer. The contacts of the GaAs NW LED were defined by standard EBL, a time calibrated wet etching with a citric acid solution was performed to remove the n-GaAs shell and expose the p-GaAs core on one end of the wire, before deposition of Pd/Ti/Au (40/10/240 nm) ohmic contacts with both p-GaAs and n-GaAs by electron beam evaporation. The metallic strip waveguides are also generated by EBL and evaporation of 200 nm of Ag. In order to avoid shorting of the junction due to Ag on top of the NW device, Al2O3 dielectric spacer (20 nm-50 nm) was deposited ahead of the waveguide fabrication via atomic layer deposition. Measurements of Light Emission and Waveguiding. The GaAs nanowire LEDs were electrically driven with a function generator (Hewlett-Packard) that provides pulses with 6−20 V amplitude, 20−160 ns pulse width at a rate of 1−2 MHz to achieve reliable and repeatable device performance. Images of the scattered light were collected through a 50× microscope objective (Nikon), an optional rotating polarizer, and detected by a thermoelectrically cooled charge-coupled device (CCD) camera (Pixis 1024). All measurements were performed at room temperature. Electromagnetic Simulations. Time-averaged Poynting vector map was calculated with full-field 2D finite-difference time-domain algorithm for the Ag-coated GaAs NW geometry excited by a horizontally oriented electric dipole placed at the center of the NW. Power flow analysis30 of the fraction of emitted power that couples to SPPs is estimated by counting the fraction of power flux lines which are guided along the Ag/ SiO2 interface. The exact coupling efficiency could be computed in full 3D simulations taking into account the ensemble dipole emission distributed around the NW. The mode indices and propagation lengths for the Ag strip on SiO2 are solved using COMSOL Multiphysics finite elements electromagnetic simulator.
Figure 3. Signal propagation along plasmonic waveguides with varying widths. (a) Spatial distribution of the scattered light intensity near the ends of the waveguides when the NW SPED is turned on. The EL image collected by a CCD camera is overlaid on top of the SEM images of strip waveguides with different widths: 700, 500, 400, and 300 nm. Scale bar is 1 μm. (b) Electromagnetic mode solver reveals mode profiles of the propagating SPPs supported by the plasmonic strip waveguides shown in panel a. The distribution of the magnetic field amplitude is plotted.
As2, and increasing the V/III ratio to 150. After the deposition of a thin intrinsic layer of about 30 nm, the n-shell is deposited using silicon as a dopant with a concentration of 5 × 1018 at/ cm3. The result is a coreshell GaAs nanowire with radial p-i-n junction.
Figure 4. GaAs NW SPED coupled to plasmonic bend and splitter elements. (a) SEM image of the fabricated structure with a 90° bend and a Ysplitter as collected by a CCD camera under white light illumination. The false colors highlight the key components: yellow is for the electrical contacts, cyan is for the GaAs nanowire, and green identifies the Ag strip waveguide bend and Y-splitter. Scale bar is 2 μm. The dashed boxes indicate the areas shown (with matching boundary colors) in (b,c) that plot the spatial distribution of scattered light intensity when NW LED is turned on. The waveguide outlines are overlapped with the maps of the scattered light intensity to help identify the relative position of the scattered light. The maps in (b) show the scattered light near the 90° bend (red boxes) for two different orientations of the transmission axis of the polarizer and (c) shows the same for the Y-shaped splitter (blue boxes). Scale bars are 1 μm and the transmission axis of the polarizer is indicated by the red arrows in each image. 4946
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ASSOCIATED CONTENT
S Supporting Information *
Additonal information and figures. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions ∥
These authors contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge support from a Multidisciplinary University Research Initiative Grant (Air Force Office of Scientific Research, FA9550-10-1-0264) and the Interconnect Focus Center, one of six research centers funded under the Focus Center Research Program (FCRP), a Semiconductor Research Corporation entity. C.C. thanks the SNF program for Prospective Researchers No. 134484 . A.F.i.M. and C.C. thank funding of ERC through Grant UpCon and SNF funding through Grants 121758/1 and 129775/1/and the NCCR QSIT. The NW growth was supported by the Danish National Advanced Technology Foundation through project 022-2009-1.
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REFERENCES
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