Letter pubs.acs.org/NanoLett
An Order of Magnitude Increase in the Quantum Efficiency of (Al)GaAs Nanowires Using Hybrid Photonic−Plasmonic Modes Sudha Mokkapati,*,† Dhruv Saxena,*,† Nian Jiang,† Li Li,‡ Hark Hoe Tan,† and Chennupati Jagadish† †
Department of Electronic Materials Engineering and ‡Australian National Fabrication Facility, Research School of Physics and Engineering, The Australian National University, Canberra, A. C. T 0200, Australia S Supporting Information *
ABSTRACT: We demonstrate 900% relative enhancement in the quantum efficiency (QE) of surface passivated GaAs nanowires by coupling them to resonant nanocavities that support hybrid photonic−plasmonic modes. This nonconventional approach to increase the QE of GaAs nanowires results in QE enhancement over the entire nanowire volume and is not limited to the near-field of the plasmonic structure. Our cavity design enables spatially and spectrally tunable resonant modes and efficient in- and out-coupling of light from the nanowires. Futhermore, this approach is not fabrication intensive; it is scalable and can be adapted to enhance the QE of a wide range of low QE semiconductor nanostructures. KEYWORDS: GaAs, nanowire, quantum efficiency, surface passivation, radiative redombination rate, Purcell factor
H
effective mode volume is also reduced,8 which results in high Purcell factors. Purcell enhancement demonstrations in various semiconductors have so far been based on electromagnetic field concentrations supported in the near field (on the order of 10 nm) of metallic structures. Near field effects require the emitter to be in close proximity to the plasmonic structure and therefore limits experimental demonstrations to organic/dye molecules deposited on plasmonic structures or thin semiconductor layers close to the surface.10−15,20−23 Increasing the QE of III−V semiconductor nanowires is essential for optoelectronic device applications and requires novel cavity configurations that would enhance electromagnetic field intensities in the entire nanowire, not just limited to the near field of the metal. Additionally, these structures should be easy to fabricate, enable efficient light coupling and extraction, and not significantly increase the nanowire footprint. In the context of III−V semiconductor nanowires, applications of the most widely studied and used material system, (Al)GaAs, have been hindered until surface passivation was used to improve its QE.3,4,24 With surface passivation, γnr
igh quantum efficiency (QE) semiconductor nanowires are promising for downscaling the physical dimensions of optoelectronic devices.1,2 The QE of a semiconductor depends on the radiative and nonradiative recombination rates (γr and γnr) via the relationship QE = (γr/(γr + γnr)). Achieving high QE in nanowires is challenging because of the large surface area to volume ratio that increases the probability of nonradiative recombination via the surface states. The most widely used approaches for increasing QE are to optimize material quality and surface passivation.3−5 These approaches increase QE by reducing γnr. An alternative approach to increasing QE is to increase γr. The recombination rate of an emitter is proportional to the local density of optical states and can be enhanced by coupling the emitter to a resonant cavity.6 The Purcell factor, which determines the recombination rate enhancement, is proportional to Q/V, where Q is the cavity quality factor, and V is the effective volume of the cavity mode. The Purcell effect has been demonstrated in high Q dielectric cavities7−9 by tuning the cavity resonance to match the emission spectrum of the emitter. However, high Q dielectric cavities have large footprints. Recently, plasmonic cavities have been studied to increase the γr of various emitters.10−15 This phenomenon has also been observed in metal cavity based nanolasers.16−19 Plasmonic cavities in general have low Q factors, but the © 2014 American Chemical Society
Received: September 17, 2014 Revised: December 13, 2014 Published: December 29, 2014 307
DOI: 10.1021/nl503593w Nano Lett. 2015, 15, 307−312
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Nano Letters
velocity calculated taking into account the nanowire diameter is the lowest reported value to date for GaAs nanowires.25 The QE of the nanowires can be estimated using the measured τmc, and the value calculated is 1−2% at an estimated carrier density of ∼1016 cm−3 (see Supporting Information). The QE of the nanowires can also be quantified from the variation of the integrated PL emission as a function of the excitation power density (see Supporting Information). The variation of the integrated PL emission from the nanowire with excitation power density and the fit to the data are shown in the inset of Figure 1, panel c. The QE extracted from the fit is shown in Figure 1, panel c and is 1.5% at an excitation power density of 0.2 kW/cm2. It increases with excitation power density because of reduced radiative recombination lifetime28 and saturates at 7.5% as the radiative recombination lifetime approaches the nonradiative recombination lifetime. We couple the surface passivated GaAs nanowires to “Ω”shaped29 Au cavities (see Supporting Information) to further enhance their optical properties. Figure 2, panel a shows the schematic of a core−shell-cap nanowire coupled to an “Ω” cavity and the experimental configuration used for optical excitation. The Ω-shaped metal nanocavities support resonant modes. The spectral position of the resonant modes depends on the nanowire shape, diameter, the refractive index of the nanowire material, and the thickness of the native oxide around the nanowire (see Supporting Information). The numerically evaluated spectral position of the resonant modes supported is shown in Figure 2, panel b for various nanowire diameters. There are four distinct resonant modes supported in the cavity, denoted as I, II, III, and IV. The dashed lines in the figure show the spectral position of resonant modes supported in a nanowire, had the nanowire been conformally coated with Au in the absence of the substrate. In this configuration, the cavity supports two distinct modes, a dipole mode at longer wavelengths and a quadrapole mode at shorter wavelengths.30 In the Ω configuration shown in Figure 2, panel a, the degeneracy of the resonant modes is lifted because of the asymmetry in the cavity design. We attribute modes I and II to the dipole resonances and modes III and IV to the quadrapole resonances based on the field intensity profiles across the cavity cross-section (see Supporting Information). The field intensity profiles of the four resonant cavity modes supported for a nanowire with a diameter of 120 nm are shown in Figure 2, panels c−f. The resonant mode IV, at 687 nm, is weakly confined in the cavity and has minimal overlap with the nanowire volume. Resonant modes I, II, and III are well confined in the cavity. These modes result in enhanced field intensities in the bulk of the nanowire and are thus expected to result in recombination rate enhancements not limited to the near field of the metal. The spectral positions of modes II and III match with the band-edge emission from the GaAs core and the AlGaAs shell of the core−shell-cap nanowires, respectively, for nanowire diameters between 110−130 nm. This range corresponds to the diameter of our core−shell-cap nanowires. Strong-field concentration in the thin oxide layer around the nanowire is observed for all resonant modes as shown in Figure 2, panels c−f. The electromagnetic flux density (magnitude of Poynting vector) across the nanowire/nanocavity cross-section is shown in Figure 3 for cavity resonance mode I. The line plots at the top and on the right-hand side of Figure 3 show the variation of the magnitude of the Poynting vector at y = 10 nm and x = 10 nm, respectively. The magnitude of the Poynting vector is maximum in the oxide layer formed between the
can be reduced by over an order of magnitude, which increases the QE of GaAs nanowires.25−27 In this work, we further enhance the QE of these surface passivated GaAs nanowires by increasing γr by coupling them to “Ω”-shaped resonant Au nanocavities. Our approach is not fabrication intensive, is scalable, results in QE enhancement over the entire nanowire volume, and is not limited to the near field of the plasmonic structure unlike all other demonstrations to date.10−12,20−23 Our cavity design also enables spatially and spectrally tunable resonant modes and efficient in- and out-coupling of light from the nanowires. We grew core−shell-cap GaAs/AlGaAs/GaAs nanowires using Au-catalyzed vapor−liquid−solid (VLS) growth by metal−organic chemical vapor deposition (MOCVD) (see Supporting Information). The nanowires were grown using 50 nm diameter Au catalyst nanoparticles, which resulted in a core diameter of 50 ± 5 nm. The AlGaAs shell passivates the GaAs core, and the GaAs cap prevents oxidation of the Al containing shell. The nanowires have a hexagonal cross-section, are free of twin defects, and have an overall diameter that varies between 111 ± 7 nm at the top to 126 ± 6 nm at the base, as shown in Figure 1, panel a. Figure 1, panel b shows a typical room-
Figure 1. (a) Transmission electron microscope (TEM) image of a whole nanowire showing variation in diameter along the nanowire. The scale bar is 1 μm. Cross-section high angle annular dark field (HAADF) scanning transmission electron microscope (STEM) images taken at two different positions along the nanowire are shown. The scale bar for the cross-section images is 50 nm. (b) Typical room-temperature PL spectrum from core−shell-cap nanowires showing band-edge emission from GaAs. Inset shows the timeresolved photon counts at the PL peak emission wavelength. A monoexponential fit to the experimental data gives a minority carrier lifetime of 1.5 ns. (c) QE of the GaAs core as a function of excitation power density. These data are extracted from the variation of the integrated PL emission from the nanowire with excitation power density, as shown in the inset.
temperature photoluminescence (PL) spectrum from a core− shell-cap nanowire. There is a single peak in the PL spectrum, which corresponds to the band-edge emission from GaAs at 870 nm. The inset in Figure 1, panel b shows a typical timeresolved PL decay at the peak of the PL spectrum. A monoexponential fit to the PL decay data gives a minority carrier lifetime (τmc) of 1.5 ns. The surface recombination 308
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Figure 2. (a) Schematic diagram of the Ω-shaped Au cavity formed around the core−shell-cap nanowire. A thin native oxide layer is present between the nanowire and the Au cavity (see Supporting Information). The nanowire is optically excited from the substrate side, and the emitted light is also collected from the substrate side. (b) Spectral position of the resonant modes supported by the cavity for varying nanowire diameters. The cavity supports four resonant modes, denoted as I, II, III, and IV. Modes I and II are dipole modes, and modes III and IV are quadrapole modes (see Supporting Information). The dashed lines show the spectral position of cavity modes supported in a nanowire conformally coated with Au, without substrate. The resonant modes for the Ω cavity split and shift in position because of the asymmetry introduced by the substrate. (c−f) Spatial mode profiles of the cavity resonances supported in a 120 nm diameter nanowire in the Ω cavity. The quadrapole modes are shown in panels c and d, and the dipole modes are shown in panels e and f. The fields are highly concentrated in the native oxide layer formed around the nanowire. The scale bar in the images is 50 nm. Panels b−f are calculated using finite-difference time-domain (FDTD) simulations.
the plasmonic nature of the resonant modes. Resonant mode I has a quality factor of ∼50 and mode volume of ∼λ3/103. The quality factor of the mode is smaller by approximately an order of magnitude than the typical value for nanowire photonic modes,4 and the mode volume is higher than that expected for purely plasmonic modes. Since the resonant modes lack the qualities of purely photonic/plasmonic modes but possess both plasmonic (electromagnetic field concentration in low index region) and photonic nature (diffraction limited mode volume), we refer to these modes as hybrid plasmonic−photonic modes. The spectral maps of luminescence from a bare nanowire and a nanowire enclosed in an Ω-shaped cavity are shown in Figure 4, panels a and b for excitation fluence ranging from 15−90 μJ/ cm2/pulse. Emissions from both nanowires show the band-edge emission from GaAs. The spectral maps are plotted on the same color scale; the emission from the nanowire enclosed in a cavity is three times stronger than that from a bare nanowire at any given excitation fluence. However, the photogenerated carrier density in a bare nanowire and a nanowire coupled to the Ω cavity may be different for the same excitation fluence due to differences in absorption cross-section, Qabs. Likewise, the probability of detecting the emitted photons (detection efficiency, ηdet) is different for the two geometries. We use FDTD simulations to calculate the absorption cross-section and detection efficiency for a bare nanowire and a nanowire coupled to the cavity. The calculated variation in absorption crosssection and detection efficiency as a function of wavelength is shown in the Supporting Information. The PL data corrected for differences in both excitation and detection efficiencies at an excitation fluence of 2 μJ/cm2/pulse are shown in Figure 4, panel c. The corrected data from the nanowire enclosed in the cavity show two peaks, the long wavelength peak corresponding
Figure 3. Variation of electromagnetic flux density (magnitude of Poynting vector) across the nanowire/nanocavity cross-section. The nanowire has a diameter of 120 nm. The solid white lines show the native oxide layer formed between the hexagonal nanowire and the Au layer. The dashed white line separates the substrate from the nanowire. The line plots on the top and on the right are the profile plots for y = 10 nm and x = 10 nm, respectively.
nanowire and the Au layer. Purely photonic modes result in electromagnetic field intensity/flux density concentrations in high refractive index regions. Strong electromagnetic field intensity/flux density concentration in the low refractive index oxide layer around the high refractive index nanowire indicates 309
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Figure 4. Room-temperature PL maps for (a) a bare core−shell-cap nanowire and (b) a core−shell-cap nanowire enclosed in an Ω cavity. The color bar represents the experimentally measured photon counts; both data sets are plotted on the same color scale. (c) PL data for a bare nanowire and a nanowire enclosed in an Ω cavity, at an excitation fluence of 2 μJ/cm2/pulse, corrected for numerically evaluated absorption and extraction efficiency. (d) Time-resolved PL data for emission at 870 nm from a bare nanowire and a nanowire enclosed in an Ω cavity at an excitation fluence of 2 μJ/cm2/pulse. The minority carrier lifetime (τmc) extracted from the fits to the experimental data are 1.5 and 0.5 ns, respectively, for a bare nanowire and a nanowire enclosed in an Ω cavity. (e) Top panel, radiative recombination rate (γr) enhancement; bottom panel, QE enhancement (QEenh) in a nanowire coupled to an Ω cavity at an excitation fluence of 2 μJ/cm2/pulse. QEenh and (γr)enh for GaAs emission are calculated from panels c and d. The AlGaAs emission peak in panel c indicates that there is enhancement in QE and γr in the AlGaAs shell, as shown in the gray region, but the enhancements cannot be quantified since there is no AlGaAs emission from the bare nanowire. (f) Average QE enhancement in a nanowire coupled to an Ω cavity as a function of excitation fluence. Inset shows the integrated PL counts for a bare nanowire and a nanowire coupled to an Ω cavity used for calculating QE enhancement.
shown in Figure 4c) and the total recombination rate enhancement, (γr)enh = (PL)enh_corrected × (γtot)enh. The calculated (γr)enh is shown in the top of Figure 4, panel e. The radiative recombination rate is enhanced by a factor of 30 at the GaAs peak emission wavelength. The increase in radiative recombination rate results in QE enhancement, QEenh, in the nanowire. The QEenh is calculated using QEenh = ((γr)enh)/ ((γtot)enh), which is essentially equal to the ratio of PL intensities from the nanowire coupled to the cavity and from a bare nanowire, corrected for differences in absorption and detection efficiencies. The QEenh data are shown in the bottom of Figure 4, panel e, and it increases by a factor of 10 at the GaAs emission peak. Taking into account the QE at low excitation in the bare nanowire, the QE at the GaAs emission peak is 10−20% in the nanowire coupled to Ω cavity. This is the highest reported QE for GaAs nanowires to date and is an order of magnitude larger than the previous reported values.25−27 The relative QE enhancement, defined as ΔQE/ QEi, where ΔQE is the change in QE, and QEi is the initial QE, is 900%. The AlGaAs emission peak in Figure 4, panel c indicates that there is enhancement in QE and γr in the AlGaAs shell, as shown in the gray region in Figure 4, panel e. However, the enhancement cannot be quantified using our approach since AlGaAs emission could not be observed from the bare nanowire. The average QEenh over the wavelength range shown in Figure 4, panels a and b is shown in Figure 4, panel f as a function of excitation fluence. The inset shows the integrated PL emission from a bare nanowire and a nanowire enclosed in
to the band-edge emission from the GaAs core and the short wavelength peak corresponding to the emission from the AlGaAs shell of the nanowire. The observation of AlGaAs emission from core−shell-cap nanowire coupled to the Ω cavity is due to an increased radiative recombination rate (γr) in the AlGaAs shell.31 The GaAs peak emission from the nanowire enclosed in the Ω cavity is ∼10 times more intense than that from a bare nanowire. Since this data is corrected for differences in excitation and detection efficiencies, the increase in PL intensity can unambiguosly be attributed to the increase in the radiative recombination rate (γr) in the nanowire coupled to an Ω-shaped cavity. The radiative recombination rate enhancement (γr)enh in the core−shell-cap nanowires coupled to the Ω cavity can be estimated from the PL enhancement data and changes in minority carrier lifetime, τmc (see Supporting Information for detailed analysis). τmc measured at the GaAs peak emission wavelength is reduced from 1.5 ns in a bare nanowire to 500 ps in a nanowire coupled to Ω-shaped cavity, as shown in Figure 4, panel d. The total recombination rate enhancement, (γtot)enh defined as the ratio of the total recombination rate in the nanowire coupled to the cavity to the total recombination rate in the bare nanowire, is estimated using (γtot)enh = ((τmc)ref)/ ((τmc)cav), where (τmc)ref is the minority carrier lifetime in a bare nanowire, and (τmc)cav is the minority carrier lifetime in the nanowire coupled to the Ω cavity. The (γr)enh is then estimated as the product of PL intensity enhancement corrected for differences in the absorption and detection efficiencies (data 310
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Nano Letters Ω-shaped cavity, corrected for differences in excitation and detection efficiencies, used for calculating QEenh. The average QEenh initially increases with increased excitation fluence. We believe this is a consequence of variations in nanowire diameter across its length and the sensitivity of spectral position of the resonant modes to the nanowire diameter (see Supporting Information). A factor of 10 QEenh is observed over a broad excitation range. The two peaks in the (γr)enh and QEenh data shown in Figure 4, panel e correspond to the cavity resonance positions for an average nanowire diameter of 120 nm. The calculated spatial maps of the recombination rate enhancement, γenh, in the core− shell-cap nanowires due to the resonant dipole and quadrapole cavity modes are shown in Figure 5, panels a and b,
to nanocavities involves minimal postgrowth processing and is scalable. More importantly, our cavity design enables QE enhancement in the entire volume of the nanowires, is spatially and spectrally tunable, and facilitates efficient coupling of light into and out of the nanowires. This one-step postgrowth process of coupling nanowires to resonant nanocavities can be used to enhance the QE of a wide range of semiconductor nanostructures, not limited to III−Vs, for development of efficient optoelectronic devices. This technique is especially promising for intrinsically low QE semiconductors, such as metal dichalcogenides, being currently developed for optoelectronic applications.
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ASSOCIATED CONTENT
S Supporting Information *
Additional information on nanowire growth, cavity fabrication, structural and optical characterization of nanowires/cavities, numerical analyses to identify the nature of cavity modes, and estimates of the Purcell factor. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Author Contributions
S.M. conceived the experiments. S.M. and D.S. did the modeling and analysis. N.J. grew the nanowires. L.L. and N.J. prepared the cross-section TEM samples, and N.J. imaged the TEM samples. S.M. and D.S. carried out the experiments and wrote the manuscript. H.T. and C.J. supervised the work. All authors contributed to the manuscript. S.M. and D.S. contributed equally to this work. Notes
Figure 5. Spatial map of recombination rate enhancement factor for the nanowire enclosed in the Ω cavity. The data is calculated for 120 nm diameter nanowire at the cavity resonance of (a) 870 nm and (b) 755 nm. The color scale shows the magnitude of the recombination rate enhancement factor with respect to a bare nanowire. (c, d) Spatial map of recombination rate enhancement factor (shown in panels a and b) superimposed on the cross-sectional HAADF-STEM image of the core−shell-cap nanowire enclosed in the Ω cavity. The images in panels a and b are rotated in panels c and d to match the orientation of the HAADF-STEM images. The scale bars are 20 nm.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge the Australian Research Council (ARC) for financial support, the Australian National Fabrication Facility (ANFF) for access to facilities, and the National Computational Infrastructure (NCI) for providing access to computational resources. The authors acknowledge professor Leigh Smith for the insightful discussions and Dr. Kaushal Vora for the technical support.
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respectively, and are superimposed on the cross-sectional HAADF-STEM image of the core−shell-cap nanowire enclosed in the Ω cavity in Figure 5, panels c and d. Both cavity modes result in large recombination rate enhancements not limited to the near-field of the metal-dielectric boundary. The dipole and quadrapole mode result in strong recombination rate enhancement, spatially and spectrally overlapping with the GaAs layers and the AlGaAs shell, respectively, in the nanowire. This helps us to unambiguously attribute the QE enhancement at the GaAs emission peak to the dipole and the AlGaAs emission peak to the quadrapole resonant modes supported by the Ω cavity. The multiple resonant modes supported in the Ω cavity offer the possibility of enhancing the QE of multiple bandgap emitters incorporated into a single nanowire. In conclusion, we have demonstrated substantial increase in the QE of GaAs nanowires beyond what has been achieved using growth optimization and surface passivation. Our approach to increase the QE of nanowires by coupling them
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