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Tuning Electroluminescence from a Plasmonic Cavity-Coupled Silicon Light Source Sebastian Glassner, Hamid Keshmiri, David J. Hill, James F Cahoon, Bruno Fernandez, Martien Ilse Den Hertog, and Alois Lugstein Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b03391 • Publication Date (Web): 10 Oct 2018 Downloaded from http://pubs.acs.org on October 11, 2018

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Nano Letters

Tuning Electroluminescence from a Plasmonic Cavity-Coupled Silicon Light Source

S. Glassner 1, H. Keshmiri 1,2, D.J. Hill 3, J.F. Cahoon3, B. Fernandez4, M. I. den Hertog 4,

A. Lugstein 1,*

1

Institute of Solid State Electronics, TU Wien, Gußhausstraße 25, 1040 Vienna, Austria

2

Vienna Biocenter Core Facilities GmbH, Dr. Bohr-Gasse 3, 1030 Vienna, Austria

3

Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-

3290, United States 4

Institut NEEL CNRS/UGA UPR2940, 25 avenue des Martyrs, 38042 Grenoble, France

1 ACS Paragon Plus Environment

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The combination of Moore's law and Dennard's scaling rules have constituted the fundamental guidelines for the silicon-based semiconductor industry for decades. Furthermore, the enormous growth of global data volume has pushed the demand for complex and densely packed devices. In recent years, it has become clear that wired interconnects impose increasingly severe speed and power limitations onto integrated circuits as scaling slows towards a halt. To overcome these limitations, there is a clear need for optical data processing. Despite significant progress in the development of silicon photonics, light sources remain challenging owing to the indirect bandgap of group IV materials. It is therefore highly desirable to develop new concepts for a silicon light source that meets efficiency and footprint requirements similar to their electronic counterparts. Here, we demonstrate an electrically-driven and tunable silicon light source by matching the resonant modes of a silver nanocavity with the hot luminescence spectrum of an avalanching p-n junction. The cavity significantly enhances phonon-assisted recombination of hot carriers by tailoring the local density of states at the sizetunable resonance. Such tunable nanoscale emitter may be of great interest for short-reach communications, micro-displays or lab-on-chip applications. Keywords:

Silicon

nanowire,

plasmonic

cavity,

electroluminescence,

Purcell

enhancement. For decades the search for an efficient silicon-based light source to be integrated with microelectronic devices has been described as looking for the holy grail of the IC technology.1,2 Due to the upcoming limitations in device speed, performance and power constraints of CMOS technology, there have been extensive studies on silicon-based photodetectors3, modulators4, and optical waveguides5 to enable on-chip ultrafast optical data processing. However, owing to its indirect bandgap, silicon is hardly ever considered as a candidate for light emitting devices. The large momentum mismatch between electrons and holes prevents their direct radiative recombination as the excited carriers are prone to relaxation and may radiate only with the assistance of phonons. To increase light emission efficiency in silicon, several approaches have been proposed including quantum confined nanostructures6 like silicon nanocrystals7,8, silicon quantum dots9 or 2 ACS Paragon Plus Environment

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rare earth doped silicon structures10,11. Lasing action in group IV semiconductors12 has been demonstrated by alloying germanium with tin13 or germanium quantum dots14 on silicon or by exploiting stimulated Raman scattering15,16. Electrically driven silicon-based light sources have previously been demonstrated from boron doped silicon17, amorphous silicon films18, porous silicon19,20 or erbium-doped superlattices21. In a common diode configuration, that is, a conventional forward biased p-n junction, the rate of radiative electron-hole recombination in silicon is extremely low. In contrast, in 1955, Newman reported electroluminescence (EL) from reverse-biased silicon diodes under high electric fields22 and more recently ultraviolet and visible light emission has been observed from hot carrier injection by Zener tunneling23. The main photon generation mechanisms of visible avalanche EL are intraband transitions of hot carriers and phonon-assisted interband recombination.24 However, as hot carriers in silicon thermalize on a ps timescale8 while the radiative lifetime is in the 10 ns regime25, the internal quantum efficiency for hot luminescence is rather low (~1×10-4)26. Thus, for efficient light emission via recombination of non-thermalized carriers across the -point, the radiative lifetime should be comparable with the hot carriers’ relaxation time. Recently, Cho et al. have shown that highly concentrated electromagnetic fields inside plasmonic nanocavities promote phonon-assisted, radiative recombination from hot carriers before their thermalization.27 According to Fermi’s golden rule, the rate of spontaneous emission is proportional to the local density of states (LDOS).28 The enhancement of the decay rate of an excited dipolar emitter resonantly coupled to a nanocavity is thus determined by the cavity properties and can be expressed independently of the emitter properties by the well-known Purcell factor29 3

𝜆 3 𝑄 , 𝑉 𝑚) 𝑛

()(

𝐹𝑃 = 4𝜋2

(1)

with n the refractive index of the material, λ the emission wavelength, Q the quality factor and Vm the effective coupling modal volume. Hence, efficient light-matter interaction is achieved either due to a high quality factor or low modal volume, that is, either for a narrow resonance, or a deeply confined mode. Over the last decade so called sub-wavelength plasmonic cavities have been explored enabling the squeezing of optical fields into a volume as small as 10-3 (λ3).30 Hence, even if the Q factors of plasmonic cavities are 3 ACS Paragon Plus Environment

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generally low, the extremely small effective mode volumes of nanoplasmonic cavities facilitate a significant reduction of the radiative lifetime accompanied by a huge enhancement of the spontaneous emission rate of the emitter.31 Recently, quasi-1D nanowires, enabling ultra-compact device architectures have appeared as promising structures for optoelectronic devices scaled beyond the diffraction limit of visible light.26,27,30,32–34 Thus, the next logical step to realize CMOS-compatible, monolithically integrated light sources is to electrically interface plasmon cavity-coupled silicon nanowires, exploiting the extreme confinement to enhance the range of lightmatter interactions. Here, we demonstrate the plasmonic enhancement and tunability of visible EL from non-thermalized carrier recombination in reverse-biased silicon nanowire p-n junctions at room temperature. To enable optical and electrical characterization of plasmon augmented hot carrier EL, vapor-liquid-solid grown silicon nanowires with axial p-n junctions and diameters ranging from 60 to 80 nm were deposited on a widely transparent 150 nm thick Si3N4 membrane. The surfaces of such self-assembled nanowires are typically superior to those of top-down fabricated nanowires with respect to non-radiative recombination via surface states, which is crucial for quasi 1D structures owing to the large surface-to-volume ratio35. After electrical contact formation to individual p-n junctions, the nanowires were passivated with 8 nm of Al2O3 via atomic layer deposition. Thus, the Al2O3 interlayer was thick enough to reduce non-radiative surface exciton recombination, but still thin enough to maintain strong emitter-to-cavity coupling. The Ag cavity, enabling the plasmonic enhancement was patterned directly atop of the nanowire p-n junction (for nanowire growth and device processing details see Methods). Figure 1a shows a schematic illustration of a fully featured plasmonic electroluminescent (PEL) device with the Ag nanocavity enveloping the p-n junction. The scanning electron microscopy (SEM) image in Figure 1b depicts the Ag cavity in between two Ni contacts biasing the p-n junction and a third contact intended for reference measurements.


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Figure 1. Plasmonic electroluminescent device. (a) Schematic illustration of the PEL device architecture seen from the backside, with an Al2O3 coated silicon nanowire on a 150 nm thick transparent Si3N4 membrane and the plasmonic Ag nanocavity enveloping the axial p-n junction. (b) SEM image of a PEL device with a 65 nm thick and 12 µm long silicon nanowire. The two highlighted Ni contacts are used for reverse biasing of the axial p-n diode located directly below the 1.2 µm long Ag cavity. The scale bar is 2 µm. (c) Simulated spectral position of the resonant modes supported by the Ag nanocavity as a function of the nanowire diameter. The simulated resonance at 604 nm of a nanowire with a diameter of 65 nm is indicated along with the resonant plasmonic mode accompanied by two photonic modes at shorter wavelengths. (d) and (e) TM and TE modes in a cross-sectional view for a nanowire with D = 65 nm at the simulated resonance wavelength of 604 nm.

To study the corresponding electromagnetic field distribution and tunability of the cavity resonance frequencies, numerical simulations based on the finite-difference timedomain (FDTD) method were performed for the fully scaled PEL devices with various nanowire diameters. As discussed in the following, the spectral position of the plasmon-


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coupled nanocavity modes was simulated as a function of the optical properties, shape and diameter of the nanowire and its dielectric cladding. Figure 1c shows distinct modes supported by the nanocavity for nanowires with diameters varied from 50 to 100 nm. Based on the field intensity distribution of the nanocavity cross-section, the resonant plasmonic dipole mode dominates in the 550-750 nm wavelength window, with two associated photonic modes being weakly confined at shorter wavelengths. The magnetic field intensity distributions at both coupled transversal electric (TE) and transversal magnetic (TM) polarizations are shown for a nanowire with a diameter of 65 nm in Figure 1d and e respectively, signifying the strongly coupled modes in the TM-polarized case. As a result, the plasmonic nanocavity modes, can be coupled to highly emissive hot carrier states of the avalanching p-n junction. Therefore, a change in the diameter of the nanowire shifts the Eigenfrequency of the structured nanocavity and thus the spectral position of the resonant emission. As shown in Figure 1d, the dipole mode at the simulated resonant wavelength of 604 nm is tightly confined in the nanocavity, resulting in enhanced field intensity and recombination rate in the system. The field, however, is more concentrated in the low-index, thin, conformal layer of aluminum oxide around the nanowire due to increased field intensity (electromagnetic flux density as the magnitude of Poynting vector) in the structure. The resonant plasmonic mode of the simulated Purcell factor demonstrates a lower quality factor than that of the higher energetic photonic mode, and manifests as a hybrid plasmonic-photonic mode30. The plasmonic mode has a quality factor of 9.13 at 608 nm for a diameter of 65 nm, and is accompanied by a photonic mode at 428 nm with a quality factor of 12.95. Figure 1d demonstrates the hybrid nature of such mode, where both the confined plasmonic mode in the dielectric as well as the concentrated photonic mode in the high-refractive nanowire can be observed. The simulated Purcell factor of the cavitycoupled device configuration (D = 65 nm) indicates an enhancement by a factor of 5.6 at the 608 nm resonant wavelength compared to the pure photonic mode excited in the bare nanowire. This translates into an enhanced recombination rate in the structure that is dictated by the nanowire dimensions (Figure 1c). All nanowires have hybrid plasmonicphotonic modes. However, the emergence of multiple photonic modes in larger nanowires 6 ACS Paragon Plus Environment

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(D > 85 nm) reduces the intensity of the plasmonic mode. The enhancement rate decreases by about 50% of its magnitude for a nanowire with a diameter of 90 nm with a second photonic mode emerging at the lower scale boundary. The modal field distribution profiles of the supported modes are presented in Supplementary Figure S1. To prove the effectiveness of the Ag plasmon nanocavity experimentally, both µphotoluminescence (PL) and EL measurements were carried out on individual devices before and after plasmon nanocavity formation. The particular device architecture on the transparent Si3N4 membrane enables PL characterization by optical excitation with a green laser ( = 532 nm) from the backside as well as spectroscopy of the EL by electrically biasing the device. Figure 2a compares the EL spectra of the very same, reverse-biased Si p-n junction nanowire device with a diameter of 65 nm before and after the formation of the Ag nanocavity. The bare p-n junction nanowire device shows moderate EL covering the visible spectrum and extending toward the near infrared spectral region with two distinct peaks in the visible at 442 and 657 nm. In contrast, the EL of the plasmon cavity-coupled device is markedly enhanced with the main peak at 618 nm and smaller side peaks at 586 nm and 649 nm. The drive current was 10 µA, corresponding to a reverse voltage of 4.95 V for the EL device, which is slightly increased to 5.07 V for the PEL device due to a gating effect of the floating Ag cavity. Figure S2 compares the EL characteristics for three PEL devices with diameters of 60, 65 and 80 nm. The I-V characteristics of the corresponding PEL device are depicted in Supplementary Figure S3. In the reverse bias region, breakdown occurs at a critical reverse voltage of about 4.6 V, causing an avalanche-like increase in current. At this point the carriers undergo collisions with bound carriers, generating additional electron-hole pairs before being accelerated again. Thereby, the maximum energy individual carriers can gain, is limited by the ionization threshold of 2.3 eV36, equal for electrons and holes37. This limitation also confines the EL due to intraband transitions to 2.3 eV emphasizing the major contribution to the overall emission by phonon-assisted interband recombination. Moreover, no indications of direct interband recombination were observed at the high energy side of the spectrum. 7 ACS Paragon Plus Environment

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Figure 2. EL and PL spectra. (a) EL spectra of the bare reverse-biased p-n junction nanowire with a diameter of D = 65 nm (red) compared to the enhanced EL of the same nanowire coupled with a plasmonic cavity (black), both at a drive current of 10 µA. The peak Purcell enhancement of the device is 500% at a wavelength of 618 nm. The inset shows the polarization-resolved EL spectra for the PEL device detected parallel (blue) and perpendicular (green) to the nanowire axis. The region around 532 nm, which manifests as a notch originating from the measurement setup, is not shown for clarity (see Methods) (b) PL spectra of the bare (dashed) and PEL (solid) device when excited parallel (red) and perpendicular (black) to the nanowire axis. The peaks at 573 nm and 581 nm, especially pronounced for perpendicular excitation (black, solid line), reflect the hot luminescence bands of the cavity. Spectral features at 547 nm and 560 nm correspond to the Raman signal of Si.

Compared to the bare p-n junction nanowire, the emission intensity of the PEL device (i.e. the same nanowire after Ag nanocavity formation), is markedly increased due to the Purcell enhancement. For recombining electron-hole pairs in the depletion region 8 ACS Paragon Plus Environment

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close to the cavity of the PEL device, the LDOS will have a maximum at the plasmon resonance wavelength, providing a new, strong decay channel for the hot electrons. The overall signal enhancement and easily identifiable resonance peaks show unambiguous plasmonic enhancement of the phonon-assisted hot carrier recombination in the PEL device. Thus, taking advantage of the nano-localized electromagnetic field of hybrid plasmonic modes, the experimentally determined Purcell enhancement at the resonance wavelength of 618 nm is 500%. Polarization-sensitive spectral characterization was performed to examine the interaction of phonon and cavity plasmon modes leading to enhanced hot carrier emission in the PEL device. Figure 2b shows the PL spectra for the bare p-n junction silicon nanowire and the same nanowire coupled with the Ag nanocavity when excited with a green laser (532 nm) parallel and perpendicular to the long nanowire axis. For the bare nanowire, the overall shape of the emission spectrum is similar to the EL emission spectrum with little structure but a broad luminescence band in the visible peaking at about 600 nm and extending toward the near infrared spectral region. Such broad luminescence is expected when hot carriers excited by the laser beam emit photons through a phonon-assisted recombination process competing with intraband relaxation.26 The bare nanowire device shows the highest PL counts with the exciting laser polarized parallel to the nanowire axis. The absorption as well as the emission properties of nanostructures can be altered through their shape. At nanowire diameters smaller than the wavelength of light, the polarization anisotropy can be explained in terms of the large dielectric contrast between the bare quasi-1D Si nanowire and its surroundings.38 In good agreement with the work of Cho et al.26 we observed highly enhanced bright visible PL from the same silicon nanowire after the formation of a plasmonic nanocavity. Similar to the bare p-n junction device, the PEL device shows broad hot luminescence, but with much higher counts and distinct bands, attributed to the interplay of cavity plasmons and phonon-assisted hot carrier emission. Therefore, the peaks at 573 nm and 581 nm reflect the hot luminescence bands of the cavity39 and resemble the peak structure of the EL spectra in Figure 2a. In contrast to the bare nanowire device, the PL characteristics of the PEL device show higher counts when excited perpendicular to the long nanowire axis. Overall, the peak emission from the PEL device with the excitation laser light polarized 9 ACS Paragon Plus Environment

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normal or parallel to the nanowire axis is 34 or 4 times more intense than that from the bare nanowire. Surface plasmon polaritons propagating along the metal-to-dielectric interface have a transverse magnetic character,40 which requires the electric field perpendicular to the nanowire axis. In the PL case, this effect seems to overwhelm the effect of the dielectric contrast between the nanowire and its surroundings. In contrast, the inset in Figure 2a shows that light emitted from the electrically-driven PEL device is polarized predominantly parallel to the nanowire axis. The room temperature emission of the PEL device presented in this work proved efficient enough to become observable with the naked eye as shown in the photograph of Figure S4. The µ-EL image map in Figure 3a, recorded at the resonance wavelength and overlaid with the optical image of a PEL device shows clearly that the maximum intensity is emitted from the region covered by the cavity at the location of the reverse biased p-n junction. Figure 3b depicts the EL spectra of the PEL device for various drive currents as well as the reference EL spectrum of the bare nanowire at a drive current of 10 µA. Increasing the drive current through the reverse-biased p-n junction of the PEL device up to 20 µA results in a nearly linear increase of the spectral power. Increasing the current above 20 µA resulted in a sublinear increase of the spectral power, whereas the overall shape of the emission remained widely unchanged. Up to 60 µA, the quasi1D p-n junction prevents current concentration and resulting hot spots, so that the diode remains undamaged by the operation in the breakdown regime. Moreover, the emission appeared to be remarkably stable over a timescale of hours. The reverse breakdown voltage slightly increases for higher currents; hence, the main peak is slightly blue-shifted from 618 nm (at 10 µA) to 615 nm (at 60 µA) as the increased electric field in the nanowire enhances the high-energy photon emission. However, raising the current above 60 µA was accompanied by a large increase of the breakdown voltage in combination with an irreversible red-shift of the emission spectrum. In accordance with Meneghini et al.41, we assume that the obvious degradation of the diode is subject to the generation/propagation of point defects due to the injection of highly accelerated carriers.


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Figure 3. Electroluminescence from PEL device. (a) Image of a PEL device recorded with an optical microscope and a µ-EL map as an overlay. The contours of the silicon nanowire, the current-carrying contacts, and the Ag cavity are highlighted. The scale bar is 2 µm. (b) EL spectra of the PEL device with D = 65 nm for drive currents of 10, 20, 40, 60 and 80 µA and for comparison, the bare nanowire device at the lowest drive current of 10 µA.

For the PEL device, enhanced emission is expected when the plasmonic cavity modes are resonant with efficient emission channels of the phonon-assisted indirect interband recombination of hot carriers. As the modes of a cavity and their resonant wavelengths depend on the geometry of the cavity, the resonance is closely related to the nanowire diameter. Hence, changing the nanowire diameter tunes the Eigenfrequency of the plasmonic nanocavity. Figure 4a shows the EL spectra and the respective simulated field intensities for PEL devices with nanowire diameters of 60, 65 and 80 nm. The distinct resonance peaks observed in the emission spectra shift to longer wavelengths with increasing nanowire diameter and are reproduced by the simulated frequency-dependent electromagnetic field intensity in the cavity. The EL spectra of both p-n junction and PEL devices with diameters of 60, 65 and 80 nm are presented in Supplementary Figure S2. The plasmonic nanocavities proved most effective for diameters ranging from 60 – 85 nm (Figure 1c). The corresponding wavelength range of 550 to 650 nm, is close to the maximum emission of the bare nanowire p-n junctions. Figure 4b depicts the experimentally determined Purcell factors as a function of the wavelength - defined as the ratio ΦPEL/ΦEL, where ΦPEL and ΦEL are the spectral power 11 ACS Paragon Plus Environment

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of the PEL and the bare p-n junction device, respectively - and compares it to the simulated Purcell factors. A small peak at about 430 nm, additionally appearing for the 80 nm thick nanowire is a result of the higher energetic photonic mode for thicker nanowires in agreement with the simulation in Figure 1c. The unambiguous Purcell enhancement of the EL at the resonance frequencies of the respective PEL devices and the red-shift with increasing nanowire diameter (and thus increasing cavity size) proves the effective functionality and tunability of the plasmonic nanocavity.

Figure 4. Diameter dependence of the resonantly enhanced EL Purcell factor. (a) EL (solid) and superimposed simulated field intensity (dashed) of PEL devices with diameters of 60 (red), 65 (black) and 80 nm (blue). (b) Purcell enhancement calculated from the experimental results of the individual devices (solid) compared to the simulated Purcell factor (dashed).

In conclusion, this work demonstrates that light emission from reverse-biased, p-n junction silicon nanowire devices can be enhanced and tuned when the intrinsic phononassisted hot carrier recombination is resonantly coupled with the modes of a plasmonic 12 ACS Paragon Plus Environment

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nanocavity. Therefore, the diameter of the nanowire and the related geometry of the cavity modulates the wavelength of the emitted light. Enwrapping the p-n junction nanowire with an Al2O3-Ag plasmonic cavity constructs an ultrasmall volume with increased LDOS, reducing the time for radiative recombination. Moreover, the hot-carrier population during avalanche breakdown enables the PEL devices to act as a fast spontaneous emitter. In addition to their compatibility with high frequency applications, the PEL devices address the footprint challenge, set by the diffraction limit of light. Combining these properties, the devices may be of interest for short-reach communications operating at ultrafast speeds42 and low energy consumption,43 in microdisplays44 or as sensing element in lab-on-chip applications45,46. The device concept is also suitable to be transferred to other indirect semiconductor materials and may be of general interest for the analytic investigation of hot carrier distribution.47 METHODS Nanowire synthesis Nanowires were grown in a low pressure chemical vapor deposition system. Au colloids (BBI International) as growth promoting catalyst were immobilized on Si/SiO2 wafers with polylysine (Sigma-Aldrich) and cleaned in a UV-ozone cleaner (SAMCO UV-1). Multiple colloid sizes ranging from 60 to 80 nm were chosen to achieve a greater variety in nanowire diameters. The substrate was then inserted into a 1 in. hot-wall tube furnace (Lindberg Blue M) and heated to the growth temperature of 510°C. Silane (SiH4, Voltaix), diborane (B2H6, 1000 ppm in H2, Voltaix), and phosphine (PH3, 1000 ppm in H2, Voltaix) served as the precursors for the NWs. HCl (Matheson TriGas, 5 N) was used for chlorination, and H2 (Matheson TriGas, 5N semiconductor grade) was used as the carrier gas. NWs were grown at 510 °C and a total pressure of 20 Torr, with 2 sccm SiH4, 4.2 sccm HCl, and 180 sccm H2, with a SiH4 partial pressure of ∼200 mTorr. In p-type segments, the B2H6 flow rate was 15 sccm while n-type segments were grown with 20 sccm PH3. All gas flow rates were controlled and measured using model P4B mass-flow controllers from MKS Instruments. The typical length of the 112 oriented nanowires was 12 µm consisting of alternating p- and n- segments with lengths of 2 µm each.


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Device fabrication Arrays of homemade 150 nm thick Si3N4 membranes with a window size of 200 μm were fabricated starting from a 450 μm thick silicon (100) wafer with a 200 nm thick layer of thermal SiO2 followed by a 150 nm thick layer of stoichiometric Si3N4 on each side deposited by low-pressure chemical vapor deposition (LioniX BV, The Netherlands). Then, windows and lines were opened on one side in the Si3N4 and SiO2 layers by UV lithography and a sequential reactive ion etching step was performed to locally remove the Si3N4 and SiO2 layers. Subsequently, silicon was etched through the backside in a KOH bath at 80 °C for several hours, until the opposite Si3N4 layer was reached. The sodefined membrane arrays were cleaned in 65% HNO3 at 80 °C for 1 h. Macroscopic contact pads (Ti/Au) on the top side were patterned in a further photolithographic step. The nanowires were removed from the growth substrate by ultrasonic treatment in isopropyl alcohol and dropcasted onto the Si3N4 membranes. Nickel contacts were formed using electron-beam lithography, electron-beam assisted evaporation of a 200 nm thick layer and lift-off techniques. A 7 s dip in buffered HF was used to remove the native oxide prior to the metal deposition. Individual nanowires were contacted in three-terminal configurations to interface two p-n junctions per wire, allowing for reference measurements. The contact separation was 2 µm and a 2 µm long p- or n- segment was assumed at the tip or base of the nanowire. The p-n characteristics were pre-checked by electrical characterization and µ-EL image maps verified the exact location of the junction. The exact location of the junction The whole structure was then passivated with 8 nm of Al2O3 by atomic layer deposition. After optical and electrical characterization of the bare nanowire device, the plasmonic nanocavity was finally patterned directly on top of one p-n junction by another electron-beam lithography step, evaporation of 150 nm of Ag and a subsequent lift-off. It should be noted that the functionality of the plasmonic Ag cavity degraded after about three weeks upon exposure to atmospheric conditions. Electrical/Optical characterization To enable the electrical/optical characterization, the individual devices were bonded and glued to a sample holder, allowing to turn the membrane chips upside down for optical 14 ACS Paragon Plus Environment

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characterization from the backside. Biasing of the chips during optical characterization was accomplished by using a Keithley 6430 fA SourceMeter. Spectrographic measurements were carried out in a WITec alpha300 RAS system in combination with an Andor iDus DV401A-BV detector for the range of 380-950 nm and an Andor iDus DU491A detector for the range of 950 to 1600 nm and a Zeiss objective (LD EC Epiplan) featuring a magnification of 100x and a numerical aperture of 0.75. The spectral efficiency of the complete system including the optical components was determined using a calibrated light source (Ocean Optics HL-3P-CAL-EXT). All EL data presented in here were corrected with respect to the system’s transmission characteristics. The system characteristics comprise a 20 nm wide notch centred at 532 nm produced by the filter characteristics of a holographic beam splitter. To provide clarity in the displayed EL spectra, the immediate vicinity of this wavelength is not shown. PL measurements were performed using a linearly polarized 532 nm frequency-doubled Nd:YAG laser at a laser power of 200 µW and a spot size of about 865 nm. Numerical methods The surface plasmon-coupled silicon nanowire cavities were theoretically analyzed in a fully featured device by the finite-difference time-domain method implemented in Lumerical Inc. (Canada). By using the Fourier transform on the time signals, the resonant nanocavity modes and their associated spatial electromagnetic field distributions obtained from the time domain solution were solved within a two-dimensional model by exciting a sum of randomly located point magnetic dipoles inside the nanocavity as a pulsed source. In addition, the models of the Purcell factor and mode volume were directly established by a single infinitesimal magnetic dipole source parallel to the nanowire axis. The boundary conditions were assumed to be perfectly matched layers placed at a distance of 180 nm above the Ag cavity with the same impedance as the medium, and as a result, no reflection occurred at the boundaries. The radiated energy towards the silicon nitride membrane was collected by a reflectivity monitor, placed below the nanowire, where the resonant wavelengths agreed with the Purcell factor calculations. The Purcell factor was calculated from the radiating dipole, and is defined in Equation (1), The effective mode volume (Vm) was calculated from the magnetic field distribution in the 15 ACS Paragon Plus Environment

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nanocavity, expressed as Vm = ʃ ɛ(r)E2(r)d3r / [ɛ(r)E2(r)]dip, where ɛ(r) is the material’s dielectric constant. It should be noted that the longitudinal confinement in the simulated nanowire can effectively modify the spontaneous emission rate upon the spatial distribution of the field, and is therefore taken into account.30 ASSOCIATED CONTENT Supporting Information Supporting information is available in the online version of the paper at https://pubs.acs.org/journal/nalefd: Modal field distribution profiles at larger nanowire diameter; Detailed EL characteristics of Nanowires with different diameters before and after cavity formation; I-V characteristics; Photograph of the PEL device. AUTHOR INFORMATION Corresponding Author * E-Mail : [email protected]

Author Contributions S.G. and A.L. conceived and designed the experiments. S.G. fabricated the nanostructures and performed the electrical and optical measurements. S.G. and A.L. analyzed the results and prepared the manuscript. H.K. performed the numerical simulations and contributed to the manuscript. M.I.d.H. and B.F. designed and fabricated the membrane chips, D.J.H and J.F.C provided the p-n junction nanowires and all together provided helpful feedback and commented on the manuscript. All authors analyzed and discussed the results extensively. Notes The authors declare no competing financial interest Acknowledgements 16 ACS Paragon Plus Environment

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S.G and A.L gratefully acknowledge financial support by the Austrian Science Fund (FWF) project No.: P28175-N27 and further thank the Center for Micro- and Nanostructures (ZMNS) for providing the cleanroom facilities. J.F.C. and D.J.H. acknowledge support by the National Science Foundation (DMR-1555001). M.I.d.H. and B.F acknowledges financial support from the ANR-COSMOS (ANR-12-JS10-0002) project.

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