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1. Optical properties of plasmonic tunneling junctions

   https://doi.org/10.1063/5.0128822  

   Tang, Yuankai; Harutyunyan, Hayk (February 2023, The Journal of Chemical Physics)

   Over the last century, quantum theories have revolutionized our understanding of material properties. One of the most striking quantum phenomena occurring in heterogeneous media is the quantum tunneling effect, where carriers can tunnel through potential barriers even if the barrier height exceeds the carrier energy. Interestingly, the tunneling process can be accompanied by the absorption or emission of light. In most tunneling junctions made of noble metal electrodes, these optical phenomena are governed by plasmonic modes, i.e., light-driven collective oscillations of surface electrons. In the emission process, plasmon excitation via inelastic tunneling electrons can improve the efficiency of photon generation, resulting in bright nanoscale optical sources. On the other hand, the incident light can affect the tunneling behavior of plasmonic junctions as well, leading to phenomena such as optical rectification and induced photocurrent. Thus, plasmonic tunneling junctions provide a rich platform for investigating light–matter interactions, paving the way for various applications, including nanoscale light sources, sensors, and chemical reactors. In this paper, we will introduce recent research progress and promising applications based on plasmonic tunneling junctions. 

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2. Design and demonstration of efficient transparent 30% Al-content AlGaN interband tunnel junctions

   https://doi.org/10.1063/5.0122919  

   Dominic Merwin Xavier, Agnes Maneesha; Ghosh, Arnob; Rahman, Sheikh Ifatur; Allerman, Andrew; Arafin, Shamsul; Rajan, Siddharth (February 2023, Applied Physics Letters)

   Ultra-violet (UV) light emitting diodes operating at 339 nm using transparent interband tunnel junctions are reported. Tunneling-based ultraviolet light emitting diodes were grown by plasma-assisted molecular beam epitaxy on 30% Al-content AlGaN layers. A low tunnel junction voltage drop is obtained through the use of compositionally graded n and p-type layers in the tunnel junction, which enhance hole density and tunneling rates. The transparent tunnel junction-based UV LED reported here show a low voltage drop of 5.55 V at 20 A/cm2 and an on-wafer external quantum efficiency of 1.02% at 80 A/cm2. 

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3. Demonstration of AlGaN Tunnel Junction p-Down UV Light Emitting Diodes

   Dominic Merwin Xavier, Agnes Maneesha; Ghosh, Arnob: Rahman; Allerman, Andrew; Arafin, Shamsul; Rajan, Siddharth (June 2023, EMC)

   Ultra-violet light emitting diodes (UV-LEDs) and lasers based on the III-Nitride material system are very promising since they enable compact, safe, and efficient solid-state sources of UV light for a range of applications. The primary challenges for UV LEDs are related to the poor conductivity of p-AlGaN layers and the low light extraction efficiency of LED structures. Tunnel junction-based UV LEDs provide a distinct and unique pathway to eliminate several challenges associated with UV LEDs1-4. In this work, we present for the first time, a reversed-polarization (p-down) AlGaN based UV-LED utilizing bottom tunnel junction (BTJ) design. We show that compositional grading enables us to achieve the lowest reported voltage drop of 1.1 V at 20 A/cm2 among transparent AlGaN based tunnel junctions at this Al-composition. Compared to conventional LED design, a p-down structure offers lower voltage drop because the depletion barrier for both holes and electrons is lower due to polarization fields aligning with the depletion field. Furthermore, the bottom tunnel junction also allows us to use polarization grading to realize better p- and n-type doping to improve tunneling transport. The epitaxial structure of the UV-LED was grown by plasma-assisted molecular beam epitaxy (PAMBE) on metal-organic chemical vapor deposition (MOCVD)-grown n-type Al0.3Ga0.7N templates. The transparent TJ was grown using graded n++-Al0.3Ga0.7N→ n++-Al0.4Ga0.6N (Si=3×1020 cm-3) and graded p++-Al0.4Ga0.6N →p++-Al0.3Ga0.7N (Mg=1×1020 cm-3) to take advantage of induced 3D polarization charges. The high number of charges at the tunnel junction region leads to lower depletion width and efficient hole injection to the p-type layer. The UV LED active region consists of three 2.5 nm Al0.2Ga0.8N quantum wells and 7 nm Al0.3Ga0.6N quantum barriers followed by 12 nm of p- Al0.46Ga0.64N electron blocking layer (EBL). The active region was grown on top of the tunnel junction. A similar LED with p-up configuration was also grown to compare the electrical performance. The surface morphology examined by atomic force microscopy (AFM) shows smooth growth features with a surface roughness of 1.9 nm. The dendritic features on the surface are characteristic of high Si doping on the surface. The composition of each layer was extracted from the scan by high resolution x-ray diffraction (HR-XRD). The electrical characteristics of a device show a voltage drop of 4.9 V at 20 A/cm2, which corresponds to a tunnel junction voltage drop of ~ 1.1 V. This is the best lowest voltage for transparent 30% AlGaN tunnel junctions to-date and is comparable with the lowest voltage drop reported previously on non-transparent (InGaN-based) tunnel junctions at similar Al mole fraction AlGaN. On-wafer electroluminescence measurements on patterned light-emitting diodes showed single peak emission wavelength of 325 nm at 100 A/cm2 which corresponds to Al0.2Ga0.8N, confirming that efficient hole injection was achieved within the structure. The device exhibits a wavelength shift from 330 nm to 325 nm with increasing current densities from 10A/cm2 to 100A/cm2. In summary, we have demonstrated a fully transparent bottom AlGaN homojunction tunnel junction that enables p-down reversed polarization ultraviolet light emitting diodes, and has very low voltage drop at the tunnel junction. This work could enable new flexibility in the design of future III-Nitride ultraviolet LEDs and lasers. 

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4. Efficient light generation from enhanced inelastic electron tunnelling

   Qian, H; Hsu, SW; Gurunatha, K; Riley, CT; Zha, J; Lu, D; Tao, AR; Liu, Z (July 2018, Nature photonics)

   Light emission from biased tunnel junctions has recently gained much attention owing to its unique potential to create ultracompact optical sources with terahertz modulation bandwidth1,2,3,4,5. The emission originates from an inelastic electron tunnelling process in which electronic energy is transferred to surface plasmon polaritons and subsequently converted to radiation photons by an optical antenna. Because most of the electrons tunnel elastically, the emission efficiency is typically about 10−5–10−4. Here, we demonstrate efficient light generation from enhanced inelastic tunnelling using nanocrystals assembled into metal–insulator–metal junctions. The colour of the emitted light is determined by the optical antenna and thus can be tuned by the geometry of the junction structures. The efficiency of far-field free-space light generation reaches ~2%, showing an improvement of two orders of magnitude over previous work3,4. This brings on-chip ultrafast and ultracompact light sources one step closer to reality. 

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5. (Invited) Hybrid Plasmonic Nanocatalysts for Solar-Chemical Energy Conversion Systems

   https://doi.org/10.1149/MA2025-02643039mtgabs  

   Nazemi, Reza (November 2025, ECS Meeting Abstracts)

   Utilizing solar energy for chemical transformations has attracted a growing interest in promoting the clean and modular chemical synthesis approach and addressing conventional thermocatalytic systems’ limitations. Noble metal nanoparticles are known to support coherent oscillatory modes of their conduction band electrons when photoexcited with a resonant frequency, producing localized surface plasmon resonances (LSPR). Under light irradiation, noble metal nanoparticles, particularly those characterized by LSPR, commonly known as plasmonic nanoparticles, generate a strong electromagnetic field, excited hot carriers, and photothermal heating. The LSPR can be excited by UV, visible, or near-infrared (NIR) light, which depends on the nanoparticle’s size, shape, composition, structure (hollow or solid), and the dielectric function of the surrounding medium. By placing various plasmonic nanoparticles in their proximity (< 10 nm), a strong electromagnetic field is generated at a certain distance due to the coupling of the electromagnetic field of individual plasmonic nanoparticles. After plasmonic excitation of plasmonic nanoparticles with incident light, the plasmon can quickly dephase through electron-surface scattering on a few femtoseconds (fs) time scales, generating non-thermal hot electrons. After a few hundred fs, electron-electron scattering produces a distribution of electrons with temperature on the order of hundreds of degrees above the surrounding medium. Within a few picoseconds (ps) of the initial excitation, the absorbed energy is transferred to the nanoparticle’s lattice via electron-phonon coupling (τ_e-ph). The final stage of heat dissipation to the surrounding medium by thermal conduction occurs on a longer timescale ranging from several tens of ps to 10 nanoseconds (ns) through phonon-phonon coupling (τ_ph-ph). At this stage, the heated nanoparticle lattice transfers heat to the surrounding medium, resulting in increasing the medium’s temperature until the nanoparticle lattice and its surrounding medium reach an equilibrium temperature, which is higher than the temperature of the bulk medium. Both non-thermal hot electrons and the local heating could excite electronic or vibrational transitions in adsorbed molecules on the nanoparticle’s surface, thus enabling catalytic reactions. Understanding the electronic and thermal properties of plasmonic nanoparticles and their interactions at the nanoscale in a local catalytic environment with adsorbed molecules is imperative to improving photocatalytic efficiency. We aim to design and develop a plasmonic photothermal reactor to synthesize ammonia from dinitrogen and hydrogen under concentrated sunlight using plasmonic nanoparticles that absorb light in the visible and near-infrared regions (Figure 1). Despite various design strategies for enhancing the catalytic activity of plasmonic nanocatalysts, including controlling nanocatalysts’ size and shape, they have exhibited moderate activity for driving chemical reactions. We incorporate a co-catalyst made of transition metal nanoparticles that could improve the catalytic activity in the hybrid plasmonic-catalytic nanoparticles. In this hybrid system, a plasmonic nanoparticle is mainly responsible for absorbing light, generating hot carriers, and local heat, while transition metal catalytic reactors handle the chemical reactions. Figure 1 

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