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1. N-polar InGaN/GaN nanowires: overcoming the efficiency cliff of red-emitting micro-LEDs

   https://doi.org/10.1364/PRJ.450465  

   Pandey, A. ; Malhotra, Y. ; Wang, P. ; Sun, K. ; Liu, X. ; Mi, Z. ( March 2022 , Photonics Research)

   A high efficiency, high brightness, and robust micro or sub-microscale red light emitting diode (LED) is an essential, yet missing, component of the emerging virtual reality and future ultrahigh resolution mobile displays. We report, for the first time, to our knowledge, the demonstration of an N-polar InGaN/GaN nanowire sub-microscale LED emitting in the red spectrum that can overcome the efficiency cliff of conventional red-emitting micro-LEDs. We show that the emission wavelengths of N-polar InGaN/GaN nanowires can be progressively shifted from yellow to orange and red, which is difficult to achieve for conventional InGaN quantum wells or Ga-polar nanowires. Significantly, the optical emission intensity can be enhanced by more than one order of magnitude by employing anin situannealing process of the InGaN active region, suggesting significantly reduced defect formation. LEDs with lateral dimensions as small as$\sim 0.75\mathrm{\mu m}$, consisting of approximately five nanowires, were fabricated and characterized, which are the smallest red-emitting LEDs ever reported, to our knowledge. A maximum external quantum efficiency$\sim 1.2\%$was measured, which is comparable to previously reported conventional quantum well micro-LEDs operating in this wavelength range, while our device sizes are nearly three to five orders of magnitude smaller in surface area.

    

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2. Controlled Electron Leakage in Electron Blocking Layer Free InGaN/GaN Nanowire Light-Emitting Diodes

   https://doi.org/10.2298/FUEE2103393V  

   R. T. Velpula, B. Jain ( July 2021 , Facta Universitatis)

   In this study, we have proposed and investigated the effect of coupled quantum wells to reduce electron overflow in InGaN/GaN nanowire white color light-emitting diodes. The coupled quantum well before the active region could decrease the thermal velocity, which leads to a reduced electron mean free path. This improves the electron confinement in the active region and mitigates electron overflow in the devices. In addition, coupled quantum well after the active region utilizes the leaked electrons from the active region and contributes to the white light emission. Therefore, the output power and external quantum efficiency of the proposed nanowire LEDs are improved. Moreover, the efficiency droop was negligible up to 900 mA injection current. 

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3. AlGaN nanowires with inverse taper for flexible DUV emitters

   https://doi.org/10.1088/2515-7647/abf6be  

   Hartensveld, Matthew ; Melanson, Bryan ; Liu, Cheng ; Zhang, Jing ( April 2021 , Journal of Physics: Photonics)

   Deep ultraviolet (DUV) AlGaN light-emitting diodes (LEDs) are promising alternatives for production of DUV light, offering many advantages over mercury arc lamps. In this work, AlGaN nanowires with an inverse taper profile were demonstrated through a wet etching process, enabling removal of the nanowires from the growth substrate in a novel peeling process to form flexible devices. AlGaN nanowires with taper angles of ∼22° were obtained following a 70 min etch in AZ400K. Nanowire taper angle was studied as a function of etch time and nanowire top diameter. Nanowires with inverse taper were then embedded in a flexible polymer layer and removed from their growth substrate, which could enable development of high-efficiency flexible micro-LEDs. Released nanowires embedded within the polymer liftoff layer exhibit strain relaxation induced redshift due to reduction in piezoelectric polarization electric field intensity. The inverse taper structure was found to promote enhanced light extraction from the nanowire. The demonstrated flexible DUV emitters with inverse taper are shown to improve the device efficiency and allow for realization of flexible emitters through a novel fabrication process for the first time.

    

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4. Coatings for Core–Shell Composite Micro‐Lattice Structures: Varying Sputtering Parameters

   https://doi.org/10.1002/adem.202101264  

   Garcia-Taormina, Alina R. ; Kurpiers, Chantal M. ; Schwaiger, Ruth ; Hodge, Andrea M. ( December 2021 , Advanced Engineering Materials)

   Magnetron sputtering has garnered increased attention as a versatile deposition method for generating novel core–shell composite nano‐ and micro‐lattice materials spanning a wide range of properties and functionalities. Sputtering offers an expansive material workspace consisting of a wide range of ceramics, single‐element metals, and alloy systems. However, achieving uniform coatings on such fine‐featured structures remains a challenge. Thus, herein, a foundational assessment of various sputtering configurations, cathode geometries, and deposition parameters is carried out to investigate their implications on the coating thickness and uniformity of 3D micro‐lattice scaffolds. Specifically, tetrahedral‐truss structures fabricated via direct laser writing are coated by leveraging planar and inverted cylindrical magnetron cathodes at select deposition rates, sputtering powers, and Ar working pressures. Both plasma focused ion beam and microtome sectioning techniques are employed to evaluate the cross section of individual struts and assess overall uniformity. Overall, the influence of key sputtering factors on the design and development of core–shell composite nano‐ and micro‐lattice materials is highlighted and a pathway for future sputter coating optimization on these complex structures is provided.

    

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5. Characteristics of leakage currents in InGaN/AlGaN nanowire-based red microLEDs

   https://doi.org/10.1364/AO.476400  

   Shrestha, Sanju ; Velpula, Ravi Teja ; Pandey, Bed Prasad ; Muthu, Mano Balo Sankar ; Nguyen, Ngoc Thi Ai ; Nguyen, Hieu Pham Trung ( January 2023 , Applied Optics)

   III-nitride nanowire (NW) LEDs have been intensively studied for several emerging applications. However, the performance of these LEDs is still limited due to many factors. A leakage current may cause idle power consumption and affect the reliability and luminescence efficiency of the devices. Hence, it is one of the most important limiting factors from an application point of view. In this context, we have experimentally observed temperature-dependent forward and reverse leakage current–voltage characteristics of InGaN/AlGaN NW-based red microLEDs. The characteristic curves are fitted using different constant parameters such as the space charge term, zero bias current, and the characteristic energy. They are found to have error bars of less than 10%. The extra space charge term is believed to be due to inherent space charges trapped with the NWs and presents at every instance of the operation of the diode. The characteristic energy and ideality factors are compared to the reported values. An Arrhenius plot is used to calculate the thermal activation energy in the high- and low-temperature regions for both bias conditions. Our results show that the voltage-dependent activation energy is found to be about double in the case of the forward bias compared to that of the reverse bias in all voltage ranges. However, in a high voltage regime, the magnitudes of these parameters are almost four and six times greater for the forward and reverse biases, respectively, compared to those in the lower voltage regions. This study presents vital insight into the design and fabrication of high-performance NW-based LEDs.

    

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