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1. Improving carrier transport in AlGaN deep-ultraviolet light-emitting diodes using a strip-in-a-barrier structure

   https://doi.org/10.1364/AO.394149  

   Velpula, Ravi Teja ; Jain, Barsha ; Bui, Ha Quoc Thang ; Shakiba, Fatemeh Mohammadi ; Jude, Jeffrey ; Tumuna, Moses ; Nguyen, Hoang-Duy ; Lenka, Trupti Ranjan ; Nguyen, Hieu Pham Trung ( June 2020 , Applied Optics)

   This paper reports the illustration of electron blocking layer (EBL)-free AlGaN light-emitting diodes (LEDs) operating in the deep-ultraviolet (DUV) wavelength at$\sim <\#comment/>270\mathrm{n}\mathrm{m}$. In this work, we demonstrated that the integration of an optimized thin undoped AlGaN strip layer in the middle of the last quantum barrier (LQB) could generate enough conduction band barrier height for the effectively reduced electron overflow into the$p\text{-}\mathrm{G}\mathrm{a}\mathrm{N}$region. Moreover, the hole injection into the multi-quantum-well active region is significantly increased due to a large hole accumulation at the interface of the AlGaN strip and the LQB. As a result, the internal quantum efficiency and output power of the proposed LED structure has been enhanced tremendously compared to that of the conventional$p\text{-}\mathrm{t}\mathrm{y}\mathrm{p}\mathrm{e}$EBL-based LED structure.

    

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2. Enhanced hole transport in AlGaN deep ultraviolet light-emitting diodes using a double-sided step graded superlattice electron blocking layer

   https://doi.org/10.1364/JOSAB.399773  

   Jain, Barsha ; Velpula, Ravi Teja ; Velpula, Swetha ; Nguyen, Hoang-Duy ; Nguyen, Hieu Pham Trung ( August 2020 , Journal of the Optical Society of America B)

   In this paper, deep ultraviolet AlGaN light-emitting diodes (LEDs) with a novel double-sided step graded superlattice (DSGS) electron blocking layer (EBL) instead of a conventional EBL have been proposed for$\sim <\#comment/>254\mathrm{n}\mathrm{m}$wavelength emission. The enhanced carrier transport in the DSGS structure results in reduced electron leakage into the$p$-region, improved hole activation and hole injection, and enhanced output power and external quantum efficiency. The calculations show that output power of the DSGS structure is$\sim <\#comment/>3.56$times higher and electron leakage is$\sim <\#comment/>12$times lower, compared to the conventional structure. Moreover, the efficiency droop at 60 mA in the DSGS LED was found to be$\sim <\#comment/>9.1\mathrm{\%<\#comment/>}$, which is$\sim <\#comment/>4.5$times lower than the regular LED structure.

    

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3. Controlled carrier mean free path for the enhanced efficiency of III-nitride deep-ultraviolet light-emitting diodes

   https://doi.org/10.1364/AO.418603  

   Jain, Barsha ; Velpula, Ravi Teja ; Patel, Moulik ; Nguyen, Hieu Pham Trung ( April 2021 , Applied Optics)

   Electron overflow from the active region confines the AlGaN deep-ultraviolet (UV) light-emitting diode (LED) performance. This paper proposes a novel approach to mitigate the electron leakage problem in AlGaN deep-UV LEDs using concave quantum barrier (QB) structures. The proposed QBs suppress the electron leakage by significantly reducing the electron mean free path that improves the electron capturing capability in the active region. Overall, such an engineered structure also enhances the hole injection into the active region, thereby enhancing the radiative recombination in the quantum wells. As a result, our study shows that the proposed structure exhibits an optical power of 9.16 mW at$\sim <\#comment/>284\mathrm{n}\mathrm{m}$wavelength, which is boosted by$\sim <\#comment/>40.5\mathrm{\%<\#comment/>}$compared to conventional AlGaN UV LED operating at 60 mA injection current.

    

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4. High-performance nanowire ultraviolet light-emitting diodes with potassium hydroxide and ammonium sulfide surface passivation

   https://doi.org/10.1364/AO.400877  

   Bui, Ha Quoc Thang ; Velpula, Ravi Teja ; Jian, Barsha ; Philip, Moab Rajan ; Tong, Hien Duy ; Lenka, Trupti Rajan ; Nguyen, Hieu Pham Trung ( August 2020 , Applied Optics)

   Potassium hydroxide (KOH) and ammonium sulfide$({\mathrm{N}\mathrm{H}}_4{)}_2{\mathrm{S}}_x$have been used as a surface passivation treatment to improve the electrical and optical performance of AlGaN nanowire ultraviolet (UV) light-emitting diodes (LEDs). Enhancements in photoluminescence at 335 nm (49%), optical output power (65%), and electroluminescence (83%), with respect to the as-grown nanowire LED are recorded for the AlGaN nanowire UV LEDs with surface passivation. These enhancements are attributed to the reduced nonradiative recombination on the nanowire surfaces. This study provides a potential surface passivation approach to produce high-power AlGaN nanowire LEDs operating in the UV spectrum.

    

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5. Phase-factor spectra of turbulent phase screens

   https://doi.org/10.1364/JOSAA.429928  

   Muschinski, Andreas ( August 2021 , Journal of the Optical Society of America A)

   The optical phase$\mathrm{\varphi <\#comment/>}$is a key quantity in the physics of light propagating through a turbulent medium. In certain respects, however, the statistics of the phasefactor,$\mathrm{\psi <\#comment/>}=\exp <\#comment/>(i\mathrm{\varphi <\#comment/>})$, are more relevant than the statistics of the phase itself. Here, we present a theoretical analysis of the 2D phase-factor spectrum$F_{\mathrm{\psi <\#comment/>}}(\mathit{\kappa <\#comment/>})$of a random phase screen. We apply the theory to four types of phase screens, each characterized by a power-law phase structure function,$D_{\mathrm{\varphi <\#comment/>}}(r)=(r/r_c{)}^{\mathrm{\gamma <\#comment/>}}$(where$r_c$is the phase coherence length defined by$D_{\mathrm{\varphi <\#comment/>}}(r_c)=1{\mathrm{r}\mathrm{a}\mathrm{d}}^2$), and a probability density function$p_{\mathrm{\alpha <\#comment/>}}(\mathrm{\alpha <\#comment/>})$of the phase increments for a given spatial lag. We analyze phase screens with turbulent ($\mathrm{\gamma <\#comment/>}=5/3$) and quadratic ($\mathrm{\gamma <\#comment/>}=2$) phase structure functions and with normally distributed (i.e., Gaussian) versus Laplacian phase increments. We find that there is a pronounced bump in each of the four phase-factor spectra$F_{\mathrm{\psi <\#comment/>}}(\mathrm{\kappa <\#comment/>})$. The precise location and shape of the bump are different for the four phase-screen types, but in each case it occurs at$\mathrm{\kappa <\#comment/>}\sim <\#comment/>1/r_c$. The bump is unrelated to the well-known “Hill bump” and is not caused by diffraction effects. It is solely a characteristic of the refractive-index statistics represented by the respective phase screen. We show that the second-order$\mathrm{\psi <\#comment/>}$statistics (covariance function, structure function, and spectrum) characterize a random phase screen more completely than the second-order$\mathrm{\varphi <\#comment/>}$counterparts.

    

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