More Like this

1. 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. 

   more » « less
2. Advantages of concave quantum barriers in AlGaN deep ultraviolet light-emitting diodes

   https://doi.org/10.1117/12.2650411  

   Jain, Barsha T.; Muthu, Manobalasankar; Velpula, Ravi Teja; Nguyen, Ngoc T.; Nguyen, Hieu Pham (March 2023, Gallium Nitride Materials and Devices XVIII)

   Morkoç, Hadis; Fujioka, Hiroshi; Schwarz, Ulrich T. (Ed.)

   Although AlGaN-based deep ultraviolet (UV) light-emitting diodes (LEDs) have been studied extensively, their quantum efficiency and optical output power still remain extremely low compared to the InGaN-based visible color LEDs. Electron leakage has been identified as one of the most possible reasons for the low internal quantum efficiency (IQE) in AlGaN based UV LEDs. The integration of a p-doped AlGaN electron blocking layer (EBL) or/and increasing the conduction band barrier heights with prompt utilization of higher Al composition quantum barriers (QBs) in the LED could mitigate the electron leakage problem to an extent, but not completely. In this context, we introduce a promising approach to alleviate the electron overflow without using EBL by utilizing graded concave QBs instead of conventional QBs in AlGaN UV LEDs. Overall, the carrier transportation, confinement capability and radiative recombination are significantly improved. As a result, the IQE, and output power of the proposed concave QB LED were enhanced by ~25.4% and ~25.6% compared to the conventional LED for emission at ~254 nm, under 60 mA injection current. 

   more » « less
3. 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. 

   more » « less
4. Numerical investigation on the device performance of electron blocking layer free AlInN nanowire deep ultraviolet light-emitting diodes

   https://doi.org/10.1364/OME.380409  

   Velpula, Ravi_Teja; Jain, Barsha; Bui, Ha_Quoc_Thang; Pham, Tan_Thi; Le, Van_Thang; Nguyen, Hoang-Duy; Lenka, Trupti_Ranjan; Nguyen, Hieu_Pham_Trung (January 2020, Optical Materials Express)

   We report on the illustration of the first electron blocking layer (EBL) free AlInN nanowire light-emitting diodes (LEDs) operating in the deep ultraviolet (DUV) wavelength region (sub-250 nm). We have systematically analyzed the results using APSYS software and compared with simulated AlGaN nanowire DUV LEDs. From the simulation results, significant efficiency droop was observed in AlGaN based devices, attributed to the significant electron leakage. However, compared to AlGaN nanowire DUV LEDs at similar emission wavelength, the proposed single quantum well (SQW) AlInN based light-emitters offer higher internal quantum efficiency without droop up to current density of 1500 A/cm²and high output optical power. Moreover, we find that transverse magnetic polarized emission is ∼ 5 orders stronger than transverse electric polarized emission at 238 nm wavelength. Further research shows that the performance of the AlInN DUV nanowire LEDs decreases with multiple QWs in the active region due to the presence of the non-uniform carrier distribution in the active region. This study provides important insights on the design of new type of high performance AlInN nanowire DUV LEDs, by replacing currently used AlGaN semiconductors. 

   more » « less
5. Microbial biofilms for electricity generation from water evaporation and power to wearables

   https://doi.org/10.1038/s41467-022-32105-6  

   Liu, Xiaomeng; Ueki, Toshiyuki; Gao, Hongyan; Woodard, Trevor L.; Nevin, Kelly P.; Fu, Tianda; Fu, Shuai; Sun, Lu; Lovley, Derek R.; Yao, Jun (December 2022, Nature Communications)

   Abstract Employing renewable materials for fabricating clean energy harvesting devices can further improve sustainability. Microorganisms can be mass produced with renewable feedstocks. Here, we demonstrate that it is possible to engineer microbial biofilms as a cohesive, flexible material for long-term continuous electricity production from evaporating water. Single biofilm sheet (~40 µm thick) serving as the functional component in an electronic device continuously produces power density (~1 μW/cm 2 ) higher than that achieved with thicker engineered materials. The energy output is comparable to that achieved with similar sized biofilms catalyzing current production in microbial fuel cells, without the need for an organic feedstock or maintaining cell viability. The biofilm can be sandwiched between a pair of mesh electrodes for scalable device integration and current production. The devices maintain the energy production in ionic solutions and can be used as skin-patch devices to harvest electricity from sweat and moisture on skin to continuously power wearable devices. Biofilms made from different microbial species show generic current production from water evaporation. These results suggest that we can harness the ubiquity of biofilms in nature as additional sources of biomaterial for evaporation-based electricity generation in diverse aqueous environments. 

   more » « less