Title: Effects of polarized-induced doping and graded composition in an advanced multiple quantum well InGaN/GaN UV-LED for enhanced light technology
Abstract In this paper, a light-emitting diode in the ultra-violet range (UV-LED) with multiple-quantum wells (MQWs) of InGaN/GaN is designed and analyzed through Technology Computer-Aided Design (TCAD) simulations. The polarization effects in III-nitride heterojunction and the effects of graded composition in the electron blocking layer (EBL) are exploited to enhance the performance of the proposed UV-LED. It is observed that the effect of graded composition in the EBL helps to enhance the electrical and optical performance of the LED, thereby enabling the achievement of some promising results. The simulation-based results demonstrated that superior internal efficiency and an inferior leakage current are achieved by using a graded Al composition in the EBL rather than a uniform composition. The reported results also confirm the remarkable improvement of the light output power by 17% at ∼100 mA when using the graded composition and also show a reduction in series resistance leading to more current. Graded Al composition in the EBL results in the enhancement of electroluminescence spectra (i.e., an increase in the peak of the spectral density). more »« less
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.
In this paper, in order to address the problem of electron leakage in AlGaN ultra-violet light-emitting diodes, we have proposed an electron-blocking free layer AlGaN ultra-violet (UV) light-emitting diode (LED) using polarization-engineered heart-shaped AlGaN quantum barriers (QB) instead of conventional barriers. This novel structure has decreased the downward band bending at the interconnection between the consecutive quantum barriers and also flattened the electrostatic field. The parameters used during simulation are extracted from the referred experimental data of conventional UV LED. Using the Silvaco Atlas TCAD tool; version 8.18.1.R, we have compared and optimized the optical as well as electrical characteristics of three varying LED structures. Enhancements in electroluminescence at 275 nm (52.7%), optical output power (50.4%), and efficiency (61.3%) are recorded for an EBL-free AlGaN UV LED with heart-shaped Al composition in the barriers. These improvements are attributed to the minimized non-radiative recombination on the surfaces, due to the progressively increasing effective conduction band barrier height, which subsequently enhances the carrier confinement. Hence, the proposed EBL-free AlGaN LED is the potential solution to enhance optical power and produce highly efficient UV emitters.
To prevent electron leakage in deep ultraviolet (UV) AlGaN light-emitting diodes (LEDs), Al-rich p-type AlxGa(1−x)N electron blocking layer (EBL) has been utilized. However, the conventional EBL can mitigate the electron overflow only up to some extent and adversely, holes are depleted in the EBL due to the formation of positive sheet polarization charges at the heterointerface of the last quantum barrier (QB)/EBL. Subsequently, the hole injection efficiency of the LED is severely limited. In this regard, we propose an EBL-free AlGaN deep UV LED structure using graded staircase quantum barriers (GSQBs) instead of conventional QBs without affecting the hole injection efficiency. The reported structure exhibits significantly reduced thermal velocity and mean free path of electrons in the active region, thus greatly confines the electrons over there and tremendously decreases the electron leakage into the p-region. Moreover, such specially designed QBs reduce the quantum-confined Stark effect in the active region, thereby improves the electron and hole wavefunctions overlap. As a result, both the internal quantum efficiency and output power of the GSQB structure are ~2.13 times higher than the conventional structure at 60 mA. Importantly, our proposed structure exhibits only ~20.68% efficiency droop during 0–60 mA injection current, which is significantly lower compared to the regular structure.
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.
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.
Das, Samadrita, Lenka, Trupti Ranjan, Talukdar, Fazal Ahmed, Velpula, Ravi Teja, Jain, Barsha, Nguyen, Hieu Pham, and Crupi, Giovanni. Effects of polarized-induced doping and graded composition in an advanced multiple quantum well InGaN/GaN UV-LED for enhanced light technology. Retrieved from https://par.nsf.gov/biblio/10331745. Engineering Research Express 4.1 Web. doi:10.1088/2631-8695/ac4fb1.
Das, Samadrita, Lenka, Trupti Ranjan, Talukdar, Fazal Ahmed, Velpula, Ravi Teja, Jain, Barsha, Nguyen, Hieu Pham, and Crupi, Giovanni.
"Effects of polarized-induced doping and graded composition in an advanced multiple quantum well InGaN/GaN UV-LED for enhanced light technology". Engineering Research Express 4 (1). Country unknown/Code not available. https://doi.org/10.1088/2631-8695/ac4fb1.https://par.nsf.gov/biblio/10331745.
@article{osti_10331745,
place = {Country unknown/Code not available},
title = {Effects of polarized-induced doping and graded composition in an advanced multiple quantum well InGaN/GaN UV-LED for enhanced light technology},
url = {https://par.nsf.gov/biblio/10331745},
DOI = {10.1088/2631-8695/ac4fb1},
abstractNote = {Abstract In this paper, a light-emitting diode in the ultra-violet range (UV-LED) with multiple-quantum wells (MQWs) of InGaN/GaN is designed and analyzed through Technology Computer-Aided Design (TCAD) simulations. The polarization effects in III-nitride heterojunction and the effects of graded composition in the electron blocking layer (EBL) are exploited to enhance the performance of the proposed UV-LED. It is observed that the effect of graded composition in the EBL helps to enhance the electrical and optical performance of the LED, thereby enabling the achievement of some promising results. The simulation-based results demonstrated that superior internal efficiency and an inferior leakage current are achieved by using a graded Al composition in the EBL rather than a uniform composition. The reported results also confirm the remarkable improvement of the light output power by 17% at ∼100 mA when using the graded composition and also show a reduction in series resistance leading to more current. Graded Al composition in the EBL results in the enhancement of electroluminescence spectra (i.e., an increase in the peak of the spectral density).},
journal = {Engineering Research Express},
volume = {4},
number = {1},
author = {Das, Samadrita and Lenka, Trupti Ranjan and Talukdar, Fazal Ahmed and Velpula, Ravi Teja and Jain, Barsha and Nguyen, Hieu Pham and Crupi, Giovanni},
}
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