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1. ∼8.5 μ m-emitting InP-based quantum cascade lasers grown on GaAs by metal-organic chemical vapor deposition

   https://doi.org/10.1063/5.0122272  

   Xu, S.; Zhang, S.; Kirch, J. D.; Suri, S.; Pokharel, N.; Gao, H.; Kim, H.; Dhingra, P.; Lee, M. L.; Botez, D.; et al (October 2022, Applied Physics Letters)

   Room-temperature, pulsed-operation lasing of 8.5  μm-emitting InP-based quantum cascade lasers (QCLs), with low threshold-current density and watt-level output power, is demonstrated from structures grown on (001) GaAs substrates by metal-organic chemical vapor deposition. Prior to growing the laser structure, which contains a 35-stage In 0.53 Ga 0.47 As/In 0.52 Al 0.48 As lattice-matched active-core region, a ∼2  μm-thick nearly fully relaxed InP buffer with strained 1.6 nm-thick InAs quantum-dot-like dislocation-filter layers was grown. A smooth terminal buffer-layer surface, with roughness as low as 0.4 nm on a 10 × 10  μm 2 scale, was obtained, while the estimated threading-dislocation density was in the mid-range × 10 8  cm −2 . A series of measurements, on lasers grown on InP metamorphic buffer layers (MBLs) and on native InP substrates, were performed for understanding the impact of the buffer-layer's surface roughness, residual strain, and threading-dislocation density on unipolar devices such as QCLs. As-cleaved devices, grown on InP MBLs, were fabricated as 25  μm × 3 mm deep-etched ridge guides with lateral current injection. The results are pulsed maximum output power of 1.95 W/facet and a low threshold-current density of 1.86 kA/cm 2 at 293 K. These values are comparable to those obtained from devices grown on InP: 2.09 W/facet and 2.42 kA/cm 2 . This demonstrates the relative insensitivity of the device-performance metrics on high residual threading-dislocation density, and high-performance InP-based QCLs emitting near 8  μm can be achieved on lattice-mismatched substrates. 

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2. On electrical analysis of Al-rich p-AlGaN films for III-nitride UV light emitters

   https://doi.org/10.1088/1361-6641/ac3710  

   Jadhav, Aakash; Bagheri, Pegah; Klump, Andrew; Khachariya, Dolar; Mita, Seiji; Reddy, Pramod; Rathkanthiwar, Shashwat; Kirste, Ronny; Collazo, Ramon; Sitar, Zlatko; et al (November 2021, Semiconductor Science and Technology)

   Abstract In this work, an alternative scheme to estimate the resistivity and ionization energy of Al-rich p-AlGaN epitaxial films is developed using two large-area ohmic contacts. Accordingly, the resistivities measured using current–voltage measurements were observed to corroborate the Hall measurements in the Van der Pauw configuration. A free hole concentration of ∼1.5 × 10 17 cm −3 and low ionization energy of ∼65 meV in Mg-doped Al 0.7 Ga 0.3 N films is demonstrated. Nearly an order of magnitude lower hydrogen concentration than Mg in the as-grown AlGaN films is thought to reduce the Mg passivation and enable higher hole concentrations in Al-rich p-AlGaN films, compared to p-GaN films. The alternate methodology proposed in this work is expected to provide a simpler pathway to evaluate the electrical characteristics of Al-rich p-AlGaN films for future III-nitride ultraviolet light emitters. 

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3. 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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4. Towards Electrically-Pumped AlGaN UV-A Lasers with Transparent Tunnel Junctions

   https://doi.org/10.1364/CLEO_SI.2023.SF2Q.5  

   Ghosh, Arnob; Xavier, Agnes M.; Rahman, Sheikh Ifatur; Allerman, Andrew; Rajan, Siddharth; Arafin, Shamsul (January 2023, CLEO 2023, Technical Digest Series (Optica Publishing Group, 2023))

   Molecular beam epitaxy-grown GaN/AlGaN-based active regions were optimized by varying quantum-well widths to yield increased photoluminescence intensity at UV-A wavelengths. The optimized gain medium was then used in electrically-pumped laser structures with transparent tunnel junctions. 

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5. 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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