More Like this

1. Tuning Electrical and Thermal Transport in AlGaN/GaN Heterostructures via Buffer Layer Engineering

   https://doi.org/10.1002/adfm.201705823  

   Yalamarthy, Ananth Saran ; So, Hongyun ; Muñoz Rojo, Miguel ; Suria, Ateeq J. ; Xu, Xiaoqing ; Pop, Eric ; Senesky, Debbie G. ( March 2018 , Advanced Functional Materials)

   Progress in wide bandgap, III–V material systems based on gallium nitride (GaN) has enabled the realization of high‐power and high‐frequency electronics. Since the highly conductive, 2D electron gas (2DEG) at the aluminum gallium nitride (AlGaN)/GaN interface is based on built‐in polarization fields and is confined to nanoscale thicknesses, its charge carriers exhibit much higher mobilities compared to their doped counterparts. This study shows that such 2DEGs also offer the unique ability to manipulate electrical transport separately from thermal transport, through the examination of fully suspended AlGaN/GaN diaphragms of varied GaN buffer layer thickness. Notably, ≈100 nm thin GaN layers can considerably impede heat flow without electrical transport degradation. These achieve 4× improvement in the thermoelectric figure of merit (zT) over externally doped GaN, with state‐of‐the‐art power factors of 4–7 mW m^‐1K^‐2. The remarkable tuning behavior and thermoelectric enhancement, elucidated here for the first time in a polarization‐based heterostructure, are achieved because electrons are at the heterostructured interface, while phonons are within the material system. These results highlight the potential for using 2DEGs in III–V materials for on‐chip thermal sensing and energy harvesting.

    

   more » « less
2. Properties and device performance of BN thin films grown on GaN by pulsed laser deposition

   https://doi.org/10.1063/5.0092356  

   Biswas, Abhijit ; Xu, Mingfei ; Fu, Kai ; Zhou, Jingan ; Xu, Rui ; Puthirath, Anand B. ; Hachtel, Jordan A. ; Li, Chenxi ; Iyengar, Sathvik Ajay ; Kannan, Harikishan ; et al ( September 2022 , Applied Physics Letters)

   Wide and ultrawide-bandgap semiconductors lie at the heart of next-generation high-power, high-frequency electronics. Here, we report the growth of ultrawide-bandgap boron nitride (BN) thin films on wide-bandgap gallium nitride (GaN) by pulsed laser deposition. Comprehensive spectroscopic (core level and valence band x-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, and Raman) and microscopic (atomic force microscopy and scanning transmission electron microscopy) characterizations confirm the growth of BN thin films on GaN. Optically, we observed that the BN/GaN heterostructure is second-harmonic generation active. Moreover, we fabricated the BN/GaN heterostructure-based Schottky diode that demonstrates rectifying characteristics, lower turn-on voltage, and an improved breakdown capability (∼234 V) as compared to GaN (∼168 V), owing to the higher breakdown electrical field of BN. Our approach is an early step toward bridging the gap between wide and ultrawide-bandgap materials for potential optoelectronics as well as next-generation high-power electronics.

    

   more » « less
3. Bandgap and strain engineering in epitaxial rocksalt structure (Ti 0.5 Mg 0.5 ) 1−x Al x N(001) semiconductors

   https://doi.org/10.1039/d0tc03598j  

   Wang, Baiwei ; Zhang, Minghua ; Adhikari, Vijaya ; Fang, Peijiao ; Khare, Sanjay V. ; Gall, Daniel ( September 2020 , Journal of Materials Chemistry C)

   null (Ed.)

   Rocksalt structure nitrides emerge as a promising class of semiconductors for high-temperature thermoelectric and plasmonic applications. Controlling the bandgap and strain is essential for the development of a wide variety of electronic devices. Here we use (Ti 0.5 Mg 0.5 ) 1−x Al x N as a model system to explore and demonstrate the tunability of both the bandgap and the strain state in rocksalt structure nitrides, employing a combined experimental and computational approach. (Ti 0.5 Mg 0.5 ) 1−x Al x N layers with x ≤ 0.44 deposited on MgO(001) substrates by reactive co-sputtering at 700 °C are epitaxial single crystals with a solid-solution B1 rocksalt structure. The lattice mismatch with the substrate decreases with increasing x , leading to a transition in the strain-state from partially relaxed (74% and 38% for x = 0 and 0.09) to fully strained for x ≥ 0.22. First-principles calculations employing 64-atom Special Quasirandom Structures (SQS) indicate that the lattice constant decreases linearly with x according to a = (4.308 − 0.234 x ) Å for 0 ≤ x ≤ 1. In contrast, the measured relaxed lattice parameter a o = (4.269 − 0.131 x ) Å is linear only for x ≤ 0.33, its composition dependence is less pronounced, and x > 0.44 leads to the nucleation of secondary phases. The fundamental (indirect) bandgap predicted using the same SQS supercells and the HSE06 functional increases from 1.0 to 2.6 eV for x = 0–0.75. In contrast, the onset of the measured optical absorption due to interband transitions increases only from 2.3 to 2.6 eV for x = 0–0.44, suggesting that the addition of Al in the solid solution relaxes the electron momentum conservation and causes a shift from direct to indirect gap transitions. The resistivity increases from 9.0 to 708 μΩ m at 77 K and from 6.8 to 89 μΩ m at 295 K with increasing x = 0–0.44, indicating an increasing carrier localization associated with a randomization of cation site occupation and the increasing bandgap which also causes a 33% reduction in the optical carrier concentration. The overall results demonstrate bandgap and strain engineering in rocksalt nitride semiconductors and show that, in contrast to conventional covalent semiconductors, the random cation site occupation strongly affects optical transitions. 

   more » « less
4. Tunable bandgap and Si-doping in N-polar AlGaN on C-face 4H-SiC via molecular beam epitaxy

   https://doi.org/10.1063/5.0173637  

   Mondal, Shubham ; Wang, Ding ; Anhar Uddin Bhuiyan, A F ; Hu, Mingtao ; Reddeppa, Maddaka ; Wang, Ping ; Zhao, Hongping ; Mi, Zetian ( October 2023 , Applied Physics Letters)

   N-polar AlGaN is an emerging wide-bandgap semiconductor for next-generation high electron mobility transistors and ultraviolet light emitting diodes and lasers. Here, we demonstrate the growth and characterization of high-quality N-polar AlGaN films on C-face 4H-silicon carbide (SiC) substrates by molecular beam epitaxy. On optimization of the growth conditions, N-polar AlGaN films exhibit a crack free, atomically smooth surface (rms roughness ∼ 0.9 nm), and high crystal quality with low density of defects and dislocations. The N-polar crystallographic orientation of the epitaxially grown AlGaN film is unambiguously confirmed by wet chemical etching. We demonstrate precise compositional tunability of the N-polar AlGaN films over a wide range of Al content and a high internal quantum efficiency ∼74% for the 65% Al content AlGaN film at room temperature. Furthermore, controllable silicon (Si) doping in high Al content (65%) N-polar AlGaN films has been demonstrated with the highest mobility value ∼65 cm2/V-s observed corresponding to an electron concentration of 1.1 × 1017 cm−3, whereas a relatively high mobility value of 18 cm2/V-s is sustained for an electron concentration of 3.2 × 1019 cm−3, with an exceptionally low resistivity value of 0.009 Ω·cm. The polarity-controlled epitaxy of AlGaN on SiC presents a viable approach for achieving high-quality N-polar III-nitride semiconductors that can be harnessed for a wide range of emerging electronic and optoelectronic device applications.

    

   more » « less
5. WIDE-BANDGAP SEMICONDUCTORS Gallium Nitride, GaN - Silicon Carbide, SiC

   Michael Haralambous, Chrysanthos Panayiotou ( January 2022 , WIDE-BANDGAP SEMICONDUCTORS)

   About the LASER-TEC Laser and Fiber Optics Educational Series This series was created for use in engineering technology programs such as electronics, photonics, laser electro-optics, etc. This series of publications has three goals in mind: 1) to create educational materials for areas of laser electro-optics technology in which no materials exist, 2) to work with industry to use, adapt, and enhance available industry-created materials, 3) to make these materials available at no cost which, in turn, would generate more accessible education to everyone (including technicians). The Laser and Fiber Optics Educational Series is available for free online at www.laser-tec.org. About Wide-Bandgap Semiconductors New semiconductors based on silicon carbide (SiC) and gallium nitride (GaN) are now commercially available, which has been instrumental in removing obstacles that legacy silicon bipolar and metal-oxide-semiconductor field-effect transistor (MOSFET) devices could not overcome. These new devices have superior power-handling abilities due to their better thermal properties, higher switching frequencies, and lower conduction losses. Collectively, these properties make wide-bandgap devices the preferred technology for high-power conversion applications with efficiencies approaching 99%. For this reason, this new technology must be introduced to existing curricula, preparing engineers and technicians to tackle today’s and tomorrow’s power electronics challenges. This module is intended for use in technical programs after coverage of basic semiconductor theory and discrete devices such as silicon diodes, bipolar junction, field effect, and MOSFET transistors. 

   more » « less