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1. Schottky contacts to N-polar GaN with SiN interlayer for elevated temperature operation

   https://doi.org/10.1063/5.0083588  

   Khachariya, Dolar; Szymanski, Dennis; Reddy, Pramod; Kohn, Erhard; Sitar, Zlatko; Collazo, Ramón; Pavlidis, Spyridon (April 2022, Applied Physics Letters)

   In this Letter, we unveil the high-temperature limits of N-polar GaN Schottky contacts enhanced by a low-pressure chemical vapor deposited (LPCVD) SiN interlayer. Compared to conventional Schottky diodes, the insertion of a 5 nm SiN lossy dielectric interlayer in-between Ni and N-polar GaN increases the turn-on voltage ( V ON ) from 0.4 to 0.9 V and the barrier height ( ϕ B ) from 0.4 to 0.8 eV. This modification also reduces the leakage current at zero bias significantly: at room temperature, the leakage current in the conventional Schottky diode is >10 3 larger than that observed in the device with the SiN interlayer, while at 200 °C, this ratio increases to 10 5 . Thus, the rectification ratio (I ON /I OFF ) at ±1.5 V reduces to less than one at 250 °C for the conventional Schottky diode, whereas for SiN-coated diodes, rectification continues until 500 °C. The I–V characteristics of the diode with an SiN interlayer can be recovered after exposure to 400 °C or lower. Contact degradation occurs at 500 °C, although devices are not destroyed yet. Here, we report N-polar GaN Schottky contact operation up to 500 °C using an LPCVD SiN interlayer. 

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2. Improving vertical GaN p–n diode performance with room temperature defect mitigation

   https://doi.org/10.1088/1361-6641/ad10c3  

   Sultan Al-Mamun, Nahid; Gallagher, James; Jacobs, Alan G.; Hobart, Karl D.; Anderson, Travis J.; Gunning, Brendan P.; Kaplar, Robert J.; Wolfe, Douglas E.; Haque, Aman (December 2023, Semiconductor Science and Technology)

   Abstract Defect mitigation of electronic devices is conventionally achieved using thermal annealing. To mobilize the defects, very high temperatures are necessary. Since thermal diffusion is random in nature, the process may take a prolonged period of time. In contrast, we demonstrate a room temperature annealing technique that takes only a few seconds. The fundamental mechanism is defect mobilization by atomic scale mechanical force originating from very high current density but low duty cycle electrical pulses. The high-energy electrons lose their momentum upon collision with the defects, yet the low duty cycle suppresses any heat accumulation to keep the temperature ambient. For a 7 × 10⁵A cm⁻²pulsed current, we report an approximately 26% reduction in specific on-resistance, a 50% increase of the rectification ratio with a lower ideality factor, and reverse leakage current for as-fabricated vertical geometry GaN p–n diodes. We characterize the microscopic defect density of the devices before and after the room temperature processing to explain the improvement in the electrical characteristics. Raman analysis reveals an improvement in the crystallinity of the GaN layer and an approximately 40% relaxation of any post-fabrication residual strain compared to the as-received sample. Cross-sectional transmission electron microscopy (TEM) images and geometric phase analysis results of high-resolution TEM images further confirm the effectiveness of the proposed room temperature annealing technique to mitigate defects in the device. No detrimental effect, such as diffusion and/or segregation of elements, is observed as a result of applying a high-density pulsed current, as confirmed by energy dispersive x-ray spectroscopy mapping. 

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3. High-density polarization-induced 2D electron gases in N-polar pseudomorphic undoped GaN/Al0.85Ga0.15N heterostructures on single-crystal AlN substrates

   https://doi.org/10.1063/5.0107159  

   Zhang, Zexuan; Encomendero, Jimy; Kim, Eungkyun; Singhal, Jashan; Cho, YongJin; Nomoto, Kazuki; Toita, Masato; Xing, Huili Grace; Jena, Debdeep (August 2022, Applied Physics Letters)

   The polarization difference and band offset between Al(Ga)N and GaN induce two-dimensional (2D) free carriers in Al(Ga)N/GaN heterojunctions without any chemical doping. A high-density 2D electron gas (2DEG), analogous to the recently discovered 2D hole gas in a metal-polar structure, is predicted in a N-polar pseudomorphic GaN/Al(Ga)N heterostructure on unstrained AlN. We report the observation of such 2DEGs in N-polar undoped pseudomorphic GaN/AlGaN heterostructures on single-crystal AlN substrates by molecular beam epitaxy. With a high electron density of ∼4.3 ×1013/cm2 that maintains down to cryogenic temperatures and a room temperature electron mobility of ∼450 cm2/V s, a sheet resistance as low as ∼320 Ω/◻ is achieved in a structure with an 8 nm GaN layer. These results indicate significant potential of AlN platform for future high-power RF electronics based on N-polar III-nitride high electron mobility transistors. 

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4. Schottky contacts on ultra-high-pressure-annealed GaN with high rectification ratio and near-unity ideality factor

   https://doi.org/10.35848/1882-0786/acc443  

   Stein, Shane R.; Khachariya, Dolar; Mita, Seiji; Breckenridge, M. Hayden; Tweedie, James; Reddy, Pramod; Sierakowski, Kacper; Kamler, Grzegorz; Boćkowski, Michał; Kohn, Erhard; et al (March 2023, Applied Physics Express)

   Abstract We investigate the electrical characteristics of Ni Schottky contacts on n-type GaN films that have undergone ultra-high-pressure annealing (UHPA), a key processing step for activating implanted Mg. Contacts deposited on these films exhibit low rectification and high leakage current compared to contacts on as-grown films. By employing an optimized surface treatment to restore the GaN surface following UHPA, we obtain Schottky contacts with a high rectification ratio of ∼10⁹, a near-unity ideality factor of 1.03, and a barrier height of ∼0.9 eV. These characteristics enable the development of GaN junction barrier Schottky diodes employing Mg implantation and UHPA. 

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5. Systematic oxygen impurity reduction in smooth N-polar GaN by chemical potential control

   https://doi.org/10.1088/1361-6641/ac3638  

   Szymanski, Dennis; Wang, Ke; Kaess, Felix; Kirste, Ronny; Mita, Seiji; Reddy, Pramod; Sitar, Zlatko; Collazo, Ramon (November 2021, Semiconductor Science and Technology)

   Abstract Process chemical potential control and dislocation reduction were implemented to control oxygen concentration in N-polar GaN layers grown on sapphire substrates via metal organic chemical vapor deposition (MOCVD). As process supersaturation was changed from ∼30 to 3400, the formation energy of the oxygen point defect increased, which resulted in a 25-fold decrease in oxygen incorporation. Reducing dislocations by approximately a factor of 4 (to ∼10 9 cm −3 ) allowed for further reduction of oxygen incorporation to the low-10 17 cm −3 range. Smooth N-polar GaN layers with low oxygen content were achieved by a two-step process, whereas first a 1 µ m thick smooth N-polar layer with high oxygen concentration was grown, followed by low oxygen concentration layer grown at high supersaturation. 

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