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1. 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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2. 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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3. Point-defect management in homoepitaxially grown Si-doped GaN by MOCVD for vertical power devices

   https://doi.org/10.35848/1882-0786/ac6566  

   Rathkanthiwar, Shashwat; Bagheri, Pegah; Khachariya, Dolar; Mita, Seiji; Pavlidis, Spyridon; Reddy, Pramod; Kirste, Ronny; Tweedie, James; Sitar, Zlatko; Collazo, Ramón (April 2022, Applied Physics Express)

   Abstract We demonstrate controlled Si doping in the low doping range of 5 × 10 15 –2.5 × 10 16 cm −3 with mobility >1000 cm 2  V −1 s −1 in GaN films grown by metalorganic chemical vapor deposition. The carbon-related compensation and mobility collapse were prevented by controlling the electrochemical potential near the growth surface via chemical potential control (CPC) and defect quasi-Fermi level (dQFL) point-defect management techniques. While the CPC was targeted to reduce the net C N concentration, the dQFL control was used to reduce the fraction of C atoms with the compensating configuration, i.e. C N − 1 . The low compensating acceptor concentration was confirmed via temperature-dependent Hall effect analysis and capacitance–voltage measurements. 

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4. N-polar GaN p-n junction diodes with low ideality factors

   https://doi.org/10.35848/1882-0786/ac6ec5  

   Nomoto, Kazuki; Xing, Huili Grace; Jena, Debdeep; Cho, YongJin (May 2022, Applied Physics Express)

   Abstract High-quality N-polar GaN p-n diodes are realized on single-crystal N-polar GaN bulk substrate by plasma-assisted molecular beam epitaxy. The room-temperature current–voltage characteristics reveal a high on/off current ratio of >10 11 at ±4 V and an ideality factor of 1.6. As the temperature increases to 200 °C, the apparent ideality factor gradually approaches 2. At such high temperatures, Shockley–Read–Hall recombination times of 0.32–0.46 ns are estimated. The measured electroluminescence spectrum is dominated by a strong near-band edge emission, while deep level and acceptor-related luminescence is greatly suppressed. 

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5. Wafer-Scale Synthesis of WS ₂ Films with In Situ Controllable p-Type Doping by Atomic Layer Deposition

   https://doi.org/10.34133/2021/9862483  

   Yang, Hanjie; Wang, Yang; Zou, Xingli; Bai, Rongxu; Wu, Zecheng; Han, Sheng; Chen, Tao; Hu, Shen; Zhu, Hao; Chen, Lin; et al (January 2021, Research)

   Wafer-scale synthesis of p-type TMD films is critical for its commercialization in next-generation electro/optoelectronics. In this work, wafer-scale intrinsic n-type WS₂films and in situ Nb-doped p-type WS₂films were synthesized through atomic layer deposition (ALD) on 8-inchα-Al₂O₃/Si wafers, 2-inch sapphire, and 1 cm²GaN substrate pieces. The Nb doping concentration was precisely controlled by altering cycle number of Nb precursor and activated by postannealing. WS₂n-FETs and Nb-doped p-FETs with different Nb concentrations have been fabricated using CMOS-compatible processes. X-ray photoelectron spectroscopy, Raman spectroscopy, and Hall measurements confirmed the effective substitutional doping with Nb. The on/off ratio and electron mobility of WS₂n-FET are as high as 10⁵and 6.85 cm² V^-1 s^-1, respectively. In WS₂p-FET with 15-cycle Nb doping, the on/off ratio and hole mobility are 10 and 0.016 cm² V^-1 s^-1, respectively. The p-n structure based on n- and p- type WS₂films was proved with a 10⁴rectifying ratio. The realization of controllablein situNb-doped WS₂films paved a way for fabricating wafer-scale complementary WS₂FETs. 

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