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1. Effects of charge fluctuation and charge regulation on the phase transitions in stoichiometric VO2

   https://doi.org/10.1038/s41598-020-73447-9  

   Joshi, Siddharth; Smieszek, Nicholas; Chakrapani, Vidhya (December 2020, Scientific Reports)

   null (Ed.)

   Abstract Detailed electrical and photoemission studies were carried out to probe the chemical nature of the insulating ground state of VO 2 , whose properties have been an issue for accurate prediction by common theoretical probes. The effects of a systematic modulation of oxygen over-stoichiometry of VO 2 from 1.86 to 2.44 on the band structure and insulator–metal transitions are presented for the first time. Results offer a different perspective on the temperature- and doping-induced IMT process. They suggest that charge fluctuation in the metallic phase of intrinsic VO 2 results in the formation of e − and h + pairs that lead to delocalized polaronic V 3+ and V 5+ cation states. The metal-to-insulator transition is linked to the cooperative effects of changes in the V–O bond length, localization of V 3+ electrons at V 5+ sites, which results in the formation of V 4+ –V 4+ dimers, and removal of $$\pi^{*}$$ π ∗ screening electrons. It is shown that the nature of phase transitions is linked to the lattice V 3+ /V 5+ concentrations of stoichiometric VO 2 and that electronic transitions are regulated by the interplay between charge fluctuation, charge redistribution, and structural transition. 

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2. High performance AlGaN/GaN MISHEMTs using N ₂ O treated TiO ₂ as the gate dielectric

   https://doi.org/10.1088/1361-6641/ad96dc  

   Zhama, Tuofu; Cui, Peng; Chen, Zijun; Zhang, Jie; Zhao, Haochen; Wei, Lincheng; Sharma, Ashwani; Bakaul, Saidur; Zeng, Yuping (December 2024, Semiconductor Science and Technology)

   Abstract In this work, TiO₂thin films deposited by the atomic layer deposition (ALD) method were treated with a special N₂O plasma surface treatment and used as the gate dielectric for AlGaN/GaN metal insulator semiconductor high electron mobility transistors (MISHEMTs). The N₂O plasma surface treatment effectively reduces defects in the oxide during low-temperature ALD growth. In addition, it allows oxygen atoms to diffuse into the device cap layer to increase the barrier height and thus reduce the gate leakage current. These TiO₂films exhibit a dielectric constant of 54.8 and a two-terminal current of 1.96 × 10⁻¹⁰A mm⁻¹in 2μm distance. When applied as the gate dielectric, the AlGaN/GaN MISHEMT with a 2μm-gate-length shows a high on/off ratio of 2.59 × 10⁸and a low subthreshold slope (SS) of 84 mV dec⁻¹among all GaN MISHEMTs using TiO₂as the gate dielectric. This work provides a feasible way to significantly improve the TiO₂film electrical property for gate dielectrics, and it suggests that the developed TiO₂dielectric is a promising high-κgate oxide and a potential passivation layer for GaN-based MISHEMTs, which can be further extended to other transistors. 

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3. Magnetic Ni‐Nanoinclusions in VO ₂ Thin Films for Broad Tuning of Phase Transition Properties

   https://doi.org/10.1002/apxr.202300031  

   He, Zihao; Jian, Jie; Quigley, Lizabeth; Bhatt, Nirali A; Barnard, James P; Mihalko, Claire A; Wang, Haohan; Phuah, Xin Li; Lu, Juanjuan; Xu, Xiaoshan; et al (December 2023, Advanced Physics Research)

   Abstract Mott insulator VO₂exhibits an ultrafast and reversible semiconductor‐to‐metal transition (SMT) near 340 K (67 °C). In order to fulfill the multifunctional device applications, effective transition temperature (T_c) tuning as well as integrated functionality in VO₂is desired. In this study, multifunctionalities including tailorable SMT characteristics, ferromagnetic (FM) integration, and magneto‐optical (MO) coupling, have been demonstrated via metal/VO₂nanocomposite designs with controlled morphology, i.e., a two‐phase Ni/VO₂pillar‐in‐matrix geometry and a three‐phase Au/Ni/VO₂particle‐in‐matrix geometry. EvidentT_creduction of 20.4 to 54.9 K has been achieved by morphology engineering. Interestingly, the Au/Ni/VO₂film achieves a record‐lowT_cof 295.2 K (22.2 °C), slightly below room temperature (25 °C). The change in film morphology is also correlated with unique property tuning. Highly anisotropic magnetic and optical properties have been demonstrated in Ni/VO₂film, whereas Au/Ni/VO₂film exhibits isotropic properties because of the uniform distribution of Au/Ni nanoparticles. Furthermore, a strong MO coupling with enhanced magnetic coercivity and anisotropy is demonstrated for both films, indicating great potential for optically active property tuning. This demonstration opens exciting opportunities for the VO₂‐based device implementation towards smart windows, next‐generation optical‐coupled switches, and spintronic devices. 

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4. Antenna-coupled microbolometer based on VO ₂ 's non-linear properties across the metal–insulator transition region

   https://doi.org/10.1063/5.0123779  

   Chen, Shangyi; Lust, Mark; Ghalichechian, Nima (November 2022, Applied Physics Letters)

   This paper presents an antenna-coupled non-linear vanadium dioxide (VO 2 ) microbolometer operating in the non-linear metal–insulator transition (MIT) region with an ultra-high responsivity of 6.55 × 10 4 V/W. Sputtered VO 2 films used in this device exhibit 10 4 times change in resistivity between the dielectric and conductive states. The VO 2 microbolometer is coupled to a wideband dipole antenna operating at 31–55 GHz and a coplanar waveguide for probed measurement. To enhance the sensitivity, the sensor is suspended in air by micro-electro-mechanical systems process. The large thermal coefficient of resistance of VO 2 is utilized by DC biasing the device in the MIT region. Measurements for the fabricated sensor were performed, and a high responsivity was demonstrated, owing to non-linear conductivity change in the transition region. The measured sensitivity is >10 2 times higher than the state-of-the-art sensors. In addition, the concept of utilizing the proposed VO 2 sensor in a mmWave imager was demonstrated by the radiation pattern measurement of a 4 × 4 (16 elements) antenna-coupled VO 2 sensor array. The results presented in this work reveal the initial step to employ VO 2 's MIT for a hyper-sensitive sensor in future mmWave sensing and imaging applications. 

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5. Charge‐Density‐Wave Resistive Switching and Voltage Oscillations in Ternary Chalcogenide BaTiS ₃

   https://doi.org/10.1002/aelm.202300461  

   Chen, Huandong; Wang, Nan; Liu, Hefei; Wang, Han; Ravichandran, Jayakanth (November 2023, Advanced Electronic Materials)

   Abstract Phase change materials, which show different electrical characteristics across the phase transitions, have attracted considerable research attention for their potential electronic device applications. Materials with metal‐to‐insulator or charge density wave (CDW) transitions such as VO₂and 1T‐TaS₂have demonstrated voltage oscillations due to their robust bi‐state resistive switching behavior with some basic neuronal characteristics. BaTiS₃is a small bandgap ternary chalcogenide that has recently reported the emergence of CDW order below 245 K. Here, the discovery of DC voltage / current‐induced reversible threshold switching in BaTiS₃devices between a CDW phase and a room temperature semiconducting phase is reported. The resistive switching behavior is consistent with a Joule heating scheme and sustained voltage oscillations with a frequency of up to 1 kHz are demonstrated by leveraging the CDW phase transition and the associated negative differential resistance. Strategies of reducing channel sizes and improving thermal management may further improve the device's performance. The findings establish BaTiS₃as a promising CDW material for future electronic device applications, especially for energy‐efficient neuromorphic computing. 

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