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1. Heavy ion irradiation induced failure of gallium nitride high electron mobility transistors: effects of in-situ biasing

   https://doi.org/10.1088/1361-6463/accfa7  

   Rasel, Md Abu Jafar ; Schoell, Ryan ; Al-Mamun, Nahid Sultan ; Hattar, Khalid ; Harris, C. Thomas ; Haque, Aman ; Wolfe, Douglas E. ; Ren, Fan ; Pearton, Stephen J. ( May 2023 , Journal of Physics D: Applied Physics)

   While radiation is known to degrade AlGaN/GaN high-electron-mobility transistors (HEMTs), the question remains on the extent of damage governed by the presence of an electrical field in the device. In this study, we induced displacement damage in HEMTs in both ON and OFF states by irradiating with 2.8 MeV Au⁴⁺ion to fluence levels ranging from$1.72\times {10}^{10}$to$3.745\times {10}^{13}$ions cm⁻², or 0.001–2 displacement per atom (dpa). Electrical measurement is donein situ, and high-resolution transmission electron microscopy (HRTEM), energy dispersive x-ray (EDX), geometrical phase analysis (GPA), and micro-Raman are performed on the highest fluence of Au⁴⁺irradiated devices. The selected heavy ion irradiation causes cascade damage in the passivation, AlGaN, and GaN layers and at all associated interfaces. After just 0.1 dpa, the current density in the ON-mode device deteriorates by two orders of magnitude, whereas the OFF-mode device totally ceases to operate. Moreover, six orders of magnitude increase in leakage current and loss of gate control over the 2-dimensional electron gas channel are observed. GPA and Raman analysis reveal strain relaxation after a 2 dpa damage level in devices. Significant defects and intermixing of atoms near AlGaN/GaN interfaces and GaN layer are found from HRTEM and EDX analyses, which can substantially alter device characteristics and result in complete failure.

    

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2. Influence of Electrical Field on the Susceptibility of Gallium Nitride Transistors to Proton Irradiation

   https://doi.org/10.1088/1361-6463/ad3f29  

   Rasel, Md Abu Jafar ; Schoell, Ryan ; Smyth, Christopher M. ; Hattar, Khalid ; Harris, C. T. ; Lu, Tzu-Ming ; Haque, Aman ; Wolfe, Douglas E. ; Ren, Fan ; Pearton, Stephen J. ( April 2024 , Journal of Physics D: Applied Physics)

   Radiation susceptibility of electronic devices is commonly studied as a function of radiation energetics and device physics. Often overlooked is the presence or magnitude of the electrical field, which we hypothesize to play an influential role in low energy radiation. Accordingly, we present a comprehensive study of low-energy proton irradiation on Gallium Nitride High Electron Mobility Transistors (HEMTs), turning the transistor ON or OFF during irradiation. Commercially available GaN HEMTs were exposed to 300 keV proton irradiation at fluences varying from 3.76 x1012 to 3.76x1014 cm-2, and the electrical performance was evaluated in terms of forward saturation current, transconductance, and threshold voltage. The results demonstrate that the presence of an electrical field makes it more susceptible to proton irradiation. The decrease of 12.4% in forward saturation and 19% in transconductance at the lowest fluence in ON mode suggests that both carrier density and mobility are reduced after irradiation. Additionally, a positive shift in threshold voltage (0.32 V and 0.09 V in ON and OFF mode, respectively) indicates the generation of acceptor-like traps due to proton bombardment. HRTEM (High-Resolution Transmission Electron Microscopy) and EDS (Energy Dispersive X-ray Spectroscopy) analysis reveal significant defects introduction and atom intermixing near AlGaN/GaN interfaces and within the GaN layer after the highest irradiation dose employed in this study. According to in-situ Raman spectroscopy, defects caused by irradiation can lead to a rise in self-heating and a considerable increase in (~ 750 times) thermoelastic stress in the GaN layer during device operation. The findings indicate device engineering or electrical biasing protocol must be employed to compensate for radiation-induced defects formed during proton irradiation to improve device durability and reliability.

    

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3. Gamma radiation on gallium nitride high electron mobility transistors at ON, OFF, and prestressed conditions

   https://doi.org/10.1116/6.0002216  

   Rasel, Md Abu Jafar ; Stepanoff, Sergei ; Haque, Aman ; Wolfe, Douglas E. ; Ren, Fan ; Pearton, Stephen J. ( November 2022 , Journal of Vacuum Science & Technology B)

   Radiation damage in electronic devices is known to be influenced by physics, design, and materials system. Here, we report the effects of biasing state (such as ON and OFF) and pre-existing damage in GaN high electron mobility transistors exposed to γ radiation. Controlled and accelerated DC biasing was used to prestress the devices, which showed significant degradation in device characteristics compared to pristine devices under ON and OFF states after γ irradiation. The experiment is performed in situ for the ON-state to investigate transient effects during irradiation until the total dose reaches 10 Mrad. It shows that threshold voltage, maximum transconductance, and leakage current initially decrease with dosage but slowly converge to a steady value at higher doses. After 10 Mrad irradiation, the OFF-state device demonstrates larger RON and one order of magnitude increased leakage current compared to the ON-state irradiated device. The micro-Raman study also confirms that the ON-state operation shows more radiation hardness than OFF and prestressed devices. Prestressed devices generate the highest threshold voltage shift from −2.85 to −2.49 V and two orders of magnitude higher leakage current with decreased saturation current after irradiation. These findings indicate that high electric fields during stressing can generate defects by modifying strain distribution, and higher defect density can not only create more charges during irradiation but also accelerate the diffusion process from the ionizing track to the nearest collector and consequently degrade device performances.

    

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4. Tensorial stress-plastic strain fields in α - ω Zr mixture, transformation kinetics, and friction in diamond-anvil cell

   https://doi.org/10.1038/s41467-023-41680-1  

   Levitas, Valery I. ; Dhar, Achyut ; Pandey, K. K. ( September 2023 , Nature Communications)

   Various phenomena (phase transformations (PTs), chemical reactions, microstructure evolution, strength, and friction) under high pressures in diamond-anvil cell are strongly affected by fields of stress and plastic strain tensors. However, they could not be measured. Here, we suggest coupled experimental-analytical-computational approaches utilizing synchrotron X-ray diffraction, to solve an inverse problem and find fields of all components of stress and plastic strain tensors and friction rules before, during, and after α-ω PT in strongly plastically predeformed Zr. Results are in good correspondence with each other and experiments. Due to advanced characterization, the minimum pressure for the strain-induced α-ω PT is changed from 1.36 to 2.7 GPa. It is independent of the plastic strain before PT and compression-shear path. The theoretically predicted plastic strain-controlled kinetic equation is verified and quantified. Obtained results open opportunities for developing quantitative high-pressure/stress science, including mechanochemistry, synthesis of new nanostructured materials, geophysics, astrogeology, and tribology.

    

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5. The influence of crystal thickness and interlayer interactions on the properties of heavy ion irradiated MoS 2

   https://doi.org/10.1088/2053-1583/ab817b  

   Isherwood, Liam H. ; Hennighausen, Zachariah ; Son, Seok-Kyun ; Spencer, Ben F. ; Wady, Paul T. ; Shubeita, Samir M. ; Kar, Swastik ; Casiraghi, Cinzia ; Baidak, Aliaksandr ( May 2020 , 2D Materials)

   Ion irradiation is a versatile tool to introduce controlled defects into two-dimensional (2D) MoS₂on account of its unique spatial resolution and plethora of ion types and energies available. In order to fully realise the potential of this technique, a holistic understanding of ion-induced defect production in 2D MoS₂crystals of different thicknesses is mandatory. X-ray photoelectron spectroscopy, electron diffraction and Raman spectroscopy show that thinner MoS₂crystals are more susceptible to radiation damage caused by 225 keV Xe⁺ions. However, the rate of defect production in quadrilayer and bulk crystals is not significantly different under our experimental conditions. The rate at which S atoms are sputtered as a function of radiation exposure is considerably higher for monolayer MoS₂, compared to bulk crystals, leading to MoO₃formation. P-doping of MoS₂is observed and attributed to the acceptor states introduced by vacancies and charge transfer interactions with adsorbed species. Moreover, the out-of-plane vibrational properties of irradiated MoS₂crystals are shown to be strongly thickness-dependent: in mono- and bilayer MoS₂, the confinement of phonons by defects results in a blueshift of the$A_{1g}$mode. Whereas, a redshift is observed in bulk crystals due to attenuation of the effective restoring forces acting on S atoms caused by vacancies in adjacent MoS₂layers. Consequently, the$A_{1g}$frequency of tri- and quadrilayer crystals is statistically invariant on account oft competition between phonon confinement effects and interlayer interactions. The$A_{1g}$linewidth is observed to decrease in bi- and trilayer crystals after low dose irradiation and is attributed to layer decoupling. This work shows that there is a complex interplay between defect production, crystal thickness and interlayer interactions in MoS₂. Our results demonstrate that ion irradiation is an effective tool to modulate the electronic, vibrational and structural properties of MoS₂, which may prove beneficial for practical applications.

    

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