About

Kai Fu is an Assistant Professor in the Department of Electrical & Computer Engineering at the University of Utah. His research focuses on semiconductor devices, wide bandgap semiconductors such as GaN, AlN, Ga2O3, BN, and diamond, as well as material epitaxy science, nanofabrication, semiconductor characterization, reliability, heterogeneous integration, power electronics, and high-temperature electronics. His work involves developing advanced semiconductor technologies and exploring their applications in various electronic systems. As part of his academic contributions, he advances the understanding and development of novel semiconductor materials and device fabrication techniques, contributing to the fields of power electronics and high-temperature electronics.

Research topics

• Materials science
• Nanotechnology
• Electrical engineering
• Chemistry
• Engineering
• Optoelectronics
• Engineering physics
• Computer Science
• Composite material
• Physics
• Crystallography
• Optics
• Condensed matter physics

Selected publications

• <i>In situ</i> XRD study of strain evolution in AlGaN/GaN HEMT at high temperatures up to 1000 °C

  Applied Physics Letters · 2026-03-16

  articleSenior author

  The thermal stability and structural evolution of a GaN high-electron mobility transistor (HEMT) heterostructure grown on a Si (111) substrate were investigated using in situ high-temperature x-ray diffraction, reciprocal space mapping (RSM), Raman spectroscopy, and rocking-curve (RC) analysis at varying temperatures. The heterostructure, consisting of a p-GaN cap, an AlGaN barrier, and a GaN channel supported by two AlGaN/AlGaN superlattice buffer layers, maintained clear and periodic satellite peaks up to a temperature of 1000 °C, confirming structural integrity. Symmetric and asymmetric RSM results reveal that both the Si and GaN diffraction peaks shift with increasing temperature, consistent with thermal expansion, and show no significant broadening or relaxation throughout the heating process. The c-lattice constant follows the theoretical expansion predicted by the multi-frequency Einstein model, whereas the a-lattice expansion is slower due to in-plane strain constraints imposed by the underlying Si substrate and buffer layers. Irreversible strain relaxation and thermal mismatch-induced stress redistribution induce a residual compressive strain of roughly 0.3% in the GaN channel after cooling, which has been further confirmed in Raman spectra through a blue shift (∼1 cm−1) of the GaN E2 (high) phonon mode, corresponding to an in-plane strain of −0.15% ± 0.16%. RC analysis revealed an increase in both screw and edge dislocation densities of 28% and 12%, respectively. These results collectively demonstrate that the GaN HEMT heterostructure exhibits robust crystalline stability up to 1000 °C without cracking due to strain relaxation, with only minor strain redistribution and limited dislocation activity, providing experimental evidence for GaN devices' applications under high-temperature conditions.

  PublisherDOI

• Dosimetry for proton therapy using a <i>β</i> -Ga ₂ O ₃ metal–semiconductor–metal detector with low-noise amplification

  Semiconductor Science and Technology · 2026-04-01

  articleOpen accessSenior author

  Abstract Intensity-modulated proton therapy (IMPT) employs proton radiation rather than conventional x-rays to treat cancerous tumors. This approach offers significant advantages by minimizing the radiation exposure of surrounding healthy tissue, leading to improved patient outcomes and reduced side effects compared to traditional x-ray therapy. To ensure patient safety, each treatment plan must be experimentally validated before clinical implementation. However, current dosimetry devices face limitations in performing angled beam measurements and obtaining multi-depth assessments, both of which are essential for verifying IMPT treatment plans. In this study, the performance of a β -Ga 2 O 3 -based metal–semiconductor–metal detector with a low-noise amplifier has been studied and evaluated under various proton radiation doses and energy levels delivered by a MEVION S250i proton accelerator. The detector’s performance was also compared with that of an ionization chamber. The β -Ga 2 O 3 detector exhibited a linear response with proton dose for single-spot irradiations, and its response to varying proton energies closely matched both the ion chamber data and simulated dose distributions. These findings highlight the potential of β -Ga 2 O 3 -based detectors as robust dosimetry devices for IMPT applications.

  PublisherDOI

• <i>β</i> -Ga ₂ O ₃ -Based Radiation Detector for Proton Therapy

  IEEE Transactions on Nuclear Science · 2026-01-19

  articleSenior author

  Intensity-modulated proton therapy (IMPT) is an advanced cancer treatment modality that offers advantages over conventional X-ray therapies, particularly in its ability to minimize radiation dose beyond the tumor target. This reduction in unnecessary irradiation exposure lowers the risk to surrounding healthy tissue and reduces side effects compared to conventional X-ray treatments. However, due to the high complexity of intensity-modulated proton therapy plans, each plan must be independently validated to ensure the safety of the radiation exposure to the patient. While ion chambers are currently used for this purpose, their limitations—particularly in angled beam measurements and multi-depth assessments—hinder their effectiveness. Silicon-based detectors, commonly used in X-ray therapy, are unsuitable for IMPT due to their rapid degradation under proton irradiation. In this study, a β-Ga<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>O<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub>-based metal–semiconductor–metal detector was compared with a commercial ion chamber using a MEVION S250i proton accelerator. The β-Ga<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>O<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> detector demonstrated reliable detection of single-pulse proton doses as low as 0.26 MU and exhibited a linear charge-to-dose relationship across a wide range of irradiation conditions. Furthermore, its measurement variability was comparable to that of the ion chamber. These results highlight the strong potential of β-Ga<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>O<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> as a detector material for dose verification in IMPT.

  PublisherDOI

• Thermal Stability and Phase Transformation of Conductive $α$-$(\mathrm{Al}_{x}\mathrm{Ga}_{1-x})_{2}\mathrm{O}_{3}/\mathrm{Ga}_{2}\mathrm{O}_{3}$ Heterostructures on Sapphire Substrates

  arXiv (Cornell University) · 2026-02-07

  articleOpen access

  Thermal stability and phase transformation of conductive $α$-$(\mathrm{Al}_{0.16}\mathrm{Ga}_{0.84})_{2}\mathrm{O}_{3}/\mathrm{Ga}_{2}\mathrm{O}_{3}$ heterostructures on sapphire substrates were investigated using in situ high-temperature X-ray diffraction (HT-XRD), scanning electron microscopy (SEM), and atomic force microscopy (AFM). Conductive $α$-$(\mathrm{Al}_{0.16}\mathrm{Ga}_{0.84})_{2}\mathrm{O}_{3}/\mathrm{Ga}_{2}\mathrm{O}_{3}$ heterostructures with fluorine (F) doping were grown by mist chemical vapor deposition on sapphire substrates, achieving a Hall mobility of $28~\mathrm{cm^{2}\,V^{-1}\,s^{-1}}$ and an electron concentration of $1.4\times10^{20}~\mathrm{cm^{-3}}$. The heterostructures exhibited thermal stability up to approximately $550$--$

  PublisherOA PDF

• The study of the effects of interface on the HER performance of copper and copper-based catalysts

  Journal of Alloys and Compounds · 2026-04-01

  article
  PublisherDOI

• Crystallinity Evolution of MOCVD-Grown $β$-Ga$_2$O$_3$ Films Probed by In Situ HT-XRD under Different Reactor Heights

  ArXiv.org · 2026-02-05

  articleOpen accessSenior author

  The crystallinity of $β$-Ga$_2$O$_3$ thin films grown by metal-organic chemical vapor deposition (MOCVD) is strongly influenced by reactor design and the resulting growth environment. In this work, we investigate the role of reactor height on the crystallinity evolution of MOCVD-grown $β$-Ga$_2$O$_3$ films by directly comparing long- and short-chamber showerhead configurations. Structural evolution was probed by in situ high-temperature X-ray diffraction (HT-XRD) as the MOCVD-grown films were heated from 25~$^\circ$C to 1100~$^\circ$C. Temperature-dependent XRD reveals a consistent redshift of the $β$-Ga$_2$O$_3$~($-201$) reflection after HT-XRD heating and subsequent cooling to room temperature for both reactor geometries, indicating a similar thermally driven strain response. Quantitative rocking-curve analysis shows a non-monotonic temperature dependence of the ($-201$) full width at half maximum (FWHM), with minimum values of approximately 2.03$^\circ$ and 2.72$^\circ$ for the short- and long-chamber films, respectively, reflecting differences in mosaic alignment established during growth. Atomic force microscopy further shows that short-chamber-grown films exhibit smoother surfaces, with root-mean-square roughness values of approximately 7.7~nm before and 7.3~nm after HT-XRD heating, compared to 19.3~nm and 12.3~nm, respectively, for long-chamber-grown films. Overall, these results indicate that reactor height influences the initial crystalline and morphological templates of $β$-Ga$_2$O$_3$ films and modulates their elevated-temperature structural response, providing practical insights for optimizing MOCVD reactor design for high-quality $β$-Ga$_2$O$_3$ growth.

  PublisherOA PDF

• In Situ Study of p‐NiO Film Quality at High Temperatures up to 1100°C

  physica status solidi (a) · 2026-05-08

  articleSenior author

  NiO is a promising material for photovoltaics and high‐power electronics, especially as a p‐type material. However, the temperature limit of this material has not been thoroughly investigated. This work evaluates the structural evolution and thermal stability of NiO films using in situ high‐temperature X‐ray diffraction from 30°C to 1100°C in air. The film transformed from an amorphous state to a cubic NiO phase between 300°C and 400°C. The NiO (111) peak intensity follows a similar temperature‐dependent trend as the film's resistivity, indicating that higher temperatures promote increased crystallinity, which correlates with higher resistivity. At 1100°C, the formation of Ni 2 O 3 was observed, accompanied by a transition to a highly resistive film. The study presents a clear evolution of NiO film quality and crystallinity across temperatures and establishes their correlation with the material's resistive behavior.

  PublisherDOI

• Memory Phenomenon of GaN pn Junction by Engineering Its Interface, Which is Stable at High Temperatures up to 500oC

  IMAPSource Proceedings · 2026-02-04

  articleOpen access1st authorCorresponding

  Memory Phenomenon of GaN pn Junction by Engineering Its Interface, Which is Stable at High Temperatures up to 500oC

  PublisherOA PDFDOI

• Dosimetry for Proton Therapy Using a β-Ga$_2$O$_3$ Metal-Semiconductor-Metal Detector with Low-Noise Amplification

  ArXiv.org · 2026-01-21

  articleOpen accessSenior author

  Intensity-modulated proton therapy (IMPT) employs proton radiation rather than conventional X-rays to treat cancerous tumors. This approach offers significant advantages by minimizing the radiation exposure of surrounding healthy tissue, leading to improved patient outcomes and reduced side effects compared to traditional X-ray therapy. To ensure patient safety, each treatment plan must be experimentally validated before clinical implementation. However, current dosimetry devices face limitations in performing angled beam measurements and obtaining multi-depth assessments, both of which are essential for verifying IMPT treatment plans. In this study, the performance of a β-Ga$_2$O$_3$-based metal-semiconductor-metal (MSM) detector with a low-noise amplifier is studied and evaluated under various proton radiation doses and energy levels delivered by a MEVION S250i proton accelerator. The detector performance is also compared with that of an ionization chamber. The β-Ga$_2$O$_3$ detector exhibits a linear response with proton dose for single-spot irradiations, and its response to varying proton energies closely matches both the ion chamber data and simulated dose distributions. These findings highlight the potential of β-Ga$_2$O$_3$-based detectors as robust dosimetry devices for IMPT applications.

  PublisherOA PDF

• Control of extraordinary optical transmission in resonant terahertz gratings via lateral depletion in an AlGaN/GaN heterostructure

  Journal of Applied Physics · 2026-02-17

  articleOpen access

  Periodic metallic gratings on substrates can support a range of electromagnetic modes, such as leaky waveguide, guided-resonant, and Fabry–Pérot cavity modes, which can strongly modulate optical transmission under resonant excitation. Here, we investigate how this coupling can be dynamically manipulated through charge-density control in a laterally patterned AlGaN/GaN heterostructure. The structure comprises metallic stripes separated by regions containing a two-dimensional electron gas (2DEG), forming a periodically modulated interface whose electromagnetic response is governed by the charge density between the stripes. In the unbiased state, the conductive 2DEG screens the incident terahertz field and suppresses excitation of guided modes. When the 2DEG is depleted, the change in boundary conditions allows efficient coupling into substrate resonances, producing a strong modulation at particular frequencies where extraordinary optical transmission (EOT) through the structure takes place. The results highlight the sensitive dependence of guided-mode-resonance mediated EOT on inter-stripe charge distribution and demonstrate a direct interplay between carrier dynamics and resonant electromagnetic phenomena in the terahertz regime.

  PublisherDOI

Frequent coauthors

• Yuji Zhao

  Rice University

  130 shared

• Houqiang Fu

  78 shared

• Jingan Zhou

  74 shared

• Yong Cai

  Suzhou Institute of Nano-tech and Nano-bionics

  57 shared

• Guohao Yu

  Suzhou Institute of Nano-tech and Nano-bionics

  56 shared

• Shuan Li

  Peking University

  48 shared

• Baoshun Zhang

  47 shared

• Xingguo Li

  Peking University

  45 shared

Labs

• Utah Nanofab Utah Robotics Center U-Smart Energy LaboratoryPI

Education

• Ph.D.

  Suzhou Institute of Nano-tech and Nano-bionics

  2013

• B.S.

  Ocean University of China

  2008

Awards & honors

• ACES Fellow
• UTAH Nanofab System Development Award