arXiv subjects

Kai Fu

Publications and source records attributed to Kai Fu.

34 records · Page 2Linked to original sources

arXiv2025

Phase Evolution and Substrate-Dependent Nucleation of Quartz GeO$_2$ Films Grown by MOCVD on r- and c-Plane Sapphires

Ultrawide-bandgap (UWBG) semiconductors, such as GeO$_2$, are gaining significant attention for their potential in high-performance applications, particularly in piezoelectric devices. Despite extensive research, a comprehensive understanding of the growth dynamics and phase evolution of GeO$_2$ films via metal-organic chemical vapor deposition (MOCVD) remains insufficient. In this study, we investigate the growth behavior and morphological evolution of GeO$_2$ thin films on r-plane and c-plane sapphire substrates for the MOCVD growth process. The temporal evolution of crystallization and the amorphous-to-quartz phase transition are systematically elucidated for the first time. As growth time increases, the spherulitic quartz patterns expand in size, and elevated growth temperatures are found to enhance the crystallization rate. Distinct morphological symmetries emerge depending on the substrate orientation: quadrangular patterns on r-plane sapphire and hexagonal patterns on c-plane sapphire. Atomic force microscopy reveals that these spherulitic domains exhibit pyramid-like surface topography, consistent with volumetric contraction during the amorphous-to-quartz phase transition. These findings offer new insights into the phase evolution and substrate-dependent crystallization behavior of GeO$_2$ films grown by MOCVD.

cond-mat.mtrl-sci↗

arXiv2025

Carbon-Nanotube/$β$-Ga$_2$O$_3$ Heterojunction PIN Diodes

$β$-Ga$_2$O$_3$ is gaining attention as a promising semiconductor for next-generation high-power, high-efficiency, and high-temperature electronic devices, thanks to its exceptional material properties. However, challenges such as the lack of viable p-type doping have hindered its full potential, particularly in the development of ambipolar devices. This work introduces a novel heterojunction diode (HD) that combines p-type carbon nanotubes (CNTs) with i/n-type $β$-Ga$_2$O$_3$ to overcome these limitations. For the first time, a CNT/$β$-Ga$_2$O$_3$ hetero-p-n-junction diode is fabricated. Compared to a traditional Schottky barrier diode (SBD) with the same $β$-Ga$_2$O$_3$ epilayer, the CNT/$β$-Ga$_2$O$_3$ HD demonstrates significant improvements, including a higher rectifying ratio ($1.2 \times 10^{11}$), a larger turn-on voltage (1.96 V), a drastically reduced leakage current at temperatures up to 300 °C, and a 26.7% increase in breakdown voltage. Notably, the CNT/$β$-Ga$_2$O$_3$ HD exhibits a low ideality factor of 1.02, signifying an ideal interface between the materials. These results underline the potential of CNT/$β$-Ga$_2$O$_3$ heterojunctions for electronic applications, offering a promising solution to current limitations in $β$-Ga$_2$O$_3$-based devices.

physics.app-ph↗

arXiv2025

Degradation of 2.4-kV $Ga_{2}O_{3}$ Schottky Barrier Diode at High Temperatures up to 500 °C

Ga2O3 Schottky barrier diodes featuring a field plate and a composite SiO2/SiNx dielectric layer beneath the field plate were fabricated, achieving a breakdown voltage of 2.4 kV at room temperature. Electrical performance and degradation were analyzed via I-V and C-V measurements from 25 °C to 500 °C, revealing temperature-dependent transport, interface stability, and device stability. Upon returning to room temperature, the diodes exhibited nearly unchanged forward characteristics, while the breakdown voltage declined significantly from 2.4 kV to 700 V. This behavior indicates a temperature-induced reduction in the barrier height. Detailed analysis revealed that variable range hopping (VRH) dominated the leakage mechanism at moderate temperatures, while thermal emission (TE) became increasingly significant at temperatures exceeding 400 °C.

cond-mat.mtrl-sci↗

arXiv2024

A TEM Study of MOCVD-Grown Rutile GeO2 Films

Ultrawide bandgap (UWBG) semiconductors are promising for next-generation power electronics, largely attributed to their substantial bandgap and exceptional breakdown electric field. Rutile GeO2 (r-GeO2) emerges as a promising alternative, particularly because of its ambipolar dopability. However, research on r-GeO2 is still in its infancy, and further investigation into its structural properties is essential for enhancing epilayer quality. In our previous work, we identified distinct surface morphologies; square-patterned and smooth regions of epitaxial r-GeO2 films grown on r-TiO2 (001) substrates using metal-organic chemical vapor deposition (MOCVD).This research employs transmission electron microscopy (TEM) to investigate the structural characteristics of the material. The findings indicate that the square-patterned regions are crystalline, whereas the smooth regions exhibit amorphous properties. The measured lattice spacing in the (110) plane is 0.324 nm, slightly exceeding the theoretical value of 0.312 nm. This discrepancy suggests the presence of tensile strain in the r-GeO2 film, resulting from lattice mismatch or thermal expansion differences with the substrate. We also observed a threading dislocation density of 1.83*10^9 cm-2, consisting of 11.76% screw-type, 29.41% edge-type, 55.89% mixed-type dislocations, and 2.94% planar defects. These findings offer valuable insights into the growth mechanisms and defect characteristics of r-GeO2.

cond-mat.mtrl-sci↗

arXiv2024

Epitaxial Growth of Rutile GeO$_2$ via MOCVD

Rutile Germanium Dioxide (r-GeO$_2$) has been identified as an ultrawide bandgap (UWBG) semiconductor recently, featuring a bandgap of 4.68 eV, comparable to Ga$_2$O$_3$ but offering bipolar dopability, higher electron mobility, higher thermal conductivity, and higher Baliga's figure of merit (BFOM).These superior properties position GeO$_2$ as a promising material for various semiconductor applications. However, the epitaxial growth of r-GeO$_2$, particularly in its most advantageous rutile polymorph, is still at an early stage. This work explores the growth of r-GeO$_2$ using metal-organic chemical vapor deposition (MOCVD) on an r-TiO$_2$ (001) substrate, utilizing tetraethyl germane (TEGe) as the precursor. Our investigations reveal that higher growth temperatures significantly enhance crystalline quality, achieving a full width at half maximum (FWHM) of 0.181 degree at 925 degree C, compared to 0.54 degree at 840 degree C and amorphous structures at 725 degree C. Additionally, we found that longer growth durations increase surface roughness due to the formation of faceted crystals. Meanwhile, adjusting the susceptor rotation speed from 300 RPM to 170 RPM plays a crucial role in optimizing crystalline quality, effectively reducing surface roughness by approximately 15 times. This study offers a foundational guide for optimizing MOCVD growth conditions of r-GeO$_2$ films, emphasizing the crucial need for precise control over deposition temperature and rotation speed to enhance adatom mobility and effectively minimize the boundary layer thickness.

physics.app-ph↗

arXiv2024

Growth of GeO2 on R-plane and C-plane Sapphires by MOCVD

Rutile Germanium Dioxide (GeO2) has been recently theoretically identified as an ultrawide bandgap (UWBG) semiconductor with bandgap 4.68 eV similar to Ga2O3 but having bipolar dopability and ~2x higher electron mobility, Baliga figure of merit (BFOM) and thermal conductivity than Ga2O3. Bulk crystal growth is rapidly moving towards making large sized native substrates available. These outstanding material properties position GeO2 as a highly attractive UWBG semiconductor for various applications. However, the epitaxial growth in the most advantageous polymorph (rutile), ensuring controlled phase, pristine surface/interface quality, precise microstructure, and optimal functional properties, is still in its infancy. In this work, we explored growth of GeO2 by metal-organic chemical vapor deposition (MOCVD) on both C- and R-plane sapphire. Utilizing tetramethylgermane (TMGe) as a precursor, we have investigated the influences of different parameters on the film properties, including growth temperature, chamber pressure, TMGe flow rate, oxygen flow rate, shroud gas flow rate, and rotation speed. The total pressure emerged as a crucial parameter while growth attempts at low total pressure resulted in no films for a wide range of temperatures, precursor flow rate, argon flow rates, and susceptor rotation rate. A phase diagram, derived from our experimental findings, delineates the growth windows for GeO2 films on sapphire substrates. This study serves as a pioneering guide for the MOCVD growth of GeO2 films.

cond-mat.mtrl-sci↗

arXiv2023

Robust Diamond/\b{eta}-Ga2O3 Hetero-p-n-junction Via Mechanically Integrating Their Building Blocks

We report a novel approach for crafting robust diamond/\b{eta}-Ga2O3 hetero-p-n-junctions through the mechanical integration of their bulk materials. This resulting heterojunction, with a turn-on voltage of ~2.7 V at room temperature, exhibits resilient electrical performance across a temperature spectrum up to 125°C, displaying minimal hysteresis-measuring as low as 0.2 V at room temperature and below 0.7 V at 125°C. Remarkably, the ideality factor achieves a record low value of 1.28, setting a new benchmark for diamond/ \b{eta}-Ga2O3 heterojunctions. The rectification ratio reaches over 10^8 at different temperatures. This effortlessly fabricated and remarkably resilient diamond/Ga2O3 hetero-p-n-junction pioneers a novel pathway for the exploration and fabrication of heterojunctions for ultra-wide bandgap semiconductors with substantial lattice mismatch and different thermal expansion coefficients.

cond-mat.mtrl-sci↗

arXiv2022

Properties and device performance of BN thin films grown on GaN by pulsed laser deposition

Wide and ultrawide-bandgap semiconductors lie at the heart of next-generation high-power, high-frequency electronics. Here, we report the growth of ultrawide-bandgap boron nitride (BN) thin films on wide-bandgap gallium nitride (GaN) by pulsed laser deposition. Comprehensive spectroscopic (core level and valence band XPS, FTIR, Raman) and microscopic (AFM and STEM) characterizations confirm the growth of BN thin films on GaN. Optically, we observed that BN/GaN heterostructure is second-harmonic generation active. Moreover, we fabricated the BN/GaN heterostructure-based Schottky diode that demonstrates rectifying characteristics, lower turn-on voltage, and an improved breakdown capability (234 V) as compared to GaN (168 V), owing to the higher breakdown electrical field of BN. Our approach is an early step towards bridging the gap between wide and ultrawide-bandgap materials for potential optoelectronics as well as next-generation high-power electronics.

physics.app-ph↗

arXiv2022

Transfer learning to decode brain states reflecting the relationship between cognitive tasks

Transfer learning improves the performance of the target task by leveraging the data of a specific source task: the closer the relationship between the source and the target tasks, the greater the performance improvement by transfer learning. In neuroscience, the relationship between cognitive tasks is usually represented by similarity of activated brain regions or neural representation. However, no study has linked transfer learning and neuroscience to reveal the relationship between cognitive tasks. In this study, we propose a transfer learning framework to reflect the relationship between cognitive tasks, and compare the task relations reflected by transfer learning and by the overlaps of brain regions (e.g., neurosynth). Our results of transfer learning create cognitive taskonomy to reflect the relationship between cognitive tasks which is well in line with the task relations derived from neurosynth. Transfer learning performs better in task decoding with fMRI data if the source and target cognitive tasks activate similar brain regions. Our study uncovers the relationship of multiple cognitive tasks and provides guidance for source task selection in transfer learning for neural decoding based on small-sample data.

q-bio.NC↗

arXiv2021

Low-Frequency Noise Characteristics of GaN Vertical PIN Diodes -- Effects of Current and Temperature

We report low-frequency noise characteristics of vertical GaN PIN diodes, focusing on the effects of the diode design, current and temperature. The as-grown and regrown diodes, with and without surface treatment have been studied. The noise in most of the GaN devices had a characteristic 1/f spectrum at high and moderate currents, while some devices revealed generation-recombination bulges at low currents (f is the frequency). The predominant trend of the noise spectral density, S, dependence on the current was S ~ I. All tested GaN PIN diodes had rather low normalized noise spectral densities of 10^-18 cm2/Hz -- 10^-16 cm2/Hz (f=10 Hz) at the current density J=1 A/cm2 at room temperature. The noise temperature dependences at different currents revealed peaks at T=375 K -- 400 K. Temperature, current, and frequency dependences of noise suggest that the noise mechanism is of the recombination origin. We argue that the noise measurements at low currents can be used to efficiently assess the quality of GaN PIN diodes.

cond-mat.mtrl-sci↗

arXiv2019

Demonstration of beta-Ga2O3 Optical Waveguides and the Analysis of Their Propagation Losses in the UV-Visible Spectra

This paper reports the first demonstration of beta-phase gallium oxide as optical waveguides on sapphire substrates grown by metal-organic chemical vapor deposition (MOCVD). The propagation losses from visible to ultraviolet spectra were comprehensively studied. By optimizing the fabrication processes, minimum propagation loss was identified to be 3.7 dB/cm at the wavelength of 810 nm, which is comparable to other wide bandgap materials within the III-N family (GaN, AlN). To further reveal the underlying loss mechanisms, several physical mechanisms such as two-photon absorption, sidewall scattering, top surface scattering, and bulk scattering were taken into consideration. The results obtained from this work suggest that beta-Ga2O3 is promising for ultraviolet-visible spectrum integrated photonic applications.

physics.optics↗

arXiv2019

On-chip directional octave-spanning supercontinuum generation from high order mode in near ultraviolet to infrared spectrum using AlN waveguides

On-chip ultraviolet to infrared (UV-IR) spectrum frequency metrology is of crucial importance as a characterization tool for fundamental studies on quantum physics, chemistry, and biology. Due to the strong material dispersion, traditional techniques fail to demonstrate the device that can be applied to generate coherent broadband spectrum that covers the full UV-IR wavelengths. In this work, we explore several novel techniques for supercontinuum generation covering near-UV to near-IR spectrum using AlN micro-photonic waveguides, which is essential for frequency metrology applications: First, to create anomalous dispersion, high order mode (TE10) was adopted, together with its carefully designed high efficiency excitation strategies. Second, the spectrum was broadened by soliton fission through third order dispersion and second harmonic generation, by which directional energy transfer from near-IR to near-UV can be obtained. Finally, high quality single crystalline AlN material was used to provide broadband transparency from UV to IR. Under decently low pulse energy of 0.36 nJ, the experimental spectrum from supercontinuum generation covers from 490 nm to over 1100 nm, with a second harmonic generation band covering from 405 nm to 425 nm. This work paves the way towards UV-IR full spectrum on-chip frequency metrology applications.

physics.optics↗

arXiv2018

A Novel PN junction between Mechanically Exfoliated \b{eta}-Ga2O3 and p-GaN

Several pn junctions were constructed from mechanically exfoliated ultrawide bandgap (UWBG) beta-phase gallium oxide (\b{eta}-Ga2O3) and p-type gallium nitride (GaN). The mechanical exfoliation process, which is described in detail, is similar to that of graphene and other 2D materials. Atomic force microscopy (AFM) scans of the exfoliated \b{eta}-Ga2O3 flakes show very smooth surfaces with average roughness of 0.647 nm and transmission electron microscopy (TEM) scans reveal flat, clean interfaces between the \b{eta}-Ga2O3 flakes and p-GaN. The device showed a rectification ratio around 541.3 (V+5/V-5). Diode performance improved over the temperature range of 25°C and 200°C, leading to an unintentional donor activation energy of 135 meV. As the thickness of exfoliated \b{eta}-Ga2O3 increases, ideality factors decrease as do the diode turn on voltages, tending toward an ideal threshold voltage of 3.2 V as determined by simulation. This investigation can help increase study of novel devices between mechanically exfoliated \b{eta}-Ga2O3 and other materials.

physics.app-ph↗

arXiv2018

Direct numerical study of speed of sound in dispersed air-water two-phase flow

Speed of sound is a key parameter for the compressibility effects in multiphase flow. We present a new approach to do direct numerical simulations on the speed of sound in compressible two-phase flow, based on the stratified multiphase flow model (Chang & Liou, JCP 2007). In this method, each face is divided into gas-gas, gas-liquid, and liquid-liquid parts via reconstruction of volume fraction, and the corresponding fluxes are calculated by Riemann solvers. Viscosity and heat transfer models are included. The effects of frequency (below the natural frequency of bubbles), volume fraction, viscosity and heat transfer are investigated. With frequency 1 kHz, under viscous and isothermal conditions, the simulation results satisfy the experimental ones very well. The simulation results show that the speed of sound in air-water bubbly two-phase flow is larger when the frequency is higher. At lower frequency, for the phasic velocities, the homogeneous condition is better satisfied. Considering the phasic temperatures, during the wave propagation an isothermal bubble behavior is observed. Finally, the dispersion relation of acoustics in two-phase flow is compared with analytical results below the natural frequency. This work for the first time presents an approach to the direct numerical simulations of speed of sound and other compressibility effects in multiphase flow, which can be applied to study more complex situations, especially when it is hard to do experimental study.

physics.flu-dyn↗

arXiv2017

Implementation and validation of two-phase boiling flow models in OpenFOAM

Prediction of two-phase boiling flows using the computational fluid dynamics (CFD) approach is very challenging since several sub-models for interfacial mass, momentum and energy transfer in such flows are still not well established and require further development and validation. Once validating a particular model, it is important that all key parameter involved in the model are carefully verified. Such verification is typically performed by separate effect tests, where one parameter at a time is compared to a measured or otherwise known value. Needless to say that for complex models, which are typical for CFD applications to two-phase flow, the number of independent parameters that need to be verified can be quite high. This particular feature makes the validation process of complex CFD models in open source codes very attractive, since full access to the implementation details is possible. This paper is concerned with implementation and validation of two-phase boiling bubbly flow models using the OpenFOAM, open source environment. The model employs the two-fluid formulation of the conservation equations with the Reynolds-averaged treatment of the turbulent terms. The model consists of six conservation equations for the liquid and the vapor phase, allowing for the thermodynamic non-equilibrium and compressibility of both phases. In addition, the model includes two transport equations for the turbulence kinetic energy and energy dissipation and one transport equation for the interfacial area concentration. New models for wall heat partitioning as well as for the phase change terms in nucleate boiling have been implemented. Sensitivity studies as well as validation of the model against measured data available in the open literature have been performed and it has been shown that a reasonable agreement between predictions and experiments has been achieved.

physics.flu-dyn↗

arXiv2015

Statistical Constraints on the Error of the Leptonic CP Violation of Neutrinos

A constraint on the error of leptonic CP violation, which require the phase $δ_{CP}$ to be less than $π/4$ for it to be distinguishable on a $2π$ cycle, is presented. Under this constraint, the effects of neutrino detector 's distance, beam energy, and energy resolution are discussed with reference to the present values of these parameters in experiments. Although an optimized detector performances can minimize the deviation to yield a larger distinguishable range of the leptonic CP phase on a $2π$ cycle, it is not possible to determine an arbitrary leptonic CP phase in the range of $2π$ with the statistics from a single detector because of the existence of two singular points. An efficiency factor $η$ is defined to characterize the distinguishable range of $δ_{CP}$. To cover the entire possible $δ_{CP}$ range, a combined efficiency factor $η^*$ corresponding to multiple sets of detection parameters with different neutrino beam energies and distances is proposed. The combined efficiency factors $η^*$ of various major experiments are also presented.

hep-ph↗