arXiv subjects

Houqiang Fu

Publications and source records attributed to Houqiang Fu.

11 recordsLinked to original sources

arXiv2025

Acoustic Phonon Characteristics of Gallium Oxide Single Crystals Investigated with Brillouin-Mandelstam Light Scattering Spectroscopy

We report an investigation of the bulk and surface acoustic phonons in gallium oxide ultra-wide bandgap single crystals along various crystallographic directions using Brillouin-Mandelstam spectroscopy. Pronounced anisotropy in the acoustic phonon dispersion and velocities was observed across different crystal orientations. The measured average acoustic phonon velocities for the crystallographic directions of interest are 5,250 m/s and 4,990 m/s. The surface acoustic phonons propagate approximately twice as slowly as the bulk acoustic phonons. Our results suggest that the anisotropy of heat conduction in gallium oxide results from the difference in phonon velocities rather than the phonon lifetime. The obtained information for bulk and surface acoustic phonons can be used for developing accurate theoretical models of phonon scattering and optimization of thermal and electrical transport in this technologically important ultra-wide bandgap semiconductor.

cond-mat.mtrl-sci↗

arXiv2024

Acoustic and Optical Phonon Frequencies and Acoustic Phonon Velocities in Silicon-Doped Aluminum Nitride Thin Films

We report the results of the study of the acoustic and optical phonons in Si-doped AlN thin films grown by metalorganic chemical vapor deposition on sapphire substrates. The Brillouin - Mandelstam and Raman light scattering spectroscopies were used to measure the acoustic and optical phonon frequencies close to the Brillouin zone center. The optical phonon frequencies reveal non-monotonic changes, reflective of the variations in the thin-film strain and dislocation densities with the addition of Si dopant atoms. The acoustic phonon velocity decreases monotonically with increasing Si dopant concentration, reducing by ~300 m/s at the doping level of 3 x 10^19 1/cm3. Knowledge of the acoustic phonon velocities can be used to optimize ultra-wide bandgap semiconductor heterostructures and minimize the thermal boundary resistance of high-power devices.

cond-mat.mtrl-sci↗

arXiv2024

Over 600 V Lateral AlN-on-AlN Schottky Barrier Diodes with Ultra-Low Ideality Factor

This letter reports the demonstration of lateral AlN Schottky barrier diodes (SBDs) on single-crystal AlN substrates by metalorganic chemical vapor deposition (MOCVD) with an ultra-low ideality factor (η) of 1.65, a breakdown voltage (BV) of 640 V, and a record high normalized BV by the anode-to-cathode distance (LAC). The homoepitaxially grown AlN epilayers had much lower defect densities and excellent surface morphology, and the AlN ohmic contacts also showed improvements. At forward bias, the devices exhibited ultra-low η of 1.65 and high Schottky barrier height of 1.94 eV. The device current was dominated by thermionic emission, while most previously reported AlN SBDs suffered from defect-induced current with much higher η of >4. Additionally, the devices also had excellent rectifying characteristics with ON/OFF ratios on the order of 10^7 to 10^9 and excellent thermal stability from 298 to 573 K. At reverse bias, the devices showed a high BV of 640 V and record-high normalized breakdown voltage (BV/LAC) in lateral AlN SBDs. This work represents a big step towards high-performance ultra-wide bandgap AlN-based high-voltage and high-power devices.

physics.app-ph↗

arXiv2023

3 kV AlN Schottky Barrier Diodes on Bulk AlN Substrates by MOCVD

This letter reports the first demonstration of AlN Schottky diodes on bulk AlN substrates by metalorganic chemical vapor phase deposition (MOCVD) with breakdown voltages exceeding 3 kV. The devices exhibited good rectifying characteristics with ON/OFF ratios on the order of 10^6 to 10^8 and excellent thermal stability from 298 to 623 K. The device Schottky barrier height increased from 0.89 to 1.85 eV, and the ideality factor decreased from 4.29 to 1.95 with increasing temperatures, which was ascribed to the inhomogeneous metal/AlN interface. At reverse bias of -3 kV, the devices showed a low leakage current of 200 nA without the incorporation of any field plate structures or passivation techniques. This work demonstrates the potential of AlN as an ultra-wide bandgap semiconductor and represents a big step toward the development of multi-kV AlN high-voltage and high-power devices.

physics.app-ph↗

arXiv2023

Temperature Dependent Low-Frequency Noise Characteristics of NiO$_x$/Ga$_2$O$_3$ p-n Heterojunction Diodes

We report on the temperature dependence of the low-frequency electronic noise in NiO$_x$/Ga$_2$O$_3$ p-n heterojunction diodes. The noise spectral density is of the 1/f-type near room temperature but shows signatures of Lorentzian components at elevated temperatures and at higher current levels (f is the frequency). We observed an intriguing non-monotonic dependence of the noise on temperature near T = 380$^\circ$ K. The Raman spectroscopy of the device structure suggests material changes, which results in reduced noise above this temperature. The normalized noise spectral density in such diodes was determined to be on the order of 10$^{-14}$ cm$^2$/Hz (f = 10 Hz) at 0.1 A/cm$^2$ current density. In terms of the noise level, NiO$_x$/Ga$_2$O$_3$ p-n diodes occupy an intermediate position among devices of various designs implemented with different ultra-wide-band-gap (UWBG) semiconductors. The obtained results are important for understanding the electronic properties of the UWBG heterojunctions and contribute to the development of noise spectroscopy as the quality assessment tool for new electronic materials and device technologies.

physics.app-ph↗

arXiv2023

Low-Frequency Electronic Noise in the Aluminum Gallium Oxide Schottky Barrier Diodes

We report on the low-frequency electronic noise in (Al$_x$Ga$_{1-x}$)$_2$O$_3$ Schottky barrier diodes. The noise spectral density reveals 1/f dependence, characteristic of the flicker noise, with superimposed Lorentzian bulges at the intermediate current levels (f is the frequency). The normalized noise spectral density in such diodes was determined to be on the order of 10$^{-12}$ cm$^2$/Hz (f=10 Hz) at 1 A/cm$^2$ current density. At the intermediate current regime, we observed the random telegraph signal noise, correlated with the appearance of Lorentzian bulges in the noise spectrum. The random telegraph signal noise was attributed to the defects near the Schottky barrier. The defects can affect the local electric field and the potential barrier, and correspondingly, impact the electric current. The obtained results help to understand noise in Schottky barrier diodes made of ultra-wide-band-gap semiconductors and can be used for the material and device quality assessment.

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↗

arXiv2017

Effect of crystalline anisotropy on vertical (-201) and (010) beta-Ga2O3 Schottky barrier diodes on EFG single-crystal substrates

Vertical (-201) and (010) beta-Ga2O3 Schottky barrier diodes (SBDs) were fabricated on single-crystal substrates grown by edge-defined film-fed growth (EFG) method. High resolution X-ray diffraction (HRXRD) and atomic force microscopy (AFM) confirmed good crystal quality and surface morphology of the substrates. The electrical properties of both devices, including current-voltage (I-V) and capacitance-voltage (C-V) characteristics, were comprehensively measured and compared. The (-201) and (010) SBDs exhibited on-resistances (Ron) of 0.56 and 0.77 mΩcm2, turn-on voltages (Von) of 1.0 and 1.3 V, Schottky barrier heights (SBH) of 1.05 and 1.20 eV, electron mobilities of 125 and 65 cm2/(Vs), respectively, with a high on-current of ~1.3 kA/cm2 and on/off ratio of ~109. The (010) SBD had a larger Von and SBH than (-201) SBD due to anisotropic surface properties (i.e., surface Fermi level pinning and band bending), as supported by X-ray photoelectron spectroscopy (XPS) measurements. Temperature-dependent I-V also revealed the inhomogeneous nature of the SBH in both devices, where (-201) SBD showed a more uniform SBH distribution. The homogeneous SBH was also extracted: 1.33 eV for (-201) SBD and 1.53 eV for (010) SBD. The reverse leakage current of the devices was well described by the two-step trap-assisted tunneling model and the one-dimensional variable range hopping conduction (1D-VRH) model. The (-201) SBD showed larger leakage current due to its lower SBH and smaller activation energy. These results indicate the crystalline anisotropy of beta-Ga2O3 can affect the electrical properties of vertical SBDs and should be taken into consideration when designing beta-Ga2O3 electronics.

cond-mat.mtrl-sci↗

arXiv2017

Characterizations of the Nonlinear Optical Properties for (010) and (-201) Beta-phase Gallium Oxide

We report, for the first time, the characterizations on optical nonlinearities of beta-phase gallium oxide (\b{eta}-Ga2O3), where both (010) \b{eta}-Ga2O3 and (-201) \b{eta}-Ga2O3 were examined for two-photon absorption (TPA) coefficient, Kerr refractive index, and their polarization dependence. The wavelength dependence of the TPA coefficient and Kerr refractive index was estimated using a widely used analytical model. \b{eta}-Ga2O3 exhibits a TPA coefficient of 1.2 cm/GW for (010) \b{eta}-Ga2O3 and 0.58 cm/GW for (-201) \b{eta}-Ga2O3. The Kerr refractive index is -2.14*10^(15) cm2/W for (010) \b{eta}-Ga2O3 and -2.89*10^(15) cm2/W for (-201) \b{eta}-Ga2O3. In addition, \b{eta}-Ga2O3 shows stronger nonlinear optical anisotropy on the (-201) plane than on the (010) plane, possibly due to highly asymmetric crystal structure. Compared with that of gallium nitride (GaN), the TPA coefficient of \b{eta}-Ga2O3 is 20 times smaller, and the Kerr refractive index of \b{eta}-Ga2O3 is also found to be 4 to 5 times smaller. These results indicate that \b{eta}-Ga2O3 has the potential for use in ultra-low loss waveguides and ultra-stable resonators and integrated photonics, especially in the UV and visible wavelength spectral range.

physics.app-ph↗