arXiv subjects

Hongping Zhao

Publications and source records attributed to Hongping Zhao.

28 records · Page 2Linked to original sources

arXiv2021

Laser-Assisted Metalorganic Chemical Vapor Deposition of GaN

Ammonia (NH3) is commonly used as group V precursor in gallium nitride (GaN) metalorganic chemical vapor deposition (MOCVD). The high background carbon (C) impurity in MOCVD GaN is related to the low pyrolysis efficiency of NH3, which represents one of the fundamental challenges hindering the development of high purity thick GaN for vertical high power device applications. This work uses a laser-assisted MOCVD (LA-MOCVD) growth technique to address the high-C issue in MOCVD GaN. Carbon dioxide (CO2) laser with wavelength of 9.219 um was utilized to facilitate NH3 decomposition via resonant vibrational excitation. The LA-MOCVD GaN growth rate (as high as 10 um/hr) shows a strong linear relationship with the trimethylgallium (TMGa) flow rate, indicating high effective V/III ratios and hence efficient NH3 decomposition. Pits-free surface morphology of LA-MOCVD GaN was demonstrated for films with growth rate as high as 8.5 um/hr. The background [C] in LA-MOCVD GaN films decreases monotonically as the laser power increases. A low [C] at 5.5E15 cm-3 was achieved in LA-MOCVD GaN film grown with the growth rate of 4 um/hr. Charge transport characterization of LA-MOCVD GaN films reveals high crystalline quality with room temperature mobility >1000 cm2/Vs. LA-MOCVD growth technique provides an enabling route to achieve high quality GaN epitaxy with low-C impurity and fast growth rate simultaneously. This technique can also be extended for epitaxy of other nitride-based semiconductors.

physics.app-ph↗

arXiv2021

Large-Size Free-Standing Single-crystal b-Ga2O3 Membranes Fabricated by Hydrogen Implantation and Lift-Off

In this paper, we have demonstrated the large-size free-standing single-crystal b-Ga2O3 NMs fabricated by the hydrogen implantation and lift-off process directly from MOCVD grown b-Ga2O3 epifilms on native substrates. The optimum implantation conditions were simulated with a Monte-Carlo simulation to obtain the high hydrogen concentration with a narrow ion distribution at the desired depth. Two as grown b-Ga2O3 samples with different orientation ([100] and [001]) were used and successfully create 1.2 um thick b-Ga2O3 NMs without any physical damages. These b-Ga2O3 NMs were then transfer-printed onto rigid and flexible substrates such as SiC substrate and polyimide substrate. Various material characterizations were performed to investigate the crystal quality, surface morphology, optical property, mechanical property, and bandgap before and after the lift-off and revealed that good material quality is maintained. This result offers several benefits in that the thickness, doping, and size of b-Ga2O3 NMs can be fully controlled. Moreover, more advanced b-Ga2O3-based NM structures such as (AlxGa1-x)2O3/Ga2O3 heterostructure NMs can be directly created from their bulk epitaxy substrates thus this result provides a viable route for the realization of high performance b-Ga2O3 NM-based electronics and optoelectronics that can be built on various substrates and platforms.

cond-mat.mtrl-sci↗

arXiv2021

Experimental determination of the valence band offsets of $ZnGeN_2$ and $ZnGe_{0.94}Ga_{0.12}N_2$ with $GaN$

A predicted type-II staggered band alignment with an approximately $1.4 eV$ valence band offset at the $ZnGeN_2/GaN$ heterointerface has inspired novel band-engineered $III-N/ZnGeN_2$ heterostructure-based device designs for applications in high performance optoelectronics. We report on the determination of the valence band offset between metalorganic chemical vapor deposition grown $(ZnGe)_{1-x}Ga_{2x}N_2$, for $x = 0$ and $0.06$, and $GaN$ using X-ray photoemission spectroscopy. The valence band of $ZnGeN_2$ was found to lie $1.45-1.65 eV$ above that of $GaN$. This result agrees well with the value predicted by first-principles density functional theory calculations using the local density approximation for the potential profile and quasiparticle self-consistent GW calculations of the band edge states relative to the potential. For $(ZnGe)_{0.94}Ga_{0.12}N_2$ the value was determined to be $1.29 eV$, $~10-20\%$ lower than that of $ZnGeN_2$. The experimental determination of the large band offset between $ZnGeN_2$ and $GaN$ provides promising alternative solutions to address challenges faced with pure III-nitride-based structures and devices.

cond-mat.mtrl-sci↗

arXiv2020

Direct observation of site-specific dopant substitution in Si doped (AlxGa1-x)2O3 via Atom Probe Tomography

In this work, the interaction of n-type dopants in Si doped (AlxGa1-x)2O3 films with varying Al content over the entire composition range (x = 0-100%) was analyzed using atom probe tomography. An almost uniform dopant distribution with dopant density in the range of 1018 cm-3 was obtained in all (AlxGa1-x)2O3 layers containing different Al contents. We have demonstrated that for the single phase \b{eta}-(AlxGa1-x)2O3 films with Al content of x<0.30, dopants prefer to occupy on Ga sites while Al site is preferred for high Al content (x>0.50) (AlxGa1-x)2O3 layers. It was also observed for Al content, x = 0.30-0.50, no specific cationic site occupancy was observed, Si occupies either Al or Ga sites. This can be attributed to highly inhomogeneous layers within this composition range due to which dopant Si atoms are either in the Al-rich or Al-depleted regions.

physics.app-ph↗

arXiv2020

Electrostatic Engineering using Extreme Permittivity Materials for Ultra-wide Bandgap Semiconductor Transistors

The performance of ultra-wide band gap materials like $β$-Ga$_\mathrm{2}$O$_\mathrm{3}$ is critically dependent on achieving high average electric fields within the active region of the device. In this report, we show that high-k gate dielectrics like BaTiO$_\mathrm{3}$ can provide an efficient field management strategy by improving the uniformity of electric field profile in the gate-drain region of lateral field effect transistors. Using this strategy, we were able to achieve high average breakdown fields of 1.5 MV/cm and 4 MV/cm at gate-drain spacing (L$_\mathrm{gd}$) of 6 um and 0.6 um respectively in $β$-Ga$_\mathrm{2}$O$_\mathrm{3}$, at a high channel sheet charge density of 1.8x10$^\mathrm{13}$cm$^\mathrm{-2}$. The high sheet charge density together with high breakdown field enabled a record power figure of merit (V$^\mathrm{2}$$_\mathrm{br}$/R$_\mathrm{on}$) of 376 MW/cm$^\mathrm{2}$ at a gate-drain spacing of 3 um.

physics.app-ph↗

arXiv2020

Probing charge transport and background doping in MOCVD grown (010) $β$-Ga$_{2}$O$_{3}$

A new record-high room temperature electron Hall mobility ($μ_{RT} = 194\space cm^{2}/V\space s$ at $n\sim 8\times 10^{15}\space cm^{-3}$) for $β$-Ga2O3 is demonstrated in the unintentionally doped thin film grown on (010) semi-insulating substrate via metalorganic chemical vapor deposition (MOCVD). A peak electron mobility of $\sim 9500\space cm^{2}/V\space s$ is achieved at 45 K. Further investigation on the transport properties indicate the existence of sheet charges near the epi-layer/substrate interface. Si is identified as the primary contributor to the background carrier in both the epi-layer and the interface, originated from both surface contamination as well as growth environment. Pre-growth hydrofluoric acid cleaning of the substrate lead to an obvious decrease of Si impurity both at interface and in epi-layer. In addition, the effect of MOCVD growth condition, particularly the chamber pressure, on the Si impurity incorporation is studied. A positive correlation between the background charge concentration and the MOCVD growth pressure is confirmed. It is noteworthy that in a $β$-Ga2O3 film with very low bulk charge concentration, even a reduced sheet charge density can play an important role in the charge transport properties.

physics.app-ph↗

arXiv2019

Clustering Bioactive Molecules in 3D Chemical Space with Unsupervised Deep Learning

Unsupervised clustering has broad applications in data stratification, pattern investigation and new discovery beyond existing knowledge. In particular, clustering of bioactive molecules facilitates chemical space mapping, structure-activity studies, and drug discovery. These tasks, conventionally conducted by similarity-based methods, are complicated by data complexity and diversity. We ex-plored the superior learning capability of deep autoencoders for unsupervised clustering of 1.39 mil-lion bioactive molecules into band-clusters in a 3-dimensional latent chemical space. These band-clusters, displayed by a space-navigation simulation software, band molecules of selected bioactivity classes into individual band-clusters possessing unique sets of common sub-structural features beyond structural similarity. These sub-structural features form the frameworks of the literature-reported pharmacophores and privileged fragments. Within each band-cluster, molecules are further banded into selected sub-regions with respect to their bioactivity target, sub-structural features and molecular scaffolds. Our method is potentially applicable for big data clustering tasks of different fields.

q-bio.BM↗

arXiv2018

Donors and Deep Acceptors in $β$-Ga2O3

We have studied the properties of Si, Ge shallow donors and Fe, Mg deep acceptors in $β$-Ga2O3 through temperature dependent van der Pauw and Hall effect measurements of samples grown by a variety of methods, including edge-defined film-fed (EFG), Czochralski (CZ), molecular beam epitaxy (MBE), and low pressure chemical vapor deposition (LPCVD). Through simultaneous, self-consistent fitting of the temperature dependent carrier density and mobility, we are able to accurately estimate the donor energy of Si and Ge to be 30 meV in $β$-Ga2O3. Additionally, we show that our measured Hall effect data are consistent with Si and Ge acting as typical shallow donors, rather than shallow DX centers. High temperature Hall effect measurement of Fe doped $β$-Ga2O3 indicates that the material remains weakly n-type even with the Fe doping, with an acceptor energy of 860 meV relative to the conduction band for the Fe deep acceptor. Van der Pauw measurements of Mg doped Ga2O3 indicate an activation energy of 1.1 eV, as determined from the temperature dependent conductivity.

cond-mat.mes-hall↗

arXiv2018

Ultrawide Band Gap β-Ga2O3 Nanomechanical Resonators with Spatially Visualized Multimode Motion

Beta gallium oxide (β-Ga2O3) is an emerging ultrawide band gap (4.5 - 4.9 eV) semiconductor with attractive properties for future power electronics, optoelectronics, and sensors for detecting gases and ultraviolet radiation. β-Ga2O3 thin films made by various methods are being actively studied toward such devices. Here, we report on the experimental demonstration of single-crystal β-Ga2O3 nanomechanical resonators using β-Ga2O3 nanoflakes grown via low-pressure chemical vapor deposition (LPCVD). By investigating β-Ga2O3 circular drumhead structures, we demonstrate multimode nanoresonators up to the 6th mode in high and very high frequency (HF / VHF) bands, and also realize spatial mapping and visualization of the multimode motion. These measurements reveal a Young's modulus of E_Y = 261 GPa and anisotropic biaxial built-in tension of 37.5 MPa and 107.5 MPa. We find that thermal annealing can considerably improve the resonance characteristics, including ~40% upshift in frequency and ~90% enhancement in quality (Q) factor. This study lays a foundation for future exploration and development of mechanically coupled and tunable β-Ga2O3 electronic, optoelectronic, and physical sensing devices.

physics.app-ph↗

arXiv2018

β-Ga2O3 NEMS Oscillator for Real-Time Middle Ultraviolet (MUV) Light Detection

We report on the first beta gallium oxide (β-Ga2O3) crystal feedback oscillator built by employing a vibrating β-Ga2O3 nanoresonator as the frequency reference for real-time middle ultraviolet (MUV) light detection. We fabricate suspended β-Ga2O3 nanodevices through synthesis of β-Ga2O3 nanoflakes using low-pressure chemical vapor deposition (LPCVD), and dry transfer of nanoflakes on microtrenches. Open-loop tests reveal a resonance of the β-Ga2O3 device at ~30 MHz. A closed-loop oscillator is then realized by using a combined optical-electrical feedback circuitry, to perform real-time resonant sensing of MUV irradiation. The oscillator exposed to cyclic MUV irradiation exhibits resonant frequency downshifts, with a measured responsivity of $\mathscr{R}$ = -3.1 Hz/pW and a minimum detectable power of δPmin = 0.53 nW for MUV detection.

physics.app-ph↗