arXiv subjects

Huili Grace Xing

Publications and source records attributed to Huili Grace Xing.

At least 37 records · Page 2Linked to original sources

arXiv2024

Leveraging both faces of polar semiconductor wafers for functional devices

Unlike non-polar semiconductors such as silicon, the broken inversion symmetry of the wide bandgap semiconductor gallium nitride leads to a large electronic polarization along a unique crystal axis. This makes the two surfaces of the semiconductor wafer perpendicular to the polar axis dramatically different in their physical and chemical properties. In the last three decades, the cation (gallium) face of gallium nitride has been used for photonic devices such as LEDs and lasers. Though the cation face has also been predominantly used for electronic devices, the anion (nitrogen) face has recently shown promise for high electron mobility transistors due to favorable polarization discontinuities. In this work we introduce dualtronics, showing that it is possible to make photonic devices on the cation face, and electronic devices on the anion face, of the same semiconductor wafer. This opens the possibility for leveraging both faces of polar semiconductors in a single structure, where electronic, photonic, and acoustic properties can be implemented on opposite faces of the same wafer, dramatically enhancing the functional capabilities of this revolutionary semiconductor family.

physics.app-ph↗

arXiv2023

Epitaxial lattice-matched Al$_{0.89}$Sc$_{0.11}$N/GaN distributed Bragg reflectors

We demonstrate epitaxial lattice-matched Al$_{0.89}$Sc$_{0.11}$N/GaN ten and twenty period distributed Bragg reflectors (DBRs) grown on c-plane bulk n-type GaN substrates by plasma-enhanced molecular beam epitaxy (PA-MBE). Resulting from a rapid increase of in-plane lattice coefficient as scandium is incorporated into AlScN, we measure a lattice-matched condition to $c$-plane GaN for a Sc content of just 11\%, resulting in a large refractive index mismatch $\mathrm{Δn}$ greater than 0.3 corresponding to an index contrast of $\mathrm{Δn/n_{GaN}}$ = 0.12 with GaN. The DBRs demonstrated here are designed for a peak reflectivity at a wavelength of 400 nm reaching a reflectivity of 0.98 for twenty periods. It is highlighted that AlScN/GaN multilayers require fewer periods for a desired reflectivity than other lattice-matched Bragg reflectors such as those based on AlInN/GaN multilayers.

cond-mat.mtrl-sci↗

arXiv2023

Excitonic and deep-level emission from N- and Al-polar homoepitaxial AlN grown by molecular beam epitaxy

Using low-temperature cathodoluminescence spectroscopy, we study the properties of N- and Al-polar AlN layers grown by molecular beam epitaxy on bulk AlN{0001}. Compared to the bulk AlN substrate, layers of both polarities feature a suppression of deep-level luminescence, a total absence of the prevalent donor with an exciton binding energy of 28 meV, and a much increased intensity of the emission from free excitons. The dominant donor in these layers is characterized by an associated exciton binding energy of 13 meV. The observation of excited exciton states up to the exciton continuum allows us to directly extract the $Γ_{5}$ free exciton binding energy of 57 meV.

cond-mat.mtrl-sci↗

arXiv2023

Broken Symmetry Effects due to Polarization on Resonant Tunneling Transport in Double-Barrier Nitride Heterostructures

The phenomenon of resonant tunneling transport through polar double-barrier heterostructures is systematically investigated using a combined experimental and theoretical approach. On the experimental side, GaN/AlN RTDs are grown by MBE. In-situ electron diffraction is employed to monitor the number of monolayers incorporated into each tunneling barrier. Using this precise epitaxial control at the monolayer level, we demonstrate exponential modulation of the resonant tunneling current as a function of barrier thickness. Both the peak voltage and characteristic threshold bias exhibit a dependence on barrier thickness as a result of the intense electric fields present in the polar heterostructures. To get further insight into the asymmetric tunneling injection, we present an analytical theory for tunneling transport across polar heterostructures. A general expression for the resonant tunneling current with contributions from coherent and sequential tunneling processes is introduced. After applying this theory to the case of GaN/AlN RTDs, their experimental current-voltage characteristics are reproduced over both bias polarities, with tunneling currents spanning several orders of magnitude. This agreement allows us to elucidate the role played by the internal polarization fields on the magnitude of the tunneling current and broadening of the resonant line shape. Under reverse bias, we identify new tunneling features originating from highly attenuated resonant tunneling phenomena, which are completely captured by our model. Our analytical model, provides a simple expression which reveals the connection between the polar RTD design parameters and its current-voltage characteristics. This new theory paves the way for the design of polar resonant tunneling devices exhibiting efficient resonant current injection and enhanced tunneling dynamics, as required in various practical applications.

cond-mat.mes-hall↗

arXiv2023

Fighting Broken Symmetry with Doping: Toward Polar Resonant Tunneling Diodes with Symmetric Characteristics

The recent demonstration of resonant tunneling transport in nitride semiconductors has led to an invigorated effort to harness this quantum transport regime for practical applications. In polar semiconductors, however, the interplay between fixed polarization charges and mobile free carriers leads to asymmetric transport characteristics. Here, we investigate the possibility of using degenerately doped contact layers to screen the built-in polarization fields and recover symmetric resonant injection. Thanks to a high doping density, negative differential conductance is observed under both bias polarities of GaN/AlN resonant tunneling diodes (RTDs). Moreover, our analytical model reveals a lower bound for the minimum resonant-tunneling voltage achieved via uniform doping, owing to the dopant solubility limit. Charge storage dynamics is also studied by impedance measurements, showing that at close-to-equilibrium conditions, polar RTDs behave effectively as parallel-plate capacitors. These mechanisms are completely reproduced by our analytical model, providing a theoretical framework useful in the design and analysis of polar resonant-tunneling devices.

physics.app-ph↗

arXiv2023

Defeating Broken Symmetry with Doping: Symmetric Resonant Tunneling in Noncentrosymetric Heterostructures

Resonant tunneling transport in polar heterostructures is intimately connected to the polarization fields emerging from the geometric Berry-phase. In these structures, quantum confinement results not only in a discrete electronic spectrum, but also in built-in polarization charges exhibiting a broken inversion symmetry along the transport direction. Thus, electrons undergo highly asymmetric quantum interference effects with respect to the direction of current flow. By employing doping to counter the broken symmetry, we deterministically control the resonant transmission through GaN/AlN resonant tunneling diodes and experimentally demonstrate the recovery of symmetric resonant tunneling injection across the noncentrosymmetric double-barrier potential.

cond-mat.mes-hall↗

arXiv2022

Molecular Beam Homoepitaxy of N-polar AlN on bulk AlN substrates

N-polar AlN epilayers were grown on the N-face of single crystal bulk AlN substrates by plasma assisted molecular beam epitaxy (PA-MBE). A combination of in situ thermal deoxidation and Al-assisted thermal desorption at high temperature helped in removing native surface oxides and impurities from the N-polar surface of the substrate enabling successful homoepitaxy. Subsequent epitaxial growth of AlN layer on the in situ cleaned substrates, grown in sufficiently high Al droplet regime, exhibited smooth surface morphologies with clean and wide atomic steps. KOH etch studies confirmed the N-polarity of the homoepitaxial films. Secondary ion mass spectrometry profiles show Si and H impurity concentrations below the noise levels, whereas O and C impurities concentrations of ~ 8x10^{17} atoms/cm^3 and ~ 2x10^{17} atoms/cm^3 are observed respectively. Though the structural defect densities are low, they interestingly appear as inversion domains of different dimensionalities.

cond-mat.mtrl-sci↗

arXiv2022

N-polar GaN p-n junction diodes with low ideality factors

High-quality N-polar GaN p-n diodes are realized on single-crystal N-polar GaN bulk substrate by plasma-assisted molecular beam epitaxy. The room-temperature current-voltage characteristics reveal a high on/off current ratio of 10^11 at 4 V and an ideality factor of 1.6. As the temperature increases to 200 C, the apparent ideality factor gradually approaches 2. At such high temperatures, Shockley-Read-Hall recombination times of 0.32-0.46 ns are estimated. The measured electroluminescence spectrum is dominated by a strong near-band edge emission, while deep level and acceptor-related luminescence is greatly suppressed. A relatively high reverse breakdown field of 2.4 MV/cm without field-plates is achieved. This work indicates that the quality of N-polar GaN diodes is now approaching to that of their state-of-the-art Ga-polar counterparts.

cond-mat.mtrl-sci↗

arXiv2022

Molecular beam homoepitaxy of N-polar AlN: enabling role of Al-assisted surface cleaning

N-polar aluminum nitride (AlN) is an important building block for next-generation high-power RF electronics. We report successful homoepitaxial growth of N-polar AlN by molecular beam epitaxy (MBE) on large-area cost-effective N-polar AlN templates. Direct growth without any in-situ surface cleaning leads to films with inverted Al-polarity. It is found that Al-assisted cleaning before growth enables the epitaxial film to maintain N-polarity. The grown N-polar AlN epilayer with its smooth, pit-free surface duplicates the structural quality of the substrate as evidenced by a clean and smooth growth interface with no noticeable extended defects generation. Near band-edge photoluminescence peaks are observed at room temperature on samples with MBE-grown layers but not on the bare AlN substrates, implying the suppression of non-radiative recombination centers in the epitaxial N-polar AlN. These results are pivotal steps towards future high-power RF electronics and deep ultraviolet photonics based on the N-polar AlN platform.

cond-mat.mtrl-sci↗

arXiv2021

Infrared dielectric functions and Brillouin zone center phonons of $α$-Ga$_2$O$_3$ compared to $α$-Al$_2$O$_3$

We determine the anisotropic dielectric functions of rhombohedral $α$-Ga$_2$O$_3$ by far-infrared and infrared generalized spectroscopic ellipsometry and derive all transverse optical and longitudinal optical phonon mode frequencies and broadening parameters. We also determine the high frequency and static dielectric constants. We perform density functional theory computations and determine the phonon dispersion for all branches in the Brillouin zone, and we derive all phonon mode parameters at the Brillouin zone center including Raman-active, infrared-active, and silent modes. Excellent agreement is obtained between our experimental and computation results as well as among all previously reported partial information from experiment and theory. We also compute the same information for $α$-Al$_2$O$_3$, the binary parent compound for the emerging alloy of $α$-(Al$_{x}$Ga$_{1-x}$)$_2$O$_3$, and use results from previous investigations [Schubert, Tiwald, and Herzinger, Phys. Rev. B 61, 8187 (2000)] to compare all properties among the two isostructural compounds. From both experimental and theoretical investigations we compute the frequency shifts of all modes between the two compounds. Additionally, we calculate overlap parameters between phonon mode eigenvectors and discuss the possible evolution of all phonon modes into the ternary alloy system and whether modes may form single mode or more complex mode behaviors.

cond-mat.mtrl-sci↗

arXiv2021

High conductivity Polarization-induced 2D hole gases in Undoped GaN/AlN Heterojunctions enabled by Impurity Blocking Layers

High-conductivity undoped GaN/AlN 2D hole gases (2DHGs), the p-type dual of the AlGaN/GaN 2D electron gases (2DEGs), have offered valuable insights into hole transport in GaN and enabled the first GaN GHz RF p-channel FETs. They are an important step towards high-speed and high-power complementary electronics with wide-bandgap semiconductors. These technologically and scientifically relevant 2D hole gases are perceived to be not as robust as the 2DEGs because structurally similar heterostructures exhibit wide variations of the hole density over $Δp_s >$ 7 x 10$^{13}$ cm$^{-2}$, and low mobilities. In this work, we uncover that the variations are tied to undesired dopant impurities such as Silicon and Oxygen floating up from the nucleation interface. By introducing impurity blocking layers (IBLs) in the AlN buffer layer, we eliminate the variability in 2D hole gas densities and transport properties, resulting in a much tighter-control over the 2DHG density variations to $Δp_s \leq$ 1 x 10$^{13}$ cm$^{-2}$ across growths, and a 3x boost in the Hall mobilities. These changes result in a 2-3x increase in hole conductivity when compared to GaN/AlN structures without IBLs.

cond-mat.mes-hall↗

arXiv2021

Anisotropic dielectric functions, band-to-band transitions, and critical points in α-Ga2O3

We use a combined generalized spectroscopic ellipsometry and density functional theory approach to determine and analyze the anisotropic dielectric functions of an $α$-Ga$_2$O$_3$ thin film. The sample is grown epitaxially by plasma-assisted molecular beam epitaxy on $m$-plane sapphire. Generalized spectroscopic ellipsometry data from multiple sample azimuths in the spectral range from 0.73 eV to 8.75 eV are simultaneously analyzed. Density functional theory is used to calculate the valence and conduction band structure. We identify, for the indirect-bandgap material, two direct band-to-band transitions with $M_0$-type van Hove singularities for polarization perpendicular to the $c$ axis, $E_{0,\perp}=5.46(6)$ eV and $E_{0,\perp}=6.04(1)$ eV, and one direct band-to-band transition with $M_1$-type van Hove singularity for polarization parallel with $E_{0,||}=5.44(2)$ eV. We further identify excitonic contributions with small binding energy of 7 meV associated with the lowest ordinary transition, and a hyperbolic exciton at the $M_1$-type critical point with large binding energy of 178 meV.

cond-mat.mtrl-sci↗

arXiv2020

$γ$-phase Inclusions as Common Defects in Alloyed $β$-(Al$_x$Ga$_{1\text{-}x}$)$_2$O$_3$ and Doped $β$-Ga$_2$O$_3$ Films

$β$-Ga$_2$O$_3$ is a promising ultra-wide bandgap semiconductor whose properties can be further enhanced by alloying with Al. Here, using atomic-resolution scanning transmission electron microscopy (STEM), we find the thermodynamically-unstable $γ$-phase is a ubiquitous defect in both $β$-(Al$_x$Ga$_{1\text{-}x}$)$_2$O$_3$ films and doped $β$-Ga$_2$O$_3$ films grown by molecular beam epitaxy. For undoped $β$-(Al$_x$Ga$_{1\text{-}x}$)$_2$O$_3$ films we observe $γ$-phase inclusions between nucleating islands of the $β$-phase at lower growth temperatures (~400-600 $^{\circ}$C). In doped $β$-Ga$_2$O$_3$, a thin layer of the $γ$-phase is observed on the surfaces of films grown with a wide range of n-type dopants and dopant concentrations. The thickness of the $γ$-phase layer was most strongly correlated with the growth temperature, peaking at about 600 $^{\circ}$C. Ga interstitials are observed in $β$-phase, especially near the interface with the $γ$-phase. By imaging the same region of the surface of a Sn-doped $β$-(Al$_x$Ga$_{1\text{-}x}$)$_2$O$_3$ after ex-situ heating up to 400 $^{\circ}$C, a $γ$-phase region is observed to grow above the initial surface, accompanied by a decrease in Ga interstitials in the $β$-phase. This suggests that the diffusion of Ga interstitials towards the surface is likely the mechanism for growth of the surface $γ$-phase, and more generally that the more-open $γ$-phase may offer diffusion pathways to be a kinetically-favored and early-forming phase in the growth of Ga$_2$O$_3$.

cond-mat.mtrl-sci↗

arXiv2020

N-polar GaN/AlN resonant tunneling diodes

N-polar GaN/AlN resonant tunneling diodes are realized on single-crystal N-polar GaN bulk substrate by plasma-assisted molecular beam epitaxy growth. The room-temperature current-voltage characteristics reveal a negative differential conductance (NDC) region with a peak tunneling current of 6.8$\pm$ 0.8 kA/cm$^2$ at a forward bias of ~8 V. Under reverse bias, the polarization-induced threshold voltage is measured at ~$-$4 V. These resonant and threshold voltages are well explained with the polarization field which is opposite to that of the metal-polar counterpart, confirming the N-polarity of the RTDs. When the device is biased in the NDC-region, electronic oscillations are generated in the external circuit, attesting to the robustness of the resonant tunneling phenomenon. In contrast to metal-polar RTDs, N-polar structures have the emitter on the top of the resonant tunneling cavity. As a consequence, this device architecture opens up the possibility of seamlessly interfacing$-$via resonant tunneling injection$-$a wide range of exotic materials with III-nitride semiconductors, providing a route to explore new device physics.

cond-mat.mtrl-sci↗

arXiv2020

Thermionic emission or tunneling? The universal transition electric field for ideal Schottky reverse leakage current in $β$-Ga$_{2}$O$_{3}$

The reverse leakage current through a Schottky barrier transitions from a thermionic-emission dominated regime to a barrier-tunneling dominated regime as the surface electric field increases. In this study, we evaluate such transition electric field ($E_{\rm T}$) in $β$-Ga$_{2}$O$_{3}$ using a numerical reverse leakage model. $E_{\rm T}$ is found to have very weak dependence on the doping concentration and barrier height, thus a near-universal temperature dependence suffices and is given by a simple empirical expression in Ga$_{2}$O$_{3}$. With the help of a field-plate design, we observed experimentally in Ga$_{2}$O$_{3}$ Schottky barrier diodes a near-ideal bulk reverse leakage characteristics, which matches well with our numerical model and confirms the presence of the transition region. Near the transition electric field, both thermionic emission and barrier tunneling should be considered. The study provides important guidance toward accurate design and modeling of ideal reverse leakage characteristics in $β$-Ga$_{2}$O$_{3}$ Schottky barrier diodes.

physics.app-ph↗

arXiv2020

Spin-Orbit-Torque Field-Effect Transistor (SOTFET): Proposal for a New Magnetoelectric Memory

Spin-based memories are attractive for their non-volatility and high durability but provide modest resistance changes, whereas semiconductor logic transistors are capable of large resistance changes, but lack memory function with high durability. The recent availability of multiferroic materials provides an opportunity to directly couple the change in spin states of a magnetic memory to a charge change in a semiconductor transistor. In this work, we propose and analyze the spin-orbit torque field-effect transistor (SOTFET), a device with the potential to significantly boost the energy efficiency of spin-based memories, and to simultaneously offer a palette of new functionalities.

physics.app-ph↗

arXiv2019

Materials Relevant to Realizing a Field-Effect Transistor based on Spin-Orbit Torques

Spin-orbit torque is a promising mechanism for writing magnetic memories, while field-effect transistors are the gold-standard device for logic operation. The spin-orbit torque field effect transistor (SOTFET) is a proposed device that couples a spin-orbit-torque-controlled ferromagnet to a semiconducting transistor channel via the transduction in a magnetoelectric multiferroic. This allows the SOTFET to operate as both a memory and a logic device, but its realization depends on the choice of appropriate materials. In this report, we discuss and parametrize the types of materials that can lead to a SOTFET heterostructure.

cond-mat.mtrl-sci↗

arXiv2019

Oxygen Incorporation in the MBE growth of ScxGa1-xN and ScxAl1-xN

Secondary-ion Mass Spectrometry (SIMS) is used to determine impurity concentrations of carbon and oxygen in two scandium containing nitride semiconductor multilayer heterostructures: ScxGa1-xN/GaN and ScxAl1-xN/AlN grown by molecular beam epitaxy (MBE). In the ScxGa1-xN/GaN heterostructure grown in metal rich conditions on GaN-SiC template substrates with Sc contents up to 28 atomic percent, the oxygen concentration is found to be below 1x1019/cm3, with an increase directly correlated with the Scandium content. In the ScxAl1-xN-AlN heterostructure grown in nitrogen rich conditions on AlN-Al2O3 template substrates with Sc contents up to 26 atomic percent, the oxygen concentration is found to be between 1019 to 1021/cm3, again directly correlated with the Sc content. The increased oxygen and carbon arises during the deposition of scandium alloyed layers.

cond-mat.mtrl-sci↗