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Yuewei Zhang

Publications and source records attributed to Yuewei Zhang.

33 records · Page 2Linked to original sources

Magnitude-and-phase-aware Speech Enhancement with Parallel Sequence Modeling

In speech enhancement (SE), phase estimation is important for perceptual quality, so many methods take clean speech's complex short-time Fourier transform (STFT) spectrum or the complex ideal ratio mask (cIRM) as the learning target. To predict these complex targets, the common solution is to design a complex neural network, or use a real network to separately predict the real and imaginary parts of the target. But in this paper, we propose to use a real network to estimate the magnitude mask and normalized cIRM, which not only avoids the significant increase of the model complexity caused by complex networks, but also shows better performance than previous phase estimation methods. Meanwhile, we devise a parallel sequence modeling (PSM) block to improve the RNN block in the convolutional recurrent network (CRN)-based SE model. We name our method as magnitude-and-phase-aware and PSM-based CRN (MPCRN). The experimental results illustrate that our MPCRN has superior SE performance.

eess.AS↗

Strain and composition dependencies of the near bandgap optical transitions in monoclinic (Al$_x$Ga$_{1-x}$)$_2$O$_3$ alloys with coherent biaxial in-plane strain on (010) Ga$_2$O$_3$

The bowing of the energy of the three lowest band-to-band transitions in $β$-(Al$_{x}$Ga$_{1-x}$)$_2$O$_3$ alloys was resolved using a combined density functional theory (DFT) and generalized spectroscopic ellipsometry (GSE) approach. The DFT calculations of the electronic band structure of both, $β$-Ga$_2$O$_3$ and $θ$-Al$_2$O$_3$, allow extracting of the linear portion of the energy shift in the alloys, and provide a method for quantifying the role of coherent strain present in the $β$-(Al$_{x}$Ga$_{1-x}$)$_2$O$_3$ thin films on (010) $β$-Ga$_2$O$_3$ substrates. The energies of band-to-band transitions were obtained using the spectroscopic ellipsometry eigenpolarization model approach [A. Mock et al., Phys. Rev. B 95, 165202 (2017)]. After subtracting the effects of strain which also induces additional bowing and after subtraction of the linear portion of the energy shift due to alloying, the bowing parameters associated with the three lowest band-to-band transitions in monoclinic $β$-(Al$_{x}$Ga$_{1-x}$)$_2$O$_3$ are found.

cond-mat.mtrl-sci↗

Zeeman Spin-Splitting in the (010) $β$-Ga2O3 Two-Dimensional Electron Gas

Through magneto-transport measurements and analysis of the observed Shubnikov de Haas oscillations in (010) (AlxGa1-x)2O3/Ga2O3 heterostructures, spin-splitting of the Landau levels in the (010) Ga2O3 two-dimensional electron gas (2DEG) has been studied. Analysis indicates that the spin-splitting results from the Zeeman effect. By fitting the both the first and second harmonic of the oscillations as a function of magnetic field, we determine the magnitude of the Zeeman splitting to be 0.4$\hbarω_c$, with a corresponding effective g-factor of 2.7, for magnetic field perpendicular to the 2DEG.

cond-mat.mes-hall↗

Compositionally graded contact layers for MOCVD grown high Al-content AlGaN transistors

In this letter, we design and demonstrate an improved MOCVD grown reverse Al-composition graded contact layer to achieve low resistance contact to MOCVD grown ultra-wide bandgap (UWBG) Al0.70Ga0.30N channel metal semiconductor field-effect transistors (MESFETs). Increasing the thickness of the reverse graded layer was found to improve contact layer resistance significantly, leading to contact resistance of 3.3x10^-5 Ohm.cm2. Devices with gate length, LG, of 0.6 microns and source-drain spacing, LSD, of 1.5 microns displayed a maximum current density, IDSMAX, of 635 mA/mm with an applied gate voltage, VGS, of +2 V. Breakdown measurements on transistors with gate to drain spacing, LGD, of 770 nm had breakdown voltage greater than 220 , corresponding to minimum breakdown field of 2.86 MV/cm. This work provides a framework for the design of low resistance contacts to MOCVD grown high Al-content AlxGa1-xN channel transistors.

physics.app-ph↗

High Mobility 2DEG in modulation-doped \b{eta}-(AlxGa1-x)2O3/Ga2O3 heterostructures

Beta-phase Ga2O3 has emerged as a promising candidate for a wide range of device applications, including power electronic devices, radio-frequency devices and solar-blind photodetectors. The wide bandgap energy and the predicted high breakdown field, together with the availability of low-cost native substrates, make \b{eta}-Ga2O3 a promising material compared to other conventional wide bandgap materials, such as GaN and SiC. Alloying of Al with \b{eta}-Ga2O3 could enable even larger band gap materials, and provide more flexibility for electronic and optoelectronic device design. In this work, we demonstrate a high mobility two-dimensional electron gas (2DEG) formed at the \b{eta}-(AlxGa1-x)2O3/Ga2O3 interface through modulation doping. Shubnikov-de Haas oscillation was observed for the first time in the modulation-doped \b{eta}-(AlxGa1-x)2O3/Ga2O3 structure, indicating a high-quality channel formed at the heterojunction interface. The formation of the 2DEG channel was further confirmed by a weak temperature-dependence of the carrier density, and the peak low temperature mobility was found to be 2790 cm2/Vs, which is significantly higher than can be achieved in bulk-doped \b{eta}-Ga2O3. The demonstrated modulation-doped \b{eta}-(AlxGa1-x)2O3/Ga2O3 structure lays the foundation for future exploration of quantum physical phenomena as well as new semiconductor device technologies based on the \b{eta}-Ga2O3 material system.

cond-mat.mtrl-sci↗

Modulation-doped beta-(Al0.2Ga0.8)2O3/Ga2O3 Field-Effect Transistor

Modulation-doped heterostructures are a key enabler for realizing high mobility and better scaling properties for high performance transistors. We report the realization of modulation-doped two-dimensional electron gas (2DEG) at beta(Al0.2Ga0.8)2O3/ Ga2O3 heterojunction using silicon delta doping. The formation of a 2DEG was confirmed using capacitance voltage measurements. A modulation-doped 2DEG channel was used to realize a modulation-doped field-effect transistor. The demonstration of modulation doping in the beta-(Al0.2Ga0.8)2O3/ Ga2O3 material system could enable heterojunction devices for high performance electronics.

cond-mat.mes-hall↗

Reflective Metal/Semiconductor Tunnel Junctions for Hole Injection in AlGaN UV LEDs

In this work, we investigate the use of nanoscale polarization engineering to achieve efficient hole injection from metals to ultra-wide band gap AlGaN, and we show that UV-reflective aluminum (Al) layers can be used for hole injection into p-AlGaN. The dependence of tunneling on the work function of the metal was investigated, and it was found that highly reflective Al metal layers can enable efficient hole injection into p-AlGaN, despite the relatively low work function of Al. Efficient tunneling hole injection was confirmed by light emission at 326 nm with on-wafer peak external quantum efficiency and wall-plug efficiency of 2.65% and 1.55%, respectively. A high power density of 83.7 W/cm2 was measured at 1200 kA/cm2. The metal/semiconductor tunnel junction structure demonstrated here could provide significant advantages for efficient and manufacturable device topologies for high power UV emitters.

physics.app-ph↗

Tunnel-injected sub-260 nm ultraviolet light emitting diodes

We report on tunnel-injected deep ultraviolet light emitting diodes (UV LEDs) configured with a polarization engineered Al0.75Ga0.25N/ In0.2Ga0.8N tunnel junction structure. Tunnel-injected UV LED structure enables n-type contacts for both bottom and top contact layers. However, achieving Ohmic contact to wide bandgap n-AlGaN layers is challenging and typically requires high temperature contact metal annealing. In this work, we adopted a compositionally graded top contact layer for non-alloyed metal contact, and obtained a low contact resistance of Rc=4.8x10-5 Ohm cm2 on n-Al0.75Ga0.25N. We also observed a significant reduction in the forward operation voltage from 30.9 V to 19.2 V at 1 kA/cm2 by increasing the Mg doping concentration from 6.2x1018 cm-3 to 1.5x1019 cm-3. Non-equilibrium hole injection into wide bandgap Al0.75Ga0.25N with Eg>5.2 eV was confirmed by light emission at 257 nm. This work demonstrates the feasibility of tunneling hole injection into deep UV LEDs, and provides a novel structural design towards high power deep-UV emitters.

cond-mat.mtrl-sci↗

Design and Demonstration of Ultra Wide Bandgap AlGaN Tunnel Junctions

Ultra violet light emitting diodes (UV LEDs) face critical limitations in both the injection efficiency and light extraction efficiency due to the resistive and absorbing p-type contact layers. In this work, we investigate the design and application of polarization engineered tunnel junctions for ultra-wide bandgap AlGaN (Al mole fraction higher than 50%) materials towards highly efficient UV LEDs. We demonstrate that polarization-induced 3D charge is beneficial in reducing tunneling barriers especially for high composition AlGaN tunnel junctions. The design of graded tunnel junction structures could lead to low tunneling resistance below 10-3 Ohm cm2 and low voltage consumption below 1 V (at 1 kA/cm2) for high composition AlGaN tunnel junctions. Experimental demonstration of 292 nm emission was achieved through non-equilibrium hole injection into wide bandgap materials with bandgap energy larger than 4.7 eV, and detailed modeling of tunnel junctions shows that they can be engineered to have low resistance, and can enable efficient emitters in the UV-C wavelength range.

cond-mat.mtrl-sci↗

AlGaN Channel Field Effect Transistors with Graded Heterostructure Ohmic Contacts

We report on ultra-wide bandgap (UWBG) Al0.75Ga0.25N channel metal-insulator-semiconductor field-effect transistors (MISFET) with heterostructure engineered low-resistance ohmic contacts. The low intrinsic electron affinity of AlN (0.6 eV) leads to large Schottky barriers at metal-AlGaN interface, resulting in highly resistive ohmic contacts. In this work, we use reverse compositional graded n++ AlGaN contact layer to achieve upward electron affinity grading, leading to a low specific contact resistance of 1.9x10-6 ohm.cm2 to n-Al0.75Ga0.25N channels (bandgap ~ 5.3 eV) with non-alloyed contacts. We also demonstrate UWBG Al0.75Ga0.25N channel MISFET device operation employing compositional graded n++ ohmic contact layer and 20 nm atomic layer deposited Al2O3 as the gate-dielectric.

cond-mat.mes-hall↗

Design of P-Type Cladding Layers for Tunnel-Injected UVA Light Emitting Diodes

We discuss the engineering of p-AlGaN cladding layers for achieving efficient tunnel-injected III-Nitride ultraviolet light emitting diodes (UV LEDs) in the UV-A spectral range. We show that capacitance-voltage measurements can be used to estimate the compensation and doping in p-AlGaN layers located between the multi-quantum well region and the tunnel junction layer. By increasing the p-type doping concentration to overcome the background compensation, on-wafer external quantum efficiency and wall-plug efficiency of 3.37% and 1.62% were achieved for tunnel-injected UV LEDs emitting at 325 nm. We also show that interband tunneling hole injection can be used to realize UV LEDs without any acceptor doping. The work discussed here provides new understanding of hole doping and transport in AlGaN-based UV LEDs, and demonstrates the excellent performance of tunnel-injected LEDs for the UV-A wavelength range.

cond-mat.mes-hall↗

High Current Density 2D/3D Esaki Tunnel Diodes

The integration of two-dimensional materials such as transition metal dichalcogenides with bulk semiconductors offer interesting opportunities for 2D/3D heterojunction-based novel device structures without any constraints of lattice matching. By exploiting the favorable band alignment at the GaN/MoS2 heterojunction, an Esaki interband tunnel diode is demonstrated by transferring large area, Nb-doped, p-type MoS2 onto heavily n-doped GaN. A peak current density of 446 A/cm2 with repeatable room temperature negative differential resistance, peak to valley current ratio of 1.2, and minimal hysteresis was measured in the MoS2/GaN non-epitaxial tunnel diode. A high current density of 1 kA/cm2 was measured in the Zener mode (reverse bias) at -1 V bias. The GaN/MoS2 tunnel junction was also modeled by treating MoS2 as a bulk semiconductor, and the electrostatics at the 2D/3D interface was found to be crucial in explaining the experimentally observed device characteristics.

physics.ins-det↗

Low-resistance GaN tunnel homojunctions with 150 kA/cm^2 current and repeatable negative differential resistance

We report GaN n++/p++ interband tunnel junctions with repeatable negative differential resistance and low resistance. Reverse and forward tunneling current densities were observed to increase as Si and Mg doping concentrations were increased. Hysteresis-free, bidirectional negative differential resistance was observed at room temperature from these junctions at a forward voltage of ~1.6-2 V. Thermionic PN junctions with tunnel contact to the p-layer exhibited forward current density of 150 kA/cm^2 at 7.6 V, with a low series device resistance of 1 x 10^-5 ohm.cm^2.

cond-mat.mtrl-sci↗

Enhanced Light Extraction in Tunnel Junction Enabled Top Emitting UV LEDs

The efficiency of ultra violet LEDs is critically limited by the absorption losses in p-type and metal layers. In this work, surface roughening based light extraction structures are combined with tunneling-based top-contacts to realize highly efficient top-side light extraction efficiency in UV LEDs. Surface roughening of the top n-type AlGaN contact layer is demonstrated using self-assembled Ni nano-clusters as etch mask. The top surface roughened LEDs were found to enhance external quantum efficiency by over 40% for UV LEDs with a peak emission wavelength of 326 nm. The method described here can enable highly efficient UV LEDs without the need for complex manufacturing methods such as flip chip bonding.

cond-mat.mtrl-sci↗

Interband Tunneling for Hole Injection in III-Nitride Ultra-violet Emitters

Ultra-violet emitters have several applications in the areas of sensing, water purification, and data storage. While the III-Nitride semiconductor system has the band gap region necessary for ultraviolet emission, achieving efficient ultraviolet solid state emitters remains a challenge due to the low p-type conductivity and high contact resistance in wide band gap AlGaN-based ultra-violet light emitters. In this work, we show that efficient interband tunneling can be used for non-equilibrium injection of holes into ultraviolet emitters. Polarization-engineered tunnel junctions were used to enhance tunneling probability by several orders of magnitude over a PN homojunction, leading to highly efficient tunnel injection of holes to ultraviolet light emitters. This demonstration of efficient interband tunneling introduces a new paradigm for design of ultra-violet light emitting diodes and diode lasers, and enables higher efficiency and lower cost ultra-violet emitters.

cond-mat.mtrl-sci↗