Showing papers in "Applied Physics Letters in 2023"

Thickness scaling down to 5 nm of ferroelectric ScAlN on CMOS compatible molybdenum grown by molecular beam epitaxy

Abstract: We report on the thickness scaling behavior of ferroelectric Sc0.3Al0.7N (ScAlN) films grown on Mo substrates by molecular beam epitaxy. Switchable ferroelectricity is confirmed in ScAlN films with thicknesses ranging from 100 to 5 nm. An increase in coercive field and a significant diminution of remnant polarization are found when the ferroelectric layer is scaled down to below 20 nm. Notably, a switching voltage of 2–3.8 V and saturated remnant polarization of ∼23 μC/cm2 are measured in 5 nm thick ScAlN. X-ray diffractions and transmission electron microscopy studies indicate that the increase in coercive field and diminishment in switchable polarization can be closely linked to the surface oxidation and strain state in ultrathin ScAlN films. This work sheds light on the fundamental thickness scaling fingerprints of ScAlN thin films and represents an important step for next-generation compact and power-efficient devices and applications based on nitride ferroelectrics.

Piezoelectric thin films for MEMS

Abstract: First Page

Enhanced out-of-plane piezoelectricity and carrier mobility in Janus γ- Sn2XY ( X /Y= S, Se, Te) monolayers: A first-principles prediction

Abstract: In this Letter, we design Janus γ-[Formula: see text] ([Formula: see text] S, Se, Te) monolayers and predict their piezoelectricity and carrier mobility by using first-principles simulations. Janus γ-[Formula: see text] are found to be indirect semiconducting characteristics with a camel's back-like dispersion in the top valence band. We discovered that Janus γ-[Formula: see text] are piezoelectric with high out-of-plane piezoelectric coefficients. Our calculated results for the piezoelectricity demonstrate that the out-of-plane piezoelectric coefficient d31 of Janus γ-Sn2STe is calculated to be 1.02 pm/V, larger than that of other 2D structures. Moreover, our calculations for the transport features reveal that while the carrier mobility of γ-Sn2SSe is directionally isotropic, the electron mobility of both γ-Sn2STe and γ-Sn2SeTe exhibit high anisotropy along the two transport directions. The Janus γ-[Formula: see text] monolayers have high electron mobility, especially the electron mobility of γ-Sn2STe exceeds 105 cm2 V–1 s–1, which is potential for nanoelectronic applications.

Ultrafast response self-powered UV photodetectors based on GaS/GaN heterojunctions

Abstract: Self-powered ultraviolet (UV) photodetectors (PDs) based on GaN have been of great importance in the application of UV communication, thanks to its wide direct bandgap and strong resistance to irradiation. However, current self-powered GaN-based heterojunction UV photodetectors could not meet the requirement of fast photoresponse. Herein, type-II pn heterojunction GaS/GaN-based self-powered PDs have been proposed with a naturally p-type doping GaS thin film grown on n-type GaN via chemical vapor deposition. The electronic and optical properties of GaS/GaN heterojunction were investigated via experiments and the density functional theory. Afterward, as-prepared GaS/GaN-based PDs reveal an excellent self-powered photosensitivity/detectivity of 6.26 mA W−1/8.29 × 109 Jones at 0 V at 365 nm, ultrafast response speed with a rise/fall time of 48/80 μs as well as an amazing rejection ratio ( R365 nm/ R500 nm) of 3.42 × 104, and a fine rectification ratio of 105.9. This work provides a feasible method to synthesize high-performance GaS/GaN heterojunctions and demonstrates their enormous potential in ultrafast response self-powered UV photodetection.

Enhanced dielectric and energy storage properties of P(VDF-HFP) through elevating β-phase formation under unipolar nanosecond electric pulses

Abstract: Structural manipulation of electroactive β-phase of poly(vinylidene fluoride- co-hexafluoropropylene) [P(VDF-HFP)] is of great importance in high-energy-density polymer devices. In this Letter, an efficient way to improve dielectric and energy storage properties of P(VDF-HFP) films by inducing a high β-phase content and lowering the crystallite size through repetitive unipolar nanosecond electric pulses (nsEP) is proposed. It is found that the percentage of the β-phase in P(VDF-HFP) can be significantly enhanced to ∼84% under a low unipolar nsEP of 5 V/mm vs only 35% in pristine P(VDF-HFP). Meanwhile, the orientation of the amorphous chains is also achieved, which improves the dielectric constant, electric breakdown, and energy storage properties of P(VDF-HFP). Specifically, the P(VDF-HFP) film processed under nsEP of 5 V/mm exhibits a high breakdown field of 541 MV/m, and discharged energy density of 14 J/cm3, which is 28.8% and 127% higher than those of the pristine polymer, respectively. This work provides a facile approach to optimize the crystalline morphology of P(VDF-HFP) polymers for dielectric energy storage applications.

Order controllable enhanced stimulated Brillouin scattering utilizing cascaded diamond Raman conversion

Abstract: We report on the design and operation of a laser, which outputs wavelengths in the 1.2 and 1.5 μm ranges by leveraging two non-linear processes of stimulated Raman scattering and stimulated Brillouin scattering in diamond. By precisely controlling characteristics of the laser resonator formed around the diamond crystal, we are able to selectively control the onset of each non-linear process so as to tailor laser output characteristics both in way of wavelength and output power. This work demonstrates the high degree of flexibility and power-handling capacity of diamond for wavelength conversion of common laser wavelengths (such as 1064 nm as used in this work) and the generation of a span of discrete wavelengths (with up to eight cascaded orders being demonstrated in this work).

A highly sensitive broadband superconducting thermoelectric single-photon detector

Abstract: We propose a passive single-photon detector based on the bipolar thermoelectric effect occurring in tunnel junctions between two different superconductors thanks to spontaneous electron–hole symmetry breaking. Our superconducting thermoelectric detector (STED) converts a finite temperature difference caused by the absorption of a single photon into an open circuit thermovoltage. Designed with feasible parameters, our STED is able to reveal single photons of frequency ranging from ∼15 GHz to ∼150 PHz depending on the chosen design and materials. In particular, this detector is expected to show values of the signal-to-noise ratio SNR ∼ 15 at ν = 50 GHz when operated at a temperature of 10 mK. Interestingly, this device can be viewed as a digital single-photon detector, since it generates an almost constant voltage VS for the full operation energies. Our STED can reveal single photons in a frequency range wider than four decades with the possibility to discern the energy of the incident photon by measuring the time persistence of the generated thermovoltage. Its broadband operation suggests that our STED could find practical applications in several fields of quantum science and technology, such as quantum computing, telecommunications, optoelectronics, THz spectroscopy, and astro-particle physics.

Investigation of lithium (Li) doping on the resistive switching property of p-Li:NiO/n- β-Ga2O3 thin-film based heterojunction devices

Abstract: Li-doped NiO/[Formula: see text]-Ga2O3 polycrystalline bilayer thin-film pn-heterojunctions with different Li-doping concentrations are grown on Si-substrates using the pulsed laser deposition technique. Resistive switching property of these devices has been investigated in detail. This study shows that the Li-doping concentration in NiO layer significantly influences the performance of these devices. For an optimum Li-doping of 1.5%, a stable memory window of ∼102 with endurance of more than 100 cycles and long retention time can be achieved. The coefficient of variation ([Formula: see text]) of SET and RESET voltages also found to ∼ 20% and ∼ 40%, respectively, satisfying the acceptability benchmark. A transition from complementary resistive switching (CRS) to bipolar resistive switching (BRS) after multiple sweeping operations has been observed in devices with intermediate Li-doping concentrations. Observation of CRS has been explained in terms of the formation of Li-rich metallic layer at the NiO/Ga2O3 interface as a result of out-diffusion of Li. Redistribution of the Li-ions from the Li-rich interfacial zone to whole of the NiO layer after first few sweeping cycles must be the reason for CRS-to-BRS transition. Results further suggest that return to high resistive state via Poole–Frenkel (PF) pathway during the RESET process might be the key to achieve high performance in p–n junction based resistive switching devices. This study, thus, presents Li-doping as a possible route to modulate the resistive switching property of bilayer Li:NiO/Ga2O3 based memory devices.

631% room temperature tunnel magnetoresistance with large oscillation effect in CoFe/MgO/CoFe(001) junctions

Abstract: We demonstrate tunnel magnetoresistance (TMR) ratios of up to 631% at room temperature (RT) using CoFe/MgO/CoFe(001) epitaxial magnetic tunnel junctions (MTJs). The TMR ratio increased up to 1143% at 10 K. The large TMR ratios resulted from fine-tuning of atomic-scale structures of the MTJs, such as crystallographic orientations and MgO interface oxidation by interface insertion of ultrathin CoFe and Mg layers, which are expected to enhance the well-known Δ1 coherent tunneling transport. Interestingly, the TMR oscillation effect, which is not covered by the standard coherent tunneling theory, also became significant. A 0.32-nm period TMR oscillation with increasing MgO thickness dominates the transport in a wide range of MgO thicknesses; the peak-to-valley difference of the TMR oscillation exceeds 140% at RT, which is attributed to the appearance of large oscillatory components in the resistance area product.

Multiple dimension-component designed Co/Co9S8/Ti3C2Tx MXene composite for enhanced microwave absorption

Abstract: The two-dimensional (2D) transition metal carbide Ti3C2T x MXene is a potential candidate for efficient electromagnetic wave absorbers due to its excellent intrinsic conductivity and structural machinability. However, Ti3C2T x MXene also has some problems (such as self-stacking and single loss mechanism) that limit its practical electromagnetic wave absorption. Based on the electromagnetic wave absorption mechanism, electromagnetic responsiveness of absorbers can be modulated by designing the composition and structure. Herein, a 1D/2D Co/Co9S8/Ti3C2T x composite has been synthesized by assembling 2D Ti3C2T x MXene with the designed 1D magnetic structure. The 1D Co/Co9S8 was designed as a core-sheath structure that avoids magnetic agglomeration, and the assembly with 2D Ti3C2T x sheets alleviates the self-stacking problem of Ti3C2T x MXene sheets. More importantly, the magnetic component enriches the electromagnetic wave dissipation mechanism, and the multiple heterojunction surfaces provide strong polarization loss capability for the Ti3C2T x MXene-based absorber. Benefiting from the unique structure and dielectric-magnetic synergistic loss, the Co/Co9S8/Ti3C2T x composite shows an effective absorption bandwidth of 5.36 GHz (10.08–15.44 GHz) at 2.1 mm and the optimal RLmin value of −52.02 dB at 1.8 mm. This work provides an innovative idea for the design of effective Ti3C2T x MXene-based absorbers.

Characterization of a low-noise superconductor–insulator–superconductor-based microwave amplifier with local oscillator phase-adjusting architecture

Abstract: This paper describes a low-noise microwave amplifier based on up- and down-frequency-conversion processes in quasiparticle superconductor–insulator–superconductor (SIS) tunnel junctions. The SIS amplifier was configured with two SIS frequency-converter modules and a cryogenic millimeter-wave isolator inserted between them. Moreover, a local oscillator (LO) using millimeter-wave attenuators and a phase shifter was considered. This setup allowed the control of individual LO power and differential phase in these SIS frequency converters to optimize the amplifier performance. The SIS amplifier showed noise temperatures as low as 11 K and a 6–8 dB gain from nearly DC to 5 GHz. The attained microwave performance is promising for obtaining large-format arrays, such as multibeam heterodyne receivers. Moreover, this two-frequency-converter concept based on SIS junctions might enable microwave applications, such as wideband non-reciprocal circuits in isolators, gyrators, and circulators, which are essential devices in the quantum computing and radio astronomy fields.

Josephson parametric circulator with same-frequency signal ports, 200 MHz bandwidth, and high dynamic range

Abstract: We demonstrate a 3-port Josephson parametric circulator, matched to 50 Ohm using second order Chebyshev networks. The device notably operates with two of its signal ports at the same frequency and uses only two out-of-phase pumps at a single frequency. As a consequence, when operated as an isolator it does not require phase coherence between the pumps and the signal, thus simplifying the requirements for its integration into standard dispersive qubit readout setups. The device utilizes parametric couplers based on a balanced bridge of rf-SQUID arrays, which offer purely parametric coupling and high dynamic range. We characterize the device by measuring its full 3x3 S-matrix as a function of frequency and the relative phase between the two pumps. We find up to 15 dB nonreciprocity over a 200 MHz signal band, port match better than 10 dB, low insertion loss of 0.6 dB, and saturation power exceeding -80 dBm.

Highly compressible and thermal insulative conductive MXene/PEDOT:PSS@melamine foam for promising wearable piezoresistive sensor

Abstract: With the rapid development of intelligent wearable electronic devices, highly compressible porous piezoresistive sensors are in imperative demand. However, the robustness of conductive coating that affects the stability and durability of porous piezoresistive sensors still needs to be solved urgently. In this work, a flexible conductive MXene/PEDOT:PSS@Melamine foam (MPMF) piezoresistive sensor was designed and prepared by simply dip-coating it in MXene and PEDOT:PSS mixed solution. Here, foam skeleton was first treated with PDA to improve its hydrophilicity and enhance the interfacial interaction with the functional groups of MXene nanosheets. More importantly, the usage of PEDOT:PSS can fix the MXene nanosheets tightly and construct synergistic conductive network between them, obtaining stable, robust, and highly conductive coating. Based on the contact effect between the adjacent conductive skeleton, the prepared MPMF sensor displays excellent piezoresistive sensing performances, which includes a wide working range (up to 80% compression strain, 60 kPa pressure), high sensitivity (0.30 kPa−1 in the pressure range of 12–60 kPa), and stable sensing pattern over 1000 compression cycles. All these merits make the sensor capable of detecting various human motions and pressure/location distribution of different items when assembled into an electronic skin. In addition, excellent thermal insulation property under different temperature conditions was also observed for MPMF due to the existence of special porous structures, providing necessary thermal protection when served as a wearable sensor. This research provides a convenient, simple, and cost-effective method for the manufacture of high-performance porous piezoresistive sensor.

Fully epitaxial, monolithic ScAlN/AlGaN/GaN ferroelectric HEMT

Abstract: In this Letter, we demonstrated fully epitaxial ScAlN/AlGaN/GaN based ferroelectric high electron mobility transistors (HEMTs). Clean and atomically sharp heterostructure interfaces were obtained by utilizing molecular beam epitaxy. The fabricated ferroelectric gate HEMTs showed counterclockwise hysteretic transfer curves with a wide threshold voltage tuning range of 3.8 V, a large ON/OFF ratio of 3 × 107, and reconfigurable output characteristics depending on the poling conditions. The high quality ferroelectric gate stack and effective ferroelectric polarization coupling lead to improved subthreshold performance, with subthreshold swing values approaching 110 and 30 mV/dec under forward and backward gate sweeps, respectively. The results provide fundamental insight into the ferroelectric polarization coupling and threshold tuning processes in ferroelectric nitride heterostructures and are promising for nitride-based nonvolatile, multi-functional, reconfigurable power, and radio frequency devices as well as memory devices and negative capacitance transistors for next-generation electronics.

Wide-angle and broadband nonreciprocal thermal emitter with cascaded dielectric and Weyl semimetal grating structure

Abstract: The recent review [Phys. Rev. Appl. 18, 027001 (2022)] has considered that the existing schemes of nonreciprocal radiation are greatly limited by the narrow-operated bandwidth and small angular range. To address these key challenges, here, the wide-angle and broadband nonreciprocal radiation based on cascaded dielectric and Weyl semimetal (WS) grating atop a thick continuous metal film is investigated. It is shown that strong nonreciprocal radiation with nonreciprocity larger than 0.9 is achieved in the spectral range of 14.77–16.175 μm for the angle of 59°. The physical origin behind this broadband nonreciprocal radiation is revealed through investigating the magnetic field distributions at several selected wavelengths and is also confirmed by the impedance matching theory. In addition, the broadband nonreciprocal radiation performance remains stably in a wide parameter space. Furthermore, it is found that the broadband spectral nonreciprocity can be maintained well in a wide angular range, in particular, above 0.7 nonreciprocity can be realized in the wavelength range of 14.5–16.5 μm for the angle between 36° and 64.5°. Both features make the proposed scheme very attractive for real production. Finally, the broadband spectral nonreciprocity can be flexibly controlled through change in the axial vector of the WS. We believe that the conclusions will pave the way for designing energy harvesting and conversion devices with improved efficiency.

Extremely low excess noise avalanche photodiode with GaAsSb absorption region and AlGaAsSb avalanche region

Abstract: An extremely low noise Separate Absorption and Multiplication Avalanche Photodiode (SAM-APD), consisting of a GaAs 0.52 Sb 0.48 absorption region and an Al 0.85 Ga 0.15 As 0.56 Sb 0.44 avalanche region, is reported. The device incorporated an appropriate doping profile to suppress tunneling current from the absorption region, achieving a large avalanche gain, ∼130 at room temperature. It exhibits extremely low excess noise factors of 1.52 and 2.48 at the gain of 10 and 20, respectively. At the gain of 20, our measured excess noise factor of 2.48 is more than three times lower than that in the commercial InGaAs/InP SAM-APD. These results are corroborated by a Simple Monte Carlo simulation. Our results demonstrate the potential of low excess noise performance from GaAs 0.52 Sb 0.48 /Al 0.85 Ga 0.15 As 0.56 Sb 0.44 avalanche photodiodes.

Strain induced power enhancement of far-UVC LEDs on high temperature annealed AlN templates

Abstract: High temperature annealed AlN/sapphire templates exhibit a reduced in-plane lattice constant compared to conventional non-annealed AlN/sapphire grown by metalorganic vapor phase epitaxy (MOVPE). This leads to additional lattice mismatch between the template and the AlGaN-based ultraviolet-C light emitting diode (UVC LED) heterostructure grown on these templates. This mismatch introduces additional compressive strain in AlGaN quantum wells resulting in enhanced transverse electric polarization of the quantum well emission at wavelengths below 235 nm compared to layer structures deposited on conventional MOVPE-grown AlN templates, which exhibit mainly transverse magnetic polarized emission. In addition, high temperature annealed AlN/sapphire templates also feature reduced defect densities leading to reduced non-radiative recombination. Based on these two factors, i.e., better outcoupling efficiency of the transverse electric polarized light and an enhanced internal quantum efficiency, the performance characteristic of far-UVC LEDs emitting at 231 nm was further improved with a cw optical output power of 3.5 mW at 150 mA.

High peak power quantum cascade lasers monolithically integrated onto silicon with high yield and good near-term reliability

Abstract: High peak power, room-temperature operation in the long wave infrared spectral region is reported for double-channel, ridge waveguide quantum cascade lasers (QCLs) monolithically integrated onto a silicon substrate. The 55-stage laser structure with an AlInAs/InGaAs core and InP cladding was grown by molecular beam epitaxy directly onto an 8-in. diameter germanium-coated silicon substrate template via a III–V alloy metamorphic buffer. Atomic force microscope imaging demonstrated a good quality surface for the full QCL structure grown on silicon, with improved roughness over wider areas compared to the previous work. Fabricated 3 mm × 26 μm lasers operate at room temperature, deliver more than 3 W of peak (6 mW of average) optical power, and show approximately 3% wall plug efficiency and 4.3 kA/cm2 threshold current density with emission wavelength centered at 11.5 μm. The lasers had a high yield with only around 15% max power deviation and no signs of performance degradation were observed over a 10 h burn in period at maximum power. Singled-lobed high quality output beam with M2 = 1.36 was measured for 3 mm × 22 μm devices, demonstrating that it is possible to produce high-brightness quantum cascade lasers on silicon with standard ridge waveguide processing paving the way for low-cost production of integrated mid-infrared platforms.

Compact optomechanical accelerometers for use in gravitational wave detectors

Abstract: We present measurements of an optomechanical accelerometer for monitoring low-frequency noise in gravitational wave detectors, such as ground motion. Our device measures accelerations by tracking the test-mass motion of a 4.7 Hz mechanical resonator using a heterodyne interferometer. This resonator is etched from monolithic fused silica, an under-explored design in low-frequency sensors, allowing a device with a noise floor competitive with existing technologies but with a lighter and more compact form. In addition, our heterodyne interferometer is a compact optical assembly that can be integrated directly into the mechanical resonator wafer to further reduce the overall size of our accelerometer. We anticipate this accelerometer to perform competitively with commercial seismometers, and benchtop measurements show a noise floor reaching 82 pico- g Hz−1/2 sensitivities at 0.4 Hz. Furthermore, we present the effects of air pressure, laser fluctuations, and temperature to determine the stability requirements needed to achieve thermally limited measurements.

An insight into lithium-ion transport in germanium-doped lithium titanate anode through NMR spectroscopy and post-carbonization for anode applications in lithium-ion battery

Abstract: Adapting toward lithium titanate as a negative electrode for lithium-ion batteries led to the safest and long-lasting battery technology, especially for electric vehicle applications. However, the poor conductivity and lithium-ion diffusion of lithium titanate have to be addressed for widespread usage in next-generation E-mobility. The lithium-ion motion inside lithium titanate and germanium-doped lithium titanate was investigated through pulsed-field gradient nuclear magnetic resonance spectroscopy and temperature-dependent ionic conductivity studies. The superior charge carrier mobility of germanium enhanced the lithium-ion diffusion in lithium titanate significantly to 1.48 × 10−8 cm2 s−1 in Li4Ge0.1Ti4.9O12 at 500 °C. While germanium improves the ionic diffusion, an ex situ carbon coating was adapted over the sample for electronic conductivity enhancement. Samples with two different carbon contents (5 and 10 wt. %) were examined for electrochemical analysis. Significant improvements in battery performance were observed on carbon-coated germanium-doped lithium titanate. The carbon-coated sample gave superior initial performance (191 and 178 mAh g−1 for 10 and 5 wt. % carbon at 0.1C) than the pristine lithium titanate and preserved the exceptional capacity retention over a thousand cycles at 1C rate.

Flexible self-powered DUV photodetectors with high responsivity utilizing Ga2O3/NiO heterostructure on buffered Hastelloy substrates

Abstract: In this research, β-Ga2O3/NiO heterostructures were grown directly on CeO2 buffered Hastelloy flexible substrates. With pulsed laser deposition under high temperatures, as-grown β-Ga2O3 and NiO thin films have a preferred out-of-plane orientation along the ⟨−201⟩ and ➎111➉ directions. This is due to the ideal epitaxial ability of the CeO2 buffer layer, which serves as a perfect template for the epitaxial growth of single-oriented NiO and β-Ga2O3 by creating a constant gradient from CeO2 (2.7 Å along ➎001➉) to NiO (2.9 Å along ➎110➉), and eventually to β-Ga2O3 (3.04 Å along ➎010➉). The Hastelloy substrates endow photodetectors with good deformability and mechanical robustness. Moreover, owing to the type-II band alignment of β-Ga2O3/NiO heterostructures, the photodetectors have a good photocurrent at zero bias under 284 nm of light illumination. In addition, the photocurrent is significantly higher than when using an analogous heterostructure (as described in some previous reports), because the β-Ga2O3 and NiO thin films are crystalized along a single orientation with fewer defects.

Etching-enabled ultra-scalable micro and nanosculpturing of metal surfaces for enhanced thermal performance

Abstract: Incorporation of micro- and nanostructures on metals can improve thermal performance in a variety of applications. In this work, we demonstrate two independent highly scalable and cost-effective methods to generate micro- and nanostructures on copper and stainless steel, two widely used metals in energy and thermal applications. The performance of the developed structures, fabricated using scalable chemical etching techniques, is compared against their respective base metals. Our results demonstrate significant flow boiling heat transfer coefficient improvements up to 89% for etched copper and 104% for etched stainless steel. Mercury porosimetry is used to demonstrate that the varying pore-size distributions and presence of micro/nanoscale channels help to regulate heat transfer mechanisms, such as nucleate and convective flow boiling. Furthermore, structure integrity after 7-day flow boiling tests demonstrate surface structure resiliency to damage, a key challenge to implementation. This work combines advances in thermal performance with surface structure durability to provide guidelines for broader application of similar chemical etching methods to scalably create micro- and nanosculptured surfaces.

Si doping of β-Ga2O3 by disilane via hybrid plasma-assisted molecular beam epitaxy

Abstract: Obtaining uniform silicon concentration, especially with low concentrations (ranging from 1 × 1016 to 1 × 1018 cm−3) by molecular beam epitaxy, has been challenging due to oxidation of a silicon solid source in the oxide environment. In this work, Si doping of β-Ga2O3 (010) films by diluted disilane as the Si source is investigated using hybrid plasma-assisted molecular beam epitaxy. The impact of growth temperature, disilane source concentration, and disilane flow rate on Si incorporation was studied by secondary ion mass spectrometry. Uniform Si concentrations ranging from 3 × 1016 to 2 × 1019 cm−3 are demonstrated. Si-doped β-Ga2O3 films with different silicon concentrations were grown on Fe-doped β-Ga2O3 (010) substrates. The electron concentration and mobility were determined using van de Pauw Hall measurements. A high mobility of 135 cm2/V s was measured for an electron concentration of 3.4 × 1017 cm−3 at room temperature.

Enhanced operating temperature in terahertz quantum cascade lasers based on direct phonon depopulation

Abstract: Room temperature operation of terahertz quantum cascade lasers (THz QCLs) has been a long-pursued goal to realize compact semiconductor THz sources. In this paper, we report on improving the maximum operating temperature of THz QCLs to ∼ 261 K as a step toward the realization of this goal.

Topological rainbow trapping and acoustic energy amplification in two-dimensional gradient phononic crystals

Abstract: In this work, we numerically and experimentally investigate topological rainbow trapping and energy amplification of acoustic waves in a gradient phononic crystal (PC) structure. Thanks to the acoustic valley Hall effect, topological interface states (TISs) are generated along the interface between two PCs with different topological phases. To achieve rainbow trapping, we introduce the gradient into a 3D-printed PC structure by varying the geometrical parameter of scatterers along the interface. The incident acoustic waves at different frequencies split, stop, and, hence, are significantly amplified at different positions. Notably, the rainbow trapping of TISs is immune to random structural disorders. The topological rainbow trapping is promising for the design of broadband energy harvesters with excellent robustness.

NiO junction termination extension for high-voltage (>3 kV) Ga2O3 devices

Abstract: Edge termination is the enabling building block of power devices to exploit the high breakdown field of wide bandgap (WBG) and ultra-wide bandgap (UWBG) semiconductors. This work presents a heterogeneous junction termination extension (JTE) based on p-type nickel oxide (NiO) for gallium oxide (Ga2O3) devices. Distinct from prior JTEs usually made by implantation or etch, this NiO JTE is deposited on the surface of Ga2O3 by magnetron sputtering. The JTE consists of multiple NiO layers with various lengths to allow for a graded decrease in effective charge density away from the device active region. Moreover, this surface JTE has broad design window and process latitude, and its efficiency is drift-layer agnostic. The physics of this NiO JTE is validated by experimental applications into NiO/Ga2O3 p–n diodes fabricated on two Ga2O3 wafers with different doping concentrations. The JTE enables a breakdown voltage over 3.2 kV and a consistent parallel-plate junction field of 4.2 MV/cm in both devices, rendering a power figure of merit of 2.5–2.7 GW/cm2. These results show the great promise of the deposited JTE as a flexible, near ideal edge termination for WBG and UWBG devices, particularly those lacking high-quality homojunctions.

Accelerating ultrafast processes in hydrogen-bonded complexes under pressure

Abstract: Acceleration of ultrafast processes is vital in hydrogen-bonded coumarin–methanol complexes for improving the photoelectric conversion efficiency of dye-sensitized solar cells (DSSCs). The traditional methods expedite ultrafast processes individually related to electron injection in DSSCs, namely, internal conversion (IC) or intermolecular charge transfer (inter-CT), by adjusting molecular topologies. We introduce pressure as an external drive to realize the acceleration of both processes simultaneously without changing the configuration. In the definite hydrogen-bonded complexes, the acceleration of IC and inter-CT processes is visualized by in situ high-pressure femtosecond transient absorption spectroscopy. In liquid-phase methanol, the IC and inter-CT processes are actuated effectively from 150.20 to 59.21 fs and 93.95 to 29.05 ps, respectively. The quickening of both processes is attributed to the enhancement of intermolecular hydrogen bonds under pressure. After the pressure-induced methanol phase transition, the rates of IC and inter-CT processes at 3.67 GPa are increased by 36.42% and 80.55% compared to at 1.00 atm. Our study results open an enlightening avenue for boosting the photoelectric conversion efficiency of DSSCs.

N-polar GaN/AlGaN/AlN high electron mobility transistors on single-crystal bulk AlN substrates

Abstract: Recent observation of high density polarization-induced two-dimensional electron gases in ultra-thin N-polar GaN layers grown on single-crystal AlN has enabled the development of N-polar high electron mobility transistors (HEMTs) on AlN. Such devices will take advantage of thermal and power handling capabilities of AlN, while simultaneously benefitting from the merits of N-polar structures, such as a strong back barrier. We report the experimental demonstration of N-polar GaN/AlGaN/AlN HEMTs on single-crystal AlN substrates, showing an on-current of 2.6 A/mm with a peak transconductance of 0.31 S/mm. Small-signal RF measurements revealed speeds exceeding ft/ fmax = 68/100 GHz. These results pave the way for developing RF electronics with excellent thermal management based on N-polar single-crystal AlN.

Doping effects on the ferroelectric properties of wurtzite nitrides

Abstract: Ferroelectric materials have been explored for a long time for easy integration with state-of-the-art semiconductor technologies. Doped wurtzite nitrides have been reported as promising candidates due to their high stability, compatibility, and scalability. We investigate doping effects on ferroelectric properties of Sc-doped AlN (AlScN) and B-doped AlN (AlBN) by first-principles methods. The energy barrier against polarization switching is observed to decrease with increasing doping concentration at low concentration ranges, which is the origin of the emerging ferroelectricity in doped AlN. Further increasing the doping concentration to a critical value, the ferroelectric wurtzite phase transforms into paraelectric phases (a rock salt phase for AlScN and a zinc blende phase for AlBN), making it invalid to decrease the coercivity by increasing the doping concentration. Furthermore, it is revealed that different nonpolar structures (a hexagonal phase for AlScN and a [Formula: see text]-BeO phase for AlBN) appear in the ferroelectric switching pathway, generating different switching features in doped AlN. Our results give a microscopic understanding of the ferroelectricity in doped wurtzite materials and broaden the route to improve their ferroelectric properties.

Growing fields in a temporal photonic (time) crystal with a square profile of the permittivity ε(t)

Abstract: We investigate a band structure [Formula: see text] of a photonic time crystal with periodic square (step) modulation in time of its permittivity [Formula: see text], oscillating between the value [Formula: see text] (sustained for a fraction of time τ of the period) and the value [Formula: see text] [fraction (1 − τ)]. The strength of modulation is [Formula: see text]. We find that [Formula: see text] can be periodic in a wave number k (in addition to the frequency ω), provided that a certain function [Formula: see text] of the parameters m and τ is an irreducible rational number. However, even for arbitrary values of m and τ, [Formula: see text] can be approximated by a fractional number to any desired degree of periodicity. Hence, for square modulation, a photonic band structure is necessarily periodic or quasi-periodic in the wave number. Moreover, for appropriate sets of the parameters m and τ, the modes associated with k values within the band gaps can have identical values of the imaginary part of ω. For simultaneous excitation of these modes, all the fields would grow in time at the same rate, resulting in powerful amplification.