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Using the spin-polarized first-principles calculations, we revealed that two-dimensional transition metal nitride MXenes Cr2NX2 (X = O, F, OH) are excellent two-dimensional half-metallic ferromagnetic materials. Their structures and ferromagnetic ground states are stable well above room temperature. In addition, their large half-metal bandgaps are enough to prevent spin reversal and ensure high spin filtering efficiency and large spin mean free paths. The half-metallic property of these functionalized Cr2NX2 systems is robust and can be maintained under tensile strains up to 10%. These predictions suggest that the functionalized Cr2NX2 is of great significance for the development of highly efficient spintronic devices for room temperature applications. [ABSTRACT FROM AUTHOR]

An A- and B-site ordered quadruple perovskite oxide LaCu3Co2Re2O12 was synthesized at 9 GPa and 1323 K. The compound possesses a Pn-3 space group, where both A and B sites are orderly occupied by different cations with a nearly 100% degree of order. Bond valence sum calculations and x-ray absorption spectroscopy confirm the charge distribution to be LaCu2+3Co2+2Re5.5+2O12. A ferrimagnetic phase transition is found to occur around 150 K due to the Cu2+(↑)-Co2+(↑)-Re5.5+(↓) spin coupling found by x-ray magnetic circular dichroism at Cu-, Co-, and Re-L2,3 edges. The magnetoresistance effects as well as the first-principle calculations indicate the half-metallic nature for LaCu3Co2Re2O12 with a wider energy gap at the up-spin channel and a conducting band at the down-spin channel. [ABSTRACT FROM AUTHOR]

This letter presents a study on the nucleation and microstructural evolution of m-plane GaN epilayers on m-plane SiC substrates using high-temperature AlN buffer layers. Controlled growth interruptions were carried out to render snapshots of heteroepitaxial dynamics. It was discovered that island coalescence results in an inhomogeneous mosaic tilt along the c-axis. Mesoscopic study of nucleation evolution helps elucidate the origin of commonly observed surface undulation and striation, which is attributed to concave growth due to the coalescence of trapezoidal islands upon contact. A model correlating microstructural defects with optical properties is proposed to explain the observed pattern in spatially resolved cathodoluminescence mapping. [ABSTRACT FROM AUTHOR]

In this work, AlN polarization-enhancement interlayer (AlN-PEL) is adopted to enhance two-dimensional hole gas (2DHG) density in a p-GaN/AlN-PEL(∼2 nm)/AlGaN(<6 nm)/GaN heterostructure, aiming at monolithic integration of p/n-channel field effect transistors (p-FETs) on GaN-on-Si substrate. Owing to the strong built-in polarization of the AlN-PEL, high density 2DHG over 2.3 × 1013 cm−2 with good immunity to thermal freeze out effect is realized. Assisted by a two-step gate trench etching process, enhancement-mode (E-mode) buried-channel GaN p-FETs with temperature independent ON-resistance RON, and ON/OFF current ratio ION/IOFF (>108), have been fabricated. The fabricated p-FETs also deliver thermally stable subthreshold swing as well as threshold voltage Vth, and smaller Vth shift than that of p-FETs without the AlN-PEL, which is primarily due to enhanced 2DHG confinement by the AlN-PEL. The proposed structure is an attractive platform for monolithic integration of GaN-based logic and power devices for cryogenic applications as low as 10 K. [ABSTRACT FROM AUTHOR]

A HfO2/Al2O3/AlN stack deposited in a plasma-enhanced atomic layer deposition system is used as the gate dielectric for GaN metal–insulator–semiconductor structures. Prior to the deposition, in situ remote plasma pretreatments (RPPs) with O2, NH3, and NH3–O2 (in sequence) were carried out. Frequency-dependent capacitance–voltage (C–V) measurements reveal that for all the RPP-treated samples, the interface trap density Dit was significantly reduced compared to the sample without RPP. In particular, the NH3-treated sample exhibits an extremely low Dit of ∼1011 cm−2·eV−1 from EC − 0.31 eV to EC − 0.46 eV. To characterize the interface traps at deeper levels, the transfer pulsed I–V measurement was employed further. In the energy range of EC − ET > 0.54 eV, the interface trap charge density Qit of various samples demonstrates the following order: NH3 RPP < NH3–O2 RPP < O2 RPP < without RPP. By adjusting multiple pulse widths, the Dit derived from the Qit result matches the C–V measurement precisely. Moreover, X-ray photoelectron spectroscopy analysis of the Ga 3d core-level indicates that NH3 RPP facilitates the conversion of Ga2O into GaN at high RF power of 2500 W, whereas O2 RPP mainly oxidizes GaN to relatively stable Ga2O3. Combined with the C–V and pulsed I–V measurements, it is confirmed that a strong positive correlation exists between Ga2O and the interface traps, rather than Ga2O3. [ABSTRACT FROM AUTHOR]

High efficiency color-tunable organic light-emitting diodes (CT-OLEDs) have been demonstrated by combining non-doped ultra-thin red and green phosphorescent emitters with a blue phosphorescence-doped exciplex system. The resultant devices deliver a sunlight-style emission with a wide correlated color-temperature span from 2143 to 7563 K and achieve the maximum current efficiency, the power efficiency, and the external quantum efficiency of 34.4 cd A−1, 39.5 lm W−1, and 18.6%, respectively, without any out-coupling enhancements. Furthermore, the CT-OLEDs exhibit low efficiency roll-off that the external quantum efficiencies still remain 18.5% and 17.1% even at 1000 and 5000 cd m−2, respectively, which are more efficient than those of state-of-the-art CT-OLEDs. [ABSTRACT FROM AUTHOR]

This Letter reports low-temperature (700 °C) growth of heavily Si-doped GaN (n++GaN) by metal-organic chemical vapor deposition with a resistivity as low as 1.9 × 10−4 Ω·cm and an atomically smooth surface. Indium adatoms added during the growth of n++GaN play an important role in improving both the surface morphology and free electron concentration. On the one hand, acting as surfactant, they greatly boost the adatoms surface mobility at low growth temperature and mitigate Si-induced anti-surfactant effect. On the other hand, they can effectively suppress the formation of compensating defects, thus contributing to an extremely high electron concentration of 2.8 × 1020 cm−3. This high-quality n++GaN was further applied to the realization of Ohmic contacts with an ultra-low contact resistance for AlGaN/GaN high electron mobility transistors. The carrier gas was carefully modulated for the selective area epitaxy (SAE) of n++GaN to facilitate the nucleation of GaN on the dielectric mask, which effectively suppressed the undesired mass transport and resulted in a uniform SAE of n++GaN in the recessed source/drain regions. A nearly defect-free interface between the n++GaN and two-dimensional electron gas channel has been also realized, and the resistance induced by the interface was only 0.03 Ω·mm. As a result, an ultra-low contact resistance of 0.07 Ω·mm has been realized. This work lays a solid foundation for further improving the performance of GaN-based RF and power devices. [ABSTRACT FROM AUTHOR]

This article was originally published online on 27 December 2023 with an error in the affiliation. The affiliation appears correct above. All online versions of this article were corrected on 4 April 2024.By Yingjie Wang; Sen Huang; Qimeng Jiang; Xinhua Wang; Zhongchen Ji; Jie Fan; Haibo Yin; Ke Wei; Xinyu Liu; Qian Sun and Kevin J. ChenReported by Author; Author; Author; Author; Author; Author; Author; Author; Author; Author; Author [Extracted from the article]

Threshold voltage (VTH) instability has been studied in the as-fabricated p-GaN gated enhancement-mode high electron mobility transistors (p-GaN E-HEMTs) under a positive gate stress. A negative VTH shift (ΔVTH) obtained by dynamic measurement has been observed more severely when compared to the static one. The VTH deviation is attributed to the complicated influence of carrier transport behaviors in the p-GaN gate. The impacts of hole accumulation, trapping, and consumption on the VTH instability and drain current variation can be effectively distinguished according to the transfer characteristics obtained from the pulse I–V measurement. Moreover, the density of hole traps in the p-GaN gate is estimated to be around ∼2 × 1011 cm−2 by the capacitance–voltage measurement, and the energy level is calculated to be around EV + 0.62 eV by fitting the recovery curve of gate current after positive gate bias. This study focusing on the in-depth influence of different carrier behaviors on the gate performance can help with the understanding and further improvement of the dynamic instability and reliability of the GaN-based HEMTs. [ABSTRACT FROM AUTHOR]

This Letter studies the reverse leakage and breakdown mechanisms of vertical GaN-on-Si Schottky barrier diodes (SBDs) with and without argon-implanted termination (ArIT). The electrical leakage characteristics in the vertical GaN-on-Si SBD without edge termination sequentially go through the thermionic field emission, variable range hopping (VRH), and trap-assisted tunneling conduction mechanisms as the reverse bias increases gradually. Its leakage and breakdown mechanisms are limited by the edge electric field crowding effect. While for the vertical GaN-on-Si SBD with ArIT (ArIT-SBD), the electrons conduction at a low reverse bias, following the space-charge-limited conduction (SCLC) model, is limited by the damage-induced traps in the implanted GaN. As the reverse bias increases up to the occurrence of breakdown, the VRH and SCLC dominate the leakage mechanism of the ArIT-SBD, which stem from intrinsic traps in GaN grown on Si. A rapidly growing leakage under a low reverse bias and enhanced breakdown voltage performance in the ArIT-SBD is attributed to the charging of the damage-induced traps in implanted GaN. This Letter not only gives in-depth insights of vertical GaN-on-Si SBDs but also provides a useful design guidance of implanted termination for high-voltage power devices. [ABSTRACT FROM AUTHOR]

A room-temperature method to determine the trap energy levels in the carbon-doped (Al)GaN buffers is developed via a transient current measurement on the AlGaN/GaN high electron mobility transistors under back-gate voltages combined with a measurement of the buffer vertical leakage. Under high back-gate voltages, a linear relationship is obtained between the trap energy levels and the square roots of electric field strength, suggesting that the vertical conduction in the C-doped buffer follows the Poole–Frenkel law. The trap energy level in C-doped Al0.07Ga0.93N is finally determined to be 1.1 eV through the established room-temperature approach, while that in C-doped GaN is extracted to be 0.9 eV, both of which are related to the carbon impurities. [ABSTRACT FROM AUTHOR]

We observed the formation of oxide precipitates (bcc-In2O3) in InN nanostructures formed during metal-organic chemical vapor deposition (MOCVD) and/or subsequent postgrowth procedures in H2 ambient. It was found that InN is extremely unstable in H2 ambient and the activation energy of N2 desorption of InN is measured to be ∼0.28 eV, which is one order of magnitude smaller than that of reported value of InN in vacuum. Instability of InN nanostructures under H2 ambient together with residual oxidant in the reactor facilitates the formation of indium oxide precipitates in the nanostructure matrix during MOCVD or the oxidation of residual indium at the surface, resulting in indium oxide dots. [ABSTRACT FROM AUTHOR]

We investigated the light-induced backward scattering in doubly doped lithium niobate crystals and observed an intensity threshold effect. It will be shown that scattering microregions are locally recorded in the sample by holographic interaction due to the existence of the threshold effect. Experimental results of multilayer recording point out that this property could be used for high-density multilayer-like bit data optical storage while keeping high signal-to-noise ratio. [ABSTRACT FROM AUTHOR]

The thermal expansion of a scanning tunneling microscope tip induced by femtosecond laser is investigated with various parameters including laser power, modulation frequency, illumination spot, and laser wavelength. The magnitude of tip expansion is measured to be proportional to the laser power. The response bandwidth is closely related to the length of the tip cone section, which is consistent with a two-rod model simulation. While visible lasers produce significant tip expansion, deep ultraviolet and near infrared lasers result in significantly reduced expansion magnitude, which can be explained with the tip induced surface plasmon in the tunneling junction. [ABSTRACT FROM AUTHOR]