Boosting the Curie temperature of GaN monolayer through van der Waals heterostructures.

The pursuit of van der Waals (vdW) heterostructures with high Curie temperature and strong perpendicular magnetic anisotropy (PMA) is vital to the advancement of next generation spintronic devices. First-principles calculations are used to study the electronic structures and magnetic characteristics of GaN/VS2vdW heterostructure under biaxial strain and electrostatic doping. Our findings show that a ferromagnetic ground state with a remarkable Curie temperature (477 K), much above room temperature, exists in GaN/VS2vdW heterostructure and 100% spin polarization efficiency. Additionally, GaN/VS2vdW heterostructure still maintains PMA under biaxial strain, which is indispensable for high-density information storage. We further explore the electron, magnetic, and transport properties of VS2/GaN/VS2vdW sandwich heterostructure, where the magnetoresistivity can reach as high as 40%. Our research indicates that the heterostructure constructed by combining the ferromagnet VS2and the non-magnetic semiconductor GaN is a promising material for vdW spin valve devices at room temperature.

Effect of Fe Doping Profile on Current Collapse in GaN-based RF HEMTs.

Using computer-aided design (TCAD) simulation, the impact of the Fe doping profile, including concentration, decay rate, and depth of the doping region on current-collapse magnitude (▵CC) in 0.5-µm gated GaN-based high electron mobility transistors (HEMTs) is systematically investigated. Accurate simulation models are established and developed to facilitate the fabrication of electronics. It is elucidated that the intricate interplay between trapping and de-trapping of Fe-related traps at the gate-drain edge is responsible for current collapse. The concentration and decay rate of the doping region have a more significant impact on current collapse than the depth. Increased trap state density near two-dimensional electron gas (2DEG) channel caused by deep-level acceptors would boost ▵CC. However, a minor dynamic reduction in 2DEG density (nT) induces a relatively small ▵CC. By adjusting the concentration, decay rate, and depth of the doping region, ▵CC of GaN-based Radio Frequency (RF) HEMTs can be reduced by approximately 50.3 %. The optimized distribution of Fe doping discussed in this work helps to prepare GaN-based RF HEMTs with a limited current collapse effect.

ß-Ga2O3nanotube arrays for high-performance self-powered ultraviolet photoelectrochemical photodetectors.

Self-powered ultraviolet (UV) photodetectors (PDs) are critical for future energy-efficient optoelectronic systems due to their low energy consumption and high sensitivity. In this paper, the vertically alignedß-Ga2O3nanotube arrays (NTs) have been prepared on GaN/sapphire substrate by the thermal oxidation process combined with the dry etching technology, and applied in the UV photoelectrochemical photodetectors (PEC-PDs) for the first time. Based on the large specific surface area ofß-Ga2O3NTs on GaN/sapphire substrates and the solid/liquid heterojunction, the PEC-PDs exhibit excellent self-powered characteristics under 255 nm (UVA) and 365 nm (UVC) light illumination. Under 255 nm (365 nm) light illumination, the maximum responsivity of 49.9 mA W-1(32.04 mA W-1) and a high detectivity of 1.58 × 1011Jones (1.01 × 1011Jones) were achieved for theß-Ga2O3NTs photodetectors at 0 V bias. In addition, the device shows a fast rise/decay time of 8/4 ms (4/2 ms), which is superior to the level of the previously reported self-powered UV PEC-PDs. This high-performance PEC-PD has potential applications in next-generation low-energy UV detection systems.

Al0.1Ga0.9N p-i-n ultraviolet avalanche photodiodes with suppressed surface leakage current and uniform avalanche breakdown.

We report high-performance Al0.1Ga0.9N p-i-n ultraviolet (UV) avalanche photodiodes (APDs) based on sapphire substrates with stable breakdown voltages (VBR) around 113.4 V, low dark current densities (JBR) below 9 × 10-4 A/cm2 and a high avalanche gain over 2 × 106. A two-step deposition method was employed to reduce passivation-induced plasma damage while maintaining high dielectric film quality. Consistent JBR for various mesa sizes at the VBR are demonstrated, which reveals the suppression of the surface leakage current. Uniform electroluminescence (EL) distributions during the avalanche multiplication processes are displayed, which confirms the elimination of edge breakdown. Pure bulk leakage current distributions and uniform body avalanche breakdown behaviors are observed for the first time in AlGaN APDs. The emission spectra of the EL at various current levels are also presented.

An Analytical Model of Dynamic Power Losses in eGaN HEMT Power Devices.

In this work, we present an analytical model of dynamic power losses for enhancement-mode AlGaN/GaN high-electron-mobility transistor power devices (eGaN HEMTs). To build this new model, the dynamic on-resistance (Rdson) is first accurately extracted via our extraction circuit based on a double-diode isolation (DDI) method using a high operating frequency of up to 1 MHz and a large drain voltage of up to 600 V; thus, the unique problem of an increase in the dynamic Rdson is presented. Then, the impact of the current operation mode on the on/off transition time is evaluated via a dual-pulse-current-mode test (DPCT), including a discontinuous conduction mode (DCM) and a continuous conduction mode (CCM); thus, the transition time is revised for different current modes. Afterward, the discrepancy between the drain current and the real channel current is qualitative investigated using an external shunt capacitance (ESC) method; thus, the losses due to device parasitic capacitance are also taken into account. After these improvements, the dynamic model will be more compatible for eGaN HEMTs. Finally, the dynamic power losses calculated via this model are found to be in good agreement with the experimental results. Based on this model, we propose a superior solution with a quasi-resonant mode (QRM) to achieve lossless switching and accelerated switching speeds.

The Study on the Lasing Modes Modulated by the Dislocation Distribution in the GaN-Based Microrod Cavities.

Low-threshold lasing under pulsed optical pumping is demonstrated in GaN-based microrod cavities at room temperature, which are fabricated on the patterned sapphire substrates (PSS). Because the distribution of threading dislocations (TDs) is different at different locations, a confocal micro-photoluminescence spectroscopy (µ-PL) was performed to analyze the lasing properties of the different diameter microrods at the top of the triangle islands and between the triangle islands of the PSS substrates, respectively. The µ-PL results show that the 2 µm-diameter microrod cavity has a minimum threshold of about 0.3 kW/cm2. Whispering gallery modes (WGMs) in the microrod cavities are investigated by finite-difference time-domain simulation. Combined with the dislocation distribution in the GaN on the PSS substrates, it is found that the distribution of the strongest lasing WGMs always moves to the region with fewer TDs. This work reveals the connection between the lasing modes and the dislocation distribution, and can contribute to the development of low-threshold and high-efficiency GaN-based micro-lasers.

High-responsivity dual-band ultraviolet photodetector based on Ga2O3/GaN heterostructure.

Ultraviolet photodetectors have aroused wide concern based on wide-band-gap semiconductors, such as GaN and Ga2O3. Exploiting multi-spectral detection provides unparalleled driving force and direction for high-precision ultraviolet detection. Here we demonstrate an optimized design strategy of Ga2O3/GaN heterostructure bi-color ultraviolet photodetector, which presents extremely high responsivity and UV-to-visible rejection ratio. The electric field distribution of optical absorption region was profitably modified by optimizing heterostructure doping concentration and thickness ratio, thus further facilitating the separation and transport of photogenerated carriers. Meanwhile, the modulation of Ga2O3/GaN heterostructure band offset leads to the fluent transport of electrons and the blocking of holes, thereby enhancing the photoconductive gain of the device. Eventually, the Ga2O3/GaN heterostructure photodetector successfully realizes dual-band ultraviolet detection and achieves high responsivity of 892/950 A/W at the wavelength of 254/365 nm, respectively. Moreover, UV-to-visible rejection ratio of the optimized device also keeps at a high level (∼103) while exhibiting dual-band characteristic. The proposed optimization scheme is anticipated to provide significant guidance for the reasonable device fabrication and design on multi-spectral detection.

Enhanced Thermoelectric Properties of Coated Vanadium Oxynitride Nanoparticles/PEDOT:PSS Hybrid Films.

Thermoelectric (TE) materials transform thermal energy into electricity, which can play an important role for global sustainability. Conducting polymers are suitable for the preparation of flexible TE materials because of their low-cost, lightweight, flexible, and easily synthesized properties. Here, we fabricate organic-inorganic hybrids by combining vanadium oxynitride nanoparticles coated with nitrogen-doped carbon (NC@VNO) and poly(3,4-ethylenedioxy thiophene):poly(styrene sulfonate) (PEDOT:PSS). We find that the electrical conductivity, Seebeck coefficient, and power factor of the NC@VNO/PEDOT:PSS film can be enhanced up to 4158 S/cm, 45.8 µV/K, and 873 µW/mK2 at 380 K, respectively. The large enhancement of the power factor may be due to the facilitation of the interfacial charge transport tunnel between the NC@VNO nanoparticles and PEDOT:PSS. The improvement of the Seebeck coefficient may be due to the energy filter effect as induced by interfacial contact and internal electric field between the NC@VNO nanoparticles and PEDOT:PSS. Our measurement suggests that the high binding energy of pyrrolic-N enhances the Seebeck coefficient, and the high binding energy of oxide-N increases electrical conductivity.

Simulation Study on the Structure Design of p-GaN/AlGaN/GaN HEMT-Based Ultraviolet Phototransistors.

This work investigates the impacts of structural parameters on the performances of p-GaN/AlGaN/GaN HEMT-based ultraviolet (UV) phototransistors (PTs) using Silvaco Atlas. The simulation results show that a larger Al content or greater thickness for the AlGaN barrier layer can induce a higher two-dimensional electron gas (2DEG) density and produce a larger photocurrent. However, they may also lead to a larger dark current due to the incomplete depletion of the GaN channel layer. The depletion conditions with various Al contents and thicknesses of the AlGaN layer are investigated in detail, and a borderline between full depletion and incomplete depletion was drawn. An optimized structure with an Al content of 0.23 and a thickness of 14 nm is achieved for UV-PT, which exhibits a high photocurrent density of 92.11 mA/mm, a low dark current density of 7.68 × 10-10 mA/mm, and a large photo-to-dark-current ratio of over 1011 at a drain voltage of 5 V. In addition, the effects of other structural parameters, such as the thickness and hole concentration of the p-GaN layer as well as the thickness of the GaN channel layer, on the performances of the UV-PTs are also studied in this work.

Low Leakage Current and High Breakdown Field AlGaN/GaN MIS-HEMTs Using PECVD-SiNx as a Gate Dielectric.

In this paper, SiNx film deposited by plasma-enhanced chemical vapor deposition was employed as a gate dielectric of AlGaN/GaN high electron mobility transistors (HEMTs). We found that the NH3 flow during the deposition of SiNx can significantly affect the performances of metal-insulator-semiconductor (MIS) HEMTs. Compared to that without using NH3 flow, the device with the optimized NH3 flow exhibited three orders of magnitude lower gate leakage current, two orders of magnitude higher ON/OF drain current ratio, and an increased breakdown field by 69%. In addition, an in situ N2 plasma surface treatment prepared prior to SiNx deposition can further improve DC performances of MIS-HEMTs to a very low gate leakage current of 10-9 mA/mm and a high ON/OFF drain current ratio up to 109 by reducing the interface state density. These results demonstrate the great potential for using PECVD-SiNx as a gate dielectric in GaN-based MIS-HEMTs.

Evaluation on Temperature-Dependent Transient VT Instability in p-GaN Gate HEMTs under Negative Gate Stress by Fast Sweeping Characterization.

In this work, temperature-dependent transient threshold voltage (VT) instability behaviors in p-GaN/AlGaN/GaN HEMTs, with both Schottky gate (SG) and Ohmic gate (OG), were investigated systematically, under negative gate bias stress, by a fast voltage sweeping method. For SG devices, a concave-shaped VT evolution gradually occurs with the increase in temperature, and the concave peak appears faster with increasing reverse bias stress, followed by a corresponding convex-shaped VT recovery process. In contrast, the concave-shaped VT evolution for OG devices that occurred at room temperature gradually disappears in the opposite shifting direction with the increasing temperature, but the corresponding convex-shaped VT recovery process is not observed, substituted, instead, with a quick and monotonic recovery process to the initial state. To explain these interesting and different phenomena, we proposed physical mechanisms of time and temperature-dependent hole trapping, releasing, and transport, in terms of the discrepancies in barrier height and space charge region, at the metal/p-GaN junction between SG and OG HEMTs.

High Power Figure-of-Merit, 10.6-kV AlGaN/GaN Lateral Schottky Barrier Diode with Single Channel and Sub-100-µm Anode-to-Cathode Spacing.

GaN-based lateral Schottky barrier diodes (SBDs) have attracted great attention for high-power applications due to its combined high electron mobility and large critical breakdown field. However, the breakdown voltage (BV) of the SBDs are far from exploiting the material advantages of GaN at present, limiting the desire to use GaN for ultra-high voltage (UHV) applications. Then, a golden question is whether the excellent properties of GaN-based materials can be practically used in the UHV field? Here, UHV AlGaN/GaN SBDs are demonstrated on sapphire with a BV of 10.6 kV, a specific on-resistance (RON,SP ) of 25.8 mΩ cm2 , yielding a power figure-of-merit (P-FOM = BV2 /RON,SP ) of 4.35 GW cm-2 . These devices are designed with single channel and 85-µm anode-to-cathode spacing, without other additional electric field management, demonstrating its great potential for the UHV application in power electronics.

Self-Assembly Nanopillar/Superlattice Hierarchical Structure: Boosting AlGaN Crystalline Quality and Achieving High-Performance Ultraviolet Avalanche Photodetector.

As a burgeoning wide-band gap semiconductor material, AlxGa1-xN alloy has attracted great attention for versatile applications due to its superior properties. However, its poor crystalline quality has restricted the employment of AlGaN on electronic devices for a long time. Herein, we proposed a nanopillar/superlattice hierarchical structure for AlGaN epitaxy to boost the crystalline quality. The scale-controllable AlN nanopillar template is fabricated from a nickel self-assembly process. AlGaN initiates the epitaxial laterally overgrowth mode based on the nanopatterned template. In addition, the AlxGa1-xN/AlyGa1-yN superlattice structure could effectively block the propagation of threading dislocation segments. The kinetics of the dislocation and epitaxy process in the hierarchical structure is intuitively demonstrated and analyzed. Consequently, the dislocation density of AlGaN grown by this method is significantly reduced by more than 30 times compared to the AlN template. No threading dislocation segments were observed in the 4 µm TEM field of view. Moreover, based on the hierarchical structure, we also fabricated an AlGaN ultraviolet avalanche photodiode (APD). The APD exhibits superior performance, achieving a maximum gain of 1.3 × 105 and high responsivity of 1.46 A/W at 324 nm. The reliability of the nanopillar/superlattice AlGaN epitaxial procedure is anticipated to shed new light on the nitride semiconductor material, further bringing a breakthrough to wide-band gap electronic devices.

Tunable tunneling magnetoresistance in in-plane double barrier magnetic tunnel junctions based on B vacancy h-NB nanoribbons.

Magnetic tunnel junctions (MTJs) have attained new opportunities due to the emergence of two-dimensional (2D) magnetic materials after they were proposed more than forty years ago. Here, an in-plane double barrier magnetic tunnel junction (IDB-MTJ) based on B vacancy h-NB nanoribbons has been proposed firstly, and the transport properties have been studied using density functional theory combined with the nonequilibrium Green's function method. Due to its unique structural characteristics, the tunneling magnetoresistance (TMR) ratio can be tuned and the maximum TMR can reach 1.86 × 105. The potential applications of the IDB-MTJ in magnetic random-access memories and logical computation have also been discussed. We find that the IDB-MTJs have great potential in magnetic random-access memories and logical computation applications.

Progress on AlGaN-based solar-blind ultraviolet photodetectors and focal plane arrays.

Solar-blind ultraviolet (UV) photodetectors (PDs) have attracted tremendous attention in the environmental, industrial, military, and biological fields. As a representative III-nitride material, AlGaN alloys have broad development prospects in the field of solar-blind detection due to their superior properties, such as tunable wide bandgaps for intrinsic UV detection. In recent decades, a variety of AlGaN-based PDs have been developed to achieve high-precision solar-blind UV detection. As integrated optoelectronic technology advances, AlGaN-based focal plane arrays (FPAs) are manufactured and exhibit outstanding solar-blind imaging capability. Considering the rapid development of AlGaN detection techniques, this paper comprehensively reviews the progress on AlGaN-based solar-blind UV PDs and FPAs. First, the basic physical properties of AlGaN are presented. The epitaxy and p-type doping problems of AlGaN alloys are then discussed. Diverse PDs, including photoconductors and Schottky, metal-semiconductor-metal (MSM), p-i-n, and avalanche photodiodes (APDs), are demonstrated, and the physical mechanisms are analyzed to improve device performance. Additionally, this paper summarizes imaging technologies used with AlGaN FPAs in recent years. Benefiting from the development of AlGaN materials and optoelectronic devices, solar-blind UV detection technology is greeted with significant revolutions. Summarizing recent advances in the processing and properties of AlGaN-based solar-blind UV PDs and FPAs as well as AlGaN growth and doping techniques.

Study on the self-absorption of InGaN quantum wells at high photon density.

In this paper, the self-absorption of InGaN quantum wells at high photon density is studied based on a rectangular ridge structure. The ridge structure was fabricated based on a standard GaN-based blue LED wafer grown on (0001) patterned sapphire substrate. The high-density photons were obtained by a high-power femtosecond laser with high excitation of 42kW/cm2 at room temperature. Based on the analysis of the photoluminescence intensities of the InGaN quantum wells, we found that the absorption coefficient of the InGaN quantum wells varies with the background photon density. The results revealed that the final absorption coefficient of the InGaN quantum well decreases with the increase of photon density, which can be 48.7% lower than its normal value under our experimental conditions.

Back-illuminated AlGaN heterostructure solar-blind avalanche photodiodes with one-dimensional photonic crystal filter.

AlGaN heterostructure solar-blind avalanche photodiodes (APDs) were fabricated on a double-polished AlN/sapphire template based on a separate absorption and multiplication (SAM) back-illuminated configuration. By employing AlGaN heterostructures with different Al compositions across the entire device, the SAM APD achieved an avalanche gain of over 1×105 at an operated reverse bias of 92 V and a low dark current of 0.5 nA at the onset point of breakdown. These excellent performances were attributed to the acceleration of holes by the polarization electric field with the same direction as the reverse bias and higher impact ionization coefficient of the low-Al-content Al0.2Ga0.8N in the multiplication region. However, the Al0.2Ga0.8N layer produced a photocurrent response in the out of the solar-blind band. To retain the solar-blind detecting characteristic, a periodic Si3N4/SiO2 photonic crystal was deposited on the back of the AlN/sapphire template as an optical filter. This significantly improved the solar-blind characteristic of the device.

The influence of an AlN seeding layer on nucleation of self-assembled GaN nanowires on silicon substrates.

Gallium nitride (GaN)-based nanowires (NWs) have attracted much attention for the fabrication of novel nanostructured devices. In this paper, the influence of an AlN seeding layer on the nucleation of self-assembled GaN NWs grown by plasma-assisted molecular beam epitaxy (MBE) on Si (111) substrates has been investigated. Not only is the formation of a two-dimensional compact GaN layer at the bottom of the NWs suppressed, but also a high density of vertically aligned well-separated GaN NWs originating from GaN islands are successfully obtained after introducing annealing and nitridation processes. Scanning electronic microscope and transmission electron microscope measurements show that the NWs have a high crystalline wurtzite structure nearly free of dislocations and stacking faults and the NW diameter remains constant over almost the entire length. Due to the temperature-dependent diffusion length of Ga adatoms during the nucleation process, the formation of well-separated NWs relies on the distribution and morphology of the underlying AlN seeding layer. Moreover, the SiNx layer served as mask to inhibit coalescence at the nucleation sites. The developed growth processes and the obtained results provide a viable path facilitating the use of MBE growth techniques to fabricate III-nitride NW-based materials and related devices on Si substrates.

Electron-Beam-Driven III-Nitride Plasmonic Nanolasers in the Deep-UV and Visible Region.

Plasmonic nanolasers based on wide bandgap semiconductors are presently attracting immense research interests due to the breaking in light diffraction limit and subwavelength mode operation with fast dynamics. However, these plasmonic nanolasers have so far been mostly realized in the visible light ranges, or most are still under optical excitation pumping. In this work, III-nitride-based plasmonic nanolasers emitting from the green to the deep-ultraviolet (UV) region by energetic electron beam injection are reported, and a threshold as low as 8 kW cm-2 is achieved. A fast decay time as short as 123 ps is collected, indicating a strong coupling between excitons and surface plasmon. Both the spatial and temporal coherences are observed, which provide a solid evidence for exciton-plasmon coupled polariton lasing. Consequently, the achievements in III-nitride-based plasmonic nanolaser devices represent a significant step toward practical applications for biological technology, computing systems, and on-chip optical communication.