Gallium Nitride Research Articles

Optoelectronic synapses with chemical-electric behaviors in gallium nitride semiconductors for biorealistic neuromorphic functionality

Optoelectronic synapses, leveraging the integration of classic photo-electric effect with synaptic plasticity, are emerging as building blocks for artificial vision and photonic neuromorphic computing. However, the fundamental working principles of most optoelectronic synapses mainly rely on physical behaviors while missing chemical-electric synaptic processes critical for mimicking biorealistic neuromorphic functionality. Herein, we report a photoelectrochemical synaptic device based on p-AlGaN/n-GaN semiconductor nanowires to incorporate chemical-electric synaptic behaviors into optoelectronic synapses, demonstrating unparalleled dual-modal plasticity and chemically-regulated neuromorphic functions through the interplay of internal photo-electric and external electrolyte-mediated chemical-electric processes. Electrical modulation by implementing closed or open-circuit enables switching of optoelectronic synaptic operation between short-term and long-term plasticity. Furthermore, inspired by transmembrane receptors that connect extracellular and intracellular events, synaptic responses can also be effectively amplified by applying chemical modifications to nanowire surfaces, which tune external and internal charge behaviors. Notably, under varied external electrolyte environments (ion/molecule species and concentrations), our device successfully mimics chemically-regulated synaptic activities and emulates intricate oxidative stress-induced biological phenomena. Essentially, we demonstrate that through the nanowire photoelectrochemical synapse configuration, optoelectronic synapses can be incorporated with chemical-electric behaviors to bridge the gap between classic optoelectronic synapses and biological synapses, providing a promising platform for multifunctional neuromorphic applications.

Direct Growth of Wafer-Scale Self-Separated GaN on Reusable 2D Material Substrates.

Free-standing gallium nitride has been prepared using various methods; however, the removal of the original substrate is still challenging with low success rates. In this work, 2-inch free-standing GaN films are obtained by direct growth on a fluoro phlogopite mica by hydride vapor-phase epitaxy. Depending on the van der Waals (vdW) interaction between GaN and mica, the effect of the significant lattice mismatch is effectively reduced; thus, enabling the production of a high-quality wafer-scale GaN film on mica. The vdW-induced cracks at GaN-mica interface are found to be initiated near the interface so that GaN can easily separate from mica during rapid cooling. Owing to the hydrophilic nature of mica, the residual GaN on the mica can be lifted off by following deionized water treatment, and the mica substrate can be repeatedly used to grow free-standing GaN films. The self-separated GaN films grown on both pristine and used mica substrates are single crystallinity and strain-free. Additionally, a fully functional ultraviolet light-emitting diode is demonstrated to show that the self-separated GaN films are of device quality. The proposed approach achieves epitaxial growth of wafer-scale single-crystalline GaN on 2D materials and provides a new substrate option in the technology of III-V materials.

Understanding the machining mechanism in ultrasonic vibration-assisted nanogrinding of GaN

As a typical difficult-to-machine material, gallium nitride (GaN) crystals have the merits of excellent chemical stability and large bandwidth, which can be used in fields of space optical components, photoelectric devices, and electronic communication. The smooth surface of GaN is the basis of these applications. Grinding of GaN has thus drawn increasing attention. Furthermore, vibration-assisted nanogrinding has the potential to reduce grinding forces compared with conventional nanogrinding. In this study, ultrasonic-vibration-assisted nanogrinding of GaN with a single grit is conducted based on molecular dynamic (MD) simulations. The plastic deformation mechanism of GaN single crystals as well as the influence of grinding parameters, such as vibration period and amplitude, on the grinding outcomes are investigated. The MD simulations reveal that plastic deformation of GaN in the vibration-assisted nanogrinding process is dominated by amorphous transformation, dislocation, stacking faults, lattice distortion, and formation of nanocrystals. Furthermore, amorphous GaN atoms are mainly induced in front of the grit and at either side of the grinded groove. In the grinding process, the periodic fluctuation of grinding force components, which include the normal, tangential, and horizontal forces, are stable. A smaller grinding force is obtained when grinding with a small vibration period. Moreover, a small vibration period and large vibration amplitude lead to improved grinding efficiency, suppressing the grinding force and subsurface damage, and improving the grinding quality. The findings in this study facilitate an understanding of the plastic deformation mechanism of GaN single crystals and also provide guidance for parameter optimization during vibration-assisted nanogrinding.

Facile formation of van der Waals metal contact with III-nitride semiconductors

Metal–semiconductor contacts play a pivotal role in controlling carrier transport in the fabrication of modern electronic devices. The exploration of van der Waals (vdW) metal contacts in semiconductor devices can potentially mitigate Fermi-level pinning at the metal–semiconductor interface, with particular success in two-dimensional layered semiconductors, triggering unprecedented electrical and optical characteristics. In this work, for the first time, we report the direct integration of vdW metal contacts with bulk wide bandgap gallium nitride (GaN) by employing a dry transfer technique. High-angle annular dark-field scanning transmission electron microscopy explicitly illustrates the existence of a vdW gap between the metal electrode and GaN. Strikingly, compared with devices fabricated with electron beam-evaporated metal contacts, the vdW contact device exhibits a responsivity two orders of magnitude higher with a significantly suppressed dark current in the nanoampere range. Furthermore, by leveraging the high responsivity and persistent photoconductivity obtained from vdW contact devices, we demonstrate imaging, wireless optical communication, and neuromorphic computing functionality. The integration of vdW contacts with bulk semiconductors offers a promising architecture to overcome device fabrication challenges, forming nearly ideal metal–semiconductor contacts for future integrated electronics and optoelectronics.

Enhanced breakdown voltage for p-GaN gate AlGaN/GaN HEMT on AlN/Si with triple trenches: A simulation study

This study presents an analysis of an AlGaN/GaN high-electron-mobility transistor (HEMT) with a triple-trench structure (TT-HEMT) for the enhancement of breakdown voltage, which is often limited by the high electric field near the gate edge. Using COMSOL Multiphysics, simulations were run, with trenches made of gallium nitride (GaN) constructed between the GaN and aluminum nitride (AlN) nucleation layers. Each trench was positioned directly beneath the source, gate, and drain contacts. Under the metal gate, a p-type doped GaN cap is added to produce an enhancement mode device, which is more desirable in high-power applications. The results indicate that the effective dispersion of the electric field distribution enhances the breakdown voltage of the device by 70 % in comparison to a conventional HEMT. Additionally, the impact of various design parameters, such as the p-GaN doping concentration and the metal gate work function on the electrical performance, was also investigated. Overall, the proposed device structure exhibits superior electrical properties for high-power applications.

The role of gallium nitride in the evolution of electric vehicles: Energy applications, technology, and challenges

The role of gallium nitride in the evolution of electric vehicles: Energy applications, technology, and challenges

Mechanism of glycine and H2O action in tribochemical mechanical polishing of single-crystal gallium nitride substrate

With the rapid development of the third-generation semiconductors, environmentally friendly tribochemical mechanical polishing (TCMP) is in urgent need and has been a research hot-spot. In this work, the mechanism of glycine (Gly) and H2O action in TCMP of single-crystal gallium nitride (SC-GaN) substrate is systematically studied. The experimental results indicate that the TCMP of SC-GaN substrate with 0.75 wt% Gly exhibits a higher surface quality and a 3-fold improvement in material removal rate than that with H2O. Based on first-principle calculations, the geometric configurations and interfacial interactions of the Gly/SC-GaN and H2O/SC-GaN heterostrutures are further explored. Charge transfer analysis shows that SC-GaN with lattice distortion of 0.4 Å transfers more electrons into Gly (0.202 e) than into H2O (0.128 e), which suggests the reaction between SC-GaN and Gly is more intense under friction by abrasives. The research of differential charge density and geometric properties (bond angles etc.) demonstrates that the –COOH functional group (Gly) has a higher reactivity than the –OH functional group (H2O). Based on the above findings, a microscopic material removal model for the TCMP of SC-GaN substrate with Gly is established. These results have potential applications in the semiconductor and microelectronics industries.

Wafer-scale N-polar GaN heterogeneous structure fabricated by surface active bonding and laser lift-off

N-polar Gallium nitride (GaN) technology is one of the most important technical routes for millimeter-wave devices. This paper introduces a new approach based on reverse epitaxial layer transfer technology to fabricate N-polar GaN materials. By combining low-damage surface-activated bonding and laser lift-off techniques, a wafer-scale N-polar GaN heterogeneous structure with low O impurities, high two-dimensional electron gas density, and high carrier mobility was obtained. The thin (∼2 nm) crystal bonding interlayer, coupled with the low thermal boundary resistance of only 6.8 m2K/G·W, provides evidence for the low interfacial damage caused by this approach. These results demonstrate its superiority as an attractive method for fabricating high-performance N-polar GaN millimeter-wave devices.

Self-bias Mo–Sb–Ga multilayer photodetector encompassing ultra-broad spectral response from UV–C to IR–B

Achieving an ultra-broad spectral response in a self-bias detector is a formidable challenge that persists in optoelectronics that necessitates innovative solutions. We propose a unique ultra-broadband photodetecting device, utilizing a multilayer structure comprising Molybdenum Di-sulfide (MoS2), Antimony Tri-selenide (Sb2Se3), and Gallium Nitride (GaN), which exhibits the unique capability of detecting photons without applied bias. The fabricated device demonstrates exceptional sensitivity to a wide range of illumination wavelengths, spanning from ultraviolet–C (UV–C) to infrared–B (IR–B). The design detector displays the highest photo-responsivity of 665 mAW−1 in photovoltaic mode and 3.89 × 105 mAW−1 in photoconductive mode. The designed detector also exhibits a minimal dark current of 90 nA and an extremely weak signal detection capability of ∼12 femto watt-hertz−1/2 at 6 V bias. Additionally, the thermal stability of the MoS2-Sb2Se3-GaN (Mo-Sb-Ga) multi-layer-based self-bias detector was explored. Under the self-bias conditions, the photodetector exhibits a stable behavior up to 250°C with a peak responsivity of 635 mAW−1. The thermal durability of the self-bias ultra-broadband photodetector indicates excellent potential for developing futuristic optoelectronic devices. Further, the performance of the developed detector was examined using Technology Computer-Aided Design (TCAD) simulations, providing valuable insights into the device behavior and the transport of photo-generated carriers, enhancing our understanding of the device operation and enabling performance optimization for diverse applications.

Temperature- and gate voltage-dependent I–V modeling of GaN HEMTs based on ASM-HEMT

Abstract In this paper, a temperature and gate voltage dependent current-voltage (I-V) model for Gallium nitride (GaN) high electron mobility transistors (HEMTs) devices is studied. In order to help researchers and designers simulate devices in the absence of experimental data and improve parameter extraction efficiency, the temperature and gate voltage dependence of mobility are discussed and incorporated into the proposed model which is revised from the standard advanced SPICE model (ASM) for GaN HEMTs. The mobility variations with gate voltage from -2 V to 6 V and temperature from 10 K to 1000 K are analyzed. The simulation results of our model are compared with the standard ASM-HEMT model, and the output and transfer characteristics of the device from 270 K to 420 K are simulated. Thereby, our model may simulate various applications of GaN HEMTs at different gate voltages and temperatures in the early stages of design and research.

Characterization of Growth Sectors in Gallium Nitride Substrate Wafers

During crystal growth processes, growth sectors are formed due to growth along different crystallographic directions. Although the crystal structure in the different growth sectors is unchanged, strain induced topography contrast is observed by synchrotron X-ray topography. In this study, synchrotron monochromatic beam X-ray topography (SMBXT), synchrotron X-ray plane wave topography (SXPWT), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and secondary ion mass spectrometry (SIMS) are used to characterize growth sectors in gallium nitride (GaN) substrate wafers grown by patterned hydride vapor phase epitaxy (HVPE). The SMBXT images reveal the boundaries of {0001} and {1122} type growth sectors. Strain maps generated from SXPWT shows that the out-of-plane strains in different growth sectors have a difference of the order of 10-5. SEM images from SE2 signal shows no contrast of growth sector boundaries while images from Robinson detector (RBSD) show different growth sectors as different grey scale contrast, indicating a strain effect. SIMS analysis shows that the different oxygen impurity levels in the growth sectors, which is the origin of the strain. A formation mechanism of growth sectors in patterned HVPE grown GaN wafers is proposed.

Utilizing full-spectrum sunlight for ammonia decomposition to hydrogen over GaN nanowires-supported Ru nanoparticles on silicon

Photo-thermal-coupling ammonia decomposition presents a promising strategy for utilizing the full-spectrum to address the H2 storage and transportation issues. Herein, we exhibit a photo-thermal-catalytic architecture by assembling gallium nitride nanowires-supported ruthenium nanoparticles on a silicon for extracting hydrogen from ammonia aqueous solution in a batch reactor with only sunlight input. The photoexcited charge carriers make a predomination contribution on H2 activity with the assistance of the photothermal effect. Upon concentrated light illumination, the architecture significantly reduces the activation energy barrier from 1.08 to 0.22 eV. As a result, a high turnover number of 3,400,750 is reported during 400 h of continuous light illumination, and the H2 activity per hour is nearly 1000 times higher than that under the pure thermo-catalytic conditions. The reaction mechanism is extensively studied by coordinating experiments, spectroscopic characterizations, and density functional theory calculation. Outdoor tests validate the viability of such a multifunctional architecture for ammonia decomposition toward H2 under natural sunlight.

Study of GaN solubility in ammonothermal alkaline solution

The solubility of gallium nitride in supercritical ammonia containing an alkaline mineralizer, sodium amide, was investigated. Twenty dissolution processes were carried out at constant ammonia pressure and constant mineralizer concentration. In each process, the solubility of four ammonothermal GaN crystals was analyzed at four different temperatures and five different times. By analyzing the total mass loss of all crystals in a given process, the time interval at which the solubility of GaN stopped changing was determined, indicating the saturated state of the solution. Based on these data, the solubility curve was determined. Retrograde solubility was observed. The impact of surface preparation of GaN samples and its correlation with the quantity of dissolved material was investigated. It was shown that as-grown and optically smooth surfaces dissolved slower than mechanically treated (000-1) GaN surfaces. These are important data for ammonothermal crystallization processes using native GaN seeds with differently prepared (000-1) surfaces.

Soft‐switching boundary and piecewise control of LLC converter with hybrid modulation

AbstractThe voltage gain range of LLC converter can be effectively broadened by using the hybrid modulation of phase‐shift modulation (PSM) and pulse frequency modulation (PFM). However, the harmonic of resonant tank increases dramatically in PSM mode, and the first harmonic approximation is no longer suitable for solving the voltage gain. Moreover, the conditions for soft switching in PSM mode are not clear. Meanwhile, the small‐signal models of the converter in different modulation modes are quite different, and it is difficult for a fixed controller to ensure a good dynamic performance of the converter in both modulation modes. To address above issues, a voltage gain modeling method of PSM mode LLC converter is proposed. By establishing the function between input current and resonant current, the voltage gain model of the converter is obtained according to the power conservation. Moreover, considering the parasitic capacitance charge and resonant current commutation time, the condition for soft switching is derived. Furthermore, a control strategy of LLC converter in hybrid modulation is proposed. The piecewise proportional integral (PI) controller is adopted, and the parameters of the controller are dynamically adjusted with the modulation mode, so that the converter has excellent dynamic performance in both modes. And the hysteresis control is used to ensure the stability of mode switching. An experimental prototype with 200–340 V input and 28 V/1 kW output was built based on gallium nitride (GaN) devices. The peak efficiency was 97.82%, and the power density was 65.76 w/in3.

Unveiling 5-fluorouracil anti-cancer drug encapsulation within pristine and silver-doped boron, aluminum, and gallium nitride nanotubes: A DFT investigation for advancing nanomedicine

Nanomaterials represent a promising frontier in drug delivery systems for combating cancer, offering precise targeting of tumour cells. This study delves into the encapsulation capabilities of 5-fluorouracil (5-FU) on pristine armchair (6,6) single-wall boron-, aluminium-, and gallium-nitride nanotubes (BNNT, AlNNT, GaNNT), both in their natural state and doped with silver (Ag) atoms under gas phase and water solution conditions. Employing density functional theory calculations at the M06-2X level, we scrutinise the geometric stabilities, encapsulation efficiencies, and electronic interactions between 5-FU molecules and NNTs. Our results reveal that a shorter distance between the drug and nanotube cavity enhances encapsulation ability and solubility while facilitating larger charge transfers, especially within Ag-doped NNTs. The Ag doping significantly augments the reactivity of NNTs towards 5-FU molecules, with thermodynamic analyses indicating exothermic and spontaneous interactions with Ag-doped NNTs. The decapsulation of 5-FU from Ag-BNNT, Ag-AlNNT, and Ag-GaNNT necessitates recovery time 6.42E+00, 3.21E+05, and 2.19E−04 s, with corresponding adsorption energies of −38.70, −49.52, and −28.42 kcal/mol, respectively. Electronic analyses demonstrate notable changes in states’ energy gap and density post-5-FU encapsulation in Ag-doped NNTs, suggesting enhanced sensitivity. Moreover, our investigation delves into topological features, molecular stability, bond strength, and intermolecular interactions using various analytical techniques, including MEPs, ELF maps, NBO, QTAIM, Hirshfeld surface, and RDG analyses. These comprehensive findings underscore the potential of Ag-doped NNTs as promising components for drug delivery systems and biosensors in nanomedicine applications, particularly in sensing and catalytic functions for 5-FU molecule encapsulation and detection.

Near zero-field magnetoresistance and defects in gallium nitride pn junctions

As gallium nitride (GaN) grows in importance, an atomic scale understanding of device performance is of significant relevance. In this paper, we present the first observations of near zero-field magnetoresistance (NZFMR) detecting electrically active defects in GaN-based devices. Our observations involve recombination current in GaN pn junction diodes. We attribute the NZFMR response to gallium vacancies.

Supercontinua from integrated gallium nitride waveguides

Supercontinua are broadband spectra that are essential to optical spectroscopy, sensing, imaging, and metrology. They are generated from ultrashort laser pulses through nonlinear frequency conversion in fibers, bulk media, and chip-integrated waveguides. For any generating platform, balancing the competing criteria of strong nonlinearity, transparency, and absence of multiphoton absorption is a key challenge. Here, we explore supercontinuum generation in integrated gallium nitride (GaN) waveguides, which combine a high Kerr nonlinearity, mid-infrared transparency, and a large bandgap that prevents two- and three-photon absorption in the technologically important telecom C-band, where compact erbium-based pump lasers exist. Using this type of laser, we demonstrate tunable dispersive waves and gap-free spectra extending to almost 4 µm in wavelength, which is relevant to functional group chemical sensing. Additionally, leveraging the material’s second-order nonlinearity, we implement on-chip f-to-2f interferometry to detect the pump laser’s carrier-envelope offset frequency, which enables precision metrology. These results demonstrate the versatility of GaN-on-sapphire as a platform for broadband nonlinear photonics.

Sapphire-Based Optrode for Low Noise Neural Recording and Optogenetic Manipulation.

Electrophysiological recordings of neurons in deep brain regions using optogenetic stimulation are essential to understanding and regulating the role of complex neural activity in biological behavior and cognitive function. Optogenetic techniques have significantly advanced neuroscience research by enabling the optical manipulation of neural activities. Because of the significance of the technique, constant advancements in implantable optrodes that integrate optical stimulation with low-noise, large-scale electrophysiological recording are in demand to improve the spatiotemporal resolution for various experimental designs and future clinical applications. However, robust and easy-to-use neural optrodes that integrate neural recording arrays with high-intensity light emitting diodes (LEDs) are still lacking. Here, we propose a neural optrode based on Gallium Nitride (GaN) on sapphire technology, which integrates a high-intensity blue LED with a 5x2 recording array monolithically for simultaneous neural recording and optogenetic manipulation. To reduce the noise interference between the recording electrodes and the LED, which is in close physical proximity, three metal grounding interlayers were incorporated within the optrode, and their ability to reduce LED-induced artifacts during neural recording was confirmed through both electromagnetic simulations and experimental demonstrations. The capability of the sapphire optrode to record action potentials has been demonstrated by recording the firing of mitral/tuft cells in the olfactory bulbs of mice in vivo. Additionally, the elevation of action potential firing due to optogenetic stimulation observed using the sapphire probe in medial superior olive (MSO) neurons of the gerbil auditory brainstem confirms the capability of this sapphire optrode to precisely access neural activities in deep brain regions under complex experimental designs.

Rolling mechanism profundities on material removal mechanism of surface-textured GaN using Molecular dynamics simulation

Molecular dynamics simulation examines how the polishing tool's rotating velocity and axes affect surface nanotribological properties and material removal mechanism of patterned gallium nitride (GaN) substrates. Frictional coefficient and average contact area affect material removal rate (MRR) variance. Rotating speed increases the frictional coefficient and contact area, elevating MRR. Anticlockwise abrasives have substantially higher root-mean-square roughness (RMS) than clockwise ones. After polishing, increasing the rotating angle increases the frictional coefficient, average contact area, MRR, and RMS. MRR enhancement is maximum at −15 rad/ns, the only spinning velocity that improves MRR. RMS improvement ratio is highest when the polishing tool spins clockwise or the rotational axis orientation is lowered. Particularly, the MRR and RMS improvements after the polishing process can reach 103.8 % and 223.5 %, respectively. These findings help explain atomic-scale GaN-based material removal and deformation with frictional resistance and erosion by polishing.

First-principles study of recombination-enhanced migration of an interstitial magnesium in gallium nitride

First-principles study of recombination-enhanced migration of an interstitial magnesium in gallium nitride