Gallium Nitride Research Articles

Investigation of Persistent Photoconductivity of Gallium Nitride Semiconductor and Differentiation of Primary Neural Stem Cells.

A gallium nitride (GaN) semiconductor is one of the most promising materials integrated into biomedical devices to play the roles of connecting, monitoring, and manipulating the activity of biological components, due to its excellent photoelectric properties, chemical stability, and biocompatibility. In this work, it was found that the photogenerated free charge carriers of the GaN substrate, as an exogenous stimulus, served to promote neural stem cells (NSCs) to differentiate into neurons. This was observed through the systematic investigation of the effect of the persistent photoconductivity (PPC) of GaN on the differentiation of primary NSCs from the embryonic rat cerebral cortex. NSCs were directly cultured on the GaN surface with and without ultraviolet (UV) irradiation, with a control sample consisting of tissue culture polystyrene (TCPS) in the presence of fetal bovine serum (FBS) medium. Through optical microscopy, the morphology showed a greater number of neurons with the branching structures of axons and dendrites on GaN with UV irradiation. The immunocytochemical results demonstrated that GaN with UV irradiation could promote the NSCs to differentiate into neurons. Western blot analysis showed that GaN with UV irradiation significantly upregulated the expression of two neuron-related markers, βIII-tubulin (Tuj-1) and microtubule-associated protein 2 (MAP-2), suggesting that neurite formation and the proliferation of NSCs during differentiation were enhanced by GaN with UV irradiation. Finally, the results of the Kelvin probe force microscope (KPFM) experiments showed that the NSCs cultured on GaN with UV irradiation displayed about 50 mV higher potential than those cultured on GaN without irradiation. The increase in cell membrane potential may have been due to the larger number of photogenerated free charges on the GaN surface with UV irradiation. These results could benefit topical research and the application of GaN as a biomedical material integrated into neural interface systems or other bioelectronic devices.

In-situ investigation into the atomic-scale removal behavior of gallium nitride

The process of elimination at the atomic level stands as the paramount step in crafting ultra-smooth gallium nitride (GaN) substrates. However, this delicate removal mechanism presents a notable challenge. In this study, we employed a scraping technique utilizing atomic force microscope (AFM) probe manipulation to conduct an in-situ examination of atomic-scale removal behaviors. Through in-situ regular imaging, we uncovered the principles underlying atomic-scale removal. We systematically explored how scraping and imaging techniques impacted surface topography and the behavior of atomic step-terraces removal. Our findings indicated that abrasives knocking facilitates the emergence of atomic step-terraces, while sliding removal techniques hinder the structure. During the scraping process, both the upper and lower sections of the atomic step-terraces were simultaneously eliminated, with material removal rate (MRR) being slower at the bottom. To validate our in-situ observations, we conducted polishing experiments. Furthermore, the coefficient of friction (COF) recorded during chemical mechanical polishing offered insights into the frictional disparities of abrasives on the GaN surface, thereby shedding light on the atomic-scale removal mechanism.

Selective incorporation of antimony into gallium nitride

Dilute concentrations of antimony (Sb) incorporation into GaN induce strong bandgap bowing and tunable room-temperature photoluminescence from the UV to the green spectral regions. However, the atomistic details of the incorporation of Sb into the GaN host remain unclear. In this work, we use first-principles calculations to understand the thermodynamics of Sb substitution into GaN and its effect on the optical and Raman spectra. Although it is empirically considered that Sb is preferentially incorporated as an anion (Sb3−) into the N sublattice, we demonstrate that Sb can also be incorporated as a cation (Sb3+, Sb5+) into the metal sublattice. Our thermodynamic analysis demonstrates that SbN0, SbGa2+, and SbGa0 can co-exist under Ga-rich conditions in n-type samples. We further confirm the dual incorporation of Sb by calculating the vibrational frequencies of different anionic and cationic substitutions to explain the origins of experimentally observed additional Raman peaks of Sb-doped GaN. Moreover, the calculated band structures of different Sb substitutions into GaN explain the experimental photoluminescence and optical absorption spectra. Overall, our analysis suggests that the coexistence of Sb3−, Sb3+, and Sb5+ substitutions into GaN explains the totality of experimental measurements. Our results demonstrate that the selective incorporation of Sb into GaN (and potentially other group-V elements such as As, P, or Bi) by tuning the growth conditions can drastically modify the electronic properties, for applications in visible light emitters and photocatalysis.

Counterbalancing effects of bowing in gallium nitride templates by epitaxial growth on pre-strained sapphire substrates

To minimize bowing in a gallium nitride (GaN) template, which is caused by differences in the properties of the GaN film and the sapphire substrate, we studied how to offset bowing by directly growing GaN using the hydride vapor phase epitaxy (HVPE) method on the frontside of a pre-strained sapphire substrate prepared using a backside grinding process. The relationships between the respective thicknesses and stresses of the undamaged sapphire, damaged sapphire, and GaN layers were analyzed by deriving the bow formula, which is a modification of Freund's equation, and the theoretical formula-derived results were compared with experimentally determined values. The bow of the GaN template was measured using a micrometer, and the residual stresses in the GaN and sapphire, carrier mobilities, and GaN crystal qualities were measured using a micro-Raman spectrophotometer. The results confirmed the annealing effect on the damaged sapphire layer, and the introduction of pre-strained GaN templates facilitated easier bow control compared to that afforded by a conventional GaN template. Additionally, we identified trends and optimal conditions for changes in residual stress, carrier mobility, and crystal quality in GaN and the damaged sapphire layer based on layer thickness and stress variations. We explain the mechanisms responsible for changes in bow, stress, carrier mobility, and crystal quality of the GaN template.

Efficient access to non-damaging GaN (0001) by inductively coupled plasma etching and chemical–mechanical polishing

As third-generation semiconductors advance, gallium nitride (GaN)-based devices are gaining significant attention. However, the processing of GaN substrates directly affects device performance. This paper proposes an ICP etching-chemical–mechanical polishing (CMP) strategy to efficiently produce GaN single-crystal substrates with non-destructive, atomic-level surfaces. The process time is significantly reduced from 15 h to 1.5 h. The wet oxidation reaction during CMP eliminates impurities and defects from dry etching. FIB-STEM tests confirm that this method is non-destructive, as the ICP technique enables isotropic etching, and CMP introduces no new damage to the substrate. This approach enhances the productivity of large-size GaN single-crystal substrates, promoting their broader application in laser displays, RF devices, and power electronics.

Quasi-passive modulation for monolithic GaN photoelectronic circuit

Due to the overlap between the electroluminescence spectrum and spectral responsivity curve, gallium nitride (GaN)-based multi-quantum well (MQW) diodes can modulate and detect light emitted by another diode with the same MQW structure. This enables the realization of a monolithic integrated GaN optoelectronic circuit, integrating an MQW-based transmitter, waveguide, modulator, and receiver on a tiny GaN chip. It is well known that the active region of MQW absorbs high-energy photons within the plane, generating electron-hole pairs and forming photogenerated carriers. The change in free carrier concentration causes variations in the refractive index and absorption characteristics of the waveguide, thereby manipulating light propagation within the waveguide. Based on this physical phenomenon, a quasi-passive modulation scheme is proposed for a monolithic GaN photoelectronic circuit. This scheme achieves optical modulation by connecting a variable resistor between the modulator electrodes, avoiding the need for high-precision external electric fields and complex control circuits. The performance of the quasi-passive modulation was tested through on-chip data transmission and real-time audio signal transmission. The results indicate that quasi-passive modulation is highly feasible in optoelectronic systems and shows great promise as a competitive core module for future large-scale photonic integrated circuits.

Influence of the electric field on the electronic structure of flat hexagonal two-dimensional GaN bilayers

Two-dimensional gallium nitride materials have recently garnered significant attention due to their promising optoelectronic properties, chemical stability, and mechanical strength. These attributes make them attractive for various technological applications, particularly optoelectronics, photonics, sensors, and more recently for high-power electronic applications. Our research, using first-principles calculations based on density functional theory (DFT) considering different exchange–correlation functionals, including van der Waals interaction, investigated the electronic properties of a single GaN monolayer and five different stacking configurations of GaN bilayers. The aim is to characterize the electronic properties of 2D-GaN-based materials and explore the impact of external electric fields on the bilayer stacking bandgap. We report the energetically most favorable among the bilayer configurations analyzed. Additionally, we confirmed that it is possible to modulate the energy bandgap both by the type of bilayer stacking and by the effect of the electric field. The ability to tune the energy bandgap (Eg) in 2D-GaN-based materials by adjusting their geometric configuration or applying an external electric field could inspire new applications in various technological fields.

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.