GaN Structures Research Articles

Semi-polar GaN(11-22) on nano-structured Si(113): a structure for reducing thermal stresses

The article reports the growth of semi-polar GaN(11-22) layers using epitaxy from metal organic compounds on a nanostructured NP-Si(113) substrate. It was shown that upon the emergence of an island layer, elastic deformed structures of GaN(11-22)/NP-Si(113) form a nano-meter compliant silicon layer on a substrate while elastic stresses conditioned by the difference of temperature coefficients of GaN and Si in such a structure decrease

Cross-band spectrum prediction algorithm based on data conversion using generative adversarial networks

Spectrum prediction is one of the new techniques in cognitive radio that predicts changes in the spectrum state and plays a crucial role in improving spectrum sensing performance. Prediction models previously trained in the source band tend to perform poorly in the new target band because of changes in the channel. In addition, cognitive radio devices require dynamic spectrum access, which means that the time to retrain the model in the new band is minimal. To increase the amount of data in the target band, we use the GAN to convert the data of source band into target band. First, we analyze the data differences between bands and calculate FID scores to identify the available bands with the slightest difference from the target predicted band. The original GAN structure is unsuitable for converting spectrum data, and we propose the spectrum data conversion GAN (SDC-GAN). The generator module consists of a convolutional network and an LSTM module that can integrate multiple features of the data and can convert data from the source band to the target band. Finally, we use the generated target band data to train the prediction model. The experimental results validate the effectiveness of the proposed algorithm.

A Physics-Informed Generative Car-Following Model for Connected Autonomous Vehicles.

This paper proposes a novel hybrid car-following model: the physics-informed conditional generative adversarial network (PICGAN), designed to enhance multi-step car-following modeling in mixed traffic flow scenarios. This hybrid model leverages the strengths of both physics-based and deep-learning-based models. By taking advantage of the inherent structure of GAN, the PICGAN eliminates the need for an explicit weighting parameter typically used in the combination of traditional physics-based and data-driven models. The effectiveness of the proposed model is substantiated through case studies using the NGSIM I-80 dataset. These studies demonstrate the model's superior trajectory reproduction, suggesting its potential as a strong contender to replace conventional models in trajectory prediction tasks. Furthermore, the deployment of PICGAN significantly enhances the stability and efficiency in mixed traffic flow environments. Given its reliable and stable results, the PICGAN framework contributes substantially to the development of efficient longitudinal control strategies for connected autonomous vehicles (CAVs) in real-world mixed traffic conditions.

Dispersive response of polar optical phonon modes on the dielectric and size effects in a nitride nanostructure

Within the framework of the macroscopic dielectric continuum model, the dispersive spectra of polar optical phonon modes of III-nitride based nanostructures with different inner and outer dielectric media are investigated in this paper. The dielectric and quantum size effects on the dispersive frequencies of phonon modes are focused and emphasized. In the case of GaN quantum rings (QRs), we found that the dielectric constant of the inner/outer medium could significantly influence the dispersive spectra of the surface optical (SO) phonon mode as the high/low-frequency SO phonon modes are mainly localized at the inner/outer surface of QRs, respectively. Moreover, such effects associated with different crystallographic structures of GaN, i.e., the wurtzite (WZ) and zinc blende (ZB) crystal structures, are also studied in detail, showing the different degenerating behavior and electron–phonon coupling strength. The results obtained in this study provide a beneficial opportunity to independently adjust dispersive spectra of the two branches of SO modes in GaN nanostructures; thus, it could be used to design and develop phonon-based detectors and sensors.

Effect of GaN-Based Distributed Bragg Reflector on Optical Properties of CH3NH3PbBr3 Crystals

As a photoelectric material, the luminescent efficiency improvement of organic–inorganic perovskite material is a hot topic. This work fabricated a nanoporous distributed Bragg reflector based on GaN with a periodic structure using electrochemical etching methods. Considering the fact that hybrid perovskite materials are difficult to prepare on an inorganic GaN-based substrate, ultraviolet ozone treatment was implemented to enhance the surface activity of the prepared distributed Bragg reflector substrate. Cubic CH3NH3PbBr3 crystals with smooth surfaces and precise edges were successfully prepared on the ozone-treated distributed Bragg reflector substrate by a two-step immersion method in the air environment. The structural property of the prepared CH3NH3PbBr3 crystals was investigated using X-ray diffraction, scanning electron microscopy and Fourier-transform infrared spectroscopy. The structural analysis results showed that CH3NH3PbBr3 crystals grown on the prepared distributed Bragg reflector and reference substrates have the same structure, with a good crystal quality. The photoluminescence intensity of CH3NH3PbBr3 crystals grown on the distributed Bragg reflector was significantly enhanced. The enhancement is approximately 3.11-fold compared with CH3NH3PbBr3 crystals grown on the unetched reference GaN substrate. Moreover, there is a 7.2 nm spectral blue shift. The enhancement of the photoluminescence intensity could be attributed to the out-coupling of emission light in the prepared crystals, and the blue shift could be attributed to the stress relaxation caused by the nanoporous GaN structure of the distributed Bragg reflector substrate.

High carrier mobilities and broad absorption spectra in twisted and non-twisted bilayer GaN, BN, and ZnO structures

Stacking is effective in modulating the properties of two-dimensional (2D) materials, and more interesting properties exist in twisted stacking systems. The electronic and optical properties of twisted-13.17°, twisted-21.78°, and non-twisted-AA’ structures of GaN, BN, and ZnO have been studied by using first-principles. The calculated results indicate that all structures are stable, and the band gaps of the three structures are modulated effectively. Compared with the monolayer, the twisted structures of GaN, BN, and ZnO can enhance absorption peaks and broaden the absorption spectra. The carrier mobilities of the twisted and non-twisted structures have been investigated by using DP theory. The twisted-13.17° and twisted-21.78° structures of the BN exhibit much high hole mobilities along the y direction of 55.92 and 44.30 @103 cm2 V−1 s−1 respectively. The twisted-13.17° structure of ZnO exhibits much high electron mobilities of 56.81 (x direction) and 16.57 (y direction) @103 cm2 V−1 s−1. The band gap, carrier mobility, and absorption spectrum of the twisted and non-twisted GaN, BN, and ZnO stacking structures have been modulated effectively, and these stacking structures will promise important application prospects in the field of ultraviolet broadband electronic devices and photoelectric devices.

Magnetic properties of nickel electrodeposited on porous GaN substrates with infiltrated and laminated connectivity

We studied the magnetic properties of ferromagnetic-semiconductor composites based on nickel and porous-GaN, motivated by the effort to couple magnetic and semiconductor functionality. Nickel-infiltrated and nickel-coated (laminated thin-film) porous GaN structures were fabricated by electrodeposition, and their magnetic properties were subsequently examined collectively, by vibrating sample magnetometry and on the nanoscale, by magnetic force microscopy. We successfully demonstrated the ability to realize nickel infiltrated porous GaN, where the magnetic properties were dominated by the infiltrated material without a measurable surface contribution. We found that the structure and magnetization of electrodeposited porous-GaN/Ni composites depended on GaN degree of porosity and the amount of deposited nickel. The magnetization evolves from a nearly isotropic response in the infiltrated structures, to a shape-anisotropy controlled magnetic thin-film behavior. Furthermore, both infiltrated and thin-film nickel electrodeposited on porous GaN were found to have low (< 0.1%) strain and corresponding low coercivity: < 6.4 and < 2.4 kA/m for infiltrated and thin-film, correspondingly. An unconventional trend of increased remanence at room temperature, compared to cryogenic temperature, was observed in the thin film. The most likely cause for these findings is increased compliance of the porous GaN compared to bulk, allowing strain relaxation at room temperature. These results encourage deeper investigation of magnetic nanostructure property tuning and of magnetic property coupling to GaN and similar materials.

Theoretical study on Cs-activation mechanism of two-dimensional GaN photocathode

Based on the first principles, this paper studied the structure and photoelectric properties of two-dimensional GaN adsorbed by Cs atoms with different concentrations. The results show that the most stable adsorption position is located above Ga atom. The adsorption concentration increases and the system become unstable. With the increase of Cs adsorption atoms, the energy band moves towards showing stronger metal properties. The work function of 2D GaN is reduced after adsorption of Cs atom, which is beneficial to photoelectron escape and exhibits better photoelectric efficiency. The optical properties reveal that peak value of adsorption coefficient, reflectivity and loss function of 2Cs-adsorbed 2D GaN system is lower than that of 1Cs-adsorbed system, which means that 2Cs-adsorbed 2D GaN have a better optical performance.

Epitaxial growth of AlGaN/GaN HEMTs on patterned Si substrate for high voltage power switching applications

In this work, a series of AlGaN/GaN high electron mobility transistors (HEMTs) have been grown on patterned (Al)GaN-on-Si seed layers by metalorganic chemical vapor deposition (MOCVD) to study the effect of the (Al)GaN buffer thickness and its aluminum content on the vertical breakdown voltage. The atomic force microscopy (AFM) analysis performed on the GaN seed layers shows a perfectly smooth, MOCVD-like morphology with a threading dislocation density (TDD) around 2 × 109 cm−2. We succeeded to regrow up to 10 μm and 5 μm crack-free GaN + HEMT and Al0.30G0.70N + HEMT structures, respectively and scanning electron microscopy (SEM) revealed limited lateral growth on mesas with few microscopic defects while AFM show the same surface morphology of HEMT structures as ones grown on planar substrates. NiAu Schottky contacts were deposited on the regrown (Al)GaN HEMTs as well as on (Al)GaN seed layer structures. No breakdown was noticed up to 200 V for the regrown Al0.30Ga0.70N (3 μm) HEMT and the GaN seed layer structures and the leakage current remained in the range of 1 nA. However, the leakage current in the regrown GaN (5 μm) structure reached rapidly 1 mA. Such a high leakage was not noticed for non-patterned devices. Thus, it is possibly attributed to silicon contamination in the regrown GaN layer. On the other hand, the absence of noticeable leakage in the regrown 3 μm thick Al-rich AlGaN buffer HEMT opens the way to a new competitive solution for high power electronics applications on silicon substrate.

Luminescence Characteristics of the MOCVD GaN Structures with Chemically Etched Surfaces.

Gallium nitride is a wide-direct-bandgap semiconductor suitable for the creation of modern optoelectronic devices and radiation tolerant detectors. However, formation of dislocations is inevitable in MOCVD GaN materials. Dislocations serve as accumulators of point defects within space charge regions covering cores of dislocations. Space charge regions also may act as local volumes of enhanced non-radiative recombination, deteriorating the photoluminescence efficiency. Surface etching has appeared to be an efficient means to increase the photoluminescence yield from MOCVD GaN materials. This work aimed to improve the scintillation characteristics of MOCVD GaN by a wet etching method. An additional blue photo-luminescence (B-PL) band peaking at 2.7-2.9 eV and related to dislocations was discovered. This B-PL band intensity appeared to be dependent on wet etching exposure. The intensity of the B-PL was considerably enhanced when recorded at rather low temperatures. This finding resembles PL thermal quenching of B-PL centers. The mechanisms of scintillation intensity and spectrum variations were examined by coordinating the complementary photo-ionization and PL spectroscopy techniques. Analysis of dislocation etch pits was additionally performed by scanning techniques, such as confocal and atomic force microscopy. It was proved that this blue luminescence band, which peaked at 2.7-2.9 eV, is related to point defects those decorate dislocation cores. It was shown that the intensity of this blue PL band was increased due to enhancement of light extraction efficiency, dependent on the surface area of either single etch-pit or total etched crystal surface.

Obtaining of Mg-Zn Co-Doped GaN Powders via Nitridation of the Ga-Mg-Zn Metallic Solution and Their Structural and Optical Properties.

Mg-Zn co-dopedGaN powders via the nitridation of a Ga-Mg-Zn metallic solution at 1000 °C for 2 h in ammonia flow were obtained. XRD patterns for the Mg-Zn co-dopedGaN powders showed a crystal size average of 46.88 nm. Scanning electron microscopy micrographs had an irregular shape, with a ribbon-like structure and a length of 8.63 µm. Energy-dispersive spectroscopy showed the incorporation of Zn (Lα 1.012 eV) and Mg (Kα 1.253 eV), while XPS measurements showed the elemental contributions of magnesium and zinc as co-dopant elements quantified in 49.31 eV and 1019.49 eV, respectively. The photoluminescence spectrum showed a fundamental emission located at 3.40 eV(364.70 nm), which was related to band-to-band transition, besides a second emission found in a range from 2.80 eV to 2.90 eV (442.85-427.58 nm), which was related to a characteristic of Mg-doped GaN and Zn-doped GaN powders. Furthermore, Raman scattering demonstrated a shoulder at 648.05 cm-1, which could indicate the incorporation of the Mg and Zn co-dopants atoms into the GaN structure. It is expected that one of the main applications of Mg-Zn co-doped GaN powders is in obtaining thin films for SARS-CoV-2 biosensors.

Characterization of GaN structures for power electronics by secondary ion mass spectrometry and atomic force microscope approach

This scientific paper describes the chemical study of different Al2O3/GaN interfaces found in the new generation of high electron mobility transistors developed for power electronics applications. The stake for these interfaces is the limitation, ideally the absence of an oxidized gallium layer hampering the good electrical behavior of the semiconductor. These structures have been studied through time-of-flight secondary ion mass spectrometry (ToF-SIMS), magnetic SIMS (M-SIMS), and atomic force microscopy (AFM) analyses. Two structures were considered: a bilayer Al2O3 10 nm–GaOx 2 nm on GaN and a Al2O3 10 nm single layer deposited on preliminary etched GaN. The first sample was used as a dedicated reference sample where an actual gallium oxide layer was grown, whereas the second one was a technologically relevant structure. Several experimental conditions were compared for the ToF- and M-SIMS analyses; in particular, three angles of incidence for the primary Cs+ sputter beam (65°, 61°, 45°) were used, leading to diverse depth resolutions and roughnesses, as revealed by surface topography analysis provided by AFM. Among the different incidence angles, it was found that the best experimental conditions were those obtained by ToF-SIMS analysis with an incidence angle of 45°, generating the least roughness.

Artificial Intelligence-Based Digital Media Design Effect Enhancement Mechanism

The advancement of AI technology has promoted the development speed of digital multimedia and brought a new experience to the digital media experience effect. In this paper, we aim to using artificial intelligence methods to enhance the digital media design experience. Specifically, we propose a method for low-light image enhancement using generative adversarial networks as a model framework. To better solve the problem, we design the following strategies in our proposed method. First, we preprocess the images into patches with a proper size. Second, we introduce the overall network structure of GAN. Third, we designed a multifeature extraction module with different sizes of convolution kernels to enhance the model’s ability for extracting features. Fourth, we propose a loss that combines the mean square error distance function with the adversarial error function to enable the model to learn the distributional features of the image. Finally, we verify the effectiveness of the proposed method on two datasets, namely, PASCAL VOC and MIT-Adobe FiveK. The results show that our proposed method performs well in the process of evaluation.

InGaN/GaN QWs on tetrahedral structures grown on graphene/SiC

In recent years, flexible nitride LEDs have been developed for applications in fields such as lighting, displays but also medicine and biology. Said nanowire-based flexible LEDs are made of InGaN/GaN core-shell nanowires encapsulated in a polymer matrix, which is peeled off and constitutes the final flexible LED structure. However, the current peel off process is complex and hardly compatible with industrial applications. One solution to this issue could be the Van Der Waals epitaxy of the LED structures on graphene, as this approach is known to ease the separation from the original substrate. However, to realize Van der Waals epitaxy and prevent nucleation of GaN on defects located within the graphene layer, the quality of the graphene needs to be optimized. In this work we employ graphene grown by Chemical Vapor Deposition on SiC, known to be notably less defective than graphene grown on metals or sapphire. In this work, tetrahedral structures of GaN have been grown on monolayer and multilayer graphene on SiC. X-Ray Diffraction measurements confirmed that the GaN followed the crystallographic directions of SiC, implying growth through remote-epitaxy mechanism thanks to the presence of graphene in both cases. Last, InGaN/GaN structures were realized by growth of a 5 period of InGaN/GaN quantum wells on these GaN tetrahedra. Cathodoluminescence as well as photoluminescence measurements revealed the presence of stacking faults and Zinc Blende inclusion, as well as a broad visible emission near 480 nm of quantum wells.

Tight-binding analysis of the effect of strain on the band structure of GaN

The effects of strain on the band structure of GaN are investigated by using an empirical tight-binding method. The impacts on its bandgap, carrier effective mass, and group velocity are discussed. By analyzing the orbital components at the top of the valence band, the cause of the variation of the band structure including effective-mass exchange is discussed. Analysis of the average group velocity indicates that tensile uniaxial or compressive biaxial strain may be beneficial for achieving higher breakdown voltage in vertical GaN devices due to the smaller group velocity of the valence band. For the same reason, we also predict higher breakdown voltages due to tensile biaxial strain for horizontal devices.

Electronic and optical properties of GaN/MoSe2 and its vacancy heterojunctions studied by first-principles

Heterojunction GaN/MoSe2 has recently piqued the interest of researchers due to its exceptional electronic and optical properties. Despite this, the higher rate of photogenerated carrier recombination limits their technical application. Implementing a promising approach to the formation of the 2D heterostructure with vacancies may improve photocatalytic activity. By first-principles, the stability, electronic structure, and optical properties of monolayer GaN and MoSe2, GaN/MoSe2 (GN/MS), GaN/MoSe2 with a Ga vacancy (VGa-GN/MS), GaN/MoSe2 with an N vacancy (VN-GN/MS), and GaN/MoSe2 with vacancies of Ga and N (VGa@N-GN/MS) are systematically computed. Compared with monolayer GaN, MoSe2, and GaN/MoSe2, the results show that vacancies do not destroy the stability of heterojunctions and cause a decrease in their bandgaps and a redshift of the absorption spectra. Type-II band alignment is observed through Perdew–Burke–Ernzerhof calculations in all heterostructures. VGa-GN/MS and VGa@N-GN/MS are p-type semiconductors, while VN-GN/MS is an n-type semiconductor. Based on the analysis of Mulliken occupancy, work function, and optical properties, we speculate that vacancies of Ga and N cause GN/MS to be energetically favorable for water splitting.

InkGAN: Generative Adversarial Networks for Ink-And-Wash Style Transfer of Photographs

In this work, we present a novel approach for Chinese Ink-and-Wash style transfer using a GAN structure. The proposed method incorporates a specially designed smooth loss tailored for this style transfer task, and an end-to-end framework that seamlessly integrates various components for efficient and effective image style transferring. To demonstrate the superiority of our approach, comparative results against other popular style transfer methods such as CycleGAN is presented. The experimentation showcased the notable improvements achieved with our proposed method in terms of preserving the intricate details and capturing the essence of the Chinese Ink-and-Wash style. Furthermore, an ablation study is conducted to evaluate the effectiveness of each loss component in our framework. We conclude in the end and anticipate that our findings will inspire further advancements in this domain and foster new avenues for artistic expression in the digital realm.

Towards the quantification of the chemical mechanism of light-driven water splitting on GaN photoelectrodes.

We present results from a study addressing the unbiased water-splitting process and its side reactions on GaN-based photoelectrodes decorated with NiOx, FeOx, and CoOx nanoparticles. Observations involving physicochemical analyses of liquid and vapour phases after the experiments were performed in 1 M NaOH under ambient conditions. A water-splitting process with GaN-based photoelectrodes results in the generation of hydrogen gas and hydrogen peroxide. Quantification of the water-splitting chemical mechanism gave numerical values indicating an increase in the device performance and restriction of the GaN electrocorrosion with surface modifications of GaN structures. The hydrogen generation efficiencies are ηH2(bare GaN) = 1.23%, ηH2(NiOx/GaN) = 4.31%, ηH2(FeOx/GaN) = 2.69%, and ηH2(CoOx/GaN) = 2.31%. The photoelectrode etching reaction moieties Qetch/Q are 11.5%. 0.21%, 0.26% and 0.20% for bare GaN, NiOx/GaN, FeOx/GaN, and CoOx/GaN, respectively.

Point Cloud Adversarial Perturbation Generation for Adversarial Attacks

In recent years, 3D model analysis has made a revolutionary development. Point cloud contains rich 3D object geometry information, which is an important 3D object data format widely used in many applications. However, the irregularity and disorder of the point cloud also cause its vulnerability to environmental impact, which may bring security risks to safety-critical 3D applications such as self-driving tasks. Recently, there are only a few methods engaged to attack the point cloud models to improve the robustness of point cloud analysis models. Most of them only focus on the attack by adjusting the points but ignore learning the perturbation’s distribution characteristics. In this work, we propose a novel framework to attack point cloud models. By introducing the GAN structure, we train a generator to produce slight point-to-point perturbations according to the sample’s raw classification, which can effectively boost the attack performance. Meanwhile, we propose an outlier removal module to constrain the magnitude of the generated perturbation. The goal is to guarantee the visual quality of generation samples to improve the difficulty of training and further improve the robustness of 3D analysis models. Finally, we carry out extensive attack experiments, and the related results demonstrate the effectiveness of our proposed method.

Fabrication of GaN-air channels for embedded photonic structures

Refractive index engineering is a key element in the design of optoelectronic device structures. In this work we present a method for the fabrication of buried hollow channels, periodically arranged inside GaN structures. The method is based on selective area doping and electrochemical etching. Channels width is defined by the selective area ion implantation mask geometry while the channel vertical dimension depends on the implantation energy and annealing conditions. In this work we used implantation with Si 100 keV and dose 2.6·1015 cm−2 into GaN. Next, a 100 nm GaN layer was grown by plasma-assisted molecular beam epitaxy (PAMBE). After annealing at 1070 °C for 5 min, selectively doped regions were removed in the process of electrochemical etching (ECE), forming periodically arranged empty volumes. Annealing resulted in partial strain removal as observed by X-Ray diffraction. Surface roughness was evaluated at every stage of the process: after implantation, after regrowth and after annealing, and it is promising for the integration of the proposed technology with device fabrication and processing. The flexibility in the mask design for selective ion implantation and a high refractive index contrast between GaN and air, make the proposed technology attractive for the fabrication of buried photonic structures such as diffraction gratings.