Microstructure Of GaN Research Articles

Influence of oxygen impurities in generating ferromagnetism in GaN doped with Mn, Fe, and Cr

We report the influence of oxygen impurities in generating ferromagnetism in GaN doped with Mn, Fe, and Cr through superexchange involving Mn–O–Mn, Fe–O–Fe, and Cr–N–Cr atomic configurations. Density Functional Theory (DFT) calculations demonstrated that these configurations originate within VGa-ON\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${V}_{\ ext{Ga}}-{O}_{\ ext{N}}$$\\end{document} complex defects by incorporating Mn2+, Fe2+, and Cr3+ ions into gallium vacancies (VGa\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${V}_{\ ext{Ga}}$$\\end{document}) sites promoted by the presence of oxygen as a substitutional impurity (ON\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${O}_{\ ext{N}}$$\\end{document}). These co-doped GaN microstructures were synthesized using the thermal evaporation method. Cathodoluminescence (CL) and photoluminescence (PL) measurements confirmed the presence of VGa-ON\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${V}_{\ ext{Ga}}-{O}_{\ ext{N}}$$\\end{document} complex defects in all GaN-grown samples. X-ray photoelectron spectroscopy (XPS) measurements confirmed the successful incorporation of Mn2+, Fe2+, and Cr3+ ions in GaN samples. SQUID measurements demonstrated room-temperature ferromagnetism with respective saturation magnetization (Ms) and coercive field (Hc) values of ± 1.5 × 10−5 emu and 4 mT for GaN:O,Mn, ± 4.4 × 10−5 emu and 2.6 mT for GaN:O,Fe, and ± 1.7 × 10−5 emu and 2.6 mT for GaN:O,Cr.

Microstructure and reflectance of porous GaN distributed Bragg reflectors on silicon substrates

Distributed Bragg reflectors (DBRs) based on alternating layers of porous and non-porous GaN have previously been fabricated at the wafer-scale in heteroepitaxial GaN layers grown on sapphire substrates. Porosification is achieved via the electrochemical etching of highly Si-doped layers, and the etchant accesses the n+-GaN layers through nanoscale channels arising at threading dislocations that are ubiquitous in the heteroepitaxial growth process. Here, we show that the same process applies to GaN multilayer structures grown on silicon substrates. The reflectance of the resulting DBRs depends on the voltage at which the porosification process is carried out. Etching at higher voltages yields higher porosities. However, while an increase in porosity is theoretically expected to lead to peak reflectance, in practice, the highest reflectance is achieved at a moderate etching voltage because etching at higher voltages leads to pore formation in the nominally non-porous layers, pore coarsening in the porous layers, and in the worst cases layer collapse. We also find that at the high threading dislocation densities present in these samples, not all dislocations participate in the etching process at low and moderate etching voltages. However, the number of dislocations involved in the process increases with etching voltage.

An Approach Toward Broader Emission Bands in Semipolar InGaN Quantum Wells on Convex Lens‐Shaped GaN Microstructures via Lower‐Temperature Growth

InGaN quantum wells (QWs) are grown at different temperatures on convex lens‐shaped GaN microstructures formed on the () plane. Microlens QWs grown at different temperatures have centrosymmetric convex lens shapes, and the wavelength spatial distributions within the structures exhibit similar tendencies. However, lowering the growth temperature broadens the wavelength range of room‐temperature cathodoluminescence spectra from the microlens QWs as ≈70 nm for ≈700 °C, ≈100 nm for ≈660 °C, and ≈150 nm for ≈650 °C. Peak wavelength profiles of the two orthogonal lines along [] and [] indicate that the broader emission bands are mainly due to the significant spatial distribution of the emission wavelength along the [] direction. Because the QW thickness variations are nearly the same along [] and [], the observed difference in the emission wavelength distributions along those directions is attributed to the difference in In incorporation, which is determined by the competition among the growth rate, crystal orientation, and growth temperature.

A combined AIMD and DFT study of the low-energy radiation responses of GaN.

Although GaN is a promising candidate for semiconductor devices, degradation of GaN-based device performance may occur when the device is bombarded by high-energy charged particles during its application in aerospace, astronomy, and nuclear-related areas. It is thus of great significance to explore the influence of irradiation on the microstructure and electronic properties of GaN and to reveal the internal relationship between the damage mechanisms and physical characteristics. Using a combined density functional theory (DFT) and ab initio molecular dynamics (AIMD) study, we explored the low-energy recoil events in GaN and the effects of point defects on GaN. The threshold displacement energies (Eds) significantly depend on the recoil directions and the primary knock-on atoms. Moreover, the Ed values for nitrogen atoms are smaller than those for gallium atoms, indicating that the displacement of nitrogen dominates under electron irradiation and the created defects are mainly nitrogen vacancies and interstitials. The formation energy of nitrogen vacancies and interstitials is smaller than that for gallium vacancies and interstitials, which is consistent with the AIMD results. Although the created defects improve the elastic compliance of GaN, these radiation damage states deteriorate its ability to resist external compression. Meanwhile, these point defects lead the Debye temperature to decrease and thus increase the thermal expansion coefficients of GaN. As for the electronic properties of defective GaN, the point defects have various effects, i.e., VN (N vacancy), Gaint (Ga interstitial), Nint (N interstitial), and GaN (Ga occupying the N lattice site) defects induce the metallicity, and NGa (N occupying the Ga lattice site) defects decrease the band gap. The presented results provide underlying mechanisms for defect generation in GaN, and advance the fundamental understanding of the radiation resistances of semiconductor materials.

Anisotropic emission wavelength distribution of semipolar InGaN quantum wells on symmetric convex lens-shaped GaN microstructures

We fabricated InGaN quantum wells (QWs) on convex lens-shaped GaN microstructures formed on semipolar (1¯1¯22¯) GaN substrates. Despite the centrosymmetric lens-like shapes, the overgrown InGaN QWs exhibit an anisotropic emission wavelength distribution. This behavior is distinct from the concentric wavelength distribution found in microlens QWs on the polar (0001) plane. The anisotropic distribution is mainly attributed to the In composition variations, which are influenced by the asymmetry of the (1¯1¯22¯) crystallographic plane. The obtained results demonstrate that the (1¯1¯22¯) microlens QWs serve as multi-wavelength light emitters and allow us to investigate unstable planes around the (1¯1¯22¯) plane, which have remained unexplored.

Highly sensitive and low detection limit NO2 gas sensor based on In2O3 nanoparticles modified peach kernel-like GaN composites

The effective detection of ppb-level NO2 is of great significance to the natural environment and human health. In this work, a series of In2O3 nanoparticles modified peach kernel-like GaN for NO2 detection was synthesized by a concise two-step method. The gas-sensing results showed that 10 mol% In2O3-decorated GaN (In: Ga) exhibited a response as high as 1070.9–1 ppm NO2 at the working temperature of 100 ℃, as well as a low limit of detection (1 ppb). Furthermore, the sensor also possessed excellent selectivity, repeatability and fast response-recovery speed. The outstanding sensing performance was contributed to the rough and porous microstructure of GaN, abundant negative oxygen ion species originating from In2O3, and the heterojunction effect between GaN and In2O3. The sensor proposed herein provide a useful reference for reliable detection of ppb-level NO2 gas.

InGaN-based LEDs on convex lens-shaped GaN arrays toward multiwavelength light emitters

InGaN-based LEDs on convex lens-shaped GaN microstructures are fabricated. The gently sloping microstructures enable us to employ simple device processes similar to those for conventional planar LEDs. The fabricated LEDs show a clear rectifying property and broadband electroluminescence spectra ranging from ∼380 to ∼500 nm due to the multiwavelength emission properties of the InGaN quantum wells. Furthermore, the peak wavelength continuously changes from ∼470 to ∼410 nm within a particular current range.

Multiwavelength-emitting InGaN quantum wells on convex-lens-shaped GaN microstructures

We fabricated InGaN quantum wells on GaN microlens structures by employing a thermal reflow method. The peak emission wavelengths shift from ∼490 nm on the top of the microstructure to ∼400 nm on the bottom. This variation is attributed to the In composition distribution caused by the continuously changing off-angles from the (0001) plane. The obtained results suggest that not only stable planes exposed in microstructures by selective area growth technique but also unstable planes can be useful for InGaN-based microstructures with multiwavelength emission properties.

ICP etching of GaN microstructures in a Cl2–Ar plasma with subnanometer-scale sidewall surface roughness

Substrate temperature, RF power, and ICP power were investigated for their effects on GaN micropillar sidewall roughness and etch characteristics. Elevated substrate temperature was shown to improve the sidewall etch morphology at low RF powers (reduced physical bombardment) and low ICP powers (lower plasma densities). Increased lateral etching is observed with both increased ICP power and substrate temperature, which both act to increase the chemical component of the etch. Etch conditions with a high chemical driving force resulted in faceting along the a-plane on the sidewalls. This faceting produced extremely smooth surfaces with root-mean-square roughness (Rq) as low as 0.20 nm which is comparable to typical epitaxy-ready surfaces and smaller than the a-plane lattice spacing of 0.3186 nm. The smooth surfaces produced in this study enable possibilities for laser facets or for new device structures that require high quality surfaces for GaN regrowth.

Activated hybrid g-C3N4/porous GaN heterojunction for tunable self-powered and broadband photodetection

Top-down contamination-free uniform etching techniques for preparing porous micro-structure of GaN to enhance its optical absorption has long been explored to arrive at a convenient and efficient strategy. Besides, appropriate heterointerfacing to enhance the photoexcited charge transport with minimal recombination and quenching for consistent and efficient photodetection has been a challenging issue. Herein, we demonstrate one-step solvothermal etching for preparing macroporous GaN (m-GaN), followed by its activation with 2D g-C3N4 to fabricate type II heterojunction for broadband and self-powered (360–635 nm) photodetector (PD). Additionally, tunability in photoresponse and self-powered operation is achieved by impregnating g-C3N4 in thermally reduced graphene oxide (rGO) and chemically modified graphene oxide (CMGO) in separate experiments. With the optimized thickness and composition of CMGO:g-C3N4 on m-GaN, the developed PD displays remarkable photoresponse with broadened photosensitivity and estimated responsivity, detectivity, and external quantum efficiency of 3 A/W, ~1014 Jones, and 103%, respectively. CMGO treated devices exhibit better self-powered characteristics compared to rGO due to improved carrier transport from g-C3N4 and higher built-in potential developed with m-GaN. Under laboratory testing environment, the devices' stability has been recorded for 40 days without noticing any decay in dark current and photoresponse. This work suggests the application of hybrid g-C3N4/m-GaN heterojunction for stable, high-performance, and self-powered photodetection. Further, the modification in band-structure and optical properties of g-C3N4 with graphene oxides of different reduction levels can be leveraged in functional optoelectronics.

Flexible and monolithically integrated multicolor light emitting diodes using morphology-controlled GaN microstructures grown on graphene films

We report flexible and monolithically integrated multicolor light-emitting diode (LED) arrays using morphology-controlled growth of GaN microstructures on chemical-vapor-deposited (CVD) graphene films. As the morphology-controlled growth template of GaN microstructures, we used position-controlled ZnO nanostructure arrays with different spacings grown on graphene substrates. In particular, we investigated the effect of the growth parameters, including micropattern spacings and growth time and temperature, on the morphology of the GaN microstructures when they were coated on ZnO nanostructures on graphene substrates. By optimizing the growth parameters, both GaN microrods and micropyramids formed simultaneously on the graphene substrates. Subsequent depositions of InGaN/GaN quantum well and p-GaN layers and n- and p-type metallization yielded monolithic integration of microstructural LED arrays on the same substrate, which enabled multicolor emission depending on the shape of the microstructures. Furthermore, the CVD graphene substrates beneath the microstructure LEDs facilitated transfer of the LED arrays onto any foreign substrate. In this study, Cu foil was used for flexible LEDs. The flexible devices exhibited stable electroluminescence, even under severe bending conditions. Cyclic bending tests demonstrated the excellent mechanical stability and reliability of the devices.

Influence of micro-patterning of the growth template on defect reduction and optical properties of non-polar GaN

We investigate the influence of different types of template micro-patterning on defect reduction and optical properties of non-polar GaN using detailed luminescence studies. Non-polar (or a-plane) GaN exhibits a range of different extended defects compared with its more commonly used c-plane counterpart. In order to reduce the number of defects and investigate their impact on luminescence uniformity, non-polar GaN was overgrown on four different GaN microstructures. The micro-patterned structures consist of a regular microrod array; a microrod array where the −c-side of the microrods has been etched to suppress defect generation; etched periodic stripes and finally a subsequent combination of etched stripes and etched microrods (double overgrowth). Overall the presence of extended defects, namely threading dislocations and stacking faults (SFs) is greatly reduced for the two samples containing stripes compared with the two microrod samples. This is evidenced by more uniform emission and reduction in dark regions of non-radiative recombination in room temperature cathodoluminescence imaging as well as a reduction of the SF emission line in low temperature photoluminescence. The observed energy shifts of the GaN near band edge emission are related to anisotropic strain relaxation occurring during the overgrowth on these microstructures. A combination of stripes and microrods is a promising approach for defect reduction and emission uniformity in non-polar GaN for applications in light-emitting devices as well as power electronics.

Molecular beam epitaxy of GaN on 2H–MoS2

Recent interest in the heterogeneous integration of two-dimensional (2D) materials with three-dimensional (3D) semiconductors has resulted in the implementation of direct heteroepitaxial growth via various epitaxy techniques. In this study, we demonstrated the synthesis of wafer-scale direct growth of GaN on MoS2 using plasma-assisted molecular beam epitaxy. Systematic studies were employed to understand the growth diagram of GaN on large area multi-layer MoS2, and various growth parameters such as substrate temperature and Ga to N flux ratio were investigated to study the degradation mechanism of the underlying MoS2 layer. The growth diagram of GaN on MoS2 was proposed based on the studies. The microstructure of GaN (i.e., N-rich and 550 °C) was evaluated by scanning transmission electron microscopy. Phase segregation was found in the N-rich GaN layer. A two-step growth process involving N-rich condition at a lower temperature and Ga-rich condition at a higher growth temperature was attempted to obtain the smooth and high crystalline quality of GaN. While the two-step growth method led to substantial improvement in the film quality, the removal of MoS2 was observed and the degradation of MoS2 substrate was attributed to Ga diffusion through dislocation lines based on energy-dispersive x-ray spectroscopy mapping. This work helps illuminate critical issues in the epitaxial growth of conventional semiconductors on 2D crystals for various device applications.

Analyzing oxygen and silicon incorporation in GaN microstructures composed of c-planes and angled facets by confocal magneto-photoluminescence microscopy

Unintentionally doped impurities formed in the microstructures of free-standing GaN grown with facets were studied using confocal magneto-photoluminescence (PL) microscopy. Donor-bound exciton related peaks in PL spectra and their magnetic behavior allowed us to distinguish typical donor impurity atoms, such as silicon and oxygen. Combining this technique with confocal microscopy also revealed the spatial distribution of the impurities. The results showed that angled facets tend to incorporate oxygen. Moreover, even facets angled at a few degrees with respect to the (0001) surface cause a noticeable change in oxygen incorporation on the order of 1 × 1016 cm−3.

GaN microstructure light-emitting diodes directly fabricated on tungsten-metal electrodes using a micro-patterned graphene interlayer

We report on the selective-area growth of GaN microstructures on tungsten (W)-metal electrodes using a micro-patterned graphene intermediate layer between GaN and W, and demonstrate their use as light-emitting diodes (LEDs). Prior to the GaN growths, the cm-scale graphene layer was transferred on W and then further patterned into a regular array of few-μm-sized graphene microdots using conventional lithography. The graphene microdots served as a seed layer for selectively growing crack-free GaN microstructures with regular diameter and spacing. Each microstructure displayed a microdisk morphology, exhibiting a single crystalline phase from epitaxial lateral overgrowth (ELOG). We observed ohmic behavior between the as-grown GaN microdisks and underlying W film, facilitating the fabrication of LED microarrays. Using the underlying W layer as an ohmic contact, we fabricated p-n junction GaN microdisk LEDs, consisting of three periods of InxGa1–xN/GaN multiple quantum wells. Uniform electroluminescence was observed across the microdisks. These results open up novel strategies for streamlined fabrication of high-performance and high-resolution LEDs.

Strain of M-plane GaN epitaxial layer grown on β-LiGaO2 (100) by plasma-assisted molecular beam epitaxy

We have grown a high-quality single-phase M-plane GaN on β-LiGaO2 (100) by plasma-assisted molecular beam epitaxy. The Hooke’s law for M-plane GaN was derived by a stress tenor transformation. From the analysis of M-plane GaN microstructure, the lattice strain of M-plane GaN along the [112¯0] and [0001] directions have been estimated. Based on the Hooke’s law, we calculated the ratio of anisotropic stress which was consistent with the ratio of thermal expansion-mismatch between GaN and LiGaO2 (100). We demonstrated that the thermal expansion mismatch was the major factor to degrade the quality of M-plane GaN on β-LiGaO2 (100).

Influence of mask material on the electrical properties of selective area epitaxy GaN microstructures

The authors have investigated the effect of dielectric (SiO2/SiNx) and metal (W) masks on impurity incorporation and electrical properties of selective area epitaxy (SAE) GaN microstructures. It is shown that SAE growths result in highly conductive n-type material. Carrier concentrations greater than metal-nonmetal transition level and low resistivity in the range of 0.18–0.29 mΩ cm were observed from Hall measurements for these structures. Two terminal current-voltage measurements showed a 40× increase in current for SAE GaN microstructure devices compared to that of conventional planar GaN devices. Secondary ion mass spectroscopy (SIMS) measurements of unintentional Si and O dopants in these structures showed dependency on the mask type. Similar dopant and carrier concentrations were obtained from SIMS and Hall data, indicating low compensation from acceptors in the SAE growths.

Micro-structure changes induced by femtosecond laser on the surface of GaN multilayer film grown on Si substrate

The light-emitting-diode (LED) surface of GaN epilayers grown on Si substrate was irradiated by a femtosecond laser (λ = 800 nm). The morphology of LED surface after laser irradiation was measured by optical microscope and scanning electron microscopy. It was found that many radial cracks appeared in the film on account of thermal expansion and then became more numerous as the number of laser pulses increased. In addition, the changes of the micro structure of GaN and Si substrate were studied by means of Micro Raman Spectroscopy. The results showed the presence of polycrystalline silicon and the decomposition of GaN had taken place in the laser spot or the region with the large laser fluence. Comparatively, in the areas with the low laser fluence or the edge of laser spot, the characteristic peaks of GaN films and the AO phonon peak of Si had an obvious blueshift, which most likely was the result of compressive stress between GaN layers and Si substrate. The maximum values of the compressive stress were 0.74 GPa and 1.4 GPa, respectively. Furthermore, with the increase of the laser fluence, the original Si crystalline structure recovered gradually. The above study showed that the stress, the transformation and the deformation play a predominant role in the femtosecond laser etching process of LED.

Ultradeep electron cyclotron resonance plasma etching of GaN

Ultradeep (≥5 μm) electron cyclotron resonance plasma etching of GaN micropillars was investigated. Parametric studies on the influence of the applied radio-frequency power, chlorine content in a Cl2/Ar etch plasma, and operating pressure on the etch depth, GaN-to-SiO2 selectivity, and surface morphology were performed. Etch depths of >10 μm were achieved over a wide range of parameters. Etch rates and sidewall roughness were found to be most sensitive to variations in RF power and % Cl2 in the etch plasma. Selectivities of >20:1 GaN:SiO2 were achieved under several chemically driven etch conditions where a maximum selectivity of ∼39:1 was obtained using a 100% Cl2 plasma. The etch profile and (0001) surface morphology were significantly influenced by operating pressure and the chlorine content in the plasma. Optimized etch conditions yielded >10 μm tall micropillars with nanometer-scale sidewall roughness, high GaN:SiO2 selectivity, and nearly vertical etch profiles. These results provide a promising route for the fabrication of ultradeep GaN microstructures for use in electronic and optoelectronic device applications. In addition, dry etch induced preferential crystallographic etching in GaN microstructures is also demonstrated, which may be of great interest for applications requiring access to non- or semipolar GaN surfaces.

Growth behavior of hexagonal GaN on Si(100) and Si(111) substrates prepared by pulsed laser deposition

In this study, we investigated the microstructure and optical properties of hexagonal GaN (h-GaN) films grown by high-temperature pulsed laser deposition (PLD) on Si(100) and Si(111) substrates. The growth mechanism, crystallization, and surface morphology of h-GaN deposition on both Si(100) and Si(111) substrates were monitored by transmission electron microscopy (TEM) and scanning electron microscopy at various times in the growth process. Our results indicated that the h-GaN grown on Si(111) has better crystalline structure and optical properties than that on Si(100) owing to the smaller mismatch of the orientations of the Si(111) substrate and h-GaN film. On the Si(100) substrate, the growth principles of PLD and N2 plasma nitridation are the main contributions to the conversion of the cubic GaN into h-GaN. Moreover, no significant Ga–Si meltback etching was observed on the GaN/Si surface with the PLD operation temperature of 1000 °C. The TEM images also revealed that an abrupt GaN/Si interface can be obtained because of the suppression of substrate–film interfacial reactions in PLD.