Gallium Nitride Substrates Research Articles

Ammonothermal GaN substrates: Growth accomplishments and applications

AbstractWe are presenting some physical and chemical basis of ammonothermal method of bulk gallium nitride (GaN) synthesis in ammonobasic route. The substrates of polar, non‐polar, and semi‐polar orientation can be obtained by this method. Excellent structural and wide spectrum of electrical parameters of truly bulk GaN crystals obtained this way are revealed. Large progress in manufacturing of large size (1‐in.) non‐polar ammonothermal crystals is reported. A preliminary results on performance of the devices grown on ammonothermal GaN substrates will be shown.

Dissociative Chemisorption of Trimethylgallium, Trimethylindium, and Ammonia on Gallium and Indium Nitride Substrates. A Computational Study

The methyl or hydrogen dissociations of trimethylgallium [Ga(CH3)3], trimethylindium [In(CH3)3], and ammonia (NH3), adsorbed onto model substrates for gallium nitride (GaN) and indium nitride (InN), were studied using the hybrid program ONIOM. The model substrates consisted of 44 GaN units and 35 InN units, with no capping hydrogen atoms, and were simulated using a quantum mechanical semiempirical approach (PM6). The adsorbates were represented using density functional theory (DFT) and basis sets containing polarization functions [B3LYP/6-311G(d,p)/3-21G(d,p)]. Adsorption rate constants, as well as methyl and hydrogen unimolecular homolytic dissociation rate constants, were computed using a semiclassical procedure, for pressures between 1 and 100 atm and for temperatures ranging from 300 to 1400 K. Time-dependent dissociative chemisorption processes of Ga(CH3)3, In(CH3)3, and NH3 adsorbing onto GaN and InN were modeled by solving sets of coupled differential equations using stiff methods.

ZINC OXIDE NANOPARTICLES FOR ULTRAVIOLET PHOTODETECTION

Zinc Oxide ( ZnO ) nanoparticles were created by a top-down wet-chemistry synthesis process ( ZnO - A ) and then coated with polyvinyl-alcohol (PVA) ( ZnO - U ). In ZnO - U , strong UV emission was apparent while the parasitic green emission, which normally appears in ZnO suspensions, was suppressed. A standard lift-off process via e-beam lithography was used to fabricate a detector by evaporating Aluminum ( Al ) as ohmic electrodes on the ZnO nanoparticle film. Photoconductivity experiments showed that linear current-voltage response were achieved and the ZnO - U nanoparticles based detector had a ratio of UV photo-generated current more than 5 times better than that of the ZnO - A based detector. In addition, non-linear current-voltage responses were observed when interdigitated finger Gold ( Au ) contacts were deposited on ZnO - U . The UV generated current to dark current ratios were between 4 and 7 orders of magnitude, showing better performance than the photodetector with Al contacts. ZnO - U were also deposited on Gallium Nitride ( GaN ) and Aluminum Gallium Nitride ( AlGaN ) substrates to create spectrally selective photodetectors. The responsivity of detector based on AlGaN is twice that of commercial UV enhanced Silicon photodiodes. These results confirmed that ZnO nanoparticles coating with PVA is a good material for small-signal, visible blind, and wavelength selective UV detection.

Chemical Mechanical Polishing of Gallium Nitride with Colloidal Silica

In this report, chemical mechanical polishing (CMP) of gallium nitride (GaN) with colloidal silica was studied. It was confirmed that colloidal silica based slurry could be used for gallium face of GaN. Removal rate of GaN was 17 nm/h under typical polishing conditions. An atomically flat surface with Ra = 0.1 nm was achieved after CMP. Detailed observation of the scratch density was done during the CMP process. Cathode luminescence (CL) imaging was used to understand the sub-surface damage induced by mechanical polishing process and its removal by CMP with colloidal silica based slurry. Combining the optical microscope, atomic force microscope (AFM), CL imaging, and reflection high energy electron diffraction (RHEED) observations before, after and at intermediate stages of the CMP process, the schematic model of the removal of scratch and damage layer from Ga-faced GaN substrate by CMP process was proposed.

Strain Dependence of Formation Mechanism of Growth Layer in Molecular Beam Epitaxy of Gallium Nitride

In this study, the epitaxial growth of gallium nitride (GaN) on a GaN substrate is investigated by a molecular dynamics (MD) method. Furthermore, the difference between the surface diffusion of atoms of a strained substrate and an unstrained substrate is examined. From the results of this examination, it is found that the diffusion characteristic in the unstrained case is higher than that in the strained case. Therefore, in the unstrained case, GaN grows layer-by-layer. On the other hand, in the strained case, multiple layers of GaN grow simultaneously. Furthermore, it is also found that the wurtzite structure of GaN differs between the strained case and the unstrained case.

Stress and Defect Distribution of Thick GaN Film Homoepitaxially Regrown on Free-Standing GaN by Hydride Vapor Phase Epitaxy

A 220-µm-thick Gallium nitride (GaN) layer was homoepitaxially regrown on the Ga-polar face of a 200-µm-thick free-standing c-plane GaN by hydride vapor-phase epitaxy (HVPE). The boundary of the biaxial stress distribution in the GaN substrate after regrowth was clearly distinguished. One half part, the regrown GaN, was found to be more compressive than the other half part, the free-standing GaN. Additionally, the densities of the screw and mixed dislocations reduced from 2.4 ×107 to 6 ×106 cm-2 after regrowth. Furthermore, the yellow band emission almost disappeared, accompanied by a peak emission at approximately 380 nm related to the edge dislocation was under slightly improved in regrown GaN. We conclude that the reduction of the dislocation defects and Ga vacancies and/or O impurities are the two main reasons for the higher compressive stress in the regrown GaN than in the free-standing GaN, causing the curvature of the GaN substrate to be twice concave after regrowth.

Heteroepitaxial Patterned Growth of Vertically Aligned and Periodically Distributed ZnO Nanowires on GaN Using Laser Interference Ablation

AbstractA simple two‐step method of fabricating vertically aligned and periodically distributed ZnO nanowires on gallium nitride (GaN) substrates is described. The method combines laser interference ablation (LIA) and low temperature hydrothermal decomposition. The ZnO nanowires grow heteroepitaxially on unablated regions of GaN over areas spanning 1 cm2, with a high degree of control over size, orientation, uniformity, and periodicity. High resolution transmission electron microscopy and scanning electron microscopy are utilized to study the structural characteristics of the LIA‐patterned GaN substrate in detail. These studies reveal the possible mechanism for the preferential, site‐selective growth of the ZnO nanowires. The method demonstrates high application potential for wafer‐scale integration into sensor arrays, piezoelectric devices, and optoelectronic devices.

Screw Dislocation-Driven Epitaxial Solution Growth of ZnO Nanowires Seeded by Dislocations in GaN Substrates

In the current examples of dislocation-driven nanowire growth, the screw dislocations that propagate one-dimensional growth originate from spontaneously formed highly defective "seed" crystals. Here we intentionally utilize screw dislocations from defect-rich gallium nitride (GaN) thin films to propagate dislocation-driven growth, demonstrating epitaxial growth of zinc oxide (ZnO) nanowires directly from aqueous solution. Atomic force microscopy confirms screw dislocations are present on the native GaN surface and ZnO nanowires grow directly from dislocation etch pits of heavily etched GaN surfaces. Furthermore, transmission electron microscopy confirms the existence of axial dislocations. Eshelby twist in the resulting ZnO nanowires was confirmed using bright-/dark-field imaging and twist contour analysis. These results further confirm the connection between dislocation source and nanowire growth. This may eventually lead to defect engineering strategies for rationally designed catalyst-free dislocation-driven nanowire growth for specific applications.

Enhancement of resolution and quality of nano-hole structure on GaN substrates using the second-harmonic beam of near-infrared femtosecond laser

By using a second harmonic of near infrared femtosecond (fs) laser (λ=387 nm, 150 fs) with high NA objective lens, fabrication resolution has been greatly improved in nano-fabrication of wide band-gap semiconductor gallium nitride (GaN). We have carried out a wet-chemical-assisted fs laser ablation method, in which the laser beam is focused onto a single-crystal GaN substrate immersed in a concentrated hydrochloric (HCl) acid solution. A two-step processing involving irradiation with a fs laser beam in air followed by wet chemical treatment is also performed for comparison. In the wet-chemical-assisted ablation, theoretical diameters of ablation craters are calculated as a function of pulse energy by assuming that the reaction is based on two-photon absorption. In lower energy, the calculated curve is close to the experimental value, while the actual measured diameters in the region of higher energy are larger than calculated values. In the condition of the highest fabrication resolution, we obtained ablation craters smaller than 200 nm at full width at half maximum. We have also demonstrated the fabrication of two-dimensional (2D) periodic nanostructures on surface of a GaN substrate using the second harmonic single fs-laser pulse. Uniform ablation craters with the size as small as 410 nm in diameter are arranged with a periodicity of 1 μm. Such structures are applicable to 2D photonic crystals which improve the light extraction efficiency for blue LEDs in the near future.

The Ammonothermal Crystal Growth of Gallium Nitride—A Technique on the Up Rise

Gallium nitride (GaN) is one of the most important wide band gap semiconductor materials in modern technology with even higher expectations for future applications it is ought to play a crucial role. Among this, the growth of lattice and thermally matched GaN substrates for the GaN device technology takes an essential piece. This paper is reporting on the achievements in the ammonothermal growth technique of GaN bulk crystals. Important features specific to the ammonothermal technique are focused on. Although only a few groups (currently <; 10 worldwide) are directly involved in the development of the ammonothermal bulk crystal growth technology, partly due to the extreme technological challenges, tremendous progress over the last decade has recently resulted in the fabrication of 2 inch large, free-standing, single crystalline GaN with excellent structural perfection.

An improved passive inductor microwave performance on gallium nitride substrates using ion implantation technology

Recently, gallium nitride (GaN) substrates have been widely used in high-power monolithic microwave integrated circuit applications, which work because of the high breakdown voltage associated with the wider energy band-gap. The objective of this article is to investigate the spiral inductors that have been used to implant various ions on GaN substrates, demonstrated separately by N+, Mg+ and O+. The measured substrate resistance (R sub) results were 85 Ω and 360 Ω for undoped GaN substrates and N+ doped substrates, respectively. This ion implantation technology could substantially improve the substrate resistance and provide a higher quality factor value (Q-value) for spiral inductors. The low parasitic effect extracted by the scattering parameter (S-parameter) for various substrates indicated that the spiral inductor value can be applied stably at high frequency. Consequently, the high GaN substrate quality adopting high-isolation ion implantation technology, demonstrates its huge potential for high-power and high-efficiency amplifier applications.

Comparison of dimethylzinc and diethylzinc as precursors for monocrystalline zinc oxide grown by atomic layer deposition method

The new trend in semiconductor technology is search of new materials, which could replace silicon in specialized applications. One of them is zinc oxide (ZnO), a II–VI semiconductor. In this work we show how to obtain at relatively low temperature monocrystalline layers of ZnO using atomic layer deposition (ALD) method with reactive dimethylzinc (DMZn) and diethylzinc (DEZn) precursors. ZnO films were deposited on gallium nitride substrates at 300 °C. We compare the results obtained for these two organic zinc precursors. High resolution X-ray diffraction analysis proves that our ZnO layers are monocrystalline with FWHM of (00.2) peak 250 arcsec for DEZn and 335 arcsec for DMZn precursors.

Influence of the UV Light Intensity on the Photoelectrochemical Planarization Technique for Gallium Nitride

We have developed a novel planarization technique for gallium nitride (GaN) substrates using a photo-electro chemical process and solid acid catalyst. In this method, a GaN surface is oxidized by ultraviolet (UV) light irradiation, and the oxide layer is chemically removed by a solid acid catalyst. In the current work, the dependence of the removal rate on the UV light intensity was investigated.

Gain analysis of blue nitride-based lasers by small signal modulation

With a small signal frequency-modulation of the driving current, the resonance frequency and the damping factor of the optical output power response of blue nitride-based ridge lasers grown on [0001]-plane gallium-nitride substrates were investigated with a network analyzer setup. From the linear dependence of the squared resonance frequency on the driving current, the gain coefficients of the logarithmic gain model could be extracted being 7680 cm−1 for blue nitride-based lasers. For this purpose, additional parameters such as the carrier density and the confinement factor were assigned by carrier lifetime and quantum efficiency measurements and one dimensional transfer matrix simulations, respectively.

Study of Temperature Dependence of Island Formation and Structure of Gallium Nitride in MBE Growth

Homoepitaxial growth on gallium nitride substrate is simulated by molecular dynamics (MD) method. Crystal growth configuration is evaluated qualitatively according to the atomic configuration and the radial distribution function. Moreover, crystal structure is evaluated quantitatively by considering variance of local atomic density as a monitoring index. As a result, we found that the growth layer is formed two-dimensionally and like film formation when the substrate temperature is high. It is found that the dynamics of crystal growth can be understood from not only variance value but also gradient of temporal evolution of variance.

Initial growth of AllnGaN on Polar Gallium Nitride Substrates under Biaxial Strain: First-principle Simulations

Initial growth of AllnGaN on Polar Gallium Nitride Substrates under Biaxial Strain: First-principle Simulations

Nitrogen-polar gallium nitride substrates as solid-state pH-selective potentiometric sensors

This paper reports pH-selective potentiometric responses of epitaxial nitrogen-polar (0001) GaN films on sapphire substrates. The potential of a nitrogen-polar GaN substrate increased with decreasing solution pH from 10 to 2 in a Nernstian manner, whereas it did not significantly change upon increasing the concentrations of the cationic and anionic interfering species. In particular, the negligible responses to anions marked a sharp contrast with previously reported results on gallium-polar GaN that exhibited Nernstian responses to anions. The potentiometric response to pH probably originates from the adsorption of H+ onto oxide-coated nitrogen-polar GaN substrates having polarization-induced negative surface charge.

Fabrication of microchannels in single-crystal GaN by wet-chemical-assisted femtosecond-laser ablation

We investigated micro- and nano-fabrication of wide band-gap semiconductor gallium nitride (GaN) using a femtosecond (fs) laser. Nanoscale craters were successfully formed by wet-chemical-assisted fs-laser ablation, in which the laser beam is focused onto a single-crystal GaN substrate in a hydrochloric acid (HCl) solution. This allows efficient removal of ablation debris produced by chemical reactions during ablation, resulting in high-quality ablation. However, a two-step processing method involving irradiation by a fs-laser beam in air followed by wet etching, distorts the shape of the crater because of residual debris. The threshold fluence for wet-chemical-assisted fs-laser ablation is lower than that for fs-laser ablation in air, which is advantageous for improving fabrication resolution since it reduces thermal effects. We have fabricated craters as small as 510 nm by using a high numerical aperture (NA) objective lens with an NA of 0.73. Furthermore, we have formed three-dimensional hollow microchannels in GaN by fs-laser direct-writing in HCl solution.

Development of Bulk GaN Crystals and Nonpolar/Semipolar Substrates by HVPE

AbstractThe remarkable progress in nonpolar and semipolar devices based on gallium nitride (GaN) in recent years has been driven by not only advancements in the epitaxial growth technique but also improvements in the quality of bulk nonpolar and semipolar GaN substrates. At present, high-quality nonpolar/semipolar substrates are only made by slicing thick bulk GaN crystals grown by hydride vapor-phase epitaxy (HVPE). Although HVPE is currently the most successful method for obtaining high-quality bulk GaN crystals, it is still difficult to obtain uniform crystals with large diameters and thicknesses. The size of the nonpolar/semipolar substrates has been limited by the growth thickness along the c-axis of bulk GaN crystals. Here we review the growth of bulk GaN crystals by HVPE to achieve high-quality and large-sized nonpolar and semipolar substrates.

Electrical Characterization of High-k Anodic Aluminum Oxide Gate Dielectrics on Gallium Nitride Substrate MOS Capacitors

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