Gallium Nitride Template Research Articles

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.

Effective Chemical Lift-Off for Air-Tunnel GaN on a Trapezoid-Patterned Sapphire Substrate

We fabricated an air-tunnel structure between a gallium nitride (GaN) layer and trapezoid-patterned sapphire substrate (TPSS) through the in situ carbonization of a photoresist layer to enable rapid chemical lift-off (CLO). A trapezoid-shaped PSS was used, which is advantageous for epitaxial growth on the upper c-plane when forming an air tunnel between the substrate and GaN layer. The upper c-plane of the TPSS was exposed during carbonization. This was followed by selective GaN epitaxial lateral overgrowth using a homemade metal organic chemical vapor deposition system. The air tunnel maintained its structure under the GaN layer, whereas the photoresist layer between the GaN layer and TPSS disappeared. The crystalline structures of GaN (0002) and (0004) were investigated using X-ray diffraction. The photoluminescence spectra of the GaN templates with and without the air tunnel showed an intense peak at 364 nm. The Raman spectroscopy results for the GaN templates with and without the air tunnel were redshifted relative to the results for free-standing GaN. The CLO process using potassium hydroxide solution neatly separated the GaN template with the air tunnel from the TPSS.

Fabrication method of GaN template for high-speed chemical lift-off

In this study, a gallium nitride (GaN) template fabrication method for efficient chemical lift-off (CLO) is developed. CLO is slower than other lift-off methods. An air tunnel structure is formed using a photoresist to reduce the process time and improve the etchant penetration rate. Furthermore, an aluminum nitride (AlN) sacrificial layer is mounted on a trapezoid-shaped patterned sapphire substrate. GaN epitaxial growth is observed on the AlN sacrificial layer. The basic physical properties of the grown GaN are investigated using x-ray diffraction, atomic force microscopy, and photoluminescence measurement. The GaN template with the air tunnel structure is depleted by CLO, and the peeling rate is found to be 2.3–6.45 times higher than that reported in the literature.

Polymorphism and Faceting in Ga2O3 Layers Grown by HVPE at Various Gallium‐to‐Oxygen Ratios

Herein, the influence of the oxygen flow on the formation of metastable polymorphs of gallium oxide (Ga2O3) grown by halide vapor phase epitaxy (HVPE) on c‐plane patterned sapphire substrates (PSS), on gallium nitride (GaN) templates, and on m‐plane smooth sapphire substrates is reported. X‐ray diffraction, scanning electron microscopy, and cathodoluminescence are used to identify different polymorphic phases. The samples deposited on bare PSS exhibit faceted growth of the α‐Ga2O3 on the cones of the sapphire substrate and the formation of the κ‐Ga2O3 between the cones. In contrast, growth on GaN templates results in hexagonal columns of κ‐Ga2O3 which produce a continuous smooth layer upon coalescence. The growth of Ga2O3 on m‐plane sapphire substrates results in overgrown pyramids of the α‐phase. For all types of substrates, the variation of the oxygen flow affects only the thickness and coalescence of Ga2O3 layers. Thus, the growth of Ga2O3 metastable polymorphs is mainly influenced by the form, orientation, and symmetry of the substrate and shows a weak dependence on stoichiometry.

HVPE growth of bulk GaN with high conductivity for vertical devices

The electrical properties of gallium nitride (GaN) substrate are crucial to the performance of vertical power devices. Bulk GaN substrates with carrier concentrations in the range from 6.7 × 1017 to 1.7 × 1019 cm−3 are grown by hydride vapor phase epitaxy. All samples show no obvious tensile stress regardless of the carrier concentration. Moreover, the mobility of Si-doped high-quality bulk GaN is superior to the GaN template with higher dislocation density at the same carrier concentration. The influence of carrier concentration on the performance of ohmic contact on N-face of Si-doped GaN is also carefully studied by circular transfer length measurement and rapid thermal annealing methods. The specific contact resistivity decreases monotonically with increase of carrier concentration, while it increases with the annealing temperature. The N-face contact becomes non-ohmic when the annealing temperature exceeds the limit value, which increases with the carrier concentration. The sample with carrier concentration of 1.7 × 1019 cm−3 still showed ohmic behavior after annealing at 450 °C. These results are not only useful to improve the electrical properties of N-type bulk GaN substrate, but also provide a potential solution for improving the efficiency of vertical devices in the future.

Multiple epitaxial lateral overgrowth of GaN thin films using a patterned graphene mask by metal organic chemical vapor deposition

Single-crystal gallium nitride (GaN) thin films were grown using a graphene mask via multiple epitaxial lateral overgrowth (multiple-ELOG). During the growth process, the graphene mask self-decomposed to enable the emergence of a GaN film with a thickness of several hundred nanometres. This is in contrast to selective area growth of GaN using an SiO2 mask leading to the well known hexagonal-pyramid shape under the same growth conditions. The multiple-ELOG GaN had a single-crystalline wurtzite structure corresponding to the crystallinity of the GaN template, which was confirmed with electron backscatter diffraction measurements. An X-ray diffraction rocking curve of the asymmetric 102 reflection showed that the FWHM for the multiple-ELOG GaN decreased to 405 from 540′′ for the underlying GaN template. From these results, the self-decomposition of the graphene mask during ELOG was experimentally proven to be affected by the GaN decomposition rather than the high-temperature/H2 growth conditions.

Selective mask formation and gallium nitride template fabrication on patterned sapphire substrates for light-emitting diodes

Patterned sapphire substrates are composed of multiple planes, leading to the undesirable polycrystalline growth of gallium nitride (GaN) during template deposition. However, patterned sapphire substrates and templates with good crystalline quality would be useful for increasing the light extraction efficiency in light-emitting diodes. In this study, a silicon dioxide (SiO2) mask was selectively introduced onto the lenses of a patterned sapphire substrate to suppress the growth of polycrystalline GaN. The SiO2 mask prevented the negative influence of the patterned sapphire substrate on GaN growth under various conditions. High-resolution x-ray diffraction analysis revealed that the GaN template grew as a single crystal in the presence of the SiO2 mask. Furthermore, the compressive stress generated in the GaN template was relieved due to the suppression of polycrystalline growth.

Influence of Temperature‐Dependent Substrate Decomposition on Graphene for Separable GaN Growth

AbstractGraphene has been adopted in III−V material growth since it can reduce the threading dislocations and the III−V epilayer can easily be separated from the substrate due to the weak chemical bond. However, depending on the substrate supporting the graphene, some substrates decompose in the III−V material growth environment, which results in the problem that no graphene remains. In this study, the influence of temperature‐dependent substrate decomposition on graphene through an annealing process that resembles conventional growth conditions in metal–organic chemical vapor deposition (MOCVD) is investigated. It is also confirmed that trimethylgallium, hydrogen, and ammonia gases do not directly affect the graphene loss through gallium nitride (GaN) growth on a graphene/sapphire. In addition, GaN grown on graphene/sapphire could separate, but GaN grown on a graphene/GaN template could not be separated due to GaN template decomposition and related graphene damage. Through further investigation for graphene/gallium arsenide, it is deduced that the gallium generated by substrate decomposition does not play a major role in damage to the graphene but instead the nitrogen generated by substrate decomposition is closely related to it. These results suggest that it is very important to adopt a decomposition‐free substrate that do not damage graphene during GaN growth in MOCVD.

Metal-covered van der Waals epitaxy of gallium nitride films on graphitic substrates by ECR-MBE

We demonstrated that repetition of gallium deposition and nitrogen-rich supply is a superior approach to fabricate gallium nitride (GaN) thin films on graphene by electron-cyclotron-resonance plasma-excited molecular beam epitaxy. The gallium layer, which covered the surface at the initial growth stage, reduced nitrogen-plasma-induced damage to the graphene. It thereby might suppress the nucleation of misoriented crystals and enabled the growth of c-axis-oriented GaN films. This approach was more effective on graphene transferred onto a GaN template, which suggests the possibility of remote-homoepitaxy reflecting the electric potential of the GaN template unscreened by graphene.

GaN growth via tri-halide vapor phase epitaxy using solid source of GaCl3: investigation of the growth dependence on NH3 and additional Cl2

Gallium nitride (GaN) growth via a tri-halide vapor phase epitaxy method using a solid source of GaCl3 and gaseous NH3 was investigated both on Ga-polar and N-polar GaN templates. The relationship between gallium precursor molecule and growth polarity was clarified; it was found that a small amount of GaCl3 could be reduced by H2 originating from the decomposition of NH3 to produce GaCl, and additional Cl2 could suppress the reduction process. Accordingly, GaCl3 was found to be a proper Ga precursor for N-polar GaN growth, whereas GaCl was the proper precursor for Ga-polar GaN growth. The state of the Ga precursor molecule could be predicted by thermodynamic analysis. Furthermore, the decomposition ratio of NH3 could be determined by a combination of the experimental results and the calculated value of the thermodynamic analysis.

Monolithic Integration of GaN Nanowire Light-Emitting Diode With Field Effect Transistor

Gallium nitride (GaN)-based light-emitting diodes (LEDs) are being investigated for the next generation display technology. The persistent issue, however, has been the lack of ability to integrate transistors with LEDs for control. Here, a novel vertical integration scheme is utilized to fabricate nanowire LEDs with nanowire field effect transistors (FETs) for the first time. This approach utilizes the unintentionally doped GaN template layer which is common to LED growth for the fabrication of nanowire FETs. The demonstrated voltage-controlled light-emitting unit provides area savings, scaling, and easier fabrication due to the vertical integration. For these initial nanowire devices, light modulation is demonstrated with LED turn OFF at −10 V. Due to the nanowire approach, these devices show over two times improvement in the $I_{ \mathrm{\scriptscriptstyle ON}}$ to $I_{ \mathrm{\scriptscriptstyle OFF}}$ ratio compared with the alternative integration schemes.

Surface Analysis of Plasma Pretreated Sapphire Substrate for Aluminum Nitride Buffer Layer

Recently, the use of an aluminum nitride(AlN) buffer layer has been actively studied for fabricating a high quality gallium nitride(GaN) template for high efficiency Light Emitting Diode(LED) production. We confirmed that AlN deposition after N2 plasma treatment of the substrate has a positive influence on GaN epitaxial growth. In this study, N2 plasma treatment was performed on a commercial patterned sapphire substrate by RF magnetron sputtering equipment. GaN was grown by metal organic chemical vapor deposition(MOCVD). The surface treated with N2 plasma was analyzed by x-ray photoelectron spectroscopy(XPS) to determine the binding energy. The XPS results indicated the surface was changed from Al2O3 to AlN and AlON, and we confirmed that the thickness of the pretreated layer was about 1 nm using high resolution transmission electron microscopy(HR-TEM). The AlN buffer layer deposited on the grown pretreated layer had lower crystallinity than the as-treated PSS. Therefore, the surface N2 plasma treatment on PSS resulted in a reduction in the crystallinity of the AlN buffer layer, which can improve the epitaxial growth quality of the GaN template.

Reduction of Dislocations in GaN on Silicon Substrate Using In Situ Etching

A gallium nitride (GaN) epitaxial layer with a low density of threading dislocations is successfully grown on a silicon substrate by using in situ gas etching. Silicon nitride (SiNx) film is used as a mask, and ammonia is intermittently supplied in hydrogen ambient during the etching. After etching, high‐density deep pits appeared on the surface of a GaN template layer and corresponded to the threading dislocations in the layer. In this novel method, before growing an additional GaN layer on the template GaN layer, a second SiNx layer is deposited after the etching process, and this layer prevents GaN nuclei from growing on the upper side‐walls of the pits. By using this method, the density of threading dislocations of the GaN surface is reduced to 6.7 × 107 cm−2. This method is cost effective, completing all the necessary processes in one growth run without taking samples out from a metalorganic chemical vapor deposition (MOCVD) reactor.

Nearly perfect GaN crystal via pit-assisted growth by HVPE

We demonstrated 5 mm-thick bulk gallium nitride (GaN) with nearly perfect crystal quality. To achieve this, intentional etch pit formation by in situ dry HCl etching and ex situ wet etching in H3PO4 solution was employed on freestanding GaN templates followed by regrowth by hydride vapor phase epitaxy (HVPE). This consecutive etching gave rise to a number of large and deep etch pits on the whole surface of the freestanding GaN. We believe that these can reveal the etch pits originating from the dislocations with the edge, and mixed components. Intentionally formed etch pits were partially covered with the fresh regrown bulk GaN layer, being transformed into voids. Dislocations cannot propagate into new GaN layers through the voids, thus resulting in the reduction of dislocation density. 5 mm-thick bulk GaN exhibits a smooth, transparent surface and extremely high crystal quality with full width at half maximum (FWHM) of 21 arcsec in (0002) X-ray rocking curve and etch pit density (EPD) of 3 × 102 per cm2. This method can provide a promising way to fabricate bulk GaN with extremely low dislocation density, suitable for the fabrication of high-performance devices.

Effect of the Ammonia Flow on the Formation of Microstructure Defects in GaN Layers Grown by High-Temperature Vapor Phase Epitaxy

High-temperature vapor phase epitaxy (HTVPE) is a physical vapor transport technology for a deposition of gallium nitride (GaN) layers. However, little is known about the influence of the deposition parameters on the microstructure of the layers. In order to fill this gap, the influence of the ammonia (NH3) flow applied during the HTVPE growth on the microstructure of the deposited GaN layers is investigated in this work. Although the HTVPE technology is intended to grow GaN layers on foreign substrates, the GaN layers under study were grown on GaN templates produced by metal organic vapor phase epitaxy in order to be able to separate the growth defects from the defects induced by the lattice misfit between the foreign substrate and the GaN layer. The microstructure of the layers is characterized by means of high-resolution x-ray diffraction (XRD), transmission electron microscopy and photoluminescence. In samples deposited at low ammonia flow, planar defects were detected, along which the nitrogen atoms are found to be substituted by impurity atoms. The interplay between these planar defects and the threading dislocations is discussed. A combination of XRD and micro-Raman spectroscopy reveals the presence of compressive residual stress in the samples.

Scalable semipolar gallium nitride templates for high-speed LEDs

Scalable semipolar gallium nitride templates for high-speed LEDs

Fabrication of high-quality GaN substrates using the Na flux method

Gallium nitride (GaN) substrates fabricated along the nonpolar and semipolar directions are the most promising materials for realizing optical and electronic devices with low power consumption. In this study, we carry out the Na flux growth on -plane GaN templates grown heteroepitaxially on sapphires. The GaN crystals are grown at low supersaturation using the Na flux method with the dipping technique. The crystallinity of the grown GaN crystals is improved compared to that of the seed substrates. Then it improves further by lowering the supersaturation. Finally, we succeed in fabricating a 2-in. -plane GaN single crystal with high transparency and crystallinity.

TECHNOLOGY GROWTH, MORPHOLOGICAL, STRUCTURAL, ELECTRICAL AND OPTICAL PROPERTIES OF NITROGEN DOPED ZINC OXIDE THIN FILM ON POROUS GALLIUM NITRIDE TEMPLATE

Gallium nitride (GaN) is susceptible of producing efficient display and lighting devices. Low cost hybrid heterostructured lighting devices are developed by combining zinc oxide (ZnO) with GaN that has gained much more research interest, nowadays. Porous GaN receives a great deal of attraction by its excellent and improved properties compared with its bulk counterpart. Several potential applications have been realized, including for serving as a strain-relaxed substrates for the growth of superior quality heteroepitaxial thin films and it is quite interesting. UV assisted photo electrochemical etching nanostructuring technique was utilized for the development of GaN porous structure in this study. Nitrogen is regarded as a good candidate for acceptor doping in zinc oxide by the availability of its gaseous state compared with the other available p-type doping sources. Nitrogen doped ZnO thin film was grown on the resultant porous GaN template using radio frequency magnetron sputtering technique at room temperature. The morphological, structural and optical properties of the fabricated porous nanostructure as well as the grown nitrogen doped ZnO thin films were studied. It was found that the grown nitrogen doped ZnO thin film exhibited p-type conductivity, determined by Hall measurements and the obtained results were presented.

Efficient stress-relaxation in InGaN/GaN light-emitting diodes using carbon nanotubes.

A facile method to facilitate epitaxial lateral overgrowth (ELO) of gallium nitride (GaN) was developed by using single-walled carbon nanotubes (SWCNTs). High-quality GaN was achieved on sapphire by simply coating the SWCNTs as an intermediate layer for stress and defect mitigation. SWCNTs maintained their integrity at high reaction temperature and led to suppression of edge dislocations and biaxial stress relaxation by up to 0.32 GPa in a GaN template layer. InGaN/GaN multi-quantum-well light-emitting diodes (LEDs) on this high-quality GaN template offered enhanced internal quantum efficiency and light output power with reduced efficiency droop. The method developed here has high potential to replace current ELO methods such as patterned sapphire substrates or buffer layers like SiO2 and SiNx.

Nanoscale selective area growth of thick, dense, uniform, In-rich, InGaN nanostructure arrays on GaN/sapphire template

Uniform, dense, single-phase, 150 nm thick indium gallium nitride (InGaN) nanostructure (nanorods and nanostripes) arrays have been obtained on gallium nitride templates, by metal organic chemical vapor deposition and nanoscale selective area growth on silicon dioxide patterned masks. The 150 nm thick InGaN nanorods have a perfect hexagonal pyramid shape with relatively homogenous indium concentration up to 22%, which is almost twice as high as in planar InGaN grown in the same condition, and luminesce at 535 nm. InGaN nanostripes feature c-axis oriented InGaN in the core which is covered by InGaN grown along semi-polar facets with higher In content. Transmission electron microscope and sub micron beam X-rays diffraction investigations confirm that both InGaN nanostructures are mostly defect free and monocrystalline. The ability to grow defect-free thick InGaN nanostructures with reduced polarization and high indium incorporation offers a solution to develop high efficiency InGaN-based solar cells.