Kazuhide Kusakabe

Characterization of Overgrown GaN Layers on Nano-Columns Grown by RF-Molecular Beam Epitaxy

We report on the successful growth of free-standing GaN films on (0001) sapphire substrates by RF-molecular beam epitaxy. The key to obtain unstrained GaN layers is employing self-organized GaN nano-columns which involve an air gap structure as a footing layer of overgrown GaN. The residual strain in overgrown GaN films is evaluated by measuring the lattice constant by X-ray diffraction. It is found that the c-axis length of overgrown GaN is estimated to be 5.1848 A, which is close to the value of strain-free GaN even with a layer thickness of 2.7 µm. Overgrown GaN peeled arbitrarily from GaN nano-columns is observed due to the cleaving process.

Modeling of Reaction Pathways of GaN Growth by Metalorganic Vapor-Phase Epitaxy Using TMGa/NH3/H2 System: A Computational Fluid Dynamics Simulation Study

A model of reaction pathways of GaN growth by metalorganic vapor-phase epitaxy was studied by computational fluid dynamics simulations. We included the formation of polymers such as [Ga–N]n and [MMGaNH]n (n=2–6) in the reaction model in a TMGa/NH3/H2 system for the first time. The simulations using this reaction modeling successfully explained experimental growth rates at various temperatures, and clarified the main reaction pathway of GaN growth. The change in gas-phase chemistry due to temperature in the range of 300–1400 K was investigated. It was found that the type of reactive molecule changes with temperature, followed by the formation of different polymers in a certain temperature range, that is, [MMGaNH]n at 600–750 K and [Ga–N]n at higher temperatures.

Overgrowth of GaN layer on GaN nano-columns by RF-molecular beam epitaxy

The growth behavior of GaN layers overgrown on nano-columns on sapphire substrates by RF-plasma molecular beam epitaxy was investigated. Free-standing GaN films were obtained through the coalescence of mono-columns, showing the layered structure supported by underlying columnar GaN. Overgrown GaN layers were characterized with respect to the residual strain. As a result, it was found that overgrown GaN films with layer thickness of 2.7 μm possessed a stress-free property such as Raman frequency of high-frequency E 2 mode of 568.1 cm -1 .

Improvement of Crystal Quality of RF-Plasma-Assisted Molecular Beam Epitaxy Grown Ga-Polarity GaN by High-Temperature Grown AlN Multiple Intermediate Layers.

The effects of high-temperature-grown AlN multiple intermediate layers (HT-AlN-MILs) on the crystal quality of Ga-polarity GaN layers grown on (0001) Al2O3 substrates by molecular beam epitaxy using rf-plasma nitrogen source were investigated. The high-temperature-grown AlN intermediate layers (HT-AlN-ILs) with different thicknesses were found to play different roles in the improvement of crystal quality. The 8-nm-thick HT-AlN-ILs brought about improvement of electrical properties. On the other hand, the 2-nm-thick HT-AlN-ILs improved the surface morphology. The combination of these 8-nm-HT-AlN-ILs and 2-nm-HT-AlN-ILs improved both the electrical properties and the surface morphology concurrently.

Reduction of threading dislocations in migration enhanced epitaxy grown GaN with N-polarity by use of AlN multiple interlayer

High quality GaN layer was obtained by insertion of high temperature grown AlN multiple intermediate layers with migration enhanced epitaxy method by the RF-plasma assisted molecular beam epitaxy on (0001) sapphire substrates. The propagating behaviors of dislocations were studied, using a transmission electron microscope. The results show that the edge dislocations were filtered at the AlN/GaN interfaces. The bending propagation of threading dislocations in GaN above AlN interlayers was confirmed. Thereby, further reduction of dislocations was achieved. Dislocation density being reduced, the drastic increase of electron mobility to 668 cm 2 /Vs was obtained at the carrier density of 9.5 × 10 16 cm 3 in Si doped GaN layer.

2.6 μm/h High‐Speed Growth of GaN by RF‐Molecular Beam Epitaxy and Improvement of Crystal Quality by Migration Enhanced Epitaxy

High-quality GaN films were grown by molecular beam epitaxy (MBE) using elemental Ga and rf-plasma nitrogen as source with 1.2 μm/h growth rate. GaN films were grown on the migration enhanced epitaxy (MEE)-GaN buffers deposited on (0001) sapphire substrates. The room temperature (RT) mobility was 372 cm2/Vs at 1.2 × 1017 cm—3. Furthermore, extreme high-speed GaN growth of 2.6 μm/h was also demonstrated by increase of radical nitrogen supply at the substrates. The electron density of Si-doped GaN films was controlled in the range of 5.3 × 1015 to 4.9 × 1020 cm—3 and the room temperature mobility was 252 cm2/Vs at 3.4 × 1017 cm—3. The narrowest full width at half maximum (FWHM) of photoluminescence at 15 K was 10.5 meV.

Morphological Characteristics of a-Plane GaN Grown on r-Plane Sapphire by Metalorganic Vapor-Phase Epitaxy

We report the morphological evolution of a-plane GaN thin films grown on r-plane sapphire substrates by atmospheric metalorganic vapor-phase epitaxy. The surface flatness is improved under optimized growth conditions which are different from those of c-plane epitaxy. The peak-to-valley height of surface roughness is reduced from 4 to 0.8 µm when GaN is grown at 1120°C on a 40-nm-thick low-temperature GaN (LT-GaN) buffer layer, as well as at 1150°C on a 20-nm-thick LT-GaN. These samples show their highest electron mobility of 220 cm2/(V s) at an electron concentration of 1.1×1018 cm-3 at room temperature.

High-Quality GaN on AlN Multiple Intermediate Layer with Migration Enhanced Epitaxy by RF-Molecular Beam Epitaxy

High-speed GaN growth of 1.0 µm/h with migration enhanced epitaxy (MEE) by molecular beam epitaxy using rf-plasma nitrogen (RF-MBE) was demonstrated. The electron mobility of MEE-GaN was 362 cm2/Vs for the electron density of 1.7×1017 cm-3 at room temperature. The threading dislocation density of MEE-GaN was estimated to be 1.0–3.0×1010 cm-2 based on the cross-sectional transmission electron microscope (TEM) image. The remarkable improvement of electrical properties was obtained by the introduction of a high-temperature (750°C) grown AlN/GaN multiple intermediate layer (AlN-MIL). The cross-sectional TEM image showed that threading dislocations were bent or terminated at the AlN-MIL. The highest room temperature mobility of 668 cm2/Vs was obtained at the electron density of 9.5×1016 cm-3. The low-temperature peak mobility was 2340 cm2/Vs at 90 K.

Impurity doping effect on thermal stability of InGaN/GaN multiple quantum-well structures

Thermal stability of InGaN∕GaN multiple quantum-well (MQW) structures grown by metalorganic vapor phase epitaxy on (0001) sapphire substrates was investigated. Samples were annealed under atmospheric nitrogen ambient at 1000°C after growth. The thermal stability of MQW structures was estimated by high-resolution x-ray diffraction. It was found that thermal annealing degraded MQW periodicity in an undoped sample. This was due to the thermal diffusion of indium atoms via Ga vacancies in the GaN barrier region. It was also found that both Si doping and Mg doping improved the thermal stability of MQW structures. This mechanism was considered that Si and Mg, which were incorporated into column-III sites, prevented formation of the Ga vacancies. Thus, the thermal diffusion of indium atoms was suppressed. Room temperature photoluminescence (PL) from the Si-doped MQWs retained intense emission after annealing, while the undoped and Mg-doped MQWs showed degradation of PL intensities after annealing. It was, theref...

Improvement of electrical property and surface morphology of GaN grown by RF-plasma assisted molecular beam epitaxy by introduction of multiple AlN intermediate layer

Abstract The crystal properties of Ga-polarity GaN layers grown by molecular beam epitaxy using RF-plasma nitrogen on (0001) Al 2 O 3 substrates were remarkably improved by introduction of high-temperature grown AlN multiple intermediate layers (HT–AlN–MILs). The effects of HT–AlN–MIL on the improvement of crystal quality were found to be different for its thickness. The 8 nm-thick HT–AlN–MIL improved the electrical property and the 2 nm-thick HT–AlN–MIL improved the surface morphology. The combination of 8 nm- and 2 nm-thick HT–AlN–MILs brought about improvement of both electrical property and surface morphology, concurrently. The HT–AlN–MIL was used for RF-MBE re-growth on MOCVD–GaN template. Relatively fine step flow structure without spiral hillocks was obtained.