Session Index

Our research group has been conducting a series of studies on the solution growth of AlN crystals using Fe-based fluxes. Based on abundant thermodynamic data for Fe-based alloys and previous studies demonstrating maximum AlN growth at an Al to N molar ratio of 1:1, we calculated the AlN solubility product to determine the optimal Fe-Cr-Ni alloy compositions for AlN crystal growth. The compositions at 1773 K are expressed as xFe-(49.5-0.43x)Cr-(50.5-0.57x)Ni (x: mass% Fe). Using this flux, we investigated the growth behavior of AlN crystals and found that the growth thickness of AlN crystals increases as x decreases.

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In this work, we design composite buffer layer structures consisting of AlGaN superlattices based on electron transmission and reflection simulations to achieve low leakage current as well as low stress for high voltage power HEMTs grown on 150 mm Si substrates. Devices with a vertical breakdown voltage as high as 1600 V have been realized.

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Incorporating boron into GaN, either by doping or forming a BGaN alloy, is proposed to improve GaN channel confinement and enhance GaN buffer isolation of GaN HEMTs. A BGaN back barrier layer was implemented to suppress trap-induced current degradation. a higher vertical breakdown characteristic was demonstrated on the HEMTs with BB structure, significantly. These results demonstrate Boron’s dual role in improving both trap immunity and high-voltage robustness in GaN based HEMT devices.

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For integration of different colors nanocolumn (NC) micro-LEDs, bulk InGaN NC
arrays were fabricated by nanotemplate selective area growth (SAG) method. Control of
nanocolumn period (L) and diameter is important for the InGaN emission color. Using the
conventional Ti nanodots mask, the tapered shape and insufficient height of the nanopillars were
fabricated and no selective growth of bulk InGaN NCs was obtained. Using Ni nanodot mask
technology, uniform c-plane (0001) top nanopillars with the sufficient height were fabricated.
On the nanopillar templates, well-controlled bulk InGaN NC arrays were successfully prepared
with the controlled emission wavelength from 551 to 668 nm.

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Nanocolumns (NCs) are considered promising structures for light-emitting devices
for their advantageous properties. We aim to realize selective area growth (SAG) of GaInN NCs
directly on the substrate to further relieve strain. Therefore, this study investigated how the
underlying GaN NCs affects the SAG of GaInN using the rf-plasma-assisted molecular beam
epitaxy (RF-MBE). The results indicate that both the height of the underlying GaN NCs and the
gap width between adjacent ones affect the occurrence of spontaneous nucleation. These
findings imply that controlling the underlying structure enables the growth of GaInN NCs
without spontaneous nucleation on the substrate.

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Conductive GaN thin films were grown homoepitaxially on c-plane GaN templates by reactive pulsed sputtering using Si-doped GaN sintered targets. The N₂/Ar flow ratio influenced the surface stoichiometry; the low N2 flow ratio of 15% or less resulted in Ga droplet formation and the high N2 flow ratio of 18% or more resulted in the three -dimentional growth. Undoped films showed 181 cm²/V·s mobility and 5.9 × 10¹⁹ cm⁻³ carriers from oxygen donors. Si-doped films yielded ~1 × 10²⁰ cm⁻³ carriers. X-ray rocking curves (~1000 arcsec FWHM) indicated reduced crystallinity; optimizing surface treatment may improve quality.

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In this work, normally-off InAlGaN/GaN HEMTs were fabricated on QST substrates with a quaternary
InAlGaN barrier, employing Li-doped p-type NiOx gates deposited by ultrasonic spray pyrolysis (USPD). The
devices show high drain current, low on-resistance, a large on/off ratio, and high breakdown voltage. XPS and
electrical analyses verify effective Li incorporation and a low interface-trap density. These results demonstrate
that USPD-grown Li-doped NiOx gates are promising for reliable, high-performance GaN power devices.

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We report on the growth of smooth N-polar AlN and GaN on sapphire by MOVPE for HEMTs. High temperature nitridization followed by low V/III ratio AlN growth was critical to achieve step-flow growth mode with a roughness of about 3Å. The typical impurities in AlN were 10e19 oxygen and mid 10e16 carbon atoms/cm3, depending on growth conditions. For GaN, N2 carrier gas resulted in smooth close layers while H2 carrier gas induced step-bunching, pinned at dislocations. Both oxygen and carbon levels were above 10e17 atoms/cm3. For heterostructures, also the transition between AlN and GaN growth was investigated.

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