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.

  Preview abstract

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.

  Preview abstract

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.

  Preview abstract

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.

  Preview abstract

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.

  Preview abstract

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.

  Preview abstract

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.

  Preview abstract

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.

  Preview abstract

We have successfully fabricated high-quality AlN templates by combining sputtering deposition of AlN (Sp-AlN) with high-temperature face-to-face annealing (FFA). These templates are hereafter referred to as FFA Sp-AlN. Additionally, we achieved Al-polar and N-polar FFA Sp-AlN by annealing Sp-AlN films sputtered using a sintered AlN target and an Al metal target, respectively. By controlling the polarity, we fabricated a bilayer polarity-inverted AlN structure, where the Al-polar FFA Sp-AlN serves as the lower layer and the N-polar FFA Sp-AlN as the upper layer. In this bilayer polarity-inverted FFA Sp-AlN, an inversion domain boundary (IDB) with an abrupt interface, consisting of several monolayers (ML), and an atomically smooth surface were observed. Various devices have been proposed using polarity inversion structures, such as second harmonic generation (SHG) devices based on quasi-phase matching, which exploits the nonlinear optical properties of nitride semiconductors. Leveraging the structural characteristics of the bilayer polarity-inverted FFA Sp-AlN, an SHG device with a transverse quasi-phase matched waveguide was fabricated. This device achieves a high nonlinear coupling coefficient by placing IDBs at the nodes of the second harmonic (SH) wave electric field distribution. The fabrication of a multi-layer polarity-inverted AlN structure with more than two layers is highly desirable. In this study, we demonstrate the fabrication of a four-layer polarity-inverted AlN structure using multiple sputtering of AlN and FFA with the polarity control method of FFA Sp-AlN. It was found that the IDB structures vary depending on the sequence of polarity inversion.

  Preview abstract

(11-22) AlGaN multiple quantum wells (MQWs) grown by metalorganic vapor phase epitaxy (MOVPE) on sputter-deposited and face-to-face annealed AlN (FFA Sp-AlN) templates were structurally evaluated by transmission electron microscopy (TEM). Weak-beam dark-field TEM (WBDF-TEM) images revealed dislocation bending at the MOVPE-AlN/FFA Sp-AlN interface and the AlGaN/MOVPE-AlN interface, which is likely caused by compressive stress in FFA Sp-AlN. Most dislocations were identified as mixed a+c type, and the threading dislocation density was estimated to be 8.2 × 10⁹ cm⁻². High-angle annular dark-field scanning TEM (HAADF-STEM) images showed sharp MQW interfaces attributed to the small surface roughness.

  Preview abstract

AlGaN-based short-wavelength light-emitting devices with high Al content on polar planes suffer from reduced internal quantum efficiency due to the quantum-confined Stark effect and poor light extraction efficiency caused by preferential c-axis perpendicular emission. Nonpolar a-plane nitride semiconductor growth on r-plane sapphire substrates has attracted attention to solve these issues. This study investigates the effects of thermal treatment of r-plane sapphire substrates on crystallographic orientation and crystal quality of subsequently grown AlN films. Single-domain a-plane AlN growth was achieved on thermally treated substrates, while untreated substrates showed mixed a-plane and semipolar orientations.

  Preview abstract

Optical pyrometry-based substrate temperature calibration technique allowing reliable determination of emissivity of even nonstandard substrates such as thin metallic buffer on bulk wafer is presented. As example this approach is used to measure emissivity of metallic ZrN layers deposited on Si and Al2O3 wafers and reveals a strong dependence of substrate emissivity on thickness of the thin film. Accuracy of our technique is demonstrated by MBE growth of GaN nanowires on Si substrates with and without ZrN buffer and using nucleation kinetics of GaN nanowires as a sensitive probe of small changes in the surface temperature of the substrate.

  Preview abstract

We estimated the screw dislocation density in high-crystallinity AlN templates fabricated by sputtering and face-to-face annealing. Since the (0002) X-ray rocking curve (XRC) full widths at half maximum (FWHMs) were close to the instrumental resolution limit, estimating
the screw dislocation density solely from the (0002) XRC-FWHM was challenging. However, the (0002) XRC profiles exhibited both sharp and broad components. We found a positive correlation between the intensity of the broad component and the hillock density observed after AlGaN growth. These results indicate that screw dislocation densities can be estimated even when the (0002) XRC-FWHM approaches the instrumental resolution limit.

  Preview abstract

AlN is a key material for deep-UV LEDs and power devices. However, traditional sublimation growth is expensive due to the high temperatures required. This study focuses on low-temperature AlN crystal growth using type 304L stainless steel flux. AlN crystals were grown on sapphire substrates under different conditions. Substrate rotation improved surface smoothness and slightly increased thickness. Lower initial growth temperatures improved surface quality but reduced thickness. A slower cooling rate improved both thickness and smoothness. These results suggest that the solution growth method using stainless steel flux has the potential to produce AlN crystals efficiently and at a lower cost.

  Preview abstract

AlN is a promising substrate material, but single crystal growth is difficult due to high-temperature constraints. This study explores AlN solution growth using Fe-Nb alloy fluxes, which offer improved nitrogen solubility. Thermodynamic calculations and growth experiments were conducted with Fe-Nb alloys of varying Nb content. Results showed that AlN solubility, N solubility and crystal thickness increased with Nb content up to 4 mass%, but declined at 5 mass%, suggesting an optimal Nb range. SEM analysis confirmed growth behavior, supporting the use of Fe-Nb alloys for improved AlN crystal fabrication.

  Preview abstract

Polarity-inverted AlN structures, which are promising for wavelength conversion devices, can be fabricated through oxidation processes. However, the effect of different oxidation methods on the atomic arrangement has not yet been fully clarified. In this study, polarity-inverted AlN structures were fabricated using several oxidation techniques, and polarity inversion was verified by KOH wet etching. For the oxygen plasma treatment, voids were observed at the position of the carbon spike, corresponding to the sputtering interface. Furthermore, secondary ion mass spectrometry (SIMS) analysis indicated that the position of the inversion domain boundaries (IDB) was influenced by the oxidation process employed.

  Preview abstract