Bulk GaN Single Crystals Research Articles

Growth and Characterization of Bulk GaN Crystals at High Pressure and High Temperature

ABSTRACTWe report the growth and characterization of bulk GaN single crystals by temperature-gradient recrystallization at high pressure and high temperature (HPHT), using apparatus adapted from that used to synthesize gem-grade diamond crystals. The bulk crystals are grown on seeds that were synthesized by hydride vapor phase epitaxy (HVPE) and subsequently removed from their sapphire substrate. Our largest crystals to date are 15×18 mm in diameter; however, the process is scalable to 50 mm and above. The crystals are transparent and well faceted, and dislocation densities below 100 cm−2 have been achieved. Additional characterization of the GaN crystals is also presented.

Growth of bulk GaN single crystals by high-pressure sublimation method

Growth of bulk GaN single crystals by high-pressure sublimation method

Effect of the Buffer Layers on Lattice Polarity of GaN Epilayers Grown on the N‐Face of Bulk GaN Single Crystals by Molecular‐Beam Epitaxy

Lattice polarity of GaN epilayers grown on N-faces of bulk GaN single crystals has been investigated. The determinations of lattice polarity have been carried out by observation of surface reconstruction and surface morphology change by alkaline etching. As a result, it is found that GaN epilayers grown on N-faces using In-doped GaN buffer layers have Ga-polarity; on the other hand, GaN epilayers grown on N-faces using high-temperature (HT) AlN buffer layers have N-polarity. These results suggest that In-doping has the role of reversing the polarity of the GaN epilayers from N- to Ga-polarity, whereas HT-AlN buffer layers itself do not act as the layer reversing the polarity from N- to Al-polarity.

Bulk GaN single crystals: growth conditions by flux method

Hexagonal GaN platelet crystals with a size of 1–4 mm have been grown by a Li-based flux method. The influence of growth conditions such as the molar ratio of starting materials, temperature, pressure, the position of Li 3N in the crucible on the growth of GaN single crystals was studied. The quality of GaN single crystal was checked by optical microscope and X-ray rocking curve.

Growth of AlxGa1?xN and Mg-Doped GaN Epilayers on Ga- and N-Faces of Bulk GaN Single Crystal Substrates by Molecular-Beam Epitaxy

AlGaN and Mg-doped GaN epilayers were grown by radio-frequency molecular beam epitaxy on both polar faces of bulk GaN single crystals with single polar domain, which were prepared by the pressure-controlled solution growth method. AlGaN epilayers on Ga-faces of bulk GaN substrates exhibited higher Al composition and narrower full-width at half maximum of 2θ — ω X-ray diffraction than those values obtained for AlGaN epilayers on N-faces at the same growth condition. P-type conduction was only achieved in Mg-doped GaN on Ga-faces of bulk GaN substrates without any post-treatments. These results indicate that the growth on Ga-faces (or with Ga-polarity) is more favourable for device fabrication even using high-crystallinity bulk GaN substrates.

RF-MBE法によるバルクGaN単結晶N面上へのGa極性GaN薄膜の成長 : エピキタシャル成長II

RF-MBE法によるバルクGaN単結晶N面上へのGa極性GaN薄膜の成長 : エピキタシャル成長II

Room-temperature ferromagnetism in Cr-doped GaN single crystals

We report on the discovery of a room-temperature ferromagnetism in Cr-doped GaN single crystals with a Tc=280 K. The addition of Cr into GaN single crystals grown by the flux method induces the lattice constant increase due to the larger Cr atomic radius. In x-ray photoelectron spectroscopy measurement, Cr 2p3/2 core-level exhibited spectra near 575.7 eV. This binding energy is similar to the reported value of CrN. The coercive field by magnetization–magnetic field (M–H) hysteresis curve at 250 K was 54 Oe. We verified the presence of ferromagnetic transition in the temperature dependence of the electrical resistance measurements. We discuss the ferromagnetic ordering in Cr-doped GaN bulk single crystals excluding the contribution of the substrate crystal structure.

Influence of pressure control on the growth of bulk GaN single crystal using a Na flux

We investigated the influence of N 2 pressure control on the growth of bulk GaN crystals by using a Na flux. GaN single crystals with a maximum dimension of 3 mm could be obtained by this method, though smaller crystals of 0.5–2 mm in size have been grown without pressure control. This suggested significant effects of additional NH 3 gas on growth of GaN crystals in the Na flux method.

Energy gap in GaN bulk single crystal between 293 and 1237 K

Optical transmission measurements were performed on GaN bulk single-crystal platelet at temperatures between 293 and 1237 K. The energy bandgaps were determined from the corresponding optical absorption spectra. The bandgaps can be fit well as a function of temperature using the Varshni expression as E g ( eV)=3.556−9.9×10 −4T 2 (T+600). The shapes of the measured absorption edges were found to be dependent on the thermal treatments of the sample.

Growth of bulk GaN single crystals by the pressure-controlled solution growth method

Crystal growth of GaN by the pressure-controlled solution growth (PC-SG) method has been carried out using a high-pressure furnace. We have investigated the effect of the supersaturation of nitrogen atoms (the rate of increase of nitrogen pressure) in order to grow large GaN single crystals. It was found that the rate of increase of nitrogen pressure affected the size of a GaN single crystal and its morphology. GaN single crystals with a surface area of about 120 mm 2 and/or with good morphology were obtained at a rate less than 49 MPa/h. We have studied the effect of the supersaturation of nitrogen atoms on the crystallinity. The GaN single crystals obtained had the following good crystallinity: (1) the FWHM of the rocking curve was about 120 arcsec without any low-angle grain boundaries, (2) the dislocation density was estimated to be less than 10 5 cm −2 by TEM observations, and (3) the photoluminescence intensity of the yellowish band became very weak and the PL intensity ratio of the band-edge band to the yellowish band was greatly improved. These results clearly suggest that the rate of increase of the nitrogen pressure in the PC-SG method must be lower to grow large GaN single crystals with good crystallinity.

Photoluminescence and Raman Spectra of Flux Processed Bulk Single Crystal GaN

ABSTRACTBulk wurtzite-GaN crystal was obtained with a size more than 3 mm along the length of the crystal and with a thickness 200 ∼ 300 µm at around 750°C and 100 bar for 24 hrs in the flux growth method. The structural and compositional property of the GaN bulk single crystal was also studied by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) techniques. Photoluminescence, cathodoluminescence and micro-Raman measurements at room temperature are investigated for bulk single crystals of wurtzite GaN. The cathodoluminescence peak of near band-to-band transition at 365.5 nm and the E2(high energy, 568 cm−1) and A1(LO, 737 cm−1) Raman phonon modes were obtained according to the different position of the (0001) surface of GaN grown by flux method. Sharp line shape for the strain-sensitive E2 (high) mode is considered to be due to the high crystalline quality of the crystal.

Growth of Bulk GaN Single Crystals by the Pressure-Controlled Solution Growth Method

We are the first researchers to have developed the pressure-controlled solution growth (PC-SG) method. Using this method, supersaturation of growing crystals is realized by applying an over pressure without any temperature decrease. We applied this method to the growth of GaN crystals for examining the validity of the PC-SG method, using a high-pressure furnace. It was determined that the crystal growth rate increased as a function of temperature with a maximum at 1475°C under a nitrogen pressure of 0.98 GPa. The dependency of the growth rate on the Ga metal weight strongly supports the fact that the dominant growth process is by diffusion of nitrogen atoms in the Ga melt. GaN single crystals with a maximum size of 10 mm diameter were reproducibly grown by the PC-SG method and it was determined that these crystals had good crystallinity.

Electrical Properties of GaN Bulk Single Crystals Doped with Mg

Electrical Properties of GaN Bulk Single Crystals Doped with Mg

Electrical Properties of GaN Bulk Single Crystals Doped with Mg

We have studied the electrical properties of magnesium doped bulk GaN crystals grown by high-pressure, high-temperature method. We discovered that while undoped bulk crystals are usually highly conductive, with increasing magnesium concentration they transform to highly resistive and eventually to p-type material. For this type of samples the conductivity shows activated character with energy close to 150 meV characteristic of an Mg acceptor. Also we discovered that although annealing modifies the sample resistance it leaves the dominating transport mechanism unchanged. At lower temperatures transport seems to be governed by hopping conductivity.

24aB8 圧力制御溶液成長法によるバルクGaN単結晶の育成(バルク成長VII)

24aB8 圧力制御溶液成長法によるバルクGaN単結晶の育成(バルク成長VII)

Strain relaxation of Al Ga1−N epitaxial layers on GaN and SiC substrates

Abstract In this article, we present a study on strain relaxation in Al x Ga 1− x N layers grown by molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) on the bulk GaN single crystals (lattice mismatch of –2.5% to 0%) and on the 6H SiC crystals (lattice mismatch of 1% to 3.5%). Both synchrotron and X-ray tube radiation were used for diffractometry (double- and triple-axis) and reflectivity measurements. We found that the layers on SiC relax easier than those grown on GaN. The results show that growth of Al x Ga 1− x N layers on GaN substrates may provide a method for producing fully strained (i.e., unrelaxed) blue-laser structures.

Preparation and properties of free-standing HVPE grown GaN substrates

In this work, free-standing GaN single crystalline crackless substrates were grown by hydride vapor phase epitaxy (HVPE). The GaN substrate, having a current maximum size of 350 μm thickness and 10 mm×10 mm area, were obtained by the HVPE growth of a thick-film GaN on a sapphire substrate. Subsequently, the sapphire substrate was removed. To grow the crack-free GaN on a sapphire substrate, cracks were intentionally generated only in the sapphire substrates after 100 μm thickness GaN growth, and then the GaN thick film was re-grown. The GaN single crystalline substrate prepared through this work was found to be optically and electrically superior to those of bulk GaN single crystals. The free-standing and crack-free GaN single crystalline substrate is suitable for the homoepitaxial growth of GaN.

Hydride vapor-phase epitaxy growth of high-quality GaN bulk single crystal by epitaxial lateral overgrowth

Selective area growth (SAG) of GaN using the HVPE method was carried out to obtain a high-quality GaN bulk single crystal. Selectivity of the GaN growth was excellent and the GaN layer had no cracks. The FWHM (195arcsec) of the X-ray rocking curve for (0004) diffraction was narrower than that (348arcsec) of the sample grown by the conventional method. Overgrowth of the GaN on the SiO2 mask was enhanced in the 〈112̄0〉 direction. Cathode luminescence intensity of this overgrowth region was much stronger than that of the window region. It was found that a reduction in stress is achieved by narrowing the window area, and the epitaxial lateral overgrowth on the mask is effective in obtaining high-quality GaN layer.

Optical metastability in bulk GaN single crystals

Bulk GaN single crystals were grown from cold pressed GaN powder by sublimation in flowing ammonia. Optical transmission measurements indicated that the absorption coefficient for the transparent samples is 50 cm−1 in the wavelength region from 650 to 400 nm. Optical metastability in bulk GaN crystals was studied through time dependent photoluminescence both at room and liquid–nitrogen temperatures. The observation included decreasing output intensity of the ultraviolet emission attributed to the band edge and increasing output intensity of a new emission band centered at 378 nm at room temperature. At liquid–nitrogen temperature, the photoinduced emission band consisted of at least one LO-phonon replica of the zero-phonon line centered at 378 nm. The ratio of output intensities of the photoinduced band to the band edge increased by a factor of 10 during 27 min of exposure time. The photoinduced effect is attributed to the metastable nature of traps in bulk GaN.

Selective Area Growth of GaN by MOVPE and HVPE

AbstractRecent successful results on the selective area growth (SAG) of GaN that has been done by MOVPE and HVPE are shown. The SAG were carried out on MOVPE-grown GaN (0001) / sapphire substrates with lined or dotted SiO2 masks. Sub-micron GaN dot and line structures are fabricated by the SAG in MOVPE, so that smoothly overgrown GaN layers are successfully realized using the epitaxially lateral overgrowth (ELO) technique. The ELO structures are confirmed to be good quality GaN single crystal with a smooth surface, no grain boundaries, and low-dislocation densities. In addition, thick GaN bulk single crystals without any cracks are grown by the SAG in HVPE. Crystalline and optical properties of the GaN bulk are much improved. The reduction in the thermal strain due to the growth on the limited area as well as the ELO are found to be effective to reduce crystalline defects of the GaN bulk single crystals.