A normally- OFF AlGaN/GaN high-electron-mobility transistor (HEMT) with p-GaN gate was fabricated on free-standing GaN substrate. The self-terminated etching technology was achieved by combining with ...AlN stop layer and SF 6 -based etching gas. Compared with HEMT on Si substrate (Si-HEMT), the HEMT on GaN substrate (GaN-HEMT) demonstrated the higher current density, lower subthreshold swing (SS), lower drain leakage current, and lower static and dynamic ON-resistance that were ascribed to the lower dislocation and less defect on free-standing GaN substrate. Meanwhile, the excellent <inline-formula> <tex-math notation="LaTeX">{V}_{\mathrm{th}} </tex-math></inline-formula> stability up to 175 °C and uniformity was demonstrated. Also, higher breakdown voltage (BV) of 683 V was measured, which was higher than 507 V for Si-HEMT. Hence, a normally- OFF AlGaN/GaN HEMT on free-standing GaN substrate shows good potential in power application.
P-type conversion of n−-GaN by Mg-ion implantation was successfully performed using high quality GaN epitaxial layers grown on free-standing low-dislocation-density GaN substrates. These samples ...showed low-temperature PL spectra quite similar to those observed from Mg-doped MOVPE-grown p-type GaN, consisting of Mg related donor–acceptor pair (DAP) and acceptor bound exciton (ABE) emission. P–n diodes fabricated by the Mg-ion implantation showed clear rectifying I–V characteristics and UV and blue light emissions were observed at forward biased conditions for the first time.
A high-efficiency uniform/selective heating microwave oven was developed. Because the power amplifier requires high-efficiency characteristics to function as a microwave source, a free-standing ...Gallium Nitride (GaN) substrate was applied in this study. By applying a harmonic tuning circuit, an output power of 71 W and PAE of 73% were achieved in pulsed operation, and an output power of 63 W and PAE of 69% were achieved in CW operation. Moreover, we fabricated a prototype PA module that consists of an oscillator, a driver amplifier, PA, and other RF circuits. The output power was controlled by pulse width modulation to maintain high efficiency regardless of output power. We evaluated the arrangement of antenna polarizations to isolate each antenna. By suppressing the interference of output from adjacent antennas, it is possible to irradiate the object on the top surface of the antenna, thereby demonstrating heating characteristics with small temperature unevenness. The prototype microwave oven successfully demonstrated uniform/selective heating.
Optical characteristics of Mg-ion implanted GaN layers with various fluence ranges were evaluated. Mg ion implantation was performed twice at energies of 30 and 60keV on n-GaN layers. The first ...implantation at 30keV was performed with three different fluence ranges of 1.0×1014, 1.0×1015 and 5.0×1015cm−2. The second implantation at an energy of 60keV was performed with a fluence of 6.5×1013cm−2. After implantation, samples were annealed at 1250°C for 1min under N2 atmosphere. Photoluminescence (PL) spectrum of the GaN layer with the Mg ion implantation at the fluence range of 1.0×1014cm−2 at 30keV was similar to the one of Mg-doped p-GaN layers grown by MOVPE (Metal-Organic Vapor Phase Epitaxy) on free-standing GaN substrates and those at the fluence ranges over 1.0×1015cm−2 were largely degraded.
Free-standing GaN substrates are urgently needed to fabricate high-power GaN-based devices. In this study, 2-inch free-standing GaN substrates with a thickness of ~250 μm were successfully fabricated ...on double-polished sapphire substrates, by taking advantage of a combined buffer layer using hydride vapor phase epitaxy (HVPE) and the laser lift-off technique. Such combined buffer layer intentionally introduced a thin AlN layer, using a mix of physical and chemical vapor deposition at a relatively low temperature, a 3-dimensional GaN interlayer grown under excess ambient H
, and a coalescent GaN layer. It was found that the cracks in the epitaxial GaN layer could be effectively suppressed due to the large size and orderly orientation of the AlN nucleus caused by pre-annealing treatment. With the addition of a 3D GaN interlayer, the crystal quality of the GaN epitaxial films was further improved. The 250-μm thick GaN film showed an improved crystalline quality. The full width at half-maximums for GaN (002) and GaN (102), respectively dropped from 245 and 412 to 123 and 151 arcsec, relative to those without the 3D GaN interlayer. The underlying mechanisms for the improvement of crystal quality were assessed. This method may provide a practical route for fabricating free-standing GaN substrates at low cost with HVPE.
The authors report on the electrical properties of free-standing GaN substrates, using one typical set of low crystalline quality (type I) and high crystalline quality (type II), fabricated by ...hydride vapor-phase epitaxy and self-separation technique. Type I has a large electron concentration of ~10
18
cm
−3
and a small electron mobility of ~85 cm
2
/Vsec while type II has a small electron concentration of ~5 × 10
15
cm
−3
and a large electron mobility of ~400 cm
2
/Vsec. In type I, the electron concentration decreases with the decrease of temperature until a critical temperature and then increases at lower temperatures while in type II, the electron concentration continuously decreases with the decrease of temperature. These different electrical properties are ascribed to the difference in the structural defect densities of the two samples. These indicate that the electron-transport mechanism of the defective free-standing GaN substrates is dominated by the two-band conduction model. These results are consistent with the temperature dependence of the electron mobility and with the structural properties observed by using high-resolution X-ray diffraction and atomic-force microscopy.
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