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.
We fabricated piezoelectric energy harvesters with lead-free (K,Na)NbO3 (KNN) thin films and compared their power-generation performance with that of Pb(Zr,Ti)O3 (PZT) thin film energy harvesters. ...The KNN and PZT thin films were deposited on Pt/Ti/Si substrates by rf-magnetron sputtering. The transverse piezoelectric properties of the KNN and PZT films were essentially the same, e31,f∼−11C/m2. The peak average output power of the unimorph cantilevers of KNN/Si and PZT/Si were 1.1 and 1.0μW, respectively. Thus, the performance of lead-free KNN thin films as a piezoelectric energy harvester is comparable to that of PZT films.
The effect of the surface off‐angle toward either the a‐ or m‐axis on the defect formation is characterized using deep‐level transient spectroscopy (DLTS) in conjunction with the carrier ...concentration for Ni Schottky contacts formed on n‐GaN drift layers. In both noncontact and conventional capacitance–voltage results, off‐angle dependence on carrier concentration is observed. For all samples, a large dominant peak appears at approximately 270 K in the DLTS spectra and is attributed to E3 (EC − 0.57–0.61 eV) defects. Carbon atoms can act as carrier compensators and form E3 defects. These results can be interpreted based on how C incorporation during crystal growth depends on the off‐angle.
The effect of the surface off‐angle toward either the a‐ or m‐axis on the defect formation is characterized using deep‐level transient spectroscopy for Ni Schottky contacts formed on n‐GaN drift layers. A large dominant peak appears at 270 K and is attributed to E3 defects. Carbon atoms can act as carrier compensators and form E3 defects.
Photo-electrochemical (PEC) etching was used to fabricate deep trench structures in a GaN-on-GaN epilayer grown on n-GaN substrates. A 50-nm-thick layer of Ti used for an etching mask was not removed ...even after etching to a depth of >30 µm. The width of the side etching was less than 1 µm with high accuracy. The aspect ratio (depth/width) of a 3.3-µm-wide trench with a PEC etching depth of 24.3 µm was 7.3. These results demonstrate the excellent potential of PEC etching for fabricating deep trenches in vertical GaN devices.
We applied scanning internal photoemission microscopy (SIPM) to characterize the degradation of GaN Schottky contacts formed on a thick n-GaN layer grown on a freestanding GaN substrate by in situ ...applying reverse bias voltage (Vbias) down to −45 V. For most of the contacts, uniform distribution of the photocurrent was observed over the electrode with the visible lasers. Irregular-shape regions with 5%-25% larger photocurrent appeared with the near UV laser by applying Vbias, but the I-V characteristics were stable. On the other hand, for the contacts with a slightly larger reverse current, the photocurrent distribution was also uniform at Vbias = 0 V, but over Vbias = −36 V, the photocurrent was intensively increased at small spots. After the SIPM measurements, the I-V characteristics became leaky, and the same spots were observed in the microscope image. These results indicate that SIPM is useful for in situ monitoring of the initial stage of the degradation under applying reverse bias voltage.
Nitrogen-ion-implantation damage on SiC has been clearly imaged using scanning internal photoemission microscopy (SIPM). Ni Schottky contacts were formed on selectively N-ion-implanted n-SiC surfaces ...at 80keV with an ion dose of 1×1015cm−2. A photocurrent, Y (photoyield; defined as photocurrent per incident photon), was detected by focusing and scanning a laser beam over the contacts. The N-ion-implanted regions were clearly imaged with Y measurements. Y was detected even where the implanted region is protruding out of the electrode in the unannealed sample. We also found significant increase of Y in the periphery of the ion-implanted region. We confirmed that SIPM is a powerful tool for mapping damages due to ion implantation.
Low dislocation density and low-resistance GaN wafers are in high demand for improving the performance of vertical GaN power devices. Recently, GaN wafers with the dislocation density of 8.8 × 104 ...cm−2 and the resistivity of 7.8 × 10−4 cm, were fabricated using oxide vapor phase epitaxy (OVPE). In this study, GaN p-n diodes on GaN wafers prepared by the OVPE method were evaluated for verifying their suitability as vertical GaN power devices. An extremely low-differential specific on-resistance of 0.08 m cm2 and a high breakdown voltage of 1.8 kV were obtained from forward and reverse I-V measurements.
We investigated the properties of Ge-doped, high-quality bulk GaN crystals with Ge concentrations up to 2.4×10
19
cm
−3. The Ge-doped crystals were fabricated by hydride vapor phase epitaxy with GeCl
...4 as the dopant source. Cathodoluminescence imaging revealed no increase in the dislocation density at even the highest Ge concentration, with values as low as 3.4×10
6
cm
−2. The carrier concentration, as determined by Hall measurement, was almost identical to the combined concentration of Ge and unintentionally incorporated Si. The electron mobilities were 260 and 146
cm
2
V
−1
s
−1 for
n=3.3×10
18 and 3.35×10
19
cm
−3, respectively; these values are markedly larger than those reported in the past for Ge-doped GaN thin films. The optical absorption coefficient was quite small below the band gap energy; it slightly increased with increase in Ge concentration. Thermal conductivity, estimated by the laser-flash method, was virtually independent of Ge concentration, maintaining an excellent value around 2.0
W
cm
−1
K
−1. Thermal expansion coefficients along the
a- and
m-axes were approximately constant at 5.0×10
−6
K
−1 in the measured doping concentration range.
This report describes the fabrication and characteristics of GaN p–n junction diodes on free‐standing GaN substrates with low dislocation density. We have demonstrated GaN p–n junction diodes with a ...unique field‐plate (FP) structure. The breakdown voltage VB is further improved due to the FP structure and the low dislocation density. The breakdown voltage of a diode of 60 µm in diameter with the FP structure is over −1000 V, and the leakage current is below 10−9 A until reaching the breakdown voltage. Even in larger diodes (100 and 200 µm in diameter) with FP, the breakdown voltage is over −800 V. However, the specific on‐resistance Ron is high due to damage by the plasma process of sputtering. The specific on‐resistance is further improved due to using a low damage passivation film. As a result, a specific on‐resistance of 1.2 mΩ · cm2 is obtained.
This paper presents electroluminescence intensity mapping on a p–n junction plane of vertical GaN diodes under forward‐biased conditions for the first time. By this mapping, it has been discovered ...that current crowding existed, corresponding to the naturally formed surface stripes on epitaxial layers grown on freestanding GaN substrates. Detailed analyses by AFM and TOF‐SIMS clarified that the concentration of doped Mg acceptors on one slope of the stripe was higher than that on the other slope. The higher Mg‐concentration region should have lower electric resistance, which would cause the current crowding. By improving the surface flatness, the current crowding was suppressed and a low specific on‐resistance was obtained.
Characteristic stripe‐shaped surface morphology often appears on MOVPE‐grown layers at a certain range of c‐axis off‐angle of GaN substrates. The p–n junction diode fabricated on such surface shows a non‐uniform electroluminescence intensity pattern (reflecting its current‐density distribution) under the anode electrode matching with the morphology. This method is a strong tool for evaluating the quality of GaN epitaxial layers.
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