By using the hydride vapor phase epitaxy and a void-assisted separation method, freestanding 3-in GaN substrates were successfully fabricated for the first time, and the process showed an excellent ...reproducibility. A thick GaN layer 3.2
in in diameter was easily separated from the base substrate. No cracks were generated during the separation process. The dislocation density was of the order of 10
6
cm
–2. The carrier density was approximately 1×10
18
cm
–3 and the mobility was 3.4×10
2
cm
2
V
–1
s
–1. The concentrations of impurities, estimated by secondary-ion mass spectrometry, were below the limit of detection, except for Si. The Si concentration was approximately 1×10
18
cm
–3, which is in good agreement with the carrier density.
Current–voltage (I–V) characteristics with variations of voltage sweep speed (vsweep) and changing sweep directions, and alternating current (AC) operation of low-Mg-doped p-GaN Schottky contacts ...were analyzed. In the I–V characteristics, conventional memory effect, due to carrier capture and emission from acceptor-type deep-level defects, was seen for all the measurement conditions. In addition, proportional relationship between vsweep and current, and polarity inversion of the current were observed when the applied voltage was low, which indicates the existence of a displacement current, because the Schottky barrier height is so high and a true current is significantly small. In the AC operations under the square- and sinusoidal-wave input, a conventional rectifying operation was observed when the input voltage was relatively high. However, we found differential operation in the output current when the input voltage was low, where the displacement current was dominant. Such a unique functional operation was achieved by only one Schottky diode.
•Electrical characteristics of low-Mg-doped p-GaN Schottky contacts were analyzed.•Existence of a displacement current was confirmed.•Differential operation in alternating current operations was observed.
Advanced contactless photoelectrochemical etching for GaN was conducted under the condition that the sulfate radicals (SO4 −) as the oxidizing agent were mainly produced from the S2O82− ions by heat. ...The generation rate of SO4 − was determined from the titration curve of the pH in the mixed solutions between KOH (aq.) and K2S2O8 (aq.); it clearly increased with an increase in the S2O82− ion concentration. The highest etching rate of >25 nm min−1 was obtained in the "alkali-free" electrolyte of 0.25 mol dm−3 (NH4)2S2O8 (aq.) at 80 °C, which was approximately 10 times higher than that reported by previous studies.
Forward-current-density J F /forward-voltage V F characteristics of vertical gallium-nitride (GaN) Schottky-barrier diodes (SBDs) and p-n diodes on free-standing GaN substrates were computationally, ...as well as experimentally, investigated. Based on the thermionic emission model, electron-drift mobility μ n was used as a parameter to fit the J F - V F characteristics of both reported and fabricated GaN SBDs. At 300 K, μ n was fitted to be 600 cm 2 /V ·s when electron concentration n was 1 × 10 16 cm -3 and 750 cm 2 /V ·s when n was 5 ×10 15 cm -3 . Accordingly, the theoretical μ n - n curve for a carrier compensation ratio of 0.90 was applied in the case of n-GaN layers on GaN substrates. With respect to GaN p-n diodes, the reported J F - V F characteristics were successfully fitted with dislocation-mediated carrier lifetimes in the high-injection region and with Shockley-Read-Hall lifetimes in the generation-recombination current region.
Attempt to achieve a surface passivation of p-type GaN was conducted on low-Mg-doped p-GaN by employing SiN films depositions by an Ar-plasma-sputtering and a plasma-enhancement chemical vapor ...deposition. Process induced damages were then characterized by using a high-temperature isothermal capacitance transient spectroscopy. A large single peak, likely attributed to acceptor-type surface states, was detected in the as-grown samples. The energy level was measured to be 1.18eV above the valence band edge, which is close to a Ga-vacancy (VGa) reported elsewhere. It was suggested that a small portion of Ga atoms were missing from the surface, and a large density of VGa were created in a few surface layers. The peak intensity was found to significantly decrease by the SiN depositions, irrespective of the deposition methods, and further decreases upon annealing at 800°C. After the SiN deposition and the annealing, the peak intensity decreased: the pure Ga vacancies may transform into complex defects in the course of the SiN deposition and annealing. These results show that the present characterization method with the low-Mg-doped p-GaN Schottky contacts is effective and serves as sensitive characterization of the surface defects.
•Process induced damages on a surface passivation of p-type GaN were characterized.•Acceptor-type single surface-states were detected at 1.18eV from the valence band.•The peak intensity was found to significantly decrease by the SiN depositions.
•Mechanism of In content fluctuation in InGaN LEDs has yet to be understood.•Reported fluctuation is successfully modeled by the kink segregation model.•Equilibrium indium content at the kink site ...decreases with growth temperature.•Relaxation time for the step-edge site decreases with growth temperature.•Fluctuation in alloy semiconductors can be practically predicted.
Reported substrate-misorientation and growth-temperature dependences of alloy composition in metalorganic vapor-phase epitaxially grown c-plane InGaN were successfully analyzed by applying least-squares regression to relationships derived in the kink segregation model. The analysis gave a reasonable result that the equilibrium indium content at the kink site decreased with growth temperature T due to increased desorption of indium adatoms. The analysis also revealed that the relaxation time for the indium content at the step-edge site (i.e., the length of time before the indium content at the step-edge site reaches its equilibrium value) was reduced with increasing T (i.e., 34 ms at 780 °C and 21 ms at 820 °C). The analysis can be improved by more experimental results for various T and contribute to practical predictions of compositional variation in alloy semiconductors.
Vertical structured GaN power devices have recently been attracting a great interest because of their potential on extremely high-power conversion efficiency. This letter describes increased ...breakdown voltages in the vertical GaN p-n diodes fabricated on the free-standing GaN substrates. By applying multiple lightly Si doped n-GaN drift layers to the p-n diode, the record breakdown voltages (V B ) of 4.7 kV combined with low specific differential ON-resistance (R ON ) of 1.7 mΩcm 2 were achieved. With reducing the Si-doping concentration of the top n-GaN drift layer adjacent to the p-n junction using well-controlled metal-organic vapor phase epitaxy systems, the peak electric field at the p-n junction could be suppressed under high negatively biased conditions. The second drift layer with a moderate doping concentration contributed to the low R ON . A Baliga's figure of merit (V B 2 /R ON ) was 13 GW/cm 2 . These are the best values ever reported among those achieved by GaN p-n junction diodes on the free-standing GaN substrates.
Vertical structure GaN power devices fabricated on freestanding GaN substrates have high potentials in ideally efficient energy conversion systems. This letter describes successful acquisition of ...high avalanche capabilities for high breakdown voltage GaN p-n junction diodes. We have already reported the high avalanche capability by applying a punch-through structure in p-GaN layer, however; the structure caused a drawback of increase in on-resistance and turn-on voltage. By applying a newly developed two-step mesa structure consisting of the first inner mesa with partially thinned p-GaN layer and the second outer mesa etched to n-GaN drift layer, the high avalanche capabilities with reversible current-voltage characteristics have been realized at high breakdown voltages of 4.7-4.8 kV without sacrificing the forward I-V characteristics. The two-step mesa structure transferred the position of the peak electric field in the p-n junction from the dry-etch damaged second outer mesa to the first mesa covered by the thinned p-GaN layer, which could lead the mild breakdown. The high avalanche capability was obtained with good reproducibility regardless of the anode electrode diameter. This structure can contribute to the construction of robust power systems.
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