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For rooting the development of GaN-based optoelectronic devices, understanding the roles of midgap recombination centers (MGRCs), namely, nonradiative recombination centers and deep-state radiative recombination centers, on the carrier recombination dynamics is an essential task. By using the combination of time-resolved photoluminescence and positron annihilation spectroscopy (PAS) measurements, the origins of major MGRCs in the state-of-the-art GaN epilayers, bulk crystals, and Mg-implanted layers were identified, and their concentrations were quantified for deriving the capture coefficients of minority carriers. In this article, potential standardization of the room-temperature photoluminescence lifetime for the near-band-edge emission (τ PL RT ) as the concentration of major MGRCs well below the detection limit of PAS is proposed. For n-GaN substrates and epilayers grown from the vapor phase, τ PL RT was limited by the concentration of carbon on N sites or divacancies comprising a Ga vacancy (VGa) and a N vacancy (VN), [VGaVN], when carbon concentration was higher or lower, respectively, than approximately 1016 cm−3. Here, carbon and VGaVN act as major deep-state radiative and nonradiative recombination centers, respectively, while major MGRCs in bulk GaN crystals were identified as VGa(VN)3 vacancy clusters in Na-flux GaN and VGa or VGaVN buried by a hydrogen and/or VGa decorated with oxygen on N sites, VGa(ON)3–4, in ammonothermal GaN. The values of τ PL RT in n-GaN samples are compared with those of p-GaN, in which τ PL RT was limited by the concentration of VGa(VN)2 in Mg-doped epilayers and by the concentrations of VGaVN and (VGaVN)3 in Mg-implanted GaN right after the implantation and after appropriate activation annealing, respectively. [ABSTRACT FROM AUTHOR]

The anisotropic hole transport along [0001] and [11 2 ¯ 0] in the p-doped (10 1 ¯ 0) GaN layer was compared for layers grown on bulk (10 1 ¯ 0) GaN substrates and on (10 1 ¯ 0) sapphire. The sheet resistance along [0001] was 1.1 times larger on GaN substrates and even 1.2 times larger on sapphire than that along [11 2 ¯ 0]. The anisotropic hole transport on bulk GaN substrates is due to the anisotropy of the hole's effective mass and the different contribution of carriers in different bands, whereas the larger anisotropy for GaN on sapphire is also due to additional scattering at stacking faults. The annealing process of metal Mg applied to the m-plane p-type GaN successfully results in a robust p-type ohmic contact, functioning as a p + + layer. [ABSTRACT FROM AUTHOR]

The precise control of Mg concentration ([Mg]) in p-type GaN layers from 2.3 × 1016 to 2.0 × 1019 cm−3 was demonstrated by halide vapor phase epitaxy (HVPE) on n-type GaN (0001) freestanding substrates. [Mg] in GaN layers could be controlled well by varying the input partial pressure of MgCl2 formed by a chemical reaction between MgO solid and HCl gas under the thermodynamic equilibrium condition. In the sample with [Mg] of 2.0 × 1019 cm−3, a step-bunched surface was observed because the surface migration of Ga adatoms was enhanced by the surfactant effect of Mg atoms. The samples show high structural qualities determined from x-ray rocking curve measurements. The acceptor concentration was in good agreement with [Mg], indicating that almost all Mg atoms act as acceptors. The compensating donor concentrations in the samples were higher than the concentrations of Si, O, and C impurities. We also obtained the Mg acceptor level at a sufficiently low net acceptor concentration of 245 ± 2 meV. These results show that the HVPE method is promising for fabricating GaN vertical power devices, such as n-channel metal–oxide–semiconductor field-effect transistors. [ABSTRACT FROM AUTHOR]

As the size of micro light-emitting diodes (μLEDs) decreases, μLEDs encounter etching damage especially at the sidewalls that critically affects their properties. In this study, we investigated the influence of etching bias power (Pbias) on the performance of μLEDs and found that the current–voltage and light output–current characteristics of μLEDs were enhanced when Pbias was reduced. It was shown that at low Pbias, the chemical reaction between etching gas and gallium nitride, rather than ion sputtering, dominated the etching process, leading to low plasma damage and rough surface morphology. Additionally, to understand the etching-induced surface roughening behaviors, various substrates with different threading dislocation densities were treated at low Pbias. It was found that for the sample (with p-contact size of 10 × 10 μm2), the efficiency droop was approximately 20%, although the current reached 10 mA due most probably to the suppressed polarization effect in the quantum well. It was further observed that the external quantum efficiency (EQE) was dependent on Pbias, where the lowest Pbias yielded the highest maximum EQE, indicating that the plasma damage was mitigated by reducing Pbias. Optimization of dry etching and polarization-suppression conditions could pave the way for realizing high-performance and brightness μLEDs for next-generation displays. [ABSTRACT FROM AUTHOR]

Minority carrier properties in dopant-free p-type distributed polarization doped (DPD) AlGaN layers were investigated on the basis of the forward-biased current density–voltage (J–V) characteristics of p–n+ diodes. The fabricated p-DPD AlGaN/n+–AlGaN:Si diodes exhibited ideal electrical characteristics despite the absence of acceptor atoms in the p-type layer. The extracted Shockley–Read–Hall lifetime exceeded 300 ps, which was longer than that reported for p-GaN:Mg on GaN substrates with a similar acceptor concentration (20–50 ps). Moreover, the electron diffusion coefficient was about 20 cm2 s−1 at any temperature, which was convincing in terms of the electron mobility in DPD layers. The results suggest that p-DPD AlGaN has more desirable minority carrier properties than conventional p-GaN:Mg, particularly for bipolar device applications. [ABSTRACT FROM AUTHOR]

In this study, vertical-type thick BGaN PIN diodes were successfully fabricated to improve the neutron capture rate of BGaN diodes by improving the BGaN epitaxial growth technique. In this technique, 5-μm-thick epitaxial growth was achieved using trimethylboron as a B metal-organic source, which suppressed the gas-phase reaction. The α-particle energy spectrum, the neutron pulse signal, and the residual energy of particles emitted from a neutron capture reaction were measured using the fabricated BGaN diodes by performing radiation detection measurements. The detected signal position in the neutron detection signal spectrum was similar to that of 2.3 MeV α-particles. These results indicate that vertical-type BGaN diodes can be used as effective neutron detectors. [ABSTRACT FROM AUTHOR]