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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]

Temperature-dependent cathodoluminescence spectra were measured for rock salt-structured MgxZn1−xO films with x = 0.95–0.61. The Mg0.95Zn0.05O film exhibited the shortest deep UV peak wavelength of 199 nm (6.24 eV) at 6 K. Relatively high equivalent internal quantum efficiencies of 0.9%–11% were obtained. The Tauc plots, which were obtained from temperature-dependent optical transmittance measurements, exhibited large Stokes-like shifts of 0.7–0.9 eV at 6–300 K. Time-resolved photoluminescence (PL) signals at 7 K exhibited fast and slow decay components. The fast decay component had PL lifetimes of 2.59–3.08 ns, and the slow decay component far exceeded the measurement time range of 12.5 ns. The fast decay constant reflected the transfer lifetime of the photoexcited carriers to certain trapping centers. These centers were tentatively ascribed to Zn-related isoelectronic trapped-hole centers and may be a cause of the large Stokes-like shifts. The signals at 300 K exhibited very short PL lifetimes of 120–180 ps. The PL lifetimes were mainly attributed to the nonradiative recombination lifetime. Simultaneous decreases in the Zn-related isoelectronic trapped-hole centers and the nonradiative recombination centers were found to be necessary to improve the DUV emission properties of RS-MgxZn1−xO films. [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]

Ga-rich zones created along macrosteps in n-AlGaN plausibly function as electron pathways of AlGaN-based deep-ultraviolet (DUV) LEDs fabricated on AlN templates using 1.0°-miscut c(0001) sapphire substrates toward the m[1-100] axis. This work was performed to clarify AlN mole fractions (xAl) of Ga-rich zones. xAl ≃ (7/12, 6/12, and 5/12) was observed in Ga-rich zones in AlαGa1−αN (α ≃ 0.63, 0.55, and 0.43, respectively) by the method proposed in our previous article in which we showed that Ga-rich zones of Al8/12Ga4/12N were created in Al0.7Ga0.3N. xAl in the Ga-rich zones obtained from an energy-dispersive x-ray signal by scanning transmission electron microscopy calibrated by Rutherford backscattering well agreed with xAl obtained by cross-sectional cathodoluminescence (CL) spectroscopy using scanning electron microscopy. A weak CL shoulder peak corresponding to Al4/12Ga8/12N was also observed for Al0.43Ga0.57N. In addition, xAl ≃ n/12 (n = 6–9) in Al-rich zones appeared in the rest of the Ga-rich zones. Furthermore, nanobeam electron diffraction patterns of the Ga-rich zones indicated a high possibility of a regular configuration of Ga and Al atoms on the c(0001) plane in our samples. Consequently, xAl values in nonflat AlGaN layers with macrosteps were often determined to be near n/12 (n: integer). Thus, Ga-rich zones (xAl = n/12: n = 4–8) formed in our nonflat AlGaN layers, which originated from the macrosteps along [11–20] edgelines normal to the m[1–100] axis, are suggested to be metastable. The creation of discrete xAl in Ga-rich zones should contribute to the stable production of DUV-LEDs using high-miscut sapphire substrates. [ABSTRACT FROM AUTHOR]

Vacancy-type defects in AlN films were probed by using monoenergetic positron beams. The AlN films were deposited on sapphire substrates by using a radio-frequency sputtering technique. Epitaxial films grown by metalorganic vapor phase epitaxy on the sputtered AlN films were also characterized. For the sputtered AlN, the major defect species was identified to be complexes between Al-vacancy and oxygen atoms located at nitrogen sites. Vacancy clusters were introduced after annealing at 1300 °C in the N2 atmosphere but their concentration decreased with a higher annealing temperature. The vacancy–oxygen complexes, however, still existed in the AlN film after annealing at 1700 °C. For the AlN epitaxial films, the concentration of vacancy clusters increased as the growth temperature increased up to 1300 °C but it decreased with the post-growth annealing at 1700 °C. [ABSTRACT FROM AUTHOR]

Wurtzite ZnO and related MgxZn1−xO alloys are attractive semiconductors for the use in radiation-resistant and/or visible-light-transparent transistors and ultraviolet light-emitters. As free-carrier lifetime controls the device performances, the accurate understanding of the carrier capture-coefficients of dominant nonradiative recombination channels is essential. In this paper, the hole capture-coefficient (C p) at room temperature of major intrinsic nonradiative recombination centers (NRCs) that commonly exist in various low dislocation density n-type epitaxial films and nearly dislocation-free bulk single crystals of ZnO with and without irradiation by an 8 MeV proton beam is determined. A two-component density functional theory calculation with positron annihilation measurement reveals that major vacancy-type defects are divacancies comprised of a Zn-vacancy and an O-vacancy (VZnVO). Because the weak-excitation nonradiative photoluminescence lifetime (τ NR ) decreases with increasing VZnVO concentration ([VZnVO]), VZnVO are assigned as major NRCs in n-type ZnO. From the relationship between τ NR and [VZnVO], the values of C p and hole capture-cross section of VZnVO are obtained to be 3 × 10 − 7 c m 3 s − 1 and 2 × 10 − 14 c m 2 , respectively, according to the Shockley–Read–Hall approach. These values are an order of magnitude larger than those of 3d transition metals such as Ni or Mn but are comparable to those of major intrinsic NRCs in n-type GaN, i.e., divacancies comprised of a Ga-vacancy and a N-vacancy (VGaVN), being 6 × 10 − 7 c m 3 s − 1 and 7 × 10 − 14 c m 2 , respectively [S. F. Chichibu, A. Uedono, K. Kojima, H. Ikeda, K. Fujito, S. Takashima, M. Edo, K. Ueno, and S. Ishibashi, J. Appl. Phys. 123, 161413 (2018)]. [ABSTRACT FROM AUTHOR]

AlGaN-based deep-ultraviolet light-emitting diodes (LEDs) incorporating uneven multiple quantum wells (MQWs) with inclined and terrace zones, which were fabricated on an AlN template with dense macrosteps, have exhibited a high internal quantum efficiency (IQE). To investigate the microscopic structure of uneven MQWs, cathodoluminescence (CL) mapping characterization was carried out, and the maps of the CL intensity at 300 K relative to that at 38 K were obtained for uneven MQWs that targeted 265 and 285 nm LEDs. At an electron beam current of less than 1.0 nA, the signals from inclined and terrace zones of the uneven MQWs were confirmed to satisfy the nonsaturated excitation condition at 300 K. Nonradiative recombination (NR) was insufficiently frozen even at 38 K, specifically on the terraces in the 265 nm MQW, suggesting high concentrations of NR centers due to point defects (PDs). In contrast, NR in the 285 nm MQW at 38 K was closer to freeze-out. The concentration of PDs in the 285 nm MQW was likely to be lower than that in the 265 nm MQW. Finally, the ratios of the CL intensity at 300 K to those at 38 K were mapped, demonstrating an approach to creating an approximate map of IQE. The values in the CL intensity ratio maps are discussed by considering the analytical error factors. The results support the model of localized current injection through Ga-rich stripe zones in the n-AlGaN cladding layer. [ABSTRACT FROM AUTHOR]

The polarity dependence of the radiation hardness of single-crystalline ZnO bulk crystals is studied by irradiating the Zn-polar and O-polar c-planes with an 8 MeV proton beam up to the fluence of 4.2 × 1016 p/cm2. To analyze the hardness, radiation-induced defects were evaluated using positron annihilation (PA) analysis, and the recovery by post-annealing was examined using continuous-wave photoluminescence (PL) and time-resolved photoluminescence (TRPL) measurements. It was suggested by the PA and PL analyses that the major defects in both polarities were VZnVO divacancies. While the PA data did not show the clear dependence on the polarity, the PL and TRPL results showed that the Zn-polar c-plane had a little higher radiation tolerance than that of the O-polar c-plane, which was consistent with the result that the increase in the electrical resistance by proton beam irradiation was smaller for the former one. Considering these results in total, the polarity dependence is considered to be not so large, but the Zn-polar c-plane has a little higher tolerance than that of the O-polar one. [ABSTRACT FROM AUTHOR]

Energy-dispersive X-ray signals calibrated by Rutherford backscattering indicated the generation of Al13/24Ga11/24N in Ga-rich stripes in a nonflat Al0.58Ga0.42N layer. Also, the CL peak wavelengths of ∼259 and 272 nm also showed the generation of Al15/24Ga9/24N and Al13/24Ga11/24N in Al-rich zones and Ga-rich stripes, respectively. The wavelength of a strong CL peak at ∼246 nm, which was observed from the Al0.7Ga0.3N layer in our previous study, is also considered to correspond to the near-band-emission wavelengths of Al17/24Ga7/24N. In particular, the stronger reproducibility of metastable Al15/24Ga9/24N generation was confirmed, in agreement with the computed predictions by other research groups. [ABSTRACT FROM AUTHOR]

Thermal properties of a red phosphor CaAlSiN3:Eu (CASN) at elevated temperatures were evaluated. A heat treatment at 800°C degraded the photoluminescence property of CASN and caused irreversible changes in both the excitation and emission intensities. The heat treatment in air simultaneously decreased the N elements and increased the O elements. Consequently, the Eu2+ luminescence center was oxidized and CASN lost its photoluminescence property. Although the crystal structure of CASN host was stable even after the heat treatments, the local structure change around the Eu2+ ions is the origin of the thermal degradation of CASN. We found that the heat treatment in N2 atmosphere suppresses the thermal degradation. This is due to the suppression of N evolutions and the incorporation of O elements, which sustains the optically active Eu2+ state. [ABSTRACT FROM AUTHOR]

The strain dependence of the free-exciton resonance energies in AlN epilayers and the values are analyzed by using an appropriated Hamiltonian assuming equibiaxial stress for the wurtzite crystal structure in order to obtain valence band parameters. The strain dependence of the valence band ordering, optical transition probability and free-exciton binding energy are analyzed.