Kohei Shima

Al x Ga 1 − x N -based deep-ultraviolet light-emitting diodes (LEDs) operating at 260–280 nm are a sustainable alternative to mercury discharge lamps for sterilization and disinfection of water, air, and surfaces. For accelerating their practical applications, the degradation mechanisms specific to 275-nm band AlxGa1−xN quantum well (QW) LEDs fabricated on a (0001) sapphire substrate were investigated. The optical output power (Po) showed a two-mode degradation: initial degradation by about 20%–30% within the operating time (top) shorter than 100 h and mid- to long-term slow degradation to about 60%–70% for top > 100 h. For elucidating the cause of the slow degradation, layer-selective time-resolved photoluminescence (TRPL) measurements on the QWs and emission microscopy observations were carried out at various top. Because the spectral lineshape of the electroluminescence did not change remarkably and QWs appear undamaged even after the slow degradation, overall degradation is attributed to the decrease in the carrier injection efficiency (ηinjection) to the QWs, most likely due to progressive depassivation of initially H-passivated preexisting nonradiative recombination centers (NRCs), namely, vacancy clusters comprising a cation vacancy and surrounding several nitrogen vacancies, in a Mg-doped Al0.85Ga0.15N electron blocking layer (EBL). During the slow degradation, the areas where the nonradiative recombination in the EBL took place tended to converge in particular zones, indicating a convergence of current paths in low ηinjection zones. By decreasing the Mg doping concentration in the EBL for the purpose of reducing NRC concentration, a device lifetime exceeded 18 000 h, keeping the TRPL lifetime unchanged at least until 16 000 h.

Roles of electron-beam (EB) irradiation in the incorporation of midgap recombination centers (MGRCs) in n-GaN are investigated using a complementary combination of photoluminescence (PL), time-resolved PL, and positron annihilation spectroscopy. The irradiation energies were 137 keV, 675 keV, and 2 MeV, corresponding to the calculated displacements of only N atoms, Ga and N atoms in a 1:1 ratio, and Ga and N atoms in a 3:1 ratio, respectively. The relative PL intensities at 12 K of the yellow and red luminescence bands associated with carbon impurities and nitrogen-vacancy (VN)-related defects, respectively, normalized to the near-band edge emission intensity, varied with irradiation energy in a manner consistent with the calculated atomic displacement ratios. The PL lifetimes for the near-band edge emission at 293 K decreased with increasing fluence, irrespective of the irradiation energy, indicating that higher fluences resulted in higher MGRC concentrations. The most probable candidates for the dominant MGRCs were vacancy clusters comprising a Ga vacancy (VGa) and one or more VNs, namely, VGa(VN)n, which act as nonradiative recombination centers. While lower irradiation energies generate lower total concentrations of VGa and VN at a constant fluence, larger values of n were obtained at lower irradiation energies, reflecting the irradiation energy-dependent atomic displacement ratios. These results indicate that the structure of vacancy complexes is strongly influenced by irradiation energy and provide a basis for controlled manipulation of intrinsic defects via EB irradiation.

Hexagonal boron nitride (hBN) is a semiconductor that crystallizes in layers of a two-dimensional honeycomb structure based on in-plane three s p 2 covalent bonds between boron and nitrogen, namely monolayer BN (mBN), which are connected by out-of-plane π bonds with a stacking sequence AA′. Since hBN exhibits high quantum efficiency near-band-edge emission at around 5.77 eV (215 nm) in spite of the indirect bandgap, layered BN polytypes have attracted attention as a far UVC light-emitting material, where UVC is an abbreviation for deep-ultraviolet lights with the wavelengths between 100 and 280 nm. Recently, syntheses and characterizations of layered BN polytypes such as mBN, AB-stacked graphitic BN (bBN), ABC-stacked rhombohedral BN (rBN), and even unstable AA-stacked BN have been reported, which included indispensable physical and electronic parameters. In this article, steady-state, temporary-resolved, and spatially-resolved luminescence properties of O ₂ -annealed hBN microcrystals (MCs) and rBN epitaxial films containing bBN segments grown by chemical vapor deposition using carbon-free precursors are reviewed to explore the excellence of s p 2 -bonded, layered BN polytypes. The hBN MCs and rBN epilayers exhibited UVC emissions in the energy range of 5.7–6.0 eV that were identified as phonon replicas of indirect excitons (iXs) with photoluminescence lifetimes ( τ PL ) of a few tens to 54 ps, which most likely represent the midgap recombination lifetimes for each iX reservoir. Because τ PL did not change while the cathodoluminescence intensity increased with temperature above 100 K, i.e. the iX emissions exhibited a retrograde thermal quenching behavior, both high joint density of states of iXs and strong exciton-phonon interaction seem significant for procreating the high quantum efficiency. Equally significant emissions at 6.035 eV at 12 K and 6.0–6.1 eV at 300 K were observed from the bBN segments. The results shown in this article are of importance for promoting BN polytypes in practical application to UVC light-emitting devices.

Spatial distributions of minority carrier lifetimes (τminority) in nanowire-based GaN and InGaN/GaN multiple quantum shells grown on a GaN template were investigated using macroarea time-resolved photoluminescence and spatio-time-resolved cathodoluminescence (STRCL) measurements. The τminority value of the GaN nanowires (approximately 101 ps) was uniformly distributed along the nanowire altitude and was longer than that of the GaN template (approximately 24 ps), reflecting the dislocation-free nature of nanowire structures. The τminority value of the InGaN wells grown under optimized conditions was also uniform along the height (approximately 86 ps). However, the value was more than one order of magnitude shorter than that of the planar c-plane single quantum well and several times shorter than that of the planar m-plane multiple quantum wells. Single τminority components observed for both GaN nanowires and InGaN wells, consistently confirmed by both macroarea time-resolved photoluminescence and STRCL measurements, indicated that spatial variations of NMGRC within individual nanowires or across the chip were likely negligible and that surface recombination effects were insignificant. Based on the quantified τminority, the concentrations of potentially dominant midgap recombination centers (MGRCs) in the GaN nanowires, which are likely divacancies comprising a Ga vacancy (VGa) and a N vacancy (VN) [VGaVN] and/or carbon impurities resulting from extremely low V/III ratios, are estimated to be in the first half of 1016 cm−3. The MGRC concentration in the InGaN wells was estimated to be several times higher than that in the planar m-plane multiple quantum wells. Reducing the MGRC concentrations is essential for achieving highly luminescent InGaN/GaN multiple quantum shells.

In an attempt to realize diamond-based p–n heterojunctions for high-power and high-frequency electronic devices, fabrication and characterization of heterojunctions of wurtzite oxynitride of AlGaON(0001) layers on diamond (111) substrates were performed. Thin layers of oxynitride of AlON (2 nm) and AlGaON (15–100 nm) were fabricated on a diamond (111) substrate using remote RF plasma with a source gas of N2(99%) and O2(1%) and Knudsen effusion sources of Al and Ga, and a solid source of Si (n-type dopant). Atomic force microscopy, reflection high energy electron diffraction, high-resolution transmission electron microscopy (TEM) images, and energy dispersive spectroscopic spectra in TEM of the oxynitride layers showed an epitaxial growth to the substrate with grain sizes of 10–100 nm. X-ray photoelectron spectroscopy results of the AlON (2 nm) and AlGaON (15 nm) layers revealed favorable type-I band alignments toward the diamond substrate. Hall-effect measurements showed an n-type nature of AlGaON(100 nm) layers and ∼1000 times larger conductivity than that of the highest conducting n-type diamond. Thus, the possibility of diamond-based p–n heterojunction for high-power and high-frequency electronic devices is shown.

The impact of heterostructure variations on the AlGaN-based ultraviolet-C light-emitting diodes (UVC-LEDs) device performance and secondary ion mass spectrometry (SIMS) profiles were studied. To improve the crystalline quality of the p-layer adjacent to the active region based on H-atom behavior from SIMS profile, reducing the Mg concentration to values 17 cm⁻³ increased considerably the device’s lifetime. The obtained long-lifetime of the UVC-LED exhibited an output power of 158 mW at 350 mA.

Annealing behaviors of vacancy‐type defects in low‐dose ion‐implanted GaN are studied by positron annihilation. N ⁺ , Al ⁺ , and Si ⁺ ions are implanted with a dose of 1 × 10 ¹² cm ⁻² . For Al‐ and Si‐implanted GaN after annealing at 1100 °C, the depth profiles of the net donor concentration ( N _D ) are close to those for implanted impurities, but N _D is 2–3 times higher than Al or Si concentrations. N _D of N‐implanted GaN is higher than that of an unimplanted sample but is lower than that of the Al‐ and Si‐implanted samples. The origin of donor‐like defects introduced by ion implantation is expected to be N‐vacancy‐related defects. A comparison between depth profiles of vacancy‐type defects and N _D reveals that Ga‐vacancy (V _Ga )‐type defects play a significant role in the annealing behavior of N _D . Photoexcitation of V _Ga ‐type defects and their electron detrapping phenomena are also studied.