Shoji Ishibashi

Annealing behavior of vacancies in N‐ and Mg‐implanted GaN was studied using a monoenergetic positron beam. For N‐implanted GaN after ultrahigh‐pressure annealing at 1100 °C, the major defect species were identified as vacancy‐type defects coupled with hydrogen atoms (such as H‐V _Ga V _N , V _Ga : Ga‐vacancy, V _N : N‐vacancy). The positron trapping rate of vacancies was increased by illumination at photon energies above 1.5 eV, which was attributed to electron capture by vacancy complexes, resulting in a charge transfer of the defects from positive to neutral (or neutral to negative). The illumination‐active defects consisted of more V _N than detectable vacancy complexes in darkness. For Mg‐implanted GaN with sequential N‐implantation, the diffusion property of Mg depended on the N implantation doses. The sequential N implantation caused recombination of vacancy complexes with excess N, which resulted in the reduction of the size of vacancies. This modification of vacancy species was attributed to the origin of suppressed Mg migration in the vacancy‐rich region.

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.

We present novel electro-optic (EO) molecular crystals suitable for future optoelectronic integration that combine high EO activity, thin-film processability, and thermal stability.

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.

Annealing behaviors of vacancy-type defects in Mg and N-implanted GaN were studied by positron annihilation. The major defect species in as-implanted samples was identified as Ga-vacancy (VGa)-type defects. For Mg-implanted GaN with sequential N-implantation after annealing above 10000C, the defect species were vacancy clusters such as (VGaVN)3. Due to the downward shift of the Fermi level position resulting from a partial activation of Mg, the charge states of defects tended to become positive. For N-implanted GaN, the size of the vacancy cluster started to decrease above 12000C annealing, which was attributed to recombinations between VNs coupled with VGas and excess N atoms. The impact of sequential N-implantations on vacancies in Mg-implanted GaN was found to be most pronounced when the ratio of the concentration of N to that of Mg was three.

Dielectric crystals with switchable electric polarizations represent the key functional materials utilized for a broad range of practical applications. They allow for academically intriguing platforms, where the use of a strong external electric field can potentially unveil hidden crystal phases. Proton–π-electron correlated bistable systems turn out to be promising for exploring such electrically induced crystal polymorphisms, mainly because strong π-electronic polarization can be sensitively switched depending on mobile hydrogen locations. Pseudo-symmetry and hydrogen disorder are utilized as clues for the data mining of the Cambridge Structural Database in the search for molecular candidates with novel switchable dielectrics. The polarization hysteresis, electrostriction, and second harmonic generation of the candidates were experimentally evaluated, together with the re-inspection of crystal structure. This feature article highlights the rich variation and competition of some candidate polarization configurations and switching modes in close relation to high and efficient electrical energy storage/discharge, large electrostriction effects, polarization rotations, and multistage switching phenomena. The experimental findings are well-reproduced by the computational optimization of crystal structure and the simulation of the switchable polarization, piezoelectric coefficients, and relative stability for each of the real or hypothetical hydrogen-ordered crystal phases. Effective prediction and strategic design are thereby guaranteed by systematically understanding the appropriate integration of experimental, computational, and data sciences.

The annealing properties of vacancies in HfO2 layers deposited on Si substrates and their role in amorphous-to-crystalline transition were studied with monoenergetic positron beams. HfO2 layers with a thickness of 4.30 nm were fabricated by the atomic layer deposition technique. The major vacancytype defects in these layers were identified as a Hf vacancy (VHf) coupled with multiple oxygen vacancies (VOs) and larger vacancy clusters. After annealing at 500°C, the concentration of vacancy clusters started to increase, which was attributed to the agglomeration of intrinsic open spaces in the amorphous phase upon the phase transition from amorphous to monoclinic crystalline phases. The amorphous-crystalline transition started near the interface between the HfO2 layer and bottom electrodes (TiN). After the crystallization, the concentration of vacancy clusters decreased as the annealing temperature increased.

Roles of Al-vacancy (VAl) complexes on the cathodoluminescence (CL) spectra of Si-doped AlN grown by halide vapor phase epitaxy (HVPE) on a physical-vapor-transported (0001) AlN substrate are described, making a connection with the results of positron annihilation measurements. A combination of HVPE and AlN substrate enabled decreasing deleterious carbon concentration and dislocation density, respectively, thus accentuating the influences of VAl-complexes on the luminescence processes. A low-temperature CL spectrum of unintentionally doped AlN exhibited predominant excitonic emissions at around 6 eV and a marginal deep-state emission band at around 3.7 eV that originates from residual carbon (<1016 cm−3) on nitrogen sites (CN). However, the sample was revealed to contain a considerable amount (∼1017 cm−3) of vacancy clusters, most likely comprising a VAl and nitrogen-vacancies (VN), namely, VAlVN1−2, which act as nonradiative recombination centers that decrease overall CL intensity at elevated temperatures. With increasing Si-doping concentration ([Si]), major vacancy species progressively changed from VAlVN1−2 to VAlON1−2, where ON is oxygen on N sites, which exhibit other deep-state emission bands ranging from 3.2 to 3.5 eV. Further increase in [Si] gave rise to the formation of donor-compensating defects comprising VAl and Si on the second-nearest-neighbor Al sites (SiAl), abbreviated by VAl−SiAln, which exhibit emission shoulders at around 2.9–3.0 eV. When [Si] exceeded 5 × 1018 cm−3, an emission band at around 4.5 eV emerged, which had been ascribed to originate from the nearest-neighbor SiAlCN complexes. Because VAl-complexes, including those containing impurities, are thermally stable, incorporation of vacancies should be blocked at the growth stage.

Behaviors of vacancies during the amorphous-to-crystalline transition of thin HfO2 layers were studied by monoenergetic positron beams. The HfO2 layers with 4‒30 nm thickness were fabricated on TiN/Si templates by atomic layer deposition technique. The major vacancy-type defects in the layers were identified as (i) Hf vacancies (VHf) coupled with oxygen vacancies (VOs) and (ii) vacancy clusters. After annealing at 500°C, the concentration of vacancy clusters increased, which was attributed to the agglomeration of intrinsic open spaces in the amorphous phase upon the phase transition from amorphous to crystalline phases. The phase transition started near the interface between the HfO2 layer and bottom electrodes (TiN). After the crystallization, the concentration of vacancy clusters decreased as the annealing temperature increased.