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We report the observation of optical polarization switching in In x Ga 1−x N / GaN quantum well active layers, using semipolar ͕1122͖ planes. When the In composition is less than ϳ30%, the emissions related to the top and second valence bands are polarized along the ͓1100͔ and perpendicular ͓1123͔ directions, respectively, similar to earlier studies. On the contrary, as the In composition increases above 30%, the polarizations switch, indicating a crossover between the two valence bands. Because the polarization degree is less sensitive to the well width, the observed polarization switch is ascribed to the InN deformation potentials.

Large electron capture-cross-section of the major nonradiative recombination centers in Mg-doped GaN epilayers grown on a GaN substrateThe nonradiative lifetime (s NR ) of the near-band-edge emission in various quality GaN samples is compared with the results of positron annihilation measurement, in order to identify the origin and to determine the capture-cross-section of the major intrinsic nonradiative recombination centers (NRCs). The room-temperature s NR of various n-type GaN samples increased with decreasing the concentration of divacancies composed of a Ga vacancy (V Ga ) and a N vacancy (V N ), namely, V Ga V N . The s NR value also increased with increasing the diffusion length of positrons, which is almost proportional to the inverse third root of the gross concentration of all point defects. The results indicate that major intrinsic NRC in n-type GaN is V Ga V N . From the relationship between its concentration and s NR , its hole capture-cross-section is estimated to be about 7 Â 10 À14 cm 2 . Different from the case of 4H-SiC, the major NRCs in p-type and n-type GaN are different: the major NRCs in Mg-doped p-type GaN epilayers are assigned to multiple vacancies containing a V Ga and two (or three) V N s, namely, V Ga (V N ) n (n ¼ 2 or 3). The ion-implanted Mgdoped GaN films are found to contain larger size vacancy complexes such as (V Ga ) 3 (V N ) 3 . In analogy with GaN, major NRCs in Al 0.6 Ga 0.4 N alloys are assigned to vacancy complexes containing an Al vacancy or a V Ga . Published by AIP Publishing. https://doi.

Gibberellin (GA) 20-oxidase (GA20ox) is a key enzyme that normally catalyzes the penultimate steps in GA biosynthesis. One of the GA20ox genes in rice (Oryza sativa L.), OsGA20ox2 ( SD1 ), is well known as the "Green Revolution gene", and loss-of function mutation in this locus causes semi-dwarfism. Another GA20ox gene, OsGA20ox1, has also been identified, but its contribution to plant stature has remained unclear because no suitable mutants have been available. We isolated a mutant, B142, tagged with a T-DNA containing three CaMV 35S promoters, which showed a tall, GA-overproduction phenotype. The final stature of the B142 mutant reflects internode overgrowth and is approximately twice that of its wild-type parent. This mutant responds to application of both GA3 and a GA biosynthesis inhibitor, indicating that it is a novel tall mutant of rice distinct from GA signaling mutants such as slr1 . The integrated T-DNAs, which contain three CaMV 35S promoters, are located upstream of the OsGA20ox1 open reading frame (ORF) in the B142 mutant genome. Analysis of mRNA and the endogenous GAs reveal that biologically active GA level is increased by up-regulation of the OsGA20ox1 gene in B142. Introduction of OsGA20ox1 cDNA driven by 35S promoter into the wild type phenocopies the morphological characteristics of B142. These results indicate that the elongated phenotype of the B142 mutant is caused by up-regulation of the OsGA20ox1 gene. Moreover, the final stature of rice was reduced by specific suppression of the OsGA20ox1 gene expression. This result indicates that not only OsGA20ox2 but also OsGA20ox1 affects plant stature.

Vacancy‐type defects in Mg‐implanted GaN are probed using monoenergetic positron beams. Mg+ ions are implanted to provide a 500‐nm‐deep box profile with Mg concentrations, [Mg], of 1 × 1017–1 × 1019 cm−3 at room temperature. In the as‐implanted samples, the major defect species is a complex of a Ga vacancy (VGa) and a nitrogen vacancy (VN). After annealing above 1000 °C, the major defect species is changed to vacancy clusters due to vacancy agglomeration. This agglomeration is suppressed, and the agglomeration onset temperature is decreased with a decreasing [Mg]. For samples with [Mg] ≥ 1 × 1018 cm−3, the trapping rate of positrons by vacancy‐type defects decrease after annealing above 1100–1200 °C. This decreases is attributed to the change in the defect charge states from neutral to positive due to a downward shift of the Fermi level. The carrier trapping/detrapping properties of the vacancy‐type defects and their time dependences are also revealed.