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Notes: Threading dislocations (TDs) in (Al,In,Ga)N semiconductors are known to affect theluminescence efficiency of near-band-edge (NBE) emissions in bulk films and quantum structures.However, the principal role of point defects such as vacancies on the luminescent properties has notbeen fully understood. In this article, impacts of point defects on the luminescence quantumefficiency of NBE emissions and on the intensity of deep emission bands will be described, based onthe results of steady-state and time-resolved photoluminescence (TRPL) and positron annihilationmeasurements. The room temperature nonradiative lifetime (τNR) of the NBE excitonicphotoluminescence (PL) peak in polar (0001) and (000-1) , nonpolar (11-20) and (10-10), andzincblende (001) GaN layers prepared by various growth techniques was shown to increase with thedecrease in concentration or size of Ga vacancies (VGa) and with the decrease in gross concentrationof point defects including complexes, leading to an increase in the NBE PL intensity. As the edge TDdensity decreased, the concentration or size of VGa tended to decrease and τNR tended to increase.However, there existed remarkable exceptions. The results indicate that the nonradiativerecombination process is governed not by single point defects, but by certain defects introduced withthe incorporation of VGa, such as VGa-defect complexes. Similar relations were found in AlxGa1-xNalloy films grown by metalorganic vapor phase epitaxy: i. e. τNR at room temperature increased withthe decrease in the concentration of cation vacancies (VIII) and with the decrease in grossconcentration of point defects. In addition to nonradiative processes, the VIII concentration was foundto correlate with the intensity ratio of characteristic deep emission band to the NBE emission(Ideep/INBE). For example, Ideep/INBE at low temperature for the deep emission bands at 4.6, 3.8, and 3.1eV of AlN epilayers grown by NH3-source molecular beam epitaxy had a linear correlation with theconcentration or size of Al vacancies (VAl). Since the relative intensities of 3.1 eV and 3.8 eV bandsincreased remarkably with lowering the supply ratio of NH3 to Al (V/III ratio) and growthtemperature (Tg), they were assigned to originate from VAl-O as well as VAl-shallow donor complexes.The VAl concentration could be decreased by adjusting the V/III ratio and Tg. In the case of AlxGa1-xNalloys, the concentration or size of VIII and Ideep/INBE at 300 K increased simultaneously with theincrease in x up to approximately 0.7. Similar to the case for GaN and AlN, the deep emission bandwas assigned as being due to the emission involving VIII-O complexes

Notes: To improve the spatial resolution of positron annihilation spectroscopy (PAS), a system to produce an intense positron microbeam was developed in AIST. A slow positron beam, which was produced by an electron linear accelerator, was focused by a lens onto a remoderator to enhance its brightness. The brightness-enhanced beam with an intensity of ≈1 × 106 e+/s was extracted from the remoderator and focused onto the sample by a lens. The beam size at the sample was 25 μm, which is more than two and half orders of magnitude smaller than that in the magnetic transport system (≈10 mm). Hence, the spatial resolution of PAS with an AIST positron microbeam can be drastically improved relative to PAS using conventional methods

Notes: An interlaboratory comparison for positron annihilation lifetime (PAL) measurements for pure nickel and fused silica was performed. Based on the reported positron (for nickel) and positronium (for fused silica) lifetimes, the uncertainties in the PAL measurements were estimated