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The annealing properties of open spaces in 90-nm-thick SiO2 deposited from tetraethylorthosilicate (TEOS) using plasma-enhanced chemical vapor deposition (PECVD) were studied with monoenergetic positron beams. From the lifetime of positronium (Ps) and an empirical model assuming a spherical open space, the mean diameter of open spaces was estimated to be 0.45 nm for PECVD-SiO2 before annealing. In the annealing temperature range below 350 °C, the size of the open spaces and their concentration increased as the temperature increased. Because initial water desorption from PECVD-SiO2 occurred in this temperature range, the observed increases in the size and concentration of spaces were attributed to the detrapping of water from such regions. Above 400 °C annealing, Ps formation was suppressed due to carrier traps introduced by the desorption of gas incorporated during TEOS decomposition. The size of the open spaces reached its maximum value (0.61 nm) after 800 °C annealing and started to decrease above 900 °C. After 1000 °C annealing, although the size of the spaces was close to that in thermally grown SiO2, their concentration remained low, which was attributed to residual impurities in the SiO2 network. [ABSTRACT FROM AUTHOR]

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]

Positron annihilation was used to probe vacancy-type defects in electrodeposited Cu films after nanosecond pulse laser annealing. For the as-deposited Cu film, we identified the coexistence of two different vacancy-type defects, vacancy clusters (such as V16) and monovacancy-type defects, coupled with impurities. An enlargement in the vacancy size was observed after the laser annealing process. The size of these defects was estimated to be close to V30, and such defects could not be formed by conventional furnace annealing. After furnace annealing at 400 °C, the size of the larger vacancy clusters decreased, but that of the smaller vacancies increased. The observed change in the sizes of vacancies is considered to be related to interactions between vacancies and impurities. The depth profile of the defects varied by changing the laser energy density and the number of laser shots. The impact of laser annealing on the vacancy-type defects was observed even after furnace annealing at 800 °C. Because the presence of point defects in electroplated Cu directly correlates with electromigration and grain growth, the ability of laser annealing to introduce large vacancy clusters in the localized region shows the potential of nanosecond laser annealing as a low-thermal budget process tool for back-end-of-line materials. [ABSTRACT FROM AUTHOR]

Vacancy-type defects in AlInN(10 nm)/AlN(1–2 nm)/GaN were probed by using a positron annihilation technique. The crystal quality of the AlInN layer and atomic diffusion near heterointerfaces were also studied by x-ray diffraction reciprocal space mapping, transmission electron microscopy, and energy-dispersive x-ray spectroscopy. For an as-deposited sample without an AlN spacer layer (AlInN/GaN), Ga atoms diffused into the AlInN layer, and as a result, the concentration of Ga-vacancy-type defects in the GaN layer increased. The vacancy concentration was decreased by inserting the AlN layer, which was attributed to the suppression of out-diffusion of Ga from the GaN layer. The effect of the thickness of the AlN layer on the mobility of two-dimensional electron gas is discussed in terms of the introduction of vacancies into the channel region. The annealing behaviors of vacancies in the GaN layer and atomic exchange near heterointerfaces are also discussed. [ABSTRACT FROM AUTHOR]

We carried out atomic-scale observations of Mg-ion-implanted GaN by transmission electron microscopy (TEM) and atom probe tomography (APT) to clarify the crystallographic structures of extended defects and Mg agglomerations that form during post-implantation annealing. The complementary TEM and APT analyses have shown that Mg atoms agglomerate at dislocations that bound extended defects. The concentration of Mg is higher at the dislocations with a larger Burgers vector. This indicates that Mg agglomeration is caused by the pressure at the dislocations. Mg concentration in highly Mg-rich regions is 1 at. %, which exceeds the solubility limit of Mg in GaN. We investigated isothermal and isochronal evolution of the defects by TEM, cathodoluminescence analysis, and positron annihilation spectroscopy. The results indicated that the intensity of donor–acceptor pair emission increases with the annealing temperature and duration and reaches a maximum after elimination of the extended defects with highly Mg-rich regions. These results strongly suggest that such extended defects reduce the acceptor formation and that they as well as the previously reported compensating centers, such as N-related vacancies, can inhibit the formation of p-type GaN. The mechanism by which the extended defects reduce acceptor formation is discussed. [ABSTRACT FROM AUTHOR]

In this study, we performed a systematic search for dopants to stabilize the tetragonal structure of HfO2 and ZrO2 by using high-throughput first-principles calculations. By exhaustively exploring all possible impurity configurations for over 12,000 systems, we obtained the expected most stable doping status for various dopants at different doping levels. The stabilization effect of dopants are investigated and compared. The results show that Si or Ge significantly stabilize the tetragonal phase. [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]

Voids in SiO2 films deposited on GaN were probed by using monoenergetic positron beams. The films were fabricated on GaN substrates by using plasma-enhanced chemical vapor deposition. The size and density of the voids in the films increased up to an annealing temperature of 800 °C and then decreased at 1000 °C. The observed annealing behaviors of the voids were attributed to the desorption of impurities incorporated during the deposition process and the shrinkage of the Si–O matrix by high-temperature annealing. Vacancy-type defects were introduced into the GaN substrate after 1000 °C annealing in O2 atmosphere due to the diffusion of Ga from the substrate to the SiO2 film. No out-diffusion of Ga into the SiO2 film was observed for the annealing in N2 atmosphere. Thus, the observed out-diffusion of Ga was attributed to the enhanced oxidation of GaN during the annealing in O2 atmosphere. The diffusion of positrons implanted into the GaN substrate toward the SiO2 film was suppressed by annealing, suggesting a decrease in the negative charges in the SiO2 film or near the SiO2/GaN interface. [ABSTRACT FROM AUTHOR]

GaN films were grown on hydride vapor phase epitaxy (HVPE) AlN/SiC templates by metalorganic CVD (MOCVD) without annealing the reactor to eliminate the memory effect. A step-terrace structure and smooth surface were obtained for the GaN film, which had a thickness of ∼200 nm. Subsequently, AlGaN/GaN heterostructures for application in high electron mobility transistors (HEMTs) with thin GaN channels were fabricated without a C- or Fe-doped GaN buffer layer. The interface quality at the AlGaN/GaN heterostructure was good enough for a two-dimensional electron gas to exhibit Shubnikov–de Haas oscillation in a magnetic field at 1.8 K. The GaN HEMTs with a thin channel on the AlN/SiC templates exhibited both a pinch-off character and conventional properties. In view of both the shorter epitaxial growth time and higher thermal conduction, HVPE AlN/SiC templates are applicable to the fabrication of GaN HEMTs by MOCVD. [ABSTRACT FROM AUTHOR]