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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]

Temperature-dependent cathodoluminescence spectra were measured for rock salt-structured MgxZn1−xO films with x = 0.95–0.61. The Mg0.95Zn0.05O film exhibited the shortest deep UV peak wavelength of 199 nm (6.24 eV) at 6 K. Relatively high equivalent internal quantum efficiencies of 0.9%–11% were obtained. The Tauc plots, which were obtained from temperature-dependent optical transmittance measurements, exhibited large Stokes-like shifts of 0.7–0.9 eV at 6–300 K. Time-resolved photoluminescence (PL) signals at 7 K exhibited fast and slow decay components. The fast decay component had PL lifetimes of 2.59–3.08 ns, and the slow decay component far exceeded the measurement time range of 12.5 ns. The fast decay constant reflected the transfer lifetime of the photoexcited carriers to certain trapping centers. These centers were tentatively ascribed to Zn-related isoelectronic trapped-hole centers and may be a cause of the large Stokes-like shifts. The signals at 300 K exhibited very short PL lifetimes of 120–180 ps. The PL lifetimes were mainly attributed to the nonradiative recombination lifetime. Simultaneous decreases in the Zn-related isoelectronic trapped-hole centers and the nonradiative recombination centers were found to be necessary to improve the DUV emission properties of RS-MgxZn1−xO films. [ABSTRACT FROM AUTHOR]

Polymerization by‐products with the formula (SiCl2)n or SinHxCl2n–x are accumulated at the reactor exhaust of SiC chemical vapor infiltration (CVI) from CH3SiCl3/H2, which occur as an explosive hazard. Thermodynamic constants of the chlorinated silane polymers (SiCl2)n and Cl(SiCl2)mCl (n ≤ 5, m ≤ 4) were obtained by quantum chemical calculations using the CBS‐QB3 method. Polymerization reaction pathways of SiCl2 to (SiCl2)n and Cl(SiCl2)mCl (n ≤ 3, m ≤ 3) were also determined using the CBS‐QB3 method, with higher accuracy than in previous reports. The thermodynamic and kinetic constants were incorporated into an existing elementary reaction model of CH3SiCl3 (methyltrichlorosilane; MTS) reacting with H2 to derive the UT2019 model. This enabled the decomposition of MTS into SiCl2 to be studied. Chemical equilibrium calculations and gas‐phase kinetic simulations were performed. The chemical equilibrium calculations revealed that the chlorinated silane polymers, which are explosive by‐products of CVI of SiC, are unstable in the gas phase at any temperature under reduced pressure conditions, suggesting that their formation could be inhibited by controlling the gas‐phase reactions. The gas‐phase kinetic simulations showed that by‐products with a polymerization degree of 3 are not produced above 400°C, but are rapidly produced in the gas phase below 400°C. These findings also apply to the chemical vapor deposition of Si from chlorinated silane precursors. [ABSTRACT FROM AUTHOR]

We established a kinetic model (the UT2017 model) for chemical vapor deposition of silicon carbide (SiC) from methyltrichlorosilane (CH3SiCl3, MTS)/H2, and quantitatively identified CH2SiCl3 as one of the SiC film‐forming species. In a previous study, we established a kinetic model (the UT2014 model), which reproduced the overall decomposition of MTS, but had not validated it in terms of radicals. In the present study, we first validated the UT2014 model by comparing it with the experimental results of radical production from MTS by Lemieux et al, without H2 present. The UT2014 model did not reproduce the production of CH2Cl from MTS at 1247°C. We found that the reactions of CH2SiCl3 isomerization to CH2ClSiCl2 and CH2ClSiCl2 decomposition to CH2Cl and SiCl2 were important for the production of CH2Cl from MTS. We re‐calculated those constants in pressure‐dependent formulas using the Rice‐Ramsperger‐Kassel‐Marcus method at the CBS‐QB3 level. These chemistries were added to the UT2014 model to yield the UT2017 model, which reproduced the production of radicals, including CH2Cl. Finally, using the UT2017 model, we simulated the gas composition under typical SiC chemical vapor infiltration conditions, which comprised mainly MTS and H2. A comparison of the simulation results with the partial pressure of film‐forming species in our previous report suggested that CH2SiCl3 was the possible film‐forming species of SiC from MTS/H2. For quantitative verification, we estimated the distribution of the partial pressure of CH2SiCl3 in our reactor while considering the consumption of CH2SiCl3 on the SiC surface. The values from this simulation result were almost identical to our experimental results for all positions in the reactor. Thus, we showed quantitatively that CH2SiCl3 is the film‐forming species of SiC. [ABSTRACT FROM AUTHOR]

Seeded ammonothermal growths of a few-mm-thick GaN crystals on a 2 inch diameter c-plane and a 45 mm long m-plane GaN wafers were carried out by using an NH4F mineralizer in a 60 mm diameter Ag-lined autoclave. As a result of dynamic control of the temperature profile, low dislocation density and nearly bowing-free m-plane GaN was grown: i.e. the full-width at half-maximum values for the X-ray rocking curves of the 10 2 reflections were smaller than 28 arcsec and the radius of curvature was estimated to be 1460 m. In addition, its low temperature photoluminescence spectrum exhibited free and neutral donor-bound exciton emission peaks. [ABSTRACT FROM AUTHOR]

Immiscible semiconductors are of premier importance since the source of lighting has been replaced by white light-emitting-diodes (LEDs) composed of thermodynamically immiscible InxGa1−xN blue LEDs and yellow phosphors. For realizing versatile deep-ultraviolet to near-infrared light-emitters, Al1−xInxN alloys are one of the desirable candidates. Here we exemplify the appearance and self-formation sequence of compositional superlattices in compressively strained m-plane Al1−xInxN films. On each terrace of atomically-flat m-plane GaN, In- and Al-species diffuse toward a monolayer (ML) step edge, and the first and second uppermost < 1 - 1 - 20 > cation-rows are preferentially occupied by Al and In atoms, respectively, because the configuration of one In-N and two Al-N bonds is more stable than that of one Al-N and two In-N bonds. Subsequent coverage by next < 1 - 1 - 20 > Al-row buries the < 1 - 1 - 20 > In-row, producing nearly Al0.5In0.5N cation-stripe ordering along [0001]-axis on GaN. At the second Al0.72In0.28N layer, this ordinality suddenly lessens but In-rich and In-poor < 1 - 1 - 20 >-rows are alternately formed, which grow into respective {0001}-planes. Simultaneously, approximately 5-nm-period Al0.70In0.30N/Al0.74In0.26N ordering is formed to mitigate the lattice mismatch along [0001], which grow into approximately 5-nm-period Al0.70In0.30N/Al0.74In0.26N { 10 1 - 2 } superlattices as step-flow growth progresses. Spatially resolved cathodoluminescence spectra identify the emissions from particular structures. [ABSTRACT FROM AUTHOR]

Polymerization by‐products with the formula (SiCl2)n or SinHxCl2n–x are accumulated at the reactor exhaust of SiC chemical vapor infiltration (CVI) from CH3SiCl3/H2, which occur as an explosive hazard. Thermodynamic constants of the chlorinated silane polymers (SiCl2)n and Cl(SiCl2)mCl (n ≤ 5, m ≤ 4) were obtained by quantum chemical calculations using the CBS‐QB3 method. Polymerization reaction pathways of SiCl2 to (SiCl2)n and Cl(SiCl2)mCl (n ≤ 3, m ≤ 3) were also determined using the CBS‐QB3 method, with higher accuracy than in previous reports. The thermodynamic and kinetic constants were incorporated into an existing elementary reaction model of CH3SiCl3 (methyltrichlorosilane; MTS) reacting with H2 to derive the UT2019 model. This enabled the decomposition of MTS into SiCl2 to be studied. Chemical equilibrium calculations and gas‐phase kinetic simulations were performed. The chemical equilibrium calculations revealed that the chlorinated silane polymers, which are explosive by‐products of CVI of SiC, are unstable in the gas phase at any temperature under reduced pressure conditions, suggesting that their formation could be inhibited by controlling the gas‐phase reactions. The gas‐phase kinetic simulations showed that by‐products with a polymerization degree of 3 are not produced above 400°C, but are rapidly produced in the gas phase below 400°C. These findings also apply to the chemical vapor deposition of Si from chlorinated silane precursors. [ABSTRACT FROM AUTHOR]