Strontium tantalum oxynitrides were prepared within the nominal composition range of 1.0 ≤ x ≤ 2.0, where x = Sr/Ta atomic ratio. A gradual structural transition was observed between the perovskite SrTaON and the Ruddlesden-Popper phase SrTaON with increasing SrO content. X-ray diffraction analyses showed that a single-phase perovskite was obtained up to x = 1.1, after which SrTaON gradually appeared at x ≥ 1.25. High-resolution scanning transmission electron microscopy observations identified the gradual intergrowth of a Ruddlesden-Popper SrTaON type planar structure interwoven with the perovskite crystal lattice upon increasing x. The crystal lattice at x = 1.4 was highly defective and consisted primarily of perovskite intergrown with a large amount of the Ruddlesden-Popper phase structure. This Ruddlesden-Popper phase layer intergrowth is a characteristic of an oxynitride perovskite rather than the Ruddlesden-Popper defects previously reported in oxide perovskites. Partial substitution of Ta with Sr was also evident in this perovskite lattice. Just below x = 2, a perovskite-type structure was intergrown as defects in the Ruddlesden-Popper SrTaON. Characterization of SrTaON in ambient air was challenging due to its moisture sensitivity. Thermal analysis demonstrated that this material was relatively stable up to approximately 1400 °C in comparison with SrTaON perovskite, especially under nitrogen. SrTaON could keep its structure in a sealed tube, and some amount of SrCO was observed in XRD after 10 days of exposure to 75% relative humidity under prior ambient conditions. A compact of this material had a relative density of 96% after sintering at 1400 °C under 0.2 MPa of nitrogen, even though a drastic loss of nitrogen was previously reported for a SrTaON perovskite under these same conditions. Postammonolysis of the SrTaON ceramics was not required prior to studying its dielectric behavior. This is in contrast to the SrTaON perovskite, which requires postammonolysis to recover its stoichiometric composition and electrical insulating properties.
The effect of an Al buffer layer on the growth of AlN on a Si (111) substrate was investigated to develop an all-sputtered GaN film on a Si (111) template substrate. The X-ray diffraction method revealed an obvious improvement in the crystallinity of an AlN layer on the initial layer. At the interface structure, AlN film without the Al buffer layer exhibited surface nitridation of the Si surface, which degraded the AlN crystal growth. After investigating various growth conditions such as substrate temperature and layer thickness, we achieved the all-sputtered epitaxial growth of a GaN/AlN/Si substrate. The substrate temperature was below 650 °C, and the total thickness was less than 200 nm, which is beneficial as regards the cost efficiency of the template substrate for nitride semiconductors.
The ability to control
the polarity of an all-sputtered epitaxial
GaN/AlN/Al film on a Si(111) substrate via intermediate oxidization
was investigated. A stable surface of GaN on a Si substrate is a N-terminated
surface (−c surface); hence, for electric device applications,
the Ga-terminated surface (+c surface) is preferable. The GaN/AlN/Al
film on Si(111) showed a −c surface, as confirmed by time-of-flight
low-energy atom scattering spectroscopy (TOFLAS) and X-ray photoelectron
spectroscopy (XPS). The AlN layer was intentionally oxidized via air
exposure during film growth. The GaN surface subjected to the oxidization
process had the +c surface. Secondary-ion mass spectrometry measurements
indicated a high oxygen concentration after the intentional oxidization.
However, the intentional oxidization degraded the crystallinity of
the GaN/AlN layer. By changing the oxidization point and repeating
the GaN/AlN growth, the crystallinity of GaN was recovered. Such polarity
control of GaN on Si grown by sputtering shows strong potential for
the fabrication of large-diameter +c-GaN template substrates at low
cost.
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