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Abstract Epitaxial (Ti1−
x Mgx )0.25Al0.75N(0001)/Al2O3(0001) layers are used as a model system to explore how Fermi‐level engineering facilitates structural stabilization of a host matrix despite the intentional introduction of local bonding instabilities that enhance the piezoelectric response. The destabilizing octahedral bonding preference of Ti dopants and the preferred 0.67 nitrogen‐to‐Mg ratio for Mg dopants deteriorate the wurtzite AlN matrix for both Ti‐rich (x < 0.2) and Mg‐rich (x ≥ 0.9) alloys. Conversely,x = 0.5 leads to a stability peak with a minimum in the lattice constant ratioc /a , which is caused by a Fermi‐level shift into the bandgap and a trend toward nondirectional ionic bonding, leading to a maximum in the expected piezoelectric stress constante 33. The refractive index and the subgap absorption decrease withx , the optical bandgap increases, and the elastic constant along the hexagonal axisC 33= 270 ± 14 GPa remains composition independent, leading to an expected piezoelectric constantd 33= 6.4 pC N−1atx = 0.5, which is 50% larger than for the pure AlN matrix. Thus, contrary to the typical anticorrelation between stability and electromechanical coupling, the (Ti1−x Mgx )0.25Al0.75N system exhibits simultaneous maxima in the structural stability and the piezoelectric response atx = 0.5. -
SiC and Ga 2 O 3 are promising wide band gap semiconductors for applications in power electronics because of their high breakdown electric field and normally off operation. However, lack of a suitable dielectric material that can provide high interfacial quality remains a problem. This can potentially lead to high leakage current and conducting loss. In this work, we present a novel atomic layer deposition process to grow epitaxially Mg x Ca 1− x O dielectric layers on 4H-SiC(0001) and β-Ga 2 O 3 $\left( {\bar 201} \right)$ substrates. By tuning the composition of Mg x Ca 1− x O toward the substrate lattice constant, better interfacial epitaxy can be achieved. The interfacial and epitaxy qualities were investigated and confirmed by cross-sectional transmission electron microscopy and X-ray diffraction studies. Mg 0.72 Ca 0.28 O film showed the highest epitaxy quality on 4H-SiC(0001) because of its closest lattice match with the substrate. Meanwhile, highly textured Mg 0.25 Ca 0.75 O films can be grown on β-Ga 2 O 3 $\left( {\bar 201} \right)$ with a preferred orientation of (111).more » « less