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Abstract Epitaxial (Ti1−xMgx)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 constante33. The refractive index and the subgap absorption decrease withx, the optical bandgap increases, and the elastic constant along the hexagonal axisC33= 270 ± 14 GPa remains composition independent, leading to an expected piezoelectric constantd33= 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−xMgx)0.25Al0.75N system exhibits simultaneous maxima in the structural stability and the piezoelectric response atx= 0.5.
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This content will become publicly available on November 1, 2025
Epitaxial AlBN/β‐Nb 2 N Ferroelectric/Superconductor Heterostructures
We report the growth of AlBN/β‐Nb2N nitride epitaxial heterostructures in which the AlBN is ferroelectric, and β‐Nb2N with metallic resistivity ≈40 μ at 300 K becomes superconducting belowTC ≈ 0.5 K. Using nitrogen plasma molecular beam epitaxy, we grow hexagonal β‐Nb2N films on c‐plane Al2O3substrates, followed by wurtzite AlBN. The AlBN is in epitaxial registry and rotationally aligned with the β‐Nb2N, and the hexagonal lattices of both nitride layers make angles of 30° with the hexagonal lattice of the Al2O3substrate. The B composition of the AlBN layer is varied from 0 to 14.7%. It is found to depend weakly on the B flux, but increases strongly with decreasing growth temperature, indicating a reaction rate‐controlled growth. The increase in B content causes a non‐monotonic change in the a‐lattice constant and a monotonic decrease in the c‐lattice constant of AlBN. Sharp, abrupt epitaxial AlBN/β‐Nb2N/Al2O3heterojunction interfaces and close symmetry matching are observed by transmission electron microscopy. The observation of ferroelectricity and superconductivity in epitaxial nitride heterostructures opens avenues for novel electronic and quantum devices.
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- Award ID(s):
- 2039380
- PAR ID:
- 10597663
- Publisher / Repository:
- Wiley VCH
- Date Published:
- Journal Name:
- physica status solidi (RRL) – Rapid Research Letters
- Volume:
- 18
- Issue:
- 11
- ISSN:
- 1862-6254
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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