Understanding effect of distortions and vacancies in wurtzite AlScN ferroelectric memory materials: Vacancy-induced multiple defect state types and relaxation dependence in transition energy levels

Abstract

Energy-efficient compact alternatives to fully digital computing strategies could be achieved by implementations of artificial neural networks (ANNs) that borrow analog techniques. In-memory computing based on crossbar device architectures with memristive materials systems that execute, in an analog way, multiply-and-accumulate operations prevalent in ANN is a notable example. Ferroelectric (FE) materials are promising candidates for achieving ANN thanks to their excellent down-scalability, improved electrical control, and high energy efficiency. However, it remains challenging to develop a crossbar device architecture using FE materials. The difficulty stems from decreasing the leakage current of FE hardware and, simultaneously, reducing the film thickness for achieving compact systems. Here, we have performed density-functional-theory calculations to investigate the electronic, energy-based, and structural signatures of wurtzite FE material Al0.75Sc0.25N with a nitrogen vacancy ( VN) in different charge states. We find that VN can introduce two defect states, viz., the singlet state above the valence band maximum (VBM) and a triplet state below the conduction band minimum in wurtzite AlScN models. The calculations reveal that the group of transition levels E3+/2+/ E2+/1+ with small formation energies occur at ∼0.78/1.03 eV above the VBM in the wurtzite AlScN with a relaxed configuration, which may shift by a large degree to lower energy levels if atoms surrounding the defect are not fully relaxed. Theoretical studies elucidate the vacancy-enhanced increase in the leakage current utilizing large AlScN supercells. These findings render atomistic insights that can provide a path forward for the design of next-generation portable low-power electronic systems.

Observation of Photodielectric Effect in Aluminum Nitride Piezoelectric Films

“…It is noted that a similar PDE phenomenon was observed in Sc 0.15 Al 0.85 N films (Figure 7a,b), where the change in Δε values of the film after irradiation with different wavelengths of light became significantly larger compared to those of AlN, while the amount of change in tan δ became smaller instead. We propose that Sc doping enables nitrogen vacancies to introduce defect states above the valence band maximum (VBM) 49 and results in a decrease in optical bandgaps. 44 Therefore, photogenerated charges would be trapped by the deep-gap states and then show electron polarization at the trap level.…”

Strain-Controlled Formation Energy and Migration of Nitrogen Vacancy in Al1–xScxN: A First-Principles Study

“…The lattice constant a and c/a ratio of Al 11 Sc 5 N 16 are calculated to be 3.277 Å and 1.553, which are in agreement with the previously reported values. 35,36 Under ab-biaxial strain, all supercells are relaxed with the ion positions and cell shape. Figure S1 in the Supporting Information shows that the lattice parameters a, c, and c/a change with strain.…”

“…In first-principles calculations, SQS is a common method for generating doped structures, as it is used to investigate the properties of doped and disordered structures with representativeness. , In this work, all calculations are performed using the same parameter setup. The lattice constant a and c / a ratio of Al 11 Sc 5 N 16 are calculated to be 3.277 Å and 1.553, which are in agreement with the previously reported values. , Under ab -biaxial strain, all supercells are relaxed with the ion positions and cell shape. Figure S1 in the Supporting Information shows that the lattice parameters a , c , and c / a change with strain.…”

Band Gap and Polarization Tuning of Ion-Doped XNbO3 (X = Li, K, Na, Ag) for Photovoltaic and Energy Storage Applications

“…Finally, it could be mentioned that in order to understand the properties of ferroelectric materials and make a comparative analysis with other theoretical works, it could be of interest to study the intricate interplay of distortions and vacancies in these materials [ 35 ] or to investigate their pyroelectric properties with potential applications in energy harvesting and microelectronic devices [ 36 ].…”