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Transition metal dichalcogenides (TMDC) are currently drawing significant interest from the scientific community as 2D materials that have intrinsically semiconducting bandgaps. One additional advantage of TMDCs for discovering and developing materials with novel electronic, electromechanical, or optoelectronic properties is that both layer composition and registry can be readily tailored. To understand how such tailoring can expand the range of properties, here we used density functional theory calculations to determine the electronic structure and piezoelectric properties of bilayer TMDC heterostructures based on MoX2 and WX2, where X can be S, Se, or Te. For identical layers with no misorientation with respect to one another, we find that the registry of the two layers can change the bandgap type (direct vs indirect), as well as its value (by ≈0.25 eV). We report similar conclusions for bilayer heterostructures in which the composition of the two layers is different. Interlayer registry also has a pronounced effect on piezoelectric properties as the piezoelectric coefficients of the two layers either nearly cancel each other or add up to yield enhanced values for the associated TMDC bilayer heterostructures. These results may serve as a guide for enhancing electronic and piezoelectric properties by stacking TMDC layers.

 

Low-energy compute-in-memory architectures promise to reduce the energy demand for computation and data storage. Wurtzite- type ferroelectrics are promising options for both performance and integration with existing semiconductor processes. The Al1-xScxN alloy is among the few tetrahedral materials that exhibit polarization switching, but the electric field required to switch the polarization is too high (few MV/cm). Going beyond binary com- pounds, we explore the search space of multinary wurtzite-type compounds. Through this large-scale search, we identify four prom- ising ternary nitrides and oxides, including Mg2PN3, MgSiN2, Li2SiO3, and Li2GeO3, for future experimental realization and engi- neering. In >90% of the considered multinary materials, we identify unique switching pathways and non-polar structures that are distinct from the commonly assumed switching mechanism in AlN-based materials. Our results disprove the existing design principle based on the reduction of the wurtzite c/a lattice parameter ratio when comparing different chemistries while sup- porting two emerging design principles—ionicity and bond strength. 

Anomalously abrupt nucleation and growth kinetics in polarization switching of wurtzite ferroelectrics are demonstrated. The anomaly inspires an extension of the traditional model to a regime that simultaneous non-linear nucleation and growth occur.

 

A high-speed and high-power current measurement instrument is described for measuring rapid switching of ferroelectric samples with large spontaneous polarization and coercive field. Instrument capabilities (±200 V, 200 mA, and 200 ns order response) are validated with a LiTaO3 single crystal whose switching kinetics are well known. The new instrument described here enables measurements that are not possible using existing commercial measurement systems, including the observation of ferroelectric switching in large coercive field and large spontaneous polarization Al0.7Sc0.3N thin films. 

Ferroelectricity enables key modern technologies from non-volatile memory to precision ultrasound. The first known wurtzite ferroelectric Al 1− x Sc x N has recently attracted attention because of its robust ferroelectricity and Si process compatibility, but the chemical and structural origins of ferroelectricity in wurtzite materials are not yet fully understood. Here we show that ferroelectric behavior in wurtzite nitrides has local chemical rather than extended structural origin. According to our coupled experimental and computational results, the local bond ionicity and ionic displacement, rather than simply the change in the lattice parameter of the wurtzite structure, is key to controlling the macroscopic ferroelectric response in these materials. Across gradients in composition and thickness of 0 < x < 0.35 and 140–260 nm, respectively, in combinatorial thin films of Al 1− x Sc x N, the pure wurtzite phase exhibits a similar c / a ratio regardless of the Sc content due to elastic interaction with neighboring crystals. The coercive field and spontaneous polarization significantly decrease with increasing Sc content despite this invariant c / a ratio. This property change is due to the more ionic bonding nature of Sc–N relative to the more covalent Al–N bonds, and the local displacement of the neighboring Al atoms caused by Sc substitution, according to DFT calculations. Based on these insights, ionicity engineering is introduced as an approach to reduce coercive field of Al 1− x Sc x N for memory and other applications and to control ferroelectric properties in other wurtzites. 

We present a thermodynamic analysis of the recently discovered nitride ferroelectric materials using the classic Landau–Devonshire approach. Electrostrictive and dielectric stiffness coefficients of Al 1− x Sc x N with a wurtzite structure ( 6 mm) are determined using a free energy density function assuming a hexagonal parent phase (6/ mmm), with the first-order phase transition based on the dielectric stiffness relationships. The results of this analysis show that the strain sensitivity of the energy barrier is one order of magnitude larger than that of the spontaneous polarization in these wurtzite ferroelectrics, yet both are less sensitive to strain compared to classic perovskite ferroelectrics. These analysis results reported here explain experimentally reported sensitivity of the coercive field to elastic strain/stress in Al 1− x Sc x N films and would enable further thermodynamic analysis via phase field simulation and related methods. 

Density-functional theory is used to validate spin-resolved and orbital-resolved metrics of localized electronic states to anticipate ferroic and dielectric properties of [Formula: see text] and [Formula: see text] under epitaxial strain. Using previous investigations of epitaxial phase stability in these systems, trends in properties such as spontaneous polarization and bandgap are compared to trends in atomic orbital occupation derived from projected density of states. Based on first principles theories of ferroic and dielectric properties, such as the Modern Theory of Polarization for spontaneous polarization or Goodenough–Kanamori theory for magnetic interactions, this work validates the sufficiency of metrics of localized electronic states to predict trends in multiple ferroic and dielectric properties. Capabilities of these metrics include the anticipation of the transition from G-Type to C-Type antiferromagnetism in [Formula: see text] under 4.2% compressive epitaxial strain and the interval of C-Type antiferromagnetism from 3% to 7% tensile epitaxial strain in [Formula: see text]. The results of this work suggest a capability of localized electronic metrics to predict multiferroic characteristics in the Bi X[Formula: see text] systems under epitaxial strain, with single or mixed B-site occupation.