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Wurtzite $(\mathrm{Al},\mathrm{Sc})\mathrm{N}$ferroelectrics are attractive for microelectronics applications due to their chemical and structural compatibility with wurtzite semiconductors, such as $\mathrm{Ga}\mathrm{N}$and $(\mathrm{Al},\mathrm{Ga})\mathrm{N}$. However, the leakage current in epitaxial stacks reported to date should be reduced for reliable device operation. Here, we demonstrate low leakage current in epitaxial ${\mathrm{Al}}_{0.7}{\mathrm{Sc}}_{0.3}\mathrm{N}$films on $\mathrm{Ga}\mathrm{N}$with well-saturated ferroelectric hysteresis loops that are orders of magnitude lower (i.e., 0.07 A ${\mathrm{cm}}^{-2}$) than previously reported films (1–19 A ${\mathrm{cm}}^{-2}$) having similar or better structural characteristics. We also show that, for these high-quality epitaxial $(\mathrm{Al},\mathrm{Sc})\mathrm{N}$films, structural quality (edge and screw dislocations), as measured by diffraction techniques, is not the dominant contributor to leakage. Instead, the small leakage in our films is limited by thermionic emission across the interfaces, which is distinct from the large leakage due to trap-mediated bulk transport in the previously reported $(\mathrm{Al},\mathrm{Sc})\mathrm{N}$films. To support this conclusion, we show that ${\mathrm{Al}}_{0.7}{\mathrm{Sc}}_{0.3}\mathrm{N}$on lattice-matched ${\mathrm{In}}_{0.18}{\mathrm{Ga}}_{0.82}\mathrm{N}$buffers with improved structural characteristics but higher interface roughness exhibit increased leakage characteristics. This demonstration of low leakage current in heteroepitaxial $(\mathrm{Al},\mathrm{Sc})\mathrm{N}$films and understanding of the importance of interface barrier and surface roughness can guide further efforts toward improving the reliability of wurtzite ferroelectric devices. Published by the American Physical Society2025 

Wurtzite-type ferroelectrics have drawn increasing attention due to the promise of better performance and integration than traditional oxide ferroelectrics with semiconductors such as Si, SiC, and III-V compounds. However, wurtzite-type ferroelectrics generally require enormous electric fields, approaching breakdown, to reverse their polarization. The underlying switching mechanism(s), especially for multinary compounds and alloys, remains elusive. Here, we examine the switching behaviors in Al_1−xSc_xN alloys and wurtzite-type multinary candidate compounds we recently computationally identified. We find that switching in these tetrahedrally coordinated materials proceeds via a variety of nonpolar intermediate structures and that switching barriers are dominated by the more-electronegative cations. For Al_1−xSc_xN alloys, we find that the switching pathway changes from a collective mechanism to a lower-barrier mechanism enabled by inversion of individual tetrahedra with increased Sc composition. Our findings provide insights for future engineering and realization of wurtzite-type materials and open a door to understanding domain motion. 

Abstract The development of tandem photovoltaics and photoelectrochemical solar cells requires new absorber materials with bandgaps in the range of ≈1.5–2.3 eV, for use in the top cell paired with a narrower‐gap bottom cell. An outstanding challenge is finding materials with suitable optoelectronic and defect properties, good operational stability, and synthesis conditions that preserve underlying device layers. This study demonstrates the Zintl phosphide compound CaZn₂P₂as a compelling candidate semiconductor for these applications. Phase‐pure, ≈500 nm‐thick CaZn₂P₂thin films are prepared using a scalable reactive sputter deposition process at growth temperatures as low as 100 °C, which is desirable for device integration. Ultraviolet‐visible  spectroscopy shows that CaZn₂P₂films exhibit an optical absorptivity of ≈10⁴ cm⁻¹at ≈1.95 eV direct bandgap. Room‐temperature photoluminescence (PL) measurements show near‐band‐edge optical emission, and time‐resolved microwave conductivity (TRMC) measurements indicate a photoexcited carrier lifetime of ≈30 ns. CaZn₂P₂is highly stable in both ambient conditions and moisture, as evidenced by PL and TRMC measurements. Experimental data are supported by first‐principles calculations, which indicate the absence of low‐formation‐energy, deep intrinsic defects. Overall, this study shall motivate future work integrating this potential top cell absorber material into tandem solar cells. 

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.