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1. From prediction to experimental realization of ferroelectric wurtzite Al1− x Gd x N alloys

   https://doi.org/10.1063/5.0251168  

   Lee, Cheng-Wei; Smaha, Rebecca W; Brennecka, Geoff L; Haegel, Nancy M; Gorai, Prashun; Yazawa, Keisuke (February 2025, APL Materials)

   AlN-based alloys find widespread application in high-power microelectronics, optoelectronics, and electromechanics. The realization of ferroelectricity in wurtzite AlN-based heterostructural alloys has opened up the possibility of directly integrating ferroelectrics with conventional microelectronics based on tetrahedral semiconductors, such as Si, SiC, and III–Vs, enabling compute-in-memory architectures, high-density data storage, and more. The discovery of AlN-based wurtzite ferroelectrics has been driven to date by chemical intuition and empirical explorations. Here, we demonstrate the computationally-guided discovery and experimental demonstration of new ferroelectric wurtzite Al1−xGdxN alloys. First-principles calculations indicate that the minimum energy pathway for switching changes from a collective to an individual switching process with a lower overall energy barrier, at a rare-earth fraction x with x > 0.10–0.15. Experimentally, ferroelectric switching is observed at room temperature in Al1−xGdxN films with x > 0.12, which strongly supports the switching mechanisms in wurtzite ferroelectrics proposed previously [Lee et al., Sci. Adv. 10, eadl0848 (2024)]. This is also the first demonstration of ferroelectricity in an AlN-based alloy with a magnetic rare-earth element, which could pave the way for additional functionalities such as multiferroicity and opto-ferroelectricity in this exciting class of AlN-based materials. 

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2. Thermophysical property measurement of GaN/SiC, GaN/AlN, and AlN/SiC epitaxial wafers using multi-frequency/spot-size time-domain thermoreflectance

   https://doi.org/10.1063/5.0245381  

   Walwil, Husam; Song, Yiwen; Shoemaker, Daniel C; Kang, Kyuhwe; Mirabito, Timothy; Redwing, Joan M; Choi, Sukwon (March 2025, Journal of Applied Physics)

   Gallium nitride (GaN)-based high electron mobility transistors (HEMTs) are essential components in modern radio frequency power amplifiers. In order to improve both the device electrical and thermal performance (e.g., higher current density operation and better heat dissipation), researchers are introducing AlN into the GaN HEMT structure. The knowledge of thermal properties of the constituent layers, substrates, and interfaces is crucial for designing and optimizing GaN HEMTs that incorporate AlN into the device structure as the barrier layer, buffer layer, and/or the substrate material. This study employs a multi-frequency/spot-size time-domain thermoreflectance approach to measure the anisotropic thermal conductivity of (i) AlN and GaN epitaxial films, (ii) AlN and SiC substrates, and (iii) the thermal boundary conductance for GaN/AlN, AlN/SiC, and GaN/SiC interfaces (as a function of temperature) by characterizing GaN-on-SiC, GaN-on-AlN, and AlN-on-SiC epitaxial wafers. The thermal conductivity of both AlN and GaN films exhibits an anisotropy ratio of ∼1.3, where the in-plane thermal conductivity of a ∼1.35 μm thick high quality GaN layer (∼223 W m−1 K−1) is comparable to that of bulk GaN. A ∼1 μm thick AlN film grown by metalorganic chemical vapor deposition possesses a higher thermal conductivity than a thicker (∼1.4 μm) GaN film. The thermal boundary conductance values for a GaN/AlN interface (∼490 MW m-2 K−1) and AlN/SiC interface (∼470 MW m−2 K−1) are found to be higher than that of a GaN/SiC interface (∼305 MW m−2 K−1). This work provides thermophysical property data that are essential for optimizing the thermal design of AlN-incorporated GaN HEMT devices. 

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3. Ferroelectric behavior of sputter deposited Al0.72Sc0.28N approaching 5 nm thickness

   https://doi.org/10.1063/5.0147224  

   Zheng, Jeffrey X.; Fiagbenu, Merrilyn Mercy; Esteves, Giovanni; Musavigharavi, Pariasadat; Gunda, Akhil; Jariwala, Deep; Stach, Eric A.; Olsson, Roy H. (May 2023, Applied Physics Letters)

   Ferroelectric Al1−xScxN has raised much interest in recent years due to its unique ferroelectric properties and complementary metal oxide semiconductor back-end-of-line compatible processing temperatures. Potential applications in embedded nonvolatile memory, however, require ferroelectric materials to switch at relatively low voltages. One approach to achieving a lower switching voltage is to significantly reduce the Al1−xScxN thickness. In this work, ferroelectric behavior in 5–27 nm films of sputter deposited Al0.72Sc0.28N has been studied. We find that the 10 kHz normalized coercive field increases from 4.4 to 7.3 MV/cm when reducing the film thickness from 27.1 to 5.4 nm, while over the same thickness range, the characteristic breakdown field of a 12.5 μm radius capacitor increases from 8.3 to 12.1 MV/cm. The 5.4 nm film demonstrates ferroelectric switching at 5.5 V when excited with a 500 ns pulse and a switching speed of 60 ns. 

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4. Bulk-like dielectric and magnetic properties of sub 100 nm thick single crystal Cr2O3 films on an epitaxial oxide electrode

   https://doi.org/10.1038/s41598-020-71619-1  

   Vu, N. M.; Luo, X.; Novakov, S.; Jin, W.; Nordlander, J.; Meisenheimer, P. B.; Trassin, M.; Zhao, L.; Heron, J. T. (December 2020, Scientific Reports)

   null (Ed.)

   Abstract The manipulation of antiferromagnetic order in magnetoelectric Cr 2 O 3 using electric field has been of great interest due to its potential in low-power electronics. The substantial leakage and low dielectric breakdown observed in twinned Cr 2 O 3 thin films, however, hinders its development in energy efficient spintronics. To compensate, large film thicknesses (250 nm or greater) have been employed at the expense of device scalability. Recently, epitaxial V 2 O 3 thin film electrodes have been used to eliminate twin boundaries and significantly reduce the leakage of 300 nm thick single crystal films. Here we report the electrical endurance and magnetic properties of thin (less than 100 nm) single crystal Cr 2 O 3 films on epitaxial V 2 O 3 buffered Al 2 O 3 (0001) single crystal substrates. The growth of Cr 2 O 3 on isostructural V 2 O 3 thin film electrodes helps eliminate the existence of twin domains in Cr 2 O 3 films, therefore significantly reducing leakage current and increasing dielectric breakdown. 60 nm thick Cr 2 O 3 films show bulk-like resistivity (~ 10 12 Ω cm) with a breakdown voltage in the range of 150–300 MV/m. Exchange bias measurements of 30 nm thick Cr 2 O 3 display a blocking temperature of ~ 285 K while room temperature optical second harmonic generation measurements possess the symmetry consistent with bulk magnetic order. 

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5. Sputter deposition of epitaxial manganese boride Mn2B(100) films

   https://doi.org/10.1116/6.0005221  

   Rahman, Sanzida; Shu, Rui; Gall, Daniel (March 2026, Journal of Vacuum Science & Technology A)

   Mn2B thin film deposition is explored because of the predicted promise of Mn2B as directional conductor for interconnects in integrated circuits. MnxB thin films, 40–80 nm thick, are deposited on Al2O3(0001), Al2O3(112¯0), and Al2O3(11¯02) substrates by combined d.c. and r.f. cosputtering from Mn and B targets, respectively. Energy dispersive spectroscopy analyses indicate that the Mn content decreases with increasing deposition temperature Ts = 600–800 °C, which is attributed to Mn evaporation from the substrate and growing film. Raising Ts from 600–700 °C also results in an increase in crystalline quality and surface roughness, while a further increase to Ts = 800 °C causes dewetting that results in discontinuous films. MnxB films on Al2O3(0001) and Al2O3(112¯0) form polycrystalline microstructures with random grain orientations, while Al2O3(11¯02) substrates facilitate epitaxial Mn2B(100) growth with Mn2B(100) || Al2O3(11¯02) and Mn2B[010] || Al2O3[1¯101], as determined by x-ray diffraction methods. However, the epitaxial Mn2B(100) matrix contains MnB or Mn impurity phase grains for films with B-rich or Mn-rich compositions, respectively. The room temperature resistivity ρ of polycrystalline MnxB films decreases from 153 to 70 μΩ cm with increasing x = 1.3–5.6 and is further reduced for epitaxial Mn2B(100)/Al2O3(11¯02) films with measured ρ = 44 and 107 μΩ cm for x = 2.9 and 1.7, respectively. These values are affected by electron scattering at residual impurity grains such that the Mn2B bulk resistivity along the high-conductivity [001]-direction is likely considerably smaller and may be competitive for the envisioned interconnect application. 

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