Aluminum scandium nitride (Al 1 − x Sc _x N) is known for its tremendous enhancement of piezoelectric properties. This material can improve the energy efficiency of a wide range of applications, for example bulk acoustic wave (BAW) resonators. Such devices heat up during operation and are thus sensitive to temperature-dependent material properties. As a result, device characteristics change with temperature, causing undesirable shifts in achievable frequency and performance. These shifts are well known for AlN-based resonators, where the frequency shifts are compensated without increasing the bandwidth of the frequency filter. For the ternary alloy Al 1 − x Sc _x N, many temperature-dependent structural, mechanical and piezoelectric properties have not or have been insufficiently investigated. This review provides an overview of relevant temperature dependent properties of hexagonal wurtzite (x < 0.5) and cubic rock salt Al 1 − x Sc _x N (x > 0.5) crystals up to 2000 K. The properties studied are the thermal expansion coefficient α, the elastic tensor C i j , the piezoelectric coefficient d ₃₃, the piezoelectric stress coefficient e ₃₃ and the permittivity ɛ ₃₃. Based on these material properties, the derived characteristics for BAW resonators are determined from 0 to 1400 K, which are the resonance frequency f _R and the electromechanical coupling coefficient k t 2 .

This study explores the effects of a sputtered Al0.7Sc0.3N layer deposited onto metalorganic chemical vapor deposition-grown Al0.3Ga0.7N/GaN heterostructures, with a focus on structural and electrical properties, including DC characteristics of high-electron-mobility transistors (HEMTs). Structural analysis reveals that a 25 nm AlScN layer preserves surface roughness and barrier/channel interface quality, although relaxation occurs due to non-ideal growth conditions. Capacitance–voltage profiling indicates a sharp and uniform depletion, with reduced hysteresis post-deposition, suggesting effective passivation. Incorporation of AlScN increases the sheet carrier density from 1.4 to 1.89 × 1013 cm−2, although this is accompanied by lower mobility and a modest rise in sheet resistance from 406 to 441 Ω/sq. Devices with a 0.7 μm gate length demonstrate a saturation current of 0.98 A/mm, approximately 30% higher than reference AlGaN/GaN HEMTs. The composite barrier configuration further exhibits reduced leakage and improved insulation, but results in decreased transconductance and a pronounced threshold voltage shift from −0.8 to −7.8 V, attributed to increased barrier thickness. On-resistance also rises from 2 to 2.5 Ω mm likely due to sputter-induced contact damage. Optimization with a thinner 15 nm AlScN layer achieves a favorable balance, enhancing saturation current and soft-breakdown voltage while minimizing drawbacks in transconductance and on-resistance. Overall, sputtered AlScN serves as both a high-quality barrier and passivation layer, yielding enhanced device performance and robustness, and highlighting the future potential of sputter epitaxy to further advance GaN technology.

Lead zirconate titanate (PZT) is a widely used material for applications in microsensors, actuators, and transducers. Due to its high piezoelectric coefficient, large dielectric constant, and strong polarization capability near the morphotropic phase boundary (Zr/Ti ≈ 52/48), it is considered one of the most attractive materials for micro-electromechanical systems (MEMS). These advantageous material properties strongly depend on the PZT layer’s microstructure and crystallinity, which are primarily determined by the choice of seed layer, deposition conditions, and the post-deposition annealing treatment that promotes the formation of the PZT’s perovskite phase. In this contribution, sputter-deposited PZT thin films were crystallized by conventional furnace annealing (CFA) to evaluate the effect of heating/cooling rates (1 °C·min⁻¹–7 °C·min⁻¹) within a temperature range of 450 °C to 700 °C on structural, electrical, and ferroelectric properties, with consideration of the seed layer preparation. We characterized the materials’ properties by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), and measurements of the ferroelectric hysteresis, capacitance, and leakage current. All samples annealed at temperatures of at least 500 °C fully crystallized into the perovskite phase, independently of the heating/cooling rate. The best ferroelectric performance was achieved at 550 °C with a 1 °C·min⁻¹ heating/cooling rate, yielding a saturation polarization of 82.8 µC·cm⁻² and a remnant polarization of 36.9 µC·cm⁻² under a maximum applied field of 300 kV·cm⁻¹.