ICMCTF 2026 Session MB2-3-ThM: Thin Films for Emerging Electronic and Quantum Photonic Devices III

Nowadays, flexible magnetoelectric (ME) heterostructures comprising lead-free piezoelectric are of considerable interest for commercializing wearable electronic devices such as energy harvesters, nonvolatile memory, implantable medical diagnostics, and sensors. Here, we fabricate a highly flexible, cost-effective, nanostructured magnetic field sensor comprising an AlN/Ni-Mn-In ME heterostructure over Ni foils. The functionality of the AlN/Ni-Mn-In/Ni heterostructure has been investigated by measuring the magnetodielectric MD (%) and magnetoelectric coupling (α_ME) coefficient with Ni-Mn-In thickness, anisotropy, and flexibility. The thickness ratio of piezoelectric AlN (∼400 nm) and magnetostrictive Ni-Mn-In (∼385 nm) layers has been optimized to achieve the high performance of the magnetic sensor. The encapsulation of the Ni-Mn-In layer drastically enhances the performance of the fabricated heterostructure. The highest MD ∼ 2.95% and α_ME∼ 3.2 V/cm·Oe have been recorded with an equal thickness ratio of AlN and Ni-Mn-In layers. It could be ascribed to the large magnetostrictive strain transferred to the AlN piezoelectric layer, which enhances the induced ME voltage. Moreover, the nonzero value of α_ME at zero bias magnetic field has been observed and related to the piezomagnetic coefficient (q) grading in the Ni-Mn-In(+q)/Ni(−q) ferromagnetic system, which enhances the strength of magnetoelectric coupling. The fabricated device has easily detected the ultralow magnetic field of up to or less than ∼1 μT. In addition, the anisotropic functionality of the device has been explained by measuring the magnetodielectric and magnetoelectric characteristics in parallel and perpendicular dc bias fields. Further the surface acoustic wave (SAW) delay line-based piezo resonator has been fabricated over highly flexible AlN/Ni-Mn-In/Kapton for flexible MEMS application. The fabricated device resonates at ~1.40 GHz. The effect of the external magnetic field on the resonance frequency (f_R) of the device has been investigated and tunability (∆fR/fR) ~9% was observed. The device displays high sensitivity of ~0.94 Hz/nT at room temperature.

Keywords:Ferromagnetic shape memory alloys, flexible magnetic sensor, lead-free piezoelectric, magnetostrictive effect, surface acoustic waves (SAW).

Wurtzite AlN-based ternary nitride alloys are a promising platform to realize functional materials, particularly ferroelectrics and optical emitters, that can smoothly integrate with conventional microelectronics. Here, we explore substitution of multiple elements into AlN forming quaternary nitride alloys as a strategy for enabling new multifunctional materials. Through a computationally-guided approach, we successfully predict the phase diagram of these pseudo-ternary heterostructural AlN–ScN–GdN alloys as a function of effective temperature and experimentally grow Al_1−x−ySc_xGd_yN thin films via combinatorial sputter synthesis for the first time. We find that for x + y ≲ 0.35, Al_1−x−ySc_xGd_yN crystallizes into a wurtzite structure which is consistent with the calculated phase diagram. Further, we computationally probe whether co-substitution induces cooperative effects on these alloys’ piezoelectric and ferroelectric properties, finding that it is beneficial for reducing the polarization switching barrier. We also calculate that Al_1−x−ySc_xGd_yN thin films should display ferroelectric switching that could be realized experimentally in the future. This is supported by our experimental measurements of a high optical band gap, enhanced piezoelectric coefficient, and a change in the calculated polarization switching mechanism. Overall, our work sets the foundation toward quaternary wurtzite-nitride-based multifunctional materials, including piezoelectrics, ferroelectrics, and possibly multiferroics.

The metal-insulator transition (MIT) in vanadium dioxide (VO₂) thin films is a topic of great interest for applications in smart windows, memristors, and neuromorphic computing applications. VO₂ thin films are accompanied by substantial changes in the electronic and optical properties across the MIT, and these changes can be induced by external stimuli such as voltage, strain, or temperature. While several studies have shown that flexible and freestanding VO₂ films can be achieved, complex pre- or post-growth processing is required. In this work, we show that direct sputter deposition of VO₂ on flexible Kapton substrates results in a straightforward methodology to achieve flexible MIT films. A pre-deposited Al₂O₃ layer on Kapton enhances film adhesion, and the resulting flexible VO₂ films show up to 4 decades of change in resistance across the MIT. Temperature and substrate-induced strain during growth affect substantially the quality of the films. The resulting VO₂ flexible devices show ultra-low power consumption for resistive switching, up to two orders of magnitude lower than in samples grown on traditional substrates. We also demonstrate that the VO₂ MIT characteristics can be governed by mechanical deformation, resulting in a novel method to induce resistive switching and decrease power consumption. This study reveals a straightforward approach for direct growth of high-quality flexible VO₂ films exhibiting robust MIT, with promising applications in tactile sensors and electromechanical devices.

Funding Acknowledgment: This material is based upon work supported by the Air Force Office of Scientific Research under award number FA9550-22-1-0135.

The emerging hydrogen industry is stimulating efforts in developing new materials for various purposes, including the quest for efficient, sustainable, and low-power hydrogen detectors. Many such devices rely on metal oxide semiconductor materials, which are easily integrable into devices and relatively cheap but suffer from some challenges, such as low sensitivity and selectivity.

This study explores the possibility of exploiting a Glancing Angle (GLAD) sputter deposition of Cu-doped TiO₂ and WO_x films, targeting the enhancement of active surface area and, therefore, sensor sensitivity improvements.

Cu-doped TiO₂ and WO₃ films were deposited using conventional reactive DC magnetron sputtering, employing circular titanium and tungsten targets in a mixture of argon and oxygen. Cu-doping was achieved by using a composite target.Films were post-annealed prior to sensing characterization. The Glancing Angle Deposition (GLAD) technique was employed to induce a characteristic columnar nanostructure, thereby increasing the films' porosity and so leading to a desired increase in active surface area. Multiple parameters were tuned to enhance the sensing response, including the angle of deposition (80°, 85°, 88°), thickness (50–300 nm), and reactive sputtering parameters.

Synthesized films were thoroughly analyzed by SEM and XRD. Sensing response measurements revealed an interesting fact: that neither the surface roughness nor the surface area improves the response to the sensing gas monotonically. In the presented paper, we discuss the geometrical reasons as well as the synthesis parameters that influence the sensing characteristic. The comparison of the two materials, WO₃ and TiO₂, is also given.