Piezoelectric and ferroelectric HfAlN layers for the energy-efficient generation and propagation of high frequency acoustic waves (Acronym: Tsunami)

• Applicants: Professor Dr. Oliver Ambacher; Dr. Simon Fichtner
• Subject Area: Electronic Semiconductors, Components and Circuits, Integrated Systems, Sensor Technology, Theoretical Electrical Engineering ; Synthesis and Properties of Functional Materials ; Communication Technology and Networks, High-Frequency Technology and Photonic Systems, Signal Processing and Machine Learning for Information Technology
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 553113037

The scientific work program of the project is motivated by the constantly increasing requirements for the structural, elastic, and piezo/ferroelectric properties of active layers in acoustic high-frequency filters for mobile communication applications. For high carrier frequencies, the filters are implemented using “bulk acoustic wave” devices (BAW), which have to be particularly space-saving and energy-efficient due to the flat design and the intensive use of smartphones. The aim of the project is the first theoretical and experimental demonstration of piezoelectric and ferroelectric properties of disordered HfAlN alloys with a hexagonal crystal structure. The focus of the basic science project is on the deposition, characterization, and optimization of HfAlN thin films, which have an improved combination of piezoelectric coupling coefficient and elastic stiffness compared to AlN and ScAlN and have an enhanced combination of high remanent polarization and low coercivity. If the piezoelectric stress coefficient (e33) of hexagonal AlN is maximized by the alloy with cubic HfN, the corresponding stiffness coefficient (C33) decreases with the increasing number of Al atoms substituted by Hf. In contrast, with increasing alloy the permittivity (ε33) and the mass density of the disordered wz-HfAlN alloys will increase. These changes in material parameters become particularly drastic when the alloy induces a structural transition from a wurtzite to a rock salt or layered hexagonal crystal lattice. Since the piezoelectric coefficient is squarely included in the coupling coefficient and thus in the figure of merit (FOM) of BAW devices, wz-HfAlN alloys can outperform the state of art defined by wz-ScAlN, especially the energy efficiency of BAWs. However, the accompanying decrease in stiffness leads to both a reduction in the natural frequency and a reduced quality of the devices. If an attempt is made to compensate for the resonance frequency by reducing the thickness of the piezoelectric layer, the quality of the BAW will be reduced. We would like to address this problem in a combination of experiments and simulations by striving to keep the resonance frequency constant by adjusting the layer thickness and thereby maximizing the FOM-relevant factor e33^2⁄ε33 without C33 falling too much. For the first time, we will compare the influence of the alloy of wz-AlN and rs-HfN with the properties of ScAlN solid solution, as well as carefully examine the influence of the phase transitions that occur with different alloys. We theoretically and experimentally analyze the structural, elastic, and piezo/ferroelectric material parameters of HfAlN alloys, determine the BAW-relevant parameters on this basis, and make a recommendation for the selection of mixed crystals and component designs that are best based on our results to further increase the performance of BAWs for mobile communications applications.

• DFG Programme: Research Grants