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Piezo- and Pyroelectric Materials

The main focus of the project was on a detailed understanding of the piezo- and pyroelectric properties (PE) in thin doped HfO2 layers. These effects were characterized for Si doped HfO2 and a mixed HfxZr1-xO2.

Fig. 1
Fig. 1: PFM point spectroscopy data obtained on Si doped HfO2 films.

Temperature- and field-induced phase transitions in piezo- and pyroelectric nanoscale TiN/Si:HfO2/TiN capacitors with 3.8 to 5.6  mol% Si content are investigated for energy conversion and storage applications. Films with 5.6 mol% Si concentration exhibit an energy storage density of ~40 J/cm3 with a very high efficiency of ~80% over a wide temperature range useful for supercapacitors.
Details of the piezo response are determined by piezo force microscopy (PFM): Static domain structures and polarization dynamics on free and electroded surfaces of silicon doped HfO2 are explored (Fig. 1). Electrical polarization methods, grazing incidence X-ray diffraction, transmission electron microscopy are combined with the piezoresponse force microscopy to obtain insight into the structure, macroscopic and local piezoelectric properties. Variations in the piezoelectric behavior can be related to the polycrystalline structure of the layer with a typical grain size in the order of 20-30 nm. Here, we demonstrate that the properties can be detected on a pristine, poled and electroded surface, providing conclusive evidence to intrinsic behavior of the material.
Furthermore, giant pyroelectric coefficients of up to ​-1300 µC/(m2K) are observed due to temperature dependent ferroelectric to paraelectric phase transitions. The phase transition is experimentally confirmed using in-situ X-ray diffraction.

Fig. 2
Fig. 2: (a) Temperature dependent in-situ X-ray diffraction patterns of 40 nm thick Si-doped HfO2 and (b-g) according polarization-electric field characteristics.

(Fig. 2) This enables pyroelectric energy harvesting with the highest harvestable energy density ever reported of 20.3 J/cm3 per Olsen cycle. Possible applications in infrared sensing were evaluated. Inversely, through the electrocaloric effect an adiabatic temperature change of up to 9.5 K and the highest refrigerant capacity ever reported of 19.6 J/cm3 per cycle is achievable. This might enable energy efficient on-chip electrocaloric cooling devices. The phase transition in HfO2 thin films doped with various dopants and their electrocaloric effect is studied, and broader phase transition could be achieved in acceptor (Al and Gd) doped films.

Fig. 3
Fig. 3: The schematic diagram for the broadening of phase transition in acceptor doped HfO2.

(Fig. 3) From the result, Si and Zr are considered more promising for pyroelectric and electrocaloric applications. Additionally, low cost fabrication of doped HfO2 films is feasible by existing semiconductor process technology.
Future studies will focus on the structural basis of the piezo- and pyroelectric properties and their impact on the switching behavior.




Contact: Dr. Uwe Schroeder


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