Ferroelectric phase transitions in nanoscale HfO2 films enable giant pyroelectric energy conversion and highly efficient supercapacitors

Temperature- and field-induced phase transitions in ferroelectric 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 similar to 40 J/cm(3) with a very high efficiency of similar to 80% over a wide temperature range useful for super capacitors. Furthermore, giant pyroelectric coefficients of up to -1300 mu C/(m(2) K) are observed due to temperature dependent ferroelectric to paraelectric phase transitions. The broad transition region is related to the grain size distribution and adjustable by the Si content. This strong pyroelectricity yields electrothermal coupling factors k(2) of up to 0.591 which are more than one order of magnitude higher than the best values ever reported. This enables pyroelectric energy harvesting with the highest harvestable energy density ever reported of 20.27 J/cm(3) per Olsen cycle. Possible applications in infrared sensing are discussed. 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/cm(3) per cycle is achievable. This might enable energy efficient on-chip electrocaloric cooling devices. Additionally, low cost fabrication of these films is feasible by existing semiconductor process technology. (C) 2015 Elsevier Ltd. All rights reserved.