Roadmap on Energy Harvesting Materials

This section is focused on wurtzitic nitrides, i.e. aluminum nitride (AlN) and aluminum scandium nitride (Al1-xScxN, AlScN) as well as other similar ternary and quaternary AlN-based alloys. Piezoelectric nitrides are attractive for various applications, including the energy harvesting due to low dielectric constants, good mechanical properties, CMOS compatibility, as well as high temperature stability. The main method of synthesis is magnetron sputtering, which is very scalable and inexpensive. Furthermore, in contrast to PZT-like materials, piezoelectric nitrides with non-centrosymmetric wurtzite structure do not need to be poled as they exhibit permanent polarization along the c-axis (Fig.1(a)). Until 2009, when the first paper about enhanced piezoelectric properties of aluminium scandium nitride (AlScN) was published [1], aluminium nitride (AlN) was the main nitride considered for its piezoelectric properties. However, it suffered from relatively low d33 value of ~4-6 pC/N. High acoustic velocity made it worthwhile for RF applications, but in energy harvesting it was not very competitive with lead zirconium titanate (Pb(ZrxTi1-x)O3, PZT)-like materials. Now, with the emergence of AlScN (up to d33 = 31.6 pC/N reported for Al0.59Sc0.41N [2]), nitrides are also being considered a viable option for energy harvesting, especially where CMOS integration plays a role or where high-temperature poling step would damage a soft substrate so PZT can’t be used. Power generation output of AlScN harvesters can reach relatively high values, e.g. >1 mW in the 25-55 Hz range at Sc concentration of 39% [3]. An additional advantage is the possibility for use in high temperature applications, as the Curie temperature of PZT materials is not so high (TC of PZT < 400 °C, much lower for "soft" material) whereas AlN or AlScN are stable at much higher temperatures [4]. AlScN and other ternary or quaternary nitrides with enhanced piezoelectric properties is a relatively new and very actively growing scientific field.