Ferroelectric, Pyroelectric and Piezoelectric Effects of Hafnia and Zirconia Based Thin Films

Ferroelectric materials are of great interest for several applications. On the one hand, the ferroelectric field effect transistor (FeFET) is a promising candidate for future high density, nonvolatile memory devices. On the other hand, in the recent years the energy related applications such as pyroelectric and piezoelectric energy harvesting as well as electrocaloric cooling and electrostatic energy storage attracted wide interest. The conventional ferroelectric materials such as lead zirconatetitanate (PZT) are not completely CMOS compatible and therefore a high-density integration for memory application could not be realized up to date. Furthermore, PZT has environmental issues due to the contained lead.

Ferroelectric hafnium oxide, which was first reported in 2011, can overcome the mentioned drawbacks of the conventional ferroelectrics, since it is fully CMOS compatible. The ferroelectric phase is stabilized by doping with various dopants. Furthermore, a mixture of hafnium and zirconium oxide (Hf1-xZrxO2) does also stabilize the ferroelectric phase.

In this thesis, hafnia and zirconia based ferroelectrics are deposited by a novel CSD (chemicals solution deposition) process and are characterized in respect to their ferroelectric, piezoelectric and pyroelectric properties. The ferroelectric nature of hafnium oxide is shown for several dopants as well as for Hf1-xZrxO2 with different compositions and for pure ZrO2. Especially in the case of ZrO2 this is very surprising since ZrO2 was studied for many years and for several applications without revealing ferroelectric properties. In contrast to atomic layer deposition (ALD), which is most commonly used for the deposition of hafnia and zirconia based ferroelectric film, the CSD technique is appropriate for deposition of thicker films without a strong reduction of the ferroelectric response. This makes hafnia and zirconia based ferroelectrics suitable for applications, where larger film thicknesses are unavoidable such as piezoelectric and electrocaloric cooling devices.