| AVS 65th International Symposium & Exhibition | |
| Thin Films Division | Monday Sessions |
| Session TF+EM+MI+PS-MoA |
| Session: | Thin Films for Advanced Memory Applications and Magnetics |
| Presenter: | Adrian Acosta, University of California at Los Angeles |
| Authors: | J. Chang, University of California at Los Angeles A. Acosta, University of California at Los Angeles J.P. Chang, University of California at Los Angeles |
| Correspondent: | Click to Email |
Multiferroic materials that exhibit the coexistence and coupling between ferroelectricity and magnetism are of great interest due to their potential for enabling next-generation memories. To overcome the scarcity and weak response of intrinsic multiferroics, composite strategies were proposed to realize robust multiferroic behavior by coupling the properties from constituent ferroelectric and magnetic phases. However, additional challenges for an applicable multiferroic composite are present in the ferroelectric phase since conventional perovskite-based ferroelectrics lack the necessary electrical stability and silicon-compatibility for device integration.
Orthorhombic ferroelectric HfO2 (FE-HfO2) based thin films have emerged in the field of microelectronics research owing to its superior compatibility with CMOS technology as well as desirable electrical properties. In this work, multiferroic integration of undoped FE-HfO2 thin films and ferrimagnetic CoFe2O4 (CFO) on Si substrates via radical-enhanced atomic layer deposition (RE-ALD) are first demonstrated. For the RE-ALD process, atomic oxygen was utilized in conjunction with TDMAHf and TMHD-based metalorganic precursors for the growth of HfO2 and CFO respectively. In the composite design, CFO acts as a mechanical constraint to stabilize FE-HfO2 as well as an active magnetic layer.
Composite ferroelectricity was studied as a function of FE-HfO2 film thickness as well as post-deposition annealing temperatures. Film crystallinity was investigated through the use of a synchrotron beam source to understand the structural evolution. The induced ferroelectricity was observed to correlate with HfO2 orthorhombic phase and was maximized when HfO2 is ~6 nm and after annealing at ~700-800 °C. CFO/FE-HfO2 composites showed ferroelectric behavior with remnant polarization ~5.5 μC/cm2 and electrical coercivity ~340-2000 kV/cm, with the potential to be further enhanced via the inclusion of dopants. Comparable magnetism was observed with out-of-plane anisotropy, a saturation magnetization of ~155 emu/cm3, and a magnetic coercivity ranging from ~1000-3400 Oe. Piezoresponse force microscopy (PFM) verified the strain interaction in the CFO/FE-HfO2 design. Lastly, a magnetoelectric coupling coefficient of ~5.5×10-8 s/m (~55 Oe cm/kV) was obtained from the multiferroic structure with 6-nm thick HfO2 layer via an ex situ poling SQUID magnetometer setup. This work not only highlights the potential of FE-HfO2 based multiferroic composites in realizing magnetoelectric spintronic devices but also unveils the possibility of utilizing alternative capping layers for achieving multifunctional composite heterostructures.