Compositional Dependence of Ferroelectric Properties in Thin Film Zn1-xMgxO
Restricted (Penn State Only)
- Author:
- Goodling, Devin
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- December 05, 2022
- Committee Members:
- Jon-Paul Maria, Thesis Advisor/Co-Advisor
Ismaila Dabo, Committee Member
John Mauro, Program Head/Chair
Venkatraman Gopalan, Thesis Advisor/Co-Advisor - Keywords:
- ZnO
ferroelectric
ZnMgO
RF sputtering - Abstract:
- ZnO is a II-VI semiconducting oxide that crystallizes in the wurtzite crystal structure and has been explored in a wide range of electronic applications. It has received increased research interest in the last two decades fueled largely by optoelectronic applications and the discovery of ferroelectricity in Mg substituted ZnO. The discovery of ferroelectricity in Zn1-xMgxO directly follows the discovery of ferroelectricity in the other wurtzite structured materials including Al1-xScxN and Al1-xBxN.1,2,3 These findings have prompted considerable interest in developing Si-compatible ferroelectric memory and devices based on wurtzite ferroelectrics. Previous efforts to incorporate ferroelectrics with Si complementary metal oxide semiconductor (CMOS) technology have experienced some success, but limitations with existing material systems leave room for measurable advances. Ferroelectric materials historically integrated with Si in commercially available microelectronics include lead zirconate titanate (PZT), bismuth titanate (BiT), and strontium bismuth tantalate (SBT).1 However, these materials suffer polarization losses at thin film thicknesses and are not robust against etching processes when creating small feature sizes. These limitations restrict thickness scaling in the devices which incorporate these materials for ferroelectric layers. Orthorhombic hafnia is another promising ferroelectric material which has been discovered and adapted to commercial processes in the last decade. Ferroelectric hafnia shows promise in thickness scaling and films have been successfully scaled below 10 nm in laboratory experiments.4 Features such as these have led to the recent development of commercial devices utilizing ferroelectric hafnia, but limitations still exist. Hafnia suffers from limited polarization retention and high annealing temperatures also exclude some device applications. The existing compatibility of ZnO with CMOS processing could provide a realizable pathway for the adoption of the ferroelectric Zn1-xMgxO in semiconductor devices. This document works to expand the knowledge of the material properties and synthesis science of Zn1-xMgxO to take steps towards this adoption. The adoption of ferroelectric Zn1-xMgxO to Si-based electronic devices would follow the successful adoption of Zn1-xMgxO to several applications in the field of optoelectronics. This wide application space may enable Zn1-xMgxO to be a material of great utility in emerging electronic applications. Structural characterization of ferroelectric thin films is essential for progress to be made towards creating functional devices. Determining the orientation of thin film growth is essential as this defines the polarization direction for a certain crystal structure. Other structural data is useful in defining lattice constants, strain, and film mosaicity, all of which are helpful in defining the crystalline quality of a ferroelectric film. Capacitor stacks in this study consisted of roughly 300 nm (0002)-oriented Zn1-xMgxO deposited on approximately 120 nm (111)-Pt on (0001)-Al2O3 with a 10 nm Ti adhesion layer. Ferroelectric layers were grown using reactive, RF magnetron sputtering. The c-axis oriented Zn1-xMgxO possesses a polarization direction perpendicular to the substrate surface. It is surprising to note that two discrete distributions of lattice constants are apparent in these films. Films with less than 0.16 mole fraction Mg have a c-axis lattice constant larger than 5.26 Å and are well oriented films with full width half maximum (FWHM) values of omega rocking curves about the (0002) diffraction condition of less than 1.5°. In contrast, films with greater than 0.28 mole fraction Mg have a c-axis lattice constant smaller than 5.24 Å and are less well oriented with FWHM values of omega rocking curves exceeding 4.5°. Definitive trends in ferroelectric properties vs. Mg mole fraction were quantified in this work. Zn1-xMgxO remanent polarization was found to decrease from 87 to 75 µC/cm2 when x is varied from 0.16 to 0.40. Over the same composition range, coercive field values increased from 2.85 to 3.39 MV/cm. The breakdown margin or the amount of accessible field over the coercive field also increased monotonically over this composition range. These findings provide insights into the mechanisms leading to ferroelectricity in Zn1-xMgxO and are useful in motivating future experiments.