As the dielectric film thickness shrinks to ~10 nm, some traditional wurtzite piezoelectric materials demonstrate ferroelectricity through element doping. Among them, Sc doped AlN and Mg doped ZnO are the most famous examples. While it is widely acknowledged that the dopant atoms effectively reduce the coercive field, enabling ferroelectric polarization switching, the material genome of these wurtzite (WZ) ferroelectrics is still less understood. In this work, we analyze the features of WZ ferroelectrics, ascribing them to five-coordination (5C) ferroelectrics, which may be compared with 6C ferroelectrics (perovskite-type) and 7C ferroelectrics (hafnia-like). In particular, the exact reason for their adopting the hexagonal WZ structure instead of the zinc blende structure is studied. Emphasis is paid to the degree of ionicity in promoting the hexagonal arrangement, and the phenomenon of layer distance compression is discovered and explained in WZ ferroelectrics. The role of element doping in coercive field reduction is understood within this context.
Ferroelectricity enables key integrated technologies from non-volatile memory to precision ultrasound. Wurtzite ferroelectric Al1-xScxN has recently attracted attention because of its robust ferroelectricity and Si process compatibility in addition to being the first known ferroelectric wurtzite. However, the origin and control of ferroelectricity in wurtzite materials is not yet fully understood. Here we show that the local bond ionicity, rather than simply the change in tetrahedral distortion, is key to controlling the macroscopic ferroelectric response, according to our coupled experimental…
Ferroelectric polarization switching is one common example of a process that occurs via nucleation and growth, and understanding switching kinetics is crucial for applications such as ferroelectric memory. Here we describe and interpret anomalous switching dynamics in the wurtzite nitride thin film ferroelectrics Al0.7Sc0.3N and Al0.94B0.06N using a general model that can be directly applied to other abrupt transitions that proceed via nucleation and growth. When substantial growth and impingement occur while nucleation rate is increasing, such as in these wurtzite ferroelectrics under high el…
Light-facilitated ferroelectric switching in wurtzite crystals
Instituto Politécnico Nacional（国立理工学院）；Institut de Ciència de Materials de Barcelona (ICMAB-CSIC)（巴塞罗那材料科学研究所（ICMAB-CSIC））；Universitat Politècnica de Catalunya（加泰罗尼亚理工大学）；Institució Catalana de Recerca i Estudis Avançats (ICREA)（加泰罗尼亚研究与高等研究学院（ICREA））
The first epitaxial ferroelectric wurtzite film with clear polarization-electric field hysteresis behavior is presented. The coercive field of this epitaxial Al0.7Sc0.3N film on W/c-sapphire substrate is 0.4 +- 0.3 MV cm-1 (8 %) smaller than that of a conventional fiber textured film on a Pt/TiOx/SiO2/Si substrate, attributed to the 0.01 +- 0.007 Å smaller c-axis lattice parameter in the epitaxial film. The strain and decrease of the coercive field most likely originate from epitaxial strain rather than the mismatch in thermal coefficient of expansion. These results provide an insight for furt…
High-Sc Al1-xScxN thin films are of tremendous interest because of their attractive piezoelectric and ferroelectric properties, but overall film quality and reproducibility are widely reported to suffer as x increases. In this study, we correlate the structure and electrical properties of Al0.6Sc0.4N with in-situ observations of glow discharge optical emission during growth. This in-situ technique uses changes in the Ar(I) and N2(I) emission lines of the glow discharge during growth to identify films that subsequently exhibit unacceptable structural and electrical performance. We show that a s…
Origin and Reduction of Coercive Fields in ZnO-based Wurtzite Ferroelectrics
Bredesen Center for Interdisciplinary Research, University of Tennessee（田纳西大学布雷德森跨学科研究中心）；Center for Nanophase Materials Sciences, Oak Ridge National Laboratory（橡树岭国家实验室纳米材料科学中心）；Department of Mechanical Engineering, The Pennsylvania State University（宾夕法尼亚州立大学机械工程系）；Department of Materials Science and Engineering, The Pennsylvania State University（宾夕法尼亚州立大学材料科学与工程系）
Recent experiments demonstrated robust ferroelectricity in Mg-doped ZnO (ZMO) of the wurtzite structure, hinting at a promising strategy to substantially expand the list of ferroelectrics by doping conventional piezoelectrics. We investigate the origin of ferroelectricity in ZMO with first-principles density functional theory (DFT). The general argument that the Mg alloying could soften the ionic potential energy surface of ZMO for polarization reversal is overly simplified. Our DFT calculations reveal that even at a high Mg concentration, the energy difference ($ΔU$) between the polar and non…
Interaction between point defects and vertical inversion domain walls in wurtzite AlN
Université de Toulouse, CNRS, CEMES（图卢兹大学，法国国家科学研究中心，材料科学与化学研究所）；Smart Ferroic Materials Center, Physics Department and Institute for Nanoscience and Engineering, University of Arkansas（阿肯色大学智能铁电材料中心、物理系与纳米科学与工程研究所）；Université Paris-Saclay, CentraleSupélec, CNRS, Laboratoire SPMS（巴黎萨克雷大学，中央理工-高等电力学院，法国国家科学研究中心，SPMS实验室）；Department of Materials Science and Engineering, Tel Aviv University（特拉维夫大学材料科学与工程系）；CEMES-CNRS, Université de Toulouse（材料科学与化学研究所-法国国家科学研究中心，图卢兹大学）
Towards Extended Active Learning for Modelling Ferroelectric Switching: the Need for 'Gold Standards'
International Centre for Quantum and Molecular Structures and the School of Physics, Shanghai University（上海大学量子与分子结构国际中心及物理学院）；University of Technology Sydney（悉尼科技大学）；The University of Queensland（昆士兰大学）；Griffith University（格里菲斯大学）；Materials Genome Institute, International Centre for Quantum and Molecular Structures, Shanghai University（上海大学量子与分子结构国际中心材料基因组研究院）
