hafnium(IV) oxide

Hafnia typically adopts the same structure as zirconia (ZrO₂). Unlike TiO₂, which features six-coordinate Ti in all phases, zirconia and hafnia consist of seven-coordinate metal centres. A variety of other crystalline phases have been experimentally observed, including cubic fluorite (Fm{{overline|3}}m), tetragonal (P4₂/nmc), monoclinic (P2₁/c) and orthorhombic (Pbca and Pnma).{{cite journal|at=Table III |author=V. Miikkulainen|display-authors=etal|year=2013 |journal=Journal of Applied Physics |title=Crystallinity of inorganic films grown by atomic layer deposition: Overview and general trends |volume=113 |issue= 2 |doi=10.1063/1.4757907 |bibcode=2013JAP...113b1301M}} It is also known that hafnia may adopt two other orthorhombic metastable phases (space group Pca2₁ and Pmn2₁) over a wide range of pressures and temperatures,{{cite journal |author1=T. D. Huan |author2=V. Sharma |author3=G. A. Rossetti, Jr. |author4=R. Ramprasad |year=2014 |journal=Physical Review B |title=Pathways towards ferroelectricity in hafnia |volume=90 |issue=6 |page=064111 |doi=10.1103/PhysRevB.90.064111|arxiv=1407.1008 |bibcode=2014PhRvB..90f4111H |s2cid=53347579 }} presumably being the sources of the ferroelectricity observed in thin films of hafnia.{{cite journal |author= T. S. Boscke |year=2011 |journal=Applied Physics Letters |title=Ferroelectricity in hafnium oxide thin films |volume=99 |issue=10 |page=102903 |doi=10.1063/1.3634052|bibcode=2011ApPhL..99j2903B}}

Thin films of hafnium oxides deposited by atomic layer deposition are usually crystalline. Because semiconductor devices benefit from having amorphous films present, researchers have alloyed hafnium oxide with aluminum or silicon (forming hafnium silicates), which have a higher crystallization temperature than hafnium oxide.{{cite journal |author=J.H. Choi|year=2011 |journal=Materials Science and Engineering: R |title=Development of hafnium based high-k materials—A review |volume=72 |issue=6 |pages=97–136 |doi=10.1016/j.mser.2010.12.001 |display-authors=etal}}

Hafnia is used in optical coatings, and as a high-κ dielectric in DRAM capacitors and in advanced metal–oxide–semiconductor devices.{{cite journal |author1=H. Zhu |author2=C. Tang |author3=L. R. C. Fonseca |author4=R. Ramprasad |year=2012 |journal=Journal of Materials Science |title=Recent progress in ab initio simulations of hafnia-based gate stacks |volume=47 |issue=21 |pages=7399–7416 |doi= 10.1007/s10853-012-6568-y|bibcode=2012JMatS..47.7399Z |s2cid=7806254 }} Hafnium-based oxides were introduced by Intel in 2007 as a replacement for silicon oxide as a gate insulator in field-effect transistors.{{cite web |author=Intel|title=Intel's Fundamental Advance in Transistor Design Extends Moore's Law, Computing Performance |url=https://www.intel.com/pressroom/archive/releases/2007/20071111comp.htm |date = 11 November 2007 |author-link=Intel Corporation }} The advantage for transistors is its high dielectric constant: the dielectric constant of HfO₂ is 4–6 times higher than that of SiO₂.{{cite journal|doi=10.1063/1.1361065 | volume=89 | issue=10 | title=High-κ gate dielectrics: Current status and materials properties considerations | year=2001 | journal=Journal of Applied Physics | pages=5243–5275 | author1=G. D. Wilk | author2=R. M. Wallace | author3=J. M. Anthony | bibcode=2001JAP....89.5243W }}, Table 1 The dielectric constant and other properties depend on the deposition method, composition and microstructure of the material.

Hafnium oxide (as well as doped and oxygen-deficient hafnium oxide) attracts additional interest as a possible candidate for resistive-switching memories{{cite journal |author=K.-L. Lin|year=2011 |journal=Journal of Applied Physics |title=Electrode dependence of filament formation in HfO2 resistive-switching memory |volume=109 |issue=8 |pages=084104–084104–7 |doi=10.1063/1.3567915 |display-authors=etal|bibcode=2011JAP...109h4104L}} and CMOS-compatible ferroelectric field effect transistors (FeFET memory) and memory chips.{{cite web |author=Imec|title=Imec demonstrates breakthrough in CMOS-compatible Ferroelectric Memory |url=https://www.imec-int.com/en/articles/imec-demonstrates-breakthrough-in-cmos-compatible-ferroelectric-memory |date = 7 June 2017 |author-link=Imec }}{{cite web |author=The Ferroelectric Memory Company|title=World's first FeFET-based 3D NAND demonstration |url=https://ferroelectric-memory.com/2017/06/08/worlds-first-fefet-based-3d-nand-demonstration/ |date = 8 June 2017 }}{{cite book|author=T. S. Böscke |author2=J. Müller |author3=D. Bräuhaus |title=2011 International Electron Devices Meeting |chapter=Ferroelectricity in hafnium oxide: CMOS compatible ferroelectric field effect transistors |publisher=IEEE |doi=10.1109/IEDM.2011.6131606 |pages=24.5.1–24.5.4 |date=7 Dec 2011 |isbn=978-1-4577-0505-2 }}{{Cite book|title=Mit HFO2 voll CMOS-kompatibel|author=Nivole Ahner |publisher=Elektronik Industrie| date=August 2018|language=de}}

Because of its very high melting point, hafnia is also used as a refractory material in the insulation of such devices as thermocouples, where it can operate at temperatures up to 2500 °C.[http://www.omega.co.uk/ppt/pptsc.asp?ref=XTA-W5R26&Nav=tema06 Very High Temperature Exotic Thermocouple Probes product data], Omega Engineering, Inc., retrieved 2008-12-03

Multilayered films of hafnium dioxide, silica, and other materials have been developed for use in passive cooling of buildings. The films reflect sunlight and radiate heat at wavelengths that pass through Earth's atmosphere, and can have temperatures several degrees cooler than surrounding materials under the same conditions.{{Cite web|title = Aaswath Raman {{!}} Innovators Under 35 {{!}} MIT Technology Review|url = http://www.technologyreview.com/lists/innovators-under-35/2015/pioneer/aaswath-raman/|access-date = 2015-09-02|date = August 2015}}

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{{Hafnium compounds}}

{{Oxides}}

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Category:Hafnium compounds

Category:High-κ dielectrics

Category:Transition metal oxides

Category:Ferroelectric materials