Hafnium(IV) oxide

Hafnium(IV) oxide is the inorganic compound with the formula HfO
₂. Also known as hafnium dioxide or hafnia, this colourless solid is one of the most common and stable compounds of hafnium. It is an electrical insulator with a band gap of 5.3~5.7 eV.^[2] Hafnium dioxide is an intermediate in some processes that give hafnium metal.

Hafnium(IV) oxide

Names
IUPAC name Hafnium(IV) oxide
Other names Hafnium dioxide Hafnia
Identifiers
CAS Number	12055-23-1 Y
3D model (JSmol)	Interactive image
ChemSpider	258363 Y
ECHA InfoCard	100.031.818
EC Number	235-013-2
PubChem CID	292779
UNII	3C4Z4KG52T Y
CompTox Dashboard (EPA)	DTXSID70893204
InChI InChI=1S/Hf.2O Y Key: CJNBYAVZURUTKZ-UHFFFAOYSA-N Y InChI=1/Hf.2O/rHfO2/c2-1-3 Key: CJNBYAVZURUTKZ-MSHMTBKAAI
SMILES O=[Hf]=O
Properties
Chemical formula	HfO2
Molar mass	210.49 g/mol
Appearance	off-white powder
Density	9.68 g/cm3, solid
Melting point	2,758 °C (4,996 °F; 3,031 K)
Boiling point	5,400 °C (9,750 °F; 5,670 K)
Solubility in water	insoluble
Magnetic susceptibility (χ)	−23.0·10−6 cm3/mol
Thermochemistry
Std enthalpy of formation (ΔfH⦵298)	–1117 kJ/mol[1]
Hazards
Flash point	Non-flammable
Related compounds
Other cations	Titanium(IV) oxide Zirconium(IV) oxide
Related compounds	Hafnium nitride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N verify (what is YN ?) Infobox references

Hafnium(IV) oxide is quite inert. It reacts with strong acids such as concentrated sulfuric acid and with strong bases. It dissolves slowly in hydrofluoric acid to give fluorohafnate anions. At elevated temperatures, it reacts with chlorine in the presence of graphite or carbon tetrachloride to give hafnium tetrachloride.

Structure

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 (Fm3m), tetragonal (P4₂/nmc), monoclinic (P2₁/c) and orthorhombic (Pbca and Pnma).^[3] 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,^[4] presumably being the sources of the ferroelectricity observed in thin films of hafnia.^[5]

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.^[6]

Applications

Hafnia is used in optical coatings, and as a high-κ dielectric in DRAM capacitors and in advanced metal–oxide–semiconductor devices.^[7] Hafnium-based oxides were introduced by Intel in 2007 as a replacement for silicon oxide as a gate insulator in field-effect transistors.^[8] The advantage for transistors is its high dielectric constant: the dielectric constant of HfO₂ is 4–6 times higher than that of SiO₂.^[9] 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^[10] and CMOS-compatible ferroelectric field effect transistors (FeFET memory) and memory chips.^{[11][12][13][14]}

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.^[15]

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.^[16]

References

1. doi:10.1016/0021-9614(75)90076-2
2. Bersch, Eric; et al. (2008). "Band offsets of ultrathin high-k oxide films with Si". Phys. Rev. B. 78 (8): 085114. Bibcode:2008PhRvB..78h5114B. doi:10.1103/PhysRevB.78.085114.
3. V. Miikkulainen; et al. (2013). "Crystallinity of inorganic films grown by atomic layer deposition: Overview and general trends". Journal of Applied Physics. 113 (2). Table III. Bibcode:2013JAP...113b1301M. doi:10.1063/1.4757907.
4. T. D. Huan; V. Sharma; G. A. Rossetti, Jr.; R. Ramprasad (2014). "Pathways towards ferroelectricity in hafnia". Physical Review B. 90 (6): 064111. arXiv:1407.1008. Bibcode:2014PhRvB..90f4111H. doi:10.1103/PhysRevB.90.064111. S2CID 53347579.
5. T. S. Boscke (2011). "Ferroelectricity in hafnium oxide thin films". Applied Physics Letters. 99 (10): 102903. Bibcode:2011ApPhL..99j2903B. doi:10.1063/1.3634052.
6. J.H. Choi; et al. (2011). "Development of hafnium based high-k materials—A review". Materials Science and Engineering: R. 72 (6): 97–136. doi:10.1016/j.mser.2010.12.001.
7. H. Zhu; C. Tang; L. R. C. Fonseca; R. Ramprasad (2012). "Recent progress in ab initio simulations of hafnia-based gate stacks". Journal of Materials Science. 47 (21): 7399–7416. Bibcode:2012JMatS..47.7399Z. doi:10.1007/s10853-012-6568-y. S2CID 7806254.
8. Intel (11 November 2007). "Intel's Fundamental Advance in Transistor Design Extends Moore's Law, Computing Performance".
9. G. D. Wilk; R. M. Wallace; J. M. Anthony (2001). "High-κ gate dielectrics: Current status and materials properties considerations". Journal of Applied Physics. 89 (10): 5243–5275. Bibcode:2001JAP....89.5243W. doi:10.1063/1.1361065., Table 1
10. K.-L. Lin; et al. (2011). "Electrode dependence of filament formation in HfO2 resistive-switching memory". Journal of Applied Physics. 109 (8): 084104–084104–7. Bibcode:2011JAP...109h4104L. doi:10.1063/1.3567915.
11. Imec (7 June 2017). "Imec demonstrates breakthrough in CMOS-compatible Ferroelectric Memory".
12. The Ferroelectric Memory Company (8 June 2017). "World's first FeFET-based 3D NAND demonstration".
13. T. S. Böscke; J. Müller; D. Bräuhaus (7 Dec 2011). "Ferroelectricity in hafnium oxide: CMOS compatible ferroelectric field effect transistors". 2011 International Electron Devices Meeting. IEEE. pp. 24.5.1–24.5.4. doi:10.1109/IEDM.2011.6131606. ISBN 978-1-4577-0505-2.
14. Nivole Ahner (August 2018). Mit HFO2 voll CMOS-kompatibel (in German). Elektronik Industrie.
15. Very High Temperature Exotic Thermocouple Probes product data, Omega Engineering, Inc., retrieved 2008-12-03
16. "Aaswath Raman | Innovators Under 35 | MIT Technology Review". August 2015. Retrieved 2015-09-02.