Hafnium dioxide

Hafnium dioxide
Names
IUPAC name
Hafnium(IV) oxide
Other names
Hafnium dioxide
Hafnia
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.031.818
Properties
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)
insoluble
−23.0·10−6 cm3/mol
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).
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Infobox references

Hafnium(IV) oxide is the inorganic compound with the formula HfO2. Also known as 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.[1] Hafnium dioxide is an intermediate in some processes that give hafnium metal.

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 adopts the same structure as zirconia (ZrO2). Unlike TiO2, which features six-coordinate Ti in all phases, zirconia and hafnia consists of seven-coordinate metal centres. A variety of crystalline phases have been experimentally observed, including cubic (Fm-3m), tetragonal (P42/nmc), monoclinic (P21/c) and orthorhombic (Pbca and Pnma).[2] 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,[3] presumably being the sources of the ferroelectricity recently observed in thin films of hafnia.[4]

Thin films of hafnium oxides, used in modern semiconductor devices, are often deposited with an amorphous structure (commonly by atomic layer deposition). Possible benefits of the amorphous structure have led researchers to alloy hafnium oxide with silicon (forming hafnium silicates) or aluminium, which were found to increase the crystallization temperature of hafnium oxide.[5]

Applications

Hafnia is used in optical coatings, and as a high-κ dielectric in DRAM capacitors and in advanced metal-oxide-semiconductor devices.[6] Hafnium-based oxides were introduced by Intel in 2007 as a replacement for silicon oxide as a gate insulator in field-effect transistors.[7] The advantage for transistors is its high dielectric constant: the dielectric constant of HfO2 is 4–6 times higher than that of SiO2.[8] The dielectric constant and other properties depend on the deposition method, composition and microstructure of the material.

In recent years, hafnium oxide (as well as doped and oxygen-deficient hafnium oxide) attracts additional interest as a possible candidate for resistive-switching memories[9] and CMOS-compatible ferroelectric memory.[10] [11]

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.[12]

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.[13]

References

  1. ↑ Bersch, Eric; et al. "Band offsets of ultrathin high-k oxide films with Si". Phys. Rev. B. 78: 085114. doi:10.1103/PhysRevB.78.085114.
  2. ↑ Table III, V. Miikkulainen; et al. (2013). "Crystallinity of inorganic films grown by atomic layer deposition: Overview and general trends". Journal of Applied Physics. 113: 021301. doi:10.1063/1.4757907.
  3. ↑ T. D. Huan; V. Sharma; G. A. Rossetti, Jr.; R. Ramprasad (2014). "Pathways towards ferroelectricity in hafnia". Physical Review B. 90: 064111. doi:10.1103/PhysRevB.90.064111.
  4. ↑ T. S. Boscke (2011). "Ferroelectricity in hafnium oxide thin films". Applied Physics Letters. 99: 102903. doi:10.1063/1.3634052.
  5. ↑ 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.
  6. ↑ 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: 7399. doi:10.1007/s10853-012-6568-y.
  7. ↑ Intel (11 November 2007). "Intel's Fundamental Advance in Transistor Design Extends Moore's Law, Computing Performance".
  8. ↑ Review article by Wilk et al. in the Journal of Applied Physics, Table 1
  9. ↑ K.-L. Lin; et al. (2011). "Electrode dependence of filament formation in HfO2 resistive-switching memory". Journal of Applied Physics. 109: 084104. doi:10.1063/1.3567915.
  10. ↑ Imec (7 June 2017). "Imec demonstrates breakthrough in CMOS-compatible Ferroelectric Memory".
  11. ↑ FMC (8 June 2017). "World’s first FeFET-based 3D NAND demonstration".
  12. ↑ Very High Temperature Exotic Thermocouple Probes product data, Omega Engineering, Inc., retrieved 2008-12-03
  13. ↑ "Aaswath Raman | Innovators Under 35 | MIT Technology Review". August 2015. Retrieved 2015-09-02.
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