Hafnium(IV) oxide is the
inorganic compound
In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as '' inorganic chemi ...
with the
formula . Also known as hafnium dioxide or hafnia, this colourless solid is one of the most common and stable compounds of
hafnium
Hafnium is a chemical element with the symbol Hf and atomic number 72. A lustrous, silvery gray, tetravalent transition metal, hafnium chemically resembles zirconium and is found in many zirconium minerals. Its existence was predicted by D ...
. It is an electrical insulator with a
band gap
In solid-state physics, a band gap, also called an energy gap, is an energy range in a solid where no electronic states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference ( ...
of 5.3~5.7
eV. 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
Hydrofluoric acid is a solution of hydrogen fluoride (HF) in water. Solutions of HF are colourless, acidic and highly corrosive. It is used to make most fluorine-containing compounds; examples include the commonly used pharmaceutical antidepr ...
to give fluorohafnate anions. At elevated temperatures, it reacts with
chlorine
Chlorine is a chemical element with the symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine is ...
in the presence of
graphite
Graphite () is a crystalline form of the element carbon. It consists of stacked layers of graphene. Graphite occurs naturally and is the most stable form of carbon under standard conditions. Synthetic and natural graphite are consumed on la ...
or
carbon tetrachloride
Carbon tetrachloride, also known by many other names (such as tetrachloromethane, also recognised by the IUPAC, carbon tet in the cleaning industry, Halon-104 in firefighting, and Refrigerant-10 in HVACR) is an organic compound with the chemi ...
to give
hafnium tetrachloride.
Structure
Hafnia typically adopts the same structure as
zirconia
Zirconium dioxide (), sometimes known as zirconia (not to be confused with zircon), is a white crystalline oxide of zirconium. Its most naturally occurring form, with a monoclinic crystalline structure, is the mineral baddeleyite. A dopant stab ...
(ZrO
2). 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
Fluorite (also called fluorspar) is the mineral form of calcium fluoride, CaF2. It belongs to the halide minerals. It crystallizes in isometric cubic habit, although octahedral and more complex isometric forms are not uncommon.
The Mohs sca ...
(Fmm), tetragonal (P4
2/nmc), monoclinic (P2
1/c) and orthorhombic (Pbca and Pnma). It is also known that hafnia may adopt two other orthorhombic metastable phases (space group Pca2
1 and Pmn2
1) over a wide range of pressures and temperatures, presumably being the sources of the ferroelectricity observed in thin films of hafnia.
Thin films of hafnium oxides deposited by
atomic layer deposition
Atomic layer deposition (ALD) is a thin-film deposition technique based on the sequential use of a gas-phase chemical process; it is a subclass of chemical vapour deposition. The majority of ALD reactions use two chemicals called precursors (a ...
are usually crystalline. Because
semiconductor
A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way. ...
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.
Applications
Hafnia is used in
optical coatings, and as a
high-κ dielectric The term high-κ dielectric refers to a material with a high dielectric constant (κ, kappa), as compared to silicon dioxide. High-κ dielectrics are used in semiconductor manufacturing processes where they are usually used to replace a silicon di ...
in
DRAM capacitors and in advanced
metal-oxide-semiconductor devices. Hafnium-based oxides were introduced by
Intel
Intel Corporation is an American multinational corporation and technology company headquartered in Santa Clara, California, Santa Clara, California. It is the world's largest semiconductor chip manufacturer by revenue, and is one of the devel ...
in 2007 as a replacement for
silicon oxide Silicon oxide may refer to either of the following:
*Silicon dioxide
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many part ...
as a gate insulator in
field-effect transistors
The field-effect transistor (FET) is a type of transistor that uses an electric field to control the flow of current in a semiconductor. FETs ( JFETs or MOSFETs) are devices with three terminals: ''source'', ''gate'', and ''drain''. FETs cont ...
. The advantage for transistors is its high
dielectric constant
The relative permittivity (in older texts, dielectric constant) is the permittivity of a material expressed as a ratio with the electric permittivity of a vacuum. A dielectric is an insulating material, and the dielectric constant of an insulat ...
: the dielectric constant of HfO
2 is 4–6 times higher than that of SiO
2. 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 and CMOS-compatible ferroelectric field effect transistors (
FeFET memory
A ferroelectric field-effect transistor (Fe FET) is a type of field-effect transistor that includes a ferroelectric material sandwiched between the gate electrode and source-drain conduction region of the device (the channel). Permanent electric ...
) and memory chips.
Because of its very high melting point, hafnia is also used as a refractory material in the insulation of such devices as
thermocouple
A thermocouple, also known as a "thermoelectrical thermometer", is an electrical device consisting of two dissimilar electrical conductors forming an electrical junction. A thermocouple produces a temperature-dependent voltage as a result of th ...
s, where it can operate at temperatures up to 2500 °C.
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.
References
{{oxygen compounds
Hafnium compounds
High-κ dielectrics
Transition metal oxides
Ferroelectric materials