Hafnium (IV) oxide

Crystal structure
__ Hf 4+      __ O 2−
Space group	P 2 1 / c (No. 14)
General
Surname	Hafnium (IV) oxide
other names	Hafnium dioxide, hafnia
Ratio formula	HfO 2
Brief description	colorless solid
External identifiers / databases
CAS number 12055-23-1 EC number 235-013-2 ECHA InfoCard 100,031,818 PubChem 292779 Wikidata Q418740
properties
Molar mass	210.49 g mol −1
Physical state	firmly
density	9.68 g cm −3
Melting point	2812 ° C
boiling point	5400 ° C
Vapor pressure	low
solubility	almost insoluble in water
Refractive index	2.00 (500 nm )
safety instructions
GHS labeling of hazardous substances no GHS pictograms H and P phrases H: no H-phrases P: no P-phrases
As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions . Refractive index: Na-D line , 20 ° C

Hafnium dioxide is a chemical compound made up of the elements hafnium and oxygen . It belongs to the oxide class .

Extraction and presentation

Hafnium dioxide is represented by calcining the hydroxide , oxalate , oxide chloride or sulfate at 600-1000 ° C. The hydrolysis of Hf (OR) ₄ (R = i-amyl) yields high-purity hafnium dioxide.

properties

Hafnium (IV) oxide

In its pure state, hafnium dioxide is a white, microcrystalline powder with a monoclinic crystal structure that has a very high melting and boiling point and a density of 9.68 g · cm ⁻³ . The refractive index is 1.95 to 2.00, the dielectric constant of the amorphous form is in the range from 20 to 25. The oxide is practically insoluble in water and organic solvents.

It has high hardness, low thermal expansion and is chemically very similar to zirconium dioxide (ZrO ₂ ). The compound has a monoclinic crystal structure with the space group P 2 ₁ / c (space group no. 14) . At 1720 ° C it changes into a modification with a tetragonal crystal system with the space group P 4 ₂ / nmc (No. 137) and at 2600 ° C into a cubic form with the space group Fm 3 m (No. 225) . The tetragonal shape can be stabilized by doping (for example with Si, Ge, Sn, Ti, P, Al). The shapes also differ in the dielectric constant. The monoclinic shape has a dielectric constant of 16-18, the tetragonal shape of 30 and the cubic shape of 70. Some sources also report a further orthorhombic shape.

Ferroelectric hafnium oxide

A ferroelectric orthorhombic crystal phase can be generated in thin dielectric hafnium dioxide layers by means of doping and the generation of layer voltages . Due to the widespread use of hafnium oxide as a high- k dielectric and its very good CMOS compatibility, there are new applications as semiconductor memories (FeFET, FRAM , FeCap). Such ferroelectric hafnium oxide layers enable the production of very fast and energy-efficient non-volatile semiconductor memories, especially for use in mobile devices and the Internet of Things . The first ferroelectric memories based on the ferroelectric compound lead-zirconate-titanate (PZT) could not gain acceptance due to a lack of scalability and the difficult integration of lead into the CMOS process. Ferroelectric hafnium oxide enables a significant reduction in the structure size compared to PZT using already established materials, so that ferroelectric field effect transistors (FeFET) with a gate length of 28 nm could already be manufactured.

Formation of the ferroelectric crystal phase

Amorphous thin hafnium dioxide layers (<20 nm) produced with atomic layer deposition crystallize in the non-ferroelectric monoclinic phase after tempering . With the help of metal electrodes that enclose the hafnium dioxide layer, a voltage of several GPa can be induced in the layer . This enables the transformation of the monoclinic crystal phase into the orthorhombic and tetragonal phases when it cools down . This transformation can be supported by doping the layer with, for example, silicon , yttrium and zirconium . The orthorhombic phase has a non-centrosymmetric crystal axis . This enables the position of oxygen ions to be changed between two stable lattice sites in the crystal lattice through the action of an electric field and generates a dielectric displacement current ( polarization ). The influence of the layer tension on the crystal phase decreases with increasing layer thickness, so that thick layers crystallize in the monoclinic phase despite metal electrodes and the remanent polarization decreases. Thin layers in the 10 to 15 nm range, on the other hand, have the best ferroelectric properties. With these layer thicknesses, however, the influence of a possible leakage current through the layer should not be neglected.

Zirconia-doped hafnium oxide

The mixed oxide from the monoclinic hafnium oxide and zirconium oxide shows ferroelectric behavior over a wide mixing range of ~ 25-75% zirconium oxide in the hafnium oxide. A maximum remanent polarization is achieved with 50% zirconium oxide. Starting with pure hafnium oxide, with increasing admixture of zirconium, a phase transformation takes place from the paraelectric monoclinic phase of the hafnium oxide into the ferroelectric orthorhombic phase of the hafnium zirconium oxide. A further increase in the zirconium concentration leads to a further phase center transformation into the antiferroelectric tetragonal phase, which the pure zirconium oxide also shows.

use

Hafnium dioxide is used in semiconductor production as a high-k dielectric or as a coating or mirror material in the optical industry.

Due to its increased dielectric constant compared to silicon dioxide , hafnium dioxide can be used as a material, for. B. in the production of semiconductor components with 45 nm structure replace the silicon dioxide previously used to manufacture CMOS-compatible memories and FeFETs.

Individual evidence

1. ↑ ^{a b c d e f g h} data sheet Hafnium (IV) oxide, Spectrographic Grade, 99.9% (metals basis excluding Zr), Zr typically <80ppm from AlfaAesar, accessed on December 7, 2019 ( PDF )(JavaScript required) .
2. ↑ ltschem.com: Hafnium Oxide ( Memento of the original from March 4, 2016 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. , accessed November 2, 2014. 
3. ↑ ^{a b c} Georg Brauer (Ed.): Handbook of Preparative Inorganic Chemistry . 3., reworked. Edition. tape II . Enke, Stuttgart 1978, ISBN 3-432-87813-3 , p. 1371 .
4. ^ Robert Ruh, HJ Garrett, RF Domagala, NM Tallan: The System Zirconia-Hafnia. In: Journal of the American Ceramic Society. Volume 51, 1968, pp. 23-27.
5. ↑ Pan Kwi Park, Sang-Won Kang: Enhancement of dielectric constant in HfO [sub 2] thin films by the addition of Al [sub 2] O [sub 3]. In: Applied Physics Letters. 89, 2006, p. 192905, doi : 10.1063 / 1.2387126 .
6. ↑ MN Jones, YW Kwon, DP Norton: Dielectric constant and current transport for HfO2 thin films on ITO. In: Applied Physics A. 81, 2005, pp. 285-288, doi : 10.1007 / s00339-005-3208-2 .
7. ^ Daniel Cunningham: "A First-Principles Examination of Dopants in HfO2". In: Honors Scholar Theses. Paper 359. University of Connecticut, February 5, 2014, p. 25 , accessed February 8, 2015 . 
8. ↑ P. Rauwel and E. Rauwel: Probing the Electronic Structure of HfO ₂ polymorphs via Electron Energy Loss Spectroscopy , accessed on February 8, 2015.
9. ↑ ^{a b} Ferroelectricity in hafnium oxide thin films . In: Applied Physics Letters . tape 99 , no. 10 , September 5, 2011, p. 102903 , doi : 10.1063 / 1.3634052 .
10. ↑ J. Müller, E. Yurchuk, T. Schlösser, J. Paul, R. Hoffmann: Ferroelectricity in HfO ₂ enables nonvolatile data storage in 28 nm HKMG . In: 2012 Symposium on VLSI Technology (VLSIT) . June 1, 2012, p. 25-26 , doi : 10.1109 / VLSIT.2012.6242443 .
11. ↑ Ferroelectric Zr _0.5 Hf _0.5 O ₂ thin films for nonvolatile memory applications . In: Applied Physics Letters . tape 99 , no. 11 , September 12, 2011, p. 112901 , doi : 10.1063 / 1.3636417 .
12. ↑ ferroelectricity in yttrium-doped hafnium oxide . In: Journal of Applied Physics . tape 110 , no. 11 , December 1, 2011, p. 114113 , doi : 10.1063 / 1.3667205 .
13. ^ Terence Mittmann, Franz PG Fengler, Claudia Richter, Min Hyuk Park, Thomas Mikolajick: Optimizing process conditions for improved Hf1 - xZrxO2 ferroelectric capacitor performance . In: Microelectronic Engineering (=  Special issue of Insulating Films on Semiconductors (INFOS 2017) ). tape 178 , June 25, 2017, p. 48–51 , doi : 10.1016 / j.mee.2017.04.031 ( sciencedirect.com [accessed June 28, 2017]).
14. ^ Terence Mittmann, Franz PG Fengler, Claudia Richter, Min Hyuk Park, Thomas Mikolajick: Optimizing process conditions for improved Hf1 - xZrxO2 ferroelectric capacitor performance . In: Microelectronic Engineering (=  Special issue of Insulating Films on Semiconductors (INFOS 2017) ). tape 178 , June 25, 2017, p. 48–51 , doi : 10.1016 / j.mee.2017.04.031 ( sciencedirect.com [accessed June 28, 2017]).
15. ↑ NZZ Online: (Still) no limits for Moore's Law , 23 May 2007.
16. ↑ Nivole Ahner: Fully CMOS compatible with HFO2 . Ed .: electronics industry. August 2018.
17. ↑ Ferroelectricity in hafnium oxide: CMOS compatible ferroelectric field effect transistors . IEEE.