Platinum silicide

Platinum silicide
Names

IUPAC name Platinum silicide
Identifiers
CAS Number	12137-83-6^Y
3D model (JSmol)	Interactive image
PubChem CID	9859339
InChI InChI=1S/Pt.Si Key: XRZCZVQJHOCRCR-UHFFFAOYSA-N
SMILES [Si].[Pt]
Properties
Chemical formula	PtSi
Molar mass	223.169 g·mol⁻¹
Appearance	Orthorhombic crystals^[1]
Density	12.4 g/cm³^[1]
Melting point	1,229 °C (2,244 °F; 1,502 K)^[1]
Structure
Crystal structure	Orthorhombic^[2]
Space group	Pnma (No. 62), oP8
Lattice constant	a = 0.5577 nm, b = 0.3587 nm, c = 0.5916 nm
Formula units (Z)	4
Hazards
Flash point	Non-flammable
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N (what is ^YN ?) Infobox references

Chemical compound

Platinum silicide, also known as platinum monosilicide, is the inorganic compound with the formula PtSi. It is a semiconductor that turns into a superconductor when cooled to 0.8 K.^[3]

Structure and bonding

The crystal structure of PtSi is orthorhombic, with each silicon atom having six neighboring platinum atoms. The distances between the silicon and the platinum neighbors are as follows: one at a distance of 2.41 angstroms, two at a distance of 2.43 angstroms, one at a distance of 2.52 angstroms, and the final two at a distance of 2.64 angstroms. Each platinum atom has six silicon neighbors at the same distances, as well as two platinum neighbors, at a distance of 2.87 and 2.90 angstroms. All of the distances over 2.50 angstroms are considered too far to really be involved in bonding interactions of the compound. As a result, it has been shown that two sets of covalent bonds compose the bonds forming the compound. One set is the three center Pt–Si–Pt bond, and the other set the two center Pt–Si bonds. Each silicon atom in the compound has one three center bond and two center bonds. The thinnest film of PtSi would consist of two alternating planes of atoms, a single sheet of orthorhombic structures. Thicker layers are formed by stacking pairs of the alternating sheets. The mechanism of bonding between PtSi is more similar to that of pure silicon than pure platinum or Pt₂Si, though experimentation has revealed metallic bonding character in PtSi that pure silicon lacks.^[4]

Synthesis

Methods

PtSi can be synthesized in several ways. The standard method involves depositing a thin film of pure platinum onto silicon wafers and heating in a conventional furnace at 450–600 °C for a half an hour in inert ambients. The process cannot be carried out in an oxygenated environment, as this results in the formation of an oxide layer on the silicon, preventing PtSi from forming.^[5]

A secondary technique for synthesis requires a sputtered platinum film deposited on a silicon substrate. Due to the ease with which PtSi can become contaminated by oxygen, several variations of the methods have been reported. Rapid thermal processing has been shown to increase the purity of PtSi layers formed.^[6] Lower temperatures (200–450 °C) were also found to be successful,^[7] higher temperatures produce thicker PtSi layers, though temperatures in excess of 950 °C formed PtSi with increased resistivity due to clusters of large PtSi grains.^[8]

Kinetics

Despite the synthesis method employed, PtSi forms in the same way. When pure platinum is first heated with silicon, Pt₂Si is formed. Once all the available Pt and Si are used and the only available surfaces are Pt₂Si, the silicide will begin the slower reaction of converting into PtSi. The activation energy for the Pt₂Si reaction is around 1.38 eV, while it is 1.67 eV for PtSi.

Oxygen is extremely detrimental to the reaction, as it will bind preferably to Pt, limiting the sites available for Pt–Si bonding and preventing the silicide formation. A partial pressure of O₂ as low at 10⁻⁷ has been found to be sufficient to slow the formation of the silicide. To avoid this issue inert ambients are used, as well as small annealing chambers to minimize amount of potential contamination.^[5] The cleanliness of the metal film is also extremely important, and unclean conditions result in poor PtSi synthesis.^[7]

In certain cases an oxide layer can be beneficial. When PtSi is used as a Schottky barrier, an oxide layer prevents wear of the PtSi.^[5]

Applications

PtSi is a semiconductor and a Schottky barrier with high stability and good sensitivity, and can be used in infrared detection, thermal imaging, or ohmic and Schottky contacts.^[9] Platinum silicide was most widely studied and used in the 1980s and 90s, but has become less commonly used, due to its low quantum efficiency. PtSi is now most commonly used in infrared detectors, due to the large size of wavelengths it can be used to detect.^[10] It has also been used in detectors for infrared astronomy. It can operate with good stability up to 0.05 °C. Platinum silicide offers high uniformity of arrays imaged. The low cost and stability makes it suited for preventative maintenance and scientific infrared imaging.

References

^ ^a ^b ^c Haynes, William M., ed. (2016). CRC Handbook of Chemistry and Physics (97th ed.). CRC Press. p. 4.79. ISBN 9781498754293.
^ Graeber, E. J.; Baughman, R. J.; Morosin, B. (1973). "Crystal structure and linear thermal expansitivities of platinum silicide and platinum germanide". Acta Crystallographica Section B: Structural Crystallography and Crystal Chemistry. 29 (9): 1991–1994. doi:10.1107/S0567740873005911.
^ Haynes, William M., ed. (2016). CRC Handbook of Chemistry and Physics (97th ed.). CRC Press. p. 12.68. ISBN 9781498754293.
^ Kelpeis, J.E.; Beckstein, O.; Pankratoc, O; Hart, G.L.W. (2001). "Chemical bonding, elasticity, and valence force field models: A case study for α−Pt₂Si and Pt'Si". Physical Review B. 64 (15): 155110. arXiv:cond-mat/0106187. doi:10.1103/PhysRevB.64.155110. S2CID 2857031.
^ ^a ^b ^c Pant, A.K.; Muraka, S.P.; Shepard, C.; Lanford, W. (1992). "Kinetics of platinum silicide formation during rapid thermal processing". Journal of Applied Physics. 72 (5): 1833–1836. Bibcode:1992JAP....72.1833P. doi:10.1063/1.351654.
^ Naem, A.A. (1988). "Platinum silicide formation using rapid thermal processing". Journal of Applied Physics. 64 (8): 4161–4167. Bibcode:1988JAP....64.4161N. doi:10.1063/1.341329.
^ ^a ^b Crider, C.A.; Poate, J.M.; Rowe, J.E.; Sheng, T.T. (1981). "Platinum silicide formation under vacuum and controlled impurity ambients". Journal of Applied Physics. 52 (4): 2860–2868. doi:10.1063/1.329018.
^ "The properties of this platinum silicide films". Platinum Metals Review. 20 (1): 9. 1976.
^ "Platinum Silicide (PtSi) Semiconductors". AZO Materials. Archived from the original on 2014-12-22. Retrieved 2014-04-28.
^ US 5648297, Lin, True-Lon; Park, Jin S. & Gunapala, Sarath D. et al., "Long wavelength PTSI infrared detectors and method of fabrication therof.", published 1997-07-15, assigned to NASA