Platinum silicide

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]

Platinum silicide
Names
IUPAC name
Platinum silicide
Identifiers
3D model (JSmol)
Properties
PtSi
Molar mass 223.17 g/mol
Appearance Orthorhombic crystals[1]
Density 12.4 g/cm3[1]
Melting point 1,229 °C (2,244 °F; 1,502 K)[1]
Structure
Orthorhombic[2]
Pnma (No. 62), oP8
a = 0.5577 nm, b = 0.3587 nm, c = 0.5916 nm
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).
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Infobox references

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 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 IR imaging.

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See also

References
  1. Haynes, William M., ed. (2016). CRC Handbook of Chemistry and Physics (97th ed.). CRC Press. p. 4.79. ISBN 9781498754293.
  2. 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.
  3. Haynes, William M., ed. (2016). CRC Handbook of Chemistry and Physics (97th ed.). CRC Press. p. 12.68. ISBN 9781498754293.
  4. 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). arXiv:cond-mat/0106187. doi:10.1103/PhysRevB.64.155110.
  5. 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.
  6. 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.
  7. 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.
  8. "The properties of this platinum silicide films". Platinum Metals Review. 20 (1): 9. 1976.
  9. "Platinum Silicide (PtSi) Semiconductors". AZO Materials. Archived from the original on 2014-12-22. Retrieved 2014-04-28.
  10. US5648297A, Del Castillo, H.M.; S.D. Gunapala & E.W. Jones et al., "Long wavelength PTSI infrared detectors and method of fabrication therof.", issued 1997-07-15
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