Platinum silicide

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Platinum silicide, also known as platinum monosilicide, is the inorganic compound with the formula PtSi and forms an orthorhombic crystalline structure when synthesized.

Contents

Chemical FormulaPtSi
Molecular Weight223.17 g/mol
CAS #12137-83-6
ShapeOrthorhombic crystals
Electrical Resistivity25-35 μΩ cm [1]
Density12.4 g/cm³
Melting Point1229 °C

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. [2]

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. [3] 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. [4] Lower temperatures (200–450 °C) were also found to be successful [5] , 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. [6]

Sputtering

In physics, sputtering is a phenomenon in which microscopic particles of a solid material are ejected from its surface, after the material is itself bombarded by energetic particles of a plasma or gas. It occurs naturally in outer space, and can be an unwelcome source of wear in precision components. However, the fact that it can be made to act on extremely fine layers of material is exploited in science and industry -- there, it is used to perform precise etching, carry out analytical techniques, and deposit thin film layers in the manufacture of optical coatings, semiconductor devices and nanotechnology products.

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. [3] The cleanliness of the metal film is also extremely important, and unclean conditions result in poor PtSi synthesis. [5]

However, in certain cases an oxide layer can be beneficial. When PtSi is used as a Schottky barrier, an oxide layer has been shown to be protective and prevent wear of the PtSi. [3]

Schottky barrier potential energy barrier in metal-semiconductor junctions

A Schottky barrier, named after Walter H. Schottky, is a potential energy barrier for electrons formed at a metal–semiconductor junction. Schottky barriers have rectifying characteristics, suitable for use as a diode. One of the primary characteristics of a Schottky barrier is the Schottky barrier height, denoted by ΦB. The value of ΦB depends on the combination of metal and semiconductor.

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. [7] Platinum silicide was most widely studied and used in the 1980s and 90’s, 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. [8] 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.

A semiconductor material has an electrical conductivity value falling between that of a conductor, such as metallic copper, and an insulator, such as glass. Its resistance decreases as its temperature increases, which is behaviour opposite to that of a metal. Its conducting properties may be altered in useful ways by the deliberate, controlled introduction of impurities ("doping") into the crystal structure. Where two differently-doped regions exist in the same crystal, a semiconductor junction is created. The behavior of charge carriers which include electrons, ions and electron holes at these junctions is the basis of diodes, transistors and all modern electronics. Some examples of semiconductors are silicon, germanium, gallium arsenide, and elements near the so-called "metalloid staircase" on the periodic table. After silicon, gallium arsenide is the second most common semiconductor and is used in laser diodes, solar cells, microwave-frequency integrated circuits and others. Silicon is a critical element for fabricating most electronic circuits.

See also

Related Research Articles

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References

  1. "Platinum Silicide (PtSi) Semiconductors". AZO Materials. Retrieved 2015-01-22.
  2. 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.
  3. 1 2 3 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.
  4. 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.
  5. 1 2 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.
  6. "The properties of this platinum silicide films". Platinum Metals Review. 20 (1): 9. 1976.
  7. "Platinum Silicide (PtSi) Semiconductors". AZO Materials.
  8. Del Castillo, H.M.; Gunapala, S.D.; Jones, E.W.; Lin, T (1997). "Long wavelength PTSI infrared detectors and method of fabrication therof". US Patent.