Amorphous solid

"Amorphous" redirects here. For amorphousness in computational systems, see amorphous computing. For amorphousness in science fiction, see amorphous creature. For amorphousness in set theory, see amorphous set.

In condensed matter physics, an amorphous (from the Greek a, without, morphé, shape, form) or non-crystalline solid is a solid that lacks the long-range order characteristic of a crystal.

In part of the older literature, the term has been used synonymously with glass. Nowadays, "amorphous solid" is considered to be the overarching concept, and "glass" the more special case: A glass is an amorphous solid that transforms into a liquid upon heating through the glass transition.^[1]

Other types of amorphous solids include gels, thin films, and nanostructured materials.

Nanostructured materials

It is difficult to make a distinction between truly amorphous solids and crystalline solids if the size of the crystals are very small. Even amorphous materials have some short-range order at the atomic length scale due to the nature of chemical bonding. Furthermore, in very small crystals a large fraction of the atoms are located at or near the surface of the crystal; relaxation of the surface and interfacial effects distort the atomic positions, decreasing the structural order. Even the most advanced structural characterization techniques, such as x-ray diffraction and transmission electron microscopy, have difficulty in distinguishing between amorphous and crystalline structures on these length scales.

Amorphous thin films

Amorphous phases are important constituents of thin films, which are solid layers of a few nm to some tens of µm thickness deposited upon an underlying substrate. So-called structure zone models were developed to describe the microstructure and ceramics of thin films as a function of the homologous temperature T_h that is the ratio of deposition temperature over melting temperature.^[2][3] According to these models, a necessary (but not sufficient) condition for the occurrence of amorphous phases is that T_h has to be smaller than 0.3, that is the deposition temperature must be below 30% of the melting temperature. For higher values, the surface diffusion of deposited atomic species would allow for the formation of crystallites with long range atomic order.

Regarding their applications, amorphous metallic layers played an important role in the discussion of a suspected superconductivity in amorphous metals.^[4] Today, optical coatings made from TiO₂, SiO₂, Ta₂O₅ etc. and combinations of them in most cases consist of amorphous phases of these compounds. Much research is carried out into thin amorphous films as a gas separating membrane layer.^[5] The technologically most important thin amorphous film is probably represented by few nm thin SiO₂ layers serving as isolator above the conducting channel of a metal-oxide semiconductor field-effect transistor (MOSFET). Also, hydrogenated amorphous silicon, a-Si:H in short, is of technical significance for thin film solar cells. In case of a-Si:H the missing long-range order between silicon atoms is partly induced by the presence by hydrogen in the percent range.

The occurrence of amorphous phases turned out as a phenomenon of particular interest for studying thin film growth. Remarkably, the growth of polycrystalline films is often preceded by an initial amorphous layer, the thickness of which may amount to only a few nm. The most investigated example is represented by thin multicrystalline silicon films, where such an initial amorphous layer was observed in many studies.^[6] Wedge-shaped polycrystals were identified by transmission electron microscopy to grow out of the amorphous phase only after the latter has exceeded a certain thickness, the precise value of which depends on deposition temperature, background pressure and various other process parameters. The phenomenon has been interpreted in the framework of Ostwald's rule of stages^[7] that predicts the formation of phases to proceed with increasing condensation time towards increasing stability.^[4][6] Experimental studies of the phenomenon require a clearly defined state of the substrate surface and its contaminant density etc., upon which the thin film is deposited.

References

1. ^ J. Zarzycki: Les verres et l'état vitreux. Paris: Masson 1982. English translation available.
2. ^ B. A. Movchan and A. V. Demchishin (1969). "Study of the structure and properties of thick vacuum condensates of nickel, titanium, tungsten, aluminium oxide and zirconium dioxide". Phys. Met. Metallogr. 28: 83–90. 
3. ^ J.A. Thornton (1974). "Influence of apparatus geometry and deposition conditions on the structure and topography of thick sputtered coatings". J. Vac. Sci. Tech. 11: 666–670. Bibcode 1974JVST...11..666T. doi:10.1116/1.1312732. 
4. ^ ^{a b} Buckel, W. (1961). "The influence of crystal bonds on film growth". Elektrische en Magnetische Eigenschappen van dunne Metallaagies. Leuven, Belgium. 
5. ^ R.M. de Vos, H. Verweij (1998). "High-Selectivity, High-Flux Silica Membranes for Gas Separation". Science 279 (5357): 1710–1. Bibcode 1998Sci...279.1710D. doi:10.1126/science.279.5357.1710. PMID 9497287. 
6. ^ ^{a b} M. Birkholz, B. Selle, W. Fuhs, S. Christiansen, H. P. Strunk, and R. Reich (2001). "Amorphous-crystalline phase transition during the growth of thin films: the case of microcrystalline silicon". Phys. Rev. B 64: 085402. Bibcode 2001PhRvB..64h5402B. doi:10.1103/PhysRevB.64.085402. http://www.mariobirkholz.de/PRB2001.pdf. 
7. ^ W. Ostwald (1897). "Studien über die Umwandlung fester Körper". Z. Phys. Chem. 22: 289–330. 

Further reading

• R. Zallen (1998). The Physics of Amorphous Solids. Wiley Interscience. 
• S.R. Elliot (1990). The Physics of Amorphous Materials (2nd ed.). Longman. 
• N. Cusack (1987). The Physics of Structurally Disordered Matter: An Introduction. IOP Publishing. 
• N.H. March, R.A. Street, M.P. Tosi, Eds., (1985). Amorphous Solids and the Liquid State. Springer. 
• D.A. Adler, B.B. Schwartz, M.C. Steele, Eds. (1985). Physical Properties of Amorphous Materials. Springer. 
• A. Inoue, K. Hasimoto, Eds. (1985). Amorphous and Nanocrystalline Materials. Springer. 

External links

• Journal of non-crystalline solids (Elsevier)