Metastable semiconductor alloys

Metastable alloys have been deposited as epitaxial thin films on a variety of substrates by a variety of techniques. Deposition methods (see Chapters 10-12) have included sputtering, metal-organic vapor-phase epitaxy (MOVPE), molecular beam 

The general approach to obtaining a metastable alloy is to deposit the material one atomic layer at a time under conditions in which atoms land in random locations (producing an alloy) but where atomic mobility is sufficiently hmited or nucleation of second phases is so suppressed that phase separation on the surface is not possible. Once the surface atoms are buried in a well-mixed state under other atoms, their mobility is greatly reduced. Hence phase separation can be avoided if it can be prevented on the surface. 

Because the (GaSb)].xGe2x was deposited by a vapor-phase growth technique where atoms strike the surface in random distributions, the material is created as a well-mixed alloy. To initiate phase separation, separate phases nucleate. In other words, sufficiently large composition fluctuations must develop to be stable. 

Diffusion kinetic limitations must certainly also be important as evidenced by the presence of the mixed region including domains of both the metastable alloy and the two equilibrium phases. If nucleation were the only problem, then this region would not exist because the equilibrium phases provide nuclei for further phase separation. The only explanation is that even when nucleation has occurred in some locations, diffusion prevents atoms in the alloy from reaching their preferred phases in adjacent grains. Furthermore, it is clear that even under these conditions nucleation is sufficiently rare that it does not occur everywhere. 

Typically in sputtering and related processes in which energetic particles are common the particles striking the surface with high kinetie energies are a few percent of all incident atoms. If more of the particles can be aeeelerated then the energy needed in each particle can be redueed. This is the basis of teehniques such as ionized physical vapor deposition where one intentionally ionizes as many of the gas phase species as possible but in which these are accelerated only to modest energies. 

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J.E. Greene, A review of recent research on the growth and physical properties of single crystal metastable elemental and alloy semiconductors, J. Vac. Sci. Technol. B1 (2) (1983) 229. 

We prepared solar cells by growth of CuInSe2 on the Cu-Mo alloys and observed large increases in adhesion between the semiconductor film and the metal alloy. In tape tests in which a piece of common office tape was applied to the semiconductor surface and then peeled off virtually all of the semiconductor was removed from a pure Mo metal layer by the tape, while virtually all of the semiconductor remained in place in the base of the metastable alloy contact. Investingating the mechanism showed that the residual stress in the film was almost completely relieved as one heated the sample to deposit the semiconductor. Our solar cells were correspondingly improved by the reduction in the density of defects due to adhesion failures. 

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