Thermodynamics of Crystal Growth from the Vapor

Growth from the vapor is the preferred phase transition for the production of thin epitaxial layers, while growth of bulk crystals from the vapor is rather the exception, only applied if unavoidable. An example is the growth of semiconductor-grade Sic by the so-called modified Lely method (MLM), a sublimation technique. The physical reason for the avoidance of vapor growth techniques for bulk crystals is the huge difference in the particle densities between the two states of aggregation. 

In vapor growth, the formation of a crystal is the result of a phase transition gas soUd. The constituents of the crystal to be grown are fed into the deposition 

CVD processes can be characterized as the formation of a crystal (mostly a crystalline layer) as the result of a chemical reaction involving one or more reagent gas(es) containing constituents of the material to be deposited. The mean free path of the vapor phase species is shorter than the distance between inlet into the reaction chamber and substrate surface. 

Consequently, PVD processes usually proceed under high-vacuum conditions (P 10 Pa), while CVD processes usually take place at atmospheric or only slightly reduced pressure (10 Pa P 10 Pa). 

The borders between the two classes of vapor phase growth techniques are not sharp. Even in the processes generally considered as typical physical vapor deposition processes, in most cases chemical reactions take place. The most striking example is molecular beam epitaxy (MBE). 

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The ideas described previously for understanding film morphology in terms of the local equilibrium and in terms of the surface tension are useful, but film growth occurs far from equilibrium (ex vi termini). Thus, kinetic processes control the details of film growth and the final film morphology. According to the paper by Burton, Cabrera, and Frank [13], the kinetic rates and processes are controlled by the thermodynamic driving force Ap, defined as the positive difference between the chemical potential of a molecule in the vapor phase and that in the crystal phase. 

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