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Growth rates in Atomic Layer Epitaxy (ALE)
(page designed by Thorsten Volkmann)

Atomic Layer Epitaxy (ALE) is a common technique for growing thin solid films composed of two or more material types on a certain substrate. Contrary to MBE (molecular beam epitaxy) where the species are sent to the substrate simultaneously, in ALE growth the reactants are brought to the substrate as alternating pulses with dead times in between. ALE makes use of the fact that the incoming material is bound strongly until all sites available for chemisorption are occupied. The surplus material then binds only very weakly and evaporates quickly during the dead times. Thus in principle self-regulated growth of a whole monolayer (ML) of the compound per ALE cycle may be achieved.
However, a variety of II-VI semiconductors (e.g. CdTe, ZnTe, ZnSe) exhibit growth rates which differ from the "ideal" value (1 ML/cycle). This behavior is due to the occurence of surface reconstructions during the growth process. For instance, the vacancy structures of Cd-terminated CdTe(001) surfaces do not allow for a complete filling of the surface layer during the Cd pulse. This leads to a growth rate of only 1/2 ML per ALE cycle for a wide range of temperatures. At low enough temperatures, though, each cycle adds a complete layer of CdTe.


KMC simulation of ALE growth

Within the framework of our lattice gas model of II-VI(001) surfaces we are able to explain the experimentally observed temperature dependence of the ALE growth rate of CdTe(001). We demonstrate that the presence of weakly bound excess Te at low temperatures is crucial for the transition between the different growth rate regimes. Details of the simulations can be found here.

The following pictures show sections of the surface at the end of the Cd or Te pulses in the KMC simulations. Cd atoms are green and Te atoms are blue. The parameters for the simulations are the same as described in the reference.

Two and a half ALE cycles for a particle flux F=5ML/s and temperature T=0.44 (high temperature regime). Two ALE cycles are needed to deposit one monolayer of CdTe.

after 1st Cd pulse, high T after 1st Te pulse, high T after 2nd Cd pulse, high T after 2nd Te pulse, high T after 3rd Cd pulse, high T

Movie: MPEG-4 (8.3 MB), Windows Media Video (4.4 MB)


Two and a half ALE cycles for a particle flux F=5ML/s and temperature T=0.36 (low temperature regime). A growth rate of approx. one monolayer CdTe per ALE cycle is observed.

after 1st Cd pulse, low T after 1st Te pulse, low T after 2nd Cd pulse, low T after 2nd Te pulse, low T after 3rd Cd pulse, low T

Movie: MPEG-4 (8.9 MB), Windows Media Video (4.3 MB),



The comparison of the temperature dependence of the ALE growth rate shows very good qualitative agreement between experimental observations and the results from our KMC simulations. For low temperatures there is a plateau with a growth rate of approx. one layer per ALE cycle. At a certain transition temperature the growth rate drops to about 1/2 monolayer per ALE cycle. The transition temperature depends crucially on the particle fluxes but is rather insensitive to the variation of other parameters. For very high temperatures, sublimation of the crystal dominates over the incoming flux and growth becomes impossible.

Experimentally observed ALE growth rates of CdTe(001) Simulated ALE growth rates

Left: Experimentally observed ALE growth rates of CdTe(001) (plot generated after W. Faschinger and H. Sitter, J. Cryst. Growth 99 (1990), 566).
Right: Average growth rates obtained from our KMC simulations of ALE growth. Parameters as described in the reference.



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