Limited thickness epitaxy of semiconductors and Si MBE down to room temperature

Si MBE on smooth Si(100) surfaces is shown to occur at temperatures down to room temperature. We demonstrate that Si deposition at constant temperature becomes amorphous after growth of a limiting epitaxial thickness h_epi. At room temperature, h_epi is ≈ 10-30 Å and increases rapidly at higher temperatures with a rate-dependent activation energy in the range 0.4-1.5 eV. Experiments suggest that neither impurity segregation nor defect build-up cause the ultimate nucleation of the amorphous phase, and the effect may be linked to surface roughening during growth at low temperatures. The possibility that some step arrangements may be sufficient to trigger nucleation of the amorphous phase is discussed. We show that dopants are active even at very low growth temperatures, so that limited-thickness epitaxy provides a solution to the "doping problem" of thermally-activated surface segregation of dopants in Si MBE. Our observations of Si/Si(111), Ge/Si(100), and GaAs/GaAs(100) suggest that limited-thickness epitaxy may occur in MBE deposition of almost any semiconductor.