Mechanism of structural changes of Si(111) surfaces subjected to low-energy ion pulses during molecular-beam epitaxy

Reflected high-energy electron diffraction (RHEED) and detection of the int ensity oscillations of the specular reflection have been used to investigat e morphological changes in Si(111) associated with the two-dimensional laye r-by-layer mechanism of silicon growth from a molecular beam under conditio ns of pulsed (0.25-1 s) bombardment with low-energy (80-150 eV) Kr ions in the interval of small total radiative fluxes (10(11) - 10(12) cm(-2)), for which the density of radiation-generated defects is small in comparison wit h the surface density of the atoms. After pulsed ion bombardment an increas e in the intensity of the specular reflection is observed if the degree of filling of the monolayer satisfies 0.5 < theta < 1. No increase in the inte nsity occurs during the initial stages of filling of the monolayer. The max imum amplitude increment of the oscillations is reached at theta approximat e to 0.8. The magnitude of the amplitude increment of the RHEED oscillation s increases with temperature up to 400 degrees C and then falls. At tempera tures above 500 degrees C amplification of the reflection intensity essenti ally vanishes. Experiments on multiple ion bombardment of each growing laye r showed that the magnitude of the amplitude increment of the oscillations decreased as a function of the number of deposited layers (the order of the RHEED oscillation). A mechanism for the observed phenomena is proposed, ba sed on the concept of surface reconstruction by pulsed ion bombardment acco mpanied by formation of a (7 X 7) superstructure, which corresponds to a de crease of the activation energy of surface diffusion of the adatoms. Within the framework of the proposed mechanism the results of Monte Carlo modelin g agree with the main experimental data. (C) 1998 American Institute of Phy sics. [S1063-7761(98)01112- 3].