We have previously proposed a Si sub-atomic layer epitaxy (SALE) method with which Si digital epitaxy from non-cracked Si2H6 is realized at a growth rate of similar to0.63 ML/cycle by repeating submonolayer Si saturation adsorption and adatom migration induced by surface thermal excitation. By the use of thermally-cracked Si2H6 in gas phase, the Si saturation coverage on Si(0 0 1) approaches similar to1 ML and Si atomic layer epitaxy on Ge(0 0 1) has also been achieved (thermally-cracked hydride molecule (TCH)-atomic layer epitaxy (ALE)) in the initial ALE cycles. The adsorption mechanisms are analyzed by scanning tunneling microscope (STM) and hydrogen temperature-programmed desorption (TPD) techniques together with the Si coverage measurement and :SiH2 production rate analysis. The results are systematically understood and :SiH2 produced through gas-phase Si2H6 thermal cracking nearly self-limitedly adsorbs on Si(0 0 1) by similar to1 ML. From the STM image analysis, the :SiH2 is interpreted to orderly adsorb on each of the two dangling bonds of the dimer. (C) D 2003 Elsevier Science B.V. All rights reserved.