Ε½
. We employed surface sensitive photoelectron spectroscopy PES using synchrotron radiation to investigate the strain of both deposited InAs and GaAs substrates with and without Se-termination. The two distinct chemical components `comprising Ga As bonding and In As bonding states in the As 3d spectrum are clearly observed for the first time, which indicates that the strain in both the deposited InAs and GaAs substrates can be separately evaluated using the core-level energy difference between the respective core-level chemical components. This difference between the chemical components of Ga 3d and As 3d levels for both two kinds of samples is found to be independent of InAs deposition time, and to be almost the same as that of a bulk GaAs, implying that no strain is generated in both two type GaAs substrates. On the other hand, in the case of the deposited InAs, this value increases with an increase in the layer thickness of InAs for the InAsrGaAs system and approaches to a bulk InAs value, whereas, there exits only a slight change for the InAsrSerGaAs system at the very early stages of InAs growth and is almost the same as a bulk InAs value. These results suggest that the driving force for the formation of InAs nanocrystals in the InAsrGaAs system is the elastic strain, whereas, in the InAsrSerGaAs system, passivation-induced self-organizing mechanism is crucial.