Study of interfaces using ultraviolet photoelectron spectroscopy (UPS)

Ultraviolet photoelectron spectroscopy (UPS) is a powerful tool to investigate the interfaces where two materials contact. The large excitation probability for valence electrons and the high surface sensitivity of UPS enable us to obtain the information of interaction between two materials at the interface. A UPS study on Al/Sn interface is introduced as an example to get insight to the origin of eutectic system formation. A valence band spectrum with two increased and two decreased regions of photoemission intensity was observed only for a very thin Al film deposited on a Sn substrate. This phenomenon suggested that valence bands of Al and Sn caused the interorbital interaction and formed new bonding and antibonding bands. According to the molecular orbital theory, the formation of such an electronic structure has an energy disadvantage. The Al-Sn system is known to be a eutectic binary system which is separated into two phases under the eutectic temperature. The energy disadvantage of the valence electrons was considered to be the origin of phase separation in metallic eutectic binary systems through positive free energy of mixing.