Quantum well states and film structure

Abstract

Pronounced quantum size effects in photo-electron spectroscopy from 5 to 26 monolayer thick Au films deposited on Nb(1 0 0) occur only when the deposition is made at low temperatures (150 K). The low deposition temperature causes the long-range order of the frozen Au surface to be quite poor, making the film structure determination with standard methods such as RHEED or LEED impossible. Also techniques which do not need long-range order, such as X-ray photo-electron diffraction, cannot be used in this case to determine the structure of the Au films. The XPD measurements only show that the hexagonal dense-packed Au layers are situated perpendicularly to the substrate surface, but cannot reveal their stacking sequence. Using an ARUPS electron spectrometer and the properties of the quantum well states of the film we can determine, even in this case, the periodicity of the film states in the reciprocal space, and, consequently, the structure of the Au films. The structure turns out to be hexagonal (hcp and/or dhcp) and not fcc.