Determination of growth modes of Cu on O/Ni(1 0 0) and NiO(1 0 0) surfaces by SIMS and secondary electron emission measurements

The growth modes of Cu overlayers deposited on oxygen-predosed Ni(1 0 0) surfaces in the Cu coverage range 0–8 monolayers (ML) have been studied using secondary ion mass spectrometry (SIMS) and ion-induced secondary electron emission measurements. On a c(2×2)-O/Ni(1 0 0) surface prepared by 40 L O2 exposure of Ni(1 0 0), Cu grows according to the Frank-van der Merwe (FVDM, or layer-by-layer) mode up to at least 2 ML Cu coverage. In contrast, Cu grows by the Volmer–Weber mode on a NiO(1 0 0) surface prepared by 400 L O2 exposure of Ni(1 0 0) up to at least 8 ML Cu coverage; three-dimensional growth of Cu clusters commences at about 0.2 ML Cu coverage. The characterisation of Cu overlayer growth modes from the measurement of SIMS intensity variations hinges largely on the behaviour of the Ni2+ and Cu2+ cluster ions which mainly originate from the surface layer of the sample. For example, the deposition of 1 ML Cu on c(2×2)-O/Ni(1 0 0) completely quenches Ni2+ emission, whereas on NiO(1 0 0) this process is not complete even after deposition of 8 ML Cu. This behaviour can be formally described with a simple analytical model. On the basis of comparisons between the secondary ion and secondary electron emission properties of O/Cu/Ni(1 0 0) and Cu/O/Ni(1 0 0) surfaces, it is concluded that the Cu layers deposited on the c(2×2)-O/Ni(1 0 0) substrate are covered by an adsorbed layer of oxygen.