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Description
Magnesium oxide (MgO) thin films are widely used in surface science as insulating decoupling layers, substrates for metal particles, and model systems for studying charge transfer phenomena. While their functional properties have been extensively studied and several works on the growth of islands and films have been published, there still seems to be some uncertainty about the influence of different preparation parameters on the films' morphological and electronic properties.
In this work, we systematically investigate the early stages of MgO growth on Ag(001) using Scanning Tunneling Microscopy (STM) and X-ray Photoemission Spectroscopy (XPS). The influence of varying substrate temperature during growth under constant oxygen pressure is investigated, and vice versa. In submonolayer preparations, the optimal parameters yielding the stoichiometric growth of rectangular bilayer islands with edges running along the ⟨100⟩ directions have been identified. Under these conditions, the MgO islands are always embedded one layer deep in the Ag substrate. If the growth proceeds in magnesium excess – which is achieved by either lower O$_2$ pressure or elevated temperatures – island growth favours ⟨110⟩ edges with additional Mg appearing at the MgO/Ag interface. This regime is additionally examined through Density Functional Theory (DFT), which provides further insights into edge stabilization mechanisms for stoichiometric and Mg-rich systems.
XPS in combination with observation of the secondary electron cutoff of XPS spectra have been applied to investigate the evolution of film thickness and workfunction from the early stages of island growth to the formation of thick, closed films. These investigations reveal the appearance of two growth regimes: (i) heteroepitaxial growth of islands until coalescence to a closed film occurs at a thickness of 3 ML, and (ii) homoepitaxial growth of MgO for thicknesses > 3 ML.