Effect of alkanethiol chain length on the oxidation resistance of self-assembled monolayer passivated Ge(100) surfaces

Abstract

• The resistance to oxidation is dependent on the length of thiol chain • Oxidation resistance increases with increasing chain length. • Acceptable oxidation resistance down the C4 chain length. • Van der Waals interactions are significant for SAM stability. The use of self-assembled monolayers (SAMs) of heteroalkanes as passivation layers to protect against oxidation has been studied on a variety of materials. However, typically heteroalkanes with C8 and longer have been used, since the contributing van der Waals between the carbon chains is a significant stabilising force and the longer the chain the greater the force. The industry requirement for passivating semiconducting materials is that they remain oxide free for a queue time of 24 h. The remit of this study is to explore whether passivation using shorter chain alkanethiols, which would be beneficial for reducing carbon, will work to prevent re-oxidation over this time frame. A series of 1-alkanethiols, with chain lengths from C2 to C12 are used to create SAMs on Ge(100) and a study of the re-oxidation of the passivated Ge upon exposure to ambient conditions is undertaken in an effort to determine how chain length effects oxidation resistance of the passivated Ge. X-ray photoelectron spectroscopy is used, complimented by water contact angle measurements to show that the longer thiol molecules outperform their shorter-chain counterparts at inhibiting re-oxidation over 168 h of exposure to ambient. Nonetheless, Ge surfaces passivated by the short-chain thiols, down to C4, still display acceptable resistance to re-oxidation. Finally, a detailed summary of density functional theory simulations whereby the most stable SAM structures and coverages are explored.