Surface passivation of (001) GaAs with self-assembled monolayers of long-chain thiols

Passivation of (001) GaAs surface was investigated with self-assembled monolayers (SAMs) of a variety of thiols having various methylene chain length and terminal groups. The effect of passivation was monitored by measuring the intensity of the GaAs-related photoluminescence (PL) signal excited with lasers operating either at 683 or 248 nm wavelengths. Generally, for each case of the thiol treated surface the PL signal was more intense than that from non-treated samples. Additionally, it was found that the thiol terminal groups play an important role in determining the methylene chain orientation in the SAMs and consequently the efficiency of the passivation. The methyl terminated methylene chain formed a layer of a closely packed and relatively thick film, which resulted in a significantly increased PL signal. In contrast, carboxylic acid group (-CO2H) terminated methylene chains formed thin and less compacted films, leading to only a slightly increased PL signal and less efficient passivation of the GaAs surface.