For a semiconductor based biosensor, functionalization of the surface and the stability of the semiconductor-biomolecule interface are the primary issues to be addressed by researchers. We have investigated a variety of strategies to passivate (001) GaAs surface with a long chain hexadecanethiol (C<sub>16</sub>H<sub>33</sub>SH: T16). GaAs substrates were cleaned and etched either with ArF excimer laser irradiation in an atmospheric environment or with conventional wet etchants. The effect of surface passivation and stability of the interface were evaluated using photoluminescence (PL) measurements. Significant cleaning of the (001) GaAs surface has been achieved with an ArF laser, as evidenced by the up to 4-fold increase of the PL signal. This compares to the 12-fold enhancement of the PL signal from samples that were alternately etched in solutions of NH<sub>3</sub>/H<sub>2</sub>O and HCl/ethanol. A combination of a diluted base and an acid possibly provides the cleanest surface and therefore the highest surface functionalization efficacy and long term stability upon thiolation.
We have investigated the deposition of biotinylated nano-beads on the surface of GaAs. The deposition procedure involved either direct coating of (001) GaAs with nano-beads, or binding the nano-beads with avidin immobilized on the surface of (001) GaAs through the interface of biotin and the NH<sub>2</sub> terminal group of 11-amino-1-undecanethiol (HS(CH<sub>2</sub>)<sub>11</sub>NH<sub>2</sub>). The efficiency of binding was tested by washing the samples in a solution of a commercial detergent and by subjecting them to a deionized water ultrasonic bath. The results indicate that nano-beads deposited directly on the surface of (001) GaAs withstand the detergent washing test but they are easily removed by ultrasonic washing. In contrast, the nano-beads attached to (001) GaAs through the avidin-biotin-thiol interface survive the ultrasonic washing tests.
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 (-CO<sub>2</sub>H) 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.