A Photoemission Analysis of Gold on Silicon Regarding the Initial Stages of Nanowire Metal-Catalyzed Vapor-Liquid-Solid Growth

When semiconducting nanowires are grown by vapor-liquid-solid mechanism using gold as catalyst, the first stages, i.e., gold deposition and subsequent annealing, are of prime importance as they determine nanowire size and repartition. In this paper, we aim at identifying key factors which drive the first stages when growing nanowires, viz., surface preparation of the silicon wafer, gold deposition, and subsequent annealing. As silicon wafer surface preparation may or may not include the etching of the silica protecting overlayer, we have investigated the surface composition (a) after in situ gold deposition on Si(001) substrates that are either clean or "epiready" (i.e., SiO2-covered) and (b) during the subsequent in situ annealing from room temperature up to 600 degrees C, using soft X-ray photoelectron spectroscopy at the TEMPO beamline (SOLEIL synchrotron radiation facility). When Au is deposited directly on clean Si(001), our results reveal the formation of a AuSi alloy at surface, even when Au deposition is performed at room temperature. Postdeposition annealing prompts a complex dewetting/demixing of this initial AuSi alloy. When gold is deposited at room temperature on SiO2-covered Si(001) substrate, it slightly sinks into silica. During postdeposition annealing, a complex dynamic process takes place: As the temperature increases, gold gradually sinks into the silica and catalyzes its decomposition, which starts at 365 degrees C thus lower than usual (840 degrees C). This etching process is concomitant with Au dewetting on silica, which acts as a barrier against alloying. This gold dewetting implies local changes in surface potential and therefore local shifts of the Au-Si region toward higher binding energies. The result is a superposition of spectra, which can be used to monitor Au dewetting and therefore substrate coverage by Au. When Au finally reaches the substrate after completing the catalytic desorption of silica, it undergoes mixing with Si as on a clean Si substrate. All these results give new insights in the VLS mechanism, which is widely used for growing nanowires.