Assessment of the passivation capabilities of two different covalent chemical modifications on GaP(100)

Gallium phosphide is a semiconductor material that can be used for the fabrication of optoelectronic devices. The report compares the ability of two similar organic molecules to form covalent bonds with the GaP(100) surface. Undecenoic acid (UDA) is a terminal alkene that can potentially form Ga-C bonds, and mercaptoundecanoic acid (MUA) is a thiol that can be used to generate Ga-S bonds. The chemical passivation capabilities of the functionalized surfaces exposed to different media were investigated by contact angle measurements, atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). Toxicity levels, which are important for sensing applications, were evaluated by inductively coupled plasma mass spectrometry (ICP-MS) on the media in which surfaces were stored in order to identify any gallium leaching from the substrates. Both molecules formed fairly disordered monolayers demonstrated by comparable oxide thicknesses. The UDA molecules demonstrated better stability compared to MUA molecules based on contact angle measurements and tilt angle data extracted from XPS results. According to the XPS data, the UDA molecules formed a more dense adlayer compared to MUA molecules. With respect to toxicity, the UDA-functionalized GaP provided better passivation which was confirmed by less gallium leaching into water and saline solutions. Overall, the superior passivation provided by UDA demonstrates that alkene grafting has better potential for modifying GaP based devices such as implantable sensors.