Assessment of the Passivation Capabilities of Two Different Covalent Chemical Modifications on GaP(100)

Abstract

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.

Comparison of the Stability of Functionalized GaN and GaP

“…Additionally, lower pH values tend not to result in as great of a decrease in water contact angle post hydration as has been seen for neutral or basic conditions. [43] …”

“…Unlike GaN, which devolves into insoluble gallium oxide species and ammonia (no N-O peak seen to indicate formation of nitrogen oxide-N x O y or hydroxyl termination), GaP forms gallium oxide as well as phosphorous oxide (P x O y ). [28,43,66,67] Whereas post-soaking resulted in the formation of gallium-rich species for primarily uncapped GaN, the soaking produced gallium oxides of almost equal gallium content regardless of the treatments. There is a slight variation, but when observing the degree of phosphorous oxide, the difference is made up with increased phosphorous content (SI, S7).…”

“…[43,68] Understanding that the surface areas contribute more to the stability than the bulk volume, a ratio taken between total gallium leached (ppb) versus the total area exposed (mm 2 ) provides a mechanism to compare the two material types, which has not yet been explored. The use of gallium terminated GaN and GaP morphologies (SI, S8) allows for ].…”

“…The decrease in Ga 2 O 3 indicates a mechanism for increased leaching of Ga from GaP via the more soluble P x O y species. [17,43,73,74] Given enough time, gallium oxides would have limited subsequent leaching because by simply soaking the surface, it becomes more gallium-rich. Despite the reported insolubility of gallium oxide formations, the fact that water spontaneously dissociates when in contact with these materials [63,65] ultimately controls the degree of stability that appears similar to one another.…”

“…[32] However, this does not account for the predominantly formed Ga 2 O 3 seen from GaN as well as phosphorous oxides (P x O y ) that have been reported in aqueous environments. [43,44] It is expected that phosphorous forms a phosphoric derivative (depending on the degree of hydroxyl termination) to maximize solubility in low pH environments. [41] In addition to preferential removal of group-V species, defects play a crucial role as they present preferential etching locations due to increased energy at the crystal defects.…”