External Control of GaN Band Bending Using Phosphonate Self-Assembled Monolayers

We report on the optoelectronic properties of GaN(0001) and (1 (1) over bar 00) surfaces after their functionalization with phosphonic acid derivatives. To analyze the possible correlation between the acid's electronegativity and the GaN surface band bending, two types of phosphonic acids, n-octylphosphonic acid (OPA) and 1H,1H,2H,2H-perfluorooctanephosphonic acid (PFOPA), are grafted on oxidized GaN(0001) and GaN(1 (1) over bar 00) layers as well as on GaN nanowires. The resulting hybrid inorganic/organic heterostructures are investigated by X-ray photoemission and photoluminescence spectroscopy. The GaN work function is changed significantly by the grafting of phosphonic acids, evidencing the formation of dense self-assembled monolayers. Regardless of the GaN surface orientation, both types of phosphonic acids significantly impact the GaN surface band bending. A dependence on the acids' electronegativity is, however, only observed for the oxidized GaN(1 (1) over bar 00) surface, indicating a relatively low density of surface states and a favorable band alignment between the surface oxide and acids' electronic states. Regarding the optical properties, the covalent bonding of PFOPA and OPA on oxidized GaN layers and nanowires significantly affects their internal quantum efficiency, especially in the nanowire case due to the large surface-to-volume ratio. The variation in the internal quantum efficiency is related to the modification of both the internal electric fields and surface states. These results demonstrate the potential of phosphonate chemistry for the surface functionalization of GaN, which could be exploited for selective sensing applications.

Automated summary

The optoelectronic properties of GaN surfaces were studied after functionalization with phosphonic acid derivatives, showing significant changes in GaN work function and impacting surface band bending. The internal quantum efficiency of GaN layers and nanowires was significantly affected by the covalent bonding of phosphonic acids, with modifications to internal electric fields and surface states. Phosphonate chemistry shows potential for selective sensing applications in surface functionalization of GaN.