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

Abstract

We report on the optoelectronic properties of GaN(0001) and (11̅00) 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(11̅00) 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(11̅00) 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.

Enhanced Radiative Efficiency in GaN Nanowires Grown on Sputtered TiNx: Effects of Surface Electric Fields

“…Since the nonradiative decay is mediated by the field ionization of excitons, the radial electric fields should be eliminated or at least reduced. This task can be achieved by either reducing unintentional doping or, perhaps more practically, by a suitable surface passivation. − On the contrary, the growth of a thin shell with a larger band gap [e.g., made of (Al,Ga)­N] is generally inadequate to cancel the surface electric fields. We stress that the mechanism described in this work is not restricted to GaN NWs, but applies to nano- and microstructures in general.…”

Silicon Doping by Polymer Grafting: Size Distribution Matters

“…Organophosphonates have been reported as a versatile solution to form stable SAM on a broad class of semiconductor and dielectric surfaces, with potential applications on several different nanostructures and novel emerging materials as well . Organophosphonate chemistry has attracted a growing interest because SAM formation does not require tight control of environmental conditions and the strong phosphonate-oxide bond guarantees stable anchoring of the molecules over the substrate. − The stability of organophosphonate interfaces on technologically relevant semiconductors has been widely investigated, providing important insights into the parameters that govern their grafting and organization over the surface. ,− …”

“…In particular, SAMs of organophosphonates have been proposed as a viable and mild solution for the ex situ doping of a large variety of semiconductor materials. ,− Doping of 2D materials, such as graphene, has been achieved using SAM of organophosphates. For instance, arylphosphonic acids have been used to engineer the electronic properties of atomically thin sheets of semiconducting transition-metal dichalcogenides .…”

Enhanced radiative efficiency in GaN nanowires grown on sputtered TiN$_{\boldsymbol{x}}$: effects of surface electric fields

“…Since the nonradiative decay is mediated by the field ionization of excitons, the radial electric fields should be eliminated or at least reduced. This task can be achieved by either reducing unintentional doping or, perhaps more practical, by a suitable surface passivation [43,44]. We finally note that the mechanism described in this work is not restricted to GaN NWs, but applies to NWs in general and may affect materials with lower exciton binding energy even more severely.…”