Plasma-assisted Atomic Layer Deposition of Monolayer AlO_x on GaN for Surface Functionalization and Low-Resistance Contacts

Atomic layer deposition (ALD) is an essential tool in semiconductor device fabrication that allows the growth of ultrathin and conformal films to precisely form heterostructures and tune interface properties. The self-limiting nature of the chemical reactions during ALD provides excellent control over the layer thickness and film properties. However, in contrast to idealized growth models, it is experimentally challenging to create continuous monolayers by ALD because surface inhomogeneities and precursor steric interactions result in island growth during film nucleation. Thus, the ability to create closed monolayers by ALD would offer new opportunities for controlling interfacial charge and mass transport in semiconductor devices, as well as for tailoring surface chemistry. Here, we report full encapsulation of <i>c</i>-plane gallium nitride (GaN) with an ultimately thin (~3 Å) aluminum oxide (AlO_x) monolayer, which is enabled by the transformation of the GaN surface oxide into AlO_x using a combination of trimethylaluminum (TMA) deposition and hydrogen plasma exposure.[1] The introduction of ultrathin AlO_x significantly modifies the physical and chemical properties of the surface, decreasing the work function and introducing new chemical reactivity. The AlO_x-terminated surface strongly reacts with phosphonic acids under standard conditions, which we exploited to create functional self-assembled monolayers with densities approaching the theoretical limit. Moreover, we show that surface band bending of <i>n</i>-type GaN is reduced by over 0.4 eV following the transformation of the gallium oxide layer into AlO_x. As a consequence, we achieved a low contact resistance and Ohmic behavior before annealing of titanium-contacted <i>n</i>-doped GaN with interfacial AlO_x. Given the high reactivity of TMA with surface oxides, the presented monolayer AlO_x deposition scheme likely can be extended to other dielectrics and III-V-based semiconductors, with significant relevance for applications in optoelectronics, chemical sensing, and (photo)electrocatalysis.<br/>[1] Henning, A.; Bartl, J. D.; Zeidler, A.; Qian, S.; Bienek, O.; Jiang, C.-M.; Paulus, C.; Rieger, B.; Stutzmann, M.; Sharp, I. D. <i>Adv. Funct. Mater.</i> 2021, 31, (33), 2101441.