Abstract
Contact property is now becoming to be a key factor for achieving high performance and high reliability in GaN-based III-V semiconductor devices. Energetic ion sputter, as an effective interface probe, is widely used to profile the metal/GaN contacts for interfacial analysis and process optimization. However, the details of ion-induced interfacial reaction, as well as the formation of sputter by-products at the interfaces are still unclear. Here by combining state-of-the-art Ar+ ion sputter with in-situ X-ray photoelectron spectroscopy (XPS) and ex-situ high resolution transmission electron microscopy (HRTEM), we have observed clearly not only the ion-induced chemical state changes at interface, but also the by-products at the prototypical Ti/GaN system. For the first time, we identified the formation of a metallic Ga layer at the GaOx/GaN interface. At the Ti/GaOx interface, TiCx components were also detected due to the reaction between metal Ti and surface-adsorbed C species. Our study reveals that the corresponding core level binding energy and peak intensity obtained from ion sputter depth profile should be treated with much caution, since they will be changed due to ion-induced interface reactions and formation of by-products during ion bombardment.
Highlights
Ion-sputter depth profiling is currently a ubiquitous technique applied in various areas, and widely used to resolve layered information of heterostructure[1,2], such as metal/GaN and AlxGa1-xN/GaN
We focus on Ar+ ion sputter experiment on Pd/Ti/GaN sample with inductively coupled plasma (ICP) treatment (Labeled as N2)
The crater roughness at the Ti/GaN interface is ~ 4 nm as measured by ex-situ atomic force microscopy (AFM), indicating that deeper sputter leads to larger roughness and poorer depth resolution
Summary
Ion-sputter depth profiling is currently a ubiquitous technique applied in various areas, and widely used to resolve layered information of heterostructure[1,2], such as metal/GaN and AlxGa1-xN/GaN. As surface roughness is one of the physically well-defined factors to induce distortions of the original in-depth distribution[1], we designed a contact structure on purpose with a thin metal layer (i.e. 10 nm Ti layer here) with a thin cap layer (i.e. 10 nm Pd layer here), rather than the common Ohmic contact structure with thick metal layers[8,9] Benefit from this special design, the roughness at interface is nearly the same as the initial surface, promising us credible XPS results during the ion sputter. We found that ion sputter induced the formation of metallic Ga layer and TiCx compounds at the metal/GaN interface These ion-sputter-induced interfacial reactions and products alter the chemical states and compositions on GaN surface, which could be characterized by the corresponding core level peak intensity. This phenomenon must be considered with caution either for data analysis of depth profiling or properly understanding of the contact properties
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