Ionic Bombardment to Tune the Electrochemical Properties of a Semiconductor.
Authors: Aureau Damien , Frégnaux Mathieu , Bouttemy Muriel , Vigneron Jackie , Simon Nathalie , Etcheberry Arnaud , Gonçalves Anne-Marie 
Published: February 2020

The purpose of this work is to study in detail the effects of monoatomic and gas cluster ion beam (GCIB) on a semiconducting electrode (InP) thanks to the correlation between X-Ray Photoelectron Spectroscopy (XPS) and electrochemistry. Such approach allows estimating the overall change in the electrical, chemical and structural properties of the modified semiconductor (SC). Recent developments in ion sputtering source enable a better control of the perturbations induced at the surface of the material. Furthermore, anodic dissolution will be presented as a tool to recover the initial characteristics of the modified electrode.

The interaction of ions with matter and especially surfaces has been an active topic for both fundamental and applied research over the past decades and is still relevant today. Thanks to ion bombardments, various types of surfaces can be chemically, mechanically, optically or electrically modified. However, the understanding of the inherent phenomena and physical/chemical mechanisms involved remains incomplete. When ions, whose energy range between 1 and 10 000 eV, interact with a surface, many processes can occur and the energy distribution (dissipation) could differ markedly from one material to another. Nevertheless, whatever the source of modifications considered, the common fundamental question is the nature of the effects induced by the perturbation and its evaluation (area, depth). The consequences of ion bombardment on the surface of a SC may have repercussions: formation of ordered nanostructures via low-energy ion-beam irradiation as a bottom-up nano-manufacturing technique; ion implantation for SC device technology; optical lithography using ion beam or even functionalization of 2D materials. This topic has gained specific interest in the field of surface analysis where depth-profiling is required (e.g. XPS, Auger Spectroscopy or Secondary Ion Mass Spectroscopy). In contrast with monoatomic ions (e.g. Ar+), cluster ions (e.g. Arn
+) do not penetrate deeply into target materials. Therefore, the energy of their impact is dissipated within the first few nanometers of the surface, resulting in low sputtering yields and minimal surface damage. However, the effects of these new generations of GCIB are not yet fully understood.

The strong originality of our approach is to investigate the surface modifications induced not only by UHV characterization technique (XPS) but also with the use of solid/liquid interface electrochemical response for which SC have specific behavior. These efficient and quantitative tools provide relevant information on the modification performed at the SC surface: metallization, amorphization, preferential sputtering. The high number of parameters that govern the interfacial electrochemistry of the SC/electrolyte system (band bending, SC doping level, double layer capacitance, space charge layer, SC electrical impedance, photopotential) make this approach pertinent and sensitive to evaluate the perturbation amplitude. An example is given on the figure where the metallization of an InP electrode can be observed in vacuum after successive Arn
+ etching sequences by the shift of the Fermi level (XPS valence band region) and in solution by the narrowing of the depletion region and the flattening of the Mott-Schottky plot.

In addition, a very important characteristic of SC interfacial electrochemistry is its availability to generate in situ anodic dissolution at nanoscale. Thereby, it represents an accurate and quantitative tool to estimate the thickness of the modified region. Moreover, using well-controlled dissolution, the initial properties can be retrieved. These results are compared to those obtained after annealing under UHV. Thus, the electrochemical responses will be associated directly to surface chemical information such as: loss of stoichiometry, Ar implantation, shift of the Fermi Energy level position, photo-peak broadening.

Figure 1