Enhanced wetting of Cu on ZnO by migration of subsurface oxygen vacancies

Abstract

Metal adhesion on metal oxides is strongly controlled by the oxide surface structure and composition, but lack of control over the surface conditions often limits the possibilities to exploit this in opto- and micro-electronics applications and heterogeneous catalysis where nanostructural control is of utmost importance. The Cu/ZnO system is among the most investigated of such systems in model studies, but the presence of subsurface ZnO defects and their important role for adhesion on ZnO have been unappreciated so far. Here we reveal that the surface-directed migration of subsurface defects affects the Cu adhesion on polar ZnO(0001) in the technologically interesting temperature range up to 550 K. This leads to enhanced adhesion and ultimately complete wetting of ZnO(0001) by a Cu overlayer. On the basis of our experimental and computational results we demonstrate a mechanism which implies that defect concentrations in the bulk are an important, and possibly controllable, parameter for the metal-on-oxide growth.

Band alignment at Ag/ZnO(0001) interfaces: A combined soft and hard x-ray photoemission study

“…7-a). While ∆V bb < 0 favors a charge transfer toward the bulk, the surface dipole (∆I > 0) suggests anionic Ag in a similar way as for Cu 65 . The obtained dipole amounts to ∼ 3 Debye at the beginning of the growth; the corresponding charge is ∼ 0.3 electron per Ag atom with a charge separation of 2.5Å, the sum of Ag and of Zn 2+ radii.…”

“…After an inital coarsening, the wetting of Cu/ZnO is enhanced by annealing 65 in contrast to the sintering commonly observed for metals on oxides 60 up to an entrenching. Migration of subsurface defects (in particular positively charged O vacancies) due to the space charged layer induced by the charge transfer between Cu and ZnO was invoked 65 .…”

“…The present work aims at exploring the band alignment between vapor-deposited silver films and ZnO polar faces on (i) high quality single crystals (ii) prepared in ultrahigh vacuum to minimize contamination and interface roughness (iii) without thermal processing of the layer to avoid diffusion 65 . In situ Ultraviolet Photoemission Spectroscopy (UPS) is adjoined to HArd X-Ray PhotoEmission Spectroscopy 70 (HAXPES) to fully picture the band profile from surface to bulk.…”

Activating a Cu/ZnO : Al Catalyst – Much More than Reduction: Decomposition, Self‐Doping and Polymorphism

“…The data are normalized to the white‐line and thus relative changes, also compared to a Cu reference foil, become visible. Flattened and not well resolved features‐ III and ‐ IV represent a charge transfer to the Cu metal from the metal oxide moieties (ZnO : Al) ,. This effect weakens with increasing dwell time at 250 °C and 10 % H 2 in Ar at ambient pressure but still remains remarkably pronounced.…”

Ultrathin Metal films for Transparent Electrodes of Flexible Optoelectronic Devices

“…There are numerous original research results and reviews that support the influence of oxide‐supporting materials and their surface states on wetting of the objective metals subsequently deposited on the oxides . Even if the choice of oxide materials is limited to highly transparent semiconductors and conductors for OMO applications, the oxide supporting materials still include many candidates, such as ITO and SnO x :M (where M is a metal dopant), ZnO and ZnO:M, MoO 3 , TiO 2 , WO 3 , and ZnS .…”