Self Assembled Metamaterials Formed via Plasma Enhanced ALD of Ag Thin Films

            Plasma enhanced atomic layer deposition (PEALD) of Ag thin films using Ag(fod)(PEt₃) as the precursor has been investigated. Although a regular film of Ag does not have any plasmonic activity, when silver is grown by PEALD, it exhibits unexpected plasmonic behavior, shown in Fig.1 characterized by the Surface Enhanced Raman (SERS) intensity.  We show that these films consist of a mosaic structure (inset, Fig. 1) characterized by 2D Ag islands separated by very narrow air gaps (<8nm), which are parallel and extend down to the substrate.  These parallel air gaps are an inherent property of the PEALD film, and the source of the plasmonic behavior.  We have modeled the ALD-Ag film using FDTD E-M simulations, representing the film as a ring shaped air gap in Ag.  The result of the model is shown in figure 2 where we display spatial maps of the electric fields at the resonance wavelength above the film and in cross section.  The maps show a very strong confinement of optical fields in the gaps with an enhancement >10x.  When PEALD Ag is combined with an active SERS material, such as dielectric core nanowires, the SERS enhancement can be enhanced by > 100x.  We have also calculated the time evolution of these fields and have found that the confined plasmonic fields persist long after the excitation pulse.  These results indicate that the ALD-Ag is a metamaterial with unique optical constants. 

            Since the air gaps are critical to the plasmonic behavior of this material, we will also discuss the effect of surface modification of the Ag PEALD.  This was achieved by examining the effect of Ag oxide removal by H₂ plasma as well as by ALD deposition of Al₂O₃, where the conformal nature of the coverage is critical.  Ellipsometry data indicate a blue shift of the plasmonic resonance with the widening of the air gap (achieved by oxide removal) and with the addition of 2 cycles of Al₂O₃, which result in the lowering of the refractive index with the addition of the Al₂O₃.  We will discuss the source of this blue shift and show how ellipsometry combined with electric field simulations can be used to obtain detailed chemical information on the surface and in the air gaps of this material, which would not be possible by experiment alone.