Understanding the origins of the intrinsic dead layer effect in nanocapacitors

Thin films of high-permittivity dielectrics are considered ideal candidates for realizing high charge-density nanosized capacitors for use in next generation energy storage and nanoelectronic applications. The experimentally observed capacitance of such film nanocapacitors is, however, 1 order of magnitude lower than expected. This dramatic drop in capacitance is attributed to the so-called dead layer-a low-permittivity layer at the metal-dielectric interface in series with the high-permittivity dielectric. The exact nature of the dead layer and the reasons for its origin still remain somewhat unclear. Based on insights gained from recently published ab initio work on SrRuO3/SrTiO3/SrRuO3 and our first-principles simulations on Au/MgO/Au and Pt/MgO/Pt nanocapacitors, we construct an analytical model that isolates the contributions of various physical mechanisms to the intrinsic dead layer. In particular we argue that strain-gradients automatically arise in very thin films even in absence of external strain inducers and, due to flexoelectric coupling, are dominant contributors to the dead layer effect. Our theoretical results compare well to existing as well as our own ab initio calculations and suggest that inclusion of flexoelectricity is necessary for qualitative reconciliation of atomistic results. Our results also hint at some remedies for mitigating the dead layer effect.