Single layer of Ge quantum dots in HfO₂ for floating gate memory capacitors

High performance trilayer memory capacitors with a floating gate of a single layer of Ge quantum dots (QDs) in HfO₂ were fabricated using magnetron sputtering followed by rapid thermal annealing (RTA). The layer sequence of the capacitors is gate HfO ₂/floating gate of single layer of Ge QDs in HfO ₂/tunnel HfO ₂/p-Si wafers. Both Ge and HfO₂ are nanostructured by RTA at moderate temperatures of 600-700 °C. By nanostructuring at 600 °C, the formation of a single layer of well separated Ge QDs with diameters of 2-3 nm at a density of 4-5 × 10¹⁵ m^-2 is achieved in the floating gate (intermediate layer). The Ge QDs inside the intermediate layer are arranged in a single layer and are separated from each other by HfO₂ nanocrystals (NCs) about 8 nm in diameter with a tetragonal/orthorhombic structure. The Ge QDs in the single layer are located at the crossing of the HfO₂ NCs boundaries. In the intermediate layer, besides Ge QDs, a part of the Ge atoms is segregated by RTA at the HfO₂ NCs boundaries, while another part of the Ge atoms is present inside the HfO₂ lattice stabilizing the tetragonal/orthorhombic structure. The fabricated capacitors show a memory window of 3.8 ± 0.5 V and a capacitance-time characteristic with 14% capacitance decay in the first 3000-4000 s followed by a very slow capacitance decrease extrapolated to 50% after 10 years. This high performance is mainly due to the floating gate of a single layer of well separated Ge QDs in HfO₂, distanced from the Si substrate by the tunnel oxide layer with a precise thickness.