Ge quantum dots in HfO2 for floating gate memory capacitors

What is it about?

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 HfO2/floating gate of single layer of Ge QDs in HfO2/tunnel HfO2/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 4–5×10^15 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.

Featured Image

Why is it important?

The novelty of this work is that the floating gate consists of a single layer of isolated Ge QDs in HfO2 that is separated from the Si substrate by the tunnel oxide layer with a precise thickness. Additionally, the intermediate layer containing the single layer of Ge QDs in HfO2 has sharp interfaces with the adjacent HfO2 tunnel and gate layers, as demonstrated by XPS spectroscopy and capacitance–voltage (C–V ) characteristics. We show that these capacitors have good memory characteristics that we attribute to the use of Ge-HfO2 alloy instead of pure Ge in the deposition of the intermediate layer and to RTA at 600 °C resulting in a better control of Ge QD formation in a single layer that in turn produces the reduction of lateral charge dissipation that is beneficial for long-term memory effects.

Perspectives