How morphology determines the charge storage properties of Ge nanocrystals in HfO2

What is it about?

The strong correlation between morphology and charge storage properties of HfO2/Ge/HfO2/Si trilayer structures was evidenced. The morphology of structures deposited by magnetron sputtering (MS) and electron beam evaporation (EBE) was tailored by rapid thermal annealing and investigated by TEM, Raman and XPS. The best charge storage properties are obtained on trilayer capacitors (MS, annealed at 600 C for 8 min and EBE annealed at 850 C for 4 min) having similar morphology (high density Ge nanocrystals (NCs) at fixed position, i.e. in the position of as-deposited Ge layer). By changing the trilayers morphology with the increase of annealing temperature, the density of Ge NCs is decreased producing a narrowing of memory windows. So, in MS trilayer structures, Ge NCs formation is hindered due to the spreading of Ge atoms into the HfO2 matrix, while in EBE structures Ge atoms are expulsed from the HfO2 lattice during crystal growth process toward HfO2 NCs surface.

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Why is it important?

We show that the charge storage properties of trilayers are strongly influenced by their morphology. We demonstrate that the only contributors to the charge storage properties (C – V hysteresis loops) are Ge NCs embedded in crystallized HfO2, located in the position of as-deposited Ge layer and having a high density. The understanding of the relationship between morphology and charge storage properties is useful for the preparation of materials with improved properties for NVMs applications.

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