Metal nanocluster for application in floating gate memory devices

Abstract

The motivation for integrating metal nanoclusters (metal NCs) with high-k gate dielectrics is its potential for implementation in the new generation terabit/cm2 density non-volatile memory. The quantum confined structure of metal NCs provides superior storage characteristics over semiconductors; and high-k dielectrics can greatly improve the trade-off between data retention and program efficiency. In this project, trilayer floating gate memory capacitors with Au NCs embedded in HfAlO high-k dielectrics are fabricated and their memory characteristics are studied. A memory structure containing a control gate, self-organized Au NCs, and an HfAlO tunnel layer has been fabricated by pulsed-laser deposition (PLD), as this method provides excellent control on cluster size, density and crystallinity at various growth ambients. Owing to the charging effect of Au particles, obvious memory characteristics of the trilayer structure were demonstrated by the presence of hysteresis loops in the capacitance-voltage (C-V) curves with maximum storage charge density of up to 1014 cm-2. The physical morphology of the Au NCs was characterized by cross-section and plane-view transmission electron microscopy (TEM). The memory characteristics of the capacitors were characterized by using C-V and capacitance-time (C-t) measurements. The effect of growth temperature, gaseous ambient and deposition time on the size/density of the Au NCs to the charge storage and thus the memory characteristics has been studied. Fowler-Nordheim tunneling, which is the major mechanism for write in non-volatile floating gate memory, has been further studied by verifying the memory properties of HfAlO/Au NCs/HfAlO trilayer structure. Apart from the conventional measurement of non-volatile memory structure, low temperature current-voltage (I-V) measurement has also been carried out. The Coulomb blockade and resonant tunneling effects have been observed in the trilayer floating gate memory structure due to the quantum confinement effect in the sub-5 nm Au NCs. This suggests the feasibility in turning the trilayer floating gate memory structure into single electron device, which is a future flash memory technology. To further study the tunneling mechanism, a five-layer structure of HfAlO/Au NCs/ HfAlO/Au NCs/ HfAlO has been fabricated. It is discovered that instead of a single memory window, a double loop has been revealed due to the two quantum wells formed by the double layers of Au NCs. The memory effect, tunneling mechanism and implications of this multilayer structure will be discussed briefly.