Nanocrystal formation for non-volatile memory application

Abstract

i ABSTRACT This dissertation focuses on the formation of nanocrystals and integration with high-k dielectrics to address the gate stack and voltage scaling issues for future generation flash memory. Several concepts for improved device performance were discussed, including the introduction of new materials, process development and novel device structures.The first part of the work introduces a simple technique for the formation of Ge nanocrystals embedded in Lu 2 O 3 high-k dielectric using pulsed laser deposition followed by rapid thermal annealing. The nanocrystal formation mechanism was discussed, which elucidates the low temperature formation of nanocrystals. The feasibility of tuning the nanocrystal density was further demonstrated with adequate size control. The size-dependent properties of nanocrystals were also examined, which shows the charge confinement effects in the small-size nanocrystals. The fabricated capacitor devices show promising potential for low voltage memory application, and a charge storage model was proposed. Further enhancement of the memory performance was demonstrated with the realization of a lanthanide-based graded high-k barrier structure, which shows simultaneous improvement in charge storage and retention.The second part of the work explores a solution-based chemical synthesis approach to provide adequate control on the size, density and surface properties of the nanocrystals. A sonochemical reduction method was introduced for the synthesis of Ge nanocrystals, without the need of high temperature and pressure. A reduction of nanocrystal size and a more narrow size distribution was achieved, with effective surface passivation of the Ge nanocrystals. Self-assembly of Ge nanocrystals was further demonstrated by using functionalized substrate surfaces for chemical grafting of the nanocrystals. The fabrication of a colloidal nanocrystal / high-k dielectric charge storage memory device suggests a potential approach in overcoming the difficulties inherent in physical techniques for well-defined key device features.