Local Self-Order Observed During Chemical Vapor Deposition of Silicon Quantum Dots for Application in Nanocrystal Memori
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Local Self-Order Observed During Chemical Vapor Deposition of Silicon Quantum Dots for Application in Nanocrystal Memories Rosaria A. Puglisi, Giuseppe Nicotra, Salvatore Lombardo, Barbara De Salvo1, Cosimo Gerardi2. CNR-IMM, Sezione di Catania, Str.le Primosole 50 95121 Catania, Italy 1 CEA-LETI, Grenoble, France 2 STMicroelectronics, Str.le Primosole 50 95121 Catania, Italy. ABSTRACT A systematic study on the Si dot formation after chemical vapor deposition on silicon oxide substrates is presented. The process has been followed from the early stages of the dot formation up to 25% of coverages. Structural characterization has been performed by means of energy filtered transmission electron microscopy, which allowed us to observe dot sizes down to 0.5 nm in radius. The nanodots are shown to be surrounded by a depleted zone, where no new Si dots are observed to nucleate. This has been attributed to the adatoms capture mechanism by pre-existing dots, during the deposition. The dot radius and the capture size are shown to collapse onto the same curve, thus indicating the scaling behavior of the process. The adatom diffusion process is shown to restrict the number of nucleation sites, the final dot size and the dot position, thus driving the process toward partial self-order. INTRODUCTION The storage of electrical charge in silicon nanodots has stimulated considerable effort to understand its mechanism and utilise it to fabricate non volatile memory (NVM) devices [1-5] The main advantage of using discrete charge storage nodes, respect to the conventional continuous floating gates, is the high reliability associated with the localized traps. In discrete trap memories, in fact, a single leakage path due to a defect (intrinsic or stress induced) in the oxide can only discharge a single storage node. Many different types of memory cells with discrete storage nodes have been demonstrated in literature. In these devices the storage medium consists of natural traps inside a nitride layer, or semiconductor nanodots. In this paper we will focus on silicon nanodots. Among the other methods of synthesis, like ion implantation or aerosol, a well established way to obtain Si nanodots for application in NVM is the chemical vapor deposition (CVD), because it is fully compatible with standard integrated circuit technology and the deposition parameters are well controlled. Moreover the stoichiometric SiO2 matrix allows one to obtain electrically isolated storage nodes. Many results exist in literature on the tunability of the CVD process, through the deposition parameters [6, 7], but so far, the application of CVD to nanocrystal synthesis is still under development, and the optimisation of deposition parameters is still under study. In an earlier study we have showed the evolution of the grain size distribution at several deposition temperatures and times over silicon dioxide substrates [8]. The results indicated that the Si dots continuosly nucleate, even after long deposition times, hence at high substrate coverage. The result
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