Spatial separation mechanism in Si quantum dots deposited by chemical vapour deposition on SiO 2
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Spatial separation mechanism in Si quantum dots deposited by chemical vapour deposition on SiO2 Rosaria A. Puglisi, Giuseppe Nicotra, Salvatore Lombardo, Corrado Spinella, Cosimo Gerardi1 CNR-IMM, Sezione di Catania, Str.le Primosole 50 95121 Catania, Italy 1 STMicroelectronics, Str.le Primosole 50 95121 Catania, Italy ABSTRACT A systematic study on the Si inter-dot distance after nucleation on silicon oxide substrates is presented. The process has been followed from the very 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. Silicon nanodots are shown to be surrounded by a depleted zone, where no new Si dots are observed to nucleate. The average size of such a zone ranges between 4 and 9 nm, depending on the deposition conditions. The dot radius is shown to be proportional to the depleted region size, thus indicating the scaling behaviour of the process. INTRODUCTION Si nanostructures synthesized on oxidized Si substrates have received intense study over the last decade, due to their fundamental physical properties, but also for application in novel devices for microelectronics [1-3] and photonics [4]. Specifically, in the non-volatile memory (NVM) technology, the use of nanocrystals as storage-nodes has emerged as an important alternative to conventional floating gates, because of the high reliability associated with the discrete-trap structure [5, 6]. Several methods to synthesize the Si dots have been investigated in the past [4, 7], but the use of CVD has demonstrated to be a convenient technique because of its immediate implementation in the ULSI processing, a good control on the deposition parameters, and because of the possibility to obtain isolated storage nodes, immersed in stoichiometric SiO2. The good control on the dot size is of great importance for the device application, so the study on the dot nucleation parameters is strategic both from the scientific and technological point of view. Several papers are present in literature on the synthesis of Si nanodots by CVD [8, 9], 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 paper we showed the evolution of the grain size distribution at several deposition temperatures and times over silicon dioxide substrates [10]. The results indicated that the Si dots continuosly nucleate, even after long deposition times, hence at high substrate coverage. The results have been well fitted by adopting a model of continuous nucleation in the approximation of capillarity [11], modified to take into account the dot coalescence contribution. Indeed, in the nanocrystal NVM technology a fundamental parameter is the distance between the dots, which has a direct impact on the robustness of the device with respect to defects in the tunnel oxide [12]. As a consequence, it is
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