Charging effects in Si quantum dots for Non Volatile Memories applications monitored by Electrostatic Force Microscopy
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Charging effects in Si quantum dots for Non Volatile Memories applications monitored by Electrostatic Force Microscopy R.A. Puglisi, G. Nicotra, S. Lombardo, C. Spinella, G. Ammendola*, C. Gerardi* Consiglio Nazionale delle Ricerche - Istituto per la Microelettronica e Microsistemi, Sezione di Catania, Str.le Primosole 50, 95121 Catania, Italy *STMicroelectronics, Str.le Primosole 50, 95121 Catania, Italy ABSTRACT Nanoscale structures have been recently proposed as charge storage nodes due to their potential applications for future nanoscale memory devices. Our approach is based on the idea of using Si nanodots as discrete floating gates. To experimentally investigate such potential, we have fabricated MOS structures with Si nanocrystals. The dots have been deposited onto an ultra-thin tunnel oxide by chemical vapour deposition, and then annealed at 1000 °C for 40 s, to crystallize all the dots. After deposition the dots have been covered by a CVD SiO2 layer, thus resulting in dots completely embedded in stoichiometric silicon oxide. The nanocrystal density and size have been studied by energy filtered TEM (EFTEM) analysis. An electrostatic force microscope has been used to locally inject the charge. By applying a relatively large tip voltage a few dots have been charged, and the shift in the tip phase has been monitored. The shift in the phase is attributed to the presence of the charge in the sample. A comparison between n and p type samples is also shown. INTRODUCTION Si quantum dots 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) device technology, the use of Si nanocrystals as storage nodes has emerged, over the last years, as a very important alternative to conventional floating gates, because of the high reliability associated with the discrete-trap structures [5-7]. Several methods to synthesize the Si dots have been proposed and investigated in the last years: ion implantation [4, 7], aerosol [8], and chemical vapour deposition (CVD) [1, 9-14]. Specifically the use of CVD methods to synthesise the nanodots was 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 a stoichiometric silicon dioxide matrix. So far, the electrical local properties of the nanocrystal memory have not been yet fully studied, because of the nanometric dimensions of the single storage node. However, in the nanocrystal NVM technology, fundamental parameters ruling the electronic transport, such as the Coulomb blockade and the energy quantization due to the carrier confinement are strong functions of the local characteristics of the dots, such as dot size and shape [15], and particularly the relative dot distance [16, 17]. The charge transfer mechanism between the dots, in fac
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