Characterisation of Nanocrystals by Scanning Capacitance Force Microscopy
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Characterisation of Nanocrystals by Scanning Capacitance Force Microscopy Grazia Tallarida, Sabina Spiga, and Marco Fanciulli Laboratorio MDM – INFM, Via Olivetti 2, 20041 Agrate Brianza, Milan, Italy. ABSTRACT Scanning capacitance force microscopy is used to localise Sn nanometer sized structures embedded in a silicon oxide thin film. The capacitance variation occurring between probe and sample in presence of a metallic cluster modifies the oscillation amplitude of the AFM probe at twice the frequency of the applied voltage. The extreme localisation of the interaction due to the small geometries involved allows a lateral resolution of few nm. Issues related to the contrast mechanism are discussed with the support 2D finite element calculation of the electric field distribution between probe and sample. INTRODUCTION Metallic and semiconducting nanoclusters embedded in amorphous matrix are currently extensively studied because of their peculiar optical, electrical and magnetic properties, as well as for their potential technological application [1]. Of particular interest are applications related to nanoelectronics, where nanoclusters can be used, for example, in memory devices [2] as charge storage elements. Moreover, Single Electron Devices based on metallic nanoparticles have been produced [3], ferromagnetic nanoparticles may be used for ultra-high-density magnetic data storage and metallic or semiconducting nanoclusters in a glass matrix are promising for the fabrication of optical devices [4]. For all these applications a proper control on the nanoclusters composition, size, and position inside the dielectric film is required to optimise the formation process and to relate their morphology with physical properties. On the other hand, the characterisation of such nanostructures is a true challenge: techniques with high sensitivity and appropriate spatial resolution are required to analyse properties at the nanoscale. For the determination of the nanocluster size, density and position into the oxide, transmission electron microscopy (TEM) is an incomparable technique. However, it requires an accurate preparation of the sample, is time consuming and destructive. In this work, imaging of Sn nanoclusters embedded in a thin SiO2 film is accomplished by scanning capacitance force microscopy. The configuration adopted was proposed by Abraham et al [5], who suggested it as an alternative to conventional scanning capacitance microscopy (SCM) for the dopant profiling of silicon. The main difference with SCM is that it is based on a non-contact measurement and the detected signal is the force induced on the conductive AFM probe by capacitance variations occurring in the sample, rather than the capacitance signal itself. This is obtained by detecting the amplitude variation at twice the frequency of the applied ac voltage. To differentiate the two techniques, we will refer to the employed configuration as SCFM, as recently proposed by Kobayashi et. al. [6]. Obtained results well compare with cross sectional and plan
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