Oxidation of Silicon Nanocrystals

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Oxidation of Silicon Nanocrystals Kristen C. Scheer1,2, Sucharita Madhukar2, Ramachandran Muralidar2, Jose Lozano1, David O’Meara2, Sandeep Bagchi2, James Conner2, Carlos Perez2, Michael Sadd2, R. E. Jones2, and B. E. White, Jr.2 1 Department of Chemistry and Biochemistry A5300, University of Texas at Austin, Austin, TX 78712, U.S.A. 2 Digital DNA Laboratories, Motorola, Austin, TX 78721, U.S.A. ABSTRACT Silicon nanocrystals have made a recent appearance in the literature because of their potential importance in microelectronic[1] and optoelectronic [2] devices. For example, replacing the traditional Si floating gate in nonvolatile memory field effect transistors with nanocrystals has been shown to produce more reliable and lower power memory devices than traditional transistors [1]. Controlled oxidation of Si nanocrystals is desirable in the processing of Si quantum dots for these devices. If controlled oxidation is attainable, then it could be used to control nanocrystal size, achieve high nanocrystal density, and achieve a high quality interface between Si dot and SiO2 gate oxide layer in nanocrystal based memory. In this work, we report the oxidation properties of various sized silicon nanocrystals, deposited by low-pressure chemical vapor deposition, in different oxidizing environments. In the literature, when a Si column or dot is oxidized below the viscoelastic temperature of SiO2 (950 oC), the oxidation will self-limit to a Si core size that is dependent on the oxidizing conditions and the initial particle size. This self-limiting phenomenon is said to occur because of the presence of a compressive stress in the oxide layer that limits the diffusion of the oxidizing agent through the SiO2 to the SiSiO2 interface. This compressive stress is present at the interface because of the difference in the density of the oxide and the Si and is enhanced by the radius of curvature of the dot. This selflimiting oxidation phenomenon will be studied experimentally using microscopy techniques and the effect of the constrained structure will be characterized. INTRODUCTION Silicon nanocrystals are interesting because they have potential importance in memory [1] and optoelectronic [2] devices. For example, if the traditional Si floating gate in a nonvolatile memory field effect transistor is replaced with an array of Si nanocrystals, then the tunnel oxide can be thinned. The overall result is a low power, reliable device with long retention times [1]. Controlled oxidation of the Si nanocrystals could be valuable in the processing of these devices, potentially forming high quality interfaces between the Si nanocrystals and the SiO2 control oxide and passivating the nanocrystals to prevent further oxidation or diffusion of impurities into the nanocrystals. Oxidation of Si nanocrystals could potentially be important for optoelectronic applications, where it has been reported that the emission wavelength of Si nanocrystal photoluminescence can be tuned by controlled oxidation [3]. On a fundamental level, the oxidation of planar s