Control of Nucleation to Realize High Density Si Nanoparticles on SiO 2 Thin Films
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Control of Nucleation to Realize High Density Si Nanoparticles on SiO2 Thin Films Jian-hong Zhu, W. Thomas Leach, and John G. Ekerdt Department of Chemical Engineering, The University of Texas at Austin, Austin, TX 78712, U.S.A. ABSTRACT A non-thermal method to facilitate nucleation for subsequent thermal chemical vapor deposition of Si nanoparticles on SiO2/Si(001) with high density and uniform size is demonstrated. Submonolayers of Si adatoms are predeposited on SiO2/Si(001) substrates by hotwire chemical vapor deposition with disilane in an UHV chamber. The nanoparticles are grown with a disilane pressure of 1×10–4 Torr at 550 °C. The Si nanoparticles density is increased and size distribution is narrowed by predeposition of Si adatoms when compared to thermal growth on bare SiO2/Si(001). The nanoparticles density can be controlled by the amount of Si adatom predeposition. 1.2×1012 cm-2 density and 5.5 nm size are demonstrated on SiO2/Si(001) under UHV-CVD conditions. INTRODUCTION Growth of semiconductor quantum dots (QDs) on dielectric surfaces has attracted increasing interest in recent years. QDs can be used as discrete charge storage elements replacing the conventional continuous poly Si floating gate in nonvolatile flash memory applications [1] allowing for high speed and low voltage operation. Separation of the QDs minimizes electron leakage through defects in the surrounding oxide, allowing the use of a thin tunneling oxide to achieve direct tunneling of charge carriers. With the continuous shrinking of the size of microelectronic devices, high-density (~1×1012 cm-2, equivalent to 5 nm diameter QDs separated by 5 nm) of QDs is highly desired for this application. Various techniques have been reported for growing Si nanoparticles on dielectric surfaces, the dominant method being chemical vapor deposition (CVD) [2-6]. Nucleation is one of the key steps in controlling nanoparticle density and enhanced nucleation has been realized by: using disilane rather than silane [7]; growing the nanoparticles on Si3N4 rather than SiO2 to reduce the surface oxygen [6]; physically damaging the dielectric surface by ion bombardment [5,8]; terminating the SiO2 surface with –OH groups by pretreatment with dilute HF or pure water [2,9]; and, predepositing SiClxHy by exposing SiO2 to dichlorosilane [10]. Si nanoparticles with densities of 4×1011 cm-2 have been obtained on SiO2 [9], and 9.3×1011 cm-2 densities have been realized on Si3N4 [6]. We demonstrate a new method involving hot wire-CVD (HW-CVD) to facilitate nucleation on SiO2/Si(001). HW-CVD, also called catalytic-CVD, is a recently developed deposition technique [11], which uses a high-temperature filament to crack precursors. We show predeposition of submonolayer Si on SiO2/Si(001) by HW-CVD followed by ultra-high vacuum (UHV)-CVD growth with disilane at 550 °C leads to a high density of Si nanoparticles with a narrow size distribution. Si nanoparticles with a density of 1.2×1012 cm-2 and a small size of 5.5 nm are fabricated; the density can be controlled by the amoun
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