Si(100) Surface Cleaning Using Sr and SrO
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Si(100) Surface Cleaning Using Sr and SrO Yi Wei1, Xiaoming Hu, Yong Liang, D.C. Jordan, Brad Craigo, Ravi Droopad, Z. Yu, Alex Demkov, John L. Edwards Jr., Karen Moore, and William J. Ooms Physical Science Research Laboratory, Motorola Labs, Motorola Inc. 7700 S. River Parkway, Tempe, AZ 85284 ABSTRACT A method for removing SiO2 and producing an ordered Si(100) surface using Sr or SrO has been developed. In this technique, a few monolayers of Sr or SrO are deposited onto the as received Si(100) wafer in a ultrahigh vacuum molecular beam epitaxy system. The substrate is then heated to ~800oC for about 5 minutes, the SiO2 is removed to leave behind a Sr terminated Si(100) surface. This Sr terminated Si(100) surface is well suited for the growth of crystalline high k dielectric SrTiO3 films. Temperature programmed desorption measurements were carried out to understand the mechanism of removing SiO2 from Si(100) using Sr or SrO. The species we observed coming off the surface during the temperature cycle was mainly SiO and O, no significant amount of Sr containing species was observed. We conclude that the SiO2 removal is due to the catalyst reaction SiO2 + Sr (or SrO) → SiO (g) + O + Sr (or SrO). The reaction happened through several intermediate steps. The reaction SiO2 + Si → 2SiO (g) at the SiO2/Si interface is limited and the pit formation is suppressed. The main roles that Sr or SrO play during the oxide removal process are catalysts promoting SiO formation and preventing further etching and the formation of pits in the substrate. INTRODUCTION Perovskite-type oxide SrTiO3 (STO) is a very promising candidate for alternative dielectric material [1-8]. STO has a simple cubic crystal structure and a high bulk k value of 300 at room temperature. The lattice mismatch between STO (a0=3.905 Å) and Si (a0=5.431 Å) is also fairly small (~1.7%) with STO rotated 45o around Si surface normal [100] axis. For device applications, high quality Epitaxial oxide films are expected to offer better uniformity, lower leakage and higher reliability than amorphous and polycrystalline ones [9]. An epitaxial oxide thin film also serves as an excellent buffer layer for subsequent heteroepitaxy of various functional oxides including ferroelectric and high-Tc superconductor thin film on Si substrate, and single crystalline oxide thin films are expected to play an important role in silicon-oninsulator (SOI) technology [9-11]. To grow high quality crystalline oxides, the first important step is to remove amorphous native Si oxide and other contaminants from Si substrates (de-oxidation process). The starting Si(100) surfaces need to be chemically clean and morphologically ordered and smooth. A technique that uses Sr or SrO to remove SiO2 from Si(100) surface has been developed for STO growth. In this technique, the UV ozone cleaned Si(100) wafer was transferred into the growth chamber of ultra high vacuum (UHV) molecular beam epitaxy (MBE) system and heated to about 500oC. ~ 1 - 2 ML of Sr, or 3-4 ML of SrO was deposited onto the substrate surfa
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