Surface Science Letters Complete dissociation of water on hot silicon (1 1 1)–7  7 surface––direct observation of hopping oxygen atom Rong-Li Lo a,b, * , Ing-Shouh Hwang a , Tien. T. Tsong a a Institute of Physics, Academia Sinica, Nankang 115, Taipei, Taiwan, ROC b Department of Physics, National Tsing-Hua University, No. 101, Sec. 2, Kuang Fu Road, Hsinchu 300, Taiwan, ROC Received 20 September 2002; accepted for publication 13 February 2003 Abstract The adsorption of water molecules on Si(1 1 1)–7  7 surface is investigated by scanning tunneling microscopy (STM) at elevated temperatures. With further scission of OH bond above 290 °C, the complete decomposition of water molecule into two H atoms and one O atom can now be observed directly. Thus surface diffusion of single O atom is studied quantitatively for the first time with diffusion parameters derived from Arrhenius plots. Two main adsorption states for O atom appear as protruded spots but with different image brightness. They can switch from one state to the other randomly at an adatom site. These two states are proposed to be the insertion of an oxygen atom into different Si adatoms backbonds. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: Molecule–solid reactions; Scanning tunneling microscopy; Surface diffusion; Silicon; Water Oxidation of silicon surface is an important fabrication process in semiconductor industry. Especially, as the structure dimension shrinks, formation of controllable thickness and high uni- formity thin layers of silicon oxide is now an exciting research objective for commercial semi- conductor devices. Two well-known procedures for formation of a silicon oxide layer are exposure of a hot silicon surface to oxygen molecules (dry oxidation) and to gaseous water (wet oxidation) [1]. In both procedures diffusion of oxygen atom plays the key role. Therefore, not only of its great technological importance, diffusion of oxygen on silicon surfaces and into the bulk is itself a very interesting subject for fundamental research. Es- pecially, diffusion of oxygen atoms at surfaces is related to the very early stage of oxidation, which is of crucial importance when the silicon oxide layer reduces to near its insulator thickness limit of five-silicon-atom thickness [2]. In this letter, we present a study of water ad- sorption on hot Si(1 1 1)–7  7 surfaces. A water molecule decomposes first into H and OH upon adsorbing on the surfaces. The OH radical can hop between neighboring adatom sites. It can further * Corresponding author. Address: Department of Physics, National Tsing-Hua University, No. 101, Sec. 2, Kuang Fu Road, Hsinchu 300, Taiwan, ROC. Tel.: +886-3-5742273; fax: +886-3-5723052. E-mail address: rllo@phys.nthu.edu.tw (R.-L. Lo). 0039-6028/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0039-6028(03)00330-3 Surface Science 530 (2003) L302–L306 www.elsevier.com/locate/susc