Reconstruction From an Oxidized Si(111) Surface Studied by High-Temperature STM
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ABSTRACT We have investigated the transitions that take place at high-temperature from the oxidized state to the 7x7 phase on the Si (111) surface using high-temperature (HT) scanning tunneling microscopy (STM). The oxidized surface was transformed at 580 'C after oxidation by exposing the surface to 45 L of oxygen at 4800 C. The structural modification was directly observed by HT-STM. The comer adatom was created at the initial stage of the reconstruction, and then the disordered, 5x5 and 7x7 phases successively appeared on the surface. By etching Si atoms, the island structure was formed on the surface because of the reduced Si density. Finally, the completed 7x7 terrace was created.
INTRODUCTION The oxidation and etching of Si(1 11) surfaces by oxygen have been studied previously with the STM [1-4]. Both ex-situ and in-situ experiments have been conducted. The recent ex-situ experiments were performed at room temperature after the sample had been exposed to oxygen at high-temperature and subsequently quenched to room-temperature [2,3]. The sample preparation temperature was respectively between 400 'C and 600 'C [2] and between 640 'C and 800 TC [3]. Results of in-situ STM experiments under various oxygen pressures and temperatures have been previously obtained by Feltz et al. [1,4]. In this paper we present the results of in-situ experiments where the phase transition of the
surface was followed directly by STM during the reaction. The transition from the oxidized phase to the 707 phase was observed on the Si(1 11) surface. At the initial stage of transition, the formation of comer adatoms took place around the corner hole in the dimer-adatom-stacking fault (DAS) structure. Previously published results report on the etching processes [5]. Details of the instrument used in this experiment have been published elsewhere [6,7].
EXPERIMENTAL We used an HT-STM, the JSTM-4500XT manufactured by JEOL Ltd., Akishima, Japan. This instrument allows direct high-temperature observation (above 900 °C) at ultrahigh vacuum (< 3x 10-8 Pa). The samples were rectangular plates 1x7xO.38 mm 3 cut from a P-doped Si(1 11) wafer (resistivity 0.1-1.0 ohm • cm). The sample was outgassed in the vacuum chamber for 12 hours and finally flashed repetitively at 1250 TC to obtain a clean 707 surface. The tip used for imaging was an electrochemically polished tungsten tip. STM images were acquired in the constant current mode at a tunnel voltage of approximately +2 volts applied to the sample. The scan frequency was 18 s per frame. The sample temperature was measured with an infrared pyrometer. The oxygen pressure was determined by a nude ion gauge. Samples were prepared as follows: The sample was oxidized by exposing the Si(1 11) stabilized 707 surface to 45 L of oxygen (20 min at 5x10- 6 Pa) at 480 'C. The subsequent surface reduction took place by increasing the surface temperature to 580 'C. An alternate etching procedure is described subsequently. 259
Mat. Res. Soc. Symp. Proc. Vol. 355
C 1 99 5
Materials Research Society
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