Zusammenfassung
Changes in Si(111) 7 × 7 pattern with air exposure are studied by RHEED. The relation between exposure time and air-oxidized film thickness is obtained using an ellipsometer. As a result, it is found that the 7 × 7 surface after air exposure (760 Torr, 20 h) still exhibits several 7 × 7 spots near normal ones in the RHEED pattern. The air-oxidized film thickness corresponding to this exposure is about 13 Å. The fact that the 7 × 7 spots do not disappear with HF etching of the oxide film indicates that the superstructure remains not at the oxide surface but at the substrate selvedge. The 7 × 7 spots gradually disappear after longer exposure time and no 7 × 7 spots can be observed after 40 h (about 15 Å in oxide thickness). For a sample exposed longer than 40 h, no 7 × 7 spots reappear, even if the oxide is removed by HF. Therefore, it is concluded that the structure of the substrate selvedge changes with exposure time.
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