Layer-by-layer oxidation of Si(001) surfaces has been studied by scanning reflection electron microscopy (SREM). The oxidation kinetics of the top and second layers were independently investigated from the change in oxygen Auger peak intensity calibrated from the SREM observation. A barrierless oxidation of the first subsurface layer, as well as oxygen chemisorption onto the top layer, occurs at room temperature. The energy barrier of the second-layer oxidation was found to be 0.3 eV. The initial oxidation kinetics are discussed based on first-principles calculations.[S0031-9007(97)04959-4] PACS numbers: 81.65. Mq, Oxidation of Si surfaces is important for technological application of electronic devices [1][2][3]. Although many kinds of surface analyses have been used to study oxygen adsorption kinetics onto Si surfaces [4][5][6][7][8][9], the oxidation kinetics of subsurface layers, which determine oxide film growth, have not been studied in detail. This is because of the difficulty of experimentally and independently analyzing the oxidation processes of specific subsurface layers. In this Letter we used scanning reflection electron microscopy (SREM [10]) combined with Auger electron and x-ray photoelectron spectroscopy (AES and XPS) to investigate the initial oxidation of Si(001) surfaces. Our combined analysis has a great advantage for observing layer-by-layer oxidation of subsurface layers, as well as the step and terrace configurations buried with oxide layers. We report, for the first time, reaction barriers of the uppermost and second layer oxidations, and discuss our experimental results based on first-principles calculations.Our experiments were carried out using an ultrahighvacuum surface analysis system that performs SREM, AES, and XPS [11]. This system is equipped with a thermal field emission electron gun, a precision energy analyzer (a spherical capacitor analyzer), and a conventional x-ray source (Mg Ka excitation). A 30-keV electron beam with a 2-nm diameter was used for the SREM with a low incident angle of about 2 ± to the surface. The AES measurement could be performed simultaneously by using the electron gun for SREM at an incident angle and detection angle of about 2 ± and 73 ± to the sample surface, respectively. The XPS was performed with a 60 ± takeoff angle with respect to the normal to the surface. A Si(001)-͑2 3 1͒ surface was prepared by flash heating with a direct current. Oxidation of the surfaces was carried out by introducing molecular oxygen into the analysis chamber. Since the electron gun and the energy analyzer were independently evacuated, the AES measurement could be performed under oxygen pressure on the order of 10 26 Torr.As we previously reported, since SREM images of SiO 2 ͞Si systems are obtained by recording the intensity change in reflection spots from a crystal Si substrate covered with an amorphous oxide layer, the interfacial structure can be observed without the need to remove the SiO 2 overlayer [12][13][14]. Figures 1(a)-1(d) show SREM images of Si(001) surfaces...

Self-limitation in the surface segregation of Ge atoms in the Si epitaxial overlayers arising from the surface bond geometry on Si(100) during molecular beam epitaxy has been investigated theoretically. It was found that Ge surface segregation is strongly limiting when the Ge concentration exceeds 0.01 monolayer. As a result of this self-limitation, segregation profiles of Ge in Si overlayers are found to decay nonexponentially in the growth direction with a kink in the profile around 3×1020 cm−3, which is in close agreement with the experimental observation. The kinetic barrier of the Ge surface segregation is estimated to be 1.63±0.1 eV.

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