Article pubs.acs.org/JPCC
Intitial Oxidation of Si(100) Surface at Cryogenic Temperature Ryutaro Souda* International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan ABSTRACT: The interaction of oxygen with the Si(100) surface has been investigated based on time-of-flight secondary ion mass spectrometry and ultraviolet photoelectron spectroscopy as a function of temperature. The probability for dissociative adsorption of oxygen increases with decreasing surface temperature. The O2 molecules deposited at 20 K tend to charge negatively during desorption, suggesting that a chemisorption state is entered transiently. No metastable molecular precursors are identified on the surface after desorption of physisorbed O2. The nascent oxide layer is metastable as confirmed not only from a drastic change in photoemission spectra but also from a surface compositional change at 140−150 K. The result is explainable as oxygen atoms chemisorbed initially at the dimer bridge site move to the most stable backbond site at this temperature. Hydrogen atoms segregate to the surface during the structural transformation of oxides.
1. INTRODUCTION The initial oxidation of Si in dry O2 has been a subject of both fundamental and technological importance. The formation of a well-controlled SiO2/Si interface or a highly reliable ultrathin oxide is prerequisite for the development of ultralarge scale integrated circuits. The dissociative adsorption of O2 molecules is quite inefficient,1 thereby enabling better control of oxide film thickness and thinner films. The oxidation proceeds via various surface elementary processes, such as adsorption, diffusion, and dissociation of O2 molecules, penetration and diffusion of atomic oxygen, and making and breaking bonds between the oxygen and silicon atoms near the surface and at the interface. It is known that oxidation schemes change from passive oxidation to active oxidation at 700−800 °C depending on the O2 pressure.1,2 To date, the oxidation mechanism of Si(100) and (111) surfaces in the low-temperature region has been studied extensively by static and dynamic methods,3−13 as well as first principle calculations.14−20 Molecular beam scattering experiments from Si(100) revealed that two distinct adsorption regimes exist.9,10 At low kinetic energy ( 145 K. Thus, it is found that the Si− O bond character also changes considerably with increasing temperature. The work function of the surface is plotted in Figure 7 as a function of temperature. It should be noticed that the work function decreases gradually during and after desorption of O2 molecules. This behavior can be explained as the negatively charged oxygen on the surface moves into subsurface sites with increasing temperature. The work function turns to increase at T > 140 K, which is associated with the change in the Si−O bond character or the surface segregation of hydrogen.
Figure 6. Temperature evolutions of UPS spectra from the Si(100) surface that was exposed to 1 L O2 at 20 K.
4. DISCUSSION The TOF-SIMS analysis revealed that physisorbed O 2 molecules play a key role in oxidation of the Si(100) surface. It was also found that the nascent oxide layer is metastable: significant changes occur in structure and bonding of the oxide layer at 140−150 K, where the Si−O−Si species disappears from the surface and the hydrogen atoms segregate to the surface as observed in TOF-SIMS. Schell-Sorokin and Demuth6 reported complicated behavior of the EELS spectra of oxygenated Si(111) surface due to hydroxyl contamination. They concluded that the contaminant comes from the vacuum during deposition of oxygen. On the other hand, it is known that hydrogen can be introduced into crystalline Si unintentionally during crystal growth and a number of processing and device operation steps.27−30 The incorporated hydrogen is distributed in the near surface region (
