STM study of the initial oxidation stage of Ge(100) 2×1

Abstract

The initial stage of oxygen chemisorption on the Ge(100) surface was studied by scanning tunneling microscopy. By using a defect-free surface and in situ oxidation, oxygen-induced products could be unambiguously determined. Two types of bright products and two types of dark products were identified. One of the bright products is a major product and it protrudes at the center of the dimer. Since this product was observed even after annealing at 300 • C, it is a stable product. The other bright product is a bright spot at one of the dimer atoms, and dimer buckling is stabilized near this product. The two dark products are similar to the missing dimer defects in filled-state images, but they appeared as bright spots in empty-state images. After annealing of the oxygen-chemisorbed surfaces, the dark product aligned in the dimer row with a 2× periodic structure inside.The invention of scanning tunneling microscopy and its descendants opened up a new era for semiconductor technology. One fruitful application of these microscopies is the atomistic investigation of chemical reactions on semiconductor surfaces with gaseous molecules. Oxidation is one of the key technologies for semiconductors and a detailed understanding of oxidation is vital for studying the interface formation between substrates and oxide layers. On the Si(111) 7 × 7 surface, direct imaging of the reaction sites by STM [1-3] has proved the existence of several types of chemisorption geometries. Recent tip-induced chemical-bond breaking [4] unambiguously identified the presence of the molecular precursor state, which was previously indicated by high-resolution electron energy loss spectroscopy [5], photoelectron spectroscopy [1, 6], and electron-stimulated desorption [7] studies. Although the Si(100) surface is more important from a technological point of view and has a much simpler surface reconstruction, i.e., the 2 × 1 dimer structure, STM has not been able to provide atomically resolved images of oxygeninduced structures. Over the past 10 years, several authors * To whom correspondence should be addressed have attempted to identify the initial chemisorption geometries on the Si(100) surface by using STM. However, since oxygen first reacts with surface defects rather than with perfect dimers [8,9] and since the substrate atoms are ejected by the oxygen reaction [10], it has been difficult to acquire a clear picture of chemisorption products that is comparable to the theoretically predicted structure.We present here an STM study of the initial oxygen chemisorption on the Ge(100) 2 × 1 surface. The Ge(100) 2 × 1 surface has atomic and electronic structures quite similar to those of the Si(100) 2 × 1 surface [11,12], and therefore we would expect to see similar oxygen adsorption. A theoretical calculation suggests almost the same chemisorption configurations, but it gives only 25% of the adsorption energy compared to Si dimers [13]. Therefore, one would expect that the exothermic effect [14] would be less pronounced on the Ge(100) surface. Experimentally...

Comparative study of dimer-vacancies and dimer-vacancy lines on Si() and Ge()

“…Most notably, the formation energy of a single-DV line on Si(001) is negative, while for the case of Ge (001) it is positive. While there is a possibility that this sign difference between Si(001) and Ge(001) might be due to artifacts of the empirical potential, it is consistent with the observation of self assembly of DVs on Si(001) [26] and not on Ge(001) [18,16,43,44]. Some of these trends are related to the formation and binding of individual dimer vacancies, as will be shown in the next section.…”

“…To our knowledge, the only observation of vacancy lines on Ge(001) is for the case of Bi deposition (1ML) [45]. No vacancy lines have so far been observed on clean Ge(001) [16,18,43,44], while two different groups report the observation of vacancy lines on the clean Si(001) surface [26,24,25].…”

Initial Stages of Oxygen Chemisorption on the Ge(001) Surface

“…Assignment of this structure to the type-B feature would be consistent with the observations by Fleischmann et al that insertion into the back-bonds is not favored. The type-B features observed by Fukuda and Ogino , were found to exhibit enhanced brightness on the same side of the dimer row in both filled and empty state. Such behavior is exhibited by the dimer-bridging, back-bond up and dangling bond configurations (Figure a,c,d), but not the back-bond down structure of Figure b.…”

“…Such behavior is exhibited by the dimer-bridging, back-bond up and dangling bond configurations (Figure a,c,d), but not the back-bond down structure of Figure b. Thus, only three of our single oxygen chemisorption structures are potential candidates for the type-B feature of Fukuda and Ogino. , …”

The Adsorption and Decomposition of H2S on Ge(001): An STM Study

“…Secondly, hydrogen is desorbed at around 325 • C, which corresponds with the loss of the Ge-H feature seen in both HREELS and UPS. Thirdly, GeS is desorbed at around 425 • C. It is conceivable that this molecule is a mobile complex on the surface at lower temperatures; comparison can be made with GeO, as discussed by Fukuda and Ogino [8]. Certainly, STM shows that the surface has begun to re-structure at a much lower temperature.…”