High-k/Ge MOSFETs for future nanoelectronics

Oxidation of Ge(100) and Ge(111) surfaces: an UPS and XPS study

We have studied the kinetics and mechanism of oxidation of SiGe alloys deposited epitaxially onto Si substrates by low‐temperature chemical vapor deposition. Ge is shown to enhance oxidation rates by a factor of about 3 in the linear regime, and to be completely rejected from the oxide so that it piles up at the SiO2/SiGe interface. We demonstrate that Ge plays a purely catalytic role, i.e., it enhances the reaction rate while remaining unchanged itself. Electrical properties of the oxides formed under these conditions are presented, as well as microstructures of the oxide/substrate, Ge‐enriched/SiGe substrate, and SiGe/Si substrate interfaces, and x‐ray photoemission studies of the early stages of oxidation. Possible mechanisms are discussed and compared with oxidation of pure silicon.

Oxidation studies of SiGe

Angle-resolved photoemission is shown to be a very versatile technique which probes electronic structure as well as geometric structure. All quantum numbers of an electronic state can be determined. In particular, energy and momentum of electrons in a solid are probed by measuring energy and momentum of photoelectrons. Thus, energy versus momentum band dispersions can be measured. Since photoelectrons have a small but tunable escape depth, it is possible to see surface and adsorbate states as well as bulk states. A completely different application of angle-resolved photoemission concerns the atomic positions and bond orientations at surfaces. Photoelectrons from a well-defined atom serve as source for an electron diffraction experiment which is localized to a surface. Another type of experiment uses the emission pattern from adsorbed molecules to determine their orientation.

Angle-resolved measurements of the photoemission of electrons in the study of solids

In 1973 the oil crisis prompted considerable world interest in the production of synfuels which, as a result, led to the development of the Mobil Methanol to Gasoline Process [1] and more recently to the Shell Middle Distillate Process [2]. Synfuels have been a subject of both scientific and political interest for most of the 20th century. With the recent Gulf crisis, which has resulted in a surge in crude oil prices, it is clear that the search to find economic alternatives to the use of petroleum as an energy source will continue. This observation is based on the fact that the world reserves of coal and natural gas far exceed those of oil, and this fact alone implies that the conversion of coal and natural gas to transportation fuels is almost certain to become more widespread within the next 20 years.

Mechanistic Aspects of the Formation of Hydrocarbons and Alcohols from CO Hydrogenation

Surface oxidation states of germanium

The adsorption of chlorine on Si(111)-(7\ifmmode\times\else\texttimes\fi{}7) has been studied with various photoemission techniques with the use of synchrotron radiation. At room temperature, bonding of one, two, and three chlorine atoms to a Si surface atom was observed with Si 2p core-level shifts of 930, 1830, and 2730 meV to higher binding energy, respectively. After annealing to 400 \ifmmode^\circ\else\textdegree\fi{}C only monochloride species remained. For the annealed (400 \ifmmode^\circ\else\textdegree\fi{}C) ordered Cl/Si(111)-(7\ifmmode\times\else\texttimes\fi{}7) system four chlorine-induced valence states could be distinguished with polarization-dependent angle-resolved photoelectron spectroscopy, i.e., two nondispersive bonding Cl-Si states: \ensuremath{\sigma} (${E}_{B}$=7.21 eV below the top of the valence band), s (${E}_{B}$=8.7 eV), and two dispersing Cl-Cl \ensuremath{\pi} states of different symmetry ${\ensuremath{\pi}}^{+}$${\ensuremath{\pi}}^{\mathrm{\ensuremath{-}}}$). The dispersion of the ${\ensuremath{\pi}}^{+}$ (${\ensuremath{\pi}}^{\mathrm{\ensuremath{-}}}$) band is \ensuremath{\Delta}${E}^{+}$=1000 meV (\ensuremath{\Delta}${E}^{\mathrm{\ensuremath{-}}}$=320 meV) along \ensuremath{\Gamma}\ifmmode\bar\else\textasciimacron\fi{} M\ifmmode\bar\else\textasciimacron\fi{} and \ensuremath{\Delta}${E}^{+}$=900 meV (\ensuremath{\Delta}${E}^{\mathrm{\ensuremath{-}}}$=450 meV) along \ensuremath{\Gamma}\ifmmode\bar\else\textasciimacron\fi{} K\ifmmode\bar\else\textasciimacron\fi{}. The photoemission angular distribution of the \ensuremath{\sigma} and s states shows maximum intensity in the gaps of the projected Si band structure and attenuated emission where the chlorine bands overlap with Si bands. The attenuation is explained by hybridization with the substrate bands.

Electronic properties and bonding sites for chlorine chemisorption on Si(111)-(7 x 7).

The interaction of elementary sulfur with Ge(100)(2\ifmmode\times\else\texttimes\fi{}1) surfaces was investigated using low-energy electron diffraction, Auger-electron spectroscopy, and photoemission core-level spectroscopy. Chemisorption of sulfur results in a binding-energy shift of the Ge 3d core-level electrons of \ensuremath{\Delta}E=0.33\ifmmode\pm\else\textpm\fi{}0.01 eV per S---Ge bond. The adsorbed sulfur dissociates and leads to an ordered (1\ifmmode\times\else\texttimes\fi{}1) reconstruction with one S atom per surface unit cell. The adsorbed sulfur atoms are bonded on bridge sites. Sulfur deposition exceeding one monolayer is possible, probably due to defects; the excess can be selectively thermodesorbed. During room-temperature adsorption, no S islands are formed. Thermodesorption is destructive, possibly etching.

Photoemission surface core-level study of sulfur adsorption on Ge(100).

The intensity and energy shift of the Ge 3d surface core-level photoemission is determined with the use of synchrotron radiation. Cl- and Br-terminated Ge(111)?(hy(1\ifmmode\times\else\texttimes\fi{}1) surfaces are used to calibrate the absolute coverage CTHETA and to obtain accurate line shapes. We find binding-energy shifts \ensuremath{\Delta}E=+0.57 eV for Cl/Ge(111)-(1\ifmmode\times\else\texttimes\fi{}1) and for Cl/Ge(100)-(2\ifmmode\times\else\texttimes\fi{}1), \ensuremath{\Delta}E=+0.47 eV for Br/Ge(111)-(1\ifmmode\times\else\texttimes\fi{}1), \ensuremath{\Delta}${E}_{1}$=-0.26 eV and \ensuremath{\Delta}${E}_{2}$=-0.76 eV for clean Ge(111)-c(2\ifmmode\times\else\texttimes\fi{}8), and \ensuremath{\Delta}E=-0.43 eV for clean Ge(100)-c(4\ifmmode\times\else\texttimes\fi{}2). The coverage of special surface atoms (in monolayers) is ${\mathrm{CTHETA}}_{1}$=0.93, ${\mathrm{CTHETA}}_{2}$=0.25 for clean Ge(111)-c(2\ifmmode\times\else\texttimes\fi{}8), CTHETA=0.62 for clean Ge(100)-c(4\ifmmode\times\else\texttimes\fi{}2), and CTHETA=1.03 for Cl/Ge(100)-(2\ifmmode\times\else\texttimes\fi{}1). The electron mean free path is 5.4 A\r{} at a kinetic energy of 25 eV. With regard to structural models we conclude that the asymmetric dimer model fits the Ge(100) data best, with a possible contribution of 24% symmetric dimers. For Ge(111)-c(2\ifmmode\times\else\texttimes\fi{}8) two new models are proposed with adatoms and tetramer building blocks, respectively.

R.D. Schnell

Papers

Surface oxidation states of germanium

Electronic properties and bonding sites for chlorine chemisorption on Si(111)-(7 x 7).

Photoemission surface core-level study of sulfur adsorption on Ge(100).

Surface core-level shifts for clean and halogen-covered Ge(100) and Ge(111).

Angular distribution patterns of photoelectrons from orbitals of CO adsorbed on Ni(100), Pt(111) and Pt(110)