Dissociative Chemisorption at Hyperthermal Energies: Benchmark Studies in Group IV Systems
- PDF / 399,789 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 108 Downloads / 188 Views
EXPERIMENTAL PROCEDURES The experiments were conducted in a custom-designed multiple-stage stainless steel ultrahigh vacuum (UHV) chamber (base pressure < 10-10 Torr) that is coupled to a triply differentially pumped supersonic beam line, which is described in detail elsewhere [9]. The supersonic beam is generated via a heatable nozzle source (orifice dia.= 150-250 gm), and passes through two intermediate stages before striking the sample. The main UHV chamber houses a differentially pumped, rotatable quadrupole mass spectrometer (QMS) that may be employed to measure angular distributions of species scattered from the surface, or in direct (e.g., time-offlight) analysis of the incoming molecular beam. Both the Si(1 00) and Si(l 11) samples (cut to 1.9 x 2.3 cm from 0.04 cm. thick, 1-10 Q-cm Si wafers) were prepared by standard RCA clean, inserted into vacuum, and annealed in situ to 1000°C [I 150°C for Si(1 11)] for 4 minutes to remove the surface oxide. Surface cleanliness and order is verified in situ via x-ray photoelectron
spectroscopy (XPS) and low energy electron diffraction. The substrates are mounted on a precision manipulator, the axis of which is coincident with the molecular beam, that permits both polar rotation and tilt, the latter providing variation of the angle of incidence (±1°*) of the molecular beam with respect to the surface normal. Substrate temperatures are measured by a calibrated infrared pyrometer, and the sample is heated via radiation by a hot filament on the back side of the wafer. Primarily due to safety concerns the reactants were provided in pre-diluted mixtures (1% reactant in ultrahigh purity H2 or He) directly from the vendor. The translational energies of the reactant gases were varied by employing a combination of seeding techniques (i.e., H2 or He carrier) and heating the nozzle, and were measured directly via time-of-flight (TOF) techniques employing the rotatable QMS. Reaction probabilities were measured by the beam reflectivity technique pioneered by King and Wells [101. Briefly, the reaction probability is calculated from the change in partial pressure of the reactant produced by the scattered flux when the molecular beam is either impinging upon the Si substrate or a non-reactive beam flag (Macor). In our approach the beam is additionally modulated prior to entering the scattering chamber to clearly distinguish between the partial pressure rise produced by the collimated supersonic beam and that produced from the small effusive component (
Data Loading...
