Growth of Transition Metal Films on AL(110) Surfaces
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surface peak (SP) associated with ions backscattered from the topmost layers of the solid. The SP areas are converted to areal densities of visible target atoms (atoms/cm2 ) using the Rutherford scattering cross section, the solid angle subtended by the detector, and the integrated incident ion flux. Increases in the SP area for substrate atoms can be associated with displacements of those atoms off of lattice sites, a characteristic of reconstruction or alloy formation. A decrease in the SP area of substrate atoms is associated with shadowing of those atoms by the atoms in the overlayer. X-ray photoemission experiments (XPS) are used to further characterize the overlayer. All of the experiments were done in an ultrahigh vacuum chamber at a base pressure of 1 x 1010 Torr, attached to a 2 MV Van de Graaff accelerator via a differentially pumped beam line [3]. The standard dose of He" ions for one spectrum was 1.6 x 1015 ions/cm2 . Energy analysis of the backscattered He' ions for HEIS was performed using a bakeable silicon detector at a scattering angle of 1050. The sample was kept in the channeling position during film deposition and XPS measurements to eliminate problems with reproducing the ion beam-sample alignment. The Al crystals were mechanically polished and then chemically etched to remove mechanical damage. In the vacuum chamber the crystals were cleaned by Ar ion sputtering using a beam energy of 1500 eV and ion current of 1 itA/cm 2. Cleaning the sample required repeated cycles of several hours of sputtering at room temperature, followed by annealing of the samples at about 500 'C for 20 minutes. The cleaning procedure was repeated until the photopeak associated with Al-oxide was completely removed from the XPS spectrum of Al. Metals were evaporated onto the Al substrates at room temperature from resistively heated wires. Typical deposition rates ranged from 0.5 to 0.9 monolayers per minute. Metal coverage was determined using the ion backscattering spectrum. The uncertainty in the conversion of measured SP areas to ion yields in these experiments was calculated to be 5.6%, with the largest contribution to the uncertainty coming 1200 from the determination of the detector solid angle. Fe on A1(11O)
Figure 1 shows the HEIS channeling spectra of backscattered ions in the regions of the Al and Fe surface
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peaks. Each curve in the figure is
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identified by the Fe coverage. One monolayer (ML) here is equal to the atomic density of the AI(1 10) plane, 0.862 x 1015 atoms/cm 2 . The arrows and the dashed lines in the figure indicate the position on the energy axis for surface Al and Fe atoms. From the figure it is clear that the Al surface
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peak increases as the Fe deposition increases, indicating that more Al atoms are visible to the incident ion beam in the presence of Fe atoms at the surface.
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RESULTS
600 700 800 900 1000 Backscattered Ion energy (keV)
Figure 1 Ion scattering spectra for 0.96 MeV He' ions inci
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