1. Alloy Formation at the Co-Al Interface for Thin Co Films Deposited on Al(001) and Al(110) Surfaces at Room Temperature* N.R. Shivaparan, M.A. Teter, and R.J. Smith, Physics Dept., Montana State University, Bozeman MT 59717  Goal: Understand epitaxial growth of metals on metals  Contributing factors include: lattice matching, surface energies, formation energies, strain energy  Related work: "small" atoms (Ni, Fe, Pd,) form alloys at the Al surface; "large" atoms ( Ag, Ti ) tend to form an overlayer at room temperature (kinetic barrier)  What about Co ? Alloy or overlayer?  Co-Al phase diagram very similar to Ni-Al  15% lattice mismatch with Al  metallic radii of Co and Ni both smaller than Al  surface energy of Co larger than for Al  negative formation energy: -54 kJ/mole for CoAl  Motivation: recent interest in GMR and tunneling devices  Co/Al 2 O 3 /Co structures may have diffuse interfaces which affect electron (spin) transport  Present work: XPS, HEIS, LEED for Co deposition using resistively heated Co wires. Figure 1. (left) HEIS Channeling spectra for 0.84 ML Co Measure an increase of Al surface peak, so Co atoms cause Al atoms to move off lattice sites (alloy formation) Coverage of Co is determined from area of Co peak Figure 2. (right) HEIS Al surface peak vs. Co coverage Number of visible Al atoms increases up to 3 ML Slope of 2:1 suggest a stoichiometry of Al 2 Co for the interface but Al 2 Co is not found in the bulk phase diagram Interface formation stops after 3 ML and Co metal covers the interface Figure 3. (left) HEIS Co surface peak (aligned) vs. Co coverage  No decrease in Co surface peak area for aligned geometry as compared to a random incident direction  Thus no ordered Co structure aligned with the substrate (100) axis. Figure 4. (right) XPS Intensity for Al 2p and Co 2p 3/2 vs Co coverage  Al intensity is normalized to clean surface value; Co intensity is normalized to measured value at 7.4 ML  Al yield is attenuated, but more slowly than expected for growth of a Co metal adlayer; Co yield grows rapidly, non-linearly  Lines are from model calculations for growth of CoAl compound with (100) orientation, followed by Co metal island growth after 3 ML  Co binding energy shifted to larger values in interface, ~ 0.1 eV Figure 5. (far right) LEED patterns for Co on Al(001) at 42.8 eV  (a) Clean Al(001)  (b) 0.5 ML Co destroys pattern completely  (c) 7.6 ML A hint of some long range order (1x1) is seen. Figure 6. HEIS Al yield vs. Co coverage on Al(110) Number of visible Al atoms increases up to 5 ML Slope of 2.3:1 suggests a stoichiometry of Al 2 Co or Al 5 Co 2. Al 5 Co 2 exists in bulk phase diagram, with formation energy of -41 kJ/mole, but a more complex crystal structure, so less likely to form at room temperature. Interface formation stops after 5 ML and Co metal covers the interface. Figure 7. (right) XPS Intensity for Al 2p and Co 2p 3/2 vs Co coverage on Al (110)  Al intensity normalized to clean surface value; Co intensity normalized to thick film value  Al yield is attenuated very slowly at first, but rapidly after 5 ML; suggests compound growth followed by Co metal in a layer-by -layer mode  Lines are from model calculations for growth of CoAl compound with (110) orientation, then Co metal growth in a layer-by-layer mode Conclusions:  Co deposition leads to interface alloy formation on both Al surfaces, similar to that seen for Ni on Al(110) V. Shutthanandan, et al., Surf. Sci 450 (2000) 204  Behavior seen here for Co-Al differs from that for Ni-Al in that the Co-Al interface is more abrupt (thinner) and has a single growth stage, i.e. a single compound.  No strong evidence observed for an ordered epitaxial compound on either surface. Hint of order on (100).  XPS intensities are fit well using a CoAl stoichiometry but HEIS data suggests more Al rich interface. May be related to dechanneling below the interface.  No diffusion of Co into Al is observed at room temperature. *Work supported by NSF Grant DMR-9409205 and 0077534, and by NASA EPSCoR grant NCCW-0058. A copy of this poster is available at http://www.physics.montana.edu/Ionbeams/ionbeams.html