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/Al2O3/Co structures may have diffuse

interfaces which affect electron (spin) transportPresent 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 Al2Co for the

interface but Al2Co 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 2p3/2 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 Al2Co or Al5Co2.

• Al5Co2 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 2p3/2 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