Ultra-Low Coverage Spontaneous Etching and Hyperthermal Desorption of Aluminum Chlorides from Cl 2 /Al(111) Tyler J. Grassman, Gary C. Poon, and Andrew C. Kummel University of California, San Diego Gordon Research Conferences: Dynamics at Surfaces – August 10-15, 2003 Time-of-Flight Distributions Normalized Al + Intensity (arb.) (Inc. Cl 2 /Ne T s = 500 K) Time (  sec) Observed AlCl 3 TOF Expected 500 K AlCl 3 TOF (b) Normalized Al + Intensity (arb.) (Inc. Cl 2 /Ne T s = 100 K) Time (  sec) (a) Observed Al 2 Cl 6 TOF Expected 100 K Al 2 Cl 6 TOF Maxwell-Boltzmann-like time-of-flight distribution curves for the etch products from the 0.27 eV incident Cl 2 beam on the (a) 100 K and (b) 500 K Al(111) surface. The solid curves show the experimentally observed desorption distribution, and the dashed curves show the expected thermal desorption time-of-flight distribution for the etch product mass and surface temperature of interest, as indicated in the figures. The most probable time-of-flights are indicated by the vertical, single-headed arrows. The full-width half-max of the experimental distributions are indicated by the horizontal, double-headed arrows. Values extracted from experimental time-of-flight spectra compared against values taken from expected thermal distributions (flux-weighted Maxwell-Boltzmann for density detector): Thermal distribution should exhibit most-probably velocity to width ratio (v p :w) of about 1. An experimental ratio smaller than unity indicates a wider distribution than would be expected for a purely thermal desorption mechanism. “most-probable time of flight” “most-probable velocity” “most-probable translational energy” Non-resonant multiphoton ionization (MPI) and time-of-flight mass spectrometry (TOF-MS) have been used to monitor the desorption of aluminum chloride (Al x Cl y ) etch products from the Al(111) surface at 100 K and 500 K during low-coverage ( 1 monolayer) and subsurface Cl diffusion. Density functional theory calculations yield results that are consistent with both our experimental findings and mechanistic descriptions.Abstract Experimental Methods Very low Cl 2 flux (~ 2x10 14 cm -2 sec -1 ) pulsed molecular beam MPI (210.2 nm) and TOF-MS → photodissociate Al x Cl y, detect Al + King & Wells type sticking measurements with QMS DFT-GGA with Vanderbuilt ultrasoft pseudopotentials and plane-wave basis, 7×6×1 21 k-point Monkhorst-Pack, 250 eV plane-wave cut-off for Cl/Al(111) adsorbate system (Vienna Ab-initio Simulation Package, VASP) QMS TOF-MS Pulsed Nozzle (10Hz) (Cl 2 /He, Cl 2 /Ne pure Cl 2 ) LEED AES Al(111) Skimmer Mechanical Chopper (7  s) Collimator Ionizer (210.2 nm) Measured velocities, translational energies, and peak-to-width ratios for desorbing aluminum chlorides at all incident Cl 2 translational energies and Al(111) surface temperatures studied. a: Translational energy calculated from mass of AlCl 3 etch product and measured v p b: Translational energy calculated from mass of Al 2 Cl 6 etch product and measured v p c: Total dosing time ≤ 10 sec d: Total dosing time > 10 sec T s (K) E inc (eV) pure Cl 2 Cl 2 /Ne Cl 2 /He Cl 2 /He: high d Cl 2 /He: low c Dose Species ― ± ― ± ― ± E (eV) b Al 2 Cl ± ± ― ± ― ± ― E (eV) a AlCl 3 ― ± ± ± ± ± ± 9.2 v p (m/s) ± ± ± ± ± ± ± ± 0.06 v p :wResults Exit vs. Incident Velocities/Energies (b) Cl 2 Incident Energy, E inc (eV) Exit Energy, E exit (eV) Thermal AlCl 3 (exp) Al 2 Cl 6 (exp) (T s = 100 K) (c) Cl 2 Incident Velocity, v inc (m/s) Exit Velocity, v exit (m/s) Experimental AlCl (thermal) AlCl 3 (thermal) (T s = 500 K) (a) Cl 2 Incident Velocity, v inc (m/s) Exit Velocity, v exit (m/s) Experimental AlCl 3 (thermal) Al 2 Cl 6 (thermal) (T s = 100 K) (d) Cl 2 Incident Energy, E inc (eV) Exit Energy, E exit (eV) Thermal AlCl (exp) AlCl 3 (exp) (T s = 500 K) Etch product exit velocities and energies plotted against incident velocities and energies, respectively, for surface temperatures of 100 K and 500 K. The open symbols (squares, circles, triangles, diamonds) represent experimental data, while the filled symbols represent the expected values from a purely thermal desorption mechanism. The data clearly shows that the etch products are exiting the surface at hyperthermal velocities. Etch Rate Profiles Etch Rate (arb.) Time (sec) Begin surface Cl 2 exposure m/s Al 2 Cl 6 component (T s = 100 K) (a) Time (sec) Etch Rate (arb.) 550 m/s AlCl 3 component (T s = 500 K) Begin surface Cl 2 exposure (b) Etch rate profiles of the 0.27 eV incident energy Cl 2 on (a) the 100 K and (b) the 500 K Al(111) surface. The insets are blow-ups of the shaded regions and show the first 5 seconds of data. As seen in the figures, etching begins immediately upon exposure of the Al(111) surface to the low-flux Cl 2 molecular beam, at surface coverages of < 5% monolayer. Such results indicate fast time-scale surface agglomeration of adsorbed Cl atoms/molecules and submolecular aluminum chlorides. DFT: Bonding to Al Adatoms Non-bonded Bonded  E = -2 eV DFT slab calculations of Cl adsorbates on the Al(111) surface near an Al adatom indicate a strong energetic preference for Cl atoms to bond to Al adatoms rather than remain dispersed on the surface (out of bonding range). This is likely to be the case for regrowth islands and step edges, as well. The surface reaction, 3Cl (ad.) + Al (ad.) → AlCl 3(ad.), is computationally found to be about 2 eV thermodynamically favorable. These AlCl 3 adsorbates are also likely be highly mobile on the surface, as the surface binding potential for the molecules (with respect to the different possible adsorption sites) was found to have a maximum corrugation of only 0.1 eV. Al Cl Al (ad.) DFT: Bonding on Terraces DFT slab calculations of the Cl/Al(111) adsorbate/terrace system meant to examine the possibility of adsorbate clustering do not show a thermodynamic preference for clustering geometries. However, the calculations do indicate a strong preference for ontop adsorption sites, with differences in total energies of -0.4 to -0.9 eV compared to other sites. Ontop-site Cl adsorbates are also found to pull the Al terrace atom to which they were bonded out of the surface plane by 0.4 Å, thereby likely making them more vulnerable to attack by other atomic or molecular adsorbates, and helping to replenish nucleation sites. Al Cl Dispersed Clustered Conclusion The unusual desorption phenomena observed in this work is consistent with a model consisting of a combination of fast surface agglomeration of Cl and Al x Cl y adsorbates (as seen in the etch rate profiles, as well as the computational data) and the existence of activated aluminum chloride chemisorption states, with potential wells above the vacuum level. The activated chemisorption state model is diagrammed in the figure above (b), and is compared with the standard activated chemisorption model (a) in which the chemisorption well is below the vacuum level and desorbing species must surmount an activation barrier. Distance From Surface (arb.) Energy (arb.) vacuum level chemisorption well exit kinetic energy (a) Distance From Surface (arb.) Energy (arb.) vacuum level activated chemisorption state chemisorption well exit kinetic energy (b) Standard Activated Chemisorption Model Activated Chemisorption State Model