We investigated the Au growth on clean and Au-covered Ir(111) surfaces using scanning tunneling microscopy (STM). Au was deposited on a clean Ir(111) surface at a rate of 1×10-3 ML/s and sample temperatures in the range from 45 to 300 K, and subsequently the surfaces were examined by STM at the deposition temperatures. We found that the island density on Ir(111) is much lower than that on the first Au layer grown on Ir(111). Using the mean-field nucleation theory [1] that connects temperature dependence of the island density with its diffusion coefficient, we derived the diffusion barrier and prefactor of Au diffusion on Ir(111) to be 0.08±0.01 eV and 1.8×1012±0.3 1/s, respectively. On the other hand, the diffusion barrier of Au adatoms on the first Au layer grown on Ir(111) is estimated to be 0.16 eV by assuming the same prefactor as that on Ir(111).
As the Au coverage is increased, we furthermore found that the second and higher Au layers grow in a multilayer fashion, while the first Au layer undergoes a layer growth. This indicates that the step-down diffusion of Au atoms from the second and higher Au layers is strongly hindered compared with that from the first Au layer. We analyzed the step-down diffusion barrier on each Au layer on the basis of the critical island radius theory [2]. The step-down diffusion barrier on the first Au layer is estimated to be smaller than 0.03 eV by assuming the same prefactor as that on Ir(111). On the other hand, the step-down diffusion barrier on the second Au layer is estimated to be higher than 0.04 eV. This deference in the step-down diffusion barriers would be related to the lattice mismatch between Au and Ir. Since the lattice constant of Au is larger than that of Ir, there is a compressive strain in the Au islands grown on Ir. The islands preferentially relieve their strain at the island edge. This relaxation can facilitate the exchange process that is reported to be energetically preferred for the step-down diffusion. Due to the larger compressive strain, the step-down diffusion barrier becomes lower on the first Au layer.
[1] J.A. Venables, Philos. Mag. 27 (1973) 697.
[2] J. Tersoff et al., Phys. Rev. Lett. 72 (1994) 266.
|