HOMOEPITAXIAL GROWTH ON PD(100)

We have studied the temperature- and coverage-dependent variation of d iffraction-spot profiles during growth of Pd on Pd(100), using convent ional low-energy electron diffraction (LEED). The crystal temperature is varied from 100 to 500 K during deposition, and the coverage is var ied from 0 to 5 monolayers. The diffraction spot profiles are separabl e into two components: the Bragg peak, reflecting layer occupations; a nd a broad ''foot'', reflecting the distribution of particles within l ayers. Between 200 and 400 K, the broad component adopts a ring-like a ppearance, with intensity maxima bracketing the Bragg peak. Developmen t of the ring structure is associated with the onset of diffusion at 1 70-200 K (activation barrier of 12-14 kcal/mol). The diameter of the r ing indicates an average separation between island centers of 28 +/- 5 angstrom at 300 K and at coverages below the onset of coalescence. Os cillations in the ring intensity as a function of coverage occur out-o f-phase with oscillations in the Bragg intensity, and are associated w ith the successive filling of individual layers. By 500 K both oscilla tions disappear, apparently due to the advent of step propagation as a major mechanism for film growth when diffusional processes accelerate .