Energetic and entropic contributions to surface diffusion and epitaxial growth

For Pd/Pd(111) an exceptionally high barrier (350 meV) for surface self-diffusion and a negative additional energy DeltaE = -53 meV for step-down diffusion are measured. Both findings agree with the proposed role of free-electron-like surface states. Despite the negative value of DeltaE layer-by-layer growth is not observed. This is related to the low preexponential factor for step-down diffusion. Preadsorption of oxygen increases DeltaE but flattens the films. Again this is due to the prefactors for diffusion. The present results demonstrate the importance of entropic effects for diffusion and growth.