Kinetic Energy Effects on Morphology Evolution During Pulsed Laser Deposition of Metal-On-Insulator Films

Abstract

We report an experimental comparison of Volmer-Weber metal-on-insulator growth morphology in pulsed laser deposition (PLD) and thermal deposition under identical thermal, background, and surface preparation conditions for Ag on SiO₂ and mica. Films exhibit a characteristic morphological progression from isolated three-dimensional islands to elongated clusters to a percolating, electrically conducting film to a pinhole-free film. We observed this same progression for films deposited by PLD. Kinetic Monte Carlo (KMC) simulations that take into account only the pulsed nature of the flux predict that PLD films should advance to percolation with less deposition than thermally deposited films under otherwise identical conditions. At low substrate temperatures, this prediction is confirmed. However, <i>in situ</i> resistance measurements and <i>ex situ</i> atomic force microscopy measurements demonstrate that at high substrate temperatures, PLD films require more deposition to reach percolation. PLD experiments performed at varying kinetic energy of the depositing Ag species suggest a regime in which increasing kinetic energy can delay the percolation transition. Comparison was made with KMC simulations of two-island coalescence in the presence of adatom-vacancy pair creation, which occurs with a greater-than-unity yield per incident ion at kinetic energy >50 eV. A mechanism controlling the delayed percolation of PLD films in the high-temperature regime is proposed: (1) the energetic deposition results in a net uphill flux from adatom-vacancy pair creation, inducing a vertical shape change; (2) taller-than-equilibrium islands coalesce more rapidly; (3) the result is an extended time period over which coalescence is efficient compared to island-island impingement; (4) the percolation transition is delayed