Transition from compact to porous films in deposition with temperature activated diffusion

TL;DR

This study models thin film growth with temperature-activated diffusion, revealing how porosity and surface roughness evolve with temperature and diffusion parameters, and identifying a critical percolation transition affecting pore structure.

Contribution

It introduces a detailed simulation of thin film growth incorporating overhangs and pores, analyzing the effects of temperature and diffusion ratios on porosity and surface morphology.

Findings

Porosity exhibits a minimum at a specific temperature and increases at higher temperatures.

The deposit structure at high temperature resembles a critical percolation cluster.

Pore size distribution follows a power law with an exponent around 1.45.

Abstract

We study a thin film growth model with temperature activated diffusion of adsorbed particles, allowing for the formation of overhangs and pores, but without detachment of adatoms or clusters from the deposit. Simulations in one-dimensional substrates are performed for several values of the diffusion-to-deposition ratio R of adatoms with a single bond and of the detachment probability epsilon per additional nearest neighbor (NN), respectively with activation energies are Es and Eb. If R and epsilon independently vary, regimes of low and high porosity are separated at 0.075 < epsilon_c < 0.09, with vanishingly small porosity below that point and finite porosity for larger epsilon. Alternatively, for fixed values of Es and Eb and varying temperature, the porosity has a minimum at Tc, and a nontrivial regime in which it increases with temperature is observe above that point. This is related to the large mobility of adatoms, resembling features of equilibrium surface roughening. In this high-temperature region, the deposit has the structure of a critical percolation cluster due to the non-desorption condition. The pores are regions enclosed by blobs of the corresponding percolating backbone, thus the distribution of pore size s is expected to scale as s^{-tau} with tau ~ 1.45, in reasonable agreement with numerical estimates. Roughening of the outer interface of the deposits suggests Villain-Lai-Das Sarma scaling below the transition. Above the transition, the roughness exponent alpha ~ 0.35 is consistent with the percolation backbone structure via the relation alpha = 2-d_B, where d_B is the backbone fractal dimension.