Influence of Surface Roughness on the Dynamics and Crystallization of Vapor-Deposited Thin Films

Abstract

The substrate roughness is a very important parameter that can influence the properties of supported thin films. In this work, we investigate the effect of surface roughness on the properties of a vapor-deposited glass (celecoxib, CXB) both in its bulk and in confined states. Using dielectric spectroscopy, we provide experimental evidence depicting a profound influence of surface roughness on the α-relaxation dynamics and the isothermal crystallization of this vapor-deposited glass. Besides, we have verified the influence of film confinement on varying values of surface roughnesses as well. At a fixed surface roughness value, the confinement could alter both the dynamics and crystallization of vapor-deposited CXB.

Figure 1

The molecular structure of CXB compound investigated in this study.

Figure 2

Schematic diagram showing the experimental setup used to modify the surface roughness of the Si wafers. The wafers were exposed to HF for different amounts of time to obtain various surface roughness values.

Figure 3

AFM 3D topographic images showing Si substrates treated with HF for different amounts of time, (a) 20, (b) 40, and (c) 60 min, which produced varying surface roughnesses R_rms ∼ 0.5 nm, R_rms ∼ 1.5 nm, and R_rms ∼ 5 nm, respectively.

Figure 4

PSD profile of a Si wafer surface with R_rms ∼ 0.5, 1.5, and 5 nm, respectively (20, 40, and 60 min of HF treatment). The green line represents the ABC fit for the respective data.

Figure 5

Mean α-relaxation time (τ_α) plotted as a function of the roughness of the silicon substrate for vapor-deposited CXB films of thicknesses of 30, 70, and 200 nm.

Figure 6

Mean α-relaxation time (τ_α) plotted as a function of inverse temperature (T) for vapor-deposited CXB films with varying film thicknesses deposited on Si substrates with roughness values of (a) ∼0.5, (b) ∼1.5, and (c) ∼5 nm.

Figure 7

Crystallization rate k as a function of film thickness obtained for vapor-deposited CXB films on substrates with varying surface roughness measured at 368 K. The dashed lines are just a guide for the eyes.