Studying the kinetics of microstructure formation through dewetting of As-Se thin films

The chalcogenide glasses have always drawn attention in photonics for being the most suitable materials for applications in the infrared region. Although their use in integrated photonics is promising, there are yet many challenges to integrating optical components in nano- and microscale. The dewetting of thin films is a mostly unexplored and attractive alternative to produce self-assembled structures in solid substrates. Chalcogenide glasses are among the few materials to present dewetting below 300 °C. We report the thermal-induced dewetting of ${\mathrm{As}}_x{\mathrm{Se}}_{100\text{–}x}$ thin films, deposited by electron beam. The activation energy for dewetting is obtained by analyzing the kinetics of dewetting at different temperatures. This energy is greatly affected by the number of homopolar bonds and the glass dimensionality. As a consequence, the higher activation energy is found in the composition with fewer degrees of freedom $\left({\mathrm{As}}_{40}{\mathrm{Se}}_{60}\right)$. The rupture mechanism and the size of the droplets are also greatly affected by the glass composition. This study provides an insight on how to control and use dewetting as an alternative route for nano- and microfabrication in photonics.

Figure 1

Raman spectra of ${\mathrm{As}}_x{\mathrm{Se}}_{100\text{–}x}$ thin films with $x=20$, 30, 40, and 50. The scatter curve represents the experimental data while continuous lines correspond to deconvoluted peaks and cumulative fit peak. Peak 1 corresponds to vibrations of Se-Se between $\mathrm{As}{\mathrm{Se}}_{3/2}$ units, while peaks 2 and 3 are associated to vibrations of Se chains and $\mathrm{As}{\mathrm{Se}}_{3/2}$ structural units, respectively. Peaks 4 and 5 are related to vibrations of the structural units ${\mathrm{As}}_4{\mathrm{Se}}_3$ and ${\mathrm{As}}_4{\mathrm{Se}}_4$, respectively.

Figure 2

Thermal activated dewetting of ${\mathrm{As}}_{20}{\mathrm{Se}}_{80}$ thin film (200 nm) when submitted at 300 °C ($T_g+199{}^{\circ }\mathrm{C}$). The holes appear at randomly dispersed sites in the substrate (a), and grow in time (b), (c) until they collide with each other (c), (d) and form the ribbons of material that eventually decay into droplets (e). The total dewetting of this film is achieved after 3 s at 300 °C.

Figure 3

Optical microscope images of dewetting droplets in ${\mathrm{As}}_x{\mathrm{Se}}_{100\text{–}x}$ thin films (200 nm thickness) after thermal treatment at 300 °C. The images are presented in two different magnifications (upper and bottom rows).

Figure 4

Droplet size distribution of dewetted 200-nm films of ${\mathrm{As}}_x{\mathrm{Se}}_{100\text{–}x}$, $x=20$, 30, 38, 50 glasses. The films are submitted at 300 °C until total dewetting. The figures inset show the fast Fourier transform (FFT) and radially averaged correlation function of droplet position.

Figure 5

SEM images of dewetted droplets from ${\mathrm{As}}_x{\mathrm{Se}}_{100\text{–}x}$, $x=20$, 30, 38, 50 films treated at 300 °C until total dewetting. The scale bar represents 5 μm.

Figure 6

Deconvoluted Raman spectra of ${\mathrm{As}}_{50}{\mathrm{Se}}_{50}$ thin film and droplets of dewetting at $T_g+150{}^{\circ }\mathrm{C}$ and $T_g+76{}^{\circ }\mathrm{C}$.

Figure 7

Dewetting transformation curves of ${\mathrm{As}}_x{\mathrm{Se}}_{100\text{–}x}$ ($x=20,30,40,50$) thin films with 500 nm thickness, when submitted to constant temperatures between 180 °C and 300 °C.

Figure 8

(a) Arrhenius plot of the velocity of dewetting K as a function of $1/k_{B}T$, for each glass composition. The inset shows the dependency of the K value with temperature for film ${\mathrm{As}}_{30}{\mathrm{Se}}_{70}$. (b) Effective activation energy for surface dewetting in films of 500 nm (solid circles) and 200 nm thickness (open circles). (c) Characteristic dewetting time for ${\mathrm{As}}_x{\mathrm{Se}}_{100\text{–}x}$ glasses of thickness 200 nm. The dashed lines are used as a guide to the eye.

Figure 9

Mean size of dewetting droplets for 500-nm ${\mathrm{As}}_x{\mathrm{Se}}_{100\text{–}x}$ ($x=20$, 30, 38, 50) films dewetted at different temperatures.