Anomalous scaling behavior and surface roughening in molecular thin-film deposition

The thin film growth dynamics of a molecular semiconductor, free-base phthalocyanine $\left({\mathrm{H}}_2\mathrm{Pc}\right)$, deposited by organic molecular beam deposition, has been studied by atomic force microscopy (AFM) and height difference correlation function (HDCF) analysis. The measured dynamic scaling components (${\alpha }_{\mathit{loc}}=0.61\pm 0.12$, $\beta =1.02\pm 0.08$, and $1∕z=0.72\pm 0.13$) are consistent with rapid surface roughening and anomalous scaling behavior. A detailed analysis of AFM images and simple growth models suggest that this behavior arises from the pronounced upward growth of crystalline ${\mathrm{H}}_2\mathrm{Pc}$ mounds during the initial stages of thin film growth.

Figure 1

(Color online) (a) Schematic of the structure of the ${\mathrm{H}}_2\mathrm{Pc}$ $\alpha$-herringbone unit cell; the molecular planes are slightly tilted with $\tau =\pm 8.6°$. (b)–(f) AFM images for ${\mathrm{H}}_2\mathrm{Pc}$ thin films deposited on glass substrates with various thicknesses $\left(D\right)$ of (b) $651\mathrm{\AA }$, (c) $930\mathrm{\AA }$, (d) $1628\mathrm{\AA }$, (e) $3720\mathrm{\AA }$, and (f) $7440\mathrm{\AA }$.

Figure 2

(a) Plot of averaged $g(R,D)$ vs $R$ for ${\mathrm{H}}_2\mathrm{Pc}$ films with various $D$ ($●=651\mathrm{\AA }$, $*=930\mathrm{\AA }$, $∎=1628\mathrm{\AA }$, $▵=3720\mathrm{\AA }$, and $◆=7440\mathrm{\AA }$). Also shown are plots of (b) ${\alpha }_{\mathit{loc}}$, (c) $\sigma$, (d) $\xi$, and (e) $d$ as a function of film thickness, $D$. The horizontal dashed line in (c) denotes the average value of ${\alpha }_{\mathit{loc}}$.

Figure 3

(Color online) Schematic of the two growth models used for HDCF analysis. In case A, the mounds grow in every direction, whereas in B, they grow only in an upward direction. The starting surface is the same for both models.

Figure 4

Plots of averaged $g(R,D)$ vs $R$ for growth models A (a) and B (b). The insets show the corresponding plots of ${\alpha }_{\mathit{loc}}$, $\sigma$, and $\xi$ as a function of $D$.