Polycrystalline domain structures and epitaxial structures ofPn thin film crystals on a H-Si(111) surfaceSusumu Nishikata1, Gen Sazaki1,2, Jerzy T. Sadowski1, Abdullah Al-Mahboob1,Shozo Suto3, Toshio Sakurai1, Kazuo Nakajima1
1

IMR, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan,
2CIR, Tohoku University, Aramaki, Aoba-ku, Sendai 980-8578, Japan,
3
Department of Physics, Graduate School of Science, Tohoku University, Aramaki,Aoba-ku, Sendai 980-8578, Japan
Introduction To establish the general method to grow high-quality organic semiconductor thin film
crystals, one has to observe changes in morphology and measure kinetics of nucleation and growth, etc
in situ. Low-energy electron microscopy (LEEM) is a method of great promise to observe such processes during the growth of organic thin film crystals. In this study, we observed the dendrite growth
process of pentacene (Pn) thin film crystals on a H-Si(111) surface by LEEM in situ, and demonstrated
the multi-domain structures inside the Pn thin film crystals and their epitaxial structures.
Experiment A H-Si(111) surface was prepared by chemical etching, and transferred to an observationchamber (base pressure: 5 x 10-10 Torr) and heated up at 150 °C for 12 hrs to degas the substrate. Pn
thin film crystals were grown by thermal evaporation, and observed by LEEM and low-energy electron diffraction (LEED) modes.
(b)(a)
Results Figs. 1(a)-(d) show LEEM images and LEED patternsof Pn thin film crystals on a H-Si(111) surface. Since a Pn islandwith a dendritic shape grew on a H-Si(111) surface (Fig. 1(a)),
surface diffusion of Pn molecules on a H-Si(111) surface limited
6.0µm
the growth of Pn thin film crystals. A LEED pattern taken from aSi
1.1eV
14.7eVPn
Pn island using a selected area of five micron in diameter (a cir- (c)(d)
cle in Fig. 1(a)) presents 24 diffraction spots (Fig. 1(b)). Hence, a
Pn island in Fig. 1(a) is a polycrystalline dendrite. Using a tilted
bright-field mode and changing an acceleration voltage, we succeeded in observing domain structures inside the Pn island withPn
three kinds of contrasts (white, gray and black) (Fig. 1(c)). In
6.0µm
Si
2.1eV
14.7eV
addition, a micro-LEED pattern taken from a single-contrast
Fig. 1. LEEM images and LEED patterns of Pn
area using a selected area of two micron in diameter (a circle in thin film crystals on H-Si(111). (a) A bright-field
image. (b) A LEED pattern from the area
Fig. 1(c)) presents four diffraction spots of a single-crystalline LEEMof the circle in Fig. 1(a). (c) A tilted bright-fielddomain (Fig. 1(d)). From the analyses of systematic LEED ob- LEEM image. (d) A micro-LEED pattern fromservations, we concluded that Pn molecules grow in the “stand- the single-contrast area of the circle in Fig. 1(c).
ing-up” orientation with the Pn(001) plane parallel to the substrate surface and there are two kinds ofepitaxial relations between Pn thin film crystals and H-Si(111). We found that each epitaxial relation
has six equivalent orientations of two-dimensional unit cells, and the two kinds of epitaxial relations
have almost same molecular packing but their angular relations with a H-Si(111) surface are different.
Conclusion: We succeeded in visualizing the domain structures inside the polycrystalline Pn dendrite insitu by LEEM. From the systematic LEED observations, we determined the epitaxial structures of Pn
thin film crystals on a H-Si(111) surface.