Photon Factory Activity Report 2005 #23Part B (2006)
Surface and Interface

7A/2003G259

Molecular orientation of the pentacene ultrathin films grown on SiO2 substrates
11
1
Genki YOSHIKAWA*, Tetsuhiko MIYADERA,3 Ryo ONOKI2, Keiji 4UENO2, Ikuyo NAKAI,
3
3
1
Shiro ENTANI , Susumu IKEDA , Dong GUO,Manabu
KIGUCHI,
Hiroshi
KONDOH,Toshiaki OHTA1, Koichiro SAIKI1,3
1
Graduate School of Science, The University of Tokyo, Bunkyo, Tokyo 113-0033, Japan
2
Faculty of Science, Saitama University, Shimo-Okubo 255, Saitama, 338-8570, Japan3Graduate School of Frontier Sciences, The University of Tokyo, Bunkyo, Tokyo 113-0033, Japan
4
Graduate School of Science, Hokkaido University, Sapporo, 060-0810, Japan

Introduction
Pentacene has been attracting much attention recently
because it shows the highest FET mobility. The
orientation of pentacene molecules in the first few
monolayers has large influence on the mobility. There
were some ex situ measurements of the orientation ofpentacene grown on SiO2 substrates and showed theStanding-mode growth (Fig. 1). The atmosphere would,
however, have influence on the orientation or morphologyof pentacene films as shown in the previous study [1].
Accordingly, we carried out all experimental processes
from deposition of pentacene to characterization of films
under vacuum condition without exposure to theatmosphere in order to reveal whether the observed
structure is truly as-grown one or changed by theinfluence of the atmosphere.
ExperimentThree types of SiO2 substrates were used; prepared byoxidation of a Si wafer with Shiraki method, Shiraki method
and following anneal in dry O2, and a commercially obtained
Si wafer with thermally oxidized amorphous SiO2 layer.
Pentacene was evaporated from a Knudsen cell in acustom-designed all-in-vacuum system with a base pressure-7of 10 Pa. C K-edge NEXAFS and AFM measurements
were carried out under vacuum condition without exposing
the specimens to the atmosphere.

Figure 1: Schematic illustrations of the possible
configurations of pentacene molecules on a substratewith each π* orbitals. The incident directions of X-ray
with its electric vector are also shown.
*present address: Institute for Materials Research,Tohoku University; yoshik-0@imr.tohoku.ac.jp

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Users' Report

Results and DiscussionFigure 2 shows the NEXAFS spectra of pentacene films
deposited on the SiO2 substrate prepared byShiraki-method. Pentacene films deposited on SiO2substrates prepared by other methods also showed similar
NEXAFS spectra. Clear polarization dependence wasobserved in all films with different thickness, that is,
π*-peak was most enhanced at normal X-ray incidence(90°), while almost no peak was observed at grazing
incidence (15°), indicating that pentacene molecules grewwith Standing-mode. Detailed curve fitting analysis of
π*-peaks revealed that the inclination angle of π* orbital to
the substrates was almost 0° in the sub-monolayer film, andless than 10° ± 5° even in the film with 10 nm thick (shownas ‘multi layer’ in Fig. 2). We also measured AFM and theheight of each islands showed about 1.6 nm, indicating theStanding-mode growth. The results of NEXAFS and AFM
revealed that pentacene grows in Standing-mode from theinitial stage of the growth in vacuum condition irrespectiveof the type of SiO2
substrates preparedbydifferent procedures. Thus,
our study elucidated that thepentacene islands withstanding molecules were notformed by the effect ofexposure to the atmospherebut by the nature of thepentacene on SiO2. The
growth mechanism could be
explained by the interaction
between the molecules andFigure 2: C K-edge NEXAFS spectra of pentacene
films grown on SiO2 prepared by Shiraki-method.
Peaks observed around 283-288 eV correspond to thetransition from C 1s to π* orbital of pentacene.
the substrates, and the thermodynamic behavior of growing
molecules [2,3].
References[1] Y. Qiu et al., Appl. Phys. Lett. 83 (2003) 1644.
[2] A. Kubono et al., Mol. Cryst. Liquid Cryst. 378 (2002) 167.
[3] G. Yoshikawa et al., Surf. Sci. in press.