Orientation Analysis of Organic Thin Film on Single Crystal Sub by In-plane XRD

Introduction

Organic TFT (thin film transistor) displays hold great promise as they can be produced in large volumes relatively cheaply. They can also be fabricated on flexible substrates resulting in thin, lightweight flexible displays. The response time is a function of the carrier mobility of the organic TFT material. By orienting the organic molecule in a specific direction, its performance can be significantly improved. Next to pentacene, the sexithiophene (6T) molecule is perhaps the most promising organic semiconductor due to its high carrier mobility. In this report we evaluate the orientation of 6T thin films using X-ray diffraction (XRD).

Measurements and results 

A 6T thin film was grown on a KBr 100 single crystal substrate. Figures 1 and 2 show the out-of-plane XRD (θ/2θ scan) and the in-plane XRD⁽¹⁾ profiles respectively. 

Figure 1: The out-of-plane XRD (θ/2θ scan) profile for the 6T thin film (The diffraction from lattice plane parallel to the sample surface). 

Figure 2: The in-plane XRD profile for the 6T thin film (The diffraction from lattice plane perpendicular to the  sample surface). Top: 2θχ/φ scan, Bottom: φ scan for 6T 020 reflection. 

From Fig. 1 and 2, it is evident that 6T molecules are oriented three-dimensionally and the orientation relationship with the KBr substrate is summarized below:

[100] 6 T // [001] KBr (lattice plane parallel to the sample surface).
[010] 6T, [001] 6 T // <100> KBr (crystal axis perpendicular to the sample surface).

Furthermore, the AFM image suggests this three-dimensional orientation is due to nucleation and growth in the <100> step of KBr⁽¹⁾. 

Samples provided by: Saiki laboratory, The University of Tokyo.

Reference

(1) S.Ikeda, M.Kiguchi, Y,Yoshida, Y,Kiyoshi,T.Mitsunaga, K,Inaba, K.Saiki: J. Crystal Growth., 265 (2004) 296-301.