Saturday, May 31, 2014

A new technique for fabricating high-quality epitaxial oxide thin films on amorphous substrates

A new technique for fabricating high-quality epitaxial oxide thin films on amorphous substrates such as glass has been developed by Japaneese reserachers from University of Tokyo, Kanagawa Academy of Science and Technology, Japan Science and Technology Agency and National Institute for Materials Science. The new manufacturing method called lateral solid-phase epitaxy, could help realise applications of oxide-based thin film devices. This is especially interesting for large scale production of flexible electronics on foil or large glass substrates used in e.g. display technology. The results has been published in ACS Nano (abstract and supporting information below).

Lateral Solid-Phase Epitaxy of Oxide Thin Films on Glass Substrate Seeded with Oxide Nanosheets
Kenji Taira, Yasushi Hirose, Shoichiro Nakao, Naoomi Yamada, Toshihiro Kogure, Tatsuo Shibata, Takayoshi Sasaki, and Tetsuya Hasegawa
ACS Nano, Article ASAP, DOI: 10.1021/nn501563j, Publication Date (Web): May 27, 2014
 
 
Pictures from graphical abstratct (ACS Nano).

Abstract: We developed a technique to fabricate oxide thin films with uniaxially controlled crystallographic orientation and lateral size of more than micrometers on amorphous substrates. This technique is lateral solid-phase epitaxy, where epitaxial crystallization of amorphous precursor is seeded with ultrathin oxide nanosheets sparsely (≈10% coverage) deposited on the substrate. Transparent conducting Nb-doped anatase TiO2 thin films were fabricated on glass substrates by this technique. Perfect (001) orientation and large grains with lateral sizes up to 10 μm were confirmed by X-ray diffraction, atomic force microscopy, and electron beam backscattering diffraction measurements. As a consequence of these features, the obtained film exhibited excellent electrical transport properties comparable to those of epitaxial thin films on single-crystalline substrates. This technique is a versatile method for fabricating high-quality oxide thin films other than anatase TiO2 and would increase the possible applications of oxide-based thin film devices.

[ACS Nano free Supporting information] An alkaline-free glass substrate sparsely covered with Ca2Nb3O10 nanosheets was prepared by the same process described in the main text. Amorphous SrTiO3 (STO) precursor films were fabricated on the unheated substrate by pulsed laser deposition (PLD) with a single crystalline STO plate target. Partial oxygen gas pressure (PO2) was set at 10−3 Torr during the deposition. A 1-nm-thick STO secondary seed layer was also fabricated by PLD at TS = 400 °C prior to the deposition of the precursor film. The precursor film was crystallized by post-deposition annealing at 600 °C for 1 hour under H2 atmosphere (1 atm) in an infrared image furnace. After the annealing, the crystallographic structure and orientation of the film were determined by X-ray diffraction (XRD) measurements with a two-dimensional area detector. Figure S1a shows the θ-2θ XRD profile of the STO thin film fabricated on a glass substrate by NS-LSPE with the 1 nm-secondary seed layer. Only 100 and 200 diffraction peaks with spot-like shapes were recognizable, which indicates perfectly (100)-oriented growth of STO, as expected from good lattice-matching with Ca2Nb3O10 nanosheets (−1.0%). In contrast, in case of STO film fabricated directly on bare glass by solid phase crystallization, only Debye rings of 110 and 200 diffractions from randomly oriented grains were observed (Fig. S1b). These results verify the versatility of the NS-LSPE technique for oxide thin films other than TiO2.
 

Figure S1. θ-2θ XRD profile of STO thin films fabricated on glass substrate (a) by the NS-LSPE and (b) by conventional solid phase crystallization without nanosheets. The corresponding two dimensional area detector images are also shown. [ACS Nano free Supporting information]