Superior local conductivity in self-organized nanodots on indium-tin-oxide films induced by femtosecond laser pulses

Chih Wang,Jihperng Leu,Hsuan-I Wang,Wei-Tsung Tang,Takayoshi Kobayashi,Chih-Wei Luo

doi:10.1364/oe.19.024286

Abstract

Large-area surface ripple structures of indium-tin-oxide films, composed of self-organized nanodots, were induced by femtosecond laser pulses, without scanning. The multi-periodic spacing (~800 nm, ~400 nm and ~200 nm) was observed in the laser-induced ripple of ITO films. The local conductivity of ITO films is significantly higher, by approximately 30 times, than that of the as-deposited ITO films, due to the formation of these nanodots. Such a significant change can be ascribed to the formation of indium metal-like clusters, which appear as budges of ~5 nm height, due to an effective volume increase after breaking the In-O to form In-In bonding.

Highlights

Indium-tin-oxide (ITO) is an important transparent conducting oxide (TCO)
ITO has been extensively used as a good ohmic contact material in GaN-base light emitting diodes (LEDs), because ITO shows excellent ohmic behavior in terms of high surface current, which is evidenced by the excellent surface conductivity [6, 7]
Following normal-incidence irradiation with fs laser pulses, a periodic structure was clearly observed on the surface of ITO films, as shown in the scanning electron microscope (SEM) images of Figs. 1(a-f)

Summary

Introduction

Indium-tin-oxide (ITO) is an important transparent conducting oxide (TCO). ITO films have been widely used as transparent electrodes in optoelectronic devices, such as solar cells [1] and organic light-emitting devices (OLEDs) [2], because of their high electrical conductivity (∼10−4 Ω-cm), coupled with their high transmission (∼90%) in the visible range [3, 4]. A thermal annealing process (≥ 200 ◦C) is frequently used to change the crystallinity of ITO from amorphous (a-ITO) to crystalline (c-ITO), which results in a diminution of resistivity and an increase in transparency [8]. This conventional method of thermal annealing at high temperatures does not work for flexible devices, due to their inherently poor thermal stability and the constraint of a low glass transition temperature (Tg), for flexible substrates based on plastic materials. To overcome the constraint of avoiding a high-temperature processing step for flexible polymer substrates, ultraviolet (UV) lasers, such KrF and XeCl excimer lasers with nanosecond pulses, have been reported to anneal ITO films and modify the crystallinity, without a marked rise in the sample temperature [9,10,11]

Methods

Results

Conclusion