Effects of thermal annealing and film thickness on the structural and optical properties of indium-tin-oxide thin films

TL;DR

This study examines how thermal annealing and film thickness influence the structural and optical properties of ITO thin films, revealing significant changes in crystallization and color that impact optoelectronic applications.

Contribution

It provides a detailed analysis of the effects of annealing temperature and film thickness on ITO's properties, highlighting considerations for reconfigurable display technologies.

Findings

Crystallization occurs at 250°C for films ≥15 nm.

Color change is limited for 5 nm films even after annealing.

A colormap of annealing temperature versus film thickness is provided.

Abstract

Indium-tin oxide (ITO) is a crucial functional layer for the optoelectronic applications, such as non-volatile color display thin films based on the ITO/phase-change material (PCM)/ITO/reflective metal multilayer structures on a silicon substrate. In addition to non-volatile color tuning by PCMs, thermally induced crystallization may alter the optical properties of ITO layers as well. But the potential change in color of the ITO layers is not considered so far. In this work, we investigate the structural and optical properties of ITO thin films via X-ray diffraction, spectroscopic ellipsometry and ultraviolet-visible spectrophotometry measurements. After thermal annealing at 250 {\deg}C, the ITO thin films of 15-100 nm get crystallized with strong changes in refractive index n and extinction coefficient k in the visible light range. However, for the 5-nm ITO thin film, crystallization is only observed after thermal annealing at 350 {\deg}C and the change in color is limited upon phase transition. We provide a colormap of the ITO/platinum/silicon structure in terms of the annealing temperature (150-350 {\deg}C) and ITO film thickness (5-100 nm). Our work suggests that the intrinsic change in colors of ITO layers should also be considered for the PCM-based reconfigurable display application.