Structural, optical, and mechanical properties of TiO2 nanolaminates

Lilit Ghazaryan,Adriana Szeghalmi,Shiti Handa,Vivek Beladiya,Andreas Tünnermann,Vladimir Roddatis,Paul Schmitt

doi:10.1088/1361-6528/abcbc1

Abstract

The structural, optical, and mechanical properties of TiO2 nanolaminate films grown by plasma-enhanced atomic layer deposition are discussed. Several TiO2/Al2O3 and TiO2/SiO2 compositions have been investigated to study the effect of the relative number of ALD oxide cycles on the film properties to obtain a high refractive index coating with low optical losses, low roughness, and low mechanical stress. The formation of crystalline TiO2 observed at high deposition temperature, or film thickness was inhibited by periodically introducing ultra-thin amorphous layers into the film. Only 4 ALD cycles of Al2O3 (corresponding to ca. 0.5 nm) between 335 ALD cycles of TiO2 (ca. 11 nm) form a closed, distinct layer suppressing the crystallization in TiO2 film. Consequently, the roughness of the pure TiO2 film is reduced from ca. 20 nm rms to 1 nm rms in the 335/4 nanolaminate, with only a slight decrease of the refractive index from 2.46 to 2.44 in 100 nm pure TiO2 and the nanolaminate, respectively. The refractive indices of the nanolaminates in various compositions vary between 2.38 and 2.50 at 632 nm, and the corresponding optical losses from the films are low. The mechanical stress was reduced to about 140 MPa in several TiO2/Al2O3 nanolaminates; however, lower mechanical stress has not been obtained with the studied compositions. The nanolaminate structure is preserved up to 600 °C annealing temperature. After annealing at 800 °C, the individual layers interdiffuse into each other so that no distinct nanolaminate structure is detected. By using TiO2/Al2O3 nanolaminates with reduced mechanical stress, a narrow bandpass filter was realized on various substrates, including half-ball and aspherical lenses.

Highlights

Titanium dioxide (TiO2) thin films have extensively been investigated as non-toxic, chemically stable, low-cost semiconducting coatings in a wide area of applications such as solar cells [1], photocatalysis [2], batteries [3], wire grid polarizers [4], sensors [5], etc
With only a few cycles of Al2O3, the crystallization of TiO2 could be suppressed to achieve optical TiO2 coatings with properties superior to that of the pure TiO2. These properties depend on the growth temperature and individual layer thicknesses, which have a substantial impact on the surface roughness and residual stress
Thin films with the lowest residual stress of 136 MPa was achieved in the TiO2/Al2O3 84/16 nanolaminate deposited at 200 °C

Summary

Introduction

Titanium dioxide (TiO2) thin films have extensively been investigated as non-toxic, chemically stable, low-cost semiconducting coatings in a wide area of applications such as solar cells [1], photocatalysis [2], batteries [3], wire grid polarizers [4], sensors [5], etc. TiO2 is considered to be a promising candidate among dielectrics as high refractive index optical coating in multilayer optical systems [6, 7]. The important material properties of the coatings are, among others, the density, the refractive index, the surface roughness, and the mechanical stress. The mechanical stress of thin coatings is essential for numerous applications, such as in microelectromechanical systems (MEMS), semi-conductors, or optical devices [9]. High and low refractive index multilayer stacks, e.g. broadband antireflection coatings, dichroic mirrors, and narrow bandpass filters (NBF) are examples of such coatings in optics. High residual stress in single layers leads to severe cracking of the films in the optical element [10]. Further improvement of the mechanical properties of single layers is essential for such optical coatings

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