Elsevier

Thin Solid Films

Volume 411, Issue 1, 22 May 2002, Pages 1-5
Thin Solid Films

Transparent and conducting ITO films: new developments and applications

https://doi.org/10.1016/S0040-6090(02)00163-3Get rights and content

Abstract

We review data on transparent and electrically conducting films of ITO (i.e. In2O3:Sn). A survey is given on the most recent progress, in 2001, of fundamental electronic bandstructure, techniques to boost the conductivity, and alternative dopants and manufacturing techniques. We then outline quantitative theories for the optical properties and their applications. Finally, we consider ITO films for uses in optimized electrochromic window coatings capable of yielding indoor comfort and energy efficiency.

Introduction

Thin films with optical transparency and electrical conductivity find many applications in contemporary and emerging technology, such as in displays of various kinds, solar cells, electrochromic devices, and heatable glass. If the conductivity is large enough, the thermal emittance is low, which opens applications to thermal insulation of windows, prevention of radiative cooling, etc.

A combination of transparency and conduction can be achieved in different types of materials [1], [2]. One of these comprises extremely thin films of metals, especially of Ag, Au or Cu. Their luminous transmittance can be up to ∼50% for a single film, but much larger if the film is embedded between non-absorbing layers serving, essentially, so as to anti-reflect the metal. Glass with coatings including Ag are in widespread use in modern fenestration technology for providing thermal insulation and, in climates requiring space cooling, for diminishing throughputs of near-infrared solar radiation.

A second materials category—of primary concern for this article—is found among the wide-bandgap oxide semiconductors [3]. The first observation appears to have been in Cd oxide, for which it was reported in 1951, i.e. 50 years ago, that electrical conductivity and optical transparency could co-exist [4], [5]. Similar behavior was reported [6], [7] in In oxide 5 years later; applications were found for heated windows. Then a concentrated research effort, approximately a decade later, led to films of doped SnO2 and In2O3:Sn (known as indium tin oxide or ITO) with excellent electrical and optical properties [8], [9]. The application of these films was primarily to diminish heat losses from sodium lamps.

ITO, along with SnO2:F and ZnO:Al, are still the most used and widely studied transparent conductors among the oxides. However, a large number of alternative ternary oxides have been explored, including Cd2SnO4, Zn2SnO4, MgIn2O4, ZnSnO3, GaInO3, Zn2In2O5, and In4Sn3O12 [10]. A large number of dopants has been investigated in oxides based on Zn, Cd, In and Sn [10]. Doping by oxygen deficiency is also possible. All of the materials mentioned thus far exhibit n-type conduction. However, p-type conduction has been discovered recently in materials such as CuAlO2, CuGaO2, SrCu2O2, AgInO2 and ZnO:N [11].

The literature on ITO and other transparent conducting materials is vast, as expected from their great technological significance. A survey of the field was given in 2000 [3]. Much progress has occurred recently—not the least as late as in 2001—and the field of research is notable for its current vitality. Some of this most recent work is outlined in Section 2 below. We then continue in Section 3 with theoretical modeling of the optical properties of ITO, essentially following work [12] published a number of years ago. Finally, in Section 4, we discuss advances in fenestration which enables electrochromic ‘smart windows’ to provide indoor comfort together with energy efficiency [13]; high-performance transparent conductors are essential for these applications.

Section snippets

Some recent advances for ITO and related materials

Our theoretical understanding of ITO has been limited, especially on the electronic bandstructure level, even if some results have been known of electronic structure, defect chemistry, and carrier generation mechanisms [14]. This lack of detailed knowledge is not surprising, considering that crystalline In2O3 exhibits a bixbyite structure with a unit cell containing 40 atoms and two non-equivalent cation sites, and that features near the bandgap have to be elucidated. However, important

Theoretical modeling of ITO's optical properties

This section outlines the theories that enable detailed quantitative modeling of the optical properties of ITO in the wavelength range from the ultraviolet and through the visible and thermal infrared regions. Our exposition is based on work carried out several years ago [12], although still apparently representing state-of-the-art. One reason why this early work is still of interest is its foundation on many-body representations of the electron gas, i.e. on theories that were thoroughly

ITO films in electrochromic devices

Electrochromic devices are able to modulate their optical properties upon charge insertion/extraction [30], [31]. They are already used for some applications, including for ‘smart windows’ with a variable luminous transmittance, and the level of present interest makes it likely that the range of applications will widen in the future. A critical component in an electrochromic device is the transparent conductor; it should enable rapid charge transport, and hence ρ should be low especially for

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