2. Background Theory and Literature Review

2.1 Indium Tin Oxide (ITO)

Interest in transparent conductors can be traced back to 1907 when reports of transparent and conductive cadmium oxide (CdO) films first appeared. Since then there has been a growing technological interest in materials with these unique properties as evidenced by not only their increased numbers but also the large variety of techniques that have been developed for their deposition. It is now known that non-stoichiometric and doped films of oxides of tin, indium, cadmium, zinc and their various alloys exhibit high transmittance and nearly metallic conductivity is achievable [1]. However, tin doped indium oxide (ITO), with reported transmittance and conductivity as high as 95% and 1e4 W-1cm-1 respectively, is among the most popular of these thin films which have found a host of electronic, opto-electronic and mechanical applications. Hence, some of the physical and technological aspects behind ITO films will now be reviewed and discussed.

2.1.1 Introduction and Uses of ITO

Although partial transparency, with acceptable reduction in conductivity, can be obtained for very thin metallic films, high transparency and simultaneously high conductivity cannot be attained in intrinsic stoichiometric materials. The only way this can be achieved is by creating electron degeneracy in a wide bandgap (Eg > 3eV or more for visible radiation) material by controllably introducing non-stoichiometry and/or appropriate dopants. These conditions can be conveniently met for ITO as well as a number of other materials previously mentioned.

Uses of ITO have traditionally ranged from transparent heating elements of aircraft and car windows, antistatic coatings over electronic instrument display panels, heat reflecting mirrors, antireflection coatings and even in high temperature gas sensors. Early electro-optic devices using ITO include CCD arrays, liquid crystal displays and as transparent electrodes for various display devices. More recently, ITO has been used as a transparent contact in advanced optoelectronic devices such as solar cells, light emitting and photo diodes, photo transistors and lasers - some for the first time as a result of this investigation [2]. Thus it is soon becoming an integral part of modern electronic technology wherever there is a potential for improving optical sensitivity of light detecting devices or quantum efficiency of light emitting devices.

However, with increased development in electronic technology has come the need for a greater understanding of the optical and electrical properties of ITO. As a result some of the solid state physics of ITO has also emerged. Although no concise and accurate knowledge is available, the literature survey indicates that many of these properties can be tailored by careful control of the deposition parameters.

2.1.2 Physical Structure and Properties of ITO

Indium Tin Oxide is essentially formed by subsititutional doping of In2O 3 with Sn which replaces the In3+ atoms from the cubic bixbyte structure of indium oxide [3]. Sn thus forms an interstitial bond with oxygen and exists either as SnO or SnO2 - accordingly it has a valency of +2 or +4 respectively. This valency state has a direct bearing on the ultimate conductivity of ITO. The lower valence state results in a net reduction in carrier concentration since a hole is created which acts as a trap and reduces conductivity. On the other hand, predominance of the SnO2 state means Sn4+ acts as a n-type donor releasing electrons to the conduction band. However, in ITO, both substitutional tin and oxygen vacancies contribute to the high conductivity and the material can be represented as In2-xSn xO3-2x. ITO films have a lattice parameter close to that of In2O3 and lie in the range 10.12 to 10.31Å [4].

A summary of electrical and optical properties of typical ITO films deposited using various techniques is shown in Table 2.1. Variations in film properties can be easily noted; these are attributable to both pre- and post-deposition treatments as well as the techniques themselves.

The high conductivity, s, of ITO films is said to be due to high carrier concentration, N, rather than high Hall mobility, m H [1] bearing in mind that resistivity, r = 1/s = 1/(qN mH) according to Ohm's law. The observed low mobility of ITO, compared to bulk In2O3 , and its dependence on carrier concentration and substrate temperature has been explained in terms of scattering mechanisms due to ionized impurities or grain boundaries. Mobility is said to increase due to enhanced crystallinity of films deposited at higher substrate temperatures [10]. TEM and electron diffraction studies of r.f. sputtered ITO films on glass substrates by Sreenivas et al suggest that films grown at room temperature have large stacking faults and represent an amorphous structure [6]; increasing this temperature to 200°C leads to a polycrystalline structure and finally annealing results in near single crystallinity with uniform grain size which leads to increased conductivity. The authors further suggest that deposition of ITO on single crystal substrates, rather than amorphous glass, can enhance the grain growth process.

The direct optical bandgap of ITO films is generally greater than 3.75 eV although a range of values from 3.5 to 4.06 eV have also been reported in the literature [3,13]. The high optical transmittance, Tr, of these films is a direct consequence of their being a wide bandgap semiconductor. The fundamental absorption edge generally lies in the ultraviolet of the solar spectrum and shifts to shorter wavelengths with increasing carrier concentration, N. This is because the bandgap exhibits an N2/3 dependence due to the Moss-Burstein shift [14]. The band structure of ITO is assumed to be parabolic as shown in Figure 2.1:

Figure 2.1: Assumed parabolic band structure of undoped In2O 3 and the effect of tin doping; (After Gupta et al [14])

The conduction band is curved upwards, the valence band is curved downwards and the Fermi level is located at mid bandgap for the undoped material; addition of Sn dopants results in the formation of donor states just below the conduction band. As the doping density is increased, these eventually merge with the conduction band at a critical density, nc, which was calculated to be 2.3e19 cm-3 by Gupta et al [14]. Free electron properties are exhibited by the material when the density of electrons from the donor atoms exceeds this value. As Table 2.1 shows, all reported values of carrier concentration are greater than nc. Hence all ITO films are expected to be degenerate in nature. Once the material becomes degenerate, the mutual exchange and coulombic interactions shift the conduction band downwards and the valence band upwards - effectively narrowing it from Eg to Eg' - as shown in Figure 2.1 earlier. The bandgap increase by the Burstein-Moss shift is partially compensated by this effect.

The reported value for the refractive index of ITO is 1.96 [7]. The transmittance of ITO films is also influenced by a number of minor effects which include surface roughness and optical inhomogeneity in the direction normal to the film surface. Inadvertently grown dark brown (effectively translucent) metallic films of ITO have also been reported. This opaqueness has been attributed to unoxidised Sn metal grains on the ITO surface as a result of instability due to absence of sufficient oxygen during deposition [3,6].

2.1.3 ITO Deposition Techniques

Sputtering, of one form or another, is by far the most extensively used technique for the deposition of ITO. This is closely followed by thermal evaporation - which can also be achieved using several different techniques. ITO has also been prepared by other methods such as Spray Pyrolysis and Screen Printing. The choice of deposition technique is dictated by a number of factors such as quality and reproducibility of the ITO film, homogeneity over a wide cross section, capacity, ease and cost of use as well as detrimental side effects and limitations specific to each technique. In addition, since the properties of ITO depend strongly on the microstructure, stoichiometry and the nature of the impurities present, it is inevitable that each deposition technique with its associated controlling parameters should yield films with different characteristics. Some of these issues will now be discussed briefly.

2.1.3.1 Sputtering

Sputtering involves knocking an atom or molecule out of a target material by accelerated ions from an excited plasma and condensing it on the substrate either in its original or in a modified form. When this modification is induced by a chemical reaction during the transit from the target to the substrate, the process is referred to as reactive sputtering. In general, most ITO sputter sources consist of hot pressed 90% In2O3 : 10% SnO2 compound targets. The sputtering can be achieved by a number of ways which include accelerating the plasma ions by a d.c. field [9] or a d.c. field combined with a magnet (to direct the high velocity emitted electrons away from the substrate), r. f. (with its self induced bias) as well as by ion beams [11]. Hence names such as magentron [8] and reactive r.f. sputtering [5,6] reflect on the process that has been used for the deposition of the ITO film.

The technique used in the course of this study involves reactive r. f. sputtering in an Ar/O 2 plasma. This method is reputed for its excellent uniformity, high conductivity and high transparency. The r. f. field ensures that sputtering of non-conductive materials can also be achieved at a practical rate. Parameters known to influence ITO quality include sputtering pressure, pre-conditioning, film thickness and r. f. power amongst others. The control of oxygen partial pressure is particularly critical in determining the conductivity and transmittance. However, without the ability to direct unwanted high velocity electrons away from the substrate, damage is associated with this technique; on the other hand magentron sputtering yields high deposition rates and minimises this damage. Detailed description of the system used here is presented in section 3.2 while the detailed optimisation of the deposition parameters is discussed in section 5.1 and elsewhere in the thesis.

2.1.3.2 Thermal Evaporation

Thermal evaporation involves vaporising a solid by heating the material to sufficiently high temperatures and recondensing it on a cooler substrate. The high temperature can be achieved by resistively heating or by firing an electron or ion beam at the boat containing the material to be evaporated. Similarly, reactive thermal evaporation is achieved by introducing oxygen into the chamber during deposition and is one of the most widely and successfully used techniques for good quality ITO depositions [10]. A 95% In - 5% Sn alloy (by weight) is commonly used as the source.

There is no damage associated with resistive thermal evaporation since there are no high velocity particles. Film properties strongly depend on oxygen partial pressure and film thickness [15], deposition rate, substrate temperature and tin concentration [13]. Reports of substrate temperatures being raised from 300°C up to 450°C during evaporation in order to enhance conductivity and transmittance are available in the literature [16].

2.1.3.3 Spray Pyrolysis Technique

Pyrolysis refers to the thermal decomposition of gaseous species at a hot susceptor surface. The spray deposition scheme is particularly attractive because of its relatively fast rate (> 1000 Å/sec) and because it does not require a vacuum. The ITO spray is obtained from an alcoholic solution of anhydrous indium chloride (InCl3) and tin chloride (SnCl 4.5H2O) with nitrogen acting as the carrier gas. The spraying is carried out in a furnace, held at 400°C. Critical parameters including positioning of the substrate and the chemical composition of the spray solution. Ashok et al [17] have reported resisitivities of 1e-3 Wcm for a 4200Å thick ITO film with transmission greater than 90% at 550 nm while corresponding values obtained by Haitjema et al are 3e-4 Wcm and 85% respectively [12]. As for most other techniques, this demonstrates an apparent trade-off between the conductivity and the transmittance of ITO films.

2.1.3.4 Screen Printing Technique

This technique is suitable for large scale non-device orientated applications where relatively thick layers of ITO are required such as in liquid crystal displays, blackwall contacts and anti reflection coatings for solar cells [18]. Typically, the deposited thickness varies in the range 10 to 30 mm and the post deposition crystallization temperature can be as high as 600°C for a period exceeding an hour. Although the resistivity (> 4e-4 Wcm) of the ITO film is said to be comparable to those obtained by other deposition techniques, its transparency is markedly lower (< 80%).

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Acknowledgements

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