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Figure 2.4 The three basic modes of thin film growth: island (or Volmer-Weber), layer (or Frank-van der Merwe) and Stranski-Krastanov (combination of layer and island growth). The layer by layer (Frank-van der Merwe) growth is favoured in PLD growth.

2.4 Pulsed Laser Deposition of ITO thin films

The substrate temperature (T_s) and oxygen pressure (P(O₂)) can be easily controlled during deposition to obtain ITO thin films with the required properties. However, parameters also affecting film properties such as film thickness (t) and stress are much more difficult to control. These four parameters are reviewed in the next sections.

2.4.1 Substrate temperature

Much work has been done on ITO films using commonly known fabrication technologies such as sputtering, evaporation, chemical vapour deposition and spray pyrolysis. This will be detailed in the next section. Nevertheless, most of these techniques require elevated temperatures (T_s > 300^◦C) to grow reasonably low resistivity and high transparency films [24]. This requirement may be restrictive for a number of applications such as liquid crystal displays which require low deposition temperature, solar cells (especially photovoltaic devices based on amorphous silicon which may deteriorate seriously at elevated temperature [66]) and in situations where coatings need to be made on poly-merized materials (flexible substrates). PLD is then extremely suitable since it allows lower deposition temperatures to be used.

Even at RT, high quality indium oxide thin films can be grown [41]. At RT and 10 mT oxygen pressure, better conductivity films (2.5 × 10⁻⁴Ω cm) could be obtained from

2.4. Pulsed Laser Deposition of ITO thin films 22

undoped In₂O₃ material as compared with ITO. Optical transmittance in the visible range in excess of 90% was obtained and the presence of Sn was less critical to the transmittance value. This behaviour is consistent with the fact that it has been reported [38] [24] that at RT, the presence of Sn atoms in the ITO films does not contribute free carriers to the conduction band, rather they act as scattering centres in the films. Sn has to be thermally

“activated” effectively to substitute for In and provide free carriers to the matrix [67]

resulting in lower resisitity ITO thin films at higher substrate temperatures.

Regarding ITO thin films, the decrease in resistivity with an increase in T_s is also explained in terms of enhanced crystallisation. Grain growth is favoured at higher T_s resulting in better optical properties.

The plasma wavelength (λ_p), related to free carrier cocnentration, was observed to de-crease initially with increasing Ts up to 100^◦C and then increase slightly up to 300^◦C [25].

Usually, post deposition heat-treated films or films deposited on heated substrates (at least above 300^◦C) exhibits high near-infrared reflectance. This parameter is extremely important to tailor the optical properties of applications like window layer coatings and solar cells. Such a high near-infrared reflectance was also obtained for films deposited by PLD at temperatures as low as 200^◦C [68][69]. T_s also affects the direct optical band gap. It was found [25] that the direct band gap of ITO films increases from 3.89 to 4.21 eV as T_s increases from RT to 300^◦C owing to increase in carrier concentration. In another paper [70], Adurodija et al. found that the best films were obtained for oxygen pressures around 10 mT. For this oxygen pressure, resistivities as low as 5.35 × 10⁻⁴Ωcm and 1.75 × 10⁻⁴Ωcm were obtained at RT and 200^◦C respectively. Optical transmittance in the visible region of 85% was also achieved.

The substrate temperature is sometimes found to have an effect on thickness [71].

The authors noticed that the thickness decreases from 90 to 44 nm as the substrate temperature increases from RT to 300^◦C.

2.4.2 Oxygen pressure

The oxygen flow during the deposition influences the chemical composition, film density, growth rate and layer morphology. P(O₂) is found to affect the PLD deposition rate and hence the film thickness during growth. The deposition rate reduced significantly as P(O₂) increased. It was found [70] to reduce from 15 nm/min under a P(O₂) of 5 mT to 5 nm/min under a P(O₂) of 50 mT. Such a reduction in the growth rate was attributed

2.4. Pulsed Laser Deposition of ITO thin films 23

primarily to increased collisions of the ablated ITO particles with the ambient oxygen gas.

This is a widely shared view among researchers where similar effects of P(O2) on the growth rate and thickness have been reported, not only for PLD but also for sputtering and electron beam evaporation [72] [73]. Another effect of P(O₂) is to reduce surface mobility. This reduction is so important that at very high P(O2), ITO films were found to be completely amorphous [68].

The electrical properties are also strongly impacted by oxygen pressure changes. At very low P(O₂), there is a very large number of oxygen vacancies; above a certain threshold, the crystallinity can be greatly impacted, reducing the mobility of the free carriers. At very high P(O₂), the mobility also tends to decrease for two main reasons: oxygen atoms accumulate mainly at grain boundaries and tend to form defects because an Sn⁴⁺ ion pair may attract an additional oxygen atom, producing a neutral cluster (SnO₂)₂ in which the additional oxygen atoms play the role of electron traps and so reduce the carrier mobility [13]. In between, there is a narrow window of oxygen pressure (at a given substrate temperature) where low resistivity thin films can be grown. For instance, several groups found that for RT PLD deposited indium oxide and ITO thin films, the optimal oxygen pressure that yielded the highest conductivity films lies within a narrow range from 10 to 15 mT [73] [25] [69] [70]. This range is about the same for films grown at higher substrate temperatures.

The carrier concentration also plays a role in the increase/decrease in resistivity. The incorporation of oxygen leads to a decrease in oxygen vacancies in the films and hence to a fall in the carrier concentration. Therefore, the resistivity of the ITO film is strongly cor-related with the chamber oxygen pressure and the resulting film stoichiometry. Oxygen provides the background gas necessary for optimal PLD growth of complex oxides, and equilibrates the energetic species of the emerging atomic/ionic constituents ablated from the target [74]. A uniform velocity distribution is found to be a key parameter in growing high quality thin films. This gas-dynamic equilibration is even more necessary in the low temperature case, where there is not sufficient surface diffusivity to allow formation of the film after the atomic/ionic species impinge on the substrate and where it mediates enhanced ITO cluster formation within the plume. Zheng et al. [74] observed that at low and high P(O₂), small particles covered the surfaces of their films. They suggested that these small particles are precipitates of indium or of tin. Nonetheless, due to the very small size of the particle, they could not perform accurate energy dispersive X-ray analysis in the SEM. They also noticed that there were no particles on the surface for the films

2.4. Pulsed Laser Deposition of ITO thin films 24

grown at the optimized pressure. At low P(O₂), ITO films are substoichiometric, [75]

which leads to metallic inclusions (metallic indium or tin) and hence to opaque films.

These metal-rich dark ITO films also exhibit high carrier concentrations but very low mobilities with the presence of different phases. Optical properties are also impacted because of light scattering.

The density of ITO films is reported [76] to increase as the deposition pressure increases.

The incorporation of oxygen leads to a decrease in oxygen vacancy concentration and hence the density increases. Optical properties are affected by this density change: less light is scattered as the film becomes denser, increasing its transparency. This oxygen vacancy concentration change impacts as well on the refractive index n of In₂O₃ films.

A decrease in the refractive index with increasing P(O₂) was observed in the thickness range 200-300 nm for pulsed laser deposited indium oxide films [77] and for reactively evaporated indium oxide films[78]. It is suggested that this is due to a change in the number of conduction electrons [79].

2.4.3 Film Thickness

Film thickness is an important parameter for the properties of ITO films. It is directly proportional to the number of laser shots. When the film thickness decreases below a critical value, the electrical properties of the indium oxide and ITO films deteriorate drastically. As films get thinner, the effect of surface scattering becomes more and more predominant over grain boundary [80] than ionised/neutral impurity scattering and tend to be discontinuous. The critical thickness, below which electro-optical properties degrade strongly, depends on the control parameters of the process. Jan et al. [81] also studied the critical thickness of ITO thin films deposited by directly coevaporating metallic In and Sn onto a heated glass substrate. A rapid increase of resistivity with decreasing film thickness below 50 nm was again attributed to 1) the increased importance of carrier scattering from the outer surface of the film due to surface roughness as seen previously but also to: 2) the greater density of grain boundaries due to decrease in grain size [25] and 3) film discontinuities attributed to the formation of islands [82] and extensive voids. Similar studies for pulsed laser deposited films have been published.

Kim et al. [25] observed that the resistivity of the ITO thin films initially decreased with an increase in the film thickness, up to 220 nm, but it remains almost constant with further increases in the film thickness up to 870 nm. The resistivity does not change very much for films thicker than 300 nm because both carrier density and mobility become independent of film thickness.

2.4. Pulsed Laser Deposition of ITO thin films 25

The same authors [25] also reported the effect of thickness on optical transmission.

They found that the transmission was not exponentially related to the film thickness as expected from the following relation: I = I₀exp(−αt) where α is the absorption coefficient, t the film thickness, I the transmitted intensity at a particular wavelength and I0 the incident intensity, taken to be 100%. Therefore, other parameters affect the optical properties. The one found to have the greatest impact was the grain size, which grew larger with film thickness. But surface roughness might play a role as well as the films grew thicker [83]. It has also been shown that the refractive index depends on the film thickness up to 100 nm, after which it is almost constant [79].

In the range of extremely thin films ( a few nanometres), it was reported [84] that reactively evaporated indium oxide films were amorphous below 4 nm, while films thicker than 4 nm were polycrystalline. Finally, adjusting the film thickness has a practical use in applications where the ITO serves as an antireflective layer. In that specific case, the reflectivity losses can be reduced when the layer thickness is adjusted to the wavelength of the light used [85].

2.4.4 Stress in thin films

Virtually all vacuum-deposited coatings are in a state of stress. The total stress is composed of a thermal stress and an intrinsic stress. The thermal stress is due to the difference in the thermal expansion coefficients of the coating and the substrate and is generally independent of the deposition process. The intrinsic stress is due to the cumulative effect of the crystallographic flaws that are built into the coating during deposition.

2.4.4.1 Thermal stress

Strain is invoked due to thermal mismatch between the ITO film (7.2×10⁻⁶/^◦C [86]) and the substrate when the latter is a polymer substrate (from 12×10⁻⁶/^◦C for PET substrates [87] to 39×10⁻⁶/^◦C for polycarbonate substrates [76]). This thermal stress is somewhat less important when ITO films are grown on glass substrates, since the thermal expansion coefficient of glass (4.6×10⁻⁶/^◦C) is close to that of ITO.

2.4.4.2 Intrinsic stress

In PLD, the tailoring of stress in the film becomes possible through the bombardment of the depositing film with the energetic species during film growth. It is indeed possible