**2. Pulsed laser deposition of ITO films**

A typical experimental setup of PLD is shown in **Figure 2** [4] where ablation can be obtained by laser with different wavelengths or pulse duration, in either background gas or vacuum. Deposition of ITO is normally performed in O2 environment to preserve the stoichiometry, similar to the deposition of superconducting oxide. In addition, the substrates can be heated

**Figure 2.** PLD setup for ITO growth. Deposition is often carried out in O2 background.


atmospheric pressure CVD or APCVD method is applied for transition metal oxide thin film

88 Applications of Laser Ablation - Thin Film Deposition, Nanomaterial Synthesis and Surface Modification

sition mainly because they employ direct vaporization to produce the required deposition flux. In particular, PLD has attracted much interest mainly due to its ability to deposit of high

 superconducting films with necessary stoichiometry. While PLD can be operated between high vacuum and low pressure, the sputtering method is usually operated at low pressures with a continuous gas flow. For both methods, the production of a wide range of kinetic energy for the depositing specie ensures thin film depositions from different types of solid targets including doped target such as ITO. The film stoichiometry or multicomponent control is often obtained with the use of background gases for moderation of kinetic energy. While a suitable energy range can facilitate the growth of more uniform and crystalline thin films, the presence of background gas often prevents the film damage by energetic ion bombardment. For PLD, the background gases at suitable pressures may also introduce gas-phase reactions which may generate new species and/or formation of nanoclusters prior to the deposition on the substrate.

by laser with different wavelengths or pulse duration, in either background gas or vacuum.

similar to the deposition of superconducting oxide. In addition, the substrates can be heated

2

2

background.

3] and sputtering are considered more "versatile" methods for normal thin film depo


4] where ablation can be obtained

environment to preserve the stoichiometry,

deposition.

**2. Pulsed laser deposition of ITO films**

Deposition of ITO is normally performed in O

A typical experimental setup of PLD is shown in **Figure 2** [

**Figure 2.** PLD setup for ITO growth. Deposition is often carried out in O

PLD [

Tc

for films growth, and targets with different compositions can be sintered and used. In 1993, Zheng and Kwok [5] first reported the deposition of ITO by the pulsed laser ablation technique using an excimer laser at wavelength of 193 nm. The ITO target used was a 90 wt% In2 O3 + 10 wt% SnO2 sintered ceramic. The best electrical resistivity of the ITO films grown on glass substrate was 5.6 × 10−4 Ω cm and 1.4 × 10−4 Ω cm at substrate temperature of 20 and 310°C, respectively. The optical transmittance was greater than 90% for the optimized film between the wavelength ranges of 600 and 800 nm.

After the first successful deposition of ITO thin films on glass using the PLD technique was reported by Zheng and Kwok [5, 6], highly conductive ITO was deposited on InP substrates by PLD. The sample was then fabricated the ITO/p-InP device for the photovoltaic solar cell applications [7]. Thereafter, numerous studies on the ITO film initial growth and electrical conduction mechanism on different substrates using the same laser were reported [8–10]. When 248 nm KrF laser was used, ITO films with 85% optical transmittance in the visible and sheet resistance less than 1 Ω/sq were obtained at a substrate temperature of 200°C [11]. The properties of the ITO films were similar to the work done by using a 193 nm ArF laser. The properties of ITO films deposited at different conditions by using a KrF laser are summarized in **Table 2**.

An obvious disadvantage with excimer lasers is that its operation requires the handling of reactive gases. As an alternative, Nd:YAG solid-state laser has also been used for ITO deposition, especially after frequency doubled at 532 nm and frequency tripled at 355 nm. However, when the wavelength and pulse length are varied, it affects the laser penetration depth and the degree of absorption in laser-material interaction [3]. **Table 3** summarizes the experimental conditions and ITO thin films properties deposited by using an Nd:YAG pulsed laser at 355 and 532 nm in our previous reports as compared to others.

As a comparison, the effects due to the difference in laser wavelengths between 193 and 248 nm on the properties of the deposited ITO films are fairly small. A larger difference is observed for 193/248 nm deposition of ITO as compared to 355 nm deposition at room temperature. The resistivity that can be obtained by 193/248 nm laser is in the range of 10−4 Ω cm, while optimization in terms of ITO doping concentration [24] or substrate temperature [4, 26, 29] is needed in order to achieve the resistivity in the same range for 355 nm laser deposition.

Besides the need for sufficient Sn dopant in the ITO film, the deposition parameters: substrate temperature and background pressure are crucial to achieve the desired films properties. In the early reports, the resistivity of the films grown at lower substrate temperature was affected greatly by the background pressure as compared to those grown at higher temperature [5, 6]. The main reason for such an observation was ascribed to the film growth mechanisms in PLD. Upon ablation, the ablated plasma plume, consisting of Sn, In, and O species, undergoes collisions with the background gas that resultant in the final velocity distribution for each species. At higher substrate temperature, the low kinetic energy species will still be able to rearrange to form uniform films because of thermally induced migration from the heated substrate. Thus, deposition of high-quality films is possible covering a wider pressure range. However, at low substrate temperature, uniform films can only be obtained at an optimal velocity distribution which is affected greatly by the degree of collisions with the background gas. Thus, film


for films growth, and targets with different compositions can be sintered and used. In 1993, Zheng and Kwok [5] first reported the deposition of ITO by the pulsed laser ablation technique using an excimer laser at wavelength of 193 nm. The ITO target used was a 90 wt% In2

90 Applications of Laser Ablation - Thin Film Deposition, Nanomaterial Synthesis and Surface Modification

substrate was 5.6 × 10−4 Ω cm and 1.4 × 10−4 Ω cm at substrate temperature of 20 and 310°C, respectively. The optical transmittance was greater than 90% for the optimized film between

After the first successful deposition of ITO thin films on glass using the PLD technique was reported by Zheng and Kwok [5, 6], highly conductive ITO was deposited on InP substrates by PLD. The sample was then fabricated the ITO/p-InP device for the photovoltaic solar cell applications [7]. Thereafter, numerous studies on the ITO film initial growth and electrical conduction mechanism on different substrates using the same laser were reported [8–10]. When 248 nm KrF laser was used, ITO films with 85% optical transmittance in the visible and sheet resistance less than 1 Ω/sq were obtained at a substrate temperature of 200°C [11]. The properties of the ITO films were similar to the work done by using a 193 nm ArF laser. The properties of ITO films deposited at different conditions by using a KrF laser are summarized

An obvious disadvantage with excimer lasers is that its operation requires the handling of reactive gases. As an alternative, Nd:YAG solid-state laser has also been used for ITO deposition, especially after frequency doubled at 532 nm and frequency tripled at 355 nm. However, when the wavelength and pulse length are varied, it affects the laser penetration depth and the degree of absorption in laser-material interaction [3]. **Table 3** summarizes the experimental conditions and ITO thin films properties deposited by using an Nd:YAG pulsed laser at

As a comparison, the effects due to the difference in laser wavelengths between 193 and 248 nm on the properties of the deposited ITO films are fairly small. A larger difference is observed for 193/248 nm deposition of ITO as compared to 355 nm deposition at room temperature. The resistivity that can be obtained by 193/248 nm laser is in the range of 10−4 Ω cm, while optimization in terms of ITO doping concentration [24] or substrate temperature [4, 26, 29] is needed in order to achieve the resistivity in the same range for 355 nm laser deposition. Besides the need for sufficient Sn dopant in the ITO film, the deposition parameters: substrate temperature and background pressure are crucial to achieve the desired films properties. In the early reports, the resistivity of the films grown at lower substrate temperature was affected greatly by the background pressure as compared to those grown at higher temperature [5, 6]. The main reason for such an observation was ascribed to the film growth mechanisms in PLD. Upon ablation, the ablated plasma plume, consisting of Sn, In, and O species, undergoes collisions with the background gas that resultant in the final velocity distribution for each species. At higher substrate temperature, the low kinetic energy species will still be able to rearrange to form uniform films because of thermally induced migration from the heated substrate. Thus, deposition of high-quality films is possible covering a wider pressure range. However, at low substrate temperature, uniform films can only be obtained at an optimal velocity distribution which is affected greatly by the degree of collisions with the background gas. Thus, film

355 and 532 nm in our previous reports as compared to others.

sintered ceramic. The best electrical resistivity of the ITO films grown on glass

+ 10 wt% SnO2

in **Table 2**.

the wavelength ranges of 600 and 800 nm.

O3

Pulsed Laser Deposition of ITO: From Films to Nanostructures http://dx.doi.org/10.5772/65897 91

growth is rather sensitive to the background gas pressure. Similar trend has been observed in the deposition using excimer lasers [7, 13, 15] and the depositions using 355 nm laser [22, 30].

**Figure 3.** Effects of substrate temperature on film resistivity, carrier density, and Hall mobility deposited by (a) 248 nm laser at 10 mTorr [12] and (b) 355 nm laser at 30 mTorr [26].

**Figure 4.** Resistivity of ITO films deposited in O2 at RT as a function of laser wavelength.

growth is rather sensitive to the background gas pressure. Similar trend has been observed in the deposition using excimer lasers [7, 13, 15] and the depositions using 355 nm laser [22, 30].

92 Applications of Laser Ablation - Thin Film Deposition, Nanomaterial Synthesis and Surface Modification

**Figure 3.** Effects of substrate temperature on film resistivity, carrier density, and Hall mobility deposited by (a) 248 nm

laser at 10 mTorr [12] and (b) 355 nm laser at 30 mTorr [26].

In addition to the formation of a uniform film, substrate temperature affects the carrier density of the films and thus the resistivity of the ITO films. **Figure 3** shows the resistivity and the Hall effects measurement results of ITO films deposited by 248 nm [12] and a 355 nm laser [26]. As the substrate temperature was changed from room temperature (RT) to 300°C, the carrier density was doubled for the ITO films deposited by using 248 nm laser, while it was increased by 14 times for ITO deposited by using the 355 nm laser. At the same time, the Hall mobility was also increased by 2 times for ITO deposited by 248 nm laser, but the value was decreased by ~3 times for ITO deposited by 355 nm laser. Thus, effectively, the resistivity of ITO films in both cases was reduced at higher substrate temperature to ~2 × 10−4 Ω cm at 300°C. The results indicate that 248 nm laser was superior to 355 nm laser in the pristine deposition condition where the substrate was not heated, possibly due to the higher photon energy of 248 nm laser and its more congruent ablation with a smaller penetration depth [3]. Thus, ITO film properties that were inferior to those obtained by the shorter laser wavelengths were also obtained in 532 nm deposition [25, 28]. The resistivity of ITO films deposited at different laser wavelengths at RT is shown in **Figure 4**. However, substrate heating successfully improved the thin film property, and lower resistivity can be achieved for the case of 355 nm laser. The crystallinity of ITO films is also affected by substrate heating during the deposition process. The ITO films

**Figure 5.** Effects of substrate temperature on the crystallinity of the ITO films deposited by (a) 248 nm laser [12] and (b) 355 nm laser [26].

are amorphous when deposited at RT by both 248 and 355 nm lasers. Crystalline films are obtained when deposited at above 100°C as shown in the XRD spectra in **Figure 5**.

ITO films deposited by laser ablation of ITO target have been applied in various applications such as solar cell [7, 31, 32], organic light-emitting devices [4, 12, 26], and more recently as low loss alternative plasmonic materials in the near-infrared region [33]. Our results show that ITO films with a range of properties can be obtained, controlled by the deposition parameters. PLD is capable of growing low resistivity, high transmittance ITO films at relatively low temperature or even room temperature that are beneficial for device application onto delicate polymer substrates or active materials.
