2 – 1. INCLUSIÓN SOCIAL

An attempt is made to correlate the electrical and optical data of ITO thin films grown for chambers 1 and 2 since the transposition of the growth parameters from chamber 1 to chamber 2 resulted in films an order of magnitude more resistive than those in chamber 1. Then, it was noticed that at given growth conditions, thin films grown in chamber 2 had a resistivity ρ very close to the films grown in chamber 1 (under exactly the same conditions) provided a static atmosphere is used in chamber 2. Here “static” means that no pumping took place in the chamber after the introduction of a certain amount of pure O2 gas. Consequently, a comparison between chamber 1, where a dynamic atmosphere

(i.e. pumping plus continuous introduction of O2 gas) was used, and chamber 2, under

a static atmosphere, is made. (The differences in set-up between the two chambers was illustrated in section 4.1, Fig. 4.1). All other parameters used in chamber 2 were set to be exactly the same as those used in chamber 1, i.e. 10 mT of pure oxygen was introduced to the chamber and then pumping was stopped. It was found necessary to inject regulary some oxygen inside the chamber as gas was consumed by the thin film during its growth. Table 5.8 summarises the growth conditions used in chamber 2.

Table 5.8 Summary of ITO growth parameters in chamber 2. P(O2) was kept at 10 mT.

The parameters were unchanged as compared with chamber 1, although the atmosphere was static. Temperature RT - 400◦C T-S distance 63 mm Laser fluence 7.5 J/cm2 Laser frequency 10 Hz Oxygen pressure 10 mT

Electro-optical and structural properties are studied and compared.

5.2.3.1 Electrical properties

Fig. 5.47 shows Hall effect measurements where the mobility µ, the carrier concentration N and the resistivity ρ are plotted versus Ts.

The resistivities are similar over the range RT to 400◦_{C. Here the carrier concentration,}

agreement is satisfactory (less than 25% difference) up to 200◦_{C, after which the curves}

diverge increasingly. Conversely, the agreement in mobility is not good except above 300◦_C.

5.2. Influence of substrate temperature 127

Figure 5.47 Comparison of electrical properties between chambers 1 and 2 in terms. Tsis varied from RT to 400◦C.

5.2.3.2 Structural properties

XRD patterns are presented and compared at RT, 200◦_{C, 300}◦_{C and 400}◦_{C (see Fig.}

5.48).

It is interesting to observe that the agreement between the mobility measurements is best (at 300◦C and 400◦C) when both films are textured along the <111> direction.

5.2.3.3 Optical properties

Fig. 5.49 shows a comparison of optical properties in transmission between specimens grown at RT, 200, 300 and 400◦_{C in chambers 1 and 2.}

Films grown in chamber 1 show a systematic absorption edge shift towards higher wavelengths as compared with chamber 2. Since the thickness of the films compared is similar, this shift was explained on the basis of free carrier concentration. Indeed, carrier concentration measurements (Fig. 5.47) showed that films grown at RT, 200, 300 and

5.2. Influence of substrate temperature 128

Figure 5.48 Comparison between XRD patterns of films grown in chamber 1 (dynamic atmosphere) and 2 (static atmosphere) at four different Ts: RT, 200, 300 and 400◦C.

400◦_{C in chamber 1 contain a lower amount of free carriers than those grown in chamber}

2 in a static atmosphere. The change in carrier concentration is particularly large at 400◦_{C, which seems to induce the slightly larger shift observed at this temperature (Fig.}

5.49). At RT, the shift is reversed owing to the higher carrier density in chamber 1. Hence, electro-optical measurements are self-consistent. It is also interesting to note that the absorption edge slope at RT is much less steep than at higher Ts due to the lower

crystallinity. Both absorption edges are similar in both shape and slope showing that the crystallinity might be about the same in both chambers.

5.2.3.4 Summary

Reconciling results from chambers 1 and 2 comes down to reconcile with static and dynamic pumping. Comparing films grown at different Tsin chamber 2 under a static

atmosphere with those grown initially in chamber 1 under a dynamic atmosphere yielded mixed results. As regards the electrical properties, µ and N do not compare particulary well showing opposite behaviour in the two chambers, although ρ does compare well, a

5.2. Influence of substrate temperature 129

Figure 5.49 Comparison between chambers 1 and 2 for transmittance over the 300 -1000 nm wavelength range. A static atmosphere was used in chamber 2 where P(O2)

was kept at 5 mT. In chamber 1, there was a constant pressure of P(O2)=10 mT in a

dynamic atmosphere (pumping system operating). Ts was varied from RT to 400◦C.

fact that had initially lead us to make the present comparisons. The XRD results compare relatively well with an observed (222) preferred orientation, although at Ts=200◦C, the

agreement is not so good since films in chamber 2 do not show any preferred orientation. Optical properties agree very well, especially at RT where the slope of the absorption edge was reproduced in chamber 2. A systematic absorption edge shift towards lower wavelength is also noticed between chambers 1 and 2 and is attributed to the previously discussed Burstein-Moss effect due to the difference in N. The fact that using a static atmosphere in chamber 2 makes it possible to grow ITO thin films with similar physical properties to those initially grown in chamber 1 demonstrates the very strong impact of oxygen flow and the resulting gas dynamics in a PLD chamber during growth. Very few studies have been undertaken regarding that matter. One of the most detailed study coming from Spencer et al. [198] for a magnetron sputtering system. They monitored the film and pump consumption of oxygen and showed that the best way to obtain a stable deposition system (i.e. a constant P(O2) on the substrate surface so that the arrival