5.3 Post Deposition Treatments

The dependence of ITO film properties on a number of growth parameters for each deposition method has previously been discussed in the earlier section and Chapter 2. Film properties can also be modified by annealing in either oxidising or reducing atmospheres as well as vacuum or nitrogen ambients [151,152,139].

Hence, it is necessary to exploit and develop the understanding of the effects of film structure and stoichiometry on electrical conductivity and optical transmission. Hence, the effects of annealing on the electrical and optical properties of ITO films were investigated.

5.3.1 Effects of Heat Treatment on the Electrical Properties of ITO

The two major types of ITO contacts used in this investigation are Schottky and ohmic. These contacts themselves have very different annealing conditions dictated by material and device processing factors and are thus discussed separately. As in section 5.1, electrical measurements on the films were done using the TLM method. A schematic diagram of this set-up can be seen in Figure 5.2. ITO was the mesa in the TLM pattern and for metal contacts to the ITO, in the following cases the Ni/AuGe/Ni/Au metalisation system was used.

5.3.1.1 Schottky Annealing Scheme

Because Schottky junctions diffuse at high temperatures resulting in lowered barrier heights and degraded performance, it is necessary to employ low but prolonged annealing schemes. In addition, sputter induced damage has to be recovered by the overall annealing procedures. Thus this scheme is dictated by the following factors:

• integrity of the Schottky junction
• adequate removal of sputter damage
• adequate annealing of the ITO to improve electrical properties of the film

Keeping the above in mind, the following temperature and times were used consecutively: (a) 5 hours at 240 �C followed by (b) another 5 hours at 340 �C. Similar schemes have also been used by other investigators [9]. The results are summarised in Table 5.3.

The results shown in the above table are similar to those reported by Sreenivas et al [6] who have used a cracked ammonia and N2 annealing ambient for ITO films deposited using a very similar set-up to ourselves. They report that following the anneal, the carrier concentration increased from 1e18 cm-3 to 1e20 cm-3 while the mobility increased from 2 cm2V-1s-1 to 4 cm2V-1s-1 in N2 and to 12 cm2V-1s-1 in cracked ammonia resulting in Rsh decreasing from 1e4 W/� to 40 W/� and 11 W/� respectively. They attribute this significant reduction in the Rsh to the large increase in the mobility and further suggest that this may be associated with the increase in grain size of the film.

Figure 5.16: Effect of the Schottky annealing scheme on the sheet resistance, Rsh, of various ITO films

The Rsh and r from Table 5.3 have been plotted in Figure 5.16 and Figure 5.17 as a function of annealing time. It is noticed that the maximum change in Rsh and r occur within the first hour of annealing at 240 �C and the curve becomes near linear indicating minimal change after subsequent annealing for a given temperature. There is little but noticeable further change following an increase in the anneal temperature to 340 �C; the electrical properties of the film following additional anneal at this temperature also level off as before.

All three films grown at different r.f. powers - thus having varying initial Rsh - show similar decreasing resistivity and sheet resistance as the annealing scheme progresses. The final value of sheet resistance, however, is very much dependent on the film growth parameters rather than the annealing conditions.

Figure 5.17: Effect of the Schottky annealing scheme on the resistivity, r, of various ITO films

The change in the resistivity, r, of each of the three films with annealing as shown in Figure 5.17, is very similar to the corresponding change in their sheet resistances. The final value for the r of (15.3 � 0.5) x 1e-4 Wcm for sample SAB-5 (grown at 150W) is in good agreement with those reported in the literature. Table 5.4 compares the annealed results from this work with those in the literature.

5.3.1.2 Ohmic Annealing Scheme

This scheme is less restrictive in terms of the temperatures that can be used because the aim is to form a diffused junction between the underlying highly doped semiconductor and the ITO film. A test ITO sample deposited at 150W (with film thickness = 2,683 � 118�) was put through the same annealing scheme as those for producing ITO/semiconductor contacts; this scheme is described in section 6.2.1 in greater detail. A forming gas ambient (5% H2 + 95% N2) with a flow rate of 10 sccm was used during the alloying experiment. Table 5.5 shows the results.

The above table suggests that the electrical properties improve by an order of magnitude following alloying in forming gas ambient under the afore mentioned conditions.

5.3.2 Effects of Heat Treatment on the Optical Properties of ITO

Figure 5.18 shows the effect of the ohmic annealing scheme on the transmittance of a typical ITO film.

Figure 5.18: Effect of the ohmic annealing scheme on the transmittance of a typical ITO film

It is seen from Figure 5.18 that there is a minimal change in transparency following the first annealing stage and no noticeable change thereafter. This change is a shift of the transmittance curve to the left of the x-axis, i.e. towards shorter wavelength/higher energy. This shift is most likely due to the increased carrier concentration, N, (also giving rise to better electrical properties) which is known to cause an increase in the absorption edge of ITO [144].

5.3.3 Conclusion

Both the Schottky and the ohmic annealing experiment results show resistivity as low as 14e-4 Wcm - similar to those reported in the literature. This suggests that there is minimal structural change to the ITO films following annealing used in this work. Other investigators have reported conflicting effects of heating on the transmittance of ITO: while Haines et al [152] suggest a decrease in transmission due to heating, Higuchi et al [9] and Sreenivas et al [6] observe an increase. These conflicts are most likely due to their differing methods of ITO growth and post deposition treatments.

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