Indium tin oxide (ITO) is tin-doped In2O3, a n-type wide-bandgap heavily doped (or degenerate) semiconductor with certain metal-like properties. Therefore it is a transparent conductor, and broadly used in variety of industries.
ITO film can be sputtered on glass or plastic films, e.g. PET or PEN. The composition of ITO target is In2O3 (90 wt%)/SnO2(10wt%): molar ratio is 4.86:1. Due to Moss–Bursting effect (as shown in Figure 1), heavily doped by Sn enlarges its In2O3’s energy gap (good transparency).
During sputtering process (In2O3/SnO2 target), the following parameters are critical in determine ITO’s properties:
1) Substrate (e.g. glass) temperature. it can be up to 400C; surface resistance of less than 10ohm/sq can be achieved
2)PO2. O2/Ar ratio determines resistance and transparency. Deficiency of oxygen enhances the absorption in visible range (Urbach tail, Figure 2), leading to reduced transparency. Therefore, increasing of PO2 increases transparency (Figure 3); while the resistance decreases first and then increase with increasing PO2. It can be explained by charge carrier concentration times mobility. Charge concentration is affected by oxygen vacancy in addition to Sn doping; mobility is affected by voids, crystalline size, boundary, defects etc.
3) Total pressure. It affects nanostructure (voids, crystalline size, boundary, defects)
4) Annealing
5) Power density, process method, equipment setup, and others
A good commercially available ITO glass can have over 85% transparency in Vis and less than 10ohm/sq surface resistance.