Electrical, optical, and structural properties of indium-tin-oxide thin films for organic light-emitting devices

High-quality indium-tin-oxide (ITO) thin films (200-850 nm) have been grown by pulsed laser deposition (PLD) on glass substrates without a postdeposition annealing treatment. The structural, electrical, and optical properties of these films have been investigated as a function of target composition, substrate deposition temperature, background gas pressure, and film thickness. Films were deposited from various target compositions ranging from 0 to 15 wt % of SnO₂ content. The optimum target composition for high conductivity was 5 wt % SnO₂+95 wt % In₂O₃. Films were deposited at substrate temperatures ranging from room temperature to 300 °C in O₂ partial pressures ranging from 1 to 100 mTorr. Films were deposited using a KrF excimer laser (248 nm, 30 ns full width at half maximum) at a fluence of 2 ^J/cm2. For a 150-nm-thick ITO film grown at room temperature in an oxygen pressure of 10 mTorr, the resistivity was 4×10^-4 Ω cm and the average transmission in the visible range (400-700 nm) was 85%. For a 170-nm-thick ITO film deposited at 300 °C in 10 mTorr of oxygen, the resistivity was 2×10^-4 Ω cm and the average transmission in the visible range was 92%. The Hall mobility and carrier density for a 150-nm-thick film deposited at 300 °C were 27 cm²/V s and 1.4×10^{21 cm-3}, respectively. A reduction in the refractive index for ITO films can be achieved by raising the electron density in the films, which can be obtained by increasing the concentration of Sn dopants in the targets and/or increasing deposition temperature. Atomic force microscopy measurements of these ITO films indicated that their root-mean-square surface roughness (∼5 Å) was superior to that of commercially available sputter deposited ITO films (∼40 Å). The PLD ITO films were used to fabricate organic light-emitting diodes. From these structures the electroluminescence was measured and an external quantum efficiency of 1.5% was calculated.