Growth-controlled surface roughness in Al-doped ZnO as transparent conducting oxide

The surface morphology of Al2O3-doped ZnO (AZO, 2 wt%) thin films varies from a uniform layer to nanorod structure by simply controlling oxygen pressure during growth. All AZO films were deposited on sapphire(0001) substrates using a pulsed laser deposition (PLD) technique. In the low oxygen pressure regime (vacuum ∼50 mTorr), AZO films grow as a smooth and uniform layer. In the high oxygen pressure regime (100-250 mTorr) AZO thin films with nanorods have formed. Detailed cross-sectional transmission electron microscopy (TEM) and x-ray diffraction (XRD) studies reveal that, besides the obvious variation in the film morphology, the in-plane d spacing of AZO film increases and the out-of-plane d spacing decreases, as oxygen pressure increases. A bilayer AZO film with a nanorod structure on top of a uniform layer was demonstrated by controlling the oxygen pressure for the two layers. Electrical resistivity and optical transmittance measurements were carried out to correlate with the microstructures obtained under different oxygen pressures. The bilayer AZO films could find applications as a transparent conducting oxide (TCO) with a unique light trapping function in thin film solar cells.