Effects of post-deposition rapid thermal annealing on aluminum-doped ZnO thin films grown by atomic layer deposition

Aluminum-doped ZnO (AZO) thin films with alternating stacks of ZnO and Al2O3 were grown by atomic layer deposition (ALD) on c-face sapphire substrates. Post-deposition rapid thermal annealing (RTA) at 950 degrees C for 5 min was conducted in all AZO samples. The X-ray diffraction patterns demonstrate that the intensity of cubic ZnAl2O4 (311) peaks grow with the increase of Al content, which implies the diffusion of Al atoms into ZnO. The reduction of dominant peak intensity of cubic ZnO (111) and the increase of hexagonal ZnO (100) and (101) peaks intensity suggest that there were variations of crystal structures for the samples with Al content above 6%. Two orders of magnitude of electron concentration raises in samples with 2 and 4% Al content compared with the as-grown without RTA-treated samples. It also infers that RTA facilitates diffusion of Al atoms in AZO material structures and activation of Al dopants. When Al content is above 6%, the variations of crystal structures with the enhancement of biaxial compressive strain result in blue shift in energy of photoluminescence peak and in frequency of E-2(high) phonon mode of micro-Raman spectra. The deterioration of crystal quality due to the increase of strain-induced defects hinders the electrical and optical performance when Al doping concentration is above 6% in AZO materials. (C) 2011 Elsevier B.V. All rights reserved.