Abstract
Al-doped ZnO thin films were prepared on the (0001) sapphire (c-Al2O3) substrates by atomic layer deposition (ALD) using alternating pulses of Zn(C2H5)2, Al(CH3)3 and H2O precursors and post-deposition high-temperature annealing. Photoluminescence (PL) spectroscopy showed that the threshold of stimulated emission decreases with increasing Al concentration, from 49.2 kW/cm2 of the ZnO film to 12.2 kW/cm2 of the ZnO film nominally containing 4% Al. This reduction is attributable to the increase in the optical scattering resulting from segregation of excess Al in heavily Al-doped ZnO films. The structure of these films was investigated by analytical scanning-transmission electron microscopy (STEM) as well as X-ray diffraction (XRD). It was revealed that a single crystal ZnO layer containing a small amount of Al is formed with the orientation relation with respect to the c-Al2O3: [0001]ZnO || [0001]Al2O3 and [01\\bar10]ZnO || [2\\bar1\\bar10]Al2O3, and that a polycrystalline ZnAl2O4 layer is formed between the ZnO layer and the c-Al2O3 substrate. The electron microscopy observation accounts for the results of the electro-optical experiments. The growth mechanism of the observed two layers is discussed.
