Electron transport in Al-doped ZnO nanolayers obtained by atomic layer deposition

Al-doped ZnO thin films with different Al content were prepared by atomic layer deposition (ALD). To carry out thermal ALD, diethyl zinc (DEZ) and tri-methyl aluminium (TMA) were used as Zn and Al precursors, respectively, and water vapor as oxidant. Various numbers n of DEZ and m TMA cycles was used to obtain different [ZnO]_n[Al₂O₃]_m films, where n = 100 – 95, m = 1 – 5. The X-ray diffraction analysis showed a predominantly (100) oriented polycrystalline phase for the ZnO:Al films with a low Al content (m = 1 – 3) and an amorphous structure for pure Al₂O₃. In ZnO:Al with a higher Al content (m = 4 – 6) the (100) reflection disappeared and the (002) peak increased. The resistivity of the films decreased with the increase in the Al content, reaching a minimum of 3.3×10^-3 Ω cm at about 1.1 % Al₂O₃ for the [ZnO]₉₉[Al₂O₃]₂ sample; for higher dopant concentrations, the resistivity increased because of the increased crystal inhomogeneity due to axis reorientation.