Synthesis and characterization of Al-doped ZnO and Al/F co-doped ZnO thin films prepared by atomic layer deposition

Deposition of Al-doped ZnO (AZO) and Al/F co-doped ZnO (FAZO) thin films in an ALD "batch-type" reactor at temperature of 200 degrees C was presented. AZO films with various Al content were investigated in terms of their morphological, textural, optical and electrical properties, leading to assessment of the most suitable ALD process parameters, which enabled fabrication of high quality AZO films. For the best performance of AZO-20 sample containing 5 % of cycles of Al precursor, the lowest resistivity of 1.36 x 10-3 omega center dot cm was achieved, proving that obtained film can be used in photovoltaic applications. This was correlated with increased share of preferred crystallites growth orientation along c-axis. Moreover, all AZO samples exhibited high transparency in the visible light (>80 %), and optical band gap energy values ranging from 3.32 eV to 3.75 eV. The latter increased with raising Al doping level. The potential of further improvement by addition of fluorine was investigated as well. Simultaneous incorporation of Al and F ions into ZnO, aiming to obtain FAZO thin films of constant Al content and ascending fluorine concentrations, resulted in slight increase of resistivities of FAZO samples compared to AZO-20. Raising the fluorine content in FAZO samples caused simultaneous decrease of carrier concentration and decrease of their mobility, which may arise from enhanced formation of lattice defects due to F- ions substitution. In the case of highly doped FAZO sample, HF etching was observed, which resulted in the reduction of the overall film thickness. Al/F co-doped ZnO films retained the optical properties of original AZO-20 film, as confirmed by measured high transparency and band gap energies of 3.53 - 3.58 eV. It was shown that simultaneous Al/F codoping of zinc oxide at temperature of 200 degrees C and batch type reactor, is is not the process of choice, when considering simple pulses sequences.