Growth of Al Doped ZnO Thin Films by Atomic Layer Deposition Process

ZnO is a transparent semiconductor with a wide   and direct band gape of 3.37 eV, large excitonic binding energy of ~60 meV, high mechanical and thermal stabilities, and good electrical conductivity. Therefore, ZnO based materials are extensively investigated in spintronics, gas and biological sensors, solar cells, catalysts, electronics, piezoelectric devices and high-efficiency short-wavelength optoelectronic nanodevices. In solar cell application, especially Al doped ZnO (AZO) thin films have been extensively developed and investigated due to their low cost, high thermal stability, high optical transparency and high conductivity [1, 2 , 3].

 Several methods have been applied to deposit ZnO thin films in the past, such as sol gel spin coating, magnetron sputtering, pulsed laser deposition (PLD), chemical vapor deposition (CVD), and atomic layer deposition (ALD). Among these, ALD is considered as a novel technique to deposit ZnO thin films, because it possesses self-limiting surface reactions in each ALD cycle, which consequently control the layer thickness with unprecedented atomic level precision, stoichiometry, composition, uniformity of large area and maintain excellent conformity of complex nanostructures. In addition, ALD process is able to introduce separately each precursor to react on the surface of the structure. The concentration of dopant in the ZnO films can be precisely controlled by the number of ALD cycles, so that the physical properties of AZO films, such as carrier density and conductivity, can be adjusted to meet the requirements of different applications.

 In this study, Al doped ZnO thin films were deposited by the ALD technique with trimethylaluminium (TMA), diethylzinc (DEZ) and water vapor as aluminium, zinc and oxygen precursors, respectively. The films were grown in Savannah 100 cross-flow thermal ALD reactor. The growth temperature was maintained at 250 ^oC during the reaction.  N₂ was used as a carrier gas with a flow rate of 20 sccm. The ALD AZO films with various Al doping level were prepared by introducing TMA cycles once in every 30 pulses of DEZ and H₂O injection.

 The structural, optical and electrical properties of the AZO films prepared by using ALD processes were investigated. The crystal structure of ZnO was analyzed by X-ray diffraction (XRD). The AZO film surface morphology was inspected using a field emission scanning electron microscopy (FE-SEM). The surface roughness was analyzed by atomic force microscopy (AFM). The sheet resistivity of AZO films was determined by using four-point probe measurements. The optical reflection was carried out by UV-VIS-NIR spectrometer.

References:

 P. Genevée, F. Donsanti, G. Renou, D. Lincot, Applied Surface Science 264,  464 (2013)

1. G. Luka,  T. A. Krajewski, B. S. Witkowski, G. Wisz, I. S. Virt, E. Guziewicz, M. Godlewski, J. Mater. Sci: Mater. Electron 22, 1810 (2011)
2. J. W. Elam, D. Routkevitch, S. M. George, Journal of The Electrochemical Society, 150 (6) G339 (2003).