Elaboration of ZnMgO thin films by ultrasonic spray pyrolysis for optoelectronic applications using water-based solutions

International audienceIn this work were elaborated thin films of Zinc Magnesium Oxide (ZnMgO) by ultrasonic spray pyrolysis. Zn(1-x)Mg(x)O is a wide bandgap compound semiconductor with useful properties for optoelectronic devices, mainly ultraviolet “solar-blind” detectors, as an active layer, and solar cells where it can be used as a buffer layer. The main difficulties related to the elaboration of Zn(1-x)Mg(x)O using solution-based and spray pyrolysis techniques are the wide miscibility gap between MgO and ZnO and the use of toxic and unsafe solvents such as methanol. In this study, we were able to elaborate Zn(1-x)Mg(x)O with varying Mg content up to 70% with water-based solutions only. The difficulty using only water-based solutions is related to the inherent unfavorable properties such as strong hydrogen bonds with high boiling point. However, we managed to optimize the precursors solution concentration, the number of scans, the flow rate, the pressure and the temperature, to obtain homogeneous and good quality films. The solid composition in the thin films was measured using X-ray fluorescence to assess the Mg solubility limit in Zn(1-x)Mg(x)O and its incorporation efficiency. High transmittance of about 90 % in the visible region was obtained in all the films with a very small variation with the thickness, suggesting a highly controlled process of the elaboration.The bandgap and the Urbach energy variation with the composition were investigated and linked to the morphology and structural properties. A miscibility limit of 30 % of Mg was obtained, confirmed by the X-ray fluorescence measurements and correlated to the bandgap variation with the Mg composition extracted from the transmittance spectra analysis. The structural properties were investigated using confocal micro-Raman spectroscopy which permitted to identify the wurtzite structure as the only phase in all the films with composition less than 30 %, with a phase separation occurring beyond this limit. The electrical properties were characterized using the van der Pauw and Hall effect technique using indium to form the ohmic contacts. The n-type carrier concentration dramatically decreases from about 2*10^(19) to 5*10^(15) cm^(-3) when increasing Mg composition from 0 % to 7 %. The thin films become semi-insulating for Mg composition higher than 7 %. The correlation of the electrical results with the optical and structural properties suggests that, in addition to the increase of the bandgap, a compensation mechanism related to the defects introduced in the alloy plays a central role in the decrease in the carrier concentration. The mobility slightly increases with the Mg composition suggesting that the films crystal quality was not significantly degraded by the increase of the magnesium composition in the considered range. The modulation of the conductivity, in addition and partially related to the modulation of the bandgap, is of high interest to the detector or the solar cell device