Effect of sapphire substrate orientations on the microstructural, optical and NO2 gas sensing properties of Zn(1−x)CdxO thin films synthesized by sol gel spin-coating method

Abstract

A simple and cost-effective sol–gel technique was employed to elaborate ZnO and Zn(1−x)CdxO thin films deposit by spin coating onto the c- and r-plane sapphire substrates. The deposited films were characterized for their structural, morphological and optical properties using high resolution X ray diffraction (HRXRD), atomic force microscopy (AFM) and scanning electron microscopy (SEM) as function of Cd incorporation and employed substrate. Higher Cd incorporation (4.5%) is obtained for ZnCdO layer deposited on r-plane sapphire, which is confirmed by the greatest energy shift (110 meV) to lower energy measured by low temperature photoluminescence. X-ray diffraction study revealed that all films are polycrystalline with a hexagonal wurtzite structure. A preferred orientation along [001] and [110] direction is obtained respectively for layer deposited on c- and r-plane sapphire. However, the (002) and (110) XRD layers peak were shifted towards the lower 2θ values after Cd incorporation showing a slight variation of cell parameters. SEM and AFM image show no very significant variation in the morphology of the layers depending on the substrate orientation and Cd content incorporated. A mixture of large and small hexagonal grains are obtained which are more pronounced for ZnCdO deposited on r-plane sapphire and their agglomeration leaves more empty space in films.The gas sensing performances were tested in NO2 containing air for different operating temperatures as function of Cd incorporation and sapphire substrate orientation. The experimental result exhibited that ZnCdO sensors deposited on r-plane sapphire shows a more better gas response with fast response and recovery time at moderate operating temperatures as Cd contend increase.