Grain size and composition effects on the cathodoluminescent characteristics of sprayed zinc oxide thin films

The cathodoluminescent (CL) property of 1?µm-thick sprayed ZnO films is here reported versus the sample mean grain size (D) resulting from the spraying flow rate (f) parameter. The deposition process, performed at T _s?=?723?K, leads to hexagonal packed structure of 002] preferred growth orientation, regardless of the f value. The CL-spectra are typical of the film's growth rate (r) and the intensity (I) of the three main emission lines (λ?≈?386, 497 and 644?nm) is greatly affected by D that is ranged between 0.40 and 0.90?µm. Their (I???D) curves have a similar profile that presents a common critical point, D _c?=?0.57?µm, depicting two different luminescent behaviours of the material. Smaller grains exhibit optimal intensities. Their band-edge near UV line (λ?≈?386?nm) proceeds from a phonon replica phenomenon; excess of interstitial Zn explains the result of their blue–green line (λ?≈?497?nm) while deep-level interactions effect is the relevant interpretation for the red one (λ?≈?644?nm). A decrease of luminescent properties from D?=?0.45?µm till D _c is assigned to the sample microstructure degradation. The bell-shaped profile of CL-intensity for D>D _c reflects a competition between the increase of the material reactive part and the collapse of crystallites quality due to higher content of Cl and S impurities in bigger grains. A limit of the two approaches appears at D?=?0.65?µm.