Poly-crystalline Indium Nitride deposited by reactive sputtering presents a room-temperature direct apparent optical bandgap around 1.8 eV, making it very attractive for optoelectronic applications like high-efficency low cost solar cells, optical coatings and optical sensors. In this work, we report on a detailed study of the influence of deposition parameters on the crystalline quality, surface morphology and optical properties of InN films grown on GaN-on-sapphire substrates by reactive radio frequency (RF) sputtering. InN samples present a wurzite crystallographic structure and a highly c-axis preferred growth orientation, as confirmed by X-ray diffraction measurements. In a pure N₂ atmosphere the growth conditions under study are the substrate temperature, RF power and N₂ pressure. The optical properties were studied through transmission measurements. The apparent optical bandgap was estimated close to 690 nm (1.79 eV) for InN samples deposited at 450ºC and 3.5 mTorr. For samples deposited at temperatures above 450ºC a slight blueshift of 0.02 eV of the bandgap was observed. The sputtering pressure seems to strongly influence the InN absorption edge due to a change of the species in the plasma composition with a damage of the InN surface. However, RF power does not appear to affect the bandgap position. The surface morphology of the InN films was analyzed by atomic force microscopy in a surface of 5x5 µm². The morphological quality of the InN samples was highly improved at the optimized conditions. At low substrate temperatures (400°C) and low RF powers (20W and 30W) high surface quality InN films were achieved, with root-mean-square surface roughness as low as ~0.4 nm, comparable to the surface roughness of the GaN-template substrate. Our results indicate that the reactive RF sputtering is a promising deposition technique for obtaining high quality InN layers on GaN-template substrates with low cost and good compatibility with other optoelectronic devices.

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