Nitrogen and oxygen annealing effects on properties of aluminum-gallium oxide films grown by pulsed laser deposition

Aluminum-gallium oxide (AGO) films on c-plane sapphire substrates by pulsed laser deposition are described. Both nitrogen and oxygen annealing effects on the structural and optical properties of AGO films are investigated. The AGO film shows an amorphous structure when deposited at low temperatures (<= 400 degrees C) while a crystalline structure at 800 degrees C. After post annealing at 900 degrees C, an amorphous-to-crystalline phase transformation for the 400 degrees C-deposited film occurs and shows the preferred f3 phase. The corresponding optical bandgap also increases from 5.14 eV to 5.41-5.46 eV depending on the annealing ambience. From Raman measurements, the 800 degrees C-deposited AGO sample possesses a more stable O-Ga-O bonding compared to that of the 400 degrees C-deposited one after annealing. Unusually, an evident increase in the nitrogen content is observed for the samples after post annealing at 900 degrees C in nitrogen atmosphere. The rapid dissociation of oxygen atoms may accelerate the disintegration of crystals and rearrangement, which makes the AGO film adsorb nitrogen atoms and cause the grain size to be significantly reduced. However, the extent of the nitrogen incorporation seems to have no apparent effect on the optical properties. All the AGO films show the optical transmittance over 80% in the ultraviolet-visible region with the calculated bandgaps more than 5.4 eV. Details of the mechanism about the nitrogen incorporation into the annealed AGO films via the oxygen vacancies or micro-pores will be discussed.