GaN films have been grown on quartz and (011̄2) sapphire substrates using combined ultraviolet excimer laser and microwave plasma enhanced metalorganic chemical vapour deposition (MOCVD) at a substrate temperature of 500°C. Film compositions were analysed by X-ray photoelectron spectroscopy (XPS) and less than 5% residual impurity, principally carbon and oxygen was found. Films grown on quartz were polycrystalline wurtzite with a preferential (0002) orientation, while (0002) and (21̄1̄0) orientations were both found on (011̄2) sapphire. Electron carrier concentration was found to be controllable between 1017 and 1014 cm-3 via control of ammonia plasma injection rate, whilst electron mobility also increased proportionally with the flow rate of the plasma. A room-temperature mobility of 95 cm2 V-1 s-1 was obtained for films on (011̄2) sapphire, saturating at a plasma flow rate of 100 ml/min. The results are interpreted as showing a reduction of nitrogen vacancies by an increase in the reacting species liberated in the plasma.