Top-view SEM analysis showed that the films deposited at 250  C is composed of many small crystalline grains with little variation in grain size (Figure ). The little variation in grain size indicates that the film grows in a layer-by-layer fashion without any observable secondary nucleation. The chemical bonding environment and atomic composition in the GaN films deposited at 250 °C were analysed by XPS (Figure ). High resolution XPS spectra were used to analyse the chemical environment of the Ga 2p3/2 and N 1s regions. Two peaks were used to obtain a good fit for the Ga 2p3/2 at 1117.5 eV and 1119.4 eV, which were attributed to Ga-N and Ga-O bonds, respectively (Figure ). Three peaks were used to fit the N 1s at 397.1 eV, 394.8 eV and 392.7 eV, which were assigned to N-Ga and two Ga Auger peaks, respectively (Figure ). This is in accordance with previous XPS measurements on GaN thin films. After sputter cleaning the film surface, XPS measurements gave an initial overall composition of 25.0 at.% Ga, 74.5 at.% N, 0.5 at.% O with no detectable C. It should be noted that the N 1s peak overlaps with Ga Auger peaks, which results in a broad N 1s peak and an overestimate of the N content. To obtain the Ga/N ratio in the film without Ga Auger interference, the Auger peaks were subtracted from the contribution to the N content. The N-Ga peak contribution is 33% of the overall N content, which gives 24.6 at.% of N that is attributed to the actual N content of the film. This gives a corrected Ga/N ratio of 1.02, which is close to stochiometric GaN. The high resolution XPS peak of Ga 2p3/2 can be fitted with contributions from Ga-N and Ga-O bonds while the high resolution XPS N 1s peak cannot be fitted with a N-O peak, suggesting that all oxygen in the film is bonded to Ga. If oxygen is subtracted from Ga 2p3/2 spectra and only the Ga -N and N-Ga bonds are considered, a near stoichiometric Ga/N ratio of 0.99 is obtained. To investigate if the subtraction of the Ga Auger peaks from the N 1s XPS region is a good strategy, RBS combined with ERDA was conducted on the GaN film. The RBS/ERDA measurement showed that the GaN film contained 45.7 at.% Ga, 47.2 at.% N, 2.8 at.% C, 3.1 at.% O and 1.2 at.% H giving a Ga/N ratio of 0.97, which indicates a slightly understochiometric film composition. This shows that even when subtracting the Ga Auger peak from the N 1s XPS region, the film composition and Ga/N ratio is not reliable and other techniques are required to determine an accurate atomic content. In addition, C impurities were not detected by XPS after clean sputtering whilst RBS/ERDA showed C present in the film, showing the latter is preferred for detection of impurities. In addition, sputtering was used in the XPS measurement, which could cause preferential sputtering and change the composition and impurity levels in the film. Nonetheless, XPS is highly important for accessing the chemical environment of each element. (2)