“…The feature of sapphire located at 630 cm -1 does not exist in as-grown GaN thin film spectrum, suggesting that the GaN layer is thick (5 µm, as determined by cross-sectional SEM). Thus, the feature is dominant by GaN layer instead of sapphire substrate [11]. Apart from that, the IR spectrum for the as-grown GaN thin film exhibited a dominant reflectance peak at 559 cm -1 , as well as a small, broad dip around 734 cm -1 .…”