The absorption coefficient of GaN immediately above the band gap is equal to 1.0x10⁵ cm^-1. It implies that GaN layers of thickness ranging from 200 to 500 nm are required for optical transmission measurements with light of energy larger than the band-gap. A few hundred nm thick GaN layers grown on sapphire have a very high defect density due to lattice mismatch between GaN and sapphire. Therefore, the growth of GaN layers of thickness and quality suitable for measurements of optical absorption is very difficult. In order to overcome this obstacle, heterostructures consisting of a 2 μm thick Al_0.08Ga_0.92N layer and, on top of it, of a thin GaN layer have been grown in a single run of a MOVPE reactor. The absorption spectra of these samples taken at 12 K show three well resolved excitons on the onset of the fundamental absorption edge of GaN. These spectra have the property of light-induced narrowing. The spectral range of absorption spectra of GaN grown on AlGaN layer is limited by the absorption edge of AlGaN. In order to measure the spectra in a broader spectral range and to investigate the influence of the growth temperature on properties of GaN, a 500 nm thick layers of GaN have been grown. The layers grown at 500C, 800C, and 1100C have mirror-like surfaces, and their thicknesses were determined from the spacing of the interference fringes in transmission spectra. The layer grown at 1100C has a step-like absorption edge at 3.5 eV with an excitonic structure. The layer grown at 800C has broad absorption edge at 3.5 eV and lacks the excitonic structure. The absorption coefficient of the layer grown at 500C increases approximately linearly with energy above 2.8 eV. The absorption coefficient for energies larger than 4.0 eV of the layers grown at 500C and 800C is much larger than that of the layer grown at 1100C. It seems that it is due to a relaxation of momentum selection rule for optical absorption in the amorphous GaN layers.

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