The fundamental absorption edge of high crystalline quality GaN and that of amorphous GaN grown at low temperature

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An amorphous and two crystalline GaN layers have been grown by metalorganic vapor phase epitaxy on sapphire substrates at the growth temperature of 500, 800, and 1075 °C, respectively. The fundamental absorption edge of amorphous GaN starts at 2.8 eV. The absorption coefficient of that layer increases approximately linearly with energy, which is qualitatively different from the sharp excitonic absorption edge starting at 3.5 eV in crystalline GaN. The fundamental absorption edge of GaN has also been investigated in a 200 nm thick GaN layer grown on a 2 µm thick Al0.08Ga0.92N layer. A broad excitonic line is observed in this GaN layer after the sample has been cooled in darkness. A metastable narrowing occurs and three excitonic absorption lines are observed after the sample has been illuminated with UV light. Measurements of this effect have been made as a function of temperature and photon energy. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)