Optical characterization of AlNÕGaN heterostructures

AlN/GaN/sapphire heterostructures with AlN gate film thickness of 3–35 nm are characterized using photoreflectivity ~PR! and photoluminescence ~PL! spectroscopy. Under a critical AlN film thickness, the luminescence from the GaN channel layer near the interface proves to be excitonic. No luminescence related to the recombination of the two-dimensional electron gas ~2DEG! is observed, in spite of high 2DEG parameters indicated by Hall-effect measurements. The increase of the AlN gate film thickness beyond a critical value leads to a sharp decrease in exciton resonance in PR and PL spectra as well as to the emergence of a PL band in the 3.40–3.45 eV spectral range. These findings are explained taking into account the formation of defects in the GaN channel layer as a result of strain-induced AlN film cracking. A model of electronic transitions responsible for the emission band involved is proposed. © 2003 American Institute of Physics. @DOI: 10.1063/1.1609048#