High resistivity AlxGa1-xN layers grown by MOCVD

Undoped AlXGa1-XN layers with good surface morphology and very low electron concentration have been grown by MOCVD on sapphire substrates. The observed electrical and optical properties depend strongly on the growth temperature. Layers grown at 1000 degrees C exhibited low resistivity and strong optical absorption below the bandgap. In contrast,layers grown at 1050 degrees C had low carrier concentrations and good mobilities. Virtually no optical absorption near the band edge was observed as opposed to the usual situation in AlXGa1-XN. The electrical properties of these layers can be explained by the presence of donor centers whose energy increases with composition, and deeper lying compensating defects. The interaction of these centers renders the samples with x<0.2 highly resistive, with room temperature resistivity higher than 10(6) ohm-cm. SIMS data strongly suggest that the electrically active centers in our AlGaN layers are native defect-related. Implantation of Si ions into Al0.12Ga0.88N, and subsequent annealing at 1140 degrees C resulted in layers with electron concentration of 4.6 x 10(17) cm(-3).