High-efficiency AlGaN deep-UV LEDs fabricated on a- and m-axis oriented c-plane sapphire substrates

We demonstrated high-efficiency 270 nm-band AlGaN deep-ultraviolet (DUV) light-emitting diodes (LEDs) fabricated on 0.15 off a-and m-axis oriented c-plane (0001) sapphire substrates grown by low-pressure metal organic chemical-vapor deposition (LP-MOCVD). An approximately 4.5 mu m-thick low threading dislocation density (TDD) AlN template was grown on the sapphire substrate by using an NH3 pulsed-flow multilayer (ML) growth method. We found that atomically flat surface can be easily obtained for AlN layer grown on a-axis oriented sample. On the other hand, a step-bunching was observed for the surface of AlN layer grown on m-axis oriented sample. We found, from these results, that a-axis oriented (0001) sapphire is more suitable for obtaining a flat sur-face in AlN template without step-bunching. We also found that shorter wavelength LED can be obtained by using an a-axis oriented (0001) sapphire, because an Al incorporation in AlGaN is slightly high for the growth on a-axis oriented (0001) sapphire. We achieved highefficiency AlGaN quantum well (QW) DUV LEDs on AlN templates grown on both a- and m-axis oriented sapphire substrates. The peak wavelengths of the LEDs fabricated on a- and m-axis oriented sapphire were 270 and 277 nm, respectively. The maximum external quantum efficiencies (EQEs) of the LEDs fabricated on a- and m-axis oriented sapphire were 3.8 and 3.2%, respectively, measured under room temperature (RT) continuous wave (CW) operations. (C) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim