Growth and fabrication of AlGaN-based ultraviolet light emitting diodes on 6H-SiC(0 0 0 1) substrates and the effect of carrier-blocking layers on their emission characteristics

Growth, fabrication, and the electrical and optical characterization of ultraviolet light emitting diodes and their components, including AlxGa1-xN films, quantum wells (QWs), and ohmic contacts, and the problems encountered in the process integration of these components have been investigated. Ni/Au ohmic contacts with specific contact resistivities of 2.2 * 10^-4 and 2.0 * 10^-2 (omega) cm^2 were achieved on annealed, Mg-doped ([Mg] ~5 × 10^19 cm^-3), p-type GaN layers that had been cleaned in HCl at 85 °C and on the backside of the SiC substrates after annealing in nitrogen, respectively. The emission intensity of the diodes increased with an increase in the number of Al0.06Ga0.94N/Al0.10Ga0.90N QWs and with the use of Si-doped n-type barrier layers. The highest intensities of the principle emission at 353 nm were measured at all values of the injection current in the device with a p-type carrier-blocking layer at the top of the QWs; this device also exhibited the highest values of light output power. Growth of an n-type carrier-blocking layer at the bottom of the QWs had an adverse effect on their characteristics. A broad peak centered at ~540 nm exhibited yellow luminescence and was present in the spectra acquired from all the devices. This peak is attributed to absorption of the ultraviolet emission by and re-emission from the p-GaN and/or to the luminescence from the AlGaN within QWs by current injection.