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 Al xGa1-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 x 10-4 and 2.0 x 10-2 Ω cm2 were achieved on annealed, Mg-doped ([Mg] ∼ 5 x 1019 cm-3), p-type GaN layers that had been cleaned in HCl at 85 deg. 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/Al0.1Ga0.9N 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