UVA and UVB light emitting diodes with Al_yGa_1−yN quantum dot active regions covering the 305–335 nm range

Ultra-violet (UV) light emitting diodes (LEDs) using III-N quantum dot (QD) active regions have been fabricated by molecular beam epitaxy on (0001)-oriented sapphire substrates. By using the epitaxial compressive stress between the QD material and the template/barrier layers, leading to a 2D–3D growth mode transition, self-assembled Al_yGa_1−yN QDs with a nominal Al composition of 10% and 20% have been fabricated on Al_0.6Ga_0.4N. Atomic force microscopy and transmission electron microscopy measurements show high QD densities, ranging between 2 × 10¹¹–5 × 10¹¹ cm⁻², and height and diameter distributions between 1.5–3 nm and 5–20 nm. LED structures including two different Al_yGa_1−yN/Al_0.6Ga_0.4N (0001) QD active regions have then been fabricated and processed using a standard planar geometry and squared mesa structures. Current–voltage characteristics and electroluminescence (EL) measurements have been performed at room temperature. In particular, the EL properties are investigated in terms of spectral range and wavelength shift as a function of the injection current density. Typically, an emission between 325 and 335 nm is obtained for Al_0.1Ga_0.9N QDs and between 305 and 320 nm for Al_0.2Ga_0.8N QDs. The LED characteristics (EL wavelength and broadening) are then correlated to the QD structural properties and the results, supported by calculations, show the main influence of the QD height dispersion and composition fluctuations. Finally, the light output intensity variation as a function of the injection current density has also been investigated and is discussed in terms of injection and recombination mechanisms in the devices. (paper)