Development of aluminum gallium nitride-based emitters in the form of graded-index separate confinement heterostructure (GRINSCH)
The development of ultraviolet semiconductor emitters (LEDs and lasers) will enable a large number of industrial and medical applications. AlGaN alloys are ideally suited for the development of such devices since their energy gap can be tuned from the near UV (365 nm) to deep UV (200 nm). However, the doping of such materials n- and p-type is difficult. Another problem is the generally poor light extraction efficiency from both UV and visible LEDs. This research addressed the first problem by developing UV emitters in the form of graded-index-separate-confinement-heterostructure (GRINSCH). In these device the active region is embedded in two compositionally graded wave guiding layers. Due to the polar nature of nitride semiconductors these compositionally graded AlGaN films are doped p- or n-type if the grading changes from high to low concentration or from low to high concentration respectively. Thus, a p-n junction is automatically formed without the incorporation of dopants. The polarization induced doping level in these structures was calculated to be 1018cm-3 for the p- and n-sides. A number of devices, whose active region is either 75 nm Al0.72Ga0.28N bulk film or multiple QWs have been grown on 6H-SiC substrates by Molecular-Beam Epitaxy (MBE) and investigated. The emission properties of these structures were investigated by cathodoluminescence (CL) and by measuring their optical gain. A maximum net modal gain in excess of 80 cm-1 was measured with an optical gain threshold of 14 µJ / cm2. Some of these structures, emitting in the near UV, were also electrically pumped. The second problem was addressed by incorporating dielectric (TiO2) photonic crystals on the phosphor plates of white LEDs in order to increase the light extraction efficiency upon illumination with blue LEDs. The two-dimensional (2D) hexagonal-lattice of TiO2 photonic crystal was formed by e-beam lithography on low-scattering (Y1-xCex)3Al5O12 (YAG:Ce) ceramic phosphor plates. Yellow light extraction enhancement by a factor of 4.4 was achieved with a 2D photonic crystal nano-cylinders having diameter 430 nm, lattice constant of 580 nm and height of 350 nm. Simulations using a three-dimensional finite difference time domain are consistent with our measured data.