Strain-compensated type-II InGaN-ZnGeN2-AlGaN quantum wells (QWs) are studied as improved active regions for light-emitting diodes (LEDs). Both the band gap and the lattice parameters of ZnGeN2 are very close to those of GaN. The recently predicted large band offset between GaN and ZnGeN2 allows the formation of a type-II heterostructure. The deep confinement of holes in the ZnGeN2 layer allows the use of a low In-content InGaN QW to extend the emission wavelength into the green wavelength region. A thin layer of AlGaN surrounding the QW is used as a strain compensation layer. Simulation studies of the proposed type-II QW indicate an enhancement of 5.6-6.8 times the spontaneous emission rate compared to InGaN-GaN QWs emitting in the green wavelength region.
GaN microdomes are studied as a broadband omnidirectional anti-reflection structure for high efficiency multi-junction concentrated photovoltaics. Comprehensive studies of the effect of GaN microdome sizes and shapes on the light collection efficiency were studied. The three dimensional finite difference time domain (3-D FDTD) method was used to calculate the surface reflectance of GaN microdomes as compared to that of the flat surface. Studies indicate significant reduction of the surface reflectance is achievable by properly designing the microdome structures. Formation of the GaN microdomes with the flexibility to tune the size and shape has been demonstrated by using reactive ion etching (RIE) of both GaN and the self-assembled silica monolayer microspheres. Characterizations of the angle-dependence light surface reflectance for both micro-domes and flat surface show the similar trend as the simulation.
The enhancement of light extraction efficiency for thin-film-flip-chip (TFFC) InGaN QWs LEDs with GaN microdomes
on n-GaN layer was studied. The three dimensional FDTD method was used to calculate the light extraction
efficiency for the TFFC InGaN QWs LEDs emitting at visible spectral regime, as compared to that of the conventional
TFFC InGaN QWs LEDs. The calculation indicates significant dependence of the p-GaN layer thickness on the light
extraction efficiency. Significant enhancement of the light extraction efficiency (2.5-2.7 times for λpeak=460nm and 2.7-
2.8 times for λpeak=550nm) is achievable from LEDs with GaN micro-domes with optimized micro-dome diameter and