Session Index

Light emitting diodes (LEDs) are the emerging next generation lighting technology but obtaining high performance green and longer wavelength LEDs, require InGaN alloys with higher indium content has proven difficult. One major problem is that the lattice mismatch between InGaN active quantum wells and the surrounding GaN layers worsens with increasing indium content. In addition, localization mechanism associated with compositional fluctuations that allow bright LEDs in the blue is no longer valid. In this work ZnO substrates are used for GaN and InGaN growth by metalorganic chemical vapor deposition (MOCVD). ZnO offers many advantages for GaN and InGaN due to its closely matched lattice constant, similar thermal expansion coefficients, and its ability to also be easily chemically etched, which results in improved light extraction. However, H2 etching of the ZnO substrate at high temperatures, Zn diffusion out of the substrate, and the stability of an oxide substrate in a highly reducing atmosphere still cause many issues during MOCVD growth. The direct nucleation of GaInN on ZnO is considered in this work including some native and non-native III-Nitride transition layers. The transition layer was grown on the substrates before or during the MOCVD growth to prevent Zn diffusion, protect the ZnO substrate from H2 back etching, and promote high quality nitride growth on the ZnO substrates. It was found that Al2O3 grown by Atomic Layer Deposition (ALD) on ZnO was a particularly effective transition layer. High resolution x-ray diffraction (HRXRD) measurements revealed that the thin Al2O3 layers after optimal annealing showed distinct wurtzite GaN and InGaN films grown epitaxially on ZnO with a mirror-like surface, no etched pits, and no peeling off. This work was extended to LEDs grown on Si with an ALD Al2O3 transition layer and these devices had a turn on voltage of <5V.

 

Confocal Raman spectroscopy is adopted to analyze strain distribution of GaN layer on patterned sapphire substrate. The Raman mapping results show that strain distribution is strongly correlated to the pattern on sapphire substrate.

 

We simulated AlGaN-based UVC LEDs with alloy fluctuation model. The alloy fluctuation model helps to form a permeation path for carrier injection, but also reduces carrier confinement within the quantum wells. The extension of the wave function into quantum barriers enhances TM emission, which might lead to poor light extraction.

 

Green InGaN-based LED with an embedded nanopipe GaN layer was demonstrated. The EC-etched pipe structure can be observed through the electroluminescence emission images through a polarizer. The polarized EL emission light with 537nm and 558nm peak wavelengths was observed in the EC-LED.

 

Abstract: 20 pair n-type GaN:Si/undoped-GaN stack structure were transformed into the porous-GaN/u-GaN distributed Bragg reflectors (DBR) structure through the doping-selective electrochemically etching process. Polarized reflectivity and central wavelength of the porous GaN DBR were measured as the values of 93.8%/440nm perpendicular to the pipe and 93.0%/465nm along the pipe structure.

 

286nm AlGaN-based UVC light-emitting diodes (LED) with an embedded nanoporous AlGaN layer was demonstrated. The n-type AlGaN:Si layer was transformed into the conductive porous AlGaN layer with triangle-shaped air void structure through the electrochemical wet etching process. Far-field radiative patterns of the TE- and TM-modes EL spectra were analyzed.