Nano-structured InGaN Light-Emitting Diodes for Solid-State Lighting.

Abstract

Solid-state lighting can potentially reduce the electricity consumption by 25%. It requires high efficiency light-emitting diodes across the visible spectrum. GaN and related materials have direct bandgap across the entire visible spectrum and are ideal for future solid-state lighting applications. However, materials defects, polarization charges, and total internal reflection have thus far limited the efficiencies of InGaN LEDs, in particular InGaN LEDs in the green/yellow wavelength range, which are critical in achieving highly efficient LED luminaires with an excellent color-rendering index. In this Thesis, we have developed and demonstrated that novel in situ nano-structured GaN processes in MOCVD are effective in improving the efficiencies of InGaN LEDs. InGaN LEDs grown on quasi-planar semi-polar GaN templates were proven to exhibit three times higher internal quantum efficiencies and negligible quantum confined Stark effect using selective area epitaxy. InGaN LEDs grown on nano-structured semi-polar GaN templates are also effective to improve the internal quantum efficiency by 31%. The same in situ processes are also effective in reducing the defect density by an order of magnitude and increasing the photon extraction efficiency as a factor of two. The in situ processes include in situ silane treatment and high temperature overgrowth. Both processes require only standard MOCVD tools and hence are cost effective and suitable for mass-production. In situ silane treatment treats c-plane GaN samples with silane under ammonia environment, generating nano-scale truncated cone structures with up to 200 nm scale. These truncated cone structures can be subsequently transformed into pyramidal nanostructures comprising of only (10-11) and (11-22) semi-polar planes using high temperature overgrowth. These processes were applied to both InGaN active region and the LED surface to improve the internal quantum efficiency and the photon extraction efficiency, respectively. Extensive materials, device, and optical characterizations have been carried out in this research.