Lateral epitaxial overgrowth of aluminum nitride and near ultraviolet LEDs for white lighting applications

In recent years, substantial efforts have been made to develop deep ultraviolet AlGaN-based LEDs (200-280 nm) for specialized applications such as bio-detection and non-line-of-sight (NLOS) communications. One of several factors limiting the performance of these devices is the high threading dislocation (TD) density of ∼5x109 cm-2 that results from growing the required AlN base layer on either a SiC or sapphire substrate. Lateral epitaxial overgrowth (LEO) of AlN, the first topic of this dissertation, is a promising technology for growing low TD density AlN templates.;Conventional LEO methods relying on selective area growth (SAG) have not been effective for AlxGa1-xN with x > 0.2, because of the high aluminum sticking coefficient for the mask materials and/or contamination of the film by the mask. Therefore, maskless AlN LEO was investigated using metal organic chemical vapor deposition (MOCVD) and hydride vapor phase epitaxy (HVPE). Cracked AlN films with TD densities of < 1x107 cm-2 in the wing regions were grown by HVPE on patterned 6H-SiC templates. Cracking was attributed to the unfavorable thermal expansion mismatch between AlN and 6H-SiC. Crack-free AlN films with TD densities of < 1x108 cm-2 in the wing regions were grown by HVPE on patterned sapphire templates. An MOCVD AlN LEO process relying on SAG over an in situ deposited SiNx nano-mask was also investigated. However, this process did not yield useful defect reduction.;The second topic of this dissertation is 365-400 nm near-ultraviolet (NUV) LEDs for white lighting applications. Currently, cool white LEDs consisting of a blue GaN/InGaN LED and the YAG:Ce3+ yellow phosphor are available with 107 lm/W efficacy, but have have high correlated color temperatures (CCTs) of ∼5,500 K and poor color rendering indices (CRIs) of ∼75. The alternative approach of combining a NUV LED with suitable NUV-excitation phosphors (e.g., red, green, and blue phosphors) can theoretically allow for high CRI white lighting with relatively good efficacy and a variety of CCTs. When this project began in late 2007, the lack of suitable blue-excitation phosphors suggested that this was the only viable approach to attaining very high CRI white lighting.;NUV LEDs with AlN buffers on 6H-SiC substrates and AlGaN/InGaN active regions were first developed to target white phosphors with excitation peaks near 365 nm. Later, NUV LEDs with GaN buffers on sapphire substrates and GaN/InGaN active regions were developed to diagnose problems with the AlGaN/InGaN LEDs and to target white phosphors with excitation peaks near 400 nm. The best device produced in this study was a 410 nm GaN/InGaN LED which emitted 7.4 mW at 20 mA, with a maximum external quantum efficiency (EQE) of 16.7% at 100 mA (40 A/cm2) using pulsed current.