UC Santa Barbara

The group III-nitride material system has emerged as the preferred choice for a wide range of semiconductor optoelectronic applications, ranging from light-emitting diodes (LEDs) for general illumination to red-green-blue micro-LEDs for next generation displays. Properties such as direct bandgaps spanning the visible spectrum, high defect tolerance, and excellent efficiency make InGaN-based LEDs attractive for replacing conventional lighting and display technologies. However, the optical and electrical efficiency decreases significantly when increasing the emission wavelength of nitride LEDs from blue to red, which can be explained in part by materials quality degradation and larger polarization-induced electric fields. This dissertation focuses on the epitaxial growth and characterization of long-wavelength group III-nitride LEDs emitting in the green to red spectral range. The origin of low electrical efficiency in c-plane green LEDs is first explored using a combined experimental and simulation-based approach. LEDs are grown using metal organic chemical vapor deposition, fabricated into devices, and electrically tested. Three-dimensional LED device simulations based on the Localization Landscape theory of disorder are then conducted to relate nitride materials properties to device performance. These studies demonstrate that polarization barriers at the GaN/InGaN interface and sequential filling of quantum wells (QWs) contribute to the low electrical efficiency in green LEDs.

The use of InGaN quantum barriers (QBs) is introduced as an approach to overcome barriers to carrier transport in green LEDs. Structures incorporating InGaN QBs have a reduced polarization discontinuity at the QW/QB interface and contribute an additional source of alloy disorder. Through experiments and three-dimensional simulations, it is shown that InGaN QBs improve electrical efficiency and introduce preferential pathways for vertical transport through regions of high indium content. Next, an approach using engineered V-defects, which have lower total polarization discontinuities for InGaN/GaN interfaces than c-plane, is proposed to mitigate polarization-induced potential barriers in green LEDs. Lastly, efforts to improve green LED performance are extended to red micro-LEDs, where materials degradation and polarization-induced electric fields become more severe. The results presented in this dissertation expand the understanding of long-wavelength LED efficiency and provide a path to realize high-performance green and red LEDs and micro-LEDs through careful heteroepitaxial growth and design.