High-Efficiency Al(Ga)N Ultraviolet Optoelectronics and Integrated Photonics

Abstract

Al(Ga)N semiconductors have attracted tremendous research interest due to their wide direct bandgap that enables passive/active optoelectronic, photonic, and quantum functionalities in UV, visible, and infrared wavelengths. In addition, high-power handling properties and intrinsic 2nd and 3rd order nonlinear optical characteristics make AlGaN a unique platform for photonic integrated circuit (PIC) at telecom and UV/visible wavelengths. Despite the attractive characteristics, the performance of AlGaN-based optoelectronic devices has been severely limited by the presence of large densities of defects/dislocations in the active region, extremely inefficient p-type doping, and poor light extraction due to predominantly TM polarized emission for Al-rich AlGaN. For the emerging AlN-based PIC platform, the growth and fabrication processes have remained underdeveloped prior to this work. Furthermore, the 2nd order nonlinear susceptibility of Al(Ga)N required for wavelength conversion and basis of linear electro-optic effect, Pockels effect, is relatively weak, limiting the performance of photonic and quantum functionalities. In this thesis, we focus on the design and development of optoelectronic and photonic devices based on high Al composition Al(Ga)N on a sapphire substrate, including deep UV light-emitting diodes (LEDs) and microring resonators (MRRs). Regarding deep UV LEDs, we investigate the epitaxy, design, fabrication, and characterization of high-efficiency deep UV LEDs emitting at ~265nm, which can be utilized for sterilization of pathogens such as current COVID-19 and preventing future pandemics. First, we apply polarization-engineered tunnel junctions to improve hole injection efficiency. The demonstrated bottom-emitting UV LEDs with an optimized GaN tunnel junction show the maximum EQE of 11%, the highest value reported at this wavelength. We also investigate and improve the light-emitting efficiency (LEE) by utilizing photonic crystal (PhC) as a diffraction grating layer. For this study, we investigate the top-emitting UV LEDs where LEE is the dominant limiting factor. The demonstrated LED shows EQE of 5.4% and WPE of 3.5%, which are approximately 2.5 times higher than identical LED structures without PhC. Next, we focus on the AlN-based microring resonator, one of the widely investigated components of PIC platform offering unique features due to cavity-based resonance characteristics. Initially, the fabrication process is optimized to provide ultra-high Q AlN MRRs using single-crystal AlN on the sapphire platform. The demonstrated AlN MRR showed intrinsic Q (Qint) of 2.8×106 at telecom wavelength, the highest Qint reported for fully etched AlN-on-sapphire resonators. Subsequently, we extend our focus to shorter wavelengths, especially green wavelength (~532nm), to demonstrate and characterize MRRs where tunable lasers are less available. Here, we utilize a new technique, thermo-optic tuning with an integrated microheater, to characterize the demonstrated microring resonator, which shows a Qint of 147,000, first measured Qint at this wavelength. Finally, we achieve significantly enhanced Pockels coefficient of AlN by utilizing AlGaN/AlN multiple quantum wells (MQWs). The demonstrated AlN microring resonator modulators (MRMs) with MQWs show ~2.16 times and ~1.56 times higher effective Pockels effect than MRMs without MQWs at telecom and ~780nm wavelengths, respectively. These enhancements can be attributed to an order of magnitude higher Pockels coefficient of the AlGaN/AlN MQWs than that of AlN. This work provides a viable and unique path to realize high-efficiency deep UV LEDs extended to other wavelengths. In addition, based on demonstrated high-Q microring resonators at UV to telecom wavelength along with enhanced Pockels effect, our studies show a new opportunity to achieve a high-performance monolithic PIC platform.