High power gallium nitride light-emitting diodes for efficient solid state lighting and displays

• Main Author: Ji, Yun
• Other Authors: Sun Xiaowei
• Format: Theses and Dissertations
• Language: English
• Published: 2014
• Subjects: DRNTU::Engineering::Electrical and electronic engineering::Microelectronics
• Online Access: https://hdl.handle.net/10356/61778
• Institution: Nanyang Technological University

Nitride based light-emitting diodes (LEDs) are considered to be the next generation lighting source, owing to their superior advantages including high brightness, high energy conversion efficiency, long life span, compact size, fast response, and low maintenance cost. Tremendous work has been devoted to improving the performance of InGaN/GaN multiple quantum well LEDs in the past decades. The GaN based white LED has currently achieved reasonably high efficacy and displays a huge potential of replacing the incandescent and fluorescent lamps in the current market. However, the performance of InGaN/GaN LED light sources is still limited by some technical issues. One of the challenges is to reduce the efficiency droop, the phenomenon of reduction of external quantum efficiency at high current levels, such that the device could maintain high luminous efficacy for high brightness lighting applications. The root cause of the efficiency droop is still under debate, although there have been more convincing reports lately. Also, the lighting source should provide high chromaticity quality comparable to the conventional lighting sources to generate comfortable visual perception. Tremendous research work has been contributed to the improvement of InGaN/GaN LED lighting quality. This thesis research work focuses on the design, growth and fabrication of InGaN/GaN LED structures for enhanced efficiency of light emission. InGaN/GaN multiple quantum wells LED wafers with high crystal quality and high uniformity were grown using metal-organic chemical vapor deposition technique. The fabrication processes of LED chips were developed, and characterization methods for both wafer-level and chip-level electrical and optical performance were employed. Based on these standard growth and fabrication procedures, new device epi-structures have been designed for the performance improvement of InGaN/GaN LEDs. Excess electron overflow and hole injection shortage into the quantum wells cause the non-uniform carrier distribution and carrier crowding within the MQWs. Hence, all the carriers are involved in the radiative recombination process to generate photons, suppressing the optical output power as well as the energy conversion efficiency of LED devices. In this thesis, to enhance the hole transport depth, a partially p-type doped quantum barriers structure has been proposed. In order to investigate the carrier transport behavior within the active region, the quantum wells are intentionally grown at different temperatures, to incorporate different indium content. Through examining the emission intensity at different wavelengths, it is found that, in conventional LED structures, the holes could only be injected into shallow quantum wells close to the p-GaN layer. By inserting p-type doped GaN layer into the quantum barriers close to the p-GaN, the potential energy barriers for holes are reduced. Hence, the holes are able to reach deeper quantum wells close to the n-GaN side, as indicated by the increased light emission from deeper quantum wells. As a benefit, the hole distribution is more uniform within the MQWs, and more QWs are involved in the light emission. The conventional LED employs a p-type doped AlGaN layer as the electron blocking layer (EBL) to confine electrons within the MQWs region and prevent them from overflowing into the p-GaN layer. However, the p-EBL, inserted between the MQWs and the p-GaN layer, also suppresses the hole injection from the p-GaN to the MQWs, since the large band gap AlGaN layer creates an energy barrier for both electrons and holes. In contrast, when an n-type AlGaN EBL is adopted instead, the EBL blocks excess electrons before they enter the MQWs region. Hence, the electron crowding is avoided. Also, since the n-EBL is not on the path of hole transport into the active region, the amount of holes injected into the QWs is not suppressed. Simulation results suggest that the n-EBL structure results in more uniform electron and hole distributions, a higher hole concentration, and a higher radiative recombination rate in each individual QW, which agrees with the experimentally measured electroluminescence emission intensity and optical power output. The study on the influence of polarization fields on the device efficiency has also been carried out. So far, most research on the improvement of GaN LED performance focus on the structures grown on (0001) c-plane sapphire substrates. Due to the strong polarization field caused by the spontaneous and piezoelectric polarizations, the electron and hole wave functions are spatially separated, hence reducing the radiative recombination rates within the QWs. LED structures grown on nonpolar and semipolar planes have been proposed to eliminate this problem. However, it is still unclear how the polarities of different growth planes affect the carrier recombination dynamics and device performance. In this thesis work, the MQW LED structures grown on (0001) polar and (11-22) semipolar planes are investigated for the comparative investigation of carrier dynamics, in collaboration with the University of California, Santa Barbara (UCSB). The underlying physics behind the performance difference is revealed through photoluminescence and electroabsorption measurements and energy band analysis. Finally, a GaN based white light emitting source with tunable optical parameters is demonstrated for the extended application of GaN LED as lighting and display sources. The blue light emitting GaN LED chip has been designed to yield a current-dependent dual-wavelength emission centered at 425 and 460 nm. Nano-sized phosphor particles with photoluminescence emission peaks at 560 and 625 nm are coated on the LED chip to convert the blue light into green and red color light. As the operating current of the device varies, the blue emission intensity of the two peaks from the GaN LED chip changes, leading to intensity change of the green and red light. Hence, the color temperature and color rendering index could be adjusted accordingly. The proposed device structure provides a new method for generating high quality tunable white light source for indoor lighting and display applications. In summary, this dissertation includes the epitaxial growth, device fabrication, and theoretical studies of InGaN/GaN LEDs. Novel epitaxial structures have been proposed and realized in LED devices for performance enhancement, and the physics behind the performance improvement has been revealed for each proposed structure. The thesis work has provided insights important for design, growth and fabrication of high performance GaN LED based solid state lighting and display sources.