Growth and fabrication of low thermal-mass gallium nitride light-emitting diodes
Date of Issue2017-08-14
School of Electrical and Electronic Engineering
Over the past two decades, the technology of InGaN/GaN-based light-emitting diodes (LEDs) has made tremendous progress. The optical performance has been extensively studied to get higher luminous efficacy within a desired cost structure. Today, the efficacy of LEDs has already surpassed that of the incandescent and fluorescent luminaires. In addition, LED lighting has many advantages such as high brightness, long lifetime, high reliability, small size, high efficiency, and low power consumption. Owing to these benefits, LEDs have been widely used in a myriad of applications. Nevertheless, there is still room for improvement of the optical performance for unique applications. For applications on mobile display backlighting, micro-displays, medical devices, visible light communication, and so on, LEDs with larger chip sizes are not effective in achieving higher power densities, higher light extraction efficiency, and faster pulsed operation even when operating at higher current densities. To fulfill the requirements of these different applications, smaller sized low thermal-mass LEDs (LTM-LEDs) are proposed in this thesis. LTM-LED has less thermal mass and better thermal conductivity which enables a lower junction temperature to be maintained. As a result of the reduced thermal mass and better thermal conductivity, LTM-LEDs can sustain much higher current density, higher power density, and faster response speed. In this dissertation, LTM-LEDs in various sizes and geometries were demonstrated and studied based on our standard flip-chip LED fabrication technique to identify the optimal power density performance. Furthermore, partitioned growth LTM-LEDs in different sizes are also grown by the metal-organic chemical-vapor deposition (MOCVD) system for enhanced optical output power performance. Heat is a critical factor for the efficiency droop. Moreover, the distance between the n-contact and the LED mesa, which acts as a conductive path, has a substantial influence on current spreading and output power of LEDs, especially for the smaller sized LEDs. Hence, to increase the output power density of LTM-LEDs, it is important to reduce the thermal mass and decrease the distance between the n-contact and the LED mesa. One of the effective ways is to make LTM-LEDs smaller. Experimental results show that output power density is improved by the decreasing chip size. Our model suggests that smaller size has better uniformity on current density and also generates less heat. As is well known, high light extraction efficiency (LEE) and better current spreading are important criteria for LEDs operating in the higher power density regime. Different shapes always show different characteristics on LEE and current spreading. To increase the power density of LTM-LEDs, triangle-, circle-, and square-shaped LTM-LEDs with the same mesa area are designed and realized in the flip-chip configuration. It was revealed that the circle-shaped LTM-LEDs show the lowest electrical and optical properties, while the triangle-shaped LTM-LEDs deliver the highest power density versus current density. However, our numerical simulations demonstrated that the LEE of triangle-shaped LTM-LEDs is only 0.06% higher than the lowest circle-shaped LTM-LEDs due to only 800 nm depth of the sidewalls. On the other hand, our model and simulation results show that the lower resistance at the mesa edge and the shorter n-GaN current paths, which not only reduce the self-heating but also contribute to higher average radiative recombination rate, account for the superior performance of triangular LTM-LEDs. The quantum-confined Stark effect (QCSE) induced by the lattice mismatch between gallium nitride and the sapphire substrate significantly hinders the optical performance of LEDs. To reduce the QCSE effect, LTM-LEDs with different sizes were grown on patterned c-plane sapphire substrate with the MOCVD technique, i.e., partitioned growth, and the size effect on the optical properties and the indium concentration for the quantum wells is studied experimentally. It is revealed that the optical properties can be improved by decreasing the chip size which subsequently reduces the in-plane compressive stress. With the decreasing chip size (from 1,000 µm to 100 µm), the device performance is enhanced. However, the 50 × 50 µm2 device shows a decreasing of output power which is attributed to more defects induced by the higher indium incorporation in the quantum wells. The underlying mechanisms of these observations are discussed based on different methods of characterization, and furthermore, it is proven that for a specific partitioned growth process, the ideal size for LTM-LEDs with the optimal power performance is identified. In summary, both growth and fabrication techniques are used to study the LTM-LEDs to improve their optical performance. Optimized shape and size for LTM-LEDs with the highest optical performance are found based on certain fabrication conditions, respectively. The optimal size for partitioned growth LTM-LEDs with the highest optical power is found. This provides a good rule of thumb on how to choose the size and geometry to obtain LTM-LEDs with an optimal output power performance.