Investigation, Fabrication, and Characterization of Metal-Insulator-Semiconductor InGaN/GaN Micro-LEDs with Superior Efficiency

Abstract

Since the invention of high-efficiency Indium Gallium nitride/Gallium nitride (InGaN/GaN) light-emitting diodes (LEDs), they have become the most widely used visible light sources in the first two decades of the 21st century. Despite the “efficiency droop” problem, InGaN/GaN LEDs offer key advantages for illumination and display applications in electronics, including high brightness, high efficiency, and long lifetime. As the size of mobile electronics decreases, high-resolution displays and miniaturized light sources must also scale down. InGaN/GaN micro-LEDs were considered the most promising candidates for meeting the market demand. However, in recent years, micrometer-scale organic LEDs (OLEDs) have dominated the portable electronics market. The main limitation of InGaN/GaN micro-LEDs is their low efficiency, particularly at low working current densities. Unlike the damage-free inkjet printing techniques used to pattern OLEDs, the dry etching techniques required for patterning InGaN/GaN micro-LEDs cause unavoidable sidewall damage and reduce efficiency. Nonetheless, InGaN/GaN micro-LEDs retain incomparable advantages, such as high-speed response and long lifespan, which provide strong motivation to develop high-efficiency InGaN/GaN micro-LEDs. In this thesis, the mechanisms and solutions to address the low efficiency of InGaN/GaN micro-LEDs are discussed. Chapter 1 provides an overview of the fundamental knowledge of InGaN/GaN heterostructures. Chapter 2 introduces the widely used ABC model to establish the relationship between the Shockley-Read-Hall (SRH) recombination and the internal quantum efficiency (IQE) of micro-LEDs. The properties of surface defects in InGaN/GaN heterostructures are studied. More importantly, nearly all reported practical fabrication techniques, which can effectively suppress surface recombination, are summarized and assessed for applicability. Chapter 3 investigates a widely-neglected mechanism, defects-assisted tunneling recombination, which needs be further studied for InGaN/GaN micro-LEDs. Temperature-dependent external quantum efficiency (EQE) measurements demonstrate that surface defects-assisted tunneling recombination becomes predominant at low temperatures as device size decreases. Quantitative analysis reveals that the role of surface defects-assisted tunneling recombination cannot be overlooked in InGaN/GaN micro-LEDs, even at room temperature. In Chapter 4, a unique device structure is proposed to achieve high-efficiency InGaN/GaN micro-LEDs. A metal-insulator-semiconductor (MIS) structure is fabricated on the sidewall of the device mesa, enabling the application of sidewall bias to adjust the sidewall surface potential of the micro-LED. Quantitative analysis demonstrates that the variation in surface recombination velocity is proportional to the sidewall bias applied to the MIS micro-LEDs. Further calculations indicate that the increase or decrease in IQE is proportional to the sidewall bias and remains constant at low current densities with a fixed sidewall bias. MIS micro-LEDs with 10 μm diameter mesas, fabricated by our collaborator, Vuereal, show promising but modest EQE improvements. The simulated data from numerical simulations match the measured data, verifying the efficiency enhancement of micro-LEDs with MIS structures. The limited efficiency improvement observed in these devices suggests some constraints in the current implementation of MIS micro-LEDs. To fully demonstrate the advantages of the MIS structure, further optimization of both the device structures and fabrication processes is necessary. Therefore, the fabrication and characterization of the MIS micro-LEDs with superior EQE performance are proposed in Chapter 5. Compared to the MIS micro-LEDs provided by our collaborator, the high-efficiency MIS micro-LEDs address the poor ohmic p-contacts problem, replace the insulator in the MIS structure with a 100 nm layer of Al2O3, and employ advanced fabrication methods detailed in Chapter 2. The current density-voltage (JV) characteristics of MIS micro-LEDs with various dimensions show remarkably low leakage current densities, indicating that surface recombination has been minimized. Furthermore, the increase or decrease in EQE of MIS micro-LEDs is observed to be approximately proportional to the magnitude of the positive or negative sidewall bias applied to the MIS micro-LEDs in the range of -20 V to +20 V. This phenomenon is well explained by the analytical model discussed in Chapter 4 and 5. The EQE of the MIS micro-LED with a mesa dimension of 8 μm can be enhanced from 20% to 30.7% (an increase of 10.7%) at a low injection current density of 0.625 A/cm2 by applying a +20 V sidewall bias, which is comparable to the performance of state-of-the-art OLEDs. The maximum EQE of the 8 μm MIS micro-LED was measured to be ~53.9% at an injection current density of 23.3 A/cm2, which is the highest reported EQE to the best of our knowledge.