Atomic-Scale Insights into Light Emitting Diode

In solid-state lightning, GaN-based vertical LED technology has attracted tremendous attention because its luminous efficacy has surpassed the traditional lightning technologies, even the 2014 Nobel Prize in Physics was awarded for the invention of efficient blue LEDs, which enabled eco-friendly and energy-saving white lighting sources. Despite today’s GaN-based blue VLEDs can produce IQE of 90% and EQE of 70-80%, still there exist a major challenge of efficiency droop. Nonetheless, state-of-the-art material characterization and failure analysis tools are inevitable to address that issue. In this context, although LEDs have been characterized by different microscopy techniques, they are still limited to either its semiconductor or active layer, which mainly contributes towards the IQE. This is also one of the reason that today’s LEDs IQE exceeded above 80% but EQE of 70-80% remains. Therefore, to scrutinize the efficiency droop issue, this work focused on developing a novel strategy to investigate key layers of the LED structure, which play the critical role in enhancing the EQE = IQE x LEE factors. Based on that strategy, wafer bonding, reflection, GaN-Ag interface, MQWs and top-textured layers have been systematically investigated under the powerful advanced microscopy techniques of SEM-based TKD/EDX/EBSD, AC-STEM, AFM, Raman spectroscopy, XRD, and PL. Further, based on these correlative microscopy results, optimization suggestions are given for performance enhancement in the LEDs. The objective of this doctoral research is to perform atomic-scale characterization on the VLED layers/interfaces to scrutinize their surface topography, grain morphology, chemical composition, interfacial diffusion, atomic structure and carrier localization mechanism in quest of efficiency droop and reliability issues. The outcome of this research advances in understanding LED device physics, which will facilitate standardization in its design for better smart optoelectronics products

Thin film technology for optoelectronics and their thermal management

Thin-film semiconductor optoelectronics are important for applications from optical communication, solid-state lighting, and wearable electronics to biomedical sensors. It is now possible to separate the micrometer-thick device layers from their native substrates and transfer them onto new platforms to optimize system performance and integration. The understanding of thermal management for such devices is very important in order to control the junction temperature effectively. Here, the laser-lift-off (LLO) technique was theoretically and experimentally studied. The temperature distribution at the III-nitride/sapphire interface induced by absorption of 248-nm KrF excimer energetic laser pulses was simulated to verify the experimental results. A 1.5-m-thick n-type Al0.6Ga0.4N membrane was separated from a c-plane sapphire substrate and then bonded to a Si substrate. The electrical behaviour of Ti/Al/Ti/Au contacts on the N-polar n-Al0.6Ga0.4N membrane was characterized. Furthermore, free-standing semipolar InGaN/GaN light-emitting diodes (LEDs) emitting at 445 nm were first realized by separation from patterned r-plane sapphire substrate using LLO. The LEDs showed a turn-on voltage of 3.6 V and output power of 0.87 mW at 20 mA. Electroluminescence measurements showed stronger emission intensity along the inclined c-direction. The -3 dB bandwidth of the LEDs is in excess of 150 MHz at 20 mA and a back-to-back data transmission rate at 300 Mbps is demonstrated. This indicates that the LEDs can be used for high bandwidth visible light communications. For thermal management of thin-film optoelectronics, a GaAs based laser diode (LD) was investigated. The 2-dimensional temperature distribution of the transfer-bonded LD was simulated; where the power dissipation, the thermal resistance of different cavity lengths and configurations were investigated. This can be utilized to optimize the device design and the choice of carrier substrate for efficient thermal management of thin-film optoelectronics

Wide Bandgap Based Devices

Emerging wide bandgap (WBG) semiconductors hold the potential to advance the global industry in the same way that, more than 50 years ago, the invention of the silicon (Si) chip enabled the modern computer era. SiC- and GaN-based devices are starting to become more commercially available. Smaller, faster, and more efficient than their counterpart Si-based components, these WBG devices also offer greater expected reliability in tougher operating conditions. Furthermore, in this frame, a new class of microelectronic-grade semiconducting materials that have an even larger bandgap than the previously established wide bandgap semiconductors, such as GaN and SiC, have been created, and are thus referred to as “ultra-wide bandgap” materials. These materials, which include AlGaN, AlN, diamond, Ga2O3, and BN, offer theoretically superior properties, including a higher critical breakdown field, higher temperature operation, and potentially higher radiation tolerance. These attributes, in turn, make it possible to use revolutionary new devices for extreme environments, such as high-efficiency power transistors, because of the improved Baliga figure of merit, ultra-high voltage pulsed power switches, high-efficiency UV-LEDs, and electronics. This Special Issue aims to collect high quality research papers, short communications, and review articles that focus on wide bandgap device design, fabrication, and advanced characterization. The Special Issue will also publish selected papers from the 43rd Workshop on Compound Semiconductor Devices and Integrated Circuits, held in France (WOCSDICE 2019), which brings together scientists and engineers working in the area of III–V, and other compound semiconductor devices and integrated circuits

III-Nitride Vertical-Cavity Surface-Emitting Lasers: Growth, Fabrication, and Design of Dual Dielectric DBR Nonpolar VCSELs

Vertical-cavity surface-emitting lasers (VCSELs) have a long history of development in GaAs-based and InP-based systems, however III-nitride VCSELs research is still in its infancy. Yet, over the past several years we have made dramatic improvements in the lasing characteristics of these highly complex devices. Specifically, we have reduced the threshold current density from ~100 kA/cm2 to ~3 kA/cm2, while simultaneously increasing the output power from ~10 µW to ~550 µW. These developments have primarily come about by focusing on the aperture design and intracavity contact design for flip-chip dual dielectric DBR III-nitride VCSELs. We have carried out a number of studies developing an Al ion implanted aperture (IIA) and photoelectrochemically etched aperture (PECA), while simultaneously improving the quality of tin-doped indium oxide (ITO) intracavity contacts, and demonstrating the first III-nitride VCSEL with an n-GaN tunnel junction intracavity contact. Beyond these most notable research fronts, we have analyzed numerous other parameters, including epitaxial growth, flip-chip bonding, substrate removal, and more, bringing further improvement to III-nitride VCSEL performance and yield. This thesis aims to give a comprehensive discussion of the relevant underlying concepts for nonpolar VCSELs, while detailing our specific experimental advances. In Section 1, we give an overview of the applications of VCSELs generally, before describing some of the potential applications for III-nitride VCSELs. This is followed by a summary of the different material systems used to fabricate VCSELs, before going into detail on the basic design principles for developing III-nitride VCSELs. In Section 2, we outline the basic process and geometry for fabricating flip-chip nonpolar VCSELs with different aperture and intracavity contact designs. Finally, in Section 3 and 4, we delve into the experimental results achieved in the last several years, beginning with a discussion on the epitaxial growth developments. In Section 4, we discuss the most noteworthy accomplishments related to the nonpolar VCSELs structural design, such as different aperture and intracavity contact developments. Overall, this thesis is focused on the nonpolar VCSEL, however our hope is that many of the underlying insights will be of great use for the III-nitride VCSELs community as a whole. Throughout this report, we have taken great effort to highlight the future research fronts that would advance the field of III-nitride VCSELs generally, with the goal of illuminating the path forward for achieving efficient CW operating III-nitride VCSELs

Feature Papers in Electronic Materials Section

This book entitled "Feature Papers in Electronic Materials Section" is a collection of selected papers recently published on the journal Materials, focusing on the latest advances in electronic materials and devices in different fields (e.g., power- and high-frequency electronics, optoelectronic devices, detectors, etc.). In the first part of the book, many articles are dedicated to wide band gap semiconductors (e.g., SiC, GaN, Ga2O3, diamond), focusing on the current relevant materials and devices technology issues. The second part of the book is a miscellaneous of other electronics materials for various applications, including two-dimensional materials for optoelectronic and high-frequency devices. Finally, some recent advances in materials and flexible sensors for bioelectronics and medical applications are presented at the end of the book

Scaling the Power and Tailoring the Wavelength of Semiconductor Disk Lasers

Optically pumped semiconductor disk lasers (SDLs) provide a unique combination of high output power, high beam quality and possible emission wavelengths spanning from the ultraviolet to the mid-infrared spectral range. In essence, SDLs combine the wavelength versatility of semiconductor gain media with the power scaling principles of optically pumped solid state disk lasers. The emission wavelength of SDLs can be tailored to match the desired application simply by altering the composition of the gain material. High power operation, however, requires efficient thermal management, which can be realized using thin structures that are integrated with industrial diamond heat spreaders. The main objective of this thesis was to develop methods for increasing the output power of optically pumped SDLs, especially in challenging wavelength regions. The work included integrating SDL gain elements onto diamond heat spreaders using thin intermediate gold layers. This configuration enabled 45–50 % higher output powers than conventional bonding with indium solder. In addition, the reflectivity of the SDL gain mirror was enhanced using a semiconductor-dielectric-metal compound mirror. This procedure enabled 30 % thinner mirror structures when compared with the conventional design, where the reflectivity of the semiconductor mirror is enhanced with a metal layer. Finally, thin GaAs-based semiconductor mirrors were integrated with InP-based active regions. Such integration is necessary for high power operation in the spectral range 1.3–1.6 µm, because InP-based compounds for a highly reflective thin mirror section are not available. The configuration enabled record-high output powers of 6.6 W and 4.6 W at the wavelengths of 1.3 µm and 1.58 µm, respectively. The second objective of this thesis was to generate high output powers in single-frequency operation and via intracavity frequency-doubling. In single-frequency operation, record-high output powers of 4.6 W and 1 W were demonstrated at the wavelengths of 1.05 µm and 1.56 µm, respectively. Such light sources are required for numerous applications including free-space communications and high resolution spectroscopy. In addition, second-harmonic generation was demonstrated with SDLs emitting at 1.3 µm and 1.57 µm. The output powers reached 3 W at 650 nm and 1 W at 785 nm, which represent record-high output powers from SDLs in this wavelength range. These types of lasers could be especially useful in biophotonics and medical applications

Novel Materials and Fabrication Techniques for Enhanced Current Spreading and Light Extraction in High Efficiency Light-emitting Diodes

Although solid-state lighting based on III-nitride light-emitting diodes (LEDs) is on track to become the dominant technology for lighting our world, we have yet to reach the efficacy limit in these devices. Many challenges remain to be solved to achieve this goal. In this thesis, we discuss novel materials and fabrication techniques which can be used to enhance the efficiency of LEDs through improving lateral current spreading through p-type GaN and increasing light extraction. Presented in the first part of this thesis is the growth and characterization of hydrothermal ZnO thin films. The effect of growth conditions on the morphology and optoelectronic properties of the films will be discussed. In particular, we studied the effects that group III dopants (Al, Ga, and In) have when introduced into the hydrothermal ZnO thin films. The Ga doped film showed the lowest resistivity, with a resistivity of 1.94 mΩ cm for films doped with 0.4 at.% Ga. Undoped ZnO films showed the lowest optical absorption coefficient of 441 cm-1 at 450 nm. This study represents the first time all three dopants have been systematically compared to one another. The second part of the thesis focuses on an alternative approach to forming transparent contacts to p-GaN. This involved the regrowth of highly doped n-type GaN using ammonia molecular beam epitaxy (NH3 MBE) to form epitaxial tunnel junction contact. We discuss several aspects of the growth and performance of regrown TJ contacts on p-n diode structures as well as InGaN/GaN LEDs. Improved turn-on voltages and reduced series resistances have been realized by depositing highly doped Si-doped n-type GaN using MBE on polarization enhanced p-type InGaN contact layers grown using MOCVD. We compared the effects of different Si doping concentrations, and the addition of p-type InGaN on the forward voltages of p-n diodes and LEDs. It was found that increasing Si concentrations from 1.9x1020 to 4.6x1020 cm-3 and including a highly doped p-type InGaN at the junction both contribute to the narrowing of the depletion width, lowering series resistance from 4.2x10-3 to 3.4x10-3 Ω cm2 and decreasing turn-on voltages of the diodes.The third, and last part, of this thesis, will focus device results utilizing the alternative current spreading contacts discussed in the previous chapters. LED devices fabricated using doped ZnO films will be analyzed as well as LEDs utilizing MBE n-GaN TJ contacts on blue InGaN flip-chip triangular LEDs grown on both free-standing GaN and patterned sapphire substrates. The performance of LEDs with Ga-doped ZnO (Ga:ZnO) and Sn-doped In2O3 (ITO) current-spreading layers (CSLs) has been evaluated at high injection current densities. LEDs with electron beam-hydrothermally deposited Ga:ZnO transparent CSLs showed improved performance compared to electron beam deposited ITO at all current densities. External quantum efficiency and wall plug efficiency were both higher for blue emitting LEDs with ZnO. Luminous efficacy increased greatly for the ZnO-based CSL with a peak value of 113 lm/W compared to 82 lm/W for the ITO-based CSL, a 37% improvement. Issues with metal organic chemical vapor deposition (MOCVD), device processing, and device performance are discussed as well