Fabrication and Characterisation of Nitride DBRs and Nitride Membranes by Electrochemical Etching Techniques

A Distributed Bragg Reflector (DBR) is an important component for semiconductor microcavities and optoelectronic devices, such as vertical cavity surface emitting lasers (VCSELs), resonant cavity light-emitting diodes (RCLEDs). In the past thirty years, epitaxially grown GaAs-based DBRs have made great achievements of the application of III-V VCSELs in communications and mobile applications. At the same time, III-nitrides have demonstrated excellent performance in solid-state lighting and advanced optoelectronic devices due to the wide bandgap and unique properties. In recent years, GaN-based semiconductors have made great progress in the application of blue VCSELs. However, the absence of high-performance DBRs is a challenge for developing higher-power GaN-based VCSELs. Currently, the typical epitaxial GaN-based DBRs are limited by a long growth period, low optical performance, and poor quality of growth. Therefore, this project proposes a method to fabricate nanoporous (NP)/GaN-based DBR by electrochemical etching (EC), which are grown using metalorganic vapour-phase epitaxy (MOVPE). The heavily silicon doped GaN layer is transformed into an NP structure by selective etching, resulting in a higher refractive index contrast in each periodic layer. Moreover, a lateral etching method is proposed to further improve the EC etching of DBRs. This method can confine the etching in each sacrificial layer and make the etching aperture directions highly uniform. The corresponding characterizations have been carried out to explore the mechanisms of different etching methods, by optical microscopy, scanning electron microscopy (SEM) and reflectance measurements. It further confirms that the laterally etched NP GaN-based DBRs exhibit a higher reflectivity and wider stopband. The GaN sacrificial layers required for the EC etching are typically heavily silicon doped (>1019cm-3), resulting in a rough surface and saturated conductivity. On the other hand, the heavily silicon doped AlGaN with a low Al content (≤5%) exhibits an atomically flat surface and an enhanced electrical conductivity. Therefore, in this work, we introduced multiple pairs of heavily doped n++-Al0.01Ga0.99N/GaN to replace the widely used multiple pairs of heavily doped n++-GaN/GaN to fabricate lattice-matched NP DBRs by EC etching. Consequently, the epitaxially grown n++-Al0.01Ga0.99N/GaN-based DBR demonstrates a smoother surface than the n++-GaN/GaN-based DBR. Moreover, the NP-Al0.01Ga0.99N/GaN-based DBR exhibits higher reflectivity and wider stopband after lateral EC etching compared to the NP-GaN/GaN-based DBR. This method has been successfully applied to fabrication of high-performance DBR structures with the wavelength range from blue to deep yellow by modifying the epitaxial growth conditions. Furthermore, it is found that a very thin Al-Si diffusion layer is formed at the interface between an AlN buffer layer and a silicon substrate when growing the low-temperature AlN buffer layer on the n-doped silicon substrate by MOVPE. The diffusion layer exhibits high conductivity and can be EC-etched and polished as a sacrificial layer. Therefore, this method is proposed for stripping large-area GaN membranes by EC etching. A sample with AlN/AlGaN/GaN layers is first epitaxially grown by MOVPE on an n-doped (111) silicon substrate, and then bonded upside-down to a new glass host substrate and EC etched. Finally, lift-off of a large size GaN-based membrane has been realized with an area of 2.625cm2 and a crack-free and nanoscale smooth surface. Compared to other lift-off methods such as laser lift-off (LLO), chemical lift-off (CLO), and mechanical release techniques, this method does not involve bulky and expensive equipment, which can be used to fabricate high-performance III-nitride devices on the membrane at low cost in the future

Heterogeneous integration of InP etched facet lasers to silicon photonics by micro transfer printing

Photonics Integrated Circuits allow optical functionalities and interconnects with small footprint, large band -width and -density, low heat generation. The silicon photonics platform (SOI) offers excellent waveguiding properties, large-area wafers and a highly developed CMOS infrastructure matured with electronics. Nevertheless, the key function of light amplification is missing due to the indirect band-gap of silicon. The light has to be provided to the SOI from a separate direct band-gap III-V material. InP based devices work in the infrared optical window of the electromagnetic spectrum and can be heterogeneously integrated to the SOI. This research deals with the development of the first stand-alone InP Fabry-Perot lasers heterogeneously integrated to SOI by Micro Transfer Printing (µTP). The lasers are pre-fabricated and tested before transfer and are optimized to reach excellent optical, electrical and thermal performance. Lasers printed on Si substrates emit over 20 mW optical power, have threshold current of 16 mA and series resistance of 6 Ω; the thermal impedance of 38 K/W is half of that for the same laser printed directly on the SOI. The transfer printable InP ridge lasers have been designed as rectangular coupons with both contacts at the top and etched facets at the sidewalls. Two main release technologies based on the FeCl3:H2O (1:2) solution and a InGaAs or a InAlAs sacrificial layer were developed for releasing the devices from the original InP substrate with selectivity to InP greater than 4000 at 1 ◦C. The working principle of a polymer anchor system which restrains the devices to the substrate during the undercut were determined. The devices were printed on different silicon photonic substrates with excellent adhesion, with and without adhesive layers. A process for creating recesses into the SOI was developed to allow edge coupling the laser waveguide to the SOI or a polymer waveguide. High alignment accuracy along the three spatial directions can be achieved with alignment markers, reference walls and the interposition of a metal layer beneath the devices. This work shows a possible path for the achievement of a laser source for silicon photonics and it has been the basis for the integration of others InP devices to PICs by micro transfer printing

Integrated Circuits/Microchips

With the world marching inexorably towards the fourth industrial revolution (IR 4.0), one is now embracing lives with artificial intelligence (AI), the Internet of Things (IoTs), virtual reality (VR) and 5G technology. Wherever we are, whatever we are doing, there are electronic devices that we rely indispensably on. While some of these technologies, such as those fueled with smart, autonomous systems, are seemingly precocious; others have existed for quite a while. These devices range from simple home appliances, entertainment media to complex aeronautical instruments. Clearly, the daily lives of mankind today are interwoven seamlessly with electronics. Surprising as it may seem, the cornerstone that empowers these electronic devices is nothing more than a mere diminutive semiconductor cube block. More colloquially referred to as the Very-Large-Scale-Integration (VLSI) chip or an integrated circuit (IC) chip or simply a microchip, this semiconductor cube block, approximately the size of a grain of rice, is composed of millions to billions of transistors. The transistors are interconnected in such a way that allows electrical circuitries for certain applications to be realized. Some of these chips serve specific permanent applications and are known as Application Specific Integrated Circuits (ASICS); while, others are computing processors which could be programmed for diverse applications. The computer processor, together with its supporting hardware and user interfaces, is known as an embedded system.In this book, a variety of topics related to microchips are extensively illustrated. The topics encompass the physics of the microchip device, as well as its design methods and applications

Optoelectronics – Devices and Applications

Optoelectronics - Devices and Applications is the second part of an edited anthology on the multifaced areas of optoelectronics by a selected group of authors including promising novices to experts in the field. Photonics and optoelectronics are making an impact multiple times as the semiconductor revolution made on the quality of our life. In telecommunication, entertainment devices, computational techniques, clean energy harvesting, medical instrumentation, materials and device characterization and scores of other areas of R&D the science of optics and electronics get coupled by fine technology advances to make incredibly large strides. The technology of light has advanced to a stage where disciplines sans boundaries are finding it indispensable. New design concepts are fast emerging and being tested and applications developed in an unimaginable pace and speed. The wide spectrum of topics related to optoelectronics and photonics presented here is sure to make this collection of essays extremely useful to students and other stake holders in the field such as researchers and device designers

Optimized grating coupler designs for integrated photonics

A wide range of expertise is enclosed in the expression: “Photonics Packaging” with the common aim of interfacing a Photonic Integrated Circuit (PIC), either optically, electrically, thermally, and mechanically, with the surrounding environment. Thus, a multi-physics approach is essential to tackle the many challenges that Photonics Packaging poses. In this context, the design process is a crucial and vital step to overcome the majority of the issues that can potentially arise during the packaging assembly procedure. As a consequence, a single person cannot manage all the aspects behind the design process, but a multidisciplinary team needs to work together engineering and optimizing the different types of connections. The current thesis work is oriented on the optical packaging branch with particular focus on the design of the optical connections needed to deliver, in an efficiently and controlled manner, the light signal from a specific external light source or waveguide into the PIC. This work aims to show the importance of the design process in photonics packaging, and how it can be carefully exploited and tailored to optimize coupling schemes to obtain high efficient and packaging compatible optical connectors, which can constitute the building-blocks of future photonic devises. In this context, my research deals with the optimization of complex grating couplers for SOI platforms to couple light from a specific coupling scheme and it is divided in 4 sections. First, Chapter 2 presents in details the customized design routine, developed during my PhD, based on a Particle Swarm Optimization, an iterative algorithm, implemented using a commercial Finite Different Time Domain (FDTD) software. Then, in Chapter 3, the routine is exploited and tested to optimize the structural features of non-uniform grating coupler designs, characterized by a non-constant pitch. The aim is boosting their Coupling-Efficiency (CE) under a horizontal fibre coupling scheme, which is of particular interest in photonics packaging. The optimal designs were then fabricated in Cornerstone, the Silicon photonics foundry of the University of Southampton, and eventually packaged and tested at Tyndall National Institute. High CE values, up to 83% at 1550nm, are demonstrated and the results are shown to be in excellent agreement with the computational predictions. Due to the high efficiency, these designs were requested by the foundry as part of their official Process Design Kit, which is now on offer. In Chapter 4, the first experimental and FDTD comparative analysis of the multiwavelength response, in terms of bandwidth and asymmetry of the CE curve, is conducted and reported for such optimized non-uniform grating couplers. Here, the bandwidth is shown to be inversely proportional to the dimension of the impinging mode field diameter, which affects, together with the energy dispersion curve of each grating structure, its CE curve. Grating coupling is not only a suitable packaging solution for fiber-to-PIC coupling, but also for direct laser-to-PIC coupling. Chapter 5 shows how millimetre-scale FDTD simulations can be used to carefully design a Micro Optical Bench, made of a micro ball-lens and a micro prism. The Optics are used to reshape the laser emission making it compatible with a single mode fiber emission, thus suitable for grating coupling. Here, a uniform grating coupler is used to couple the impinging electromagnetic field. A laser-to-PIC performance penalty of only 1dB, with respect to the fiber-to-PIC coupling scheme, is shown under perfect alignment conditions. The origin of the higher loss is carefully analysed suggesting that 0.6dB are due to back-reflection from the optics interfaces, and 0.4dB due to spherical aberrations. Moreover, a detailed analysis of the manufacturing and alignment tolerances is conducted demonstrating their compatibility with current standard packaging processes

Oberflächenemittierende Laser mit vertikaler Kavität (VCSELs) und VCSEL-Arrays für Kommunikation und Sensorik

Future generations of optical wireless communication and sensing systems require compact, low-cost, reliable, and highly efficient light sources capable of transmitting modulated beams across free space at gigabit per second (Gbps) data rates and pulsed beams with sub-nanosecond rise and fall times. The infrared vertical cavity surface emitting laser (VCSEL) is exactly one such light source. Fifth generation (5G) systems promise to connect billions of people and trillions of Internet of Things gadgets and sensors at 1 to beyond 20 Gbps via newly auctioned millimeter wave (30 GHz to 300 GHz) spectral bands. By circa 2030 sixth generation (6G) systems envision vast broadband capacity with zero latency – enabling real-time virtual and mixed realities, human-machine interfaces, autonomous vehicles, and much more. The 6G technology adds terahertz wave emitters including infrared VCSELs and VCSEL arrays to vastly increase data rates, boost energy and spectral efficiency, and take advantage of available and unregulated spectral bands. I design, fabricate, and test new experimental VCSEL diodes and novel two-dimensional (2D) VCSEL diode arrays. I study the physics and performance trade-offs of VCSEL light emitters aimed at 5G and 6G optical wireless communication and sensing applications. Via in-house computer modeling and simulation programs, I design VCSEL epitaxial structures – composed of nanometer-thick aluminum-gallium-arsenide, indium-gallium arsenide, and gallium-arsenide-phosphide layers – with peak target emission wavelengths of 940 and 980 nanometers. A commercial foundry grows my experimental VCSEL epitaxial wafers by metal-organic vapor phase epitaxy on 3-inch diameter gallium-arsenide substrates. In my university cleanroom, I fabricate my VCSELs as quarter wafer test pieces using a new VCSEL Array 2018 mask set which contains single VCSELs, and several variations of novel 2D electrically parallel triple (3-element), septuple (7-element), and novemdecuple (19-element) geometric device designs. My fabricated devices feature high frequency, coplanar ground-signal-ground metal contact pads, and top-epitaxial-surface emission. I perform all device tests in my university laser diode laboratory via direct, on-wafer electrical probing under computer control, starting with continuous wave light output power-current-voltage sweeps via a calibrated photodiode-integrating sphere and variable current source. For emission spectra and small-signal frequency response measurements, I collect the emitted VCSEL light with a standard OM1 multiple mode optical fiber (MMF) – connected to either an optical spectrum analyzer or a photoreceiver. For on-wafer data transmission tests across OM1 MMF patch cords, I modulate my VCSELs with nonreturn to zero, pseudorandom bit patterns in the form of 2-level pulse amplitude modulation. I achieve record combinations of optical output power, bandwidth, and efficiency for my large oxide aperture diameter (larger than 20 micrometers) VCSELs and for my VCSEL arrays. For example, I demonstrate 200 milliwatts of optical output power, a bandwidth of 18 GHz, and a wall plug efficiency of 35 percent with a 19-element VCSEL array. I set several records for error free data transmission, for example, 40 Gbps for my triple and septuple VCSEL arrays and 25 Gbps for my novemdecuple VCSEL arrays, well beyond the previous record of 10 Gbps. My work is the first to investigate trade-offs in the highly nontrivial physics of VCSEL arrays aimed at high power and high bandwidth arrays for free space data transmission – producing new guiding principles for further device optimization and product development.Zukünftige Generationen optischer drahtloser Kommunikations- und Sensorsysteme erfordern kompakte, kostengünstige, zuverlässige und hocheffiziente Lichtquellen, die modulierte Strahlen mit Datenraten von Gigabit pro Sekunde (Gbps) und gepulste Strahlen mit Anstieg- und Abfallzeiten im Sub-Nanosekundenbereich über den freien Raum übertragen können. Infrarote, oberflächenemittierende Laser mit vertikaler Kavität (VCSEL) sind genau eine solche Lichtquelle. Systeme der fünften Generation (5G) versprechen, Milliarden von Menschen und Billionen von Geräten und Sensoren für das Internet der Dinge mit 1 bis über 20 Gbps über neu versteigerte Millimeterwellen-Spektralbänder (30 GHz bis 300 GHz) zu verbinden. Bis etwa 2030 sehen Systeme der sechsten Generation (6G) eine enorme Breitbandkapazität ohne Latenzzeit vor – sie ermöglichen virtuelle und gemischte Realitäten in Echtzeit, Mensch-Maschine-Schnittstellen, autonome Fahrzeuge und vieles mehr. Die 6G-Technologie fügt Terahertz-Wellensender hinzu, einschließlich Infrarot-VCSELs und VCSEL-Arrays, um die Datenraten signifikant zu erhöhen, die Energie- und Spektraleffizienz zu steigern und die verfügbaren und noch unregulierten Spektralbänder zu nutzen. In der vorliegenden Arbeit werden neue experimentelle VCSEL-Dioden und neuartige zweidimensionale (2D) VCSEL-Diodenarrays entworfen, hergestellt und getestet. Die Physik der VCSEL-Lichtemittern, welche auf 5G- und 6G-optische drahtlose Kommunikations- und Sensoranwendungen ausgerichtet sind, wird untersucht und Performance-Tradeoffs für die angedachten Anwendungen werden identifiziert und analysiert. Über hauseigene Computermodellierungs- und Simulationsprogramme wurden epitaktische VCSEL-Strukturen – bestehend aus nanometerdicken Aluminium-Gallium-Arsenid-, Indium-Gallium-Arsenid- und Gallium-Arsenid-Phosphid-Schichten – mit Peak-Zielemissionswellenlängen von 940 und 980 Nanometern entworfen. Ein kommerzieller Hersteller hat die experimentellen VCSEL-Epitaxiewafer durch metallorganische Gasphasenepitaxie auf Gallium-Arsenid-Substraten mit einem Durchmesser von 3 Zoll gewachsen. In einem Reinraum an der Universität wurden die VCSELs als Viertelwafer-Teststücke mit einem neuen VCSEL Array 2018-Maskensatz gefertigt, der einzelne VCSELs und mehrere Variationen von neuartigen elektrisch parallelen 2D-Tripel- (3-Element), Septuple- (7-Element) und Novemdecuple- (19-Elemente) Strukturdesigns enthält. Bei den prozessierten Strukturen handelt es sich um Top-Emitter mit hochfrequenzkompatiblen koplanare Masse-Signal-Masse-Metallkontaktpads. Alle Device-Tests wurden computergesteuert in einem universitären Laserdiodenlabor durch direktes elektrisches On-Wafer Probing durchgeführt, beginnend mit Dauerstrich-Lichtausgangsleistung-Strom-Spannungs-Sweeps über eine kalibrierte Photodioden-Integrationskugel und eine variable Stromquelle. Für Emissionsspektren und Kleinsignal-Frequenzgangmessungen wurde das emittierte VCSEL-Licht mit einer standardmäßigen OM1-Multimode-Glasfaser (MMF) eingesammelt – verbunden mit einem optischen Spektrumanalysator oder einem Fotoempfänger. Für On-Wafer-Datenübertragungstests über OM1-MMF-Patchkabel wurden die VCSELs mit pseudozufälligen Bitmustern im Non-Return-To-Zero Format mit 2-Level-Pulsamplitudenmodulation moduliert. In dieser Arbeit werden bisher unerreichte Kombinationen von optischer Ausgangsleistung, Bandbreite und Effizienz für VCSEL und VCSEL-Arrays mit großer Oxid-Apertur (größer als 20 Mikrometer) demonstriert. Beispielsweise werden 200 Milliwatt optische Ausgangsleistung, eine Bandbreite von 18 GHz und eine Konversionseffizienz elektrischer zu optischer Leistung von 35 Prozent mit einem 19-Element-VCSEL-Array erreicht. Zudem werden mehrere Rekorde für fehlerfreie Datenübertragung aufgestellt, zum Beispiel 40 Gbps für Triple- und Septuple-VCSEL-Arrays und 25 Gbps für Novemdecuple-VCSEL-Arrays, weit über den bisherigen Stand der Technik von 10 Gbps hinaus. Diese Arbeit ist die erste, die Trade-Offs in der hochgradig nichttrivialen Physik von VCSEL-Arrays untersucht, die auf Arrays mit hoher Leistung und hoher Bandbreite für die Datenübertragung im freien Raum abzielen – und damit neue Leitprinzipien für die weitere Bauelementoptimierung und Produktentwicklung schafft.DFG, 43659573, SFB 787: Halbleiter - Nanophotonik: Materialien, Modelle, Bauelement