Nanomaterial integration in micro LED technology: Enhancing efficiency and applications

The micro-light emitting diode (µLED) technology is poised to revolutionise display applications through the introduction of nanomaterials and Group III-nitride nanostructures. This review charts state-of-the-art in this important area of micro-LEDs by highlighting their key roles, progress and concerns. The review encompasses details from various types of nanomaterials to the complexity of gallium nitride (GaN) and III nitride nanostructures. The necessity to integrate nanomaterials with III-nitride structures to create effective displays that could disrupt industries was emphasised in this review. Commercialisation challenges and the economic enhancement of micro-LED integration into display applications using monolithic integrated devices have also been discussed. Furthermore, different approaches in micro-LED development are discussed from top-down and bottom-up approaches. The last part of the review focuses on nanomaterials employed in the production of micro-LED displays. It also highlights the combination of III-V LEDs with silicon LCDs and perovskite-based micro-LED displays. There is evidence that efficiency and performance have improved significantly since the inception of the use of nanomaterials in manufacturing these

Data availability and the need for research to localize, quantify and recycle critical metals in information technology, telecommunication and consumer equipment

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.The supply of critical metals like gallium, germanium, indium and rare earths elements (REE) is of technological, economic and strategic relevance in the manufacturing of electrical and electronic equipment (EEE). Recycling is one of the key strategies to secure the long-term supply of these metals. The dissipation of the metals related to the low concentrations in the products and to the configuration of the life cycle (short use time, insufficient collection, treatment focusing on the recovery of other materials) creates challenges to achieve efficient recycling. This article assesses the available data and sets priorities for further research aimed at developing solutions to improve the recycling of seven critical metals or metal families (antimony, cobalt, gallium, germanium, indium, REE and tantalum). Twenty-six metal applications were identified for those six metals and the REE family. The criteria used for the assessment are (i) the metal criticality related to strategic and economic issues; (ii) the share of the worldwide mine or refinery production going to EEE manufacturing; (iii) rough estimates of the concentration and the content of the metals in the products; (iv) the accuracy of the data already available; and (v) the occurrence of the application in specific WEEE groups. Eight applications were classified as relevant for further research, including the use of antimony as a flame retardant, gallium and germanium in integrated circuits, rare earths in phosphors and permanent magnets, cobalt in batteries, tantalum capacitors and indium as an indium–tin-oxide transparent conductive layer in flat displays.BMBF, 033R087A, r³ - Strategische Metalle, Verbundvorhaben: UPGRADE - Integrierte Ansätze zur Rückgewinnung von Spurenmetallen und zur Verbesserung der Wertschöpfung aus Elektro- und Elektronikaltgeräten, TP1: Übergreifendes Stoffstrommanagement und Design für Recyclin

Untersuchungen an AlGaInN-basierten Laserdioden im sichtbaren Spektralbereich

Ziel dieser Arbeit war die physikalischen Ursachen des unterschiedlichen Verhaltens grüner Laserdioden im Vergleich zu blauen Bauelementen zu identifizieren. Aus diesem Grund wurden zunächst die elektro-optischen Eigenschaften von blauen und grünen InGaN-basierten Laserdioden mit Hilfe gepulster L-I-Kennlinien analysiert und miteinander verglichen. Dabei zeigte sich, dass der grüne Laser Abweichungen von dem idealen Verhalten eines Halbleiterlasers aufweist. Während weder die internen Verluste noch die Injektionseffizienz (ηinj) beider Lasertypen eine explizite Temperaturabhängigkeit im Bereich von 10-90°C aufweisen, zeigt sich im Bezug auf die Stromabhängigkeit der Injektionseffizienz ein unterschiedliches Verhalten. Bei blauen InGaN-Laserdioden kann die Injektionseffizienz als konstant angesehen werden, wohingegen ηinj bei grünen Bauteilen eine stromabhängige Abnahme aufweist. Diese konnte mit einem unzureichenden Einfang der Ladungsträger in die Quantenfilme korreliert werden. Anschließend wurde mit Hilfe von Teststrukturen untersucht, ob die Reduktion der Injektionseffizienz durch ein Überschießen von Elektronen oder Löchern verursacht wird. Durch die Variation des Aluminiumgehaltes in der Elektronenbarriere (EBL) wurde der unzureichende Einfang von Elektronen in die Quantenfilme nachgewiesen. Obwohl die Qualität der EBL einen drastischen Einfluss auf die Absolutwerte der Injektionseffizienz hat, zeigte sich jedoch keine Auswirkung auf die relative Abnahme von ηinj als Funktion des Stroms. Stattdessen konnte durch eine Teststruktur mit n-seitiger InGaN-Detektionsschicht ein stromabhängiges Überschießen von Löchern nachgewiesen werden. Um den Einfluss der stromabhängigen Injektionseffizienz auf die Laserschwelle, insbesondere den temperaturabhängigen Anstieg, zu analysieren, wurden die entsprechenden Einflussgrößen im weiteren Verlauf der Arbeit experimentell quantifiziert und es wurde ein empirisches Modell für die Laserschwelle hergeleitet. Die optische Verstärkung wurde für unterschiedliche Temperaturen und Betriebsströme mit Hilfe von Hakki-Paoli-Messungen untersucht. Für die Herleitung einer Schwellbedingung ist jedoch die Verstärkung als Funktion der Ladungsträgerdichte notwendig. Die für die Umrechnung des Stroms in Ladungsträgerdichte erforderlichen Rekombinationsparameter wurden für Temperaturen von 25 bis 80°C bestimmt. Die experimentellen Daten der Hakki-Paoli-Messungen wurden genutzt, um die physikalischen Parameter eines linearen Gewinnmodells zu bestimmen, insbesondere die Temperaturabhängigkeit der Transparenzladungsträgerdichte und des differentiellen Gewinns. Auf der Grundlage dieses Parametersatzes wurden dann die Einflussgrößen des temperaturabhängigen Schwellanstiegs anhand der Schwellbedingung, basierend auf dem linearen Gewinnmodell, hergeleitet. Somit konnte die Ladungsträgerlebensdauer, welche in dem betreffenden Operationsregime maßgeblich durch Auger-Verluste dominiert ist, als Hauptursache für den temperaturabhängigen Schwellstromanstieg identifiziert werden. Um die Langzeitstabilität der Injektionseffizienz zu untersuchen, wurden zunächst die Beschleunigungsfaktoren der Degradation grüner Laserdioden untersucht. Es zeigte sich, dass die Alterung der Bauteile elektro-thermisch aktiviert ist und sich damit vergleichbar zu dem Degradationsmechanismus von Blu-Ray Lasern verhält. Durch die Alterung einer grünen Laserdiode im Wechsel zwischen zwei unterschiedlichen Betriebszuständen, bei denen die Temperatur der aktiven Zone konstant gehalten wurde, konnte der Strom als dominierender Einflussfaktor identifiziert werden. Während des elektrischen Betriebs zeigt die Schwelle einen wurzelförmigen Anstieg, welcher bereits von blauen Laserdioden bekannt ist. Die Steilheit und damit auch die Injektionseffizienz oberhalb der Schwelle nehmen jedoch während der Degradation nicht ab. Auch die optische Verstärkung, welche durch Hakki-Paoli-Messungen vor bzw. nach Degradation charakterisiert wurde, bleibt unverändert. Allerdings konnte nachgewiesen werden, dass die Ladungsträgerdichte in den Quantenfilmen während des Betriebs abnimmt. Basierend auf den im Rahmen dieser Arbeit bestimmten Rekombinationsparametern konnte abgeschätzt werden, dass sich die Rate der defekt-assistierten Rekombinationsprozesse in den Quantenfilmen verdreifachen müsste, um die experimentell beobachtete Zunahme der Schwelle um 20% zu erklären. Dies ist unwahrscheinlich und konnte durch einen Vergleich des experimentell bestimmten EL-Verhaltens einer grünen Laserdiode unterhalb der Schwelle vor bzw. nach der Alterung mit berechneten Kennlinien als Ursache ausgeschlossen werden. In den Untersuchungen des Ladungsträgertransportes wurde gezeigt, dass auch außerhalb der Quantenfilme eine nicht zu vernachlässigende Ladungsträgerdichte existiert. Die Degradation muss somit nicht auf die Quantenfilme beschränkt sein

Study of the III-nitride materials grown by mixed-source HVPE for white LED applications emitting multi spectrum range

The purpose of this study is to explore the possibility of phosphor-free white-emitting LED？s based in the gallium nitride material system. The structures are to be grown using mixed source hydride vapor phase epitaxy (MS-HVPE). It is unique crystal growth technology different from conventional HVPE and MOCVD system using mixed metal source of aluminum, indium and gallium. The first step in this project is the optimization of MS-HVPE growth process. This was achieved successfully, as binary, ternary and quaternary films are demonstrated. Successful n and p-type doping are also demonstrated introducing Te and Mg. The second step in this project is fabricating broadband spectrum emitting device of phosphor-free white LED by MS-HVPE. The device structure consisted of conventional double-hetero (DH) structure, which was the undoped InAlGaN active layer and n, p-AlGaN cladding layers. We observed that the device of AlInGaN quarternary active grown by MS-HVPE emitted multi spectrum from UV to red area. We also found that its spectrum was variable as indium mole fraction and controllable. It was nano phase epitaxy phenomenon being only observed in HS-HVPE process. An extensive growth study of GaN based material was also carried out. The effects of several growth parameters on emission characteristics were presented. PL emission wavelengths for each structure were demonstrated. And EL emission wavelengths were also demonstrated after wafer fabrication process. Additionally, x-ray diffraction and x-ray photoelectron spectroscopy (XPS) showed to verify crystal quality of MS-HVPE. The dissertation presented herein demonstrates achieving phosphor-free solid-state white lighting. But it still has unknown physical characteristics. Continuation of this study will lead to future industry. And hopefully it will be commercialized and applied to residential illumination due to this technology.Chapter 1. Introduction 1 1.1. Overview of LED 1 1.2. Wide bandgap compound semiconductor 6 1.3. Overview of white LED 10 1.4. Purpose and outline of this project 15 Chapter 2. Fundamentals of Gallium Nitride 21 2.1. Introduction 21 2.1.1. Current Issues in GaN-based LED 23 2.2. Crystallography of Gallium Nitride 26 2.3. Characteristics of Gallium Nitride 32 2.3.1. Doping of Gallium Nitride 33 2.3.2. Optical Properties of Gallium Nitride 35 2.3.3. Polarity in Gallium Nitride 38 2.4. Substrates for GaN Epitxial Growth 40 2.4.1. Substrate issues 40 2.4.2. Sapphire 41 2.4.3. SiC 45 Chapter 3. Overview of Epitaxial Growth Experimental 57 3.1. Hydride vapor phase epitaxy 57 3.1.1. Introduction to HVPE 57 3.1.2. Mixed source HVPE system 59 3.1.3. Some parameters for optimized GaN growth 62 3.2. Wafer fabrication process 63 3.2.1. Selective area growth 63 3.2.2.Metallization of GaN 64 3.3. Measurements 66 3.3.1. Photoluminescence 66 3.3.2. DXRD 67 3.3.3. SEM/CL 70 3.3.4. E-CV 75 3.3.5. Hall measurement 77 Chapter 4. Mixed Source HVPE Growth Experiment for Bulk Characteristics 82 4.1. GaN growth 82 4.1.1 Buffer growth for GaN layer 83 4.1.2. Mg-doped GaN layer 87 4.2. AlGaN growth 90 4.3. InGaN growth 97 Chapter 5. Fabrication of AlInGaN-Based LED for White Emission 115 5.1. AlInGaN SAG-DH structure growth 115 5.2. Characterization of AlInGaN SAG-DH epitaxial structure 123 5.3. Device fabrication 127 Chapter 6. Experimental Results for Active layer’s Condition 135 6.1. Performance of AlInGaN white LED 136 6.2. EL characteristics of AlGaN and AlInGaN active 139 6.2.1 GaN active layer 139 6.2.2 Al(0.1g)GaN active layer 141 6.2.3 Al(0.3g)GaN active layer 144 6.2.4 Al(0.4g)GaN active layer 144 6.2.5 Al(0.5g)GaN active layer 146 6.2.6 Al(0.6g)GaN active layer 149 6.2.7 In(0.1g) Al(0.6g) GaN active layer 151 6.2.8 In(0.2g)Al(0.6g)GaN active layer 153 6.2.9 In(0.3g)Al(0.6g)GaN active layer 155 6.2.10 In(0.4g)Al(0.6g)GaN active layer 158 6.2.11 In(0.5g)Al(0.6g)GaN active layer 160 6.3. XRD characteristics 163 Chapter 7. Phosphor &#8211Free White LED Lamp 180 7.1. Manufacturing of white LED lamp 180 7.2. Analysis of White LED Spectra and Color Rendering 181 7.3. Measurement of Phospohor free white LED 189 7.4. Future research 197 Chapter 8. Conclusions 200 Publications 202 Conference 204 Biography 209 Acknowledgements 21

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

Optoelectronic study of InGaN/GaN LEDs

The quality of light emitting diodes (LEDs) has increased to a point where solid state lighting is becoming fairly common. Despite this, greater understanding of the effect of the device structure and the electric fields within them is helpful to continue improving device efficiency and uniformity and in reducing costs. In this thesis the optical and electronic properties of InGaN/GaN LEDs have been studied with a combination of luminescence spectroscopy, microscopy, conductivity mapping and efficiency measurements.A study was made of the effects of the various electric fields, and the interplay between them, on LED luminescence and conductivity. Cathodoluminescence (CL) mapping shows die to die variation across large wafers revealing the powerful effects of a induced electric field on spectral intensity/position/width, in uncontacted devices. Micron scale spots in the LED material, lower in luminescence intensity and which trap charge, were revealed by CL/EBIC mapping with the origin attributed to cluster point defects in the active region. Depth resolved CL and CL under bias reveal the extent of asymmetry in carrier transport in the p/n type GaN around the active region. LEDs grown with different active region temperature profiles were studied. Devices exposed to high temperature after quantum well growth (2T) were found to have a uniform spatial luminescence and a peak efficiency that is higher and occurs at a lower current density (0.1 W/A @ 1 Acm¯²). By contrast those with a low temperature cap (Q2T) exhibit dark spots in the luminescence, and a lower peak efficiency at a higher current density (0.04 W/A @ 10 Acm¯²). The effect of improvement in LED design and material quality on the device efficiency, uniformity and spectral characteristics was studied. The addition of an Al₀.₂₃Ga.₇₇N electron blocking layer (EBL) was found to reduce the size and strength of the dark spots by about a factor of 2, while an additional In₀.₀₅Ga₀.₉₅N underlayer (UL) removed the dark spots entirely and shifted the luminescence peak by around 100 meV. The effect on the electroluminescence efficiency of the reduction in template dislocation density was found to depend strongly on the drive current density, with defect non-radiative recombination more important at low currents. Overall device efficiency was shown to be improved with an EBL and UL. The most efficient devices were those with the 2T type growth but the relative improvements are larger in LEDs grown with the Q2T method.Together, the results present a number of factors limiting the performance of current LEDs and suggest potential routes for improvement and optimisation.The quality of light emitting diodes (LEDs) has increased to a point where solid state lighting is becoming fairly common. Despite this, greater understanding of the effect of the device structure and the electric fields within them is helpful to continue improving device efficiency and uniformity and in reducing costs. In this thesis the optical and electronic properties of InGaN/GaN LEDs have been studied with a combination of luminescence spectroscopy, microscopy, conductivity mapping and efficiency measurements.A study was made of the effects of the various electric fields, and the interplay between them, on LED luminescence and conductivity. Cathodoluminescence (CL) mapping shows die to die variation across large wafers revealing the powerful effects of a induced electric field on spectral intensity/position/width, in uncontacted devices. Micron scale spots in the LED material, lower in luminescence intensity and which trap charge, were revealed by CL/EBIC mapping with the origin attributed to cluster point defects in the active region. Depth resolved CL and CL under bias reveal the extent of asymmetry in carrier transport in the p/n type GaN around the active region. LEDs grown with different active region temperature profiles were studied. Devices exposed to high temperature after quantum well growth (2T) were found to have a uniform spatial luminescence and a peak efficiency that is higher and occurs at a lower current density (0.1 W/A @ 1 Acm¯²). By contrast those with a low temperature cap (Q2T) exhibit dark spots in the luminescence, and a lower peak efficiency at a higher current density (0.04 W/A @ 10 Acm¯²). The effect of improvement in LED design and material quality on the device efficiency, uniformity and spectral characteristics was studied. The addition of an Al₀.₂₃Ga.₇₇N electron blocking layer (EBL) was found to reduce the size and strength of the dark spots by about a factor of 2, while an additional In₀.₀₅Ga₀.₉₅N underlayer (UL) removed the dark spots entirely and shifted the luminescence peak by around 100 meV. The effect on the electroluminescence efficiency of the reduction in template dislocation density was found to depend strongly on the drive current density, with defect non-radiative recombination more important at low currents. Overall device efficiency was shown to be improved with an EBL and UL. The most efficient devices were those with the 2T type growth but the relative improvements are larger in LEDs grown with the Q2T method.Together, the results present a number of factors limiting the performance of current LEDs and suggest potential routes for improvement and optimisation

Development of III-nitride bipolar devices: avalanche photodiodes, laser diodes, and double-heterojunction bipolar transistors

This dissertation describes the development of III-nitride (III-N) bipolar devices for optoelectronic and electronic applications. Research mainly involves device design, fabrication process development, and device characterization for Geiger-mode gallium nitride (GaN) deep-UV (DUV) p-i-n avalanche photodiodes (APDs), indium gallium nitride (InGaN)/GaN-based violet/blue laser diodes (LDs), and GaN/InGaN-based npn radio-frequency (RF) double-heterojunction bipolar transistors (DHBTs). All the epitaxial materials of these devices were grown in the Advanced Materials and Devices Group (AMDG) led by Prof. Russell D. Dupuis at the Georgia Institute of Technology using the metalorganic chemical vapor deposition (MOCVD) technique. Geiger-mode GaN p-i-n APDs have important applications in DUV and UV single-photon detections. In the fabrication of GaN p-i-n APDs, the major technical challenge is the sidewall leakage current. To address this issue, two surface leakage reduction schemes have been developed: a wet-etching surface treatment technique to recover the dry-etching-induced surface damage, and a ledged structure to form a surface depletion layer to partially passivate the sidewall. The first Geiger-mode DUV GaN p-i-n APD on a free-standing (FS) c-plane GaN substrate has been demonstrated. InGaN/GaN-based violet/blue/green LDs are the coherent light sources for high-density optical storage systems and the next-generation full-color LD display systems. The design of InGaN/GaN LDs has several challenges, such as the quantum-confined stark effect (QCSE), the efficiency droop issue, and the optical confinement design optimization. In this dissertation, a step-graded electron-blocking layer (EBL) is studied to address the efficiency droop issue. Enhanced internal quantum efficiency (ɳi) has been observed on 420-nm InGaN/GaN-based LDs. Moreover, an InGaN waveguide design is implemented, and the continuous-wave (CW)-mode operation on 460-nm InGaN/GaN-based LDs is achieved at room temperature (RT). III-N HBTs are promising devices for the next-generation RF and power electronics because of their advantages of high breakdown voltages, high power handling capability, and high-temperature and harsh-environment operation stability. One of the major technical challenges to fabricate high-performance RF III-N HBTs is to suppress the base surface recombination current on the extrinsic base region. The wet-etching surface treatment has also been employed to lower the surface recombination current. As a result, a record small-signal current gain (hfe) > 100 is achieved on GaN/InGaN-based npn DHBTs on sapphire substrates. A cut-off frequency (fT) > 5.3 GHz and a maximum oscillation frequency (fmax) > 1.3 GHz are also demonstrated for the first time. Furthermore, A FS c-plane GaN substrate with low epitaxial defect density and good thermal dissipation ability is used for reduced base bulk recombination current. The hfe > 115, collector current density (JC) > 141 kA/cm², and power density > 3.05 MW/cm² are achieved at RT, which are all the highest values reported ever on III-N HBTs.PhDCommittee Chair: Shen, Shyh-Chiang; Committee Member: Dupuis, Russell; Committee Member: Jiang, Zhigang; Committee Member: Mukhopadhyay, Saibal; Committee Member: Yoder, Dougla

Degradation mechanisms of devices for optoelectronics and power electronics based on Gallium Nitride heterostructures

Gallium Nitride is rapidly emerging as a promising material for electronic devices in various fields. Since it is a direct bandgap semiconductor it can be used for highly efficient light emitting devices (Light Emitting Diodes and Laser Diodes) and the possibility of growing alloys containing Aluminum and Indium allow for the selection of the peak wavelength along the whole UV-green part of the radiation spectrum. Moreover, the high electron mobility, the ability of withstand high electric fields and the good thermal dissipation make GaN-based diodes and transistors devices with a good potential for high frequency and power applications. Before final products containing Gallium Nitride devices can permeate the international market, it is required to guarantee that they are reliable enough to have long lifetimes to appeal potential customers, and that their performance/cost relationship is superior compared to other competitors, at least in some specific fields of application. Aim of this thesis is to investigate the strong points of Gallium Nitrides by means of characterization and reliability tests on various different structures (LEDs, laser diodes, blocking diodes, HEMTs, GITs, MISs), in order to analyze the behavior of the material from different points of view. Within this work is reported a detailed study of the gradual degradation of InGaN-based laser diodes and Light-Emitting Diodes submitted to electro-thermal stress. The purpose is to compare the behavior of the two devices by means of electro-optical measurements, electroluminescence characterization, near field emission measurements and Deep-Level Transient Spectroscopy (DLTS) investigation in order to give a deeper understanding of the mechanisms involved in LD degradation. Particular attention is given to the role of injection efficiency decrease and non-radiative recombination. The comparison of the degradation kinetics and an analysis of the degradation modes of the two device structures allowed a complete study of the physical mechanisms responsible for the degradation. It was found that the degradation of the devices can be ascribed to an increase of the defect density, which has a strong impact on non radiative recombination kinetics. The activation energy of the detected deep level is 0.35 - 0.45 eV. As an effect of combined electrical and thermal stress tests on commercially-available InGaN-based blue laser diodes, it has been found that sometimes there is an initial decrease of the threshold current, which is ascribed to the increase of the activation of p-type dopant, promoted by the temperature and the flow of minority carriers. In order to investigate the effects of the creation of defects, two different commercial blue InGaN-based LEDs were submitted to 3 MeV proton irradiation at various fluencies (10^11, 10^12 and 10^13 p/cm2). The degradation process was characterized by combined current-voltage (I - V), optical power-current (L - I) and capacitance-voltage (C - V) measurements, in order to investigate the changes induced by the irradiation and the recovery after annealing time at high temperature (150 °C). The experimental data suggest the creation of non-radiative recombination centers near or into the active region of the LEDs, due to atomic displacement. This hypothesis is confirmed by the results of the recovery tests: the increase of the optical power and its correlation with the recovery of the forward current is consistent with the annealing of those defects. Part of the activity on high electron mobility transistors was devoted to the realization of measurement setups in order to carry out novel characterization techniques. Were analyzed the advantages and limitations of the current-transient method used for the study of the deep levels in GaN-based high electron mobility transistors (HEMTs), by evaluating how the procedures adopted for measurement and data analysis can influence the results of the investigation. The choice of the measurement parameters (such as the voltage levels used to induce the trapping phenomena and monitor the current transients and the duration of the filling pulses) and of the analysis procedure (the method used for the extrapolation of the time constants of the processes) can influence the results of the drain current transient investigation and can provide information on the location of the trap levels responsible for current collapse. Moreover, was collected a database of defects described in more than 60 papers on GaN and its compounds, which can be used to extract information on the nature and origin of the traps in AlGaN/GaN HEMTs. Using this newly developed technique and other more common tests, several reliability and lifetime test were carried out on various structures, in order to gain a better understanding of their problematic aspects and possible improvements. One potential variation is the composition of the gate stack. Degradation tests were performed at Vgs = -5 V and increasing Vds levels on GaN HEMTs with different gate materials: Ni/Au/Ni, ITO and Ni/ITO. At each step of the stress experiment, the electrical and optical characteristics of the transistors were measured in order to analyze the degradation process. It was found that stress induces a permanent degradation of the gate diode, consisting in an increase in the leakage current. This change is due to the generation of parasitic conductive paths, as suggested by electroluminescence (EL) mapping, and devices based on ITO showed higher reliability. These data strongly support the hypothesis that the robustness is influenced by processing parameters and/or by the gate material, since all analyzed devices come from the same epitaxial wafer. Other than varying the gate material, it is possible to add a p-type layer under the gate in order to achieve normally-off operation. This change produces a benefit in terms of performances, but can give birth to unusual trapping phenomena. It was carried out an extensive analysis of the time and field-dependent trapping processes that occur in GaN-based gate injection transistors exposed to high drain voltage levels. Results indicate that, even if the devices do not suffer from current collapse, continuous exposure to high drain voltages can induce a remarkable increase in the on-resistance (Ron). The increase in Ron can be recovered by leaving the device in rest conditions. Temperature-dependent analysis indicates that the activation energy of the detrapping process is equal to 0.47 eV. By time-resolved electroluminescence characterization, it is shown that this effect is related to the capture of electrons in the gate - drain access region. This is further confirmed by the fact that charge emission can be significantly accelerated through the injection of holes from the gate. A first-order model was developed to explain the time dependence of the trapping process. Using other deep levels characterization techniques, such as drain current transients, gate frequency sweeps and backgating, several other trap states were identified in these devices. Their activation energies are 0.13, 0.14, 0.25, 0.47 and 0.51 eV. During the accelerated lifetime tests of these devices, it was found a variation of the relative amplitude of the transconductance peaks, well correlated with the increase of the electroluminescence. This effect can be explained by the activation of the p-type dopant, a phenomenon which was detected also in laser diodes. It is possible to develop diodes able to withstand very high reverse voltages using a similar structure, deprived of the gate region and with an additional Schottky diode (Natural superjunction). In this case, the activation energies of the detected deep levels were 0.35, 0.36, 0.44 and 0.47 eV. These values are very similar to the ones found in GITs, and this fact, along with the presence of the p-dopant activation in very different devices, confirms that it is useful to study different structures based on the same material in order to gain more knowledge on its performances, possibilities and reliability aspects

Nitride semiconductors studied by atom probe tomography and correlative techniques

Optoelectronic devices fabricated from nitride semiconductors include blue and green light emitting diodes (LEDs) and laser diodes (LDs). To design efficient devices, the structure and composition of the constituent materials must be well-characterised. Traditional microscopy techniques used to examine nitride semiconductors include transmission electron microscopy (TEM), and atomic force microscopy (AFM). This thesis describes the study of nitride semiconductor materials using these traditional methods, as well as atom probe tomography (APT), a technique more usually applied to metals that provides three-dimensional (3D) compositional information at the atomic scale. By using both APT and correlative microscopy techniques, a more complete understanding of the material can be gained, which can potentially lead to higher-efficiency, longer-lasting devices. Defects, such as threading dislocations (TDs), can harm device performance. An AFM-based technique was used to show that TDs affect the local electrical properties of nitride materials. To investigate any compositional changes around the TD, APT studies of TDs were attempted, and evidence for oxygen enrichment near the TD was observed. The dopant level in nitride devices also affects their optoelectronic properties, and the combination of APT and TEM was used to show that Mg dopants were preferentially incorporated into pyramidal inversion domains, with a Mg content two orders of magnitude above the background level. Much debate has been focused on the microstructural origin of charge carrier localisation in InGaN. Alloy inhomogeneities have often been suggested to provide this localisation, yet APT has revealed InGaN quantum wells to be a statistically random alloy. Electron beam irradiation in the TEM caused damage to the InGaN, however, and a statistically significant deviation from a random alloy distribution was then observed by APT. The alloy homogeneity of InAlN was also studied, and this alloy system provided a unique opportunity to study gallium implantation damage to the APT sample caused during sample preparation by the focused ion beam (FIB). The combination of APT with traditional microscopy techniques made it possible to achieve a thorough understanding of a wide variety of nitride semiconductor materials.This work was supported by the EPSRC, Sharp Laboratories of Europe and Pembroke College, Cambridge

Estudo de propriedades estruturais e ópticas de multicamadas epitaxiais emissoras de luz baseadas em InGaN/GaN

Esta tese apresenta os resultados de uma investigação experimental em filmes epitaxiais emissores de luz baseados em InxGa1-xN. O InxGa1-xN é uma liga semicondutora ternária do grupo III-N muito utilizada como camada activa numa gama de dispositivos optoelectrónicos em desenvolvimento, incluindo díodos emissores de luz (LEDs) e díodos laser (LDs), para operação na região do visível e ultravioleta do espectro electromagnético. Neste estudo, caracterizam-se as propriedade ópticas e estruturais de camadas simples e poços quânticos múltiplos (Multiple Quantum Wells, MQWs) de InxGa1-xN/GaN, com ênfase nas suas propriedades físicas fundamentais. O objectivo central do trabalho prende-se com a compreensão mais profunda dos processos físicos que estão por trás das suas propriedades ópticas, preenchendo o fosso existente entre aplicações tecnológicas e o conhecimento científico. Nomeadamente, a tese aborda os problemas da medição da fracção de InN (x) em multicamadas ultrafinas sujeitas a tensões, a influência da composição e das tensões microscópicas nas propriedades ópticas e estruturais. A questão relativa à segregação de fases em multicamadas de InxGa1-xN/GaN é também discutida à luz dos resultados obtidos. A metodologia seguida assenta na integração de resultados obtidos por técnicas complementares através de uma análise sistemática e multidisciplinar. Esta abordagem passa pela combinação de: 1) Crescimento de amostras por deposição epitaxial em fase de vapor organometálico (MOVPE) com características específicas de forma a tentar isolar parâmetros estruturais, tais como espessura e composição; 2) Caracterização nanoestrutural por microscopia de força atómica (AFM), microscópica electrónica de varrimento (SEM), difracção de raios-X e retro-dispersão de Rutherford (RBS); 3) Caracterização óptica a escalas complementares por: espectroscopia de absorção óptica (OA), fotoluminescência (PL), catodoluminescência (CL) e microscopia confocal (CM) com análise espectral. Com base nos resultados obtidos, a tese propõe modelos de interpretação para as propriedades estruturais e ópticas, dando ênfase às suas correlações. Em particular, estabelece-se a necessidade de considerar fenómenos relacionados com tensões microscópicas na interpretação dos resultados experimentais. Com este trabalho fica clara a necessidade de um conhecimento detalhado das características nanoestruturais para interpretar as propriedades ópticas das ligas de InxGa1-xN.This thesis presents an experimental investigation of light emitting epitaxial layers based on indium gallium nitride (InxGa1-xN). This group III-nitride ternary semiconductor alloy is used as the active layer in a novel class of optoelectronic devices, including light emitting diodes (LEDs) and laser diodes (LDs), under development to operate in the visible and ultraviolet regions of the electromagnetic spectrum. The structural and optical properties of InxGa1- xN/GaN single layers and multiple quantum wells (MQWs) are characterized with an emphasis on their fundamental physical properties. The fundamental purpose of this work is to provide grounds for better understanding of the yet unclear physics of this important material system, and help to fill the gap between basic scientific knowledge and technological applications. Namely, this work addresses the issues of accurate measurement of the InN mole fraction (x), the influence of composition and strain in the structural and optical properties and the topic of phase segregation in InxGa1- xN. The approach taken in this thesis is to integrate information provided by several complementary structural and optical characterization techniques through a systematic and multidisciplinary analysis. Specifically we combine: 1) sample growth by metal organic chemical vapour deposition (MOCVD) with specific features in an attempt to isolate the influence of structural parameters, such as layer thickness and composition; 2) Structural characterization by atomic force microscopy (AFM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and Rutherford Backscattering spectrometry (RBS); 3) Optical characterisation at complementary length scales by: optical absorption (OA), photoluminescence (PL), and cathodoluminescence (CL) spectroscopy and confocal microscopy (CM) spectroscopy. Based on the results obtained, interpretation models to describe the structural and optical features are proposed, with a particular emphasis on the establishment of direct correlations between both. It is concluded that strainrelated phenomena must be taken into account to interpret the experimental results. Moreover it is also deduced that a detailed knowledge on the nanostructure is essential to explain InxGa1-xN optical properties