Synthesis and Characterization of electrodes for III-Nitride Resonant Cavity Light Emitting Diode Applications

In recent years Light Emitting Diodes (LEDs) of high efficiency and long lifetime have been produced. Gallium Nitride in particular is used in the manufacture of Blue LEDs because it is a direct band gap semiconductor which can be alloyed with AlN and InN allowing band gap energies to range from 1.9eV to 6.2eV, which allows the emission of short wavelengths including blue light. The interest in replacing conventional lighting by solid-state lighting has led to focus on the development of high brightness Gallium Nitride (GaN) LEDs. Efficient LED structures such as flip chip configurations and Resonant Cavity LEDs (RCLEDs) typically need a highly reflective ohmic contact on p-GaN. RCLEDs are of great interest due their features such as, high spectral purity, and high emission intensity when compared to conventional GaN based LEDs [1]. Au based contacts are not appropriate for the RCLEDs due to low reflectance in the blue region. Ag films have much higher reflectivity in the visible region when compared to Au films.;The first part of the thesis presents the detailed fabrication process and characterization of Ag based electrodes for III-Nitride Resonant Cavity LEDs. A GaN/AlGaN Distributed Bragg reflector (DBR) of very high reflectance is employed on the sapphire substrate below the LED, which acts as the bottom mirror. For the top mirror, low resistance and highly reflective ohmic contacts on p-type GaN were achieved using an Ag-based metallization scheme. The second part of the thesis presents, development of a transparent, conductive zinc oxide contact to a GaN/InGaN MQW LED. A process and post-deposition annealing have been optimized to obtain ZnO films with high electrical conductivity and transparency, and ZnO-based contacts with a low contact resistance. This work represents a step toward the fabrication of high-efficiency GaN-based blue and green LEDs with transparent ZnO electrodes

Design, growth, fabrication and characterization of white LEDs by monolithic on-chip epitaxial integration on (11-22) semi-polar GaN

Ultimate lighting sources for general illumination are monolithic on-chip white light emitting diodes (LEDs) containing multiple colour emissions, either red-green-blue or blue-yellow, but without involving any yellow phosphors. It is highly likely that current white LEDs fabricated by using a “blue LED+ yellow phosphors” approach will be eventually replaced by monolithic on-chip white LEDs. One of the direct routes for the fabrication of monolithic white LEDs is to utilize InGaN quantum wells (QWs) with different emission wavelengths as an active region, which will involve a number of fundamental issues, such as the design of an active region, carrier transport, etc. So far, these fundamental issues have not been understood. In this work, a systematic simulation study on these challenging issues has been carried out, achieving a full understanding of these issues and thus leading to the design of optimized white LED structures on (11-22) semi-polar substrates by taking the major advantages of semi-polar LEDs in comparison with their c-plane counterparts. Finally, the monolithic on-chip white LED epiwafer based on these designs have been successfully grown on our well-established (11-22) GaN templates with a step-change in crystal quality. Detailed device characterization has been performed on these LEDs, validating these approaches and designs. The design of dual-colour (11-22) semi-polar LEDs aiming at white LEDs and their carrier transport issues have been systemically studied by using one-dimensional drift-diffusion simulations. Due to the much heavier effective mass of holes than that of electrons and also the much larger activation energy of p-GaN than n-GaN, the distribution of injected carrier (mainly holes) is extremely uneven during LED operation. Furthermore, the residual polarization of semi-polar LEDs makes the case even more complicated. Based on a systematic study, carrier transport issues for (11-22) semi-polar white LEDs and their c-plane counterparts have been fully understood, demonstrating their major differences. In addition a novel structure utilizing an extra thin GaN spacer prior to the growth of blue InGaN quantum well, has been design to effectively improve hole transportation and a dual-colour emission LED has been achieved. A tri-chromatic emission has been subsequently designed by further optimizing two key factors, indium content in InGaN quantum wells and barrier thicknesses. In order to validate our simulation results dual-color emission LEDs have been grown on our high quality (11-22) semi-polar GaN templates. Simulations have agreed very well with experimental results demonstrating that both the growth order of the yellow and blue InGaN quantum wells and the growth of a thin GaN spacer are of vital importance. A different approach has been developed, leading to the growth and then the fabrication of monolithically integrated white light LEDs on (11-22) semi-polar GaN template. In this approach, an electrically injected semi-polar blue LED is firstly grown, followed by a yellow multiple quantum well structure as a down conversion layer. This forms a white LED. For the first time, a systematic and comprehensive study on optical polarization properties has been conducted on (11-22) semi-polar LEDs with a wide spectral region from blue to yellow as a function of indium concentration and injection current. Fundamental understanding of the polarization properties and the emission mechanisms of (11-22) semi-polar LEDs has been achieved. Detailed polarization dependent electroluminescence measurements have demonstrated that both indium content and current injection play crucial roles in the optical polarization properties of (11-22) semi-polar LEDs

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

Growth and Characterization of Graphene on Texture-Controlled Platinum Films

Department of Materials Science EngineeringIn this study, the primary purpose of this research is to grow high quality graphene on platinum (Pt) films, especially wrinkle-free graphene as a 2-dimensional membrane for transparent conductor and hydrophobic water-distillation applications by using texture-controlled Pt films that have incorporated oxygen atoms. In order to achieve the final goals, this research primary had been focused on analysis of abnormal Pt grains through annealing process and study of graphene growth kinetics through chemical vapor deposition process. Then, a new transfer method was applied to graphene transfer by reacting graphene/Pt interface, without incurring damages and unintentional doping. The wrinkle-free graphene was synthesized by using texture-controlled Pt films (200, 220) with giant grains (GGPt) via chemical vapor deposition (CVD). The Pt films on SiO2/Si substrates could be controlled by sputtering with Ar/O2 gas mixtures and abnormal grain growth was affected by the incorporated oxygen during post-annealing process. In order to analysis of graphene growth kinetics on GGPt, each films were heated at the CVD process temperature of ~975 ºC and maintained for 10 min under CH4/H2 gas mixture (5 and 50 sccm, respectively) without cleaning treatments. Enhanced surface perfectness and limited number of grain boundary (GB) of Pt induced homogeneous C-precipitation, thus the high-crystallized monolayer graphene sheets was formed. The transferred graphene shows wrinkle-free characteristics regardless of the orientation types of Pt, probably due to much lesser difference in thermal expansion coefficient (TEC, ~11 μm m-1K-1 at 1000 °C) to graphene. The wrinkles or ripples-free graphene films showed a high crystallinity and high carrier mobility at room-temperature up to ~8,500 cm2V-1s-1. To transfer graphene, a thermal-assisted transfer method was applied by a NaOH (1 M) aqueous solution at 90 °C. The thermal-assisted transfer method was only activated by the hydroxide (OH-) in NaOH solution to separate the graphene/Pt interface. The thermal-assisted transfer process allowed the complete transfer of large-scale graphene films onto arbitrary target substrates without incurring damages and unintentional doping. Compare to bare GGPt, graphene-free GGPt showed no contamination and degradation after the graphene transfer. The fact was demonstrated by XPS data, which showed almost same binding energy of Pt-4f5/2, Pt-4f7/2 (74±0.2 eV). On the basis of these results, a recycle ability of Pt was demonstrated. Also, the result of graphene on the recycled Pt showed almost same quality as the obtained graphene from 1st Pt. Furthermore, the transfer method could be applicable to the large-scale patterned graphene on Pt films with SiO2 regions. By comparing an electrochemical transfer method, the thermal-assisted transfer method have proved to be successfully transferred onto SiO2/Si substrate for the patterned Pt films. The reason is that the reaction between Si and Na+ took place in the boiled NaOH solution to react the SiO2 surface. Through a pre-annealing step in CVD process, the porous graphene membrane could be obtained from the porous Pt texture. The density and size of pore depended on the pre-annealing time in hydrogen gas. Especially, a dense pores of Pt films was obtained with controllable density (~2×105 cm-2) and ~2.5 μm of radius by pre-annealing for 5 min. Since an oxygen was inserted during Pt film sputtered as an adhesion layer between Pt and SiO2/Si substrate, Pt sintering has occurred by oxygen diffusion during pre-annealing step in H2 atmosphere. The porous graphene membrane were successfully transferred onto SiO2/Si substrate by thermal-assisted transfer method. Surprisingly, graphene was grown direct in the pores of Pt films. It was demonstrated through that Pt particles directly formed growth of the graphene. In summary, this study shows the wrinkle-free characteristic of graphene layer by using Pt thin films with preferred orientations and giant grains. In addition, large-scale and patterned graphene films can be successfully transferred onto arbitrary substrates via thermal-assisted transfer method and the Pt substrates can be repeatedly used for the proliferation of graphene applications. Furthermore, this transfer technique shows a high tolerance to variations in types and morphologies of underlying substrates, which is essential for the various applications proposed for graphene.ope

Optical and luminiscent properties of terbium / ytterbium doped aluminum oxynitride and terbium doped aluminum nitride thin films

In the present thesis the optical and light emission properties of two systems consisting of Tb3+ and Yb3+ doped amorphous AlOxNy thin films and Tb3+ doped polycrystalline AlN thin films were analyzed. In the two ions system, to obtain an adequate luminescent emission, commonly a significant effort must be made to find a suitable concentration of dopants and elemental composition of the host material. An interesting and highly efficient method is a combinatorial approach, allowing a high velocity screening of a wider range of properties. In the present work a combinatorial gradient based thin film libraries of amorphous AlOxNy:Yb3+, AlOxNy:Tb3+ and AlOxNy:Tb3+:Yb3+ have been prepared by radio frequency co-sputtering from more than one target. In the prepared libraries, the Tb and Yb concentration range spreads along with the oxygen to nitrogen ratio of the host matrix all over the substrate area. Concentrations ranges for each ion were established for producing high emission intensity samples, along with an analysis of the light emission features of Yb3+ ions with Tb3+ ions as sensitizers for cooperative down conversion process. Using different annealing temperatures the activation energy of the rare earth ions and thermal-induced activation mechanisms are evaluated. Here we show that the different oxygen to nitrogen ratios in the host composition affect the light emission intensity. According to experimental results, there is a strong enhancement of the Yb3+ related emission intensity over the Tb3+ emission in codoped films with Tb:Yb concentration ratios near to 1:2, at 850°C. Thus, suggesting the sensitization of Tb3+ ions through an AlOxNy matrix and the cooperative energy transfer between Tb3+ and Yb3+ ions as the driven mechanism for down conversion process with promising applications in silicon solar cells. At the end of this first part, the optimal elemental composition and optimal annealing temperature in the investigated ranges to achieve the highest Yb3+ emission intensity upon sensitization of Tb3+ ions is reported. The second system studied consists of Tb3+ doped AlN layers prepared by reactive magnetron sputtering and analyzed using the conventional one at a time approach. In this work, two types of thermal treatments have been applied: substrate heating during deposition of the films and post deposition rapid thermal annealing, with varying temperature from non intentional heating up to 600°C. The composition, morphology and crystalline structure of the films under different thermal processes and temperatures were investigated along with their optical and light emission properties, with the aim of maximizing the Tb3+ emission intensity. The polycrystalline nature of the films was confirmed by X-ray diffraction under grazing incidence, and the influence of substrate temperature on the crystalline structure was reported. Atomic force microscopy and scanning electron microscopy has revealed the smooth grainy surface quality of the AlN:Tb3+ films. The highest Tb3+ photoluminescence emission intensity was achieved in the film treated with rapid thermal annealing process. For a more detailed study of the post deposition annealing treatments, temperature was further increased up to 900°C, and the influence of annealing temperature on the emission properties was investigated by photoluminescence and photoluminescence decay measurements. An increase in the photoluminescence intensity and photoluminescence decay time was observed upon annealing for the main transition of Tb3+ ions at 545 nm, which was attributed to a decrease of non radiative recombination and increase of the population of excited Tb3+ ions upon annealing. Additionally, using the characterized films as active layer, direct current and alternate current thin film electroluminescence devices were designed and investigated.Tesi

Epitaxy and Device Design for High Efficiency Blue LEDs and Laser Diodes

The (Al,Ga,In)N materials system has impacted energy efficiency on the world-wide scale through its application to blue light-emitting diodes (LEDs), which were invented and developed in the 1990s. Since then, cost reductions and performance improvements have brought GaN-based LEDs into the mainstream, supplanting outdated lighting technology and improving energy efficiency.One of the main challenges that still limits commercial LEDs, however, is “efficiency droop,” which refers to the reduction in efficiency as the input current density (and with it, the carrier density) increases. This phenomenon especially plagues high power LEDs, which operate in the current density range of 100-1000 A/cm2.Few practical options exist to directly eliminate efficiency droop, however we investigated two complementary approaches to circumvent the phenomenon. The first “high power solution” would employ blue laser diodes as the engine of solid state white lighting in lieu of LEDs. When laser diodes reach the threshold current density for stimulated emission, the carrier density in the active region clamps, simultaneously clamping droop. The wall plug efficiency of the laser diodes can then continue to rise as input current density increases until another effect (usually thermal) overrides it. The second “low power solution” maintains the blue LED as the solid state lighting engine, but shifts the operation point to low current density (and low carrier density) where efficiency droop effects are negligible and other thermal and electrical constraints in the device design are alleviated, enabling designs for high wall-plug efficiency. Both approaches to circumventing efficiency droop are likely to find a home in diverse future technologies and applications for lighting and displays.The challenge to produce high performance blue laser diodes was approached from an m-plane epitaxy platform. m-Plane is a non-polar orientation of the wurtzite (Al,Ga,In)N, which is free from deleterious polarization-related electric fields in the growth direction. m Plane is a naturally occurring crystal plane with high material gain due to its non-degenerate valence band structure, and thus should be well-suited for laser diode applications. However, m plane blue emission suffers from low indium uptake and broad spontaneous emission linewidth. The use of surface “double miscut” was investigated to improve the local step structure and morphology, resulting in higher indium uptake, narrower linewidth and higher peak power in the blue spectrum.The complementary challenge to improve the wall-plug efficiency for LEDs at low power operation focuses primarily on improved light extraction efficiency and low voltage operation. The main sources of extraction efficiency losses in typical c-plane blue LEDs on patterned sapphire substrates are absorption on the metal contacts, in the current spreading layer and on the metallic reflector, which also doubles as the heat sink. With the relaxed constraints at low power operation, new designs become possible. High light extraction designs were vetted with ray tracing software prior to experimental implementation. The highest demonstrated wall-plug efficiency resulting from these designs was 78.2%, and was accompanied by a greater than unity electrical efficiency (1.03) resulting from thermoelectric pumping, suggesting a pathway for 100% or greater wall-plug efficiency

Growth, processing and chracterization of gallium nitride based coaxial LEDs grown by MOVPE

Gallium nitride (GaN) based coaxial (core-shell type) light emitting diodes (LEDs) offer a wide range of advantages. The active region of these LEDs is located on non-polar, {1-100} m-plane GaN sidewalls, which helps eliminate the quantum confined Stark effect (QCSE) and improve the radiative recombination efficiency of LEDs. The recent evolution of a catalyst free, scalable, repeatable and industrially viable device quality GaN nanowire and nanowall metal organic vapor phase epitaxy (MOVPE) growth process has enhanced the possibility of these LEDs going into production from laboratory. Previous work has shown that these nanowires exhibited an intense photoluminescence (PL), in spite of their large surface-area to volume ratio, and lasing was observed when these nanowires were optically pumped at high intensity. In this dissertation, it is shown that as long as the GaN three dimensional (3D) structures have their critical dimension below a micron, the threading defect (TD) density along the c- direction approaches zero. A TD that enters into this structure bends towards the surface vii ({1-100} m-plane side wall) in its vicinity, thereby reducing its dislocation line energy. The possibility of growing zero defect GaN templates is extremely important in the breakdown voltage improvement, the reverse bias leakage current reduction and efficiency droop reduction. This growth method has also been extended to device quality micron sized features, thereby presenting us with opportunity to study and explore LEDs of different sizes and shapes. In addition to the microstructure growth, two different repeatable approaches have been identified and demonstrated for the microelectronic processing of these micron-sized LEDs. Despite being far from perfect, the characterization results obtained from these LEDs have been encouraging. The technological challenges associated with the fabrication of the coaxial LEDs are also discussed in this dissertation