Fabrication technology for high light-extraction ultraviolet thin-film flip-chip (UV TFFC) LEDs grown on SiC

The light output of deep ultraviolet (UV-C) AlGaN light-emitting diodes (LEDs) is limited due to their poor light extraction efficiency (LEE). To improve the LEE of AlGaN LEDs, we developed a fabrication technology to process AlGaN LEDs grown on SiC into thin-film flip-chip LEDs (TFFC LEDs) with high LEE. This process transfers the AlGaN LED epi onto a new substrate by wafer-to-wafer bonding, and by removing the absorbing SiC substrate with a highly selective SF6 plasma etch that stops at the AlN buffer layer. We optimized the inductively coupled plasma (ICP) SF6 etch parameters to develop a substrate-removal process with high reliability and precise epitaxial control, without creating micromasking defects or degrading the health of the plasma etching system. The SiC etch rate by SF6 plasma was ~46 \mu m/hr at a high RF bias (400 W), and ~7 \mu m/hr at a low RF bias (49 W) with very high etch selectivity between SiC and AlN. The high SF6 etch selectivity between SiC and AlN was essential for removing the SiC substrate and exposing a pristine, smooth AlN surface. We demonstrated the epi-transfer process by fabricating high light extraction TFFC LEDs from AlGaN LEDs grown on SiC. To further enhance the light extraction, the exposed N-face AlN was anisotropically etched in dilute KOH. The LEE of the AlGaN LED improved by ~3X after KOH roughening at room temperature. This AlGaN TFFC LED process establishes a viable path to high external quantum efficiency (EQE) and power conversion efficiency (PCE) UV-C LEDs.Comment: 22 pages, 6 figures. (accepted in Semiconductor Science and Technology, SST-105156.R1 2018

Molecular beam epitaxial growth of rare-earth compounds for semimetal/semiconductor heterostructure optical devices

textHeterostructures of materials with dramatically different properties are exciting for a variety of devices. In particular, the epitaxial integration of metals with semiconductors is promising for low-loss tunnel junctions, embedded Ohmic contacts, high-conductivity spreading layers, as well as optical devices based on the surface plasmons at metal/semiconductor interfaces. This thesis investigates the structural, electrical, and optical properties of compound (III-V) semiconductors employing rare-earth monopnictide (RE-V) nanostructures. Tunnel junctions employing RE-V nanoparticles are developed to enhance current optical devices, and the epitaxial incorporation of RE-V films is discussed for embedded electrical and plasmonic devices. Leveraging the favorable band alignments of RE-V materials in GaAs and GaSb semiconductors, nanoparticle-enhanced tunnel junctions are investigated for applications of wide-bandgap tunnel junctions and lightly-doped tunnel junctions in optical devices. Through optimization of the growth space, ErAs nanoparticle-enhanced GaAs tunnel junctions exhibit conductivity similar to the best reports on the material system. Additionally, GaSb-based tunnel junctions are developed with low p-type doping that could reduce optical loss in the cladding of a 4 μm laser by ~75%. These tunnel junctions have several advantages over competing approaches, including improved thermal stability, precise control over nanoparticle location, and incorporation of a manifold of states at the tunnel junction interface. Investigating the integration of RE-V nanostructures into optical devices revealed important details of the RE-V growth, allowing for quantum wells to be grown within 15nm of an ErAs nanoparticle layer with minimal degradation (i.e. 95% of the peak photoluminescence intensity). This investigation into the MBE growth of ErAs provides the foundation for enhancing optical devices with RE-V nanostructures. Additionally, the improved understanding of ErAs growth leads to development of a method to grow full films of RE-V embedded in III-V materials. The growth method overcomes the mismatch in rotational symmetry of RE-V and III-V materials by seeding film growth with epitaxial nanoparticles, and growing the film through a thin III-V spacer. The growth of RE-V films is promising for both embedded electrical devices as well as a potential path towards realization of plasmonic devices with epitaxially integrated metallic films.Electrical and Computer Engineerin

Gruppe III-Nitrid basierte UVC LEDs und Laser mit transparenten AlGaN:Mg Schichten und Tunneldioden, hergestellt mittels MOVPE

In this work, AlGaN-based light-emitting diodes (LEDs) and lasers with emission wavelengths in the deep ultraviolet (UVC) spectral range are produced, analyzed, and optimized. Here, the focus is on the UV transparency of the structures, enabling high light extraction efficiency for UVC LEDs and being a necessary condition for UVC laser diodes, however at the same time challenging due to low electrical conductivity. AlN and AlGaN layers as well as heterostructures for devices are grown by metalorganic vapor phase epitaxy. A systematic analysis of the influence of individual layer properties on the emission properties of LEDs and lasers is provided. Defect reduced (ELO) AlN layers on sapphire and AlN substrates serve as basis for the epitaxial growth of AlN and AlGaN layers. By analyzing the influence of substrate offcut on surface morphology, atomically smooth AlN layers are reproducibly obtained on both types of substrates for offcut angles < 0.17°. For the realization of n-type AlGaN:Si cladding layers, the influence of growth parameters such as temperature, gas phase composition and growth rate was separately analyzed. Highly conductive, uniform and smooth AlGaN:Si layers were obtained by the implementation of a superlattice concept with 10 s growth interruptions to increase the diffusion length of metal adatoms. Despite high compressive strain, pseudomorphic laser structures with three-fold quantum wells were obtained with emission wavelength at 270 nm by the choice of Al0.7Ga0.3N waveguide composition, whereas lower aluminum contents lead to partial strain relaxation. In addition, the formation of V-pits acting as scattering centers in the waveguide was successfully reduced by increasing the growth temperature from 900 ℃ to 1080 ℃. Finally, the influence of these individual optimization steps on laser properties was analyzed. Optically pumped UVC lasers with laser threshold, spectral linewidth reduction, and TE polarized emission above threshold were shown near 270 nm. By reducing the surface roughness, the laser thresholds were reduced by a factor of seven. Electrical injection mechanisms were experimentally analyzed by electroluminescence measurements on transparent UVC LEDs with waveguide system, and combined with simulations of optical modes and the corresponding losses. By the variation of composition and layer thickness of waveguide and cladding layers an optimized heterostructure design for UVC laser diodes with 200 nm thick Al0.76Ga0.24N:Mg cladding layers was found. This design simultaneously enables efficient carrier injection and sufficient mode confinement with low optical losses of 40 cm-1. As an unconventional alternative to resistive AlGaN:Mg layers, tunnel junctions (TJ) in reverse bias configuration were implemented into the UVC LED heterostructure for efficient injection of holes. By the initial optimization of individual TJ components, such as doping concentrations at the TJ interface or the composition of an interlayer, the first demonstration of functional TJ-LEDs with AlGaN tunnel homojunction was achieved, as well as the first demonstration of AlGaN-based TJ-LEDs grown by metalorganic vapor phase epitaxy. Based on these devices, the interlayer thickness was varied to exploit polarization charges at the interface in order to reduce the space charge region width and enhance tunneling probabilities. Using 8 nm thick GaN interlayers, a reduction of the operation voltage by 20 V was achieved, as well as TJ-LEDs with external quantum efficiencies of 2.3% and emission powers of 6.6 mW at 268 nm and 0.26 mW at 232 nm.In dieser Arbeit werden AlGaN-basierte Leuchtdioden (LEDs) und Laser mit Emissionswellenlängen im tiefen ultravioletten (UVC) Spektralbereich hergestellt, charakterisiert und optimiert. Dabei liegt die UV-Transparenz der Strukturen im Fokus, die hohe Lichtextraktionseffizienz für UVC LEDs ermöglicht und eine notwendige Bedingung für UVC Laserdioden darstellt, gleichzeitig aber aufgrund geringer elektrischer Leitfähigkeit herausfordernd ist. AlN und AlGaN Schichten sowie Heterostrukturen für Bauelemente werden mittels metallorganischer Gasphasenepitaxie hergestellt und der Einfluss einzelner Schichteigenschaften auf die Emissionseigenschaften von LEDs und Lasern systematisch analysiert. Defektreduzierte (ELO) AlN Schichten auf Saphirsubstraten sowie AlN Substrate dienen als Basis für das epitaktische Wachstum von AlN und AlGaN Schichten. Durch die Analyse des Einflusses des Substratfehlschnittes auf die Oberflächenmorphologie konnten atomar glatte AlN Schichten auf beiden Substrattypen für Fehlschnittwinkel < 0.17° reproduzierbar hergestellt werden. Die AlGaN:Si Wachstumsparameter Temperatur, Gasphasenzusammensetzung und Wachstumsrate wurden separat variiert. Leitfähige, homogene und glatte AlGaN:Si Schichten konnten durch die Umsetzung eines Übergitterkonzeptes mit je 10 s Wachstumsunterbrechung zur Erhöhung der Diffusionslänge von Metalladatomen realisiert werden. Pseudomorphe Laserstrukturen mit Dreifach-Quantenfilmen und Emissionswellenlängen von 270 nm wurden trotz stark kompressiver Verspannung mittels Al0.7Ga0.3N Wellenleitern realisiert, wogegen geringere Aluminiumgehalte zu Teilrelaxation der Verspannung führen. Zudem konnte die Ausbildung von V-Pits als Streuzentren im Wellenleiter durch Erhöhung der Wachstumstemperatur von 900 ℃ auf 1080 ℃ erfolgreich reduziert werden. Schließlich wurde der Einfluss dieser einzelnen Optimierungsschritte auf die Lasereigenschaften analysiert. Optisch gepumpte UVC Laser mit spektraler Einschnürung, Laserschwelle sowie TE polarisierter Emission nahe 270 nm wurden gezeigt. Durch Reduktion der Oberflächenrauheit konnte die Laserschwelle schrittweise um den Faktor sieben reduziert werden. Elektrische Injektion wurde mittels Elektrolumineszenz an transparenten UVC LEDs mit Wellenleitersystem experimentell analysiert und mit Simulationen optischer Moden und deren Verluste kombiniert. Durch die Variation von Zusammensetzung und Schichtdicke von Wellenleiter- bzw. Mantelschichten konnte ein optimiertes Heterostrukturdesign für UVC Laserdioden mit 200 nm dicken Al0.76Ga0.24N:Mg Mantelschichten gefunden werden, welches gleichzeitig effiziente Ladungsträgerinjektion und ausreichenden Modeneinschluss mit geringen optischen Verlusten von 40 cm-1 ermöglicht. Als unkonventionelle Alternative zu resistiven AlGaN:Mg Schichten wurden Tunneldioden (TJ) zur Löcherinjektion implementiert. Durch die anfängliche Optimierung individueller Komponenten wie der Zusammensetzung einer Zwischenschicht oder der Dotierlevel an der Grenzfläche, wurde die erste Demonstration AlGaN-basierter TJLEDs ermöglicht, die mit metallorganischer Gasphasenepitaxie gewachsen wurden. Auf dieser Basis wurde die Zwischenschichtdicke gezielt variiert, um Polarisationsladungen an der Grenzfläche zur Reduktion der Raumladungszonenbreite auszunutzen und die Tunnelwahrscheinlichkeit zu erhöhen. Mit 8 nm GaN Zwischenschichten wurde eine Spannungsreduktion um 20 V erreicht, sowie TJ-LEDs mit externer Quanteneffizienz von 2,3% und Emissionsleistung von 6,6 mW bei 268 nm und 0,26 mW bei 232 nm

III-Nitride Nanocrystal Based Green and Ultraviolet Optoelectronics

Extensive research efforts have been devoted to III-nitride based solid-state lighting since the first demonstration of high-brightness GaN-based blue light emitting diodes (LEDs). Over the past decade, the performance of GaN-based LEDs including external quantum efficiency (EQE), wall-plug efficiency, output power and lifetime has been improved significantly while the cost of GaN substrate has been reduced drastically. Although the development of blue and near ultraviolet (UV) LED is mature, achieving equally excellent performance in other wavelengths based on III-nitrides is still challenging. Especially, the significant efficiency droop in the green wavelength, known as “green gap” and the extremely low EQE in the UV regime, known as “UV threshold”, have become two most urgent issues. Green LEDs emit light that is most sensitive to human eye, implicating its importance in a variety of applications such as screen- and projection-based displays. UV light sources have a variety of applications including water and air purification, sterilization/disinfection of medical tools, medical diagnostics, phototherapy, sensing, which make solid-state deep UV (DUV) light sources with compactness, low operating power and long lifetime highly desirable. The deterioration of performance with green LEDs originates from increased indium content of the active region, which could degrade material quality and increase quantum confined Stark effect due to the high polarization fields in c-plane InGaN/GaN quantum wells (QWs). Meanwhile, limiting factors in III-nitride UV LEDs include low internal quantum efficiency due to large densities of dislocations, poor carrier injection efficiency and low light extraction efficiency. In this dissertation, we have investigated the molecular beam epitaxial growth, structural characterization, and electrical and optical properties of low-dimensional III-nitride nanocrystals as potential solutions to above-mentioned issues. Through a combination of theoretical calculation and experimental investigation, we show that defects formation in AlN could be precisely controlled under N-rich epitaxy condition. With further optimized p-type doping, AlN nanowire-based LEDs emitting at 210 nm were fabricated. We report DUV excitonic LEDs with the incorporation of monolayer GaN with emission wavelengths of ~238 nm, and exhibit suppressed Auger recombination, negligible efficiency droop and a small turn on voltage ~5 V. To enhance the light extraction efficiency of AlGaN nanowires grown on Si substrate, we demonstrated epitaxy of AlGaN nanowires on Al coated Si(001) substrate wherein Al film functions as a UV light reflective layer to enhance the light extraction efficiency. AlGaN nanowire-based DUV LEDs on Al film were successfully grown and fabricated and measured with a turn-on voltage of 7 V and an electroluminescence emission at 288 nm. Green-emitting InGaN/GaN nanowire LEDs on Si(001) substrate were demonstrated, wherein the active region and p-contact layer consist of InGaN/GaN disks-in-nanowires and Mg-doped GaN epilayers. The incorporation of planar p-GaN layer significantly reduces the fabrication complexity of nanowire-based devices and improves the robustness of electrical connection, leading to a more stable device operation. We also demonstrated micrometer scale InGaN photonic nanocrystal green LEDs with ultra-stable operation. The emission features a wavelength of ~548 nm and a spectral linewidth of ~4 nm, which is nearly five to ten times narrower than that of conventional InGaN QW LEDs in this wavelength range. Significantly, the device performance, in terms of the emission peak and spectral linewidth, is nearly invariant with injection current. Work presented in this thesis provides a new approach for achieving high-performance green and DUV LEDs by using III-nitride nanostructures.PHDElectrical and Computer EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/163122/1/ypwu_1.pd

Wide-Bandgap III-Nitride Tunnel Junctions and Novel Approaches towards Improving Optoelectronic Devices

A combination of novel techniques, materials, and devices are explored to enhance III-nitride optoelectronics from the infrared to the deep ultraviolet wavelengths. Low-bandgap, high indium content III-nitride materials are investigated for longer wavelength applications. High indium incorporation into the crystal is achieved via plasma-assisted molecular beam epitaxy (PAMBE) at low growth substrate temperatures 2x light output power when compared to a control.Ph.D

Sputtered Gallium Nitride Tunnel Junction Contacts

Gallium nitride LEDs produced for commercialization currently use indium tin oxide (ITO) as both a current spreading layer (CSL) and a contact to p-GaN. ITO is known to absorb wavelengths in the UV and visible light regions, the primary spectrum of GaN devices [20]. Tunnel junctions (TJ) have been proposed as an alternative p-contact and CSL which would transmit more light from the active region [20] while generating holes, from the electron tunneling, to reach the quantum wells of the LEDs for additional radiative recombination [18]-[21]. The fabrication process would also be simplified because the top layer of both the p- and n-contacts would be n-GaN and the metal contacts to both could be deposited at the same time. However, until now all GaN TJs have been deposited by MOCVD or MBE systems despite sputtering machines offering a lower cost and easier to use method of depositing GaN [12] [13]. This thesis explores the application of two sputtering techniques, the electron cyclotron resonance (ECR) and radio frequency (RF) magnetron, to the creation of TJs on blue GaN LEDs.Both sputtering methods utilized silicon doped n-GaN targets with the intention of depositing the n-GaN layer of the TJs. The films were examined for their transmissivity of 440 nm light as well as their resistivity. The ECR system was observed to produce GaN that lost around 5% of the emitted light when N2 in conjunction with argon was used in the sputtering gas. Substrate heating did not meaningfully affect the transmissive property of the deposited GaN. Despite the good optical quality, the GaN remained resistive. Secondary ion mass spectrometry (SIMS) of the n-GaN target found it contain many impurities. High amounts of carbon, hydrogen, oxygen, magnesium, and calcium among other elements were discovered to be within the target. This stopped the research to produce a tunnel junction with the ECR system, but with a cleaner target it could still be a viable option.The RF magnetron machine also relied upon nitrogen within the sputtering gas mixture to produce transparent GaN films. In general, the higher the rate of N2 the less absorption of the tested 440 nm light. Substrate heating did improve the transparency of the GaN when sputtered with the RF magnetron machine. For a gas mixture of 38:25 sccm of N2:Ar and a temperature of 800 ⁰C, the GaN layer absorbed less than one percent of the blue wavelength. Hall-effect measurements showed greater substrate heating also increased the carrier concentration with films reaching the high 1019 and low 1020 cm-3 ranges.During the course of this work the RF magnetron system was modified and the maximum substrate temperature was lowered to 650 ⁰C. Some previous films had been grown at 600 ⁰C and this temperature was kept for continuity, but a silicon target was co-sputtered with the n-GaN target to raise the carrier concentration of the GaN. Hall measurements of these samples presented them to have higher mobilities compared to the GaN sputtered with only the GaN target. Sputtering the silicon target at 35 W of power was determined to give the highest carrier concentration of 1.137x1018 cm-3.A tunnel junction was created utilizing the RF magnetron sputtering system on a UCSB blue LED structure deposited on a sapphire substrate with a MOCVD machine. Silicon was co-sputtered at 35 W and the nitrogen and substrate temperature were set to 38 sccm and 650 ⁰C, respectively, to create the n-GaN layer of the TJ. LEDs with an area of 0.1 mm2 were fabricated and required only two photolithography steps compared to the three steps necessary for ITO GaN LEDs. After a chlorine etch performed by a RIE machine to create the LED mesas, the MOCVD n-GaN around the mesas was noted to be rough in some areas of the wafer. When metal contacts were deposited on these areas, the metal did not stick to the n-GaN. Many LEDs were created in the smoother regions and were observed to emit blue light. These were tested for their current-voltage relationships and were found to have turn-on voltages of approximately 6.5 V with the least resistive devices reaching almost 5 mA at 10 V. These are the first reported GaN tunnel junctions deposited onto LEDs with a sputtering system

Photonic integrated circuits for optical logic applications

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2007.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Includes bibliographical references.The optical logic unit cell is the photonic analog to transistor-transistor logic in electronic devices. Active devices such as InP-based semiconductor optical amplifiers (SOA) emitting at 1550 nm are vertically integrated with passive waveguides using the asymmetric twin waveguide technique and the SOAs are placed in a Mach-Zehnder interferometer (MZI) configuration. By sending in high-intensity pulses, the gain characteristics, phase-shifting, and refractive indices of the SOA can be altered, creating constructive or deconstructive interference at the MZI output. Boolean logic and wavelength conversion can be achieved using this technique, building blocks for optical switching and signal regeneration. The fabrication of these devices is complex and the fabrication of two generations of devices is described in this thesis, including optimization of the mask design, photolithography, etching, and backside processing techniques. Testing and characterization of the active and passive components is also reported, confirming gain and emission at 1550 nm for the SOAs, as well as verifying evanescent coupling between the active and passive waveguides. In addition to the vertical integration of photonic waveguides, Esaki tunnel junctions are investigated for vertical electronic integration. Quantum dot formation and growth via molecular beam epitaxy is investigated for emission at the technologically important wavelength of 1310 nm. The effect of indium incorporation on tunnel junctions is investigated. The tunnel junctions are used to epitaxially link multiple quantum dot active regions in series and lasers are designed, fabricated, and tested.by Ryan Daniel Williams.Ph.D

Growth and Characterization of Tandem-Junction Photovoltaic Nanowires

In order to satisfy the growing energy needs of our planet’s population, and at the same time mitigate global warming, sustainable energy sources such as solar energy are indispensable. In addition to conventional silicon-based solar cells, nanotechnology offers interesting approaches for complementary applications. Multi-junction solar cells based on III–V semiconductors hold today’s world-record efficiencies—twice as efficient as solar cells found on rooftops nowadays—but their high cost is limiting their terrestrial use.In this thesis, nanowires for photovoltaic applications are studied. Nanowire solar cells have the potential to reach the same efficiencies as the world-record III–V solar cells while only using a fraction of the material. First, InP single-junction nanowires were investigated. For solar energy harvesting, large-area nanowire solar cells have to be processed but so far only devices with less than one mm2 have been fabricated. To lay the foundation of large-area nanowire solar cells, the wafer-scale synthesis of InP nanowire arrays was systematically studied. Then the effect of embedding InP nanowires in different oxides was investigated. Due to their inherent large surface-to-volume ratio, nanowires require surface passivation. However, fixed charge carriers in the passivating layer can alter the electrostatic potential of nanowires, which was directly imaged by measuring the electron-beam-induced current. Furthermore, the current-voltage characteristics of single nanowires under in situ illumination was measured and correlated with electron-beam-induced current measurements, by using a setup that combines a nanoprobe system with an optical fiber coupled to a multi-LED setup inside a scanning electron microscope. Guided by the multi-LED and electron-beam-induced current setup, tandem-junction nanowires were developed. After identifying and subsequently preventing the occurrence of a parasitic junction when combining an InP n–i–p junction with a tunnel diode, GaInP/InP tandem-junction nanowires were synthesized. An optical and electrical bias was applied to individually measure the electron-beam-induced current of both sub-cells. Finally, axially defined, GaInP/InP/InAsP triple-junction photovoltaic nanowires optimized for light absorption exhibiting an open-circuit voltage of up to 2.37 V were synthesized. The open-circuit voltage amounts to 94 % of the sum of the respective single-junction nanowires. These results pave the way for realizing the next-generation of scalable, high-performance, and ultra-high power-to-weight ratio multi-junction, nanowire-based solar cells