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

Growth of highly conductive Al-rich AlGaN:Si with low group-III vacancy concentration

Publisher Copyright: © 2021 Author(s).The impact of AlGaN growth conditions on AlGaN:Si resistivity and surface morphology has been investigated using metalorganic chemical vapor deposition. Growth parameters including growth temperature, growth rate, and trimethylindium (TMI) flow have been systematically studied to minimize the resistivity of AlGaN:Si. We observed a strong anticorrelation between AlGaN:Si conductivity and growth temperature, suggesting increased silicon donor compensation at elevated temperatures. Secondary ion mass spectrometry and positron annihilation spectroscopy ruled out compensation by common impurities or group-III monovacancies as a reason for the observed phenomenon, in contrast to theoretical predictions. The underlying reason for AlGaN:Si resistivity dependence on growth temperature is discussed based on the possibility of silicon acting as a DX center in Al0.65Ga0.35N at high growth temperatures. We also show remarkable enhancement of AlGaN:Si conductivity by introducing TMI flow during growth. A minimum resistivity of 7.5 m? cm was obtained for n-type Al0.65Ga0.35N, which is among the lowest reported resistivity for this composition.& nbsp;(c)& nbsp;2021 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license(http://creativecommons.org/licenses/by/4.0/).& nbsp;Peer reviewe

Decreased expression of breast cancer resistance protein in the duodenum in patients with obstructive cholestasis

Background/Aims: The expression of transporters involved in bile acid homeostasis is differentially regulated during obstructive cholestasis. Since the drug efflux transporter breast cancer resistance protein (BCRP) is known to transport bile acids, we investigated whether duodenal BCRP expression could be altered during cholestasis. Methods: Using real-time RT-PCR analysis we determined mRNA expression levels in duodenal tissue of 19 cholestatic patients. Expression levels were compared to 14 healthy subjects. BCRP protein staining was determined in biopsies of 6 cholestatic and 6 healthy subjects by immunohistochemistry. Results: We found that in patients with obstructive cholestasis mean duodenal BCRP mRNA levels were significantly reduced to 53% and mean protein staining was reduced to 57%. Conclusions: BCRP, a transporter for bile acids and numerous drugs, appears to be down-regulated in the human duodenum during cholestasis. The clinical impact of these results has to be investigated in further studies. Copyright (c) 2006 S. Karger AG, Basel

The effects of spatial legacies following shifting management practices and fire on boreal forest age structure

Forest age structure and its spatial arrangement are important elements of sustainable forestry because of their effects on biodiversity and timber availability. Forest management objectives that include specific forest age structure may not be easily attained due to constraints imposed by the legacies of historical management and natural disturbance. We used a spatially explicit stochastic model to explore the synergetic effects of forest management and fire on boreal forest age structure. Specifically, we examined (1) the duration of spatial legacies of different management practices in the boreal forest, (2) how multiple shifts in management practices affect legacy duration and the spatial trajectories of forest age structure, and (3) how fire influences legacy duration and pattern development in combination with harvesting. Results based on 30 replicates of 500 years for each scenario indicate that (1) spatial legacies persist over 200 years and the rate at which legacies are overcome depends on whether new management targets are in synchrony with existing spatial pattern; (2) age specific goals were met faster after multiple management shifts due to the similar spatial scale of the preceding management types; (3) because large fires can erase the spatial pattern created by smaller disturbances, scenarios with fire had shorter lags than scenarios without fire. These results suggest that forest management goals can be accelerated by applying management at a similar spatial scale as existing spatial patterns. Also, management planning should include careful consideration of historical management as well as current and likely future disturbances

Genomic Dissection of Bipolar Disorder and Schizophrenia, Including 28 Subphenotypes

publisher: Elsevier articletitle: Genomic Dissection of Bipolar Disorder and Schizophrenia, Including 28 Subphenotypes journaltitle: Cell articlelink: https://doi.org/10.1016/j.cell.2018.05.046 content_type: article copyright: © 2018 Elsevier Inc

Highly Conductive n-Al0.65Ga0.35N Grown by MOCVD Using Low V/III Ratio

Highly conductive silicon-doped AlGaN and ohmic contacts are needed for deep-UV LEDs and ultrawide bandgap electronics. We demonstrate improved n-Al0.65Ga0.35N films grown by metal–organic chemical vapor deposition (MOCVD) on sapphire substrates using a low V/III ratio (V/III = 10). A reduced V/III ratio improves repeatability and uniformity by allowing a wider range of silicon precursor flow conditions. AlxGa1−xN:Si with x > 0.5 typically has an electron concentration vs. silicon concentration trend that peaks at a particular “knee” value before dropping sharply as [Si] continues to increase (self-compensation). The Al0.65Ga0.35N:Si grown under the lowest V/III conditions in this study does not show the typical knee behavior, and instead, it has a flat electron concentration trend for [Si] > 3 × 1019 cm−3. Resistivities as low as 4 mΩ-cm were achieved, with corresponding electron mobility of 40 cm2/Vs. AFM and TEM confirm that surface morphology and dislocation density are not degraded by these growth conditions. Furthermore, we report vanadium-based ohmic contacts with a resistivity of 7 × 10−5 Ω-cm2 to AlGaN films grown using a low V/III ratio. Lastly, we use these highly conductive silicon-doped layers to demonstrate a 284 nm UV LED with an operating voltage of 7.99 V at 20 A/cm2, with peak EQE and WPE of 3.5% and 2.7%, respectively

Ultraviolet Light Emitting Diodes Grown on Sapphire and Silicon Carbide Substrates

Germicidal ultraviolet (GUV) light with wavelengths between 250 nm and 300 nm was first discovered to have a strong disinfectant effect in the 19th century. GUV irradiation is a chemical-free, species-agnostic disinfection technology: some example applications include disinfection of medical equipment and buildings; municipal water treatment, residential water purification, and consumer devices like water bottles; air disinfection in offices, schools, planes, cruise ships, and retail spaces, and countless others. Most importantly, a low-cost, durable, portable, and efficient solid-state GUV light source could bring these disinfection applications to low-income and developing regions where most sewage enters waterways untreated, and the lack of sterile environments and equipment worsens healthcare outcomes. The current incumbent technology for GUV light emission is the mercury lamp. This technology has remained largely unchanged for the past century—it is efficient and inexpensive, and has a large market due to its use for phosphor-converted white light (“fluorescent lighting”). On the other hand, this technology relies on toxic mercury vapor contained within a hot quartz bulb, requires complex and inefficient driving electronics, and cannot be reliably dimmed or rapidly switched on/off. To meet growing demand for disinfection technologies, and for GUV to expand into residential, consumer, and smart applications, a solid-state GUV light emitter is needed. A UV LED would have many benefits, much like the now-mature solid-state blue and white LEDs which have revolutionized the lighting and display industries. Semiconductor LEDs can be produced at smaller sizes, lower costs, higher efficiencies, and longer lifetimes than their 19th-century vapor lamp counterparts; these semiconductor devices can be dimmed or switched rapidly with no warm-up or flicker, and are readily produced in a variety of target wavelengths. Currently available UV LED technologies based on the AlGaN materials system are much less efficient (3-5%) than Hg lamps (20-30%), due to a number of challenges related to AlGaN material quality, chemical purity, process technologies, and optical properties. Therefore, there is a strong desire to improve the efficiency, cost, and lifetime of UV emitters to match their visible LED counterparts, which have efficiencies well over 50% and lifetimes over 10,000 h. In this dissertation, I discuss the ways in which AlN and AlGaN defect density on foreign substrates has been greatly reduced using improved MOCVD growth methods, achieving a dislocation density of 2x108 cm-2 on SiC. Next, I will explain why n-type and p-type doping (both of which are needed for LED operation) of AlGaN are challenging, and how both have been greatly improved using growth condition modification and polarization engineering, respectively. Using the methods outlined in this dissertation, we achieved n-type AlxGa1-xN (x>0.65) resistivities below 10 m-cm in material grown on three different substrate platforms (4H-SiC, 6H-SiC, and sapphire), and on two different MOCVD reactors. I will discuss active region engineering for increased light emission power, as well as an analysis of the various LED processing methods and packaging architectures used in the devices shown. Combining improvements in material quality, electrical conductivity, and optical quality, we have increased GUV LED external quantum efficiency from 2% to 10%

Engineering Carrier Effective Masses in Ultrathin Quantum Wells of IrO2

The carrier effective mass plays a crucial role in modern electronic, optical, and catalytic devices and is fundamentally related to key properties of solids such as the mobility and density of states. Here we demonstrate a method to deterministically engineer the effective mass using spatial confinement in metallic quantum wells of the transition metal oxide IrO2. Using a combination of in situ angle-resolved photoemission spectroscopy measurements in conjunction with precise synthesis by oxide molecular-beam epitaxy, we show that the low-energy electronic subbands in ultrathin films of rutile IrO2 have their effective masses enhanced by up to a factor of 6 with respect to the bulk. The origin of this strikingly large mass enhancement is the confinement-induced quantization of the highly nonparabolic, three-dimensional electronic structure of IrO2 in the ultrathin limit. This mechanism lies in contrast to that observed in other transition metal oxides, in which mass enhancement tends to result from complex electron-electron interactions and is difficult to control. Our results demonstrate a general route towards the deterministic enhancement and engineering of carrier effective masses in spatially confined systems, based on an understanding of the three-dimensional bulk electronic structure. © 2018 American Physical Societ

Connections between spin-orbit torques and unidirectional magnetoresistance in ferromagnetic-metal–heavy-metal heterostructures

We investigate the connections between current-induced spin-orbit torques and unidirectional magnetoresistance (UMR) by performing second harmonic longitudinal resistance measurements on Co/Pt bilayer with magnetic-field μ0H up to 10 T and temperature T down to 2 K. The fieldlike torque hFL changes sign with varying Co thickness tCo, which indicates that competing mechanisms, i.e., the spin Hall effect (SHE) and the inverse spin galvanic effect (iSGE), are responsible for the generation of hFL. The sign of hFL coincides with the sign of UMR induced by spin-dependent scattering. However, the dampinglike torque hDL is proportional to the inverse Co thickness, with no sign reversal for all measured temperatures and tCo, indicating that hDL originates solely from the SHE. The generation of hDL via iSGE can be further excluded because of the observation of an H-linear dependence of UMR, which, in turn, indicates the negligible exchange coupling between spin accumulation and ferromagnetic metal