Showing papers in "ECS Journal of Solid State Science and Technology in 2016"

Review—Ionizing Radiation Damage Effects on GaN Devices

Abstract: GalliumNitridebasedhighelectronmobilitytransistors(HEMTs)areattractiveforuseinhighpowerandhighfrequencyapplications, with higher breakdown voltages and two dimensional electron gas (2DEG) density compared to their GaAs counterparts. Specific applications for nitride HEMTs include air, land and satellite based communications and phased array radar. Highly efficient GaNbased blue light emitting diodes (LEDs) employ AlGaN and InGaN alloys with different compositions integrated into heterojunctions and quantum wells. The realization of these blue LEDs has led to white light sources, in which a blue LED is used to excite a phosphor material; light is then emitted in the yellow spectral range, which, combined with the blue light, appears as white. Alternatively, multiple LEDs of red, green and blue can be used together. Both of these technologies are used in high-efficiency white electroluminescent light sources. These light sources are efficient and long-lived and are therefore replacing incandescent and fluorescent lamps for general lighting purposes. Since lighting represents 20‐30% of electrical energy consumption, and because GaN white light LEDs require ten times less energy than ordinary light bulbs, the use of efficient blue LEDs leads to significant energy savings. GaN-based devices are more radiation hard than their Si and GaAs counterparts due to the high bond strength in III-nitride materials. The response of GaN to radiation damage is a function of radiation type, dose and energy, as well as the carrier density, impurity content and dislocation density in the GaN. The latter can act as sinks for created defects and parameters such as the carrier removal rate due to trapping of carriers into radiation-induced defects depends on the crystal growth method used to grow the GaN layers. The growth method has a clear effect on radiation response beyond the carrier type and radiation source. We review data on the radiation resistance of AlGaN/GaN and InAlN/GaN HEMTs and GaN‐based LEDs to different types of ionizing radiation, and discuss ion stopping mechanisms. The primary energy levels introduced by different forms of radiation, carrier removal rates and role of existing defects in GaN are discussed. The carrier removal rates are a function of initial carrier concentration and dose but not of dose rate or hydrogen concentration in the nitride material grown by Metal Organic Chemical Vapor Deposition. Proton and electron irradiation damage in HEMTs creates positive threshold voltage shifts due to a decrease in the two dimensional electron gas concentration resulting from electron trapping at defect sites, as well as a decrease in carrier mobility and degradation of drain current and transconductance. State-of-art simulators now provide accurate predictions for the observed changes in radiation-damaged HEMT performance. Neutron irradiation creates more extended damage regions and at high doses leads to Fermi level pinning while 60 Co γ-ray irradiation leads to much smaller changes in HEMT drain current relative to the other forms of radiation. In InGaN/GaN blue LEDs irradiated with protons at fluences near 10 14 cm −2 or electrons at fluences near 10 16 cm −2 , both current-voltage and light output-current characteristics are degraded with increasing proton dose. The optical performance of the LEDs is more sensitive to the proton or electron irradiation than that of the corresponding electrical performances. © The Author(s) 2015. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any

Editors' Choice Communication—A (001) β-Ga2O3 MOSFET with +2.9 V Threshold Voltage and HfO2 Gate Dielectric

Abstract: An MOS transistor fabricated on (001) β-Ga2O3 exfoliated from a commercial (−201) β-Ga2O3 substrate is reported. A maximum drain current of 11.1 mA/mm was measured, and a non-destructive breakdown was reached around 80 V in the off state. Threshold voltage of +2.9 V was extracted at 0.1 V drain bias, and peak transconductance of 0.18 mS/mm was measured at VDS = 1 V, corresponding to a field effect mobility of 0.17 cm2/Vs. Hall effect and electron spin resonance data suggested that electron conductivity was due primarily to O vacancy donors (VO+) with an estimated density of 2.3 × 1017 (±50%) cm−3.

Review—Quantum Dots and Their Application in Lighting, Displays, and Biology

Abstract: Quantum dots have attracted considerable interest in the fields of solid state lighting, displays, and fluorescent imaging. Their tunable optical properties by changing the size and solution processability lead to commercial applications. In this review, we focus on the advancement of white light emitting nanocrystals, their usage as the emissive layer in LEDs and display backlights, and examine the increased efficiency and longevity of quantum dots based colored LEDs. In addition, we also explore recent discoveries on quantum dots as biological labels, dynamic trackers, and applications in drug delivery. (C) 2015 The Electrochemical Society. All rights reserved.

Luminescent Behavior of the K2SiF6:Mn4+ Red Phosphor at High Fluxes and at the Microscopic Level

Abstract: Phosphor-converted white light-emitting diodes (LEDs) are becoming increasingly popular for general lighting. The non-rare-earth phosphorK(2)SiF(6): Mn4+, showing promising saturated red d-d-line emission, was investigated. To evaluate the application potential of this phosphor, the luminescence behavior was studied at high excitation intensities and on the microscopic level. The emission shows a sublinear behavior at excitation powers exceeding 40 W/cm(2), caused by ground-state depletion due to the ms range luminescence lifetime. The thermal properties of the luminescence in K2SiF6: Mn4+ were investigated up to 450 K, with thermal quenching only setting in above 400 K. The luminescence lifetime decreases with increasing temperature, even before thermal quenching sets in, which is favorable to counteract the sublinear response at high excitation intensity. A second, faster, decay component emerges above 295 K, which, according to crystal field calculations, is related to a fraction of the Mn4+ ions incorporated on tetragonally deformed lattice sites. A combined investigation of structural and luminescence properties in a scanning electron microscope using energy-dispersive X-ray spectroscopy and cathodoluminescence mappings showed both phosphor degradation at high fluxes and a preferential location of the light outcoupling at irregularities in the crystal facets. The use of K2SiF6: Mn4+ in a remote phosphor configuration is discussed.

Wearable Sensor System Powered by a Biofuel Cell for Detection of Lactate Levels in Sweat

Abstract: Performance tracking technologies have become extremely popular over the past several decades. Most rely on sensing of physical attributes such as heart rate, body temperature, calories burned, or steps taken. There is a significant push to develop technologies that can accurately monitor the change in biomarkers non-invasively from biological fluids, such as sweat. The concept of utilizing biological fluids to access biomarkers is not new.1 Numerous studies have been performed employing a variety of methods for detection of ions,2–4 lactate,5 glucose,6 ethanol,7 and other biomarkers in sweat. Additionally, investigations into correlation between sweat expressed biomarker and their blood levels have been conducted.8,9 Other biological fluids, such as urine, saliva, and tears, have also been investigated for their potential use in human performance sensing.10–14 Lactate is a key biomarker of stress and indicator of the health state of an individual. Lactate is a product of both anaerobic and aerobic glucose metabolism, via glycolysis and plays an important role in maintaining cellular and tissue homeostasis.15 At the cellular level, energy production (in the form of ATP) occurs through the break down of glucose to pyruvate (oxidized lactate), through glycolysis yielding 2 ATP, followed by either transport to the mitochondria for further processing through the citric acid cycle and oxidative phosphorylation (OxPhos, producing 36 ATP per 1 glucose) or reduced to lactate for storage of fuel. Under stress conditions, glycolysis responds by rapidly producing large amount of pyruvate greater than that what the mitochondria can sustain resulting in an inhibitory accumulation of pyruvate. To prevent negative feedback inhibition excess pyruvate is then converted to lactate and exported into the blood for transport to other tissues for processing or storage. This increase of lactate levels in the blood can correlate to lactate levels in extracorporeal liquids including saliva, urine and sweat.5,9,16,17 Additionally, the increase of blood lactate levels as a response to stress and the commercial availability of redox-active lactate oxidizing enzymes makes following this chemical reaction system attractive for development of sensing technology for the monitoring of human performance. Amperometric biosensors are known to be inexpensive, reproducible, sensitive, selective, and typically composed of chemically modified electrode material and biological recognition elements (BRE).16 The most widely used BRE for amperometic lactate sensing are oxidoreductase enzymes that catalyze the oxidation or reduction of a substrate. In regards to lactate sensing the most used redox enzymes are lactate oxidase followed by NAD+-dependent lactate dehydrogenase (LDH). In general, oxidases catalyze an oxidation reaction in the presence of oxygen and water and yield a product and H2O2. In this case, the H2O2 is detected at the electrode surface. Dehydrogenases comprise the largest group of oxidoreductases and require an additional co-enzyme, NAD(P)+ that can be immobilized on the electrode or added to the buffer to catalyze the oxidation of the substrate.18 It is NAD(P)+ that is detected quantitatively at the electrode surface. A disadvantage of using these oxidoreductases is the need of high applied potentials to detect the oxidase electro-active product at the electrode surface or to regenerate the oxidized co-enzyme from the reduced form (NAD(P)(H)). The high applied potentials introduce interferences from the oxidation of contaminating species within the solution. To overcome the overpotential, the electrode surface can be modified with mediator like Prussian Blue or polymethylene green (PMG).19–22 With ever-increasing interest in monitoring human performance a significant amount of research has been conducted in the area of wearable sensor development that employ close bodily contact.23–27 Metabolite sensing through Band-Aid like RFID sensor patches and temporary tattoo-based sensors have been developed for electrolyte and lactate sensing in sweat as part of on-body sensing technology.27–29 To date the main focus was on the development of a sensor, discounting the need for a power source to operate the sensor, and to communicate the sensor information. In the majority of the cases a simple coin cell battery could be employed as a power source. Others have looked at utilizing flexible solid state batteries. One main disadvantage of using traditional power sources for sensor operation is their short operational lifetime, which is related to limited amount of active material and excess of packaging material to overcome safety concerns. Accessibility and popularity of smart phones has influenced a move toward production of sensor technology that reads and transmits data wirelessly by Android smart phone and custom-apps.27 A major problem of wireless sensors is the requirement for electronic power to run individual sensors in a sustainable and maintenance-free way.30 Using batteries to supply power is impractical as multiple sensor networks would likely be used and replacing individual batteries would be a tremendous task along with the toxicity of material from batteries that would require recycling of the batteries to reduce unfriendly ecosystem hazards.30 Alternative, green energy technologies are necessary to supply the demands for sensors, nanarobotics, microelectronic networks, and wearable electronics.31 An alternate battery-free solution comes in a form of a biofuel cell (BFC), which utilizes a biomimetic approach to power generation employing commonly available, safe, and energy dense fuels such as glucose. The idea of enzyme-based biofuel cells for self-powered biosensors was first discussed in 2001 and has gained momentum in recent years.32–34 Information technology has impacted the trends of economic development in the last 20 years.30 If trends continue then development of self-powered sensing systems may have an impact on the world economy.30 Here we present a complete wearable system, designed for lactate sensing in sweat. The overall system is comprised of a lactate biosensor,35,36 a glucose oxidase BFC power source, an energy harvester and a micropotentiostat. The following sections describe the development of individual components, component combination into an integrated system, and system performance.

Review—Two-Dimensional Layered Materials for Energy Storage Applications

Abstract: Rechargeable batteries are most important energy storage devices in modern society with the rapid development and increasing demand for handy electronic devices and electric vehicles. The higher surface-to-volume ratio two-dimensional (2D) materials, especially transition metal dichalcogenides (TMDCs) and transition metal carbide/nitrite generally referred as MXene, have attracted intensive research activities due to their fascinating physical/chemical properties with extensive applications. One of the growing applications is to use these 2D materials as potential electrodes for rechargeable batteries and electrochemical capacitors. This review is an attempt to summarize the research and development of TMDCs, MXenes and their hybrid structures in energy storage systems. © The Author(s) 2016. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives 4.0 License (CC BY-NC-ND, http://creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial reuse, distribution, and reproduction in any medium, provided the original work is not changed in any way and is properly cited. For permission for commercial reuse, please email: oa@electrochem.org. [DOI: 10.1149/2.0051611jss] All rights reserved.

Editors' Choice—On the Radiation Tolerance of AlGaN/GaN HEMTs

Abstract: The radiation tolerance of AlGaN/GaN high electron mobility transistors (HEMTs) fabricated on high quality, low threading dislocation density (TDD) ammonothermal GaN and hydride vapor phase epitaxy GaN substrates was studied and compared to the radiation response of devices on SiC substrates where the TDD is 104 times higher. Hall and transport measurements were performed as a function of 2 MeV proton fluence. The threading dislocation density had no effect on the radiation response. Comparing the results with published data reveals that almost all irradiated GaN-based HEMTs respond to radiation damage similarly regardless of differences in initial film quality, device structure, aluminum mole fraction, etc. AlGaAs/GaAs HEMTs are also shown to behave similarly but are around ten times more sensitive to radiation damage than GaN-based HEMTs. Known values of the displacement energy thresholds in GaN and GaAs are used to calculate that 36% fewer defects are created in GaN than in GaAs, which is too small to cause a 1000% difference in radiation sensitivity between GaN- and GaAs-based HEMTs. An alternative explanation is proposed in which the piezoelectric field at the AlGaN/GaN interface causes scattered carriers to be reinjected into the 2DEG channel, thereby mitigating some of the harmful radiation effects.

Editors' Choice—Rb2SiF6:Mn4+ and Rb2TiF6:Mn4+ Red-Emitting Phosphors

Abstract: Rb-based hexafluoride red-emitting phosphors, Rb2SiF6:Mn4+ and Rb2TiF6:Mn4+, were synthesized by the coprecipitation method. Optical microscopy observation, X-ray diffraction (XRD) measurement, photoluminescence (PL) analysis, PL excitation (PLE) spectroscopy, and luminescence decay characteristics measurements were used to study the structural and optical properties of the phosphors. The photographs of the bulk samples showed clear crystallographic habits originating from the cubic and trigonal symmetries of the Rb2SiF6 and Rb2TiF6 hosts, respectively, in agreement with the XRD results. The phosphors exhibited an intense narrow-band Mn4+ (2Eg → 4A2g) red emission with internal quantum efficiencies higher than 90% upon blue light excitation. The Franck−Condon analysis of the PLE data yielded the Mn4+ intra-d-shell transitions to occur at ~2.47 eV (~2.34 eV) for the 4A2g → 4T2g transition and at ~2.86 eV (~2.83 eV) for the 4A2g → 4T1g transition in the Rb2SiF6:Mn4+ (Rb2TiF6:Mn4+) phosphor. Temperature dependence of the PL spectra from T = 20 to 500 K gave the quenching temperature values (Tq's) at which the PL intensity has fallen to half its maximum value to be Tq ~ 490 and ~450 K for Rb2SiF6:Mn4+ and Rb2TiF6:Mn4+, respectively, promising for use as high-temperature stable phosphors in solid-state lighting applications.

Combined Surface Activated Bonding Technique for Low-Temperature Cu/Dielectric Hybrid Bonding

Abstract: Cu/dielectric hybrid bonding at low temperatures of no more than 200◦C remains challenging because of the different features of CuCu and dielectric-dielectric (such as SiO2-SiO2) bonding. This paper reports a combined surface activated bonding (SAB) technique for low-temperature Cu-Cu, SiO2-SiO2, and SiO2-SiNx bonding. This technique involves a combination of surface irradiation using a Si-containing Ar beam and prebonding attach-detach process prior to bonding in vacuum. Wafer bonding experiments were conducted at either room temperature or 200◦C. Results of bonding strength measurements, transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS) observations, and X-ray photoelectron spectroscopy (XPS) analysis were reported and discussed to understand the present combined SAB technique. © 2016 The Electrochemical Society. [DOI: 10.1149/2.0201607jss] All rights reserved.

Using DFT Methods to Study Activators in Optical Materials

Abstract: Density functional theory (DFT) calculations of various activators (ranging from transition metal ions, rare-earth ions, ns2 ions, to self-trapped and dopant-bound excitons) in phosphors and scintillators are reviewed. As a single-particle ground-state theory, DFT calculations cannot reproduce the experimentally observed optical spectra, which involve transitions between multi-electronic states. However, DFT calculations can generally provide sufficiently accurate structural relaxation and distinguish different hybridization strengths between an activator and its ligands in different host compounds. This is important because the activator-ligand interaction often governs the trends in luminescence properties in phosphors and scintillators, and can be used to search for new materials. DFT calculations of the electronic structure of the host compound and the positions of the activator levels relative to the host band edges in scintillators are also important for finding optimal host-activator combinations for high light yields and fast scintillation response. Mn4+ activated red phosphors, scintillators activated by Ce3+, Eu2+, Tl+, and excitons are shown as examples of using DFT calculations in phosphor and scintillator research.

Efficient light-emitting electrochemical cells using small molecular weight, ionic, host-guest systems

Abstract: This work has been supported by the Spanish Ministry of Economy and Competitiveness (MAT2014-55200).

Improved Vertical GaN Schottky Diodes with Ion Implanted Junction Termination Extension

Abstract: The realization of selectively implanted p-type regions in GaN as well as an understanding of processing effects that cause carrier type conversion are key enabling steps for vertical GaN devices. Here, GaN Schottky barrier diodes (SBDs) with edge termination formed by either a field plate or junction termination extension (JTE) achieved by ion implanted and symmetric multicycle rapid thermal annealing (SMRTA) are presented. The devices with JTE exhibited substantially reduced leakage currents and improved turn-on characteristics. This is attributed to the elimination of the plasma process steps associated with the deposition and patterning of the field oxide layer required in a field plate process. The breakdown characteristics were studied by electroluminescence imaging, and is indicative of avalanche behavior. The realization of vertical GaN devices with low reverse leakage and ion implanted termination regions represents a key step for future power electronic devices.