Secondary Ion Mass Spectrometry Results Research Articles

A High‐Resolution Versatile Focused Ion Implantation Platform for Nanoscale Engineering

The ability to spatially control and modify material properties on the nanoscale, including within nanoscale objects themselves, is a fundamental requirement for the development of advanced nanotechnologies. The development of a platform for nanoscale advanced materials engineering (P‐NAME) designed to meet this demand is demonstrated. P‐NAME delivers a high‐resolution focused ion beam system with a coincident scanning electron microscope and secondary electron detection of single‐ion implantation events. The isotopic mass‐resolution capability of the P‐NAME system for a wide range of ion species is demonstrated, offering access to the implantation of isotopes that are vital for nanomaterials engineering and nanofunctionalization. The performance of the isotopic mass selection is independently validated using secondary ion mass spectrometry (SIMS) for a number of species implanted into intrinsic silicon. The SIMS results are shown to be in good agreement with dynamic ion implantation simulations, demonstrating the validity of this simulation approach. The wider performance capabilities of P‐NAME, including sub‐10 nm ion beam imaging resolution and the ability to perform direct‐write ion beam doping and nanoscale ion lithography, are also demonstrated.

Comparison of AMS, TIMS, and SIMS techniques for determining uranium isotope ratios in individual particles.

The determination of isotope ratios in individual uranium particles is very important for nuclear safeguards. In this work, accelerator mass spectrometry (AMS), thermal ionization mass spectrometry (TIMS), and secondary ion mass spectrometry (SIMS) were applied to isotope ratio analysis of individual uranium particles and compared in terms of background, measurement accuracy, and efficiency. Several individual uranium particles (1-7μm) from certified reference materials were used as samples. The results show that the average values of blank counting rate of 235 U for AMS, FT-TIMS (FT: fission track), SEM-TIMS (SEM: scanning electron microscope), and SIMS were 7.3, 7.8, 2.7 and 2.2cps, respectively. The relative error of 234 U/235 U and 234 U/236 U isotope ratios of the particles from U200 for AMS were within 10% and 20%, whereas the results of FT-TIMS and SIMS were within 5% and 10%, respectively. The relative error and external precision of 234 U/238 U and 235 U/238 U of the particles from U850 for the method of AMS, SEM-TIMS, and SIMS were within 10% and 5%, respectively. For 236 U/238 U, the average values of the relative error and external precision measured by AMS were within 5%, which measured by SEM-TIMS and SIMS were all within 10%. AMS has advantages in measuring 236 U/238 U. The measurement time of AMS and SEM-TIMS was shorter than that of FT-TIMS and longer than that of SIMS. It is considered that AMS and SEM-TIMS have a certain development prospect, and it is necessary to research deeply.

Development towards stable chlorine isotope measurements of astromaterials using the modified Middleton source of an accelerator mass spectrometer

The volatile element Cl can be lost during the formation and evolution of planetary bodies, leading to fractionation of its two stable isotopes 35Cl and 37Cl. Chlorine isotope variations (reported as δ37Cl in parts per thousand (‰) relative to Standard Mean Ocean Chloride, SMOC) are documented to exceed 80‰ between different lunar rock samples and have been variably interpreted as the fingerprint of degassing during accretion, magma ocean, or volcanic portions of lunar history. The large intersample and intrasample variations observed by both bulk isotope ratio mass spectrometry (IRMS) and in-situ secondary ion mass spectrometry (SIMS) methods are difficult to interpret in part because of a paucity of bulk Cl isotope measurements. This lack of high-precision bulk data is due to the relative rarity of IRMS laboratories capable of making these high precision measurements on small samples of precious planetary materials such as those returned by human or robotic exploration. Here we present a new method for performing high precision δ37Cl measurements using the modified Middleton ion source of an existing accelerator mass spectrometer. For samples with as little as 1 μg Cl–the equivalent of 2–4 mm3 of a typical lunar rock sample–the average cathode accuracy is ∼1‰. Cathode reproducibility is typically ∼1‰ (2σ) for samples with at least 10 μg of Cl, increasing to ∼3–6‰ for aliquots with ∼1–2 μg Cl, similar to published SIMS results and sufficient to study astromaterials from the Moon, Mars, or 4 Vesta, which have tens of ‰ observed variations.

Correlation between electrical properties and growth dynamics for Si-doped Al-rich AlGaN grown by metal-organic chemical vapor deposition

Correlation between electrical properties and growth dynamics for Si-doped AlGaN with Al mole fraction above 60% has been investigated. It is found that the electron concentration decreases significantly when decreasing the growth rate, while the electron mobility experiences a non-monotonic process of increasing at first and then decreasing. Combination of secondary ion mass spectroscopy and panchromatic cathodoluminescence results, reveals that the evolution of electrical properties mainly originates from compensation of III vacancy (VIII) to Si dopant, making VIII-nSi complexes, i.e., the concentrations of VIII-nSi complexes increase with decreasing the growth rate, implying high growth rate principle is vital for n-AlGaN.

Metabolism in action: stable isotope probing using vibrational spectroscopy and SIMS reveals kinetic and metabolic flux of key substrates.

Microbial communities play essential functions which drive various ecosystems supporting animal and aquatic life. However, linking bacteria with specific metabolic functions is difficult, since microbial communities consist of numerous and phylogenetically diverse microbes. Stable isotope probing (SIP) combined with single-cell tools has emerged as a novel culture-independent strategy for unravelling microbial metabolic roles and intertwined interactions in complex communities. In this study, we applied Raman and Fourier-transform infrared (FT-IR) spectroscopies, secondary ion mass spectrometry (SIMS) with SIP to probe the rate of 13C incorporation in Escherichia coli at 37 and 25 °C. Our results indicate quantitative enrichment and flow of 13C into E. coli at various time points. Multivariate and univariate analyses of Raman and FT-IR data demonstrated distinctive 13C concentration-dependent trends that were due to vibrational bands shifting to lower frequencies and these shifts were a result of incubation time and metabolic rate. SIMS results were in complete agreement with the spectroscopy findings, and confirmed the detected levels of 13C incorporation into microbial biomass at the investigated conditions. Having established that FT-IR and Raman spectroscopy with SIP can measure metabolism kinetics in this simple system, we have applied the kinetics concept to study the metabolism of phenol by Pseudomonas putida and metabolic interactions within a two-species consortia with E. coli that could not degrade phenol. Raman spectroscopy combined with SIP identified quantitative shifts in P. putida due to temporal assimilation of phenol. Although E. coli was unable to grow on phenol, in co-culture with P. putida, general metabolic probing using deuterated water for SIP revealed that E. coli displayed increasing metabolic activity, presumably due to cross feeding from metabolites generated by P. putida. This study clearly demonstrates that Raman and FT-IR combined with SIP provide rapid and sensitive detection of carbon incorporation rates and microbial interactions. These novel findings may guide the identification of primary substrate consumers in complex microbial communities in situ, which is a key step towards the characterisation of novel genes, enzymes and metabolic flux analysis in microbial consortia.

Growth and Characterizations of GeSn Films with High Sn Composition by Chemical Vapor Deposition (CVD) Using Ge2H6 and SnCl4 for Mid-IR Applications

Epitaxial GeSn films present several advantages which can provide the semiconductor devices with higher frequency operation [1-2]. GeSn films have been theoretically and experimentally shown to become direct bandgap materials with about 6-10% of Sn composition [3-7]. This opens the room for potential optoelectronic applications such as amplifiers, lasers, photodetectors [8-13]. The red shift in the absorption due to the reduction of band-gap of the GeSn film enables the optoelectronic devices to operate in the mid-infrared (MIR) region [14-16]. However, it is very challenging to incorporate Sn into Ge crystal lattice due to the extremely limited solid solubility of Sn [17]. Therefore, low temperature growth techniques were developed to create non-equilibrium growth conditions so as to incorporate higher Sn content in high quality GeSn layer. In this work, the epitaxial growth of GeSn alloys with Sn content varying from 3.4 to 11% using Ge2H6 and SnCl4 as precursors in a CVD system under reduced pressure condition is reported in detail. Characterisation results are also presented. High quality GeSn film with Sn content as high as 11% has been achieved.The growth process consists of Ge buffer layer growth followed by GeSn epitaxial growth: 1 µm of Ge buffer layer was grown on 6“ Si wafers in the CVD reactor. Prior to actual growth of Ge, the wafer was baked in H2 at 1000°C for 2 min to remove the native oxide. 10% Ge2H6 diluted in H2 was used as Ge precursor, and H2 worked as the carrier gas. The Ge growth was then carried out at 400°C for ~10 min to target a thickness of 1 µm. The wafers were annealed soon after growth at ~850°C for another 20 min in H2 environment to reduce the threading dislocation density (TDD). Subsequently, SnCl4 was introduced into the chamber. The chamber temperature was kept below 350°C. Table I summarizes the growth conditions for some of the grown GeSn films. GeSn film with Sn content as high as 11% has been achieved successfully.Atomic force microscope (AFM) images in Figure 1(a)-(d) show that the RMS value of the surface roughness of the GeSn films is 0.52, 1.09, 0.95 and 0.91 nm for sample 1 to 4, respectively. The comprehensive studies showed that higher SnCl4flow rate, lower Ge2H6flow rate or higher growth temperature resulted in higher surface roughness (Figure 1(e)-(g)). Secondary ion mass spectrometry (SIMS) results shown in Figure 2 confirm the existence of Sn element and the Sn content. For sample 1 to 4, the Sn content is 3.4, 8, 10 and 11%, respectively.The X-Ray Diffraction (XRD) results of GeSn films are shown in Figure 3(a). It revealed that the growth temperature played the most important role in varying the Sn content in the GeSn film. The Sn content value extracted from rocking curve aligned well with the SIMS data. From the RSM mapping shown in Figure 3(b), sample 3 has a film relaxation of ~49% and compressive strain of ~0.7% based on models reported in the literatures [18-20].Etching pits density (EPD) measurements were carried out to study the threading dislocation density (TDD) of the GeSn films. Figure 4(a) shows the scanning electron microscope (SEM) image of sample 3 (Ge0.9Sn0.1) after EPD. The TDD of Ge0.9Sn0.1 is ~3.33 x 107cm-2. Figure 4(b) summarised that the TDD increases as Sn content increases which is due to the increased lattice mismatch, moreover, rapid thermal annealing (RTA) does not effectively reduce the TDD of the GeSn film.Raman analysis results are shown in Figure 5. The peak position of sample 3 (Ge0.9Sn0.1) is at 293.7 cm-1. Sn content and compressive strain were calculated to be 10 and 0.7%, respectively, by using the models in literature [21]. The values are consistent with those from XRD RSM and SIMS results. Figure 6 shows the absorption coefficient of GeSn films from ellipsometry analysis. Sn-induced red shift is observed. The cut off wavelength of the absorption extends from ~1,500 nm for bulk Ge to ~2,175 nm for sample 2 (Ge0.92Sn0.08), and it enables the devices fabricated to work in the MIR region.High resolution transmission electron microscopy (HRTEM) images in Figure 7(a)-(b) confirm that Ge buffer layer has a thickness of ~916.4 nm, while the thickness of GeSn film in sample 3 is ~175.9 nm. Figure 7(c) shows the selected area electron diffraction (SAED) pattern of the GeSn film and it concludes that the film is single crystalline.We would like to acknowledge support from the National Research Foundation Singapore under the Competitive Research Program (NRF-CRP19-2017-01). Figure 1

LIBS study of ITER relevant tungsten–oxygen coatings exposed to deuterium plasma in Magnum-PSI

We discuss the applicability of laser induced breakdown spectroscopy (LIBS) for deuterium retention analysis in compact and porous tungsten-oxide (W-O) coatings. Deuterium loading was performed by exposing the coatings to deuterium plasma in Magnum-PSI linear plasma device.The deuterium signals obtained by ex-situ LIBS had sufficiently good signal-to-noise ratio for reliable separation of essentially broadened hydrogen and deuterium lines as well as for comparison of the lateral and depth distributions of deuterium in the coatings. Strong deuterium signal was obtained for the first laser shot which corresponded to the surface layer of the W-O coatings whereas deeper in the coating the signal decreased to noise level. In addition, the deuterium signal was highest in the central region of compact W-O coating. For both coatings, depth profiles of elements obtained by LIBS match with those recorded by secondary ion mass spectroscopy (SIMS) in the lateral direction along the sample surface. The results of LIBS and SIMS results were supported by Scanning Electron Microscopy (SEM) and X-ray diffraction (XRD) data which showed that the exposure to deuterium plasma resulted in remarkable changes in the surface morphology along the sample surface. The study demonstrates the LIBS potential in deuterium retention measurements in plasma facing components.

Pressure-Dependent Behavior of Defect-Modulated Band Structure in Boron Arsenide.

The recent observation of unusually high thermal conductivity exceeding 1000 W m-1 K-1 in single-crystal boron arsenide (BAs) has led to interest in the potential application of this semiconductor for thermal management. Although both the electron/hole high mobilities have been calculated for BAs, there is a lack of experimental investigation of its electronic properties. Here, a photoluminescence (PL) measurement of single-crystal BAs at different temperatures and pressures is reported. The measurements reveal an indirect bandgap and two donor-acceptor pair (DAP) recombination transitions. Based on first-principles calculations and time-of-flight secondary-ion mass spectrometry results, the two DAP transitions are confirmed to originate from Si and C impurities occupying shallow energy levels in the bandgap. High-pressure PL spectra show that the donor level with respect to the conduction band minimum shrinks with increasing pressure, which affects the release of free carriers from defect states. These findings suggest the possibility of strain engineering of the transport properties of BAs for application in electronic devices.

Band Alignment at the Al2O3/β-Ga2O3 Interface with CHF3 Treatment**Supported by the Guangdong Province Key Technologies Research and Development Program (Grant No. 2019B010128001), the National Natural Science Foundation of China (Grant No. 61774041), and the Shanghai Science and Technology Innovation Program (Grant No. 19520711500).

The energy band alignment at the atomic layer deposited Al2O3/β-Ga2O3 interface with CHF3 treatment was characterized by x-ray photoelectron spectroscopy and secondary ion mass spectrometry (SIMS). With additional CHF3 plasma treatment, the conduction band offset increases from 1.95±0.1 eV to 2.32±0.1 eV; and the valence band offset decreases from 0.21±0.1 eV to −0.16±0.1 eV. As a result, the energy band alignment changes from type I to type II. This energy band alignment transition could be attributed to the downshift of the core-level of Ga 3d, resulting from the Ga–F bond formation in the F-rich interfacial layer, which is confirmed by the SIMS results.

BBr3 as a boron source in plasma-assisted molecular beam epitaxy

Boron is a difficult material to use in a molecular beam epitaxy (MBE) reactor due to its high melting point as a pure compound. Consequently, there is interest in exploring alternative sources for B in MBE. In this paper, the authors detail the construction and operation of a novel BBr3 injection system for plasma-assisted MBE growth and show results for BGaN thin films grown using readily available low purity BBr3 as a proof of concept for the source. The BBr3 system enables the growth of coherent BGaN films with a concentration up to 3% B on the group III site and thicknesses up to 280 nm as determined by high resolution x-ray diffraction. Atom probe tomography and secondary ion mass spectroscopy results of a B0.03Ga0.97N film indicate a high level of Br impurity on the order of 1 × 1019 atoms/cm3 and atmospheric contamination consistent with a low purity source. BBr3 is successful as a B source for high crystal quality BGaN films; however, the Br incorporation from the source limits the applications for this material.

Mechanism of surface uranium hydride formation during corrosion of uranium

Uranium is widely used in the nuclear industry and it is well known that uranium hydride, UH3, forms when uranium is exposed to air. The associated volume change during this transition can cause the surface region to crack, compromising structural integrity. Here, hydriding regions beneath hydride surface craters are studied by secondary ion mass spectroscopy (SIMS) and X-ray photoelectron spectroscopy (XPS). Our results indicate that strain transition regions exist, which are induced by hydrogen with a certain thickness between hydride craters and the uranium bulk. The SIMS and XPS results suggest that hydrogen exists covalently with uranium and oxygen in these transition regions. A micro-scale induction period model based on the transition region and previous hydriding models is developed. Within the so-called micro-scale induction period, hydrogen diffuses and accumulates at particular sites before reaching the critical concentration required to form stoichiometric UH3 in the transition regions.

Crystallisation Phenomena of In₂O₃:H Films.

The crystallisation of sputter-deposited, amorphous In2O3:H films was investigated. The influence of deposition and crystallisation parameters onto crystallinity and electron hall mobility was explored. Significant precipitation of metallic indium was discovered in the crystallised films by electron energy loss spectroscopy. Melting of metallic indium at ~160 °C was suggested to promote primary crystallisation of the amorphous In2O3:H films. The presence of hydroxyl was ascribed to be responsible for the recrystallization and grain growth accompanying the inter-grain In-O-In bounding. Metallic indium was suggested to provide an excess of free electrons in as-deposited In2O3 and In2O3:H films. According to the ultraviolet photoelectron spectroscopy, the work function of In2O3:H increased during crystallisation from 4 eV to 4.4 eV, which corresponds to the oxidation process. Furthermore, transparency simultaneously increased in the infraredspectral region. Water was queried to oxidise metallic indium in UHV at higher temperature as compared to oxygen in ambient air. Secondary ion mass-spectroscopy results revealed that the former process takes place mostly within the top ~50 nm. The optical band gap of In2O3:H increased by about 0.2 eV during annealing, indicating a doping effect. This was considered as a likely intra-grain phenomenon caused by both (In0)O•• and (OH−)O• point defects. The inconsistencies in understanding of In2O3:H crystallisation, which existed in the literature so far, were considered and explained by the multiplicity and disequilibrium of the processes running simultaneously.

An in-depth investigation on the grain growth and the formation of secondary phases of ultrasonic-sprayed Cu2ZnSnS4 based thin films assisted by Na crystallization catalyst

Up to date, the absorber low grain growth and the secondary compound formation remain the main factors causing the poor performance of kesterite solar cells based on spray pyrolysis derived Cu2ZnSnS4 (pure sulfide CZTS = PS-CZTS) compared with other deposition techniques. As the sodium (Na) inclusion is known to play a crucial role in affecting the grain growth, we evaluated in this report a thorough investigation in depth of the effect of Na doping content on crystallinity and phase segregation. The PS-CZTS was deposited by ultrasonic spray pyrolysis while the Na incorporation was carried out by soaking the pre-heated sprayed films in a solution of an environmentally-friendly NaCl salt. The change in the investigated parameters was followed throughout the absorber by scanning electron microscopy, Raman spectroscopy with different excitation wavelengths, grazing incidence X-ray diffraction (GIXRD) with various incidence angles, energy dispersive X-ray spectroscopy (EDS), secondary ion mass spectrometry (SIMS) and UV–Vis spectrometry. Increasing Na content significantly improves the crystallinity and the compactness simultaneously, resulting in smoothest surfaces with grains up to 2 µm in size. The Na was uniformly diffused leading to similar grain growth throughout the films. Whatever the Na content, the deposited materials are mostly consisting of ketserite-type PS-CZTS, whereas the use of different Raman excitations reveal the presence of Cu2S, ZnS and ZnO as secondary compounds in trace forms. The depth profile analysis with GIXRD showed that no further phase segregation has occurred while the EDS and SIMS results showed uniform distribution of PS-CZTS′ constituents along the surface and within the deposited films. By combining Raman, GIXRD and EDS analysis, the secondary compounds were found to be mostly gathered in the upper absorber layer. The absorption coefficient and the band gap range around 104 cm−1 and 1.55 eV respectively, the optimal values required for photovoltaic energy conversion.The Na doping developed in this study has a promising potential to yield high quality sprayed absorbers and opens up access to opportunities in enhancing of sprayed PS-CZTS thin film solar cell performances.

Peculiarities of the current-voltage and capacitance-voltage characteristics of plasma etched GaN and their relevance to n-GaN Schottky photodetectors

The impact of reactive ion etching (RIE) induced damage on the optoelectronic properties of GaN epitaxial layers and the photoresponse of Schottky detectors is investigated. Plasma induced surface damage in epitaxial layers is noticed which leads to a significant reduction of the intensity of the photoluminescence signal and also the photoresponse of detector devices post dry etching process. Electrical characterization of Au/Ni/GaN Schottky diodes along with secondary ion mass spectroscopy results indicate that the ion bombardment induced damage is mostly confined close to the surface of the GaN layer. It is found that the current-voltage characteristics of Schottky contacts on pristine n-GaN layers can be understood by considering a model based on the thermionic emission of carriers across the junction. However, the same is not possible in the case of plasma etched samples where the involvement of the thermionic field emission of carriers is essential. It is proposed that the RIE process leads to the generation of nitrogen vacancies in strongly localized domains near the surface. Such vacancies act as shallow donors shifting the Fermi level into the conduction band, thus enabling the tunnelling of carriers across the junction. However, this is not evident in capacitance-voltage characteristics since the damage is much prior to the depletion edge and is confined to extremely small domains. A method for the recovery of dry etch induced damage through O2 plasma treatment is demonstrated which is found to be very effective in improving the post-etch surface morphology and also the optoelectronic properties of etched GaN epitaxial layers. The spectral response of the Schottky photodetector is seen to degrade by 90% due to the plasma etching process. However, the same can be recovered along with an enhancement of the deep ultraviolet response of the detector after O2 plasma treatment of etched layers within the RIE chamber. The understanding developed here is crucial for the optimization of the RIE process and is found to be very helpful in recovery of damage caused by the dry etching process.

Constructing Desired Vertical Component Distribution Within a PBDB-T:ITIC-M Photoactive Layer via Fine-Tuning the Surface Free Energy of a Titanium Chelate Cathode Buffer Layer.

Rationally controlling the vertical component distribution within a photoactive layer is crucial for efficient polymer solar cells (PSCs). Herein, fine-tuning the surface free energy (SFE) of the titanium(IV) oxide bis(2,4-pentanedionate) (TOPD) cathode buffer layer is proposed to achieve a desired perpendicular component distribution for the PBDB-T:ITIC-M photoactive layer. The Owens-Wendt method is adopted to precisely calculate the SFE of TOPD film jointly based on the water contact angle and the diiodomethane contact angle. We find that the SFE of TOPD film increases as the annealing temperature rises, and the subtle SFE change causes the profound vertical component distribution within the bulk region of PBDB-T:ITIC-M. The results of secondary-ion mass spectroscopy visibly demonstrate that the TOPD film with an SFE of 48.71 mJ/cm2, which is very close to that of the ITIC film (43.98 mJ/cm2), tends to form desired vertical component distribution. Consequently, compared with conventional bulk heterojunction devices, the power conversion efficiency increases from 9.00 to 10.20% benefiting from the short circuit current density increase from 14.76 to 16.88 mA/cm2. Our findings confirm that the SFE adjustment is an effective way of constructing the desired vertical component distribution and therefore achieving high-efficiency PSCs.

Ionizing radiation effects on the thermal stability of deuterium trapping in reaction bonded SiC

SiC materials are prime candidates for flow channel inserts in the dual coolant lithium lead blanket concept. Flow channel inserts made of SiC will be exposed to tritium from the Li transmutation, as well as to neutron and gamma radiation. Hence a critical issue for future fusion devices is to clarify hydrogen isotope behaviour in SiC under such conditions. The objective of the work presented here is to study the effect of ionizing radiation on the deuterium trapping in Reaction Bonded SiC in similar conditions as reactor materials. This effect is evaluated by studying the influence of ionizing radiation on deuterium trapping (for both implanted and loaded SiC samples). Moreover, it is investigated how deuterium trapping may be modified by displacement damage. The ionizing radiation effect on absorption has also been evaluated for samples pre-damaged by self-ion irradiation. The irradiation and implantation experiment have been carried out at the CMAM-UAM accelerator, and the Danfysik implanter and 2 MeV Van de Graaff electron accelerator at CIEMAT. Samples are analysed by thermal desorption spectroscopy and secondary ion mass spectrometry (SIMS) to clarify the mechanisms involved in the trapping processes, depending on the different experimental conditions. The results for the deuterium loaded samples indicate that absorption is increased by ionizing radiation. When samples are pre-damaged by C+4 ions, deuterium absorption is increased in the form of Si-D according to SIMS results. Furthermore, the effect of ionizing radiation after deuterium implantation is an enhancement of the deuterium released from SiC. The deuterium release observed in this case is forming hydrocarbons during irradiations.

Depth analysis of Al/ZrC interfaces using SIMS and x‐ray reflectivity

Al/ZrC/Al/W multilayer structure is suitable for waveguide applications in the hard X‐ray region in order to confine the wave field in a nanometer‐thick layer. Intermixing of Al at the interfaces is a serious problem to achieve the expected performances from an experimentally grown multilayer. In the present study, the effect of a carbon capping layer in a C/Al/ZrC/Al/W waveguide structure is determined using time of flight secondary ion mass spectrometry (SIMS) and soft X‐ray reflectivity techniques. Structural parameters i.e., density, thickness, and roughness of the layers are determined using soft X‐ray reflectivity data. SIMS results indicate that the Al diffusion towards the top of the stack is responsible for the formation of a wide and asymmetric interfaces in the waveguide structure.

(Electronics and Photonics Division Award Address) Technological Issues and Design Rules of Electrodes for High-Efficiency GaN-Based Light-Emitting Diodes

Group III-nitride-based LEDs are of great technological importance because of their applications including display, solid-state lighting, automotive headlight, and water/air purification. For these applications, the attainment of high external quantum efficiency (EQE) of LEDs is essential. On the one hand, their applications necessitate the fabrication of different geometry LEDs, such as lateral-type, vertical-type, and flip-chip LEDs, which require different design rules of electrodes and current spreaders. To maximize the EQE, it is important to increase the light extraction efficiency (LEE). In addition, the improvement of current injection (by forming low contact resistivity) and current spreading is vital for the fabrication of high-efficiency LEDs. It is well known that lateral LEDs suffer from the light extraction limit due to the absorption of the metal contacts, such as n and p contacts, and bond pads. In addition, p-GaN has characteristic problems, which make it difficult to achieve device-quality ohmic contacts with the specific contact resistance lower than ~10–4 Ωcm2 because of the difficulty in growing a highly doped p-GaN and the absence of appropriate metals or conducting oxides having work function larger than that of p-GaN. Thus, for the formation of ohmic and reflective contacts to p-GaN, also serving as light extractor and/or current spreader, transparent conducting oxides (TCOs)-based and Ag-based schemes have been extensively investigated. In this talk, we present methods of enhancing current injection efficiency through the formation of transparent and reflective ohmic electrodes. Design rules are suggested to enhance current injection efficiency, such as the control of surface Fermi-level through surface treatment, barrier tunneling by forming interfaces with inhomogeneous Schottky barriers, the modification of Schottky barrier heights (SBHs), control of native defects in GaN surface regions. Furthermore, nanostructures, such as Ag nano-dots (NDs) and Ag nano-wires (NWs), are used to enhance the light output powers of GaN-based LEDs. Finally, on the basis of scanning transmission electron microscopy, X-ray photoemission spectroscopy, secondary ion mass spectroscopy and Auger electron spectroscopy results, possible ohmic formation and electrical degradation mechanisms are described and discussed.

Oxygen isotope analysis of the eyes of pelagic trilobites: Testing the application of sea temperature proxies for the Ordovician

The oxygen isotope composition of well-preserved trilobite eye calcite, retaining its original optical properties, represents a possible source of information on Paleozoic sea temperatures. Species of the epipelagic telephinid genera Carolinites and Opipeuterella from strata of Early to Middle Ordovician age in Spitsbergen and Australia were analyzed, and compared with benthic asaphid species. Scanning electron microscope (SEM), cathodoluminescence (CL), electron microprobe and Electron Backscatter Diffraction (EBSD) techniques were used to assess eye preservation prior to isotope analysis. Some apparently well-preserved eyes are identified from the Valhallfonna (Spitsbergen) and Emanuel (Australia) formations. The eyes show a wide variation in δ18O values: −6.2‰ to −9.8‰ for the Valhallfonna Formation, −3.2‰ to −10.4‰ for the Emanuel Formation, and −3.6‰ to −7.4‰ for the Horn Valley Siltstone (Australia). Intra-eye Secondary Ion Mass Spectrometry (SIMS) isotope results reveal an even larger range in δ18O in some specimens (δ18O of −2.4‰ to −10.4‰), suggesting that the trilobite eyes have undergone cryptic recrystallization. A sub-set of trilobite cuticle from the three formations were analyzed for their carbonate clumped isotope compositions (Δ47), and yielded crystallization temperatures above 50°C, consistent with diagenetic alteration. The SIMS and Δ47 results suggest that classic preservation assessment protocols for the stable isotope study of deep-time carbonate samples may be insufficient, especially for these techniques. There is a need for extensive microstructural characterization of lower Paleozoic biogenic carbonates, by techniques including EBSD, SIMS and Δ47, before their stable isotope signatures can be used with certainty in paleoclimate studies.

Tunnel-type β-FeOOH cathode material for high rate sodium storage via a new conversion reaction

We have investigated a tunnel-type β-FeOOH cathode material for rapid sodium storage. Rietveld refinement of the X-ray diffraction (XRD) data obtained for β-FeOOH indicated that the structure was stabilized into [2 × 2] hollandite tunnel structure, and the adhesion of the β-FeOOH onto carbon nanotubes (CNTs) led to a high electrical conductivity of 3Scm−1. As a result, the β-FeOOH/CNTs composite electrode showed excellent electrode performance, with a discharge capacity of 205 mAh g−1 and a coulombic efficiency of 88.5% in the voltage range of 1.1–4V during the first cycle, 131 mAh g−1 after 200 cycles, and a capacity retention of 70% over 300 cycles at 10C (1920mAg−1). Based on the XRD, X-ray absorption, and time-of-flight secondary-ion mass spectroscopy results, we suggest a new conversion mechanism for the β-FeOOH cathode material in Na cells, namely, FeOOH + Na+ + e- → FeO + NaOH. This conversion reaction produces NaOH as a byproduct, and the reaction is fairly reversible even at 10 C-rates.