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Hydrogen is the key element to accomplish a carbon-free based economy. Here, the first evidence of plasmonic gallium (Ga) nanoantennas is provided as nanoreactors supported on sapphire (α-Al2O3) acting as direct plasmon-enhanced photocatalyst for hydrogen sensing, storage, and spillover. The role of plasmon-catalyzed electron transfer between hydrogen and plasmonic Ga nanoparticle in the activation of those processes is highlighted, as opposed to conventional refractive index-change-based sensing. This study reveals that, while temperature selectively operates those various processes, longitudinal (LO-LSPR) and transverse (TO-LSPR) localized surface plasmon resonances of supported Ga nanoparticles open selectivity of localized reaction pathways at specific sites corresponding to the electromagnetic hot-spots. Specifically, the TO-LSPR couples light into the surface dissociative adsorption of hydrogen and formation of hydrides, whereas the LO-LSPR activates heterogeneous reactions at the interface with the support, that is, hydrogen spillover into α-Al2O3 and reverse-oxygen spillover from α-Al2O3. This Ga-based plasmon-catalytic platform expands the application of supported plasmon-catalysis to hydrogen technologies, including reversible fast hydrogen sensing in a timescale of a few seconds with a limit of detection as low as 5 ppm and in a broad temperature range from room-temperature up to 600 °C while remaining stable and reusable over an extended period of time. The authors thank all of the students and colleagues in their groups who were actively involved with nanoparticles research. M.L., Y.G., and F.M. have received funding from the European Union's Horizon 2020 Research and Innovation Program under Grant Agreement No. 899598—PHEMTRONICS. F.M. acknowledges MINECO (Spanish Ministry of Economy and Competitiveness, project PGC2018-096649-B-100).

Reconfigurable plasmonics is driving an extensive quest for active materials that can support a controllable modulation of their optical properties for dynamically tunable plasmonic structures. Here, polymorphic gallium (Ga) is demonstrated to be a very promising candidate for adaptive plasmonics and reconfigurable photonics applications. The Ga sp-metal is widely known as a liquid metal at room temperature. In addition to the many other compelling attributes of nanostructured Ga, including minimal oxidation and biocompatibility, its six phases have varying degrees of metallic character, providing a wide gamut of electrical conductivity and optical behavior tunability. Here, the dielectric function of the several Ga phases is introduced and correlated with their respective electronic structures. The key conditions for optimal optical modulation and switching for each Ga phase are evaluated. Additionally, we provide a comparison of Ga with other more common phase-change materials, showing better performance of Ga at optical frequencies. Furthermore, we first report, to the best of our knowledge, the optical properties of liquid Ga in the terahertz (THz) range showing its broad plasmonic tunability from ultraviolet to visible-infrared and down to the THz regime. Finally, we provide both computational and experimental evidence of extension of Ga polymorphism to bidimensional twodimensional (2D) gallenene, paving the way to new bidimensional reconfigurable plasmonic platforms. F.M. acknowledges MICINN (Spanish Ministry of Science and Innovation) through project PGC2018-096649-B-100.

In order to exploit gallium’s (Ga) rich polymorphism in the design of phase-change plasmonic systems, accurate understanding of the dielectric function of the different Ga-phases is crucial. The dielectric dispersion profiles of those phases appearing at atmospheric pressure have been reported in the literature, but there is no information on the dielectric function of the high-pressure Ga-phases. Through first principles calculations we present a comprehensive analysis of the interdependence of the crystal structure, band structure, and dielectric function of two high-pressure Ga phases (Ga(II) and Ga(III)). The plasmonic behavior of these high-pressure Ga-phases is compared to those stable (liquid- and α-Ga) and metastable (β-, γ- and δ-Ga) at atmospherics pressure. This analysis can have important implications in the design of pressure-driven phase-change Ga plasmonic devices and high-pressure SERS substrates. European Commission (GA692034); Army Research Laboratory (W911NF-17-2-0023); Ministerio de Economía, Industria y Competitividad, Gobierno de España (PGC2018-096955-B-C41); Ramón y Cajal Grant (RyC-2013-12515); SODERCAN through the Research Vice-rectorate of the Universidad de Cantabria (4JU2864661). M.L. acknowledges the support of the European Commission under the H2020 grant TWINFUSYON (GA692034). Y.G. and F.M. acknowledges the support by the Army Research Laboratory under Cooperative Agreement Number W911NF-17-2-0023 and by SODERCAN (Sociedad para el Desarrollo de Cantabria) and the Research Vicerrectorate of the University of Cantabria through project 4JU2864661. Y.G. thanks the University of Cantabria for her FPU grant. P.G.-F. and J.J. acknowledge financial support from the Spanish Ministry of Economy and Competitiveness through grant number PGC2018-096955-B-C41, and P.G.-F. acknowledges support from Ramón y Cajal grant no. RyC-2013-12515.

Plasmonic metal nanoparticle (NP)&ndash, graphene (G) systems are of great interest due their potential role in applications as surface-enhanced spectroscopies, enhanced photodetection, and photocatalysis. Most of these studies have been performed using noble metal NPs of silver and gold. However, recent studies have demonstrated that the noble metal&ndash, graphene interaction leads to strong distortions of the graphene sheet. In order to overcome this issue, we propose the use of Ga NPs that, due to their weak interaction with graphene, do not produce any deformation of the graphene layers. Here, we analyze systems consisting of Ga NP/G/metal sandwich coupling structures, with the metal substrate being, specifically, copper (Cu) and nickel (Ni), i.e., Ga NP/G/Cu and Ga NPs/G/Ni. We experimentally show through real-time plasmonic spectroscopic ellipsometry and Raman spectroscopy measurements of the quenching of the Ga NP localized surface plasmon resonance (LSPR) depending on the wetting of the graphene by the Ga NPs and on the electron transfer through graphene. Theoretical finite-difference time-domain (FDTD) simulations supportively demonstrate that the LSPR in such sandwich structures strongly depends on the contact angle of the NP with graphene. Finally, we also provide evidence of the electron transfer from the Ga NPs into the graphene and into the metal substrate according to the work function alignments. These considerations about the contact angle and, consequently, geometry and wetting of the metal NPs on graphene, are useful to guide the design of those plasmonic systems to maximize electromagnetic enhancement.

In x Al(1−x)N (InAlN) thin films, lattice-matched to GaN with an In composition of ∼17%, are of interest for GaN-based devices. However, InAlN thin films grown by molecular beam epitaxy exhibit a characteristic lateral composition modulation, or nanocolumnar microstructure, with an Al-rich center region and an In-rich boundary. The mechanism driving the formation of this microstructure remains unknown. To date, the structural characterization of the nanocolumn domain size and its associated compositional variation is challenging, requiring either transmission electron microscopy or atomic probe microscopy. We show that the nanocolumnar microstructure can be characterized using x-ray diffraction and is associated with increased diffuse scattering. Using this technique, we show that the development of the microstructure is dependent on growth temperature.

Gallium nanoparticles have been deposited on various substrates of technological interest. The interaction with the substrate leads to two plasmon resonance modes that can be tuned from the UV to the visible and infrared. Large polarization-dependent splitting of plasmon resonance modes arises from the broken symmetry of hemispherical gallium nanoparticles on substrates; this splitting depends on interaction with the substrate, on the angle of illumination and on the polarization state of light. These splitting plasmon modes are also dependent on the liquid and solid phases of gallium.

We describe the purification of xenon from traces of the radioactive noble gas radon using a cryogenic distillation column. The distillation column is integrated into the gas purification loop of the XENON100 detector for online radon removal. This enabled us to significantly reduce the constant $^{222}$Rn background originating from radon emanation. After inserting an auxiliary $^{222}$Rn emanation source in the gas loop, we determined a radon reduction factor of R > 27 (95% C.L.) for the distillation column by monitoring the $^{222}$Rn activity concentration inside the XENON100 detector.

Magnesium-based films and nanostructures are being studied in order to improve hydrogen reversibility, storage capacity, and kinetics, because of their potential in the hydrogen economy. Some challenges with magnesium (Mg) samples are their unavoidable oxidation by air exposure and lack of direct in situ real time measurements of hydrogen interaction with Mg and MgO surfaces and Mg plasmonic nanoparticles. Given these challenges, the present article investigates direct interaction of Mg with hydrogen, as well as implications of its inevitable oxidation by real-time spectroscopic ellipsometry for exploiting the optical properties of Mg, MgH2 and MgO. The direct hydrogenation measurements have been performed in a reactor that combines a remote hydrogen plasma source with an in situ spectroscopic ellipsometer, which allows optical monitoring of the hydrogen interaction and results in optical property modification. The hydrogen plasma dual use is to provide the hydrogen-atoms and to reduce barriers to heterogeneous hydrogen reactions. European Commission under the H2020 grant TWINFUSYON (GA692034). Army Research Laboratory under Cooperative Agreement Number W911NF-17-2-0023. SODERCAN (Sociedad para el Desarrollo de Cantabria) through the Research Vicerrectorate of the University of Cantabria.

Gallium (Ga), a group III metal, is of fundamental interest due to its polymorphism and unusual phase transition behaviours. New solid phases have been observed when Ga is confined at the nanoscale. Herein, we demonstrate the stable coexistence, from 180 K to 800 K, of the unexpected solid γ-phase core and a liquid shell in substrate-supported Ga nanoparticles. We show that the support plays a fundamental role in determining Ga nanoparticle phases, with the driving forces for the nucleation of the γ-phase being the Laplace pressure in the nanoparticles and the epitaxial relationship of this phase to the substrate. We exploit the change in the amplitude of the evolving surface plasmon resonance of Ga nanoparticle ensembles during synthesis to reveal in real time the solid core formation in the liquid Ga nanoparticle. Finally, we provide a general framework for understanding how nanoscale confinement, interfacial and surface energies, and crystalline relationships to the substrate enable and stabilize the coexistence of unexpected phases.

Single-stranded DNA immobilized on an III-V semiconductor is a potential high-sensitivity biosensor. The chemical and electronic changes occurring upon the binding of DNA to the InAs surface are essential to understanding the DNA-immobilization mechanism. In this work, the chemical properties of DNA-immobilized InAs surfaces were determined through high-resolution X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS). Prior to DNA functionalization, HF- and NH(4)OH- based aqueous etches were used to remove the native oxide from the InAs surface. The initial chemical state of the surface resulting from these etches were characterized prior to functionalization. F-tagged thiolated single-stranded DNA (ssDNA) was used as the probe species under two different functionalization methods. The presence of DNA immobilized on the surface was confirmed from the F 1s, N 1s, and P 2p peaks in the XPS spectra. The presence of salt had a profound effect on the density of immobilized DNA on the InAs surface. To study the interfacial chemistry, the surface was treated with thiolated ssDNA with and without the mercaptohexanol molecule. An analysis of the As 3d and In 3d spectra indicates that both In-S and As-S are present on the surface after DNA functionalization. The amount of In-S and As-S was determined by the functionalization method as well as the presence of mercaptohexanol during functionalization. The orientation of the adsorbed ssDNA is determined by polarization-dependent NEXAFS utilizing the N K-edge. The immobilized ssDNA molecule has a preferred tilt angle with respect to the substrate normal, but with a random azimuthal distribution.

This paper reports on the growth of Si nanowires (NWs) by SiH 4 /H 2 plasmas using the non-noble Ga-nanoparticles (NPs) catalysts. A comparative investigation of conventional Si-NWs vapour–liquid–solid (VLS) growth catalyzed by Au NPs is also reported. We investigate the use of a hydrogen plasma and of a SiH 4 /H 2 plasma for removing Ga oxide shell and for enhancing the Si dissolution into the catalyst, respectively. By exploiting the Ga NPs surface plasmon resonance (SPR) sensitivity to their surface chemistry, the SPR characteristic of Ga NPs has been monitored by real time spectroscopic ellipsometry in order to control the hydrogen plasma/Ga NPs interaction and the involved processes (oxide removal and NPs dissolution by volatile gallium hydride). Using in situ laser reflectance interferometry the metal catalyzed Si NWs growth process has been investigated to find the effect of the plasma activation on the growth kinetics. The role of atomic hydrogen in the NWs growth mechanism and, in particular, in the SiH 4 dissolution into the catalysts, is discussed. We show that while Au catalysts because of the re-aggregation of NPs yields NWs that do not correspond to the original size of the Au NPs catalyst, the NWs grown by the Ga catalyst retains the diameter dictated by the size of the Ga NPs. Therefore, the advantage of Ga NPs as catalysts for controlling NWs diameter is demonstrated.

We investigate the chemistry and kinetics of the surface functionalization of InAs with hemin aimed at demonstrating novel hemin-functionalized InAs planar resistors or molecularly controlled resistors (MOCSERs) with subparts per billion sensitivity to NO at 300 K. The high performance is a result of the strong coupling of the intrinsic surface-confined two-dimensional electron gas (2DEG) at the InAs surface with the surface-attached hemin molecules and their selective interactions with NO. The presence of the surface 2DEG in InAs is highly advantageous for sensing and obviates the need for high surface area to volume geometries conferred by nanowire structures. The chemistry and kinetics of surface attachment processes are investigated using X-ray photoelectron spectroscopy, atomic force microscopy, and spectroscopic ellipsometry, showing that the covalent attachment of hemin to both In and As sites modifies the band-bending of the InAs. In addition, the selective, reversible electronic interaction between hemin and NO molecules increases the InAs resistance, reducing the NO during the interaction. In addition to presenting the sensor performance characteristics, a chemical model based on the electron transfer from the 2DEG of InAs to the iron center of hemin, which reduces Fe(III) to the higher NO affinity Fe(II) state, is proposed.

Gallium–magnesium (Ga–Mg) core–shell nanoparticles were synthesized by sequential evaporation of gallium and magnesium molecular beams on sapphire substrates. The surface plasmon resonance properties of this novel bimetallic system are demonstrated and investigated in a broad spectral range, from the near-UV to the visible and to the near-IR by in situ real-time spectroscopic ellipsometry. Both the mean NP ensemble diameter and the relative core–shell radii affect the resultant plasmon resonance, and this study shows that the resonance can be red-shifted and tuned into the near-IR by increasing the extent of the Mg-shell, while limiting the overall nanoparticle size, promoting this novel, nanomaterials system for applications of bifunctional plasmonic-catalytic nanoparticles with optical properties tunable across a broad spectral range. Experimental real-time plasmonic results are corroborated by discrete dipole approximation calculations applied to core–shell nanoparticles supported on the substrate, whi...

Manipulating the properties of well understood materials systems for novel technological applications can be achieved by creating nanoscale structures and mixed compounds. The design of novel nanomaterials that underpin plasmonic applications drives the rapid progress in advancing plasmonic materials over the last few years. [ 1 , 2 ] Control of metal nanoparticle shape, [ 3 , 4 ] density, and spacing [ 5 ] is continually improving with novel synthetic techniques. Nevertheless, to continue advancing plasmonics, new metallic nanostructure systems must be developed that offer unique properties and are superior to Ag, [ 6 ] Au, [ 3 , 7 ] and mixtures thereof—the most widely exploited metals [ 8–10 ] and bimetallic systems. [ 11 ]

We report on enhanced control of the growth of lateral GaAs nanowires (NWs) embedded in epitaxial (100) GaAsBi thin films enabled by the use of vicinal substrates and the growth-condition dependent role of Bi as a surfactant. Enhanced step-flow growth is achieved through the use of vicinal substrates and yields unidirectional nanowire growth. The addition of Bi during GaAsBi growth enhances Ga adatom diffusion anisotropy and modifies incorporation rates at steps in comparison to GaAs growth yielding lower density but longer NWs. The NWs grown on vicinal substrates grew unidirectionally towards the misorientation direction when Bi was present. The III/V flux ratio significantly impacts the size, shape and density of the resulting NWs. These results suggest that utilizing growth conditions which enhance step-flow growth enable enhanced control of lateral nanostructures.

We investigate the molecular beam epitaxy (MBE) growth of GaAs1-xBix film using density functional theory with spin-orbit coupling to understand the growth of this film, especially the mechanisms of Bi incorporation. We study the stable adsorption structures and kinetics of the incident molecules (As₂ molecule, Ga atom, Bi atom, and Bi₂ molecule) on the (2 x 1)-Gasub||Bi surface and a proposed q(1 x 1)-Gasub||AsAs surface has a quasi-(1 x 1) As layer above the Ga-terminated GaAs substrate and a randomly oriented As dimer layer on top. We obtain the desorption and diffusion barriers of the adsorbed molecules and also the reaction barriers of three key processes related to Bi evolution, namely, Bi incorporation, As/Bi exchange, and Bi clustering. The results help explain the experimentally observed dependence of Bi incorporation on the As/Ga ratio and growth temperature. Furthermore, we find that As₂ exchange with Bi of the (2 x 1)-Gasub||Bi surface is a key step controlling the kinetics of the Bi incorporation. Finally, we explore two possible methods to enhance the Bi incorporation, namely, replacing the MBE growth mode from codeposition of all fluxes with a sequential deposition of fluxes and applying asymmetric in-plane strain to the substrate.

Graphene/metal heterojunctions are ubiquitous in graphene-based devices and, therefore, have attracted increasing interest of researchers. Indeed, the literature on the field reports apparently contradictory results about the effect of a metal on graphene doping. Here, we elucidate the effect of metal nanostructuring and oxidation on the metal work function (WF) and, consequently, on the charge transfer and doping of graphene/metal hybrids. We show that nanostructuring and oxidation of metals provide a valid support to frame WF and doping variation in metal/graphene hybrids. Chemical vapour-deposited monolayer graphene has been transferred onto a variety of metal surfaces, including d-metals, such as Ag, Au, and Cu, and sp-metals, such as Al and Ga, configured as thin films or nanoparticle (NP) ensembles of various average sizes. The metal-induced charge transfer and the doping of graphene have been investigated using Kelvin probe force microscopy (KPFM), and corroborated by Raman spectroscopy and plasmonic ellipsometric spectroscopy. We show that when the appropriate WF of the metal is considered, without any assumption, taking into account WF variations by nanostructure and/or oxidation, a linear relationship between the metal WF and the doping of graphene is found. Specifically, for all metals, nanostructuring lowers the metal WF. In addition, using gold as an example, a critical metal nanoparticle size is found at which the direction of charge transfer, and consequently graphene doping, is inverted.

Gallium has recently been demonstrated as a phase-change plasmonic material offering UV tunability, facile synthesis, and a remarkable stability due to its thin, self-terminating native oxide. However, the dense irregular nanoparticle (NP) ensembles fabricated by molecular-beam epitaxy make optical measurements of individual particles challenging. Here we employ hyperspectral cathodoluminescence (CL) microscopy to characterize the response of single Ga NPs of various sizes within an irregular ensemble by spatially and spectrally resolving both in-plane and out-of-plane plasmonic modes. These modes, which include hybridized dipolar and higher-order terms due to phase retardation and substrate interactions, are correlated with finite difference time domain (FDTD) electrodynamics calculations that consider the Ga NP contact angle, substrate, and native Ga/Si surface oxidation. This study experimentally confirms previous theoretical predictions of plasmonic size-tunability in single Ga NPs and demonstrates that the plasmonic modes of interacting Ga nanoparticles can hybridize to produce strong hot spots in the ultraviolet. The controlled, robust UV plasmonic resonances of gallium nanoparticles are applicable to energy- and phase-specific applications such as optical memory, environmental remediation, and simultaneous fluorescence and surface-enhanced Raman spectroscopies.

We have estimated the threading dislocation density and type via X-ray diffraction and Williamson–Hall analysis to elicit qualitative information directly related to the electrical and optical quality of GaN epitaxial layers grown by PAMBE on 4H- and 6H-SiC substrates. The substrate surface preparation and buffer choice, specifically: Ga flashing for SiC oxide removal, controlled nitridation of SiC, and use of AlN buffer layers all impact the resultant screw dislocation density, but do not significantly influence the edge dislocation density. We show that modification of the substrate surface strongly affects the screw dislocation density, presumably due to impact on nucleation during the initial stages of heteroepitaxy.

GaN is grown on Si-face 4H-SiC(0 0 0 1) substrates using remote plasma-assisted methods including metalorganic chemical vapour deposition (RP-MOCVD) and molecular beam epitaxy (MBE). Real time spectroscopic ellipsometry is used for monitoring all the steps of substrate pre-treatments and the heteroepitaxial growth of GaN on SiC. Our characterization emphasis is on understanding the nucleation mechanism and the GaN growth mode, which depend on the SiC surface preparation.

The growth of GaN by plasma assisted molecular beam epitaxy on GaN template substrates (GaN on sapphire) is investigated with in-situ multi-channel spectroscopic ellipsometry. Growth is performed under various Ga/N flux ratios at growth temperatures in the range 710–780 °C. The thermal roughening of the GaN template caused by decomposition of the surface is investigated through the termporal variation of the GaN pseudodielectric function over the temperature range of 650 °C to 850 °C. The structural, morphological, and optical properties are also discussed. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

GaN has been grown directly on the Si-face 4H-SiC(0001) substrates using remote plasma-assisted metal-organic chemical vapour deposition (RP-MOCVD) with UV-light irradiation. The effects of substrate pre-treatments and UV-photoirradiation of the growth surface on GaN nucleation and film morphology are investigated. Optical data from spectroscopic ellipsometry measurements and morphological data show an improvement in nucleation and material quality with UV-light irradiation.

Inverted metal-semiconductor-metal (I-MSM) photodetectors, which are thin-film MSMs with the growth substrate removed and fingers on the bottom of the device (to eliminate finger shadowing to enhance responsivity), are reported herein for high-speed high-efficiency large-area photodetectors. Reported herein are the highest speed vertically addressed large-area (40-/spl mu/m diameter) photodetectors reported to date, which operate with a responsivity of 0.16 A/W and a full-width half-maximum of less than 5 ps. Materials, fabrication processes, heterogeneous integration, and characterization of I-MSM photodetectors are presented in this paper, as measured using a fiber-based electrooptic sampling system. These large-area photodetectors are ideal for vertically addressed high-speed optical links which need alignment-tolerant packaging for cost sensitive applications.

GaN epitaxial layers have been grown by molecular beam epitaxy (MBE). Low (200 degreesC) and high (700 degreesC) temperature nitrided sapphire substrates, and GaN and AlN buffer layers have been used as the platforms for investigating polarity selection processes of GaN grown by RF-MBE, aimed at presenting a comprehensive understanding of the issues of GaN polarity and growth conditions. The results show that Ga-polar GaN results from the use of AlN buffer layers, and when the substrate/bulk interface has a Ga- or Al-rich interface. In contrast, a large density of N-polar domains is found when GaN epitaxial films are grown on GaN buffer layers on sapphire when nitrided at 200 degreesC. The impact of the polarity of GaN on the interaction of GaN surfaces with atomic hydrogen is also studied. A different reaction rate and reaction extent is found for N- and Ga-polar GaN with atomic hydrogen, with N-polar GaN exhibiting greater reactivity. It is also demonstrated that the reactivity of GaN with atomic hydrogen coupled with surface potential measurements exploiting Kelvin probe microscopy is an alternative approach to determining the polarity of GaN films, and identifying the presence of inversion domains. Finally, the impact of GaN polarity on electrical properties of AlGaN/GaN heterojunction structures is also shown

While growth of wide bandgap semiconductor materials on crystalline oxides (sapphire, lithium gallate, lithium aluminate, zinc oxide and others) has become routine, growth of crystalline oxides on wide bandgap materials remains challenging and minimally explored. The potential payoff in terms of enhanced device performance, increased functionality and reliability warrants examining this option. This presentation aims at targeting key areas, where crystalline oxides could improve wide bandgap semiconductor device performance. Some of these include the use of ferroelectric oxides for power switching applications, oxides with anisotropic dielectric constants for high voltage termination and oxides with large electric flux density near breakdown. Unique polarization engineered structures are described that are enabled by using lithographically defined poled regions in a ferroelectric substrate. The desired crystalline oxide properties, potential implementation challenges and potential pitfalls will be discussed.

Statistical experimental design was used to explore the effects of the HEMT channel growth parameters on device performance. A 2/sup 2/ full-factorial central composite circumscribed Box-Wilson design with three center points was implemented. The growth parameters under investigation were the channel growth temperature and interface formation, both of which greatly impact device operation. Interface formation was defined as the method used to form the InAs/AlSb interface.

Strained AlxIn1-xAs/Ga0.47In0.53As heterojunction N-p+ diodes and heterojunction bipolar transistors (HBTs) have been grown on InP substrates by solid-source molecular-beam epitaxy, fabricated, and characterized. To determine the effects of the conduction-band discontinuity at the emitter-base hetero-junction on turn-on voltage and ideality factor, a strained Al0.7In0.3As layer is inserted in the emitter near the base. Changes in transport across the junction are observed as a function of the strained-layer position and thickness. These results were used to implement strained emitter HBTs.

The InP-based AlInAs-GaAsSb-GaInAs heterojunction bipolar transistors (HBTs) have been grown by solid-source molecular-beam epitaxy (SSMBE). Since the AlInAs-GaAsSb heterojunction has a type-II (staggered) band lineup, the conduction-band discontinuity is negligible at 300 K (10 meV). Thus, the turn-on voltage is significantly lower than that of an AlInAs-GaInAs HBT even without compositional grading of the emitter-base junction. A self-aligned process was used to fabricate large area devices. The measured turn-on voltage and collector-emitter offset were 0.36 V and 0.23 V, respectively, with a DC current gain of approximately 25 and ideality factors of ηC = 1.01 and ηB = 1.1 at JC = 10 kA/cm2 collector-current density.

Metal nanoparticle (NP)-graphene multifunctional platforms are of great interest for exploring strong light graphene interactions enhanced by plasmons and for improving performance of numerous applications, such as sensing and catalysis. These platforms can also be used to carry out fundamental studies on charge transfer, and the findings can lead to new strategies for doping graphene. There have been a large number of studies on noble metal Au-graphene and Ag-graphene platforms that have shown their potential for a number of applications. These studies have also highlighted some drawbacks that must be overcome to realize high performance. Here we demonstrate the promise of plasmonic gallium (Ga) nanoparticle (NP) graphene hybrids as a means of modulating the graphene Fermi level, creating tunable localized surface plasmon resonances and, consequently, creating high-performance surface-enhanced Raman scattering (SERS) platforms. Four prominent peculiarities of Ga, differentiating it from the commonly used noble (gold and silver) metals are (1) the ability to create tunable (from the UV to the visible) plasmonic platforms, (2) its chemical stability leading to long-lifetime plasmonic platforms, (3) its ability to n-type dope graphene, and (4) its weak chemical interaction with graphene, which preserves the integrity of the graphene lattice. As a result of these factors, a Ga NP-enhanced graphene Raman intensity effect has been observed. To further elucidate the roles of the electromagnetic enhancement (or plasmonic) mechanism in relation to electron transfer, we compare graphene-on-Ga NP and Ga NP-on-graphene SERS platforms using the cationic dye rhodamine B, a drug model biomolecule, as the analyte.

The role of graphene in enabling deoxidation of silver nanostructures, thereby contributing to enhance plasmonic properties and to improve the temporal stability of graphene/silver hybrids for both general plasmonic and metamaterials applications, as well as for surface enhanced Raman scattering (SERS) substrates, is demonstrated. The chemical mechanism occurring at the graphene-silver oxide interface is based on the reduction of silver oxide triggered by graphene that acts as a shuttle of electrons and as a kind of catalyst in the deoxidation. A mechanism is formulated, combining elements of electron transfer, role of defects in graphene, and electrochemical potentials of graphene, silver, and oxygen. Therefore, the formulated model represents a step forward from the simple view of graphene as barrier to oxygen diffusion proposed so far in literature. Single layer graphene grown by chemical vapor deposition is transferred onto silver thin fi lms, a periodic silver fi shnet structure fabricated by nanoimprint lithography, and onto silver nanoparticle ensembles supporting a localized surface plasmon resonance in the visible range. Through the study of these nanostructured graphene/Ag hybrids, the effectiveness of graphene in preventing and reducing oxidation of silver plasmonic structures, keeping silver in a metallic state over months at air exposure, is demonstrated. The enhanced and stable plasmonic properties of the silver-fi shnet/graphene hybrids are evaluated through their SERS response for detecting benzyl mercaptane.

Self-assembled, irregular ensembles of hemi, spherical Ga nanoparticles (NPs) were deposited on sapphire by molecular beam epitaxy. These samples, whose constituent unimodal or bimodal distribution of NP sizes was controlled T deposition time, exhibited. localized surface plasmon resonances tunable from the ultraviolet to the visible (UV/vis) spectral range. The optical response of each sample was characterized using a variable-angle spectroscopic ellipsometer, and the dielectric response of the ensemble of NPs on each sample was parametrized using Lorentz oscillators. From this, a relationship was found between NP size and the deduced Lorentzian parameters (resonant frequency, damping, oscillator strength) for most unimodal and bimodal samples at most frequencies and angles of incidence. However, for samples with a bimodal size distribution, Mueller matrix ellipsometry revealed nonspecular scattering at particular frequencies and angles suggesting a resonant interparticle coupling effect consistent with recently observed strong local field enhancements in the ultraviolet.

The growth and properties of alloys in the alternative quaternary alloy system GaAs1−y−zPyBiz were explored. This materials system allows simultaneous and independent tuning of lattice constant and band gap energy, Eg, over a wide range for potential near- and mid-infrared optoelectronic applications by adjusting y and z in GaAs1−y−zPyBiz. Highly tensile-strained, pseudomorphic films of GaAs1−yPy with a lattice mismatch strain of ∼1.2% served as the host for the subsequent addition of Bi. Lattice-matched alloy materials to GaAs were generated by holding y ∼ 3.3z in GaAs1−y−zPyBiz. Epitaxial films with both high Bi content, z ∼ 0.0854, and a smooth morphology were realized with measured band gap energies as low as 1.11–1.01 eV, lattice-matched to GaAs substrates. Density functional theory calculations are used to provide a predictive model for the band gap of GaAs1−y−zPyBiz lattice-matched to GaAs.

InAlN films deposited by plasma-assisted molecular beam epitaxy exhibited a lateral composition modulation characterized by 10-12 nm diameter, honeycomb-shaped, columnar domains with Al-rich cores and In-rich boundaries. To ascertain the effect of this microstructure on its optical properties, room temperature absorption and photoluminescence characteristics of InxAl(1-x)N were comparatively investigated for indium compositions ranging from x = 0.092 to 0.235, including x = 0.166 lattice matched to GaN. The Stokes shift of the emission was significantly greater than reported for films grown by metalorganic chemical vapor deposition, possibly due to the phase separation in these nanocolumnar domains. The room temperature photoluminescence also provided evidence of carrier transfer from the InAlN film to the GaN template. (C) 2014 AIP Publishing LLC.

The drive for the heterogeneous integration of materials has led to significant advances in materials and device processing, and in the understanding of defect production and control during epitaxy. Heterogeneous integration is driven by microelectronic and packaging trends, and the advent of new materials, such as GaN, that do not possess native substrates. During the last decade, these objectives led to research in the development of compliant substrates. While the ideal compliant substrate concept and implementation may be flawed, this research has certainly advanced materials integration technology. This paper will provide an overview of recent results in compliant substrate experiments and interpretation, and the related advancement of materials and device integration and packaging deriving from some of this research.

The microstructure of solid source molecular beam epitaxy (MBE) lattice-matched GaInP-GaAs heterostructures has been studied by transmission electron microscopy (TEM). It is shown that atomic-scale roughening occurs in the first several (∼five) interfaces, and beyond which roughening is developed into micrometer-scale. The {113} faceted roughening occurs in the GaInP-on-GaAs interfaces, leading to the formation of V-shaped grooves. Dislocation dipoles are observed in the top, facet surface and bottom regions of the grooves, and there is no obvious difference. For GaInP layers exhibiting atomic-scale roughening, a small number of dislocations, mostly 60°-type and in a dipole configuration, are confined in a zone of approximately 5.0 nm in width along the interface, and dislocations are rare deep inside the layers. For GaInP layers, in contrast, there is a high density of dislocations in both the regions, and it has remarkable interfacial roughening. Based on their distribution and configurations, the dislocations are believed to result from the compositional modulation occurred in the GaInP layers. The interplay of roughening, dislocations and compositional modulation has been discussed in the light of the morphological features.

We present a comprehensive study of the electrical, optical, and structural properties of wurtzite GaN films grown under various initial growth conditions The GaN films were grown directly on sapphire substrates using GaN nucleation layers by a Riber 3200 system with a radio-frequency plasma source. In situ reflection high-energy electron diffraction (RHEED) reveals a strong correlation between nucleation conditions, including the nitridation step, and the final surface reconstruction of the GaN thin film. Well-defined reconstruction patterns, primarily (2 � 2) and(4 � 4), are observed for some of the nucleation conditions. Hall mobility, photoluminescence (PL), X-ray rocking curve data, and transmission electron microscopy (TEM) measurements are usedto interpret the observedrelationship. The results show that for the conditions investigated, an unreconstructed (1 � 1) surface after growth correlates with improved electrical, optical, andstructural properties. The surface reconstructedthin film exhibits a strong columnar growth with inversion domains (IDs). We attribute the degraded characteristics to the presence of a mixture of both polarities in the films with reconstruction. r 2001 Publishedby Elsevier Science B.V.

The structural properties and surface morphology of AlGaN/GaN structures grown on LiGaO2 (LGO), sapphire, and hydride vapor phase epitaxy (HVPE)-grown GaN templates are compared. AlGaN grown on LGO substrates shows the narrowest x-ray full width at half maximum (FWHM) for both symmetric 〈00.4〉 and asymmetric 〈10.5〉 reflections. Atomic force microscopy (AFM) analysis on AlGaN surfaces on LGO substrates also show the smoothest morphology as determined by grain size and rms roughness. The small lattice mismatch of LGO to nitrides and easily achievable Ga-polarity of the grown films are the primary reasons for the smoother surface of AlGaN/GaN structure on this alternative substrate. Optimizations of growth conditions and substrate preparation results in step flow growth for an AlGaN/GaN structure with 300 A thick Al0.25Ga0.75N on 2.4 μm thick GaN. A high III/V flux ratio during growth and recently improved polishing of LGO substrates aids in promoting two dimensional step flow growth. The GaN nucleation layer directly on the LGO substrate showed no evidence of mixed phase cubic and hexagonal structure that is typically observed in the nucleation buffer on sapphire substrates. Cross-sectional high-resolution transmission electron microscopy (HRTEM) was performed on an AlGaN/GaN heterostructure grown on LGO. The atomic arrangement at the AlGaN/GaN interface was sharp and regular, with locally observed monolayer and bilayer steps.

Compliant substrates offer significant promise as a new approach for strain management in semiconductors. The primary application is to produce device-quality highly mismatched materials on dissimilar substrates. Various implementations and processes for achieving substrate compliancy have been proposed and demonstrated. These include the use of twist-, glass-, and metal-bonds, and the use of engineered templates realized with laterally oxidized Al-containing alloys. A recent focus in our work has been on the growth of GaN on a novel and easily removable substrate — lithium gallate — for regrowth on a bonded GaN template.

Lithium gallate is receiving ever increasing attention as a possible candidate for III-nitride material growth due to its superior lattice match. This paper summarizes the recent, promising results from material growths on lithium gallate including aluminum, gallium and indium-nitride alloys. Structural, optical and electrical diagnostics are presented. The use of this material could result in significant advantages for use in power electronic applications. Specifically, the present material quality is in some ways, superior to all other current substrate technologies since very high quality, extremely thin nitride material can be produced. The areas for which lithium gallate still lags other technologies is also discussed. Issues that directly effect electronic devices, such as overcoming the limited thermal conductivity via heat pipes or substrate removal, the possibilities of achieving higher p-type doping and higher indium concentrations through the use of low growth temperatures, obtaining single polarity material with nearly no mosaic spread in grain orientation, along with issues of substrate cost are discussed.

X-ray reflectivity studies of gallium arsenide quantum well structures have been undertaken to establish the effectiveness of the technique as a non-destructive means of investigating interdiffusion between layers in these structures. Layer thicknesses were obtained from the reflectivity data both directly and by the application of modelling techniques. In addition to the layers expected from the fabrication of the samples, the reflectivity data showed that a surface layer was present on the as-produced samples which was consistent with a native oxide. Anodic oxidation and annealing of the samples gave rise to marked changes in the reflectivity profiles consistent with interdiffusion at the interfaces. The feasibility of using anomalous dispersion to highlight the gallium distribution in the samples was demonstrated. It has been demonstrated that X-ray reflectivity can give information about the layers and the interdiffusion between layers of significant value to scientists interested in semiconductor device fabrication.

We report on the optical and structural properties of strained InAlAs/InAsxP1−x heterostructures grown by solid source molecular beam epitaxy. The incorporation of As4 into strained InAsP has been studied using x-ray diffraction and photoluminscence measurements. We find that the incorporation of As decreases as x in the InAsxP1−x increases due, presumably, to increased strain. In addition, we have examined the effect of interface growth conditions and barrier material on the quantum well properties. The 4.2 K photoluminescence spectra of InAlAs/InAsP multi-quantum wells grown with different As and P flux switching conditions at the interfaces were compared with that of higher quality InP/InAsP multi-quantum wells. This comparison shows that the interface quality of InAlAs/InAsP multi-quantum wells is highly dependent on the switching conditions at the interfaces, especially the exposure time of InAsP surface to an As4 overpressure. This sensitivity is due to the As–P exchange reaction at the interfaces of InAlAs/InAsP.

We present for the first time a three-dimensional (3D) Si CMOS interconnection system consisting of three layers of optically interconnected hybrid integrated Si CMOS transceivers. The transceivers were fabricated using 0.8-/spl mu/m digital Si CMOS foundry circuits and were integrated with long wavelength InP-based emitters and detectors for through-Si vertical optical interconnections. The optical transmitter operated with a digital input and optical output with operation speeds up to 155 Mb/s. The optical receiver operated with an external optical input and a digital output up to 155 Mb/s. The transceivers were stacked to form 3D through-Si vertical optical interconnections and a fabricated three layer stack demonstrated optical interconnections between the three layers with operational speed of 1 Mb/s and bit-error rate of 10/sup -9/.

The effect of peripheral substitution on the surface activity of poly(aryl ether) dendrimers on water are being examined by measurements of the II-A isotherms and by neutron reflectometry. Contrast variation is used to define the molecular film structure at the air-water interface at high compressions. Monolayers are present for the methyl ester-substituted and cyanide-substituted dendrimers, whilst a partial second layer is formed for the unsubstituted [G-4]-OH dendrimer. The effect of temperature on these molecular films is also being investigated. © 1998 Elsevier Science B.V. All rights reserved.

InAs heterojunction bipolar transistors (HBTs) are promising candidates for low power and high frequency (THz) device applications due to their small bandgap, high electron mobility, and high saturation drift velocity. However, doping limits such as the trade-off between desired low intentional n-type concentrations and unintentional doping, and the realization of high p-type concentrations, must still be considered in device design and synthesis. In order to observe the impact of intentional and unintentional n-type doping on diode electrical properties, InAs-based homojunction diodes have been grown on InAs substrates by solid-source molecular beam epitaxy (SSMBE) and were subsequently fabricated and characterized.

Self-assembled arrays of hemispherical gallium nanoparticles deposited by molecular beam epitaxy on a sapphire support are explored as a new type of substrate for ultraviolet plasmonics. Spin-casting a 5 nm film of crystal violet upon these nanoparticles permitted the demonstration of surface-enhanced Raman spectra, fluorescence, and degradation following excitation by a HeCd laser operating at 325 nm. Measured local Raman enhancement factors exceeding 10(7) demonstrate the potential of gallium nanoparticle arrays for plasmonically enhanced ultraviolet detection and remediation.

Nanoscale charge transfer is important to both the frontier of fundamental science and to applications in molecular electronics, photonic, electronic, optical, imaging, catalysis, sensing devices, photovoltaics, and energy savings and storage. For many of those applications, plasmonic metal nanoparticles are with molecules and/or semiconductors, where nanoparticles act as an electron-bridge. Metal nanoparticles experience charge transfer either by a hopping mechanism involving transient charging of the nanoparticle and/or by electron storage and delocalization among/in the nanoparticles. This electron transfer affects the electron density in the metal, and the plasmon resonance, and therefore, can be detected spectroscopically. This chapter discusses examples of exploitation of spectroscopic ellipsometry as a real time research tool that advance description and understanding of charge transfer phenomena involving (i) chemisorption of metals on semiconductor surfaces, (ii) growth of plasmonic nanoparticles on polar semiconductors, (iii) coupling plasmonic nanoparticles to graphene, and (iv) charge transfer between plasmonic nanoparticles and biomolecules, activating sensing processes.

The practical efficacy of technologically promising metals for use in ultraviolet plasmonics (3–6 eV) is assessed by an exhaustive numerical analysis. This begins with estimates of the near- and far-field electromagnetic enhancement factors of isolated hemispherical and spherical metallic nanoparticles deposited on typical dielectric substrates like sapphire, from which the potential of each metal for plasmonic applications may be ascertained. The ultraviolet plasmonic behavior of aluminum, chromium, copper, gallium, indium, magnesium, palladium, platinum, rhodium, ruthenium, titanium, and tungsten was compared with the well-known behavior of gold and silver in the visible. After exploring this behavior for each metal as a function of nanoparticle shape and size, the deleterious effect caused by the metal’s native oxide is considered, and the potential for applications such as surface-enhanced Raman spectroscopy, accelerated photodegradation and photocatalysis is addressed.

Compliant substrates allow a new approach to the growth of strained epitaxial layers, in which part of the strain is accommodated in the substrate. In this article, compliant substrates are discussed and a new compliant substrate technology based on bonded thin film substrates is introduced. This technology has several advantages over previously published methods, including the ability to pattern both the top and bottom of the material. A new concept enabled by this compliant substrate technology,strain-modulated epitaxy, will be introduced. Using this technique, the properties of the semiconductor material can be controlled laterally across a substrate. Results of two experiments are presented in which low composition InxGa1−xAs was grown by molecular beam epitaxy on GaAs compliant substrates at thicknesses both greater than and less than the conventional critical thickness. It was found that for t > tc, there was an inhibition of defect production in the epitaxial films grown on the compliant substrates as compared to those grown on conventional reference substrates. For t < tc, photoluminescence and x-ray diffraction show the compliant substrates to be of excellent quality and uniformity as compared to conventional substrates.