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Localized surface phonon polaritons (LSPhPs) can be implemented to engineer light-matter interactions through nanoscale patterning for a range of midinfrared application spaces. However, the polar material systems studied to date have mainly focused on simple designs featuring a single element in the periodic unit cell. Increasing the complexity of the unit cell can serve to modify the resonant near-fields and intra- and inter-unit-cell coupling as well as to dictate spectral tuning in the far-field. In this work, we exploit more complicated unit-cell structures to realize LSPhP modes with additional degrees of design freedom, which are largely unexplored. Collectively excited LSPhP modes with distinctly symmetric and antisymmetric near-fields are supported in these subarray designs, which are based on nanopillars that are scaled by the number of subarray elements to ensure a constant unit-cell size. Moreover, we observe an anomalous mode-matching of the collective symmetric mode in our fabricated subarrays that is robust to changing numbers of pillars within the subarrays as well as to defects intentionally introduced in the form of missing pillars. This work therefore illustrates the hierarchical design of tailored LSPhP resonances and modal near-field profiles simultaneously for a variety of IR applications such as surface-enhanced spectroscopies and biochemical sensing.

ACS photonics 4(7), 1753-1760 (2017). doi:10.1021/acsphotonics.7b00321, Published by ACS, Washington, DC

Gap surface plasmons (GSPs) serve a diverse range of plasmonic applications, including energy harvesting, communications, molecular sensing, and optical detection. GSPs may be realized where tightly spaced plasmonic structures exhibit strong spatial overlap between the evanescent fields. We demonstrate that within similar, nested geometries that the near-fields of the GSPs within the individual nanostructures are hybridized. This creates two or more distinct resonances exhibiting near-field distributions extended over adjacent spatial regions. In contrast, dissimilar, nested structures exhibit two distinct resonances with nominally uncoupled near-fields, resulting in two or more individual antenna resonance modes. We deploy plasmonic band structure calculations to provide insight into the type and degree of hybridization within these systems, comparing the individual components. This understanding can be used in the optimized design of polaritonic metamaterial structures for desired applications.

Monolayers of α,ω-dithiol oligo(phenyleneethynlene) molecules are critical to the field of molecular electronics because of their abilities to form bonds with many metallic surfaces and rectify current. In this study Fourier Transformation-Raman, surface-enhanced Raman scattering (SERS) spectroscopy and Tip-enhanced Raman Spectroscopy (TERS) were used to characterize a selectively oriented self-assembled monolayer of 4″-trimethylsilylethylsulfanyl-4,4′-bis-(phenyleneethynylene)benzenethiol (OPE′) on silver coated nanospheres. Selective orientation was achieved by synthesizing 4″-trimethylsilylethylsulfanyl-4,4′-bis-(phenyleneethynylene)benzene disulfide, which undergoes oxidative dissociation and covalently bonds to the metal surface. The Ag coated nanosphere surfaces were characterized by scanning electron microscopy (SEM), which showed a large area of surface charging. The SERS and TERS spectra show similar results; however, a greater enhancement was achieved with the TERS relative to the SERS spectra. Assignments of vibrational bands were based on DFT calculations performed at the B3LYP level with good agreement between theoretical and experimental values. An average percent difference of 2.5 cm −1 was obtained for the non-CH stretching frequencies and a scaling factor was not applied to theoretically generated frequencies. A red shift of the ν(C–S) peak at 1087 cm −1 was observed when OPE′ was adsorbed on a Ag surface. Vibrations specific to the trimethylsilylethyl (TMSE) group were visible in the TERS spectra, and disappear upon deprotection.

We have investigated the novel optical behavior of Ag metal deposited via plasma enhanced atomic layer deposition (PEALD). This material exhibits unexpected plasmonic properties and the plasmonic enhancement is dependent on the unique microstructure of the Ag film. Electromagnetic field simulations indicate that this optical behavior is due to air gaps that are an inherent property of the mosaic-like microstructure of the PEALD-grown Ag film. We have also investigated the effect of modifying these air gaps, and the development of flexible plasmonic materials, using this unique metamaterial.

Plasmonics provides great promise for nanophotonic applications. However, the high optical losses inherent in metal-based plasmonic systems have limited progress. Thus, it is critical to identify alternative low-loss materials. One alternative is polar dielectrics that support surface phonon polariton (SPhP) modes, where the confinement of infrared light is aided by optical phonons. SiC nanopillar arrays support such modes, exhibiting a dipolar resonance transverse to the nanopillar axis and a monopolar resonance associated with the longitudinal axis dependent upon the SiC substrate. Both exhibit exceptionally narrow linewidths (7–24 cm−1), with quality factors of 40–135, which exceed the theoretical limit of plasmonic systems, with extreme subwavelength confinement of (λres3/Veff)1/3 = 50–200. These observations promise to reinvigorate research in SPhP phenomena and their use for nanophotonic applications. Another approach is the use of hyperbolic materials, which have been a focus of the nanophotonics community for their potential to realize sub-diffractional imaging and focusing of light, and novel optical properties, such as a negative index of refraction. The recent observation that hexagonal boron nitride (hBN) is a natural, high efficiency hyperbolic material has led to a surge in research within this field. Due to the low-loss nature, van der Waals bonding and extreme crystal anisotropy, the hyperbolic polaritons within hBN are not only promising for novel applications within the mid-infrared, but is also extremely well suited for fundamental investigations into their resonant behaviors. We have used scattering near-field optical microscopy (s-SNOM) to directly probe the local surface electromagnetic fields of three-dimensionally confined nanostructures of hBN, reporting the first experimental observation of frequency dependent internal angular reflection within a hyperbolic nanostructure, a phenomenon previously theoretically predicted.

At low frequencies, metals behave as a perfect electric conducting (PEC) material and no longer exhibit plasmonic properties. By “spoofing” a metal, through the use of grooves or perforations, with feature sizes much smaller than the incident wavelength, metals can have an effective dielectric that allows plasmonic responses even in ranges outside the optical range. Additionally, while on a smooth metal film, surface plasmons will propagate along the surface, a patterned film can have localized plasmon resonance, as in the case of PEALD Ag, which has small gaps. Thus, spoof hybrid plasmonics can occur (a) in large conformal coatings (b) can allow for multi mode transmission, and (c) allow for both traveling and localized modes. Here, we explore the effects of randomness in ALD Ag, and also modify the optical response in metamaterial structures by changing the unit cell. This is explored using reflectance measurements and calculations, including band structure. This research has applications in energy harvesting, communications, and sensing.

It has been demonstrated that nanoresonators fabricated on the surface of polar dielectric materials, such as silicon carbide, are able to sustain plasmonic-like effects in the mid- to long- wave infrared spectral range with impressive figures of merit (Hillenbrand R, Taubner T, Keilmann F, Nature 418:159–162, 2002; Caldwell JD, Glembocki OJ, et al., Nano Lett 13:3690–3697, 2013; Wang T, Li P, et al., Nano Lett 13:5051–5055, 2013). Such phenomena is achieved by exploiting the TO and LO phonons to resonantly excite collective oscillations of bound lattice (Caldwell JD, Lindsay L, et al., Nanophotonics 4:2192–8614, 2015). The fact that these excitations are mediated by bound charges, rather than free charges - such as the case with plasmonic metals, results in extremely low optical losses and enhanced resonant phenomena. As such, polar dielectric nanoresonators may play a role in improving infrared nanophotonic technologies, such as waveguides, sources, near-field optics, solar cells, chemical sensors, biosensors, and photonic circuitry. However, fully realizing this potential, hinges on the ability to precisely control the near-field behavior of polar dielectric nanoresonators. In this work, we use a combination of optical measurements and finite element method simulations to investigate the far- and near-field resonant behavior of structurally-asymmetric, cuboidally-shaped, 4H-SiC nanoresonators with fixed height (\( h=950\ \mathrm{nm} \)), fixed length (\( l=400\ \mathrm{nm} \)), and varying width (\( w=400-6400\ \mathrm{nm} \)). Overall, we observe over 12 polarization-sensitive resonances that can be tuned across the Reststrahlen band of 4H-SiC (\( 796-964\ {\mathrm{cm}}^{-1} \)) (Ellis, C.T. et al. Scientific Reports 6:32959, 2016) by changing the nanopillar aspect ratio (\( AR=w/l=1-16 \)). Futhermore, we find that these resonances exhibit a wide range of near-field radiation patterns that vary from a simple transverse dipole mode that is preserved for all ARs to complex, high-order multipoles with modal profiles that evolve with aspect ratio.

Polar dielectrics have garnered much attention as an alternative to plasmonic metals in the mid- to long-wave infrared spectral regime due to their low optical losses. As such, nanoscale resonators composed of these materials demonstrate figures of merit beyond those achievable in plasmonic equivalents. However, until now, only low-order, phonon-mediated, localized polariton resonances, known as surface phonon polaritons (SPhPs), have been observed in polar dielectric optical resonators. In the present work, we investigate the excitation of 16 distinct high-order, multipolar, localized surface phonon polariton resonances that are optically excited in rectangular pillars etched into a semi-insulating silicon carbide substrate. By elongating a single pillar axis we are able to significantly modify the far- and near-field properties of localized SPhP resonances, opening the door to realizing narrow-band infrared sources with tailored radiation patterns. Such control of the near-field behavior of resonances can also impact surface enhanced infrared optical sensing, which is mediated by polarization selection rules, as well as the morphology and strength of resonator hot spots. Furthermore, through the careful choice of polar dielectric material, these results can also serve as the guiding principles for the generalized design of optical devices that operate from the mid- to far-infrared.

Nanowires (NWs)/Ag sheath composites were produced to investigate plasmonic coupling between vertically aligned NWs for surface-enhanced Raman scattering (SERS) applications. In this investigation, two types of vertical NW arrays were studied; those of ZnO NWs grown on nanosphere lithography patterned sapphire substrate via vapor-liquid-solid (VLS) mechanism and Si NW arrays produced by wet chemical etching. Both types of vertical NW arrays were coated with a thin layer of silver by electroless silver plating for SERS enhancement studies. The experimental results show extremely strong SERS signals due to plasmonic coupling between the NWs, which was verified by COMSOL electric field simulations. We also compared the SERS enhancement intensity of aligned and random ZnO NWs, indicating that the aligned NWs show much stronger and repeatable SERS signal than those grown in nonaligned geometries.

Periodic arrays of plasmonic nanopillars have been shown to provide large, uniform surface-enhanced Raman scattering (SERS) enhancements. We show that these enhancements are the result of the combined impact of localized and propagating surface plasmon modes within the plasmonic architecture. Here, arrays of periodically arranged silicon nanopillars of varying sizes and interpillar gaps were fabricated to enable the exploration of the SERS response from two different structures; one featuring only localized surface plasmon (LSP) modes and the other featuring LSP and propagating (PSP) modes. It is shown that the LSP modes determine the optimal architecture, and thereby determine the optimum diameter for the structures at a given incident. However, the increase in the SERS enhancement factor for a system in which LSP and PSP cooperatively interact was measured to be over an order of magnitude higher and the peak in the diameter dependence was significantly broadened, thus, such structures not only provide larger enhancement factors but are also more forgiving of lithographic variations.

The structure of inclusions and their influence on surface morphology, local strain, and basal plane dislocations were investigated in silicon carbide (SiC) epitaxial layers grown on 4° offcut substrates. On high-resolution x-ray topography images, strain fields were observed surrounding the inclusions. Ultraviolet photoluminescence images revealed the presence of strain-induced dislocations around the inclusions. Micro-Raman and microphotoluminescence spectroscopy showed that the inclusions exhibited a complex structure that consisted of 3C polytype regions and misoriented 4H polytype regions. The resulting lattice deformation typically propagates in the step-flow growth direction and causes distorted surface morphology.

α,ω-Dithiols are a useful class of compounds in molecular electronics because of their ability to easily adsorb to two metal surfaces, producing a molecular junction. We have prepared Ag nanosphere/oligo(phenyleneethynylene)/Ag sol (AgNS/OPE/Ag sol) and Ag nanowire/oligo(phenyleneethynylene)/Ag sol (AgNW/OPE/Ag sol) sandwiches to simulate the architecture of a molecular electronic device. This was achieved by self-assembly of OPE on the silver nanosurface, deprotection of the terminal sulfur, and deposition of Ag sol atop the monolayer. These sandwiches were then characterized by surface-enhanced Raman scattering (SERS) spectroscopy. The resulting spectra were compared to the bulk spectrum of the dimer and to the Ag nanosurface/OPE SERS spectra. The intensities of the SERS spectra in both systems exhibit a strong dependence on Ag deposition time and the results are also suggestive of intense interparticle coupling of the electromagnetic fields in both the AgNW/OPE/Ag and the AgNS/OPE/Ag systems. Three previously unobserved bands (1219, 1234, 2037 cm(-1)) arose in the SER spectra of the sandwiches and their presence is attributed to the strong enhancement of the electromagnetic field which is predicted from the COSMOL computational package. The 544 cm(-1) disulfide bond which is observed in the spectrum of solid OPE but is absent in the AgNS/OPE/Ag and AgNW/OPE/Ag spectra is indicative of chemisorption of OPE to the nanoparticles through oxidative dissociation of the disulfide bond.

Exposure to light or heat, or simply a dearth of fingerprint material, renders some latent fingerprints undetectable using conventional methods. We begin to address such elusive fingerprints using detection targeting photo- and thermally stable fingerprint constituents: surface-enhanced Raman spectroscopy (SERS). SERS can give descriptive vibrational spectra of amino acids, among other robust fingerprint constituents, and good sensitivity can be attained by improving metal-dielectric nanoparticle substrates. With SERS chemical imaging, vibrational bands' intensities recreate a visual of fingerprint topography. The impact of nanoparticle synthesis route, dispersal methodology-deposition solvent, and laser wavelength are discussed, as are data from enhanced vibrational spectra of fingerprint components. SERS and Raman chemical images of fingerprints and realistic contaminants are shown. To our knowledge, this represents the first SERS imaging of fingerprints. In conclusion, this work progresses toward the ultimate goal of vibrationally detecting latent prints that would otherwise remain undetected using traditional development methods.

Since it was determined that the formation and expansion of intrinsic stacking faults (SFs) induced a drift in the forward voltage (Vf) in 4H-SiC bipolar devices, significant effort has been made to understand the driving force causing SF motion as well as the various associated luminescence processes. The observation that annealing of faulted SiC devices and epilayers induced SF contraction and a recovery of the Vf drift enabled the studying of the impact of various parameters such as temperature, injection level and operation time upon SF motion, the Vf drift and luminescence within the same device. However, these observations in many cases contradicted the previously reported driving force models. Here we report on a basic driving force model explaining SF expansion in hexagonal SiC as well as discuss the observation of green luminescence from C-core partial dislocations bounding the SFs that may indicate an enhanced mobility of point defects within forward biased SiC pin diodes.

The structure of various inclusions in SiC epitaxial layers grown on 4o offcut substrates was investigated using three advanced techniques. Using micro-Raman spectroscopy, the observed inclusions exhibited a complex structure having either different SiC polytypes like 3C or 6H or they were misoriented 4H-SiC inclusions. The UVPL images showed dislocations and other extended defects around the inclusion-related defects, and strain fields were observed in the x-ray topographs near the defect sites. Spectral UVPL imaging shows features with varying luminescence inside the inclusion related defects which propagate and may cause deformation in the crystalline structure and lead to non-radiative recombination centers within the defect.

To understand the nature of various extended defects and their impact on the electronic or optoelectronic characteristics of semiconductor devices, the investigation of spectral properties is required. However, electroluminescence spectroscopy does not provide spatial or structural information. The lack of such information can lead to incorrect assignment of a luminescence band and therefore misinterpretation of the nature of the emitting defect. Here we report on the collection and analysis of real-color and spectrally selective monochromatic electroluminescence (EL) images from 4H-SiC PiN diodes. The former provides the approximate spectral properties from the color of the various defects with high spatial resolution, while the latter enables simultaneous collection of both structural and spectral properties from extended defects, thereby assisting in providing the correct assignment of the various spectral features observed. This effort enabled the observation of the formation of a green EL emission located at C-core partial dislocations (PDs) that occurred during the stacking fault (SF) expansion process.

We have investigated the growth of a number of nanowire systems, including Ga2O3, ZnO, InAs, as well as Ga2O3/ZnO hierarchical structures and dielectric core/metal sheath composites. We show that the growth of nanowires can occur by a number of growth mechanisms, even in the same system, and how this process can be tailored for specific applications, including plasmonics. We will show that dielectric/metal composite nanowires can exhibit very strong surface enhanced Raman (SERS) signals, when arranged in a random 3D geometry, which is due to nanowire crossings, and we will show the effects of crossing geometries on the strength of the SERS signal, which is compared with electric field calculations.

The nature of the background emission in surface enhanced Raman scattering is elucidated. A simple group of benzene ring based thiols is used to demonstrate the introduction of optically active interface states upon adsorption on Ag coated dielectric core nanowires. Excellent agreement is obtained with density of state (DOS) calculations for benzenethiol adsorbed on Ag. We also present evidence for photoluminescence enhanced resonant Raman scattering as an important component of surface enhanced Raman scattering.

Confocal μ-Raman was used to measure the operating temperatures in SiC MESFETS, AlGaN/GaN/SiC HEMT’s and 4H-SiC PiN diodes. Temperatures obtained from thermal imaging of the MESFETS compared well with those measured from Raman scattering. Operating temperatures were also obtained for large area PiN diode and it was shown that a single point at the center of the device can be used to measure the average temperature.

Oligo(phenyleneethynylene) (OPE) compounds have been identified as promising molecular electronic bridges. Self-assembled monolayers of 4″-trimethylsilylethylsulfanyl-4,4′-phenyleneethynylenebenzene thiol (OPE′) on Au were characterized by surface-enhanced Raman scattering (SERS). The FT-Raman spectrum of OPE′ shows three C–S bands at 834, 1086, and 1131 cm −1 . From the FT-Raman to the SERS spectra, the 1086 cm −1 band exhibits a 9 cm −1 red shift. Chemisorption of OPE′ to the gold surface occurs via oxidative cleavage of the disulfide bond and the formation of the Au–S bond. The Au–S vibration is visible in the SERS spectra at 257 cm −1 . Peaks due to the S–S and S–H stretch are observed at 544 and 2519 cm −1 , respectively, in the FT spectrum, but are unobserved in the surface-enhanced spectra. The C–H stretching region (2700–3350 cm −1 ) in the spectrum of neat OPE′ shows three distinct bands, whereas the SERS spectra show a single broad band. Assignments of vibrational bands were based on DFT calculations performed at the B3LYP level with good agreement between theoretical and experimental values. An average percent difference of 2.52 was obtained for the non-CH stretching frequencies.

The surprising room-temperature phase conversion of anatase to rutile TiO2, a process that normally requires temperatures well in excess of 700 °C, is facilitated through the use of transition metal ions with highly unpaired electron spin. Raman spectroscopy is used to demonstrate this unexpected process, facilitated through the use of high-spin cobalt (CoII) and nickel (NiII) transition metal ion seeding into a TiO2 or TiO2-xNx nanocolloid lattice. In distinct contrast, the effect is not manifest upon seeding with low-spin ions including CuII. The Raman spectra also demonstrate the concomitant incorporation of a spinel-like structure into the titania lattice, which is suggested to influence the high-spin cobalt interaction.

While Shockley stacking fault (SSF) creation and expansion within 4H-SiC bipolar devices is well known, only recently was it observed that this expansion and the associated increase in the forward voltage drop (V f) could be completely reversed via low-temperature annealing. Here we report the temperature dependence of the recovery rate of the V f drift via annealing, reporting an activation energy of 1.3 ± 0.3 eV. The V f drift was observed to saturate following extended electrical stressing, and it was observed that the value of V f at this saturation was inversely proportional to the stressing temperature. We also observed that SSF and V f drift recovery could occur in highly stressed diodes at elevated temperatures even under high current injection conditions (14 A/cm2).

Shockley stacking faults (SSFs) in hexagonal SiC are known to induce an increase in the forward voltage drop of bipolar devices, such as pin diodes. Recent annealing studies have illustrated that propagated SSFs can not only propagate under current or optical injection conditions, but under low temperature (210-7000C) annealing can contract completely. Associated with this contraction is the complete and repeatable recovery of the drift. These observations have brought about questions as to the driving force for SSF propagation and contraction. Here we report that following extended periods of forward bias operation, the drift of 4H-SiC 10kV pin diodes increased until an eventual saturation occurred. At this point, further current injection no longer induced any further SSF propagation or increases in . Furthermore, the maximum attainable drift was inversely proportional to the stressing temperature, meaning that at elevated temperatures, the degree to which the SSFs may propagate is significantly limited.

A novel rare-earth chloride seed was employed as a catalyst for growth of GaN nano- and micro-crystals on c-, a- and r-plane sapphire. The ErCl3 seed on the substrate surface enhanced the growth rate and density of the GaN crystals. Distinctive green photoluminescence was measured, confirming that Er 3+ ions were active in the GaN matrix. This technique can be adapted to selectively grow GaN crystals with emission tailored to

Stacking faults within 4H-SiC PiN diodes are known to be detrimental to device operation. Here, we present electroluminescence (EL) images of 4H-SiC PiN diodes providing evidence that electrically and optically stimulated Shockley stacking fault (SSF) propagation is a reversible process at temperatures as low as 210°C. Optical beam induced current (OBIC) images taken following complete optical stressing of a PiN diode and that lead to a small number of completely propagated SSFs provide evidence that such defects propagate across the n–/p+ interface and continue to grow throughout the p+ layer. These observations bring about questions regarding the validity of the currently accepted driving force mechanism for SSF propagation.

The field of nanophotonics is based on the ability to confine light to sub-diffractional dimensions. Up until recently, research in this field has been primarily focused on the use of plasmonic metals. However, the high optical losses inherent in such metal-based surface plasmon materials has led to an ever-expanding effort to identify, low-loss alternative materials capable of supporting sub-diffractional confinement. One highly promising alternative are polar dielectric crystals whereby sub-diffraction confinement of light can be achieved through the stimulation of surface phonon polaritons within an all-dielectric, and thus low loss material system. Both SiC and hexagonal BN are two exemplary SPhP systems, which along with a whole host of alternative materials promise to transform nanophotonics and metamaterials in the mid-IR to THz spectral range. In addition to the lower losses, these materials offer novel opportunities not available with traditional plasmonics, for instance hyperbolic optical behavior in natural materials such as hBN, enabling super-resolution imaging without the need for complex fabrication. This talk will provide an overview of the SPhP phenomenon, a discussion of what makes a ‘good’ SPhP material and recent results from SiC and the naturally hyperbolic material, hBN from our research group.

Metamaterials designed for the visible or near IR wavelengths require patterning on the nanometer scale. To achieve this, e-beam lithography is used, but it is extremely difficult and can only produce 2D structures. A new alternative technique to produce 2D and 3D structures involves laser fabrication using the Nanoscribe 3D laser lithography system. This is a direct laser writing technique which can form arbitrary 3D nanostructures on the nanometer scale and is based on multi-photon polymerization. We are creating 2D and 3D metamaterials via this technique, and subsequently conformally coating them using Atomic Layer Deposition of oxides and Ag. We will discuss the optical properties of these novel composite structures and their potential for dual resonant metamaterials.

Two approaches to achieving reduced-defect active regions in III-N devices are discussed - confined epitaxy and heteroepitaxy on step-free SiC surfaces. In confined epitaxy, sapphire substrates (either GaN coated or not) are patterned with a dielectric mask and then III-N device structures are selectively and vertically grown in the openings using metalorganic chemical vapor deposition (MOCVD). In heteroepitaxy on step-free SiC surfaces, SiC mesas are created that have surfaces completely free of atomic steps and then used as substrates for conventional MOCVD III-N growth. In both approaches significant reductions in extended defect densities (10-100x) are observed and manifest in improved electroluminescence efficiency of UV emitters and leakage currents in UV detectors. Extensions of these efforts and other structural characterization results will be presented. Modeling results suggesting directions for future efforts will also be discussed.

We report improvements in ultraviolet (380 nm) electroluminescence (EL) output of GaN pn homojunctions grown on 4H-SiC mesas with step-free surfaces of 49% on average in comparison to their stepped counterparts. The GaN on step-free 4H-SiC growth process has been previously reported to lead to 100-fold reductions in threading dislocation densities in III-N heterofilms. Despite the intense EL from the GaN on step-free SiC devices, significant current leakage was observed through the periphery of the device, possibly due to the lack of GaN junction isolation processing.

Presented is the use of a commercial optical scanner for the mapping of doping density ( ) D N of SiC substrates and as a local probe for D N in different regions. This method provides a fast and cost effective method for determining D N homogeneity, examining local electrical characteristics, and recognizing defect sites including areas of different polytypes or polycrystallinity. Micro-Raman spectroscopy was used to calibrate the transmission intensity with D N . It is shown that features presented in the calculated D N maps strongly correlate to those observed in Lehighton resistivity maps.

Bulk n+-4H-SiC wafers (n=1-2×1019 cm-3) containing annealing-induced stacking faults were examined by Raman scattering. The coupled plasmon-LO mode was observed to shift in a manner consistent with 1018 cm-3 doping in the 4H-SiC. Numerical simulations were performed using a self-consistent Poisson-Schrödinger solver and agree well with the experimental observations of carrier transfer from the 4H-SiC into the 3C-SiC stacking faults. The Raman data also shows that the 3C stacking faults induce a tensile strain on the surrounding 4H-SiC regions.

Zirconia has attracted considerable attention because of its diverse practical applications in fuel-cell technology, its use as a catalyst or catalyst support, its ability to combine with sulfuric acid to form extremely strong acids, and its use as an oxygen sensor and as a possible high-dielectric-constant material for large-scale integrated circuits or as a gate dielectric in metal-oxide semiconductor devices. However, the optical properties of this semiconductor, especially its photoluminescence (PL) properties, are seldom reported. Because these PL properties, if properly harnessed, might play an important role in the future improvement of informationstorage devices, the characterization and enhancement of PL from ZrOx may have important implications. Here, we expand the potential of these optical properties. We report not only on the formation of nanometer-sized ZrOx structures which combine into fractal forms, but also on the first synthesis procedure for preparing zirconia-based nanoshells and hollow nanospheres. Our synthesis procedure complements recent spray-pyrolysis studies, which produce hollow nanospheres in the size range 1–10 lm. The formation and growth of nanoshells and nanospheres can greatly dominate the formation of other less-defined zirconia-based nanostructures under certain experimental conditions. The nanoshells produced do not result from solvent-induced nanosphere disruption and they have smooth, as opposed to fractured, surfaces. In contrast to zirconia-based nanoparticles and nanospheres, we find that these “metal-oxide” nanoshell configurations demonstrate a significantly enhanced PL that may result from the geometry of the ultrathin nanoshell walls. In Figures 1a–c, we present representative transmission electron microscopy (TEM) images corresponding to the irregularly shaped and distorted spheroidal structures typically formed under a wide range of experimental conditions. These spheroidal structures readily combine into the fractal-like structure depicted in Figure 1c. In contrast, Figures 2a–e present representative TEM images corresponding to the ZrOx nanoshell and nanospherical forms, which we generated under a set of specific conditions outlined in the Experimental section. These structures should be compared to the nanostructures depicted in Figure 1, which are formed over a much wider range of conditions. Figure 2a represents an overview of nanoshell and nanosphere formation accompanied by the formation of a few hollow elliptical forms. Figure 2b presents a closer view obtained from a spectrum taken of material formed under similar conditions demonstrating both thick(∼ 20 nm) and thin(∼ 5 nm) walled shell formation. We found that thick-walled shells can be generated and grown in regions of slower condensation between a source reactant oxidant nozzle(ZrCl4) containing crucible and a downstream cold finger at ∼ 20 °C. More rapid condensation at the cold finger, enhanced by a larger temperature gradient relative to the reactant oxidation zone, produces thinner-walled shells and some hollow spheres. Figure 2c represents the arrested formation of a hollow nanosphere as it grows from a thick-walled nanoshell in the region (downstream) intermediate to the reaction/oxidation zone and the system cold finger. By examining product formation across this region under appropriate conditions, it is possible to observe several degrees of growth. Provided that initially forming shells diffuse to the cold finger they can be surrounded by a thin-walled shell (Fig. 2d). Figure 2d exemplifies several multishelled/-walled configurations oriented about a central nanosphere where the outer nanoshell is thin-walled. Figure 2e demonstrates a smaller nanosphere located within a thick-walled, nearly nanospherical shell-like structure. A closer view of the hollow nanosphere groupings reveals that they can be interconnected by a permeable channel. In Figures 3a–c Raman spectra taken for the ZrOx nanoshelland hollow-nanosphere configurations can be compared. Figure 3a corresponds to the Raman spectrum and curve fit for ZrOx nanoshells (see Fig. 2b). The curve-fit spectrum is dominated by features associated with crystalline tetragonal (472 620 cm) and monoclinic ZrOx. The features associated with the spectrum of crystalline tetragonal nanoshells are shifted from those of ZrOx powder (Fig. 3c), where they are located at 477.5 and 616.45 cm. The Raman spectrum observed for samples generated under conditions that favor the formation of nanospheres (Fig. 3b) appears to correspond to a broad peak located near 570 cm. This result suggests that the crystallinity associated with the nanoshells is lost in sphere formation (Figs. 2c–e), as these structures are amorphous in character. We have not yet obtained evidence that a crystalline structure can be associated with either the irregular nanostructures of Figure 1 or the formed nanospheres. For comparison, Figure 3c shows the corresponding Raman spectrum of bulk ZrOx powder. The observed features are in agreement with the observations of previous workers. Not C O M M U N IC A TI O N S

Silicon carbide was deposited homoepitaxially at high growth rates (up to 90 μm/h) with methyltrichlorosilane (MTS) as the precursor in chemical vapor deposition. The substrates were 6H- and 4H-SiC (0 0 0 1) wafers with different misorientation ( 1 1 2 ¯ 0 ). At a growth temperature of 1400 °C, the grown film was composed of faceted grains and the surface was very rough. As the growth temperature increased, the crystal grains were enlarged and the films’ surfaces became smoother and smoother. At 1600 °C, on 8° misoriented substrate, a very smooth film with mirror-like surface was generated. Raman spectroscopy showed that the polytypes of the films were affected by the substrate misorientation. When the 6H-SiC substrates were well oriented, i.e. with the offset angle of less than 0.5°, 3C-SiC was deposited with numerous triangular stacking faults (TSFs). When the offset angle of substrates was 3.5°, mixtures of 3C- and 6H-SiC polytypes were deposited. When the offset angle was 8°, the epitaxial growth perfectly replicated the substrates’ polytypes, i.e. high-quality 6H- and 4H-SiC epilayers were generated on the 6H- and 4H-SiC substrates, respectively. The full-width half-maximum (FWHM) of X-ray diffraction rocking curves for the c -plane of pure smooth 6H- and 4H-SiC layers was in the range of 15–20 arcsecs. Photoluminescence (PL) confirmed the polytypes of 6H- and 4H-SiC layers: Both exhibited clear near-band edge emission at room temperature. These results demonstrate that MTS is a suitable precursor for the rapid, high quality SiC epitaxy.

Cross-polarized imaging with an optical scanner and confocal μ-Raman spectroscopy have been used to reveal and to study structural defects in n-4H-SiC. We introduce a new method of polarized imaging that uses a combination of linear and circular polarization, which enhances contrast in images of defect regions. Regions that show intensity contrast have been observed to have characteristics of either low-angle grain boundaries or sharp lines of delineation between low and high strain areas. Different types of polytype inclusions have also been observed, and the material within the inclusions has been shown to be either 3C-SiC or 6H-SiC. Polytype inclusions occur as isolated features or form cores of holes, and they can form at any point of the growth.

We have formed Mn clusters on Si(1 1 1) and Si(1 1 2) surfaces and we have examined their magnetic behavior using circularly polarized optical scattering. We show that Mn clusters of diameter 15–25 nm exhibit a temperature dependent ferromagnet-like behavior for temperatures below 10 K. As we use magnetic fields perpendicular to the plane of the surface, our results indicate that the magnetic moment of some of these clusters is out of the plane. In addition, our experiments verify previous theoretical suggestions that polarized optical scattering can be used to detect magnetism in small clusters.

The organometallic vapor phase epitaxial growth of GaAs double-drift Read impact ionization avalanche transit time diodes where the p-type layers were doped with carbon is described. Ka-band oscillation testing yielded average performance of 3.5 W with 16% efficiency for pulse lengths >1 µs and 10.5 W with 13% efficiency for pulse lengths

We report the first measurement of room temperature carrier lifetimes at a magnetic metal--semiconductor interface, $\mathrm{Fe}/\mathrm{GaAs}(001)\ensuremath{-}(2\ifmmode\times\else\texttimes\fi{}4)$. The lifetimes are significantly enhanced relative to uncoated or sulfur-passivated surfaces, or the $\mathrm{Al}/\mathrm{GaAs}(001)\ensuremath{-}(2\ifmmode\times\else\texttimes\fi{}4)$ interface. These enhanced lifetimes correlate with a corresponding decrease in the density of midgap states which otherwise dominate the character of the GaAs(001) surface. The epitaxial Fe film provides both a ferromagnetic contact and a superior surface-passivating layer which does not pin the Fermi energy.

In an effort to monitor ion-driven surface chemistry in the high density plasma etching of GaAs by Cl2/Ar plasma chemistries, we have applied mass spectrometry and careful substrate temperature control. Etch product chlorides were mass analyzed while the substrate temperature was monitored by optical bandgap thermometry and as pressure (neutral flux), microwave power (ion flux) and rf bias of the substrate (ion energy) were varied. By ensuring that the substrate temperature does not deviate during process variations, the changes in product mass peak intensities are a direct measure of changes in the ionassisted surface chemistry which promotes anisotropic etching. Experimental results show that ion-assisted surface chemistry is optimum when sufficient Cl and Cl+ are present in the incident plasma flux. These conditions are met at low coupled microwave powers ( 200 eV. Photoreflectance measurements of the surface Fermi level show significant damage for ion energies >75 eV. However, in situ and ex situ surface passivations can recover the surface Fermi level for up to 200 eV ion energies, in good correlation to the onset of sputtering and subsurface damage. Thus, anisotropic, low damage pattern transfer is possible for ion energies between 50 and 200 eV.

Physical Review Letters, February 24, 1997. College Park, MD: American Physical Society. Vol.78, No.8, pp. 1584-1587.

Optical resonances of crystalline Si nanopillar arrays on a Si substrate are studied using optical reflectivity and Raman spectroscopy. When the nanopillars are arranged in a two-dimensional lattice, a collective resonance is observed in the reflection spectra which is absent for randomly distributed nanopillars. The resonance is due to coherent oscillations in nanopillars, can be tuned spectrally by the nanopillar diameter and lattice period, and strongly suppresses reflection from the Si surface. Raman scattering demonstrates that the reduced reflectivity is accompanied by increased electromagnetic field confined in Si, thus suggesting potential application of the lattice resonance in surface enhanced spectroscopy and thin film solar cells.

We have employed plasma-enhanced atomic layer deposition (PEALD) as a means to create multi-layered nanocomposite structures in order to enhance the plasmonic behavior and SERS response in the detection of benzenethiol (BZT). Ag PEALD films were deposited within nanoporous anodic aluminum oxide (AAO) templates of various pore depths, using Ag(fod)(PEt 3 )(fod=2,2-dimethyl-6,6,7,7,8,8,8-heptafluorooctane-3,5-dionato) as the precursor. We have examined the polycrystalline microstructure and conformality of the Ag films across the surface of an AAO template as well as into the pores, which varies significantly as thicknesses decrease. Furthermore, we investigated the plasmonic behavior of these films by performing SERS as a function of the Ag microstructure and conformality within the nanopores, using a 785 nm laser excitation and BZT as a test molecule, which forms a self-assembled monolayer on the Ag surface.

We have deposited Ag metal via plasma enhanced atomic layer deposition (PEALD) and we investigated the novel optical behavior of this material. We have found that as-deposited flat PEALD Ag films exhibit unexpected plasmonic properties and the plasmonic enhancement can differ markedly, depending on the microstructure of the Ag film. Electromagnetic field simulations indicate that this plasmonic behavior is due to air gaps that are an inherent property of the mosaiclike microstructure of the PEALD-grown Ag film. We show that this material is plasmonic by itself, and when combined with previously developed dielectric core nanowires, it can produce enhancements which are two orders of magnitude greater than those reported using electroless Ag or Ag produced by e-beam deposition. We have also investigated the effect of substrate on the plasmonic enhancement, as well deposition on fabric, which results in a flexible plasmonic material.

Our measurements of surface enhanced Raman scattering (SERS) on Ga2O3dielectric nanowires (NWs) core/silver composites indicate that the SERS enhancement is highly dependent on the polarization direction of the incident laser light. The polarization dependence of the SERS signal with respect to the direction of a single NW was studied by changing the incident light angle. Further investigations demonstrate that the SERS intensity is not only dependent on the direction and wavelength of the incident light, but also on the species of the SERS active molecule. The largest signals were observed on an NW when the incident 514.5 nm light was polarized perpendicular to the length of the NW, while the opposite phenomenon was observed at the wavelength of 785 nm. Our theoretical simulations of the polarization dependence at 514.5 nm and 785 nm are in good agreement with the experimental results.

Multi-wavelength Raman scattering from benzene-thiol adsorbed on randomly distributed silver-coated plasmonic nanowires was measured in the visible optical range to probe the density of states of the chemical, changed due to adsorption.

We have investigated the growth of ZnO nanowires on curved BaTiO3 retroreflector beads, as well as growth of ZnO nanowires on flat substrates. Results indicate that the growth of ZnO aligned nanowire arrays occurs farther away from the Zn source in the retroreflectors, while the results are opposite for the flat Si substrates. In the case of the ZnO nanowires on flat Si, the nanowires formed in nearly aligned arrays are short and significantly thicker, suggesting that the growth occurs both longitudinally and laterally in this process, which is not the case for the growth on the retroreflector beads. The SERS response of the nanowire arrays on the retroreflectors has been compared to random nanowires on flat Si substrates, and results show that the signal strength is 29 times greater in the case of the wires grown on the retroreflectors. Since one would only expect a factor of 4 enhancement due to the light reflecting properties of the retroreflector, it is believed that the enhancement in the SERS signal is due to light channeling by the aligned nanowire arrays.

We demonstrated a fast and easy way to synthesize Ag nanoparticles (NPs) on ZnO nanowires (NWs) and silicon substrates by an electroless (EL) plating approach. ZnO NWs used here were grown via vapor-solid (VS) mechanism at 560 °C for 30 min. The stability to oxidation of these EL-produced homogeneous Ag NPs on ZnO nanowires was investigated by surface enhanced Raman spectroscopy (SERS), showing that the attachment of thiol to the Ag surface can slow down the oxidation process, and the SERS signal remains strong for more than ten days. Furthermore, we examined the surface oxidation kinetics of the Ag NPs as a function of NPs size and size distribution by monitoring the oxygen amount in the composites using energy dispersive x-ray (EDX). Results indicate that the EL plated Ag NPs show faster oxidation rates than those produced by e-beam (EB) evaporation in air. We attribute this to the fact that the EL produced silver particles are very small, in the 20 nm range, and thus have high surface energy, thus enhancing the oxidation. These studies provide extensive information related to the Ag NP oxidation rates, which can help in extending the Ag lifetime for various applications.

The remarkably high electromagnetic fields that can be induced by the optical stimulation of surface plasmons in plasmonic nanostructures can induce large enhancements to Raman scattering. The Surface-enhanced Raman Scattering (SERS) effect offers great potential for the development of molecular sensors with low false alarm rates and high sensitivity. Although research in this field has grown at a very fast pace in recent years, most of this work has focused on attaining the highest enhancements at highly localized hot-spots, which while providing large peak enhancements, exhibit very low average enhancement factors, making their use for most sensing applications unlikely. Here we report on Au-coated Si nanopillar arrays where we probe the dependence of the SERS enhancement on both the nanopillar diameter and the interpillar gap over a range extending from 30 to 245 nm and 20 to 165 nm, respectively. This approach allows for the optimization of the SPR condition relative to the incident laser wavelength chosen, enabling an optimized SERS sensor. As the interparticle gaps approach 20 nm, we also explored arrays of nanopillars where interparticle plasmonic coupling should exist, however, it remains unclear if any such collective effects are present. The arrays created do illustrate very large highly uniform (