Semiconductor Thermal and Electrical Properties Decoupled by Localized Phonon Resonances.

Thermoelectric materials convert heat into electricity through thermally driven charge transport in solids or vice versa for cooling. To compete with conventional energy-conversion technologies, a thermoelectric material must possess the properties of both an electrical conductor and a thermal insulator. However, these properties are normally mutually exclusive because of the interconnection between scattering mechanisms for charge carriers and phonons.

Extreme Ultraviolet Radiation Pulsed Atom Probe Tomography of III-Nitride Semiconductor Materials.

Laser-pulsed atom probe tomography (LAPT) is a materials characterization technique that has been widely applied in the study and characterization of III-nitride semiconductors. To date, most of these studies have used light sources ranging from the visible to the near-ultraviolet region of the spectrum. In this manuscript, we demonstrate that a recently developed extreme ultraviolet (EUV) radiation pulsed atom probe tomograph can trigger controlled field ion evaporation from III-nitride samples.

Crystallographic polarity measurements in two-terminal GaN nanowire devices by lateral piezoresponse force microscopy.

Lateral piezoresponse force microscopy (L-PFM) is demonstrated as a reliable method for determining the crystallographic polarity of individual, dispersed GaN nanowires that were functional components in electrical test structures. In contrast to PFM measurements of vertically oriented (as-grown) nanowires, where a biased probe tip couples to out-of-plane deformations through the d piezoelectic coefficient, the L-PFM measurements in this study were implemented on horizontally oriented nanowires that coupled to shear deformations through the d coefficient. L-PFM phase-polarity relationships were determined experimentally using a bulk m-plane GaN sample with a known [0001] direction and further indicated that the sign of the d piezoelectric coefficient was negative.

The role of Si in GaN/AlN/Si(111) plasma assisted molecular beam epitaxy: polarity and inversion.

The microstructure, polarity and Si distribution in AlN/GaN layers grown by plasma assisted molecular beam epitaxy (PAMBE) on Si(111) was assessed by scanning transmission electron microscopy (STEM). Samples grown under both metal- and nitrogen-rich conditions contained defects at the AlN/Si interface which suggest formation of an Al-Si eutectic. Correlated with this, interfacial segregation of Si was found in the samples.

UV LEDs based on p-i-n core-shell AlGaN/GaN nanowire heterostructures grown by N-polar selective area epitaxy.

Ultraviolet light-emitting diodes fabricated from N-polar AlGaN/GaN core-shell nanowires (NWs) with p-i-n structure produced electroluminescence at 365 nm with ∼5× higher intensities than similar GaN homojunction LEDs. The improved characteristics were attributed to localization of spontaneous recombination to the NW core, reduction of carrier overflow losses through the NW shell, and elimination of current shunting. Poisson-drift-diffusion modeling indicated that a shell Al mole fraction of x = 0.

Eutectic Formation, V/III Ratio and Controlled Polarity Inversion in Nitrides on Silicon.

The crystallographic polarity of AlN grown on Si(111) by plasma assisted molecular beam epitaxy is intentionally inverted from N-polar to Al-polar at a planar boundary. The position of the inversion boundary is controlled by a two-step growth process that abruptly changes from Al-rich to N-rich growth conditions. The polarity inversion is induced by the presence of Si, which is incorporated from an Al-Si eutectic layer that forms during the initial stages of AlN growth and floats on the AlN surface under Al-rich growth conditions.

Black GaAs with Sub-Wavelength Nanostructures Fabricated via Lithography-Free Metal-Assisted Chemical Etching.

A practical nanofabrication process is detailed here for the generation of black GaAs. Discontinuous thin films of Au nanoparticles are electrodeposited onto GaAs substrates to catalyze site-specific etching in a solution of KMnO and HF according to the metal-assisted chemical etching mechanism. This provides a solution-based and lithography-free method for fabricating sub-wavelength nanostructure arrays that exhibit solar-weighted reflectance approaching 4 %.

An optical Bragg scattering readout for simultaneous detection of all low-order mechanical modes of gallium nitride nanowires in nanowire arrays.

We report the use of optical Bragg scattering and homodyne interferometry to simultaneously measure all the first order cantilever-mode mechanical resonance frequencies and quality factors (Q) of gallium nitride nanowires (GaN NWs) in 100 NW periodic selected-area growth arrays. Hexagonal 2D arrays of 100 GaN NWs with pitch spacings of 350-1100 nm were designed and prepared to allow optical Bragg scattering. The NWs studied have diameters ranging from 100-300 nm, lengths from 3-10 μm, resonance frequencies between 1-10 MHz, and Q-values near 10,000 at 300 K.

GaN Nanowire MOSFET with Near-Ideal Subthreshold Slope.

Wrap-around gate GaN nanowire MOSFETs using AlO as gate oxide have been experimentally demonstrated. The fabricated devices exhibit a minimum subthreshold slope of 60 mV/dec, an average subthreshold slope of 68 mV/dec over three decades of drain current, drain-induced barrier lowering of 27 mV/V, an on-current of 42 μA/μm (normalized by nanowire circumference), on/off ratio over 10, an intrinsic transconductance of 27.8 μS/μm, for a switching efficiency figure of merit, Q=g/SS of 0.

Core-shell GaN nanowire LEDs by N-polar selective area growth.

GaN nanowire LEDs with radial junctions were grown by molecular beam epitaxy using N-polar selective area growth on Si(111) substrates. The N-polar selective area growth process facilitated the growth of isolated and high-aspect-ratio -type NW cores that were not subject to self-shadowing effects during the subsequent growth of a conformal low-temperature Mg:GaN shell. LED devices were fabricated from single-NW and multiple-NW arrays in their as-grown configuration by contacting the -type core through an underlying conductive GaN layer and the -type NW shell via a metallization layer.

Selective Area Growth and Structural Characterization of GaN Nanostructures on Si(111) Substrates.

Selective area growth (SAG) of GaN nanowires and nanowalls on Si(111) substrates with AlN and GaN buffer layers grown by plasma-assisted molecular beam epitaxy was studied. For N-polar samples filling of SAG features increased with decreasing lattice mismatch between the SAG and buffer. Defects related to Al-Si eutectic formation were observed in all samples, irrespective of lattice mismatch and buffer layer polarity.

Comparison of convergent beam electron diffraction and annular bright field atomic imaging for GaN polarity determination.

A comparison of two electron microscopy techniques used to determine the polarity of GaN nanowires is presented. The techniques are convergent beam electron diffraction (CBED) in TEM mode and annular bright field (ABF) imaging in aberration corrected STEM mode. Both measurements were made at nominally the same locations on a variety of GaN nanowires.

Near-Field Control and Imaging of Free Charge Carrier Variations in GaN Nanowires.

Despite their uniform crystallinity, the shape and faceting of semiconducting nanowires (NWs) can give rise to variations in structure and associated electronic properties. Here we develop a hybrid scanning probe-based methodology to investigate local variations in electronic structure across individual n-doped GaN NWs integrated into a transistor device. We perform scanning microwave microscopy (SMM), which we combine with scanning gate microscopy (SGM) to determine the free-carrier SMM signal contribution and image local charge carrier density variations.

Periodic dynamics, localization metastability, and elastic interaction of colloidal particles with confining surfaces and helicoidal structure of cholesteric liquid crystals.

Nematic and cholesteric liquid crystals are three-dimensional fluids that possess long-range orientational ordering and can support both topological defects and chiral superstructures. Implications of this ordering remain unexplored even for simple dynamic processes such as the ones found in so-called "fall experiments," or motion of a spherical inclusion under the effects of gravity. Here we show that elastic and surface anchoring interactions prompt periodic dynamics of colloidal microparticles in confined cholesterics when gravity acts along the helical axis.

GaN nanowire coated with atomic layer deposition of tungsten: a probe for near-field scanning microwave microscopy.

GaN nanowires were coated with tungsten by means of atomic layer deposition. These structures were then adapted as probe tips for near-field scanning microwave microscopy. These probes displayed a capacitive resolution of ~0.

Analysis of contact interfaces for single GaN nanowire devices.

Single GaN nanowire (NW) devices fabricated on SiO2 can exhibit a strong degradation after annealing due to the occurrence of void formation at the contact/SiO2 interface. This void formation can cause cracking and delamination of the metal film, which can increase the resistance or lead to a complete failure of the NW device. In order to address issues associated with void formation, a technique was developed that removes Ni/Au contact metal films from the substrates to allow for the examination and characterization of the contact/substrate and contact/NW interfaces of single GaN NW devices.

On-chip optical interconnects made with gallium nitride nanowires.

In this Letter we report on the fabrication, device characteristics, and optical coupling of a two-nanowire device comprising GaN nanowires with light-emitting and photoconductive capabilities. Axial p-n junction GaN nanowires were grown by molecular beam epitaxy, transferred to a non-native substrate, and selectively contacted to form discrete optical source or detector nanowire components. The optical coupling demonstrated for this device may provide new opportunities for integration of optical interconnects between on-chip electrical subsystems.

Noncontact measurement of charge carrier lifetime and mobility in GaN nanowires.

The first noncontact photoconductivity measurements of gallium nitride nanowires (NWs) are presented, revealing a high crystallographic and optoelectronic quality achieved by use of catalyst-free molecular beam epitaxy. In comparison with bulk material, the NWs exhibit a long conductivity lifetime (>2 ns) and a high mobility (820 ± 120 cm(2)/(V s)). This is due to the weak influence of surface traps with respect to other III-V semiconducting NWs and to the favorable crystalline structure of the NWs achieved via strain-relieved growth.

Microstructure evolution and development of annealed Ni/Au contacts to GaN nanowires.

The development of Ni/Au contacts to Mg-doped GaN nanowires (NWs) is examined. Unlike Ni/Au contacts to planar GaN, current-voltage (I-V) measurements of Mg-doped nanowire devices frequently exhibit a strong degradation after annealing in N(2)/O(2). This degradation originates from the poor wetting behavior of Ni and Au on SiO(2) and the excessive void formation that occurs at the metal/NW and metal/oxide interfaces.

Controlled dielectrophoretic nanowire self-assembly using atomic layer deposition and suspended microfabricated electrodes.

Effects of design and materials on the dielectrophoretic self-assembly of individual gallium nitride nanowires (GaN NWs) onto microfabricated electrodes have been experimentally investigated. The use of TiO(2) surface coating generated by atomic layer deposition (ALD) improves dielectrophoretic assembly yield of individual GaN nanowires on microfabricated structures by as much as 67%. With a titanium dioxide coating, individual nanowires were placed across suspended electrode pairs in 46% of tests (147 out of 320 total), versus 28% of tests (88 out of 320 total tests) that used uncoated GaN NWs.

Methanol, ethanol and hydrogen sensing using metal oxide and metal (TiO(2)-Pt) composite nanoclusters on GaN nanowires: a new route towards tailoring the selectivity of nanowire/nanocluster chemical sensors.

We demonstrate a new method for tailoring the selectivity of chemical sensors using semiconductor nanowires (NWs) decorated with metal and metal oxide multicomponent nanoclusters (NCs). Here we present the change of selectivity of titanium dioxide (TiO(2)) nanocluster-coated gallium nitride (GaN) nanowire sensor devices on the addition of platinum (Pt) nanoclusters. The hybrid sensor devices were developed by fabricating two-terminal devices using individual GaN NWs followed by the deposition of TiO(2) and/or Pt nanoclusters (NCs) using the sputtering technique.

Unconventional structure-assisted optical manipulation of high-index nanowires in liquid crystals.

Stable optical trapping and manipulation of high-index particles in low-index host media is often impossible due to the dominance of scattering forces over gradient forces. Here we explore optical manipulation in liquid crystalline structured hosts and show that robust optical manipulation of high-index particles, such as GaN nanowires, is enabled by laser-induced distortions in long-range molecular alignment, via coupling of translational and rotational motions due to helicoidal molecular arrangement, or due to elastic repulsive interactions with confining substrates. Anisotropy of the viscoelastic liquid crystal medium and particle shape give rise to a number of robust unconventional trapping capabilities, which we use to characterize defect structures and study rheological properties of various thermotropic liquid crystals.

Homoepitaxial n-core: p-shell gallium nitride nanowires: HVPE overgrowth on MBE nanowires.

We present the homoepitaxial growth of p-type, magnesium doped gallium nitride shells by use of halide vapor phase epitaxy (HVPE) on n-type gallium nitride nanowires grown by plasma-assisted molecular beam epitaxy (MBE). Scanning electron microscopy shows clear dopant contrast between the core and shell of the nanowire. The growth of magnesium doped nanowire shells shows little or no effect on the lattice parameters of the underlying nanowires, as measured by x-ray diffraction (XRD).

Highly selective GaN-nanowire/TiO2-nanocluster hybrid sensors for detection of benzene and related environment pollutants.

Nanowire-nanocluster hybrid chemical sensors were realized by functionalizing gallium nitride (GaN) nanowires (NWs) with titanium dioxide (TiO(2)) nanoclusters for selectively sensing benzene and other related aromatic compounds. Hybrid sensor devices were developed by fabricating two-terminal devices using individual GaN NWs followed by the deposition of TiO(2) nanoclusters using RF magnetron sputtering. The sensor fabrication process employed standard microfabrication techniques.

Three dimensional optical manipulation and structural imaging of soft materials by use of laser tweezers and multimodal nonlinear microscopy.

We develop an integrated system of holographic optical trapping and multimodal nonlinear microscopy and perform simultaneous three-dimensional optical manipulation and non-invasive structural imaging of composite soft-matter systems. We combine different nonlinear microscopy techniques such as coherent anti-Stokes Raman scattering, multi-photon excitation fluorescence and multi-harmonic generation, and use them for visualization of long-range molecular order in soft materials by means of their polarized excitation and detection. The combined system enables us to accomplish manipulation in composite soft materials such as colloidal inclusions in liquid crystals as well as imaging of each separate constituents of the composite material in different nonlinear optical modalities.