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. In all cases the two techniques gave the same polarity result. An important aspect of the study was the calibration of the CBED pattern rotation relative to the TEM image. Three different microscopes were used for CBED measurements. For all three instruments there was a substantial rotation of the diffraction pattern (120 or 180°) relative to the image, which, if unaccounted for, would have resulted in incorrect polarity determination. The study also shows that structural defects such as inversion domains can be readily identified by ABF imaging, but may escape identification by CBED. The relative advantages of the two techniques are discussed.

A functionalization method for the specific and selective immobilization of the streptavidin (SA) protein on semiconductor nanowires (NWs) was developed. Silicon (Si) and silicon carbide (SiC) NWs were functionalized with 3-aminopropyltriethoxysilane (APTES) and subsequently biotinylated for the conjugation of SA. Existence of a thin native oxide shell on both Si and SiC NWs enabled efficient binding of APTES with the successive attachment of biotin and SA as was confirmed with x-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and atomic force microscopy. Fluorescence microscopy demonstrated nonspecific, electrostatic binding of the SA and the bovine serum albumin (BSA) proteins to APTES-coated NWs. Inhibition of nonspecific BSA binding and enhancement of selective SA binding were achieved on biotinylated NWs. The biofunctionalized NWs have the potential to be used as biosensing platforms for the specific and selective detection of proteins.

Strains in GaN nanowires with InGaN quantum wells (QW) were measured from transmission electron microscope (TEM) images. The nanowires, all from a single growth run, are single crystals of the wurtzite structure that grow along the <0001> direction, and are approximately 1000 nm long and 60 nm to 130 nm wide with hexagonal cross-sections. The In concentration in the QWs ranges from 12 to 15 at %, as determined by energy dispersive spectroscopy in both the transmission and scanning electron microscopes. Fourier transform (FT) analyses of <0002> and <1100> lattice images of the QW region show a 4 to 10 % increase of the c-axis lattice spacing, across the full specimen width, and essentially no change in the a-axis value. The magnitude of the changes in the c-axis lattice spacing far exceeds values that would be expected by using a linear Vegard's law for GaN – InN with the measured In concentration. Therefore the increases are considered to represent tensile strains in the <0001> direction. Visual representations of the location and extent of the strained regions were produced by constructing inverse FT (IFT) images from selected regions in the FT covering the range of c-axis lattice parameters in and near the QW. The present strain values for InGaN QW in nanowires are larger than any found in the literature to date for other forms of InxGa1-xN (QW)/GaN.

We have grown a variety of isolated GaN nanowires using gas-source molecular beam epitaxy (MBE) and characterized their structural and optical properties. The nanowires have demonstrated a number of promising materials characteristics, including low defect density and high luminescent intensity. Well-separated nanowires formed spontaneously on Si(111) substrates after deposition of a thin AlN buffer layer. Metal catalysts were not used. X-ray diffraction indicates that the c and a lattice parameters are within 0.01 % of the lattice parameters of bulk GaN. Transmission electron microscopy (TEM) revealed the nanowires to be free of dislocations and stacking faults, although a GaN matrix layer growing at the base of the wires was found to have a high density of basal plane stacking faults. The room temperature photoluminescence (PL) intensity compared favorably with a free-standing, thick film of high quality GaN. Several features of the low temperature PL spectra also indicated that the nanowires had few structural defects or chemical impurities. Finally, electrical characterization of dispersed nanowires demonstrated that efficient electrical contacts could be made and that the resistivity of the nanowires was comparable to that of bulk material.

We are using cavity ring-down spectroscopy (CRDS) to measure concentrations of water in nitrogen and, for the first time to our knowledge, in phosphine. Water vapor concentrations have been measured in purified and unpurified phosphine, indicating a water mole fraction of (22.0 ± 1.0) x 10-6 in unpurified phosphine. After purification with an in-line, chemically-reactive purifier, the mole fraction of water in phosphine was less than 0.1 × 10-6. Mole fractions as high as (730 ± 60) × 10-6 have been measured in unpurified phosphine, suggesting that the H2O vapor concentration increases substantially with time as the gas is stored in a cylinder. The materials properties of AlInP grown by molecular beam epitaxy (MBE) with CRDS-characterized PH3 were found to be relatively insensitive to water contamination at the 22 mol/mol level.