Precision optical astrometry of quasars and active galaxies can provide important insight into the spatial distribution and variability of emission in compact nuclei. SIM — the Space Interferometry Mission — will be the first optical interferometer capable of precision astrometry on quasars. Although it is not expected to resolve the emission, it will be very sensitive to astrometric shifts, for objects as faint as R magnitude 20. In its wide-angle mode, SIM will yield 4 microarcsecond absolute positions, and proper motions to about 2 microarcsecond/yr. A variety of AGN phenomena are expected to be visible to SIM on these scales, including time and spectral dependence in position offsets between accretion disk and jet emission. SIM should be able to answer the following questions. Does the most compact optical emission from an AGN come from an accretion disk or from a relativistic jet? Do the relative positions of the radio core and optical photocentre of quasars used for the reference frame tie change on the timescales of their photometric variability? Do the cores of galaxies harbour binary supermassive black holes remaining from galaxy mergers? In this paper we briefly describe the operation of SIM and the quasar measurements it will make. We estimate the size of the astrometric signatures which may be expected, and we discuss prospects for using astrometry as a fundamental tool for understanding quasar nuclei.

Chloride ion concentration can be used as a biomarker for the level of pollen exposure in allergic asthma, chronic cough and airway acidification related to respiratory disease. AlGaN/GaN high electron mobility transistor (HEMT) with an InN thin film in the gate region was used for real time detection of chloride ion detection. The InN thin film provided surface sites for reversible anion coordination. The sensor exhibited significant changes in channel conductance upon exposure to various concentrations of NaCl solutions. The sensor was tested over the range of 100 nM to 100 μM NaCl solutions. The effect of cations on the chloride ion detection was also studied.

Optical astrometry of quasars and active galaxies can provide key information on the spatial distribution and variability of emission in compact nuclei. The Space Interferometry Mission (SIM PlanetQuest) will have the sensitivity to measure a significant number of quasar positions at the microarcsecond level. SIM will be very sensitive to astrometric shifts for objects as faint as V=19. A variety of AGN phenomena are expected to be visible to SIM on these scales, including time and spectral dependence in position offsets between accretion disk and jet emission. These represent unique data on the spatial distribution and time dependence of quasar emission. It will also probe the use of quasar nuclei as fundamental astrometric references. Comparisons between the time-dependent optical photocenter position and VLBI radio images will provide further insight into the jet emission mechanism. Observations will be tailored to each specific target and science question. SIM will be able to distinguish spatially between jet and accretion disk emission; and it can observe the cores of galaxies potentially harboring binary supermassive black holes resulting from mergers.

Layers of GaN deposited on Si substrates have been studied using Thermal Admittance Spectroscopy (TAS) and Optical Admittance Spectroscopy (OAS). Transparent front-side contacts were used to facilitate the optical measurements. Six specimens were cut from the same location in two different wafers, and three samples were randomly chosen from other growths. A shallow level at EC − 0.051 eV was found in all the specimens using TAS. In some specimens this peak was asymmetric, indicating more than one level near this energy. Deeper levels were also seen in the high-temperature portion of the spectra, but were poorly resolved in most specimens. Illuminating the specimen with UV light at 25 K resulted in the thermal position of the peak shifting to a lower temperature, and hence, the calculated energy, of the peak shifting lower. The amplitude of the peak also decreased. Transient OAS measurements revealed the interesting phenomenon of negative persistent photo conductance at room temperature in some of the specimens when the illumination photon energy was less than the bandgap. The negative response time was very short. At lower temperatures, below 100 K, the negative response diminished, but the response time was still short. At wavelengths above the bandgap energy, normal transient response was seen, with a longer time constant.

Au-gated AlGaN/GaN High Electron Mobility Transistor (HEMT) structures were functionalized in the gate region with label free 3′-thiol modified oligonucleotides. This serves as a binding layer to the AlGaN surface for hybridization of matched target Deoxyribonucleic Acid (DNA). X-ray photoelectron spectroscopy (XPS) shows that immobilization of thiol modified DNA covalently bonded with gold on the gated region. Hybridization between probe DNA and matched or mismatched target DNA on the Au-gated HEMT was detected by electrical measurements. The HEMT drain-source current showed a clear decrease of 115 μA as this matched target DNA was introduced to the probe DNA on the surface, showing the promise of the DNA sequence detection approach for biological sensing.

InGaN films in the 0–50% InN composition range have been analyzed for the occurrence of phase separation. The ñ0.5 jum thick InGaN films were grown by metalorganic chemical vapor deposition (MOCVD) in the 690 to 780°C temperature range and analyzed by θ−20 x-ray diffraction (XRD), transmission electron microscopy (TEM), and selected area diffraction (SAD). As-grown films with up to 21% InN were single phase. However, for films with 28% InN and higher, the samples showed a spinodally decomposed microstructure as confirmed by TEM and extra spots in SAD patterns that corresponded to multiphase InGaN. An explanation of the data based on the GaN-InN pseudo-binary phase diagram is discussed.

We report on the growth and characterization of InGaN bulk films and AlGaN/InGaN/AlGaN double heterostructures (DHs). Good quality bulk InGaN films have been grown by metalorganic chemical vapor deposition (MOCVD) with up to 40% InN as characterized by x-ray diffraction. The effect of hydrogen in the growth ambient on the lnN% incorporation in the InGaN films is presented. Photoluminescence (PL) spectra of AlGaN/InGaN/AlGaN DHs exhibit emission wavelengths from the violet through yellow depending on the growth conditions of the active InGaN layer. The PL spectra are fairly broad both at room temperature and 20 K, and could be a result of native defects or impurity related transitions. We also observed a linear dependence between the PL intensity and excitation power density in the 0.001 W/cm2 to 10 MW/cm2 range. Time resolved PL of one of these DHs suggest a recombination lifetime on the order of 520 ps.

InGaN ternary alloys can be the basis for light emission from the near UV to the red region of the electromagnetic spectrum. When InGaN/AIGaN double heterostructures emitting different colors are stacked in a single structure, simultaneous emission of different wavelengths will be achieved. If the color and the intensity of emission for each well are adjusted properly, tailored emission spectra, including white light, will be feasible. We demonstrate this concept with two wells emitting at different wavelengths that are stacked between AIGaN barrier layers. The emitted PL spectra for the stacked structure is found to be the superposition of the emission from the individual double heterostructures that were grown separately.

The development of high quality indium based III-nitride compounds is lagging behind the corresponding aluminum and gallium based compounds. Potential problems confronting the growth of epitaxial and double heterostructure InGaN will be discussed. A mass balance model is presented describing the competing reaction pathways occurring during the growth of indium containing compounds. Atomic layer epitaxy and metalorganic chemical vapor deposition grown InGaN films will be used to explain this model. Also, the growth parameters leading to the attainment of high InN percentages, reduced indium metal formation, and improved structural and optical properties of indium containing nitrides will be discussed.

High quality InGaN films have led to the development of LEDs and blue lasers. The quaternary AlInGaN however, represents a more versatile material since the bandgap and lattice constant can be independently varied. We report on such films grown on (0001) sapphire substrates in an atmospheric pressure MOCVD reactor at 750-800°C. The ternary films have a composition of up to 40% InN and the quaternary films were grown in the composition range of 0 to 20% AlN and 0 to 20% InN. The quaternary compositions studied by EDS and the lattice constants from double crystal XRD followed Vegard's law indicating solid solubility for the range studied. Room temperature PL is dominated by band edge emission for InGaN and AlInGaN, at low AlN%. Higher AlN alloys of AlInGaN had PL dominated by deep levels. AlInGaN/InGaN and AlGaN/InGaN heterostructures were grown with abrupt interfaces. We emphasize the most important growth parameters for the growth of high quality ternary and quaternary thin films. The structural, electrical and optical properties of these In-based ternary and quaternary films and their lattice matched and strained heterostructures will also be presented.

The emission wavelength of the InxGa1−xN ternary system can span from the near ultraviolet through red regions of the visible spectrum. High quality double heterostructures with these InxGa1−xN active layers are essential in the development of efficient optoelectronic devices such as high performance light emitting diodes and laser diodes. We will report on the MOCVD growth and characterization of thick and thin InGaN films. Thick InxGa1−xN films with values of x up to 0.40 have been deposited and their photoluminescence (PL) spectra measured. AlGaN/InGaN/AlGaN double heterostructures (DHs) have been grown that exhibit PL emission in the violet, blue, green and yellow spectral regions, depending on the growth conditions of the thin InGaN active layer. Preliminary results of an AllnGaN/InGaN/AllnGaN DH, with the potential of realizing a near-lattice matched structure, will also be presented.

InGaN based optical devices can cover from the violet through orange regions of the visible spectrum. Difficulties in the growth of this alloy, which have impeded its applications, include problems such as the high vapor pressure of In, weak In-N bonds and lack of sufficient nitrogen during growth. We report on the MOCVD growth of InxGa1−xN (0 < x < 0.4) on sapphire substrates in the 750 - 800 °C temperature range. X-ray diffraction data show full width at half maximum line widths as narrow as 250 arcsec for low values of x, while films with higher lnN% exhibit broader line widths. Room temperature photoluminescence spectra exhibit band edge emission, with emission from deep levels increasing with x. Preliminary investigations of AlGaN/lnGaN/AlGaN double heterostructures have been conducted.

AlGaInN quaternary alloy based devices can cover the emission wavelength from deep UV to red. This Quaternary alloy also offers lattice matched heterostructures for both optical and microwave devices. We will report on the MOCVD growth of AlxGa1−x-yInyN (0<x<0.12), (0<y<0.15) at 750 °C on sapphire substrates, using TMG, EDMIn, TMAl and NH3 precursors. Chemical composition, lattice constants and bandgaps of the grown films were determined by EDS, X-ray diffraction and room temperature PL. Data indicates that the lattice constants can also be deduced using Vegard's law, indicating a solid solution of this alloy. PL showed band edge emission, however emission from deep levels was also observed. Optimized growth conditions and heterostructures using this quaternary alloy will be presented.