Noise in gallium nitride-based quantum well structures used for nanometer devices in the frequency range 1 Hz--3 Mhz and temperature range 77K--324K

Electronic noise has been investigated in AlxGa1−x N/GaN Modulation-Doped Field Effect Transistors (MODFETs) of submicron dimensions, grown for us by MBE (Molecular Beam Epitaxy) techniques at Virginia Commonwealth University by Dr. H. Morkoç and coworkers. Some 20 devices were grown on a GaN substrate, four of which have leads bonded to source (S), drain (D), and gate (G) pads, respectively. Conduction takes place in the quasi-2D layer of the junction (xy plane) which is perpendicular to the quantum well (z-direction) of average triangular width ∼3 nm. A non-doped intrinsic buffer layer of ∼5 nm separates the Si-doped donors in the AlxGa1−xN layer from the 2D-transistor plane, which affords a very high electron mobility, thus enabling high-speed devices. Since all contacts (S, D, and G) must reach through the AlxGa1−xN layer to connect internally to the 2D plane, parallel conduction through this layer is a feature of all modulation-doped devices. While the shunting effect may account for no more than a few percent of the current IDS, it is responsible for most excess noise, over and above thermal noise of the device. ^ The excess noise has been analyzed as a sum of Lorentzian spectra and 1/f noise. The Lorentzian noise has been ascribed to trapping of the carriers in the AlxGa1−xN layer. A detailed, multitrapping generation-recombination noise theory is presented, which shows that an exponential relationship exists for the time constants obtained from the spectral components as a function of 1/kT. The trap depths have been obtained from Arrhenius plots of log (τT2) vs. 1000/T. Comparison with previous noise results for GaAs devices shows that: (a) many more trapping levels are present in these nitride-based devices; (b) the traps are deeper (farther below the conduction band) than for GaAs. Furthermore, the magnitude of the noise is strongly dependent on the level of depletion of the AlxGa1−xN donor layer, which can be altered by a negative or positive gate bias VGS. ^ Altogether, these frontier nitride-based devices are promising for bluish light optoelectronic devices and lasers; however, the noise, though well understood, indicates that the purity of the constituent layers should be greatly improved for future technological applications. ^

Novel insights into the mechanisms of regulation of tyrosine kinase receptors by Ras Interference 1

Receptor-tyrosine kinases (RTKs) are membrane bound receptors characterized by their intrinsic kinase activity. RTK activities play an essential role in several human diseases, including cancer, diabetes and neurodegenerative diseases. RTK activities have been regulated by the expression or silencing of several genes as well as by the utilization of small molecules. Ras Interference 1 (Rin1) is a multifunctional protein that becomes associated with activated RTKs upon ligand stimulation. Rin1 plays a key role in receptor internalization and in signal transduction via activation of Rab5 and association with active form of Ras. This study has two main objectives: (1) It determines the role of Rin1 in the regulation of several RTKs focusing on insulin receptor. This was accomplished by studying the Rin1-insulin receptor interaction using a variety of biochemical and morphological assays. This study shows a novel interaction between the insulin receptor and Rin1 through the Vps9 domain. Two more RTKs (epidermal growth factor receptor and nerve growth factor receptor) also interacted with the SH2 domain of Rin1. The effect of the Rin1-RTK interaction on the activation of both Rab5 and Ras was also studied during receptor internalization and intracellular signaling. Finally, the role of Rin1 was examined in two differentiation processes (adipogenesis and neurogenesis). Rin1 showed a strong inhibitory effect on 3T3-L1 preadipocyte differentiation but it seems to show a modest effect in PC12 neurite outgrowth. These data indicate a selective function and specific interaction of Rin1 toward RTKs. (2) It examines the role of the small molecule Dehydroleucodine (DhL) on several key signaling molecules during adipogenesis. This was accomplished by studying the differentiation of 3T3-L1 preadipocytes exposed to different concentrations of DhL in different days of the adipocyte formation process. The results indicate that DhL selectively blocked adipocyte formation, as well as the expression of PPARγ, and C/EBP&agr;. However, DhL treatment did not affect Rin1 or Rab5 expression and their activities. Taken together, the data indicate a potential molecular mechanism by which proteins or small molecules regulate selective and specific RTK intracellular membrane trafficking and signaling during cell growth and differentiation in normal and pathological conditions.

Ionized cluster beam deposition, and thin film analysis

In 1972 the ionized cluster beam (ICB) deposition technique was introduced as a new method for thin film deposition. At that time the use of clusters was postulated to be able to enhance film nucleation and adatom surface mobility, resulting in high quality films. Although a few researchers reported singly ionized clusters containing 10$\sp2$-10$\sp3$ atoms, others were unable to repeat their work. The consensus now is that film effects in the early investigations were due to self-ion bombardment rather than clusters. Subsequently in recent work (early 1992) synthesis of large clusters of zinc without the use of a carrier gas was demonstrated by Gspann and repeated in our laboratory. Clusters resulted from very significant changes in two source parameters. Crucible pressure was increased from the earlier 2 Torr to several thousand Torr and a converging-diverging nozzle 18 mm long and 0.4 mm in diameter at the throat was used in place of the 1 mm x 1 mm nozzle used in the early work. While this is practical for zinc and other high vapor pressure materials it remains impractical for many materials of industrial interest such as gold, silver, and aluminum. The work presented here describes results using gold and silver at pressures of around 1 and 50 Torr in order to study the effect of the pressure and nozzle shape. Significant numbers of large clusters were not detected. Deposited films were studied by atomic force microscopy (AFM) for roughness analysis, and X-ray diffraction.^ Nanometer size islands of zinc deposited on flat silicon substrates by ICB were also studied by atomic force microscopy and the number of atoms/cm$\sp2$ was calculated and compared to data from Rutherford backscattering spectrometry (RBS). To improve the agreement between data from AFM and RBS, convolution and deconvolution algorithms were implemented to study and simulate the interaction between tip and sample in atomic force microscopy. The deconvolution algorithm takes into account the physical volume occupied by the tip resulting in an image that is a more accurate representation of the surface.^ One method increasingly used to study the deposited films both during the growth process and following, is ellipsometry. Ellipsometry is a surface analytical technique used to determine the optical properties and thickness of thin films. In situ measurements can be made through the windows of a deposition chamber. A method for determining the optical properties of a film, that is sensitive only to the growing film and accommodates underlying interfacial layers, multiple unknown underlayers, and other unknown substrates was developed. This method is carried out by making an initial ellipsometry measurement well past the real interface and by defining a virtual interface in the vicinity of this measurement. ^

Temperature dependence and characterization of gold thin film actuated membranes for radio frequency microelectromechanical switches

Over the last 10 years, the development and the understanding of the mechanical properties of thin film material have been essential for improving the reliability and lifetime in operation of microelectromechanical systems (MEMS). Although the properties of a bulk material might be well characterized, thin-film properties are considerably different from those of the bulk and it cannot be assumed that mechanical properties measured using bulk specimens will apply to the same materials when used as a thin film in MEMS. For many microelectronic thin films, the material properties depend strongly on the details of the deposition process and the growth conditions on its substrate. ^ The purpose of this dissertation is to determine the temperature dependence of a gold thin film membrane on the pull down voltage of a MEMS switch as the temperature is varied from room temperature (300 K) to cryogenic temperature (10 K). For this purpose, an RF MEMS shunt switch was designed and fabricated. The switch is composed of a gold coplanar waveguide structure with a gold bridge membrane suspended above an area of the center conductor which is covered by a dielectric (BaTiO3). The gold membrane is actuated by an electrostatic force acting between the transmission line and the membrane when voltage is applied. ^ Material characterization of the gold evaporated thin film membrane was obtained via AFM, SEM, TEM and X-ray diffraction analyses. A mathematical relation was used to estimate the pull down voltage of the switch at cryogenic temperature and results showed that the mathematical theory match the experimental values of the tested MEMS switches. ^

Insulin-like growth factor-I gene expression as a growth indicator in Nile tilapia Oreochromis niloticus L.

Somatic growth in fishes is regulated by a variety of hormones. A central step in this hormonal network is the growth hormone-insulin-like growth factor-I (IGF-I) axis. Studies conducted evaluated the relationship of hepatic IGF-I (hIGF-1) mRNA with growth as affected by feeding regimes (satiation or restricted level; daily or alternate-day feeding), temperatures (high, ambient, low) and by social stress. To develop a cellular means for the quantification of hIGF-I mRNA levels in O. niloticus, hIGF-I cDNA was isolated and cloned. The partial sequence of IGF-I cDNA encodes for signal peptide, mature protein and a portion of the E-domain. A sensitive TaqMan quantitative reverse transcription-polymerase chain reaction (qRT-PCR) assay was developed based on the mature IGF-I. Using the developed qRT-PCR assay a significant positive correlation was observed between hIGF-I mRNA levels and growth rate of fish reared under different feeding regimes (r = 0.64) and temperature conditions (r = 0.64). On the dynamics of hIGF-I gene expression in response to elevated temperature, hIGF-I mRNA levels were significantly elevated after at least 2 days of exposure to warm temperature. This validates the concept that hIGF-I gene expressions are sufficiently sensitive to be used as a rapid growth rate indicator for O. niloticus. The hIGF-I levels have a significant positive correlation with specific growth rate (length; r = 0.92), and with condition factor (r = 0.55). On the effect of social stress, differential alterations in growth rates between the dominant and subordinates were observed which was attributed more to behavioral changes as transduced by physiological regulators. The fish's relative position in the social hierarchy was consistently reflected in the levels of hIGF-I mRNA and the eye color pattern. Subordination depressed hIGF-I levels while dominance stimulated it. These findings have shown that hGF-I level remained positively correlated to growth rate as affected by feeding regime, temperature and social stress. This suggests that hIGF-I plays a key role in controlling growth in O. niloticus and indicates that IGF-I mRNA quantification could prove useful for the rapid assessment of growth rate in this species of fish.