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The low frequency noise of individual ZnO nanowire (NW) field effect transistors (FETs) exposed to air is systematically characterized. The measured noise power spectrum shows a classical 1/f type. The noise amplitude is independent of source-drain current and inversely proportional to gate voltage. The extracted Hooge's constant of ZnO NW is found to be 6.52×10−3. In addition, the low frequency noise of ZnO NW according to NW resistance and contact property are investigated. The noise amplitude is proportional to the square of ZnO NW resistance. If a sample shows a nonlinear current-voltage (I-V) characteristic due to a poor electrical contact, the noise power spectrum is proportional to the third power of current instead of the square of current.
Field effect transistors(FETs) made of ZnO nanowires are very sensitive to the gas environment, so that the passivation can be a good way to get reliable nanowire FETs with longer lifetime and the better mobility. The studies on the passivation effects with the positive electron-beam resist was investigated by selectively covering the part of nanowire devices between the electrodes. Reproducible electrical characteristics were recorded, reflecting the stable electrical properties by the passivation which deters the degradation of a device. Considering the defect states of oxide nanowires dominate the charge states, the pre-state just before the passivation process will be crucial to understand the reproducible and controllable device characteristics of nanowire devices.
(001) oriented (Ba, Sr)TiO3 (BST) thin films were deposited on MgO (001) single crystal substrates by the pulsed laser deposition method. Structural properties of BST films were investigated using X-ray diffractometer. Coplanar waveguide (CPW) device based on BST/MgO layer structure was fabricated by dc sputtering deposition, photolithography and etching process. To study the geometrical factor dependent microwave performance of the CPW phase shifter based on (001) oriented BST film, the CPW devices having various gap and width were fabricated. The microwave dielectric properties of BST CPW phase shifter devices were examined by calculating the scattering parameter obtained using a HP 8510C vector network analyzer with the frequency range 0.5 ∼ 20 GHz at room temperature under the dc bias field of 0 ∼ 40V. The measured return loss and insertion loss at 10 GHz with no dc bias were about -12 ∼ -4 dB and -14 ∼ -3 dB, respectively, which mainly depended on the impedances of the CPW transmission lines. The measured differential phase shift values were about 20 ° ∼ 140 ° at 10 GHz with 40 V dc bias variations, which depended on the gap size.
Microwave properties of coplanar waveguide (CPW) transmission lines fabricated on high dielectric materials, such as ferroelectric Ba1−xSrxTiO3 films, are highly sensitive on the dimension and shape of electrodes. A small change in device dimension affects the total electrical length of the CPW, which may mislead the effective dielectric constant of the dielectric layer. Furthermore, extracting dielectric constant of high-k thin films from the measured microwave properties, such as S-parameters, is very difficult. The well known a modified conformal mapping method frequently exhibits an inconsistent dielectric constant for CPW on high-k materials. CPW transmission lines were fabricated on high-k thin films, ferroelectric Ba0.6Sr0.4TiO3, which were deposited by the pulsed laser deposition with partial oxygen backgrounds. A large phase shift angle of 100° at 10 GHz was observed from the CPW (gap = 4 μm, length = 3 mm) with a 40 V of dc bias, which supports that the idea of the tunable microwave device application using ferroelectrics films. The dielectric constant of the thin ferroelectric film was extracted from the dimension of the CPW (gap, width, length) and the measured S-parameters by a modified conformal mapping. However, the dielectric constant of the ferroelectric thin film calculated by a modified conformal mapping exhibits a gap dependency; dielectric constant (990 ∼ 830) decreases with increasing gap size (4 ∼ 19 μm, respectively). For comparison, dielectric properties have been extracted by extensive EM-simulation using a HFSS™ (Ansoft) with observed dimensions of CPW devices. Total phase, which is closely related with the dielectric constant of the film, is strongly affected by gap size, film thickness, and slanted angle of CPW.
The effects of anisotropic dielectric properties of ferroelectric Ba1-xSrxTiO3 (BST) films on the characteristics of phase shifter have been studied in microwave regions at room temperature. Ferroelectric BST films with (001) and (011) orientation were epitaxially grown on (001) and (011) MgO substrates, respectively, by pulsed laser deposition method. The structures of BST films were investigated using x-ray diffraction measurement. The microwave properties of orientation engineered BST films were investigated using coplanar waveguide transmission lines that were fabricated on BST films using a thick metal layer by photolithography and etching process. The measured differential phase shift and insertion loss (S21) for (011) BST films are larger than those for (001) BST films. Dielectric constants of the ferroelectric BST films are calculated from the measured S21 using a modified conformal-mapping model.
The ferroelectric (Ba0.6Sr0.4)TiO3 (BST) films were prepared on (001) MgO single crystals by pulsed laser deposition. Coplanar waveguide (CPW) type phase shifters controlled by external dc bias field were fabricated on BST films using a 2 μm thick metal layer to reduce metal loss. Microwave properties of the CPW phase shifter were measured using a HP 8510C vector network analyzer from 0.1 – 20 GHz. The fabricated CPW phase shifters (8 mm long) exhibited differential phase angle of 100 ° at 10 GHz with a dc bias field of less than 80 kV/cm between center and ground conductors. Furthermore, a stable differential phase angle (102 ± 3.5 o) was observed from another CPW while changing the power of incident microwave from -10 to +30 dBm. Gap size dependent dielectric constant of the BST film was observed and a simple correction method was suggested in the paper. These results demonstrate the possible application of ferroelectric tunable devices on a high power tunable wireless telecommunication.
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