| Summary: | 博士 === 國立中興大學 === 物理學系所 === 95 === In this study, we examine the properties of the low-frequency excess noise of the GaN nanowires, which are grown by Vapor-Liquid-Solid method. The diameter of these nanowires is about 80 to 150nm. The 300-400 nm-wide Al electrodes to the nanowires are defined by e-beam lithography. The two-wire resistances of the samples are from few kohms to hundreds of kohms at room temperature.
At room temperature, the behavior of the 1/f noise follows the Hooge''s equation in both two-wire and four-wire measurements. The Lorentzian noise is observed at the sufficient applied bias current from 303 K to 135 K. The characteristic relaxation time associated with the Lorentzian noise is temperature dependent.
From the characteristic time and the power spectral density of the Lorentzian noise, we can obtain the activation energy of the carrier trapping process.
In the result of two-wire temperature-dependent measurement, we find there exist two temperature regimes where activation behavior is observed. From the Arrhenius plot, we find that activation energy, Ea, decreases from 818 meV to 745.8 meV in the temperature range from 300 K to 265 K, and Ea decreases from 129.6 meV to 105.9 meV for T from 205 K to 135 K with different bias currents, from 2 nA to 8 nA.
The activation energy Ea from the four-wire measurement is
41.7 meV, and E0 = 37.7 meV and E1 = 41.9 meV from
Levinshrein''s model. For the nanowires, the activation energy of the trap is close to the ionization energy of the wutzite GaN bulk material. We also observed that the Lorentzian noise in the two-wire measurement is much more significant than that in four-wire measurement. The noise magnitude of the 1/f noise in two-wire measurements is also resistance dependent. This indicate that most of the noise comes from the metal-semiconductor contact region.
We also find the correlation of noise between adjacent nanowire sections might be caused by the strong voltage fluctuations under the contact region in between, which may contain complicated alloy or defects consisting of GaN and Al or Ti/Au. Such kind of fluctuations may generate coherent current or voltage signals in the nanowire nearby.
From our investigation, the low-frequency excess noise of GaN nanowires is smaller than that of carbon nanotubes . This makes GaN nanowire a promising material of nanodevices, such as photodetectors, sensors, and low-frequency transistors.
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