In
this study, the impact of moisture on the electrical characteristics
of an amorphous In–Ga–Zn–O thin-film transistor
(a-IGZO TFT) was investigated. In commercial applications of such
TFTs, high stability and quality performance in humid environments
are essential. During TFT operation under ambient moisture, the electrolysis
of water molecules occurs via the tip electric field effect. Hydrogen
diffuses from the etch-stop layer or back-channel into the main channel
under a negative electric field. The hydrogen atoms act as shallow
donors (which causes the carrier concentration in the channel to rise),
causing the threshold voltage (VTH) to shift in the negative
direction. Hydrogen diffusion from the overlap of the source/drain
and gate electrodes to the channel center caused by the tip electric
field induces a significant barrier lowering and VTH shifts
in a short-channel device. However, under negative bias stress (NBS)
in ambient moisture, the negative VTH shift is more obvious
in short- than in long-channel devices, indicating suppressed hydrogen
diffusion in long-channel devices. This is attributed to the electrolysis
of water by the tip electric field at the source, drain, and gate
electrodes, which causes hydrogen to diffuse to the center of the
channel. Here, a novel physical model of the capacitance–voltage
(C–V) electrical property
changes under ambient moisture is proposed, based on the early appearance
of abnormalities in the C–V measurements. The electrolysis of water caused by the tip electric
field and electrical abnormalities caused by hydrogen diffusion into
the a-IGZO active layer are explained by this model. A secondary-ion
mass spectrometry analysis shows that hydrogen content in the channel
generally increases under NBS in ambient moisture. The degradation
behavior due to moisture in a-IGZO is clarified. Thus, inhibiting
the tip electric field may benefit future flexible-display and gas-sensing
applications.
The ferroelectric material KTaO3 (KTO) has a very high refractive index, which is advantageous to the photonic crystal (PC) design. KTO polycrystalline crystal has a high extinction coefficient. In this work, we perform a theoretical study of the transmission properties of a PC bandpass filter made of polycrystalline KTO at terahertz (THz) frequencies. Our results show that the defect modes of usual PC narrowband filters no longer exist because of the existence of the high loss. We provide a new PC structure for the high-extinction materials and show that it has defect modes in its transmittance spectra, providing a possible bandpass filter design in the THz region.
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