Chia-Tsung Chang

Pillar Height Dependence of Field-Emission Properties in an Array of Carbon Nanotube Pillars

Carbon nanotube pillars with optimal field-emission properties, including a high field enhancement factor β of 5384 and a low turn-on field Eto of 0.84 V/µm, have been achieved when the ratio of interpillar spacing to pillar height is 2. However, when this ratio exceeds 2, the field enhancement factor increases with increasing pillar height since the field can be enhanced by increasing the aspect ratio. When the ratio is smaller than 2, the field enhancement factor decreases with increasing pillar height owing to the increased field-screening effect. A simulation has been performed to verify the experimental results.

Sensitivity Enhancement of Ultraviolet Photodetectors With the Structure of p-NiO/Insulator-SiO 2 /n-ZnO Nanowires

A high-performance photodetector with the structure of NiO/SiO₂/ZnO nanowires has been proposed. The devices with 6-nm-thick SiO₂ exhibited a better rectification ratio (Jforward/Jreverse) of 246 at ±2 V, lower dark current density (Jdark) of 3.5 × 10^-7 A/cm² at a reverse bias of 2 V, and superior ultraviolet (UV) sensitivity (IUV/Idark) of 16.23 than those without the SiO₂ layer (J_forward/J_reverse = 44, Jdark = 4.7 × 10^-6 A/cm², and IUV/Idark = 5.5). The improved performance was mainly due to the ultrathin inserted SiO₂ layer that builds a barrier height to minimize the transmission probability of low-energy carriers, leading to the enhancement of the UV sensing characteristics.

Comparison of poly-Si films deposited by UHVCVD and LPCVD and its application for thin film transistors

The ultra-high vacuum chemical vapor deposition (UHVCVD) system can deposit poly-Si film without any laser or furnace annealing. The uniformity of threshold voltage and mobility is superior to that deposited by low-pressure chemical vapor deposition (LPCVD) system. However, due to the deposition in polycrystalline phase for UHVCVD, the film surface is rough and results in low field effect mobility compared to that obtained by LPCVD using disilane (Si2H6) in amorphous phase followed by solid phase crystallization (SPC). The on–off current ratio for UHVCVD deposited poly-Si thin film transistors (TFTs) is approximately one order smaller, however, the leakage current for LPCVD SPC TFTs is higher. In this experiment, NH3 was introduced to both of the two samples to improve the device performance. It can be shown that improvements on device characteristics are more significant for UHVCVD deposited poly-Si TFTs, e.g. threshold voltage decreased dramatically and the on–off current ratio improved by two orders of magnitude. r 2002 Elsevier Science Ltd. All rights reserved.

High-sensitivity extended-gate field-effect transistors as pH sensors with oxygen-modified reduced graphene oxide films coated on different reverse-pyramid silicon structures as sensing heads

A high-performance extended-gate field-effect transistor (EGFET) as pH sensor with its microstructured sensing head composed of an oxygen-modified reduced graphene oxide film (RGOF) on a reverse-pyramid (RP) Si structure was developed to achieve a high sensitivity of 57.5 mV/pH with an excellent linearity of 0.9929 in a wide pH sensing range of 1–13. These features were ascribed to the large amount of sensing sites and large sensing area. In contrast, the planar Si substrate with the oxygen-plasma-treated RGOF (OPT-RGOF) at the optimal bias power showed a sensitivity of 52.9 mV/pH compared with 45.0 mV/pH for that without plasma treatment. It reveals that oxygen plasma can produce oxygen-containing groups as sensing sites, enhancing proton sensing characteristics. However, oxygen plasma treatment at high bias powers would cause damage to the RGOFs, resulting in poor conducting and sensing properties. On the other hand, the use of the RP structures could increase the effective sensing area and further promote the sensing performance.

Annealing effect on the photoluminescence characteristics of ZnO-nanowires and the improved optoelectronic characteristics of p-NiO/n-ZnO nanowire UV detectors

Transparent ultraviolet (UV) detectors with nanoheterojunctions (NHJs) of p-type NiO and n-type ZnO nanowires (ZnO-NWs) were successfully fabricated using a DC sputtering system and a hydrothermal process, respectively. After annealing in nitrogen ambient, the near-band-edge emission to deep level emission ratio (NBE/DLE) of ZnO-NWs gradually increased as the temperature increased and reached a maximum of 28.9 at a temperature setting of 500 °C. In contrast, after annealing in oxygen atmosphere, the NBE/DLE of ZnO-NWs initially increased from 1.2 to 5.9 and then decreased to 3.2. At a reverse bias of 2 V, the devices with the 500-°C-N2-annealed ZnO-NWs exhibited better sensitivity (JUV/JDark = 5.65; JVisible/JDark = 1.35) to UV light (365 nm, 0.3 mW/cm2) than those with the as-grown ZnO-NWs (JUV/JDark = 4.98; JVisible/JDark = 3.82) because the structural defects in ZnO-NWs were effectively eliminated after annealing in nitrogen ambient at 500 °C.

Highly Sensitive pH Sensors of Extended-Gate Field-Effect Transistor With the Oxygen-Functionalized Reduced Graphene Oxide Films on Reverse Pyramid Substrates

The oxygen-plasma-treated reduced graphene oxide films (OPT-RGOFs) as the pH sensing membranes for the extended-gate field-effect transistors were demonstrated to achieve the higher pH sensitivity of 52 mV/pH and better linearity of 0.996 in a wide sensing range of pH 1-13 than those without plasma treatment. It was attributed to the oxygen-containing functional groups on the RGOF induced from the plasma treatment. In addition, the OPT-RGOFs sprayed on the reverse pyramid substrates were also proposed to further enhance the sensing sites, leading to a superior pH sensitivity of 57 mV/pH with an excellent linearity of 0.996.

High Field-Emission Stability of Offset-Thin-Film Transistor-Controlled Al-Doped Zinc Oxide Nanowires

Aluminum-doped zinc oxide (AZO) nanowire (NW) arrays incorporating an offset thin-film transistor (offset-TFT) have been proposed to achieve high field-emission (FE) stability. The AZO NW field emission arrays (FEAs) were hydrothermally grown at a low temperature of 85 °C. The uncontrolled AZO NW FEAs demonstrated superior FE characteristics (i.e., turn-on field of ~2.17 V/µm and threshold field of ~3.43 V/µm) compared with those of the conventional CNT FEAs grown at a temperature below 600 °C. However, uncontrolled AZO NW FEAs show a larger current fluctuation of 15.6%. Therefore, the offset-TFTs were used to control the AZO NW FEAs. Consequently, the fluctuation of AZO NW FEAs could be significantly reduced to less than 2%. This novel field emission device exhibits good emission stability, low-voltage controllability, low-temperature processing, and structural simplicity, making it promising for applications in flat panel displays.

A W-spacer GOLD TFT with high performance and high reliability

In this paper, we successfully fabricated Gate-Overlapped Lightly-doped Drain (GOLD) polycrystalline silicon thin-film transistors (poly-Si TFTs) with selectively deposited spacers. Under appropriate deposition conditions, tungsten (W) films can be selectively deposited on poly-Si gate electrodes to form spacers without any additional etching process. Compared to the conventional poly-Si TFTs without LDD structures, our devices effectively suppress the kink effect and the punch-through phenomenon in short-channel devices. The hot-carrier reliability of our devices is also improved due to the reduced electric field on the drain side. In addition, the transconductance of our devices is compatible to that of conventional devices. This is because the W-spacer acts as a part of gate electrode to induce channel when the device is operated under ON state.

A novel density control of carbon nanotubes by partial oxidation of catalyst metal and its field emission enhancement

A novel density control of carbon nanotubes is fabricated using partial oxidation of catalyst metal prior to the CNTs growth. The results show that CNTs are aligned, closely spaced, and divided into two groups with some long nanotubes protruding among short ones. Field emission improvement is achieved due to those long nanotubes are subjected less field screening effect from the surrounding nanotubes. The obtained results show that a low turn-on field (1.9 V/μm) and an ultra high field emission current density (160mA/cm2 at 6 V/μm) can be achieved through this novel morphology of CNTs.

Photoresponse of Zinc oxide thin-film transistors with location-controlled crystal grains fabricated by low-temperature hydrothermal method

Zinc oxide (ZnO) bottom-gate (BG) thin-film transistors (TFTs) with single vertical grain boundary in the channel have been successfully fabricated by a novel low-temperature (i.e. 85 °C) hydrothermal method. The proposed devices demonstrated the high field-effect mobility of 9.07 cm2/V·s, low threshold voltage of 2.25 V, high on/off current ratio above 106, and superior current drivability, attributed to the high-quality ZnO channel with single grain boundary. Moreover, a stable and repeatable operation of dynamic photoresponse is observed for the location-controlled hydrothermally grown ZnO BG-TFTs.