Han-Wen Liu

A Novel Coaxial-Structured Amorphous-Silicon p-i-n Solar Cell With Al-Doped ZnO Nanowires

A novel coaxial-structured amorphous-silicon (a-Si) p-i-n solar cell with 1-μm-long low-temperature hydrothermally synthesized Al-doped-ZnO (AZO) nanowires was demonstrated for the first time. The conversion efficiency η increased from 3.92% to 4.27% when the intrinsic a-Si thickness was increased from 25 to 150 nm and then decreased to 3.66% when the intrinsic layer thickness was further increased to 250 nm. It was attributed to an excessively thick intrinsic a-Si layer that would decrease the internal electrical field and interfere with charge separation. With the optimum intrinsic a-Si thickness of 150 nm, the conversion efficiency increased from 4.27% to 4.73% when the AZO wire length was increased from 1 to 2 μm. Moreover, the proposed coaxial-structured solar cell exhibited a nearly 46% efficiency enhancement over a conventional a-Si thin-film solar cell.

Gate-All-Around Poly-Si TFTs With Single-Crystal-Like Nanowire Channels

The gate-all-around (GAA) poly-Si thin-film transistors (TFTs) with single-crystal-like nanowire (NW) channels (SCLNCs) are demonstrated and characterized. Via the nanoscale nitride spacer, the Si NW can be easily transformed within one crystalline grain of the two-shot sequential-lateral-solidification poly-Si film. As compared with the planar ones, the GAA-SCLNC TFTs showed more excellent characteristics. The results clearly show that the variations of device characteristics can be reduced by increasing the numbers of NWs in the channels and an average mobility above 410 cm2/V·s with a low standard deviation can be achieved for the GAA-SCLNC TFTs with 20-NW channels.

High-Performance Single-Crystal-Like Nanowire Poly-Si TFTs With Spacer Patterning Technique

In this letter, high-performance single-crystal-like nanowire poly-Si TFTs with simple spacer patterning technique were demonstrated and characterized. Due to the nanoscale dimension formed by spacer patterning technique, each nanowire is easily transformed within one crystalline grain of the standard sequential-lateral-solidification (SLS) poly-Si film with the regularly arranged grains and thus performed with a single-crystal like device channel. Due to the high-crystallinity channel, together with the tri-gated structure, the fabricated devices revealed good device integrity of high field-effect mobility of 477 cm2/V · s and good ON/OFF current ratio of 1.07 × 108.

Physical properties of ZnO thin films codoped with titanium and hydrogen prepared by RF magnetron sputtering with different substrate temperatures

Transparent conducting titanium-doped zinc oxide (TZO) thin films were prepared on glass substrates by RF magnetron sputtering using 1.5 wt% TiO2-doped ZnO as the target. Electrical, structural, and optical properties of films were investigated as a function of H2/(Ar + H2) flow ratios (RH) and substrate temperatures (TS). The optimal RH value for achieving high conducting TZO:H thin film decreased from 10% to 1% when TS increased from RT to 300°C. The lowest resistivity of 9.2 × 10-4 Ω-cm was obtained as TS = 100°C and RH = 7.5%. X-ray diffraction patterns showed that all of TZO:H films had a hexagonal wurtzite structure with a preferred orientation in the (002) direction. Atomic force microscopy analysis revealed that the film surface roughness increased with increasing RH. The average visible transmittance decreased with increasing RH for the RT-deposited film, while it had not considerably changed with different RH for the 300°C-deposited films. The optical bandgap increased as RH increased, which is consistent with the Burstein-Moss effect. The figure of merits indicated that TS = 100°C and RH = 7.5% were optimal conditions for TZO thin films as transparent conducting electrode applications.

Improve the properties of p-i-n α-Si:H thin-film solar cells using the diluted hydrochloric acid-etched GZO thin films

Gallium-doped zinc oxide (GZO) thin films were deposited on glass, and the process parameters are RF power of 50W and working pressure of 5mTorr, and the substrate temperature was changed from roomtemperature to 300°C. At first, the thickness was around 300 nm by controlling the deposition time. The effects of substrate temperature on the crystallinity, lattice constant (c), carrier mobility, carrier concentration, resistivity, and optical transmission rate of the GZO thin films were studied. The 200°C-deposited GZO thin films had the best crystallinity, the larger carrier concentration and carrier mobility, and the lowest resistivity. For that, the thickness of the GZO thin films was extended to around 1000 nm. Hydrochloric (HCl) acid solutions with different concentrations (0.1%, 0.2%, and 0.5%) were used to etch the surfaces of the GZO thin films, which were then used as the substrate electrodes to fabricate the p-i-n α-Si:H thin-film solar cells. The haze ratio of the GZO thin films increased with increasing HCl concentration, and that would effectively enhance light trapping inside the absorber material of solar cells and then improve the efficiency of the fabricated thin-film solar cells.

New Current Programmed Pixel Circuit for Active-Matrix Organic Light-Emitting-Diode Displays

A new current programmed pixel circuit for active-matrix organic light-emitting-diode displays (AMOLED) was developed by using low temperature polycrystalline silicon (LTPS) technology. The new pixel circuit adopted not only asymmetric current mirror to reduce pixel charging time but two storage capacitors to increase OLED current holding ratio. The proposed current programmed pixel circuit kept high aperture ratio of the pixel and accurate OLED current despite parasitic resistance of power lines. Therefore, this pixel circuit is suitable for large size and high resolution AMOLED displays.

Superior Reliability of Gate-All-Around Polycrystalline Silicon Thin-Film Transistors with Vacuum Cavities Next to Gate Oxide Edges

The electrical characteristics and reliability of n-type gate-all-around (GAA) polycrystalline silicon thin-film transistors (poly-Si TFTs) with vacuum cavities next to the gate oxide edges are investigated. This novel structure is successfully fabricated by spacer formation, partial wet etching of a gate oxide, and in situ vacuum encapsulation. The electrical characteristics of the GAA poly-Si TFTs with vacuum cavities are superior to those of traditional GAA poly-Si TFTs because the vacuum cavity serves as an offset region to decrease the leakage current in the OFF state and as a field-induced drain (FID) to sustain the on-current in the ON state. In addition, regardless of whether static or dynamic electrical stress is imposed on these devices, the GAA poly-Si TFTs with vacuum cavities exhibit superior reliability to traditional ones owing to the simultaneous reduction of vertical and lateral electric fields near the drain junction during bias stressing due to the greater equivalent gate oxide thickness on the gate electrode edges.

Reduction of Kink Effect of Poly-Si TFT With Bottom Field Plate by Dispersing Current Density Technique

A new multiple-gate poly-Si thin-film transistor (TFT) with a bottom field plate (FP) is proposed. The FP disperses the high current density away from the top corner of the spacer channel with the highest electric field, leading to an improved kink effect. Moreover, owing to an inversion layer induced by the FP at the bottom region of spacer poly-Si channel, a higher on-state current is achieved. In addition, the electric field near drain area is reduced, leading to a lower off-state current. Overall, the new multiple-gate poly-Si TFT shows improved characteristics as compared with the counterpart without a bottom FP.

An abnormal threshold voltage variation of the p-type thin-film transistors under DC bias stress

The abnormal turnaround phenomenon of threshold voltage for the p-type low temperature poly-silicon thin film transistors (LTPS TFTs) stressed under a specific negative DC bias condition, which the gate voltage is about one half of the drain voltage, is investigated. There are two turnaround points for the TFT stressed with prolonged time. The sampling current of the TFT under the biasing stress is used to confirm the turnaround phenomenon. We propose the three-staged degradation models to explain the abnormal threshold voltage variation of LTPS TFTs under a specific DC bias stress.

Comparative Study on Physical Properties of ZnO:Al Thin Films by in Situ Hydrogen Doping and Hydrogen Plasma Post-Treatment

ZnO:Al (AZO) thin films were prepared on glass substrates by radio frequency magnetron sputtering at 300℃. Effects of H_2/(H_2 + Ar) flow rate ratio (R_H) during sputtering and post-deposition hydrogen plasma treatment on structural, electrical, and optical properties of AZO thin films were investigated and compared. For the films deposited with different R_H, all the films exhibited a (002) preferred orientation only along the c-axis and their 2θ angles decreased with increasing R_H. The lowest resistivity was obtained for the samples sputtered with R_H of 2%. The average optical transmittance in the visible wavelength region (400~700 nm) was over 85% and it slightly increased with increasing R_H. For the hydrogen plasmatreated films, the 2θ position of the (0 0 2) peak shifted to high angle side. The electrical resistivities of the hydrogen plasma-treated AZO (R_H =0) and AZO:H (R_H =2%) thin films decreased by 78% and 32%, respectively, compared to those without plasma treatment. It reveals that the hydrogen plasma treatment improves the conductivity of the non-hydrogen-doped AZO film more effectively than the hydrogen-doped one. Besides, hydrogen plasma treatment nearly did not change the optical transmittance in the visible wavelength region but broadened the optical bandgap of the film. These results suggest that the postdeposition hydrogen plasma treatment is more effective than in situ hydrogen doping in improving electrooptical properties of the AZO thin films.