Sheng-Po Chang

Electroluminescence from n-ZnO nanowires/p-GaN heterostructure light-emitting diodes

The investigation explores the fabrication and characteristics of ZnO nanowire (NW)/p-GaN/ZnO NW heterojunction light-emitting diodes (LEDs). Vertically aligned ZnO NWs arrays were grown on the p-GaN substrate. The n-p-n heterojunction LED was fabricated by combining indium tin oxide/glass substrate with the prepared ZnO NWs/p-GaN substrate. The symmetrical rectifying behavior demonstrates that the heterostructure herein was formed with two p-n junction diodes and connected back to back. The room-temperature electroluminescent emission peak at 415 nm was attributed to the band offset at the interface between n-ZnO and p-GaN and defect-related emission from ZnO and GaN. Finally, the photograph indicated the LED clearly emitted blue light.

Ultraviolet photodetectors with ZnO nanowires prepared on ZnO:Ga/glass templates

Vertically well-aligned ZnO nanowire ultraviolet (UV) photodetectors were fabricated by spin-on-glass technology on ZnO:Ga/glass templates. With 2V applied bias, it was found that dark current density of the fabricated device was only 2.0×10−7A∕cm2. It was also found that UV-to-visible rejection ratio and quantum efficiency of the fabricated ZnO nanowire photodetectors were more than 1000 and 12.6%, respectively.

Highly Sensitive ZnO Nanowire Acetone Vapor Sensor With Au Adsorption

In this study, the growth of high-density single-crystalline ZnO nanowires on patterned ZnO:Ga/ SiO2/Si templates was reported. We also adsorbed Au onto nanowire surfaces and fabricated ZnO nanowire acetone vapor sensors. With 200-ppm acetone vapor concentration, it was found that we could enhance the device sensitivities at 300deg C from 18.5% to 82.5% by Au adsorption. It was also found that measured responses at 300degC were around 52%, 61%, 71%, 77%, and 82% when the accumulative acetone vapor concentration reached 5, 10, 50, 100, and 200 ppm, respectively, for the ZnO nanowire sensor with Au adsorption.

ZnO Nanowire-Based Oxygen Gas Sensor

We report growth of vertically well-aligned ZnO nanowires on ZnO:Ga/glass templates and the fabrication of resistive ZnO nanowire-based oxygen gas sensor. It was found that the ZnO nanowires are grown preferred oriented in the (002) direction with a small X-ray diffraction full-width-half-maximum. From high resolution transmission electron microscopy, scanning electron microscopy and micro-Raman measurements, it was found that the ZnO nanowires prepared in this study are single crystalline with good crystal quality. It was also found that measured sample resistance increased logarithmically as the oxygen gas pressure in the chamber was increased. Such a relationship suggests that the device is potentially useful for resistive oxygen gas sensing at room temperature.

Electrical and Optical Characteristics of UV Photodetector With Interlaced ZnO Nanowires

A zinc oxide (ZnO) nanowire (NW) photodetector was fabricated with a simple method by bridging the gap of interdigitated gallium-doped ZnO pattern deposited on a silicon oxide (SiO₂) thin film with interlaced ZnO NWs. With an incident wavelength of 375 nm, it was found that measured responsivity was 0.055 A/W for the interlaced ZnO NWs photodetector with a 1 V applied bias. The transient time constants measured during the turn-on and turn- off states were τ_ON = 12.72 ms and τ_OFF = 447.66 ms , respectively. Furthermore, the low-frequency noise spectra obtained from the ultraviolet photodetector were purely due to the 1/<i>f</i> noise. Besides, the noise equivalent power and normalized detectivity (<i>D</i>^*) of the ZnO NW photodetector were 2.32 ×10^-9 W and 7.43 ×10⁹ cm·Hz^0.5·W^-1, respectively.

Inserting a p-InGaN layer before the p-AlGaN electron blocking layer suppresses efficiency droop in InGaN-based light-emitting diodes

In this study, we observed a dramatic decrease in the efficiency droop of InGaN/GaN light-emitting diodes after positioning a p-InGaN insertion layer before the p-AlGaN electron-blocking layer. The saturated external quantum efficiency of this device extended to 316 mA, with an efficiency droop of only 7% upon increasing the operating current to 1 A; in contrast, the corresponding conventional light-emitting diode suffered a severe efficiency droop of 42%. We suspect that the asymmetric carrier distribution was effectively mitigated as a result of an improvement in the hole injection rate and a suppression of electron overflow.

Simple Fabrication Process for 2D ZnO Nanowalls and Their Potential Application as a Methane Sensor

Two-dimensional (2D) ZnO nanowalls were prepared on a glass substrate by a low-temperature thermal evaporation method, in which the fabrication process did not use a metal catalyst or the pre-deposition of a ZnO seed layer on the substrate. The nanowalls were characterized for their surface morphology, and the structural and optical properties were investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), and photoluminescence (PL). The fabricated ZnO nanowalls have many advantages, such as low growth temperature and good crystal quality, while being fast, low cost, and easy to fabricate. Methane sensor measurements of the ZnO nanowalls show a high sensitivity to methane gas, and rapid response and recovery times. These unique characteristics are attributed to the high surface-to-volume ratio of the ZnO nanowalls. Thus, the ZnO nanowall methane sensor is a potential gas sensor candidate owing to its good performance.

High-Efficiency Si Solar Cell Fabricated by Ion Implantation and Inline Backside Rounding Process

We introduce a novel, high-throughput processing method to produce high-efficiency solar cells via a backside rounding process and ion implantation. Ion implantation combined with a backside rounding process is investigated. The ion implantation process substituted for thermal POCl3 diffusion performs better uniformity (<3%). The U-4100 spectrophotometer shows that wafers with backside rounding process perform higher reflectivity at long wavelengths. Industrial screen printed (SP) Al-BSF on different etching depth groups was analyzed. SEMs show that increasing etch depth improves back surface field (BSF). The - measurement revealed that etching depths of 6 μm ± 0.1 μm due to having the highest and , it has the best performance. SEMs also show that higher etching depths also produce uniform Al melting and better BSF. This is in agreement with IQE response data at long wavelengths.

Noise Characteristics of ZnO-Nanowire Photodetectors Prepared on ZnO:Ga/Glass Templates

In this paper, we report the fabrication of vertically well-aligned ZnO nanowire ultraviolet (UV) photodetectors on ZnO:Ga/glass templates. With 1 V applied bias, it was found that dark current density of the device was only 1.37times10^-7 A/cm². It was also found that UV-to-visible rejection ratio of the fabricated photodetector was around 1000 with a maximum quantum efficiency of 12.6%. It was also found that noise equivalent power and normalized detectivity of the ZnO nanowire photodetector were 5.73times10^-11 W and 6.17times10⁹ cmHz^0.5W^-1, respectively.

A lateral ZnO nanowire UV photodetector prepared on a ZnO:Ga/glass template

We report the lateral growth of ZnO nanowires on a ZnO:Ga/glass template. By reducing the oxygen flow ratio, it was found that we could change the growth direction of ZnO nanowires from vertical to lateral. ZnO nanowire-based photodetectors were also fabricated using the laterally grown ZnO nanowires. It was found that the detector current increased by more than 12 times upon ultraviolet illumination. It was also found that the corresponding time constant of our lateral ZnO nanowire photodetector was around 452 ms.