Hong-Ming Chang

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.

EFFECT OF HEAT TREATMENT AND CO ON THE GIANT MAGNETORESISTANCE OF FE-60 CR-XCO ALLOY THIN FILMS

This article presents the giant magnetoresistance (GMR) effect and microstructure of Fe-60Cr-XCo ternary heterogeneous alloy films. After ion beam sputtering, the films are heat treated at a temperature within the miscibility gap. The separated phases and lattice images of the α1 (Fe–Co rich phase) and α2 (Cr rich phase) are further determined by high resolution transmission electron microscopy. In the granular alloy, the GMR effect is obtained by the interface scattering between the phases of α1 and α2. By isothermal aging at 550 °C for 80 min, the largest MR ratio of −62% appears for the film with the composition of Fe-60Cr-15Co at 77 K under a field of 14 kOe. For the same treatment, the MR ratios of Fe-60Cr-10Co and Fe-60Cr-20Co are −39% and −43%, respectively. The Co content and heat treatment condition in the alloy system obviously affect the MR ratio.

Gallium nitride metal-semiconductor-metal photodetectors prepared on silicon substrates

Gallium nitride (GaN) ultraviolet metal-semiconductor-metal photodetectors (PDs) grown on Si substrates were demonstrated. The dark current of PDs fabricated on Si substrates was substantially smaller in magnitude compared to identical devices prepared on sapphire substrates. With an incident wavelength of 359nm, the maximum responsivities of the n−‐GaN MSM photodetectors with TiW and Ni∕Au contact electrodes were 0.187 and 0.0792A∕W, corresponding to quantum efficiencies of 64.7% and 27.4%, respectively. For a given bandwidth of 1kHz and a given bias of 5V, the corresponding noise equivalent powers of our n−‐GaN MSM photodetectors with TiW and Ni∕Au electrodes were 1.525×10−12 and 5.119×10−12W, respectively. Consequently, the values of detectivity (D*) determined for devices with TiW and Ni∕Au electrodes were then calculated to be 1.313×1012 and 3.914×1011cmHz0.5W−1, respectively.

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.

Low-Frequency Noise Characteristics of Epitaxial ZnO Photoconductive Sensors

We report the fabrication of epitaxial ZnO photoconductive sensors on sapphire substrates. With an incident light wavelength of 370 nm and a 5 V applied bias, we achieved a sensor responsivity of 20.5 mA/W. It was also found that low-frequency and high-frequency noises in the fabricated sensors were dominated by I/f type and shot noises, respectively. With a 5 V applied bias, it was found that noise equivalent power and normalized detectivity of the fabricated sensors were 1.83 X 10 -6 W and 6.91 X 10 5 cm Hz 0.5 W -1 , respectively.

Optical and Electrical Characteristics of ZnO Films Grown on Nitridated Si (1 0 0) Substrate with GaN and ZnO Double Buffer Layers

The optical and electrical characteristics of zinc oxide (ZnO) films grown by molecular-beam epitaxy (MBE) on Si substrates were investigated. ZnO epitaxial layer was successfully grown on nitridated Si(100) substrate initially covered with high-temperature GaN and low-temperature ZnO double buffer layers using MBE. X-ray diffraction and photoluminescence results both indicated that a reasonable quality of ZnO epitaxial layer was obtained. As the CV measurement had indicated, the carrier concentration was reduced virtually in a linear fashion from ZnO surface down to GaN buffer layer. A reduction in electron concentration was caused by the carrier depletion due to the presence of the Schottky barrier of Ni/ZnO. The large density of electron accumulated at the ZnO/GaN interface was due to the large conduction band discontinuity and offset.

The magnetoresistance and microstructure of Fe–xCr–10Co alloy thin films

Large negative magnetoresistance (MR) is observed in Fe–xCr–10Co (x=10,20,30,40,50,‐ 60,70, and 80 wt %) ternary heterogeneous alloy films prepared by the IBS (ion beam sputter) deposition process. All the as‐deposited films consisted of the α phase alone. After aging treatment, the α phase is decomposited into two phases: the α1 (Fe–Co rich phase) and α2 (Cr rich phase). In the granular alloy, the GMR effect is obtained by virtue of the interface scattering between α1 and α2. After isothermal aging at 400 °C for 1 h, the largest MR of −27% appears around the Cr content of 60 wt % at 77 K and 14 kOe. The MR ratio of Fe–50Cr–10Co and Fe–70Cr–10Co is −12% and −20%, respectively, at the same condition. The MR ratio is also obviously effected by the Cr (or Fe) content in the alloy system.

Giant magnetoresistance and phase decomposition of Fe–XCr–Co (WFe/WCo=1) thin film granular alloys

In this study, the giant magnetoresistance (MR) effect was observed in Fe–XCr–Co (WFe/WCo=1, X=27, 40, 50, 60, 70, and 80 wt %) ternary heterogeneous alloy films. After aging, the separated phases and lattice images of the α1 (Fe–Co rich phase) and α2 (Cr rich phase) were further determined by high resolution transmission electron microscope. Experimental results indicate that the MR variations in Fe–Cr–Co thin films alloys were associated with the heterogeneous microstructure. By isothermal aging at 550 °C for 80 min, the largest MR of −43% appears for the film with the composition of 40(Fe/Co)–60Cr at 77 K under a field of 14 kOe. At the same treatment condition, the MR ratios of 50(Fe/Co)–50Cr and 30(Fe/Co)–70Cr are −13.2% and −21.3%, respectively. Cr content, therefore, significantly affected the MR ratio.