Cheng-Zhi Wu

Ultraviolet photodetectors based on selectively grown ZnO nanorod arrays

Metal-semiconductor-metal (MSM) ultraviolet (UV) photodetectors with ZnO nanorods (NRs) have been fabricated and characterized in this investigation. The NR arrays were selectively grown on the gap of interdigitated electrodes by chemical solution method through a photolithography process. Compared to a traditional ZnO MSM photodetector with no NRs, the fabricated NR UV photodetector showed much higher photoresponsity. As a result, it can be attributed to high surface-to-volume ratio of ZnO NRs and such a high photoresponse could strongly depend on oxygen adsorption/desorption process in the presence of trap states at the NR surface.

Ultraviolet ZnO Nanorod Photosensors

This study fabricates and characterizes ultraviolet (UV) photosensors with ZnO nanorods (NRs). The NR arrays were selectively grown in the gap between interdigitated (IDT) electrodes of devices using hydrothermal solution processes and a lithography-based technique. Compared with a conventional ZnO photosensor without NRs, the proposed UV NR photosensors have much higher photoresponse in the UV region. Additionally, the photoconductive gain of an NR photosensor increased as UV illumination time increased; it varied at 34.45-5.32 x 10(2) under illumination by 18.28 mW/cm(2) optical power. Consequently, the substantial photoconductive gain can be attributed to high surface-to-volume ratio of ZnO NRs. The high density of hole-trap states on NR surfaces lead to a persistent photoconductivity (PPC) state, promoting the transport of carriers through devices.

Semitransparent Field-Effect Transistors Based on ZnO Nanowire Networks

This investigation demonstrates the fabrication of semitransparent field-effect transistors with self-assembling ordered ZnO nanowire (NW) networks, using a high-k HfO₂ gate. The devices exhibit excellent optical transparency and transistor performance at on/off ratios of >;10⁵, a mobility of ~7.59 cm² V^-1 s^-1, and threshold voltages of 4 V. Under UV illumination (3.65 eV), the devices exhibit the highest relative photoconductivity (2.08 10^5), corresponding to a photoresponsivity of 3.96 A/W at low operating voltages ( V_GS = 0 V and V_DS = 1 V). The result suggests that the NW-based devices have low power consumption and high photosensivity when used in photodetection.

Photoelectrical and Noise Characteristics of ZnO Nanowire Networks Photosensor

This study describes the photoresponse and noise characteristics of UV photosensors with ZnO nanowire networks. The fabrication approach, which combines conventional photolithography with sidewall nucleation sites, achieves site specificity and the self-assembly of an ordered ZnO nanowire networks. The photoresponse of the device is approximately three orders of magnitude, because the high surface-to-volume ratio enables efficient light absorption. These results reveal that the reproducible photocurrent response, high-photosensivity, internal gain and detectivity suggest that the device has potential applications in UV photodetection.

Transparent ZnO Nanowire-Network Ultraviolet Photosensor

This paper demonstrates the fabrication of transparent ultraviolet (UV) photosensors with a self-assembling ordered ZnO nanowired network. The average optical transmission of the entire networked photosensor structure in the visible range of the spectrum is about 70%. At an applied bias of 5 V and 340-nm irradiation, the photoresponsivity and the ratio of UV-to-visible rejection was 175.58 and 207.63 A/W for the transparent UV photosensors. For a bandwidth of 100 Hz and an applied bias of 5 V, the noise equivalent power and normalized detectivity of the devices were 2.32 × 10-10 W and 4.36 × 108 cm · Hz0.5/W , respectively.

Photosensitivity of Field-Effect Transistors Based on ZnO Nanowire Networks

Self-assembling ordered ZnO nanowire (NW) network-based field-effect transistors (FETs) were fabricated by bottom-up photolithography. The devices had on/off ratios of >; 104, mobilities of ~1.31 cm2 V-1 s-1, mobilities of and threshold voltages of ~-1 V. Under UV treatment (340 nm, 57.46 ), the devices exhibited relative photoconductivity ratio increases of at a depletion state of 8 V gate bias (1.56 × 103 A/V). The fabricated FETs exhibit a broad range of electrical characteristics because of variation in the contact quality of the metal/NW, the dielectric/NW, and the NW/NW interfaces. However, the fabricated approach offers a cost-effective route to integrate self-assembled ZnO NW network-based FETs.

Characteristics of ZnO-based MSM photodetectors with ZnO cap layer on flexible substrates

In this work, both of ZnO based metal-semiconductor-metal photodetectors with and without ZnO cap layer were fabricated on flexible substrates (polyethylene terephthalate, PET) for comparative analysis. The ZnO films were prepared by a low temperature sputtering process. The photodetector with ZnO cap layer (stack structure: ZnO/Ag/ZnO/PET) shows a much higher UV-to-visible rejection ratio of 1.56×103 than one without. It can be attributed to the photocurrents significantly enhanced under UV light illumination in such a novel structure. With an incident wavelength of 370 nm and 3 V applied bias, the responsivities of both photodetectors with and without ZnO cap layer are 3.80×10−2 and 2.36×10−3 A/W, respectively, which correspond to quantum efficiencies of 1.13 % and 0.07 %. The Schottky barrier height on Ag/ZnO interfaces is also obtained to be 0.782 eV.