Fabrication OF ZnO and ZnMgO Thin Films and UV Photodetectors by Mist Chemical Vapor Deposition Method
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Fabrication of ZnO and ZnMgO Thin Films and UV Photodetectors by Mist Chemical Vapor Deposition Method Yudai Kamada1, Toshiyuki Kawaharamura1, Hiroyuki Nishinaka1, and Shizuo Fujita2 1 Dept. Electronic Science and Engineering, Kyoto University, Katsura, Kyoto, 615-8520, Japan 2 International Innovation Center, Kyoto University, Katsura, Kyoto, 615-8520, Japan ABSTRACT Deposition of Zn1-xMgxO thin films on glass substrates has been investigated by the simple and cost-effective mist CVD technique. A water solution of zinc acetate and magnesium acetate was used as the source of Zn and Mg. The solution was ultrasonically atomized, and the aerosols hence formed were supplied by the N2 carrier gas to the substrates. The band gap energy of ZnMgO was successfully controlled from 3.25 eV (ZnO) to 3.75 eV with the concentration ratio [Mg]/([Zn]+[Mg]) in the solution. The transparency in the visible region was higher than 90% and the surface RMS roughness was 7.5 nm (an example for ZnO) despite the polycrystalline structure; they are satisfactory for the optical applications. A UV photodetctor with interdigital electrode structure on the ZnMgO surface was fabricated, where the photoresponsivity of 2.6 A/W at 350 nm and the lowest detectable power of about 1 µW were obtained. Although these values are satisfactory for the simple UV detection but the existence of deep defects is deteriorating the dynamic response of the detector device. INTRODUCTION The evolution of ZnO-based devices has been and is to be indebted to the development of growth/deposition techniques of ZnO-based materials. So far, pulsed laser deposition (PLD), molecular beam epitaxy (MBE), metal organic chemical vapor deposition (MOCVD), or sputtering has been considered to be promising for the achievement of semiconducting and high quality ZnO and related materials. However, more safe and economical deposition techniques are becoming important especially for utilizing the unique functions of ZnO-based materials on large-area substrates, that is, for such as transparent conductors, UV absorbers, and UV image sensors. For this reason, we have developed a technique called "mist CVD" for the deposition of ZnO[1], ZnMgO[2], and other oxide thin films. In this paper we show the fundamental preformance of UV photodetectors fabricated with ZnMgO thin films deposited by the mist CVD method and discuss the potential of this deposition technique. EXPERIMENTS Mist CVD system The mist CVD method, previously named as mist deposition or spray pyrolysis[3-9], is an environmental friendly and cost-effective technique for the deposition of thin oxide films. This features that a liquid solution of constituent elements is used as a source, and it is ultrasonically atomized, then the aerosols or mists hence formed are transferred by a carrier gas to the reaction area to the deposition of thin films. The schematic illustration of an atomizer is
Aerosols N2 gas
Source solution Water
polyethylene film Ultrasonic transducer (2.4 MHz)
Figure 1. Schematic illustration of
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