산화주석 박막 및 나노와이어 기반 고성능 자가-전력 자외선 광검출기

Abstract

We report on the performance of self-powered ultraviolet (UV) photodetectors composed of SnO2 nanowire (NW) networks. SnO2 NWs with a length of several hundred micrometers can be synthesized by thermal chemical vapor deposition (CVD) using SnO powder as the raw material, which has the advantages of a low process temperature and no requirement for a reducing agent. Based on the characterization results obtained through synchrotron X-ray diffraction (XRD), scanning electron microscopy, and transmission electron microscopy (TEM), we determined the growth behavior of SnO2 NWs via a vapor-liquid-solid mechanism with Au nanoparticles. The NW growth switched from an initial in-plane growth to subsequent vertical growth, forming NW cotton. This was started at a process temperature of 600 ℃ and optimized at 800 ℃. Moreover, the XRD and TEM results indicate that the NWs mainly grew in the form of SnO2, although the formation of SnO NWs was also possible. Metal-SnO2 NW-metal type photodetectors were fabricated, and their photoresponsivity to UV light in the range from 200 to 400 nm was investigated. The device exhibited a photo-to-dark current ratio of ∼ 2.17 × 106 at an applied bias of 10 V and 254 nm UV exposure. A maximum responsivity of about 1100 A/W was estimated at a wavelength of 270 nm, and the cutoff edge wavelength appeared around 350 nm. In particular, the self-powered photoresponse at nominal zero bias was ∼ 1.23 nA. The results of this study support the idea that SnO2 NWs are promising candidates for self-powered deep-UV photodetectors and that thermal CVD using SnO powder is suitable for synthesizing SnO2 NWs at temperatures as low as 800 °C. But, the device structure and characteristics of the photodetector are not suitable for practical applications. Thus, the UV photodetectors composed of SnO2 thin films were produced. The SnO2 thin film UV photodetectors were fabricated by RF sputtering in an Ar atmosphere. But, due to the oxygen vacancy, the time-dependent photorepsonse time curve indicated the decay time of slow speed reaction. Therefore we fabricated the SnO2 thin films having thickness thinner than 15 nm. To reduce the native oxygen vacancies, the samples were grown in a pure O2 gas atmosphere with a working pressure of 5 x 10 -3 torr. Based on the characterization results through synchrotron X-ray diffraction (XRD) and atomic force microscopy (AFM), We determined the structural properties of SnO2 thin films as a function of film thickness. At 2.5 nm thick sample, the surface of the thin film is relatively smooth. As for ϴ-rocking curve, it was observed that as thickness of SnO2 thin film increased, the diffuse scattering increased. This is because of the surface state effect. Surface state effect are responsible for the relatively slow response speed. Metal-SnO2 thin film-metal type photodetectors were fabricated. The time scale was excellent in the 4.4 nm thin film, and the photocurrent was measured the highest in the 14.2 nm thin film. The photoresponsivity to UV light of 254 nm was investigated. The device of 8.8 nm thickness exhibited a photo-to-dark current ratio ∼ 7 × 104 at an applied bias of 10 V. A maximum responsivity was estimated at a wavelength of 260 nm, and the cutoff edge wavelength appeared around 350 nm. On these results, We have successfully demonstrated that SnO2 thin film of 8.8 nm was suitable for candidate of UV photodetector.