1. Introduction
The interest in ultraviolet (UV) light sensors have been stimulated in the fields of ozone layer and ultraviolet radiation monitoring, satellite communications and UV astronomy. UV sensors based on metalsemiconductor-metal phototransistor, photoconducting layers [1, 2] and Schottky Barrier [3] have been successfully demonstrated. The sensing mechanism for these kinds of PN junction device is to monitor the variations between dark current and photocurrents that produced by the illumination of UV light. Besides, surface acoustic wave (SAW) devices were also introduced for UV sensing application because they were well-known as a good candidate for the field of various sensing applications [4-11]. Most of the acoustic waves propagate in the vicinity of the thin film surface within a depth of one wavelength, so SAW devices are very sensitive to any slight changes in the physical characteristics of the surface. In this study, thin film bulk acoustic wave (TFBAW) devices were employed due to the fact that TFBAW devices have advantages of low insertion loss, high power handling capability, small size, and high operating frequency, compared with SAW devices. TFBAW devices operated with a short wavelength and therefore more sensitive to the changes in material properties of films so they were expected to create a UV sensor with a considerably high sensitivity and better performance. Zinc oxide (ZnO) thin film has attracted much interest because of its excellent piezoelectric, optical, and semiconducting properties. The film has several applications, such as in SAW devices, thin-film bulk acoustic wave resonators, optoelectronic devices and sensors. ZnO is an attractive material candidate for acoustic wave device and ultraviolet optoelectronics, since it has a high coupling coefficient and a direct energy band gap of approximately 3.2 eV at room temperature. Therefore, ZnO film in this study was adopted simultaneously as the UV sensing layer and the piezoelectric layer.
1. Introduction The interest in ultraviolet (UV) light sensors have been stimulated in the fields of ozone layer and ultraviolet radiation monitoring, satellite communications and UV astronomy. UV sensors based on metalsemiconductor-metal phototransistor, photoconducting layers [1, 2] and Schottky Barrier [3] have been successfully demonstrated. The sensing mechanism for these kinds of PN junction device is to monitor the variations between dark current and photocurrents that produced by the illumination of UV light. Besides, surface acoustic wave (SAW) devices were also introduced for UV sensing application because they were well-known as a good candidate for the field of various sensing applications [4-11]. Most of the acoustic waves propagate in the vicinity of the thin film surface within a depth of one wavelength, so SAW devices are very sensitive to any slight changes in the physical characteristics of the surface. In this study, thin film bulk acoustic wave (TFBAW) devices were employed due to the fact that TFBAW devices have advantages of low insertion loss, high power handling capability, small size, and high operating frequency, compared with SAW devices. TFBAW devices operated with a short wavelength and therefore more sensitive to the changes in material properties of films so they were expected to create a UV sensor with a considerably high sensitivity and better performance. Zinc oxide (ZnO) thin film has attracted much interest because of its excellent piezoelectric, optical, and semiconducting properties. The film has several applications, such as in SAW devices, thin-film bulk acoustic wave resonators, optoelectronic devices and sensors. ZnO is an attractive material candidate for acoustic wave device and ultraviolet optoelectronics, since it has a high coupling coefficient and a direct energy band gap of approximately 3.2 eV at room temperature. Therefore, ZnO film in this study was adopted simultaneously as the UV sensing layer and the piezoelectric layer.
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