| Summary: | 博士 === 國立成功大學 === 微電子工程研究所碩博士班 === 97 === In this dissertation, we report the investigations of novel SiCN ultraviolet light detectors and LTPS (low temperature polysilicon) low cost hydrogen sensors. Four type structures including n-SiCN/p-PS/p-Si heterojunction, n-SiCN/p-SiCN homojunction, and n-SiCN/i-SiCN/p-SiCN and n-SiCN/i-SiCN/p-Si junctions, have been developed.
Firstly, we study the n-SiCN/p-silicon heterojunction with porous silicon (PS) buffer layer for low cost and high temperature ultraviolet (UV) detecting applications. The PS layer and the cubic crystalline n-SiCN film were formed on the top of p-(100) silicon substrate by the electrochemical anodization and rapid thermal chemical vapor deposition (RTCVD) sequentially. The PS layer has a high resistivity to suppress the dark current, and provides sponge-like structure to limit strain and cracks development after the post growth cooling, thus favoring nucleation to result a better single crystal SiCN film. Consequently, the developed optical sensing device has a high room temperature (25 °C) photo/dark current ratio (PDCR) 85.4 with and without irradiation of and 254 nm UV light with 0.5 mW/cm2 optical power. Even though at 200 °C the ratio is still equal to 7.42. These results are better than the reported ZnO on GaAs substrate or β-SiC on Si substrate UV detectors without porous treatment.
Next, a novel n-SiCN/p-SiCN homojunction was developed on Si substrate with RTCVD for low cost and high performance ultraviolet detecting applications. The current ratio of the junction under -5 V bias, with and without irradiation of 254 nm UV light are 1940, and 96.3 at room temperature, and 175 °C, respectively. Compared to the reported UV detectors with material of 4H-SiC or β-SiC, the developed n-SiCN/p-SiCN homojunction has better current ratio in both room and elevated temperature.
To raise the detector’s PDCR, both n-SiCN/i-SiCN/p-SiCN and n-SiCN/i-SiCN/p-Si junctions were developed. The measured PDCR of n-SiCN/i-SiCN/p-SiCN and the n-SiCN/i-SiCN/p-Si junctions with and without irradiation of 254 nm UV light under -5 V bias and 0.5 mW/cm2 are 150.26 and 5.42, respectively. Compared to the reported UV detectors with 4H-SiC or β-SiC (3C-SiC), the developed n-SiCN/i-SiCN/p-SiCN homojunction has the better current ratio in both room and high temperatures. Additionally, the Pd/n-LTPS (MS) and Pd/TiO2/n-LTPS (MOS) Schottky diodes fabricated on a glass substrate for hydrogen sensing are reported.
On the other hand, we also investigate the hydrogen detecting performance of Pd/n-LTPS/glass thin film Schottky diodes. The n-LTPS (n-type low temperature polysilicon) is an excimer laser annealed (ELA) and PH3 gas plasma treated amorphous silicon (a-Si) thin film. In addition, we used a TiO2 interface layer to improve hydrogen sensing ability significantly. At room temperature and -2 V bias, the developed MOS Schottky diode exhibited a high signal ratio of 1539.6% to 50 ppm of hydrogen gas, with a fast response time of 40 sec, respectively. The signal ratio is better or comparable with that of other reported MOS type hydrogen gas sensors prepared on Si or III-V compound substrate. Furthermore, the signal ratio is 7.6, 14, and 30 times over other interfering gases of C2H5OH, C2H4, and NH3 at room temperature and a concentration of 8000 ppm at -2 V bias, respectively. Thus, the developed MOS Schottky diode shows promise for the future development and commercialization of a low cost hydrogen sensor.
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