| Summary: | 碩士 === 國立清華大學 === 電子工程研究所 === 100 === Liver cancer is usually the No. 1 of the top ten causes of death in Taiwan. Many studies have found that the breath ammonia level is significantly higher in chronic patients with liver disease（> 0.7 ppm）than that in normal people. Therefore, the detection of ammonia concentration in exhaled breath becomes more and more important. In this work, we used ultrathin indium nitride (InN) epilayer（~10 nm）to fabricate gas sensors for the detection of ammonia with sub-ppm level. Furthermore, we used curve fitting to distinguish different ammonia concentration and tried to establish a method of rapid diagnosis for liver disease.
Firstly, ultrathin InN-based gas sensors with and without a thin catalytic platinum（Pt）layer atop have been compared. For the bare InN sensor, the current variation of 10 ppm ammonia in air ambience (21 % O2 + 79 % N2) at 200 °C is 19 % and the simplified response time is 1180 s. For the Pt-coated InN sensor, the sensor shows a higher current variation ratio of 32 % and a shorter response time of 922 s under the same gas exposure environment. Compared with the results of these two devices, we found that catalytic Pt layer can shorten the simplified response time by 1.3 times and increase the current variation ratio by 1.7 times. In addition, there is a linear relationship between current variation and logarithmic concentration of ammonia（0.2 ppm – 10 ppm）, and its slope is 17.76.
We further analyzed the response of Pt-InN gas sensors. Since the main composition of human's breath is nitrogen, oxygen and carbon dioxide, and reaction rate of gas molecules is related to activation energy as well as reactant concentration, we discussed the activation energy of the above-mentioned gases. The activation energy can be derived by operating devices at different temperature in N2 ambience. The activation energy of oxygen, carbon dioxide, and ammonia is 0.79 eV, 0.45 eV, and 0.33 eV respectively. In order to obtain more precise analysis of dynamic response of gas sensors, the derived formulas for gas adsorption and desorption were employed. We redefined the simplified response time by the curve fitting of measured current response and discussed the relation between response time and gas concentration. Besides, we used two different methods to distinguish different gases and concentrations: 1. Dynamic relationship between time and current variation, 2. Stable relationship between temperature modulation and radar chart. In the simulated concentration range of human breath (oxygen: 16 %－18 % and carbon dioxide: 3 %－5 %), we have achieved to calculate the concentration of oxygen ,carbon dioxide and ammonia. Currently, the lowest detectable response to ammonia of Pt-coated InN sensors is 0.06 ppm, which shows that the InN gas sensors have very high sensitivity to ammonia and can be applied to diagnose liver disease successfully.
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