本文討論以約10奈米磊晶氮化銦（indium nitrite）超薄膜製成之電阻式氫氣感測器之偵測反應特性，並比較氮化銦裸面與氮化銦表面鍍上一層白金（Pt）薄膜於偵測反應上的差異。裸面銦極化（In-polar）之氮化銦感測器在空氣或氮氣的環境中，當溫度升高至150度時，開始對氫氣有反應，而於225度時有最大的反應。在溫度225度、空氣（氮氣）的環境中，1000 ppm氫氣會造成電阻下降16.2歐姆(4.6歐姆），對應之電流變化率為6.6%（2.5%），反應時間為182秒（620秒），電流變化的對數與氫氣濃度（50-10000 ppm）的對數呈現線性關係，其斜率為0.69。在氮氣的環境中，可推導出氫氣吸附於銦極化之氮化銦表面之活性能於溫度區間150度至250度為0.91 eV。另外，裸面氮極化（N-polar）之氮化銦感測器在不同溫度及不同氫氣濃度下，與銦極化之感測器有極相似的反應特性，這樣的結果可能與氮化銦表面的銦吸附層（In adlayer）有關。因為不論銦極化或氮極化的氮化銦表面的穩定重構結構，皆存在一銦吸附層，而此最表面之銦吸附層主宰氫氣感測反應行為。另一方面，白金催化之銦極化氮化銦感測器對氫氣反應所需的溫度顯著地降低，且反應之電流變化顯著地提升。即使在室溫、空氣中，1000 ppm 氫氣即會造成64%的電流變化率，當溫度升高至150度、同樣的氣體環境下，電流變化率上升至177%，反應及回復時間分別為134及296秒。相較於裸面之銦極化氮化銦感測器於同樣溫度及氫氣濃度下，白金催化增進電流變化值約635倍。同時，於此溫度下，氫氣偵測極限濃度小於5 ppm，電流變化的對數與氫氣濃度（5-2000 ppm）的對數呈現線性關係，其斜率為0.74。感測器之氫氣吸附活性能於溫度區間25度至150度為0.31 eV。另外，白金催化之氮極化氮化銦感測器對1000 ppm氫氣，在不同溫度、空氣中，與白金催化之銦極化氮化銦感測器有相似的反應特性。但是，感測器對不同濃度氫氣的反應則相當不同，其電流變化的對數與氫氣濃度（50-2000 ppm）的對數的斜率明顯較高，為1.34，此乃由於在低濃度氫氣中（<=250 ppm），其反應顯著偏低所造成。然而，整體來說，相較於裸面氮化銦感測器，白金催化的氮化銦感測器對於氫氣有如此靈敏的反應，可能與由白金催化產生的氫原子擴散進入氮化銦表面附近形成施子（donor），進而增加表面的導電電流有關。在氮氣的環境中，白金催化之銦極化氮化銦感測器由於缺乏氧去消耗白金催化產生之氫原子，故其對氫氣的反應速率遠較於空氣的環境中快，且反應的電流變化也顯著地提升。例如，於溫度150度時，對100 ppm氫氣反應時間僅為14秒，且有170%電流變化率，接近飽和變化值。然而，也由於缺乏氧去消耗氫原子，即使氫氣關掉後，氫原子於氮氣的環境中也會存在白金內部一段時間，因而造成其恢復時間相當長。

Hydrogen gas (H2) sensors based on ultrathin (~10 nm) indium nitride (InN) epilayers with and without a thin catalytic platinum (Pt) layer atop have been fabricated and demonstrated. The bare In-polar InN sensor exhibits a detectable response to hydrogen at temperature of >=150 C and a maximum response at 225 C in both air and N2 ambiences. The maximum response of resistance variation under 1000 ppm H2 exposure in air (N2) ambience at 225 C is 16.2 ohm (4.6 ohm), corresponding to a variation ratio of 6.1% (2.5%), with a response time of 182 s (620 s). The plot of log (DI) vs. log (concentration of H2) reveals a linear relationship with a slope of 0.69 at a wide H2 concentration range from 50 ppm to 10000 ppm (1%) in air ambience. The hydrogen adsorption activation energy for the In-polar InN surface in N2 ambience at the temperature range from 150 C to 250 C is derived to be 0.91 eV. Moreover, the bare N-polar InN sensor exhibits a very similar response to its In-polar counterpart at various temperatures (150-250 C) and at different H2 concentrations (50-10000 ppm) in both air and N2 ambiences. The similar response to hydrogen for bare InN sensors with different InN polarities was explained as a result of the formation of an In-adlayer at InN reconstructed surfaces regardless of their polarities, which dominates the hydrogen sensing behavior. On the other hand, the Pt-coated In-polar InN sensor exhibits a much higher response to hydrogen even at lower temperatures (<=150 C). At room temperature, the current variation ratio of 64% is observed while exposed to 1000 ppm H2/air. At 150 C, the sensor shows a high current variation ratio of 177% and a short response/recovery time of 134 s/358 s under the same gas exposure environment. In comparison with a bare In-polar InN sensor tested in the same conditions, the Pt catalytic layer enhances the current variation by ~635 times. The detection limit of the sensor is experimentally found to be less than 5 ppm. The linear relationship of log (DI) vs. log (concentration of H2) displays a slope of 0.74 at a H2 concentration range from 5 ppm to 2000 ppm. In addition, the activation energy of the sensor at the temperature range from 25 to 150 C is derived to be 0.31 eV. For the Pt-coated N-polar InN sensor, it exhibits a similar response to its Pt-coated In-polar counterpart upon 1000 ppm H2/air exposure at various temperatures (250-150 C). However, the response to different H2 concentration at 150 C is rather different. The slope of log (DI) vs. log (concentration of H2) is as high as 1.34 at a H2 concentration range from 50 ppm to 2000 ppm due to the lower response at lower H2 concentration (<=250 ppm). The response is much higher than those of catalytic metal-gated HEMT-based H2 sensors operated in a normally-on mode with an unbiased gate under the similar gas exposure conditions. The higher response of hydrogen detection for the Pt-coated InN sensors might be associated with the incorporation of the catalytically active hydrogen atoms into the near-surface region of InN, which will act as donors and thus enhance the surface conduction current. In N2 ambience, due to the absence of oxygen to consume the catalytic hydrogen atoms, the Pt-coated In-polar InN sensor exhibits a much higher and faster response to hydrogen than that tested in air ambience. At 150 C, the response time is 14 s and the current variation ratio is 170% upon 100 ppm H2 exposure, which is close to the saturated response. In the meantime, a slow recovery rate is found because the hydrogen atoms would remain in the Pt bulk for a long time due to the same mechanism (the absence of oxygen to consume the catalytic hydrogen atoms).

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