本研究中，吾人嘗試以無電鍍技術研製鈀／氮化鋁鎵(EP Pd/ AlGaN)，以及鈀／二氧化矽／氮化鋁鎵 (EP Pd/SiO2/AlGaN)蕭特基二極體氫氣感測器並且探討其氫氣感測特性。文中針對元件之電流-電壓特性加以探討，並且分析濃度在(5ppm-1%)及溫度範圍為(303-523K)下電流-電壓之氫氣感測表現。文中以理論模式及實驗結果探討元件界面品質，並計算氫氣吸附熱力學以及動力學參數。此外，熱蒸鍍式元件(TE Pd/AlGaN)亦被研製與無電鍍式元件( EP Pd/ AlGaN )作感測性能比較。

由無電鍍鈀/氮化鋁鎵元件結果得知，以無電鍍法製備之蕭特基界面並無熱破壞情形，並且可有效消弭費米能階釘住效應。依據熱離子放射模式，無電鍍元件具較低之反向漏電流且較高之蕭特基能障（905meV）。並且，於303K，無電鍍式元件可達四個數量級變化 (1%H2/air)，甚至展現低偵檢極限5ppm H2/air，其靈敏度即達3.3 (0.2V)；暫態響應方面，最快響應速率出現在423K，當氫氣濃度50ppm提高至1% H2/air，其響應時間由50秒減少至2.5秒，且回復時間由12.1秒減少至3.5秒。

以無電鍍鈀/二氧化矽/氮化鋁鎵元件探討以氧化層作為中間層對於感測表現之影響，由於氧化層扮演緩衝層之角色，因而蕭特基界面品質更佳，其反向漏電流特性較無電鍍鈀／氮化鋁鎵元件為小，且蕭特基能障值更高可達1064meV；於溫度303K且氫氣濃度為1%之條件下，其靈敏度更可高達五個數量級變化。然而，於暫態響應結果顯示，其響應時間相較於無電鍍鈀/氮化鋁鎵元件來得大。

實驗結果顯示，氫氣吸附可以Langmuir isotherm模式描述，並以一階之動力模式描述氫氣初始響應表現。氫氣吸附熱（ H○）於無電鍍鈀／氮化鋁鎵元件和無電鍍鈀／二氧化矽／氮化鋁鎵元件分別為-56.4 kJ mole-1 (423-523K) 和-45.9 kJ mole-1(373- 473K)，且其相對應之活化能（Ea）分別為27.5 kJ mole-1和15.5 kJ mole-1。

綜合以上結果，以無電鍍技術鍍覆鈀膜之氮化鋁鎵蕭特基二極體氫氣感測器，具低偵檢極限(<5ppm)，寬偵檢範圍(5ppm～1%H2/air)，寬溫度範圍(303-523K)，高靈敏度，以及快速響應速率以及回復速率，其優異之感測性能應可滿足於市場趨勢與未來發展需求。

In this study, the Pd/AlGaN and Pd/SiO2/AlGaN Schottky diode hydrogen sensors were fabricated by electroless-plating technique. The dependence of steady current–voltage (I-V) characteristics of the devices were measured under various hydrogen concentrations (5ppm-1%H2/air) and temperatures (303-523K). Besides, the Schottky interface quality, thermodynamic and kinetic parameters were investigated and estimated from the theoretical models and experimental results. For EP Pd/AlGaN device, the same structure fabricated by thermal evaporation method (TE Pd/AlGaN) was employed for further comparison of the sensing performance.

From the experimental results, it revealed that the interfacial quality of EP Pd/AlGaN device was more perfect with no thermal damage, resulting in effectively preventing the Fermi-level pinning effect. Based on the thermionic emission model, the EP Pd/AlGaN device processed lower reverse leakage current and higher Schottky barrier height (905meV). In addition, at 303K, the sensitivity of EP Pd/AlGaN device reached up to four-order magnitude. Even at extremely low hydrogen concentration, e.g., 5ppm H2/air, the sensitivity approached to 3.3 at a bias of 0.2 V. For transient state analysis, a maximum rate was occurred at temperatures of 423K. The time required for response time is decreased from 50 to 2.5 sec and the recovery time is decreased from 12.1 to 3.5 sec with the increase of hydrogen concentration from 50ppm to 1%H2/air.

To investigate the influence of SiO2 on sensing performance, it revealed that the oxide intermediate layer can be served as the passivation layer for achieving better interfacial quality. EP Pd/SiO2/AlGaN device also exhibited the lower reverse leakage current and the higher Schottky barrier height (1064meV) as compared with EP Pd/AlGaN device. Besides, it even reached up to five-order magnitude for 1% H2/air at 303K. However, for transient state analysis, it indicated the response time of EP Pd/SiO2/AlGaN device was larger than that of EP Pd/AlGaN one at the operating temperature in this study.

For the hydrogen adsorption analysis, the equilibrium state of hydrogen adsorption could be described based on the Langmuir isotherm model, and the initial rate of transient detection could be expressed by using a first-order kinetic model. It revealed the enthalpy, H○, were calculated as -56.4 kJ mole-1 (423-523K) and -45.9 kJ mole-1 (373-473K) for EP Pd/AlGaN device, and EP Pd/SiO2/AlGaN device, respectively, also, the corresponding activation energy, Ea, were determined as 27.5 kJ mole-1 and 15.5 kJ, respectively.

Experimental results indicated that the Pd-AlGaN Schottky diode hydrogen sensors fabricated by electroless-plating exhibited excellent hydrogen sensing performances with wide detection range (5ppm~1%H2/air), wide temperature range (303-523K), high sensitivity, and fast response and recovery rate. Therefore, the electroless plating technique demonstrated promising developments on the fabrication of Pd/AlGaN device for versatile applications.

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