選擇性修飾多晶矽奈米帶元件在氫氣感應器的應用

Abstract

本論文主要研究選擇性修飾的雙接面(double-junction n+/n–/n+)多晶矽奈米帶(Poly-Si Nanobelt, PNB)元件在氫氣感測器的應用。利由被焦耳熱燒蝕 (ablation)的聚甲基丙烯酸甲酯做為奈米基板(template)，表面再修飾容易吸附氫氣催化劑在n–上，來分析元件對氫氣的響應。藉由不同的焦耳熱參數，也可調整此感測器感測區修飾的大小。我們用COMSOL Multiphysics 軟體模擬感測器的熱分佈情形。調整在n– region中的四種不同的雜質參雜濃度 (從 2.5 × 1013 到 2.5 × 1014 cm–2) ，我們合出四種不同的感測器，並測量出不同的自主性加熱溫度對應的感測結果。研究發現在室溫下，低參雜濃度的感測效果最好，推測可能在室溫下，參雜濃度越低，半導體的Debye 長度越長。除此之外，我們也運感測器的I–V特性，來分析不同奈米帶元件不同焦耳熱的操作下，溫度下得氫氣誘導的表面電位差異，並依此找到最佳工作元件。 在上述基礎上我們用三種催化劑Pt、Pd及Pt/Pd ，分別修飾在PNB感測器上，並用來偵測氫氣。結果發現，Pt修飾的PNB感測器，在全氮氣環境下，偵測氫氣的靈敏度比在一般空氣的環境下高。若改用Pd修飾PNB感應器，在一般空氣的環境下，其偵測氫氣靈敏度比Pt修飾的PNB感測器高。然而，Pd一接觸氫氣就會脹裂(Blistering)，阻礙其長時間偵測氫氣的應用。而Pt 和Pd共同修飾在PNB上的感測器，則可以減少Pd脹裂情形。雖然目前Pt/Pd修飾的PNB感應器缺點是必須在低濕度的環境下操作，不過相較於用Pt或Pd修飾PNB感應器，Pt/Pd修飾PNB感應器已經在偵測氫氣上跨出一大步。

In this thesis, double-junction n+/n–/n+ polysilicon nanobelt devices (PBN) featuring selectively deposited sensing materials have been investigated for application as hydrogen (H2) gas sensors. The selective modification as well as the H2 sensing measurement of the devices was performed utilizing self-heating feature of the nanobelts. A nano template, formed through Joule-heated ablation of poly(methyl methacrylate), served as a mask for the site-specific functionalization of the PNB with a H2 catalyst. The so-formed activation layer, whose size can be controlled by Joule-heating parameters, significantly modified the performance of PNB devices. Thermal distribution and maximum dissipated temperature of the PNB devices were simulated using COMSOL Multiphysics Software. Four nanobelt devices were fabricated with different doping concentrations at the n– region (from 2.5 × 1013 to 2.5 × 1014 cm–2). Their sensing behaviors with respect to self-heating temperature were analyzed. A low doping concentration improved the response at room temperature, owing to a longer Debye length. The variation in the H2-induced potential energy associated with temperature, accounting for the degradation in the response with Joule-heating bias, was analyzed from the I–V characteristics of the double-junction device. The PNB devices were activated with various types of catalyst, namely, Pt, Pd, or Pt/Pd. The response to H2 as well as other practical factors of a gas sensor (i.e., the selectivity, the impact of relative humidity, and the durability) was analyzed. While a Pt-decorated PNB exhibited a high sensitivity to H2 in N2 with an excellent durability, its response was seriously hindered in air ambient. The Pd-functionalized PNB, on the other hand, was more sensitive to H2 in air ambient. However, the surface deformation of Pd upon exposure to H2 has restrained it from a long-term operation. The coexistence of an ultrathin film Pt and Pd catalyst efficiently prevented the blister while still generated a high response to H2. Although the response of a Pt/Pd-catalyzed PNB was impaired with relative humidity, it appeared to be a good negotiate over single ultrathin film Pt or Pd ones.