選擇性飛秒雷射結構技術於碳化矽基材之氣體檢測元件研究

Abstract

本研究旨是利用選擇性超快飛秒雷射製程技術(Selective femtosecond laser structuring technology)，其超短脈衝之非線性吸收及極低的熱影響區(Heat-affected zone, HAZ)加工特性，在碳化矽(Silicon carbide, SiC)基材進行多尺度複合結構之探討及氣體檢測元件開發。首先，本研究採用飛秒脈衝雷射於碳化矽表面進行製程，在剝離閥值(Threshold)為1.51 J/cm2，探討多發脈衝行為所產生之孵化效應(Incubation effect)，其孵化係數為S=0.8667±0.035。同時，本研究使用不同能量密度進行雷射誘導週期性表面結構(Laser induced periodic surface structures, LIPSS)，該結果顯示隨著能量密度提高，奈米波紋狀結構逐漸亂序排列；隨後以拉曼光譜量測不同能量密度對材料所產生之特性變化，當載流子密度(Carrier density)隨能量密度上升而增加時，所量測到的特徵峰向更高波數側移動且峰形變寬、峰值強度降低，表明分子的化學鍵長度與結構分佈發生變化。進一步，本研究描述了雷射誘導的載流子失衡行為，利用福克-普朗克方程式(Fokker–Planck equation)修改的時間相依雙溫模型(Two-temperature model, TTM)，分析雷射剝離行為、電子溫度、晶格溫度與載子密度的暫態變化。在氣體檢測元件製備方面，本研究會利用選擇性飛秒雷射製作石墨烯(Graphene) SiC基材之加熱元件，在高溫度為132.9 °C，進行該複合檢測元件應用於一氧化氮(Nitric oxide, NO)檢測，其氣體響應值(Response)於50 ppm與300 ppm分別為6.5 %與19.2 %。最後，本研究利用石墨烯電極結構摻雜二硫化鉬(MoS2)之二維材料，使其產生高比表面積，提供更高的吸附能力進而提升檢測元件性能，相較於室溫環境下之檢測，顯示提升2.08倍的靈敏度(Sensitivity)，完成飛秒雷射技術於碳化矽基材之氣體檢測應用研究。關鍵詞：飛秒雷射、碳化矽、週期性表面結構、石墨烯微熱元件、二硫化鉬、氣體偵測The purpose of this study is to use the selective femtosecond laser structuring technology with the characteristics of its nonlinear absorption of ultra-short pulses and extremely low heat-affected zone (HAZ) to investigate the multi-scale composite structures of silicon carbide (SiC) and develop the sensing devices for gas detection. First, the femtosecond laser of this study can be employed by multiple pulses on the surface of SiC where the threshold fluence and incubation coefficient (S) are 1.51 J/cm2 and S=0.8667±0.035, respectively. At the same time, the different fluence of this study can be used to generate the laser induced periodic surface structures (LIPSS), showing that the nano-corrugated structures were gradually arranged out of order while the laser fluence increased. Subsequently, the Raman spectroscopy was used to measure the characteristics of the material under the different laser fluence. When the carrier density of SiC surface increased with the increase of laser fluence, the characteristic peak of Raman shift towards higher wavenumber side, resulting in the broader peak and lower peak intensity for the change of chemical bond length and structural distribution of molecules. Furthermore, study described the behavior of laser-induced carrier imbalance, which is used by the time-dependent two-temperature model (TTM) modified from the Fokker–Planck equations to analyze the transient state of laser-ablated process, electron temperature, lattice temperature, and carrier density for material removal. In this study, a selective femtosecond laser can be used to fabricate the graphene heating device based on the SiC substrate. When the composite sensing device was for applied to nitric oxide (NO) detection at the high temperature of 132.9 °C, the electrical response at 50 ppm and 300 ppm were 6.5 %, and 19.2 %, respectively. Finally, gas sensing device of this study was decorated with molybdenum disulfide (MoS2), two-dimensional material, on the graphene electrode structures which can provide high surface-to-volume ratio and higher adsorption capacity to enhance the sensing performance. Compared with the device detection at room temperature, it can be shown the 2.08 times increase in sensitivity, and be able to perform the application of gas detection based on SiC substrate in femtosecond laser process.Keywords: Femtosecond laser; Silicon carbide; Periodic surface structures; Graphene microheater; Molybdenum disulfide; Gas detection