硫屬合金奈米複合薄膜於非揮發性浮動閘極記憶體及氫氣感測器之研究

Abstract

本論文研究硫屬合金（Chalcogenide）奈米複合薄膜於非揮發性浮動閘極記憶體元件（Nonvolatile Floating Gate Memory，NFGM）與氫氣感測器之應用。硫屬合金奈米複合薄膜係以貼靶式濺鍍法將AgInSbTe（AIST）奈米微粒鑲埋於二氧化矽（SiO2）之基地中，在NFGM之研究部分，電子顯微鏡（Transmission electron Microscopy，TEM）及X光電子能譜儀（X-ray Photoelectron Spectroscopy，XPS）分析發現約5 nm大小的AIST奈米晶均勻分散SiO2基地中，其可做為NFGM元件之電荷儲存位置；含單層AIST-SiO2奈米複合薄膜之NFGM研究中，電容-電壓（Capacitance-voltage，C-V）特性量測呈現一逆時針遲滯曲線，此代表有電荷從基板注入於奈米晶中而具備訊號儲存（Data Retention）能力，顯示AIST-SiO2奈米複合薄膜能應用於NFGM元件。實驗亦得知在濺鍍製程中摻雜適當的氮氣（N2）以及在400°C、大氣環境之後退火處理有助於提升NFGM特性，其可抑制AIST的氧化及促使氧化銻（Antimony Oxides）還原成金屬態的Sb2Te相，因而提高電荷儲存於奈米複合薄膜中的能力。 NFGM之阻障氧化物層研究顯示，含AIST-SiO2奈米複合薄膜之NFGM可藉由覆蓋一HfO2/SiO2複合阻障層提高其記憶效能，在施加±23 V操作電壓下，可獲得30.7 V之平帶電壓差（VFB Shift）及2.3×10^13 cm^-2之有效電荷密度。高密度之AIST奈米晶與HfO2/SiO2阻障層的高能障高度使大量電荷能儲存於AIST奈米晶的深捕捉層（Deep Trap Sites）中，因而提升其記憶效應及良好的資料儲存能力。僅鍍有HfO2層雖亦可提升NFGM之效能，但HfO2會擴散至奈米複合薄膜層並產生氧缺陷而引致電荷洩漏，從而惡化電荷儲存的能力；以電漿輔助化學氣相沉積法（Plasma-enhanced Chemcial Vapor Deposition，PECVD）嵌入一SiO2薄膜於HfO2及AIST-SiO2奈米複合薄膜之間不僅可抑制HfO2之擴散，且可強化庫倫阻塞效應（Coulomb Blockade Effect）及降低漏電流。相較於先前之研究報導，本研究成功驗證AIST-SiO2奈米複合薄膜可簡化元件結構與製程，並可在較低溫退火完成性質絕佳之NFGM元件。 氫氣感測器的研究發現約30奈米厚之AIST-SiO2奈米複合薄膜在75°C、200 ppm的氫氣環境中，其敏感度可達61.3%，而與氫氣反應時間與大氣氣氛下回覆時間各為75秒與50秒。從相關的微觀結構與成分分析中進一步得知奈米複合薄膜呈現n-type感測行為，其係覆蓋在AIST奈米晶表層的Sb2O5相所致。當暴露於氫氣中，吸附的氫分子可使Sb2O5還原成金屬態的Sb2Te相或引發Sb2O5的非化學劑量比反應而釋放出電子，而增加奈米複合薄膜之導電性；由於AIST奈米晶的高比表面積（Specific Surface Area，SSA）特徵，故可獲得一高氫氣感測之特性；本研究亦闡明氫氣感測器可由傳統的濺鍍法製備完成，可大幅簡化元件結構與製程方法。

This thesis study investigates the applicability of chalcogenide nanocomposite thin films to the nonvolatile floating gate memory (NFGM) and the hydrogen (H2) gas sensor. The AgInSbTe (AIST)-SiO2 nanocomposite thin films were prepared by the target-attachment sputtering method. Tranmission electron microscopy (TEM) and x-ray photoelectron microscopy (XPS) revealed the AIST nanocrystals (NCs) about 5 nm in diameter uniformly dispersed in SiO2 matrix and, in the part regarding of the study of NC-based NFGM, the AIST NCs may serve as the charge trapping sites in the programming layer of NFGM. The capacitance-voltage (C-V) measurement observed a counterclockwise hysteresis loop in the device containing a sole AIST-SiO2 nanocomposite layer, indicating the saturated substrate injection of charges into the AIST NCs. Moreover, the good data retention property also illustrated the feasibility of AIST-SiO2 nanocomposite to NC-based NFGM. Analytical results also found that the appropriate nitrogen incorporation during the sputtering deposition and the post annealing at 400°C benefit the NFGM characteristics. It not only suppressed the oxidation of AIST phase, but also promoted the reduction of antimony oxides to form the metallic Sb2Te phase to improve the charge-storage capacity in nanocomposite layer. The study of blocking oxide layer of NFGMs found that the capping of HfO2/SiO2 composite layer may achieve an extremely large memory window shift (VFB shift) about 30.7 V and high charge storage density of 2.3 ×10^13 cm^-2 at ±23 V gate voltage sweep. Due to the deep trap sites formed by high-density AIST NCs in the nanocomposite layer and high barrier height feature of composite blocking oxide layer, the good retention property and low leakage current were also achieved. Though the capping of a sole HfO2 layer might also improve the NFGM performance, diffusion of HfO2 into the nanocomposite layer generated the HfSiO2 phase and oxygen defects. This induced the current leakage paths and deteriorated the charge trapping efficiency of AIST NCs. The Hf diffusion could be inhibited by inserting the SiO2 layer deposited by plasma-enhanced chemical vapor deposition (PECVD), leading to the enhancement of Coulomb blockade effect and charge storage capacility of AIST NCs. Analytical results demonstrated not only the feasibility of AIST-SiO2 nanocomposite layer to NFGM, but also a simplified device structure and processing method in comparison with previous NC-based NFGM studies. In the part regarding of the H2 gas sensor, the device containing about 30-nm thick AIST-SiO2 nanocomposite layer exhibited a maximum sensitivity of 61.3 % with fast 90% response time about 75 sec and recovery time about 50 sec in a 200-ppm H2 gas ambient at 75°C. The n-type sensing behavior of the AIST-SiO2 nanocomposite layer was ascribed to the presence of Sb2O5 clad on the nano-scale AIST phase as revealed by relevant microstructure and composition analyses. The adsorbed H2 molecules might induce the reduction of Sb2O5 into the metallic Sb2Te or the non-stoichiometry reaction of Sb2O5 to increase the electrical conduction in nanocomposite layer when it is exposed to the H2 gas. Due to the high specific surface area (SSA) feature of nano-scale AIST phase embedded in SiO2 matrix, a high H2 sensing capability could thus be achieved. A simplified device structure and fabrication process for H2 gas sensor is also illustrated by this study since the sensing layer could be easily prepared by conventional sputtering process.