利用原子層沉積與陽極氧化鋁膜製備一維二氧化鈦奈米結構陣列及其光學特性研究

Abstract

利用原子層化學氣相沉積和陽極氧化鋁模板，在400 □C p-type矽基材上，製備垂直排列的TiO2奈米管與TiO2薄膜。TiO2奈米管的管壁厚度與TiO2薄膜的膜厚可藉由沉積循環數精準地控制。經由XRD與擇區繞射影像圖分析，TiO2奈米管與TiO2薄膜為多晶銳鈦礦(anatase)結構。TiO2奈米管與TiO2薄膜的吸收能力分別隨管壁厚度與膜厚增加而上升。相較於TiO2薄膜，TiO2奈米管擁有較大的表面積，因此展現出優異的Photoluminescence光譜特性。 由於吸收能力隨厚度增加而上升，因此金屬-半導體-金屬結構的Ti/TiO2/Ti電導偵測器展現出與厚度相關的光反應。對ITO/TiO2/Si光二極體而言，光電流由TiO2/Si異質接面(PN接面)或是ITO/TiO2異質接面(蕭特基接點，Schottky contact)所控制，但是其方向相反。在短路迴路下，不論正負極接在ITO上，由於TiO2/Si異質接面擁有較大的空乏區與電位勢，因此主宰光載子傳輸方向。在0 V至-1 V偏壓下，詳細的光電流源轉換過程，由TiO2/Si異質接面轉換至ITO/TiO2接面，可經由光響應圖形偵測得知。 研究蕭特基接點與歐姆接點(Ohmic contact)對於ITO/TiO2/Si與Ti/TiO2/Si光二極體光電響應之影響。結果顯示Ti/TiO2/Si光二極體為一個與厚度相關的光電反應。這是由於Ti/TiO2為歐姆接點對光電流量沒有任何貢獻，TiO2/Si異質接面控制光電流量之大小與傳輸方向。對ITO/TiO2/Si光二極體而言，蕭特基接點的ITO/TiO2接面調節TiO2/Si所控制的光電流量值，不再呈現和Ti/TiO2/Si光二極體一致的管壁厚度正比線性關係。 研究異質接面對於TiO2奈米管與TiO2/AAO奈米管光電反應之影響。相較於TiO2奈米管，由於空間侷限效應，TiO2/AAO奈米管展現較優異的PL光譜特性。在短路迴路下，由於光產生電子-電洞對在PL光譜上的再結合，TiO2/AAO奈米管的量子效率表現不夠傑出。藉由在高電場下抑制光產生電子-電洞對的再結合，與AAO空乏區的光產生電子注入，使得TiO2/AAO奈米管的量子效率獲得改善。 在紫外光開關照射下，奈米尺寸架構中，傳統的能帶圖觀念很成功地被運用解釋載子傳輸方向機制。相較於材料本身光導特性，這些研究工作強調異質接面也擁有相同的重要性。

Vertically aligned TiO2 nanotubes and TiO2 thin film are fabricated on p-type Si substrates by using atomic layer deposition system with anodic aluminum oxide template at 400 □C. The wall-thickness of TiO2 nanotubes and the thickness of TiO2 thin films can be controlled precisely by controlling the deposition cycle number. A polycrystalline anatase structure for TiO2 nanotubes and TiO2 thin film was confirmed by XRD and selected area diffraction pattern. The absorbability for TiO2 nanotubes and TiO2 thin film was improved as an increase of the film thickness. Due to the larger surface area, the fabricated TiO2 nanotubes exhibit an excellent performance on Photoluminescence characteristics, compared with TiO2 thin film. Metal-semiconductor-Metal (MSM) structured Ti/TiO2/Ti detectors exhibited a highly thickness-dependent photoresponse due to the absorbability was enhanced as an increase of thickness. For the ITO/TiO2/Si diode, the photocurrent is controlled by either the TiO2/Si hetero-junction (p-n junction) or the ITO/TiO2 hetero-junction (Schottky contact), which is in the opposite direction. In short circuit, no matter positive or negative electrode applied on ITO, the TiO2/Si hetero-junction dominates the photocarrier transportation direction due to the larger space charge region and potential gradient. For photocurrent sources, the detail transfer process from TiO2/Si hetero-junction to ITO/TiO2 hetero-junction was examined in the time-depended photoresponse at the biases of 0 V to -1 V. Schottky-contact and Ohmic-contact effects upon the photoresponses of ITO/TiO2/Si and Ti/TiO2/Si nanotube-based photodiodes were investigated. Results show that the Ti/TiO2/Si diode exhibits a highly thickness-dependent photoresponse. This is because the photocurrent is driven by the p-n junction at TiO2/Si alone and it faces no retarding at the Ohmic contact of Ti/TiO2. For the ITO/TiO2/Si diode, the Schottky contact at ITO/TiO2 regulates photocurrent overriding TiO2/Si as a result of higher efficiency in photogeneration, leading to the opposite response compared with the Ti/TiO2/Si diode. The hetero-junction effects on the photoresponses of TiO2 and TiO2/AAO nanotube-based photodiodes were investigated. Due to the space confinement effect, TiO2/AAO nanotubes revealed excellent PL characteristics, compared with TiO2 nanotubes. In short circuit, due to the recombination of photogenerated electron-hole pairs upon PL spectra, the performance of quantum efficiency in TiO2/AAO nanotubes was not splendid. It was improved for TiO2/AAO nanotubes on quantum efficiency by restraining the recombination in high field and the injecting the photogenerated electron from AAO depletion region. For the nano-scale framework, the classical knowledge of energy band diagram was successfully used to explain the mechanism of the carrier transportation direction under UV on/off illumination. These works emphasize the equal importance of the hetero-junctions as compared with materials’ photoconductive properties.