鈣鈦礦氧化物薄膜層狀堆疊結構之半導體與鐵電特性對其光電流表現之影響

Photocurrent Response of Layered Structures of Perovskite Oxide Thin Films with Semiconducting and Ferroelectric Properties

陳思瑋(Szu-Wei Chen)

指導教授 ： 謝宗霖

國立臺灣大學/工學院/材料科學與工程學研究所/碩士(2013年)

https://doi.org/10.6342/NTU.2013.02999

摘要

本研究的主旨在於探討鈣鈦礦氧化物薄膜所形成的層狀堆疊結構的半導體與鐵電特性對其光電流表現之影響。本研究中所使用的鈣鈦礦氧化物有：SrTiO3、AgNbO3以及(Pb0.86La0.14)TiO3 (簡稱為PLT)，其中，SrTiO3與AgNbO3為光觸媒材料，而PLT則為晶種層(seeding layer)用以使SrTiO3與AgNbO3薄膜能夠於低溫(500 oC)形成穩定的結晶相。
本研究將能隙值較大(3.3 eV)的SrTiO3與能隙值較小(2.8 eV)的AgNbO3接合形成異質接面(heterojunction)以製備光電極，並將光電極應用於PEC (photoelectrochemical cell)裝置中的photoanode。異質接面乃是當兩不同能帶結構之半導體接合時，於兩者接面處會因為能帶結構的差異(band offset)而使光致激發的電子與電洞能夠快速分離，藉以提升材料的光電流表現。製備光電極的基材主要選用具備良好導電性以及高透光度的ITO導電玻璃，本研究利用溶膠-凝膠法以及旋轉塗佈法將三種鈣鈦礦氧化物薄膜堆疊於ITO上形成SrTiO3/AgNbO3/PLT/ITO的層狀結構，並以此層狀結構作為基礎模型，來與其他光電流量測結果進行比較。
本研究利用紫外光/可見光光譜儀(ultraviolet-visible spectrophotometer，UV-Vis)以及紫外光電子能譜儀(ultraviolet photoelectron spectroscopy，UPS)來鑑定SrTiO3、AgNbO3以及PLT的能帶結構，並且藉由電流-電壓曲線的量測來觀察SrTiO3/AgNbO3與AgNbO3/PLT兩異質接面的半導體特性。結果顯示，SrTiO3/AgNbO3界面呈現近似二極體(diode)的曲線，電子傾向由SrTiO3的導帶傳遞至AgNbO3的導帶，而AgNbO3/PLT界面則呈現歐姆接觸(Ohmic contact)的線性曲線，表示PLT薄膜由於厚度較薄(約20 nm)而存在著較多的缺陷，使得電子得以經由缺陷跳躍(hopping)傳遞至ITO。
除了半導體異質接面外，本研究尚利用另外兩種方法來提升SrTiO3/AgNbO3/PLT/ITO層狀堆疊結構的光電流表現。其一為貴金屬金的添加，本研究將金奈米顆粒添加於上述層狀堆疊結構中的不同位置，並藉由不同面入射的光來觀察並比較金奈米顆粒的添加對於層狀堆疊結構光電流表現之影響。實驗結果顯示，當光由SrTiO3面垂直射入時，金顆粒的存在對於層狀結構的光電流表現有著顯著的影響，由於金的費米能階位置較正(more positive)，因此當金顆粒與半導體接合時，電子傾向由半導體的導帶傳遞至金顆粒上，使得金顆粒添加於PLT/ITO界面的層狀堆疊結構(SrTiO3/AgNbO3/PLT/Au/ITO)有著最佳的光電流表現。另一方面，當光由ITO面垂直射入時，金顆粒會因表面電漿(surface plasma)效應而造成光的散射，使得含有金顆粒的層狀堆疊結構的光電流表現較差。此外，本研究尚嘗試增加金顆粒的大小以觀察不同大小的金顆粒對於層狀堆疊結構光電流表現的影響，結果顯示，含有粒徑較大的金顆粒的層狀堆疊結構有著較佳的光電流表現，原因在於含有粒徑較大的金顆粒的層狀結構能降低電子於高介電PLT薄膜中傳遞的有效路徑，使得電子較容易由AgNbO3傳遞至ITO，提升光電流值。
第二種提升SrTiO3/AgNbO3/PLT/ITO層狀堆疊結構光電流表現的方法為極化鐵電晶種層PLT，藉以觀察不同PLT電偶極方向對於層狀堆疊結構光電流表現的影響。結果顯示，經極化後，PLT的電偶極方向(負電指向正電)指向AgNbO3薄膜的試片有著最佳的光電流表現。由於極化的影響，AgNbO3/PLT界面存在著正電荷，使得界面處AgNbO3的能帶產生向下翹曲(bend downward)的現象，此現象能加速電子由AgNbO3傳遞至ITO，故使光電流值有顯著的提升。此外，本研究尚將SrTiO3、AgNbO3以及PLT薄膜製備於已鍍有銀電極的PZT鐵電陶瓷基材上，形成SrTiO3/AgNbO3/PLT/Ag/PZT的層狀堆疊結構，藉由使PZT產生不同方向的電偶極來觀察極化方向與光電流表現的關係。結果顯示，PZT的電偶極方向指向層狀堆疊結構的試片有著最佳的光電流表現，原因與上述相同，然而，由於PZT有著較佳的極化量，故使極化後所造成的光電流值上升幅度較大。

關鍵字

光電流 ； SrTiO3 ； AgNbO3 ； PLT ； 異質接面 ； 金奈米顆粒 ； 極化

並列摘要

The goal of this study is to investigate photocatalytic semiconductor systems which are layered thin film composites built from perovskite oxide materials with characteristics such as small and large band gaps and/or ferroelectricity. In order to improve the efficiency of photocatalysis, semiconductor heterojunctions within the developed composites have been designed to possess electronic band offsets favoring the separation of photo-induced electron and hole (e-/h+) pairs. Furthermore, the remanent polarization of the ferroelectric component within the composites has been utilized to induce favorable band bending at the material interface, lowering the potential barrier for electron transfer. The band offsets and ferroelectric polarization could be considered as built-in electric fields; how they interact with photo-induced e-/h+ would greatly affect the photocatalytic properties of the composites. In this study, various perovskite oxide thin film materials – large band gap strontium titanate (SrTiO3), small band gap silver niobate (AgNbO3) and ferroelectric lead lanthanum titanate (PLT) – were combined to form layered thin film composites. The composites were then adopted as photoanodes in a photoelectrochemical cell and detailed characterization of their photocurrent response was carried out under different light irradiation and ferroelectric polarization conditions. Electronic band offsets at the material interface (i.e., heterojunction) were determined by ultraviolet-visible spectrophotometry and ultraviolet photoelectron spectroscopy. Electric field poling of the ferroelectric component was achieved by non-contact corona charging.
Our results have shown that the band offsets at the SrTiO3-AgNbO3 heterojunction were about 1.0 eV in conduction band edge and 0.4 eV in valence band edge, promoting the rapid separation of photo-induced charge carriers; i.e., the flow of e- from SrTiO3 to AgNbO3 and the flow of h+ from AgNbO3 to SrTiO3. It was found that ferroelectric PLT could be used as a seeding layer for the low-temperature (500 oC) growth of SrTiO3/AgNbO3 thin film composites on ITO/glass substrates, forming a layered structure of SrTiO3/AgNbO3/PLT/ITO. In addition, the photocurrent density of the composites could be increased by depositing gold nanoparticles at the PLT-ITO interface. When the polarization of the PLT layer was poled toward the AgNbO3 layer, the potential barrier associated with the flow of e- to the ITO electrode was reduced by favorable band bending created at the AgNbO3-PLT interface. This resulted in a significant increase in photocurrent density.

並列關鍵字

Photocurrent ； SrTiO3 ； AgNbO3 ； PLT ； Heterojunction ； Gold nanoparticles ； Polarization

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