熱氧化法製作垂直二氧化鈦奈米柱及其光伏效能研究

Abstract

本研究利用鍍有鈦膜之矽基板，在氧氣氛下進行高溫熱氧化處理，成功地製備出具有高表面積特性之二氧化鈦奈米柱。當製程條件為600oC、鈦膜厚度300奈米、腔體壓力2×10-2 torr、氧氣流量10 sccm下反應3小時，可得到長度350奈米、直徑20~25奈米且高密度之最佳化二氧化鈦奈米柱。根據SEM、XRD、ESCA及TEM分析結果顯示，此方法製備之奈米柱外觀為筆直柱狀，屬於單晶金紅石相二氧化鈦結構，並且垂直成長於基板。而根據UV-Vis穿透吸收光譜分析也顯示，此二氧化鈦奈米柱之能隙能量為3.1 eV，是為紫外光波段。此外，由TEM側視圖結果，推測此二氧化鈦奈米柱之生成與鈦膜氧化成二氧化鈦過程中，體積膨脹所產生之應力有關。而試片經氨電漿表面改質處理後，在二氧化鈦奈米柱之表面形成一層TiN或TiOxNy結構之氮化合物，使其在光伏效應中之光電流密度提高至120 μA/cm2；相較於未處理前之二氧化鈦奈米柱，增強了2.4倍。由於此導電度較高之氮化物層存在，使得二氧化鈦奈米柱受光激發後產生之光載子可以快速並且有效地傳輸至電極，形成光電流，而降低電子電洞對再復合之機率，提升光電轉換效率。

Vertically aligned TiO2 nanorods with a high rod density (~3.5×109 rods/cm2) were spontaneously grown on the Ti/SiO2/Si (100) substrate in oxygen ambient at high temperature without catalyst. The density and size of the nanorods were sensitive to preparation parameters, such as the growth temperature and working pressure. TiO2 nanorods prepared at 600oC and 2 10-2 torr had a diameter and height of 15-20 nm and 350 nm, respectively. Rutile was the major TiO2 polymorph in the nanorods according to x-ray diffraction analysis. The growth of the TiO2 nanorods is likely to result from the development of the large volume expansion stress during the thermally oxidation process. The high density and high aspect ratio (~21) of the nanorod array provide a large surface area important for photovoltaic applications. After being treated by NH3 plasma, the TiO2 nanorods exhibited a significant improvement on photovoltaic performance. The plasma treated TiO2 nanorods had a photocurrent density about 120 □A, 2.4 times larger than that of the as-grown TiO2 nanorods. The improvement is ascribed to the formation of a N-contained compound layer on the nanorod surface after the NH3 plasma treatment. The presence of the nitride layer can make the carrier transfer more efficient and therefore, reduce the recombination rate.