低溫合成高效能透明p型氧化鎳/n型氧化鋅奈米線異質接面於紫外光偵測器之研究

Abstract

近年來，由於大氣中的臭氧層含量逐年減少，過量的紫外線曝曬導致生物傷害，紫外光偵測技術引起了廣泛的興趣。氧化鋅擁有寬能隙、成本低廉、抗輻射、和優異的熱及化學穩定性，因此氧化鋅基紫外光偵測器逐漸受到重視。本論文利用低溫水熱法成長垂直排列氧化鋅奈米線，配合直流磁控濺鍍法氧化鎳之沉積，形成透明p型氧化鎳結合n型氧化鋅奈米異質接面之紫外光偵測器。此方法沉積之氧化鎳均勻包覆於氧化鋅奈米線表面，大幅提升接面面積進以提升光電流特性。 本文首先探討不同厚度之氧化鎳對紫外光偵測器之光電特性影響，氧化鎳厚度為250 nm之元件於紫外光照射下(365 nm, 0.3 mW/cm2)具有最佳之光電感測特性(IUV/IDark = 4.98 @ -2V)，然而，該元件於可見光照射下亦有相當之光響應現象( IVisible/IDark = 3.82 @ -2V)。 因此，為抑制元件對可見光的響應，本論文利用熱處理方式修補氧化鋅缺陷以進一步減低紫外光偵測器對可見光之感測能力。從實驗結果得知，以500 ℃下熱退火10分鐘之氧化鋅奈米線所堆疊形成氧化鎳/氧化鋅奈米異質接面紫外光偵測器，擁有較低的漏電流及較佳的光電流特性 (IUV/IDark = 5.65 @ -2V, IVisible/IDark = 1.35 @ -2V) 。 另一方面，為進一步抑制紫外光偵測器之感測特性，本論文提出於氧化鎳/氧化鋅奈米異質接面間以電漿輔助化學汽相沉積法沉積超薄二氧化矽，探討不同二氧化矽厚度對元件特性之影響。由分析結果發現，氧化鎳(250 nm)/二氧化矽(6 nm)/氧化鋅奈米線堆疊形成之異質接面紫外光偵測器，於2 V反向偏壓操作下，其紫外光感測特性由原本IUV/IDark = 5.65大幅的提升至16.21，可將之歸功於超薄二氧化矽層利用穿隧機制使之對於高能量光激發載子不致造成抑制效果，對於低能量非光激發載子具顯著抑制特性所致。 本研究所提出之新型p型氧化鎳/二氧化矽/n型氧化鋅奈米線堆疊異質接面紫外光偵測器因具有優異的感測特性、熱穩定特性、抗輻射能力以及可以重複操作的特性，於未來光電元件發展中深具潛力。

In recent years, the detection of ultraviolet (UV) radiation has attracted extensively interest due to the great decline of the stratospheric ozone layer. Because of its wide band gap, low cost, strong radiation hardness and high thermal and chemical stability, ZnO has become the most promising candidate for UV photodetector. In this thesis, a transparent UV detector was fabricated using nano-heterojunctions (NHJs) composed of p-type NiO and n-type ZnO nanowires. Via DC-magnetron sputtering system, the p-type NiO was deposited onto the vertically-aligned n-type ZnO nanowires which were grown by low-temperature hydrothermal growth process. The as-sputtered NiO on the ZnO nanowires showed the great junction area of the NHJs. Thus, the optoelectronic characteristics were improved. In the beginning of this thesis, the effect of NiO thickness on the optoelectronic characteristics of the UV detector was investigated. It demonstrated that the as-prepared device with 250 nm thick NiO nanostructures showed good sensitivity (IUV/IDark = 4.98 @ -2V) under UV light (365 nm, 0.3 mW/cm2) illumination. However, the device also represented a great photo-response to visible light (IVisible/IDark = 3.82 @ -2V). Hence, in order to suppress the photo-response of the device under visible light illumination, thermal treatments were utilized to reduce the structural defects of ZnO nanowires. It revealed that the p-NiO/ n-ZnO nanowires devices with ZnO nanowires which were annealed in N2 ambience at 500 ℃ for 10 minutes exhibited lower structural defects, lower leakage current, and superior sensitivity (IUV/IDark = 5.65 @ -2V, and IVisible/IDark = 1.35 @ -2V). Moreover, for the propose of reducing the leakage current to further improve the sensitivity of the UV detector, an ultrathin SiO2 layer was deposited by plasma enhanced chemical vapor deposition (PECVD) as interlayer between NiO and ZnO nanowires was proposed in this study. And the dependence of the optoelectronic characteristics on the SiO2 thickness was also discussed. Encouragingly, as the reverse bias 2 V applied, the IUV/IDark increased substantially from 5.65 to 16.21 as the SiO2 thickness increased from 0 to 6 nm. The phenomenon was attributed to the ultrathin SiO2 layer which can not seriously affect the photo-excited high-energy carriers but remarkably depress the dark-state low-energy carriers due to the tunneling mechanism. In this work, the novel UV detector composed of the p-NiO/ insulator-SiO2/ n-ZnO nanowires demonstrated the excellent sensitivity, thermal stability, radiation hardness and repeatable photo-response to UV light, making it promising for the future developments of optoelectronic applications.