低溫製備氧化鋅奈米線及還原氧化石墨烯薄膜應用於紫外光偵測器暨酸鹼感測器之研究

Abstract

本論文旨在探討低溫製備氧化鋅(ZnO)奈米線及還原氧化石墨烯薄膜(Reduced Graphene Oxide Films)應用於高效能紫外光偵測器及酸鹼值感測元件之研究。 首先，本論文提出一種新穎全透明p型氧化鎳與n型氧化鋅奈米線之奈米異質接面紫外光偵測器，利用低溫水熱法成長n型氧化鋅奈米線，配合直流磁控濺鍍法沉積p型氧化鎳(NiO)於氧化鋅奈米線上。此外，我們也探討不同氧化鎳厚度對紫外光偵測器特性之影響。於紫外光(365 nm, 0.3 mW/cm2)照射下，當氧化鎳厚度從150 nm增加到250 nm時，紫外光響應(JUV/JDark)從3.24增加到4.98，但是當氧化鎳厚度增加到450 nm時，紫外光響應卻降至2.48。此因當氧化鎳厚度不足時，氧化鎳層被完全空乏，但是隨著氧化鎳厚度增加，雖形成足夠的空乏區能產生光載子，卻因串聯阻值上升而抑制光電流之傳輸，造成紫外光響應之下降。 接著，由於低溫成長之氧化鋅奈米線具有許多結構缺陷，導致元件具有高的可見光響應，因此為了抑制紫外光偵測器對可見光的響應，本章第一部分是利用光激螢光光譜分析來探討氧化鋅奈米線經過氮氣及氧氣環境下退火後對結構缺陷之影響，其結果顯示氧化鋅奈米線經過500 oC氮氣環境下退火10分鐘後，其近帶隙發光(Near-Band Edge Emission)與缺陷發光(Deep-Level Emission)之比值可由1.19大幅提升至28.88，氧化鋅內之缺陷密度大幅降低。進一步量測500 oC氮氣退火後之氧化鋅奈米線與氧化鎳(250 nm)結合所形成奈米異質接面紫外光偵測器之特性，其紫外光響應從4.98 些微提升至5.65，可見光響應(JVisible/JDark)從3.82顯著降至1.35。另一方面，為了抑制暗電流以提升紫外光感測特性，本章第二部分提出於氧化鎳及氧化鋅奈米線間以電漿輔助化學汽相沉積技術沉積超薄二氧化矽，探討不同二氧化矽厚度對元件特性的影響。量測結果發現氧化鎳(250 nm)/二氧化矽(6 nm)/氧化鋅奈米線堆疊所形成之紫外光偵測器，其紫外光響應可以大幅提升至16.21，這是由於適當厚度的二氧化矽層足以建立一能障，使多數高能光激載子能順利通過該能障，並有效抑制低能非光激載子所致。 針對延伸式閘極場效電晶體(Extended-Gate Field-Effect Transistor, EGFET)之酸鹼感測膜之開發，過去氧化鋅作為酸鹼感測薄膜時，在極酸/鹼環境下會發生感測膜被蝕刻的問題，因此於生醫感測應用上受到限制，為提高酸鹼感測膜之化學穩定性，本論文提出利用還原氧化石墨烯薄膜來做為感測膜，並利用氧電漿處理噴塗於矽基板上之還原氧化石墨烯薄膜，使得薄膜表面能夠提供更多的感測位置(Sensing Sites)，成功地製備出具有優異電壓感測度達52.9 mV/pH及線性度0.997之延伸式閘極場效電晶體之酸鹼感測器，在廣泛的酸鹼感測區間(pH 1 ~ 13)。與未經電漿處理之還原氧化石墨烯薄膜相比(45 mV/pH)，經適當偏壓功率氧電漿處理後之還原氧化石墨烯薄膜，其電壓感測度有近18%的提升且磁滯寬度亦從19.2降至13.1 mV。 為進一步提升還原氧化石墨烯薄膜之酸鹼感測特性，本論文提出利用具微米結構之倒金字塔基板，將還原氧化石墨烯薄膜製備於其上，輔以適當氧電漿處理，以提升元件之有效感測位置及面積。藉由調變相鄰倒金字塔間距(R)與其固定深度(H=37 um)之比值發現，當R/H為0.5時，未經電漿處理之還原氧化石墨烯薄膜之電壓感測度為52.0 mV/pH，輔以電漿處理後，展現出更優異之電壓感測度達57.5 mV/pH、良好的感測線性度0.993及更小的磁滯寬度10.5 mV，相較於未經電漿處理噴塗於平面基板之還原氧化石墨烯薄膜有28%的提升。 最後，亦提出論文結論與針對未來研究可著重的工作方向。

In this thesis, the low-temperature fabricated ZnO nanowires (NWs) and reduced graphene oxide thin films (RGOFs) have been demonstrated for the applications in the high-performance ultraviolet (UV) detectors and pH sensors. At first, a visible-transparent UV detector with p-type nickel oxide (NiO)/ n-type ZnO NWs nanoheterojunction (NHJ) structure was proposed by means of a low-temperature hydrothermally-grown ZnO NWs followed by the NiO deposition via a DC sputtering system. Moreover, the thickness effect of NiO on the performance of UV sensors was also investigated systematically. Under UV light illumination (365 nm, 0.3 mW/cm2), the UV sensitivity (JUV/JDark) was increased from 3.24 to 4.98 when the NiO thickness increased from 150 to 250 nm and then decreased to 2.48 when the NiO thickness further increased to 450 nm. Such p-n junction with thinner or thicker NiO layer led to worse UV detecting characteristics mainly due to the fully-depleted NiO layer and larger series resistance, respectively. Secondly, a great quantity of structural defects in the low-temperature-synthesized ZnO NWs led the ZnO-based UV sensors to high visible response. Hence, to suppress the visible response, the first part of this chapter focused on the annealing effect on the photoluminescence characteristics of ZnO NWs and the resultant optoelectronic characteristics of p-NiO/n-ZnO NW UV detectors. After annealing in the nitrogen (N2) ambient at 500 oC for 10 minutes, the near-band-edge emission to deep-level emission ratio (NBE/DLE) of the ZnO NWs substantially increased from 1.19 to 28.88. The devices with 500 oC-N2-annealed ZnO NWs UV sensors exhibited higher sensitivity (JUV/JDark=5.65) to UV light and lower response (JVisible/JDark=1.35) to visible light than those compose of unannealed ZnO NWs (JUV/JDark=4.98; JVisible/JDark=3.82). On the other hand, for the propose of reducing dark current and further promoting UV sensitivity, a novel structure of NiO/insulator-SiO2/500 oC-N2-annealed ZnO NWs were first proposed and the thickness effect of SiO2 on the UV sensing properties was discussed in the second part of this chapter. It was found the UV sensitivity could greatly promote to 16.21 for the devices with a 6-nm-thick SiO2 interlayer. It was mainly due to the thin SiO2 interlayer built a barrier height to minimize the transmission probability of low-energy carriers, leading to higher UV sensitivity. For the development of the extended-gate field-effect transistor (EGFET) pH sensors, ZnO as the sensing membranes of EGFETs exhibited well pH sensing properties but was nonresistant to the acid/alkali circumstances, restricting its applications in the biosensors. To avoid this issue, in this thesis, an EGFET with RGOFs as the sensing membrane was demonstrated and the oxygen-plasma treatment (OPT) was utilized to effectively decorate a sufficient number of sensing sites on the RGOFs. The sensing characteristics of pH-EGFET sensors based on these oxygen-plasma-treated RGOFs (OPT-RGOFs) with a higher voltage sensitivity of 52.9 mV/pH and a better voltage linearity of 0.997 in the wide sensing range of pH 1-13 were accomplished. The voltage sensitivity of the OPT-RGOFs showed a nearly 18% enhancement and the hysteresis of the OPT-RGOFs decreased from 19.2 to 13.1 mV. To further improve the pH sensing properties of RGOFs, the micro-structured reverse pyramid (RP) substrates with the OPT-RGOFs were first utilized as the sensing heads. The optimal conditions for the pH sensing of the OPT-RGOFs on RP substrates were the RP’s depth (H) of 37 μm and the R/H ratio of 0.5, where the R/H ratio was the interspacing (R) between the neighbor RPs against the RP’s depth. The sensing characteristics of pH-EGFET sensors with these OPT-RGOFs coated on the RP substrates (R/H=0.5) achieved a highest voltage sensitivity of 57.5 mV/pH with a good linearity of 0.993 and a smallest hysteresis of 10.5 mV. A nearly 28% enhancement in voltage sensitivity was acquired compared with the as-sprayed RGOFs on the planar substrate, resulting from higher effective sensing sites and area. Finally, conclusions as well as prospects for the further research are also proposed.