適用於大面積製程氧化鋅鎂/氧化鋅異質結構紫外光感測器

Abstract

本研究的第一部分針對以射頻濺鍍沉積氧化鋅鎂/氧化鋅(MgZnO/ZnO)異質結構於軟性PI塑膠基板和不鏽鋼基板的薄膜性質進行研究。回顧先前研究，要在室溫濺鍍的高缺陷密度MgZnO/ZnO異質結構形成二維量子氣體(2DEG)效應，ZnO層的退火是關鍵的步驟，且可藉由大氣電漿縮短處理時間。因此本研究採用大氣電漿快速處理此二種基板上的ZnO，使其介面形成2DEG效應。而由於PI基板不耐高溫，針對PI基板做300°C氮氣的長時間退火，以彌補溫度不足。而結果顯示，在300 °C退火， 3小時以上的退火，可增強2DEG效應，使MgZnO/ZnO之片阻值下降。並且藉由施以機械外力的彎曲和拉伸測試下，找出可穩定操作的極限應變大小，故此研究結果指出，射頻濺鍍之MgZnO/ZnO異質結構可廣泛應用於大面積的軟性電子元件中。 第二部分則是將氧化鋅及氧化鋅鎂/氧化鋅異質結構薄膜作為紫外光感測器，並且比較在不同熱退火處理下(包含大氣噴射電漿和傳統高溫爐熱退火處理)，對於ZnO及MgZnO/ZnO紫外光感測性能的影響。而由於大氣電漿溫度較高，故基板的選擇為玻璃基板；而在傳統高溫爐處理下，則同時使用玻璃及PI塑膠基板。而結果顯示，極短時間的大氣電漿處理，對於感測器的光反應性即可有明顯的改善，但過長時間的處理與異質結構所形成的高導電層反而會使其光電流值難以恢復至原始值。同樣情形也可以在傳統高溫爐後處理上觀察到，隨著薄膜結晶性越好，暗電流值越高，其在照光反應下雖然會比較快，但在結束光照後，其光電流的消逝就變得非常困難，故在選擇紫外光感測器之處理條件時，必須將各項性質做綜合比較，選出適當的參數。以大氣電漿處理部分來說，對氧化鋅薄膜做30秒的處理，可改善其光反應性且同時保持良好的反應及恢復速度；而在傳統爐管退火上，對氧化鋅做300 °C的氮氣退火30分鐘對電流在照光反應和恢復上影響最小，需要的恢復時間最短。相較起來，異質結構由於兩材料間極化效應的影響，皆會導致光電流難以回復至暗電流值，讓回復過程非常緩慢。而我們也意外發現，沉積氧化鋅鎂於未經任何處理的氧化鋅上，雖然不會形成高導電介面，但是由於氧化鋅鎂的覆蓋效應影響下，光反應性可以增幅數倍且同時保有快速的反應和恢復過程，為一良好的改善方法。

In the first part of the experiment, we investigate the electromechanical properties of RF-sputtered MgZnO/ZnO heterosructures on flexible polyimide (PI) and stainless steel 304 (StSt304) substrates. By subjecting ZnO to ultra-short (30–40 s) atmospheric-pressure plasma jet (APPJ) treatment and prolonged (>3 h) thermal annealing at 300 oC, highly conductive interfaces are induced in rf-sputtered MgZnO/ZnO heterostructures on flexible PI and StSt304 substrates. The electrical properties of on-StSt MgZnO/ZnO annealed at 400 °C for 30 min are evaluated under the inward and outward bending conditions. Furthermore, the electrical properties of on-PI MgZnO/ZnO heterostructures annealed at 300 °C for 3 h are examined under the bending and stretching conditions. Compared with ZnO, MgZnO/ZnO heterostructures show better electrical stability under mechanical flexing; deviations in the electrical properties of MgZnO/ZnO heterostructures occur under larger strain levels. Piezoelectric polarization is induced under flexing, resulting in an increase or decrease in the resistance of MgZnO/ZnO heterostructures The second part of the research is the comparison of rf-sputtered ZnO and MgZnO/ZnO heterostructures based photodetectors (PDs) with atmospheric pressure plasma jet (APPJ) treatment and traditional furnace annealing on ZnO layer. Because the temperature of APPJ exceeds the working temperature of PI substrates, Corning glass was chosen as the substrates. For the UV detectors with furnace annealing on ZnO, both films on PI and glass substrates were investigated. ZnO films under different APPJ treatment duration and different furnace annealing parameters were characterized, I-V curves and time-dependent responses of detectors after UV-illumination were shown. The mechanisms were also discussed. It is well known that the photo-conductivity of ZnO is greatly influenced by the surface adsorption and desorption of oxygen. In the past few years, M. Liu et al had the ZnO-based PD treated with oxygen plasma and found a massive enhancement in UV detection properties [1]. Thus, the APPJ treatment was chosen as a surface modification method for our ZnO PDs in this work and compared with furnace annealing. The photoresponsivity could be dramatically improved after the APPJ treatment or the thermal annealing process of furnace, but the resultant high conductivity results in a long-time recovery of photocurrent after UV-illumination. A 30-s APPJ treatment duration is the optimized APPJ treatment condition for ZnO PD. The responsivity was enhanced with little influence on the time-dependent photo-response. Moreover, depositing MgZnO on APPJ-treated and furnace-annealed ZnO films to form MgZnO/ZnO heterostructures can induce highly conductive interfaces and increase photocurrent. However, the interface may trap the excited electrons such that the photocurrent will not recover to initial value unless placing the UV diode in dark for a very long time. Capping MgZnO on as-deposited ZnO films does not induce highly conductive layer but can enhance the photocurrent level without deterioration of the response time. Our experimental results indicate that the optimized PDs are MgZnO on as-deposited ZnO and pure ZnO with a 30-s APPJ treatment; these PDs can have improved photoresponsivity without the deterioration of response time.