以低溫電漿成長之低缺陷非晶矽於可撓式光電子應用

Abstract

在電子元件追求輕薄與可攜性的趨勢下，基體材料逐漸由傳統的矽晶圓與玻璃，朝向薄型玻璃、薄型金屬與塑膠基板發展。其中聚醯亞胺(Polyimide, PI)基板因具有較高的玻璃轉移溫度、較低的熱膨脹係數與優異的彎折特性，而成為相當具有前景的軟性基板。由於傳統的高熱預算製程技術無法應用於軟性基板上，因此具低熱預算製程技術與元件的開發，將成為主要的核心發展技術。在本研究中，以製作低缺陷氫化非晶矽材料為研究核心，開發電漿薄膜沈積、雷射結晶及雷射活化等低熱預算製程，應用於非晶矽能量採集元件及多晶矽場效電晶體之製作。 在非晶矽能量採集元件方面，利用低熱預算感應耦合電漿(140-200 oC)製作低缺陷p-i-n型非晶矽薄膜光伏電池，並引入寬能隙碳化矽窗口層與緩衝層，增加紫外-可見光(UV-visible)波段的光捕捉效率。寬能隙碳化矽窗口層與緩衝層的搭配，可緩和p/i介面處的陡變介面，降低介面與吸收層缺陷，改善紫外-藍光波段的光吸收，使薄膜光伏元件具備高開路電壓、短路電流與填充因子。吸收層厚度亦需隨照射光源與照射強度而進行優化，其光電轉換效率與輸出功率在1-Sun太陽模擬光源與500 lx室內日光燈照射下，分別可達9.58 %與25.56 μW/cm2。 此低熱預算技術亦可製作n-i-p型能量採集元件於可撓性PI基板上，其輸出功率在600 lx室內光照射下可達20.64 μW/cm2。在張力彎曲測試下，其輸出功率幾乎可維持一致，直到彎曲半徑低於4 mm。在7小時1-Sun太陽模擬光源與600 lx室內日光燈照射下長照度測試下，其輸出功率只減少了0.8%與11.4%。此外，本研究亦開發了特殊的交錯電極結構以進行電池之串並聯。將兩顆電池以此技術並聯後，其輸出功率在600 lx室內光照射下可達41.76 μW/cm2。 在可撓式場效多晶矽場效電晶體方面，本研究所開發之氧化矽/鋁/氧化矽三明治結構可避免PI基板於雷射退火製程時遭受破壞。在雷射製程中以355 nm藍光尖峰退火雷射結晶，將感應耦合電漿沈積的非晶矽薄膜轉變為結晶大小約800 nm的多晶矽薄膜，並搭配532 nm綠光尖峰退火雷射結晶進行源極/汲極活化。雷射退火時，鋁/氧化矽接面處形成的熱累積有助於結晶成長與摻雜活化。藉由低溫薄膜沉積技術與雷射退火技術，我們成功製作出元件大小為400 nm之多晶矽場效電晶體，其開關比達5x106，且次臨界擺幅達190 mV/dec。此外，在張力彎曲測試下，其轉移特性幾乎可維持一致，直至彎曲半徑低於15 mm。 透過本研究所開發的低熱預算材料、製程與元件，未來將可有廣泛應用於可撓式電子元件開發、自供電元件整合與物聯網科技。

The demand for system on flexible (SoF) electronics is increasing for realizing devices with high portability and low power consumption. Polyimide (PI) is the potential flexible substrate due to its high glass-transition temperature (Tg) and low coefficient of thermal expansion (CTE). However, the conventional high thermal processes constrain this realization; thus developing low thermal budget processes is essential. In this thesis, we investigated the material characteristics of low-defect amorphous Si (a-Si) thin films and developed low thermal budget processes, such as plasma-deposited thin films, laser crystallization, and laser activation to fabricate a-Si thin film energy harvesters and poly-Si field-effect transistors (FETs). For a-Si thin film energy harvesters, highly efficient p-i-n type a-Si thin film photovoltaics were fabricated through inductively coupled plasma chemical vapor deposition at 140-200oC. The wide-bandgap p-type hydrogenated amorphous silicon carbide (p-a-SiC:H) window layer of the p-i-n solar cells increased in the ultraviolet (UV)-visible region. Besides, the p-a-SiC:H window layer/p-a-Si:H buffer layer scheme moderates the abrupt band bending across the p/i interface for the enhancement of Voc, Jsc and FF in short wavelength regime. The thickness of absorptive layer was also optimized to achieve high efficiency of 9.58% and 25.56 μW/cm2 under 1-Sun and 500 lx illumination. We also demonstrate the fabrication of flexible n-i-p type a-Si thin film photovoltaics on PI substrate with high output power of 20.64 μW/cm2 under 600 lx illumination. Various reliability measurement also been carried out to qualify the stability and lifetime in a real application. The performance of flexible a-Si thin film photovoltaics by tension stress can be kept until bending-radius over 4 mm. The output power drops by 0.8% and 11.4% while the cell exposed to 1-Sun and 600 lx illumination for 7 hours, respectively. Furthermore, we proposed a separated contact structure to envision the connection of individual cell. The output power of parallel-connected two cells of 1 cm2 achieved 41.76 μW/cm2 under 600 lx illumination For flexible field effect transistor, the SiO2/Al/SiO2 sandwiched buffer layer was introduced to prevent the damage on PI substrate in the following laser annealing process. A 355 nm nanosecond laser spike annealing was used to transform a-Si to poly-Si with 800 nm grain size, and a 532 nanosecond laser spike annealing was used to source/drain activation. The heat accumulation and laser reflection at the Al/SiO2 interface facilitates the grain growth and dopant activation. The feature size of poly-Si fFET shrunk to 400 nm via laser annealing performs the on/off current-ratio exceeding 5×106 and subthreshold swing of 190 mV/dec. Moreover, the transfer characteristics of fFET by tension stress can be kept until bending-radius over 15 mm. These low thermal budget materials, processes and devices can be widely used in SoF electronics, self-powered device integration and Internet of Things.