寬波段與低耗能之光-電-熱偵測與轉換元件研究

Development of Broadband Operation and Low Power-Consumption of Photo-Electronic-Thermo Detection and Conversion Devices

林耕德(Keng-Te Lin)

指導教授 ： 陳學禮

國立臺灣大學/工學院/材料科學與工程學研究所/博士(2015年)

https://doi.org/10.6342/NTU.2015.02541

摘要

隨著時代的演進與科技發展，光電熱轉換元件已被廣泛的運用於影像感測、光通訊、環境監控、太空偵測以及太陽能源回收等領域。然而現今市售之光電熱元件中，大多面臨著需操作偏壓過大、可偵測之頻譜範圍受限與偵測能力不足等問題，因而嚴重地限制了這些元件實際的應用性。因此，本論文主要目的在於開發高效率、寬波段、低耗能之光電熱轉換元件。
為了開發高效率之光電熱轉換元件，直接改善元件之材料品質將可快速的提升整體光電元件之轉換效率。論文第一個部分，我們利用氟化氪準分子雷射來對於硫化鎘光導體元件進行表面之改質，可大量的製作出低成本、低耗能、高光電響應之硫化鎘光偵測器。透過準分子雷射照射於市售低價之光敏電阻元件上，藉由雷射劑量與照射次數之調控，進而使得硫化鎘於極淺層之區域產生差排減少與結晶性變好等特性，我們亦發現此具有良好結晶性之硫化鎘元件將可於外加極低偏壓的情況下於弱光下仍具有極佳的光電響應能力與偵測能力；此外，本論文更發現於雷射退火前後，此硫化鎘元件於紫外光波段之光電轉換增益將較可見光來的顯著。於本論文中，此表面改質之硫化鎘元件將可於施加1毫伏特時，便可對於極弱強度(10-2 fW µm-2)之可見光(540 nm)表現出7200 A W-1之光電響應能力與1015 Jones之偵測能力；而對於極弱光強度(1.9×10-2 fW µm-2)之紫外光(365 nm)照射下時，表現出光電響應能力與偵測能力分別為74.7 A W-1與1014 Jones。此外，此表面處理之硫化鎘元件於極弱光強度照射時所需的反應時間僅需40毫秒，此反應時間相較於傳統數位相機於收集極弱光強度影像時來得更快。因此，於本論文中所提出之極淺區域表面改質之硫化鎘元件將非常適用於低耗能之紫外光至可見光波段弱光偵測相關應用上。
論文第二個部分，我們開發了一種元件架構簡單之表面電漿彩色影像偵測器。整體元件結構之組成為將具有週期性孔洞陣列結構之單層鋁膜製作於矽基材上以形成蕭特基接面，進一步達成彩色影像偵測之目的。此方法可以大幅度的降低繁複的元件結構設計與製程上的難度。此外，週期性孔洞陣列結構的單層鋁膜於彩色影像偵測之過程中將具多種功能特性，如：分光、光電轉換與光激載子收集；且此種元件設計架構更克服了於傳統彩色光偵測器中光傳輸層所造成的干涉現象，可避免分光後之彩色頻譜再次的被扭曲。而於光電轉換特性上，此具有表面電漿特性之蕭特基元件將可運用金屬結構於近場電場強度增強之效應以提升矽之光學吸收，進而使得於不施加偏壓之情況下亦可擁有極高的電流輸出與光電響應能力；此外，此元件更可將光所激發之載子直接轉換為電壓之型式輸出，以做為判斷光強度之依據，縱使於極弱之光強度(fW µm-2)照射下，此元件於電壓之輸出上仍有幾微伏特，相較於傳統之彩色影像偵測器，此種方法將可減少於後續電路設計中所需要的電容數量，進而縮小電路設計所需的面積。因此，本論文所提出具有表面電漿特性之蕭特基彩色影像偵測器將可更進一步的與互補式金屬氧化物半導體相容之電路設計進行整合，以期可利用成熟之半導體技術開發出具有極高像素之彩色影像偵測器。
論文的第三部分提出了一種利用金屬天線結構，其所產生之表面電漿子衰逝可進而產生熱電子以跨躍金屬與矽基材所形成之蕭特基接面，以作為紅外線偵測之方法，開創了利用矽基半導體材料來偵測通訊波段之紅外光的道路。本論文所提出的金屬奈米天線結構為一種將深溝槽薄金屬週期性陣列結構製作於矽基材上，將可利用表面電漿共振現象、共振腔效應以充分的收集任一偏振方向入射之紅外光源，再透過大面積的金屬/矽基材接面大幅地接收由金屬所產生的熱電子，以提升整體元件於紅外光波段之光電流輸出。本論文所提出之深溝槽薄金屬矽基元件將可於不施加偏壓的情況下，便表現出極佳的光電轉換能力；此外，此元件不論於強光(3 mW)與弱光(20.5 µW)之通訊波長紅外光(1550 nm)照射下，其光電流之輸出皆可具有極佳的線性程度。因此，本論文所提出之深溝槽薄金屬矽基元件將可突破傳統矽基光偵測器所能偵測之波長極限，可低耗能地於寬波段(紫外光至光通訊波段)中收集各種偏振方向之入射光以轉換為電訊號之輸出來做為光源強度判斷之依據，並可與成熟之半導體製程技術整合以用於矽光晶體晶片之許多通訊波段相關應用上。
論文的第四部分更進一步探討單層溝槽狀薄層金屬結構所具有的特殊光學性質及其光熱轉換特性。本論文提出一個由負載效應所誘發的單層溝槽狀薄層金屬結構來達成寬波段的光能回收。此由負載效應所誘發的單層溝槽狀薄層金屬結構將可利用表面電漿現象與共振腔效應於紫外光至近紅外光區間提供高吸收、可輕易調控吸收波段且對於光的極化偏振方向不敏感等光學特性，且此結構之最佳化參數大小約為所欲設計波長的一半，如此縱使所欲設計之波長於紫外光或可見光波段，此結構之大小仍可維持數百奈米的尺度，可大幅降低於製程上之難度。於本論文中，此結構所具有的寬波段高吸收特性將可符合太陽光能於地表之光譜輻照度(AM 1.5)分佈，如此便可充分的吸收太陽光能，以於太陽光能之回收應用上有所助益；此外，本論文亦已證實此種由負載效應所誘發的單層溝槽狀薄層金屬結構具有極佳的光熱轉換能力，並觀察到此結構於紅外光(5–10 µm)與可見光波段之光學行為大不相同，因此此結構將可充分的吸收具有高光子能量之紫外光、可見光與近紅外光以產生熱能，而後可藉由連續金屬薄膜層將熱能有效的傳導與收集，並防止所產生的熱藉由熱輻射之方式而散失，如此便可充分的回收與利用光能，以期可於光能回收利用與感測等相關領域應用上有所助益。
於論文的最後一部份，我們提出了一種無需彩色濾光片且無接面(junctionless)之彩色影像偵測器。此元件之組成僅為具有單層溝槽狀結構之薄層鋁膜，此結構將可利用近場之表面電漿特性與共振腔效應大幅度的提升於鋁金屬膜與空氣介面間之電場強度，進而使得此單層結構鋁膜具有極大的吸收；而於鋁金屬膜吸收光能後將會更進一步的將光能轉換為熱能，進而使得鋁金屬薄膜之溫度有所提升；而後，於鋁金屬薄膜之溫度上升與此溝槽狀金屬薄膜結構周圍所形成的高電場強度之影響下，金屬膜內部之電子將會於移動的過程中受到阻礙，因而導致此薄層鋁膜之電訊號輸出(如：電阻、電流)於固定偏壓下發生改變，以作為判斷光強度的依據。此外，此單層溝槽狀結構之薄層金屬膜於光偵測的過程中，將具有多功能之特性，如：顏色選擇吸收(僅有於設計波段之光能會被此結構所大幅吸收)、產生熱與電場增強來影響金屬膜內部電子之移動行為與收集所產生的電訊號差異。於本論文中，我們已證實此元件縱使於低偏壓操作下，仍可表現出極佳的光熱電轉換特性，其彩色分光頻譜響應之反應相當快速。相較於過往所發表的基於彩色濾光片且具有接面特性之彩色影像偵測器，本論文中所提出之方法為一種全新的彩色影像偵測器設計概念，將可把此種結構建構於任意之基板上以達成彩色影像之偵測，我們預期此方法將可應用於許多影像感測與光偵測之相關應用上。

關鍵字

氟化氪準分子雷射 ； 硫化鎘光導體 ； 表面電漿共振 ； 表面電漿彩色影像偵測器 ； 矽基紅外光偵測器 ； 完美吸收體 ； 光電熱轉換元件

並列摘要

Photo-electronic-thermo conversion devices have been applied in a variety of applications, such as image sensing, optical communication, environmental monitoring, astronomical studies, and solar energy harvesting. However, insufficient detection capability, limited detection bandwidth, and requiring large bias voltage restricted the practical applications of devices. Therefore, the goal of the researches in this thesis is to develop photo-electronic-thermo conversion devices featuring high conversion efficiency, detection over broad bandwidth, and low power consumptions.
Direct improving the quality of materials can intrinsically enhance the conversion efficiencies of devices. In the first part of thesis, we describe a low-cost cadmium sulfide (CdS) photoconductor that behaves as a highly sensitive and rapidly responding detector toward low-intensity light. Through the observation of transmission electron microscopy images and analysis of photoluminescence and Raman spectra, the degree of crystallization of CdS increased and defects were removed effectively in the region of a few tens nanometers beneath the surface of CdS after treatment with several shots/distinct intensities from a krypton fluoride (KrF) excimer laser. Moreover, we found the improvements of detection capability of CdS devices in the ultraviolet (UV) regime are much larger than that in the visible regime. At a low bias voltage of 1 mV, the treated CdS device provided a record high responsivity of 74.7/7200 A W-1 and a detectivity of 1014/1015 Jones in UV/visible regime. The measured response time of the treated CdS device from the dark to illumination at 10-2 fW µm-2 was only 40 ms—much faster than the shutter speed or exposure time required for a professional digital camera for such low-light image detection. This strategy appears to hold great potential for ultralow-light image detection with ultralow power consumption.
In the second part of thesis, we developed a compact surface plasmonics-based color-image sensor comprised only a multifunctional electrode based on a single-layer structured aluminum (Al) film and an underlying silicon (Si) substrate. This approach significantly simplifies the device structure and fabrication processes. Moreover, such Schottky-based plasmonic electrodes perform multiple functions, including color splitting, optical-to-electrical signal conversion, and photogenerated carrier collection for color-image detection. The device took advantage of the near-field surface plasmonic effect around the Al–Si junction to enhance the optical absorption of Si, resulting in a significant photoelectric current output even under zero bias voltage. These plasmonic Schottky-based color-image devices could convert a photocurrent directly into a photovoltage and provided sufficient voltage output for color-image detection even under a light intensity of only several femtowatts per square micrometer. Therefore, this strategy has great potential for direct integration with complementary metal–oxide–semiconductor (CMOS)-compatible circuit design, increasing the pixel density of imaging sensors developed using mature Si-based technology.
In the third part of thesis, we propose the concept of deep-trench/thin-metal (DTTM) active antenna that take advantage of surface plasmon resonance (SPR) phenomena, three-dimensional (3D) cavity effects, and large-area metal/semiconductor junctions to effectively generate and collect hot electrons arising from plasmon decay and, thereby, increase photocurrent for photodetection well below the semiconductor band edge. The DTTM-based devices exhibited superior photoelectron conversion ability and high degrees of detection linearity under infrared light of both low and high intensity. Therefore, these DTTM-based devices have the attractive properties of high responsivity, extremely low power consumption, and polarization-insensitive detection over a broad bandwidth, suggesting great potential for use in photodetection and on-chip Si photonics in many applications of telecommunication fields.
In the fourth part of thesis, we describe a broadband perfect absorber based on loading effect–induced single-layer/trench-like thin metallic (LISTTM) structures. These LISTTM structures take advantage of both SPR and 3D cavity effects to provide efficient, tunable, and polarization-insensitive absorption from the UV to the infrared (IR) regime. The optimized hole-width of the LISTTM arrays was approximately one half of the designed wavelength. Therefore, even when the designed absorption band was in the visible regime, the feature sizes of the LISTTM structure could remain on the order of several hundred nanometers—dimensions much larger, and structures much simpler, than those of metamaterial-based absorbers. Moreover, because the absorbance of the specific LISTTM structure was similar to the air mass 1.5 solar spectrum, much solar energy was absorbed by the LISTTM arrays. Besides, these LISTTM structures exhibited superior photothermal performance; they also displayed very low emissivity, thereby decreasing heat dispersion through thermal radiation. Therefore, the LISTTM arrays could efficiently absorb light of higher photon energy in the UV, visible, and near-IR regimes, effectively conduct the generated heat through the continuous metal films, and barely disperse any heat through thermal radiation. Accordingly, these attractive properties suggest that such LISTTM absorbers might have promising applications in many fields related to energy harvesting.
In the last, we developed a filter-free, junctionless color image sensor that exhibited superior photo–thermo–electrical response under a low bias voltage and a short response time. Although our compact sensor had a simple single-layer trenchlike aluminum (Al) structure, it could perform multiple functions, including light harvesting, color-selective absorption, photo–thermo–electrical transformation, and the ability to collect of photoinduced differences in electrical signals. This device exploited near-field surface plasmon resonance and cavity effects to enhance the intensity of the electric field and the color-selective absorption, ultimately resulting in significant current signals in its structured Al film. With its ability to provide functional filter-free, junctionless structures, this strategy has great potential for the production of devices that operate on different kinds of substrates, thereby bridging various applications of color imaging technologies.

並列關鍵字

krypton fluoride excimer laser ； cadmium sulfide photoconductor ； surface plasmon resonance ； plasmonics-based color image sensor ； Si-based infrared ray detector ； perfect absorber ； photo-electronic-thermo conversion device

參考文獻

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