ELECTRONIC AND OPTOELECTRONIC PROPERTIES OF NANOPORE-BASED SINGLE NANOPARTICLE DEVICES

結合奈米孔洞結構的單一奈米顆粒元件之電子傳輸與光電特性研究

阮林南(Nguyen Linh Nam)

指導教授 ： 陳啟東 ； 張廖貴術

國立清華大學/原子科學院/工程與系統科學系/博士(2013年)

摘要

Most of polycrystalline or amorphous structured semiconductor devices suffer from low carrier mobility due to undesirable carrier scatterings with structural defects or grain boundaries which prevents their commercialization. In order to eliminate grain boundary scattering and to explore the intrinsic mobility of semiconducting materials, devices containing an individual single grain are desirable. Further, these devices have several advantages over 1D and 2D structures including a large surface-to-volume ratio and short traveling distance, which can greatly prolong the carrier lifetime and offer a short transferring time. We thus expect that devices containing only a single nanoparticle can exhibit extraordinary enhancements in carrier mobility and can lead to new promising electronic and/or optoelectronic applications.
In this thesis, we design and fabricate a solid platform for exploring the electronic and optoelectronic properties of single nanoparticles. The platform consists of a robust volcano-shaped gated Al/Al2O3 nanopore made on a Si3N4 membrane with well-controlled pore sizes ranging from 60 nm to 10 nm. We further demonstrate a technique to manipulate a single nanoparticle by using a nano-manipulator equipped inside a field-emission scanning electron microscope. Optic fiber tips with diameter as small as 50 nm are employed as the manipulation probes. Owing to its unique electronic and optoelectronic properties, ZnO nanoparticles are selected to illustrate the use of the nanopore platform as well as manipulation technique. A single ZnO nanoparticle is picked up and placed onto the bottom side of the nanopore. Aluminum electrodes are then evaporated from top and bottom sides to make electrical contacts to the nanoparticle for electrical measurement. Under gate modulation, the device shows the n-type behavior with room temperature electron mobility as high as 285 cm2/Vs, which is 1~2 orders magnitude higher than those of ZnO polycrystalline films, high quality ZnO crystal, other low-dimension ZnO structures field-effect-transistors. This high value can be attributed to the absence of inter-particle boundary scattering. It is also found that a normalized transconductance reaches to 27.2 μS/μm due to strong capacitive coupling from a surrounded gate electrode to nanoparticle. With this high mobility, the devices exhibit record high external quantum efficiency values of 5.6×107 upon illumination with a 365 nm light source. Upon cooling, the asymmetric device structure facilitates the rectifying current-voltage characteristic that enables photovoltaic capacity. Upon illumination, the device shows open circuit voltage as well as short circuit current. The fill factor is found to increase at low temperatures and reaches 48.6% at 100 K. This approach can be applied not only for inorganic nanoparticles but also for those of organic semiconducting materials. We show that devices containing only a single PTCDA nanoparticle would indeed give a mobility 2~3 orders higher than that of conventional film-structured polycrystalline organic semiconductor transistors. The devices contain a single PTCDA nanoparticle embedded inside a nanopore structure and the nanoparticle is surrounded by a gate electrode and is connected to top and bottom electrodes. Owing to the absence of inter-grain scattering, we obtain record high mobility values of 0.08 cm2/Vs at room temperature and 0.5 cm2/Vs at 80 K. With this high mobility, the device when illuminated shows a record high external quantum efficiency of 3.5×106, which is the highest report value thus far for optoelectronic devices made of organic single crystals. Since the single nanoparticle device eliminates the deleterious effects of defect and grain-boundary recombination and facilitates fast transferring time due to a short traveling distance, it opens a door to electronic and optoelectronic applications of semiconductor materials.
Here, for the first time, we report perpendicular electron transport through a highly crystalline trilayer MoS2 structure in nanometer area. Devices contain trilayer MoS2, which is transferred on the top of nanopore and sandwiched by the top and bottom electrode. Our experiment provides evidence of inter-MoS2 layer charge tunneling and suggests a wave-function overlap between the sulfide atomic layers of the two stacked MoS2 layers. The interlayer tunneling current is found to be modulated by transverse electric fields as well as perpendicular magnetic fields. The observed result indicates existence of quantized energy levels in each MoS2 layers. The resonant tunneling behavior reveals the transport taking place through two coupled quantum energy states. The proposed model is confirmed by gate-induced electric field and external magnetic field modulations of resonance peak. This experiment opens the door to develop atomic-layered devices for different applications in the future.

關鍵字

奈米孔洞 ； 單一奈米顆粒元件 ； 載子遷移率 ； 外部量子效率 ； ZnO ； PTCDA ； MoS2

並列摘要

多晶或非晶結構的半導體元件其載子遷移率大多數都非常低，這是由於結構上的缺陷或晶界造成載流子的散射，而這個致命缺點讓他們無法被實際應用。由單一顆單晶半導體所構成的元件，將可以完全避免由晶界造成載子散射問題，所以非常適合用來研究半導體材料的最本質載子遷移率特性。另外，與一般一維或二維結構的元件比較起來，由於這種結構的表面積與體積比值很大，這將大大延長載子的壽命，另外由於載子在半導體單晶內移動距離非常短，則使載子的傳輸時間非常短暫。因此，我們預期單一顆奈米單晶元件，將具有快速的載子遷移率，而這將使其在電子或光電應用上有所發揮。

在本篇論文中，我們設計並製作一個全新的平台，用以從事單一奈米單晶顆粒的電子和光電特性研究。該平台是在一個建構在氮化矽薄膜上的火山形狀奈米孔洞，這個孔洞是由表面具有密緻氧化鋁的鋁金屬所形成，而孔徑大小可以控制在60奈米到10奈米範圍。我們進一步證明藉由使用裝配在場發射式掃描電子顯微鏡內部的微型機械手臂來操控單獨一顆奈米顆粒的技術，直徑為50奈米的光纖探頭。ZnO奈米顆粒由於其獨特的電子和光電特性，被選擇來展示奈米孔以及操作技術。首先利用光纖探針吸取一顆ZnO奈米顆粒，再將其放置到的奈米孔洞內。然後從奈米孔洞的上方與下方，以熱蒸鍍方式的鍍上鋁金屬，如此便能量測奈米顆粒的電性傳輸特性。在閘極研究中，元件表現出的n型半導體行為而且在室溫環境下其電子遷移率高達285 cm2/Vs，這比一般氧化鋅薄膜或是高品質的單晶氧化鋅，或其它低維度結構的氧化鋅場效應電晶體等等都要高出兩個量級。這種高的遷移率可以歸因於載子與邊界沒有散射效應。同時還發現，歸一化的跨導達到27.2 μS/μm，這是由於三圍環繞式閘極電極與奈米顆粒之間有巨大的電容耦合。有了這樣高的載子遷移速率，該元件在波長365 nm的光源照射下，表現出創紀錄的高外部量子效率值5.6×107。在低溫環境下，改善非對稱結構的電流-電壓特性而進一步提升光伏容量。由該元件表現出開路電壓和短路電流，在100K填充因子達到48.6％。這種方法不僅適用於無機奈米顆粒，也可以用於有機半導體材料。
另外我們展示只含有單一個PTCDA奈米顆粒的元件，的確其遷移率會比常規薄膜結構的多晶有機半導體電晶體，要遠遠高出2〜3個數量級。該元件只包含單一顆PTCDA嵌在奈米孔洞結構內，這個奈米PTCDA顆粒的周圍分別是閘極電極與連接到頂部和底部源極與汲極電極。由於沒有晶粒間散射的情況下，而其載子遷移率在室溫與80 K環境下，分別得到高達0.08 cm2/Vs和0.5 cm2/Vs。具有如此高的載子遷移率，顯示出這個元件相當高的外部量子效率其值為3.5×106，這是目前世界最高值，並遠高於其他以有機單晶製成的光電元件。由於單一奈米顆粒的裝置不但排除缺陷和晶界的有害影響，另外由於奈米等級的元件長度，使其具有非常高的傳輸速度，相信它可以在半導體材料的電子和光電應用領域開啟一扇大門。
最後，我們報告在一個奈米大小面積內，電子以垂直方式通過高度結晶二硫化鉬三層結構的傳輸行為。這個元件結構是在奈米孔洞上放置了三層二硫化鉬，而源極和汲極電極分別包覆在它們上下兩端。我們實驗觀察到二硫化鉬層與層之間的電荷隧穿現象，而這顯示在堆疊的二硫化鉬層之間，硫化物原子層的波函數是具有相當程度的重疊。層與層之間的隧道電流大小可以被橫向的電場以及垂直的磁場所控制。我們實驗觀察到的結果說明，在每個二硫化鉬層確實存在量子化的能階，而共振穿隧行為更進一步揭示兩個耦合量子能階的電子傳輸特性。另外我們也透過模擬方式，來進一步驗證，外加閘極電場與外部磁場對共振波峰的影響。透過這個實驗，我們可以說是替未來要發展的原子等級元件及其應用預鋪了一條大道。

並列關鍵字

nanopore ； single nanoparticle devices ； high mobility ； high external quantum efficiency ； ZnO ； PTCDA ； MoS2

參考文獻

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