光敏化石墨烯於光電元件之應用

Abstract

石墨烯是一種單層碳原子以sp2鍵結方式排列而成的二維奈米材料，由於此特殊的幾何結構使得石墨烯有著非常特別的能帶結構、機械性質及熱導性質，尤其在電性的部分，石墨烯具有相當高的室溫載子遷移率>20000cm2V-1S-1，使其具有相當大的潛力去建構下個世代的高速電路。由於目前電路主要由互補式金屬氧化物半導體所組成，不管是p型或n型石墨烯都是相當重要的，因此精準地控制摻雜程度在未來元件製作是相當重要的。 在我之前碩班的實驗中，我發現TiOx對石墨烯是一個相當有效的n型摻雜物質，就像大部分的摻雜物質一樣，TiOx僅可以大略的用濃度來控制摻雜程度。然而在此篇論文中，我們發現TiOx具有光敏化摻雜的特性，n型摻雜程度可以進一步用光的入射劑量來精準調變，在第四章中我們利用石墨烯電晶體和光調變穿隧式顯微技術來研究TiOx的光敏化摻雜機制，最後我們將TiOx在石墨烯上的摻雜分成表面摻雜和光調變摻雜兩種機制，由於光調變摻雜法的摻雜位置是在TiOx薄膜內部(遠離傳輸層)，因此可以藉由入射光激發不同的數量摻雜位置來精準調控摻雜的程度在極大的範圍內，並且不會大幅破壞石墨烯的載子遷移率。由於此特殊的摻雜技術，我們將TiOx導入雙層石墨烯電晶體的結構裡當作光閘極，藉由另一個電控制閘極與TiOx光閘極去引發雙層石墨烯的能隙，進而製造出高整流的石墨烯電機體。另一方面，由於TiOx本身能夠在石墨烯底層和上層都能有效摻雜，所以能夠應用的範疇較其他摻雜物更為廣泛，在接下來的幾個章節，我們會將此光調變摻雜技術導入各式不同的光電元件中。 在第五章裡利用TiOx在石墨烯底部的結構，加上表面覆蓋一層有機高分子當作p型光敏化物摻雜物，我們建構出互補式光敏化石墨烯摻雜平台，其內部載子可以相當容易地由外界光波長來決定為p型或是n型傳導。之後再將此平台和光罩及白光結合，成功地製造出石墨烯的p-n接面電晶體，使得此技術在光激發石墨烯電路的設計將有極大的影響力。 在第六章中則是利用TiOx在石墨烯頂部的應用。由於石墨烯在大氣下以p型的形式存在，所以利用n型石墨烯來當作是太陽能電池的陰極是相當困難的。然而TiOx摻雜的石墨烯在照光的時候會藉由吸收少部分的紫外光來提升n型的摻雜程度，是一個理想的概念(光敏化陰極)可以應用在太陽能電池上，在此章中我們將它應用在和矽晶形成的蕭基接面太陽能電池上，達到之前從未有的10.5%高效率(現階段沒有其他團隊超過1%) 在第七章中我們整合了p型和n型摻雜的技術，加上石墨烯的背表面清潔及抗反射技術，我們成功地製造出高效率的石墨烯陰極和陽極基底的蕭基接面太陽能電池，效率可分別達到10.5%(陰極)和14.2%(陽極)。 在第八章裡我們將利用之前的光敏化摻雜以及蕭基接面的原理，將傳統的蕭基接面太陽能電池做結構上的小改變，成功地製造出三極式石墨烯/矽光偵測器。其水平向和鉛直向分別利用光敏化摻雜和蕭基介面來形成光電流，此兩種方式可以分別得到超高的光學增益和高對比的特性，由於此兩方式可以自由選取或是同時使用，將使此元件在未來有極大可能被實際應用或取代現有光偵測器。 在第九章中我們對之前研究關於光敏化石墨烯/TiOx或矽異質接面做個簡單總結，由於光敏化特性使得此異質接面有可調變得光電特性，在光電元件上可以有相當多樣的應用。此外對於光敏化石墨烯/TiOx的系統中，我們提出了光敏化調變摻雜技術，使的石墨烯能夠在高摻雜後仍能擁有相當高的載子遷移率。此外，由於TiOx和其他二維材料的能帶圖也顯示光激發載子可以在此材料間轉移，因此我們預測TiOx也會和其他二維材料有光敏化調變摻雜現象，並將再接下來補充章節(第十、十一章)中提到。 在第十章中我們將TiOx應用於另一個二維的層狀材料系統中─黑磷，由於黑磷本身在大氣中相當容易被氧化而被p型摻雜或破壞，因此目前所以的研究絕大多數都只能達成p型黑磷電晶體且在大氣下無法持續運作即被破壞。在此研究中我們利用所有之前研究提到TiOx的優越特性包含自我封裝、光敏化、超薄以及電子傳輸特性，我們成功製造出高穩定、光調控n型摻雜黑磷電晶體，對於發展以黑磷構築的互補式金屬氧化物電晶體是一個相當大的突破。 在第十一章中我們進一步TiOx和另一種已知的二維材料二硫化鉬做結合。由於一般的二硫化鉬電晶體的載子遷移率受限於二硫化鉬和金屬電極的接面電阻，所以重n型摻雜此步驟在製造高效率二硫化鉬電晶體是必要的。在此研究中我們將TiOx覆蓋在二硫化鉬電晶體上當作是自我封裝及光調變摻雜層，我們成功製造出高載子遷移率、可調變的重n型摻雜二硫化鉬電晶體，使得此TiOx和二硫化鉬異質接面在以後構造互補式氧化物電晶體中將有很大的發揮空間。此外，由於TiOx的光調變摻雜步驟是和劑量成正比，我們同時也利用此現象來當作是入射光劑量偵測器，使得TiOx和二硫化鉬異質接面在未來光電元件中能有更多的應用。

Graphene, which consists of a single atom-thick plane of carbon atoms arranged in a honeycomb lattice, has attracted a large amount of research because of its novel electronic, mechanical, and thermal properties arising from its unique 2D energy dispersion. The most attractive properties of graphene is its ultrahigh mobility (>20000cm2V-1S-1 in room temperature), which has potential to apply for the next post-silicon generation. Therefore, the precisely controllable doping graphene is crucial to construct the graphene based circuits with complementary-metal-oxide-semiconductor (CMOS) device. In my previous thesis, TiOx was found to be an effective n-type dopant which only approximately controlled the doping level with various concentrations as conventional chemical dopant. In this thesis, a photoinduced surface charge transfer at TiOx/graphene heterostructure was discovered, which could further doped graphene with assistance of incident light. The first part (chapter 4) of this thesis investigates the light-induced surface charge transfer at graphene/TiOx heterostructure. By analyzing mechanism with a light-modulated scanning tunneling microscopy (STM) and thin film transistors, the doping mechanism of TiOx/graphene heterostructure could be divided into an interfacial doping and a photoinduced modulation doping. The photoinduced modulation doping precisely controlled the doping to ultra-high level with dopants mainly in bulk TiOx, retaining high mobility of graphene. Consequently, with such unique light-sensitized doping effect, the TiOx was first applied as a photogate in a bilayer graphene transistor. The combination of a photogate and an electric gate successfully opened the band gap of bilayer graphene, resulting in a high/off bilayer graphene transistor. By the way, because TiOx could form n-type doping either on the top of at the bottom of graphene, the photo-sensitized TiOx/graphene heterostructure may provide a considerable flexibility for designing optoelectronic devices as described in the following sections. Chapter 5 is an application based on TiOx bottom doping structure. By encapsulating graphene with TiOx and another p-type photoactive material on top, an organic-inorganic hybrid doping platform (OIHD) is first fabricated with this structure. The transport type of OIHD can easily be tuned into p-type or n-type with selective-wavelength illumination. In addition, by using a pattern color filter and white light, a photoinduced p-n junction is carried out with this spatially pattern illumination, which indicated that the OIHD would have potential to construct the photonic circuits based on graphene. Chapter 6 is another application based on TiOx top doping. Owing to intrinsic p-type properties of graphene, the graphene based cathodes for solar cells are usually tough to be fabricated. With only costing a small amount of ultraviolet, the TiOx/graphene would demonstrate excellent n-type properties under illumination, which is an ideal concept for fabricating high performance graphene based cathode. With this special concept, we are the first one to fabricate high performance (10.5%) n-type graphene/p-Si Schottky solar cell, which is usually <1% in previous work. Chapter 7 is the application of doping technique for graphene based transparent conducting electrodes. With introduce stable p-type (TFSA) and n-type (TiOx) dopants, combing with surface modification and antireflective technique. Both high efficiency n-graphene/p-Si and p-type/n-Si Schottky solar cell are successfully achieved with 10.5 and 14.2% power conversion efficiency, respectively. In chapter 8, by integrating the concepts of previous photoactive properties and Schottky devices, we cleverly refit the structure of Schottky solar cell to turn into a 3-probe photodetector, which can perform with horizontal and vertical modes. Either ultrahigh gain in horizontal mode and photo-switch application in vertical mode can be manipulated separately or simultaneously, making this device ideal to for the next generation photodetectors. In chapter 9, we make a brief conclusion of previous chapters about photoactive graphene heterostructure, either for graphene/TiOx or graphene/Si respectively. The photoinduced charge transfer effect results in tunable electronic properties of graphene-based electronic devices. In addition, the photoinduced modulation doping concept is advocated to explain the mechanism in graphene/TiOx heterostructure, preserving high-mobility property of graphene even at high doping level. Owing to the preferred band alignment for charge transfer in TiOx and other 2D materials, we simultaneously predict that the photoinduced charge transfer would also occur in other TiOx/2D-materials heterostructure, which is discussed in the following supplementary chapter 10 and 11. Chapter 10 (appendix A) is another application of TiOx for another two-dimensional material - black phosphorus (BP). Generally, BP is quite vulnerable in air, resulting in oxidized product which full of electron traps. Therefore, most of reported BP transistor shows p-type dominated property and the performance can be damaged after placing in air for few hours. In this work, ultrathin TiOx acted as a self-encapsulated, an electrode modified layer, and a photoactive material for BP, which protected the as-exfoliated BP and resulted in precisely controlled n-type doping level with light modulation. A stable, n-type BP transistor is first demonstrated by a TiOx/BP heterostructure. Chapter 11 (appendix B) is another application of TiOx in 2D materials- molybdenum disulfide (MoS2), which is suffered from large contact resistance between contact metal electrodes and MoS2 channel. A strong and controllable n-type doping is needed to fabricate high performance transistor for CMOS circuit. Here, TiOx is used as a self-encapsulated n-type photoactive for MoS2, which precisely controlled the n-type doping level and preserved high mobility of MoS2 with photoinduced modulation doping process. In addition, the cumulative trap-mediated doping process makes it useful as a UV dose detector, which gives more applications for this TiOx/MoS2 heterostructure in future optoelectronic device.