臺灣博碩士論文加值系統

　本篇論文的研究主軸是結合鹵化甲基銨鉛鈦鈣礦(methylammonium lead halide peroskite)以及二維材料場效電晶體的光電傳輸特性。為了延伸現今莫爾定律至10奈米以下元件製程，近十年以來，二維材料為發展迅速的新穎奈米材料，它擁有許多獨特的傳輸性質，如最廣為人知的二維材料-石墨烯(graphene)具有極高的載子遷移律、透明度及可撓性，這些特性都有助於未來新型態電晶體或是光電元件的運用。鹵化甲基銨鉛鈦鈣礦為一種鈦鈣礦結構(perovskite)的物質，其具有直接能隙、良好的吸光律以及夠長的載子平均自由徑，使得此材料能在吸光之後能輕易的將載子傳遞出去，在近幾年中被廣泛的運用在太陽能電池上。
　　文內我們將先探討石墨烯(graphene)以及二硫化鉬(MoS2)的傳輸特性，石墨烯的高載子遷移律(mobility)以及二硫化鉬的高電流開關比(on-off ratio)都很適合使用於光偵測器，然而此兩種材料的低吸收律卻限制了其光電元件的表現，因此，我們初步利用鈦鈣礦的修飾來增進元件吸收，進而改善整體的光學特性。
　　我們採用熱蒸鍍以及氣相合成的兩階段法來合成鈦鈣礦，此種合成法能夠較為有效的控制鈦鈣礦的厚度，同時可以藉由控制成長環境，在二維材料上長出特殊的鈦鈣礦結構。而由於鈦鈣礦將對原本的二維材料元件產生載子參雜(doping)的效應，且參雜行為由鈦鈣礦主導，因此便可以在元件上探討鈦鈣礦的物理特性，我們將以鈦鈣礦在溫度變化下產生的相變化來討論此種參雜行為。最後，由於鈦鈣礦容易吸收空氣中的水氣、氧氣而變質，我們利用同為太陽能電池常見的聚(3-己烷基噻吩)(P3HT)作為保護層，阻隔鈦鈣礦元件與外界接觸，進而提昇其使用壽命，同時我們也觀察到此一保護層亦能夠改善元件的特性。

　　Two-dimensional (2D) materials have attracted intense attention due to their remarkable properties. For example, high carrier mobility, high optical transparency, and high mechanical flexibility can be demonstrated on graphene, a well-known 2D material, for applications. Methylammonium lead triiodide (CH3NH3PbI3, perovskite) is a new material that has direct bandgap, strong absorption corresponding to an onset near 800 nm and carrier diffusion length up to micrometer scale. It has attracted intensive interest for its diverse optoelectronic applications in the field of solar cells, photodetectors, lasers, and light-emitting diodes (LEDs). The main objective of this thesis is investigating the photocurrent properties of perovskite/2D-material hybrid structures.
　　First, we demonstrate a fabrication of perovskite/2D-material hybrid system. It is based on 2D material field effect transistors (FETs), which are made by a resist-free method without chemical etching process. Perovskite is grown by a two-step process, involving physical vapor deposition and a vapor transport chemical deposition system. By control growth condition of perovskite, different shape of perovskite can be formed on 2D materials. From the result of photocurrent measurement on the perovskite/2D-material hybrid system, it is shown that the introduction of perovskite can improve the optoelectronic performance of graphene- or MoS2- based photodetector. At low temperature, we observe characteristics of a phase transition of perovskite from electrical properties of doped 2D materials. In other words, we can use 2D-material-based FETs as a platform to demonstrate phase transition of perovskite. Finally, perovskite is easily reaction with water and oxygen in air, how to keep its quality becomes an important task for application. We improve the stability of perovskite by poly(3-hexylthiophene) (P3HT) as a protection layer. The result shows that P3HT can improve not only stability but performance of the device based on perovskite/2D-material structures.

Chapter.1 Introduction 1
1.1 Background of 2D materials 1
1.2 Basic properties of 2D materials 3
1.2.1 Graphene 3
1.2.2 Molybdenum disulfide (MoS2) 5
1.3 Basic properties of Lead Halide Perovskite 7
1.4 Motivation 9
1.5 Thesis organization 10
Chapter.2 Theoretical background 12
2.1 Characters of 2D material electronics 12
2.1.1 Mobility and Field-effect mobility 12
2.1.2 Ion/Ioff ratio and threshold voltage 15
2.1.3 Schottky barrier height 16
2.2 Characters of 2D material optoelectronics 17
2.2.1 Photoluminescence 18
2.2.2 Responsivity 18
2.2.3 Quantum efficiency 18
2.2.4 Response time 19
2.3 Photodetection mechanism in 2D materials 20
2.3.1 Photoconductive effect 20
2.3.2 Photogating effect 21
2.3.3 Photovoltaic effect 23
2.3.4 Photo-thermoelectric effect 24
Chapter.3 Experiment methods 27
3.1 2D material FET fabrication 27
3.1.1 2D material preparation 27
3.1.2 Electrode deposition 28
3.1.3 Thermal Annealing Process 29
3.2 Perovskite formation 30
3.2.1 PbI2 growth by thermal evaporation 34
3.2.2 Perovskite formation by CVD 35
3.3 Measurement systems 36
3.3.1 Optoelectronic measure system 36
3.3.2 Scanning electron microscope (SEM) 40
3.3.3 Atomic force microscope (AFM) 41
Chapter.4 Result and Discussion 43
4.1 Perovskite formation on 2D materials 43
4.2 Hybrid System of perovskite and graphene 48
4.2.1 Optoelectronic properties 48
4.2.2 Phase Transition of perovskite on graphene 52
4.2.3 Improvement of stability and performance on perovskite/graphene device by P3HT 55
4.3 Hybrid System of perovskite and MoS2 60
Chapter.5 Conclusion 67
References 69

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