過渡金屬二硫屬化物(transition metal dichalcogenides, TMDs)的化學式可表為MX2，其中M為第16族的硫族元素，例如硫、硒、碲；X為過渡金屬元素，例如鎢、鉬、鉑。二維的二硫屬化物的性質對於基板種類與其表面懸鍵、表面缺陷很敏感，因此有需要了解二硫屬化物與基板間介面的性質。在本論文研究中，我們將鉑(Pt)濺鍍在C、M、A三種晶面的單晶藍寶石基板（Al2O3）上，控制幾種濺鍍厚度後，再藉由常壓化學氣相沉積法(atmospheric pressure chemical vapor deposition, APCVD)將其硒化成不同厚度的二硒化鉑(PtSe2)薄膜。藉由拉曼光譜的結果可以確認二硒化鉑薄膜有成長在基板上，並得知薄膜厚度。X射線反射繞射量測(x-ray reflectivity, XRR)可得知薄膜厚度、粗糙度與密度；原子力顯微鏡可知表面形貌與粗糙度。以上種種量測結果顯示，本論文中生長的二硒化鉑薄膜層數介於單層至20層(單層厚度5.08埃)。層數較少會受到基板原子尺度台階(atomic terraces)影響，基板/薄膜、薄膜/空氣的介面粗糙度(RMS roughness, σRMS)介於約2~4埃；超過20層，基板台階影響效應消失，介面粗糙度則相較於層數較少的多一個數量級。若成長於M面藍寶石基版，即使薄膜成長層數超過40層，還是會受到基板退火造成的原子尺度波浪狀形貌影響，其表面粗糙度約為~ 60 - 80 Å。從霍爾電性量測可以知道，本研究成長的二硒化鉑薄膜的主要載子為電子，霍爾電子遷移率(hall mobility, μHall)為155 cm2V-1sec-1 ~ 4 cm2V-1sec-1；載子濃度(carrier concentration, CC)為1.79 x 1015 cm-3 ~ 5.84 x 1018 cm-3，較薄的薄膜有較高的載子遷移率與較低的載子濃度。從電阻率對溫度的量測結果可以了解，較厚薄膜的載子導電機制(carrier conduction mechanism)為二維變程跳躍(2D variable range hopping, 2D - VRH)。通過X射線光電子能譜（x-ray photoelectron spectroscopy, XPS）進一步研究了這些界面，以了解化學均勻性和界面能帶排列對觀察到的電性能的影響。

Transition metal dichalcogenides (TMDs) are chemical compounds expressed in the form of MX2 where M is a transition metal atom while X is a group 16 chalcogenide atom like sulfur, selenium, or tellurium. The 2D nature of transition metal dichalcogenides (TMDs) makes it extremely sensitive to dangling bonds or surface imperfections from the substrates onto which they are grown. Thus it is imperative to understand this TMD/substrate interface’s effects on the electrical properties of 2D layers itself. Herein, we explore the physical properties of TMD – platinum diselenides (PtSe2) interfaces by selenizing Pt films of various thicknesses grown on single crystal sapphire (Al2O3) substrates of different orientations (C, M, and A). Raman spectroscopy reveals the 1T-PtSe2 phase is formed on all these substrates. Film thickness, composition and roughness is verified by X-Ray Reflectivity (XRR) and surface roughness is also probed by atomic force microscopy (AFM). These studies reveal on annealed C and A sapphire, PtSe2 films with thickness ranging from 1 L to 20 L (1 L = 5.08Å) mimic the substrate’s atomic terraces and the two interfaces – substrate/PtSe2 and PtSe2/air interfaces exhibiting RMS roughness (σRMS) of ~ 2 to 4 Å. Beyond 20 L, these atomic features vanish and σRMS for PtSe2/air increase by one order. On M sapphire, annealing creates a nanorippled/sawtooth surface, which in turn, is mimicked by the overlaying PtSe2 layers, even if film’s thickness is 40 L with σRMS ~ 60 - 80 Å. The effect of such varied morphological features on the electrical properties was probed using Hall Effect measurements which revealed (N) type conduction in all film’s behavior. The Hall mobility (μHall) and carrier concentration (CC) ranging from 155 cm2V-1sec-1 to 4 cm2V-1sec-1, and 1.79 x 1015 cm-3 to 5.84 x 1018 cm-3, respectively, with the thinnest layers showing the highest μHall and lowest CC. Based on temperature dependent resistivity results, the carrier conduction mechanism for thicker films was established to be 2D variable range hopping (2D - VRH). These interfaces are further studied by X-ray photoelectron spectroscopy (XPS) to understand the effect of chemical homogeneity and the interfacial band alignments on the observed electrical properties.

Oral examination certificate………………………………………………....………….i
摘要…………………………………………………………………………………….ii
Abstract…...……….…………………………………………………………………...iii
Table of Contents…………………………………………………………….…………v
List of Abbreviations…………………………………………………………………viii
Chapter 1 Introduction………………………………………………………………….1
1.1 Review of physical properties of TMDs, platinum diselenide (PtSe2)……. 1
1.2 Purpose of this work…………………………….…………………………12
Chapter 2 Growth of PtSe2 ….....................…………………………………………...13
2.1 Substrate preparation……...…………………………………………….....13
2.1.1 Cleaning…………………………..………………………………..13
2.1.2 Pre-Annealing….…………………………………………………..14
2.2 Deposition of platinum thin film and parameters………………………….17
2.2.1 Magnetron sputtering of films……………………………………..17
2.2.2 Sputtering parameter list for samples……………………………...18
2.3 Deposition of contacts …………………………………………………….19
2.3.1 Hall effect contacts………………………………………………...19
2.3.2 FET contacts……………………………………………………….21
2.4 Atmospheric Pressure Chemical Vapor Deposition (APCVD)…………....23
2.4.1 Description of APCVD…………………………………………….23
2.4.2 Explanation of our APCVD system..………………………………24
2.4.3 Selenization Process………………………………………………..28
Chapter 3 Characterization of thin films ………………………………………………32
3.1 X-ray reflectivity (XRR)…………………………………………………...32
3.1.1 Description of XRR………………………………………………...32
3.1.2 XRR data…………………………………………………...………36
3.2 Scanning electron microscopy (SEM) & Energy dispersive x-ray spectroscopy (EDX)........39
3.2.1 Description of SEM and EDX….…………………………………..39
3.2.1.1 SEM data…………………………………………………40
3.2.2 Description of EDX .……………………………………………….41
3.2.2.1 EDS data………………………………………………….42
3.3 Atomic force microscopy (AFM)………………………………………….43
3.3.1 Description of AFM.………………………………………………..43
3.3.2 AFM data…………………………………………………………...46
3.4 Raman spectroscopy......................................................48
3.4.1 Description of Raman………………………………………………48
3.4.2 Raman data…………………………………………………………51
3.5 X-ray Photoelectron Spectroscopy (XPS)………………………………….54
3.5.1 Description of XPS.………………………………………………...54
3.5.2 XPS data……………………………………………………………56
3.6 Magnetotransport measurements…………………………………………..58
3.6.1 Room temperature Hall effect measurement….……………………58
3.6.2 Temperature dependent resistivity… ………………………………63
Chapter 4 Discussion & Conclusion …………………………………………………...67
4.1 Conclusion....……………………………………………………………….67
4.2 Future Work..……………………………………………………………….68
Chapter 5 References…………………………………………………………………...69
Chapter 6 Appendix…………………………………………………………………….73

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