摻雜鑭系元素(鏑,釓)氧化鋅與單層二(硫,硒)化鎢薄膜的光譜性質研究

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(1)國立臺灣師範大學物理研究所碩士論文指導教授：劉祥麟博士. Optical studies of lanthanide (Dy, Gd)-doped ZnO and monolayer W(S, Se)2 thin films 摻雜鑭系元素(鏑,釓)氧化鋅與單層二(硫,硒)化鎢薄膜的光譜性質研究. 研究生：葉秦維撰中華民國一百零四年六月.

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(3) Acknowledgements 轉眼間兩年的碩士生涯,即將劃下休止符,在這段學習的過程裡,面臨著研究的壓力、課業的繁忙，雖然艱辛，但這讓我學會如何有效的安排做事的方法，以及面對問題時,該如何用積極的態度來解決。做研究、寫論文的日子雖然艱辛，但受到許多老師及朋友的幫助及鼓勵下，讓我能夠順利地取得碩士學位。首先，由衷感謝我的指導教授劉祥麟老師，在這段期間不厭其煩及細心的教導，讓我學會許多做事的方法與解決問題的能力，對於光譜研究有很深層的理解。再來，感謝我的口試委員駱芳鈺老師及林昭吟老師給予我許多寶貴的建議，讓我的論文能夠更加的充實。此外，我要感謝實驗室裡的稚強學長、孟哲學長、美君學姐因為有你們的引領下，讓剛進入實驗室的我可以更快速的融入到實驗室的生活節奏中。感謝一智學長、松勳學長、沄蓁學姐提供我在研究及實驗上的建議以及實驗技巧。萬分感謝我的夥伴孝文，每當我有實驗及研究上的問題時都會從旁協助我，還有在口試準備期間上的幫助。還要感謝雅婷、嬿婷、承緯，有你們的陪伴，讓我的碩士班生活更加多彩多姿。最後感謝駱芳鈺老師實驗室的一介學長、宗均學長以及楷傑同學，謝謝你們提供我研究的樣品以及研究上的建議。最後，感謝我親愛的家人，謝謝你們的支持與鼓勵，謝謝雅坤這期間的陪伴，使我能夠堅持到最後，並順利的完成碩士學位。 i.

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(5) 摘要我們量測摻雜（鏑,釓）氧化鋅薄膜的拉曼散射光譜、穿透光譜及橢圓偏振光譜，進而研究不同摻雜量對氧化鋅薄膜光譜性質的影響。此外，我們量測單層過渡金屬二硫屬化物薄膜（二硫化鎢和二硒化鎢）的拉曼散射光譜及橢圓偏振光譜，進而探討單層二硫化鎢和二硒化鎢薄膜的光譜性質。氧化鋅薄膜是用脈衝雷射沉積法製成在藍寶石基板上，摻雜鏑離子和釓離子的濃度範圍分別為 1% ~ 10%及 3% ~ 30%。單層二硫化鎢和二硒化鎢薄膜是用化學氣相沉積法製成在藍寶石基板上。這篇論文的目的是探討上述所有材料的晶格結構和電子結構。我們發現純氧化鋅薄膜的拉曼散射光譜，顯示 2 個拉曼峰，其頻率位置為 98.7 cm-1 和 437.1 cm-1,分別為 E2low 和 E2high 對稱性。隨著鏑離子和釓離子摻雜濃度增加，拉曼峰 E2low 和 E2high 的峰值強度會逐漸下降。在穿透光譜中發現，隨著鏑離子和釓離子摻雜濃度增加，（鏑,釓）氧化鋅薄膜在紫外光區的光穿透率會提高。在吸收能譜中發現，隨著鏑離子和釓離子摻雜濃度增加，氧化鋅薄膜的直接能隙值會受到鏑離子和釓離子的影響，產生偏移，其現象可被能帶隙變窄理論和伯斯坦-莫斯位移理論解釋。我們發現單層二硫化鎢和二硒化鎢薄膜在 532 奈米雷射光激發下的拉曼散射光譜具有多種類的拉曼峰。在室溫的吸收能譜中發現，單層二硫化鎢和二硒化鎢薄膜具有明顯的激子 A 和 B 吸收峰。此外，我們分析了單層二硫化鎢和二硒化 ii.

(6) 鎢薄膜的室溫直接能隙值和激子束縛能值。其室溫直接能隙值，分別為 2.1 電子伏特和 1.72 電子伏特；其室溫激子束縛能值，分別為 0.32 電子伏特和 0.24 電子伏特。在變溫的吸收能譜中發現，單層二硫化鎢和二硒化鎢薄膜的直接能隙值會產生紅移現象，此現象是由單層二硫化鎢和二硒化鎢薄膜的晶格受到熱膨脹和電子及聲子間的交互作用所造成。. 關鍵字：氧化鋅、過渡金屬二硫屬化物、拉曼散射光譜、橢圓偏振光譜、光譜性質. iii.

(7) Abstract We report the dysprosium (Dy) and gadolinium (Gd) doping effects on optical properties of ZnO thin films and the results of Raman scattering and spectroscopic ellipsometric measurements of monolayer transition metal dichalcogenides thin film (WS2 and WSe2). The Dy doped ZnO thin films with doping concentration of 1%, 3%, 5%, and 10% were fabricated on (0001) sapphire substrates by means of the pulsed laser deposition (PLD). The ZnO thin films with doping Gd concentration ranging from 3% to 30% were deposited on (0001) sapphire substrates by PLD. Monolayer WS2 and WSe2 thin films were deposited onto sapphire substrates by chemical vapor deposition (CVD). Our purpose is to investigate the changes of lattice dynamics and electronics structures of these materials. Raman scattering spectrum of pure ZnO thin film shows both E2low and E2high phonon modes at approximately 99 and 438 cm-1. With an increase in Dy and Gd doping, the intensity of both E2low and E2high phonon modes is decreased. Optical transmission spectra show increase in both transmittance in ultraviolet region with increasing Dy and Gd doping. The Dy and Gd doping effects on optical band gap of ZnO thin films can be explained by both energy band gap narrowing and Burstein-Moss shifted. Raman scattering spectra of monolayer WS2 and WSe2 thin films excited by 532-nm laser line show full phonon modes. The room-temperature absorption spectra of monolayer WS2 and WSe2 thin films exhibit emerging A and B excitons. Additionally, monolayer WS2 and WSe2 thin films show iv.

(8) room temperature direct band gap at approximately 2.1 and 1.72 eV. The exciton binding energy of monolayer WS2 and WSe2 thin films is found to be approximately 0.32 and 0.24 eV at 300 K. With increasing temperature, the direct band gap of monolayer WS2 and WSe2 thin films shows a redshift, which can be elucidated by thermal expansion and electron-phonon interaction.. Keyword: ZnO, Transition metal dichalcogenides, Raman scattering spectroscopy, Spectroscopic ellipsometry, Optical properties. v.

(9) Contents Acknowledgements ............................................................................................................................... i 摘要........................................................................................................................................................ii Abstract................................................................................................................................................ iv Contents ............................................................................................................................................... vi List of Figures ................................................................................................................................... viii List of Tables................................................................................................................................... xxiii Chapter 1 Introduction ........................................................................................................................ 1 Chapter 2 Brief survey of (Dy, Gd) doped ZnO and transition metal dichalcogenides ................ 7 2-1 (Dy, Gd) doped ZnO............................................................................................................... 7 2-1-1 Physical properties..................................................................................................... 8 2-1-2 Optical properties .................................................................................................... 15 2-2 Transition metal dichalcogenides ......................................................................................... 20 2-2-1 Physical properties................................................................................................... 20 2-2-2 Optical properties .................................................................................................... 22 Chapter 3 Experimental techniques ................................................................................................. 51 3-1 Raman scattering spectroscopy ............................................................................................ 51 3-2 Grating spectrometer ............................................................................................................ 60 3-3 Spectroscopic ellipsometry .................................................................................................. 65 Chapter 4 Sample preparation and properties ............................................................................... 74 4-1 Sample preparation............................................................................................................... 74 4-2 Sample properties ................................................................................................................. 77. vi.

(10) Chapter 5 Optical properties of (Dy, Gd) doped ZnO thin films .................................................. 96 5-1 Raman scattering spectra...................................................................................................... 97 5-2 Optical transmission spectra............................................................................................... 100 5-3 Spectroscopic ellipsometric spectra ................................................................................... 103 5-4 Summary ............................................................................................................................ 113 Chapter 6 Optical properties of monolayer WS2 and WSe2 thin films ....................................... 156 6-1 Raman scattering spectra.................................................................................................... 157 6-2 Spectroscopic ellipsometric spectra ................................................................................... 162 6-3 Summary ............................................................................................................................ 170 Chapter 7 Thesis summary ............................................................................................................. 190 References ......................................................................................................................................... 193. vii.

(11) List of Figures Fig. 1.1 Schematic representation of ZnO crystal structures: (a) hexagonal wurtzite structure; and (b) cubic zinc blende wurtzite structure. The yellow and white (grey) spheres denote Zn and O atoms, respectively .................................................................................................................. 6 Fig. 1.2 Schematic representation of (a) side view and (b) top view of monolayer MX2 structure, with the chalcogen atoms in yellow and the transition metal atoms in black .................................. 6 Fig. 2.1 Theoretical values of the Curie temperature for various p-type semiconductors containing 5 % of Mn and 3.5 × 1020 holes per cm3 ................................................................................... 26 Fig. 2.2 X-ray diffraction pattern of Dy-doped ZnO nanowires .......................................................... 26 Fig. 2.3 High-resolution x-ray diffractometer (HRXRD) patterns of Dy:ZnO thin films deposited under different Dy concentrations (at%) ............................................................................... 27 Fig. 2.4 Variation of resistivity of Dy:ZnO thin films with Dy concentration ..................................... 27 Fig. 2.5 Variation of mobility and carrier concentration of Dy:ZnO thin films with Dy concentration ............................................................................................................................................... 28 Fig. 2.6 XRD patterns of 0, 1, 2, and 3% Dy-doped ZnO synthesized by a sonochemical method over the 2θ range of 31 ~ 38 ................................................................................................... 28 Fig. 2.7 X-ray diffraction pattern of ZnO:Gd nanowires ..................................................................... 29 Fig. 2.8 Rutherford backscattering of Zn0.95Gd0.05O ............................................................................ 29 Fig. 2.9 The magnetization versus magnetic field of the as grown and annealed ZnO:Gd nanowires with Gd 5 mol% at 77 K ........................................................................................................ 30 Fig. 2.10 The magnetization versus magnetic field of the as grown and annealed ZnO:Gd nanowires with Gd 5 mol% at 300 K ...................................................................................................... 30 Fig. 2.11 (a) The magnetic susceptibility of the as grown and the annealed ZnO:Gd nanowires at. viii.

(12) room temperature under the applied magnetic field. (b) The effective magnetic moment per Gd atom peff is a function of the applied magnetic field .................................................... 31 Fig. 2.12 XRD data for the ZnO(002) reflection of the 1.3, 4, 7, and 16 % Gd:ZnO films. The FWHM shown in the inset depends strongly on the amount of Gd in the films ................... 31 Fig. 2.13 XANES and XLD spectra at the Zn K-edge for the ion-implanted and the sputtered Gd:ZnO films ......................................................................................................................... 32 Fig. 2.14 Isotropic (a) XANES, (b) XLD, and (c) XMCD of 1.3, 4, 7, and 16 % Gd:ZnO films at the Gd L3-edge. The implanted sample is shown for comparison as a dotted line ...................... 32 Fig. 2.15 Element-specific XMCD ( H ) curves at the Gd L3-edge for the ion-implanted Gd/cm2 ZnO and the sputtered Gd:ZnO samples. The XMCD ( H ) was measured at 6.5 K ..................... 33 Fig. 2.16 XMCD ( H ) curves taken at 6.4 and 41.5 K and SQUID M ( H ) at 5 K. The Brillouin function for J= S= 7 / 2 is shown as well ......................................................................... 33 Fig. 2.17 SQUID M (T ) behavior recorded after field-cooled and zero-field-cooled conditions. The diamagnetic back-ground has been subtracted from all data sets .......................................... 34 Fig. 2.18 Relaxed structure of 3.7 % Gd at substitutional sites (a) and at interstitial sites (b) for a 3 × 3 ×3 cell of Gd:ZnO. The red atoms are O, gray atoms are Zn, and green atoms are Gd ..... 34 Fig. 2.19 Room temperature magnetic hysteresis loop of (a) pure and Gd doped ZnO, (b) 0.01, (c) 0.03, and (d) 0.05 mol%. The inset shows the enlarged view of hysteresis loops near the centre...................................................................................................................................... 35 Fig. 2.20 Spectral dependence of transmittance of Dy:ZnO thin films and its inset in the figure shows. (α hυ ). 2. vs.. ( hυ ). plot of Dy:ZnO thin film with 0.45 at% of Dy concentration ................ 35. Fig. 2.21 Variation of Dy:ZnO thin films band gap with Dy concentration ........................................ 36 Fig. 2.22 PL spectra of pure and Dy doped ZnO thin films excited at 325 nm ................................... 36. ix.

(13) Fig. 2.23 FTIR spectra of 0, 1, 2, and 3% Dy-doped ZnO synthesized by a sonochemical method ... 37 Fig. 2.24 Decolorization efficiencies of MB by 0, 1, 2, and 3% Dy-doped ZnO during irradiation with UV light ......................................................................................................................... 37 Fig. 2.25 Absorption spectra of 5, 10, and 15 mol% Gd doped ZnO nanocrystals (sample A, B, and C) measured at room temperature............................................................................................... 38 Fig. 2.26 PL spectra of the pure and 5 mol% Gd doped ZnO nanocrystals (sample A) at room temperature ............................................................................................................................ 38 Fig. 2.27 PL spectra of ZnO:Gd nanocrystals with different mole ratios of Gd in the source materials: (a) 5, (b) 10, and (c) 15 mol% Gd doped ZnO nanocrystals ................................................. 39 Fig. 2.28 Raman scattering spectra of ZnO samples prepared with different Gd amounts, (a) pure, (b) 0.01, (c) 0.03, and (d) 0.05 mol% .......................................................................................... 39 Fig. 2.29 Deconvoluted 1LO mode from Raman scattering spectra of (a) pure and Gd doped ZnO nanoparticles (b) 0.01, (c) 0.03, and (d) 0.05 mol% .............................................................. 40 Fig. 2.30 PL spectra of the pure and Gd doped ZnO, (a) 0.00, (b) 0.01, (c) 0.03, and (d) 0.05 mol% of Gd, measured at room temperature ........................................................................................ 40 Fig. 2.31 Synthesis procedure for the atomic layer deposition based WS2 nanosheets ....................... 41 Fig. 2.32 Field-effect transistor structure on the monolayer WS2 ....................................................... 41 Fig. 2.33 Transfer curve for the field-effect transistor fabricated on a monolayer WS2 nanosheets ... 42 Fig. 2.34 A schematic of the device with the principal layers shown .................................................. 42 Fig. 2.35 (Left axis) I-V curves for a device on Si/SiO2 taken under illumination at gate voltages from -20 (red) to +20 (blue) in 10-V steps, after doping. The laser illumination energy was 2.54 eV and the power was 10 μW. The curves are linear at low bias but saturate at higher bias due to limited available charge carriers. (Right axis) I-V curves for the same device taken in the dark at gate voltages from -20 (black) to +20 (green) in 20-V steps, after doping. x.

(14) ............................................................................................................................................... 43 Fig. 2.36 (A) The density of states for monolayer transition metal dichalcogenides: MoS2, WS2, and WSe2. Strong peaks are present in all three materials that lead to a strong light-matter interaction. (B) The joint density of states with the same three transition metal dichalcogenides materials ...................................................................................................... 44 Fig. 2.37 Raman spectra of WS2 bulk (dotted) and a monolayer (solid red). The inset shows the phonon vibration of E21 g and A1g modes ........................................................................... 45 Fig. 2.38 Frequencies of E21 g and A1g Raman modes (blue) and the difference in peak position ∆ω (red) as a function of number of WS2 layers ................................................................. 45 Fig. 2.39 Room-temperature Raman spectra of single-layer WS2 excited by 514 nm laser line......... 46 Fig. 2.40 Raman spectra of single-layer WS2 with (a) E21 g and (b) A1g mode measured in a temperature range from 77 to 623 K...................................................................................... 46 Fig. 2.41 Effect of temperature variation on the Raman frequencies of monolayer WS2 for (a). 2 LA ( M ) , (b) E21 g , and (c) A1g modes ............................................................................... 47 Fig. 2.42 Selected Raman scattering spectra of WSe2 samples with different number of layers registered with a wide range of excitation wavelengths. The peak 520 cm-1 comes from the silicon substrate and was used for intensity normalization ................................................... 48 Fig. 2.43 Electronic band structure (left) and total density of states (right) for the WS2 (a) bulk and (b) monolayer .............................................................................................................................. 49 Fig. 2.44 Electronic band structure (left) and total density of states (right) for the WSe2 (a) bulk and (b) monolayer......................................................................................................................... 49 Fig. 2.45 (a) The relative photoluminescence intensity of WSe2 multilayer as a function of film thickness. The inset presents photoluminescence spectra from WSe2 monolayer and bilayer xi.

(15) respectively. (b) The normalized photoluminescence spectra (with respect to the peak A) of WSe2 ultrathin films. I labels the luminescence from indirect band gap transition; A and B label the direct band gap transition from the split valence band states edge to the conduction band states edge at K points. Spectra (dashed line) in the zoom windows have been multiplied by a factor as indicated for clarity ........................................................................ 50 Fig. 2.46 Photoluminescence intensities for 1L, 2L, 3L, and bulk using the 488 nm excitation laser line. The positions for the excitons A and B as well as the indirect band gap (I) are labeled ............................................................................................................................................... 50 Fig. 3.1 A sketch of the optical setup of the micro-Raman scattering ................................................. 72 Fig. 3.2 Diagrammatic representation of an energy transfer model of Rayleigh, anti-Stokes, and stokes scattering ..................................................................................................................... 72 Fig 3.3 The sketch map of the grating spectrometer ............................................................................ 73 Fig 3.4 The schematic diagram of a rotating compensator ellipsometer ............................................. 73 Fig. 4.1 The schematics of the shadow mask....................................................................................... 85 Fig. 4.2 Schematics for the growth of WSe2 thin films on c-sapphire substrates by CVD ................. 85 Fig. 4.3 XRD diffraction patterns of pure and Dy doped ZnO thin films. They were taken by Professor Fang-Yuh Lo's research group. The asterisks represent XRD peaks from the sapphire substrate................................................................................................................... 86 Fig. 4.4 XRD diffraction patterns of pure and Dy doped ZnO thin films show over the 2θ range of. 30 ~ 46 . They were taken by Professor Fang-Yuh Lo's research group. The asterisks represent XRD peaks from the sapphire substrate ................................................................. 86 Fig. 4.5 XRD diffraction patterns of pure and Gd doped ZnO thin films taken by Professor Fang-Yuh Lo's research group ................................................................................................................ 87 Fig. 4.6 XRD diffraction patterns of pure and Gd doped ZnO thin films show over the 2θ range of. xii.

(16) 32 ~ 46 . They were taken by Professor Fang-Yuh Lo's research group............................. 87 Fig. 4.7 The magnetic moment versus magnetic field of non-annealing pure and Dy doped ZnO thin films at 5 K taken by Professor Fang-Yuh Lo's research group ............................................ 88 Fig. 4.8 Magnetic moment versus magnetic field of non-annealing pure and Dy doped ZnO thin films at room temperature taken by Professor Fang-Yuh Lo's research group ............................... 88 Fig. 4.9 Magnified magnetic moment versus magnetic field of non-annealing pure and Dy doped ZnO thin films at room temperature taken by Professor Fang-Yuh Lo's research group ...... 89 Fig. 4.10 Field-cooled and zero-field-cooled (M-T) curves of non-annealing pure and Dy doped ZnO thin films under the magnetic field of 100 Oe ((a) 1% Dy (b) 3% Dy (c) 5% Dy (d) 10% Dy). They were taken by Professor Fang-Yuh Lo's research group .............................................. 89 Fig. 4.11 The magnetic moment versus magnetic field of non-annealing pure and Gd doped ZnO thin films at 5 K taken by Professor Fang-Yuh Lo's research group ............................................ 90 Fig. 4.12 The magnetic moment versus magnetic field of non-annealing pure and Gd doped ZnO thin films at room temperature taken by Professor Fang-Yuh Lo's research group ...................... 90 Fig. 4.13 Field-cooled and zero-field-cooled (M-T) curves of pure and Gd doped ZnO thin films under the magnetic field of 100 Oe taken by Professor Fang-Yuh Lo's research group ....... 91 Fig. 4.14 PL spectra of non-annealing pure and Dy doped ZnO thin films at 20 K taken by Professor Fang-Yuh Lo's research group ............................................................................................... 91 Fig. 4.15 PL spectra of non-annealing pure and Dy doped ZnO thin films at room temperature taken by Professor Fang-Yuh Lo's research group .......................................................................... 92 Fig. 4.16 PL spectra of non-annealing pure and Gd doped ZnO thin films at 20 K taken by Professor Fang-Yuh Lo's research group ............................................................................................... 92 Fig. 4.17 PL spectra of non-annealing pure and Gd doped ZnO thin films at room temperature taken by Professor Fang-Yuh Lo's research group .......................................................................... 93. xiii.

(17) Fig. 4.18 High-resolution TEM image of monolayer WS2 .................................................................. 93 Fig. 4.19 (a) Transport characteristics of field-effect transistors fabricated on as-grown monolayer WS2 on a linear scale (right y-axis) and a log scale (left y-axis). (b) Output characteristics of the WS2 field-effect transistors .............................................................................................. 94 Fig. 4.20 High-resolution TEM image of monolayer WSe2 ................................................................ 94 Fig. 4.21 The output characteristics for both the p- and n-channels at various VR .............................. 95 Fig. 4.22 Specific capacitance and carrier mobility values measured at various VR. The drain current level was maintained at ± 0.1 V, which was in the linear regime .......................................... 95 Fig. 5.1 Room-temperature Raman scattering spectra of non-annealing (NA) and annealing (AN) of ZnO thin films and a pure sapphire substrate excited by a 532-nm laser line ..................... 121 Fig. 5.2 Room-temperature Raman scattering spectra of various Dy doped ZnO thin films without annealing excited by a 532-nm laser line. The asterisks represent Raman peaks from the sapphire substrate................................................................................................................. 121 Fig. 5.3 Room-temperature Raman scattering spectra of various Dy doped ZnO thin films with annealing excited by a 532-nm laser line. The asterisks represent Raman peaks from the sapphire substrate................................................................................................................. 122 Fig. 5.4 Room-temperature Raman scattering spectra of various Gd doped ZnO thin films without annealing excited by a 532-nm laser line. The asterisks represent Raman peaks from the sapphire substrate................................................................................................................. 122 Fig. 5.5 Room-temperature optical transmission spectrum of non-annealing ZnO thin film ............ 123 Fig. 5.6 Room-temperature optical transmission spectra of various Dy doped ZnO thin films without annealing .............................................................................................................................. 123 Fig. 5.7 Room-temperature optical transmission spectra of various Gd doped ZnO thin films without annealing .............................................................................................................................. 124. xiv.

(18) Fig. 5.8 Room-temperature optical absorption spectrum of non-annealing ZnO thin film ............... 124 Fig. 5.9 Room-temperature optical absorption spectra of various Dy doped ZnO thin films without annealing .............................................................................................................................. 125 Fig. 5.10 Room-temperature optical absorption spectra of various Gd doped ZnO thin films without annealing .............................................................................................................................. 125 Fig. 5.11 (α ⋅ E ) versus the photon energy for non-annealing ZnO thin film ............................... 126 2. Fig. 5.12 Room-temperature experimental (symbols) at 70o and 75o incidence angles and fitted (dashed lines) values of ellipsometric parameters of psi (Ψ) and delta (Δ) of non-annealing ZnO thin film ....................................................................................................................... 126 Fig. 5.13 Room-temperature experimental (symbols) at 70o and 75o incidence angles and fitted (dashed lines) values of ellipsometric parameters of psi (Ψ) and delta (Δ) of annealing ZnO thin film ............................................................................................................................... 127 Fig. 5.14 Room-temperature experimental (symbols) at 70o and 75o incidence angles and fitted (dashed lines) values of ellipsometric parameters of psi (Ψ) and delta (Δ) of non-annealing Dy 1% doped ZnO thin film ................................................................................................ 127 Fig. 5.15 Room-temperature experimental (symbols) at 70o and 75o incidence angles and fitted (dashed lines) values of ellipsometric parameters of psi (Ψ) and delta (Δ) of annealing Dy 1% doped ZnO thin film ...................................................................................................... 128 Fig. 5.16 Room-temperature experimental (symbols) at 70o and 75o incidence angles and fitted (dashed lines) values of ellipsometric parameters of psi (Ψ) and delta (Δ) of non-annealing Dy 3% doped ZnO thin film ................................................................................................ 128 Fig. 5.17 Room-temperature experimental (symbols) at 70o and 75o incidence angles and fitted (dashed lines) values of ellipsometric parameters of psi (Ψ) and delta (Δ) of annealing Dy 3% doped ZnO thin film ...................................................................................................... 129. xv.

(19) Fig. 5.18 Room-temperature experimental (symbols) at 70o and 75o incidence angles and fitted (dashed lines) values of ellipsometric parameters of psi (Ψ) and delta (Δ) of non-annealing Dy 5% doped ZnO thin film ................................................................................................ 129 Fig. 5.19 Room-temperature experimental (symbols) at 70o and 75o incidence angles and fitted (dashed lines) values of ellipsometric parameters of psi (Ψ) and delta (Δ) of annealing Dy 5% doped ZnO thin film ...................................................................................................... 130 Fig. 5.20 Room-temperature experimental (symbols) at 70o and 75o incidence angles and fitted (dashed lines) values of ellipsometric parameters of psi (Ψ) and delta (Δ) of non-annealing Dy 10% doped ZnO thin film .............................................................................................. 130 Fig. 5.21 Room-temperature experimental (symbols) at 70o and 75o incidence angles and fitted (dashed lines) values of ellipsometric parameters of psi (Ψ) and delta (Δ) of annealing Dy 10% doped ZnO thin film .................................................................................................... 131 Fig. 5.22 Room-temperature experimental (symbols) at 70o and 75o incidence angles and fitted (dashed lines) values of ellipsometric parameters of psi (Ψ) and delta (Δ) of non-annealing Gd 3% doped ZnO thin film ................................................................................................ 131 Fig. 5.23 Room-temperature experimental (symbols) at 70o and 75o incidence angles and fitted (dashed lines) values of ellipsometric parameters of psi (Ψ) and delta (Δ) of non-annealing Gd 5% doped ZnO thin film ................................................................................................ 132 Fig. 5.24 Room-temperature experimental (symbols) at 70o and 75o incidence angles and fitted (dashed lines) values of ellipsometric parameters of psi (Ψ) and delta (Δ) of non-annealing Gd 10% doped ZnO thin film .............................................................................................. 132 Fig. 5.25 Room-temperature experimental (symbols) at 70o and 75o incidence angles and fitted (dashed lines) values of ellipsometric parameters of psi (Ψ) and delta (Δ) of non-annealing Gd 15% doped ZnO thin film .............................................................................................. 133. xvi.

(20) Fig. 5.26 Room-temperature experimental (symbols) at 70o and 75o incidence angles and fitted (dashed lines) values of ellipsometric parameters of psi (Ψ) and delta (Δ) of non-annealing Gd 20% doped ZnO thin film .............................................................................................. 133 Fig. 5.27 Room-temperature experimental (symbols) at 70o and 75o incidence angles and fitted (dashed lines) values of ellipsometric parameters of psi (Ψ) and delta (Δ) of non-annealing Gd 25% doped ZnO thin film .............................................................................................. 134 Fig. 5.28 Room-temperature experimental (symbols) at 70o and 75o incidence angles and fitted (dashed lines) values of ellipsometric parameters of psi (Ψ) and delta (Δ) of non-annealing Gd 30% doped ZnO thin film .............................................................................................. 134 Fig. 5.29 Stacked layer model............................................................................................................ 135 Fig. 5.30 Refractive index n and extinction coefficient k of non-annealing and annealing ZnO thin films ..................................................................................................................................... 135 Fig. 5.31 Refractive index n and extinction coefficient k of various Dy concentration doped ZnO thin films without annealing ....................................................................................................... 136 Fig. 5.32 Refractive index n and extinction coefficient k of various Dy concentration doped ZnO thin films with annealing ............................................................................................................ 136 Fig. 5.33 Refractive index n and extinction coefficient k of various Gd concentration doped ZnO thin films without annealing ....................................................................................................... 137 Fig. 5.34 Room-temperature optical absorption coefficient of ZnO thin films without annealing obtained by spectroscopic ellipsometry. The blue, green, and yellow dashed line are free exciton fitted line, Urbach tail, and absorption coefficient fitted line ................................. 137 Fig. 5.35 Room-temperature optical absorption coefficient of ZnO thin films with annealing obtained by spectroscopic ellipsometry. The blue, green, and yellow dashed line are free exciton fitted line, Urbach tail, and absorption coefficient fitted line .............................................. 138. xvii.

(21) Fig. 5.36 Room-temperature optical absorption coefficient of different Dy doped ZnO thin films without annealing obtained by spectroscopic ellipsometry ................................................. 138 Fig. 5.37 Room-temperature optical absorption coefficient of different Dy concentration doped ZnO thin films with annealing obtained by spectroscopic ellipsometry ...................................... 139 Fig. 5.38 Room-temperature optical absorption coefficient of different Gd concentration doped ZnO thin films without annealing obtained by spectroscopic ellipsometry................................. 139 Fig. 5.39. (α ⋅ E ). 2. versus the photon energy for non-annealing and annealing ZnO thin films. obtained by spectroscopic ellipsometry ............................................................................... 140 Fig. 5.40 (α ⋅ E ) versus the photon energy for pure and Dy 1% and 3% doped ZnO thin films 2. without annealing obtained by spectroscopic ellipsometry ................................................. 140 Fig. 5.41 (α ⋅ E ) versus the photon energy for Dy 5% and 10% doped ZnO thin films without 2. annealing obtained by spectroscopic ellipsometry .............................................................. 141 Fig. 5.42 (α ⋅ E ) versus the photon energy for pure and Dy 3%, 5%, and 10% doped ZnO thin 2. films with annealing obtained by spectroscopic ellipsometry ............................................. 141 Fig. 5.43 (α ⋅ E ) versus the photon energy for low Gd doped ZnO thin films without annealing 2. obtained by spectroscopic ellipsometry ............................................................................... 142 Fig. 5.44 (α ⋅ E ) versus the photon energy for high Gd doped ZnO thin films without annealing 2. obtained by spectroscopic ellipsometry ............................................................................... 142 Fig. 5.45 Variation of optical band gap energy of Dy doped ZnO as a function of Dy concentration. The PE is measured by grating spectrometer and the SE is measured by spectroscopic ellipsometry ......................................................................................................................... 143 Fig. 5.46 Variation of optical band gap energy of Gd doped ZnO as a function of Gd concentration.. xviii.

(22) The PE is measured by grating spectrometer and the SE is measured by spectroscopic ellipsometry ......................................................................................................................... 143 Fig 5.47 Temperature-dependent optical absorption coefficient of annealing ZnO thin film ........... 144 Fig 5.48 Temperature-dependent optical absorption coefficient of Dy 3% doped ZnO thin film with annealing .............................................................................................................................. 145 Fig 5.49 Temperature-dependent optical absorption coefficient of Dy 5% doped ZnO thin film with annealing .............................................................................................................................. 146 Fig 5.50 Temperature-dependent optical absorption coefficient of Dy 10% doped ZnO thin film with annealing .............................................................................................................................. 147 Fig. 5.51 The temperature-dependent integrated intensity of exciton absorption peak of ZnO thin film with annealing. The dashed line is the Arrhenius fitted line................................................ 148 Fig. 5.52 The temperature-dependent integrated intensity of exciton absorption peak of Dy 3% doped ZnO thin film with annealing. The dashed line is the Arrhenius fitted line ........................ 148 Fig. 5.53 (α ⋅ E ). 2. versus the photon energy for annealing of ZnO thin film as a function of. temperature .......................................................................................................................... 149 Fig. 5.54 (α ⋅ E ) versus the photon energy for annealing Dy 3% doped ZnO thin film as a function 2. of temperature ...................................................................................................................... 150 Fig. 5.55 (α ⋅ E ) versus the photon energy for annealing Dy 5% doped ZnO thin film as a function 2. of temperature ...................................................................................................................... 151 Fig. 5.56 (α ⋅ E ) versus the photon energy for annealing Dy 10% doped ZnO thin film as a 2. function of temperature........................................................................................................ 152 Fig. 5.57 The positions of A exciton for pure and Dy 3% doped ZnO thin films as a function of temperature .......................................................................................................................... 153. xix.

(23) Fig. 5.58 The linewidths of A exciton for pure and Dy 3% doped ZnO thin films as a function of temperature .......................................................................................................................... 153 Fig 5.59 Temperature-dependent band gap of pure and various Dy doped ZnO thin films with annealing. The dashed line is the Bose-Einstein model fitted line ...................................... 154 Fig 5.60 Temperature-dependent exciton binding energy of pure and Dy 3% doped ZnO thin films with annealing ...................................................................................................................... 155 Fig. 6.1 Room-temperature Raman-scattering spectrum of a monolayer WS2 thin film excited by a 532-nm laser line ................................................................................................................. 176 Fig. 6.2 The expanded-scale plot of the Raman scattering spectrum of a monolayer WS2 thin film. The black dashed lines are the results of fitting using the Lorentzian modal...................... 176 Fig. 6.3 Schematic representation of the structure of WX2 (X = S, Se): (a) WX2 bulk and single layer. The interlayer distance is denoted by d. The layers are separated by weak Van der Waals forces. (b) Slide view of the WX2 bulk unit cell .................................................................. 177 Fig. 6.4 Schematic showing atomic displacement of two infrared-active, four Raman-active, and four inactive modes in the bulk WX2 (X = S, Se) ....................................................................... 177 Fig. 6.5 The 2 LA ( M ) , E21 g , and A1g phonon modes observed in a monolayer WS2 thin film .... 178 Fig. 6.6 Room-temperature Raman-scattering spectra of a monolayer WS2 thin film and a sapphire substrate excited by a 785-nm laser line .............................................................................. 178 Fig. 6.7 The E21 g mode observed in a monolayer WS2 thin film excited by 785-nm laser line ...... 179 Fig. 6.8 Room-temperature Raman-scattering spectrum of a monolayer WSe2 thin film excited by a 532-nm laser line ................................................................................................................. 179 Fig. 6.9 The expanded-scale plot of the Raman scattering spectrum of a monolayer WSe2 thin film. The dashed lines are the results of fitting using the Lorentzian modal ............................... 180. xx.

(24) Fig. 6.10 The E21 g and A1g phonon modes observed in a monolayer WSe2 thin film ................... 180 Fig. 6.11 Room-temperature experimental (symbols) at 70o and 75o incidence angles and fitted (dashed lines) values of ellipsometric parameters of psi (Ψ) and delta (Δ) of a monolayer WS2 thin film ....................................................................................................................... 181 Fig. 6.12 Room-temperature experimental (symbols) at 70o and 75o incidence angles and fitted (dashed lines) values of ellipsometric parameters of psi (Ψ) and delta (Δ) of a monolayer WSe2 thin film ..................................................................................................................... 181 Fig. 6.14 Refractive index n and extinction coefficient k of a monolayer WS2 thin film ................. 182 Fig. 6.15 Refractive index n and extinction coefficient k of a monolayer WSe2 thin film ................ 183 Fig. 6.16 Room-temperature optical absorption coefficient of a monolayer WS2 thin film. The dashed line is the best fit by using the broadened Lorentzian line shape ........................................ 183 Fig. 6.17 Room-temperature optical absorption coefficient of a monolayer WSe2 thin film. The dashed line is the best fit by using the broadened Lorentzian line shape ............................ 184 Fig 6.18 A pair excitation in the scheme of valence and conduction band (a) in the exciton picture for a direct (b) and for an indirect gap semiconductor (c)......................................................... 184 Fig. 6.19 Temperature-dependent optical absorption coefficient of a monolayer WS2 thin film ...... 185 Fig. 6.20 Temperature-dependent optical absorption coefficient of a monolayer WSe2 thin film .... 186 Fig. 6.21 The positions of A and B excitons in a monolayer WS2 thin film as a function of temperature .......................................................................................................................... 187 Fig. 6.22 The linewidth of A and B excitons in a monolayer WS2 thin film as a function of temperature .......................................................................................................................... 187 Fig. 6.23 The positions of A and B excitons in a monolayer WSe2 thin film as a function of temperature .......................................................................................................................... 188 Fig. 6.24 The linewidth of A and B excitons in a monolayer WSe2 thin film as a function of xxi.

(25) temperature .......................................................................................................................... 188 Fig. 6.25 Temperature-dependent of direct band gap of monolayer WS2 and WSe2 thin films. The dashed lines are the Bose-Einstein model fitted lines ......................................................... 189 Fig. 6.26 Temperature-dependent of exciton binding energy of monolayer WS2 and WSe2 thin films ............................................................................................................................................. 189. xxii.

(26) List of Tables Table 4.1 Sample process parameters of different non-annealing Dy doped ZnO thin films .............. 83 Table 4.2 Sample process parameters of different annealing Dy doped ZnO thin films ..................... 83 Table 4.3 Sample process parameters of different non-annealing Gd doped ZnO thin films .............. 84 Table 5.1 Parameters of a Lorentizan fit for the Raman scattering spectrum of non-annealing different Dy doped ZnO thin films excited by a 532-nm laser line. All units are cm-1 ..... 115 Table 5.2 Parameters of a Lorentizan fit for the Raman scattering spectrum of annealing different Dy doped ZnO thin films excited by a 532-nm laser line. All units are cm-1 .......................... 115 Table 5.3 Parameters of a Lorentizan fit for the Raman scattering spectrum of non-annealing different Gd doped ZnO thin films excited by a 532-nm laser line. All units are cm-1 ..... 116 Table 5.4 The direct optical band gap energies of non-annealing different Dy doped ZnO thin films were determined by optical transmission measurements. All units are eV ....................... 117 Table 5.5 The direct optical band gap energies of non-annealing different Gd doped ZnO thin films were determined by optical transmission measurement. All units are eV ......................... 117 Table 5.6 Parameters of a stacked layer model fit for non-annealing pure and different Dy doped ZnO thin films ............................................................................................................................ 117 Table 5.7 Parameters of a stacked layer model fit for annealing pure and different Dy doped ZnO thin films ................................................................................................................................... 118 Table 5.8 Parameters of a stacked layer model fit for non-annealing different Gd doped ZnO thin films ................................................................................................................................... 118 Table 5.9 The direct optical band gap energies of non-annealing pure and different Dy doped ZnO thin films determined by spectroscopic ellipsometry. All units are eV ............................. 119 Table 5.10 The direct optical band gap energies of annealing pure and different Dy doped ZnO thin. xxiii.

(27) films determined by spectroscopic ellipsometry. All units are eV..................................... 119 Table 5.11 The direct optical band gap energies of non-annealing different Gd doped ZnO thin films determined by spectroscopic ellipsometry. All units are eV .............................................. 119 Table 5.12 The exciton binding energies, and exciton broadening parameters of non-annealing pure and different Dy doped ZnO thin films. All units are eV .................................................. 120 Table 5.13 The exciton binding energies, and exciton broadening parameters of annealing pure and different Dy doped ZnO thin films. All units are eV ......................................................... 120 Table 5.14 The exciton binding energies, and exciton broadening parameters of non-annealing pure and different Gd doped ZnO thin films. All units are eV .................................................. 120 Table 6.1 Parameters of a Lorentzian fit for the Raman scattering spectrum of a monolayer WS2 thin film excited by a 532-nm laser line. All units are cm-1 ...................................................... 171 Table 6.2 Parameters of lattice vibrations of WX2 (X = S, Se).......................................................... 172 Table 6.3 Correlation chart relating the irreducible representations of the site groups D3h and C3v to those of the factor group D6h ....................................................................................... 173 Table 6.4 Parameters of a Lorentzian fit for the Raman scattering spectrum of a monolayer WSe2 thin film excited by a 532-nm laser line. All units are cm-1 ...................................................... 174 Table 6.5 Parameters of a stacked layer model fit for the monolayer WS2 and WSe2 thin films ...... 174 Table 6.6 The exciton band-gap energies, and exciton binding energies, and exciton broadening parameters of monolayer WS2 and WSe2 thin films. All units are in eV........................... 175. xxiv.

(28) Chapter 1 Introduction In an era of advanced technology, semiconductors are playing an ever more important role. In recent years, there has been a wealth of research into optoelectronic technology, especially the optical and electrical properties of semiconductors [1,2]. Typical semiconductor devices include integrated circuits, transistors, light emitting diodes, and so on. Operational theories on typical semiconductor devices mainly focus on controlling the flow of carrier electrons through semiconductors using electric fields. In which case, the key parameter is the charge on electrons or holes. These devices, therefore, suffer in part from carrier velocity and energy-loss. Spintronic devices include the magnetic data storage, and memory cells in nonvolatile and magnetic random access memory (MRAM) systems, etc. In magnetic data storage, the key parameter is electron spin, as this is the fundamental origin of magnetic moment [3]. The emerging field of semiconductor spin transfer electronics (spintronics) seeks to exploit the spin of charge carriers in semiconductors. There is exceptional potential in this technology if electronic functionality can be exploited by controlling the injection, transfer, and detection of carrier spin at above room temperature [4,5]. For this reason, spintronics has become a subject of great interest in recent years. Research into diluted-magnetic semiconductor, materials possessing magnetic and semiconductor 1.

(29) properties, is a crucial aspect of spintronics [6]. The crystal structure and chemical bonding of diluted-magnetic semiconductor materials are best viewed in the context of existing electronic-component semiconductor materials. Zener model calculations [7] predict that the Curie temperature ( Tc ) of group III, IV, and V (Fe [8], Mn [9]) compound doped wideband-gap semiconductor (ZnO) materials are higher than room temperature, a feature important for room temperature operated spintronic devices. In the past 10 to 15 years, a lot of studies on magnetic [10], electrical [11], optical [12], and photocatalytic [13] properties of transition metal doped ZnO materials have been reported. The crystal structure of ZnO is zinc-blende or hexagonal wurtzite where each anion is surrounded by four zinc cations at the corners of a tetrahedron, and vice versa (Fig. 1.1) [14]. This tetrahedral coordination is typical of sp3 covalent bonding. The physical properties of ZnO, such as plastic deformation at relatively low loads (≧4 ~ 13 mN with an ~ 4.2 μm radius spherical indenter) [15], hardness modulus (~ 5.0 GPa), strong Young's modulus (~ 111.2 GPa), thermal conductivity (~ 0.87 W/cm-K) [16], and electron mobility (~ 300 cm2 V-1 s-1) [17], have been well documented. Rare-earth doping in ZnO is considered an interesting alternative to 3d transition metals since it has robust magnetic moments at room temperature due to its 4f states [18]. These 4f states of rare-earth ions are responsible for improving hole conductivity as holes in 4f states are more active than electrons [19]. Given this property, diluted-magnetic semiconductors are favored as future. 2.

(30) candidates for next generation electronic, spintronic, and optoelectronic devices. Among diluted-magnetic semiconductors, the optical properties of Dy and Gd doped ZnO thin films have been little explored [20,21]. We are interested in the changes to the optical properties of ZnO thin films doped with different concentrations of Dy and Gd. In this study, we plan to investigate such products using Raman scattering spectroscopy, grating spectrometry, and spectroscopic ellipsometry. These techniques can explore the effects of doping with Dy and Gd at different concentrations on lattice phonons, transmission, and complex optical constants of ZnO thin films. This study of Dy and Gd doped ZnO thin films is very important to our understanding of the optical and electronic properties of devices based on thin film structures. Two-dimensional materials have recently generated great interest because of their unique physical properties and potential practical applications [22-24]. Among them, there has been particular interest in graphene [25,26], transition metal dichalcogenides [27,28], transition metal oxides including titania- and perovskite- based oxides [29,30], and boron nitride [31,32]. Transition metal dichalcogenides are special in many respects. They exhibit a variety of surprising electronic, optical, mechanical, chemical, and thermal properties [33-35]. Furthermore, they render potential applications in catalysis [36], energy conversion [37,38],and optoelectronics [39,40]. The transition metal dichalcogenides have a common structural formula MX2, where M is a transition metal element like Mo, W, Ti, etc. and X is a chalcogen (S, Se, Te) [41]. These materials. 3.

(31) form layered structures of X-M-X covalently bonded hexagonal quasi-two-dimensional network stacked by weak Van der Waals forces (Fig. 1.2) [41]. Based on strong surface effects, the properties of the transition metal dichalcogenides change drastically with the number of layers in a sheet. The band gap energy increases from multilayers to a monolayer, and transfers indirect band gap to direct band gap [41-46]. These monolayer semiconductors have highly stable neutral and charged excitons [47,48]. For monolayer transition metal dichalcogenides, confinement of electrons and holes to the ±K valleys gives rise to valley excitons and trions, formed at an energy-degenerate set of non-central points in momentum space [49]. In principle, these valley excitons offer unprecedented opportunities to dynamically manipulate a valley index using optical means, as has been done for optically driven spintronics. These special properties make monolayer transition metal dichalcogenides complements or substitutes for materials currently used in optoelectronic and energy harvesting applications. Although the optical properties of transition metal dichalcogenides have been intensely studied [50,51], no studies on the temperature dependence of complex optical constants have been reported thus far. Therefore, in this study, we plan to investigate monolayer transition metal dichalcogenides using Raman scattering spectroscopy and spectroscopic ellipsometry. These spectra can explore lattice vibration phonons and the complex optical constants of monolayer transition metal dichalcogenides (TMDs). Knowledge of the optical properties of TMDs is necessary when conceiving of semiconductor device applications.. 4.

(32) The organization of this thesis is as follows: Chapter 2 presents a literature review of previous research into dysprosium and gadolinium doped zinc oxide and monolayer two-dimensional transition metal dichalcogenides such as WS2 and WSe2; Chapter 3 describes the technical and theoretical details of the experiment; Chapters 4 ~ 6 present the main experimental results; and finally, Chapter 7 gives the thesis summary.. 5.

(33) (a). (b). c b. a. Fig. 1.1 Schematic representation of ZnO crystal structures: (a) hexagonal wurtzite structure; and (b) cubic zinc blende wurtzite structure. The yellow and white (grey) spheres denote Zn and O atoms, respectively [11].. (a). (b). z y x. y. x Fig. 1.2 Schematic representation of (a) side view and (b) top view of monolayer MX2 structure, with the chalcogen atoms in yellow and the transition metal atoms in black [33].. 6.

(34) Chapter 2 Brief survey of (Dy, Gd) doped ZnO and transition metal dichalcogenides 2-1 (Dy, Gd) doped ZnO If the ideal spintronic devices might be commonly applied in the life, they must abide two conditions: (i) These devices have to be operated in the room temperature. This indicates the Curie temperature of materials of these devices is higher than the room temperature. (ii) The magnetic interaction of these devices can be explained by the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction, which refers to a coupling mechanism of nuclear magnetic moments or localized inner d or f orbital electron spins in a metal by means of an interaction through the conduction electrons. In 2000, T. Dietl et al. [7] presented the values of Curie temperature for various kinds of cubic zinc-blende p-type semiconductors, which were calculated by using Zener model. Figure 2.1 shows the theoretical values of the Curie temperature for GaN and ZnO semiconductors containing 5 % of Mn and 3.5 × 1020 holes per cm3. Based on this report, the transition metals doped ZnO, such as Fe [8,52], Co [53], Mn [9,54] into ZnO, are popularly researched in the later years. These dopant ions are ferromagnetically coupled and give rise to a spin polarization of the charge carriers of the semiconductor. However, the 3d electrons in 3d transition metals are exterior and delocalized so the 7.

(35) orbital momentum is often zero and gives a small total magnetic moment per atom. Recently, the rare-earth metals doped ZnO has provoked great interest ever since the colossal magnetic moment of Gd in GaN was reported by S. Dhar et al. [55] They reported the average value of the moment per Gd is as high as 4000 μB, which is extraordinarily high when compared to its atomic moment of 8μB. For rare-earth metals, the 4f electrons are localized and the exchange interactions are indirect via 5d or 6s conduction electrons, which result in high total magnetic moments per atom due to its high orbital momentum. Furthermore, the rare-earth metals doped ZnO semiconductors are considered to be the extremely potential material in the field of diluted magnetic semiconductors.. 2-1-1 Physical properties In 2006, G. S. Wu et al. [56] presented the structural analysis of Dy-doped ZnO nanowires prepared by sol-gel template approach. Figure 2.2 shows X-ray powder diffraction (XRD) pattern of the as-obtained Dy-doped ZnO nanowires. All the diffraction peaks of nanowires can be indexed to the hexagonal structured ZnO. No characteristic peaks were observed for the other impurities, implying the synthesis of high-purity ZnO samples. In 2010, R. S. Ajimsha et al. [20] examined the structural and electrical properties of Dy doped ZnO thin films. Dy doped ZnO thin films with different Dy concentration of 0.25, 0.5, 1, 2, and 5 wt% were deposited on the (0001) sapphire substrates by buffer assisted pulsed laser deposition. Figure. 8.

(36) 2.3 displays the typical ω − 2θ high-resolution x-ray diffractometer (HRXRD) patterns of intrinsic and Dy doped ZnO films. The Dy doped ZnO thin films exhibit only (0002) peak of ZnO, which indicates that all the as grown films show a preferred orientation with c-axis perpendicular to the surface. No characteristic peaks corresponding to Dy2O3 phase was observed, which implies that Dy substitute the Zn sites in the ZnO matrics. Figures 2.4 and 2.5 show the electrical resistivity ( ρ ) and carrier concentration of Dy:ZnO thin films with different Dy concentrations. All the samples were found to be n-type semiconductor. Dy concentration of 0.45 at% was found to be the optimum doping concentration for lowest resistivity. The dependence of the free carrier mobility ( µ ) of the film on Dy concentration is shown in Fig. 2.5. It can be seen that the carrier mobility of Dy:ZnO thin films with Dy concentration of 0.2 at% which was approximately 27.2 cm2/Vs decreased up to approximately 22.3 cm2/Vs with an increase in Dy concentration up to 4.12 at%. The decrease in carrier mobility in Dy:ZnO thin films with increasing Dy concentration can be explained from the deterioration of crystalline quality of the film and thus enhanced scattering of free carriers with defects and grain boundaries. In 2013, O. Yayapao et al. [57] presented the structural properties of Dy-doped ZnO nanostructures. ZnO nanostructures with Dy concentration of 1, 2, and 3% were synthesized by a sonochemical method. Figure 2.6 shows XRD patterns of the Dy-doped ZnO samples. For doping 1, 2, and 3% Dy in ZnO samples, their XRD patterns are the same as that of pure wurtzite hexagonal. 9.

(37) ZnO structure. No other peaks corresponding to Dy2O3, Zn(OH)2, and other impurities were detected. It should be noted that 2θ angles of the (100), (002), and (101) planes at 32.11 , 34.75 , and. 36.57 for pure ZnO were shifted to the lower diffraction values with the increasing in the doping concentrations until reaching at 32.99 , 34.65 , and 36.49 by doping with 3% Dy. This phenomenon can be illustrated by the expansion of ZnO lattice caused by the larger radius of Dy3+ (0.91 Å) than that of Zn2+ (0.74 Å). In 2012, X. Ma [58] reported the magnetic properties of Gd doped ZnO nanowires grown on Si substrates by means of a chemical vapor deposition process. Their sample was grown with a Gd 5% mole in a mixed Zn/Mn source under a constant O2/Ar gas mixture flowing at 580℃ followed by annealing at 800℃. Figure 2.7 shows the X-ray powder diffraction (XRD) pattern of the as grown ZnO:Gd nanowire sample. From the diffraction peaks, they can confirm that ZnO nanowires are a wurtzite structure. The diffraction spectrum of ZnO:Gd nanowire is almost the same as that of pure ZnO. That is to say, for a low doping concentration of Gd, the structure of ZnO remains unchanged, and Gd atoms simply replace Zn atoms. By the results of the Rutherford backscattering measurement, the concentration of Gd in the ZnO nanowires is about 1 × 1015 cm-3. The stoichiometry of Zn0.95Gd0.05O was obtained from the experimental data, as shown in the Rutherford backscattering spectrum (Fig. 2.8).. 10.

(38) Figure 2.9 shows the magnetization properties obtained from the as grown and annealed ZnO:Gd nanowires samples at 77 K. In the presence of the magnetic field H , a large opening up of the hysteresis loop is visible for as grown nanowires and is even more pronounced for the annealed sample. The two samples have qualitatively similar hysteresis loops, although the annealed sample shows a remarkable enhancement of the magnetic properties. This appearance implies that the annealing process removes some of the structural defects in ZnO nanowires and increases the magnetic properties of Gd. Figure 2.10 displays the magnetization properties obtained from the as grown and annealed ZnO:Gd nanowires samples at room temperature. The wide opening hysteresis loops indicate the presence of a ferromagnetic phase in the nanowires at room temperature, and the steep rise in magnetization reveals the samples to be intrinsic diluted magnetic semiconductors. The results for the annealed sample show a clear separation ∆M . They found that the ferromagnetism and the colossal moment of Gd observed in these samples are closely related to the interactions between the Gd ions. Notably, their ZnO:Gd nanowires show the high-temperature ferromagnetism. Figure 2.11 (a) shows the magnetic susceptibility of the as grown and annealed ZnO:Gd nanowires at room temperature and as a function of the applied magnetic field. The results of magnetic susceptibility further confirm that ZnO:Gd nanowires are highly ferromagnetic. The effective magnetic moment per Gd atom peff can be derived from the value of the saturation. 11.

(39) magnetization M s ( peff = M s / N Gd ; N Gd = 1× 1015 cm −3 ). Figure 2.11 (b) illustrates peff as a function of the applied magnetic field H . The maximum peff of the annealed and as growen sample is 3279 μB and 1284 μB, respectively. The large effective moment is closely connected with the exchange interaction modes of Gd ions. Such a colossal moment can be explained in terms of a very effective Ruderman-Kittel-Kasuya-Yosida exchange interaction. Because the moment of the rare-earth metals originates from the 4f electrons, which are confined to the inner shell with a radius of 0.3 × 10-10 m and screened by the outer shell 5s2p6d106s2 electrons, two neighboring rare-earth metals ions are unable to exchange couple directly or interact via a superexchange interaction because the space between two neighboring Gd ions is large. In the same year, V. Ney et al. [59] presented the structural and magnetic analysis of Gd-doped ZnO epitaxial films prepared by reactive magnetron sputtering. The Gd:ZnO films with Gd concentrations of nominally 1.3%, 4%, 7%, and 16% were grown on c-plane sapphire substrates. Figure 2.12 shows the wide range of XRD 2θ scans recorded for ZnO(002) of the 1.3%, 4%, 7%, and 16% Gd:ZnO films. They found that the intensity of the ZnO(002) peak is correlated to the thickness of the samples, which were measured by x-ray reflectometry (XRR). The inset of Fig. 2.12 illustrates the full width at half maximum (FWHM), depending extremely on the amount of Gd in the films.. 12.

(40) Figure 2.13 shows the isotropic x-ray absorption near edge spectra (XANES) and the x-ray linear dichroism (XLD) signals at the Zn K-edge for the sputtered 1.3%, 4%, 7%, and 16% Gd:ZnO films. XANES shows a small reduction of the spectral fine structure with increasing Gd content. The changes in the isotropic XANES indicate a change in the oxidation state of the corresponding element. The XLD spectra reveal the typical signature of a wurtzite crystal structure. The XLD also reflects the local structural environment of the Zn atom and therefore the quality of the ZnO films. The size of the XLD is strongly reduced for the higher Gd concentrations. With an increase in the Gd content, the ZnO films show the degradation of the local structural quality of the ZnO host lattice in accordance with the integral structural characterization using x-ray diffraction (XRD). Figure 2.14 (a) and (b) display the isotropic XANES and the XLD signals at the Gd L3-edge for the sputtered 1.3%, 4%, 7%, and 16% Gd:ZnO films. For all sputtered samples, the XANES are nearly identical. But a strong dependence of the Gd concentration is seen in the XLD, which is extremely reduced with increasing Gd content. As for the Zn, the presence of a characteristic Gd XLD confirms that at least a fraction of the Gd atoms are present in a wurtzite crystal environment. The fraction of substitutional Gd is further reduced for higher Gd concentrations. This phenomenon can be either due to rotated grains, or Gd atoms, which are not located on substitutional Zn lattice sites. In conclusion, with an increasing concentration of Gd, the crystal structure of the ZnO is more and more disturbed. However, the Gd:ZnO films remain essentially in the wurtzite structure.. 13.

(41) Figure 2.14 (c) shows the element-specific magnetic properties of the samples analyzed by measuring the x-ray magnetic circular dichroism (XMCD) at the Gd L3-edge. With increasing Gd concentration, the XMCD decreases, but the shape of the spectra remain the same. Figure 2.15 displays the XMCD ( H ) signal taken at a maximum of the XMCD signal. The magnetic field was swept between ± 60 kOe. For all samples, the XMCD ( H ) curves exhibit a clear S shape, which shows no hysteresis. This indicated paramagnetic behavior. The curvature of the XMCD ( H ) decreases with increasing Gd concentration, indicating a reduced effective Gd magnetic moment. Figure 2.16 shows the XMCD ( H ), SQUID M ( H ) , and Brillouin function for J= S= 7 / 2 experimental data of the 1.3% Gd:ZnO sample. Both integral and elementspecific magnetization measurements demonstrate that the 1.3% Gd:ZnO sample behaves purely paramagnetically. Figure 2.17 displays the integral magnetic characterization by the SQUID for the entire Gd concentration series. No separation between field-cooled (FC) and zero-field-cooled (ZFC) magnetization occurs at any temperature for any sample. Therefore, all samples have to be considered to behave solely paramagnetically. The geometry for Gd located on substitutional and interstitial sites is shown in Fig. 2.18. They calculated the structure of Gd sample by using the density-functional theory. They found that for small doping concentrations of nominally 1.3% Gd, a large fraction of the Gd atoms are substituional on Zn lattice sites within the wurtzite crystal structure. With higher Gd concentrations, an increasing. 14.

(42) amount of Gd is not substitutional anymore. The interstitial center becomes the most stable position, which is accompanied by lattice distortions. In 2014, S. Kumar et al. [60] presented the magnetic properties of Gd3+ incorporated ZnO nanoparticles. ZnO nanoparticles doped with Gd (0.01, 0.03, and 0.05 mol%) were synthesized by wet chemical route method. Figure 2.19 shows the magnetiztion versus magnetic field (M-H) curves for ZnO:Gd (0.01, 0.03, and 0.05 mol%) nanoparticles. Gd doped ZnO nanoparticles exhibit room temperature ferromagnetic properties which support bound magnetic polarons (BMP) model. With increasing Gd concentration, the doping in ZnO nanoparticles increases the magnetic properties of ZnO. The coercivity is also increased with Gd concentration (inset of Fig. 2.19).. 2-1-2 Optical properties In 2010, R. S. Ajimsha et al. [20] presented the transmission and photoluminescence spectra of Dy doped ZnO thin films. Dy doped ZnO thin films with different Dy concentration of 0.25, 0.5, 1, 2, and 5 wt% were deposited on the (0001) sapphire substrates by buffer assisted pulsed laser deposition. Figure 2.20 shows the room temperature optical transmission spectra of pure and Dy doped ZnO films. It can be seen that all the films are highly transparent with average transmission of approximately 85% in the visible range and sharp fundamental absorption edges corresponding to respective band gaps.. 15.

(43) Figure 2.21 represents the variation of band gap of Dy doped ZnO films with Dy concentration. It can be observed that band gap of Dy:ZnO films increased to a maximum value of approximately 3.42 eV at a Dy concentration of 0.45 at% and then decreased up to 3.3 eV at 4.12 at% of Dy doping. Variation of band gap with carrier concentration was modeled considering the combined effect of Burstein-Moss shift and band gap narrowing. Figure 2.22 shows the room temperature photoluminescence spectra of Dy doped ZnO films. It can be observed that the decreased intensity of near band edge emission with Dy concentration can be attributed to deterioration of the crystalline quality of the film and enhancement of non-radiative compensating native defects density with increase of Dy content. The luminescence emission peak in the visible spectral region at approximately 575 nm is related to 4F9/2-6H13/2 transition of Dy3+ ion. In 2013, O. Yayapao et al. [57] presented the Perkin Elmer RX Fourier transform infrared (FTIR) spectra and photocatalytic properties of Dy-doped ZnO nanostructures. ZnO nanostructures with Dy concentration of 1, 2, and 3% were synthesized by a sonochemical method. FTIR spectra of the 0, 1, 2, and 3% Dy-doped ZnO samples are shown in Fig. 2.23. The strong absorption bands at 425 ~ 565 cm-1 were specified as the Zn-O stretching vibration of wurtzite hexagonal structured ZnO crystal. The O-H stretching broad absorption bands, which is assigned as the vibration of hydroxyl group of rare earth hydroxide, were at 3013 ~ 3633 cm-1 and were increased with the increasing in Dy contents.. 16.

(44) They measured the absorption intensity of methylene blue (MB) at 664 nm for 0 ~ 300 min using the pure and Dy-doped ZnO samples as photocatalysts under UV light. Figure 2.24 shows MB degradation efficiency of the as-synthesized 0, 1, 2, and 3% Dy-doped ZnO samples. The Dy-doped ZnO samples exhibited higher photocatalytic activities than that of the pure ZnO sample. The 3% Dy-doped ZnO showed the highest photocatalytic activity. The rapid decrease of the MB concentration was mainly ascribed to the Dy dopant in ZnO. The increase of Dy doping content obviously enhanced the photocatalytic activity. In 2012, X. Ma et al. [21] examined the absorption and photoluminescence spectra of Gd doped ZnO nanocrystals fabricated by means of a thermal evaporation vapor phase deposition process. The ZnO nanocrystals were with stoichiometries of 5, 10, and 15 mol% Gd. Figure 2.25 shows the absorption spectra of 5, 10, and 15 mol% Gd doped ZnO nanocrystals (sample A, B, and C). They found the absorption intensity is enhanced, the number of absorption bands has also increased, and the bands are shifted slightly to a longer wavelength with increasing Gd doping concentration. These features are due to Gd doped into the ZnO nanocrystals in two ways, substitutional and interstitial. For low doping concentrations, the doping is usually substitution doping. For high doping concentrations, most of Gd impurities are present as interstitial atoms in the ZnO crystals.. 17.