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Magneto-optical and Electric Properties of Ho-doped ZnO Thin Films
pulsed laser deposition
magneto-optical Faraday effect
|Abstract:||本論文探討脈衝雷射蒸鍍法在c方向藍寶石基板上所製備之氧化鈥鋅薄膜(Ho: ZnO)的結構、光學、磁和磁光，及導電特性。氧化鈥鋅薄膜的製備條件為氧壓3×10-1 mbar，基板溫度525 ℃，雷射波長266 nm，雷射能量密度2.0 J/cm2，鈥的摻雜比例0~10 原子莫爾濃度(at. %)。
X光繞射光譜和拉曼散射光譜顯示氧化鈥鋅薄膜沒有其他雜質或晶相存在，代表薄膜中鈥原子成功取代了鋅原子。隨摻雜比例增加後，晶格常數與晶粒大小會變小，表示鈥原子(0.904 Å)取代鋅原子(0.74 Å)過程中產生缺陷造成薄膜結構變差。摻雜5及8 at. %之氧化鈥鋅薄膜的拉曼散射光譜還包含Ho 4f軌域5S2→5I8及5F4→5I8能階的螢光訊號。光致螢光光譜顯示純氧化鋅(Ho: 0 at. %)有很強的近能隙發光，隨摻雜比例增加，近能隙的發光變弱，缺陷的發光變強，從光致螢光光譜可以辨認出氧空缺、鋅空缺、鋅間隙等缺陷。
SQUID的結果顯示在T = 5與300 K氧化鈥鋅都呈現順磁性，飽和磁矩隨摻雜比例增加而變大，在8 at. %達最大值120 emu/cm3。磁矩和溫度關係結果表明，薄膜磁矩在2~100 K的範圍內都快速下降，在150 K後趨於平緩，且在40~60 K的地方有很強的氧退吸附訊號，若扣除掉氧退吸附的訊號，推測所有薄膜皆不具磁有序特性。磁光光譜顯示所有氧化鈥鋅薄膜皆為順磁性，其法拉第旋轉角對磁場的斜率隨波長越大而變小，此外氧化鈥鋅薄膜的Verdet常數數值隨波長增長變小，大約降低86 %。
從電流-電壓特性曲線可以看到所有氧化鈥鋅薄膜電極皆符合歐姆定律。且在摻雜之後電阻率從0.022 Ω-cm上升到0.221 Ω-cm，表示摻雜和產生的缺陷會增加電阻率。|
In this paper, holmium-doped zinc oxide (Ho:ZnO) thin films are grown by pulsed-laser deposition on c-oriented sapphire substrates with Ho concentration ranging from 0 to 10 atomic percent. During deposition, the oxygen partial pressure is 3×10-1 mbar, the substrate temperature is 525 °C, the laser wavelength and energy fluence are 266 nm and 2.0 J/cm2, respectively. The structural, luminescent, magnetic, and magneto-optical properties as well as electrical resistivity were investigated. X-ray diffraction and Raman scattering spectra show Ho incorporation into ZnO without any secondary phase. The c lattice constant and grain size becomes smaller as Ho dopant concentration increases, which is attributed to defect formation during growth. Moreover, luminescence of Ho 4f 5S2→5I8 and 5F4→5I8 transition are also observed for thin films of 5 and 8 at. % of Ho doping in Raman scattering spectra. In Photoluminescence (PL) spectra, ZnO shows strong near-band-edge (NBE) emission at both T = 5 K and T = 300 K. As Ho content increases, NBE peaks becomes weaker which defect emission peaks becomes stronger. Oxygen vacancy, zinc vacancy and zinc interstitial are identified from PL spectra of Ho-doped thin films. Magnetization loops measured by superconducting quantum interference device at T = 5 and 300 K reveal only paramagnetism from all Ho:ZnO thin films. Other than strong oxygen desorption characteristic between 40 and 60 K, temperature dependence of magnetization of Ho:ZnO thin films also not show any magnetic ordering. Magneto-optical Faraday effect at room temperature exhibit paramagnetic behavior for all Ho:ZnO thin films for wavelength between 380 and 700 nm. Verdet constant of Ho:ZnO thin films decrease with increasing wavelength. III Current-voltage curves show that all electrode on Ho:ZnO thin films are ohmic contact. The resistivity does not change with current, but increases after Ho cncoporation. This increase in resistivity is attributed to defects in Ho:ZnO thin films.
|Appears in Collections:||學位論文|
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