脈衝雷射蒸鍍法製備氧化銪鋅薄膜之探討: 結構、光學、電性與磁性研究

Abstract

本論文利用脈衝雷射沉積法在c方向的單晶藍寶石基板上沉積氧化銪鋅(Eu:ZnO)薄膜，摻雜比例為0－4.0 at.%，薄膜厚度控制在150 nm，之後檢測薄膜樣品的結構、光學、電性以及磁性。X光繞射光譜中在角度2θ = 31°－45°，我們只觀測到ZnO以及基板的特徵峰，確認了Eu成功摻雜進ZnO且沒有雜晶相。c軸常數隨摻雜比例提升從在5.21 Å降至5.18 Å，推測與電荷補償機制有關(2Eu3+ → 3Zn2+ + VZn)；晶粒尺寸與摻雜比例沒有明顯趨勢，晶粒尺寸在163-183 Å。光致螢光光譜的結果顯示Eu摻雜使得整體螢光強度下降、近能隙3.3 eV峰值訊號變寬，經分析後可推斷Eu:ZnO薄膜具有鋅間隙、鋅空缺、氧空缺、氧間隙等等的缺陷，隨著摻雜比例上升，缺陷和Eu 4f－4f軌域躍遷5D0－7F2逐漸主導螢光，當摻雜比例達到4.0 %時，可以觀察到5D0－7F1、5D0－7F0的螢光。在電性分析中，薄膜載子遷移率隨著摻雜比例上升從23 cm2/Vs降至0.1－1.0 cm2/Vs，推測與摻雜所產生的缺陷有關，缺陷變得更多、應力變大，形成更多的晶粒邊界使得電子容易被散射。在0、0.5、1.0 %摻雜比例的薄膜樣品，電阻率約在0.1 Ω⋅cm，載子濃度在1.0 %的薄膜樣品達到最大值32×10^18 cm-3，意味著少量摻雜可以改變、甚至促進薄膜電性，而當摻雜比例超過2.0 %時，電阻率急遽上升至1－10 Ω⋅cm、載子濃度則在3－5×10^18 cm-3。我們從磁光法拉第光譜觀察到薄膜樣品的法拉第旋轉角與外加磁場呈線性關係，並且薄膜的法拉第旋轉強度在近能隙的波長(340、350 nm)會有較強的響應，並且有Eu摻雜的樣品比無摻雜樣品具有更強的旋轉角強度，歸因於Eu摻雜帶來額外電子，達到增幅磁光效應的效果。磁性分析中，所有薄膜樣品在室溫下呈現順磁性，樣品磁矩在外加磁場達到1500 Oe後皆達到飽和。飽和磁化強度隨著摻雜比例上升有趨近飽和的趨勢，從7 emu/cm-3增加到13.7 emu/cm-3。

In this thesis, we utilized pulsed-laser deposition (PLD) to deposit europium-doped zinc oxide (Eu:ZnO) thin films on c-oriented sapphire substrate with Eu concentration ranging from 0 to 4.0 at.%. Thin film thickness had been controlled around 150 nm, and the structural, optical, electrical and magnetic properties of Eu:ZnO thin film samples were investigated.X-ray diffraction spectra showed only ZnO(002) and substrate characteristics without any secondary phases within scan range 2θ = 31°－45°, indicating successful incorporation of Eu ions into ZnO. The c-axis lattice constant dropped from 5.21 Å to 5.18 Å with increasing dopant concentration, which might be related to charge compensation mechanism (2Eu3+ → 3Zn2+ + VZn). The crystallite size was around 163－183 Å and showed no obvious trend with dopant concentration.Photoluminescence (PL) spectra showed that Eu incorporation caused decreased overall PL intensity, and near band edge (NBE) luminescence around 3.3 eV became weaker and wider. Defects such as zinc interstitial, zinc vacancy, oxygen vacancy and oxygen interstitial had been detected. With increasing Eu dopant concentration, PL from defects and Eu 4f-intraband transition 5D0－7F2 gradually dominated PL spectra. When the dopant concentration went up to 4.0 %, 5D0－7F1, 5D0－7F0 had also been detected.In the analysis of electrical properties, with increasing dopant concentration, carrier mobility decreased from 23 cm2/Vs to 0.1－1.0 cm2/Vs. This might bedue to more electrons scattering at increased grain boundary. The resistivity was around 0.1 Ω⋅cm in 0, 0.5, 1.0 % thin samples, and the carrier density reached maximum 32×10^18 cm-3 at 1.0 % concentration. This meant that dilute doping was able to change, even improve, electrical properties of the thin film. When dopant concentration went beyond 2.0 %, the resistivity rapidly increased to 1－10 Ω⋅cm, and the carrier density was around 3－5×10^18 cm-3.In the analysis of magneto-optical Faraday effect, for all thin film samples, Faraday rotation angle showed linear relation with external magnetic field, and the rotation strength became much stronger at wavelengths around 340－350 nm.Eu-doped samples showed stronger rotation response than those of un-doped sample, this effect could be attributed to additional electrons brought by Eu incorporation, thus amplifying magneto-optical effect.In the analysis of magnetism, all thin film samples showed paramagnetism at room temperature, and sample magnetic moments saturated at the magnetic field of 1500 Oe. The saturated magnetization showed asymptotic trend with dopant concentration, increased from 7 emu/cm3 to 13.7 emu/cm3.