脈衝雷射蒸鍍法沉積氧化釓鋅薄膜的磁光與導電特性

Abstract

本論文使用脈衝雷射蒸鍍法在c指向的藍寶石基板上沉積摻雜的氧化釓鋅薄膜，所有薄膜樣品的沉積條件皆為525 ℃與3×10^(-1) mbar的氧氣環境。X光繞射光譜中，只顯示氧化鋅和基板的特徵峰，沒有其他晶相，由此可知釓原子成功的摻雜進氧化鋅晶格中。隨著釓元素摻雜比例增加，薄膜的c軸晶格常數從無摻雜的5.20 Å下降到5.16 Å。光致螢光光譜的強度因為摻雜導致結晶品質變差而降低，所有樣品的光譜皆有近能隙發光譜線。在摻雜的樣品中，螢光光譜則可以看到鋅空缺、鋅間隙、氧間隙與中性和帶二價的氧空缺的發光峰。磁性相關的量測有室溫磁特性曲線與磁光法拉第效應。磁性檢測顯示，所有氧化釓鋅薄膜在平行磁場與垂直磁場方向皆為順磁性，並且在外加磁場約為1500 Oe時達到飽和。每一片樣品在兩個方向下的飽和磁化量幾乎相同，其量值介於0.80 emu/cm^3 〜2.84 emu/cm^3。磁光法拉第效應顯示，所有氧化釓鋅薄膜的皆在略大於氧化鋅能隙(368 nm)的近紫外光波段有較大的磁光法拉第響應，因此我們推測磁光響應在對應材料能隙的波段直接相關，Verdet constant的最大值介於56.28 rad/(T‧cm)〜362.05 rad/(T‧cm)。導電特性方面，電流-電壓特性曲線顯示所有樣品的電極與薄膜皆為歐姆接觸。利用范德堡量測法得到的氧化釓鋅薄膜電阻率介於11〜309.74 mΩ⋅cm，因為摻雜造成晶粒尺寸變小、晶粒邊界增多、缺陷類型與密度增加，導致電阻率變大。由霍爾效應得知所有氧化釓鋅薄膜皆為n型半導體。因為摻雜釓離子引入的電子，使載子濃度大幅提升，但摻雜超過1%時，氧化釓鋅薄膜的晶格缺陷變成載子陷阱，因此載子濃度隨釓摻雜濃度而下降，並在摻雜10%的釓時維持在略高於純氧化鋅的值，其值介於5.17 〜32.02×10^18 cm^(-3)。載子遷移率也從純氧化鋅的22.77 cm^2/V⋅s 大幅下降至4〜8 cm^2/V⋅s。In this study, Gd-doped ZnO(Gd:ZnO) thin films were grown by pulsed laser deposition on c-oriented sapphire substrates in an oxygen partial pressure of 3×10^(-1) mbar and at a substrate temperature of 525 ℃. From X-ray diffraction patterns, only substrate and ZnO characteristic peaks were observed, meaning that Gd ions were successfully substituted into ZnO matrix. As the Gd concentration increases, c-lattice constants decrease from 5.20 Å to 5.16 Å. The intensity of photoluminescence (PL) decreases due to doping-induced defects and decrease in crystal quality. All samples have near-band edge emission line and emission lines due to zinc vacancies, oxygen interstitials, and doubly ionized oxygen vacancies were observed in PL spectra ofthe Gd-doped thin films.For magnetic measurements, room temperature magnetization curves and magneto-optical Faraday effect were investigated. Room temperature magnetization curves in both parallel (IP) and perpendicular (OP) magnetic fields show that all thin films are paramagnetic without hysteresis. Magnetization of all thin films is saturated at about 1500 Oe and has a saturation magnetization about 0.80 〜2.84 emu/cm^3 in both IP and OP. Magneto-optical Faraday effect (MOFE) was measured at the external magnetic field of 0.9 T. All thin films have relatively strong MOFE at the wavelength corresponding to slightly larger than zinc oxide band gap (366 nm). The maximum Verdet constants of all thin films are in the range of 56.28〜362.05 rad/(T‧cm).The I-V curves show that all electrodes are ohmic contacts. The resistivity, carrier density, and carrier mobility were investigated by van der Pauw method and the Hall effect at room temperature. The resistivity of Gd:ZnO films ranges from 11 to 309.75 mΩ⋅cm, and the increase is due to the increase in defect types and density. Owing to doping introducing electrons by Gd-doping, the electron concentration increased. However, when Gd doping exceeds 1%, the lattice disorder and defects become carrier traps. Therefore, the carrier concentration decreases with Gd content and remains at a value slightly higher than that of pure zinc oxide for 10% Gd doping. The electron density ranges from 6.87×10^18 to 32.02×10^18 cm^(-3). The carrier mobility of Gd:ZnO also drops significantly from 22.77 cm^2/V⋅s of pure zinc oxide 4 to 8 cm^2/V⋅s.