磁控濺鍍高缺陷密度之氧化鋅/氧化鋅鎂超晶格之研究

The Study of Highly Defective ZnO/Mg0.4Zn0.6O Superlattice Made by RF Magnetron Sputter

李至弘(Chih-Hung Li)

指導教授 ： 陳建彰

國立臺灣大學/工學院/應用力學研究所/碩士(2010年)

https://doi.org/10.6342/NTU.2010.01865

摘要

本論文主要利用射頻磁控濺鍍(Radio frequency magnetron sputter)在室溫下成長ZnO、Mg0.4Zn0.6O及Mg0.2Zn0.8O薄膜及Mg0.4Zn0.6O/ZnO超晶格結構於矽晶圓及石英玻璃基板上，透過二次離子質譜儀（SIMS)、掃描式電子顯微鏡(SEM)、穿透式電子顯微鏡(TEM)、穿透頻譜(Transmission spectrum)、X光繞射(XRD)及光激螢光光譜(PL)等實驗對材料性質進行量測及研究。
　　由二次離子質譜儀量測，得知Mg0.4Zn0.6O薄膜成分，與原靶材相近，無成分變化的問題。對其進行在氮氣下600℃及800℃退火後，則產生了立方結構的氧化鋅鎂析出。此析出物可由SEM之結果觀察到，也表現在XRD之圖譜上。由穿透光譜所得到原始Mg0.4Zn0.6O之光學能隙(Optical Bandgap)大約為3.95eV，而在800℃退火之後 Mg0.4Zn0.6O之光學能隙有下降之趨勢，而在長時間退火後光學能隙趨近於ZnO。並且長時間退火後造成薄膜厚度下降使穿透率大幅上升，此一效應在600℃退火下並不明顯。另外Mg0.2Zn0.8O方面，800℃長時間退火下一樣會造成整體厚度下降，但幅度較Mg0.4Zn0.6O來的小，材料經長時間退火後之光學能隙也會趨近於ZnO。
　　接著成長不同單層厚度[ZnO/Mg0.4Zn0.6O]2.5之超晶格結構量測其穿透光譜，單層厚度較薄的樣品在波長200~500nm區間，可發現兩個明顯的吸收波段，分別為ZnO及Mg0.4Zn0.6O所造成。而在退火後，吸收邊界退化成一個，計算後之光學能隙值則略高於ZnO，因有部分鎂擴散入ZnO所致。
　　而對不同厚度下的高缺陷密度的超晶格結構進行光激螢光光譜量測，觀察到量子侷限效應（Quantum confinement effect）的現象。為了改善濺鍍製程下的超晶格結構之品質，改利用原子層沉積法（Atomic layer deposition, ALD）於基板沉積10nm厚之ZnO種子層(Seeding Layer)，再於上方成長超晶格結構。由光激螢光光譜之波峰半高寬分析，得知此種子層確實減少了上方超晶格的缺陷密度並改善了上方的超晶格結構品質。

關鍵字

氧化鋅 ； 氧化鋅鎂 ； 超晶格 ； 退火 ； 光學能隙

並列摘要

We studied ZnO, Mg0.4Zn0.6O, Mg0.2Zn0.8O thin films, as well as Mg0.4Zn0.6O/ZnO superlattice deposited on silicon wafer and quartz glass substrates at room temperature using radio frequency magnetron sputtering technique. The material properties were characterized by SIMS, SEM, TEM, transmission spectrum, XRD and PL.
From the results of SIMS measurement, the content of Mg0.4Zn0.6O thin film was close to that of the target. After annealing at 600 oC and 800 oC in N2, the cubic MgZnO precipitates in Mg0.4Zn0.6O films. The cubic precipitates were observed by the SEM; in addition, the XRD patterns also revealed the diffraction peaks of cubic MgZnO precipitates. The optical bandgap of as-deposited Mg0.4Zn0.6O, calculated from the transmission spectrum, was approximately 3.95 eV. When annealed at 800 oC, the optical bandgap approached to that of ZnO after long time annealing due to the decrease of the film thickness by evaporation. This evaporation of film thickness also caused the increase of transmittance. By contrast, this increase of transmittance after long time anneal did not occur at 600 oC. As for Mg0.2Zn0.8O films annealed at 800 oC, we also observed the decrease of film thickness, but the amount of thickness reduction was not as large as that of Mg0.4Zn0.6O films. The bandgap of Mg0.2Zn0.8O also approaches to that of ZnO after long time annealing.
We then studied [ZnO/Mg0.4Zn0.6O]2.5 superlattice of various thickness per layer. The transmission spectra of samples with thinner layer thickness reveal two absorption edges between the wavelengths of 200 nm and 500 nm, which are resulted from the partial absorption of ZnO and Mg0.4Zn0.6O layers. After long time annealing treatment at 800 oC, the absorption edge degenerated to one with an optical bandgap a little higher than ZnO, caused by the diffusion of Mg into ZnO layer and evaporation of the films. On the other hand, the transmission spectra remain almost the same when the superlattice is subjected to a heating process of 600 oC, indicating the evaporation and precipitation were suppressed. The PL peaks for superlattice shifted as the thickness of superlattice layer changed, indicating the quantum confinement effect. To improve the quality of superlattice structures, we adopted a 10 nm ZnO seeding layer by atomic layer deposition prior to the deposition of the superlattice structure by rf-sputtering. From the FWHM of PL peak, the seeding layer improved the quality of rf-sputtered superlattice structures.

並列關鍵字

ZnO ； MgZnO ； Superlattice ； anneal ； Optical bandgap

參考文獻

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