氧化鎘鋅/氧化鋅量子井生長、特性分析及發光二極體應用

Growth, Characterization, and Light-emitting Diode Application of CdZnO/ZnO Quantum Wells

丁紹瀅(Shao-Ying Ting)

指導教授 ： 楊志忠

國立臺灣大學/電機資訊學院/光電工程學研究所/博士(2012年)

https://doi.org/10.6342/NTU.2012.02111

摘要

在本論文中，我們首先報告使用電漿輔助式分子束磊晶技術分別於氮化鎵與氧化鋅模板上成長氧化鎘鋅/氧化鋅之量子井結構，並且變化長晶條件以探討他們特性之不同。接著我們使用變溫與變功率光激發方式來量測各樣品的發光特性，另外藉由高解析度穿透式電子顯微術與X光繞射量測來觀測他們的晶體特性。我們發現相較於氮化鎵模板，氧化鋅模板上鎘原子的附著能力較差，使得當我們在氧化鋅模板上成長氧化鎘鋅/氧化鋅量子井結構時，需要減少氧氣流量以達到理想化學分配比。另外也發現在氧化鎘鋅量子井層中，當總體的鎘濃度較高時，除了烏采結構氧化鎘鋅之外，也存在著岩鹽結構的氧化鎘鋅。在光激發光量測時，發現當鎘濃度較高時，岩鹽結構之氧化鎘鋅會比烏采結構之氧化鎘鋅有更高的放光強度。並且無論是成長在氮化鎵模板上，或者是氧化鋅模板上，隨鎘濃度增加，量子井發光效率會先增加後再降低。我們推論：在較低鎘濃度的樣品內，其發光效率較低可歸因於較弱的量子侷限效應，而在高鎘濃度樣品內，發光效率比較低則是因為在量子井結構內晶體品質較差。我們也發現在氮化鎵模板上成長的量子井其放光效率有漸漸比在氧化鋅模板上成長者高。無論是成長在氮化鎵模板上，或者是在氧化鋅模板上，量子侷限史塔克效應則是隨著鎘濃度升高而有變強的趨勢。
此外，我們於p型氮化鎵模板上成長氧化鎘鋅/n型氧化鋅多層量子井發光二極體結構，其中量子井結構與重摻雜n型氧化鋅層是使用分子束磊晶技術成長，而p型氮化鎵則是使用有機金屬化學氣相沉積法成長。我們觀測到在發光二極體結構表面有些V型凹陷產生，這顯示在其底下有貫穿式差排存在，為了降低這個漏電流通道，我們使用二氧化矽奈米顆粒將這些凹陷填滿，並製作側向電流結構之發光二極體。量測結果發現元件開啟電壓為4伏特，元件電阻為224歐姆。然而由於氧化鎘鋅/氧化鋅量子井內載子侷域效應較微弱，在低注入電流時，缺陷放光相當強，並且主導了整個發光二極體的光輸出強度。另外我們也發現此發光二極體在順向偏壓下的光輸出頻譜比光激發光頻譜有藍移現象，而且在較高注入電流下有大幅度藍移的現象，這些特性與一般氮化鎵發光二極體的特性有相當大的差異。
另外，我們也報告垂直式氧化鋅/氧化鎘鋅量子井發光二極體的製作過程與特性量測結果，其中量子井結構與重摻雜n型氧化鋅層為使用分子束磊晶技術成長，而p型氮化鎵則是使用有機金屬化學氣相沉積技術成長。我們拿它與側向發光二極體來做比較，發現垂直式發光二極體有較低的元件電阻、隨著電流增加有更大幅度的頻譜藍移、更小的漏電流、較弱的輸出強度飽和以及相對低的缺陷放光。

關鍵字

氧化鋅 ； 氧化鎘鋅 ； 量子井 ； 發光二極體

並列摘要

In this dissertation, we first demonstrate the growths of CdZnO/ZnO quantum well (QW) samples on GaN and ZnO templates under different growth conditions with the plasma-assisted molecular beam epitaxy (PA-MBE). Temperature-dependent and excitation-power-dependent photoluminescence (PL) measurements are undertaken for illustrating their different emission characteristics. The crystal characteristics of the QWs structures are measured by high-resolution transmission electron (TEM), X-ray diffraction (XRD). It is found that the Cd incorporation on the ZnO template is lower, when compared with that on the GaN template, such that the O2 flow rate needs to be reduced for stoichiometric CdZnO/ZnO QW growth on the ZnO template. Besides the wurtzite (wt) CdZnO structure, the rock-salt (rs) CdZnO structure exists in the CdZnO well layers when the total Cd content is high. The rs structures may dominate over the wt structures in PL intensity when the total Cd content is high. In either group of samples on the GaN and ZnO templates, the emission efficiency first increases and then decreases with increasing total Cd content. The low emission efficiencies at low (high) Cd contents are attributed to the weaker quantum confinement (poorer crystal quality) of the QWs. The emission efficiencies of the QW samples on the GaN template are generally higher than those on the ZnO template. The strength of the quantum-confined Stark effect generally increases with increasing Cd content in either group of samples on the GaN and ZnO templates.
Besides, a CdZnO/n-ZnO multiple-QW lateral light-emitting diode (LED) grown with the QWs and n+-ZnO capping layer with MBE on p-GaN, which is grown with metalorganic chemical vapor deposition (MOCVD), is fabricated and characterized. On the epitaxial surface, there exist some V-shape pits, which correspond to threading dislocations beneath. With SiO2 nanoparticles filling in the pits, the turn-on voltage and device resistance of this device are around 4 V and 224 Ω, respectively. However, because of the weak carrier localization mechanism in the ZnO-based LED, its defect emission is quite strong and dominates the LED output at low injection current levels. The blue shift of LED output spectrum in applying a forward-biased voltage and the large blue-shift range in increasing injection current show the different behaviors of such a ZnO-based LED from those of a nitride LED.
In addition, we also demonstrate the fabrication procedures and characterization results of a vertical light-emitting diode (VLED) with the CdZnO/n-ZnO QWs and n+-ZnO capping layer grown with MBE and the p-GaN layer grown with MOCVD. Its performances are compared with those of a lateral LED based on the same epitaxial structure to show the significantly lower device resistance, larger spectral blue-shift range in increasing injection current, smaller leakage current, weaker output intensity saturation, and relatively lower defect emission in the VLED.

並列關鍵字

ZnO ； CdZnO ； QW ； LED

參考文獻

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