使用有機金屬氣相沉積及分子束磊晶技術成長三五族氮化物半導體

Growths of III-V Nitride Semiconductors with the Techniques of Metalorganic Vapor Phase Deposition and Molecular Beam Epitaxy

黃政傑(Jeng-Jie Huang)

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(2011年)

https://doi.org/10.6342/NTU.2011.02558

摘要

在本論文中，我們首先報告使用有機金屬化學氣相沉積技術在r-面(1-102)藍寶石基板上成長無孔洞的a-面(11-20)氮化鎵薄膜的結果。為了改善a-面氮化鎵薄膜的晶體品質，我們在長晶的過程利用流量調變的技術。藉由流量調變技術，X光繞射量測結果顯示其搖幌曲線的半高全寬可以明顯的降低。另外經由原子力顯微鏡或薄膜厚度輪廓測度儀的量測結果可以得知樣品表面形態的粗糙度也可以降低。這裡所提到的流量調變技術指的是在成長氮化鎵薄膜時交替的開啟及關閉鎵原子源，然而整個成長過程並不改變氮原子的流量。在這樣的長晶技術下，我們發現最佳化的流量調變週期為分別開啟及關閉十秒鐘的三甲基鎵氣流。高鎵環境成長是達到無孔洞a-面氮化鎵薄膜的主要條件，在如此高鎵環鏡成長條件下，關閉三甲基鎵流量時候時仍然持續供給的氮原子可以使得化學計量達成鎵氮比為1:1條件生長，而且鎵原子也可以在這段期間藉由遷移而達到較平的表面。因此，a-面氮化鎵樣品的晶體品質可以明顯改善。
另外，除了使用流量調變技術之外，我們也結合了側向磊晶再生長的技術。然而，不管有沒有使用側向磊晶再生長的技術，我們都可以發現使用流量調變方式成長出來的a-面氮化鎵樣品的c-或m-mosaic條件都顯著的改善。使用側向磊晶再生長技術，若又搭配使用流量調變的技術，則在10 x 10微米平方範圍內的表面粗糙度可以由1.58奈米降低至0.647奈米。藉由光激發螢光量測結果，我們可以斷定使用流量調變技術成長的a-面氮化鎵有較佳的光學特性。另外，使用流量調變技術成長的a-面氮化鎵樣品也有較好的拉伸應變釋放。
除此之外，我們在n-型氧化鋅上成長p-型氮化鎵以製造異質接面型的氧化鋅發光二極體。為了降低後續成長p-型氮化鎵時所經歷的高溫熱劣化，p-型氮化鎵的成長是選用分子束磊晶技術而非高溫成長的有機金屬化學氣相沉積技術。我們使用p-型氮化鎵/n-型氧化鋅二極體的電流電壓量測結果來判斷p-型氮化鎵的p-型摻雜是否有效。藉由改變不同鎂蒸發源的溫度，我們發現只有當溫度大攝氏430度時，才有較好的電流整流效果。接著我們量測p-型氮化鎵/n-型氧化鋅二極體的電流激發螢光頻譜。這個非常寬的螢光頻譜包含了紫光、藍光、橙紅光、紅光及近紅外光等。然而，我們並沒觀測到預期中的近能隙邊緣紫外光頻譜。究其原因乃為p-型氮化鎵材料之高能隙尾態吸收，只有能量較小的光子可以透過此p-型氮化鎵層而被偵測。

關鍵字

氮化鎵 ； 流量調變 ； 側向磊晶再光長 ； 氧化鋅 ； 發光二極體 ； 電流激發螢光頻譜

並列摘要

In this dissertation, first pit-free a-plane GaN (11-20) growth on r-plane sapphire (1-102) substrate with metalorganic chemical vapor deposition (MOCVD) is reported. We use the flow-rate modulation epitaxy (FME) technique to improve the crystal quality of an a-plane GaN film. With the FME technique, the width of the rocking curve in X-ray diffraction measurement is significantly reduced. Also, the surface roughness based on either atomic-force-microscopy scanning or a-step profiling is decreased. Here, the FME technique means to alternatively turn on and off the supply of Ga atoms while N atoms are continuously supplied without changing the flow rate. Under the used growth conditions, the optimized FME parameters include the on/off period at 10/10 sec. During the period of closing the flow of TMGa, the continuous supply of nitrogen can lead to the formation of stoichiometry structure under the high-Ga growth condition, which is required for the growth pit-free morphology. Also, during this period, Ga atoms can further migrate to result in a flatter surface. Therefore, the crystal quality of the a-plane GaN sample can be improved.
Besides, we study the crystal quality of a-plane GaN grown on r-plane sapphire substrate based on the FME technique combined with epitaxial lateral overgrowth (ELOG). With or without epitaxial lateral overgrowth (ELOG), either c- or m-mosaic condition is significantly improved in the samples of using FME. With ELOG, the surface roughness can be reduced from 1.58 to 0.647 nm in an area of 10 x 10 square microns by using the FME technique. Based on the results of photoluminescence measurement, one can also conclude the better optical property of the FME-grown a-plane GaN thin films. Besides, it is shown that tensile strain is more relaxed in the FME samples.
In addition, we grow p-GaN layer on n-ZnO templates to fabricate a heterojunction ZnO-based LED. To prevent the thermal annealing effects of high temperature growth of the top p-GaN layer, the p-GaN layer is grown with molecular beam epitaxy (MBE) instead of high temperature MOCVD growth. The current-voltage (I-V) curves of p-GaN/n-GaN diodes are used as the indicators for the successful p-type doping of a p-GaN layer. It is found that only when the Mg effusion cell temperature is as high as 430 oC, we can obtain good current rectifying results. The electroluminescence (EL) characterization of such a p-GaN/n-ZnO diode shows broad band luminance, covering violet, blue, orange-red, red and near infra-red peaks. However, the expected near band edge ultra-violet (UV) luminance of the ZnO or GaN films cannot be observed. This is due to the high band-tail absorption of the top p-GaN layer. Only those photons with lower energy can pass through the p-GaN layer and be observed.

並列關鍵字

GaN ； FME ； ELOG ； ZnO ； LED ； EL

參考文獻

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