以有機金屬化學氣相磊晶成長之 氮化鋁鎵薄膜其光性與電性研究

Optical and Electronic Investigation of AlGaN Thin Films Grown by Metal Organic Chemical Vapor Deposition System

楊正弘(Cheng-Hung Yang)

指導教授 ： 廖森茂 ； 程亞桐 ； 林文仁

中原大學/工學院/電子工程研究所/碩士(2002年)

https://doi.org/10.6840/cycu200200161

摘要

利用有機金屬化學氣相沉積方法(MOCVD)調變不同的[TMGa/(TMGa+TMAl)]及[TMAl/(TMGa+TMAl)]流量比率，[TMAl/(TMGa+TMAl)]流量速率與增加1000 sccm NH3流量，和改變不同NH3流量比率成功地成長不同鋁含量的AlxGa1-xN/GaN薄膜。並藉由X光繞射(XRD)，顯微拉曼散射光譜儀和光激發螢光光譜(PL)描述氮化鋁鎵(AlGaN)薄膜成長在藍寶石基板上的光學性質。隨後利用I-V和C-V方法研究其相關之電特性。
實驗資料顯示，當TMGa的流量增加導致鋁組成含量隨之減少，相對地增加TMAl的流量而使鋁組成含量增加。由X光繞射光譜可以證實這些薄膜具高磊晶品質與相關鋁的含量。此論文焦點放在顯微拉曼光譜在材料特性上的應用與wurtzite (WZ)結構的聲子動力學。以純實驗為立論基礎的時候；一個模態行為的證據建立於這個有爭議的混合系統。在章節編址中結構好壞的決定，微細構造確認，應力與張力在薄膜基板界面。
使用光學顯微鏡可以看出一種網狀的龜裂。當氮化鋁鎵的磊晶層厚度超出臨界值時，龜裂可以被觀察到在靠近氮化鋁鎵與氮化鎵的介面處。在鋁含量x < 2的範圍中，除了微龜裂的樣品外在能障與鋁含量之間有著線性的關係。藉由Schottky定律得知這是與理想因子一致的。並確認這是由於晶格缺陷在AlGaN/GaN界面所造成的。
結果顯示鋁之氮化鋁鎵合併效率比鎵高，在成長氮化鋁鎵薄膜針對均勻之鋁含量是有益的。在TMAl與NH3之間的反應比TMGa和NH3之間的反應更加劇烈。
隨著不同的NH3流量比率成長氮化鋁鎵，x值從0到22.5%可分別決定其材料的特性。結果顯示，氮化鋁鎵中當NH3氣體流量比率增加，鋁的含量x也隨著增加，隨著鋁含量的增加X光繞射的半波高寬亦隨之增加。從顯微拉曼量測中，E2 mode 先向藍光位移再向紅光位移。增加鋁的組成導致更多的龜裂，結果降低了E2 mode的強度。由X-ray的量測，證明龜裂的產生與氮空缺和晶格缺陷增加有關。霍爾量測顯示出隨著NH3流量的增加使電子遷移率減小而載子濃度增加。當氮化鋁鎵薄膜超出它的臨界層厚度範圍時，鋁含量的增加亦導致微裂縫的現象。
整體言之，本論文針對一系列氮化鋁鎵薄膜做光性、電性及物性的研究，可提昇其薄膜磊晶品質，這將有助於氮化鎵系列元件特性之改善。

關鍵字

X光繞射 ； 顯微拉曼 ； I-V量測 ； 氮化鋁鎵 ； 光激發螢光 ； 霍爾 ； C-V量測 ； 有機金屬化學氣相磊晶

並列摘要

By means of tunable [TMGa/(TMGa+TMAl)] and [TMAl/(TMGa+TMAl)] flow rate, [TMAl/(TMGa+TMAl)] flux ratio and increasing 1000 sccm NH3 flow, as well as different NH3 flow rate the AlxGa1-xN/GaN epilayers were successfully grown on the sapphire using metal organic chemical vapor deposition (MOCVD). The optical properties of AlGaN thin films were characterized by X-ray diffraction (XRD), micro-Raman spectra and photoluminescence (PL). The electrical properties were investigated by I-V, and C-V measurement.
Experimental data indicated that the composition of Al content decreases with increasing the TMGa flow, and increases with increasing the TMAl flow. X-ray diffraction (XRD) spectra certified that the films are of high crystalline quality, and in relation to the composition of Al. This thesis focuses on the applications of micro-Raman spectroscopy to material characterizations and phonon dynamics of the wurtzite (WZ). On a purely experimental basis, this establishes an evidence of one-mode behavior for this controversial alloy system. Among the topics addressed are structure determination and microstructure identification, stress and strain in film substrate interfaces and in crystallites.
A network of cracks can be seen with an optical microscope. Cracking was observed near the interface of the AlGaN and GaN epilayer when the thickness of AlGaN layer exceeds a critical value. In the range of x < 2 a linear relationship between the barrier height and Al mole fraction was obtained except the microcracks samples. This was consistent with ideal factor predicted by the Schottky rule. It was evident that crystalline defects were fromed at the Ni/AlGaN interface.
The results reflect that the AlGaN incorporation efficiency of Al is as high as that of Ga, which is beneficial for the improvement of the Al composition uniformity in the AlGaN films. A model of the influences of parasitic on the Al composition in the AlGaN films is developed based on previous works. The parasitic reaction between TMAl and NH3 is much more severe than that between TMGa and NH3.
AlxGa1-xN alloy with different NH3 flow rate at various x from 0 to 22.5% has been investigated to determine their material properties. Our results revealed that while the AlxGa1-xN layers alloy composition x increases with increasing the NH3 gas flow rate, the full width at half-maximum (FWHM) of X-ray increases increasing aluminum (Al) content. From the micro-Raman measurement, the E2 mode shows blue shift first, then followed by a red shift. More cracks caused increasing alloy composition resulted in lower intensity of E2 mode. From X-ray measurement, the cracks arising agrees well with the nitrogen vacancy and crystal lattice defects. The Hall measurement shows that the electron mobility decreases and concentration increases with increasing gas flow of ammonia. The increment in aluminum (Al) content tends to result in the formation of microcracks when AlGaN film thickness is beyond its critical layer thickness.
As a whole, the incorporation of optical, electrical and physical properties will help us to improve the crystalline characterizations of a series of AlGaN thin films and to enhance the performance of the relevant nitride-based devices.

並列關鍵字

C-V ； AlGaN ； Hall ； PL ； MOCVD ； micro-Raman ； I-V ； X-ray

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