本論文，將使用氫化物氣相磊晶(HVPE)和有機氣相磊晶(MOCVD)兩種磊晶成長技術，作為閃鋅礦(Zinc-blende)結構和纖鋅礦(wurtzite)結構氮化鎵(GaN)磊晶薄膜的特性探討。在閃鋅礦(Zinc-blende)結構的探討方面，主要是在2o切面(miscut)的(001)GaAs基板上先成長一層低溫的氮化鎵(GaN)，再成長立方晶格的GaN磊晶薄膜。實驗結果發現結晶凝聚長度(crystalline coherence length)是30nm以及X-光束搖擺曲線(X-ray rocking curve)的半高寬(FWHM)是4.8o。這結果同時也由螢光激發光譜(photoluminescen)所確認，由螢光激發量測出受激發峰的強度是在388nm以及一個黃色螢光激發峰約在500nm。此外，在纖鋅礦(wurtzite)結構的研究是針對在使用不同類型的反應器來探討磊晶成長機制以及開發新的緩衝結構來減低GaN磊晶薄膜的缺陷密度(defect density)。最後，我們也成功的獲得高效率的發光元件，此高效率的發光元件是由同質接面的發光二極體(homo-junction LEDs)成長在含有多緩衝層(multiple-pair buffer layer)的藍寶石(sapphire)基板上。
在纖鋅礦(wurtzite)結構的磊晶成長，反應器的型式將是一個關鍵技術。實驗中我們發現，具備分流設計的水平式反應器比傳統的水平式的反應器能夠獲得更好的GaN磊晶薄膜品質。此外，我們也設計三種不同類型的蓋板以改變晶片支撐器(susceptor)和蓋板之間的氣體流動間距，同時也比較這三種不同的流動間距對不同H2流速的影響。這個不同的間距將影響反應物氣體的流動模式，更影響了GaN磊晶薄膜的品質。我們也發現只有在H2流量為5000 cm3/min以及在流動間距為10mm和18mm的條件下才能獲得鏡面的GaN磊晶薄膜。

除此以外，新的成長技術也更進一步的發展以減少GaN磊晶薄膜的缺陷(defect)和差排(dislocation)。因此產生了一些新的緩衝層成長方法來提升GaN磊晶薄膜的品質。其中的一個緩衝層結構是，在藍寶石基板上先成長一層6μm的GaN磊晶層、再成長一低溫的GaN成核層、最後再成長一層4μm的GaN磊晶層。這個緩衝層結構在成長不同成核層厚度時有較廣寬的成長區間；另一個緩衝層結構是，在低溫(525℃)成長300A厚的GaN成核層以及在高溫(1000℃)成長1~4μm厚的GaN磊晶層所組成。獲得最好品質的條件是成長四對4μm厚的GaN緩衝層。

在元件的製作方面，主要是顯示同質接面的發光二極體(homo-junction LEDs)成長在含有多緩衝層(multiple-pair buffer layer)的藍寶石(sapphire)基板上。具有三對緩衝層的發光二極體比傳統同質接面的發光二極體表現出較低的驅動電壓、更強的電激光(EL)強度和更高的光輸出功率，這主要是由於產生更多有效的電子與電洞再結合所導致。在電流-電壓(I-V)及光-電流(L-I)特性的低偏壓區域也一致的出現擴散復合電流的行為。我們將推論這個行為是由於磊晶薄膜中差排密度和雜質透過差排進入載子接合面的減少而使得含有三對緩衝層的發光二極體的光輸出功率更能提升。因此，在發光元件架構中加入多緩衝層(MBL)的結構將會是提高發光二極體的光效率及雷射二極體壽命的有效方法。

This dissertation uses two kinds of growth technology including hydride vapor phase epitaxy (HVPE) and metalorganic chemical vapor deposition (MOCVD) to investigate the zinc-blende and wurtzite GaN epitaxial films, respectively. To study zinc-blende, cubic epitaxial films are grown on a 2°miscut GaAs(001) substrate with a low temperature GaN buffer layer before growing the epitaxial film. Experimental results indicate that the crystalline coherence length is 30nm and the rocking curve width is 4.8°. Photoluminescence measurements confirm that a cubic GaN edge emission peak appears at 388nm, as well as a strong yellow emission in the 500nm region. Additionally, the study of wurtzite structure focuses on the investigation of growth mechanism with different reactor types and the novel buffer structure to reduce the defects of GaN epitaxial films. Finally, high-performance homo-junction light emitting diodes (LEDs) were successfully grown on the low-defect substrate consisting of multiple-pair buffer layer.
Reactor type is a crucial point in the growth of wurtzite GaN epitaxial films. According to our results, the separate-flow MOCVD horizontal reactor exhibits a better GaN-film quality than the conventional horizontal reactor. Additionally, three types of ceiling were designed to change the gas flow spacing between the susceptor and ceiling, and the three types of flow spacing are compared on different velocities of the upper stream H2 (FH2, up). This spacing affects the reactant gas flow pattern near the substrate surface and thus influences the quality of the epitaxial layers. It is also found that a mirror-like surface can only be grown for both 10 and 18mm spacing designs, and only if the H2 flow rate is adjusted to around 5000cc/min.

Furthermore, new growth techniques are developed to further reduce the defects and dislocations embedded in the grown GaN epitaxial films. Some novel approaches are available for the GaN buffer layer to improve the quality of GaN epitaxial layer. One structure consists of a GaN nucleation layer / 6μm GaN-bulk layer on the sapphire substrate. This buffer structure has a wide growth window for different nucleation-layer thicknesses. Another structure is multiple-pair buffer layer (MBL), which consists of a 300A thick GaN nucleation layer grown at a low temperature of 525℃ and a 1-4μm thick GaN epitaxial layer grown at a high temperature of 1000℃. The condition optimizing the quality of GaN epitaxial layers is to grow the four-pair buffer layer with a pair thickness of 4μm on the sapphire substrate.

In the device fabrication the homo-junction blue LEDs with MBL on sapphire substrates were demonstrated. The LEDs with three-pair buffer layer exhibit a lower turn-on voltage, a stronger EL intensity, and a higher light output due to more radiative recombinations occurring in the junction. A consistent behavior also appears in both the I-V and L-I, in which the diffusion recombination current can be observed in the low-bias regime. It is inferred that the reduction of dislocations in the epitaxial layers will result in the reduction of impurity diffusion into the junction via these dislocations. Therefore, the light output power of the LED with three-pair buffer layer will be improved by reducing impurities. The growth of MBL in the LED structure is a feasible means to fabricate high-performance LEDs and laser devices.

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