利用有機金屬化學氣相沉積法及漸變的氮化鋁鎵緩衝層於圖案化矽(111)基板上成長氮化鎵磊晶薄膜之研究

Abstract

由於氮化鎵(GaN)展示許多優異的物理特性，例如：寬能隙、高的崩潰電場與峰值電子漂移速度等，適合應用在高耐壓、高電流的電子元件。近年來利用有機金屬化學氣相沉積法對氮化鎵成長的研究多用碳化矽(SiC)，氧化鋁(藍寶石，Sapphire)做為基板，然而氧化鋁基板因散熱不良而使元件無法在高電壓與高溫下操作，碳化矽材料特性優越卻價格昂貴，所以使用矽基板一直是被期待的。但氮化鎵與矽基板之間熱膨脹係數差異過大，造成相當大的拉伸應力，導致氮化鎵降溫到室溫的過程中，產生相當多的裂痕。 在此研究中，我們使用300x300 μm2圖形化的矽基材及漸變的氮化鋁鎵(AlGaN)緩衝層結構，成功地成長厚度超過2 μm且無裂痕的氮化鎵薄膜。研究中比較了不同漸變的氮化鋁鎵緩衝層厚度與結構，隨著氮化鋁鎵緩衝層厚度增加，顯著地改善氮化鎵的光學、結構與表面形貌的特性。另一方面，藉由緩衝層的結構改變，使得圖形中央表面處應力值由1.09 GPa減少至0.749 GPa，有效地減少31%的應力值。此外，我們在4吋矽基板上使用不同圖形化尺寸與改變圖形化角度等方法並搭配漸變的氮化鋁鎵緩衝層結構，成長1 μm且無裂痕的氮化鎵薄膜，成功地將圖形化尺寸提升至1000x1000 μm2。研究中比較0°、30°、45°及60°等圖形化角度，由XRD GaN(002)及(102)面的半高寬量測結果發現，使用圖形化45°的矽基材成長氮化鎵薄膜可以得到較佳的結構特性。相較於平坦的矽基板，圖形化的矽基材藉由邊緣面的產生，有效降低氮化鎵與矽基板之間的拉伸應力，讓氮化鎵薄膜中的部分應力得到釋放，使得成長較厚且無裂痕的氮化鎵薄膜變的容易。

GaN shows a lot of excellent physical characteristics, such as wide band gap, high breakdown field and high saturated drift velocity, which are suitable for high voltage and high power electronic device applications. In the past, the research on GaN growth by metal-organic chemical vapor deposition (MOCVD) was mostly on sapphire and SiC substrates. However, sapphire has poor thermal conductivity which limited device applications at high temperature and high power. The high price of SiC is not suitable for general consumer applications. In contrast with sapphire and SiC, silicon is an attractive alternative substrate for GaN growth due to good thermal conductivity and low price. However, the thermal mismatch between GaN and Si causes large tensile stress in GaN layers when cooling from high growth temperature to room temperature. The tensile stress leads to the formation of cracks in GaN layers. In this study, we have successfully grown over 2 μm crack-free GaN with graded AlGaN buffer layers on 300x300 μm2 patterned Si (111) substrate. The different thicknesses and structures of graded AlGaN buffer layers are compared. Increasing the thickness of graded AlGaN has pronounced effect on the improvement of the optical characteristics, structure perfection and surface morphology of GaN layers. We can obtain 31% reduction of tensile stress from 1.09 GPa to 0.749 GPa for GaN substrate with different graded AlGaN buffer layers. In addition, we have grown 1 μm crack-free GaN with graded AlGaN buffer layers on multi-degree of pattern orientations and multi-window size patterned 4 inch Si (111) substrate. We have successfully increased the crack-free window size of patterned Si (111) substrate from 300x300 μm2 to 1000x1000 μm2. From the XRD FWHMs of GaN(002) and GaN(102), the 45 degree pattern orientation is best for GaN growth. In contrast to the planar Si substrate, patterned Si substrate becomes the crucial key to grow thicker and crack-free GaN layers.