| Summary: | 碩士 === 國立交通大學 === 工學院碩士在職專班半導體材料與製程設備組 === 98 === 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.
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