Study of cracking mechanism in GaN/α-Al2O3 structure

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Abstract
Imperfection (cracking, stress) in (0001)GaN/(0001)α‐Al2O3 grown by using the metalorganic chemical vapor deposition (MOCVD) or chemical vapor deposition (CVD) method is investigated. First, the epitaxial relation is confirmed by using the oscillating‐crystal method. Subsequently, imperfections, especially cracks, are observed in detail by optical microscopy and x‐ray diffraction topography. Cracking occurs at the layer thickness larger than 13 μm in the crystals having epitaxial layers of good quality, while it does not appear even at the thickness of 30 μm in crystals including inferior layers. Finally, stress in the layer and in the substrate are determined by measuring the warp of the wafer. Compressive stress in the good epitaxial layer is 1.6×109 dyne cm−2, and in this case, cracking occurs at the maximum tensile stress in the substrate less than 3×108 dyne cm−2. However, cracking does not occur even at the compressive stress of 3×109 dyne cm−2 in the inferior layer and at the tensile stress of 1×109 dyne cm−2 in the substrate. From these facts, a possible mechanism of cracking is proposed. During the epitaxial growth, microcracks occur in the substrate at the interface owing to the lattice mismatching. During the cooling, macrocracks which can be observed are grown from the already existing microcracks owing to the thermal stress. In the latter course, the cracking in the substrate must be simultaneously propagated to the epitaxial layer.

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